ASTM E2846-2011 Standard Guide for Thermocouple Verification《热电偶检定的标准指南》.pdf

1、Designation: E2846 11Standard Guide forThermocouple Verification1This standard is issued under the fixed designation E2846; 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 indicate

2、s the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONAthermocouple should be periodically verified (tested for compliance with specifications) to ensurethat it has not incurred physical, metallurgical, or chemical chan

3、ges that inhibit or prevent temperaturemeasurements with acceptable accuracy. Unlike many other sensors, the signal generated by athermocouple depends on the physical and chemical state of the region of the thermocouple wires orthermoelements where temperature gradients exist rather than the state o

4、f the measuring junction.Physical or chemical degradation of the thermocouple along only part of its length results inthermocouple inhomogeneity. Such inhomogeneity causes the measured temperature to depend on theintermediate thermal environment between the measuring and reference junctions of the t

5、hermo-couple. If a thermocouple becomes more inhomogeneous with time, the temperature measured by thatthermocouple may appear to drift from its original value, even though the actual temperature it ismeasuring is constant. If the intermediate thermal environment during use is different from that dur

6、ingcalibration, the temperature measurement of an inhomogeneous thermocouple will be inaccurate.Thermocouples used in a harsh environment often become progressively more inhomogeneous; forsuch thermocouples it is particularly important to make periodic tests of their performance. Inaddition, a therm

7、ocouple becomes unreliable if it undergoes certain other physical changes. It will notmeasure properly if its wires or the measuring junction are broken or if its thermoelements are inelectrical contact in a location other than the measuring junction. Metal-sheathed thermocouples willperform unrelia

8、bly if there is excessive electrical leakage between the sheath and the thermocouplewire; this can occur if holes have developed in the sheath or the seal of the end closure develops a leak.Periodic tests can check for these undesirable changes, allowing the user to know whether theperformance of th

9、e thermocouple can be trusted. These tests are particularly important before thecalibration of a thermocouple, because they determine whether the thermocouples performance isworthy of the effort and expense of calibration.1. Scope1.1 This guide describes tests that may be applied to new orpreviously

10、 used thermocouples for the purpose of verification.Some of the tests perform a suitable verification by themselves,but many tests merely alert the user to serious problems if thethermocouple fails the test. Some of the tests examine inho-mogeneity and others detect wire or measuring-junction break-

11、age. For Style U mineral-insulated metal-sheathed (MIMS)thermocouples with ungrounded measuring junctions, thisguide includes tests that examine the electrical isolation of thesheath as well as sheath deterioration.1.2 The first set of tests involves measurement verificationsdesigned to be performed

12、 while the thermocouple is in itsusage environment. The second set is composed of electricaltests and visual inspections designed to evaluate the function-ality of the thermocouple; these tests may be performed eitherin house or in a calibration laboratory. The third set is made upof homogeneity tes

13、ts designed to be performed in a calibrationlaboratory. Some of the tests provide simple methods toidentify some, but not all, defective thermocouples, and alonedo not suffice to verify a used thermocouple. They may need tobe complemented by other tests for a complete verification.1.3 The reader of

14、this guide should decide which of thedescribed tests need to be performed. This decision is depen-dent on whether the reader uses thermocouples for temperaturemeasurement or performs thermocouple calibrations in a labo-ratory. For users of thermocouples, it is recommended thatappropriate tests from

15、the first and second sets be performedinitially, as they provide immediate on-site verification of thethermocouples. The appropriateness of a test is dependent uponthe users temperature measurement uncertainty requirements.Some tests may have lower uncertainties in their verification1This practice i

16、s under the jurisdiction ofASTM Committee E20 onTemperatureMeasurement and is the direct responsibility of Subcommittee E20.04 on Thermo-couples.Current edition approved Nov. 1, 2011. Published August 2012. DOI: 10.1520/E284611.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West

17、Conshohocken, PA 19428-2959, United States.measurements than others. If these tests do not clearly deter-mine the suitability of the thermocouples, they should be sentto a calibration laboratory for performing appropriate testsfrom the third set, which give the most complete informationon the thermo

18、couple homogeneity. For those who performthermocouple calibrations in a laboratory, it is recommendedthat appropriate tests from the second and third sets beperformed prior to calibration. The appropriateness of a test isdependent on the calibration laboratorys capability and con-venience for perfor

19、ming the test, as well as the characteristicsof the unit under test (UUT).1.4 This guide may be used for base metal and noble metalthermocouples. Some of the methods covered may apply torefractory metal thermocouples but caution is advised assuitable reference devices at high temperatures may not be

20、readily available.1.5 This guide may involve hazardous materials, operationsand equipment. This standard does not purport to address allof the safety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices

21、and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E220 Test Method for Calibration of Thermocouples ByComparison TechniquesE344 Terminology Relating to Thermometry and Hydrom-etryE563 Practice for Preparation and Use of an Ice-Point Ba

22、thas a Reference TemperatureE585/E585M Specification for Compacted Mineral-Insulated, Metal-Sheathed, Base Metal ThermocoupleCableE608/E608M Specification for Mineral-Insulated, Metal-Sheathed Base Metal ThermocouplesE780 Test Method for Measuring the Insulation Resistanceof Mineral-Insulated, Metal

23、-Sheathed Thermocouples andThermocouple Cable at Room TemperatureE839 Test Methods for Sheathed Thermocouples andSheathed Thermocouple CableE1350 Guide for Testing Sheathed Thermocouples, Ther-mocouplesAssemblies, and Connecting Wires Prior to, andAfter Installation or ServiceE2181/E2181M Specificat

24、ion for Compacted Mineral-Insulated, Metal-Sheathed, Noble Metal Thermocouplesand Thermocouple Cable3. Terminology3.1 DefinitionsThe definitions given in TerminologyE344 apply to terms used in this guide.3.2 Definitions of Terms Specific to This Standard:3.2.1 expanded measurement uncertainty, nprod

25、uct of acombined standard measurement uncertainty and a factorlarger than the number one.3.2.1.1 DiscussionThe term “factor” in this definitionrefers to a coverage factor k. For k=2 (the most commoncoverage factor), a measurement instrument measures correctlyto within its expanded measurement uncert

26、ainty with a 95.4 %probability.3.2.2 gradient zone, nthe section of a thermocouple that isexposed during a measurement to temperatures in the rangefrom tamb+ 0.1(tm tamb)totamb+ 0.9(tm tamb), where tambisambient temperature and tmis the temperature of the measuringjunction.3.2.2.1 DiscussionThis ter

27、m is used as part of the descrip-tion of the thermal profile along the length of the thermo-couple. The gradient zone definition is intended to describe, inan approximate way, the section of thermocouple in whichmost of the emf was created.3.2.3 half-maximum heated length, nthe distance betweenthe m

28、easuring junction and the position along the length of thethermocouple wires or sheath where the temperature equals theaverage of the calibration-point and ambient temperatures.3.2.3.1 DiscussionThis term is used as part of the descrip-tion of the thermal profile along the length of the thermo-coupl

29、e.3.2.4 homogeneous, adjhaving uniform thermoelectricproperties along the length of the thermocouple or thermoele-ment.3.2.5 homogeneous Seebeck coeffcient, nthe temperature-dependent Seebeck coefficient of a thermocouple or thermo-element when it is in a homogeneous state.3.2.5.1 DiscussionThe homo

30、geneous Seebeck coefficientis usually determined from measurements of the Seebeckcoefficient of the thermocouple or thermoelement when it isnew, because then it is usually homogeneous. If segments ofthe new thermocouple or thermoelement are inhomogeneous,the homogenous Seebeck coefficient is determi

31、ned from mea-surements made on the segments demonstrated to be homoge-neous.3.2.6 inhomogeneity, nthe deviation of the Seebeck coef-ficient of a segment of a thermocouple or thermoelement at agiven temperature from its homogeneous Seebeck coefficient atthat temperature.3.2.6.1 DiscussionIn practice,

32、 only variations in the See-beck coefficient along the length of a thermocouple that isexposed to temperature gradients affect the voltage output of athermocouple. Inhomogeneity of a thermocouple is oftenreported as a fractional variation in the Seebeck coefficient.3.2.7 minimum immersion length, nt

33、he depth that a ther-mometer should be immersed, in a uniform temperatureenvironment, such that further immersion does not produce achange in the indicated temperature greater than the specifiedtolerance.3.2.8 referee thermocouple, na thermocouple made fromthe same lot of wire or MIMS cable as the U

34、UT group, usingidentical construction design and methods and identical anneal-ing methods but not having been placed into permanentservice.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume

35、 information, refer to the standards Document Summary page onthe ASTM website.E2846 1123.2.8.1 DiscussionBecause of the high value of refereethermocouples for performing verification tests by the user, itis strongly recommended that after users receive new lots ofthermocouple wire, they construct re

36、feree thermocouples alongwith the thermocouples intended for regular use.3.2.9 sensing point, nthe location on a thermometerwhere the temperature is (or is assumed to be) measured.3.2.9.1 DiscussionA thermocouples sensing point is itsmeasuring junction. A resistance temperature detector (RTD)contain

37、s 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 thetemperature is assumed to be that measured by the RTD.3.2.10 standard measurement uncertainty, nmeasurementuncertainty expressed as a s

38、tandard deviation.3.2.10.1 DiscussionA measurement instrument measurescorrectly to within its standard uncertainty with a 68.2 %probability.3.2.11 tolerance, nin a measurement instrument, the per-mitted variation of a measured value from the correct value.3.2.11.1 DiscussionIf a measurement instrume

39、nt 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 classified as“out of tolerance.”3.2.12 UUT, n

40、abbreviation for “unit under test.”3.2.13 validation, nthe process of testing a thermometerfor acceptable accuracy in its intended use.3.2.14 verification, nthe process of testing a thermometerfor compliance with specifications.3.2.14.1 DiscussionHere, “specifications” normally refersto specificatio

41、n tolerances for uncalibrated thermometers andto calibration uncertainties for calibrated thermometers. Thesame tests may be used for a less stringent verification calledvalidation, defined as “the process of testing a thermometer foracceptable accuracy in its intended use.”4. Summary of Verificatio

42、n Tests4.1 In Situ Measurement Verification:4.1.1 Verification with the Reference Thermometer in theSame Access PointAUUT is verified in situ at an appropriateconstant temperature by comparison to a known referencethermometer in the same access point. For the comparison, thethermocouple is temporari

43、ly replaced by the reference ther-mometer in the access point, making sure that the measuringpoint of the sensor is at the same immersion depth as themeasuring junction of the thermocouple. For open accesspoints, the reference thermometer may be a referee thermo-couple, a non-referee thermocouple th

44、at is new or determinedto be homogeneous, or another temperature sensor unaffectedby inhomogeneity such as a resistance temperature detector(RTD) or thermistor. If the reference thermometer is not areferee thermocouple, its minimum immersion length shall beless than the immersion depth of the UUT. F

45、or access pointsthat are thermowells or protection tubes, the reference ther-mometer shall be a referee thermocouple.4.1.2 Verification with the Reference Thermometer in anAdjacent Access PointAthermocouple is verified in situ at anappropriate constant temperature by comparison to a knownreference t

46、hermometer located in an adjacent access point. Inthis case the comparison can be made without removing theUUT. The reference thermometer may be a referee thermo-couple, a non-referee thermocouple that is new or determinedto be homogeneous, or another temperature sensor unaffectedby inhomogeneity su

47、ch as an RTD or thermistor. If thereference thermometer is not a referee thermocouple, itsminimum immersion length shall be less than the immersiondepth of the UUT.4.2 Thermocouple Functionality Tests:4.2.1 Measurement of the Loop ResistanceThe loop resis-tance of the thermocouple circuit is measure

48、d to verify that thethermoelements and welded measuring junction are continu-ous. This test may also be used to identify conditions where thethermoelements are in contact with each other at a point otherthan at the measuring junction. It may be difficult to identifymultiple contact points when they

49、occur near the measuringjunction.4.2.2 Measurement of the Insulation Resistance of Thermo-couples with Style U Measuring JunctionsThe resistance ofthe insulation between the UUT sheath and the thermoelementsis measured to determine if the electrical isolation betweenthem has deteriorated.4.2.3 Measurement of Sheath Diameter (Metal-SheathedThermocouples)Measurements of the UUT sheath diameterare made and compared to measurements made prior toinstallation to monitor metal erosion in the sensor sheath thatmay cause the UUT to perform unreliably.4.2.4