1、Designation: E2877 12Standard Guide forStandard Guide for Digital Contact Thermometers1This standard is issued under the fixed designation E2877; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number i
2、n parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This Guide describes general-purpose, digital contactthermometers (hereafter simply called “digital thermometers”)that provide temperature read
3、ings in units of degrees Celsius ordegrees Fahrenheit, or both. The different types of temperaturesensors for these thermometers are described, and their relativemerits are discussed. Nine accuracy classes are introduced fordigital thermometerhes; these classes consider the accuracy ofthe sensor/mea
4、suring-instrument unit.1.2 The proposed accuracy classes for digital thermometerspertain to the temperature interval of 200 C to 500 C, aninterval of special interest for many applications in thermom-etry. All of the temperature sensor types for the digitalthermometers discussed are able to measure
5、temperature overat least some range within this interval. Some types are alsoable to measure beyond this interval.To qualify for an accuracyclass, the thermometer must measure correctly to within aspecified value (in units of C) over this interval or over thesubinterval in which they are capable of
6、making measure-ments. Those thermometers that can measure temperature inranges beyond this interval generally have larger measurementuncertainty in these ranges.1.3 The digital thermometer sensors discussed are platinumresistance sensors, thermistors, and thermocouples. The rangeof use for these typ
7、es of sensors is provided. The measurementuncertainty of a sensor is determined by its tolerance class orgrade and whether the sensor has been calibrated.1.4 This Guide provides a number of recommendations forthe manufacture and selection of a digital thermometer. First, itrecommends that the thermo
8、meters sensor conform to appli-cable ASTM specifications. Also, it recommends minimumstandards for documentation on the thermometer and informa-tional markings on the probe and measuring instrument.1.5 The derived SI units (degrees Celsius) found in thisGuide are to be considered standard. However,
9、thermometersdisplaying degrees Fahrenheit are compliant with this guide aslong as all other guidance is followed.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate sa
10、fety and health practices and determine the applica-bility of regulatory limitations prior to use. Some specifichazards statements are given in Section 7 on Hazards.2. Referenced Documents2.1 ASTM Standards:2E230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized Thermoco
11、uplesE344 Terminology Relating to Thermometry and Hydrom-etryE563 Practice for Preparation and Use of an Ice-Point Bathas a Reference TemperatureE608/E608M Specification for Mineral-Insulated, Metal-Sheathed Base Metal ThermocouplesE644 Test Methods for Testing Industrial Resistance Ther-mometersE83
12、9 Test Methods for Sheathed Thermocouples andSheathed Thermocouple CableE879 Specification for Thermistor Sensors for General Pur-pose and Laboratory Temperature MeasurementsE1137/E1137M Specification for Industrial Platinum Resis-tance ThermometersE2181/E2181M Specification for Compacted Mineral-In
13、sulated, Metal-Sheathed, Noble Metal Thermocouplesand Thermocouple CableE2593 Guide for Accuracy Verification of Industrial Plati-num Resistance ThermometersE2846 Guide for Thermocouple Verification3. Terminology3.1 Definitions: The definitions given in Terminology E344apply to terms used in this gu
14、ide.3.2 Definitions:3.2.1 accuracy class, nclass of an item that meets certainmetrological requirements intended to keep errors withinspecified limits.1This guide is under the jurisdiction of ASTM Committee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.09 on Digi
15、talContact Thermometers.Current edition approved Nov. 1, 2012. Published December 2012. DOI:10.1520/E2877122For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the stand
16、ards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.1.1 DiscussionThis document describes accuracyclasses for digital thermometers.3.2.2 calibration uncertainty, nparameter, derived fromth
17、e analysis of a calibration of a measuring instrument, thatcharacterizes the range in which the true calibration result isestimated to lie within a given confidence level.3.2.3 digital contact thermometer, na device that mea-sures temperature through direct contact with a sensor andprovides a digita
18、l output or display of the determined value, orboth.3.2.3.1 DiscussionThis device consists of a temperaturesensor connected to a measuring instrument; this instrumentmeasures the temperature-dependent quantity of the sensor,computes the temperature from the measured quantity, andprovides a digital o
19、utput or display of the temperature, or both.The sensor is sometimes located inside the instrument.3.2.4 measuring instrument, nthe instrument in a digitalthermometer that is used to measure the temperature-dependentquantity of the sensor.3.2.5 probe, nan assembly, including the transducer(sensor),
20、that is used to position the transducer in the specificlocation at which the temperature is to be measured.3.2.6 reference-junction compensator, na device that mea-sures the temperature of a thermocouples reference junctionand adds to or subtracts from the reference-junction emf acompensating voltag
21、e that simulates a reference junctiontemperature of 0 C.3.2.6.1 DiscussionThe compensating voltage may beadded or subtracted electronically or digitally.3.2.7 response time, nthe time required for a sensor tochange a specified percentage of the total difference betweenits initial and final temperatu
22、res when the sensor is subjected toa step function change in temperature.3.2.8 sensing point, nthe location on a temperature sensorwhere the temperature is (or is assumed to be) measured.3.2.8.1 DiscussionA thermocouples sensing point is itsmeasuring junction (although the signal in the thermocouple
23、 isgenerated along the two thermocouple wires in regions wherea temperature gradient exists).Aplatinum resistance thermom-eter contains a sensing element that may be large enough toexperience spatial temperature variations; in this case thesensing point is the central point in the element where thet
24、emperature is assumed to be that measured by the platinumresistance thermometer.3.2.9 time constant, nthe 63.2 % response time of a sensorthat exhibits a single-exponential response.3.2.10 tolerance, nin a measurement instrument, the per-mitted variation of a measured value from the correct value.3.
25、2.10.1 DiscussionIf a measurement instrument is statedto measure correctly to within a tolerance, the instrument isclassified as “in tolerance” and it is assumed that measurementsmade with it will measure correctly to within this tolerance.Aninstrument that is not classified as “in tolerance” is cla
26、ssified as“out of tolerance.”4. Significance and Use4.1 Digital thermometers are used for measuring tempera-ture in many laboratories and industrial applications.4.2 For many applications, digital thermometers using ex-ternal probes are considered environmentally-safe alternativesto mercury-in-glass
27、 thermometers. (1)34.3 Some digital thermometers are also used as reference orworking temperature standards in verification and calibrationof thermometers and also in determining the conditions nec-essary for evaluating the performance of other measuringinstruments used in legal metrology and indust
28、ry.5. Description of the Instruments5.1 Basic Description of a Digital Thermometer5.1.1 A digital thermometer consists of a temperaturesensor, often mounted in a probe, connected to a measuringinstrument. The instrument measures the temperature-dependent quantity of the sensor, computes the temperat
29、urefrom that measured quantity, and provides a digital output ordisplay of the computed temperature, or both.5.2 Types of Digital Thermometer Sensors5.2.1 Platinum Resistance Thermometer (PRT). The electri-cal resistance of a PRTs platinum element increases nearlylinearly as its temperature increase
30、s, making it a temperaturesensor. A PRT sensor consists of a platinum filament of finewire or film supported by an insulating body. The sensor isusually mounted in a protective glass coating with size 2 mmto 4 mm or a sheathed probe (glass or stainless steel) with atypical outer diameter of 1.6 mm t
31、o 6.4 mm; this arrangementprotects the sensor from physical damage and chemical con-tamination but still allows thermal transfer between the sensorand its environment. This sensor package often determines thetemperature capability and accuracy of the device. The sensoris connected to a measuring ins
32、trument by electrically conduct-ing leads. The number of leads can be 2, 3, or 4. The measuringinstrument determines the resistance of the PRTs sensingelement by applying a known current through it and measuringthe voltage across it. Most measuring instruments for PRTscalculate the temperature of th
33、e sensor using the relevantresistance/temperature equations. The PRT calibration is de-fined as either a nominal resistance-temperature relationshipwith an interchangeability tolerance (for example, Specifica-tion E1137/E1137M) or a single sensor calibration with esti-mated uncertainty. A nominal re
34、lationship allows the readoutdevice to be programmed with a single resistance-temperaturerelationship for a specified PRT family. Interchangeabilitytolerances are usually greater than 0.1 C and increase astemperatures deviate from the ice-point. Alternatively, asensor-specific calibration is used wh
35、en a nominal curve doesnot exist or when the interchangeability tolerances do notsupport accuracy needs. PRT calibration uncertainties less than0.01 C are possible depending on temperature range, PRTstability and test measurement capability.3The boldface numbers in parentheses refer to a list of ref
36、erences at the end ofthis standard.E2877 122Temperature range, vibration tolerability and stability(against drift) are key characteristic to consider when selectinga PRT for a particular accuracy class. PRT designs vary widelybetween manufacturers and can be tailored to meet the needs ofspecific app
37、lications. General guidelines are summarized inTable 1.5.2.2 ThermistorThe electrical resistance of a thermistor(a semiconductor of blended metal oxides) varies with itstemperature, making it a temperature sensor. The resistance ofa thermistor can either increase as the temperature increases(positiv
38、e temperature coefficient, or PTC) or decrease as thetemperature increases (negative temperature coefficient, orNTC). Most thermistors that are used as temperature sensorsare of the NTC type.Thermistor sensors are frequently used fortemperature measurements in the range 20 to 100 C. Theyare sometime
39、s used for special applications over the ranges196 to 20 C and 100 to 150 C. Thermistors have theadvantages of high resolution, a fast response time, and lowuncertainty over their specified range. They also have excellentstability and very good vibration tolerability. Many thermistorsare either enca
40、psulated with epoxy or sealed with a protectiveglass coating, resulting in a typical bead size of 0.5 mm to 3mm. Others are mounted in a stainless steel sheath with atypical outer diameter of 0.9 mm to 6.4 mm. If the thermistoris external to the measuring instrument, it is connected to theinstrument
41、 by electrical leads that are electrically insulatedfrom the environment and from each other. An externalthermistor is often located inside a protective sheathed probe;this arrangement protects the sensor from physical damage andchemical contamination but still allows thermal transfer be-tween the s
42、ensor and its environment. Thermistors usuallyhave two leads to measure the resistance across the thermistormaterial. The measuring instrument determines the combinedresistance of the thermistor and leads by applying a knowncurrent through them and measuring the voltage across theends of the leads.
43、The instrument calculates the temperature ofthe thermistor using a specific resistance/temperature equationrelevant to the type of thermistor. The temperature calculationrequires the use of several coefficients, the values of which arestored in the instrument. For thermistor types used in clinicalla
44、boratory temperature measurements, nominal values of thesecoefficients may be obtained from Table 1 of SpecificationE879. For other thermistor types, the nominal values aregenerally obtained from the manufacturer. Use of the nominalvalues calculates temperature to within the tolerance of thethermist
45、or type. Calibration-determined coefficient values maybe entered into some instrument models, enabling more accu-rate temperature determination for an individual thermistorsensor.Asummary of the characteristics of thermistors is listedin Table 1.5.2.3 ThermocoupleA thermocouple consists of two par-a
46、llel dissimilar homogeneous metal wires, called thermoele-ments. These thermoelements, which are usually of equallength, are joined physically and electrically at one end, calledthe measuring junction. The other end is called the referencejunction. When there is a temperature difference between them
47、easuring junction and reference junction, an electromotiveforce (emf) is produced across each thermoelement, generatedin the region where temperature gradients exist. Because thethermoelements are dissimilar, an electromotive force differ-ence (called a thermocouple emf) is produced across therefere
48、nce junction. This thermocouple emf (a voltage) in-creases as the temperature difference increases, making thethermocouple a sensor for temperature differences. When thereference-junction temperature is known, the thermocouplemay be used as a temperature sensor that determines thetemperature of the
49、measuring junction. The reference junctionof the thermocouple is attached to terminals on the measuringinstrument, which determines the electromotive force (emf)across the reference junction. Thermocouple wires are oftencovered with ceramic, fiberglass, or polymer insulations, andthe measuring junction is often mounted in a sheathed stainlesssteel probe with a typical outer diameter of 0.2 mm to 6.4 mmfor additional protection of the sensor.The emf across the reference junction is used along with theknown emf/temperature relations to calculate the m
