ASTM_E_2846_-_14.pdf

Designation:E284614Standard 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 indicates 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.INTRODUCTIONAthermocouple 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 thermocouple 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 inhomogeneity.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 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 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 junction 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 sheath 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 whether 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 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 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 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 tests 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 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 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 is under the jurisdiction ofASTM Committee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.04 on Thermo-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 19428-2