ISO 9847:2023

Solar energy - Calibration of pyranometers by comparison to a reference pyranometer

This document specifies two preferred methods for the calibration of pyranometers using reference pyranometers; indoor (Type A) and outdoor (Type B). Indoor or type A calibration, is performed against a lamp source, while the outdoor method B, employs natural solar radiation as the source. Indoor calibration is performed either at normal incidence (type A1), the receiver surface perpendicular to the beam of the lamp or under exposure to a uniform diffuse lamp source using an integrating sphere (type A2). Outdoor calibration is performed using the sun as a source, with the pyranometer in a horizontal position (type B1), in a tilted position (type B2), or at normal incidence (type B3). Calibrations according to the specified methods will be traceable to SI, through the world radiometric reference (WRR), provided that traceable reference instruments are used. This document is applicable to most types of pyranometers regardless of the type technology employed. The methods have been validated for pyranometers that comply with the requirements for classes A, B and C of ISO 9060. In general, all pyranometers may be calibrated by using the described methods, provided that a proper uncertainty evaluation is performed. Unlike spectrally flat pyranometers, non-spectrally flat pyranometers might have a spectral response that varies strongly with the wavelength even within the spectral range from 300 to 1 500 nm, and therefore the calibration result may possibly be valid under a more limited range of conditions. The result of a calibration is an instrument sensitivity accompanied by an uncertainty. This document offers suggestions for uncertainty evaluation in the annexes.

Énergie solaire — Étalonnage des pyranomètres par comparaison à un pyranomètre de référence

General Information

• Status: Published
• Publication Date: 19-Jan-2023
• ICS: 27.160 - Solar energy engineering
• Technical Committee: ISO/TC 180/SC 1 - Climate - Measurement and data
• Drafting Committee: ISO/TC 180/SC 1 - Climate - Measurement and data
• Current Stage: 6060 - International Standard published
• Start Date: 20-Jan-2023
• Due Date: 13-Jun-2023
• Completion Date: 20-Jan-2023

Relations

• Revises: ISO 9847:1992 - Solar energy - Calibration of field pyranometers by comparison to a reference pyranometer
• Effective Date: 23-Apr-2020

Overview

ISO 9847:2023 - "Solar energy - Calibration of pyranometers by comparison to a reference pyranometer" specifies standardized methods to calibrate pyranometers used to measure hemispherical solar irradiance. The standard defines two preferred comparison-based approaches: indoor (Type A) calibrations against lamp sources and outdoor (Type B) calibrations using natural solar radiation. Calibration results are reported as an instrument sensitivity with an associated uncertainty, and procedures are designed to be SI-traceable via the World Radiometric Reference (WRR) when traceable reference instruments are used.

Keywords: ISO 9847:2023, pyranometer calibration, solar energy, SI traceability, WRR, sensitivity, uncertainty evaluation

Key topics and technical requirements

• Calibration methods
  • Type A (Indoor)
    • A1: Direct-beam lamp source at normal incidence (receiver perpendicular to beam)
    • A2: Uniform diffuse lamp source using an integrating sphere
  • Type B (Outdoor)
    • B1: Horizontal position (global horizontal irradiance, GHI)
    • B2: Tilted position (inclined plane)
    • B3: Normal incidence (tracking to sun)
• Traceability and reference standards
  • Calibrations are traceable to SI through the World Radiometric Reference (WRR) (WRR uncertainty < ±0.3%), provided traceable reference pyranometers are used.
• Instrument applicability
  • Applicable to most pyranometer technologies and validated for pyranometers meeting classes A, B and C of ISO 9060.
  • Spectrally flat vs non‑spectrally flat: non‑spectrally flat instruments may have wavelength-dependent response (300–1 500 nm) and calibration may be valid under more limited conditions.
• Measurement and uncertainty
  • Calibration yields a single sensitivity (for most linear-output pyranometers) plus an uncertainty. Annexes provide guidance and example calculations for uncertainty evaluation (referencing ISO/IEC Guide 98-3 and other practices).

Practical applications and users

• Calibration procedures in ISO 9847:2023 support activities requiring low measurement uncertainty:
  • Solar resource assessment and mapping
  • Performance testing and monitoring of PV and solar-thermal power plants
  • Meteorological services and climate monitoring stations
  • Instrument manufacturers and calibration laboratories
  • Building energy modeling, agriculture, materials testing
• Users: calibration labs, national metrology institutes, solar‑plant operators, research centres, instrument manufacturers and quality managers implementing ISO 9060-compliant measurement systems.

Related standards

• ISO 9060 - Specification and classification of pyranometers (classes A, B, C)
• ISO/IEC Guide 98-3 (GUM) - Guide to the expression of measurement uncertainty
• World Radiometric Reference (WRR) - SI reference for irradiance

ISO 9847:2023 provides practical, validated procedures for reliable, traceable pyranometer calibration to support high-quality solar irradiance measurements across research, industry and operations.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 9847
Second edition
2023-01
Solar energy — Calibration of
pyranometers by comparison to a
reference pyranometer
Énergie solaire — Étalonnage des pyranomètres par comparaison à
un pyranomètre de référence
Reference number
© ISO 2023
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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Pyranometer calibration . 4
4.1 General . 4
4.2 Pyranometer sensitivity, measurement equation, measurand . 4
4.3 Indoor and outdoor calibration compared . 6
4.4 Method validation . 6
4.5 Calibration uncertainty . 6
5 Measuring equipment . 7
5.1 Data acquisition and recording. 7
5.2 Instrument platforms . 8
5.3 Pyranometers . 8
6 Indoor calibration (Type A) . 8
6.1 Introductory remarks on indoor calibration . 8
6.2 Reference pyranometers for indoor calibration . 8
6.3 Indoor calibration systems . 9
6.3.1 System with a direct beam source (type A1) . 9
6.3.2 Systems with an integrating sphere source (type A2) . 9
6.4 Indoor calibration procedures . 9
6.4.1 Calibration procedure requirements (types A1 and A2). 9
6.4.2 Indoor calibration procedures (types A1 and A2) . 9
6.4.3 Calculation of the sensitivity . 10
6.4.4 Calibration conditions and optional correction of reference operating
conditions . 11
6.4.5 Uncertainty evaluation . 11
7 Outdoor calibration (Type B) .12
7.1 Introductory remarks on outdoor calibration .12
7.2 Reference pyranometers for outdoor calibration .12
7.3 Outdoor calibration systems . 12
7.3.1 Site selection for outdoor calibration .12
7.3.2 Tracking for normal incidence calibration (type B3) .13
7.4 Outdoor calibration procedures . 13
7.4.1 Calibration procedure requirements (B1, B2, B3) .13
7.4.2 Outdoor horizontal calibration procedure (type B1) .13
7.4.3 Outdoor tilted calibration procedure (type B2) . 14
7.4.4 Outdoor normal incidence calibration procedure (type B3) .15
7.4.5 Calculation of the sensitivity . 15
7.4.6 Calibration conditions and optional correction of reference operating
conditions . 16
7.4.7 Uncertainty evaluation . 16
8 Calibration certificate .17
Annex A (informative) Examples of calibration systems using artificial sources .18
Annex B (informative) Calculation of daily average zenith angle .22
Annex C (informative) Introduction of a new pyranometer sensitivity .24
Annex D (informative) Data quality review for outdoor calibration .26
Annex E (informative) Uncertainty evaluation for outdoor calibration .29
iii
Annex F (informative) Uncertainty evaluation for indoor calibration .30
Bibliography .31
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).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
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 180, Solar energy, Subcommittee SC 1,
Climate – Measurement and data.
This second edition cancels and replaces the first edition (ISO 9847:1992) which has been technically
revised.
The main changes are as follows:
— focus on current calibration practices;
— adapted recommendations for mathematical treatment of data;
[1]
— adaptation of the terminology to the revised ISO 9060:2018 and ISO Guide 99 ;
[2]
— added comments on uncertainty evaluation of the calibration with reference to ASTM G213 and
ISO/IEC Guide 98-3;
— inclusion of reference to non-spectrally-flat pyranometers, that are now also included in ISO 9060.
Annexes A, B, C, D, E and F are given for information only.
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
Pyranometers are instruments used to measure the irradiance (power per unit area) received from the
sun for many purposes.
In recent years the application of hemispherical solar radiation measurement, using pyranometers,
has risen sharply. The main application of pyranometers now is no longer scientific research, but
assessment of the performance of solar power plants.
Accurate measurements of the hemispherical solar radiation are required for
a) the determination of the energy input to solar energy systems such as photovoltaic (PV) -, and solar
thermal systems, as a basis for performance assessment,
b) the testing and assessment of solar technologies,
c) the geographic mapping of solar energy resources, and
d) other applications such as agriculture, building efficiency, material degradation and reliability,
climate, weather, health, etc.
Today’s growing solar energy performance assessment markets demand the lowest possible
measurement uncertainties. To meet this demand, a measurement requires an uncertainty evaluation
[3]
and an accurate time stamp .
Calibration of measuring instruments is an essential part of the uncertainty evaluation and part of
any quality management system. Regular instrument re-calibration according to this standard helps
attaining the required low measurement uncertainties. Calibration usually will show the instrument is
stable and then serves as:
— confirmation that the measurement data collected over the time interval from the previous to the
present calibration are reliable
— the instrument is expected to remain stable, future measurement data are expected to be reliable.
Uncertainties mentioned in this document are expanded uncertainties with a coverage factor k = 2.
vi
INTERNATIONAL STANDARD ISO 9847:2023(E)
Solar energy — Calibration of pyranometers by
comparison to a reference pyranometer
1 Scope
This document specifies two preferred methods for the calibration of pyranometers using reference
pyranometers; indoor (Type A) and outdoor (Type B).
Indoor or type A calibration, is performed against a lamp source, while the outdoor method B, employs
natural solar radiation as the source.
Indoor calibration is performed either at normal incidence (type A1), the receiver surface perpendicular
to the beam of the lamp or under exposure to a uniform diffuse lamp source using an integrating sphere
(type A2).
Outdoor calibration is performed using the sun as a source, with the pyranometer in a horizontal
position (type B1), in a tilted position (type B2), or at normal incidence (type B3).
Calibrations according to the specified methods will be traceable to SI, through the world radiometric
reference (WRR), provided that traceable reference instruments are used.
This document is applicable to most types of pyranometers regardless of the type technology employed.
The methods have been validated for pyranometers that comply with the requirements for classes A,
B and C of ISO 9060. In general, all pyranometers may be calibrated by using the described methods,
provided that a proper uncertainty evaluation is performed.
Unlike spectrally flat pyranometers, non-spectrally flat pyranometers might have a spectral response
that varies strongly with the wavelength even within the spectral range from 300 to 1 500 nm, and
therefore the calibration result may possibly be valid under a more limited range of conditions.
The result of a calibration is an instrument sensitivity accompanied by an uncertainty. This document
offers suggestions for uncertainty evaluation in the annexes.
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 9060, Solar energy — Specification and classification of instruments for measuring hemispherical solar
and direct solar radiation
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological 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
pyranometer
radiometer designed for measuring the irradiance on a plane receiver surface which results from the
radiant fluxes incident from the hemisphere above within the wavelength range 0,3 µm to 3 µm.
[SOURCE: ISO 9060:2018, 3.5, modified — Tolerances have been changed and the Note 1 to entry was
deleted.]
3.2
hemispherical solar radiation
solar radiation received by a plane surface from a solid angle of 2π sr
[SOURCE: ISO 9060:2018, 3.1, modified — Note 1 to entry was deleted.]
3.3
global horizontal solar irradiance
GHI
G
hemispherical solar radiation received by a horizontal plane surface
[SOURCE: ISO 9060:2018, 3.2, modified — Symbol G and abbreviation GHI were added and Note 1 to
entry was deleted.]
3.4
sensitivity
quotient of the change in an indication of a measuring system and the corresponding change in a value
of a quantity being measured
Note 1 to entry: See Reference [1].
3.5
calibration of a pyranometer
determination of the relationship between the pyranometer (3.1) output and the irradiance, with
associated measurement uncertainties, under well-defined operating conditions
Note 1 to entry: For most pyranometers, the output varies linearly with the irradiance and the calibration result
is expressed as a single sensitivity.
Note 2 to entry: See References [1] and [4].
3.6
reference pyranometer
pyranometer (3.1) used as reference standard, i.e. an instrument used for calibration of other
pyranometers in a given organization
3.7
test pyranometer
pyranometer (3.1) being calibrated
Note 1 to entry: Called field pyranometer in the previous version of this document.
3.8
calibration conditions
conditions, ambient- or instrument, during the calibration process
3.9
reference-operating condition
operating condition prescribed for evaluating the performance of a measuring instrument or measuring
system or for comparison of measurement results
Note 1 to entry: For practical purposes these typically are the conditions specified for the reported sensitivity.
Note 2 to entry: For measurement results, see Reference [1].
3.10
world radiometric reference
WRR
measurement standard representing the SI unit of irradiance with an uncertainty of less than ±0,3 %
Note 1 to entry: The reference was adopted by the World Meteorological Organization (WMO) and has been in
effect since 1 July 1980. The WRR is maintained by the WMO World Radiation Centre at Davos. The distinguishing
feature of traceability to WRR is that reference-operating conditions include the spectrum of natural direct solar
radiation.
3.11
sample
data acquired from a sensor or measuring device
Note 1 to entry: See Reference [5].
3.12
sampling interval
time between samples (3.11)
Note 1 to entry: See Reference [5].
3.13
record
data recorded and stored in data log, based on acquired samples (3.11)
Note 1 to entry: See Reference [5].
3.14
data series
set of selected records (3.13)
3.15
correction
value added algebraically to the uncorrected result of a measurement to compensate for systematic
error
[SOURCE: ISO Guide 98-3:2008, B.2.23]
Note 1 to entry: The correction for offsets is equal to the negative of the estimated systematic error. Since the
systematic error cannot be known perfectly, the compensation cannot be complete.
3.16
correction factor
numerical factor by which the uncorrected result of a measurement is multiplied to compensate for
systematic error
Note 1 to entry: Since the systematic error cannot be known perfectly, the compensation cannot be complete.
[SOURCE: ISO Guide 98-3:2008, B.2.24]
3.17
solar tracker
mechanical device capable of rotation around 2 axes, e.g. zenith and azimuth, following the path of the
sun
3.18
integrating sphere
sphere or hemisphere, equipped with one or more lamps, internally coated with a spectrally flat white
paint providing uniform illumination
3.19
tilt angle
angle between the horizontal plane and the plane of the pyranometer (3.1) sensor surface
3.20
angle of incidence
angle of radiation relative to the sensor measured from normal incidence (varies from 0° to 90°)
3.21
zenith angle
angle of incidence (3.20) of radiation, relative to zenith (angle between the earth’s surface normal and
the line to the sun)
Note 1 to entry: It equals the angle of incidence (3.20) for horizontally mounted instruments.
3.22
solar azimuth angle
angle between a reference direction (north or south) and the projection of beam radiation on the
horizontal plane
[6]
Note 1 to entry: Duffie and Beckman define the reference direction (zero solar azimuth angle) as south for both
the northern and southern hemisphere. In the Duffie and Beckman definition the azimuth angle ranges from
−180° to +180°, where angles east of south are negative and west of south positive. Other references and models
use north as reference direction.
4 Pyranometer calibration
4.1 General
Calibration of a pyranometer involves a test to determine the relationship between pyranometer output
and irradiance. The result is usually expressed as a single sensitivity, with associated uncertainty,
under well-defined operating conditions. Pyranometer calibration may be carried out according to
[4]
ISO 9846 , outdoors against a pyrheliometer, or according to this document, indoors or outdoors
against a reference pyranometer. Both documents describe how to transfer the sensitivity of the
reference instrument to the test instrument.
The recommended calibration interval for pyranometers differs from one manufacturer to the other.
[5]
IEC 61724-1 recommends instrument recalibration once every 2 years or more frequently according
to manufacturer recommendations for Class A monitoring systems, and according to manufacturer
recommendations for Class B systems. For Class A monitoring systems for global horizontal solar
irradiance and plane of array irradiance measurement, this document requires a calibration
uncertainty of less or equal than 2 %. Under typical but not all conditions, this uncertainty is attainable
[3]
with pyranometers having calibration uncertainties in the order of 1,5 % or better .
4.2 Pyranometer sensitivity, measurement equation, measurand
The relationship between pyranometer sensitivity, output and irradiance or measurement equation for
thermal pyranometers is given by Formula (1):
SV= G (1)
where
S is the sensitivity in output units/(W/m );
G is the global horizontal solar irradiance in W/m ;
V is the pyranometer output in arbitrary units.
Calibration of pyranometers essentially consists of a measurement at or traceable to an irradiance level
in the middle or close to the upper end of the measurement range.
Clear sky conditions are the most common calibration reference condition for calibration, so that during
calibration the measurand formally is global horizontal solar irradiance under a clear sky.
NOTE Calibration reference conditions can differ from the operation conditions in several ways, not only
spectrally, but can also differ in terms of e.g. temperature, wind, solar position, atmospheric conditions (cloud
cover, aerosols) and instrument tilt. Even the measurand can change (calibrated for global irradiance, used
for diffuse or reflected irradiance measurements). If the calibration reference conditions and the operating
conditions are different the user considers this for the uncertainty evaluation of the measurements and considers
proper corrections.
While traditional pyranometers had analogue millivolt output signals, many modern pyranometers
have different, for example digital or current-loop, outputs. These are often standardised outputs.
The calibration process of these instruments typically includes adjustment by programming a new
sensitivity into the firmware, so that the sensitivity after calibration as perceived by the user is
2 2 2
always the same; for example, 1 (W/m )/(W/m ) for digital instruments or 0 W/m = 4 mA, 1 600 W/
m = 20 mA for a pyranometer with a current-loop output. For these instruments the measurement
Formula (1) is adapted accordingly.
The calibration of instruments with standardised outputs is nevertheless expressed in V/(W/m )
because this gives a clear indication of the correction applied from one calibration to the next, and of the
stability of the sensor. For instruments with such internal signal conversion, the voltage measurement
2 2
usually is not separately calibrated. In such cases the V/(W/m ) shall be interpreted as V/(W/m ) “as
measured by the on-board analogue to digital conversion”.
In exceptional cases laboratories and users may choose to use alternative measurement equations.
They may use a correction factor acting on in S, for example accounting for temperature dependence.
ISO 9060 defines the measurement error "zero offset A". Corrections for zero offset A can be made
during outdoor calibration. It may lead a higher accuracy of the calibration. However, care should be
taken to ensure that the same correction technique used in calibration is then used for subsequent
measurements. The zero offset A is not constant. It depends on the environmental conditions for example
on cloud condition, sky temperature, wind speed (ventilator application), and thermal coupling of the
instrument to its mounting. Applying such corrections may lead to a lower measurement accuracy.
When applying corrections for offsets, the measurement Formula (1) then gets the form of Formula (2):
SV= − VG (2)
()
O
where
S is the sensitivity in output units/(W/m );
G is the irradiance in W/m ;
V is the pyranometer output in arbitrary units;
V is an offset on the output in arbitrary units.
Working with instruments calibrated with a correction for offsets according to Formula (2), users shall
also adapt their measurement equation from Formulae (1) and (2).
Additional corrections for example for temperature dependence, and outdoor solar- or indoor lamp
spectrum can also be implemented.
Annex C contains informative comment on what to do with calibration results; how to introduce a new
pyranometer sensitivity.
4.3 Indoor and outdoor calibration compared
Under this document, there are two options for pyranometer calibration: indoors, in the laboratory
using lamps as a source and outdoors under the Sun. There are the following fundamental differences:
— An indoor calibration is only the transfer of the outdoor calibration of the reference instrument to a
test instrument.
— Indoor calibration is done by comparison of the test pyranometer to a reference pyranometer of the
same model, and thus of the same class. Initial (i.e. before making optional corrections) reference-
operating conditions, the condition for which the calibration of the test instrument is valid, are the
conditions reported as valid for the calibration of the reference pyranometer.
— Outdoor calibration is done by comparison of the test pyranometer to the reference pyranometer,
where the reference pyranometer is not necessarily of the same model, typically of a higher or equal
class. Initial reference operating conditions are the outdoor conditions during this calibration.
— For both indoor and outdoor calibration, the reference-operating conditions may later, in the
calibration report, be adapted to other conditions than those to which the calibration is initially
traceable. This then leads to an adapted sensitivity and reduces the calibration accuracy.
In all cases corrections shall also be accounted for in the calibration uncertainty and be reported on the
calibration certificate.
Calibration laboratories may report multiple sensitivities valid for different reference-operating
conditions, so that users may work with a sensitivity valid for conditions as close as possible to actual
operating conditions (e.g. sensitivities for a non-spectrally flat pyranometer operating under clear and
overcast sky conditions).
The uncertainty evaluation for one instrument may be used for other instruments of the same model
as long as the conditions of testing remain the same and the method for evaluation is verified (the
identified critical influencing factors are under control). Some laboratories use statistical data of
the test or multiple tests as input to the uncertainty evaluation. In that case calibration of the same
pyranometer model may have a variable uncertainty.
For outdoor calibration, the conditions of testing (temperature, angle of incidence/airmass) usually
vary between one calibration and the next and the uncertainty of the calibration result (sensitivity)
will typically vary. For indoor calibration the conditions of testing can be kept within certain known
limits and the contribution to the uncertainty can be constant.
4.4 Method validation
The methods described in this document have been validated for pyranometers that comply with the
requirements for classes Spectrally Flat A, B and C of ISO 9060 and silicon photodiode pyranometers
complying with class C.
For new instrument designs, possibly working according to new measurement principles, the methods
may equally be applicable, but this shall be proven by testing for the individual instrument design.
4.5 Calibration uncertainty
Laboratories shall perform an uncertainty evaluation of all their calibrations and supply this
evaluation with the calibration in summary. Uncertainty evaluations shall be made according to
ISO/IEC Guide 98-3, and express uncertainties as expanded measurement uncertainties with a coverage
factor of 2 (confidence level typically representing approximately 95 % of the data points, and two
standard deviations).
The calibration uncertainty for indoor calibration depends on
— calibration method,
— pyranometer type, and
— uncertainty of the sensitivity of the reference pyranometer.
The uncertainty of indoor calibration may be based on a limited set of tests involving the pyranometer
type and the method, and calculations according to ISO/IEC Guide 98-3. It may then be treated as a
constant percentage of the sensitivity.
The uncertainty of outdoor calibrations depends on above factors and also on
— solar angle of incidence,
— instrument temperature, and
— atmospheric stability.
The uncertainty of outdoor calibration may not be based on a limited set of tests involving the
pyranometer type. Because of the variability of the environmental factors, the uncertainty shall be
analysed separately for every individual calibration. Typically, calibration conditions and uncertainty
will not be the same from one calibration to the other.
NOTE Pyranometer calibration uncertainties are relatively large, while the expected instrument drift
from one calibration to the next is typically small compared to the uncertainty of calibration. It is therefore
often more probable that perceived sensitivity changes are caused by differences associated with calibration
methods (even for application of the same method between different laboratories) or reference pyranometers
used, rather than by the non-stability of the calibrated pyranometer. This situation is exceptional. In most other
areas of metrology, the uncertainty of calibration is not a limiting factor; in these other areas it is possible to
calibrate with an uncertainty lower than the 1 % (expanded k = 2) that is attainable with pyranometers. Both
for ISO 9846 and for the procedures described in this document the uncertainties contributed by the calibration
method are in the order of 0,5 % when performed with care, where the uncertainty using the calibration using
a pyrheliometer is lower than that with a pyranometer. Combined with other uncertainties such as those of the
reference pyranometer sensitivity and the WRR scale, these lead to calibration uncertainties of commercially
available Class A instruments in the order of 1,5 %. See Annex F and References [17] to [19] for examples.
For spectrally non-flat pyranometers the influence of the calibration conditions on the measurement
uncertainty is high compared to spectrally flat pyranometers. This shall be considered when selecting
the calibration duration, the number of days used, and the conditions observed during the calibration.
The closer the calibration conditions match the conditions during the operation of the test pyranometer
the lower the measurement uncertainty in the application. For example, an instrument calibrated
under a clear sky may be used under overcasts skies, using a correction applied on the sensitivity. This
correction will have its own uncertainty.
NOTE Using the sensitivity of a silicon photodiode pyranometers obtained from a calibration under clear
[7]
sky conditions under overcast sky condition may lead to a 8 % or more overestimation of the solar radiation .
5 Measuring equipment
5.1 Data acquisition and recording
Traditionally pyranometers were passive instruments with an analogue output in the millivolt range.
Nowadays pyranometers may also have other outputs such as an amplified voltage, a current loop or a
digital signal. Laboratories should select the data acquisition based on their own requirements. When
using pyranometers with an analogue (as opposed to digital) output, the measurement specifications
of the data acquisition system are an important factor influencing the calibration accuracy. Their
contribution to the uncertainty should be no more than 0,1 %.
5.2 Instrument platforms
A platform for mounting instruments is required for all horizontal, B1 type, and tilted, B2 type,
calibrations. In case it is tilted, it shall be able to be tilted over a suitable range of angles from the
horizontal with an uncertainty of less than 0,5°.
Pyranometers have to be accurately aligned; the deviation between the tilt angle of the reference
pyranometer and that of the test pyranometer shall be not more than 0,2°. An alignment error of more
than 0,1° may have an impact on the calibration result, depending on solar zenith angle.
5.3 Pyranometers
Pyranometers shall be in clean and in good condition.
The test pyranometer should not show signs of degradation; check that the colour of the black absorber
paint or white diffuser has not changed by comparing to a reference pyranometer of the same type.
There should not be signs of corrosion on the connector pins.
The pyranometer bubble levels should be high-quality and stable. Typically, this is verified by visual
observation, and possibly by mounting on a level plate.
The reference and test pyranometers shall be internally dry. This is verified, for example by looking at
the colour of the humidity indicator or the output of internal humidity sensors. If humidity indicators
are absent this may be verified by looking for any signs of condensation inside the dome. Humidity may
also condense in the dome during outdoor measurement.
6 Indoor calibration (Type A)
6.1 Introductory remarks on indoor calibration
Indoor calibration is carried out in a controlled laboratory environment using a lamp as a source.
Indoor transfer of the sensitivity of a calibrated reference pyranometer may only be carried under the
condition that the calibration reference pyranometer is of the same model as the test pyranometer. The
calibration may then be carried out using a lamp as a source which has a different emission spectrum
than that of the sun, and at a relatively low irradiance level (typically in the range of 300 W/m to
1 000 W/m ).
[4]
The reference pyranometer will typically be calibrated outdoors according to ISO 9846 .
Since the test and reference pyranometer are of the same model, both instruments will respond in the
same way when irradiance level, tilt error and spectrum deviate from the conditions during outdoor
calibration (i.e. the linearity and spectral response of both instruments are identical, therefore these
effects will cancel). This way, the newly calibrated test instrument will obtain a sensitivity that is valid
under the same outdoor reference conditions of irradiance, tilt and spectrum under which the reference
pyranometer was calibrated. The additional step of the indoor transfer calibration only leads to a small
increase of the calibration uncertainty, it does not change the reference conditions of the calibration.
For example, in case the reference pyranometer has been calibrated under clear sky conditions (the
2 2
radiation spectrum is not specifically defined) over a range of 500 W/m to 1 000 W/m , the sensitivity
of the newly calibrated pyranometer will also be valid for an irradiance range of 500 W/m to 1 000 W/
m and for the clear sky conditions.
6.2 Reference pyranometers for indoor calibration
The reference pyranometer for indoor calibration (type A) shall be of the same model as the test
pyranometer. The same model is understood as having the same optics and thermal and detector
design, but not necessarily as having the same model name.
6.3 Indoor calibration systems
6.3.1 System with a direct beam source (type A1)
Type A1 calibration requires a lamp source, possibly combined with optical apertures and lenses
creating a stable and spatially uniform beam on one or more pyranometers. The system has positioners
or a translating- or rotating mechanism to accurately exchange instrument positions or exchange
instruments at one single position. The system has an optical shutter so that unshaded (light)
and shaded (dark) records can be taken. The apparatus is placed in a dark room or contains a dark
measurement compartment to mitigate the influence of the environment.
Annex A gives examples of possible designs of such systems.
6.3.2 Systems with an integrating sphere source (type A2)
For calibration type A2, an integrating sphere or hemisphere is required, offering a near perfect
homogeneous and diffuse source over the full field of view of the pyranometers. The system may have
positioners or a translating or rotating mechanism to accurately exchange instrument positions or
exchange instruments at one single position. Alternatively, the system may be designed such that the
pyranometers are positioned symmetrically so all have a homogeneous and identical field of view of the
source and no positional exchange is required as part of the calibration procedure. The user may make
an unexposed (dark conditions, zero irradiance) measurement and correct for zero offsets.
Annex A gives one example of a possible design of such a system.
6.4 Indoor calibration procedures
6.4.1 Calibration procedure requirements (types A1 and A2)
Calibration requirements for indoor calibration are:
— reference and test pyranometer shall be mounted in the same plane, having the same detector
height;
— exact exchange of the positions between reference and test pyranometer should be possible;
— the difference between irradiance levels at the instrument positions should be as small as possible
but shall be lower than 10 %;
— records used for calibration should be based on samples taken after more than 3 × the 95 % response
time as specified for the pyranometer type in ISO 9060, has elapsed.
6.4.2 Indoor calibration procedures (types A1 and A2)
Check that the reference and test pyranometers are of the same model.
Allow electronics and the lamp to stabilize. A stability- check is built into the mathematical treatment
of 6.4.3.
Properly position the pyranometers under the lamp or integrating sphere source. Optionally, determine
zero offsets, and compensate for these, for example by taking records of the output of the pyranometers
alternately shaded and unshaded. For instruments that do not suffer from zero offsets, such as
pyranometers equipped with photodiode sensors, the zero offset measurement does not need to be
done. Optionally, exchange instrument positions to compensate for inhomogeneity of the irradiance.
If fast-response (photodiode) pyranometers are used, the detector will not (like most thermal
pyranometers) average over several cycles of the mains power. This means - depending on the light
source used - it may be sensitive to power oscillations. Note that the power frequency is two times
the voltage frequency. Therefore, the sampling frequency should be a multiple of 100 Hz and 120 Hz
for mains frequencies of 50 Hz and 60 Hz respectively. Alternatively, an integrated value over the full
oscillation period should be recorded.
Apply the mathematical treatment described in 6.4.3.
6.4.3 Calculation of the sensitivity
Calculate the measured values, V, from the records taken using the following Formulae (3) to (6):
VV=−V (3)
rr,,ur s
VV=−V (4)
tt,,ut s
'' '
VV=−V (5)
rr,,ur s
'' '
VV=−V (6)
tt,,ut s
where
V is the pyranometer output in arbitrary units;
r is the reference pyranometer;
t is the test pyranometer;
u is the unshaded (light);
s is the shaded (dark) offset or zero offset obtained in another way;
‘ are the records taken with sensor positions exchanged at a new moment in time (or in case only
1 instrument mounting location is available: the repeat reference and test samples obtained at
the same position at different times).
Verify that the lamp source is stable by checking whether the condition
VV
rt
11 −k << +k (7)
() ()
''
VV
rt
is fulfilled, for k ≤ 0,01.
Calibration laboratories may also use a smaller value of k than 0,01, which is beneficial for the
calibration uncertainty.
If the condition is fulfilled, calculate the sensitivity, S , of the test pyranometer from:
t
'
VV+
tt
S = S (8)
t r
'
VV+
rr
or from:
′
V V S
 
t t r
S = + (9)
t  
V ′ V 2
 
r r
Under ideal conditions with perfectly stable lamps and instruments, the 2 formulae lead to the same
result. The choice of formula’s is left to the user’s scientific judgement. Using Formula 9 may offer a
more direct insight of the homogeneity of the irradiance.
6.4.4 Calibration conditions and optional correction of reference operating conditions
For a calibration laboratory performing indoor calibration there are several commonly used ways to
report calibration results, (sensitivity and its uncertainty), and the conditions in which the reported
sensitivity is valid; this can be either the calibration conditions or:
Option 1 a:
— the reported sensitivity is valid under conditions similar to the outdoor calibration conditions
under which the reference pyranometer was calibrated.
— if the relevant instrument properties (most importantly temperature response, directional response,
spectr
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