Shunt Cal is short for Shunt Calibration, and is a method for determining the full-scale output of a pressure transducer by electrically simulating a full-scale load. Either a shunt resistor is applied to the transducer circuit, or voltage is applied to an ancillary circuit designed to generate a full-scale output. That output can then be measured precisely to calibrate your control system’s reading of the pressure transducer.
Warning: technical circuit analysis coming!
Shunt Cal works on piezoresistive strain gauge-type pressure transducers. Four of these small gauges, which convert changes in mechanical strain and compression (due to pressure changes) into changes in electrical current, are connected in Wheatstone bridge formation. At minimum pressure the strain gauges produce minimal current; at full-scale pressure, the gauges allow peak current flow.
For simple, unamplified signals, Shunt Cal simulates maximum compression by placing a known resistor in parallel with one of the strain gauges (i.e., shunting the gauge), which drives the output to 80% of maximum.
This method of Shunt Calibration is quick, relatively easy, and doesn’t significantly increase the cost of manufacturing a sensor. Sizing the resistor is a matter of math, but after that, the primary hurdle is effectively and efficiently switching the resistor in and out of shunt position.
But moving pieces, especially conductive pieces, right on the gauge is quite a hurdle. Moving parts are more prone to failure than non-moving parts, and placing conductive paths right near the most sensitive electronic part of the sensor is almost asking to introduce noise and bias into the measurement signal.
Transducers with signal conditioning circuitry are also used to produce Shunt Cal. As part of the conditioning, an ancillary circuit can be added that produces a full-scale output. How that is achieved will depend on the design of the signal conditioning circuitry.
A sensor that uses conditioning circuitry for Shunt Cal will probably cost more than one that uses only a shunt resistor. But the trade-offs are substantial: greater reliability, and a full-scale Shunt Cal, rather than just 80%. Without the moving resistor, a sensor with conditioning Shunt Cal doesn’t have the breakdown concerns or noise and bias issues that a sensor with standard Shunt Cal would have. And the full-scale output is always more helpful for calibration than 80% of full-scale.
So which kind of Shunt Cal is in your pressure transducers? You’ll have to check the product literature for your sensor. And if that doesn’t tell you, call your sensor’s manufacturer. There’s a big difference between 80% and 100%.
There are two primary purposes for using Shunt Cal: one, as noted above, is calibrating your control system equipment that will be communicating with your transducer; the second is confirming that your transducer is functioning properly.
In the first case, it is important to know the full-scale output of your transducer. Performing a Shunt Cal allows you to compensate for the load resistance of the cable between your transducer and controller. Especially for unamplified signals, the difference between 19.925 mA and 20.125 mA can be significant. Shunt Calibration helps you accurately set your control system.
In the second scenario, Shunt Cal can be used for troubleshooting your transducer. If the output of your transducer is significantly short of 20 mA with Shunt Cal applied, you know that your system needs to be adjusted.
At APG, we currently offer Shunt Cal capabilities on our hammer union pressure transducers. All of our other pressure transducers can be calibrated and tested using full-scale pressure on a comparator. Because of their size and pressure range, hammer union pressure transducers are difficult to bench test. Thus, offering Shunt Cal capabilities provides a safe and easy method for testing.
If you have any questions about shunt cal or our hammer union pressure transmitter, contact us today. We'll discuss your application and point you in the right direction.
top photo credit: Brian Fitzharris via flickr cc
