ASTM E452-02(2023)

Standard Test Method for Calibration of Refractory Metal Thermocouples Using a Radiation Thermometer

SIGNIFICANCE AND USE
5.1 This test method is intended to be used by wire producers and thermocouple manufacturers for certification of refractory metal thermocouples. It is intended to provide a consistent method for calibration of refractory metal thermocouples referenced to a calibrated radiation thermometer. Uncertainty in calibration and operation of the radiation thermometer, and proper construction and use of the test furnace are of primary importance.  
5.2 Calibration establishes the temperature-emf relationship for a particular thermocouple under a specific temperature and chemical environment. However, during high temperature calibration or application at elevated temperatures in vacuum, oxidizing, reducing or contaminating environments, and depending on temperature distribution, local irreversible changes may occur in the Seebeck Coefficient of one or both thermoelements. If the introduced inhomogeneities are significant, the emf from the thermocouple will depend on the distribution of temperature between the measuring and reference junctions.  
5.3 At high temperatures, the accuracy of refractory metal thermocouples may be limited by electrical shunting errors through the ceramic insulators of the thermocouple assembly. This effect may be reduced by careful choice of the insulator material, but above approximately 2100 °C, the electrical shunting errors may be significant even for the best insulators available.
SCOPE
1.1 This test method covers the calibration of refractory metal thermocouples using a radiation thermometer as the standard instrument. This test method is intended for use with types of thermocouples that cannot be exposed to an oxidizing atmosphere. These procedures are appropriate for thermocouple calibrations at temperatures above 800 °C (1472 °F).  
1.2 The calibration method is applicable to the following thermocouple assemblies:  
1.2.1 Type 1—Bare-wire thermocouple assemblies in which vacuum or an inert or reducing gas is the only electrical insulating medium between the thermoelements.  
1.2.2 Type 2—Assemblies in which loose fitting ceramic insulating pieces, such as single-bore or double-bore tubes, are placed over the thermoelements.  
1.2.3 Type 2A—Assemblies in which loose fitting ceramic insulating pieces, such as single-bore or double-bore tubes, are placed over the thermoelements, permanently enclosed and sealed in a loose fitting metal or ceramic tube.  
1.2.4 Type 3—Swaged assemblies in which a refractory insulating powder is compressed around the thermoelements and encased in a thin-walled tube or sheath made of a high melting point metal or alloy.  
1.2.5 Type 4—Thermocouple assemblies in which one thermoelement is in the shape of a closed-end protection tube and the other thermoelement is a solid wire or rod that is coaxially supported inside the closed-end tube. The space between the two thermoelements can be filled with an inert or reducing gas, or with ceramic insulating materials, or kept under vacuum.  
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: 14-Aug-2023
• ICS: 17.200.20 - Temperature-measuring instruments
• Technical Committee: E20 - Temperature Measurement
• Drafting Committee: E20.11 - Thermocouples - Calibration

Relations

• Refers: ASTM E344-23 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Dec-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 E344-16 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Nov-2016
• Refers: ASTM E1256-15 - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-Jul-2015
• Refers: ASTM E344-13 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-May-2013
• Refers: ASTM E344-12 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-May-2012
• Refers: ASTM E1256-11a - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-May-2011
• Refers: ASTM E563-11 - Standard Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature
• Effective Date: 01-May-2011
• Refers: ASTM E1256-10 - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-Dec-2010
• Refers: ASTM E344-10 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Nov-2010
• Refers: ASTM E344-08 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 15-Nov-2008
• Refers: ASTM E563-08 - Standard Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature
• Effective Date: 01-Nov-2008
• Refers: ASTM E1256-95(2007) - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-Nov-2007
• Refers: ASTM E344-07 - Terminology Relating to Thermometry and Hydrometry
• Effective Date: 01-Jun-2007

Overview

ASTM E452-02(2023), Standard Test Method for Calibration of Refractory Metal Thermocouples Using a Radiation Thermometer, is an internationally recognized procedure developed by ASTM International. This standard outlines a consistent method for calibrating refractory metal thermocouples by referencing a calibrated radiation thermometer. It provides essential guidelines for wire producers and thermocouple manufacturers, especially when dealing with applications above 800 °C (1472 °F), where standard thermocouple calibration techniques may not be suitable due to the risk of oxidation and challenging temperature environments.

Key Topics

• Calibration Scope: This test method applies to a wide range of refractory metal thermocouple assemblies, including bare-wire, ceramic insulated, swaged, and coaxially supported types.
• Calibration Process: The calibration establishes the temperature-electromotive force (emf) relationship for a thermocouple within specific temperature and chemical conditions, using a radiation thermometer as the reference instrument.
• Radiation Thermometer Usage: The method ensures that the calibration is traceable to a radiation thermometer, emphasizing the need for the thermometer’s uncertainty and operational accuracy to be well characterized.
• Environmental Considerations: Special attention is given to avoiding exposure to oxidizing atmospheres, which can affect both the calibration and operational longevity of refractory metal thermocouples.
• Sources of Error: The standard identifies key sources of error during calibration, including improper blackbody design, temperature gradients, heat conduction losses, and alignment of the radiation thermometer.

Applications

• Thermocouple Manufacturing: Manufacturers rely on this method for certifying and calibrating high-temperature thermocouples to ensure process accuracy in critical thermal measurement.
• Wire Producers: Wire producers use the standard to verify the conformity of refractory metal wires used in thermocouples, supporting end-user confidence in sensor performance.
• High-Temperature Industrial Processes: Industries such as aerospace, metallurgy, and power generation that utilize vacuum furnaces, inert or reducing atmospheres, and require precise high-temperature measurement, benefit from standardized calibration and improved data reliability.
• Quality Assurance: The method is fundamental in quality assurance programs where accurate and reproducible temperature measurement is vital to meeting industry and safety standards.

Related Standards

Adherence to ASTM E452-02(2023) ensures compatibility and traceability with several relevant ASTM standards and guides, including:

• ASTM E344: Terminology Relating to Thermometry and Hydrometry
• ASTM E563: Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature
• ASTM E1256: Test Methods for Radiation Thermometers (Single Waveband Type)
• ASTM E1751: Guide for Temperature Electromotive Force (emf) Tables for Non-Letter Designated Thermocouple Combinations

Practical Value

By implementing ASTM E452-02(2023), organizations ensure that refractory metal thermocouples are calibrated to a recognized, reliable standard, minimizing measurement uncertainty and maximizing process control. This facilitates global trade, compliance, and technical interoperability by aligning with the WTO Technical Barriers to Trade (TBT) principles. The standard is particularly valuable for environments and applications where thermocouple performance at extreme temperatures is a critical factor for operational safety, regulatory compliance, and product quality.

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: E452 − 02 (Reapproved 2023) An American National Standard
Standard Test Method for
Calibration of Refractory Metal Thermocouples Using a
Radiation Thermometer
This standard is issued under the fixed designation E452; 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 ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.1 This test method covers the calibration of refractory
mendations issued by the World Trade Organization Technical
metal thermocouples using a radiation thermometer as the
Barriers to Trade (TBT) Committee.
standard instrument. This test method is intended for use with
types of thermocouples that cannot be exposed to an oxidizing
2. Referenced Documents
atmosphere. These procedures are appropriate for thermo-
2.1 ASTM Standards:
couple calibrations at temperatures above 800 °C (1472 °F).
E344 Terminology Relating to Thermometry and Hydrom-
1.2 The calibration method is applicable to the following
etry
thermocouple assemblies:
E563 Practice for Preparation and Use of an Ice-Point Bath
1.2.1 Type 1—Bare-wire thermocouple assemblies in which
as a Reference Temperature
vacuum or an inert or reducing gas is the only electrical
E988 Temperature-Electromotive Force (EMF) Tables for
insulating medium between the thermoelements.
Tungsten-Rhenium Thermocouples (Withdrawn 2011)
1.2.2 Type 2—Assemblies in which loose fitting ceramic
E1256 Test Methods for Radiation Thermometers (Single
insulating pieces, such as single-bore or double-bore tubes, are
Waveband Type)
placed over the thermoelements.
E1751 Guide for Temperature Electromotive Force (emf)
1.2.3 Type 2A—Assemblies in which loose fitting ceramic
Tables for Non-Letter Designated Thermocouple Combi-
insulating pieces, such as single-bore or double-bore tubes, are
nations
placed over the thermoelements, permanently enclosed and
3. Terminology
sealed in a loose fitting metal or ceramic tube.
1.2.4 Type 3—Swaged assemblies in which a refractory
3.1 Definitions:
insulating powder is compressed around the thermoelements
3.1.1 For definitions of terms used in this test method see
and encased in a thin-walled tube or sheath made of a high
Terminology E344.
melting point metal or alloy.
3.1.2 radiation thermometer, n—radiometer calibrated to
1.2.5 Type 4—Thermocouple assemblies in which one ther-
indicate the temperature of a blackbody.
moelement is in the shape of a closed-end protection tube and
3.1.2.1 Discussion—Radiation thermometers include instru-
the other thermoelement is a solid wire or rod that is coaxially
ments having the following or similar names: (1) optical
supported inside the closed-end tube. The space between the
radiation thermometer, (2) photoelectric pyrometer, (3) single
two thermoelements can be filled with an inert or reducing gas,
wavelength automatic thermometer, (4) disappearing filament
or with ceramic insulating materials, or kept under vacuum.
pyrometer, (5) dual wavelength pyrometer or ratio radiation
thermometer, (6) visual optical thermometer, (7) infrared
1.3 This standard does not purport to address all of the
thermometer, (8) infrared pyrometer, and permutations on the
safety concerns, if any, associated with its use. It is the
terms above as well as some manufacturer-specific names.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.2 Definitions of Terms Specific to This Standard:
mine the applicability of regulatory limitations prior to use.
3.2.1 automatic radiation thermometer, n—radiation ther-
1.4 This international standard was developed in accor-
mometer whose temperature reading is determined by elec-
dance with internationally recognized principles on standard-
tronic means.
1 2
This test method is under the jurisdiction of ASTM Committee E20 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Temperature Measurementand is the direct responsibility of Subcommittee E20.11 contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
on Thermocouples - Calibration. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Aug. 15, 2023. Published August 2023. Originally the ASTM website.
approved in 1972. Last previous edition approved in 2018 as E452 – 02 (2018). The last approved version of this historical standard is referenced on
DOI: 10.1520/E0452-02R23. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E452 − 02 (2023)
than many of the thermocouples used above 800 °C (1472 °F). The
3.2.2 disappearing filament pyrometer, n—radiation ther-
advantages of physical separation of the disappearing filament pyrometer
mometer that requires an observer to match visually the
from the test assembly may still justify its use over use of a standard
brightness of a heated filament mounted inside the radiation
thermocouple.
thermometer to that of the measured object.
5. Significance and Use
3.2.3 equalizing block, n—object, usually metal, that when
placed in a nonuniform temperature region, has greater tem-
5.1 This test method is intended to be used by wire
perature uniformity (due to its relatively high thermoconduc-
producers and thermocouple manufacturers for certification of
tivity and mass) than the medium surrounding the object.
refractory metal thermocouples. It is intended to provide a
consistent method for calibration of refractory metal thermo-
3.2.4 spectral emissivity, n—ratio of the spectral radiance at
couples referenced to a calibrated radiation thermometer.
a point on a particular specimen and in a particular direction
Uncertainty in calibration and operation of the radiation
from that point to that emitted by a blackbody at the same
thermometer, and proper construction and use of the test
temperature.
furnace are of primary importance.
3.2.5 spectral radiance, n—power radiated by a specimen in
a particular direction, per unit wavelength, per unit projected
5.2 Calibration establishes the temperature-emf relationship
area of the specimen, and per unit solid angle. for a particular thermocouple under a specific temperature and
chemical environment. However, during high temperature
3.2.6 spectral response, n—signal detected by a radiometer
calibration or application at elevated temperatures in vacuum,
at a particular wavelength of incident radiation, per unit power
oxidizing, reducing or contaminating environments, and de-
of incident radiation.
pending on temperature distribution, local irreversible changes
3.2.7 test thermocouple, n—thermocouple that is to have its
may occur in the Seebeck Coefficient of one or both thermo-
temperature-emf relationship determined by reference to a
elements. If the introduced inhomogeneities are significant, the
temperature standard.
emf from the thermocouple will depend on the distribution of
3.2.8 thermocouple calibration point, n—temperature, es-
temperature between the measuring and reference junctions.
tablished by a standard, at which the emf developed by a
5.3 At high temperatures, the accuracy of refractory metal
thermocouple is determined.
thermocouples may be limited by electrical shunting errors
through the ceramic insulators of the thermocouple assembly.
4. Summary of Test Method
This effect may be reduced by careful choice of the insulator
4.1 The thermocouple is calibrated by determining the
material, but above approximately 2100 °C, the electrical
temperature of its measuring junction with a radiation ther-
shunting errors may be significant even for the best insulators
mometer and recording the emf of the thermocouple at that
available.
temperature. The measuring junction of the thermocouple is
placed in an equalizing block containing a cavity which
6. Sources of Error
approximates blackbody conditions. The radiation thermom-
6.1 The most prevalent sources of error (Note 2) in this
eter is sighted on the cavity in the equalizing block and the
method of calibration are: (1) improper design of the black-
blackbody temperature or true temperature of the block,
body enclosure, (2) severe temperature gradients in the vicinity
including the measuring junction, is determined.
of the blackbody enclosure, (3) heat conduction losses along
4.2 Since the spectral emissivity of the radiation emanating
the thermoelements, and (4) improper alignment of the radia-
from a properly designed blackbody is considered unity (one)
tion thermometer with respect to the blackbody cavity and
for all practical purposes, no spectral emissivity corrections
unaccounted transmission losses along the optical path of the
need be applied to optical pyrometer determinations of the
radiation thermometer.
blackbody temperature.
NOTE 2—These are exclusive of any errors that are made in the
4.3 Although the use of a radiation thermometer (Note 1) is
radiation thermometer measurements or the thermocouple-emf measure-
less may require more effort and more complex apparatus to ments.
achieve a sensitivity equivalent to that of commonly used
7. Apparatus
thermocouples, a radiation thermometer has the advantage of
being physically separated from the test assembly; thus, its 7.1 Furnace:
calibration is not influenced by the temperatures and atmo- 7.1.1 The calibration furnace should be designed so that any
spheres in the test chamber. By comparison, a standard temperature within the desired calibration temperature range
thermocouple that is used to calibrate another thermocouple can be maintained constant within a maximum change of 1 °C
must be subjected to the temperatures at which the calibrations (1.8 °F) per minute in the equalizing block over the period of
are performed and in some cases must be exposed to the any observation. Figs. 1-3 show three types of furnaces (1 and
environment that is common to the test thermocouple. If a 2) that can be used for calibrating refractory-metal thermo-
standard thermocouple is exposed to high temperatures or couples. Fig. 4 is a detailed drawing of the upper section of the
contaminating environments, or both, for long periods of time, furnace in Fig. 3. An equalizing block containing a blackbody
its calibration becomes questionable and the uncertainty in the
bias of the calibration increases.
The boldface numbers in parentheses refer to the list of references at the end of
NOTE 1—Disappearing filament pyrometers are somewhat less sensitive this standard.
E452 − 02 (2023)
1. Caps for making vacuum tight seals around the thermoelements. A cylinder 18. Furnace shell (brass).
type neoprene gasket is compressed around the thermoelements. 19. First radiation shield. 0.020-in. (0.51 mm) tantalum sheet rolled into a cylinder
2. Kovar metal tube. and secured with tantalum rivets.
3. Dome made of No. 7052 glass providing electrical insulation for 20. Second radiation shield. (0.020-in. (0.51 mm) molybdenum.)
thermoelements. 21. Third radiation solid. (0.020-in. (0.51 mm) molybdenum.)
4. Neoprene O-ring gasket. 22. Fourth radiation shield. (0.010-in. (0.25 mm) molybdenum.)
5. Top plate extension (brass). 23. Liquid nitrogen trap.
6. Aluminum oxide radiation shield. 24. Metal baffle plates at liquid nitrogen temperature.
7. Ionization vacuum gage. 25. Liquid nitrogen chamber.
8. Thermocouple vacuum gage. 26. Vacuum chamber.
9. No. 7052 glass tube providing electrical insulation for thermoelements. 27. Borosilicate glass window.
10. Chamber for water flow during furnace operation. 28. Hole (0.045-in. (1.14 mm) diameter) for sighting with disappearing filament
pyrometer.
11. Electrically insulating spacers. 29. Molybdenum blackbody.
12. Power supply terminal. 30. Tantalum tube.
13. Removable top plate (brass). 31. Inert gas entrance.
14. Tantalum spacing ring providing electrical contact between plate and 32. Tantalum rings for electrical contact.
tantalum tube. 33. Removable copper plate for electrical contact.
15. Thermal expansion joint of tantalum tube. 34. Hex-head nut for tightening copper plate against O-ring gasket.
16. Copper tubing for water cooling. 35. Bottom plate (brass).
17. Auxiliary radiation shield.
FIG. 1 High-Temperature Furnace (Example 1)
E452 − 02 (2023)
(a) Nylon bushing, (b) stainless steel support, (c) rectangular stainless steel shutter, (d) borosilicate glass window, (e) brass shutter support, (f) shutter rotation
mechanism, (g) copper lead, (h) steel housing, (I) brass plate, (j) copper coil spring, (k) alumina closed-end tube, (l) port, (m) O-ring gaskets, (n) copper water-cooled
electrode, (o) tantalum heater element, (p) tantalum radiation shields, (q) water-cooling coils, (r) ceramic insulator, (s) tantalum radiation shield, (t) adjustable clamp,
(u) water out, (v) electrical leads, (w) water in, and (x) to vacuum system.
FIG. 2 High-Temperature Furnace (Example 2)
cavity is suspended in the central region of the furnace by 7.1.2 The blackbody cavity in the equalizing block should
means of support rods or wires. The mass of the support rods be designed in accordance with established criteria set forth in
or wires should be kept to a minimum to reduce heat losses by the literature (3-7). Such factors as interior surface texture,
conduction. When the furnace is in operation, a sufficiently diameter-to-depth ratio of the blackbody cavity opening, and
large region in the center of the furnace should be at a uniform internal geometry can have an appreciable effect on the spectral
temperature to ensure that the temperature throughout the emissivity of the cavity.
equalizing block (when all test thermocouple assemblies are in 7.1.3 Figs. 5-7 show three typical equalizing block designs
position in the block) is uniform. At temperatures greater than that are used in thermocouple calibrating furnaces. The design
2000 °C, furnace parts made from tantalum may introduce in Fig. 5 is used in furnaces where the standard radiation
contamination of exposed thermoelements. In this case, it may thermometer is sighted horizontally into the blackbody through
be desirable to fabricate heated furnace components from the hole in the side of the block. This design is particularly
tungsten. useful in the calibration of bare-wire thermocouples since the
E452 − 02 (2023)
FIG. 4 Upper Section of Furnace (Example 3)
(A) Disappearing filament pyrometer
(B) Sight window
(C) Gas inlet
(D) Water cooling
(E) Stainless steel shell
(F) Tungsten heat shield
(G) Tungsten heater
(H) Support rods
(J) Equalizer block (blackbody)
(K) Refractory brick
(L) Gallium alloy electrical contact
(M) Gas outlet
(N) Copper electrode
FIG. 3 High-Temperature Furnace (Example 3)
lid on the blackbody (or the entire blackbody) can be an
electrically insulating material such ashafnium oxide or beryl-
lium oxide. Thus, if the bare thermocouple wires should come
in contact with the equalizing block, the wires will not be
electrically shorted. If this design is used in the calibration of
Types, 2, 3, or 4 thermocouple assemblies (see 1.2), the
blackbody lid can be metal since electrical insulation between
the thermoelements is included as part of the assembly.
(Warning—Beryllium oxide should be considered a hazardous
material. Material Safety Data Sheets and precautions in
handling this toxic substance should be obtained from the
FIG. 5 Equalizing Block (Example A)
supplier.)
7.1.4 The designs in Figs. 6 and 7 are used in furnaces
where the standard radiation thermometer is sighted vertically calibrate a number of thermocouples during one calibration run
into the blackbody cavity. In cases where it is necessary to or to calibrate thermocouple assemblies that are large in
E452 − 02 (2023)
blackbody cavity and the thermocouple wells should be of
sufficient depth to ensure that the thermocouple measuring
junctions and a considerable length of the thermocouple
assemblies leading from the measuring junctions are contained
in the wall of the equalizing block.
7.1.5 In order to view the radiation emanating from the
blackbody cavity, some type of window shall be contained in
the outer structure of the furnace. It is important that this
window be properly designed to ensure that errors are not
encountered when the blackbody radiation is observed with a
radiation thermometer. Windows may be made from any
transparent glass or crystalline material of high optical quality.
7.1.6 Figs. 8 and 9 show an incorrectly designed furnace
window and a correctly designed window, respectively. In Fig.
8 the blackbody radiation emanating from the window does not
completely fill the objective lens of the radiation thermometer.
This is caused by the window opening being too small in
diameter and thus acting as an aperture stop. In this case, the
temperature indicated by the radiation thermometer may be
lower than the temperature indicated if all of the objective lens
FIG. 6 Equalizing Block (Example B) is filled with the cone of radiation. Fig. 9 shows a larger
window opening with the resulting cone of radiation com-
pletely filling the objective lens. It also can be seen that the
openings in the radiation shields can act as aperture stops if
they are too small in diameter. This may cause the same type
of error as described with the window opening. On the other
hand, if the window and the radiation shield openings are made
too large, radiation losses may produce appreciable tempera-
ture gradients in the hot zone of the furnace.
7.1.7 The transmission losses of the window should be
determined at all calibration temperatures and the appropriate
corrections applied to all radiation thermometer readings (see
8.3).
7.1.8 Figs. 10 and 11 show two types of vacuum seals (Note
3) that can be used to bring test thermocouple assemblies into
the furnace chamber. The seal shown in Fig. 10 is particularly
useful for bringing bare-wire thermocouples into the furnace.
This design makes use of a cylinder-shaped fluorocarbon
gasket that is compressed around the thermocouple wires to
form a vacuum tight seal. A small amount of high-temperature
vacuum grease should be placed on each gasket before sealing.
The thermocouple wires are inserted through a coaxial hole in
the gasket. Also, this design can be used to form a seal around
the outer sheath of swaged thermocouple assemblies (Type 3 of
1.2).
NOTE 3—If the calibration furnace design is such that the desired
furnace atmosphere is obtained by purging (see 8.2), vacuum seals are not
needed to maintain a relatively pure furnace atmosphere.
7.1.9 The seal shown in Fig. 11 can be used in arrangements
where many test thermocouples are inserted and removed from
the furnace chamber over a short period of time. This design
FIG. 7 Equalizing Block (Example C)
allows the test thermocouple to be quickly attached to or
detached from extension wires that are permanently sealed in
the metal-to-glass sealing unit. When a test thermocouple is to
diameter and mass, the equalizing block designs in Figs. 6 and
7 are appropriate. If the thermocouple assemblies being tested be removed from the furnace, the O-ring gasket seal (Seal A)
is broken and the top section of the seal is lifted upward, thus
in these types of equalizing blocks are massive and can conduct
a considerable amount of heat away from the block, the lifting the attached test thermocouple out of the furnace.
E452 − 02 (2023)
FIG. 8 Furnace Window (Incorrect Design)
NOTE 1—It should be recognized that errors resulting from incorrect furnace window design may be more significant for single wavelength automatic
radiation thermometers than either disappearing filament pyrometers or dual wavelength radiation thermometers.
FIG. 9 Furnace Window (Correct Design)
7.1.10 A metal clamp (B) containing a small screw is used 7.2 Radiation Thermometer:
to make a mechanical connection between the test thermo- 7.2.1 A well characterized and stable radiation thermometer,
couple and the extension wires. Care should be taken to with a calibration of known uncertainty, is used as the standard
eliminate any temperature gradients that might exist along the instrument for determining temperatures in this test method.
metal clamps during furnace operation. Such gradients can The radiation thermometer can be either the disappearing
cause extraneous emfs in the measuring circuit. filament type or the automatic type, depending on the accuracy
7.1.11 Fig. 12 shows the same type of seal as Fig. 10 but required for a specific test. (See Table 1 for calibration
with a Type 2 thermocouple suspended into the test furnace uncertainties.) Both types of radiation thermometers are avail-
instead of a Type 1. Fig. 13 shows a vacuum seal design that able commercially. If something other than a disappearing
can be used to bring Type 3 or 4 thermocouple assemblies into filament pyrometer is used, it shall have an operating wave-
the furnace. length less than or equal to 1.1 μm and meet both the
7.1.12 In general, any sealing unit that is used to bring temperature measurement and sighting field of view require-
thermocouple assemblies into a furnace chamber should (1) ments of the calibration apparatus. (Refer to Test Method
allow the thermocouple assembly to be easily installed or E1256 for methods to determine the target characteristics of an
removed from the furnace, (2) electrically insulate the thermo- automatic radiation thermometer in addition to the sighting
elements from each other and from any part of the furnace that cautions illustrated in Figs. 8 and 9 herein.) For radiation
is connected electrically to the furnace power supply, (3) not thermometers with significant spectral response at wavelengths
cause any physical or chemical changes in the thermoelements, that differ by more than 40 nm from the center wavelength of
and (4) not introduce any extraneous emfs in the thermocouple- the instrument, significant errors may be introduced if the
emf measuring circuit. calibration of the radiation thermometer was not performed
E452 − 02 (2023)
FIG. 10 Vacuum Seal (Example 1)
FIG. 11 Vacuum Seal (Example 2)
with a blackbody source or if the emissivity of the blackbody
used either in the calibration or in this test method depends on
the detected wavelength. from the blackbody cavity and the brightness of the radiation
7.2.2 In using a disappearing filament pyrometer in this thermometer lamp filament is made automatically by compo-
method, an observer varies the brightness of the standard nents in the radiation thermometer. In this case, either of the
source (usually a small tungsten filament lamp in the pyrom- methods for relating the lamp current and temperature men-
eter) until it matches the brightness of the radiation emanating tioned in 7.2.2 can be applied. Most radiation thermometers of
from the blackbody cavity (Fig. 14). After the match has been this type are designed to compare the brightness of the target
made, the corresponding temperature can be determined by source (in this case the radiation from the blackbody cavity)
either of two methods: (1) the temperature can be read directly and the radiation thermometer filament lamp many times each
from a meter that is connected to the pyrometer circuit, or (2) second. If the radiation thermometer detects a slight increase or
the current through the pyrometer filament lamp can be decrease in the temperature of the blackbody radiation, an
measured through the use of a standard resistor and a potenti- electronic balancing system automatically increases or de-
ometer or a digital voltmeter. creases the temperature of the radiation thermometer lamp until
7.2.3 If the meter-indication method is used, the initial it has the same apparent brightness temperature as the black-
calibration of the disappearing filament pyrometer shall be on body radiation. Thus, the brightness of the radiation thermom-
a meter-indication versus temperature basis. Likewise, the eter lamp filament is maintained continuously at the same
pyrometer shall be calibrated on current versus temperature apparent brightness temperature as the blackbody. If the
basis if the current measuring method is to be used. radiation thermometer lamp and a standard resistor are con-
7.2.4 In general, a smaller uncertainty can be obtained with nected in series, the voltage drop across the resistor can be
a disappearing filament pyrometer that has been calibrated and measured by means of a digital voltmeter or a potentiometer.
used on the current-measurement basis as opposed to the 7.2.6 If an automatic radiation thermometer is used, the
meter-indication method. This difference is due mainly to the radiance emitted from the cavity is measured automatically and
inability of the observer to read the meter scale to less than displayed as temperature, usually on a digital temperature
1 °C and to the accuracy of the meter itself. display in modern instruments (Note 4). Great care shall be
7.2.5 Certain automatic radiation thermometers utilize an taken to ensure that the optical measuring axis is aligned to be
internal filament lamp as a spectral radiance reference, and the coaxial with the centerline of the blackbody-radiation shield
comparison between the brightness of the radiation emanating assembly and that the sighting path of the thermometer is not
E452 − 02 (2023)
FIG. 13 Vacuum Seal (Example 1) with Type 3 or 4 Thermocouple
Assemblies
* Kovar is a registered trademark of CRS Holdings, Inc., a subsidiary of
Carpenter Technology Corporation.
test method. The reference junction temperature should be
FIG. 12 Vacuum Seal (Example 1 with Type 2 Thermocouple)
controlled closely enough to eliminate variations in it as a
significant source of error. A simple and relatively trouble-free
blocked anywhere along its length. Since this sighting path is
method of maintaining the reference junction of a thermo-
so important, it is equally important that the user have a
couple at 0 °C is through the use of reasonably pure crushed ice
detailed knowledge of the sighting“ cone” of the thermometer
and water. An acceptable method utilizing crushed ice and
to be certain that it can be aligned with the apparatus. This
water to maintain a 0 °C reference temperature is given in
requires a set of measurements along the sighting path of the
Practice E563.
thermometer to accurately determine the size and shape of the
7.4 Potentiometers or Voltmeters—The choice of a specific
cone. Test Method E1256 provides a description of a method of
potentiometer or voltmeter will depend upon the required
determining the target size at the focal distances of the
accuracy of the calibration being performed, but generally the
thermometer and is readily adapted for determining the target
instrument will be chosen from commercially available labo-
size
...