Pressure Transmitter (4-20 mA)

A pressure transmitter measures process pressure and converts it to a 4-20 milliamp current signal. At the minimum process pressure (0 bar, or whatever the lower range value is), the transmitter outputs exactly 4 mA. At the maximum (e.g. 10 bar), it outputs 20 mA. The current is proportional to the pressure anywhere between those extremes.

The 4-20 mA current loop is the industry standard for analog field devices because current is far more noise-immune than voltage over long cable runs. A voltage signal loses amplitude with cable resistance; a current source drives through the same resistance without losing signal integrity.

The 4 mA live zero is safety-relevant: if the signal falls below 4 mA the PLC knows the cable is broken or the transmitter has lost power — zero mA is not a valid process reading, it is a wire-fault. This lets the control system take protective action rather than treating a fault as "minimum pressure".

PLC analog input cards convert the current to a raw integer count (typically 0-27648 on Siemens, 6241-31208 for 4-20 mA on Allen-Bradley). A scaling block (FC105 in Siemens STEP 7, SCL in Studio 5000) converts the raw count to engineering units: bars, PSI, or kPa.

The internal sensing element is usually a piezo-resistive bridge or a ceramic capacitive cell. Both change their electrical properties under mechanical deformation caused by pressure, and the transmitter electronics condition this tiny signal into the standard current output.