Answer / carl_ellis

A controller (the remote device) generates an electronic
4-20mA output signal which tells the valve what its position
should be.

The 4-20mA signal is an input to an I/P
(current-to-pneumatic) transducer, which creates a pneumatic
output signal proportional to the 4-20mA electronic signal
from the controller, probably a 3-15psi pneumatic signal.

The pneumatic control signal is an input to the pneumatic
positioner, which mechanically senses the valve's position.
The positioner drives the actuator with as much supply air
pressure as is necessary to drive the valve to position
demanded by the control signal.

Carl Ellis
Measure First

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