How a Pneumatic Pressure Transmitter Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How a Pneumatic Pressure Transmitter Works

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The Foxboro 11GM pneumatic pressure transmitter is a force-balance instrument used to measure pressure and transmits it as a proportional 3 – 15psig signal.
Pneumatic Pressure Transmitter. (Photo Credit: Foxboro)

As shown in the diagram of the transmitter, the pressure being measured is applied to a bellows capsule. The force on the capsule is transmitted through a flexure to the lower end of the force bar. The metal diaphragm seal serves as both a fulcrum for the force bar and as a seal for the pressure chamber. The force is transmitted through the flexure connector to the range bar which pivots on the range wheel.

Any movement of the range bar causes a minute change in the clearance between the flapper and the nozzle. This produces a change in the output pressure from the pneumatic relay to the feedback bellows until the force on the feedback bellows balances the force on the bellows capsule.

The output pressure established by this force-balance mechanism is the transmitted signal and is proportional to the pressure applied to the bellows capsule. This signal can be transmitted to a pneumatic receiver or controller for recording/indication or control purposes.