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# LOAD CELL MEASUREMENT Resistance / Strain / Voltage – Oscar Zavaleta Weighing Industry

Interdependence of strain, resistance and signal upon load. A steady state model simplifies calculation and understanding. The mathematical model shows minimum and maximum change.

MECHANICS Load can be: force, weigh, torque for: shear, bending, or torsion. Compressive and tensile load have slightly different physical change.

ELECTRICAL Molecular displacement produces a piezoresistive effect. This effect produces a “linear” response to the load. Signal response can be correlated to a measurement.

MATH We can consider this idealized mathematical correlation. ε = Rg/(2 Rg + Gf) ((From VPG Tech note)

CALCULATIONS Capacity can be any value, here it has been portrayed as a 10,000 divisions.

Capacity    mV          Strain Gauge Ω                  ΔΩ                   µε 10,000       30.000     348.951 572 641        1.048 427 359     1,497.753 370 7 1                00.003     349.999 895 000        105 µ*                 0.105 000 173

*perhaps the limit of computational power at 12 digits

INSTRUMENTATION Digital multimeter resolution vs signal and division size

8 DMM ==>    0.285 72   µV ==> +100,000 div 7 DMM ==>    2.857 148 µV ==> ≈  10,000 div 6 DMM ==>  28.571 48   µV ==>       1,000 div

CONCLUSION Yes you can measure the resistance change on applied load. You need at least a 7 DMM or better to read 10,000 divisions. Otherwise, you can’t read resistance change with 3½ DMM.