Pressure Transmitter (4-20 mA)
Converts process pressure to a 4-20 mA current loop signal proportional to the engineering-unit range.
01 / Recognize it
What this sensor looks like
Learn the housing, active face, mounting, and connector before you meet it on a machine.
Hardware recognition
Know what to look for
Use the silhouette, active face, and connection style to identify the device before checking its part number and datasheet.
- Body and mounting
- A stainless process fitting supports a cylindrical or rectangular electronic head.
- Active face
- The wetted diaphragm sits behind the threaded or hygienic process connection.
- Cable and terminals
- An M12 plug, DIN connector, or cable carries the 4–20 mA loop.
02 / Understand the principle
Watch cause become a PLC signal
Follow the physical event through the sensing element and into the exact controller value.
Signal story / live loop
Pressure Transmitter (4-20 mA): cause to controller
Now showingPhysical event
Line pressure rises → Diaphragm deflects → %IW64 = 19,354
03 / Test and commission it
Commission it on the bench
Move the process, adjust the setpoint, invert the logic and inject faults. Watch the PLC value respond immediately.
Commissioning bench
Pressure Transmitter (4-20 mA)
PLC channel
%IW64
RAW 9677
Engineering value
3.5 bar
Loop current
Channel healthy
Signal is inside the expected operating range
Commissioning note: Confirm the transmitter range and PLC scaling use the same engineering units.
Field guide
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.
Use this when…
- Monitoring pipeline pressure in a water or compressed-air system
- Providing feedback for a PID pressure control loop
- Detecting a blocked filter or pump cavitation via differential pressure
Where you will see it
HVAC
Differential pressure transmitters across air-handling units control variable-speed drives on fans, keeping static pressure constant.
Oil and gas
Pressure transmitters on wellheads and separators give the SCADA system real-time process data for safety shut-down logic.
Next skill
Connect it to PLC logic
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