Ideally, a strain gage bonded to a test part would respond only to the applied strain in the part, and be unaffected by other variables in the environment. Unfortunately, the resistance strain gage, in common with all other sensors, is somewhat less than perfect. The electrical resistance of the strain gage varies not only with strain, but with temperature as well. In addition, the relationship between strain and resis-tance change, the gage factor, itself varies with temperature. These deviations from ideal behavior can be important under certain circumstances, and can cause signi?cant errors if not properly accounted for. When the underlying phenomena are thoroughly understood, however, the errors can be controlled or virtually eliminated by compensation or correction.In Section 2.0 of this Tech Note, thermal output (some-times referred to as “temperature-induced apparent strain”) is defined, and the causes of this effect are described. Typical magnitudes of the thermal output are then given, followed by the commonly used methods for compensa-tion and correction. Section 3.0 treats gage factor variation with temperature in a similar but briefer manner since this error source is generally much less signi?cant. Methods for the simultaneous correction of both thermal output and gage factor errors are given in Section 4.0, accompanied by numerical examples.2.0 Thermal OutputOnce an installed strain gage is connected to a strain indi-cator and the instrument balanced, a subsequent change in the temperature of the gage installation will normally produce a resistance change in the gage. This temperature-induced resistance change is independent of, and unrelated to, the mechanical (stress-induced) strain in the test object to which the strain gage is bonded. It is purely due to temperature change, and is thus called the thermal output of the gage.Thermal output is potentially the most serious error source in the practice of static strain measurement with strain gages. In fact, when measuring strains at temperatures remote from room temperature (or from the initial balance temperature of the gage circuit), the error due to thermal output, if not controlled, can be much greater than the mag-nitude of the strain to be measured. At any temperature, or in any temperature range, this error source requires careful consideration; and it is usually necessary to either provide compensation for thermal output or correct the strain mea-surements for its presence.Thermal output is caused by two concurrent and alge-braically additive effects in the strain gage installation.
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