Standard Guide for Use of Thermocouples in Creep and Stress-Rupture Testing to 1800°F (1000°C) in Air

SIGNIFICANCE AND USE
This guide presents techniques on the use of thermocouples and associated equipment for measuring temperature in creep and stress-rupture testing in air at temperatures up to 1800°F (1000°C).
Since creep and stress-rupture 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 creep and stress-rupture testing.  
Techniques are given in this guide for maintaining a stable temperature throughout the period of test.
If the techniques of this guide are followed, the difference between “indicated”3 temperature and “true”4 temperature will be reduced to the lowest practical level.
SCOPE
1.1 This guide covers the use of ANSI thermocouple Types K, N, R, and S for creep and stress-rupture testing at temperatures up to 1800°F (1000°C) in air at one atmosphere of pressure. It does not cover the use of sheathed thermocouples.
1.2 The values stated in inch-pound units are to be regarded as the standard. The values given in parentheses are for information only.
1.3This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

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Status
Historical
Publication Date
30-Apr-2005
Technical Committee
Drafting Committee
Current Stage
Ref Project

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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E633 − 00(Reapproved 2005)
Standard Guide for
Use of Thermocouples in Creep and Stress-Rupture Testing
to 1800°F (1000°C) in Air
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 measurement, and control equipment with sufficient accuracy to satisfy
the temperature requirements for creep and stress-rupture testing of materials.
1. Scope E344Terminology Relating to Thermometry and Hydrom-
etry
1.1 This guide covers the use ofANSI thermocouple Types
E574Specification for Duplex, Base Metal Thermocouple
K, N, R, and S for creep and stress-rupture testing at tempera-
Wire With Glass Fiber or Silica Fiber Insulation
tures up to 1800°F (1000°C) in air at one atmosphere of
E1129/E1129MSpecification for Thermocouple Connectors
pressure. It does not cover the use of sheathed thermocouples.
E1684Specification for Miniature Thermocouple Connec-
1.2 Thevaluesstatedininch-poundunitsaretoberegarded
tors
as the standard. The values given in parentheses are for
information only. 3. Terminology
1.3 This standard does not purport to address all of the 3.1 Definitions—Unless otherwise indicated, the definitions
safety concerns, if any, associated with its use. It is the
given in Terminology E344 shall apply.
responsibility of the user of this standard to establish appro-
4. Classification
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. 4.1 The following thermocouple types are identified in
Tables E230:
2. Referenced Documents
4.1.1 Type K—Nickel—10 % chromium ( + ) versus
nickel—5% (aluminum, silicon) (−),
2.1 ASTM Standards:
4.1.2 Type N—Nickel—14% chromium, 1.5% silicon (+)
E139Test Methods for Conducting Creep, Creep-Rupture,
versus nickel—4.5% silicon—0.1% magnesium (−),
and Stress-Rupture Tests of Metallic Materials
4.1.3 Type R—Platinum—13% rhodium (+) versus plati-
E207TestMethodforThermalEMFTestofSingleThermo-
num (−),
element Materials by Comparison with a Reference Ther-
4.1.4 Type S—Platinum—10% rhodium (+) versus plati-
moelement of Similar EMF-Temperature Properties
num (−).
E220Test Method for Calibration of Thermocouples By
Comparison Techniques
5. Summary of Guide
E230Specification and Temperature-Electromotive Force
5.1 This guide will help the user to conduct a creep or
(EMF) Tables for Standardized Thermocouples
stress-rupture test with the highest degree of temperature
E292Test Methods for ConductingTime-for-Rupture Notch
precision available. It provides information on the proper
Tension Tests of Materials
application of thermocouples that are used to measure and
control the temperature of the test specimen. It also points out
This guide is under the jurisdiction of ASTM Committee E28 on Mechanical
sources of error and suggests methods to eliminate them.
TestingandisthedirectresponsibilityofSubcommitteeE28.04onUniaxialTesting.
Current edition approved May 1, 2005. Published May 2005. Originally
6. Significance and Use
approved in 1987. Last previous edition approved in 2000 as E633–00. DOI:
10.1520/E0633-00R05.
6.1 This guide presents techniques on the use of thermo-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
couples and associated equipment for measuring temperature
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in creep and stress-rupture testing in air at temperatures up to
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. 1800°F (1000°C).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E633 − 00 (2005)
6.2 Since creep and stress-rupture properties are highly knownprimaryteststandard,traceabletotheNationalInstitute
sensitive to temperature, users should make every effort of Standards and Technology.
practicable to make accurate temperature measurements and
7.3 Temperature Control Equipment Requirements—Atem-
provide stable control of the test temperature. The goal of this
perature controller or temperature control system should be
guide is to provide users with good pyrometric practice and
selectedonthebasisofstability(variationsof 61°F(0.5°C)or
techniques for precise temperature control for creep and
less), and accuracy (uncertainty of 61.5°F (0.7°C) or less).
stress-rupture testing.
Generally, a control system with proportional band, automatic
6.3 Techniques are given in this guide for maintaining a
reset, and slow approach to final set point features should be
stable temperature throughout the period of test.
used. When employing an automatic feedback control system,
the tuning constants or control algorithm shall be optimized,
6.4 If the techniques of this guide are followed, the differ-
3 4
not only to maintain the test specimen at the set point without
ence between “indicated” temperature and “true” tempera-
excessive deviations, but to eliminate or limit the amount of
ture will be reduced to the lowest practical level.
overshoot upon initial heating.
7. Apparatus
NOTE 1—The same precautions regarding reference junction compen-
7.1 Instrumentation may be individual instruments, a data sation in the control device apply as in 7.2.1.
acquisition system (multipoint recorders or digital type), a
7.3.1 Configuration—The control configuration may take
computer-based control system, or a combination of these
one of several forms:
devices. (Warning—Since each thermocouple is “grounded”
7.3.1.1 The center thermocouple is connected to a control
by contacting the specimen, it is necessary that the instrumen-
loopthatstrivestomaintainthetemperatureofthecenterofthe
tationtreateachthermocoupleasisolatedor“floating”fromall
reduced section at set point. The upper and lower thermo-
other thermocouples. Neither leg should be connected to a
couplesareusedtomeasurethetemperaturesattheendsofthe
commongroundattheinstrumentationendofthesystem.Also,
reduced section. Means shall be provided to adjust the heating
equipment having a high common mode rejection ratio is
poweraboveandbelowthecentertoequalizethetemperatures.
necessary because of the proximity of strong electromagnetic
7.3.1.2 The bottom and top thermocouples may be con-
fields from the heating elements of the furnace.)
nectedtocontrolloopsthatregulatethepowertotheupperand
7.2 Temperature Measurement Instrumentation—The mea-
lower heaters independently. Thus, the end temperatures are
surement system should be able to resolve the thermocouple
maintained automatically. The center thermocouple is used
signal to 60.1°F (0.05°C). The temperature indication should
only as a monitor.
havenomorethan 61.0°F(0.5°C)uncertaintyforthepurposes
7.3.2 Control System Recalibration and Reliability—The
of this test. In addition, where specific corrections for the
control system should be subjected to routine recalibration, as
calibration of individual thermocouples or a thermocouple lot
circumstances and type of equipment dictate. The checking
is required, the capability of the instrumentation system to
procedure should include calibration of the controller and a
accommodate these data shall be considered.
sensitivity check. A calibration circuit, as shown in Appendix
7.2.1 Reference Junction Compensation:
X1, should be employed.
7.2.1.1 Thermocouples are usually calibrated to a 32°F
7.4 Heating Equipment—Furnaces should be appropriately
(0°C) reference temperature. Unless an ice point reference is
sized or adjusted relative to the workload and heat losses to
used, some means must be provided to compensate for the
provide a zone of uniform temperature across the specimen.
temperature where the thermoelectric circuit connects to the
Because creep and stress-rupture testing is usually done at
instrument (refer to MNL-12 on Reference Junctions ).
constanttemperatureandwithanunchangingfurnaceload,the
7.2.1.2 Reference junction compensation is usually per-
main requirement is a well-insulated furnace, capable of
formed within the instrumentation itself. Most devices or
achieving the desired temperatures. The top and bottom open-
electronic data acquisition systems measure the temperature
ings should be closed to limit convection losses, but the
where the thermoelements connect to the input terminals and
furnace should not be sealed airtight.
introduce a compensating emf to simulate the ice point.
7.2.1.3 The input connections shall be isothermal and
8. Hazards
shielded from sudden changes of temperature.
7.2.2 Recalibration—The accuracy of the temperature mea-
8.1 The duration of a creep test ranges from a few hours to
surement equipment may be affected by component aging,
several hundred hours at elevated temperatures, at least par-
environment, handling, or wear. Therefore, a periodic recali-
tially unattended by operators. Such tests are normally ended
bration of the measuring instrumentation with a checking
beforetestspecimenfailure.Stress-rupturetestsmayoperateat
instrument is necessary. The checking instrument should be of
higher stresses, higher temperatures, and for shorter times than
higher accuracy than the measurement system, and to ensure
creep tests, but they normally continue until the specimen has
conformity to national standards, it should be calibrated with a
achieved its required life or has failed.
8.2 The stability of the emf of the thermocouples and the
As defined in Practice E139 and Test Methods E292.
rapid response of the control system to any changes of
As defined in Practice E139.
temperature over the period of the test are crucial to maintain
Manual on the Use of Thermocouples in Temperature Measurement, ASTM
MNL-12. the specimen within the allowable temperature band.
E633 − 00 (2005)
8.3 Thermocouple Requirements—The requirements for (260°C) or 1600°F (870°C) forTypes K and N thermocouples,
thermocouples used for measurement are somewhat different respectively, should be discarded after one use.
from thermocouples used for control purposes (especially with
automatic feedback control systems). Of course, both require-
TABLE 1 Thermocouple and Extension Wire Tolerances and
ments may be met with one set of thermocouples that is
Calibration Uncertainties
judiciously chosen and placed.
Thermocouples, °F
8.3.1 Measurement—Thermocouples used for measurement
Typical
Temperature
are designed to represent the temperature of the specimen Type Tolerance Calibration
Range
A
Uncertainty
along its reduced section. Since only two or three thermo-
B
Standard Special
couples are used, they shall be located at places on the
K, N 32 to 2000 4 or 0.75 % 2 or 0.4 % 1.8 to 2.2
specimen that represent the average temperature of their S, R 32 to 2000 2.7 or 0.25 % 1 or 0.1 % 1.1 to 1.2
respective sections. This can be determined by a test program,
C
Extension Wire, °F
where more than the usual number of thermocouples are D
KX, NX 32 to 400 4 0.5to2
D
RX, SX 32 to 400 9 2to4
mountedalongthereducedsectiontoestablishthetemperature
Thermocouples, °C
profile.
Typical
Temperature
8.3.2 Control—Thermocouples used for control are de-
Type Tolerance Calibration
Range
A
Uncertainty
signed and placed to be sensitive to changes of or impending
B
Standard Special
changes to the temperature of the specimen. Control thermo-
K, N 0 to 1100 2.2 or 0.75 % 1.1 or 0.4 % 1 to 1.2
couple wire should be as thin as possible. However, the wire
R, S 0 to 1100 1.5 or 0.25 % 0.6 or 0.1 % 0.6 to 0.7
should not be so thin that oxidation or strain would cause emf
C
Extension Wire, °C
errors or failure during the test period.
D
KX, NX 0 to + 200 2 0.3to1
D
RX, SX 0 to + 200 5 1to2
NOTE2—Locatingacontrolthermocouplenexttotheheaterasameans
A
to limit fluctuations of temperature is not advisable. A controller with a Calibration uncertainty in an actual test depends on number of test points, media,
and reference standard used during the calibration (see Test Method E220).
wideproportionalbandandautomaticresetiscapableofcompensatingfor
B
Tolerances and uncertainties are plus or minus the indicated values expressed
the thermal lag of most furnace designs.
in degrees Fahrenheit or degrees Celsius or as a percentage of the value of the
measured temperature, whichever is greater (see Tables 1, 2, and 3 in Specifica-
9. Thermocouples
tion E230).
C
Worst case, where the temperature of the transition point differs from the
9.1 Basic Information—Information on basic thermocouple
instrument’s reference junction by 360°F (200°C).
D
characteristics and performance is available from ASTM pub- No special tolerance limits have been established, but materials with tolerances
closer than the standard limits may be available.
licationssuchasMNL-12. Stabilityoftheemfovertheperiod
of test is the most crucial requirement for a control
thermocouple, whereas accuracy of the measurement thermo-
couple is paramount for successful correlation of test results.
9.3.3 Assessment—Type K or N thermocouples should be
9.1.1 Factors affecting thermocouple selection are:
reused only after their suitability for a particular test program
atmosphere, temperature of exposure, duration of testing, and
is proven by a body of test data. Stability tests are advised,
responsetime.Thesefactorsshouldbeconsideredtodetermine
using Type R or S thermocouples as references.
type (K, N, R, or S), wire size, insulation, and installation
9.4 Types R and S Thermocouples:
methods.
9.4.1 Suitability—Types R and S are highly resistant to
9.2 Precautions—The emf output of a thermocouple is
oxidation and are therefore stable for these tests at the higher
affected by inhomogeneities in the region of a temperature
temperatures of the range. They provide the highest reproduc-
gradient. Inhomogeneities are produced by cold work,
ibility and repeatability of the several thermocouple types but
contamination, or metallurgical changes produced by tempera-
areinitiallymorecostly.Becausetheydonotdeteriorateduring
ture itself. Therefore, thermocouples should be handled care-
normal use, it is possible to reuse them. When Types R and S
fully without unnecessary stretching, bending, or twisting the
wires are no longer suitable for service, they still retain a
wires. Bending around small radii should be avoided entirely,
significant portion of their initial cost in salvage value.
especially where the wires may lie in a temperature gradient.
9.4.2
...

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