Updated: Dec 2, 2020

In this article, we will learn about the pneumatic transmitter widely used before a few decades ago and are still in use today as well.

I also have covered an article on SMART transmitter and Analog transmitter widely used in process automation nowadays.

A pneumatic transmitter which requires pneumatic supply to operate also gives you a pneumatic signal as an output.

It requires 20 psi input pressure and gives 3-15 psi as an output.

If a transmitter works with a pneumatic supply, then it is called a pneumatic transmitter.

It uses compressed air supply as a working supply. The standard input supply for a pneumatic transmitter is 20 psi (1.406 kg/cm2) and standard output of a pneumatic transmitter is 3- 15 psi (0.2 – 1 kg/cm2) of the rated span of the instrument.

You can see an internal view of any pneumatic transmitter in the above-mentioned diagram.

A transmitter has a sensing part and a transmitting part. The sensing part of a transmitter is mechanical sensors depending upon the process variable.

The transmitting part of a pneumatic transmitter is flapper nozzle feedback mechanism.

It contains two ports, span and zero for the calibration adjustment attached with bellows. If the pressure increases bellow flex allowing flapper nozzle assembly to come closer thus escaping less pressure.

Watch below-attached video from BTC instrumentation on Foxboro's pneumatic differential pressure (DP) transmitter and it's a function.

As shown in the above figure if air supply provided is zero psi then the gap between flapper and nozzle remains wide enough allow all the pressure to exit.

If provide full 20 psi supply then due to gap between flapper and nozzle reduce it creates back-pressure allow smaller pressure to exit and other pressure will pass through output chamber leaves with 15 psi.

All pneumatic type transmitter, convertor uses the same principle for measurement of instrumentation.

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