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Self-Temperature
Compensation
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An important property shared by constantan and
modified Karma strain gage alloys is their
responsiveness to special processing for
self-temperature compensation.
Self-temperature-compensated strain gages are
designed to produce minimum thermal output
(temperature-induced apparent strain) over the
temperature range from about -50 deg to +400 deg F
(-45 deg to +200 deg C). When selecting either
constantan (
A-alloy
) or modified Karma (
K-alloy
) strain gages, the self-temperature-compensation
(S-T-C) number must be specified. The S-T-C number is
the approximate thermal expansion coefficient in ppm/
deg F of the structural material on which the strain
gage will display minimum thermal output.
The accompanying Thermal Output graph illustrates
typical thermal output characteristics for A and K
alloys. The thermal output of uncompensated
isoelastic (D) alloy is included in the same graph
for comparison purposes. In normal practice, the
S-T-C number for an A- or K-alloy gage is selected to
most closely match the thermal expansion coefficient
of the test material. However, the thermal output
curves for these alloys can be rotated about the
room-temperature reference point to favor a
particular temperature range. This is done by
intentionally mismatching the S-T-C number and the
expansion coefficient in the appropriate direction.
When the selected S-T-C number is lower than the
expansion coefficient, the curve is rotated
counterclockwise. An opposite mismatch produces
clockwise rotation of the thermal output curve. Under
conditions of S-T-C mismatch, the thermal output
curves for A and K alloys (supplied with each package
of strain gages) do not apply, of course, and it will
generally be necessary to calibrate the installation
for thermal output as a function of temperature.
For additional information on strain gage
temperature effects, see Vishay Measurements Group
Tech Note TN-504,
Strain Gage Thermal Output and Gage Factor
Variation with Temperature
.
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