Accuracy Considerations

As described in the preceding sections, shunt calibration can be used for either system scaling or instrument verification purposes. In both cases, the greatest attainable accuracy with the procedure is limited by errors (deviations from the nominal or assumed value) in the variables which enter into the calibration calculations. The error sensitivity of the method can be demonstrated most easily with a generalized form of the basic shunt-calibration relationship [see Eqs. ( 514.3 ), ( 514.4 ), and ( 514.5 )].

Let in Eq. (514.3) represent the actual resistance of the strain gage, after installation. The factor , in Eq. (514.4) is replaced by a numerical constant, , to emphasize the fact that the nominal resistance of the gage is not changed by gage installation. Then, the relationships in Eqs. (514.3) and (514.4) can be re-expressed as:

  (514.3a)

  (514.4a)

Combining Eqs. (514.3a) and (514.4a), and solving for the simulated strain,

  (514.41)

The total differential of the simulated strain can be written:

After performing the partial differentiations and dividing through by

the following is obtained:

  (514.42)

For small deviations, the differentials can be replaced by finite differences, or:

  (514.43)

( continued ... )

Page 48 of 50