Approximate correction for thermal output can be accomplished most directly and easily using the graph ( Fig. 5 ) supplied in each package of self-temperature-compensated gages. This simple method of correction is based on the fact that the gage factors of A- and K- alloy gages are close to 2.0, which is the standardized gage-factor setting employed in calibrating the gages for thermal output. Adjustment of the thermal output data for a different instrument gage-factor setting is described later.

The first step in the correction procedure is to refer to the graph and read the thermal output corresponding to the test temperature. Then, assuming that the strain indicator was balanced for zero strain at room temperature (the reference temperature with respect to which the thermal output data were measured), subtract the thermal output given on the graph from the strain measurements at the test temperature, carrying all signs . This procedure can be expressed analytically as follows:

where:

= uncorrected strain measurement, as registered by the strain indicator.
= partially corrected strain indication - that is, corrected for thermal output, but not for gage factor variation with temperature.
= thermal output, in strain units, from the package technical data sheet.

As an example, assume that, with the test part under no load and at room temperature, the strain indicator was balanced for zero strain. At the test temperature of +250° F (+121° C), the indicated strain is +2300 microstrain. Referring to Fig. 5 , assuming that the graph was the one in the gage package, the thermal output at test temperature is -100 microstrain. From Eq. (504.3), the corrected strain is thus 2300 - (-100) = 2400 microstrain. Had the indicated strain been negative, the corrected strain would be: -2300 - (-100) = -2200 microstrain. If the instrument were balanced for zero strain at some temperature other than +75° F (+24° C), the value of for use in Eq. (504.3) is the net change in thermal output in going from the balance temperature to the test temperature. That is, = (T ₂ ) - (T ₁ ), carrying the sign of the thermal output in each case.

Page 9 of 17