Pneumatic Instrumentation

While electricity is commonly used as a medium for transferring energy across long distances, it is
also used in instrumentation to transfer information. A simple 4-20 mA current “loop” uses direct
current to represent a process measurement in percentage of span, such as in this example:

The transmitter senses an applied fluid pressure from the process being measured, regulates
electric current in the series circuit according to its calibration (4 mA = no pressure ; 20 mA =
full pressure), and the indicator (ammeter) registers this measurement on a scale calibrated to read
in pressure units. If the calibrated range of the pressure transmitter is 0 to 250 PSI, then the
indicator’s scale will be labeled to read from 0 to 250 PSI as well. No human operator reading that
scale need worry about how the measurement gets from the process to the indicator – the 4-20 mA
signal medium is transparent to the end-user as it should be.