ASTM E2877-12
(Guide)Standard Guide for Standard Guide for Digital Contact Thermometers
Standard Guide for Standard Guide for Digital Contact Thermometers
SIGNIFICANCE AND USE
4.1 Digital thermometers are used for measuring temperature in many laboratories and industrial applications.
4.2 For many applications, digital thermometers using external probes are considered environmentally-safe alternatives to mercury-in-glass thermometers. (1)3
4.3 Some digital thermometers are also used as reference or working temperature standards in verification and calibration of thermometers and also in determining the conditions necessary for evaluating the performance of other measuring instruments used in legal metrology and industry.
SCOPE
1.1 This Guide describes general-purpose, digital contact thermometers (hereafter simply called “digital thermometers”) that provide temperature readings in units of degrees Celsius or degrees Fahrenheit, or both. The different types of temperature sensors for these thermometers are described, and their relative merits are discussed. Nine accuracy classes are introduced for digital thermometerhes; these classes consider the accuracy of the sensor/measuring-instrument unit.
1.2 The proposed accuracy classes for digital thermometers pertain to the temperature interval of –200 °C to 500 °C, an interval of special interest for many applications in thermometry. All of the temperature sensor types for the digital thermometers discussed are able to measure temperature over at least some range within this interval. Some types are also able to measure beyond this interval. To qualify for an accuracy class, the thermometer must measure correctly to within a specified value (in units of °C) over this interval or over the subinterval in which they are capable of making measurements. Those thermometers that can measure temperature in ranges beyond this interval generally have larger measurement uncertainty in these ranges.
1.3 The digital thermometer sensors discussed are platinum resistance sensors, thermistors, and thermocouples. The range of use for these types of sensors is provided. The measurement uncertainty of a sensor is determined by its tolerance class or grade and whether the sensor has been calibrated.
1.4 This Guide provides a number of recommendations for the manufacture and selection of a digital thermometer. First, it recommends that the thermometer’s sensor conform to applicable ASTM specifications. Also, it recommends minimum standards for documentation on the thermometer and informational markings on the probe and measuring instrument.
1.5 The derived SI units (degrees Celsius) found in this Guide are to be considered standard. However, thermometers displaying degrees Fahrenheit are compliant with this guide as long as all other guidance is followed.
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. Some specific hazards statements are given in Section 7 on Hazards.
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Designation: E2877 − 12
StandardGuide for
Standard Guide for Digital Contact Thermometers
This standard is issued under the fixed designation E2877; 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 1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This Guide describes general-purpose, digital contact
responsibility of the user of this standard to establish appro-
thermometers (hereafter simply called “digital thermometers”)
priate safety and health practices and determine the applica-
thatprovidetemperaturereadingsinunitsofdegreesCelsiusor
bility of regulatory limitations prior to use. Some specific
degrees Fahrenheit, or both.The different types of temperature
hazards statements are given in Section 7 on Hazards.
sensorsforthesethermometersaredescribed,andtheirrelative
merits are discussed. Nine accuracy classes are introduced for
2. Referenced Documents
digital thermometerhes; these classes consider the accuracy of
2.1 ASTM Standards:
the sensor/measuring-instrument unit.
E230Specification and Temperature-Electromotive Force
1.2 The proposed accuracy classes for digital thermometers
(EMF) Tables for Standardized Thermocouples
pertain to the temperature interval of –200 °C to 500 °C, an
E344Terminology Relating to Thermometry and Hydrom-
interval of special interest for many applications in thermom-
etry
etry. All of the temperature sensor types for the digital
E563Practice for Preparation and Use of an Ice-Point Bath
thermometers discussed are able to measure temperature over
as a Reference Temperature
at least some range within this interval. Some types are also
E608/E608MSpecification for Mineral-Insulated, Metal-
abletomeasurebeyondthisinterval.Toqualifyforanaccuracy
Sheathed Base Metal Thermocouples
class, the thermometer must measure correctly to within a
E644Test Methods for Testing Industrial Resistance Ther-
specified value (in units of °C) over this interval or over the
mometers
subinterval in which they are capable of making measure-
E839 Test Methods for Sheathed Thermocouples and
ments. Those thermometers that can measure temperature in
Sheathed Thermocouple Cable
ranges beyond this interval generally have larger measurement
E879Specification for Thermistor Sensors for General Pur-
uncertainty in these ranges.
pose and Laboratory Temperature Measurements
1.3 The digital thermometer sensors discussed are platinum
E1137/E1137MSpecification for Industrial Platinum Resis-
resistance sensors, thermistors, and thermocouples. The range tance Thermometers
ofuseforthesetypesofsensorsisprovided.Themeasurement
E2181/E2181M Specification for Compacted Mineral-
uncertainty of a sensor is determined by its tolerance class or Insulated, Metal-Sheathed, Noble Metal Thermocouples
grade and whether the sensor has been calibrated.
and Thermocouple Cable
E2593Guide for Accuracy Verification of Industrial Plati-
1.4 This Guide provides a number of recommendations for
num Resistance Thermometers
themanufactureandselectionofadigitalthermometer.First,it
E2846Guide for Thermocouple Verification
recommends that the thermometer’s sensor conform to appli-
cable ASTM specifications. Also, it recommends minimum
3. Terminology
standards for documentation on the thermometer and informa-
3.1 Definitions: The definitions given in Terminology E344
tional markings on the probe and measuring instrument.
apply to terms used in this guide.
1.5 The derived SI units (degrees Celsius) found in this
3.2 Definitions:
Guide are to be considered standard. However, thermometers
3.2.1 accuracy class, n—class of an item that meets certain
displaying degrees Fahrenheit are compliant with this guide as
metrological requirements intended to keep errors within
long as all other guidance is followed.
specified limits.
This guide is under the jurisdiction ofASTM Committee E20 on Temperature
Measurement and is the direct responsibility of Subcommittee E20.09 on Digital For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Contact Thermometers. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Current edition approved Nov. 1, 2012. Published December 2012. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E2877–12 the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2877 − 12
3.2.1.1 Discussion—This document describes accuracy 4. Significance and Use
classes for digital thermometers.
4.1 Digital thermometers are used for measuring tempera-
3.2.2 calibration uncertainty, n—parameter, derived from
ture in many laboratories and industrial applications.
the analysis of a calibration of a measuring instrument, that
4.2 For many applications, digital thermometers using ex-
characterizes the range in which the true calibration result is
ternal probes are considered environmentally-safe alternatives
estimated to lie within a given confidence level.
to mercury-in-glass thermometers. (1)
3.2.3 digital contact thermometer, n—a device that mea-
4.3 Some digital thermometers are also used as reference or
sures temperature through direct contact with a sensor and
working temperature standards in verification and calibration
provides a digital output or display of the determined value, or
of thermometers and also in determining the conditions nec-
both.
essary for evaluating the performance of other measuring
instruments used in legal metrology and industry.
3.2.3.1 Discussion—This device consists of a temperature
sensor connected to a measuring instrument; this instrument
5. Description of the Instruments
measures the temperature-dependent quantity of the sensor,
computes the temperature from the measured quantity, and
5.1 Basic Description of a Digital Thermometer
providesadigitaloutputordisplayofthetemperature,orboth.
5.1.1 A digital thermometer consists of a temperature
The sensor is sometimes located inside the instrument.
sensor, often mounted in a probe, connected to a measuring
3.2.4 measuring instrument, n—the instrument in a digital
instrument. The instrument measures the temperature-
thermometerthatisusedtomeasurethetemperature-dependent
dependent quantity of the sensor, computes the temperature
quantity of the sensor.
from that measured quantity, and provides a digital output or
display of the computed temperature, or both.
3.2.5 probe, n—an assembly, including the transducer
(sensor), that is used to position the transducer in the specific
5.2 Types of Digital Thermometer Sensors
location at which the temperature is to be measured.
5.2.1 Platinum Resistance Thermometer (PRT).The electri-
cal resistance of a PRT’s platinum element increases nearly
3.2.6 reference-junction compensator, n—adevicethatmea-
linearly as its temperature increases, making it a temperature
sures the temperature of a thermocouple’s reference junction
sensor. A PRT sensor consists of a platinum filament of fine
and adds to or subtracts from the reference-junction emf a
wire or film supported by an insulating body. The sensor is
compensating voltage that simulates a reference junction
usually mounted in a protective glass coating with size 2 mm
temperature of 0 °C.
to 4 mm or a sheathed probe (glass or stainless steel) with a
3.2.6.1 Discussion—The compensating voltage may be
typical outer diameter of 1.6 mm to 6.4 mm; this arrangement
added or subtracted electronically or digitally.
protects the sensor from physical damage and chemical con-
3.2.7 response time, n—the time required for a sensor to
tamination but still allows thermal transfer between the sensor
change a specified percentage of the total difference between and its environment. This sensor package often determines the
itsinitialandfinaltemperatureswhenthesensorissubjectedto
temperature capability and accuracy of the device. The sensor
a step function change in temperature. isconnectedtoameasuringinstrumentbyelectricallyconduct-
ingleads.Thenumberofleadscanbe2,3,or4.Themeasuring
3.2.8 sensing point, n—the location on a temperature sensor
instrument determines the resistance of the PRT’s sensing
where the temperature is (or is assumed to be) measured.
elementbyapplyingaknowncurrentthroughitandmeasuring
3.2.8.1 Discussion—A thermocouple’s sensing point is its the voltage across it. Most measuring instruments for PRTs
measuring junction (although the signal in the thermocouple is
calculate the temperature of the sensor using the relevant
generated along the two thermocouple wires in regions where resistance/temperature equations. The PRT calibration is de-
atemperaturegradientexists).Aplatinumresistancethermom- fined as either a nominal resistance-temperature relationship
eter contains a sensing element that may be large enough to with an interchangeability tolerance (for example, Specifica-
experience spatial temperature variations; in this case the tion E1137/E1137M) or a single sensor calibration with esti-
sensing point is the central point in the element where the mated uncertainty. A nominal relationship allows the readout
temperature is assumed to be that measured by the platinum device to be programmed with a single resistance-temperature
resistance thermometer.
relationship for a specified PRT family. Interchangeability
tolerances are usually greater than 0.1 °C and increase as
3.2.9 time constant, n—the63.2%responsetimeofasensor
temperatures deviate from the ice-point. Alternatively, a
that exhibits a single-exponential response.
sensor-specific calibration is used when a nominal curve does
3.2.10 tolerance, n—in a measurement instrument, the per-
not exist or when the interchangeability tolerances do not
mitted variation of a measured value from the correct value.
supportaccuracyneeds.PRTcalibrationuncertaintieslessthan
0.01 °C are possible depending on temperature range, PRT
3.2.10.1 Discussion—If a measurement instrument is stated
stability and test measurement capability.
to measure correctly to within a tolerance, the instrument is
classifiedas“intolerance”anditisassumedthatmeasurements
madewithitwillmeasurecorrectlytowithinthistolerance.An
instrumentthatisnotclassifiedas“intolerance”isclassifiedas
The boldface numbers in parentheses refer to a list of references at the end of
“out of tolerance.” this standard.
E2877 − 12
Temperature range, vibration tolerability and stability in the region where temperature gradients exist. Because the
(against drift) are key characteristic to consider when selecting thermoelements are dissimilar, an electromotive force differ-
aPRTforaparticularaccuracyclass.PRTdesignsvarywidely ence (called a thermocouple emf) is produced across the
betweenmanufacturersandcanbetailoredtomeettheneedsof
reference junction. This thermocouple emf (a voltage) in-
specific applications. General guidelines are summarized in creases as the temperature difference increases, making the
Table 1.
thermocouple a sensor for temperature differences. When the
reference-junction temperature is known, the thermocouple
5.2.2 Thermistor—The electrical resistance of a thermistor
(a semiconductor of blended metal oxides) varies with its may be used as a temperature sensor that determines the
temperature, making it a temperature sensor. The resistance of temperature of the measuring junction. The reference junction
of the thermocouple is attached to terminals on the measuring
a thermistor can either increase as the temperature increases
(positive temperature coefficient, or PTC) or decrease as the instrument, which determines the electromotive force (emf)
temperature increases (negative temperature coefficient, or across the reference junction. Thermocouple wires are often
NTC). Most thermistors that are used as temperature sensors
covered with ceramic, fiberglass, or polymer insulations, and
areoftheNTCtype.Thermistorsensorsarefrequentlyusedfor themeasuringjunctionisoftenmountedinasheathedstainless
temperature measurements in the range −20 to 100 °C. They
steel probe with a typical outer diameter of 0.2 mm to 6.4 mm
are sometimes used for special applications over the ranges
for additional protection of the sensor.
−196 to −20 °C and 100 to 150 °C. Thermistors have the
The emf across the reference junction is used along with the
advantages of high resolution, a fast response time, and low
known emf/temperature relations to calculate the measuring
uncertaintyovertheirspecifiedrange.Theyalsohaveexcellent
junction temperature. However, these relations assume that the
stabilityandverygoodvibrationtolerability.Manythermistors
reference-junction temperature is 0 °C. This is never the case
are either encapsulated with epoxy or sealed with a protective
with a digital thermometer, so a reference-junction compensa-
glass coating, resulting in a typical bead size of 0.5 mm to 3
torinsidethemeasuringinstrumentsimulatesthisarrangement.
mm. Others are mounted in a stainless steel sheath with a
ItmeasurestheactualtemperatureofthereferencejunctionTrj
typical outer diameter of 0.9 mm to 6.4 mm. If the thermistor
and adds to or subtracts from the reference-junction emf a
is external to the measuring instrument, it is connected to the
compensating voltage that simulates a reference junction
instrument by electrical leads that are electrically insulated
temperature of 0 °C. The compensating voltage may be added
from the environment and from each other. An external
or subtracted either electronically (before the emf measure-
thermistor is often located inside a protective sheathed probe;
ment) or digitally (after the emf measurement). This compen-
thisarrangementprotectsthesensorfromphysicaldamageand
sating voltage is equivalent to that which the thermocouple
chemical contamination but still allows thermal transfer be-
would produce if the measuring junction temperature were Trj
tween the sensor and its environment. Thermistors usually
and the reference junction temperature were 0 °C. The instru-
have two leads to measure the resistance across the thermistor
mentthencalculatesthetemperatureofthemeasuringjunction
material. The measuring instrument determines the combined
using the emf/temperature equation provided in Note 2 of
resistance of the thermistor and leads by applying a known
Table 7 of Specification E230. This calculation requires use of
current through them and measuring the voltage across the
several coefficients, the values of which are stored in the
ends of the leads.The instr
...
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