ASTM E601-20

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: E601 − 15 E601 − 20
Standard Guide for
Measuring Electromotive Force (emf)(EMF) Stability of Base-
Metal Thermoelement Materials withWith Time in Air
This standard is issued under the fixed designation E601; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide provides a method for measuring the emf stability of base-metal thermoelement materials in air referenced to
platinum at specified constant elevated temperatures using dual, simultaneous, emf indicators, or using a single emf indicator, with
the test and reference emf measured alternately. This test is conducted over a period of weeks.
1.2 A calibrated platinum-rhodium/platinum thermocouple is used as a reference standard to establish the test temperature.
1.3 The useful life of a thermocouple depends on the stability of the emf generated at given temperatures for a required time
interval. This method provides a quantitative measure of the stability of individual thermoelements. By combining the results of
the positive (P) and negative (N) thermoelements, the stability of a thermocouple comprised of both P and N thermoelements may
be obtained. The emf of an individual thermoelement is measured against platinum, which may be the platinum leg of the
platinum-rhodium/platinum reference thermocouple, or an additional platinum reference.
NOTE 1—Some thermoelements may show insignificant emf drift while undergoing relatively rapid oxidation. In these cases, failure of the thermoelement
may be indicated only by a large rise in the electrical resistance between joined thermoelements, as measured at the reference junctions.
NOTE 2—See ASTM MNL 12MNL12 for recommended upper temperature limits in air.
NOTE 3—This guide is only applicable for initially new unused thermoelements. Base-metal thermoelements exposed to temperatures above 200°C 200
°C become thermoelectrically inhomogeneous, and stability testing of inhomogeneous thermoelements will give ambiguous results.
1.4 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 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.
2. Referenced Documents
2.1 ASTM Standards:
This guide is under the jurisdiction of ASTM Committee E20 on Temperature Measurement and is the direct responsibility of Subcommittee E20.11 on Thermocouples
- Calibration.
Current edition approved May 1, 2015Nov. 1, 2020. Published June 2015December 2020. Originally approved in 1977. Last previous edition approved in 20132015 as
E601 – 07a (2013).E601 – 15. DOI: 10.1520/E0601-15.10.1520/E0601-20.
Park RM, et al., Manual on the Use of Thermocouples in Temperature Measurement: Fourth Edition, ASM MNL12, ASTM International, 1993. Available from ASTM
Headquarters, 100 Barr Harbor Drive, West Conshohocken, PA 19428, www.astm.org.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E601 − 20
E220 Test Method for Calibration of Thermocouples By Comparison Techniques
E230E230/E230M Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
E344 Terminology Relating to Thermometry and Hydrometry
E563 Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature
E1159 Specification for Thermocouple Materials, Platinum-Rhodium Alloys, and Platinum
2.2 Other Referenced DocumentsNIST Document:
NIST Monograph 175
3. Terminology
3.1 Definitions—The definitions given in Terminology E344 shall apply to this guide.
3.1 Definitions:
3.1.1 The definitions given in Terminology E344 shall apply to this guide.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 emf indicator, n—an instrument that measures the emf and displays the value, for example, a digital voltmeter (DVM).
3.2.2 emf stability, n—change in emf (or in equivalent temperature) with time, with the thermocouple junctions held at fixed
temperatures and with the thermal profile along the thermoelements held constant.constant; this is also referred to in this standard
as drift.
3.2.3 half-maximum heated length, n—the distance between the tip of the temperature sensor and the position along the length of
the sensor leads or sheath where the temperature equals the average of the calibration-point and ambient temperatures.
3.2.3 gradient zone, n—the section of a thermocouple that is exposed during a measurement to temperatures in the range from
t + 0.1(t – t ) tot + 0.9(t – t ), wheret is ambient temperature andt is the temperature of the measuring junction.
amb m amb amb m amb amb m
3.2.5 reference thermocouple, n—calibrated Type S or Type R thermocouple.
3.2.4 test thermocouple, half-maximum heated length, n—thermocouple composed of the thermoelement being tested and the
platinum reference thermoelement. the distance between the tip of the temperature sensor and the position along the length of the
sensor leads or sheath where the temperature equals the average of the calibration-point and ambient temperatures.
3.2.5 normalize, v—to mathematically adjust experimental emf data acquired at a set of temperatures to values corresponding to
a common reference temperature.
3.2.6 reference thermocouple, n—calibrated Type S or Type R thermocouple.
3.2.7 test thermocouple, n—thermocouple composed of the thermoelement being tested and the platinum reference thermoele-
ment.
4. Summary of TestGuide
4.1 In this test,guide, the emf of a test thermocouple, comprised of a base-metal thermoelement relative to a platinum reference
thermoelement, is determined as a function of time for a specified test temperature and thermal profile. If care is taken to maintain
the chemical purity and annealed metallurgical state of the platinum thermoelement, the platinum will be thermoelectrically stable.
In that case, variation in this emf value is attributed to instability of the base-metal thermoelement. The emf of the reference
thermocouple (E ) is used to measure the test temperature, and the emf of the test thermocouple (E ) of the test thermocouple
ref test
is measured either simultaneously or alternately with E . The test consists of the measurement of E at specified time intervals
ref test
and at a specified constant value of E which corresponds to a specified, constant temperature, until the required time of the test
ref
is exceeded or until an open circuit in the base-metal thermoelement results.
Available from National Institute of Standards and Technology (NIST), 100 Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov.
E601 − 20
4.2 This testguide is based on Method A of Test Method E220, where the reference thermocouple of Test Method E220 becomes
the reference thermocouple used to measure the test temperature and one specified constant temperature replaces the series of
measured temperatures of Test Method temperatures. E220.
5. Significance and Use
5.1 This test is important because the accuracy of a temperature measurement by a thermocouple is directly related to the emf
stability of the thermoelements.
5.2 This test is used to verify that the tested thermoelements meet the intended requirements.
5.3 This test is useful in comparing the emf stability of two base metal thermoelements under the same conditions. The test and
reference emf may be measured either simultaneously or alternately.
5.4 The relative stabilities of base metal thermoelements determined by this test are valid only under the specified test conditions.
Results will be affected by changes in any of the following conditions: (1) temperature profile or gradient along the length of the
thermoelements;thermoelements, (2) abundance, velocity and composition of the air surrounding the test pieces;pieces, (3)
thermoelectric inhomogeneity of the test thermoelements; thermoelements, and (4) stability of the platinum thermoelement.
5.5 The test does not address the determination of base metal thermoelement stabilities over a series of temperature changes.
5.6 The reliability of this test depends on the emf stability of the reference platinum thermoelement. For testing the relative emf
stability of base-metal thermoelements, a reference element of platinum that has sufficient thermoelectric stability to determine any
significant change in emf of base-metal thermoelements shall be used. To ascertain that the experimental method protects the
platinum sufficiently from degradation, the method shall be validated by performing the procedure described in Appendix X1 prior
to the actual test.
5.7 The test result does not apply to applications in which the temperature distribution, for a given measuring junction temperature,
changes with time.
6. Apparatus
6.1 Thermocouple Used to Measure the Test Temperature—A reference Type S or Type R thermocouple with 0.50 mm diameter
(24 AWG) thermoelements or larger shall be used to measure the test temperature. The reference thermocouple shall consist of
either standard tolerance or special tolerance wire as per in accordance with Table 1 in Specification E230E230/E230M. The choice
of tolerance will not affect the determination of thermoelement drift. This thermocouple shall be of sufficient length to minimize
the effect of heat conduction along the lengths of the wires upon the measuring junction temperature. (Note: platinum (Platinum
is a better heat conductor than most base metal thermocouple wires.) The length shall be sufficient to enable the reference
thermocouple’s measuring junction to be located within the test furnace’s zone of nearly uniform temperature (refer to 6.5.2).
6.2 Platinum Reference Thermoelement—The emf of the test thermoelements shall be measured relative to a 0.50 mm diameter
(24 AWG) platinum wire. This wire may be the platinum wire of the Type S or R reference thermocouple or a second 0.50 mm
diameter (24 AWG) platinum wire. The length of this wire shall exceed that of the test specimen to minimize the transfer of heat
from the measuring junction to the reference junction during testing (see 6.3). For more information concerning a platinum
reference thermoelement, thermoelement see Specification E1159 may be consulted.
6.3 Test Specimens—The test specimens shall be lengths of wires, rods, ribbons, or strips of the coils or spools of the base-metal
thermoelements to be evaluated. Their lengths shall be adequate to minimize the transfer of heat from the measuring junctions to
the reference junctions during the period of test.testing. The lengths shall be at least 0.8 m (30(31 in.) depending on the length of
the testing medium and the transverse sizes of the thermoelements. The specimens shall be free of kinks or other defects due to
mechanical deformation, and shall be continuous without splices between the measuring and reference junctions.
6.4 Reference Junction Temperature—The reference junction ends of the test specimens, of the platinum reference element, if
used, and of the reference thermocouple shall be maintained at a known constant temperature during a measurement cycle. The
E601 − 20
uncertainty attributable to the reference junction temperature shall be less than 60.1°C. 60.1 °C. Ice point reference junction baths
provide a relatively simple and reliable means for maintaining the reference junction at 0°C (32°F) 0 °C (32 °F) when proper
precautions are exercised in their use. Practice E563 provides an acceptable method for utilizing the ice point as a reference
junction bath. Subsection 7.3 of Test Method E220 may be consulted for alternative methods of providing a reference junction
temperature.
6.5 Tube Furnace—The test shall be conducted in an electrically heated tube furnace such as described in subsectionSubsection
7.2.3 of Test Method E220. The furnace tube shall be long enough sufficient in length to permit a depth of immersion of the
thermocouple measuring junctions that is sufficient to assure that the temperature of the assuring the measuring junctions is are
not affected by heat conduction along length of the thermoelements.
6.5.1 Means shall be provided to control the temperature of the furnace to within 610°C (618°F) 610 °C (618 °F) of a nominal
temperature during the performance of the test.
6.5.2 The test shall be conducted in a uniformly heated furnace providing a nearly isothermal work zone sufficiently large of
sufficient size and stability to maintain all junctions at the same temperature.
6.5.3 To determine the uncertainty resulting from temperature non-uniformities in the work zone, measure the temperature profile
along the thermocouple axis in the vicinity of the work zone, using a platinum-rhodium alloy thermocouple or a platinum resistance
thermometer prior to commencement of the test. If the furnace temperature is not sufficiently stable to obtain a temperature profile
with a single thermometer, it may be useful to place one thermometer at a fixed half-maximum heated length, and to move a second
thermometer along the furnace-tube axis. Adjust the readings of the moveable thermometer by adding the correction –(–
(t (time) – t (initial)), where t is the temperature indication of the thermometer at fixed half-maximum heated length.
fixed fixed fixed
6.5.3.1 A thermoelement extending from ambient temperature into an isothermal zone of a furnace will come to equilibrium with
the temperature of the isothermal zone through radiative, convective, and conductive heat transfer between the thermoelement and
the surrounding furnace environment. The distance of immersion, L , into the isothermal zone neededrequired to achieve thermal
eq
equilibrium depends significantly on both the thermoelement diameter and its thermal conductivity. The characteristic length for
a wire to achieve thermal equilibrium with its surroundings is given by the approximate correlation:
1/2 1/2
d k
L 5 ~2.5 cm! (1)
S D S D
eq
1 mm 100 W/~m·K!
where:
where:
k = the thermal conductivity of the thermoelement, and
d = the diameter of the thermoelement.
Calculate the distance L for each tested thermoelement in units of W/(m · K), and d is the diameter of the thermoelement in
eq
millimeters. The equivalent equation in English units, with d in units of inches and k in units of BTU/(hr·ft·°F) is:
1/2 1/2
d k
L 5 1 in (2)
~ !S D S D
eq
0.04 in 58 BTU/ hr.·ft·°F
~ !
Calculate the distance L for each tested thermoelement and the platinum reference thermoelement. The approximate thermal
eq
conductivities listed in Table 1 may be used for this purpose. Measure the diameter d of the measuring junction assembly
mj
(see 7.1). Identify the maximum L of the set of all calculated L values and d
max eq mj.
NOTE 4—Eq 1 was derived for a temperature of 200°C, 200 °C, which is near the lower limit of observable thermoelement drift. For higher temperatures,
the value of L from Eq 1 will give an upper limit on the actual equilibration length.
eq
TABLE 1 Approximate Thermal Conductivities of Thermoelement
Materials at 200°C200 °C
Thermoelement Type k (W/(m · K))
Pt 72
EP, KP 21
EN, JN, TN 31
JP 62
KN 32
NP 19
NN 31
TP 380
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6.5.3.2 The standard uncertainty due to thermal non-uniformity is the maximum temperature variation in the profile from
subsection 6.5.3 between the measuring junction location and a distance L away from the measuring junction.
max
6.5.3.3 Alternative methods may be used to determine the standard uncertainty due to thermal non-uniformity, such as comparison
of results in the test furnace with results obtained either in fixed-point cells or in a stirred liquid bath of high temperature
uniformity; or numerical heat-transfer calculations.
6.6 Electromotive Force Indicator—The emf-measuring instrumentation shall have a measurement uncertainty of not more than
1 μV at 1 000 μV and 12 μV at 50 000 μV for this test. The emf indicators may be potentiometers or digital voltmeters. Subsections
See subsections 6.2 and 7.4 of Test Method E220 may be consulted for further discussions of thermal emf indicators and methods
of emf measurement.
6.7 Connecting Wires—Connecting wires from the reference junctions to the emf indicator or indicators shall be electrically
insulated copper. If the test is sensitive to electrostatic interference, the wires shall be electrically shielded. If electromagnetic
interference is present, the conductors shall be twisted to minimize this effect.
6.8 Selector Switches—When more than one thermoelement is to be tested, a selector switch is shall be introduced into the copper
part of the circuit between the reference junctions and the thermal-emf indicators. These switches shall comply with subsection
7.5.1 of Test Method E220.
6.9 Thermocouple Insulation—For the segment of the thermoelements exposed to temperatures above ambient, ceramic tubing
may be used to support and electrically insulate the test thermoelement, the thermocouple used to measure the test temperature,
and the platinum reference thermoelement, if used.
6.9.1 For the test thermoelements, the ceramic tubing shall be aluminum oxide (Al O ) with total impurities of less than 0.5 %
2 3
(mass), and the maximum limit for specific impurities shall be 0.04 % (mass) for Fe O .
2 3
6.9.2 For the thermocouple used to measure the test temperature and for the platinum reference thermoelement, the ceramic tubing
shall be aluminum oxide (Al O ) with total impurities of less than 0.5 % (mass), and the maximum limit for specific impurities
2 3
shall be 0.04 % (mass) for Fe O and 0.08 % (mass) for Si.
2 3
6.9.3 To avoid unnecessary mass and to minimize axial heat conduction in the region of the measuring junction, the ceramic tubing
should be relatively thin-walled and should have bore diameters large enough to allowof sufficient size to permit threading of the
thermoelements without bending or straining them and to avoid binding. If possible the test thermocouple(s) and the reference
thermocouple should be welded individually and then the measuring junctions welded all together to create a loose fitting bundle
with a common measuring junction.
6.9.4 To minimize contamination, single lengths of ceramic insulation and not short pieces shall be used. Additionally, if using
ceramics that were previously used, insert only thermocouples of the same type as previously used. Insert the positive and negative
thermoelement into the bore previously used for that thermoelement to prevent cross contamination from previous testing.
7. Procedure
7.1 Preparation of Thermocouples for Test—Thermocouples—The thermoelement junction shall be prepared by welding, using a
procedure proven by experience, or by testing, to produce junctions that are mechanically secure and electrically conductive at the
test temperature for the life of the test. The measuring junctions of all of the thermocouples may be welded together into a
common bead to provide good thermal contact between the junctions of the different thermocouples.thermoelements. Weld a
reference therm
...