Instrument Verification

The shunt-calibration procedures described previously are intended specifically for instrument scaling purposes; that is, for adjusting the instrument output to match a given simulated surface strain. They are not directly related to the question of verifying the linearity and/or absolute accuracy of a strain-measuring instrument. It is implicitly assumed in the preceding sections of this publication that the instrument involved is perfectly linear in its response characteristics and, if direct-indicating, is perfectly accurate. In practice, however, it is necessary to periodically verify the accuracy of the instrument by calibration; and methods for accomplishing this are given here.

By far the most reliably accurate means for instrument verification is through the use of a laboratory-standard calibrator such as the Vishay Measurements Group Model 1550A . This instrument, which incorporates true tension and compression strain simulation, provides precision calibration of strain indicators to an accuracy of 0.025 percent. It also eliminates errors due to the tolerances on the strain gage and shunt resistances. The calibrator is equipped with three decades of switches, which permit rapid calibration in small steps over a very wide strain range (to approximately 100 000 microstrain).

Whether verification of the strain indicator is to be done with a precision calibrator or by shunt calibration, it is important that the procedure be unaffected by leadwire resistance. When verifying instrument accuracy with the Model 1550A, for instance, the calibrator should be connected to the strain indicator with short leads of generous wire size. Similarly, with shunt calibration, the leadwire resistance in the shunted bridge arm should be negligibly small. This can be accomplished, for calibration purposes, by connecting an installed strain gage or a stable precision resistor directly across the active gage terminals of the strain indicator. Either the active or dummy arm of the bridge circuit can then be shunted to produce, correspondingly, a downscale or upscale calibration signal. If the active arm is a strain gage, and is to be shunted, the installed resistance of the gage must be known accurately.

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