ASTM E207-08(2015)
(Test Method)Standard Test Method for Thermal EMF Test of Single Thermoelement Materials by Comparison with a Reference Thermoelement of Similar EMF-Temperature Properties
Standard Test Method for Thermal EMF Test of Single Thermoelement Materials by Comparison with a Reference Thermoelement of Similar EMF-Temperature Properties
SIGNIFICANCE AND USE
5.1 This test method is designed to calibrate a thermoelement at one or more test temperatures. The data obtained are sometimes referred to as initial values of emf because the time at the test temperature is limited.
5.2 This test method is employed mainly by providers of spools or coils of wire or strip of thermoelectric material. Generally more than one specimen at a time is tested, and the resultant emf of individual thermoelements are used to match to companion thermoelements for use as thermocouples or in extension wiring.
5.3 The emf of a thermocouple comprised of two different thermoelements as tested with this test method may be determined by algebraically subtracting the emf of the negative thermoelement from the emf of the positive thermoelement at a particular temperature. The emf of a thermocouple may also be determined by the test described in Test Method E220, but Test Method E220 does not take into account the values of the emf of the individual thermoelements relative to Pt-67.
5.4 This test method is normally used for the calibration of thermocouple materials during their production or distribution, not for the accurate determination of the properties of a used thermocouple. If the test samples were subjected to previous use, the test results may not reflect the same emf as the thermocouple did while in service. For example, inhomogeneities may have been induced in the wires because of a chemical or metallurgical reaction while in service. Since emf is developed in the thermal gradient, and it is unlikely that the temperature profile along the wire under testing conditions will be the same as it was while in service, the test results may be misleading.
5.5 The test results are suitable for specification acceptance, manufacturing control, design, or research and development purposes.
SCOPE
1.1 This test method covers a test for determining the thermoelectric emf of a thermoelement versus NIST platinum 67 (Pt-67) by means of measuring the difference between the emf of the test thermoelement and the emf of a reference thermoelement (previously referred to as a secondary standard), which has a known relationship to NIST Pt-67.
1.2 This test is applicable to new thermocouple materials over the temperature ranges normally associated with thermocouples and their extension wires. The table on Suggested Upper Temperature Limits for Protected Thermocouples in Specification E230 lists the ranges associated with the letter-designated types of thermocouples. ASTM MNL-122 lists the temperature range of extension circuit materials.
1.3 This test is not applicable to stability testing or inhomogeneity testing.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.5 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 practices and determine the applicability of regulatory limitations prior to use.
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Designation: E207 − 08(Reapproved 2015)
Standard Test Method for
Thermal EMF Test of Single Thermoelement Materials by
Comparison with a Reference Thermoelement of Similar
EMF-Temperature Properties
This standard is issued under the fixed designation E207; 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.
1. Scope E220Test Method for Calibration of Thermocouples By
Comparison Techniques
1.1 This test method covers a test for determining the
E230Specification and Temperature-Electromotive Force
thermoelectric emf of a thermoelement versus NIST platinum
(EMF) Tables for Standardized Thermocouples
67 (Pt-67) by means of measuring the difference between the
E344Terminology Relating to Thermometry and Hydrom-
emf of the test thermoelement and the emf of a reference
etry
thermoelement (previously referred to as a secondary
E563Practice for Preparation and Use of an Ice-Point Bath
standard), which has a known relationship to NIST Pt-67.
as a Reference Temperature
1.2 This test is applicable to new thermocouple materials
over the temperature ranges normally associated with thermo-
3. Terminology
couples and their extension wires. The table on Suggested
Upper Temperature Limits for Protected Thermocouples in
3.1 Definitions—The terms used in this test method are
Specification E230 lists the ranges associated with the letter-
defined in Terminology E344.
designated types of thermocouples. ASTM MNL-12 lists the
3.2 Definitions of Terms Specific to This Standard:
temperature range of extension circuit materials.
3.2.1 reference facility, n—NIST, or a testing laboratory
1.3 This test is not applicable to stability testing or inhomo-
whose physical standards are traceable to NIST or another
geneity testing.
national standards laboratory.
1.4 The values stated in SI units are to be regarded as the
3.2.2 test temperature, n—the temperature of the measuring
standard. The values given in parentheses are for information
junction.
only.
3.2.2.1 Discussion—Inreportingtheresults,thevalueofthe
1.5 This standard does not purport to address all of the
test temperature may be rounded off, provided the stated test
safety concerns, if any, associated with its use. It is the
temperature is within the bounds indicated in 10.10.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Summary of Test Method
bility of regulatory limitations prior to use.
4.1 The emf of a thermoelement sample is determined by
2. Referenced Documents
comparison to a reference thermoelement that has similar
2.1 ASTM Standards:
Seebeck coefficients.
E77Test Method for Inspection and Verification of Ther-
4.2 This test is conducted on one or more lengths of
mometers
specimens connected to a single length of the reference
thermoelementatasinglepoint.Thejoinedendsareheldatthe
This test method is under the jurisdiction of ASTM Committee E20 on
test temperature, and their opposite ends are held at a constant
Temperature Measurement and is the direct responsibility of Subcommittee E20.04
reference temperature.
on Thermocouples.
Current edition approved May 1, 2015. Published May 2015. Originally
4.3 The emf of the reference thermoelement relative to
approved in 1962 . Last previous edition approved in 2008 as E207–08. DOI:
10.1520/E0207-08R15.
Pt-67 at several test temperatures are provided by a reference
Manual on the Use of Thermocouples in Temperature Measurement, ASTM
facility.
MNL-12, Fourth Edition, ASTM, April 1993. (Revision of STP 407B).
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
4.4 The emf of the test thermoelement relative to Pt-67 is
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
determined by algebraically adding the measured emf to the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. emf of the reference thermoelement at each test temperature.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E207 − 08 (2015)
5. Significance and Use 7. Reference Thermoelement
5.1 This test method is designed to calibrate a thermoele- 7.1 The reference thermoelement has its emf established
ment at one or more test temperatures. The data obtained are relative to NIST Pt-67 over the temperature range of its
sometimes referred to as initial values of emf because the time intended use. A specific lot of thermoelement material is
at the test temperature is limited. usually reserved for use as reference thermoelements.
5.2 This test method is employed mainly by providers of 7.2 The emf of the reference thermoelement versus plati-
spools or coils of wire or strip of thermoelectric material. num (Pt-67) shall conform to Specification E230 within one
Generally more than one specimen at a time is tested, and the half the standard tolerance specified for the related thermo-
resultant emf of individual thermoelements are used to match couple type. For example, the tolerance for KPversus Pt-67 is
to companion thermoelements for use as thermocouples or in 6 1°C or 6 0.375% of temperature from 0 to 1260°C,
extension wiring. whichever is greater.
5.3 The emf of a thermocouple comprised of two different 7.3 The cross section of the base metal thermoelement shall
thermoelements as tested with this test method may be deter- be sufficiently large so that oxidation caused by the tempera-
mined by algebraically subtracting the emf of the negative
tures of testing would not significantly affect its emf over the
thermoelement from the emf of the positive thermoelement at period of the test.
a particular temperature. The emf of a thermocouple may also
7.4 To provide some assurance that the reserved lot is
be determined by the test described in Test Method E220, but
uniforminemffromendtoend,itshallbemanufacturedinone
Test Method E220 does not take into account the values of the
continuous length with no in-process welds. . Cold working of
emf of the individual thermoelements relative to Pt-67.
the material after the final anneal shall be minimized.
5.4 This test method is normally used for the calibration of
7.4.1 Aspecimen from each end of the reserved lot shall be
thermocouple materials during their production or distribution, tested using this test method. The test temperatures shall
not for the accurate determination of the properties of a used
include the extremes of the intended range of use and addi-
thermocouple. If the test samples were subjected to previous tional test points that are no more than 260°C (500°F) apart.
use, the test results may not reflect the same emf as the
7.4.2 The emf difference between the specimens of 7.4.1 at
thermocouple did while in service. For example, inhomogene- eachtesttemperatureshallnotexceedtheequivalentof0.33°C
itiesmayhavebeeninducedinthewiresbecauseofachemical
(0.6°F)forthatthermocoupletypeor0.05%ofthevalueofthe
or metallurgical reaction while in service. Since emf is devel- test temperature in degrees Celsius, whichever is the greater.
oped in the thermal gradient, and it is unlikely that the
7.5 From the lot that meets the stated uniformity
temperatureprofilealongthewireundertestingconditionswill
requirements, at least one unused 1m (3-ft) section shall be
be the same as it was while in service, the test results may be
certified by a reference facility to document its emf relative to
misleading.
Pt-67.Traceability shall be required in the form of a certificate
5.5 Thetestresultsaresuitableforspecificationacceptance,
issued by the reference facility.
manufacturing control, design, or research and development
7.5.1 Emf data shall be provided every 50°C (100°F) or at
purposes.
intervalsthatdonotexceed25%ofthetesttemperaturerange,
whichever is the lesser. If fewer than the aforementioned
6. Test Specimen
numberofpointsaretaken,thenthedataareapplicableonlyat
6.1 Eachsampleshallrepresentonecontinuousspoolorcoil or near the measured temperatures, and interpolation beyond
of thermoelectric material. The sample shall consist of two them should not be attempted.
specimens, one cut from each end of the spool or coil. The 7.5.2 The emf of the reference thermoelement at intermedi-
extreme ends shall not be acceptable if they are distorted or ate values of temperature may be determined by one of the
have been subjected to processing dissimilar to the bulk of the following methods.
spool or coil.
7.5.2.1 For the letter-designated thermocouple types, emf
functions for thermoelements versus Pt-67 are given in Speci-
6.2 Insulation or covering shall be removed with care if it
fication E230. In these cases, the deviation of the reference
interferes with the test. Straining the test specimen shall be
thermoelementemffromthefunctionvalueisfirstcalculatedat
avoided.
thetesttemperaturevalues.Atanintermediatetemperature,the
6.3 The specimens shall be cleaned of any extraneous
deviation of emf is calculated either by linear interpolation or
surface contamination.
by fitting a polynomial to the deviation of emf using the
6.4 Thespecimensandthereferencethermoelementshallbe method of least squares, and evaluating the polynomial at the
long enough to extend continuously from the measuring intermediate temperature. For the least squares method, the
junction to the reference junction.Alength of 600 to 1200 mm number of data points shall equal or exceed twice the number
of parameters fitted. Addition of the deviation of emf to the
(2 to 4 ft) is generally satisfactory. The exact length depends
upon the depth of immersion in the testing medium and the function value at the intermediate temperature gives the emf
value of the reference thermoelement at the intermediate
transverse size (for example, diameter of round wire, width of
strip) of the thermoelement. temperature.
6.4.1 Heating of the measuring junctions shall not affect the 7.5.2.2 For the thermoelements for which there is no emf
temperatureofthereferencejunctionsduringtheperiodoftest. function for that thermoelement versus Pt-67, a function may
E207 − 08 (2015)
be determined by fitting a polynomial to the emf values ment at one of their ends. Welding is the preferred method of
reported by NIST for the reference thermoelement versus joining, particularly for test temperatures above 260°C
Pt-67, using the method of least squares. The number of data (500°F).
points shall equal or exceed twice the number of parameters
9.2 The number of test specimens that may be tested at one
fitted. Evaluation of the polynomial at the intermediate tem-
time is limited mainly by the thermal capacity of the system.
peraturegivestheemfofthereferencethermoelement.Incases
The thermal conduction along the assembly of test thermoele-
where the deviations of the fitted data from the polynomial are
ments must not be so large as to impair isothermal conditions
significant compared to other uncertainties in the test, a
at the measuring or reference junction.
subcomponent of uncertainty shall be added to the uncertainty
budget equal to:
10. Test Temperature Medium
10.1 Normally, both the test and reference thermoelements
u 5 Σ ~E 2 E ! (1)
ŒF G
i fit
N
i
df have the same nominal composition and consequently have
approximately the same values of Seebeck coefficients.
where:
Therefore, the measured emf is expected to be small in
u = uncertainty,
magnitude (compared to the emf relative to Pt-67) and vary
E = the emf at the ith calibration temperature value of the
i
only slightly as a function of temperature. Therefore, it is not
reference thermoelement that has been calibrated rela-
necessary to control the test temperature precisely.
tive to NIST Pt-67,
E = the emf of the fitted polynomial, and 10.2 The immersion media, insulation materials, supports,
fit
N = the number of degrees of freedom in the fit = number
and adjacent materials shall not interact with or electrically
df
of data points – number of fitted parameters.
shunt the thermoelements.
7.5.2.3 Linear interpolation of the reference thermoelement 10.3 For testing in the range of−160 to−75°C (−250
emf,ratherthanthedeviationofemf,mayalsobedone,butuse to−100°F), a liquid nitrogen bath may be used. Refer to the
of this method requires inclusion of an additional uncertainty devices and precautions inTest Method E77,Appendix X1, on
component to account for the interpolation error. This uncer- Discussion of Apparatus for Verification of Liquid-in-Glass
tainty component may be estimated by calculating the error of Thermometers and Fig. X1.3 on Comparator for Temperature
linear interpolation of the emf values obtained from the emf Range from−160 to−75°C (−256 to−103°F).
functions for thermoelements versus Pt-67 in Specification
10.4 For testing in the range of −80 to +5°C (−110
E230 or another source.This error may be as large as all other
to+40°F), use an apparatus as depicted in Test Method E77,
errors combined.
Appendix X1, on Discussion of Apparatus for Verification of
Liquid-in-Glass Thermometers and Fig. X1.4 on Comparator
7.6 Thesegmentofreferencethermoelementthatisusedfor
for Temperature Range from−80 to+5°C (−112 to+41°F),
each test shall be unaffected by a prior test. For example, any
using dry ice and a suitable liquid.
segment of KP, EP, or JP thermoelement, exposed to tempera-
turesexceeding260°C(500°F)shallnotbereused.However,if
10.5 For testing in the range of room temperature to 95°C
it shows no evidence of its test environment and no effects of
(200°F),aheatedbathusingdemineralizedwatermaybeused.
strain,theremaindermaybereused.Fornoblemetalsandtheir
10.6 In the range of 5 to 300°C (40 to 600°F), a stirred bath
alloys,thenumberofreusesdependsupontheamountofstrain
ofanoilwithaflashpointhigherthanthetesttemperaturemay
or contamination of the segment. Noble metal reference
be used. Refer to Test Method E77, Appendix X1, on Discus-
thermoelements should be checked for emf conformity after
sion of Apparatus for Verification of Liquid-in-Glass Ther-
ten uses or less against another noble metal reference segment
mometers and Fig. X1.6(b) on Alternative Designs.
that was not subjected to routine use.
10.7 For testing at or above 100°C (200°F), an electrically-
8. Reference Temperature Unit
heated laboratory-type wire-wound tube furnace is generally
used.The atmosphere inside the tube shall be air, and the ends
8.1 The reference temperature unit shall maintain the tem-
shall not both be sealed airtight. Other atmospheres may be
perature of the reference junc
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E207 − 08 E207 − 08 (Reapproved 2015)
Standard Test Method for
Thermal EMF Test of Single Thermoelement Materials by
Comparison with a Reference Thermoelement of Similar
EMF-Temperature Properties
This standard is issued under the fixed designation E207; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers a test for determining the thermoelectric emf of a thermoelement versus NIST platinum 67 (Pt-67)
by means of measuring the difference between the emf of the test thermoelement and the emf of a reference thermoelement
(previously referred to as a secondary standard), which has a known relationship to NIST Pt-67.
1.2 This test is applicable to new thermocouple materials over the temperature ranges normally associated with thermocouples
and their extension wires. The table on Suggested Upper Temperature Limits for Protected Thermocouples in Specification E230
lists the ranges associated with the letter-designated types of thermocouples. ASTM MNL-12 lists the temperature range of
extension circuit materials.
1.3 This test is not applicable to stability testing or inhomogeneity testing.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.5 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 practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E77 Test Method for Inspection and Verification of Thermometers
E220 Test Method for Calibration of Thermocouples By Comparison Techniques
E230 Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples
E344 Terminology Relating to Thermometry and Hydrometry
E563 Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature
3. Terminology
3.1 Definitions—The terms used in this test method are defined in Terminology E344.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 reference facility, n—NIST, or a testing laboratory whose physical standards are traceable to NIST or another national
standards laboratory.
3.2.2 test temperature, n—the temperature of the measuring junction.
This test method is under the jurisdiction of ASTM Committee E20 on Temperature Measurement and is the direct responsibility of Subcommittee E20.04 on
Thermocouples.
Current edition approved May 1, 2008May 1, 2015. Published June 2008May 2015. Originally approved in 1962 . Last previous edition approved in 20002008 as
E207E207 – 08.–00. DOI: 10.1520/E0207-08.10.1520/E0207-08R15.
Manual on the Use of Thermocouples in Temperature Measurement, ASTM MNL-12, Fourth Edition, ASTM, April 1993. (Revision of STP 407B).
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
3.2.2.1 Discussion—
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E207 − 08 (2015)
In reporting the results, the value of the test temperature may be rounded off, provided the stated test temperature is within the
bounds indicated in 10.10.
4. Summary of Test Method
4.1 The emf of a thermoelement sample is determined by comparison to a reference thermoelement that has similar Seebeck
coefficients.
4.2 This test is conducted on one or more lengths of specimens connected to a single length of the reference thermoelement at
a single point. The joined ends are held at the test temperature, and their opposite ends are held at a constant reference temperature.
4.3 The emf of the reference thermoelement relative to Pt-67 at several test temperatures are provided by a reference facility.
4.4 The emf of the test thermoelement relative to Pt-67 is determined by algebraically adding the measured emf to the emf of
the reference thermoelement at each test temperature.
5. Significance and Use
5.1 This test method is designed to calibrate a thermoelement at one or more test temperatures. The data obtained are sometimes
referred to as initial values of emf because the time at the test temperature is limited.
5.2 This test method is employed mainly by providers of spools or coils of wire or strip of thermoelectric material. Generally
more than one specimen at a time is tested, and the resultant emf of individual thermoelements are used to match to companion
thermoelements for use as thermocouples or in extension wiring.
5.3 The emf of a thermocouple comprised of two different thermoelements as tested with this test method may be determined
by algebraically subtracting the emf of the negative thermoelement from the emf of the positive thermoelement at a particular
temperature. The emf of a thermocouple may also be determined by the test described in Test Method E220, but Test Method E220
does not take into account the values of the emf of the individual thermoelements relative to Pt-67.
5.4 This test method is normally used for the calibration of thermocouple materials during their production or distribution, not
for the accurate determination of the properties of a used thermocouple. If the test samples were subjected to previous use, the test
results may not reflect the same emf as the thermocouple did while in service. For example, inhomogeneities may have been
induced in the wires because of a chemical or metallurgical reaction while in service. Since emf is developed in the thermal
gradient, and it is unlikely that the temperature profile along the wire under testing conditions will be the same as it was while in
service, the test results may be misleading.
5.5 The test results are suitable for specification acceptance, manufacturing control, design, or research and development
purposes.
6. Test Specimen
6.1 Each sample shall represent one continuous spool or coil of thermoelectric material. The sample shall consist of two
specimens, one cut from each end of the spool or coil. The extreme ends shall not be acceptable if they are distorted or have been
subjected to processing dissimilar to the bulk of the spool or coil.
6.2 Insulation or covering shall be removed with care if it interferes with the test. Straining the test specimen shall be avoided.
6.3 The specimens shall be cleaned of any extraneous surface contamination.
6.4 The specimens and the reference thermoelement shall be long enough to extend continuously from the measuring junction
to the reference junction. A length of 600 to 1200 mm (2 to 4 ft) is generally satisfactory. The exact length depends upon the depth
of immersion in the testing medium and the transverse size (for example, diameter of round wire, width of strip) of the
thermoelement.
6.4.1 Heating of the measuring junctions shall not affect the temperature of the reference junctions during the period of test.
7. Reference Thermoelement
7.1 The reference thermoelement has its emf established relative to NIST Pt-67 over the temperature range of its intended use.
A specific lot of thermoelement material is usually reserved for use as reference thermoelements.
7.2 The emf of the reference thermoelement versus platinum (Pt-67) shall conform to Specification E230 within one half the
standard tolerance specified for the related thermocouple type. For example, the tolerance for KP versus Pt-67 is 6 1 °C 1°C or
6 0.375% of temperature from 0 °C to 1260 °C, 1260°C, whichever is greater.
7.3 The cross section of the base metal thermoelement shall be sufficiently large so that oxidation caused by the temperatures
of testing would not significantly affect its emf over the period of the test.
7.4 To provide some assurance that the reserved lot is uniform in emf from end to end, it shall be manufactured in one
continuous length with no in-process welds. . Cold working of the material after the final anneal shall be minimized.
E207 − 08 (2015)
7.4.1 A specimen from each end of the reserved lot shall be tested using this test method. The test temperatures shall include
the extremes of the intended range of use and additional test points that are no more than 260 °C (500 °F) 260°C (500°F) apart.
7.4.2 The emf difference between the specimens of 7.4.1 at each test temperature shall not exceed the equivalent of 0.33 °C (0.6
°F) 0.33°C (0.6°F) for that thermocouple type or 0.05 % of the value of the test temperature in degrees Celsius, whichever is the
greater.
7.5 From the lot that meets the stated uniformity requirements, at least one unused 1-m1 m (3-ft) section shall be certified by
a reference facility to document its emf relative to Pt-67. Traceability shall be required in the form of a certificate issued by the
reference facility.
7.5.1 Emf data shall be provided every 50 °C (100 °F) 50°C (100°F) or at intervals that do not exceed 25 % of the test
temperature range, whichever is the lesser. If fewer than the aforementioned number of points are taken, then the data are
applicable only at or near the measured temperatures, and interpolation beyond them should not be attempted.
7.5.2 The emf of the reference thermoelement at intermediate values of temperature may be determined by one of the following
methods.
7.5.2.1 For the letter-designated thermocouple types, emf functions for thermoelements versus Pt-67 are given in Specification
E230. In these cases, the deviation of the reference thermoelement emf from the function value is first calculated at the test
temperature values. At an intermediate temperature, the deviation of emf is calculated either by linear interpolation or by fitting
a polynomial to the deviation of emf using the method of least squares, and evaluating the polynomial at the intermediate
temperature. For the least squares method, the number of data points shall equal or exceed twice the number of parameters fitted.
Addition of the deviation of emf to the function value at the intermediate temperature gives the emf value of the reference
thermoelement at the intermediate temperature.
7.5.2.2 For the thermoelements for which there is no emf function for that thermoelement versus Pt-67, a function may be
determined by fitting a polynomial to the emf values reported by NIST for the reference thermoelement versus Pt-67, using the
method of least squares. The number of data points shall equal or exceed twice the number of parameters fitted. Evaluation of the
polynomial at the intermediate temperature gives the emf of the reference thermoelement. In cases where the deviations of the fitted
data from the polynomial are significant compared to other uncertainties in the test, a subcomponent of uncertainty shall be added
to the uncertainty budget equal to:
u 5 Σ E 2 E (1)
Π~ !
F G
i fit
N
i
df
where:
u = uncertainty,
E = the emf at the ith calibration temperature value of the reference thermoelement that has been calibrated relative to NIST
i
Pt-67,
E = the emf of the fitted polynomial, and
fit
N = the number of degrees of freedom in the fit = number of data points – number of fitted parameters.
df
7.5.2.3 Linear interpolation of the reference thermoelement emf, rather than the deviation of emf, may also be done, but use
of this method requires inclusion of an additional uncertainty component to account for the interpolation error. This uncertainty
component may be estimated by calculating the error of linear interpolation of the emf values obtained from the emf functions for
thermoelements versus Pt-67 in Specification E230 or another source. This error may be as large as all other errors combined.
7.6 The segment of reference thermoelement that is used for each test shall be unaffected by a prior test. For example, any
segment of KP, EP, or JP thermoelement, exposed to temperatures exceeding 260 °C (500 °F) 260°C (500°F) shall not be reused.
However, if it shows no evidence of its test environment and no effects of strain, the remainder may be reused. For noble metals
and their alloys, the number of reuses depends upon the amount of strain or contamination of the segment. Noble metal reference
thermoelements should be checked for emf conformity after ten uses or less against another noble metal reference segment that
was not subjected to routine use.
8. Reference Temperature Unit
8.1 The reference temperature unit shall maintain the temperature of the reference junctions within 5 °C (9 °F) 5°C (9°F) of
the assumed value of reference temperature. The reference temperature unit shall be designed so that the temperatures of all the
reference junctions will be isothermal.
NOTE 1—The preferred reference junction temperature is 0 °C (32 °F). 0°C (32°F). This may be approximated with an ice bath (see Practice E563),
“automatic ice point” unit or a “zone box” (see MNL-12). Care should be exercised to maintain the reference junction temperatures for both the reference
and test thermocouples at the same temperature.
9. Measuring Junction
9.1 The measuring junction shall consist of an electrical connection of the test specimens to the reference thermoelement at one
of their ends. Welding is the preferred method of joining, particularly for test temperatures above 260 °C (500 °F).260°C (500°F).
E207 − 08 (2015)
9.2 The number of test specimens that may be tested at one time is limited mainly by the thermal capacity of the system. The
thermal conduction along the assembly of test thermoelements must not be so large as to impair isothermal conditions at the
measuring or reference junction.
10. Test Temperature Medium
10.1 Normally, both the test and reference thermoelements have the same nominal composition and consequently have
approximately the same values of Seebeck coefficients. Therefore, the measured emf is expected to be small in magnitude
(compared to the emf relative to Pt-67) and vary only slightly as a function of temperature. Therefore, it is not necessary to control
the test temperature precisely.
10.2 The immersion media, insulation materials
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