Instrument Verification

( ... continued )

Signal-Conditioning Amplifier
Instead of a strain indicator, the instrumentation may consist of a signal-conditioning amplifier. This type of instrument is normally equipped with a gain control rather than a gage-factor control. Its output is simply a voltage which can then be supplied to an oscillograph or other device for recording. In the ideal instrument, the voltage at any gain setting should be strictly proportional to the bridge output signal. Thus, corresponding to Eq. ( 514.33 ),

     (514.39)

The object of calibration in this instance is to verify the instrument linearity; that is, to test whether is, in fact, constant. Calibration is accomplished by comparing the measured instrument output voltage to the bridge output signal at a series of different signal levels. Substituting Eq. ( 514.35 ) into Eq. (514.39),

Or, in general,

  (514.40)

After shunting one arm of the bridge with a calibration resistor, , the instrument output voltage is measured, and the constant, , calculated from Eq. (514.39). This operation is repeated at two or more different signal levels by successively shunting with appropriate calibration resistors. If the instrument is linear, variations in the calculated value of the instrument constant should not be greater than the tolerances on the parameters in Eq. (514.40).

Page 47 of 50