The standard procedure for measuring the gage factor of a lot of any particular type of strain gage is performed at room temperature. It is this value of the gage factor, along with its tolerance, which is given on the technical data sheet in each package of Micro-Measurements strain gages. Thus, at any temperature other than room temperature the gage factor is different, and a correction may be needed, according to the circumstances. Also given on each data sheet is the applicable graph of gage factor variation with temperature, such as those in Figs.7 and 8 . This information is all that is required to make the correction.

In general, any strain measurement data can be corrected (or adjusted) from one gage factor to another with a very simple relationship. Assume, for instance, that a strain, , was registered with the gage factor setting of the strain indicator at , and it is desired to correct the data to a gage factor of . The corrected strain, , is calculated from:

   Eq.(504.9)

When correcting for gage factor variation with temperature, can be taken as the package-data room-temperature gage factor at which the strain indicator may have been set, and the gage factor at the test temperature. Of course, when the test temperature is known with reasonable accuracy in advance, the gage factor control of the strain indicator can be set at , initially, and no correction is necessary. It should be noted in this case, however, that if thermal output corrections are to be made from the graph (or polynomial equation) on the technical data sheet in the gage package, the thermal output data must be adjusted from a gage factor of 2.0 (at which the thermal output was measured) to the test temperature gage factor, , being used for strain measurement.

The following relationship is used to determine the gage factor at the test temperature from the tabular and graphical data supplied in the gage package:

   Eq.(504.10)

where:

= percent variation in gage factor with temperature as shown in Figs. 7 and 8.
( Note : The sign of the variation must always be included.)

As a numerical example, using Eqs. (504.9) and (504.10), assume that the room-temperature gage factor of a 13 S-T-C, K-alloy gage is 2.05 and, with the instrument set at this value, the strain indication at +450° F (+230° C) is 1820 microstrain. Referring to Fig. 8 , for this case is -3, and, from Eq. (504.10),

Substituting into Eq. (504.9),

Since gage factor variation with temperature affects both the thermal output and the stress-induced strain, and because confusion may arise in making the corrections individually and then combining them, the following section gives equations for performing both corrections simultaneously.

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