Shunt Calibration

The Shunt calibration method obtains two readings 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 is shunted by one of three selectable resistors located on a plug-in header inside the Wbk16.   Shunt calibration appears as "Shunt (RB)", "Shunt (RD)", and "Shunt (RF)" in the list of calibration methods.

Shunt calibration requires seven user entered parameters: Excitation, Gage Factor, Shunt Resistor Ohms, Bridge Resistance Ohms, Max Load, Quiescent Load, and Point 1 Units.   These parameters are described below:

Excitation -         This is 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 GF of 2 is typical.

Shunt Ohms -      The value in Ohms of the shunt resistor mounted on the header inside the Wbk16.

Bridge Ohms -     The resistance value of the gage.  Typically 120 or 350 Ohms.

Max Load -          Is the maximum load value that the gage will be 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 is applied to the gage in its quiescent state, enter zero.

Point 1 Units -    Is the minimum load value that the gage will be expected to measure.  This value could be greater than the min 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 Wbk16 channel.  The gage will be used in an environment where 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).   Refer to the Wbk16 section of the WaveBook users manual for detailed instructions on the installation of shunt resistors.  In the Wbk16 Calibration Parameters spreadsheet, the user would enter the values as follows:

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

*Note: 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.  First, install an appropriate non-precision shunt resistor of a value that creates the degree of bridge imbalance desired.   Second, press and hold the Alt-Key on the computers keyboard, then start the calibration process.  Once the calibration process has started you can release the Alt-Key.  The alternate Shunt calibration method calculates the shunted load value from shunted and un-shunted bridge voltage measurements, then performs the equivalent of a 2 Point Manual calibration.