When a strain gage is employed on a material other than that used in obtaining the manufacturer's thermal output data for that lot of gages, an S-T-C mismatch occurs. In such cases, the thermal output of the gage will differ from the curve supplied in the gage package. Consider, for example, strain measurements made at an elevated temperature on Monel with a strain gage of 06 S-T-C number, calibrated for thermal output on 1018 steel ( thermal expansion table ). The thermal expansion characteristics of Monel are somewhat different from 1018 steel, and the strain gage will produce a correspondingly different thermal output. Thus, if accurate strain measurement is required, the thermal output characteristics of the gage bonded to Monel must be measured over the test temperature range as described previously . For small temperature excursions from room temperature, the effect of the difference in expansion properties between Monel and 1018 steel is not very significant, and would commonly be ignored.

On the other hand, when the difference in thermal expansion properties between the thermal output calibration material and the material to which the gage is bonded for stress analysis is great, the published thermal output curve cannot be used directly for making corrections. Examples of this occur in constantan strain gages with S-T-C numbers of 30, 40, and 50. The principal application of these gages would normally be strain measurement on high-expansion coefficient plastics. But the thermal (and other) properties of plastics vary significantly from lot to lot and, because of formulation differences, even more seriously from manufacturer to manufacturer of nominally the same plastic. This fact, along with the general instability of plastics properties with time, temperature, humidity, etc., creates a situation in which there are no suitable plastic materials for use in directly measuring the thermal output characteristics of gages with S-T-C numbers of 30 and above. As an admittedly less-than-satisfactory alternative, the thermal output data provided with these gages are measured on 1018 steel specimens because of the stability and repeatability of this material.

As a result of the foregoing, it is always preferable when measuring strains on plastics or other materials with 30, 40, or 50 S-T-C gages (at temperatures different from the instrument balance temperature) to first experimentally determine the thermal output of the gage on the test material as described in a previous section . Using these data, corrections are then made as usual by subtracting algebraically the thermal output from the measured strain.

For use as a quick first approximation, the thermal output characteristics of 30, 40, or 50 S-T-C gages on a plastic (or on any other material) of known coefficient of expansion can be estimated by reversing the clockwise rotation of the thermal output curve which occurred when measuring the characteristics on a steel specimen. Assume, for example, that a 30 S-T-C gage is to be used for strain measurements on a plastic with a constant expansion coefficient of 35E-6/deg F (63E-6/degC) over the test temperature range. Assume also that the expansion coefficient of 1018 steel is constant at 6.7E-6/deg F (12.1E-6/deg C) over the same temperature range. With the strain indicator set at the gage factor of the strain gage, so that = , and noting that the ratio is normally close to unity for A-alloy gages, Eq.( 504.2 ) can be rewritten in simplified (and approximate) form as follows:

   Eq.(504.6)

( Note : Although the remainder of this example is carried through in only the Fahrenheit system to avoid overcomplicating the notation, the same procedure produces the equivalent result in the Celsius system.)

When specifically applied to 6.7 and 35E-6/deg F materials, Eq.(504.6) becomes:

    Eq.(504.7a)

and,

     Eq.(504.7b)

Solving Eq. (504.7a) for , and substituting into Eq.(504.7b),

       Eq.(504.8)

In other words, Eq.(504.8) states that the thermal output curve for the 30 S-T-C gage mounted on 1018 steel can be converted to that for the same gage mounted on a 35E-6/deg F plastic by adding to the original curve the product of the difference in expansion coefficients and the temperature deviation from room temperature ( always carrying the proper sign for the temperature deviation ). Figure 6 shows the thermal output curve for a 30 S-T-C gage as originally measured on a 1018 steel specimen, and as rotated counterclockwise to approximate the response on a plastic with an expansion coefficient of 35E-6/deg F.

Fig. 6 - Rotation of the thermal output function [from (A) to (B)] when a strain gage is installed on a material of higher themal expansion than that used by the manufacturer in S-T-C calibration.

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