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Remote Shunt Calibration
 



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It is common knowledge -- or at least it should be -- that shunt calibrating a remote bridge arm at the instrument binding posts will introduce a calibration error several times greater than the error anticipated from leadwire desensitization considerations alone. What probably is not common knowledge is that the indicated calibration magnitude will be increased, not decreased, as expected from desensitization considerations. The significant point is that if a remote bridge arm is shunt calibrated at the instrument terminals and attempts are made to mathematically or experimentally adjust gage factor or instrument gain to correct the error, the sensitivity will be adjusted in the wrong direction and increase the error. Worse, perhaps, is that this is an "unsafe" error, in that output from the transducer being used is larger than the instrument indicates.

To illustrate the point, two examples are shown for strain gages: a quarter-bridge and a full-bridge (which could be any type of strain gaged transducer). In both cases, the gages ( R G ) are 120 ohms and have a gage factor ( K ) of 2. The shunt value ( R SH ) of 59 880 ohms is selected because it produces an indicator reading of 1000 microinch/inch when used with 120 ohm gages, and there is no lead resistance. For each example, the indicated calibration strain is listed for:
(A) calibration at the instrument terminals
(B) calibration at the bridge arm by use of dedicated calibration leads
(C) calibration directly across the bridge arm.
Note that for high shunt values and reasonable lead resistances, (B) and (C) are equal. Further, for (C) -- and (B), if the values are approximately the same -- the error can be removed by adjusting the gage factor setting (i.e., gain) of the instrument.

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