ASTM E633-21a

Standard Guide for Use of Thermocouples in Elevated-Temperature Mechanical Testing

SIGNIFICANCE AND USE
6.1 This guide presents techniques on the use of thermocouples and associated equipment for measuring temperature in elevated-temperature mechanical testing under typical conditions. Test durations range from minutes for tension, compression, Young's modulus, and superplastic property tests to thousands of hours for creep, stress-rupture, creep rupture, notch tension, and stress-relaxation tests.  
6.2 Since elevated-temperature mechanical properties are highly sensitive to temperature, users should make every effort practicable to make accurate temperature measurements and provide stable control of the test temperature. The goal of this guide is to provide users with good pyrometric practice and techniques for precise temperature control for elevated-temperature mechanical testing.  
6.3 Techniques are given in this guide for maintaining a stable temperature throughout the period of test.  
6.4 If the techniques of this guide are followed, the difference between indicated temperature and true temperature, as used in Test Methods E21, E111, E139, E292, E328, E2448, and Practice E209 will be reduced to the lowest practical level.
SCOPE
1.1 This guide covers the use of ANSI thermocouple Types listed in Specification E230/E230M for elevated-temperature mechanical testing at temperatures typical in testing of metals.
Note 1: Typical conditions for mechanical testing are specified temperatures up to 1800 °F (982 °C) in air at one atmosphere of pressure.  
1.1.1 This guide focuses on most commonly used base metal and noble metal ANSI type K, N, R, and S thermocouples.  
1.1.2 Other ANSI thermocouple types that are listed in Specification E230/E230M may be used if the specific precautions and maximum temperatures for their use are followed.  
1.1.3 It does not cover the use of sheathed thermocouples.  
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.  
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 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.

General Information

• Status: Published
• Publication Date: 31-Oct-2021
• ICS: 17.200.20 - Temperature-measuring instruments
• Technical Committee: E28 - Mechanical Testing
• Drafting Committee: E28.04 - Uniaxial Testing

Relations

• Refers: ASTM E344-23 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Dec-2023
• Refers: ASTM E230/E230M-23a - Standard Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
• Effective Date: 01-Nov-2023
• Refers: ASTM E230/E230M-23 - Standard Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
• Effective Date: 01-May-2023
• Refers: ASTM E344-19 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Sep-2019
• Refers: ASTM E344-18 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Apr-2018
• Refers: ASTM E209-18 - Standard Practice for Compression Tests of Metallic Materials at Elevated Temperatures with Conventional or Rapid Heating Rates and Strain Rates
• Effective Date: 01-Feb-2018
• Refers: ASTM E344-16 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Nov-2016
• Refers: ASTM E1129/E1129M-15 - Standard Specification for Thermocouple Connectors
• Effective Date: 01-Dec-2015
• Refers: ASTM E1129/E1129M-14 - Standard Specification for Thermocouple Connectors
• Effective Date: 01-Sep-2014
• Refers: ASTM E220-13 - Standard Test Method for Calibration of Thermocouples By Comparison Techniques
• Effective Date: 01-Nov-2013
• Refers: ASTM E344-13 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-May-2013
• Refers: ASTM E574-13 - Standard Specification for Duplex, Base Metal Thermocouple Wire With Glass Fiber or Silica Fiber Insulation
• Effective Date: 01-May-2013
• Refers: ASTM E344-12 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-May-2012
• Refers: ASTM E2448-11 - Standard Test Method for Determining the Superplastic Properties of Metallic Sheet Materials
• Effective Date: 01-Jun-2011
• Refers: ASTM E2448-11e1 - Standard Test Method for Determining the Superplastic Properties of Metallic Sheet Materials
• Effective Date: 01-Jun-2011

Overview

ASTM E633-21a: Standard Guide for Use of Thermocouples in Elevated-Temperature Mechanical Testing provides comprehensive guidance on the application of thermocouples for accurate temperature measurement in elevated-temperature mechanical tests, primarily for metals. Developed by ASTM, this standard addresses best practices for instrumentation, thermocouple selection, calibration, installation, and temperature control, aimed at ensuring the highest test accuracy and repeatability.

Elevated-temperature testing plays a critical role in understanding the mechanical properties of materials under service conditions. Since such properties are highly sensitive to temperature fluctuations, the guide emphasizes precision in temperature measurement and control, directly impacting the reliability of test results in tension, compression, Young’s modulus, superplasticity, creep, stress-rupture, notch tension, and stress-relaxation tests.

Key Topics

• Thermocouple Types: Focuses on ANSI Type K, N, R, and S thermocouples as specified in ASTM E230/E230M. Other ANSI types may be used with proper precautions.
• Temperature Control: Details requirements for stable test temperatures, essential for both short- and long-duration tests.
• Instrumentation: Outlines requirements for measurement systems, including data acquisition, calibration, and system verification to minimize uncertainty between indicated and true temperature.
• Installation and Attachment: Provides guidance for positioning and attaching thermocouples to test specimens for optimal thermal contact and response.
• Calibration and Verification: Explains the necessity for periodic calibration of thermocouples and instrumentation, using reference equipment with higher accuracy and traceability to national or international standards.
• Insulation and Wire Handling: Stresses careful selection and use of insulation materials and correct wire handling to avoid contamination, strain, or error due to inhomogeneity.
• Safety and Best Practices: Reminds users of the responsibility to implement appropriate safety, health, and environmental safeguards along with regulatory compliance.

Applications

The guide is widely used across industries involved in the mechanical testing of metals at elevated temperatures, including:

• Aerospace and automotive sectors for testing high-performance alloys under thermal cycles
• Metallurgical research laboratories assessing material performance for infrastructure and energy applications
• Quality assurance in manufacturing processes requiring stringent temperature control in product development
• Academic research focusing on material behavior under thermal and mechanical loads

It helps ensure the reliability and validity of test data for:

• Tension and compression tests (short duration)
• Young’s modulus, superplasticity measurements
• Long-term creep, stress-rupture, and stress-relaxation tests

Proper use of thermocouples in these applications reduces test variability, supports traceability, and minimizes discrepancies between actual and recorded temperatures, enhancing data integrity for material qualification and certification.

Related Standards

ASTM E633-21a references and complements several other ASTM standards important to temperature measurement and mechanical testing, including:

• ASTM E230/E230M - Specification for standard thermocouple types and tolerance grades
• ASTM E21 - Elevated temperature tension tests for metallic materials
• ASTM E111 - Young’s modulus and modulus measurements
• ASTM E139, E292, E328, E2448 - Tests for creep, stress relaxation, and superplastic properties
• ASTM E220 - Thermocouple calibration by comparison techniques
• ASTM E574, E1129/E1129M, E1684 - Specifications for thermocouple wires and connectors
• ASTM E344 - Terminology for thermometry and hydrometry

These documents together form a robust framework for executing accurate elevated-temperature testing and reliable material performance analysis.

For organizations seeking compliance and excellence in high-temperature mechanical testing, following ASTM E633-21a ensures adoption of best practices in thermocouple use, promotes test repeatability, and enhances international recognition of test results.

Keywords: ASTM E633-21a, thermocouple guide, elevated-temperature mechanical testing, temperature measurement, metals testing, ASTM standards, temperature control, calibration, test accuracy

Standards Content (Sample)

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: E633 − 21a
Standard Guide for
Use of Thermocouples in Elevated-Temperature Mechanical
Testing
This standard is issued under the fixed designation E633; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This guide provides basic information, options, and guidelines to enable the user to apply
thermocouples, temperature-measuring systems, and temperature-control systems with sufficient
accuracy to satisfy the temperature requirements for elevated-temperature mechanical testing of
materials.
1. Scope* 2. Referenced Documents
1.1 This guide covers the use ofANSI thermocouple Types
2.1 ASTM Standards:
listed in Specification E230/E230M for elevated-temperature
E6Terminology Relating to Methods of MechanicalTesting
mechanical testing at temperatures typical in testing of metals.
E21TestMethodsforElevatedTemperatureTensionTestsof
Metallic Materials
NOTE 1—Typical conditions for mechanical testing are specified tem-
peratures up to 1800°F (982°C) in air at one atmosphere of pressure.
E111Test Method for Young’s Modulus, Tangent Modulus,
and Chord Modulus
1.1.1 This guide focuses on most commonly used base
E139Test Methods for Conducting Creep, Creep-Rupture,
metal and noble metal ANSI type K, N, R, and S thermo-
and Stress-Rupture Tests of Metallic Materials
couples.
E209PracticeforCompressionTestsofMetallicMaterialsat
1.1.2 Other ANSI thermocouple types that are listed in
Elevated Temperatures with Conventional or Rapid Heat-
Specification E230/E230M may be used if the specific precau-
ing Rates and Strain Rates
tions and maximum temperatures for their use are followed.
1.1.3 It does not cover the use of sheathed thermocouples. E220Test Method for Calibration of Thermocouples By
Comparison Techniques
1.2 Thevaluesstatedininch-poundunitsaretoberegarded
E230/E230MSpecification for Temperature-Electromotive
as standard. The values given in parentheses are mathematical
Force (emf) Tables for Standardized Thermocouples
conversions to SI units that are provided for information only
E292Test Methods for ConductingTime-for-Rupture Notch
and are not considered standard.
Tension Tests of Materials
1.3 This standard does not purport to address all of the
E328Test Methods for Stress Relaxation for Materials and
safety concerns, if any, associated with its use. It is the
Structures
responsibility of the user of this standard to establish appro-
E344Terminology Relating to Thermometry and Hydrom-
priate safety, health, and environmental practices and deter-
etry
mine the applicability of regulatory limitations prior to use.
E574Specification for Duplex, Base Metal Thermocouple
1.4 This international standard was developed in accor-
Wire With Glass Fiber or Silica Fiber Insulation
dance with internationally recognized principles on standard-
E1129/E1129MSpecification for Thermocouple Connectors
ization established in the Decision on Principles for the
E1684Specification for Miniature Thermocouple Connec-
Development of International Standards, Guides and Recom-
tors
mendations issued by the World Trade Organization Technical
E2448Test Method for Determining the Superplastic Prop-
Barriers to Trade (TBT) Committee.
erties of Metallic Sheet Materials
This guide is under the jurisdiction of ASTM Committee E28 on Mechanical
TestingandisthedirectresponsibilityofSubcommitteeE28.04onUniaxialTesting. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2021. Published January 2022. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1987. Last previous edition approved in 2021 as E633–13. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0633-21A. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E633 − 21a
3. Terminology 7. Apparatus
3.1 Definitions of terms common to mechanical testing that
7.1 Instrumentation may be individual instruments, a data
appear in Terminology E6 apply to this standard, including
acquisition system (multipoint recorders or digital type), a
accuracy, calibration, indicated temperature, metrological
computer-based control system, or a combination of these
traceability, reduced parallel section, temperature-measuring
devices.
system, specified temperature, and verification.
7.1.1 Since each thermocouple is “grounded” by contacting
the specimen, the instrumentation shall treat each thermo-
3.2 Some specific definitions of terms relating to thermom-
couple as isolated or “floating” from all other thermocouples.
etryandhydrometrythatappearinTerminologyE344applyto
Neither the extension wire nor thermoelement shall be con-
this guide. These terms are compensating extension wire, emf,
nected to a common ground at the instrumentation end of the
extension wire, ice point, measuring junction, reference
temperature-measuring system.
junction, reference temperature, sheathed thermocouple,
7.1.2 Instrumentation shall have a high common mode
thermocouple, thermocouple emf, thermoelectric properties,
rejection ratio because of the proximity of strong electromag-
thermoelement, and true value of a temperature.
netic fields from the heating elements of the furnace.
4. Classification
7.2 Temperature Measurement System—The resolution of
the temperature-measuring system should be 60.1 °F
4.1 The following thermocouple types are identified in
Tables E230/E230M: (0.05°C) or better. The uncertainty of the indicated tempera-
ture should be no more than 61.0°F (0.5°C) In addition,
4.1.1 Type K—Nickel-10 % chromium (+) versus nickel-5
% (aluminum, silicon) (−), where specific corrections for the calibration of individual
thermocouples or a thermocouple lot are required, the capabil-
4.1.2 Type N—Nickel-14 % chromium, 1.5 % silicon (+)
versus nickel-4.5 % silicon-0.1 % magnesium (−), ityofthetemperature-measuringsystemtoaccommodatethese
4.1.3 Type R—Platinum-13 % rhodium (+) versus platinum data shall be considered.
(−),
7.2.1 Reference Junction Compensation:
4.1.4 Type S—Platinum-10 % rhodium (+) versus platinum
7.2.1.1 Unless an ice point reference junction is used,
(−).
provide some means to compensate for the temperature where
the thermoelectric circuit connects to the instrumentation end
5. Summary of Guide
of the temperature-measuring system.
5.1 This guide will help the user to conduct an elevated-
NOTE 2—Thermocouples are usually calibrated to a 32°F (0°C)
temperature mechanical property test with the highest degree
reference temperature. Refer to MNL-12 on Reference Junctions.
of temperature precision available. It provides information on NOTE 3—Reference junction compensation is usually performed within
the instrumentation of the temperature-measuring system itself. Most
the proper application of thermocouples that are used to
temperature-measuring systems measure the temperature where the ther-
measure and control the temperature of the test specimen. It
moelements or extension wires connect to the input terminals and
also points out sources of error and suggests methods to
introduceacompensatingemftosimulatetheicepointreferencejunction.
eliminate them.
7.2.1.2 The input terminals shall be isothermal and shielded
from sudden changes of temperature.
6. Significance and Use
7.2.2 Calibration and Verification—The temperature-
6.1 This guide presents techniques on the use of thermo-
measuring system shall be periodically verified so that the
couples and associated equipment for measuring temperature
temperature errors are consistent with the specified require-
in elevated-temperature mechanical testing under typical con-
ments for the accuracy of the indicated temperature.
ditions. Test durations range from minutes for tension,
7.2.2.1 The accuracy of the verification instrument shall be
compression,Young’smodulus,andsuperplasticpropertytests
higher than that of the temperature-measuring system, and its
to thousands of hours for creep, stress-rupture, creep rupture,
calibration should exhibit metrological traceability to the
notch tension, and stress-relaxation tests.
International System of Units (SI).
6.2 Since elevated-temperature mechanical properties are
7.2.2.2 The period between calibration or verification of the
highlysensitivetotemperature,usersshouldmakeeveryeffort
temperature-measuring system should be based on age, use,
practicable to make accurate temperature measurements and
and the drift exhibited between calibrations.
provide stable control of the test temperature. The goal of this
NOTE 4—The accuracy of the temperature-measuring system can be
guide is to provide users with good pyrometric practice and
affected by component aging, environment, handling, or wear. Frequent
techniques for precise temperature control for elevated-
verifications can help determine the period between calibrations.
temperature mechanical testing.
7.2.2.3 Calibrations and verifications should include mul-
6.3 Techniques are given in this guide for maintaining a
tiple points and extend over the intended range of use.
stable temperature throughout the period of test.
6.4 If the techniques of this guide are followed, the differ-
ence between indicated temperature and true temperature, as
Manual on the Use of Thermocouples in Temperature Measurement, Fourth
used in Test Methods E21, E111, E139, E292, E328, E2448,
Edition, 1993. Sponsored by ASTM International Committee E20 on Temperature
andPracticeE209willbereducedtothelowestpracticallevel. Measurement.ASTM International MNL-12–4th. DOI: 10.1520/MNL12–4TH-EB.
E633 − 21a
7.3 Temperature-Control System Requirements—Atempera- 8.2.2 Control—Thermocouples used for control shall be
ture controller or temperature-control system should be se- selected and placed to be sensitive to changes of or impending
lected on the basis of stability (variations of 61°F (0.5°C) or changes to the temperature of the specimen.
less), and accuracy (uncertainty of 61.5°F (0.7°C) or less).
8.2.2.1 Control thermocouple thermoelements should be as
Generally, a temperature-control system with proportional
small in diameter as possible. However, the diameter should
band, automatic reset, and slow approach to final set point
not be so small that oxidation or strain causes thermocouple
should be used. When employing an automatic feedback-
emf errors or failure during the test period.
control system, the tuning constants or control algorithm shall
8.2.2.2 A control thermocouple should not be located next
be optimized, not only to maintain the test specimen at the set
to the heater as a means to limit fluctuations of temperature.
pointwithoutexcessivedeviations,buttoeliminateorlimitthe
NOTE 7—Atemperature-control system with a wide proportional band
amount of overshoot upon initial heating.
andautomaticresetiscapableofcompensatingforthethermallagofmost
7.3.1 Follow the requirements of 7.2.1 regarding reference
furnace designs
junction compensation in the temperature-control system.
8.2.3 If permitted by the test method standard that cites
7.3.2 Configuration—The configuration of the temperature-
E633, the measurement and control requirements may be met
control system shall be capable of meeting the indicated
with one set of thermocouples that is judiciously chosen and
temperature variation and configuration requirements of the
placed.
test method that cites E633.
8.3 Basic Information—Atmosphere, temperature of
NOTE 5— Many different configurations are used successfully. For
exposure, duration of testing, and response time shall be
example, in one configuration, the center thermocouple is connected to a
considered to determine type (K, N, R, or S), thermoelement
control loop that strives to maintain the temperature of the center of the
size, insulation, and installation methods.
reduced parallel section at the set point. The upper and lower thermo-
couples measure the indicated temperatures at the ends of the reduced
NOTE 8—Information on basic thermocouple characteristics and per-
parallel section. Independent means are provided to adjust the heating
formance is available from ASTM publications such as MNL-12.
power above and below the center to equalize the temperatures.
Stability of the emf over the period of test is the most crucial requirement
In another configuration, the upper and lower thermocouples are
for a control thermocouple, whereas accuracy of the measurement
connected to control loops that regulate the power to the upper and lower
thermocouple is paramount for successful correlation of test results.
heaters of a two-zone furnace independently. The center thermocouple
measures the indicated temperature. Furnaces with three independently
8.4 Precautions—Thermocouples should be handled care-
controlled zones are also used successfully.
fully without unnecessary stretching, bending, or twisting the
7.3.3 Temperature-Control System Calibration, Verification,
wires. Bending around small radii should be avoided entirely,
and Reliability—The temperature-control system should be especially where the wires may lie in a temperature gradient.
routinely verified, as circumstances and type of equipment
Where bending is necessary, the bend radius should be at least
dictate. The verification procedure should include verification, 20 times the diameter of the wire.
and if necessary, calibration of the temperature-control system
NOTE 9—The thermocouple emf is affected by inhomogeneities in the
and a sensitivity check. A calibration circuit, as shown in
region of a temperature gradient. Inhomogeneities are produced by cold
Appendix X1, should be employed.
work, contamination, or metallurgical changes produced by temperature
itself.
7.4 Heating Equipment—Furnaces should be well insulated
8.5 Types K and N Thermocouples:
andappropriatelysizedoradjustedrelativetotheworkloadand
8.5.1 Suitability:
heatlossestoprovideazoneofuniformtemperatureacrossthe
specimen. The top and bottom openings should be closed to
NOTE 10—Types K and N thermocouples are useful for elevated-
limit convection losses, but the furnace should not be sealed
temperaturemechanicaltestingbecausetheyprovidearelativelyhighemf
airtight.
and are relatively stable over the low and middle temperatures of the
testing range.
8. Thermocouples
8.5.2 Limitations—Any portion of Type K or Type N
thermoelement that has been exposed to temperatures above
8.1 The stability of the thermocouple emf and the rapid
500°F (260°C) or 1600°F (870°C) for Types K and N
response of the temperature-control system to any changes of
thermocouples,respectively,shouldbediscardedafteroneuse.
temperature over the period of the test shall be suitable to
maintain the specimen within the specified temperature band.
NOTE 11—Conventional Type K, and to a lesser extent, Type N
thermocouples undergo emf drift as the result of metallurgical changes
8.2 Thermocouple Requirements:
during use.
8.2.1 Measurement—Thermocouples used for measurement
8.5.3 Assessment—Type K or N thermocouples should be
shall be capable of representing the temperature of the speci-
reused only after their suitability for a particular test program
men along its reduced parallel section. Since only two or three
is proven by a body of test data. Stability tests, using Type R
thermocouples are used, they shall be located at places on the
or S thermocouples as references should be conducted to
specimen that represent the average temperature of their
establish their suitability.
respective sections.
8.6 Types R and S Thermocouples:
NOTE 6—Aprofile test program, where more than the usual number of
8.6.1 Suitability—Because they do not deteriorate during
thermocouples are mounted along the reduced parallel section can help to
establish the temperature profile along the test specimen. normal use,Type R andType S thermocouples may be reused.
E633 − 21a
NOTE 12—Types R and S are highly resistant to oxidation and are TABLE 1 Thermocouple and Extension Wire Tolerances and
Calibration Uncertainties
therefore stable for these tests at the higher temperatures of the range.
They provide the highest reproducibility and repeatability of the several
Thermocouples, °F
thermocouple types but are initially more costly. When Types R and S
Typical
Temperature
wires are no longer suitable for service, they still retain a significant
Type Tolerance Calibration
Range
A
portion of their initial cost in salvage value. Uncertainty
B
Standard Special
8.6.1.1 The calibration of Type R or Type S thermocouples
K, N 32 to 2000 4 or 0.75 % 2 or 0.4 % 1.8 to 2.2
S, R 32 to 2000 2.7 or 0.25 % 1 or 0.1 % 1.1 to 1.2
that have been exposed to or in contact with chromium-
containing alloys should be verified before they are reused.
C
Extension Wire, °F
D
KX, NX 32 to 400 4 0.5to2
NOTE 13—Type R and Type S thermocouples degrade significantly at
Compensating Extension Wire
temperatures above 1650°F (900°C) when attached to chromium-
D
RX, SX 32 to 400 9 2to4
containing alloys because the chromium vaporizes and contaminates the
Thermocouples, °C
thermocouple. The net result at 2200 °F (1200 °C) can be an error of up
Typical
Temperature
to 14 °F (8 °C).
Type Tolerance Calibration
Range
A
Uncertainty
8.6.2 Limitations—Type R and Type S thermocouples shall
B
Standard Special
be adequately supported to avoid straining them. Sufficient
K, N 0 to 1100 2.2 or 0.75 % 1.1 or 0.4 % 1 to 1.2
R, S 0 to 1100 1.5 or 0.25 % 0.6 or 0.1 % 0.6 to 0.7
slack should be provided so that the wires do not strain or tear
when the specimen elongates. Care should be exercised to C
Extension Wire, °C
D
avoidcontaminatingthethermocouple byoils,grease,orother
KX, NX 0 to + 200 2 0.3to1
Compensat
...

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: E633 − 21 E633 − 21a
Standard Guide for
Use of Thermocouples in Elevated-Temperature Mechanical
Testing
This standard is issued under the fixed designation E633; 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
This guide provides basic information, options, and guidelines to enable the user to apply
thermocouples, temperature measurement, and control equipmenttemperature-measuring systems, and
temperature-control systems with sufficient accuracy to satisfy the temperature requirements for
elevated-temperature mechanical testing of materials.
1. Scope*
1.1 This guide covers the use of ANSI thermocouple Types listed in Specification E230/E230M for elevated-temperature
mechanical testing at temperatures typical in testing of metals.
NOTE 1—Typical conditions for mechanical testing are specified temperatures up to 1800 °F (982 °C) in air at one atmosphere of pressure.
1.1.1 This guide focuses on most commonly used base metal and noble metal ANSI type K, N, R, and S thermocouples.
1.1.2 Other ANSI thermocouple types that are listed in Specification E230/E230M may be used if the specific precautions and
maximum temperatures for their use are followed.
1.1.3 It does not cover the use of sheathed thermocouples.
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 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.
This guide is under the jurisdiction of ASTM Committee E28 on Mechanical Testing and is the direct responsibility of Subcommittee E28.04 on Uniaxial Testing.
Current edition approved Jan. 1, 2021Nov. 1, 2021. Published March 2021January 2022. Originally approved in 1987. Last previous edition approved in 20132021 as
E633–13. DOI: 10.1520/E0633-21.10.1520/E0633-21A.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E633 − 21a
2. Referenced Documents
2.1 ASTM Standards:
E6 Terminology Relating to Methods of Mechanical Testing
E21 Test Methods for Elevated Temperature Tension Tests of Metallic Materials
E111 Test Method for Young’s Modulus, Tangent Modulus, and Chord Modulus
E139 Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials
E209 Practice for Compression Tests of Metallic Materials at Elevated Temperatures with Conventional or Rapid Heating Rates
and Strain Rates
E220 Test Method for Calibration of Thermocouples By Comparison Techniques
E230/E230M Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
E292 Test Methods for Conducting Time-for-Rupture Notch Tension Tests of Materials
E328 Test Methods for Stress Relaxation for Materials and Structures
E344 Terminology Relating to Thermometry and Hydrometry
E574 Specification for Duplex, Base Metal Thermocouple Wire With Glass Fiber or Silica Fiber Insulation
E1129/E1129M Specification for Thermocouple Connectors
E1684 Specification for Miniature Thermocouple Connectors
E2448 Test Method for Determining the Superplastic Properties of Metallic Sheet Materials
3. Terminology
3.1 Definitions—Unless otherwise indicated, the definitions given Definitions of terms common to mechanical testing that appear
in Terminology E6 apply to this standard, including accuracy, calibration, indicated temperature, metrological traceability, reduced
parallel section, temperature-measuring system, specified temperature, and E344 shall apply.verification.
3.2 Some specific definitions of terms relating to thermometry and hydrometry that appear in Terminology E344 apply to this
guide. These terms are compensating extension wire, emf, extension wire, ice point, measuring junction, reference junction,
reference temperature, sheathed thermocouple, thermocouple, thermocouple emf, thermoelectric properties, thermoelement, and
true value of a temperature.
4. Classification
4.1 The following thermocouple types are identified in Tables E230/E230M:
4.1.1 Type K—Nickel—10 % Nickel-10 % chromium ( + )(+) versus nickel—5 % nickel-5 % (aluminum, silicon) (−),
4.1.2 Type N—Nickel—14 % chromium, 1.5 % silicon ( + ) versus nickel—4.5 % silicon—0.1 % Nickel-14 % chromium, 1.5 %
silicon (+) versus nickel-4.5 % silicon-0.1 % magnesium (−),
4.1.3 Type R—Platinum—13 % Platinum-13 % rhodium ( + )(+) versus platinum (−),
4.1.4 Type S—Platinum—10 % Platinum-10 % rhodium ( + )(+) versus platinum (−).
5. Summary of Guide
5.1 This guide will help the user to conduct an elevated-temperature mechanical property test with the highest degree of
temperature precision available. It provides information on the proper application of thermocouples that are used to measure and
control the temperature of the test specimen. It also points out sources of error and suggests methods to eliminate them.
6. Significance and Use
6.1 This guide presents techniques on the use of thermocouples and associated equipment for measuring temperature in
elevated-temperature mechanical testing under typical conditions. Test durations range from minutes for tension, compression,
Young’s modulus, and superplastic property tests to thousands of hours for creep, stress-rupture, creep rupture, notch tension, and
stress-relaxation tests.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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6.2 Since elevated-temperature mechanical properties are highly sensitive to temperature, users should make every effort
practicable to make accurate temperature measurements and provide stable control of the test temperature. The goal of this guide
is to provide users with good pyrometric practice and techniques for precise temperature control for elevated-temperature
mechanical testing.
6.3 Techniques are given in this guide for maintaining a stable temperature throughout the period of test.
6.4 If the techniques of this guide are followed, the difference between indicated temperature and true temperature, as used in Test
Methods E21, E111, E139, E292, E328, E2448, and Practice E209 will be reduced to the lowest practical level.
7. Apparatus
7.1 Instrumentation may be individual instruments, a data acquisition system (multipoint recorders or digital type), a
computer-based control system, or a combination of these devices.
NOTE 2—Since each thermocouple is “grounded” by contacting the specimen, it is necessary that the instrumentation treat each thermocouple as isolated
or “floating” from all other thermocouples. Neither leg should be connected to a common ground at the instrumentation end of the system. Also,
equipment having a high common mode rejection ratio is necessary because of the proximity of strong electromagnetic fields from the heating elements
of the furnace.
7.1.1 Since each thermocouple is “grounded” by contacting the specimen, the instrumentation shall treat each thermocouple as
isolated or “floating” from all other thermocouples. Neither the extension wire nor thermoelement shall be connected to a common
ground at the instrumentation end of the temperature-measuring system.
7.1.2 Instrumentation shall have a high common mode rejection ratio because of the proximity of strong electromagnetic fields
from the heating elements of the furnace.
7.2 Temperature Measurement Instrumentation—System—The measurement resolution of the temperature-measuring system
should be able to resolve the thermocouple signal to 60.1 °F (0.05 °C). The temperature indication should have60.1 °F (0.05 °C)
or better. The uncertainty of the indicated temperature should be no more than 61.0 °F (0.5 °C) uncertainty for the purposes of
this test. In addition, where specific corrections for the calibration of individual thermocouples or a thermocouple lot are required,
the capability of the instrumentationtemperature-measuring system to accommodate these data shall be considered.
7.2.1 Reference Junction Compensation:
7.2.1.1 Thermocouples are usually calibrated to a 32 °F (0 °C) reference temperature. Unless an ice point reference is used,
provide some means to compensate for the temperature where the thermoelectric circuit connects to the instrument; refer to
MNL-12 on Reference Junctions.
7.2.1.1 Reference junction compensation is usually performed within the instrumentation itself. Most devices or electronic data
acquisition systems measure the Unless an ice point reference junction is used, provide some means to compensate for the
temperature where the thermoelements connect to the input terminals and introduce a compensating emf to simulate the ice
point.thermoelectric circuit connects to the instrumentation end of the temperature-measuring system.
NOTE 2—Thermocouples are usually calibrated to a 32 °F (0 °C) reference temperature. Refer to MNL-12 on Reference Junctions.
NOTE 3—Reference junction compensation is usually performed within the instrumentation of the temperature-measuring system itself. Most
temperature-measuring systems measure the temperature where the thermoelements or extension wires connect to the input terminals and introduce a
compensating emf to simulate the ice point reference junction.
7.2.1.2 The input connectionsterminals shall be isothermal and shielded from sudden changes of temperature.
7.2.2 Recalibration—Calibration and Verification—The accuracy of the temperature measurement equipment may be affected by
Manual on the Use of Thermocouples in Temperature Measurement, Fourth Edition, 1993. Sponsored by ASTM International Committee E20 on Temperature
Measurement. ASTM International MNL-12–4th. DOI: 10.1520/MNL12–4TH-EB.
E633 − 21a
component aging, environment, handling, or wear. Therefore, a periodic recalibration of the measuring instrumentation with a
checking instrument is necessary. The checking instrument should be of higher accuracy than the measurement system, and to
ensure conformity to national standards, it should be calibrated with a known primary test standard, traceable to a national
metrology institute.temperature-measuring system shall be periodically verified so that the temperature errors are consistent with
the specified requirements for the accuracy of the indicated temperature.
7.2.2.1 The accuracy of the verification instrument shall be higher than that of the temperature-measuring system, and its
calibration should exhibit metrological traceability to the International System of Units (SI).
7.2.2.2 The period between calibration or verification of the temperature-measuring system should be based on age, use, and the
drift exhibited between calibrations.
NOTE 4—The accuracy of the temperature-measuring system can be affected by component aging, environment, handling, or wear. Frequent verifications
can help determine the period between calibrations.
7.2.2.3 Calibrations and verifications should include multiple points and extend over the intended range of use.
7.3 Temperature Control Equipment Temperature-Control System Requirements—A temperature controller or temperature control
temperature-control system should be selected on the basis of stability (variations of 61 °F (0.5 °C) or less), and accuracy
(uncertainty of 61.5 °F (0.7 °C) or less). Generally, a controltemperature-control system with proportional band, automatic reset,
and slow approach to final set point features should be used. When employing an automatic feedback control feedback-control
system, the tuning constants or control algorithm shall be optimized, not only to maintain the test specimen at the set point without
excessive deviations, but to eliminate or limit the amount of overshoot upon initial heating.
NOTE 3—The same precautions regarding reference junction compensation in the control device apply as in 7.2.1.
7.3.1 Follow the requirements of 7.2.1 regarding reference junction compensation in the temperature-control system.
7.3.2 Configuration—The control configuration may take one of two forms:configuration of the temperature-control system shall
be capable of meeting the indicated temperature variation and configuration requirements of the test method that cites E633.
7.3.1.1 The center thermocouple is connected to a control loop that strives to maintain the temperature of the center of the reduced
section at set point. The upper and lower thermocouples are used to measure the temperatures at the ends of the reduced section.
Means shall be provided to adjust the heating power above and below the center to equalize the temperatures.
NOTE 5— Many different configurations are used successfully. For example, in one configuration, the center thermocouple is connected to a control loop
that strives to maintain the temperature of the center of the reduced parallel section at the set point. The upper and lower thermocouples measure the
indicated temperatures at the ends of the reduced parallel section. Independent means are provided to adjust the heating power above and below the center
to equalize the temperatures.
In another configuration, the upper and lower thermocouples are connected to control loops that regulate the power to the upper and lower heaters of
a two-zone furnace independently. The center thermocouple measures the indicated temperature. Furnaces with three independently controlled zones are
also used successfully.
7.3.1.2 The bottom and top thermocouples may be connected to control loops that regulate the power to the upper and lower
heaters independently. Thus, the end temperatures are maintained automatically. The center thermocouple is used only as a monitor.
7.3.3 ControlTemperature-Control System Recalibration Calibration, Verification, and Reliability—The controltemperature-
control system should be subjected to routine recalibration, routinely verified, as circumstances and type of equipment dictate. The
checkingverification procedure should include verification, and if necessary, calibration of the controller temperature-control
system and a sensitivity check. A calibration circuit, as shown in Appendix X1, should be employed.
7.4 Heating Equipment—Furnaces should be well insulated and appropriately sized or adjusted relative to the workload and heat
losses to provide a zone of uniform temperature across the specimen. Because elevated-temperature mechanical testing is usually
done at constant temperature and with an unchanging furnace load, the main requirement is a well-insulated furnace, capable of
achieving the desired temperatures. The top and bottom openings should be closed to limit convection losses, but the furnace
should not be sealed airtight.
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8. Thermocouples
8.1 The stability of the emf of the thermocouples thermocouple emf and the rapid response of the controltemperature-control
system to any changes of temperature over the period of the test are crucial shall be suitable to maintain the specimen within the
allowablespecified temperature band.
8.2 Thermocouple Requirements—Requirements: The requirements for thermocouples used for measurement are somewhat
different from thermocouples used for control purposes (especially with automatic feedback control systems). Of course, both
requirements may be met with one set of thermocouples that is judiciously chosen and placed.
8.2.1 Measurement—Thermocouples used for measurement are designed to represent shall be capable of representing the
temperature of the specimen along its reduced parallel section. Since only two or three thermocouples are used, they shall be
located at places on the specimen that represent the average temperature of their respective sections. This can be determined by
a test program, where more than the usual number of thermocouples are mounted along the reduced section to establish the
temperature profile.
NOTE 6—A profile test program, where more than the usual number of thermocouples are mounted along the reduced parallel section can help to establish
the temperature profile along the test specimen.
8.2.2 Control—Thermocouples used for control are designed shall be selected and placed to be sensitive to changes of or
impending changes to the temperature of the specimen. Control thermocouple wire should be as small in diameter as possible.
However, the wire should not be so small in diameter that oxidation or strain would cause emf errors or failure during the test
period.
NOTE 4—Locating a control thermocouple next to the heater as a means to limit fluctuations of temperature is not advisable. A controller with a wide
proportional band and automatic reset is capable of compensating for the thermal lag of most furnace designs.
8.2.2.1 Control thermocouple thermoelements should be as small in diameter as possible. However, the diameter should not be
so small that oxidation or strain causes thermocouple emf errors or failure during the test period.
8.2.2.2 A control thermocouple should not be located next to the heater as a means to limit fluctuations of temperature.
NOTE 7— A temperature-control system with a wide proportional band and automatic reset is capable of compensating for the thermal lag of most furnace
designs
8.2.3 If permitted by the test method standard that cites E633, the measurement and control requirements may be met with one
set of thermocouples that is judiciously chosen and placed.
8.3 Basic Information—Information on basic thermocouple characteristics and performance is available from ASTM publications
such as MNL-12.Atmosphere, temperature Stability of the emf over the period of test is the most crucial requirement for a control
thermocouple, whereas accuracy of the measurement thermocouple is paramount for successful correlation of test results.of
exposure, duration of testing, and response time shall be considered to determine type (K, N, R, or S), thermoelement size,
insulation, and installation methods.
8.3.1 Factors affecting thermocouple selection are: atmosphere, temperature of exposure, duration of testing, and response time.
These factors should be considered to determine type (K, N, R, or S), wire size, insulation, and installation methods.
NOTE 8—Information on basic thermocouple characteristics and performance is available from ASTM publications such as MNL-12. Stability of the emf
over the period of test is the most crucial requirement for a control thermocouple, whereas accuracy of the measurement thermocouple is paramount for
successful correlation of test results.
8.4 Precautions—The emf output of a thermocouple is affected by inhomogeneities in the region of a temperature gradient.
Inhomogeneities are produced by cold work, contamination, or metallurgical changes produced by temperature itself. Therefore,
thermocouples Thermocouples should be handled carefully without unnecessary stretching, bending, or twisting the wires. Bending
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around small radii should be avoided entirely, especially where the wires may lie in a temperature gradient. Where necessary, a
minimum amount of bending may be performed carefully around bend radiibending is necessary, the bend radius should be at least
20 times the diameter of the wire.
NOTE 9—The thermocouple emf is affected by inhomogeneities in the region of a temperature gradient. Inhomogeneities are produced by cold work,
contamination, or metallurgical changes produced by temperature itself.
8.5 Types K and N Thermocouples:
8.5.1 Suitability—Suitability: Types K and N thermocouples are useful for elevated-temperature mechanical testing because they
provide a relatively high emf and are relatively stable over the low and middle temperatures of the testing range.
NOTE 10—Types K and N thermocouples are useful for elevated-temperature mechanical testing because they provide a relatively high emf and are
relatively stable over the low and middle temperatures of the testing range.
8.5.2 Limitations—Conventional Type K, and to a lesser extent, Type N thermocouples undergo emf drift as the result of
metallurgical changes during use. Therefore, that portion of wire that Any portion of Type K or Type N thermoelement that has
been exposed to temperatures above 500 °F (260 °C) or 1600 °F (870 °C) for Types K and N thermocouples, respectively, should
be discarded after one use.
NOTE 11—Conventional Type K, and to a lesser extent, Type N thermocouples undergo emf drift as the result of metallurgical changes during use.
8.5.3 Assessment—Type K or N thermocouples should be reused only after their suitability for a particular test program is proven
by a body of test data. Stability tests are advised, tests, using Type R or S thermocouples as references.references should be
conducted to establish their suitability.
8.6 Types R and S Thermocouples:
8.6.1 Suitability—Types R and S are highly resistant to oxidation and are therefore stable for these tests at the higher temperatures
of the range. They provide the highest reproducibility and repeatability of the several thermocouple types but are initially more
costly. Because they do not deteriorate during normal use, it is possible to reuse them. When Types R and S wires are no longer
suitable for service, they still retain a significant portion of their initial cost in salvage value.Type R and Type S thermocouples
may be reused.
NOTE 12—Types R and S are highly resistant to oxidation and are therefore stable for these tests at the higher temperatures of the range. They provide
the highest reproducibility and repeatability of the several thermocouple types but are initially more costly. When Types R and S wires are no longer
suitable for service, they still retain a significant portion of their initial cost in salvage value.
8.6.1.1 The calibration of Type R or Type S thermocouples that have been exposed to or in contact with chromium-containing
alloys should be verified before they are reused.
NOTE 13—Type R and Type S thermocouples degrade significantly at temperatures above 1650 °F (900 °C) when attached to chromium-containing alloys
because the chromium vaporizes and contaminates the thermocouple. The net result at 2200 °F (1200 °C) can be an error of up to 14 °F (8 °C).
8.6.2 Limitations—The limiting factor for reuse of Type R or S thermocouples is error introduced by strain or contamination.
Wires of Types R and S are mechanically weaker than Types K and N, so they must Type R and Type S thermocouples shall be
adequately supported to avoid straining them. Sufficient slack should be provided so that the wires do not strain or tear when the
specimen elongates. Contamination of the wireCare should be exercised to avoid contaminating the thermocouple may be caused
by oils, grease, or other chemi
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