ASTM E377-08(2015)
(Practice)Standard Practice for Internal Temperature Measurements in Low-Conductivity Materials
Standard Practice for Internal Temperature Measurements in Low-Conductivity Materials
SIGNIFICANCE AND USE
2.1 Internal temperature measurements are made on both in-flight vehicles and on-ground test specimens; and, because of the importance of the temperature measurements to the design of various missile and spacecraft heat shields, it is essential that care be taken to minimize the sources of error in obtaining these measurements.
2.2 Over the past several years, the problems of using thermocouples to obtain accurate temperature measurements in low-conductivity specimens have been studied by various people to isolate the sources of error and to establish improved temperature measurement techniques. The major sources of error are listed in this document and recommended solutions to the problems are given.
SCOPE
1.1 This practice covers methods for instrumenting low-conductivity specimens for testing in an environment subject to rapid thermal changes such as produced by rocket motors, atmospheric re-entry, electric-arc plasma heaters, and so forth. Specifically, practices for bare-wire thermocouple instrumentation applicable to sheath-type thermocouples are discussed.
1.2 The values stated in inch-pound units are to be regarded as the standard. The metric equivalents of inch-pound units may be approximate.
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: E377 − 08 (Reapproved 2015)
Standard Practice for
Internal Temperature Measurements in Low-Conductivity
Materials
This standard is issued under the fixed designation E377; 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 forming and using thermocouples , that is (1) electric welding
to form junctions, (2) maintaining cleanliness of junction area
1.1 This practice covers methods for instrumenting low-
and lead wires, (3) proper selection of thermocouple type and
conductivityspecimensfortestinginanenvironmentsubjectto
size, corresponding to both the temperature range to be
rapid thermal changes such as produced by rocket motors,
measured and the chemical compatibility with the
atmospheric re-entry, electric-arc plasma heaters, and so forth.
environment, and (4) proper use of instrumentation for readout
Specifically, practices for bare-wire thermocouple instrumen-
of thermocouple emf.
tation applicable to sheath-type thermocouples are discussed.
NOTE 1—Reader is referred toASTM MNL12 (1), and STP492 (2), as
1.2 The values stated in inch-pound units are to be regarded
well as Kinzie, P.A., Thermocouple Temperature Measurement (3), for
as the standard. The metric equivalents of inch-pound units
needed information.
may be approximate.
3.2 The most important sources of error beyond the above
1.3 This standard does not purport to address all of the
basic areas are the following:
safety concerns, if any, associated with its use. It is the
3.2.1 The thermal disturbance produced in the low-
responsibility of the user of this standard to establish appro-
conductivitymaterialatthevicinityofthethermocouplesensor
priate safety and health practices and determine the applica-
hot junction due to the sensor size, configuration, and instal-
bility of regulatory limitations prior to use.
lation method.
3.2.2 Electrical shorting of lead wires due to the electrical
2. Significance and Use
conductivity of the virgin or charred ablation material, and
2.1 Internal temperature measurements are made on both
3.2.3 Thermocouple sensor hot junction location accuracy.
in-flight vehicles and on-ground test specimens; and, because
4. Thermal Disturbance at Vicinity of Thermocouple
of the importance of the temperature measurements to the
Sensor Hot Junction
design of various missile and spacecraft heat shields, it is
essential that care be taken to minimize the sources of error in
4.1 General—Ideally, to measure the true internal tempera-
obtaining these measurements.
ture of a solid body, it would be desirable not to have any
foreign substance present that would create a disturbance
2.2 Over the past several years, the problems of using
affecting the natural flow of heat in the body. Since it is
thermocouplestoobtainaccuratetemperaturemeasurementsin
physically impossible to exclude the temperature sensor from
low-conductivity specimens have been studied by various
theinternalconfinesofthebody,itisnecessarythatthethermal
people to isolate the sources of error and to establish improved
disturbanceintroducedbythesensorbeminimizedforaccurate
temperature measurement techniques. The major sources of
temperature measurements (See Refs (4-10)).
error are listed in this document and recommended solutions to
the problems are given.
4.2 Thermocouple Junction Bead Diameter:
4.2.1 General—Excessively large junction beads result in
3. General
lower than true temperature measurements in low-conductivity
materials (conductivity of material less than conductivity of
3.1 Before proceeding to the major sources of error, it is
thermocouple wire) because of the heat sink effect of the bead.
assumed that the reader is familiar with basic methods of
4.2.2 Recommendations—To minimize this effect, the junc-
tion bead diameter should be no larger than 1.5 wire diameters
for butt-welded junctions and 2 wire diameters for other types
This practice is under the jurisdiction of ASTM Committee E21 on Space
Simulation andApplications of Space Technology and is the direct responsibility of of welds.
Subcommittee E21.08 on Thermal Protection.
Current edition approved May 1, 2015. Published June 2015. Originally
approved in 1968. Last previous edition approved in 2008 as E377 – 08. DOI: ANSI MC96.1-1975. Temperature Measurement Thermocouples (Sponsor
10.1520/E0377-08R15. ISA).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E377 − 08 (2015)
NOTE 1—If a number of thermocouples in depth are required, drill holes at varying locations on the circumference.
NOTE 2—Eliminate air pockets in junction plane by filling hole with same or similar compound as that used to make test specimen.
NOTE 3—This is a schematic representation and is not intended to be an engineering drawing.
FIG. 1 Summary of Recommended Practices for Mounting Thermocouples—Schematic Representation for “One-Piece” Cylindrical
Specimen
4.3 Thermocouple Wire in Isothermal Surface of Hot Junc- smaller. It is recommended also that the difference in thermal
tion: conductivity between thermocouple assembly and the sur-
4.3.1 General—Because many materials have low thermal rounding material be minimized by: (1) avoiding the use of
conductivity compared with those of thermocouple assemblies, relatively conductive (thermal) insulation (such as ceramic and
it has been found that certain methods of installing sensors can fiberglass types) around the portion of wire that is located in
produce significant errors in internal temperature measurement the isothermal surface that includes the thermocouple junction,
(1-4). Errors of several hundred degrees are possible unless and (2) maintaining good thermal contact with the low-
heat conduction away from the sensor hot junction, by the conductivity material by bonding the thermocouple to the
sensor materials, is minimized. Test results show that a specimen (thus eliminating air pockets) with the same or
thermocouple having a sufficient length of bare wire in the similar compound (such as an epoxy plastic) as that used to
isothermal surface that includes the junction will minimize make the specimen.
these errors.
5. Electrical Shorting by Conductive Char Layers
4.3.2 Recommendations—It is therefore recommended that
the configuration of the thermocouple sensor be such that the
5.1 General—The char layer formed by most organic ma-
leads perpendicular to the heat flow have a length equivalent to
terials becomes highly conductive (electrically) as pyrolysis
at least 25 wire diameters on both sides of the junction in the
progresses. Care
...
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: E377 − 08 E377 − 08 (Reapproved 2015)
Standard Practice for
Internal Temperature Measurements in Low-Conductivity
Materials
This standard is issued under the fixed designation E377; 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 practice covers methods for instrumenting low-conductivity specimens for testing in an environment subject to rapid
thermal changes such as produced by rocket motors, atmospheric re-entry, electric-arc plasma heaters, and so forth. Specifically,
practices for bare-wire thermocouple instrumentation applicable to sheath-type thermocouples are discussed.
1.2 The values stated in inch-pound units are to be regarded as the standard. The metric equivalents of inch-pound units may
be approximate.
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.
2. Significance and Use
2.1 Internal temperature measurements are made on both in-flight vehicles and on-ground test specimens; and, because of the
importance of the temperature measurements to the design of various missile and spacecraft heat shields, it is essential that care
be taken to minimize the sources of error in obtaining these measurements.
2.2 Over the past several years, the problems of using thermocouples to obtain accurate temperature measurements in
low-conductivity specimens have been studied by various people to isolate the sources of error and to establish improved
temperature measurement techniques. The major sources of error are listed in this document and recommended solutions to the
problems are given.
3. General
3.1 Before proceeding to the major sources of error, it is assumed that the reader is familiar with basic methods of forming and
using thermocouples , that is (1) electric welding to form junctions, (2) maintaining cleanliness of junction area and lead wires,
(3) proper selection of thermocouple type and size, corresponding to both the temperature range to be measured and the chemical
compatibility with the environment, and (4) proper use of instrumentation for readout of thermocouple emf.
NOTE 1—Reader is referred to ASTM MNL 12 (1), and STP 492 (2), as well as Kinzie, P.A., Thermocouple Temperature Measurement (3), for needed
information.
3.2 The most important sources of error beyond the above basic areas are the following:
3.2.1 The thermal disturbance produced in the low-conductivity material at the vicinity of the thermocouple sensor hot junction
due to the sensor size, configuration, and installation method.
3.2.2 Electrical shorting of lead wires due to the electrical conductivity of the virgin or charred ablation material, and
3.2.3 Thermocouple sensor hot junction location accuracy.
4. Thermal Disturbance at Vicinity of Thermocouple Sensor Hot Junction
4.1 General—Ideally, to measure the true internal temperature of a solid body, it would be desirable not to have any foreign
substance present that would create a disturbance affecting the natural flow of heat in the body. Since it is physically impossible
This practice is under the jurisdiction of ASTM Committee E21 on Space Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection.
Current edition approved Dec. 1, 2008May 1, 2015. Published January 2009June 2015. Originally approved in 1968. Last previous edition approved in 20022008 as
E377 – 96 (2002).E377 – 08. DOI: 10.1520/E0377-08.10.1520/E0377-08R15.
ANSI MC96.1-1975. Temperature Measurement Thermocouples (Sponsor ISA).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E377 − 08 (2015)
to exclude the temperature sensor from the internal confines of the body, it is necessary that the thermal disturbance introduced
by the sensor be minimized for accurate temperature measurements (See Refs (4-10)).
4.2 Thermocouple Junction Bead Diameter:
4.2.1 General—Excessively large junction beads result in lower than true temperature measurements in low-conductivity
materials (conductivity of material less than conductivity of thermocouple wire) because of the heat sink effect of the bead.
4.2.2 Recommendations—To minimize this effect, the junction bead diameter should be no larger than 1.5 wire diameters for
butt-welded junctions and 2 wire diameters for other types of welds.
4.3 Thermocouple Wire in Isothermal Surface of Hot Junction:
4.3.1 General—Because many materials have low thermal conductivity compared with those of thermocouple assemblies, it has
been found that certain methods of installing sensors can produce significant errors in internal temperature measurement (1-4).
Errors of several hundred degrees are possible unless heat conduction away from the sensor hot junction, by the sensor materials,
is minimized. Test results show that a thermocouple having a sufficient length of bare wire in the isothermal surface that includes
the junction will minimize these errors.
4.3.2 Recommendations—It is therefore recommended that the configuration of the thermocouple sensor be such that the leads
perpendicular to the heat flow have a length equivalent to at least 25 wire diameters on both sides of the junction in the same
isothermal surface that includes the hot junction.
4.4 Disturbances in Vicinity of Thermocouple Sensor Hot Junctions (7-10):
4.4.1 General—It is important that a minimum amount of disturbance be created in the material around the thermocouple
junction.
4.4.2 Recommendations—The disturbed material removal area (for placement of the thermocouple junction and lead wires)
should be as small as possible. A maximum of No. 36 AWG gage (0.127-mm or 0.005-in.) wire should be used for the
thermocouple wire from the junction and along the isothermal surface which includes the junction. Holes drilled for placement of
thermocouple wires should be 3 wire diameters or smaller. It is recommended also that the difference in thermal conductivity
between thermocouple assembly and the surrounding material be minimized by: (1) avoiding the use of relatively conductive
(thermal) insulation (such as ceramic and fiberglass types) around the portion of wire that is located in the isothermal surface that
includes the thermocouple junction, and (2) maintaining good thermal contact with the low-conductivity material
...
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