ASTM E220-19

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: E220 − 19 An American National Standard
Standard Test Method for
Calibration of Thermocouples By Comparison Techniques
This standard is issued under the fixed designation E220; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope E77Test Method for Inspection and Verification of Ther-
mometers
1.1 Thistestmethoddescribestheprinciples,apparatus,and
E230Specification for Temperature-Electromotive Force
procedure for calibrating thermocouples by comparison with a
(emf) Tables for Standardized Thermocouples
referencethermometer.Calibrationsarecoveredovertempera-
E344Terminology Relating to Thermometry and Hydrom-
ture ranges appropriate to the individual types of thermo-
etry
coupleswithinanoverallrangefromapproximately−195°Cto
E452TestMethodforCalibrationofRefractoryMetalTher-
1700 °C (−320 °F to 3100 °F).
mocouples Using a Radiation Thermometer
1.2 In general, this test method is applicable to unused
E563Practice for Preparation and Use of an Ice-Point Bath
thermocouples. This test method does not apply to used
as a Reference Temperature
thermocouplesduetotheirpotentialmaterialinhomogeneity—
E644Test Methods for Testing Industrial Resistance Ther-
the effects of which cannot be identified or quantified by
mometers
standard calibration techniques. Thermocouples with large-
E1129/E1129MSpecification for Thermocouple Connectors
diameter thermoelements and sheathed thermocouples may
E1594Guide for Expression of Temperature
require special care to control thermal conduction losses.
E1684Specification for Miniature Thermocouple Connec-
tors
1.3 In this test method, all values of temperature are based
on the International Temperature Scale of 1990. See Guide E1751Guide for Temperature Electromotive Force (emf)
Tables for Non-Letter Designated Thermocouple Combi-
E1594.
nations
1.4 This standard does not purport to address all of the
E2846Guide for Thermocouple Verification
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
3. Terminology
priate safety, health, and environmental practices and deter-
3.1 Definitions—ThedefinitionsgiveninTerminologyE344
mine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accor- shall apply to this test method.
dance with internationally recognized principles on standard-
3.2 Definitions of Terms Specific to This Standard:
ization established in the Decision on Principles for the
3.2.1 check standard, n—ameasurementinstrumentorstan-
Development of International Standards, Guides and Recom-
dard whose repeated results of measurement are used to
mendations issued by the World Trade Organization Technical
determine the repeatability of a calibration process and to
Barriers to Trade (TBT) Committee.
verifythattheresultsofacalibrationprocessesarestatistically
consistent with past results.
2. Referenced Documents
3.2.2 isothermal block, n—a piece of solid material of high
2.1 ASTM Standards:
thermal conductivity used to promote thermal equilibrium
E1Specification for ASTM Liquid-in-Glass Thermometers
between two or more thermometers.
3.2.3 referencejunctioncompensation,n—theadjustmentof
This test method is under the jurisdiction of ASTM Committee E20 on
the indication of a thermocouple such that the adjusted
Temperature Measurement and is the direct responsibility of Subcommittee E20.11
indication is equivalent to the emf or temperature that the
on Thermocouples - Calibration.
thermocouple would indicate if the reference junctions were
Current edition approved Sept. 1, 2019. Published October 2019. Originally
maintained at 0°C.
approved in 1963. Last previous edition approved in 2013 as E220–13. DOI:
10.1520/E0220-19.
3.2.3.1 Discussion—In most cases, the thermocouple indi-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
cation is adjusted by measuring the temperature of a terminal
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
blockwherethethermocoupleisconnected,andthenaddingto
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. thethermocoupleemfanadditionalemfequaltotheemfofthe
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E220 − 19
thermocouple reference function evaluated at the temperature blocks, tube furnaces, and dry fluidized baths, properly used,
of the terminal block. Because the emf-temperature relation- are acceptable temperature comparison environments. In the
shipofanyactualthermocouplediffersslightlyfromthatofthe case of large diameter thermoelements and sheathed
reference function, reference junction compensation typically thermocouples, special attention must be given to effects of
introduces higher uncertainties compared to the use of a well thermal conduction.
prepared ice bath.
6.2 Voltage measurement instruments with sufficiently high
3.2.4 reference junction compensator, n—a device that
input impedance must be used for measuring thermocouple
implements reference junction compensation.
emf to eliminate instrument loading as a significant source of
error. The ratio of input impedance to thermocouple loop
3.2.5 reference thermometer, n—thermometer that estab-
resistance should be significantly (at least 10 ) greater than the
lishes the value of temperature in a given system containing
ratio of the measured emf to the desired emf uncertainty.
additional temperature sensors.
3.2.5.1 Discussion—In a calibration system the reference
6.3 The test method relies on the assumption that test
thermometer is a calibrated thermometer capable of indicating
thermoelementsarehomogeneous.Ifso,theiroutputvoltageat
values of temperature with known uncertainty. The reference
a given measuring junction temperature is independent of
thermometer provides the standard temperature for the system
temperature variations along the length of the thermocouple.
at the time of test.
Departures from this ideal contribute to uncertainty in the use
of test results. The effects typically are negligibly small for
3.2.6 thermocouple type, n—a standardized thermoelectric
new, unused thermocouple material, but not for used
class of thermoelement materials that, used as a pair, have a
thermocouples, especially those of base-metal composition.
normal relationship between relative Seebeck emf and tem-
The effects of inhomogeneity can be identified, but not
perature.
accuratelyquantified,bythetechniquesdescribedinAppendix
3.2.6.1 Discussion—For common, commercially available
X4 in this test method and in section 8.2 of Guide E2846.
thermocouples, a thermocouple type is identified by a letter
Descriptions of the testing of used thermocouples may be
designation (types B, C, E, J, K, N, R, S, and T). The letter
found Guide E2846 and Manual MNL 12 (1) .
designation scheme is given in Guide E2846. The tables in
E1751 give temperature-EMF relationships for a number of
6.4 Thistestmethodpresumesthatthetestedthermocouples
additionalthermocouplecompositionsthatarenotidentifiedby
are suitable for use in air throughout the range of calibration
a letter designation.
temperatures. To avoid oxidation of the thermoelements,
refractory-metal thermocouples that have not been hermeti-
4. Summary of Test Method
cally sealed in a sheath suitable for use in air should be tested
4.1 Comparisoncalibrationconsistsofmeasuringtheemfof
in an inert gas environment at temperatures above approxi-
the thermocouple being calibrated in an isothermal medium
mately 500 °C. In this case, use of this test method is
while simultaneously measuring the temperature of the me-
recommended in combination with the furnaces and related
dium with a reference thermometer. The reference thermom-
procedures described in Test Method E452.
eter may be any thermometer with sufficient accuracy at the
7. Apparatus
temperature of calibration.
7.1 The choice of apparatus used for the comparison test
5. Significance and Use
will depend primarily on the temperature range to be covered
5.1 For users or manufacturers of thermocouples, this test
and on the desired calibration uncertainty. The apparatus
method provides a means of verifying the emf-temperature
required for the application of this test method will depend in
characteristics of the material prior to use.
detailuponthetemperaturerangebeingcoveredbutinallcases
shall be selected from the equipment described as follows:
5.2 Thistestmethodcanbeusedtocalibrateathermocouple
for use as a reference, or it can be used to calibrate thermo-
7.2 Comparison Baths and Furnaces—A controlled tem-
couples representing a batch of purchased, assembled thermo-
peraturecomparisonmedium(bathorfurnace)shallbeusedin
couples.
which the measuring junction of the thermocouple to be
calibrated is brought to the same temperature as a reference
5.3 This test method can be used for the verification of the
thermometer. The spatial uniformity of temperature within the
conformance of thermocouple materials to temperature toler-
nominally isothermal calibration zone shall be established.
ancesforspecificationssuchasthetablesinSpecificationE230
Acceptable methods include measurements of the calibration
or other special specifications as required for commercial,
zone at the time of testing or the use of control charts that
military, or research applications.
display the periodic calibration of check standards or the
6. Interferences periodic characterization of the calibration zone. The fre-
quency of such testing will depend on the inherent stability of
6.1 Since the success of this test method depends largely
thebathorfurnace.Theuniformityofthecalibrationzoneshall
upon the ability to maintain the measuring junction of the
be remeasured sufficiently often such that any deviations in
thermocouple being calibrated and the reference thermometer
at the same temperature, considerable care must be taken in
choosing the media and conditions under which the compari-
The boldface numbers in parenthesis refer to the list of references at the end of
sons are made. Stirred liquid baths, uniformly heated metal this standard.
E220 − 19
uniformity may be corrected prior to significant adverse affect that enables the user to visually determine that all heaters are
on the readings. All thermocouples being calibrated and the operational and will require periodic remeasurement of the
reference thermometer must be immersed into this zone to an axial temperature profile. Single-zone furnaces may vary in
extentsufficienttoensurethatthemeasuringjunctiontempera- temperature profile slowly as the heater element ages and will
require only infrequent remapping of the temperature profile.
ture is not significantly affected by heat conduction along the
thermocouple and reference thermometer assemblies.To avoid
NOTE 1—Further discussions of suitable tube furnaces are given in
contaminating the thermoelements and insulation of un-
Appendix X1.
sheathed thermocouples, direct contact with calibration bath
7.2.4 Other Baths—The one essential design feature of any
fluids should be avoided.
bath to be used with this test method is that it brings the
7.2.1 Liquid Baths—In the range from −150°C to 630 °C
measuringjunctionofthethermocouplebeingcalibratedtothe
(−240 °F to 1170 °F) the comparator bath shall usually consist
sametemperatureasthereferencethermometer.Copperblocks
of a well stirred liquid bath provided with controls for
immersedinliquidnitrogenhavebeenusedsuccessfullyatlow
maintainingaconstantanduniformtemperature.Suitabletypes
temperatures. The blocks are provided with wells for the test
aredescribedintheappendixtoTestMethodE77.Attheliquid
thermocouples and the reference thermometer. Similarly, uni-
nitrogenboilingpoint,−196°C(−321°F),anisothermalblock
formly heated blocks have been used at high temperatures.
of copper suspended in an open dewar of liquid nitrogen can
Suchbathsarenotexcludedunderthistestmethod,butcareful
provide a very effective single-point liquid bath. In the range
explorations of existing temperature gradients must be made
between −196 °C (−321 °F) and −150 °C (−240 °F), the bath
before confidence may be placed in such an apparatus.
construction is relatively complex, and commercial systems
7.2.5 Isothermal Blocks—The use of an isothermal block
that rely on liquid nitrogen for cooling are recommended. A
can substantially reduce the temperature differences between
properly constructed liquid bath will have temperature gradi-
the reference thermometer and the test thermocouples. Such a
ents that are small relative to either fluidized powder baths or
block should be manufactured from a material of high thermal
tube furnaces. A disadvantage of liquid baths is the relatively
conductivitythatwillnotcontaminatethethermocouplesunder
small operating range of any one bath fluid. The temperature
test. High thermal conductivity reduces the spatial temperature
gradients in a liquid bath will be repeatable provided that the
variations in the block, resulting in better thermal equilibrium
bathliquiddoesnotthermallydecomposeathightemperatures
betweenthereferencethermometerandthetestthermocouples.
and that the conditions of bath heating and cooling are
An isothermal block may also be used to reduce temporal
comparable to those that existed when the bath gradients were
fluctuations of the thermometers. The fluctuations will de-
characterized. Periodic evaluation of bath gradients is neces-
crease as either the heat capacity of the block is increased or
sary when using oil baths, since oil viscosity can increase
the heat transfer to the surrounding furnace or bath is de-
significantlyafteruseathightemperatures.Bathswithmultiple creased. A consequence of this decrease in fluctuations is an
heaters require a monitoring system that enables the user to
increase in the time for the isothermal block to reach a
readily determine that all heaters are operational. steady-state temperature, so care must be exercised that the
block is neither too large nor too well insulated. The tempera-
7.2.2 Fluidized Powder Baths—Intherangefrom−70°Cto
ture differences between the test thermocouples and the refer-
980 °C (−100 °F to 1800 °F) the comparator bath may consist
ence thermometer should be evaluated over the full tempera-
of a gas-fluidized bath of aluminum oxide or similar powder.
ture range of the apparatus by performing calibrations of
Temperature equalizing blocks are almost always necessary
check-standardthermocouplesatavarietyofimmersionsinthe
within fluidized baths to minimize spatial and temporal tem-
blockandwiththevariousthermometersinsertedintodifferent
perature variations. The repeatability of thermal gradients
bores of the block. Similar temperature differences should also
withinsuchablockdependsonmaintainingaconstantfilllevel
be measured as a function of time, following an adjustment of
of powder in the bath and maintaining a uniform gas flow
thefurnaceorbathtemperature,todeterminethelengthoftime
through the powder. The thermal gradients of a fluidized
needed to reach thermal steady-state following a temperature
powder bath shall be verified by including either a second
change. Welding the measuring junctions of the test thermo-
reference thermometer or a check-standard thermocouple in
couplesandofathermocoupleusedasareferencethermometer
each comparison test.
isaspecialcaseofanisothermalblockformedbythecommon
7.2.3 Tube Furnaces—At temperatures above approxi-
measuring junction.
mately 620 °C (1150 °F) an electrically heated tube furnace
withasuitablenominallyisothermalzonewillusuallybeused. 7.3 Reference Junction Temperatures— A controlled tem-
Laboratory type tube furnaces may be used at any temperature peraturemediuminwhichthetemperatureofthethermocouple
provided that the increased uncertainty due to their spatial reference junctions is maintained constant during a measure-
temperature variance is accounted for. Any one of a wide ment cycle at a known or measured value shall be provided.A
varietyofdesignsmaybesuitable,butitshallbedemonstrated commonly used reference temperature is 0 °C (32 °F), usually
that the furnace chosen can maintain a temperature stability of realized through use of the ice point, but other temperatures
61 °C over a period of 10 min at any temperature in the range may be used if desired. Reports of data taken with reference
over which the furnace is to be used. The axial temperature temperatures other than the ice point should be corrected to
profile of a tube furnace shall be mapped to determine the reflect the results that would have been obtained if the
location of the region with the best temperature uniformity. referencejunctionhadbeenattheicepoint,andthereportshall
Furnaces with multiple heaters require a monitoring system state both the reference junction temperature and whether the
E220 − 19
correction is based on the reference function for that type of Gauge (AWG) diameter or less, and to use protection tubes
thermocoupleorontheemf-versus-temperatureresponseofthe with the smallest inner diameter that still permits ready
particular thermocouple under test. As an alternative, calibra- insertion of the reference junctions.
tiondatatakenwithareferencejunctiontemperatureotherthan
7.3.3 Isothermal and Electronic Reference Junction
the ice point may be reported without correction, but in such
Compensation—For the rapid calibration of large numbers of
cases the calibration report must clearly state the actual
thermocouples, the reference junctions can be made at an
reference junction temperature. With the exception of thermo-
isothermal multiterminal strip.This avoids the thermal loading
couples that have very small Seebeck coefficients at the
oftheicebathresultingfromthelargenumberofthermocouple
reference junction temperature (such as Type B
and copper connecting wires. The temperature and isothermal
thermocouples), a large uncertainty in the reference junction
conditionofthestripshallbeestablishedandmonitoredbythe
temperaturewillintroduceacorrespondinglargeuncertaintyin
use of a separate, reference temperature sensor. The spatial
the thermocouple calibration. Whatever reference junction
temperaturevariationacrosstheterminationsontheisothermal
technique is used, its uncertainty must be accounted for in the
unit shall be mapped and accounted for. If desired, the
uncertainty of the thermocouple calibration being performed.
thermocouple emf values obtained with use of an isothermal
An uncertainty in the reference junction temperature,
terminal strip may be compensated such that the compensated
u(t ), will introduce an uncertainty in the measured
ref
emf is equivalent to the thermocouple emf created by a
temperature, u (t):
rj
thermocouple with reference junctions at 0°C (32°F). An
u t 5 S t /S t u t electronic reference junction compensator accomplishes this
~ ! ~ ~ ! ~ !! ~ !
rj ref ref
where: S(t ) and S(t) are the Seebeck coefficients of the
ref task by accurately monitoring the temperature of the reference
thermocouple at temperatures t and t, respectively.
ref
junctions and adding to the thermocouple emf an additional
emf such that the sum is equivalent to the thermocouple emf
7.3.1 Method to Form a Reference Junction—Reference
producedwithreferencejunctionsat0°C.Theadditionofemf
junctions are formed by electrically connecting the ends of the
to the thermocouple emf may be accomplished through soft-
thermoelements opposite the measuring junction to copper
waremethods,aswellasthroughadditionofanactualemf.To
wires that lead to the emf measuring device.Any method that
minimize the uncertainty of an electronic reference junction
gives a reliable electrical connection may be used. The
emf as a source of error, the temperature equivalent of the emf
connection may be formed by welding, brazing, or soldering
produced by the electronic reference junction shall be known
thewires,byascrewclamp,bycrimpingthewirestogether,or
andmeasuredwithuncertaintylessthanthatexpectedfromthe
by a spring-loaded connector. Completely clean finished junc-
thermocouple calibration.
tions of any harmful contaminants, especially if any soldering
7.3.4 Extension of Thermoelements to Reference Tempera-
or brazing fluxes have been used.
ture —Whenever possible, the thermoelements under test shall
7.3.2 Use of an Ice-Point Bath to Maintain Reference
becontinuous,extendingfromthemeasuringpointthroughthe
Junction Temperature—An acceptable method for utilizing the
temperature gradient, to the reference junction without any
icepointasareferencejunctionisgiveninPracticeE563.Each
intermediate connections. In cases where this is not possible,
joined thermoelement/copper-wire pair leading to a reference
several options exist:
junction shall be immersed in the reference junction bath to a
7.3.4.1 Matched Thermoelements—Additional lengths of
sufficient depth that the reference junction is in thermal
thermoelement materials from the same wire lots as those
equilibrium with the ice-point bath. The wires shall not be
beingcalibratedmaybeusedtoextendthedeviceundertestto
immerseddirectlyintotheice-pointbath,butshallbekeptdry.
the reference bath. In such circumstances, no additional cor-
For this purpose, a glass, plastic, or metal protection tube is
rections are required.
usually used to contain the thermoelement/copper pair. If a
metal protection tube is used, the wires shall be fully electri- 7.3.4.2 Thermoelements of the Same Type with Known
cally insulated from the metal tube. In all cases, the thermo-
Thermoelectric Response, but from a Different Lot—
elementsandthecopperwiresabovethejunctionitselfshallbe Thermoelements being calibrated may be extended using
insulated with a water-resistant insulation. If more than one thermoelement materials of the same type as the thermocouple
reference junction is placed into a single protection tube, the under test. Such materials may be of thermocouple or exten-
junctions shall be electrically insulated from each other.Atest sion grade, but shall have a known emf versus temperature
to determine an appropriate depth of immersion into the relationshipoverthetemperatureintervaltowhichtheywillbe
ice-point bath is as follows: Prepare two ice-point baths, one subjected, and corrections for the deviations of the extension
containing the measuring junction of a thermocouple in an material relative to the material under test over that interval
appropriate protection tube, and the other containing the shall be made. In general, it will be necessary to calibrate the
reference junctions. Record the emf of the thermocouple at test wire in the temperature range spanned by the extension
varying immersions of the reference junctions, allowing 10 wire and to measure the temperature of the junctions between
minforthermalequilibrationaftereachchangeofdepth.Aplot the different materials in order to make this correction. It is
of emf-versus-immersion depth will show the minimum im- acceptable to not apply a correction if the uncertainty budget
mersion depth necessary such that further increases in depth for the calibration includes an appropriate allowance for
give negligible changes in emf. To promote good thermal temperature variations of the junction between the thermo-
equilibriumbetweentheicebathandthereferencejunctions,it couple and the extension material, and the calibration report
is desirable to use copper leads of 0.5 mm 24 American Wire specifies the range of transition junction temperatures for
E220 − 19
which the calibration is valid. This allowance may be experi- convenience, they shall be protected against the development
mentally determined by maintaining the measuring junction of of temperature gradients across the blocks.
the thermocouple at a fixed temperature, such as 0°C, and
7.6 Thermocouple Insulation and Protection Tubes —In the
varying the temperature of the transition junction over a
case where bare wire thermocouples are tested, two-hole
specified range.
insulation tubing may be used to support and electrically
7.3.4.3 Thermocouple Connectors—In all cases where there
insulate the immersed portion of the two bare thermoelements.
are junctions between the thermocouple under test and ther-
Use only insulation material that will not contaminate the
mocouple lead wires, the temperature variations across the
thermocouple (for example, clean, high-purity insulators such
junctions shall be minimized. Thermocouple connectors as
as99.8%aluminumoxide)andthatwillprovidethenecessary
described in Specifications E1684 and E1129/E1129M will
electrical insulation at the highest temperature of the calibra-
introduce no more than 1.1°C (2°F) error for a 40°C (70°F)
tion. To prevent contamination of thermocouples by residues
temperature difference across the connector. This error will be
left by previously tested thermocouples, each insulator shall
proportionately reduced for smaller temperature differences.
only be used with thermocouples of one type and the positive
7.3.4.4 Circumstances with Small Temperature
and negative thermoelements shall always be inserted in the
Differences—In special cases where the temperature differ-
same bore. The only exceptions allowed are: type R and type
ences from end-to-end along the length of the wires used to
S thermocouples may be calibrated in the same insulators, and
extend a thermocouple for calibration purposes are very small
the thermoelements of type B thermocouples may be mounted
(less than 62°C), thermocouple or extension grade wires of
in either bore. To avoid unnecessary mass and to minimize
matchingthermocoupletypemaybeusedincalibrationcircuits
axial heat conduction in the region of the measuring junction,
without correction.
the tubing should be relatively thin walled. Bore diameters
should provide a loose fit for the thermocouple wires. During
7.4 Emf-Measuring Instruments—The choice of a specific
thetest,thethermocouplesmaybeinsertedinaprotectiontube
instrument to use for measuring the thermocouple emf will
that is resistant to thermal shock, and noncontaminating to the
depend on the accuracy required of the calibration being
thermocouple materials.
performed. Generally the thermocouple emf will be measured
7.6.1 Sheathedthermocouplesmaybetestedwithoutfurther
using a digital voltmeter. For the highest level of accuracy,
protection or support in liquid or dry fluidized baths, provided
voltmeters shall have a maximum uncertainty no greater than
-4
10 times the emf reading and shall have input impedances that the bath medium is compatible with the sheath material.
Thermocouples insulated with fibrous insulation must not be
largerthanthethermocoupleloopresistancebyatleastafactor
of 10 . Reference junction compensation is required for ther- immersed directly into any bath liquid. Care must be taken to
keepthermalconductionlosseswithinthelimitsofexperimen-
mocouple measurement with voltmeters. In order to avoid
forming unintended reference junctions at voltmeter terminals talerrortypicallybyimmersingthethermocoupleintothebath
until no further indication of temperature change is noted.
whose temperature may be poorly controlled, thermocouples
must not be connected directly to the input terminals of
8. Reference Thermometers
voltmeters without the use of appropriate electronic reference
junction compensation and connection of the voltmeter to the
8.1 The reference thermometer to be used for the compari-
compensator with untinned copper wires.
son calibration of thermocouples will depend upon the tem-
perature range covered, the type of calibration apparatus, the
7.5 Connecting Wire Assembly—Connecting wires from the
accuracy desired, or in cases where more than one type of
reference junction to the voltmeter shall be insulated copper
thermometer will suffice, the preference of the calibrating
andshallbeconfiguredastwistedpairsforwirelengthsgreater
laboratory. All reference thermometers shall be calibrated to
than 0.3 m (1 ft.), to reduce electromagnetic noise pickup. If
indicate values of temperature corresponding to the Interna-
the environment contains substantial electromagnetic noise, it
tional Temperature Scale of 1990. The condition of the
may also be useful to run the wires in a grounded electrical
reference thermometer shall be verified both before and after a
shield or braided cable. Copper connections should be clean
calibration or a documented number of calibrations by check-
and free from oxides.
ing its indication at a thermometric fixed point or by using a
7.5.1 Scanner systems may be used to switch between the
comparison measurement of total uncertainty less than the
reference thermometer and the different thermocouples being
allowed uncertainty of the reference thermometer. Specific
calibrated. Such switches shall be of rugged construction and
methodsofverificationforeachtypeofreferencethermometer
designed so that both connecting wires are switched when
are described in 8.10,
switching from one thermocouple to the next, leaving thermo-
couples not in use electrically isolated. All of the scanner 8.2 Platinum Resistance Thermometers— Platinum Resis-
switches shall be constructed of the same material and shall be
tance thermometers are an excellent choice as a reference in
free of extraneous emf production (see Appendix X3). Precau- cases where the highest accuracy is required. Standard plati-
tions should be taken to protect the switches from temperature
num resistance thermometers (SPRTs) are the most accurate
fluctuations due to convection, conduction, or radiation. Scan- reference thermometers for use at temperatures from approxi-
ningperformanceshallbeevaluatedtoensureadequatesettling
mately −196 °C to 962 °C (−310 °F to 1764 °F), with
time before measurement. calibrationuncertaintiesaslowas0.001°C(0.002°F).SPRTs
7.5.2 It is preferable to use wire-to-wire connections in mustmeetasetofcriteriaspecifiedbytheITS-90.Inaddition,
calibration circuits, but if terminal blocks are used for there are a variety of platinum resistance thermometers that do
E220 − 19
TABLE 2 Calibration Uncertainties in Calibrating Thermocouples
notmeetthecriteriaforSPRTsthathavesufficientaccuracyfor
A
by the Comparison Method—Temperatures in Degrees Farenheit
useasareferencethermometerwiththistestmethod.Standard
(see Refs. 2 and 3)
platinum resistance thermometers are described in X2.1, other
Temperature Expanded Uncertainty
platinum resistance thermometers are described in X2.2, and
[°F] (k=2) [°F]
A
measurement instruments are described in X2.3.
Base metal thermocouples 400 0.4
(in a tube furnace by
800 0.8
8.3 Thermistors—For temperatures in the approximate comparison with a
1200 1.1
calibrated Type S
range −40 °C (−40 °F) to 150 °C (300 ° F), a thermistor may 1600 1.5
thermocouple
2000 1.9
serve as a reference thermometer with uncertainty of 0.001 to
A
Base metal thermocouples −321 (type E) 0.4
0.01 °C. Section X2.5 provides additional information.
(in stirred liquid baths, by
−150 (type E) 0.2
comparison with an SPRT)
32 0.04
8.4 Liquid-in-Glass Thermometers—Liquid-in-glass ther-
400 0.4
mometersmaybeusedfrom–80°C(−110°F),orlower,to400
800 0.7
1000 0.9
°C (750 °F), or even higher with special types. Generally, the
Type R and S 400 0.3
accuracy of these thermometers is less below −60 °C, where
thermocouples (in a tube
800 0.4
organic thermometric fluids are used, and above 400 °C where
furnace, by comparison 1200 0.5
with a calibrated Type S
1600 0.5
dimensional changes in the bulb glass may be relatively rapid,
thermocouple) 2000 0.4
requiring frequent calibration. Further discussion of liquid-in-
Type B thermocouples 400 1.3
glass thermometers is given in X2.4. Specifications forASTM
(in a tube furnace, by 800 1.0
comparison with calibrated 1200 0.8
thermometers are given in Specification E1.
Type S or Type B 1600 0.7
8.5 Types R and S Thermocouples (Platinum-Rhodium ver- thermocouples) 2000 1.1
2500 2.9
sus Platinum)—The platinum-10% rhodium versus platinum
A
Uncertainties for calibration of base metal thermocouples include an allowance
(Type S), or the platinum-13% rhodium versus platinum
for the inhomogeneity of the unused thermoelements.
thermocouple(TypeR)of0.5-mm(24-gauge)diameterwireis
recommended as the reference thermometer for temperatures
from 960 °C (1760 °F) to 1200 °C (2190 °F). Their use may
°F). The uncertainties of temperature measurements with this
also be extended down to room temperature. Uncertainties
type of thermocouple are given in Tables 1 and 2.
attainable with careful use are given in Tables 1 and 2.
8.7 Type T Thermocouples (Copper versus Constantan)
8.6 Type B Thermocouples (Platinum-Rhodium versus
—The type T thermocouple may serve as a useful reference
Rhodium-Platinum)—The platinum-30 % rhodium versus
thermometerintherangeof−195°Cto370°C(−320°Fto700
platinum-6% rhodium (Type B) thermocouple, of 0.5-mm
°F) in some instances, although its accuracy is, in general,
(24-gauge) or larger diameter wire, is recommended as the
limited by the stability of the wire at temperatures above
reference thermometer for temperatures above 1200 °C (2190
approximately200°C(390°F),andbytheaccuracyoftheemf
measurements and the inhomogeneity of the wire below 200
TABLE 1 Calibration Uncertainties in Calibrating Thermocouples
°C. One-half millimeter diameter (24 gauge) wire is a useful
A
by the Comparison Method—Temperatures in Degrees Celsius
compromisebetweenthelesserstabilityofsmallerwireandthe
(see Refs. 2 and 3)
greater heat conduction of large wire.
Temperature Expanded Uncertainty
[°C] (k=2) [°C]
8.8 GoldversusPlatinumThermocouples—Thegoldversus
A
Base metal thermocouples 200 0.2
platinum thermocouple is useful as a reference thermometer
(in a tube furnace by
400 0.4
comparison with a
overtherange0°Cto1000°C(32°Fto1830°F).Withproper
600 0.6
calibrated Type S
800 0.7
thermocouple construction and annealing, a gold versus platinum thermo-
1000 0.9
couplewillhaveuncertaintiesofapproximately0.01°Cto0.02
1200 1.0
A
°C (0.02 °F to 0.04 °F). To attain this performance, care in the
Base metal thermocouples −196 (type E) 0.2
(in stirred liquid baths, by
−100 (type E) 0.1
emf measurements and protection of the thermoelements from
comparison with an SPRT)
0 0.02
contamination is necessary.
200 0.2
400 0.4
8.9 Single-use Base-metal Thermocouples—For tests to el-
500 0.5
evated temperature, a base metal thermocouple taken from a
Type R and S thermocouples 200 0.2
(in a tube furnace, by 400 0.2 calibrated lot of wire of verified homogeneity may be used as
comparison with a 600 0.3
a reference thermometer. Lot homogeneity may be determined
calibrated Type S 800 0.3
by calibrating thermocouples fabricated from a statistical
thermocouple)
1000 0.3
1100 0.2
sample of the wire lot, and determining the standard deviation
Type B thermocouples 200 0.8
of emf values of the set of thermocouples, at each calibration
(in a tube furnace, by 400 0.6
temperature. In this application, the base-metal reference
comparison with calibrated 600 0.5
Type S or Type B 800 0.4
thermocouple would be used only at increasing test tempera-
thermocouples) 1100 0.3
tures.Single-usebasemetalthermocouplesarenotamenableto
1450 1.6
recalibration,andanadditionaluncertaintymustbeincludedto
A
Uncertainties for calibration of base metal thermocouples include an allowance
account for drift of the reference during the test. In particular,
for the inhomogeneity of the unused thermoelements.
type E and type K thermocouples that have not been specially
E220 − 19
heat-treated are known to exhibit shifts of up to the equivalent drift can be tolerated without unacceptable variation in the
of 4 °C (7 °F) in thermoelectric response after relatively short calibration results. A minimum of three consecutive readings
exposures to temperatures in the range 250 °C (480 °F) to 550 that yield the same emf value within measurement uncertainty
°C (1020 °F). isrequired.Insteadofemployingasinglenominallyisothermal
NOTE 2—In general, any thermometer may be employed as a reference
temperature calibration environment whose temperature is
thermometer provided that it has a known amount of measurement
changed to each calibration temperature, a series of nominally
uncertainty.
isothermal environments (for example, isothermal metal
8.10 Verification of Reference Thermometer Performance—
blocks,stirredfluidbaths,ortubefurnaces),eachmaintainedat
When platinum resistance thermometers or thermistors are
a calibration temperature, may be used provided the rates of
used as reference thermometers, the reference thermometer
immersion and extraction are not so large as to damage either
shall be verified by checking its indication at a thermometric
the thermocouples being calibrated or the reference thermom-
fixed point. The ice point or the triple point of water is
eter.After insertion of the thermocouple into each temperature
commonly used. Liquid-in-glass thermometers shall be mea-
calibration environment, time shall be allowed for steady-state
sured at the ice point after each thermal cycle to temperatures
conditions to be reached before readings are taken. The depth
exceeding 100 °C (212 °F). Measurements may be corrected
of immersion ideally shall be the same throughout the test.
for a change in ice-point reading following the procedures in
Otherwise, the depth shall not be less than any previous
Test Method E77. Thermocouples, other than Au/Pt
immersion. Techniques comparable to those described in
thermocouples, are not amenable to recalibration in an appa-
Appendix X4, but covering only the range of immersion depth
ratus different than the one used for the actual test. Therefore,
encountered in the calibration process, can be useful in
thermocouple reference thermometers shall be verified by a
determining if changes in immersion during the course of a
comparison test against a second reference thermometer of
calibrationsignificantlyaffecttheresults.Oneofthefollowing
equalorlesseruncertaintyinthesameapparatusasusedforthe
twogeneralmethodsmaybeusedinthecalibrationprocedure.
test. Au/Pt thermocouples shall be verified at a thermometric
10.2 Method A, Automated Method—Fig. 2 illustrates the
fixed-point or by a comparison measurement. For thermo-
schematic of a typical data acquisition system—assuming a
couplesofalltypes,verificationpointsattemperaturescloseto
thermocouple is used as the reference thermometer. This
the temperature of the reference junctions are not a sensitive
method permits the rapid testing of any number of thermo-
test of possible changes in the thermocouple. Verification
couples. The reference thermometer may be of any type
points should be chosen to have a temperature as far from the
meeting the uncertainty requirements for the calibration, pro-
reference junction temperature as practical
vided that the data acquisition system can accommodate the
corresponding type of signal. The thermocouples to be tested
9. Sampling
and the reference thermocouple are terminated at the reference
9.1 Sampling is normally specified in the ASTM material
junction temperature unit. This unit must meet the require-
specification that calls for the calibration. As a guideline for
ments of 7.3. A scanning unit sequentially connects each
compliance testing, a minimum of two samples are often
thermocouple to the input of a voltmeter. The voltmeter must
calibrated to ensure that a lot of wire or assembled thermo-
meet the requirements of 7.4. In the time interval between a
couples conforms to standardized emf-temperature relations
reading of the reference thermometer and of a test
within specified tolerances. In the case of wire, the samples
thermocouple, the temperature of the calibration zone will
should preferably be widely separated within the lot, for
vary. Measurements shall be made rapidly enough so that this
example,oppositeendsofacoil.Usersshouldbeawarethatin
variation in temperature is small in comparison to the uncer-
some instances compliance testing will cause changes to occur
tainty of the temperature measurement. The emf-measuring
inthethermoelectricpropertiesofthesamplesofthermocouple
system is comprised of a voltmeter, a display, and a data
wire tested.
storagesystem.Itmaybedesirabletoprovideacalibratedemf
source and a zero (or shorted) input to the data acquisition
10. General Procedures
system as references to improve the accuracy of the measure-
10.1 The calibration procedure consists of measuring the
ment. For a set of n test thermocouples, the recommended
emf of the thermocouple being calibrated at selected calibra-
order of readings is: reference thermometer, test thermocouple
tion points, the temperature of each point being measured with
1, test thermocouple 2, … test thermocouple n, reference
the selected reference thermometer. The number and choice of
thermometer, test thermocouple n, … test thermocouple 1,
test points will depend upon the type of thermocouple, the
reference thermometer. If the variations in temperature indi-
temperature range to be covered, and the accuracy required.
cated by the reference thermometer are greater than desired
Table 1 or Table 2 will serve as a guide to the selection. Both
with the above method, an alternative order of reading may be
the nominally isothermal temperature calibration environment
made, provided that the reference thermometer is read at least
and the thermocouples must be stabilized at the calibration
once for every reading of the test thermocouple and that the
temperature before readings are taken. For any particular
order of readings is symmetric in both forward and reverse
apparatus, the necessary stability may be determined by
directions.
performingcomparisonmeasurementsatavarietyofdriftrates
NOTE 3—The reference temperature unit, the scanner, and the emf-
of the reference thermometer temperature. A plot of the test
measuring system are typically combined into a single system. They are
thermocouple reading, normalized as described in 12.1.1,
described separately so that their characteristics may be understood and
versus drift rate will indicate what magnitude of temperature specified to meet the required uncertainty criteria.
E220 − 19
FIG. 1 Automated Thermocouple Data Acquisition System Layout—With Thermocouple Reference
FIG. 2 Thermocouple Assembly in Protection Tube (Multilayered Cutaway View)
NOTE 4—Any program that performs manipulations on the measured
10.4 Method C, Differential Method— When the thermo-
data shall be checked for accuracy by evaluating the output response to
couple being calibrated is of the same type as the reference
known inputs.
thermometer a differential emf measurement method may be
10.3 Method B, Manual Method—Thismethodmaybeused
employed. In this method, the emf of the reference thermo-
when one or more thermocouples are to be calibrated with
couple is measured and the differential emf between the
manually operated switches. Each thermocouple is connected
thermocouple being calibrated and the reference thermocouple
to the voltmeter in sequence. The reference thermometer
isdirectlymeasuredbyconnectingthethermocouplesinseries,
should be read just before and just after the reading of each
with the polarity of one of the thermocouples reversed. Note
thermocouple under calibration. After measuring the emf of
that in order to make a serial connection the thermocouple
eachtestthermocouple,theentiresequenceshouldberepeated
junctionsmustbeelectricallyisolated.Thistechniquecanoffer
at the same temperature.
E220 − 19
improved stability since the difference will remain relatively (2650°F).Thencoolitslowly(overaperiodofapproximately
constant over small temperature intervals. For this technique a 1 min) to 750 °C (1380 °F) and hold it at that temperature
voltmeter with uncertainty up to 0.5 µV can be used. approximately30min.Next,allowthethermocoupletocoolto
room temperature within a few minutes. Alternating current
11. Preparation of Thermocouples for Test
from an
...