ASTM E2846-11
(Guide)Standard Guide for Thermocouple Verification
Standard Guide for Thermocouple Verification
SIGNIFICANCE AND USE
These verification tests may be performed by users or calibrators of thermocouples. The methods are useful for both new and used thermocouples. They provide a means to assess the accuracy with which a thermocouple is capable of measuring temperature.
Results from these tests may be used to determine whether to use or discard a thermocouple. If the thermocouple is subsequently used, the test results may be included in the measurement uncertainty budget. In many circumstances, the results of in situ verifications may be used to recalibrate a used thermocouple. Laboratory measurements, on the other hand, may be used only to verify the original thermocouple calibration or to determine the uncertainty of temperature measurements with the tested thermocouple. Laboratory measurements generally do not suffice to determine the emf-versus-temperature response of a thermocouple found to be inhomogeneous.
SCOPE
1.1 This guide describes tests that may be applied to new or previously 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 the thermocouple fails the test. Some of the tests examine inhomogeneity and others detect wire or measuring-junction breakage. For Style U mineral-insulated metal-sheathed (MIMS) thermocouples with ungrounded measuring junctions, this guide includes tests that examine the electrical isolation of the sheath as well as sheath deterioration.
1.2 The first set of tests involves measurement verifications designed to be performed while the thermocouple is in its usage environment. The second set is composed of electrical tests and visual inspections designed to evaluate the functionality of the thermocouple; these tests may be performed either in house or in a calibration laboratory. The third set is made up of homogeneity tests designed to be performed in a calibration laboratory. Some of the tests provide simple methods to identify some, but not all, defective thermocouples, and alone do not suffice to verify a used thermocouple. They may need to be complemented by other tests for a complete verification.
1.3 The reader of this guide should decide which of the described tests need to be performed. This decision is dependent on whether the reader uses thermocouples for temperature measurement or performs thermocouple calibrations in a laboratory. For users of thermocouples, it is recommended that appropriate tests from the first and second sets be performed initially, as they provide immediate on-site verification of the thermocouples. The appropriateness of a test is dependent upon the user’s temperature measurement uncertainty requirements. Some tests may have lower uncertainties in their verification measurements than others. If these tests do not clearly determine the suitability of the thermocouples, they should be sent to a calibration laboratory for performing appropriate tests from the third set, which give the most complete information on the thermocouple homogeneity. For those who perform thermocouple calibrations in a laboratory, it is recommended that appropriate tests from the second and third sets be performed prior to calibration. The appropriateness of a test is dependent on the calibration laboratory’s capability and convenience for performing the test, as well as the characteristics of the unit under test (UUT).
1.4 This guide may be used for base metal and noble metal thermocouples. Some of the methods covered may apply to refractory metal thermocouples but caution is advised as suitable reference devices at high temperatures may not be readily available.
1.5 This guide may involve hazardous materials, operations and equipment. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health p...
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:E2846 −11
StandardGuide for
1
Thermocouple Verification
This standard is issued under the fixed designation E2846; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Athermocoupleshouldbeperiodicallyverified(testedforcompliancewithspecifications)toensure
thatithasnotincurredphysical,metallurgical,orchemicalchangesthatinhibitorpreventtemperature
measurements with acceptable accuracy. Unlike many other sensors, the signal generated by a
thermocouple depends on the physical and chemical state of the region of the thermocouple wires or
thermoelements where temperature gradients exist rather than the state of the measuring junction.
Physical or chemical degradation of the thermocouple along only part of its length results in
thermocoupleinhomogeneity.Suchinhomogeneitycausesthemeasuredtemperaturetodependonthe
intermediate thermal environment between the measuring and reference junctions of the thermo-
couple.Ifathermocouplebecomesmoreinhomogeneouswithtime,thetemperaturemeasuredbythat
thermocouple may appear to drift from its original value, even though the actual temperature it is
measuringisconstant.Iftheintermediatethermalenvironmentduringuseisdifferentfromthatduring
calibration, the temperature measurement of an inhomogeneous thermocouple will be inaccurate.
Thermocouples used in a harsh environment often become progressively more inhomogeneous; for
such thermocouples it is particularly important to make periodic tests of their performance. In
addition,athermocouplebecomesunreliableifitundergoescertainotherphysicalchanges.Itwillnot
measure properly if its wires or the measuring junction are broken or if its thermoelements are in
electrical contact in a location other than the measuring junction. Metal-sheathed thermocouples will
perform unreliably if there is excessive electrical leakage between the sheath and the thermocouple
wire;thiscanoccurifholeshavedevelopedinthesheathorthesealoftheendclosuredevelopsaleak.
Periodic tests can check for these undesirable changes, allowing the user to know whether the
performance of the thermocouple can be trusted. These tests are particularly important before the
calibration of a thermocouple, because they determine whether the thermocouple’s performance is
worthy of the effort and expense of calibration.
1. Scope 1.2 The first set of tests involves measurement verifications
designed to be performed while the thermocouple is in its
1.1 This guide describes tests that may be applied to new or
usage environment. The second set is composed of electrical
previously used thermocouples for the purpose of verification.
tests and visual inspections designed to evaluate the function-
Someofthetestsperformasuitableverificationbythemselves,
ality of the thermocouple; these tests may be performed either
but many tests merely alert the user to serious problems if the
inhouseorinacalibrationlaboratory.Thethirdsetismadeup
thermocouple fails the test. Some of the tests examine inho-
of homogeneity tests designed to be performed in a calibration
mogeneityandothersdetectwireormeasuring-junctionbreak-
laboratory. Some of the tests provide simple methods to
age. For Style U mineral-insulated metal-sheathed (MIMS)
identify some, but not all, defective thermocouples, and alone
thermocouples with ungrounded measuring junctions, this
donotsufficetoverifyausedthermocouple.Theymayneedto
guide includes tests that examine the electrical isolation of the
be complemented by other tests for a complete verification.
sheath as well as sheath deterioration.
1.3 The reader of this guide should decide which of the
described tests need to be performed. This decision is depen-
dentonwhetherthereaderusesthermocouplesfortemperature
1
ThispracticeisunderthejurisdictionofASTMCommitteeE20onTemperature
measurement or performs thermocouple calibrations in a labo-
Measurement and is the direct responsibility of Subcommittee E20.04 on Thermo-
ratory. For users of thermocouples, it is recommended that
couples.
appropriate tests from the first and second sets be performed
Current edition approved Nov. 1, 2011. Published August 2012. DOI: 10.1520/
E2846–11. initially, as they provide immediate on-site verification of the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1
---------------------- Page: 1 ----------------------
E2846−11
thermocouples.Theappropriatenessofatestisdependentupon 3.2 Definitions of Terms Specific to This Standard:
the user’s temperature measurement uncertainty requirements.
3.2.1 expanded measurement uncertainty,
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.