ASTM E452-97
(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
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.
1.2.5Type 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 452 – 97
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Calibration of Refractory Metal Thermocouples Using a
Radiation Thermometer
This standard is issued under the fixed designation E 452; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 344 Terminology Relating to Thermometry and Hydrom-
etry
1.1 This test method covers the calibration of refractory
E 563 Practice for Preparation and Use of Freezing Point
metal thermocouples using a radiation thermometer as the
Reference Baths
standard instrument. This test method is intended for use with
E 988 Temperature-Electromotive Force (EMF) Tables for
types of thermocouples that cannot be exposed to an oxidizing
Tungsten-Rhenium Thermocouples
atmosphere. These procedures are appropriate for thermo-
E 1256 Test Methods for Radiation Thermometers (Single
couple calibrations at temperatures above 800 °C (1472 °F).
Waveband Type)
1.2 The calibration method is applicable to the following
E 1751 Guide for Temperature Electromotive Force (EMF)
thermocouple assemblies:
Tables for Non-Letter Designated Thermocouple Combi-
1.2.1 Type 1—Bare-wire thermocouple assemblies in which
nations
vacuum or an inert or reducing gas is the only electrical
insulating medium between the thermoelements.
3. Terminology
1.2.2 Type 2—Assemblies in which loose fitting ceramic
3.1 Definitions:
insulating pieces, such as single-bore or double-bore tubes, are
3.1.1 For definitions of terms used in this test method see
placed over the thermoelements.
Terminology E 344.
1.2.3 Type 2A—Assemblies in which loose fitting ceramic
3.1.2 radiation thermometer, n—radiometer calibrated to
insulating pieces, such as single-bore or double-bore tubes, are
indicate the temperature of a blackbody.
placed over the thermoelements, permanently enclosed and
3.1.2.1 Discussion—Radiation thermometers include instru-
sealed in a loose fitting metal or ceramic tube.
ments having the following or similar names: (1) optical
1.2.4 Type 3—Swaged assemblies in which a refractory
radiation thermometer, (2) photoelectric pyrometer, (3) single
insulating powder is compressed around the thermoelements
wavelength automatic thermometer, (4) disappearing filament
and encased in a thin-walled tube or sheath made of a high
pyrometer, (5) dual wavelength pyrometer or ratio radiation
melting point metal or alloy.
thermometer, (6) visual optical thermometer, (7) infrared
1.2.5 Type 4—Thermocouple assemblies in which one ther-
thermometer, (8) infrared pyrometer, and permutations on the
moelement is in the shape of a closed-end protection tube and
terms above as well as some manufacturer-specific names.
the other thermoelement is a solid wire or rod that is coaxially
3.2 Definitions of Terms Specific to This Standard:
supported inside the closed-end tube. The space between the
3.2.1 automatic radiation thermometer, n—radiation ther-
two thermoelements can be filled with an inert or reducing gas,
mometer whose temperature reading is determined by elec-
or with ceramic insulating materials, or kept under vacuum.
tronic means.
1.3 This standard does not purport to address all of the
3.2.2 disappearing filament pyrometer, n—radiation ther-
safety concerns, if any, associated with its use. It is the
mometer that requires an observer to match visually the
responsibility of the user of this standard to establish appro-
brightness of a heated filament mounted inside the radiation
priate safety and health practices and determine the applica-
thermometer to that of the measured object.
bility of regulatory limitations prior to use.
3.2.3 equalizing block, n—object, usually metal, that when
2. Referenced Documents placed in a nonuniform temperature region, has greater tem-
perature uniformity (due to its relatively high thermoconduc-
2.1 ASTM Standards:
tivity 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
This test method is under the jurisdiction of ASTM Committee E-20 on from that point to that emitted by a blackbody at the same
Temperature Measurementand is the direct responsibility of Subcommittee E20.04
temperature.
on Thermocouples.
Current edition approved Nov. 10, 1997. Published May 1998. Originally
published as E 452 – 72. Last previous edition E 452 – 89.
Annual Book of ASTM Standards, Vol 14.03.
E 452
3.2.5 spectral radiance, n—power radiated by a specimen for a particular thermocouple under a specific temperature and
in a particular direction, per unit wavelength, per unit projected chemical environment. However, during high temperature
area of the specimen, and per unit solid angle. calibration or application at elevated temperatures in vacuum,
3.2.6 spectral response, n—signal detected by a radiometer oxidizing, reducing or contaminating environments, and de-
at a particular wavelength of incident radiation, per unit power pending on temperature distribution, local irreversible changes
of incident radiation. may occur in the Seebeck Coefficient of one or both thermo-
3.2.7 test thermocouple, n—thermocouple that is to have its elements. If the introduced inhomogeneities are significant, the
temperature-emf relationship determined by reference to a emf from the thermocouple will depend on the distribution of
temperature standard. temperature between the measuring and reference junctions.
3.2.8 thermocouple calibration point, n—temperature, es- 5.3 At high temperatures, the accuracy of refractory metal
tablished by a standard, at which the emf developed by a thermocouples may be limited by electrical shunting errors
thermocouple is determined. through the ceramic insulators of the thermocouple assembly.
This effect may be reduced by careful choice of the insulator
4. Summary of Test Method
material, but above approximately 2100 °C, the electrical
4.1 The thermocouple is calibrated by determining the shunting errors may be significant even for the best insulators
temperature of its measuring junction with a radiation ther- available.
mometer and recording the emf of the thermocouple at that
6. Sources of Error
temperature. The measuring junction of the thermocouple is
placed in an equalizing block containing a cavity which 6.1 The most prevalent sources of error (Note 2) in this
approximates blackbody conditions. The radiation thermom- method of calibration are: (1) improper design of the black-
eter is sighted on the cavity in the equalizing block and the body enclosure, (2) severe temperature gradients in the vicinity
blackbody temperature or true temperature of the block, of the blackbody enclosure, (3) heat conduction losses along
including the measuring junction, is determined. the thermoelements, and (4) improper alignment of the radia-
4.2 Since the spectral emissivity of the radiation emanating tion thermometer with respect to the blackbody cavity and
from a properly designed blackbody is considered unity (one) unaccounted transmission losses along the optical path of the
for all practical purposes, no spectral emissivity corrections radiation thermometer.
need be applied to optical pyrometer determinations of the
NOTE 2—These are exclusive of any errors that are made in the
blackbody temperature.
radiation thermometer measurements or the thermocouple-emf measure-
4.3 Although the use of a radiation thermometer (Note 1) is
ments.
less may require more effort and more complex apparatus to
7. Apparatus
achieve a sensitivity equivalent to that of commonly used
thermocouples, a radiation thermometer has the advantage of
7.1 Furnace:
being physically separated from the test assembly; thus, its
7.1.1 The calibration furnace should be designed so that any
calibration is not influenced by the temperatures and atmo-
temperature within the desired calibration temperature range
spheres in the test chamber. By comparison, a standard
can be maintained constant within a maximum change of 1 °C
thermocouple that is used to calibrate another thermocouple
(1.8 °F) per minute in the equalizing block over the period of
must be subjected to the temperatures at which the calibrations
any observation. Figs. 1-3 show three types of furnaces (1 and
are performed and in some cases must be exposed to the
2) that can be used for calibrating refractory-metal thermo-
environment that is common to the test thermocouple. If a
couples. Fig. 4 is a detailed drawing of the upper section of the
standard thermocouple is exposed to high temperatures or
furnace in Fig. 3. An equalizing block containing a blackbody
contaminating environments, or both, for long periods of time,
cavity is suspended in the central region of the furnace by
its calibration becomes questionable and the uncertainty in the
means of support rods or wires. The mass of the support rods
bias of the calibration increases.
or wires should be kept to a minimum to reduce heat losses by
conduction. When the furnace is in operation, a sufficiently
NOTE 1—Disappearing filament pyrometers are somewhat less sensi-
large region in the center of the furnace should be at a uniform
tive than many of the thermocouples used above 800 °C (1472 °F). The
temperature to ensure that the temperature throughout the
advantages of physical separation of the disappearing filament pyrometer
from the test assembly may still justify its use over use of a standard
equalizing block (when all test thermocouple assemblies are in
thermocouple.
position in the block) is uniform. At temperatures greater than
2000 °C, furnace parts made from tantalum may introduce
5. Significance and Use
contamination of exposed thermoelements. In this case, it may
5.1 This test method is intended to be used by wire
be desirable to fabricate heated furnace components from
producers and thermocouple manufacturers for certification of
tungsten.
refractory metal thermocouples. It is intended to provide a
7.1.2 The blackbody cavity in the equalizing block should
consistent method for calibration of refractory metal thermo-
be designed in accordance with established criteria set forth in
couples referenced to a calibrated radiation thermometer.
the literature (4-8). Such factors as interior surface texture,
Uncertainty in calibration and operation of the radiation
thermometer, and proper construction and use of the test
furnace are of primary importance.
The boldface numbers in parentheses refer to the list of references at the end of
5.2 Calibration establishes the temperature-emf relationship this standard.
E 452
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)
E 452
(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)
diameter-to-depth ratio of the blackbody cavity opening, and electrically shorted. If this design is used in the calibration of
internal geometry can have an appreciable effect on the spectral Types, 2, 3, or 4 thermocouple assemblies (see 1.2), the
emissivity of the cavity.
blackbody lid can be metal since electrical insulation between
7.1.3 Figs. 5-7 show three typical equalizing block designs
the thermoelements is included as part of the assembly.
that are used in thermocouple calibrating furnaces. The design
NOTE 3—Warning: Beryllium oxide should be considered a hazardous
in Fig. 5 is used in furnaces where the standard radiation
material. Material Safety Data Sheets and precautions in handling this
thermometer is sighted horizontally into the blackbody through
toxic substance should be obtained from the supplier.
the hole in the side of the block. This design is particularly
useful in the calibration of bare-wire thermocouples since the 7.1.4 The designs in Figs. 6 and 7 are used in furnaces
where the standard radiation thermometer is sighted vertically
lid on the blackbody (or the entire blackbody) can be an
electrically insulating material such ashafnium oxide or beryl- into the blackbody cavity. In cases where it is necessary to
lium oxide. Thus, if the bare thermocouple wires should come calibrate a number of thermocouples during one calibration run
in contact wit
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