How a current to pressure transmitter (I to P transmitter) works?

What is an I to P converter?

An I to P transducer converts an analog electrical signal (4-20mA) to corresponding pneumatic pressure signal (3-15psi). Its aim is to translate a control system’s analog output into a accurate, repeatable pressure value to control pneumatic actuators / operators, pneumatic valves, dampers, valves, etc.

Models of this device are generally accessible in direct and inverse action and can be selected with inputs or outputs of complete or divided spectrum as the situation may be.

The common application of I to P converter is to provide pneumatic control action to control valve positioner. They are installed directly on the actuator of the valve. The device is mounted remotely on instrument pipe stands in many cases to decrease vibration.

Working principle:

To convert electrical signals into pneumatic signals, the I / P converter utilizes an electromagnetic force balance principle.

Pneumatic tools constructed in such a way that bellows forces in the same line of action directly oppose each other to constrain movement are known as “force balance” structures. Instruments constructed in such a way that the forces of bellows fight each other through distinct lengths of levers rotating around the same point of fulcrum.

The converter is usually equipped with a booster relay between the nozzle backpressure chamber and the feedback bellows. In a self-balancing pneumatic scheme,

The booster relay is used for these purposes: first, it boosts the mechanism’s open-loop gain so that its general gain can be more predictable and stable; second, it offers extra flow ability to fill and empty lengthy pneumatic signal pipes needed to transmit the air pressure signal to distant places.

The coil produces a magnetic field corresponding to the input analog signal. A deflector motor inside the coil is connected with a flapper valve that operates against a precision nozzle to generate a backpressure on a booster relay’s servo diaphragm. The input current flows in the coil and creates a force between the coil and the flapper valve to control the servo pressure and the output pressure.