ASTM B70-90(2001)

Designation: B 70 – 90 (Reapproved 2001) An American National Standard
Standard Test Method for
Change of Resistance With Temperature of Metallic
Materials for Electrical Heating
This standard is issued under the fixed designation B 70; 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 dimension, in order to avoid disturbance of the temperature of
the specimen by conduction of heat to the colder parts of the
1.1 This test method covers the determination of the change
furnace.
of resistance with temperature of metallic materials for elec-
trical heating, and is applicable over the range of service
4. Electric Furnace
temperatures.
4.1 The furnace for heating the specimen shall be of such a
1.2 This standard does not purport to address all of the
type that the temperature can be controlled over the range from
safety concerns, if any, associated with its use. It is the
room temperature to the maximum desired. It shall be so
responsibility of the user of this standard to establish appro-
constructed that the specimens and the thermocouples can be
priate safety and health practices and determine the applica-
maintained at a uniform and constant temperature at desired
bility of regulatory limitations prior to use.
points within the working range. The specimen and thermo-
2. Significance and Use couples shall be so shielded as to prevent direct radiation from
hotter, or to colder, parts of the furnace.
2.1 The change in resistance with temperature for heating
4.2 In order to test the uniformity of the temperature in the
element materials is a major design factor and may influence
region to be occupied by the test specimen, a typical specimen
material selection. The measurement of this change is essential
and thermocouple shall be prepared and mounted in the center
to ensure that heating elements perform as designed. This test
of this region. The furnace shall then be heated to its maximum
method was designed to minimize the effect different manu-
temperature and maintained at this temperature until equilib-
facturing processes have on resistance change, thereby yielding
rium is reached. The specimen shall then be moved in the
results that are reproducible.
furnace in the direction of the maximum temperature gradient
3. Test Specimen and Leads
through a distance equal to the maximum dimension of the
largest specimen and thermocouple assembly which is to be
3.1 The test specimen shall be prepared from material as left
used in this furnace. The temperature of the typical specimen in
by the manufacturing process, and in a form suitable for
this position shall not differ from that in the normal position by
measuring its resistance in an electric furnace. When the
more than 10°C.
resistance is to be measured with a Kelvin bridge, potentiom-
eter, digital ohmmeter, or equivalent, a current lead shall be
5. Resistance Measurements
welded to each end of the specimen in such a manner that there
5.1 A Kelvin bridge, potentiometer, digital ohmmeter, or
will be no change of current distribution in the specimen during
equivalent shall be used when measuring specimens having
measurements. Potential leads, one at each end, shall be
resistances less than 10 V . A Wheatstone bridge may be used
attached by welding, at a distance from the corresponding
with specimens having resistances greater than 10 V. The
current lead not less than one tenth of the length of the
resistance of the specimen shall be measured with an accuracy
specimen between the potential leads.
of 0.1 %. The measuring current shall be so small that the
3.2 When the resistance is to be measured with a Wheat-
resistance of a specimen is not changed thereby as much as
stone bridge, only the current leads are required. The resistance
0.1 %. This condition may be determined experimentally or
of the leads in this case shall not exceed 1 % of the resistance
calculated from the power expended and the surface of the
of the specimen and the leads shall be made of the same type
specimen.
of alloy as the test specimen. For both methods of measure-
ment, the leads shall have a length within the heated zone of
6. Test Current
the furnace of at least 50 times their minimum transverse
6.1 To determine experimentally that the test current is not
too large, bring the specimen to a temperature (Note 1) where
This test method is under the jurisdiction of ASTM Committee B02 on
there is a relatively large uniform change of resistance with
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
temperature. Apply the test current and maintain it until the
B02.10 on Thermostat Metals.
Current edition approved May 25, 1990. Published July 1990. Originally
resistance of the specimen has become constant. Then increase
published as B 70 – 27 T. Last previous edition B 70 – 81.
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