The most common method for quick field calibration is the “shunt calibration” technique. This method applies a known, accurate resistance across one leg of the transducer, which simulates an actual physical stimulus when one is not present. Upon application of this resistance, the output of the transducer changes exactly as it would if a known pressure or load were applied.

In performing shunt calibration, the transducer should have no pressure or load applied, so that it is at “zero” initially. The data instrument’s ZERO control can then be adjusted to give a zero output on its indicator, or a zero voltage on its output terminals. (In the case of 4 mA to 20 mA outputs, this value would be a 4 mA.) The shunt calibration circuitry may then be activated by use of the front-panel SHUNT CAL button. A step change in amplifier output or reading will occur. If the amount of the step change does not agree with the expected change as indicated by the Transducer Calibration Datasheet, adjust the SPAN or GAIN control until it does. This will ensure that the amplification given by the data device will be correct, so that an actual stimulus will give correct readings.

It is advisable to recheck the zero when the shunt calibration resistance is removed, since there may be some interaction if the GAIN or SPAN control adjustments were large.

Strain gauge transducers with internal amplifiers usually have a shunt calibration resistor installed. The shunt calibration resistor may be activated by interconnecting two terminals on the connector. The wiring code section of the Transducer Calibration Datasheet will indicate which terminals are to be interconnected to activate the shunt calibration. For current output units (4 mA to 20 mA), several full cycles of adjusting the ZERO and SPAN controls may be required, since these controls interact greatly in such units.

Each product is shipped with a “Certificate of Calibration.” The certificate contains relevant calibration information for proper setup and usage in the user systems. If requested, calibration data may be recorded on the certificate.

Step-by-Step Procedure for Shunt Calibration

1. Make all required connections between the transducer and the instrument.

2. Apply power and allow ten to 20 minutes for stabilization.

3. Turn the ZERO adjustment so that the indicator reads zero. (If you are working with a psia transducer, the transducer must be evacuated to get zero. However, the unit can be shunt calibrated at atmosphere and the atmospheric reading added to the shunt calibration reading.)

4. From the Transducer Calibration Datasheet, obtain the transducer full scale output in millivolts per volt and the shunt calibration output in millivolts per volt.

5. Select and perform the proper calculation from the discussion below.

6. Depress the SHUNT CAL switch and turn the SPAN or GAIN adjustment to obtain the value calculated in Step 5.

Unamplified Transducers

Transducers with millivolt outputs usually do not have internal shunt calibration circuitry, but the effect of a known shunt calibration resistor being connected across the leads will be noted on the Transducer Calibration Datasheet. To determine the output of an unamplified transducer under shunt calibration conditions, perform the following calculation:

(Shunt Cal Factor in mV/V) x Excitation Voltage = Output Voltage

Substituting the values from the sample Transducer Calibration datasheet into the above equation provides the following:

(1,5090 mv/v) x (10 V)= 15.090 mV

Instrument with 0 V to 5 V Output

Consider next, an amplified transducer with a 0 V to 5 V output or an instrument, which has been factory calibrated with a transducer. Determining the output under shunt calibration conditions is done with the following equation:

Shunt Cal Factor in mV/V
Full-Scale Output in mV/V x Full-Scale Output = Output Voltage

Using the same datasheet as before, and assuming an amplified transducer with a 5 V full scale provides:

(1.5090 mV/V)
(3.0057 mV/V x 5 V = 2.510 V

Instrument with 4 mA to 20 mA output

Consider next, the case of a 4 mA to 20 mA output from an amplified transducer. Note that this represents a span of 16 mA, offset upward by 4 mA. To calculate the shunt cal output, use the following equation:

Using the same datasheet again, and assuming a 4 mA to 20 mA case:

Instrument Display

The following equation applies to instruments with a display:

Since the transducer shown on the Transducer Calibration Datasheet is a 1000-lb unit, the display should read: