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Shunt Calibration

# Shunt Calibration - the apparent strain that results and...

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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: ε C = D R R G F G Eq. 1 where: C = the calculated strain magnitude, R = the resistance change in the gage, R G = the gage nominal resistance (120 or 350 ), and F G = 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
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Unformatted text preview: the apparent strain that results, and comparing that with the expected value of strain associated with the imposed resistance change. The total resistance ( R T ) for two resistors (Gage and Shunt) in parallel is: R T = R G R S R G + R S Eq. 2 For example, consider a 120 Ω strain gage and a 270 k Ω shunt resistor. Using Equation 2, the total resistance for the two combined in parallel is 119.9467 Ω . This represents a change of 0.05331 Ω from the nominal 120 Ω 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 F C is: F C = e C e M Eq. 3 All subsequent measured strain values are multiplied by this correction factor to give a refined estimate of the actual strain value....
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