Sourcing and Valve calibration using Loop calibrator

valve

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What are Loop calibrators?

Loop calibrators are troubleshooting equipment for 4-20mA current loop. These versatile instruments are generally capable of not only measuring current, but also sourcing current to unpowered devices in a loop, and also simulating loop-powered 4-20 mA transmitters.

Sourcing using Loop calibrator:

The loop calibrator is used for sourcing the 4-20mA current into an indicating instrument to test the function of that instrument independently. A calibrator sents a particular amount of current to the controller, in order to check that the controller properly senses and displays the analog current signal.

  • No transmitter need be included in this illustration, because the calibrator takes its place.

  • The calibrator functions here as an active source of current rather than a passive load

  • Not only does it supply the information (i.e. regulate the current), but it also provides the energy in the circuit. The DC power source inside the controller is not used for loop power, because the calibrator in “source” mode provides the necessary power to drive current through the 250 ohm resistor.

Sourcing for valve calibration:

A very common use of a loop calibrator in “source” mode is to test a control valve for proper calibration, quick response, and to measure friction. Here, the loop calibrator takes place of the loop controller output, serving as the sole source of current to the I/P transducer.

In practical, the loop calibrator is connected to the valve positioner. In addition to converting a 4-20 mA signal into an air pressure, the positioner also actively monitors the valve stem’s position to ensure it goes to the correct position for any given 4-20 mA command signal

A very common use of a loop calibrator in “source” mode is to test a control valve for proper calibration, quick response, and to measure friction. Here, the loop calibrator takes place of the loop controller output, serving as the sole source of current to the I/P transducer.

This circuit configuration is extremely useful to any instrument technician testing the response of a control valve, because it allows the signal to be finely adjusted while in the direct presence of the valve to monitor its motion.

If a control valve is suspected of having excessive friction in its moving parts, for instance, a technician may test the valve by incrementing and decrementing the loop calibrator’s source current in progressively smaller steps.

Large step-changes in current should cause the valve to overcome friction and move, but small step-changes will fail to move the valve mechanism when frictional forces exceed the incremental forces produced by the changing pressure.