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Strain Gage Thermal Output
and Gage Factor Variation with Temperature

Gage Factor Variation With Temperature

The alloys used in resistance strain gages typically exhibit a change in gage factor with temperature. In some cases, the error due to this effect is small and can be ignored. In others, depending upon the alloy involved, the test temperature, and the required accuracy in strain measurement, correction for the gage factor variation may be necessary.



Fig. 7 - Gage factor variation with temperature for constantan (A-alloy) and isoelastic (D-alloy) strain gages.

Figure 7 shows the variation of gage factor with temperature for constantan and isoelastic (Micro-Measurements A and D alloys, respectively). It can be seen from the graph that the effect in the A alloy is essentially linear, and quite small at any temperature, typically being in the order of 1% or less per 100° F (2% or less per 100° C). Thus, for a temperature range of, say, + 100° F ( + 50° C), about room temperature, correction may not be necessary. At more extreme temperatures, when justified by accuracy requirements, the correction can be made separately, or combined with the thermal output correction.

The variation of gage factor in the D alloy, while very modest and flat between room temperature and +200° F (+90° C), steepens noticeably outside of this range. However, even for temperatures where the gage factor deviation is several percent, correction may not be practical. This is because D alloy is used primarily for purely dynamic strain measurement, under which conditions other errors in the measurement system may greatly overshadow the gage factor effect.



Fig. 8 - Variation of K-alloy gage factor with temperature and S-T-C number.

As shown above, the gage factor variation with temperature for modified Karma ( K alloy ) is distinctly different from that of the A and D alloys. The gage factor variation is nearly linear with temperature, as it is for A alloy, but the slope is negative and is a function of the S-T-C number, becoming steeper with higher numbers.



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