ASTM D6176-97(2015)
(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
4.1 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.
4.1.1 Single-level, near-surface measurements for weather observations (1)3, thermodynamic computations for industrial applications, or environmental studies (2).
4.1.2 Temperature differential or vertical gradient measurements to characterize atmospheric stability for atmospheric dispersion analyses studies (2).
4.1.3 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.
4.2 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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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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Designation: D6176 − 97 (Reapproved 2015)
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 E644Test Methods for Testing Industrial Resistance Ther-
mometers
1.1 This practice provides procedures to measure represen-
E1137/E1137MSpecification for Industrial Platinum Resis-
tative near-surface atmospheric (outdoor air) temperature for
tance Thermometers
meteorological purposes using commonly available electrical
thermometers housed in radiation shields mounted on station-
3. Terminology
ary or portable masts or towers.
3.1 Definitions:
1.2 This practice is applicable for measurements over the
3.1.1 For definitions of terms used in this practice, refer to
temperature range normally encountered in the ambient
Terminology D1356 and E344. Some definitions are repeated
atmosphere, –50 to +50°C.
in this section for the reader’s convenience.
1.3 Air temperature measurement systems include a radia-
3.1.2 connecting wires—the wires which run from the ele-
tion shield, resistance thermometer, signal cables, and associ-
ment through the cable end closure and external to the sheath.
ated electronics.
3.1.3 interchangeability—the extent to which the thermom-
1.4 Measurements can be made at a single level for various
eter matches a resistance-temperature relationship.
meteorological purposes, at two or more levels for vertical
3.1.4 inversion—the increase in potential temperature with
temperaturedifferences,andusingspecialequipment(atoneor
an increase in height (see 3.1.5 and 3.2.7).
more levels) for fluctuations of temperature with time applied
to flux or variance measurements.
3.1.5 lapse rate—the change in temperature with an in-
crease in height (see 3.1.4 and 3.2.7).
1.5 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
3.1.6 resistance thermometer—atemperature-measuringde-
standard.
vice comprised of a resistance thermometer element, internal
1.6 This standard does not purport to address all of the
connecting wires, a protective shell with or without means for
safety concerns, if any, associated with its use. It is the mounting, a connection head or connecting wire with other
responsibility of the user of this standard to establish appro-
fittings, or both (see also 3.2.3).
priate safety and health practices and determine the applica-
3.1.7 resistance thermometer element—the temperature-
bility of regulatory limitations prior to use.
sensitive portion of the thermometer composed of resistance
wire, film or semiconductor material, its supporting structure,
2. Referenced Documents
and the means for attaching connecting wires.
2.1 ASTM Standards:
3.1.8 thermistor—a semiconductor whose primary function
D1356Terminology Relating to Sampling and Analysis of
is to exhibit a monotonic change (generally a decrease) in
Atmospheres
electrical resistance with an increase in sensor temperature.
E344Terminology Relating to Thermometry and Hydrom-
etry 3.2 Definitions of Terms Specific to This Standard:
3.2.1 ambient—the portion of the atmosphere where the air
temperature is unaffected by local structural, terrain, or heat
ThispracticeisunderthejurisdictionofASTMCommitteeD22onAirQuality
source or sink influences.
and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved April 1, 2015. Published April 2015. Originally
3.2.2 sensor—used interchangeably with resistance ther-
approved in 1997. Last previous edition approved in 2008 as D6176–97 (2008).
mometer (see 3.1.6) in this practice.
DOI: 10.1520/D6176-97R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.2.3 shield—a ventilated housing designed to minimize the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
effectsofsolarandterrestrialradiationonatemperaturesensor
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. while maximizing convective heat transfer between the sensor
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6176 − 97 (2015)
and the passing air, and to protect the sensor from contact with tionsandtheU.S.EnvironmentalProtectionAgencyhandbook
liquid moisture; also known as radiation shield. on quality assurance in meteorological measurements (3)
should be consulted for calibration and performance audit
3.2.4 temperature differential—the difference between two
procedures.
or more simultaneous temperature measurements, typically
separated vertically at a single location; see 3.1.4 and 3.1.5.
5. Summary of Practice
3.2.5 temperature variance—a statistical measure, the de-
5.1 Ambient air temperature measurements using resistance
viationofindividualtemperaturemeasurementsfromthemean
thermometers are typically made using either thermistors or
of those measurements obtained over a user-defined sampling
platinumwireorfilmsensors,thoughsensorsmadefromother
period.
materials with similar resistance properties related to tempera-
3.2.5.1 Discussion—Temperature variance describes tem-
ture could also be suitable.The sensors are housed in naturally
perature variability at a fixed point in the atmosphere. The
ventilated or mechanically aspirated shields. The sensor tem-
covariance of temperature and vertical velocity defines the
perature is intended to be representative of the ambient air. To
sensible heat flux.
accomplish this, the sensor material and exposure in the shield
3.2.6 transfer function—the functional relationship between
are chosen to maximize convective heat transfer between the
temperature sensor electrical resistance and the corresponding
air and the sensor, and minimize solar or terrestrial radiation
sensor temperature.
exchange with the sensor.The resistance thermometer (sensor)
3.2.7 verticaltemperaturegradient—thechangeoftempera-
should be sufficiently rugged to withstand the operating envi-
ture with height (∆T/∆Zor δT/δZ), frequently expressed in
ronment without damage. The sensors are connected to elec-
°C/m; also known as lapse rate for temperature decrease, or
tronic circuits capable of measuring the sensor resistance, and
inversion for a temperature increase (see 3.1.4 and 3.1.5).
displaying or recording, or both, the corresponding tempera-
ture. Operational procedures containing quality control and
3.3 Symbols:
quality assurance tasks suitable to the intended measurements
agl = above ground level
are recommended (1, 2, 3, 4).
∆T = difference between two temperatures, also δT
∆Z = difference between two heights above ground level,
6. Resistance Thermometers
also δZ
6.1 Temperature Measurement Requirements—Define the
T = temperature, degrees in appropriate scale, typically
range, resolution, response time, precision, and bias suitable
Celsius, °C
forpurposesofthemeasurement.Themaximumrecommended
Z = height above ground level, typically metres
accuracy specification is an absolute error of 60.5°C over the
τ = time constant, the time for a sensor to change to
expected temperature range. For vertical temperature gradient
approximately 63.2% (1−l/e) of the value of the
measurements, there is an additional accuracy specification of
temperature change.
a relative error between sensors of 60.1°C over the range of
4. Significance and Use
expected temperature difference (2). The maximum recom-
mended resolution is 0.1°C for most single-level
4.1 Applications—Ambient atmospheric temperature mea-
measurements, and 0.01°C for vertical temperature difference
surements can be made using resistance thermometers for
and temperature fluctuation measurements. The recommended
many purposes.The application determines the most appropri-
response time should be5sor less for typical measurements.
ate type of resistance thermometer and data recording method
Use a fast response thermometer and a temperature measure-
to be used. Examples of three typical meteorological applica-
mentsystemcapableof5Hzorbetterdataratefortemperature
tions for temperature measurements follow.
flux and variance applications. The electrical components of a
4.1.1 Single-level, near-surface measurements for weather
3 temperature measurement system introduce uncertainty, noise,
observations (1) , thermodynamic computations for industrial
and drift. For example, a 13-bit analog-to-digital converter
applications, or environmental studies (2).
used with a thermometer operating over 100°C span can
4.1.2 Temperature differential or vertical gradient measure-
resolve 60.012°C, but electric noise and drift can produce a
ments to characterize atmospheric stability for atmospheric
system uncertainty of 60.05°C.
dispersion analyses studies (2).
4.1.3 Temperature fluctuations for heat flux or temperature,
NOTE 1—This practice really addresses the sensor time constant in air
in the operational mounting or shield. A response time of 30 to 60 s in
or variance computations, or both. Measurements of heat flux
aspirated airflow may be more typical in application and will meet most
and temperature variance require high precision measurements
standards and regulations.
with a fast response to changes in the ambient atmosphere.
6.2 Sensor Characteristics—Sensor characteristics to be
4.2 Purpose—This practice is designed to assist the user in
considered when specifying a system include the following
selecting an appropriate temperature measurement system for
elements.
the intended atmospheric application, and properly installing
6.2.1 The temperature-to-resistance relationship (transfer
and operating the system. The manufacturer’s recommenda-
function)needstoprovideadequatedataresolutionconsidering
the sensor installation and data processing equipment. It must
be traceable to fixed temperature points and exhibit no singu-
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. laritiesduetophysicalorchemicalproperties.Therelationship
D6176 − 97 (2015)
mustnotchangesignificantlywithsensorage.Optimumsensor tion heat sources, and a net temperature deficit during noctur-
interchangeability can be obtained if the individual sensors nal cooling periods (5).
have very similar transfer functions.
7.3 Shield Design—The shield shelters the temperature
6.2.2 The sensor must be able to repeatedly cycle through
sensor from solar and terrestrial radiation, condensation, and
the range of expected temperatures and return to any tempera-
precipitation while providing physical support and the ventila-
ture in the range with the required repeatability, minimizing
tion required for convective heat transfer between the sensor
hysteresis effects. The sensor must be able to dissipate the
and the ambient air. Shields can have either natural or forced
electrical power used in the measurement process without
aspiration and should allow air movement past the sensor as
producing unacceptable measurement bias. The sensor resis-
freeaspossiblefromcontaminationbyextraneousheatsources
tanceandradiativepropertiesshouldnotbealteredbyexternal
(such as a nearby tower, or exhaust from the aspirator blower
stresses such as humidity, corrosion, and vibration.
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
moisture from accumulating on the temperature probe, which could cause
ment; constants less than 1 min are adequate for most meteo-
it to sense a reduced temperature (also known as the wet-bulb effect).
rological applications. Time constant, τ, is often measured or
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
together in a circuit to provide for a reasonably linear transfer
PROCEDURES
function in the kilohm range at ambient temperatures, which
can be measured easily by modern data recorders.
8. Siting the Temperature Measuring System
7. Shields
8.1 Station Identification—The temperature measurement
system location shall be identified by an unambiguous label
7.1 Some of the largest error sources in air temperature
whichshallincludestationlocationandsensorelevationabove
measurements are due to solar
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6176 − 97 (Reapproved 2008) D6176 − 97 (Reapproved 2015)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. 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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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.
2. Referenced Documents
2.1 ASTM Standards:
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
E344 Terminology Relating to Thermometry and Hydrometry
E644 Test Methods for Testing Industrial Resistance Thermometers
E1137/E1137M Specification for Industrial Platinum Resistance Thermometers
3. Terminology
3.1 Definitions:For
3.1.1 For definitions of terms used in this practice, refer to Terminology D1356 and E344. Some definitions are repeated in this
section for the reader’s convenience. definitions of terms used in this practice, refer to Terminology D1356 and E344. Some
definitions are repeated in this section for the reader’s convenience.
3.1.2 connecting wires—the wires which run from the element through the cable end closure and external to the sheath.
3.1.3 interchangeability—the extent to which the thermometer matches a resistance-temperature relationship.
3.1.4 inversion—the increase in potential temperature with an increase in height (see 3.1.43.1.5 and 3.2.7).
3.1.5 lapse rate—the change in temperature with an increase in height (see 3.1.33.1.4 and 3.2.7).
3.1.6 resistance thermometer—a temperature-measuring device comprised of a resistance thermometer element, internal
connecting wires, a protective shell with or without means for mounting, a connection head or connecting wire with other fittings,
or both (see also 3.2.3).
This practice is under the jurisdiction of ASTM Committee D22 on Air Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved Oct. 1, 2008April 1, 2015. Published October 2008April 2015. Originally approved in 1997. Last previous edition approved in 20032008 as
D6176 - 97D6176 – 97 (2008).(2003). DOI: 10.1520/D6176-97R08.10.1520/D6176-97R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6176 − 97 (2015)
3.1.7 resistance thermometer element—the temperature-sensitive portion of the thermometer composed of resistance wire, film
or semiconductor material, its supporting structure, and the means for attaching connecting wires.
3.1.8 thermistor—a semiconductor whose primary function is to exhibit a monotonic change (generally a decrease) in electrical
resistance with an increase in sensor temperature.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 ambient—the portion of the atmosphere where the air temperature is unaffected by local structural, terrain, or heat source
or sink influences.
3.2.2 sensor—used interchangeably with resistance thermometer (see 3.1.53.1.6) in this practice.
3.2.3 shield—a ventilated housing designed to minimize the effects of solar and terrestrial radiation on a temperature sensor
while maximizing convective heat transfer between the sensor and the passing air, and to protect the sensor from contact with liquid
moisture; also known as radiation shield.
3.2.4 temperature differential—the difference between two or more simultaneous temperature measurements, typically separated
vertically at a single location; see 3.1.33.1.4 and 3.1.43.1.5.
3.2.5 temperature variance—a statistical measure, the deviation of individual temperature measurements from the mean of those
measurements obtained over a user-defined sampling period.
3.2.5.1 Discussion—
Temperature variance describes temperature variability at a fixed point in the atmosphere. The covariance of temperature and
vertical velocity defines the sensible heat flux.
3.2.6 transfer function—the functional relationship between temperature sensor electrical resistance and the corresponding
sensor temperature.
3.2.7 vertical temperature gradient—the change of temperature with height (ΔT/ΔZ or δT/δZ), frequently expressed in °C/m;
also known as lapse rate for temperature decrease, or inversion for a temperature increase (see 3.1.33.1.4 and 3.1.43.1.5).
3.3 Symbols:
agl = above ground level
ΔT = difference between two temperatures, also δT
ΔZ = difference between two heights above ground level, also δZ
T = temperature, degrees in appropriate scale, typically Celsius, °C
Z = height above ground level, typically metres
τ = time constant, the time for a sensor to change to approximately 63.2 % (1−l/e) of the value of the temperature change.
4. Significance and Use
4.1 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.
4.1.1 Single-level, near-surface measurements for weather observations (1) , thermodynamic computations for industrial
applications, or environmental studies (2).
4.1.2 Temperature differential or vertical gradient measurements to characterize atmospheric stability for atmospheric
dispersion analyses studies (2).
4.1.3 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.
4.2 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.
5. Summary of Practice
5.1 Ambient air temperature measurements using resistance thermometers are typically made using either thermistors or
platinum wire or film sensors, though sensors made from other materials with similar resistance properties related to temperature
could also be suitable. The sensors are housed in naturally ventilated or mechanically aspirated shields. The sensor temperature
is intended to be representative of the ambient air. To accomplish this, the sensor material and exposure in the shield are chosen
The boldface numbers in parentheses refer to the list of references at the end of this standard.
D6176 − 97 (2015)
to maximize convective heat transfer between the air and the sensor, and minimize solar or terrestrial radiation exchange with the
sensor. The resistance thermometer (sensor) should be sufficiently rugged to withstand the operating environment without damage.
The sensors are connected to electronic circuits capable of measuring the sensor resistance, and displaying or recording, or both,
the corresponding temperature. Operational procedures containing quality control and quality assurance tasks suitable to the
intended measurements are recommended (1, 2, 3, 4).
6. Resistance Thermometers
6.1 Temperature Measurement Requirements—Define the range, resolution, response time, precision, and bias suitable for
purposes of the measurement. The maximum recommended accuracy specification is an absolute error of 60.5°C over the
expected temperature range. For vertical temperature gradient measurements, there is an additional accuracy specification of a
relative error between sensors of 60.1°C over the range of expected temperature difference (2). The maximum recommended
resolution is 0.1°C for most single-level measurements, and 0.01°C for vertical temperature difference and temperature fluctuation
measurements. The recommended response time should be 5 s or less for typical measurements. Use a fast response thermometer
and a temperature measurement system capable of 5 Hz or better data rate for temperature flux and variance applications. The
electrical components of a temperature measurement system introduce uncertainty, noise, and drift. For example, a 13-bit
analog-to-digital converter used with a thermometer operating over 100°C span can resolve 60.012°C, but electric noise and drift
can produce a system uncertainty of 60.05°C.
NOTE 1—This practice really addresses the sensor time constant in air in the operational mounting or shield. A response time of 30 to 60 s in aspirated
airflow may be more typical in application and will meet most standards and regulations.
6.2 Sensor Characteristics—Sensor characteristics to be considered when specifying a system include the following elements.
6.2.1 The temperature-to-resistance relationship (transfer function) needs to provide adequate data resolution considering the
sensor installation and data processing equipment. It must be traceable to fixed temperature points and exhibit no singularities due
to physical or chemical properties. The relationship must not change significantly with sensor age. Optimum sensor
interchangeability can be obtained if the individual sensors have very similar transfer functions.
6.2.2 The sensor must be able to repeatedly cycle through the range of expected temperatures and return to any temperature in
the range with the required repeatability, minimizing hysteresis effects. The sensor must be able to dissipate the electrical power
used in the measurement process without producing unacceptable measurement bias. The sensor resistance and radiative properties
should not be altered by external stresses such as humidity, corrosion, and vibration.
6.2.3 The sensor time constant, τ, must be short enough to provide the necessary sampling rate for the intended measurement;
constants less than 1 min are adequate for most meteorological applications. Time constant, τ, is often measured or calculated in
still air, assuming that heat transfer only occurs by conduction and radiation. Proper installation in a ventilated shield will markedly
reduce the time constant, because heat transfer is dominated by convection.
6.3 Sensors Commonly Used—There are two commonly used resistance thermometers (sensors) for meteorological
applications—platinum (or other material) wires or films and thermistors. These two types of sensors differ in linearity of response
to temperature change and nominal resistance at ambient temperatures. Sensor linearity is more important when matching multiple
sensors for temperature difference measurements than for single level measurements.
6.3.1 Platinum resistance thermometer elements have a very linear transfer function (see Specification E1137/E1137M). The
nominal resistance at 0°C typically is 100 Ω, with a corresponding resistance change of about 0.4 Ω/°C. This sensitivity calls for
special care so the connecting wires and signal cables have no effect on the sensor resistance measurement.
6.3.2 Thermistors have nonlinear transfer functions. Typical sensors include two or three individual thermistors bound together
in a circuit to provide for a reasonably linear transfer function in the kilohm range at ambient temperatures, which can be measured
easily by modern data recorders.
7. Shields
7.1 Some of the largest error sources in air temperature measurements are due to solar and terrestrial radiation, and to moisture.
Improper sensor exposure can lead to errors of 5°C or more. A resistance thermometer senses only the temperature of its probe,
which is determined by the cumulative effects of the probe surroundings, including the temperature of the ambient air. There are
also adverse effects, such as direct and reflected solar radiation, thermal radiation from surrounding objects, heat conduction from
connecting wires and supports, and interference from moisture.
7.2 Solar and Terrestrial Radiation Effects—Electrical temperature sensors have different thermal properties than air. For
example, the thermal conductivity of air is three to four orders of magnitude lower than the metals used in temperature probes,
causing poor thermal contact between the probe and the ambient air. The result is a net temperature excess of the probe surface
during exposure to solar radiation or terrestrial radiation heat sources, and a net temperature deficit during nocturnal cooling
periods (5).
7.3 Shield Design—The shield shelters the temperature sensor from solar and terrestrial radiation, condensation, and
precipitation while providing physical support and the ventilation required for convective heat transfer between the sensor and the
D6176 − 97 (2015)
ambient air. Shields can have either natural or forced aspiration and should allow air movement past the sensor as fre
...
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