Introduction

The need for calibration arises frequently in the use of strain gage instrumentation. Periodic calibration is required, of course, to assure the accuracy and/or linearity of the instrument itself. More often, calibration is necessary to scale the instrument sensitivity (by adjusting gage factor or gain) in order that the registered output correspond conveniently and accurately to some predetermined input. An example of the latter situation occurs when a strain gage installation is remote from the instrument, with measurable signal attenuation due to leadwire resistance. In this case, calibration is used to adjust the sensitivity of the instrument so that it properly registers the strain signal produced by the gage. Calibration is also used to set the output of any auxiliary indicating or recording device (oscillograph, computer display, etc.) to a convenient scale factor in terms of the applied strain.

Direct vs. Indirect Calibration
There are basically two methods of calibration available - direct and indirect. With direct calibration, a precisely known mechanical input is applied to the sensing element of the measurement system, and the instrument output is compared to this for verification or adjustment purposes. For example, in the case of transducer instrumentation, an accurately known load (pressure, torque, displacement, etc.) is applied to the transducer, and the instrument sensitivity is adjusted as necessary to register the corresponding output. Direct calibration of instrument systems in this fashion is highly desirable, but is not ordinarily feasible for the typical stress analysis laboratory because of the special equipment and facilities required for its valid implementation.

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