Sensoray 518 - Manual (Page 48)

47
4. Use the NBS tables to obtain the thermocouple voltage Vsim corresponding to Tsim.
Note that Vsim is the corrected thermocouple voltage with the hot-junction at Tsim.
5. “Uncorrect” Vsim by subtracting off the correction voltage Vc. The resulting voltage
is Vtest.
6. Set up a calibrated millivolt calibrator to output Vtest. Apply Vtest to the 518 channel
under test. Verify that the 518 temperature output value is equal to Tsim.
Strain and Pressure Gages
A strain gage is often assembled into a device known as a load cell. In operation, a stress is
applied to the load cell, which in turn stresses the gage. Load cells vary widely in shape, size,
and material, depending on the application. Even so, all load cells serve the same basic purpose:
apply stress to the strain gage as an approximately linear function of the load.
Once a strain gage is mounted to a load cell, the entire assembly takes on a sensitivity
specification, typically rated in PSI units. This rating is defined in terms of the ratio of gage
output voltage to input voltage at full load. For example, a gage might be rated at “3mV/V”. This
means that the gage produces three millivolts out for every one volt of input excitation at a full
load condition.
Measurement Resolution
Strain gage excitation is supplied by the 518 coprocessor in the form of a strobed DC voltage.
This voltage varies slightly from one coprocessor board to another, but is on the order of 10 volts
in amplitude.
Knowing the bridge input voltage Vin and gage sensitivity specification K, the maximum gage
output voltage Vout may be computed:
Any 518 channel configured for a strain/pressure gage is measured with 5 microvolt resolution.
To compute the strain gage resolution N, divide the gage full load output voltage by 5 microvolts
(or multiply by 1/5uV):
Thus, the strain gage output is resolved by the coprocessor into one part in N. It follows that the
gage resolution is: