ASTM D6176-97(2003)
(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.
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.
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’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)
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Designation:D6176–97 (Reapproved 2003)
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This practice provides procedures to measure represen- 3.1 For definitions of terms used in this practice, refer to
tative near-surface atmospheric (outdoor air) temperature for Terminology D1356 and E344. Some definitions are repeated
meteorological purposes using commonly available electrical in this section for the reader’s convenience.
thermometers housed in radiation shields mounted on station- 3.1.1 connecting wires—the wires which run from the
ary or portable masts or towers. element through the cable end closure and external to the
1.2 This practice is applicable for measurements over the sheath.
temperature range normally encountered in the ambient atmo- 3.1.2 interchangeability—the extent to which the thermom-
sphere, –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—a temperature-measuring
temperaturedifferences,andusingspecialequipment(atoneor devicecomprisedofaresistancethermometerelement,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.
3.1.7 thermistor—a semiconductor whose primary function
2. Referenced Documents
is to exhibit a monotonic change (generally a decrease) in
2.1 ASTM Standards: electrical resistance with an increase in sensor temperature.
D1356 Terminology Relating to Sampling andAnalysis of
3.2 Definitions of Terms Specific to This Standard:
Atmospheres 3.2.1 ambient—the portion of the atmosphere where the air
E344 Terminology Relating to Thermometry and Hydrom-
temperature is unaffected by local structural, terrain, or heat
etry source or sink influences.
E644 Test Methods for Testing Industrial Resistance Ther-
3.2.2 sensor—used interchangeably with resistance ther-
mometers mometer (see 3.1.5) in this practice.
E1137 Specification for Industrial Platinum Resistance
3.2.3 shield—aventilatedhousingdesignedtominimizethe
Thermometers effectsofsolarandterrestrialradiationonatemperaturesensor
while maximizing convective heat transfer between the sensor
and the passing air, and to protect the sensor from contact with
This practice is under the jurisdiction of ASTM Committee D22 on Sampling
liquid moisture; also known as radiation shield.
and Analysis of Atmospheres and is the direct responsibility of Subcommittee
D22.11 on Meteorology. 3.2.4 temperature differential—the difference between two
CurrenteditionapprovedOctober1,2003.PublishedNovember2003.Originally
or more simultaneous temperature measurements, typically
approved in 1997. Last previous edition approved in 1997 as D6176M-97.
separated vertically at a single location; see 3.1.3 and 3.1.4.
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 (2003)
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—thefunctionalrelationshipbetween
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 vertical temperature gradient—the change of tem-
exchange with the sensor.The resistance thermometer (sensor)
peraturewithheight(DT/DZor dT/dZ),frequentlyexpressedin
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
3.3 Symbols:
displaying or recording, or both, the corresponding tempera-
ture. Operational procedures containing quality control and
quality assurance tasks suitable to the intended measurements
agl = above ground level
DT = difference between two temperatures, also dT are recommended (1, 2, 3, 4).
DZ = difference between two heights above ground level,
also dZ
6. Resistance Thermometers
T = temperature, degrees in appropriate scale, typically
6.1 Temperature Measurement Requirements—Define the
Celsius, °C
range, resolution, response time, precision, and bias suitable
Z = height above ground level, typically metres
forpurposesofthemeasurement.Themaximumrecommended
t = time constant, the time for a sensor to change to
accuracy specification is an absolute error of 60.5°C over the
approximately 63.2% (1−l/e) of the value of the
expected temperature range. For vertical temperature gradient
temperature change.
measurements, there is an additional accuracy specification of
a relative error between sensors of 60.1°C over the range of
4. Significance and Use
expected temperature difference (2). The maximum recom-
4.1 Applications—Ambient atmospheric temperature mea-
mended resolution is 0.1°C for most single-level measure-
surements can be made using resistance thermometers for
ments, and 0.01°C for vertical temperature difference and
many purposes.The application determines the most appropri-
temperature fluctuation measurements. The recommended re-
ate type of resistance thermometer and data recording method
sponsetimeshouldbe5sorlessfortypicalmeasurements.Use
to be used. Examples of three typical meteorological applica-
a fast response thermometer and a temperature measurement
tions for temperature measurements follow.
system capable of 5 Hz or better data rate for temperature flux
4.1.1 Single-level, near-surface measurements for weather
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
aspirated airflow may be more typical in application and will meet most
with a fast response to changes in the ambient atmosphere.
standards and regulations.
4.2 Purpose—This practice is designed to assist the user in
selecting an appropriate temperature measurement system for
6.2 Sensor Characteristics—Sensor characteristics to be
the intended atmospheric application, and properly installing
considered when specifying a system include the following
and operating the system. The manufacturer’s recommenda-
elements.
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 (2003)
6.2.2 The sensor must be able to repeatedly cycle through tion required for convective heat transfer between the sensor
the range of expected temperatures and return to any tempera- and the ambient air. Shields can have either natural or forced
ture in the range with the required repeatability, minimizing aspiration and should allow air movement past the sensor as
hysteresis effects. The sensor must be able to dissipate the freeaspossiblefromcontaminationbyextraneousheatsources
electrical power used in the measurement process without (such as a nearby tower, or exhaust from the aspirator blower
producing unacceptable measurement bias. The sensor resis- motor.)
tanceandradiativepropertiesshouldnotbealteredbyexternal
NOTE 2—Forced aspirators should include sufficient means to prevent
stresses such as humidity, corrosion, and vibration.
moisture from accumulating on the temperature probe, which could cause
6.2.3 The sensor time constant, t, must be short enough to
it to sense a reduced temperature (also known as the wet-bulb effect).
provide the necessary sampling rate for the intended measure-
7.3.1 Naturally ventilated shields require no electric power
ment; constants less than 1 min are adequate for most meteo-
and are often used at remote sites where electrical power is
rological applications. Time constant, t, is often measured or
unavailable. These shields offer less radiation protection with
calculatedinstillair,assumingthatheattransferonlyoccursby
wind speeds less than a few metres per second. Naturally
conduction and radiation. Proper installation in a ventilated
ventilated shields are often used with small, fast response
shield will markedly reduce the time constant, because heat
thermometer elements that require a minimum of ventilation.
transfer is dominated by convection.
6.3 Sensors Commonly Used—There are two commonly
NOTE 3—Temperatureerrorsatlesserwindspeedscouldapproach5°C.
used resistance thermometers (sensors) for meteorological
7.3.2 Forcedaspirationisusedtonormalizeconvectiveheat
applications—platinum (or other material) wires or films and
transfer between the resistance thermometer probe and the air
thermistors. These two types of sensors differ in linearity of
by providing a stream of ambient air moving at a reasonably
response to temperature change and nominal resistance at
constant velocity between approximately 3 and 10 m/s. Care
ambienttemperatures.Sensorlinearityismoreimportantwhen
must be taken to avoid drawing warm air from the shield
matching multiple sensors for temperature difference measure-
exhaust into the shield intake. Shielding and aspiration rates
ments than for single level measurements.
should be identical for all thermometers used for temperature
6.3.1 Platinumresistancethermometerelementshaveavery
profile measurements.
linear transfer function (see Specification E1137). The nomi-
7.3.3 The shield housing shall be made with and kept a
nal resistance at 0°C typically is 100 V, with a corresponding
reflective color, such as silver or white. Accumulations of
resistance change of about 0.4 V/°C. This sensitivity calls for
surface contaminants such as dirt or animal droppings could
special care so the connecting wires and signal cables have no
reduce the capability of the shield to reflect solar or terrestrial
effect on the sensor resistance measurement.
radiation.
6.3.2 Thermistorshavenonlineartransferfunctions.Typical
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
7.1 Some of the largest error sources in air temperature whichshallincludestationlocationandsensorelevationabove
measurements are due to solar and terrestrial radiation, and to ground level using units and resolution suitable for the pur-
moisture. Improper sensor exposure can lead to errors of 5°C poses of the measurement program, and any special purpose
ormore.Aresistancethermometersensesonlythetemperature information related to the measurement.
of its probe, which is determined by the cumulative effects of 8.2 Measurement Height—The typical measurement height
the probe surrou
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