ASTM D6176-97(2008)
(Practice)Standard Practice for Measuring Surface Atmospheric Temperature with Electrical Resistance Temperature Sensors
Standard Practice for Measuring Surface Atmospheric Temperature with Electrical Resistance Temperature Sensors
SIGNIFICANCE AND USE
Applications—Ambient atmospheric temperature measurements can be made using resistance thermometers for many purposes. The application determines the most appropriate type of resistance thermometer and data recording method to be used. Examples of three typical meteorological applications for temperature measurements follow.
Single-level, near-surface measurements for weather observations (1) , thermodynamic computations for industrial applications, or environmental studies (2).
Temperature differential or vertical gradient measurements to characterize atmospheric stability for atmospheric dispersion analyses studies (2).
Temperature fluctuations for heat flux or temperature, or variance computations, or both. Measurements of heat flux and temperature variance require high precision measurements with a fast response to changes in the ambient atmosphere.
Purpose—This practice is designed to assist the user in selecting an appropriate temperature measurement system for the intended atmospheric application, and properly installing and operating the system. The manufacturer's recommendations and the U.S. Environmental Protection Agency handbook on quality assurance in meteorological measurements (3) should be consulted for calibration and performance audit procedures.
SCOPE
1.1 This practice provides procedures to measure representative near-surface atmospheric (outdoor air) temperature for meteorological purposes using commonly available electrical thermometers housed in radiation shields mounted on stationary or portable masts or towers.
1.2 This practice is applicable for measurements over the temperature range normally encountered in the ambient atmosphere, –50 to +50°C.
1.3 Air temperature measurement systems include a radiation shield, resistance thermometer, signal cables, and associated electronics.
1.4 Measurements can be made at a single level for various meteorological purposes, at two or more levels for vertical temperature differences, and using special equipment (at one or more levels) for fluctuations of temperature with time applied to flux or variance measurements.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D6176 − 97(Reapproved 2008)
Standard Practice for
Measuring Surface Atmospheric Temperature with Electrical
Resistance Temperature Sensors
This standard is issued under the fixed designation D6176; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope tance Thermometers
1.1 This practice provides procedures to measure represen-
3. Terminology
tative near-surface atmospheric (outdoor air) temperature for
3.1 Definitions:Fordefinitionsoftermsusedinthispractice,
meteorological purposes using commonly available electrical
refer to Terminology D1356 and E344. Some definitions are
thermometers housed in radiation shields mounted on station-
repeated in this section for the reader’s convenience.
ary or portable masts or towers.
3.1.1 connecting wires—the wires which run from the ele-
1.2 This practice is applicable for measurements over the
ment through the cable end closure and external to the sheath.
temperature range normally encountered in the ambient
3.1.2 interchangeability—the extent to which the thermom-
atmosphere, –50 to +50°C.
eter matches a resistance-temperature relationship.
1.3 Air temperature measurement systems include a radia-
3.1.3 inversion—the increase in potential temperature with
tion shield, resistance thermometer, signal cables, and associ-
an increase in height (see 3.1.4 and 3.2.7).
ated electronics.
3.1.4 lapse rate—the change in temperature with an in-
1.4 Measurements can be made at a single level for various
crease in height (see 3.1.3 and 3.2.7).
meteorological purposes, at two or more levels for vertical
3.1.5 resistance thermometer—atemperature-measuringde-
temperaturedifferences,andusingspecialequipment(atoneor
vice comprised of a resistance thermometer element, internal
more levels) for fluctuations of temperature with time applied
connecting wires, a protective shell with or without means for
to flux or variance measurements.
mounting, a connection head or connecting wire with other
1.5 This standard does not purport to address all of the
fittings, or both (see also 3.2.3).
safety concerns, if any, associated with its use. It is the
3.1.6 resistance thermometer element—the temperature-
responsibility of the user of this standard to establish appro-
sensitive portion of the thermometer composed of resistance
priate safety and health practices and determine the applica-
wire, film or semiconductor material, its supporting structure,
bility of regulatory limitations prior to use.
and the means for attaching connecting wires.
2. Referenced Documents
3.1.7 thermistor—a semiconductor whose primary function
is to exhibit a monotonic change (generally a decrease) in
2.1 ASTM Standards:
D1356Terminology Relating to Sampling and Analysis of electrical resistance with an increase in sensor temperature.
Atmospheres
3.2 Definitions of Terms Specific to This Standard:
E344Terminology Relating to Thermometry and Hydrom-
3.2.1 ambient—the portion of the atmosphere where the air
etry
temperature is unaffected by local structural, terrain, or heat
E644Test Methods for Testing Industrial Resistance Ther-
source or sink influences.
mometers
3.2.2 sensor—used interchangeably with resistance ther-
E1137/E1137MSpecification for Industrial Platinum Resis-
mometer (see 3.1.5) in this practice.
3.2.3 shield—a ventilated housing designed to minimize the
This practice is under the jurisdiction of ASTM Committee D22 on Air effectsofsolarandterrestrialradiationonatemperaturesensor
Qualityand is the direct responsibility of Subcommittee D22.11 on Meteorology.
while maximizing convective heat transfer between the sensor
Current edition approved Oct. 1, 2008. Published October 2008. Originally
and the passing air, and to protect the sensor from contact with
approved in 1997. Last previous edition approved in 2003 as D6176-97(2003).
liquid moisture; also known as radiation shield.
DOI: 10.1520/D6176-97R08.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.2.4 temperature differential—the difference between two
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
or more simultaneous temperature measurements, typically
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. separated vertically at a single location; see 3.1.3 and 3.1.4.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6176 − 97 (2008)
3.2.5 temperature variance—a statistical measure, the de- 5. Summary of Practice
viationofindividualtemperaturemeasurementsfromthemean
5.1 Ambient air temperature measurements using resistance
of those measurements obtained over a user-defined sampling
thermometers are typically made using either thermistors or
period.
platinumwireorfilmsensors,thoughsensorsmadefromother
3.2.5.1 Discussion—Temperature variance describes tem-
materials with similar resistance properties related to tempera-
perature variability at a fixed point in the atmosphere. The
ture could also be suitable.The sensors are housed in naturally
covariance of temperature and vertical velocity defines the
ventilated or mechanically aspirated shields. The sensor tem-
sensible heat flux.
perature is intended to be representative of the ambient air. To
3.2.6 transfer function—the functional relationship between
accomplish this, the sensor material and exposure in the shield
temperature sensor electrical resistance and the corresponding
are chosen to maximize convective heat transfer between the
sensor temperature.
air and the sensor, and minimize solar or terrestrial radiation
3.2.7 verticaltemperaturegradient—thechangeoftempera-
exchange with the sensor.The resistance thermometer (sensor)
ture with height (∆T/∆Zor δT/δZ), frequently expressed in
should be sufficiently rugged to withstand the operating envi-
°C/m; also known as lapse rate for temperature decrease, or
ronment without damage. The sensors are connected to elec-
inversion for a temperature increase (see 3.1.3 and 3.1.4).
tronic circuits capable of measuring the sensor resistance, and
displaying or recording, or both, the corresponding tempera-
3.3 Symbols:
ture. Operational procedures containing quality control and
agl = above ground level
quality assurance tasks suitable to the intended measurements
∆T = difference between two temperatures, also δT
are recommended (1, 2, 3, 4).
∆Z = difference between two heights above ground level,
also δZ
6. Resistance Thermometers
T = temperature, degrees in appropriate scale, typically
6.1 Temperature Measurement Requirements—Define the
Celsius, °C
Z = height above ground level, typically metres range, resolution, response time, precision, and bias suitable
τ = time constant, the time for a sensor to change to
forpurposesofthemeasurement.Themaximumrecommended
approximately 63.2% (1−l/e) of the value of the
accuracy specification is an absolute error of 60.5°C over the
temperature change.
expected temperature range. For vertical temperature gradient
measurements, there is an additional accuracy specification of
4. Significance and Use
a relative error between sensors of 60.1°C over the range of
expected temperature difference (2). The maximum recom-
4.1 Applications—Ambient atmospheric temperature mea-
mended resolution is 0.1°C for most single-level
surements can be made using resistance thermometers for
measurements, and 0.01°C for vertical temperature difference
many purposes.The application determines the most appropri-
and temperature fluctuation measurements. The recommended
ate type of resistance thermometer and data recording method
response time should be5sor less for typical measurements.
to be used. Examples of three typical meteorological applica-
Use a fast response thermometer and a temperature measure-
tions for temperature measurements follow.
mentsystemcapableof5Hzorbetterdataratefortemperature
4.1.1 Single-level, near-surface measurements for weather
flux and variance applications. The electrical components of a
observations (1) , thermodynamic computations for industrial
temperature measurement system introduce uncertainty, noise,
applications, or environmental studies (2).
and drift. For example, a 13-bit analog-to-digital converter
4.1.2 Temperature differential or vertical gradient measure-
used with a thermometer operating over 100°C span can
ments to characterize atmospheric stability for atmospheric
resolve 60.012°C, but electric noise and drift can produce a
dispersion analyses studies (2).
system uncertainty of 60.05°C.
4.1.3 Temperature fluctuations for heat flux or temperature,
or variance computations, or both. Measurements of heat flux
NOTE 1—This practice really addresses the sensor time constant in air
and temperature variance require high precision measurements
in the operational mounting or shield. A response time of 30 to 60 s in
with a fast response to changes in the ambient atmosphere. aspirated airflow may be more typical in application and will meet most
standards and regulations.
4.2 Purpose—This practice is designed to assist the user in
6.2 Sensor Characteristics—Sensor characteristics to be
selecting an appropriate temperature measurement system for
considered when specifying a system include the following
the intended atmospheric application, and properly installing
elements.
and operating the system. The manufacturer’s recommenda-
tionsandtheU.S.EnvironmentalProtectionAgencyhandbook 6.2.1 The temperature-to-resistance relationship (transfer
on quality assurance in meteorological measurements (3)
function)needstoprovideadequatedataresolutionconsidering
should be consulted for calibration and performance audit
the sensor installation and data processing equipment. It must
procedures.
be traceable to fixed temperature points and exhibit no singu-
laritiesduetophysicalorchemicalproperties.Therelationship
mustnotchangesignificantlywithsensorage.Optimumsensor
3 interchangeability can be obtained if the individual sensors
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. have very similar transfer functions.
D6176 − 97 (2008)
6.2.2 The sensor must be able to repeatedly cycle through 7.3 Shield Design—The shield shelters the temperature
the range of expected temperatures and return to any tempera- sensor from solar and terrestrial radiation, condensation, and
ture in the range with the required repeatability, minimizing precipitation while providing physical support and the ventila-
hysteresis effects. The sensor must be able to dissipate the tion required for convective heat transfer between the sensor
electrical power used in the measurement process without and the ambient air. Shields can have either natural or forced
producing unacceptable measurement bias. The sensor resis- aspiration and should allow air movement past the sensor as
tanceandradiativepropertiesshouldnotbealteredbyexternal freeaspossiblefromcontaminationbyextraneousheatsources
stresses such as humidity, corrosion, and vibration. (such as a nearby tower, or exhaust from the aspirator blower
motor.)
6.2.3 The sensor time constant, τ, must be short enough to
provide the necessary sampling rate for the intended measure-
NOTE 2—Forced aspirators should include sufficient means to prevent
ment; constants less than 1 min are adequate for most meteo-
moisture from accumulating on the temperature probe, which could cause
rological applications. Time constant, τ, is often measured or it to sense a reduced temperature (also known as the wet-bulb effect).
calculatedinstillair,assumingthatheattransferonlyoccursby
7.3.1 Naturally ventilated shields require no electric power
conduction and radiation. Proper installation in a ventilated
and are often used at remote sites where electrical power is
shield will markedly reduce the time constant, because heat
unavailable. These shields offer less radiation protection with
transfer is dominated by convection.
wind speeds less than a few metres per second. Naturally
ventilated shields are often used with small, fast response
6.3 Sensors Commonly Used—There are two commonly
thermometer elements that require a minimum of ventilation.
used resistance thermometers (sensors) for meteorological
applications—platinum (or other material) wires or films and
NOTE3—Temperatureerrorsatlesserwindspeedscouldapproach5°C.
thermistors. These two types of sensors differ in linearity of
7.3.2 Forcedaspirationisusedtonormalizeconvectiveheat
response to temperature change and nominal resistance at
transfer between the resistance thermometer probe and the air
ambienttemperatures.Sensorlinearityismoreimportantwhen
by providing a stream of ambient air moving at a reasonably
matching multiple sensors for temperature difference measure-
constant velocity between approximately 3 and 10 m/s. Care
ments than for single level measurements.
must be taken to avoid drawing warm air from the shield
6.3.1 Platinumresistancethermometerelementshaveavery
exhaust into the shield intake. Shielding and aspiration rates
linear transfer function (see Specification E1137/E1137M).
should be identical for all thermometers used for temperature
The nominal resistance at 0°C typically is 100 Ω, with a
profile measurements.
corresponding resistance change of about 0.4 Ω/°C. This
7.3.3 The shield housing shall be made with and kept a
sensitivity calls for special care so the connecting wires and
reflective color, such as silver or white. Accumulations of
signal cables have no effect on the sensor resistance measure-
surface contaminants such as dirt or animal droppings could
ment.
reduce the capability of the shield to reflect solar or terrestrial
6.3.2 Thermistorshavenonlineartransferfunctions.Typical
radiation.
sensors include two or three individual thermistors bound
PROCEDURES
together in a circuit to provide for a reasonably linear transfer
function in the kilohm range at ambient temperatures, which
8. Siting the Temperature Measuring System
can be measured easily by modern data recorders.
8.1 Station Identification—The temperature measurement
7. Shields
system location shall be identified by an unambiguous label
whichshallincludestationlocationandsensorelevationabove
7.1 Some of the largest error sources in air temperature
ground level using units and resolution suitable for the pur-
measurements are due to solar and terrestrial radiation, and to
poses of the measurement program, and any special purpose
moisture. Improper sensor exposure can lead to errors of 5°C
inf
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