ASTM E452-02(2018)
(Test Method)Standard Test Method for Calibration of Refractory Metal Thermocouples Using a Radiation Thermometer
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.
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Designation: E452 − 02 (Reapproved 2018) 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 service@astm.org. For Annual Book of ASTM
on Thermocouples - Calibration. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved April 1, 2018. Published April 2018. Originally the ASTM website.
approved in 1972. Last previous edition approved in 2013 as E452 – 02 (2013). The last approved version of this historical standard is referenced on
DOI: 10.1520/E0452-02R18. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E452 − 02 (2018)
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-
ments.
less may require more effort and more complex apparatus to
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 (2018)
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 (2018)
(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 (2018)
FIG. 4 Upper Section of Fu
...
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: E452 − 02 (Reapproved 2013) E452 − 02 (Reapproved 2018)
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
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).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 safety, health, and healthenvironmental 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.
2. Referenced Documents
2.1 ASTM Standards:
E344 Terminology Relating to Thermometry and Hydrometry
E563 Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature
E988 Temperature-Electromotive Force (EMF) Tables for Tungsten-Rhenium Thermocouples (Withdrawn 2011)
E1256 Test Methods for Radiation Thermometers (Single Waveband Type)
E1751 Guide for Temperature Electromotive Force (EMF) Tables for Non-Letter Designated Thermocouple Combinations
(Withdrawn 2009)
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this test method see Terminology E344.
This test method is under the jurisdiction of ASTM Committee E20 on Temperature Measurementand is the direct responsibility of Subcommittee E20.11 on
Thermocouples - Calibration.
Current edition approved May 1, 2013April 1, 2018. Published May 2013April 2018. Originally approved in 1972. Last previous edition approved in 20072013 as
E452 – 02 (2007).(2013). DOI: 10.1520/E0452-02R13.10.1520/E0452-02R18.
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.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E452 − 02 (2018)
3.1.2 radiation thermometer, n—radiometer calibrated to indicate the temperature of a blackbody.
3.1.2.1 Discussion—
Radiation thermometers include instruments having the following or similar names: (1) optical radiation thermometer, (2)
photoelectric pyrometer, ( 3) single wavelength automatic thermometer, (4) disappearing filament pyrometer, (5) dual wavelength
pyrometer or ratio radiation thermometer, (6) visual optical thermometer, (7) infrared thermometer, (8) infrared pyrometer, and
permutations on the terms above as well as some manufacturer-specific names.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 automatic radiation thermometer, n—radiation thermometer whose temperature reading is determined by electronic
means.
3.2.2 disappearing filament pyrometer, n—radiation thermometer that requires an observer to match visually the brightness of
a heated filament mounted inside the radiation thermometer to that of the measured object.
3.2.3 equalizing block, n—object, usually metal, that when placed in a nonuniform temperature region, has greater temperature
uniformity (due to its relatively high thermoconductivity and mass) than the medium surrounding the object.
3.2.4 spectral emissivity, n—ratio of the spectral radiance at a point on a particular specimen and in a particular direction from
that point to that emitted by a blackbody at the same temperature.
3.2.5 spectral radiance, n—power radiated by a specimen in a particular direction, per unit wavelength, per unit projected area
of the specimen, and per unit solid angle.
3.2.6 spectral response, n—signal detected by a radiometer at a particular wavelength of incident radiation, per unit power of
incident radiation.
3.2.7 test thermocouple, n—thermocouple that is to have its temperature-emf relationship determined by reference to a
temperature standard.
3.2.8 thermocouple calibration point, n— temperature, established by a standard, at which the emf developed by a thermocouple
is determined.
4. Summary of Test Method
4.1 The thermocouple is calibrated by determining the temperature of its measuring junction with a radiation thermometer and
recording the emf of the thermocouple at that temperature. The measuring junction of the thermocouple is placed in an equalizing
block containing a cavity which approximates blackbody conditions. The radiation thermometer is sighted on the cavity in the
equalizing block and the blackbody temperature or true temperature of the block, including the measuring junction, is determined.
4.2 Since the spectral emissivity of the radiation emanating from a properly designed blackbody is considered unity (one) for
all practical purposes, no spectral emissivity corrections need be applied to optical pyrometer determinations of the blackbody
temperature.
4.3 Although the use of a radiation thermometer (Note 1) is less may require more effort and more complex apparatus to achieve
a sensitivity equivalent to that of commonly used thermocouples, a radiation thermometer has the advantage of being physically
separated from the test assembly; thus, its calibration is not influenced by the temperatures and atmospheres in the test chamber.
By comparison, a standard thermocouple that is used to calibrate another thermocouple must be subjected to the temperatures at
which the calibrations are performed and in some cases must be exposed to the environment that is common to the test
thermocouple. If a standard thermocouple is exposed to high temperatures or contaminating environments, or both, for long periods
of time, its calibration becomes questionable and the uncertainty in the bias of the calibration increases.
NOTE 1—Disappearing filament pyrometers are somewhat less sensitive than many of the thermocouples used above 800 °C (1472 °F).800°C (1472°F).
The advantages of physical separation of the disappearing filament pyrometer from the test assembly may still justify its use over use of a standard
thermocouple.
5. 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.
E452 − 02 (2018)
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,2100°C, the electrical shunting errors may be significant even for the best insulators available.
6. Sources of Error
6.1 The most prevalent sources of error (Note 2) in this method of calibration are: (1) improper design of the blackbody
enclosure, (2) severe temperature gradients in the vicinity of the blackbody enclosure, ( 3) heat conduction losses along the
thermoelements, and (4) improper alignment of the radiation thermometer with respect to the blackbody cavity and unaccounted
transmission losses along the optical path of the radiation thermometer.
NOTE 2—These are exclusive of any errors that are made in the radiation thermometer measurements or the thermocouple-emf measurements.
7. Apparatus
7.1 Furnace:
7.1.1 The calibration furnace should be designed so that any temperature within the desired calibration temperature range can
be maintained constant within a maximum change of 1 °C (1.8 °F)1°C (1.8°F) per minute in the equalizing block over the period
of any observation. Figs. 1-3 show three types of furnaces (1 and 2) that can be used for calibrating refractory-metal
thermocouples. Fig. 4 is a detailed drawing of the upper section of the furnace in Fig. 3. An equalizing block containing a
blackbody cavity is suspended in the central region of the furnace by means of support rods or wires. The mass of the support rods
or wires should be kept to a minimum to reduce heat losses by conduction. When the furnace is in operation, a sufficiently large
region in the center of the furnace should be at a uniform temperature to ensure that the temperature throughout the equalizing
block (when all test thermocouple assemblies are in position in the block) is uniform. At temperatures greater than
2000 °C,2000°C, furnace parts made from tantalum may introduce contamination of exposed thermoelements. In this case, it may
be desirable to fabricate heated furnace components from tungsten.
7.1.2 The blackbody cavity in the equalizing block should be designed in accordance with established criteria set forth in the
literature (3-7). Such factors as interior surface texture, diameter-to-depth ratio of the blackbody cavity opening, and internal
geometry can have an appreciable effect on the spectral emissivity of the cavity.
7.1.3 Figs. 5-7 show three typical equalizing block designs that are used in thermocouple calibrating furnaces. The design in
Fig. 5 is used in furnaces where the standard radiation thermometer is sighted horizontally into the blackbody through the hole in
the side of the block. This design is particularly useful in the calibration of bare-wire thermocouples since the lid on the blackbody
(or the entire blackbody) can be an electrically insulating material such ashafnium oxide or beryllium 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
supplier.)
7.1.4 The designs in Figs. 6 and 7 are used in furnaces where the standard radiation thermometer is sighted vertically into the
blackbody cavity. In cases where it is necessary to calibrate a number of thermocouples during one calibration run or to calibrate
thermocouple assemblies that are large in diameter and mass, the equalizing block designs in Figs. 6 and 7 are appropriate. If the
thermocouple assemblies being tested in these types of equalizing blocks are massive and can conduct a considerable amount of
heat away from the block, the 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
in
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