Measurement Computing WBK Options - Manual (Page 71)

WBK16, Strain-Gage Module 949794 WBK16, pg. 31
Shunt (Internal Shunt) Calibration
The Shunt calibration method pertains to an internal shunt. For this method two readings are acquired from a
bridge. The first reading is obtained with the bridge in its quiescent state; the second is taken with one leg of the
bridge shunted by one of three selectable resistors. The resistors are located on a plug-in header inside the
StrainBook or WBK16 module. Shunt calibration appears as "Shunt (RB)", "Shunt (RD)", and "Shunt (RF)" in the
list of calibration methods.
The Internal Shunt Calibration requires that the user enter the following 7 parameters:
Excitation - The value of the constant voltage source used to excite the gage.
Gage Factor - The Gage Factor value of the gage used in the bridge. A Gage Factor of 2 is typical.
Shunt Ohms - The value in Ohms of the shunt resistor mounted on the header inside the StrainBook or
WBK16 module.
Bridge Ohms - The resistance value of the gage. Typically 120 or 350 Ohms.
Max Load - The maximum load value the gage is expected to measure. This value could be less than the
max rated load of the gage.
Quiescent Load - The at rest value of the load applied to the gage. If no load will be applied to the gage in
its quiescent state, enter “0” zero.
Point 1 Units - The minimum load value the gage is expected to measure. This value could be greater than
the minimum rated load of the gage.
Example: Excitation voltage is set to 2 volts. A Quarter Bridge circuit employing a 350 Ohm strain gage with a
Gage Factor of 2 and a full load rating of +/- 1500 µe is connected to a StrainBook [or WBK16 channel]. The gage
will be used in an environment were the expected range of measurement is limited to -200 to +1000 µe. This gage
[in its quiescent state] has a 500 µe load. A 349,650 Ohm precision resistor is available that will be mounted on the
plug-in header in Shunt location R(B). Instructions for installing shunt resistors are provided elsewhere in the
document. In this example the user would enter the following values in the Calibration Parameters spreadsheet:
Cal Method Shunt R(B)
Excitation = 2V.
Gage Factor = 2
Shunt Ohms = 349,650
Bridge Ohms = 350
Max Load = 1000
Quiescent Load = 500
Point 1 Units = -200
The accuracy of Shunt Calibration is directly related to the tolerances of the Shunt resistor, Gage(s), and Bridge
Completion resistors used in the circuit. In the event that a precision shunt resistor is unavailable, WaveView
provides an alternate way of calculating Shunt calibration constants. This method is as follows:
(a) Install an appropriate non-precision shunt resistor of a value that creates the degree of bridge
imbalance desired.
(b) Press and hold the key on the computer’s keyboard; then start the calibration process.
(c) Once the calibration process has started you release the key.
This alternate Shunt Calibration method calculates the shunted load value from shunted and un-shunted bridge
voltage measurements; and then performs the equivalent of a 2 Point Manual calibration.