|
Compensation for Thermal Output -
Self-Temperature-Compensated Gages
The metallurgical properties of certain strain
gage alloys - in particular, constantan and modified
Karma (Micro-Measurements
A
and
K
alloys, respectively) - are such that these alloys
can be processed to minimize the thermal output over
a wide temperature range when bonded to test
materials with thermal expansion coefficients for
which they are intended. Strain gages employing these
specially processed alloys are referred to as
self-temperature-compensated
.
3-Wire Circuit
Since the advent of the self-temperature-compensated
strain gage, the requirement for a matching
unstrained dummy gage in the adjacent arm of the
Wheatstone bridge has been relaxed considerably. It
is now normal practice when making strain
measurements at or near room temperature to use a
single self-temperature-compensated gage in a
quarter-bridge arrangement (with a three-wire
hookup), completing the bridge circuit with a stable
fixed resistor in the adjacent arm (Fig. 3). Such
"bridge-completion" resistors, with very
low temperature coefficients of resistance, are
supplied by Micro-Measurements and are incorporated
in most modern strain indicators.
Fig.3 - A single self-temperature-compensated
strain gage in a 3-wire quarter-bridge circuit
exemplifies modern strain gage practice for stress
analysis measurements.
Characteristics of Compensated Gages
Figure 4 illustrates the thermal output
characteristics of typical
A-
and
K-
alloy self-temperature-compensated strain gages. As
demonstrated by the figure, the gages are designed to
minimize the thermal output over the temperature
range from about 0° to +400° F (-20°
to +205° C). When the
self-temperature-compensated strain gage is bonded to
a material having the thermal expansion coefficient
for which the gage is intended, and when operated
within the temperature range of effective
compensation, strain measurements can often be made
without the necessity of correcting for thermal
output. If correction for thermal output is needed,
it can be made as shown in the following sections.
Fig.4 - Typical thermal output variation with
temperature for self-temperature-compensated
constantan (A-alloy) and modified Karma (K-alloy)
strain gages.
Self-temperature-compensated strain gages can also
be used in the manner described
previously
. That is, when circumstances are such that a pair of
matched gages can be used in adjacent arms of the
bridge circuit, with both gages maintained at the
same temperature, and with one of the gages
unstrained (or strained at a determinate ratio to the
other gage), excellent temperature compensation can
be achieved over a wide temperature range.
S-T-C Number
The designations of Micro-Measurements
self-temperature-compensated strain gages include a
two-digit S-T-C number identifying the nominal
thermal expansion coefficient (in ppm/° F) of
the material on which the gage will exhibit optimum
thermal output characteristics as shown in Fig. 4.
Micro-Measurements constantan alloy gages are
available in the following S-T-C numbers: 00, 03, 05,
06, 09, 13, 15, 18, 30, 40, and 50. S-T-C numbers of
30 and higher are intended primarily for use on
plastics. In K alloy, the range of S-T-C numbers is
more limited, and consists of 00, 03, 05, 06, 09, 13,
and 15. For reference convenience, a
table
lists a number of engineering materials, and gives
nominal values of the Fahrenheit and Celsius
expansion coefficients for each, along with the S-T-C
number which would normally be selected for strain
measurements on that material. The table also
identifies those test materials used in determining
the published thermal output curves for
Micro-Measurements self-temperature-compensated
strain gages.
If a strain gage with a particular S-T-C number is
installed on a material with a nonmatching
coefficient of expansion, the thermal output
characteristics will be altered from those shown in
Fig. 4 by a general rotation of the curve about the
room-temperature reference point. When the S-T-C
number is lower than the material expansion
coefficient, the rotation is counterclockwise; and
when higher, clockwise. Rotation of the thermal
output curve by intentionally mismatching the S-T-C
number and expansion coefficient can be used to bias
the thermal output characteristics so as to favor a
particular working temperature range.
|
Vishay Measurements Group
