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Before going into a discussion of temperature compensation techniques, it is important to consider for a moment the kind of information a strain gage can provide under constant or variable high temperature conditions. Under constant temperature conditions, it is possible to allow time for all specimen parts to come to the same temperature before an external load is applied. If the entire specimen is made of the same material, it may be considered unstressed although the various parts of the specimen have new dimensions and the strength characteristics have changed. This assumes that the specimen is free to expand without restraint. If a load is applied to the specimen, strain gages suitably mounted on the specimen should measure strain due to applied load. Under conditions of rapid heating, we know that thinner sections will increase in temperature more rapidly than thicker sections and strains develop in the specimen due to structural restraints. These strains are to be measured as well as strains due to externally applied loads. It would seem that an acceptable device for measuring strain should provide very low, ideally zero, output with change in temperature when applied to a homogeneous specimen. It is only natural that in the development of such a gage, the greatest emphasis be placed on a resistance type strain gage, since this gage has served so well for all types of strain measurements under ordinary temperature conditions. It is also important to consider that millions of dollars have been invested in instrumentation equipment for use with resistance strain gages.
Naval Air Material Center, Aeronautical Structures Laboratory, Philadelphia, Pa.