Introduction

Commercial static strain indicators and signal conditioners vary considerably in their circuit details; and, although most of them are based upon some form of the Wheatstone bridge circuit, the bridge circuit is employed in differing ways in different instruments. Because of the many variations in instrument design, a completely general treatment of instrument nonlinearities is not practicable within the scope of this publication. There is, however, a large class of static strain indicators and signal conditioners with a more-or-less characteristic circuit arrangement (employing the "unbalanced" Wheatstone bridge), and displaying a characteristic nonlinearity. This publication has been prepared to provide a simple means for determining the magnitudes of the nonlinearity errors and for making corrections when necessary. It should be noted that the error and correction relationships given here apply only to instruments having the characteristics defined in the next section . For other strain indicators, the nonlinearity errors, if they exist, will have to be determined by direct calibration or from manufacturers' specifications.

The nonlinearity error occurs because, when strain measurements are made with an "unbalanced" Wheatstone bridge circuit (as described in the next section ), there are certain conditions under which the output of the bridge circuit is a nonlinear function of the resistance change(s) producing that output. The error due to the nonlinearity, when present, is ordinarily small, and can usually be ignored when measuring elastic strains in metals. However, the percentage error increases with the magnitude of the strain being measured, and can become quite significant at large strains (for example, the error is about 0.1% at 1000 microstrain, 1% at 10 000 microstrain, and 10% at 100 000 microstrain; or, as a convenient rule of thumb, the error, in percent, is approximately equal to the strain, in percent).

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