ASTM E2846-20

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2846 − 20 An American National Standard
Standard Guide for
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
A thermocouple should be periodically verified (tested for compliance with specifications) to ensure
that it has not incurred physical, metallurgical, or chemical changes that inhibit or prevent temperature
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
thermocouple inhomogeneity. Such inhomogeneity causes the measured temperature to depend on the
intermediate thermal environment between the measuring and reference junctions of the thermo-
couple. If a thermocouple becomes more inhomogeneous with time, the temperature measured by that
thermocouple may appear to drift from its original value, even though the actual temperature it is
measuring is constant. If the intermediate thermal environment during use is different from that during
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, a thermocouple becomes unreliable if it undergoes certain other physical changes. It will not
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; 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 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-
Some of the tests perform a suitable verification by themselves,
ality of the thermocouple; these tests may be performed either
but many tests merely alert the user to serious problems if the
in house or in a calibration laboratory. The third set is made up
thermocouple fails the test. Some of the tests examine inho-
of homogeneity tests designed to be performed in a calibration
mogeneity and others detect wire or measuring-junction break-
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
do not suffice to verify a used thermocouple. They may need to
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-
dent on whether the reader uses thermocouples for temperature
This guide is under the jurisdiction of ASTM Committee E20 on Temperature
Measurement and is the direct responsibility of Subcommittee E20.14 on Thermo-
measurement or performs thermocouple calibrations in a labo-
couples - Testing.
ratory. For users of thermocouples, it is recommended that
Current edition approved Jan. 1, 2020. Published February 2020. Originally
appropriate tests from the first and second sets be performed
approved in 2011. Last previous edition approved in 2014 as E2846 – 14. DOI:
10.1520/E2846-20. 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
E2846 − 20
thermocouples. The appropriateness of a test is dependent upon E2181/E2181M Specification for Compacted Mineral-
the user’s temperature measurement uncertainty requirements. Insulated, Metal-Sheathed, Noble Metal Thermocouples
Some tests may have lower uncertainties in their verification and Thermocouple Cable
measurements than others. If these tests do not clearly deter-
mine the suitability of the thermocouples, they should be sent
3. Terminology
to a calibration laboratory for performing appropriate tests
3.1 Definitions—The definitions given in Terminology E344
from the third set, which give the most complete information
apply to terms used in this guide.
on the thermocouple homogeneity. For those who perform
3.2 Definitions of Terms Specific to This Standard:
thermocouple calibrations in a laboratory, it is recommended
3.2.1 expanded measurement uncertainty, n—product of a
that appropriate tests from the second and third sets be
combined standard measurement uncertainty and a factor
performed prior to calibration. The appropriateness of a test is
larger than the number one.
dependent on the calibration laboratory’s capability and con-
3.2.1.1 Discussion—The term “factor” in this definition
venience for performing the test, as well as the characteristics
refers to a coverage factor, k. For k = 2 (the most common
of the unit under test (UUT).
coverage factor), a measurement instrument measures correctly
1.4 This guide may be used for base metal and noble metal
to within its expanded measurement uncertainty with a 95.4 %
thermocouples. Some of the methods covered may apply to
probability.
refractory metal thermocouples but caution is advised as
3.2.2 gradient zone, n—the section of a thermocouple that is
suitable reference devices at high temperatures may not be
exposed during a measurement to temperatures in the range
readily available.
from t + 0.1(t – t ) to t + 0.9(t – t ), where t
amb m amb amb m amb amb
1.5 This standard does not purport to address all of the
is ambient temperature and t is the temperature of the
m
safety concerns, if any, associated with its use. It is the
measuring junction.
responsibility of the user of this standard to establish appro-
3.2.2.1 Discussion—This term is used as part of the descrip-
priate safety, health, and environmental practices and deter-
tion of the thermal profile along the length of the thermo-
mine the applicability of regulatory limitations prior to use.
couple. The gradient zone definition is intended to describe, in
1.6 This international standard was developed in accor-
an approximate way, the section of thermocouple in which
dance with internationally recognized principles on standard-
most of the emf was created.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
3.2.3 half-maximum heated length, n—the distance between
mendations issued by the World Trade Organization Technical the measuring junction and the position along the length of the
Barriers to Trade (TBT) Committee.
thermocouple wires or sheath where the temperature equals the
average of the calibration-point and ambient temperatures.
2. Referenced Documents
3.2.3.1 Discussion—This term is used as part of the descrip-
2.1 ASTM Standards: tion of the thermal profile along the length of the thermo-
couple.
E220 Test Method for Calibration of Thermocouples By
Comparison Techniques
3.2.4 homogeneous, adj—having uniform thermoelectric
E344 Terminology Relating to Thermometry and Hydrom-
properties along the length of the thermocouple or thermoele-
etry
ment.
E563 Practice for Preparation and Use of an Ice-Point Bath
3.2.5 homogeneous Seebeck coeffıcient, n—the temperature-
as a Reference Temperature
dependent Seebeck coefficient of a thermocouple or thermo-
E585/E585M Specification for Compacted Mineral-
element when it is in a homogeneous state.
Insulated, Metal-Sheathed, Base Metal Thermocouple
3.2.5.1 Discussion—The homogeneous Seebeck coefficient
Cable
is usually determined from measurements of the Seebeck
E608/E608M Specification for Mineral-Insulated, Metal-
coefficient of the thermocouple or thermoelement when it is
Sheathed Base Metal Thermocouples
new, because then it is usually homogeneous. If segments of
E780 Test Method for Measuring the Insulation Resistance
the new thermocouple or thermoelement are inhomogeneous,
of Mineral-Insulated, Metal-Sheathed Thermocouples and
the homogenous Seebeck coefficient is determined from mea-
Mineral-Insulated, Metal-Sheathed Cable at Room Tem-
surements made on the segments demonstrated to be homoge-
perature
neous.
E839 Test Methods for Sheathed Thermocouples and
Sheathed Thermocouple Cable
3.2.6 inhomogeneity, n—the deviation of the Seebeck coef-
E1350 Guide for Testing Sheathed Thermocouples, Thermo- ficient of a segment of a thermocouple or thermoelement at a
couple Assemblies, and Connecting Wires Prior to, and given temperature from its homogeneous Seebeck coefficient at
After Installation or Service that temperature.
3.2.6.1 Discussion—In practice, only variations in the See-
beck coefficient along the length of a thermocouple that is
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
exposed to temperature gradients affect the voltage output of a
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
thermocouple. Inhomogeneity of a thermocouple is often
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. reported as a fractional variation in the Seebeck coefficient.
E2846 − 20
3.2.7 minimum immersion length, n—the depth that a ther- measuring junction of the thermocouple. For open access
mometer should be immersed, in a uniform temperature points, the reference thermometer may be a referee
environment, such that further immersion does not produce a thermocouple, a non-referee thermocouple that is new or
change in the indicated temperature greater than the specified determined to be homogeneous, or another temperature sensor
tolerance. unaffected by inhomogeneity such as a resistance temperature
detector (RTD) or thermistor. If the reference thermometer is
3.2.8 referee thermocouple, n—a thermocouple made from
not a referee thermocouple, its minimum immersion length
the same lot of wire or MIMS cable as the UUT group, using
shall be less than the immersion depth of the UUT. For access
identical construction design and methods and identical anneal-
points that are thermowells or protection tubes, the reference
ing methods but not having been placed into permanent
thermometer shall be a referee thermocouple.
service.
4.1.2 Verification with Reference Thermometer in Adjacent
3.2.8.1 Discussion—Because of the high value of referee
Access Point—A thermocouple is verified in-situ at an appro-
thermocouples for performing verification tests by the user, it
priate constant temperature by comparison to a known refer-
is strongly recommended that after users receive new lots of
ence thermometer located in an adjacent access point. In this
thermocouple wire, they construct referee thermocouples along
case the comparison can be made without removing the UUT.
with the thermocouples intended for regular use.
The reference thermometer may be a referee thermocouple, a
3.2.9 sensing point, n—the location on a thermometer where
non-referee thermocouple that is new or determined to be
the temperature is (or is assumed to be) measured.
homogeneous, or another temperature sensor unaffected by
3.2.9.1 Discussion—A thermocouple’s sensing point is its
inhomogeneity such as an RTD or thermistor. If the reference
measuring junction. A resistance temperature detector (RTD)
thermometer is not a referee thermocouple, its minimum
contains a sensing element that may be large enough to
immersion length shall be less than the immersion depth of the
experience spatial temperature variations; in this case the
UUT.
sensing point is the central point in the element where the
4.2 Thermocouple Functionality Tests:
temperature is assumed to be that measured by the RTD.
4.2.1 Measurement of Loop Resistance—The loop resis-
3.2.10 standard measurement uncertainty, n—measurement
tance of the thermocouple circuit is measured to verify that the
uncertainty expressed as a standard deviation.
thermoelements and welded measuring junction are continu-
3.2.10.1 Discussion—A measurement instrument measures
ous. This test may also be used to identify conditions where the
correctly to within its standard uncertainty with a 68.2 %
thermoelements are in contact with each other at a point other
probability.
than at the measuring junction. It may be difficult to identify
3.2.11 tolerance, n—in a measurement instrument, the per-
multiple contact points when they occur near the measuring
mitted variation of a measured value from the correct value.
junction.
3.2.11.1 Discussion—If a measurement instrument is stated
4.2.2 Measurement of Insulation Resistance of Thermo-
to measure correctly to within a tolerance, the instrument is
couples with Style U Measuring Junctions—The resistance of
classified as “in tolerance” and it is assumed that measurements
the insulation between the UUT sheath and the thermoelements
made with it will measure correctly to within this tolerance. An
is measured to determine if the electrical isolation between
instrument that is not classified as “in tolerance” is classified as
them has deteriorated.
“out of tolerance.”
4.2.3 Measurement of Sheath Diameter (Metal-sheathed
3.2.12 UUT, n—abbreviation for “unit under test.”
Thermocouples)—Measurements of the UUT sheath diameter
are made and compared to measurements made prior to
3.2.13 validation, n—the process of testing a thermometer
installation to monitor metal erosion in the sensor sheath that
for acceptable accuracy in its intended use.
may cause the UUT to perform unreliably (see also 7.5).
3.2.14 verification, n—the process of testing a thermometer
4.2.4 Visual Inspection of Metal-sheathed
for compliance with specifications.
Thermocouples—An inspection is made to look for holes,
3.2.14.1 Discussion—Here, “specifications” normally refers
severe pits, and creases in the sheath and for separation of the
to specification tolerances for uncalibrated thermometers and
end closure from the sheath. All of these items may cause the
to calibration uncertainties for calibrated thermometers. The
UUT to perform unreliably (see also 7.6).
same tests may be used for a less stringent verification called
4.3 Laboratory Verification of Thermocouples:
validation, defined as “the process of testing a thermometer for
acceptable accuracy in its intended use.” 4.3.1 Ice Point Test—The measuring junction and reference
junction of the UUT are both immersed in ice baths. No
4. Summary of Verification Tests
thermocouple extension wires are used. If the measured emf is
4.1 In-situ Measurement Verification: beyond a certain tolerance, the UUT is inhomogeneous. The
4.1.1 Verification with Reference Thermometer in Same immersion depth of the measuring junction may be varied to
examine for inhomogeneity in different segments of the ther-
Access Point—A UUT is verified in-situ at an appropriate
constant temperature by comparison to a known reference mocouple.
thermometer in the same access point. For the comparison, the 4.3.2 Single-point Verification—Inhomogeneity is checked
thermocouple is temporarily replaced by the reference ther- by comparing the temperature measured by the UUT with that
mometer in the access point, making sure that the measuring of a reference thermometer at a single temperature. The
point of the sensor is at the same immersion depth as the difference is compared to that from the original calibration at
E2846 − 20
that temperature. This test is not truly a measurement of ments with the tested thermocouple. Laboratory measurements
inhomogeneity, but rather a test for consistent temperature generally do not suffice to determine the emf-versus-
measurement of the UUT under one particular set of condi-
temperature response of a thermocouple found to be inhomo-
tions. While an inconsistent measurement will demonstrate that
geneous.
the UUT is inhomogeneous, a consistent measurement does not
necessarily indicate that the UUT is free from inhomogeneities.
6. In-situ Measurement Verification
4.3.3 Multiple Fixed Immersions in Furnace or Bath—
6.1 These verification tests are used to verify a UUT in its
Temperatures measured using the UUT are compared with
normal measurement environment by comparison with a ref-
those measured using a homogeneous reference thermocouple
erence thermometer. The tests in 6.3 and 6.4 are designed to
or other reference thermometer while the two are in the same
detect drift in the temperature measured by the UUT at a
thermal environment at a given immersion depth in the liquid
constant temperature. Both short-term and long-term drifts of
bath. The consistency of the temperature measured by the UUT
this sort are the direct result of changes in the Seebeck
relative to that measured by the reference thermometer at
coefficient, or inhomogeneity, so measuring this drift is an
different immersion depths provides information on the mea-
surement errors of the UUT due to inhomogeneity. indirect measure of inhomogeneity. These tests subject the
4.3.4 Single-gradient Scanning—The measuring junction of
thermocouple to minimal disturbance and do not involve
the UUT is immersed into a temperature-controlled liquid bath sending it away to a calibration laboratory.
at a constant rate or in a series of steps. The UUT passes
6.2 Any in-situ test should only be performed by trained
through a large temperature gradient near the top surface of the
personnel having the necessary qualifications to work on
liquid. The UUT emf is recorded as a function of immersion
instrumentation and electrical equipment in the usage environ-
depth into the liquid bath. The data provide information on the
ment. Precautions and measurements to ensure that thermo-
location and magnitude of the inhomogeneity.
couple sensors are not in contact with electrical circuits other
4.3.5 Double-gradient Scanning—Measurements of See-
than those intended for use with the thermocouple should be
beck coefficient variations are made along the length of the
made.
UUT using a short movable high-temperature zone. The two
gradient zones to which the UUT is exposed are at the edges of
6.3 Uncertainty and Tolerance—The verification tests de-
the high-temperature zone. The measured emf is used to
scribed below involve the concepts of measurement uncer-
determine the Seebeck coefficient variation along the segment
tainty and measurement tolerance. The terms “standard mea-
of the UUT between the two gradient zones. By scanning the
surement uncertainty,” “expanded measurement uncertainty,”
UUT along the high temperature zone, this Seebeck coefficient
and “tolerance” are defined in Section 3. Descriptions of
variation is determined as a function of position on the UUT;
uncertainties and their determination are based on the ISO
the result is used to estimate the total inhomogeneity as a
Guide to Uncertainty in Measurement (1). Standard uncertain-
function of position on the UUT.
ties are represented by the variable u, expanded uncertainties
are represented by the variable U, and tolerances are repre-
5. Significance and Use
sented by the variable τ. These variables generally are written
5.1 These verification tests may be performed by users or
with a descriptive subscript. A UUT that passes a tolerance test
calibrators of thermocouples. The methods are useful for both
that meets the requirements of ANSI/NCSL Z540.3-2006
new and used thermocouples. They provide a means to assess
standards (2) will measure correctly to within the stated
the accuracy with which a thermocouple is capable of measur-
tolerance with a probability of 98 % (Section 5.3b). A tolerance
ing temperature.
may be related to an expanded uncertainty with a coverage
5.2 Results from these tests may be used to determine
factor of k = 2.33, as both correspond to a 98 % confidence
whether to use or discard a thermocouple. If the thermocouple
interval. The relationship between a UUT’s tolerance τ and its
is subsequently used, the test results may be included in the
expanded uncertainty with k = 2 is then U (k = 2) = 0.858 τ.
UUT
measurement uncertainty budget. In many circumstances, the
6.4 UUT Criterion—The criterion for verification is that the
results of in-situ verifications may be used to recalibrate a used
thermocouple. Laboratory measurements, on the other hand, UUT measures correctly to within the specified value of either
U (k = 2) or τ. If the UUT meets this criterion, it is deemed
may be used only to verify the original thermocouple calibra-
UUT
tion or to determine the uncertainty of temperature measure- acceptable. If it does not meet this criterion, it should be
TABLE 1 Summary of In-situ Measurement Verification Tests
Test Provides Comments
Verification with the Reference Thermometer Verification of thermocouple temperature measurement Compares thermocouple with a reference thermometer.
in Same Access Point The thermocouple’s access port is used by the
reference thermometer. May not be used with
active control thermocouples.
Verification with the Reference Thermometer Verification of thermocouple temperature measurement Compares thermocouple with a reference thermometer.
in an Adjacent Access Point A nearby access port is used by the reference
thermometer. May be used with active control
thermocouples.
E2846 − 20
rejected. The first step in performing an in-situ verification is to uncertainty in the comparison, quantifying the probability that
specify these values. The three most common values are the result is wrong. This probability increases as the total
described below. measurement uncertainty increases. An advantage of tolerance
6.4.1 Specification Tolerance Criterion—The UUT mea- verification is that the test criterion may be adjusted to ensure
sures correctly to within its stated specification tolerance, τ that a minimal number of UUTs that should be rejected are
spec
(that is, τ = τ ). The expanded measurement uncertainty of accepted; however, such an adjustment greatly raises the
spec
the UUT corresponding to this tolerance is then number of acceptable UUTs that are rejected.
U (k = 2) = 0.858 τ .
UUT spec
6.6 Reference Measurement—A reference measurement
6.4.2 Calibration Uncertainty Criterion—The UUT mea-
used for in-situ verification requires the use of a reference
sures correctly to within its expanded calibration uncertainty,
thermometer. The type of reference thermometer to be used
U (that is, U (k = 2) = U ). The tolerance re-
UUT_cal UUT UUT_cal
depends on the type of access point being used.
lated to this uncertainty is τ = 1.165 U .
UUT_cal
6.6.1 Open Access Point—The reference thermometer may
6.4.3 Measurement Needs Criterion—The UUT measures
be a referee thermocouple, a non-referee thermocouple that is
correctly to within an uncertainty, U , based on the
UUT_accept
new or determined to be homogeneous, or another temperature
measurement needs of the user (that is, U (k = 2) = U
UUT UUT_
sensor unaffected by inhomogeneity, such as an RTD or
accept). The tolerance related to this uncertainty is
thermistor. The thermal cross section of the reference ther-
τ = 1.165 U .
UUT_accept
mometer shall be similar to that of the UUT. If the reference
6.5 Methods of In-situ Verification—The second step in
thermometer is not a referee thermocouple, its minimum
performing an in-situ verification is deciding which of the two immersion length shall be less than the immersion depth of the
methods of verification is needed. These methods are described
UUT.
below. 6.6.2 Thermowell or Protection Tube Access Point—The
6.5.1 Measurement Agreement—This method compares the
reference thermometer shall be a referee thermocouple. It shall
UUT measurement with a reference measurement, and deter- be placed in the thermowell or protection tube in the same
mines if the two measurements agree to within the combined
manner as the UUT.
uncertainty of the measurements. If the two measurements
6.7 Verification Test with Reference Thermometer in Same
agree, the UUT is deemed acceptable; otherwise, it should be
Access Point—In this test, a UUT is verified in-situ at an
rejected. As the uncertainty of the measurements increases, the
appropriate temperature by comparison to a known reference
probability that a UUT that should be rejected is actually
thermometer. The UUT and reference thermometer alternately
accepted increases. However, the probability that an acceptable
use the same access point, which is that normally used by the
UUT is rejected is always constant (4.6 % for k = 2).
UUT, as shown in Fig. 1.
6.5.2 Tolerance Verification—This method determines
NOTE 1—This method cannot be used to evaluate a control sensor as
whether the UUT measures temperature to within the stated
removing it would cause the system to go out of control.
tolerance, τ, based on a comparison with a reference measure-
ment. The verification test provides a result of either “pass” or 6.7.1 Measurement Protocol—The temperature of the envi-
“fail.” If the UUT passes the test, the UUT is deemed ronment shall be constant with small fluctuations about an
acceptable; otherwise, it should be rejected. The test also average value. For the comparison, the UUT performs a first
provides a calculated value, based on the total measurement set of measurements of the temperature at its measuring
In this figure, the reference thermometer is an RTD. In (a) temperature measurements are made while the UUT is placed in the access point with immersion depth D.
In (b) the UUT is replaced by the RTD with the same immersion depth and temperature measurements are repeated. The sensing point of the RTD is located at the center
of the sensing element. As a result, the end of the RTD probe is immersed further than that of the thermocouple.
FIG. 1 Verification of UUT by Reference Thermometer in Single Access Point
E2846 − 20
junction over a period of time long enough to average out the whether the earlier and present UUT measurements agree to
temperature fluctuations. A minimum of 20 equally spaced within the expanded total measurement uncertainty, consider-
measurements are made over this period of time, and these ing the verification criterion for the UUT.
measurements are used to calculate an average T (a) and
6.7.2.2 Tolerance Verification Method—The calculation for
UUT
standard deviation σ for the temperature, where the “a” in
UUT the first test determines whether the UUT and reference
parenthesis labels the measurement set. Here, the standard
thermometer measurements agree to within the UUT specified
deviation characterizes the fluctuations of the temperature
tolerance. The calculation for the second test determines
measurements over the measurement period. Afterwards, the
whether the earlier and present UUT measurements agree to
UUT is temporarily replaced by the reference thermometer in
within the UUT specified tolerance. Both calculations provide
the access point. When inserting the reference thermometer, the
a result of either “accept” or “reject” for the UUT. The
sensing point of the thermometer should be at the same
measurement uncertainty is used to quantify the chance that
immersion depth as the measuring junction of the UUT; this
this result is wrong.
may sometimes require that the end of the reference thermom-
6.7.2.3 Calculations—The equation needed for determining
eter be inserted to a greater immersion depth than the UUT, as
the expanded total measurement uncertainty from the uncer-
shown in Fig. 1. The reference thermometer makes a similar set
tainty elements is presented in X1.1. The equation used to
of temperature measurements, yielding an average T and
ref
determine measurement agreement is presented in X2.1, and
standard deviation σref for the temperature. Finally, the UUT is
includes example calculations. The equations used to deter-
placed back in the access point, ensuring that the measuring
mine tolerance verification are presented in X3.2.1 and X3.3.2.
junction is at the same immersion depth as before, and a second
As these calculations are not trivial, it is recommended that
set of temperature measurements are made to calculate an
qualified software engineers design software tools to facilitate
average T (b). The temperature measured by the UUT is
UUT
these calculations for those who must regularly perform
then represented by:
verification tests.
T 5 T a 1T b /2 (1)
@ ~ ! ~ !#
6.7.3 Description of Uncertainties—In the table, σ and
UUT UUT UUT
UUT
σ are the standard deviations of the measurements made with
ref
6.7.2 Data Analysis—The data described in Table 2 are used
the UUT and reference thermometer, respectively, and repre-
for determining whether the UUT meets the verification
sent the stability of the measurements. Also, u and
UUT_inst
criterion. It includes the temperature measurements of the UUT
u are the standard instrument measurement uncertainties,
ref_inst
and reference thermometers as well as the standard uncertainty
and u and u are the standard uncertainties of the
UUT_RJC ref_RJC
values described in the table and in 6.7.3. The verification data
reference junction compensation (if relevant), and u is the
ref_cal
may be used for one of the following tests: (1) comparison of
standard reference-thermometer calibration uncertainty (if rel-
measurements by the UUT and the reference thermometer, and
evant). The instrument measurement uncertainties and refer-
(2) comparison of earlier and present measurements by the
ence junction compensator uncertainties are described in the
UUT and the reference thermometer. The first test provides the
respective manufacturer specifications and may depend on the
best result if the reference thermometer is a referee thermo-
environment in which the measurements are made. The refer-
couple or is calibrated; otherwise, the second test may provide
ence thermometer calibration uncertainty is obtained from its
the best results (assuming earlier measurement results are
calibration report. If the comparison is made using a referee
available).
thermocouple and the user wishes to verify that the UUT
6.7.2.1 Measurement Agreement Method—The calculation
measurements are identical to those of the referee
for the first test determines whether the UUT and reference
thermocouple, then u = 0. If an ice bath is used for the
ref_cal
thermometer measurements agree to within the expanded total
reference junction by the UUT or the reference thermometer, or
measurement uncertainty, considering the verification criterion
both, instead of an electronic reference junction compensator,
for the UUT. The calculation for the second test determines
then u = 0 or u = 0, or both.
UUT_RJC ref_RJC
The uncertainty u is the uncertainty due to drift in the
drift
TABLE 2 Data Used for Verification Calculation for Test With
temperature of the environment between the measurements
Reference Thermometer in Same Access Point
T (a) and T (b). Based on the ISO Guide to Uncertainty
UUT UUT
Temperature
Description in Measurement (1), u may be estimated as:
drift
Data
T (a) First temperature measurement made by the UUT
UUT
T Temperature measurement made by the reference thermometer
u 5 T a 2 T b (2)
ref ~ ! ~ !
drift ? uut uut ?
T (b) Second temperature measurement made by the UUT 2=3
UUT
Uncertainties
σ Repeatability of measurements made by the UUT
UUT
The uncertainty u , relevant only when an RTD is used as
imm
σ Repeatability of measurements made by the reference
ref
the reference thermometer, is the uncertainty due to tempera-
thermometer
ture non-uniformities along the length of the RTD’s sensing
u Measuring instrument for the UUT
UUT_inst
u Measuring instrument for the reference thermometer
ref_inst
element; these non-uniformities make the measured tempera-
u Reference-junction compensator of the UUT (if relevant)
UUT_RJC
ture dependent on the RTD immersion depth. The value of u
imm
u Reference-junction compensator of the reference thermometer
ref_RJC
(if relevant) is estimated by first placing the RTD’s sensing point at the
u Calibration of the reference thermometer (if relevant)
ref_cal
same immersion depth D as the measuring junction of the
u Drift between T (a) and T (b)
drift UUT UUT
UUT. The RTD is then immersed further a distance ∆/2, where
u Immersion depth of the reference thermometer (RTD only)
imm
∆ is the manufacturer-estimated length of the RTD sensing
E2846 − 20
element, to measure T(D + ∆/2). Afterwards the RTD is moved switch the access points (for example, the UUT is a control
back a distance ∆ to measure T(D − ∆/2). These immersion thermocouple), the values for T , T , σ , and σ are
UUT ref UUT ref
depths are illustrated in Fig. 2. The value of u is then (1):
represented by their values in set “a.”
imm
6.8.2 Data Analysis—The data described in Table 3 are used
1 ∆ ∆
u 5 T D 1 2 T D 2 (3)
U S D S DU
imm ref ref
2 2 for determining if the UUT meets the verification criterion. It
2=3
includes the temperature measurements of the UUT and
NOTE 2—For thermocouple reference thermometers, u is omitted.
imm
reference thermometer as well as the standard uncertainty
6.8 Verification with Reference Thermometer in Adjacent
values described in the table and in 6.8.3. The verification data
Access Point:
may be used for one of the following tests: (1) comparison of
6.8.1 Measurement Protocol—The UUT is verified in-situ at
measurements by the UUT and the reference thermometer, and
an appropriate temperature by comparison to a known refer-
(2) comparison of earlier and present measurements by the
ence thermometer that is inserted in an adjacent access point,
UUT and the reference thermometer. The first test provides the
as shown in Fig. 3. The reference thermometer may be a
best result if the reference thermometer is a referee thermo-
referee thermocouple, a thermocouple that is new or deter-
couple or is calibrated; otherwise, the second test may provide
mined to be homogeneous, or another temperature sensor
the best results (assuming earlier measurement results are
unaffected by inhomogeneity, such as an RTD or thermistor.
available).
The thermal cross section of the reference thermometer shall be
similar to that of the UUT. If the reference thermometer is not 6.8.2.1 Measurement Agreement Method—The calculation
a referee thermocouple, its minimum immersion length shall be
for the first test determines whether the UUT and reference
less than the immersion depth of the UUT. The reference
thermometer measurements agree to within the expanded total
thermometer is inserted so that the sensing point of the
measurement uncertainty, considering the verification criterion
thermometer is located at the same immersion depth as the
for the UUT. The calculation for the second test determines
measuring junction of the thermocouple; this may sometimes
whether the earlier and present UUT measurements agree to
require that the end of the reference thermometer be inserted to
within the expanded total measurement uncertainty, consider-
a greater immersion depth than the thermocouple, as shown in
ing the verification criterion for the UUT.
Fig. 1. The temperature is maintained with minimal drifts and
6.8.2.2 Tolerance Verification Method—The calculation for
fluctuations.
the first test determines whether the UUT and reference
For the comparison, a first series of simultaneous tempera-
thermometer measurements agree to within the UUT specified
ture measurements are performed by the UUT and the refer-
tolerance. The calculation for the second test determines
ence thermometer over a period of time long enough to average
whether the earlier and present UUT measurements agree to
out the temperature fluctuations. A minimum of 20 equally
within the UUT specified tolerance. Both calculations provide
spaced measurements are made over this period of time, and
a result of either “accept” or “reject” for the UUT. The
these measurements are used to calculate averages T (a) and
UUT
measurement uncertainty is used to quantify the chance that
T (a) for the UUT and reference thermometer, respectively,
ref
this result is wrong.
and standard deviations σ (a) and σ (a) for the UUT and
UUT ref
6.8.2.3 Calculations—The equation needed for determining
reference thermometer, respectively. Here, the “a” in parenthe-
the expanded total measurement uncertainty from the uncer-
sis refers to the first series of measurements. If possible, the
tainty elements is presented in X1.2. The equation used to
access points for the UUT and reference thermometer are
determine measurement agreement is presented in X2.2, which
switched, and the set of measurements described above is
repeated to obtain T (b), T (b), σ (b), and σ (b). The includes example calculations. The equations used to perform
UUT ref UUT ref
tolerance verification are presented in X3.2.2 and X3.3. As
final values of T , T , σ , and σ are obtained by
UUT ref UUT ref
averaging the two sets “a” and “b.” If it is not possible to these calculations are not trivial, it is recommended that
Here, ∆ is the length of the RTD sensing element.
FIG. 2 Placement of Reference RTD at Increased and Decreased Immersion Depths for Determination of Immersion Uncertainty Compo-
nent in Verification Test
E2846 − 20
Here, the reference thermometer is a thermocouple. Temperature measurements are simultaneously made while the UUT and reference thermometer are placed in the
access points with immersion depth D. Because of the spatial separation between the sensing points, a temperature difference ∆T between them may exist and must be
estimated.
FIG. 3 Verification of UUT by Reference Thermometer Using Two Adjacent Access Points
TABLE 3 Data Used for Verification Calculation for Test With
switched, efforts shall be made to estimate ∆T, for example by
Reference Thermometer in Adjacent Access Point
placing the reference thermometer in a third nearby access
Temperature
point and determining the difference between the temperatures
Description
Data
measured in it and the second access point.
T Temperature Measurement made by the UUT
UUT
T Temperature Measurement made by the reference thermometer
ref
Uncertainties 7. Thermocouple Functionality Tests
σ Repeatability of the measurements made by the UUT
UUT
7.1 The following tests examine the functionality of a
σ Repeatability of the measurements made by the reference
ref
thermometer
thermocouple using electrical and dimensional measurements,
u Measuring instrument for the UUT
UUT_inst
as well as visual inspections. They can be performed by the
u Measuring instrument for the reference thermometer
ref_inst
user as well as in a calibration laboratory. While these tests are
u Reference-junction compensator of the UUT (if relevant)
UUT_RJC
u Reference-junction compensator of the reference thermometer
ref_RJC fast and simple, they do not by themselves verify a UUT; they
(if relevant)
are primarily useful for quickly detecting specific problems
u Calibration of the reference thermometer (if relevant)
ref_cal
that would render the UUT unsuitable for use. The tests, which
u Temperature difference between the sensing points of the UUT
∆T
and the reference thermometer
are based on those described in Test Methods E839 and Guide
u Immersion depth of the reference thermometer (RTD only)
imm
E1350, are listed in Table 4.
7.2 Electrical tests on a thermocouple performed in an
industrial environment should only be conducted by trained
qualified software engineers design software tools to facilitate
personnel having the necessary qualifications to work on
these calculations for those who must regularly perform
instrumentation and electrical equipment in such environ-
verification tests.
ments. Before performing any electrical tests on a
6.8.3 Description of Uncertainties—Most of the uncertain-
thermocouple, it should be disconnected from its temperature
ties shown in Table 3 are described in 6.7.3. The one
measurement/control electrical circuit. Precautions should be
uncertainty that is not described there, u , is the uncertainty
∆T
taken and measurements should be made to ensure that the
due to the temperature difference ∆T between the measuring
thermocouple is not in contact with live circuits other than
junction of the UUT and the sensing point of the reference
those used in the test.
thermometer; this difference is due to temperature non-
uniformities in the environment. If the access points are 7.3 Measurement of Thermocouple Loop Resistance—For
switched as described in 6.8.1, u = 0 because it is cancelled proper performance of the thermocouple, its wires should not
∆T
out by averaging sets “a” and “b”. If the access points are not be broken, its separate thermoelements should not be in
TABLE 4 Summary of Thermocouple Functionality Tests
Test Provides Comments
Loop Resistance Measurement Detection of fatal damage to thermocouple Fast, simple test. Requires multimeter.
Insulation Resistance Measurement Information to help detect damage or deterioration Fast, simple test. Requires megohmmeter.
Sheath Diameter Measurement Information to help detect deterioration Fast, simple test. Requires micrometer.
Sheath Inspections Information to help detect damage or deterioration Fast, simple test. Microscope needed. Helium mass
spectrometer needed for leak detection.
E2846 − 20
electrical contact except at the measuring junction, and the quantify inhomogeneity. The thermocouple should be exam-
weld at its measuring junction shall not be broken. These ined for the following signs of damage:
problems may be tested for by measuring ex situ the loop 7.6.1 Holes—Holes in the thermocouple sheath usually
resistance of the thermocouple while it is disconnected from result in degraded performance, as the sheath no longer
temperature-measurement instruments. The methods for this protects the thermocouple wire from oxidation and corrosion.
measurement are described in Test Methods E839. The results In addition, moisture can penetrate the sheath, leading to
of the loop resistance tests are then compared with those from lowered insulation resistance. It is recommended that thermo-
similar tests performed before the UUT was used or on an couples with sheaths containing holes be discarded.
unused thermocouple from the same manufacturing lot. If the 7.6.2 Severe Pits—While small pits are often harmless to the
loop resistance has changed significantly (for example, 20 %) thermocouple, severe pits may be the result of serious corro-
since the earlier measurements, the UUT should not be used sion and may contain small holes unnoticeable to the naked
until other tests, particularly those of Section 6, have verified it. eye. Such pits should be examined further under a microscope.
If the pits are sufficiently deep, they may degrade the insulation
NOTE 3—Before performing loop resistance measurements, the thermo-
resistance between the sheath and the thermocouple wires.
couple should be disconnected from its temperature measurement/control
electrical circuit. Such damage may be tested for by measuring the insulation
resistance between the thermocouple wires and the sheath, as
7.4 Measurement of Insulation Resistance of Style U
described in 7.4.
Mineral-Insulated Metal-sheathed (MIMS) Thermocoup
...