Download Uncertainty Analysis (adapted from Wheeler and Ganji)

OREGON STATE UNIVERSITY
ME 451 - INTRODUCTION TO INSTRUMENTATION AND MEASUREMENT SYSTEMS
Shunt Calibration Guidelines
1. Direct Resistance Measurement of Strain Gages
It is possible (but not optimal) to monitor a strain gage by direct resistance measurement. The National
Instruments 9219 C-series module does precisely that when used in “quarter bridge” mode. It sources a
current through the gage, measures the voltage drop across the gage, calculates the resistance, and
finally calculates strain using:
R
C 
RG
FG
Eq. 1
where:

C = the calculated strain magnitude,
R = the resistance change in the gage,
RG = the gage nominal resistance (120 or 350 ), and
FG = the gage factor (~ 2.0).
Shunt calibration is a means of simulating an applied strain by direct manipulation of the gage resistance.
It is used in place of a direct application of known strain to the gage, which is difficult in practice. The
resistance change is accomplished by placing a high ohm resistor in parallel with the strain gage, reading
the apparent strain that results, and comparing that with the expected value of strain associated with the
imposed resistance change.
The total resistance (RT) for two resistors (Gage and Shunt) in parallel is:
RT 

RG RS
RG  RS
Eq. 2
For example, consider a 120 strain gage and a 270 k shunt resistor. Using Equation 2, the total
resistance for the two combined in parallel is 119.9467 . This represents a change of 0.05331  from
the nominal 120  value. Using Equation 1, the strain that would be read, given perfect circuitry and a
gage factor of 2.0, for that value of resistance change is C = 0.000222. If your system actually reads a
value of M when the shunt resistor is in the circuit, then the correction factor FC is:
FC 
C
M
Eq. 3
All subsequent measured strain values are multiplied by this correction factor to give a refined estimate of
the actual strain value.
