It is worth noting, for quarter-bridge circuits, that scaling the instrument by shunting the internal dummy gage (which is usually a stable precision resistor) can offer distinct advantages in calibration accuracy. It is common practice, for instance, to calculate or select the value of the shunt calibration resistor on the basis of the nominal gage resistance. But the resistance of the installed gage generally differs from the nominal, due both to its initial resistance tolerance and to a further change in resistance during installation. When this occurs, and the active gage is shunted for compression scaling, the simulated strain magnitude is in error accordingly. The extent of the error can be approximated by the method given in the " Accuracy Considerations " section.

One technique for avoiding most of the error due to deviation in the gage resistance is to temporarily replace the active gage in the bridge circuit with a precision resistor equal to the nominal resistance of the gage. The instrument is then scaled (in compression) by shunting the fixed resistor with a calibration resistor calculated from Eq. ( 514.7 ). After scaling, the active gage is reconnected to the bridge circuit. It is usually much more convenient, however, and about equally accurate, to scale in the tension direction by simply shunting the internal dummy with a resistor calculated from Eq. ( 514.23 ). When the leadwire resistance is negligible, this procedure is exact, and independent of the installed gage resistance. Even with modest leadwire resistance (say, less than /10), the error due to a few ohms of gage resistance deviation is small enough to be ignored. In case of doubt, the installed gage resistance should be measured. If the resistance is significantly beyond the manufacturer's tolerance, one of the two foregoing procedures should always be used for shunt calibration.

Page 36 of 50