ISO/FDIS 8932-2

Meteorology — Radiosonde — Part 2: Laboratory test method for errors in radiosonde humidity sensor calibration

The purpose of this International Standard is to provide a test method for the calibration of radiosonde humidity sensors with varying temperature and relative humidity using laboratory setups. The main scope of this International Standard is as follows: a) Technical requirements for laboratory setups needed for the calibration of radiosonde humidity sensors. b) Test procedure for calibrating radiosonde humidity sensors with varying temperature and relative humidity. c) Uncertainty evaluation method for the calibration of radiosonde humidity sensors. To be more specific, in part (a), the requirements for an absolute humidity generator, a test cell, and a precision hygrometer as essential laboratory setups are proposed. In part (b), the test procedure includes the installation of radiosondes in the test cell, the operation of laboratory setups, and the comparison between the reference relative humidity and the relative humidity measurement by radiosondes. In part (c), the method to evaluate uncertainties related with the reference relative humidity and the radiosonde humidity sensor will be proposed.

Meteorology — Radiosonde — Partie 2: Méthode d'essai en laboratoire pour les erreurs d'étalonnage du capteur d'humidité dans la radiosonde

General Information

• Status: Not Published
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: ISO/TC 146/SC 5 - Meteorology
• Drafting Committee: ISO/TC 146/SC 5 - Meteorology
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 27-Feb-2026
• Completion Date: 27-Feb-2026

Overview

ISO/FDIS 8932-2: Meteorology - Radiosonde - Part 2: Laboratory test method for errors in radiosonde humidity sensor calibration is an international standard developed by ISO to establish a uniform laboratory test method for evaluating errors in radiosonde humidity sensor calibration. Radiosondes, balloon-borne instruments, are critical in atmospheric data collection for weather prediction and climate studies. Accurate calibration of their humidity sensors ensures data reliability and comparability across meteorological networks. This standard outlines technical requirements, test procedures, and uncertainty evaluation methods for laboratory calibration of radiosonde humidity sensors under controlled temperature and humidity conditions.

Key Topics

• Technical requirements for laboratory calibration
  The standard specifies essential laboratory setups for calibrating radiosonde humidity sensors. This includes:

  • Absolute humidity generators
  • Test cells with controlled and stable environments
  • Calibrated precision hygrometers
  • Reference thermometers and pressure gauges

• Calibration test procedures
  Procedures are defined for:

  • Preparing and installing radiosonde sensors within the test cell
  • Operating laboratory setups to achieve specific temperature and humidity conditions
  • Accurately controlling temperature (from -90°C to 35°C) and relative humidity (from 1%rh to 100%rh, noting the limitations below 0°C)
  • Comparing the radiosonde sensor output to the laboratory reference measurements

• Uncertainty evaluation
  The standard provides a method for evaluating uncertainties associated with:

  • Reference relative humidity values
  • Radiosonde humidity sensor measurements
  • Environmental control (temperature and pressure)
  • The entire measurement and calibration process, ensuring traceability to the International System of Units (SI)

Applications

• Meteorological instrument manufacturing
  Manufacturers use this standard to ensure that radiosonde humidity sensors meet global calibration requirements and that their products deliver accurate, SI-traceable measurements. Adopting the ISO/FDIS 8932-2 ensures compliance with international expectations and supports consistent quality control.

• National meteorological and research laboratories
  Laboratories apply this test method to independently verify manufacturer calibrations of radiosonde humidity sensors. This process is essential for:

  • Quality assurance in atmospheric data collection
  • Supporting research and process control in upper-air sounding operations
  • Submitting verifiable calibration reports for meteorological datasets

• Quality assurance and accreditation
  The documented procedures and uncertainty evaluation facilitate traceability, supporting accreditation to relevant ISO/IEC standards and satisfying requirements from organizations such as the World Meteorological Organization (WMO).

• Academic and meteorological research
  Researchers involved in climate and atmospheric science use this method to ensure the accuracy and comparability of radiosonde data, supporting robust scientific analysis and model validation.

Related Standards

For proper implementation and context, the following related standards should be considered:

• ISO/IEC Guide 99:2007 – International vocabulary of metrology (VIM)
• ISO/IEC Guide 98-1 & 98-3 – Guide to the expression of uncertainty in measurement (GUM)
• ISO/TR 12148:2009 – Calibration of chilled mirror instruments
• IEC 60068-3-6:2018 & IEC 60068-3-11:2007 – Environmental testing; confirmation and uncertainty of temperature/humidity chambers
• WMO No.182 (1992) – International Meteorological Vocabulary

Integrating procedures from ISO/FDIS 8932-2 with these standards ensures a comprehensive and robust approach to humidity sensor calibration in meteorological applications.

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Keywords: ISO 8932-2, radiosonde humidity calibration, meteorological standards, laboratory test method, uncertainty evaluation, precision hygrometer, humidity generator, atmospheric data quality, SI traceability, metrology, sensor calibration.

Standards Content (Sample)

DRAFT
International
Standard
ISO/DIS 8932-2
ISO/TC 146/SC 5
Meteorology — Radiosonde —
Secretariat: DIN
Part 2:
Voting begins on:
Laboratory test method for errors 2025-03-26
in radiosonde humidity sensor
Voting terminates on:
2025-06-18
calibration
ICS: 07.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
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STANDARDS MAY ON OCCASION HAVE TO
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Reference number
ISO/DIS 8932-2:2025(en)
DRAFT
ISO/DIS 8932-2:2025(en)
International
Standard
ISO/DIS 8932-2
ISO/TC 146/SC 5
Meteorology — Radiosonde —
Secretariat: DIN
Part 2:
Voting begins on:
Laboratory test method for errors
in radiosonde humidity sensor
Voting terminates on:
calibration
ICS: 07.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 8932-2:2025(en)
ii
ISO/DIS 8932-2:2025(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols . 4
4.1 Symbols .4
4.2 Subscript .4
5 Technical requirements for the laboratory setup . 5
5.1 General .5
5.1.1 Temperature .5
5.1.2 Pressure .5
5.2 Precision hygrometer .6
5.2.1 Type .6
5.2.2 Installation .6
5.2.3 Operation .7
5.3 Humidity generator .8
5.3.1 Type .8
5.3.2 Installation .9
5.3.3 Operation .9
6 Test preparation . 9
6.1 Laboratory environmental conditions .9
6.2 Preparation of the radiosonde .9
6.3 Examination of the laboratory setup . .10
6.4 Installation of the radiosonde .10
7 Test methods and procedures . 10
7.1 Operation .10
7.1.1 Purging the test cell .10
7.1.2 Temperature control of the test cell .10
7.1.3 Humidity from the humidity generator .11
7.1.4 Calculation of the reference relative humidity using the humidity generator .11
7.1.5 Calculation of the reference relative humidity using the precision hygrometer . 12
7.2 Test procedure . 12
8 Data processing .13
8.1 Calculation of the average values . 13
8.2 Calculation of the measurement error . 13
9 Evaluation of measurement uncertainty .13
9.1 Uncertainty evaluation for the reference relative humidity: u(RH ) .
ref
9.1.1 Uncertainty of the reference relative humidity . 13
9.2 Uncertainty of the radiosonde relative humidity: uRH .
()
rad
9.2.1 Uncertainty of the resolution of the radiosonde relative humidity:
uRH .
()
rad_res
9.2.2 Uncertainty of the repeatability of the radiosonde relative humidity:
uRH .
()
rad_rep
iii
ISO/DIS 8932-2:2025(en)
9.3 Calculation of the combined standard uncertainty of the measurement error:
uRH .
()
err
9.4 Calculation of expanded uncertainty .16
10 Method for reporting test results .16
Annex A (informative) Calculation of the reference relative humidity.18
Bibliography .22

iv
ISO/DIS 8932-2:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
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This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
A list of all parts in the ISO 8932 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
ISO/DIS 8932-2:2025(en)
Introduction
Temperature and water vapour (i.e., humidity) are two of the basic atmospheric variables and are
important for the initialization of numerical weather prediction and climate modelling. Radiosondes are
widely used to measure atmospheric parameters such as humidity and temperature up to an altitude of
approximately 40 km. A radiosonde is a balloon-borne instrument with several types of sensors for in situ
[1]
profile measurements. A radio transmitter is used to send these data to the observing station. Radiosonde
observations are often used in conjunction with other measurement techniques such as remote sensing using
satellites to provide comparative data. For radiosonde-derived data to be useful, the measurement accuracy
of the radio soundings needs to be known. From a metrological perspective, this measurement accuracy
should be expressed in terms of uncertainty that is traceable to the International System of Units (SI).
Previously, comparative soundings of dew-point hygrometers and radiosondes showed that radiosonde
humidity sensors have a daytime dry bias resulting from solar heating and a time-lag error at low
[2,3]
temperatures. The dry-bias and time-lag error can be corrected in sounding systems by creating proper
algorithms based on measurements of sensor temperature and response time, respectively. In addition,
polymeric thin-film humidity sensors adopted by most radiosondes are known to produce systematic errors
at the low temperature characteristic of the upper air environment if a calibration curve obtained at room
[4,5]
temperature only is employed for low temperatures. The capacitance of humidity sensors increases with
adsorbed water vapor in thin films. While the adsorption capacity of the films for water vapor increases
at lower temperatures, it does not fully compensate for the reduced water vapor pressure, leading to
temperature-dependent behaviour in thin-film humidity sensors. Therefore, calibration of these sensors
should be conducted at various temperatures. Thus, an essential prerequisite to resolve above issues and
to improve the measurement reliability of radiosondes is to calibrate the radiosonde sensors using ground-
[6,7]
based facilities .
The relative humidity generated by a laboratory setup or factory facility can be traceable to the SI via
temperature, pressure, gas flow rate, and/or mass measurements related to determining water vapor
pressure from the humidity generator itself and/or the measurement of the generated humidity using
calibrated precision hygrometers. The SI traceability of radiosonde humidity sensors should be established
through factory calibration by manufacturers. This document is mainly focused on testing the calibration
error of humidity sensors in radiosondes, which are randomly sampled from a series of products.
The Standing Committee on Measurements, Instrumentation and Traceability (SC-MINT) of the World
Meteorological Organization (WMO) urges users to test selected samples of radiosondes under laboratory
conditions to ensure that the calibrations supplied by the manufacturer are valid. Even if sensors can be
produced in large batches to meet an agreed upon set of standardized performance checks, it is necessary
[8]
for representative sensor samples, selected at random, to be checked more rigorously. This independent
testing would further improve the reliability of radiosonde measurements by verifying the calibration
results applied by manufacturers. While testing is crucial, the guide provided by SC-MINT only provides
limited requirements for the test setup. These requirements describe the need for stability better than
1 %rh and systematic errors less than ±1 %rh at the desired value. More detailed methodologies or test
procedures for the testing of radiosonde humidity sensors in a relevant range of temperatures have not been
reported.
The procedure presented in this document provides the technical requirements for the laboratory setup,
the test procedure for errors in radiosonde humidity sensor calibration at relevant temperatures using the
setup, and an evaluation method for the uncertainty of the test results.
Since calibration is valid at the time of calibration, this test may incorporate additional sources of uncertainty
related to transportation and storage of the sensors, which could introduce additional uncertainty in
the results. It's important to note that when considering uncertainty in soundings, other factors such as
radiation dry bias and time-lag should also be considered, as summarized in Table 5 of Reference [3]. While
all uncertainty terms affecting the results should be considered, the procedure in this document primarily
focus on testing a subset of the uncertainty factors outlined in Reference [3], specifically those related to
testing temperature-dependent calibration corrections. In uncertainty analysis, the uncertainty of the test
setup, reference devices, and other potential uncertainties affecting radiosonde measurements under static
conditions are included in this test.

vi
ISO/DIS 8932-2:2025(en)
The fundamental technique essential for this test involves the SI-traceable generation and/or measurement
of water vapor pressure to determine the reference relative humidity and assess its associated uncertainty.
Since humidity generators used in application of this document should be capable of producing multiple
dew/frost-point temperatures below the test temperature, the testing of sensors at several relative humidity
levels at the test temperature while avoiding condensation can be accomplished. An example for obtaining
multiple dew/frost-point values is presented using a saturator-based humidity generator. This approach is
chosen because of its traceability, and its documented validation by metrological and meteorological experts
[4-8]
in testing radiosonde humidity sensors .
[9-11]
It is worth noting that while the saturator-based technique is founded on Korean and US patents, the
patent holder has granted a license, free of charge, to an unlimited number of applicants globally, without
discrimination, and under reasonable terms and conditions. This license allows for the creation, use, and sale
of implementations based on this ISO document. However, it's important to state that this ISO document is
not solely based upon this patented technique. Alternative equivalent techniques, utilizing different types of
humidity generators and/or precision hygrometers, can also be employed for determining reference relative
humidity if they are SI-traceable and meet the specifications required under this document.

vii
DRAFT International Standard ISO/DIS 8932-2:2025(en)
Meteorology — Radiosonde —
Part 2:
Laboratory test method for errors in radiosonde humidity
sensor calibration
1 Scope
The document specifies testing procedures for determining calibration error for radiosonde humidity
sensors sampled from a mass production batches based on varying the levels of relative humidity at
atmospheric upper-air temperatures using a laboratory setup. Application of this document provides the
following:
a) the technical requirements for a laboratory setup to evaluate the calibration errors of radiosonde
humidity measurement;
b) a test procedure for evaluating calibration error of radiosonde humidity sensors for a temperature
range of −70 °C to 35 °C and for a relative humidity of 1 %rh to 100 %rh. Note, this document, is based
upon relative humidity defined by the percentage of water vapor pressure divided by saturation water
vapor pressure over liquid water, not over ice, even at temperatures below 0 °C; hence, the maximum
relative humidity is less than 100 %rh below 0 °C;
c) a method for evaluating the uncertainty for the measured radiosonde humidity calibration errors.
In this document, the requirements for a humidity generator, a test cell, a precision hygrometer, and a
thermometer as essential laboratory setups are proposed in a). In b), the test procedure including the test
preparation, the installation of radiosondes in the test cell, the operation of laboratory setups, and the
comparison between the reference relative humidity and the radiosonde relative humidity are presented.
In c), the method for evaluating uncertainties related to the reference relative humidity and the radiosonde
humidity sensor is discussed.
NOTE 1 The water vapor pressure required for the reference relative humidity can be determined from frost or
dew point temperature by either humidity generators or calibrated hygrometers as exemplified in Annex A. Frost/
dew-point is the temperature at which air becomes saturated with water vapor and condensation occurs.
NOTE 2 Due to potential limitations in the number of test setups or laboratories available for conducting this test,
peer-reviewed reports or papers published online or offline resulting from research activities conducted by academia
or meteorological institutes can be utilized as a test report when following this test procedure.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories
ISO/IEC Guide 98-1:2009, Uncertainty of measurement — Part 1: Introduction to the expression of uncertainty
in measurement
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
ISO/DIS 8932-2:2025(en)
ISO/TR 12148:2009, Natural gas — Calibration of chilled mirror type instruments for hydrocarbon dewpoint
(liquid formation)
IEC 60751:2022, Industrial platinum resistance thermometers and platinum temperature sensors
IEC 60068-3-11:2007, Environmental testing – Part 3-11: Supporting documentation and guidance –Calculation
of uncertainty of conditions in climatic test chambers
JCGM 200:2012, International vocabulary of metrology – Basic and general concepts and associated terms (VIM)
3 Terms and definitions
For this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
radiosonde
instrument intended to be carried by a balloon through the atmosphere, equipped with devices to measure
one or several meteorological variables (such as pressure, temperature, humidity), and provided with a
radio transmitter for sending this information to the observing station
Note 1 to entry: A series of battery-powered telemetry devices that are suspended high in the atmosphere using a
weather balloon, which measure atmospheric parameters, such as temperature and humidity, and transmit data to a
ground system using radio frequencies.
Note 2 to entry: To ensure the traceability of radiosondes as a product under WMO control, it is necessary to obtain
the WMO code (BUFR code 0-02-011). This WMO code must be unique for each radiosonde, and a single code must not
be used for more than one type of radiosonde.
Note 3 to entry: The current requirements for threshold, breakthrough, and goal uncertainty in upper-air humidity
[12]
measurements are specified in Observing Systems Capability Analysis and Review tool (OSCAR), which is a
resource developed by WMO.
3.2
radiosonde body
housing of a radiosonde comprising a circuit board with measurement chips and a data transmission section
with antennae and batteries
3.3
radiosonde sensor boom
boom connected to the radiosonde body to which the temperature and humidity sensors are attached
3.4
humidity generator
device that can generate the desired humidity (i.e. water vapour pressure). The water vapour pressure of
the generator output together with the controlled temperature can be converted into the reference relative
humidity in the test cell.
3.5
test cell
chamber in which radiosonde humidity sensors can be installed for calibration. The test cell should be
installed inside a space within which the temperature can be controlled (e.g. a climate chamber) to evaluate
the temperature-dependency of the measurements from the radiosonde humidity sensors.

ISO/DIS 8932-2:2025(en)
3.6
precision hygrometer
humidity measurement instrument that can accurately measure the humidity from low frost-points (−80 °C)
to high dew-points (35 °C). It is used to check the reference relative humidity in the test cell generated by
the humidity generator. The measurements from a precision hygrometer can also be used as the reference
relative humidity in the test cell if the precision hygrometer is calibrated, and correction and uncertainty
are employed. In general, chilled-mirror precision hygrometers are used.
Note 1 to entry: A thermometer and a vapor pressure equation are also needed to obtain relative humidity values.
Manufacturers may use a different vapor pressure equation for the factory calibration.
3.7
climate chamber
chamber or enclosed space where the internal temperature and humidity can be controlled within
specified limits
Note 1 to entry: The climate chamber can be used to control the temperature and, thus, the relative humidity as well
of the test cell.
3.8
liquid bath
equipment in which liquid is present in a container, with a specific volume, to maintain a constant
temperature. A liquid bath can be used to control the water vapour saturation temperature of a saturator-
based humidity generator.
3.9
dry gas generator
device that can generate dry gas (i.e. without water vapour). It can be used to lower the humidity by mixing
of its output with a well-defined humid gas in a humidity generator. It can also be used to purge the water
adsorbed onto the test cell. Therefore, the lowest possible frost-point of the dry gas generator should be
lower than the lowest frost-point used for the calibration of the radiosonde humidity sensor.
3.10
reference thermometer
instrument used to measure the reference temperature in the test cell for the calculation of the reference
relative humidity. Generally, calibrated platinum resistance thermometers (PRTs) are used.
3.11
platinum resistance thermometer (PRT) sensor
temperature-responsive device consisting of a sensing resistor within a protective sheath, internal
connecting wires and external terminals to permit connection of electrical measurement devices.
3.12
reference pressure gauge
instrument used to measure the reference pressure in the test cell for the calculation of the reference
relative humidity. Generally, calibrated capacitance diaphragm gauges are used.
3.13
measurement error
measured quantity value minus a reference quantity value
3.14
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
Note 1 to entry: Measurement uncertainty includes components arising from systematic effects, such as components
associated with corrections and the assigned quantity values of measurement standards, as well as the definitional
uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement
uncertainty components are incorporated.

ISO/DIS 8932-2:2025(en)
Note 2 to entry: The parameter may be, for example, a standard deviation called standard measurement uncertainty
(or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.
Note 3 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be evaluated
by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series
of measurements and can be characterized by standard deviations. The other components, which may be evaluated
by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from
probability density functions based on experience or other information.
Note 4 to entry: In general, for a given set of information, it is understood that the measurement uncertainty is
associated with a stated quantity value attributed to the measurand. A modification of this value results in a
modification of the associated uncertainty.
3.15
standard uncertainty
measurement uncertainty expressed as a standard deviation
3.16
coverage factor (k)
number larger than one by which a combined standard measurement uncertainty is multiplied to obtain an
expanded measurement
Note 1 to entry: Coverage factor is usually symbolized.
3.17
expanded uncertainty
product of a combined standard measurement uncertainty and a factor larger than the number one
Note 1 to entry: The factor depends upon the type of probability distribution of the output quantity in a measurement
model and on the selected coverage probability.
Note 2 to entry: The term “factor” in this definition refers to a coverage factor.
4 Symbols
4.1 Symbols
E
Saturated water vapour pressure at a given T and P
e
Water vapour pressure
f
Enhancement factor
k
Coverage factor
P
Pressure
RH
Relative humidity
T
Temperature
U
Expanded uncertainty
u
Standard uncertainty
v
Gas flow rate
4.2 Subscript
cal
Calibration
ISO/DIS 8932-2:2025(en)
DUT
Device under test
g
Generator
grad
Gradient
h
Hygrometer
s
Saturator
t
Test cell
w
Water
ref
Reference
rep
Repeatability
res
Resolution
5 Technical requirements for the laboratory setup
5.1 General
5.1.1 Temperature
The temperature of the test cell can be varied between −70 °C and 50 °C using a climate chamber or a liquid
bath. A climate chamber is more desirable as it minimizes handling of the radiosonde.
The reference temperature of the test cell should be measured inside the test cell using a reference
thermometer within calibration to calculate the reference relative humidity.
If the radiosonde humidity sensor operates using a heating element, both the reference thermometer and the
radiosonde temperature sensor could be affected by the humidity sensor heater. The temperature measured
by the reference thermometer should not be impacted by the radiosonde heater.
NOTE It is advisable to correct for the impact of the heater by increasing the air ventilation rate in the test cell to
-1
around (5~6) m∙s to facilitate the convective cooling of sensors as experienced during soundings.
The temperature stability of the test cell should be within ±0,1 °C. A change in the test cell temperature of
0,1 °C corresponds to a change in the relative humidity of roughly 0,6 %rh to 0,8 %rh in the temperature
range of –90 °C to 30 °C.
The temperature gradient around the radiosonde humidity sensor should be measured using a calibrated
thermometer to evaluate the uncertainty due to the spatial gradient of the reference relative humidity. It is
recommended that measurement points of two reference thermometers be above and below the humidity
sensor and the distance between thermometers be 10 mm to 50 mm. When testing multiple sensors,
the temperature difference between these sensors may be larger than for two sensors resulting in a
corresponding increase in uncertainty.
5.1.2 Pressure
The pressure of the test cell should be measured using a calibrated pressure gauge to calculate the saturation
water vapor pressure and the reference relative humidity in the test cell.
The pressure gradient between the test cell and the precision hygrometer should be measured using a
calibrated pressure gauge to evaluate the uncertainty when a calibrated precision hygrometer is used to
determine the reference relative humidity in the test cell instead of a humidity generator.

ISO/DIS 8932-2:2025(en)
5.2 Precision hygrometer
5.2.1 Type
A chilled-mirror hygrometer (CMH) measures the dew or frost point temperature at which a constant
condensation layer is kept on a mirror surface. This condensation layer is monitored with using a
photodetector. A CMH can be employed to measure frost/dew-point with an accuracy of ±0,1 °C. However,
two CMHs may be needed to cover the entire range of frost/dew-point (−80 °C to 35 °C) using this document
because the upper measurement limit of some CMHs measuring low frost points is a dew point of 20 °C.
Cavity ring-down spectroscopy (CRDS) measures the optical extinction due to water vapour in gases that
scatter and absorb near-infrared light. CRDS can measure low water vapour concentrations from 1 part per
billion (ppb) to 50 parts per million (ppm) with an accuracy in the ppb-level. However, this technique cannot
generally be employed to measure water concentrations higher than 100 ppm.
Tunable diode laser absorption spectroscopy (TDLAS) using near-infrared light wavelength determines the
concentration of water vapour in the gas phase. TDLAS can be employed to measure a wide range of frost/
dew points from −70 °C to 20 °C with an accuracy of ±0,2 °C.
Other technologies can be used provided they can meet measurement goals and data quality requirements.
5.2.2 Installation
It is desirable to minimize the path (tubing) between the humidity generating system and the test cell
because water adsorption/desorption occurs slowly on surfaces and may impact the humidity in the test cell
especially at low temperatures. It is recommended that the precision hygrometer be installed downstream
of the test cell to measure the frost/dew-point in the test cell as shown in Figure 1.
Key
1 dry air generator 10 radiosonde sensor boom
2 gas flow for humidity generation 11 precision hygrometer
3 gas flow for purging of water 12 gas out
4 humidity generating system 13 chilled mirror hygrometer
5 heat exchanger 14 cavity ring-down spectrometer
6 climate chamber 15 tunable diode laser absorption spectroscopy

ISO/DIS 8932-2:2025(en)
7 temperature sensor 16 other hygrometer
8 pressure gauge 17 SI traceable
9 test cell
Figure 1 — An example of the test configuration using an SI-traceable precision hygrometer as a
reference
When a precision hygrometer measurement is used as the reference relative humidity for the test cell, the
hygrometer must be within calibration to maintain SI traceability. In this case, the humidity generating
system depicted in Figure 1 may not be SI-traceable. Consequently, any type of humidity generating system
(such as a dry air cylinder) can be utilized as long as the stability of the generated relative humidity in the
test cell remains within 1 %rh. A pressure gradient along the tubing between the test cell and the hygrometer
may be present, which results in the water vapor pressure measured by the hygrometer will be slightly
different from that of the test cell. To compensate for this, the pressure in the hygrometer should also be
measured using a calibrated pressure gauge to calculate the reference relative humidity in the test cell using
the hygrometer measurement.
5.2.3 Operation
The measurement range for the frost/dew-point temperature using a precision hygrometer should be
from −80 °C to 50 °C. Two precision hygrometers may be needed to cover the entire range of frost/dew-
point temperatures. At an air pressure of 1 000 hPa, a frost-point of −80 °C corresponds to a water vapour
concentration of approximately 0,55 ppm.
The measurement stability of the precision hygrometer should be better than ±0,1 °C for the frost/dew point
temperature. A frost/dew point change of 0,1 °C corresponds to a relative humidity change of 0,6 %rh to
0,8 %rh in the temperature range of –90 °C to 30 °C.
The uncertainty of the precision hygrometer should be better than 0,3 °C to 0,15 °C for a frost/dew-point
temperature range of −80 °C to 35 °C. In general, the calibration uncertainty of hygrometers increases as the
frost-point of a gas decreases because the humidity standard uncertainties and hygrometer reproducibility
uncertainty also increase with frost-point decrease.

ISO/DIS 8932-2:2025(en)
Key
1 dry air generator 10 radiosonde sensor boom
2 generation 11 hygrometer
3 purging 12 gas out
4 humidity generator 13 saturator-based generator
5 heat exchanger 14 diffusion-tube generator
6 climate chamber 15 coulometric generator
7 temperature sensor 16 divided flow generator
8 pressure gauge 17 other humidity generator
9 test cell 18 SI traceable
Figure 2 — An example of the test setup using an SI-traceable humidity generator as a reference
5.3 Humidity generator
5.3.1 Type
[12]
Saturator-based frost/dew-point generator : This generator controls the saturated water vapour pressure
of the input gas flowing through the saturator at a specific temperature and pressure. In general, the
saturator is submerged in a liquid bath to control the temperature, and to control the range of humidity
generation from –100 °C to 95 °C in frost/dew-point.
[12]
Diffusion-tube humidity generator : This generator controls the diffusion of water vapour into a dry gas
stream (for which the humidity is known) through a tube connected to a water reservoir under controlled
temperature and pressure. Generally, frost/dew-point temperature in the range –100 °C to –70 °C can be
achieved.
[12]
Coulometric humidity generator : The coulometric humidity generator produces a controlled amount
of hydrogen and oxygen through the electrolysis of water governed by Faraday’s law, followed by the
recombination of the generated hydrogen and oxygen back into water and then the addition of the generated
water into the dry gas stream, for which the humidity is known. The humidity generation can cover –100 °C
to –70 °C in frost-point temperature.

ISO/DIS 8932-2:2025(en)
[13]
Divided-flow humidity generator : This type of generator controls humidity by diluting the humidity
produced by generators mentioned above through mixing with an extremely dry gas stream.
Other SI-traceable and characterized methods to generate humidity can be used provided they meet the
required humidity range and necessary uncertainty.
If the uncertainty of a humidity generator is not determined, it cannot be considered SI-traceable. In such
cases, the humidity generator can be calibrated against SI-traceable precision hygrometers to establish
traceability to the SI.
5.3.2 Installation
The humidity generator should be installed upstream of the test cell containing the radiosonde so that the
gas with the well-defined humidity flows into the test cell (Figure 2).
The temperature of the tubing connecting the humidity generator and the test cell shall be higher than the
frost/dew-point temperature of the generator to prevent the formation of frost or dew within the tubing.
If dry gas is needed for the generation of the desired humidity, the dry gas generator should be upstream of
the humidity generator. It is also recommended to flow the dry gas directly into the test cell and bypass the
humidity generator to efficiently purge water that has adsorbed onto the surface of the test cell.
5.3.3 Operation
The generated frost/dew point temperature ranges from −80 °C to 35 °C. At an air pressure of 1 000 hPa, a
frost-point of −80 °C corresponds to a water vapour concentration of approximately 0,55 ppm.
The stability of the generated humidity should be better than ±0,1 °C of the frost/dew point temperature.
A frost/dew point change of 0,1 °C corresponds to a relative humidity change of maximum 0,8 %rh in a
temperature range of −70 °C ~ 20 °C.
It may take several hours for the generated relative humidity to equilibrate. The test cell shown should be
located downstream of the humidity generator.
It is recommended that only the sensor boom be inserted into the test cell, with the remaining parts located
outside the test cell. This is to minimize water desorption from these surfaces in the test cell which may add
water vapor during testing.
If the size of the test cell is too large compared to the radiosonde sensor boom to test multiple sensors at the
same time, the gradient of the relative humidity and temperature inside the test cell should be evaluated.
The effect of water adsorption/desorption to/from the surface of the sensor boom may also impact the
humidity in the test cell, especially at cold temperatures.
6 Test preparation
6.1 Laboratory environmental conditions
Tests should be conducted in a laboratory with a room temperature of 23 ± 3 °C and a relative humidity of
50 ± 20 %rh.
6.2 Preparation of the radiosonde
Radiosonde measurements can be conducted via wi
...

FINAL DRAFT
International
Standard
ISO/TC 146/SC 5
Meteorology — Radiosonde —
Secretariat: DIN
Part 2:
Voting begins on:
2026-02-27
Laboratory test method for errors
in radiosonde humidity sensor
Voting terminates on:
2026-04-24
calibration
Meteorology — Radiosonde —
Partie 2: Méthode d'essai en laboratoire pour les erreurs
d'étalonnage du capteur d'humidité dans la radiosonde
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 146/SC 5
Meteorology — Radiosonde —
Secretariat: DIN
Part 2:
Voting begins on:
Laboratory test method for errors
in radiosonde humidity sensor
Voting terminates on:
calibration
Meteorology — Radiosonde —
Partie 2: Méthode d'essai en laboratoire pour les erreurs
d'étalonnage du capteur d'humidité dans la radiosonde
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
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Published in Switzerland Reference number
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and subscripts . 4
4.1 Symbols .4
4.2 Subscript .4
5 Technical requirements for the laboratory setup . 5
5.1 General .5
5.1.1 Temperature .5
5.1.2 Pressure .5
5.2 Precision hygrometer .6
5.2.1 Type .6
5.2.2 Installation .6
5.2.3 Operation .7
5.3 Humidity generator .8
5.3.1 Type .8
5.3.2 Installation .8
5.3.3 Operation .9
6 Test preparation . 10
6.1 Laboratory environmental conditions .10
6.2 Preparation of the radiosonde .10
6.3 Examination of the laboratory setup . .11
6.3.1 General .11
6.3.2 Examination of the dry gas generator .11
6.3.3 Examination of the liquid bath and climate chamber .11
6.3.4 Examination of the measurement system for the calculation of the reference
relative humidity in the test cell .11
6.3.5 Examination of the radiosonde measurement software and the control software
for the reference relative humidity.11
6.4 Installation of the radiosonde .11
7 Test methods and procedures .11
7.1 Operation .11
7.1.1 Purging the test cell .11
7.1.2 Temperature control of the test cell . 12
7.1.3 Humidity from the humidity generator . 12
7.1.4 Calculation of the reference relative humidity using the humidity generator . 13
7.1.5 Calculation of the reference relative humidity using the precision hygrometer . 13
7.2 Test procedure . 13
8 Data processing . 14
8.1 Calculation of the average values .14
8.2 Calculation of the measurement error .14
9 Evaluation of measurement uncertainty . 14
9.1 General .14
9.2 Uncertainty evaluation for the reference relative humidity, u(h ).
ref
9.2.1 Uncertainty of the reference relative humidity . 15
9.3 Uncertainty of the radiosonde relative humidity, uh .

rad
iii
9.3.1 Uncertainty of the resolution of the radiosonde relative humidity, uh

radr_ es
9.3.2 Uncertainty of the repeatability of the radiosonde relative humidity,
�uh .

radr_ ep
9.4 Calculation of the combined standard uncertainty of the measurement error, uh .

err
9.5 Calculation of expanded uncertainty .18
10 Method for reporting test results .18
Annex A (informative) Calculation of the reference relative humidity. 19
Bibliography .23

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
A list of all parts in the ISO 8932 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
Temperature and water vapour (i.e., humidity) are two of the basic atmospheric variables and are
important for the initialization of numerical weather prediction and climate modelling. Radiosondes are
widely used to measure atmospheric parameters such as humidity and temperature up to an altitude of
approximately 40 km. A radiosonde is a balloon-borne instrument with several types of sensors for in situ
[1]
profile measurements. A radio transmitter is used to send these data to the observing station. Radiosonde
observations are often used in conjunction with other measurement techniques such as remote sensing
satellites to provide comparative data. For radiosonde-derived data to be useful, the measurement accuracy
of the radio soundings must be known. From a metrological perspective, this measurement accuracy must
be expressed in terms of uncertainty that is traceable to the International System of Units (SI).
Previously, comparative soundings of dew-point hygrometers and radiosondes showed that radiosonde
humidity sensors have a daytime dry bias resulting from solar heating and a time-lag error at low
[2],[3]
temperatures. The dry-bias and time-lag error can be corrected in sounding systems by creating proper
algorithms based on measurements of sensor temperature and response time, respectively. In addition,
polymeric thin-film humidity sensors adopted by most radiosondes are known to produce systematic errors
at the low temperature characteristic of the upper air environment if a calibration curve obtained at room
[4],[5]
temperature only is employed for low temperatures. The capacitance of humidity sensors increases
with adsorbed water vapour in thin films. While the adsorption capacity of the films for water vapour
increases at lower temperatures, it does not fully compensate for the reduced water vapour pressure, leading
to temperature-dependent behaviour in thin-film humidity sensors. Therefore, calibration of these sensors
must be conducted at various temperatures. Thus, an essential prerequisite to resolve the above issues and
to improve the measurement reliability of radiosondes is to calibrate the radiosonde sensors using ground-
[6],[7]
based facilities.
The relative humidity generated by a laboratory setup or factory facility can be traceable to the SI via
temperature, pressure, gas flow rate, and mass measurements related to determining water vapour pressure
from either the humidity generator itself or the measurement of the generated humidity using calibrated
precision hygrometers. The SI traceability of radiosonde humidity sensors must be established through
factory calibration by manufacturers. This document is mainly focused on testing the calibration error of
humidity sensors in radiosondes, which are randomly sampled from a series of products.
The Standing Committee on Measurements, Instrumentation and Traceability (SC-MINT) of the World
Meteorological Organization (WMO) urges users to test selected samples of radiosondes under laboratory
conditions to ensure that the calibrations supplied by the manufacturer are valid. Even if sensors can be
produced in large batches to meet an agreed upon set of standardized performance checks, it is necessary
[8]
for representative sensor samples, selected at random, to be checked more rigorously. This independent
testing would further improve the reliability of radiosonde measurements by verifying the calibration
results applied by manufacturers. While testing is crucial, the guide provided by SC-MINT only provides
limited requirements for the test setup. These requirements describe the need for stability better than
1 %rh and systematic errors less than ±1 %rh at the desired value. More detailed methodologies or test
procedures for the testing of radiosonde humidity sensors in a relevant range of temperatures have not been
reported.
The procedure presented in this document provides the technical requirements for essential laboratory
setups that include a humidity generator, a test cell, a precision hygrometer, a pressure gauge, and a
thermometer. The test procedure, including test preparation, installation of radiosondes in the test cell,
operation of laboratory setups, and comparison between the reference and radiosonde relative humidity,
is presented. The method for evaluating uncertainties associated with both the reference relative humidity
and the radiosonde humidity sensor is also described.
The fundamental technique essential for this test involves the SI-traceable generation and measurement of
water vapour pressure to determine the reference relative humidity and assess its associated uncertainty.
Since humidity generators used in application of this document are capable of producing multiple dew-
point and frost-point temperatures below the test temperature, the testing of sensors at several relative
humidity levels at the test temperature while avoiding condensation can be accomplished. The water
vapour pressure required for the reference relative humidity can be determined from frost or dew point
temperature by either humidity generators or calibrated hygrometers as exemplified in Annex A. Dew-

vi
[9]
point is the temperature, where water vapour is in equilibrium with liquid water at the same pressures.
An example for obtaining multiple dew-point and frost-point values is presented using a saturator-based
humidity generator. This approach is chosen because of its traceability, and its documented validation by
[4]-[8]
metrological and meteorological experts in testing radiosonde humidity sensors. .
Since calibration is valid at the time of calibration, this test can incorporate additional sources of uncertainty
related to transportation and storage of the sensors, which can introduce additional uncertainty in
the results. It's important to note that when considering uncertainty in soundings, other factors such as
radiation dry bias and time-lag should also be considered, as summarized in Table 5 of Reference [3]. While
all uncertainty terms affecting the results should be considered, the procedure in this document primarily
focuses on testing a subset of the uncertainty factors outlined in Reference [3], specifically those related to
testing temperature-dependent calibration corrections. In uncertainty analysis, the uncertainty of the test
setup, reference devices, and other potential uncertainties affecting radiosonde measurements under static
conditions are included in this test.
[10]-[12]
It is worth noting that while the saturator-based technique is founded on Korean and US patents, the
patent holder has granted a license, free of charge, to an unlimited number of applicants globally, without
discrimination, and under reasonable terms and conditions. This license allows for the creation, use, and
sale of implementations based on this ISO document. However, it's important to state that this document is
not solely based upon this patented technique. Alternative equivalent techniques, utilizing either different
types of humidity generators or precision hygrometers, can also be employed for determining reference
relative humidity if they are SI-traceable and meet the specifications required under this document.
Due to potential limitations in the number of test setups or laboratories available for conducting this test,
peer-reviewed reports or papers published online or offline resulting from research activities conducted by
academia or meteorological institutes can be utilized as a test report when following this test procedure.

vii
FINAL DRAFT International Standard ISO/FDIS 8932-2:2026(en)
Meteorology — Radiosonde —
Part 2:
Laboratory test method for errors in radiosonde humidity
sensor calibration
1 Scope
The document specifies testing procedures for determining calibration error for radiosonde humidity
sensors sampled from a mass production batches based on varying the levels of relative humidity at
atmospheric upper-air temperatures using a laboratory setup. This document provides:
a) technical requirements for a laboratory setup to evaluate the calibration errors of radiosonde humidity
measurement;
b) a test procedure for evaluating calibration error of radiosonde humidity sensors for a temperature
1)
range of −90 °C to 35 °C and for a relative humidity of 1 %rh to 100 %rh. Note, this document, is based
upon relative humidity calculated by the percentage of water vapour pressure divided by saturation
water vapour pressure over liquid water, not over ice, even at temperatures below 0 °C; hence, the
maximum relative humidity is less than 100 %rh below 0 °C;
c) a method for evaluating the uncertainty for the measured radiosonde humidity calibration errors.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
IEC 60050-713:2021, International Electrotechnical Vocabulary (IEV) - Part 713: Radiocommunications:
transmitters, receivers, networks and operation
IEC 60068-3-6:2018, Environmental testing – Part 3-6: Supporting documentation and guidance – Confirmation
of the performance of temperature/humidity chambers
IEC 60068-3-11:2007, Environmental testing – Part 3-11: Supporting documentation and guidance –Calculation
of uncertainty of conditions in climatic test chambers
ISO/IEC Guide 98-1:2024, Guide to the expression of uncertainty in measurement — Part 1: Introduction
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
ISO/IEC Guide 99:2007, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
WMO No.182, 1992, International Meteorological Vocabulary
1) Currently, the lowest possible temperature of commercially-available climate chambers is approximately -75 °C. The
temperature range can be adjusted based on the capability of the climate chamber used.

3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC Guide 99:2007, WMO No.182,
IEC 60050-713:2021, IEC 60068-3-6:2018, and IEC 60068-3-11:2007 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
radiosonde
instrument intended to be carried by a balloon through the atmosphere, equipped with devices to measure
one or several meteorological variables (such as pressure, temperature, humidity), and provided with a
radio transmitter for sending this information to the observing station
Note 1 to entry: A series of battery-powered telemetry devices that are suspended high in the atmosphere using a
weather balloon, which measure atmospheric parameters, such as temperature and humidity, and transmit data to a
ground system using radio frequencies.
Note 2 to entry: To ensure the traceability of radiosondes as a product under WMO control, the WMO code (BUFR code
0-02-011) must be obtained. This WMO code shall be unique for each radiosonde, and a single code shall not be used
for more than one type of radiosonde.
3.2
radiosonde body
housing of a radiosonde (3.1) comprising a circuit board with measurement chips and a data transmission
section with antennae and batteries
3.3
radiosonde sensor boom
arm connected to the radiosonde body (3.2) to which the temperature and humidity sensors are attached
3.4
humidity generator
device that can produce the desired water vapour pressure
Note 1 to entry: The water vapour pressure of the generator output together with the controlled temperature can be
converted into the reference relative humidity in the test cell (3.5).
3.5
test cell
chamber in which radiosonde humidity sensors can be installed for testing
Note 1 to entry: The test cell is installed inside a space within which the temperature can be controlled [e.g. a climate
chamber (3.7)] to evaluate the temperature-dependency of the measurements from the radiosonde humidity sensors.
3.6
precision hygrometer
humidity measurement instrument that can accurately measure the humidity from low frost-points
(−100 °C) to high dew-points (35 °C)
Note 1 to entry: It is used to check the reference relative humidity in the test cell (3.5) generated by the humidity
generator (3.4). The measurements from a precision hygrometer can also be used as the reference relative humidity
in the test cell (3.5) if the precision hygrometer is calibrated, and correction and uncertainty are employed. In general,
chilled-mirror precision hygrometers are used.
Note 2 to entry: A thermometer and a vapour pressure equation are also needed to obtain relative humidity values.
Manufacturers may use a different vapour pressure equation for the factory calibration.

3.7
climate chamber
enclosed space where the internal temperature and humidity can be controlled within specified limits
Note 1 to entry: The climate chamber can be used to control the temperature and, thus, the relative humidity as well
of the test cell (3.5).
3.8
liquid bath
equipment in which liquid is present in a container, with a specific volume, to maintain a constant
temperature
Note 1 to entry: A liquid bath can be used to control the water vapour saturation temperature of a saturator-based
humidity generator (3.4).
3.9
dry gas generator
device that produces a gas with negligible water vapour content
Note 1 to entry: It can be used to lower the humidity by mixing of its output with a humid gas with known water
vapour pressure in a humidity generator (3.4).
Note 2 to entry: It can be used to purge the water adsorbed onto the test cell (3.5).
3.10
platinum resistance thermometer
PRT
temperature-responsive device consisting of a sensing resistor within a protective sheath, internal
connecting wires and external terminals to permit connection of electrical measurement devices
3.11
reference thermometer
instrument used to measure the temperature in the test cell (3.5) for the calculation of the reference relative
humidity
Note 1 to entry: Generally, calibrated PRTs (3.10) are used.
3.12
reference pressure gauge
instrument used to measure the pressure in the test cell (3.5) for the calculation of the reference relative
humidity
Note 1 to entry: Generally, calibrated capacitance diaphragm gauges are used.
3.13
measurement error
measured quantity value minus a reference quantity value
3.14
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
Note 1 to entry: Measurement uncertainty includes components arising from systematic effects, such as components
associated with corrections and the assigned quantity values of measurement standards, as well as the definitional
uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement
uncertainty components are incorporated.
Note 2 to entry: The parameter may be, for example, a standard deviation called standard measurement uncertainty
(or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.

Note 3 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be evaluated
by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series
of measurements and can be characterized by standard deviations. The other components, which may be evaluated
by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from
probability density functions based on experience or other information.
Note 4 to entry: In general, for a given set of information, it is understood that the measurement uncertainty is
associated with a stated quantity value attributed to the measurand. A modification of this value results in a
modification of the associated uncertainty.
3.15
standard uncertainty
measurement uncertainty (3.14) expressed as a standard deviation
3.16
coverage factor
k
number larger than one by which a combined standard measurement uncertainty is multiplied to obtain an
expanded measurement
Note 1 to entry: Coverage factor is usually symbolized.
3.17
expanded uncertainty
product of a combined standard measurement uncertainty and a factor larger than the number one
Note 1 to entry: The factor depends upon the type of probability distribution of the output quantity in a measurement
model and on the selected coverage probability.
Note 2 to entry: The term “factor” in this definition refers to a coverage factor (3.16).
4 Symbols and subscripts
4.1 Symbols
For the purposes of this document, the following symbols apply.
E
saturated water vapour pressure at a given T and P
e
water vapour pressure
f
enhancement factor
k
coverage factor
P
pressure
h
relative humidity
T
temperature
U
expanded uncertainty
u
standard uncertainty
v
gas flow rate
4.2 Subscript
Within the document, the following subscripts are used.

cal
calibration
DUT
device under test
g
generator
grad
gradient
h
hygrometer
s
saturator
t
test cell
w
water
ref
reference
rep
repeatability
res
resolution
5 Technical requirements for the laboratory setup
5.1 General
5.1.1 Temperature
The temperature of the test cell can be varied between −90 °C and 35 °C using a climate chamber or a liquid
bath. A climate chamber is more desirable as it minimizes handling of the radiosonde.
The reference temperature of the test cell should be measured inside the test cell using calibrated PRTs to
calculate the reference relative humidity.
If the radiosonde humidity sensor operates using a heating element, both the reference thermometer and the
radiosonde temperature sensor can be affected by the humidity sensor heater. The temperature measured
by the reference PRT should not be impacted by the radiosonde heater.
NOTE The impact of the heater can be reduced by increasing the air ventilation rate in the test cell to around
-1 -1 [13]
5 m∙s to 6 m∙s to facilitate the convective cooling of sensors as experienced during soundings.
The temperature stability of the test cell should be within ±0,1 °C. A change in the test cell temperature of
0,1 °C corresponds to a change in the relative humidity of roughly 0,6 %rh to 0,8 %rh in the temperature
range of –90 °C to 30 °C.
The temperature gradient around the radiosonde humidity sensor should be measured using a calibrated
thermometer to evaluate the uncertainty due to the spatial gradient of the reference relative humidity.
Measurement points of two reference PRTs should be above and below the humidity sensor and the distance
between PRTs be 10 mm to 50 mm. When testing multiple sensors, the temperature distance between these
sensors may be larger than for two sensors resulting in a corresponding increase in uncertainty.
5.1.2 Pressure
The pressure of the test cell should be measured using a calibrated pressure gauge to calculate the saturation
water vapour pressure and the reference relative humidity in the test cell.
The pressure gradient between the test cell and the precision hygrometer should be measured using a
calibrated pressure gauge to evaluate the uncertainty when a calibrated precision hygrometer is used to
determine the reference relative humidity in the test cell instead of a humidity generator.

5.2 Precision hygrometer
5.2.1 Type
A chilled-mirror hygrometer (CMH) measures the dew or frost point temperature at which a constant
condensation layer is kept on a mirror surface. This condensation layer is monitored with using a
photodetector. A CMH can be employed to measure frost-point and dew-point with an accuracy of ±0,1 °C.
However, two CMHs can be needed to cover the entire range of frost-point and dew-point (−100 °C to 35 °C)
using this document because the upper measurement limit of some CMHs measuring low frost points is a
dew-point of 20 °C. For the calibration of CMHs, the method given in ISO/TR 12148:2009 should apply.
Cavity ring-down spectroscopy (CRDS) measures the optical extinction due to water vapour in gases that
scatter and absorb near-infrared light. CRDS can measure low water vapour concentrations from 1 part per
billion (ppb) to 50 parts per million (ppm) with an accuracy in the ppb-level. However, this technique cannot
generally be employed to measure water concentrations higher than 100 ppm.
Tunable diode laser absorption spectroscopy (TDLAS) using near-infrared light wavelength determines the
concentration of water vapour in the gas phase. TDLAS can be employed to measure a wide range of frost-
point and dew-point from −70 °C to 20 °C with an accuracy of ±0,2 °C.
Other technologies can be used provided they can meet measurement goals and data quality requirements.
5.2.2 Installation
It is desirable to minimize the path (tubing) between the humidity generating system and the test cell
because water adsorption and desorption occurs slowly on surfaces and may impact the humidity in the test
cell especially at low temperatures. The precision hygrometer should be installed downstream of the test
cell to measure the frost-point and dew-point in the test cell as shown in Figure 1.

Key
1 dry air generator
2 gas flow for humidity generation
3 gas flow for purging of water
4 humidity generating system
5 heat exchanger
6 climate chamber
7 temperature sensor
8 pressure gauge
9 test cell
10 radiosonde sensor boom
11 precision hygrometer
12 gas out
13 chilled mirror hygrometer
14 cavity ring-down spectrometer
15 tunable diode laser absorption spectroscopy
16 other hygrometer
17 SI traceable
Figure 1 — Example of the test configuration using an SI-traceable precision hygrometer as a
reference
When a precision hygrometer measurement is used as the reference relative humidity for the test cell, the
hygrometer shall be within calibration to maintain SI traceability. In this case, the humidity generating
system depicted in Figure 1 may not be SI-traceable. Consequently, any type of humidity generating
system (such as a dry air cylinder) can be utilized as long as the stability of the generated relative humidity
in the test cell remains within 1 %rh. A pressure gradient along the tubing between the test cell and the
hygrometer may be present, which results in the water vapour pressure measured by the hygrometer to be
slightly different from that of the test cell. To compensate for this, the pressure in the hygrometer should
also be measured using a calibrated pressure gauge to calculate the reference relative humidity in the test
cell using the hygrometer measurement.
5.2.3 Operation
The measurement range for the frost-point and dew-point temperature using a precision hygrometer should
be from −100 °C to 35 °C. Two precision hygrometers can be needed to cover the entire range of frost-point

and dew-point temperatures. At an air pressure of 1 000 hPa, a frost-point of −100 °C corresponds to a water
vapour concentration of approximately 0,014 ppm.
The measurement stability of the precision hygrometer should be better than ±0,1 °C for the frost-point and
dew-point temperature. A frost-point and dew-point change of 0,1 °C corresponds to a relative humidity
change of 0,6 %rh to 0,8 %rh in the temperature range of –90 °C to 35 °C.
The uncertainty of the precision hygrometer should be better than 0,3 °C to 0,15 °C for a frost-point and
dew-point temperature range of −100 °C to 35 °C. In general, the calibration uncertainty of hygrometers
increases as the frost-point of a gas decreases because the humidity standard uncertainties and hygrometer
reproducibility uncertainty also increase with frost-point decrease.
5.3 Humidity generator
5.3.1 Type
[13]
5.3.1.1 Saturator-based frost-point and dew-point generator
This generator controls the saturated water vapour pressure of the input gas flowing through the saturator
at a specific temperature and pressure. In general, the saturator is submerged in a liquid bath to control the
temperature, and to control the range of humidity generation from –100 °C to 35 °C in frost-point and dew-
point.
[14]
5.3.1.2 Diffusion-tube humidity generator
This generator controls the diffusion of water vapour into a dry gas stream (for which the humidity is known)
through a tube connected to a water reservoir under controlled temperature and pressure. Generally, frost-
point and dew-point temperature in the range –100 °C to –70 °C can be achieved.
[14]
5.3.1.3 Coulometric humidity generator :
The coulometric humidity generator produces a controlled amount of hydrogen and oxygen through the
electrolysis of water governed by Faraday’s law, followed by the recombination of the generated hydrogen
and oxygen back into water and then the addition of the generated water into the dry gas stream, for which
the humidity is known. The humidity generation can cover –100 °C to –70 °C in frost-point temperature.
[15]
5.3.1.4 Divided-flow humidity generator
This type of generator controls humidity by diluting the humidity produced by generators mentioned above
through mixing with an extremely dry gas stream.
5.3.1.5 Other SI-traceable and characterized methods
Other SI-traceable and characterized methods to generate humidity can be used provided they meet the
required humidity range and necessary uncertainty.
If the uncertainty of a humidity generator is not determined, it cannot be considered SI-traceable. In such
cases, the humidity generator can be calibrated against SI-traceable precision hygrometers to establish
traceability to the SI.
5.3.2 Installation
The humidity generator should be installed upstream of the test cell containing the radiosonde so that the
gas with a known water vapour pressure flows into the test cell (see Figure 2).
The temperature of the tubing connecting the humidity generator and the test cell shall be higher than the
frost-point and dew-point temperature of the generator to prevent the formation of frost or dew within the
tubing.
If dry gas is needed for the generation of the desired humidity, the dry gas generator should be upstream of
the humidity generator. It is also recommended to flow the dry gas directly into the test cell and bypass the
humidity generator to efficiently purge water that has adsorbed onto the surface of the test cell.
5.3.3 Operation
The generated frost-point and dew point temperature ranges from −100 °C to 35 °C. At an air pressure of
1 000 hPa, a frost-point of −100 °C corresponds to a water vapour concentration of approximately 0,014 ppm.
The stability of the generated humidity should be better than ±0,1 °C of the frost-point and dew point
temperature. A frost-point and dew point change of 0,1 °C corresponds to a relative humidity change of
maximum 0,8 %rh in a temperature range of −90 °C ~ 35 °C.
It can take several hours for the generated relative humidity to equilibrate. The test cell shown should be
located downstream of the humidity generator.
Only the sensor boom should be inserted into the test cell, with the remaining parts located outside the test
cell. This is to minimize water desorption from these surfaces in the test cell which can add water vapour
during testing.
If the size of the test cell is too large compared to the radiosonde sensor boom to test multiple sensors at the
same time, the gradient of the relative humidity and temperature inside the test cell should be evaluated. The
effect of water adsorption and desorption on the surface of the se
...

ISO/FDIS 8932-2:2025(en)
ISO/TC 146/SC 5/WG 11
Secretariat: DIN
Date: 2026-02-12
Meteorology — Radiosonde — —
Part 2:
Laboratory test method for errors in radiosonde humidity sensor
calibration
Date: 2025-01-28
Meteorology — Radiosonde —
Partie 2: Méthode d'essai en laboratoire pour les erreurs d'étalonnage du capteur d'humidité dans la
radiosonde
FDIS stage
ISO/FDIS 8932-2:20252026(en)
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
iii
ISO/DIS FDIS 8932-2:2025 (E2026(en)
Contents
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Symbols and subscripts . 5
5 Technical requirements for the laboratory setup . 6
6 Test preparation . 12
7 Test methods and procedures . 13
8 Data processing . 16
9 Evaluation of measurement uncertainty . 16
10 Method for reporting test results . 20
Annex A (informative) Calculation of the reference relative humidity . 22
Bibliography . 27

© ISO #### 2026 – All rights reserved
iv
ISO/FDIS 8932-2:20252026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Field Code Changed
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had received notice of (a) patent(s) which
may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
Field Code Changed
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
A list of all parts in the ISO 8932 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
Field Code Changed
v
ISO/DIS FDIS 8932-2:2025 (E2026(en)
Introduction
Temperature and water vapour (i.e., humidity) are two of the basic atmospheric variables and are important
for the initialization of numerical weather prediction and climate modelling. Radiosondes are widely used to
measure atmospheric parameters such as humidity and temperature up to an altitude of approximately 40 km.
A radiosonde is a balloon-borne instrument with several types of sensors for in situ profile measurements. A
[1] [1]
radio transmitter is used to send these data to the observing station. . Radiosonde observations are often
used in conjunction with other measurement techniques such as remote sensing using satellites to provide
comparative data. For radiosonde-derived data to be useful, the measurement accuracy of the radio soundings
needs tomust be known. From a metrological perspective, this measurement accuracy shouldmust be
expressed in terms of uncertainty that is traceable to the International System of Units (SI).
Previously, comparative soundings of dew-point hygrometers and radiosondes showed that radiosonde
humidity sensors have a daytime dry bias resulting from solar heating and a time-lag error at low
[2] [2,3],[3]
temperatures. . The dry-bias and time-lag error can be corrected in sounding systems by creating
proper algorithms based on measurements of sensor temperature and response time, respectively. In
addition, polymeric thin-film humidity sensors adopted by most radiosondes are known to produce
systematic errors at the low temperature characteristic of the upper air environment if a calibration curve
[4] [4,5],[5]
obtained at room temperature only is employed for low temperatures. . The capacitance of humidity
sensors increases with adsorbed water vapour in thin films. While the adsorption capacity of the films for
water vapour increases at lower temperatures, it does not fully compensate for the reduced water vapour
pressure, leading to temperature-dependent behaviour in thin-film humidity sensors. Therefore, calibration
of these sensors shouldmust be conducted at various temperatures. Thus, an essential prerequisite to resolve
the above issues and to improve the measurement reliability of radiosondes is to calibrate the radiosonde
[6][6,7] ,[7]
sensors using ground-based facilities. .
The relative humidity generated by a laboratory setup or factory facility can be traceable to the SI via
temperature, pressure, gas flow rate, and mass measurements related to determining water vapour pressure
from either the humidity generator itself or the measurement of the generated humidity using calibrated
precision hygrometers. The SI traceability of radiosonde humidity sensors shouldmust be established through
factory calibration by manufacturers. This document is mainly focused on testing the calibration error of
humidity sensors in radiosondes, which are randomly sampled from a series of products.
The Standing Committee on Measurements, Instrumentation and Traceability (SC-MINT) of the World
Meteorological Organization (WMO) urges users to test selected samples of radiosondes under laboratory
conditions to ensure that the calibrations supplied by the manufacturer are valid. Even if sensors can be
produced in large batches to meet an agreed upon set of standardized performance checks, it is necessary for
[8] [8]
representative sensor samples, selected at random, to be checked more rigorously. . This independent
testing would further improve the reliability of radiosonde measurements by verifying the calibration results
applied by manufacturers. While testing is crucial, the guide provided by SC-MINT only provides limited
requirements for the test setup. These requirements describe the need for stability better than 1 %rh and
systematic errors less than ±1 %rh at the desired value. More detailed methodologies or test procedures for
the testing of radiosonde humidity sensors in a relevant range of temperatures have not been reported.
The procedure presented in this document provides the technical requirements for theessential laboratory
setup, the setups that include a humidity generator, a test cell, a precision hygrometer, a pressure gauge, and
a thermometer. The test procedure for errors in , including test preparation, installation of radiosondes in the
test cell, operation of laboratory setups, and comparison between the reference and radiosonde relative
humidity, is presented. The method for evaluating uncertainties associated with both the reference relative
humidity and the radiosonde humidity sensor calibration at relevant temperatures using the setup, and an
evaluation method for the uncertainty of the test results.
Since calibration is valid at the time of calibration, this test may incorporate additional sources of uncertainty
related to transportation and storage of the sensors, which can introduce additional uncertainty in the results.
© ISO #### 2026 – All rights reserved
vi
ISO/FDIS 8932-2:20252026(en)
It's important to note that when considering uncertainty in soundings, other factors such as radiation dry bias
and time-lag shouldis also be considered, as summarized in Table 5 of Reference [3]. While all uncertainty
terms affecting the results should be considered, the procedure in this document primarily focus on testing a
[3]
subset of the uncertainty factors outlined in Reference , specifically those related to testing temperature-
dependent calibration corrections. In uncertainty analysis, the uncertainty of the test setup, reference devices,
and other potential uncertainties affecting radiosonde measurements under static conditions are included in
this test.
described.
The fundamental technique essential for this test involves the SI-traceable generation and measurement of
water vapour pressure to determine the reference relative humidity and assess its associated uncertainty.
Since humidity generators used in application of this document should beare capable of producing multiple
dew-point and frost-point temperatures below the test temperature, the testing of sensors at several relative
humidity levels at the test temperature while avoiding condensation can be accomplished. The water vapour
pressure required for the reference relative humidity can be determined from frost or dew point temperature
by either humidity generators or calibrated hygrometers as exemplified in Annex A. Dew-point is the
[9]
temperature, where water vapour is in equilibrium with liquid water at the same pressures. An example for
obtaining multiple dew-point and frost-point values is presented using a saturator-based humidity generator.
This approach is chosen because of its traceability, and its documented validation by metrological and
[4]-[8][4-8]
meteorological experts in testing radiosonde humidity sensors. .
Since calibration is valid at the time of calibration, this test can incorporate additional sources of uncertainty
related to transportation and storage of the sensors, which can introduce additional uncertainty in the results.
It's important to note that when considering uncertainty in soundings, other factors such as radiation dry bias
and time-lag should also be considered, as summarized in Table 5 of Reference [3]. While all uncertainty terms
affecting the results should be considered, the procedure in this document primarily focuses on testing a
subset of the uncertainty factors outlined in Reference [3], specifically those related to testing temperature-
dependent calibration corrections. In uncertainty analysis, the uncertainty of the test setup, reference devices,
and other potential uncertainties affecting radiosonde measurements under static conditions are included in
this test.
[10] [9-11]-[12]
It is worth noting that while the saturator-based technique is founded on Korean and US patents, ,
the patent holder has granted a license, free of charge, to an unlimited number of applicants globally, without
discrimination, and under reasonable terms and conditions. This license allows for the creation, use, and sale
of implementations based on this ISO document. However, it's important to state that this document is not
solely based upon this patented technique. Alternative equivalent techniques, utilizing either different types
of humidity generators or precision hygrometers, can also be employed for determining reference relative
humidity if they are SI-traceable and meet the specifications required under this document.
Due to potential limitations in the number of test setups or laboratories available for conducting this test,
peer-reviewed reports or papers published online or offline resulting from research activities conducted by
academia or meteorological institutes can be utilized as a test report when following this test procedure.
vii
DRAFT International Standard ISO/DIS 8932-2:2025(en)

Meteorology — Radiosonde — —
Part 2:
Laboratory test method for errors in radiosonde humidity sensor
calibration
1 Scope
The document specifies testing procedures for determining calibration error for radiosonde humidity sensors
sampled from a mass production batches based on varying the levels of relative humidity at atmospheric
upper-air temperatures using a laboratory setup. Application of thisThis document provides the following:
a) a) the technical requirements for a laboratory setup to evaluate the calibration errors of
radiosonde humidity measurement;
b) b) a test procedure for evaluating calibration error of radiosonde humidity sensors for a
temperature range a test procedure for evaluating calibration error of radiosonde humidity sensors for a
1)
temperature range of −90 °C to 35 °C and for a relative humidity of 1 %rh to 100 %rh. Note, this
document, is based upon relative humidity calculated by the percentage of water vapour pressure divided
by saturation water vapour pressure over liquid water, not over ice, even at temperatures below 0 °C;
hence, the maximum relative humidity is less than 100 %rh below 0 °C;
c) c) a method for evaluating the uncertainty for the measured radiosonde humidity calibration
errors.
In this document, the requirements for a humidity generator, a test cell, a precision hygrometer, and a
thermometer as essential laboratory setups are proposed in a). In b), the test procedure including the test
preparation, the installation of radiosondes in the test cell, the operation of laboratory setups, and the
comparison between the reference relative humidity and the radiosonde relative humidity are presented. In
c), the method for evaluating uncertainties related to the reference relative humidity and the radiosonde
humidity sensor is discussed.
NOTE 1 The water vapour pressure required for the reference relative humidity can be determined from frost or dew
point temperature by either humidity generators or calibrated hygrometers as exemplified in Annex A. Dew-point is the
[12]
temperature, where water vapour is in equilibrium with liquid water at the same pressures .
NOTE 2 Due to potential limitations in the number of test setups or laboratories available for conducting this
test, peer-reviewed reports or papers published online or offline resulting from research activities conducted
by academia or meteorological institutes can be utilized as a test report when following this test procedure.

1)
Currently, the lowest possible temperature of commercially-available climate chambers is approximately -75 °C. The
temperature range can be adjusted based on the capability of the climate chamber used.

Currently, the lowest possible temperature of commercially-available climate chambers is approximately -75 °C. The temperature
range can be adjusted based on the capability of the climate chamber used.

2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/IEC Guide 99:2007, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
WMO No.182, 1992, International Meteorological Vocabulary
IEC 60050--713:2021, International Electrotechnical Vocabulary (IEV) - Part 713: Radiocommunications:
transmitters, receivers, networks and operation
IEC 60068--3-6:2018, Environmental testing – Part 3-6: Supporting documentation and guidance –
Confirmation of the performance of temperature/humidity chambers
IEC 60068--3-11:2007, Environmental testing – Part 3-11: Supporting documentation and guidance –
Calculation of uncertainty of conditions in climatic test chambers
ISO/TR 12148:2009, Natural gas — Calibration of chilled mirror type instruments for hydrocarbon dewpoint
(liquid formation)
ISO/IEC Guide 98--1:2024, Guide to the expression of uncertainty in measurement –— Part 1: Introduction
ISO/IEC Guide 98--3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
IEC 60068-3-11:2007, Environmental testing – Part 3-11: Supporting documentation and guidance –Calculation
of uncertainty of conditions in climatic test chambers
ISO/IEC Guide 99:2007, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
WMO No.182, 1992, International Meteorological Vocabulary
3 Terms and definitions
For the purposes of this document, the following terms and definitions, given in ISO/IEC Guide 99:2007, WMO
No.182, IEC 60050-713:2021, IEC 60068-3-6:2018, and IEC 60068-3-11:2007 withand the following, apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 3.1
radiosonde
instrument intended to be carried by a balloon through the atmosphere, equipped with devices to measure
one or several meteorological variables (such as pressure, temperature, humidity), and provided with a radio
transmitter for sending this information to the observing station
Note 1 to entry: A series of battery-powered telemetry devices that are suspended high in the atmosphere using a
weather balloon, which measure atmospheric parameters, such as temperature and humidity, and transmit data to a
ground system using radio frequencies.
ISO/FDIS 8932-2:20252026(en)
Note 2 to entry: To ensure the traceability of radiosondes as a product under WMO control, it is necessary to obtain the
WMO code (BUFR code 0-02-011).) must be obtained. This WMO code shall be unique for each radiosonde, and a single
code shall not be used for more than one type of radiosonde.
3.2 3.2
radiosonde body
housing of a radiosonde (3.1) comprising a circuit board with measurement chips and a data transmission
section with antennae and batteries
3.3 3.3
radiosonde sensor boom
arm connected to the radiosonde body (3.2) to which the temperature and humidity sensors are attached
3.4 3.4
humidity generator
device that can produce the desired water vapour pressure.
Note 1 to entry: The water vapour pressure of the generator output together with the controlled temperature can be
converted into the reference relative humidity in the test cell (3.5.).
3.5 3.5
test cell
chamber in which radiosonde humidity sensors can be installed for testing.
Note 1 to entry: The test cell is installed inside a space within which the temperature can be controlled ([e.g. a climate
chamber (3.7))] to evaluate the temperature-dependency of the measurements from the radiosonde humidity sensors.
3.6 3.6
precision hygrometer
humidity measurement instrument that can accurately measure the humidity from low frost-points (−100 °C)
to high dew-points (35 °C). )
Note 1 to entry: It is used to check the reference relative humidity in the test cell (3.5) generated by the humidity
generator (3.4.). The measurements from a precision hygrometer can also be used as the reference relative humidity in
the test cell (3.5) if the precision hygrometer is calibrated, and correction and uncertainty are employed. In general,
chilled-mirror precision hygrometers are used.
Note 2 to entry: A thermometer and a vapour pressure equation are also needed to obtain relative humidity values.
Manufacturers may use a different vapour pressure equation for the factory calibration.
3.7 3.7
climate chamber
enclosed space where the internal temperature and humidity can be controlled within specified limits
Note 1 to entry: The climate chamber can be used to control the temperature and, thus, the relative humidity as well of
the test cell (3.5.).
3.8 3.8
liquid bath
equipment in which liquid is present in a container, with a specific volume, to maintain a constant
temperature.
Note 1 to entry: A liquid bath can be used to control the water vapour saturation temperature of a saturator-based
humidity generator (3.4(3.4).).
3.9 3.9
dry gas generator
device that produces a gas with negligible water vapour content.
Note 1 to entry:  It can be used to lower the humidity by mixing of its output with a humid gas with known water vapour
pressure in a humidity generator (3.4(3.4). ).
Note 2 to entry: It can be used to purge the water adsorbed onto the test cell (3.5. ).
3.10 3.10
platinum resistance thermometer (
PRT) sensor
temperature-responsive device consisting of a sensing resistor within a protective sheath, internal connecting
wires and external terminals to permit connection of electrical measurement devices.
3.11 3.11
reference thermometer
instrument used to measure the temperature in the test cell (3.5) for the calculation of the reference relative
humidity.
Note 1 to entry:  Generally, calibrated PRTs (3.10) are used.
3.12 3.12
reference pressure gauge
instrument used to measure the pressure in the test cell (3.5) for the calculation of the reference relative
humidity.
Note 1 to entry:  Generally, calibrated capacitance diaphragm gauges are used.
3.13 3.13
measurement error
measured quantity value minus a reference quantity value
3.14 3.14
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand,
based on the information used
Note 1 to entry: Measurement uncertainty includes components arising from systematic effects, such as components
associated with corrections and the assigned quantity values of measurement standards, as well as the definitional
uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement
uncertainty components are incorporated.
Note 2 to entry: The parameter may be, for example, a standard deviation called standard measurement uncertainty (or
a specified multiple of it), or the half-width of an interval, having a stated coverage probability.
Note 3 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be evaluated by
Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series of
measurements and can be characterized by standard deviations. The other components, which may be evaluated by Type
B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from probability
density functions based on experience or other information.
Note 4 to entry: In general, for a given set of information, it is understood that the measurement uncertainty is associated
with a stated quantity value attributed to the measurand. A modification of this value results in a modification of the
associated uncertainty.
ISO/FDIS 8932-2:20252026(en)
3.15 3.15
standard uncertainty
measurement uncertainty (3.14) expressed as a standard deviation
3.16 3.16
coverage factor
k
number larger than one by which a combined standard measurement uncertainty is multiplied to obtain an
expanded measurement
Note 1 to entry: Coverage factor is usually symbolized.
3.17 3.17
expanded uncertainty
product of a combined standard measurement uncertainty and a factor larger than the number one
Note 1 to entry: The factor depends upon the type of probability distribution of the output quantity in a measurement
model and on the selected coverage probability.
Note 2 to entry: The term “factor” in this definition refers to a coverage factor (3.16.).
4 Symbols and subscripts
4.1 Symbols
For the purposes of this document, the following symbols apply.
𝐸𝐸 Saturatedsaturated water vapour pressure at a given T and P
𝑒𝑒 Waterwater vapour pressure
𝑓𝑓 Enhancementenhancement factor
𝑘𝑘 Coveragecoverage factor
𝑃𝑃 Pressurepressure
ℎ Relativerelative humidity
𝑇𝑇 Temperaturetemperature
𝑈𝑈 Expandedexpanded uncertainty
𝑢𝑢 Standardstandard uncertainty
𝑣𝑣 Gasgas flow rate
4.2 Subscript
Within the document, the following subscripts are used.
cal Calibrationcalibration
DUT Devicedevice under test
gg Generatorgenerator
grad Gradientgradient
hh Hygrometerhygrometer
ss Saturatorsaturator
tt Testtest cell
ww Waterwater
ref Referencereference
rep Repeatabilityrepeatability
res Resolutionresolution
5 Technical requirements for the laboratory setup
5.1 General
5.1.1 Temperature
The temperature of the test cell can be varied between −90 °C and 35 °C using a climate chamber or a liquid
bath. A climate chamber is more desirable as it minimizes handling of the radiosonde.
The reference temperature of the test cell should be measured inside the test cell using calibrated PRTs to
calculate the reference relative humidity.
If the radiosonde humidity sensor operates using a heating element, both the reference thermometer and the
radiosonde temperature sensor can be affected by the humidity sensor heater. The temperature measured by
the reference PRT should not be impacted by the radiosonde heater.
-
NOTE The impact of the heater can be reduced by increasing the air ventilation rate in the test cell to around 5 m∙s
1 -1 [13] [13]
to 6 m∙s to facilitate the convective cooling of sensors as experienced during soundings. .
The temperature stability of the test cell should be within ±0,1 °C. A change in the test cell temperature of
0,1 °C corresponds to a change in the relative humidity of roughly 0,6 %rh to 0,8 %rh in the temperature range
of –90 °C to 30 °C.
The temperature gradient around the radiosonde humidity sensor should be measured using a calibrated
thermometer to evaluate the uncertainty due to the spatial gradient of the reference relative humidity.
Measurement points of two reference PRTs should be above and below the humidity sensor and the distance
between PRTs be 10 mm to 50 mm. When testing multiple sensors, the temperature distance between these
sensors may be larger than for two sensors resulting in a corresponding increase in uncertainty.
5.1.2 Pressure
The pressure of the test cell should be measured using a calibrated pressure gauge to calculate the saturation
water vapour pressure and the reference relative humidity in the test cell.
The pressure gradient between the test cell and the precision hygrometer should be measured using a
calibrated pressure gauge to evaluate the uncertainty when a calibrated precision hygrometer is used to
determine the reference relative humidity in the test cell instead of a humidity generator.
5.2 Precision hygrometer
5.2.1 Type
A chilled-mirror hygrometer (CMH) measures the dew or frost point temperature at which a constant
condensation layer is kept on a mirror surface. This condensation layer is monitored with using a
photodetector. A CMH can be employed to measure frost-point and dew-point with an accuracy of ±0,1 °C.
However, two CMHs maycan be needed to cover the entire range of frost-point and dew-point (−100 °C to
35 °C) using this document because the upper measurement limit of some CMHs measuring low frost points
ISO/FDIS 8932-2:20252026(en)
is a dew-point of 20 °C. For the calibration of CMHs, the method given in ISO/TR 12148:2009 appliesshould
apply.
Cavity ring-down spectroscopy (CRDS) measures the optical extinction due to water vapour in gases that
scatter and absorb near-infrared light. CRDS can measure low water vapour concentrations from 1 part per
billion (ppb) to 50 parts per million (ppm) with an accuracy in the ppb-level. However, this technique cannot
generally be employed to measure water concentrations higher than 100 ppm.
Tunable diode laser absorption spectroscopy (TDLAS) using near-infrared light wavelength determines the
concentration of water vapour in the gas phase. TDLAS can be employed to measure a wide range of frost-
point and dew-point from −70 °C to 20 °C with an accuracy of ±0,2 °C.
Other technologies can be used provided they can meet measurement goals and data quality requirements.
5.2.2 Installation
It is desirable to minimize the path (tubing) between the humidity generating system and the test cell because
water adsorption and desorption occurs slowly on surfaces and may impact the humidity in the test cell
especially at low temperatures. The precision hygrometer should be installed downstream of the test cell to
measure the frost-point and dew-point in the test cell as shown in Figure 1Figure 1.
The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.

Key
1 dry air generator 10 radiosonde sensor boom
2 gas flow for humidity generation 11 precision hygrometer
3 gas flow for purging of water 12 gas out
4 humidity generating system 13 chilled mirror hygrometer
5 heat exchanger 14 cavity ring-down spectrometer
6 climate chamber 15 tunable diode laser absorption spectroscopy
7 temperature sensor 16 other hygrometer
8 pressure gauge 17 SI traceable
9 test cell
1 dry air generator
2 gas flow for humidity generation
3 gas flow for purging of water
4 humidity generating system
5 heat exchanger
6 climate chamber
7 temperature sensor
8 pressure gauge
ISO/FDIS 8932-2:20252026(en)
9 test cell
10 radiosonde sensor boom
11 precision hygrometer
12 gas out
13 chilled mirror hygrometer
14 cavity ring-down spectrometer
15 tunable diode laser absorption spectroscopy
16 other hygrometer
17 SI traceable
Figure 1 — Example of the test configuration using an SI-traceable precision hygrometer as a
reference
When a precision hygrometer measurement is used as the reference relative humidity for the test cell, the
hygrometer shall be within calibration to maintain SI traceability. In this case, the humidity generating system
depicted in Figure 1Figure 1 may not be SI-traceable. Consequently, any type of humidity generating system
(such as a dry air cylinder) can be utilized as long as the stability of the generated relative humidity in the test
cell remains within 1 %rh. A pressure gradient along the tubing between the test cell and the hygrometer may
be present, which results in the water vapour pressure measured by the hygrometer willto be slightly different
from that of the test cell. To compensate for this, the pressure in the hygrometer should also be measured
using a calibrated pressure gauge to calculate the reference relative humidity in the test cell using the
hygrometer measurement.
5.2.3 Operation
The measurement range for the frost-point and dew-point temperature using a precision hygrometer should
be from −100 °C to 35 °C. Two precision hygrometers maycan be needed to cover the entire range of frost-
point and dew-point temperatures. At an air pressure of 1 000 hPa, a frost-point of −100 °C corresponds to a
water vapour concentration of approximately 0,014 ppm.
The measurement stability of the precision hygrometer should be better than ±0,1 °C for the frost-point and
dew-point temperature. A frost-point and dew-point change of 0,1 °C corresponds to a relative humidity
change of 0,6 %rh to 0,8 %rh in the temperature range of –90 °C to 35 °C.
The uncertainty of the precision hygrometer should be better than 0,3 °C to 0,15 °C for a frost-point and dew-
point temperature range of −100 °C to 35 °C. In general, the calibration uncertainty of hygrometers increases
as the frost-point of a gas decreases because the humidity standard uncertainties and hygrometer
reproducibility uncertainty also increase with frost-point decrease.
5.3 Humidity generator
5.3.1 Type
[13][13]
5.3.1.1 Saturator-based frost-point and dew-point generator
This generator controls the saturated water vapour pressure of the input gas flowing through the saturator at
a specific temperature and pressure. In general, the saturator is submerged in a liquid bath to control the
temperature, and to control the range of humidity generation from –100 °C to 35 °C in frost-point and dew-
point.
[14][14]
5.3.1.2 Diffusion-tube humidity generator
This generator controls the diffusion of water vapour into a dry gas stream (for which the humidity is known)
through a tube connected to a water reservoir under controlled temperature and pressure. Generally, frost-
point and dew-point temperature in the range –100 °C to –70 °C can be achieved.
[14][14]
5.3.1.3 Coulometric humidity generator : :
The coulometric humidity generator produces a controlled amount of hydrogen and oxygen through the
electrolysis of water governed by Faraday’s law, followed by the recombination of the generated hydrogen
and oxygen back into water and then the addition of the generated water into the dry gas stream, for which
the humidity is known. The humidity generation can cover –100 °C to –70 °C in frost-point temperature.
[15][15]
5.3.1.4 Divided-flow humidity generator
This type of generator controls humidity by diluting the humidity produced by generators mentioned above
through mixing with an extremely dry gas stream.
5.3.1.5 Other SI-traceable and characterized methods
Other SI-traceable and characterized methods to generate humidity can be used provided they meet the
required humidity range and necessary uncertainty.
If the uncertainty of a humidity generator is not determined, it cannot be considered SI-traceable. In such
cases, the humidity generator can be calibrated against SI-traceable precision hygrometers to establish
traceability to the SI.
5.3.2 Installation
The humidity generator should be installed upstream of the test cell containing the radiosonde so that the gas
with a known water vapour pressure flows into the test cell (see Figure 2Figure 2).).
The temperature of the tubing connecting the humidity generator and the test cell shall be higher than the
frost-point and dew-point temperature of the generator to prevent the formation of frost or dew within the
tubing.
If dry gas is needed for the generation of the desired humidity, the dry gas generator should be upstream of
the humidity generator. It is also recommended to flow the dry gas directly into the test cell and bypass the
humidity generator to efficiently purge water that has adsorbed onto the surface of the test cell.
5.3.3 Operation
The generated frost-point and dew point temperature ranges from −100 °C to 35 °C. At an air pressure of
1 000 hPa, a frost-point of −100 °C corresponds to a water vapour concentration of approximately 0,014 ppm.
The stability of the generated humidity should be better than ±0,1 °C of the frost-point and dew point
temperature. A frost-point and dew point change of 0,1 °C corresponds to a relative humidity change of
maximum 0,8 %rh in a temperature range of −90 °C ~ 35 °C.
It can take several hours for the generated relative humidity to equilibrate. The test cell shown should be
located downstream of the humidity generator.
Only the sensor boom should be inserted into the test cell, with the remaining parts located outside the test
cell. This is to minimize water desorption from these surfaces in the test cell which maycan add water vapour
during testing.
ISO/FDIS 8932-2:20252026(en)
If the size of the test cell is too large compared to the radiosonde sensor boom to test multiple sensors at the
same time, the gradient of the relative humidity and temperature inside the test cell should be evaluated. The
effect of water adsorption and desorption on the surface of the sensor boom maycan also impact the humidity
in the test cell, especially at cold temperatures.

Key
1 dry air generator 10 radiosonde sensor boom
2 generation 11 hygrometer
3 purging 12 gas out
4 humidity generator 13 saturator-based generator
5 heat exchanger 14 diffusion-tube generator
6 climate chamber 15 coulometric generator
7 temperature sensor 16 divided flow generator
8 pressure gauge 17 other humidity generator
9 test cell 18 SI traceable
1 dry air generator
2 generation
3 purging
4 humidity generator
5 heat exchanger
6 climate chamber
7 temperature sensor
8 pressure gauge
9 test cell
10 radiosonde sensor boom
11 hygrometer
12 gas out
13 saturator-based generator
14 diffusion-tube generator
15 coulometric generator
16 divided flow generator
17 other humidity generator
18 SI traceable
Figure 2 — Example of the test setup using an SI-traceable humidity generator as a reference
6 Test preparation
6.1 Laboratory environmental conditions
Tests should be conducted in a laboratory with a room temperature of 23 °C ± 3 °C and a relative humidity of
50 %rh ± 20 %rh.
6.2 Preparation of the radiosonde
Radiosonde measurements can be conducted via wired or wireless communication. The communication setup
of the radiosonde should be evaluated for suitability before the test.
The acceptable operation of the data acquisition software recording the measurement data from the
radiosonde should be verified before starting testing.
If the measurements are taken via wired communication, it should be ensured by the manufacturer that the
measured relative humidity is same with that obtained by wireless communication.
Before taking measurements, the radiosonde should be prepared according to procedures applied in a typical
sounding event. Depending on the radiosonde manufacturer’s instructions, the preparational procedures may
include ground checks and corrections to the measurement.
6.3 Examination of the laboratory setup
6.3.1 General
The inner surface of the test cell should be verified as clean.
6.3.2 Examination of the dry gas generator
The dry gas generator should be examined to confirm that the lowest frost-point of the generated dry gas is –
100 °C or lower. The dry gas is used to purge water adsorbed onto the test cell before the test. The dry gas can
also be used to generate the desired humidity by dilution of the humidity generator’s output.
6.3.3 Examination of the liquid bath and climate chamber
Operation at the required temperature and stability should be confirmed.
6.3.4 Examination of the measurement system for the calculation of the reference relative
humidity in the test cell
Normal operation of the precision hygrometer, thermometer, pressure gauge, mass flow controller and digital
multi-meter should be confirmed.
6.3.5 Examination of the radiosonde measurement software and the control software for the
reference relative humidity
Normal operation should be confirmed.
ISO/FDIS 8932-2:20252026(en)
6.4 Installation of the radiosonde
A long-term stable power supply for the radiosonde is required because it can take several hours up to days
to continuously test the radiosonde humidity sensor at various temperatures.
The radiosonde humidity sensor should be installed in the central area of the test cell close to the gas flow.
The sensor boom should be inserted into the test cell while the other parts, including the radiosonde body,
remain outside the test cell to exclude the effect of water adsorption and desorption on the surface of the
radiosonde body.
After the installation of the radiosonde, the radiosonde measurement data should be transmitted to a data
acquisition system via wired or wireless communication.
7 Test methods and procedures
7.1 Operation
7.1.1 Purging the test cell
After the radiosonde humidity sensor is installed in the test cell, water vapour adsorbed onto the test cell and
the sensor boom should be purged using dry gas generated by the dry gas generator or the low frost-point gas
generated by the humidity generator.
The test cell and tubing can be slightly heated for more efficient purging if necessary.
The purging gas should be monitored using the precision hygrometer until the frost-point of the output gas is
lower than the lowest frost-point needed for the testing of the radiosonde humidity sensor. For example, if the
lowest frost-point for the test is −100 °C, the frost-point of the purging gas should be lower than −100 °C.
7.1
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