Pneumatic Instrument - Continuation

Air pressure may be used as an alternative signaling medium to electricity. Imagine a pressure
transmitter designed to output a variable air pressure according to its calibration rather than a
variable electric current. Such a transmitter would have to be supplied with a source of constantpressure
compressed air instead of an electric voltage, and the resulting output signal would be
conveyed to the indicator via tubing instead of wires:

The indicator in this case would be a special pressure gauge, calibrated to read in units of
process pressure although actuated by the pressure of clean compressed air from the transmitter
instead of directly by process fluid. The most common range of air pressure for industrial pneumatic
instruments is 3 to 15 PSI. An output pressure of 3 PSI represents the low end of the process
measurement scale and an output pressure of 15 PSI represents the high end of the measurement
scale. Applied to the previous example of a transmitter calibrated to a range of 0 to 250 PSI,
a lack of process pressure would result in the transmitter outputting a 3 PSI air signal and full
process pressure would result in an air signal of 15 PSI. The face of this special “receiver” gauge
would be labeled from 0 to 250 PSI, while the actual mechanism would operate on the 3 to 15 PSI
range output by the transmitter. Just like the 4-20 mA loop, the end-user need not know how the
information gets transmitted from the process to the indicator. The 3-15 PSI signal medium is once
again transparent to the operator.