How to apply a calibration resistor (Rcal) on a sensor already delivered ?

Preliminary remarks:

• This procedure only applies to Wheatstone bridge sensors without internal amplifier.
• The output signal (Vout) is therefore of the mV/V type.
• It requires the use of a millivoltmeter with sufficient accuracy: (eg ± 0.01 mV).
• Pay attention to the polarity of the measured signals.

The recommended value of Rcal depends on the resistance of the sensor and the target signal to be generated.

Examples:

• For a sensor with a resistance of 351 Ω, a 50 kΩ Rcal will result in an imbalance of ± 1.75 mV/V
• For a sensor with a resistance of 702 Ω, a 120 kΩ Rcal will cause an imbalance of ± 1.46 mV/V

How to proceed:

• Connect the sensor to the power supply (eg 10 VDC)
• Wait a few minutes
• Measure the supply voltage (Vexc)Measure the output signal of the sensor without load (Vo). Ex .: If Vexc = 10 V and the nominal sensitivity (Vout) = 2 mV/V, Vo must be between -0.2mV and +0.2 mV
• Connect the calibration resistor between the '' Exc.+ '' and '' Signal + '' terminals. Rem. This connection can also be done between the '' Exc.- '' and '' Signal - '' terminals, but the results will be slightly different.
• Measure the output signal (Vr)
• Calculate the variation of the output signal: Vcal (mV/V) = (Vr-Vo)/VexcThe simulated mechanical quantity (force or torque) is equal to:Full Scale (FS)* (Vcal/Vout)

This simulated value is only valid for the specific sensor, Rcal and used terminals.

It is therefore recommended to physically associate these components and to record the used terminals as well as the calculated values.

Example:

Force sensor:

• Full Scale (FS): 100 kN
• The sensitivity indicated on the control certificate (Vout): 2.000 mV / V
• The resistance indicated on the control certificate 702 Ω

Calibration resistor (Rcal): 120 kΩ

Measurements:

• Sensor supply voltage (Vexc): 10.2 V
• Signal without load(Vo): + 0.02 mV
• Sensor signal with Rcal connected between Power supply. + and Signal +: + 14.62 mV

Results:

• Variation of output signal Vcal = (Vr-Vo) / Vexc: (14.62-0.02)/10.2 = 1.431 mV/V
• Simulated force (Fcal) = FS*(Vcal / Vout): 100*(1.431 / 2.000) = 71.55 kN

Note: The accuracy of this method depends on the accuracy with which the nominal sensitivity of the sensor (Vo) is known and thus also the repeatability error of the sensor.