1、Designation: E2877 121Standard Guide forDigital 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 in parentheses indi
2、cates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEChanges were made editorially in February 2013.1. Scope1.1 This Guide describes general-purpose, digital contactthermometers (hereafter simply called “digital thermomet
3、ers”)that provide temperature readings 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 cons
4、ider the accuracy ofthe sensor/measuring-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 digitalthermomet
5、ers discussed are able to measure 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 thesubinte
6、rval in which they are capable of 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 thermocoup
7、les. The rangeof use for these types 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. F
8、irst, itrecommends that the thermometers 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
9、 be considered standard. However, 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 sta
10、ndard to establish appro-priate safety 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
11、) Tables for Standardized ThermocouplesE344 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 Indu
12、strial Resistance Ther-mometersE839 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 Spec
13、ification for Compacted Mineral-Insulated, 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
14、E344apply to terms used in this guide.1This guide is under the jurisdiction of ASTM Committee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.09 on DigitalContact Thermometers.Current edition approved Nov. 1, 2012. Published December 2012. DOI:10.1520/E2877122For r
15、eferenced 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 standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, W
16、est Conshohocken, PA 19428-2959. United States13.2 Definitions:3.2.1 accuracy class, nclass of an item that meets certainmetrological requirements intended to keep errors withinspecified limits.3.2.1.1 DiscussionThis document describes accuracyclasses for digital thermometers.3.2.2 calibration uncer
17、tainty, nparameter, derived fromthe 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
18、with a sensor andprovides a digital 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
19、 quantity, andprovides a digital output 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,
20、including the transducer(sensor), 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
21、-junction emf acompensating voltage 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 betw
22、eenits initial and final temperatures 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 (altho
23、ugh the signal in the thermocouple 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 centr
24、al point in the element where thetemperature 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 measur
25、ed value from the correct value.3.2.10.1 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 c
26、lassified as “in tolerance” is classified 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-sa
27、fe alternativesto mercury-in-glass 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
28、used in legal metrology and industry.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
29、 the sensor, computes the temperaturefrom 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 nearlyli
30、nearly as its temperature increases, 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 a
31、typical outer diameter of 1.6 mm to 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 sen
32、soris connected to a measuring instrument 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 P
33、RTscalculate the temperature of the 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 es
34、ti-mated uncertainty. A nominal relationship 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, asens
35、or-specific calibration is used when a nominal curve does3The boldface numbers in parentheses refer to a list of references at the end ofthis standard.E2877 1212not exist or when the interchangeability tolerances do notsupport accuracy needs. PRT calibration uncertainties less than0.01 C are possibl
36、e depending on temperature range, PRTstability and test measurement capability.Temperature 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 tailor
37、ed to meet the needs ofspecific applications. 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 a
38、s the temperature increases(positive 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
39、range 20 to 100 C. Theyare sometimes 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 tolerabili
40、ty. Many thermistorsare either encapsulated 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 instrumen
41、t, it is connected to theinstrument 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 all
42、ows thermal transfer be-tween the sensor 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 vo
43、ltage across theends of the leads. 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
44、thermistor types used in clinicallaboratory 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
45、within the tolerance of thethermistor 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
46、 thermocouple consists of two par-allel 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
47、temperature difference between themeasuring 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 thermocoupl
48、e emf) is produced across thereference 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 tha
49、t determines thetemperature of the 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/t
