ASTM E2846-2014 Standard Guide for Thermocouple Verification《热电偶检定的标准指南》.pdf

1、Designation: E2846 14Standard 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.NOTE- Balloted and approved Figures X2.1, X2.2, X2.3, and Tables X3.1 and X3.2 have been included in the standard and theyear date was changed on October 7, 2014.INTRODUCTIONAth

3、ermocouple should be periodically verified (tested for compliance with specifications) to ensure that it has notincurred physical, metallurgical, or chemical changes that inhibit or prevent temperature measurements withacceptable accuracy. Unlike many other sensors, the signal generated by a thermoc

4、ouple depends on the physical andchemical state of the region of the thermocouple wires or thermoelements where temperature gradients exist ratherthan the state of the measuring junction. Physical or chemical degradation of the thermocouple along only part ofits length results in thermocouple inhomo

5、geneity. Such inhomogeneity causes the measured temperature to dependon the intermediate thermal environment between the measuring and reference junctions of the thermocouple. If athermocouple becomes more inhomogeneous with time, the temperature measured by that thermocouple may appearto drift from

6、 its original value, even though the actual temperature it is measuring is constant. If the intermediatethermal environment during use is different from that during calibration, the temperature measurement of aninhomogeneous thermocouple will be inaccurate. Thermocouples used in a harsh environment

7、often becomeprogressively more inhomogeneous; for such thermocouples it is particularly important to make periodic tests oftheir performance. In addition, a thermocouple becomes unreliable if it undergoes certain other physical changes. Itwill not measure properly if its wires or the measuring junct

8、ion are broken or if its thermoelements are in electricalcontact in a location other than the measuring junction. Metal-sheathed thermocouples will perform unreliably ifthere is excessive electrical leakage between the sheath and the thermocouple wire; this can occur if holes havedeveloped in the sh

9、eath or the seal of the end closure develops a leak. Periodic tests can check for these undesirablechanges, allowing the user to know whether the performance of the thermocouple can be trusted. These tests areparticularly important before the calibration of a thermocouple, because they determine whe

10、ther the thermocouplesperformance is worthy of the effort and expense of calibration.1. Scope1.1 This guide describes tests that may be applied to new orpreviously used thermocouples for the purpose of verification.Some of the tests perform a suitable verification by themselves,but many tests merely

11、 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-age. For Style U mineral-insulated metal-sheathed (MIMS)thermocouples with ungrounded measuring junctions, thisguide includes tests that

12、examine the electrical isolation of thesheath as well as sheath deterioration.1.2 The first set of tests involves measurement verificationsdesigned to be performed while the thermocouple is in itsusage environment. The second set is composed of electricaltests and visual inspections designed to eval

13、uate 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 tests designed to be performed in a calibrationlaboratory. Some of the tests provide simple methods toidentify some, but not all, defective

14、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 this guide should decide which of thedescribed tests need to be performed. This decision is depen-dent on whether the reader uses thermoc

15、ouples for temperaturemeasurement or performs thermocouple calibrations in a labo-ratory. For users of thermocouples, it is recommended thatappropriate tests from the first and second sets be performedinitially, as they provide immediate on-site verification of thethermocouples. The appropriateness

16、of a test is dependent uponthe users temperature measurement uncertainty requirements.Some tests may have lower uncertainties in their verification1This practice is under the jurisdiction ofASTM Committee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.04 on Thermo

17、-couples.Current edition approved Oct. 7, 2014. Published October 2014. Originallyapproved in 2011. Last previous edition approved in 2011 as E284611. DOI:10.1520/E284614.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1measurements th

18、an 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 thermocouple homogeneity. For those who performthermocouple cali

19、brations 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 performing the test, as well as the characteristicsof the unit u

20、nder 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 bereadily available.1.5 This guide may involve hazardous mat

21、erials, 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 and determine the applica-bility of regulatory limitations

22、 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 Bathas a Reference TemperatureE585/E585M Specification for C

23、ompacted 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-Sheathed Thermocouples andThermocouple Cable at Room Temp

24、eratureE839 Test Methods for Sheathed Thermocouples andSheathed Thermocouple CableE1350 Guide for Testing Sheathed Thermocouples, Thermo-couples Assemblies, and Connecting Wires Prior to, andAfter Installation or ServiceE2181/E2181M Specification for Compacted Mineral-Insulated, Metal-Sheathed, Nobl

25、e Metal Thermocouplesand Thermocouple Cable3. Terminology3.1 DefinitionsThe definitions given in Terminology E344apply to terms used in this guide.3.2 Definitions of Terms Specific to This Standard:3.2.1 expanded measurement uncertainty, nproduct of acombined standard measurement uncertainty and a f

26、actorlarger 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 uncertainty with a 95.4 %probability.3.2.2 gradient zone, nthe

27、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 tambis ambient temperature and tmis the temperature of themeasuring junction.3.2.2.1 DiscussionThis term is used as part of the descrip-tion of the thermal pro

28、file 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 measuring junction and the position along the length of t

29、hethermocouple 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-couple.3.2.4 homogeneous, adjhaving uniform thermoelectricpro

30、perties 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 homogeneous Seebeck coefficientis usually determined from me

31、asurements 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 determined from mea-surements made on the segments demonstrated

32、 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, only variations in the See-beck coefficient along the l

33、ength 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, nthe depth that a ther-mometer should be immersed, in a un

34、iform 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 UUT group, using2For referenced ASTM standards, visit the

35、 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.E2846 142identical construction design and methods and identical anneal-ing methods but not having bee

36、n placed into permanentservice.3.2.8.1 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 referee thermocouples alongwith the thermocouples intended

37、 for regular use.3.2.9 sensing point, nthe location on a thermometer wherethe temperature is (or is assumed to be) measured.3.2.9.1 DiscussionA thermocouples sensing point is itsmeasuring junction. A resistance temperature detector (RTD)contains a sensing element that may be large enough toexperienc

38、e 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 standard deviation.3.2.10.1 DiscussionA measurement instr

39、ument 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 instrument is statedto measure correctly to within a tolerance,

40、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, nabbreviation for “unit under test.”3.2.13 validation, nt

41、he 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 specification tolerances for uncalibrated thermometers andto calibra

42、tion 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 Verification Tests4.1 In Situ Measurement Verification:4.1.1 Verifi

43、cation 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 temporarily replaced by the reference ther-mometer in the access

44、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 refereethermocouple, a non-referee thermocouple that is new ordetermined to be homogeneous, or another tempe

45、rature sensorunaffected by inhomogeneity such as a resistance temperaturedetector (RTD) or thermistor. If the reference thermometer isnot a referee thermocouple, its minimum immersion lengthshall be less than the immersion depth of the UUT. For accesspoints that are thermowells or protection tubes,

46、the referencethermometer 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 thermometer located in an adjacent access point. Inthis case

47、the comparison can be made without removing theUUT. The reference thermometer may be a refereethermocouple, a non-referee thermocouple that is new ordetermined to be homogeneous, or another temperature sensorunaffected by inhomogeneity such as an RTD or thermistor. Ifthe reference thermometer is not

48、 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 measured to verify that thethermoelements and welded measuring juncti

49、on 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 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 Sheat