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Pressure Transmitter (4-20 mA)

Converts process pressure to a 4-20 mA current loop signal proportional to the engineering-unit range.

PLC address%IW64
SignalLoop current
BenchLive + faults
Try it: use this signal in PLC logic Learn the sensor first
FIELD DEVICE / 24 VDC

01 / Recognize it

What this sensor looks like

Learn the housing, active face, mounting, and connector before you meet it on a machine.

Representative real-world Pressure Transmitter (4-20 mA) hardware on an industrial workbench
Representative field appearance · form factors vary by manufacturer

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.
Field rule: identify by appearance, then verify the exact wiring, range, approvals, and output type from the device label and datasheet.

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

Paused

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)

24 VDC%IW64
9.60 mA
3.5 bar
7 bar

PLC channel

%IW64

RAW 9677

Engineering value

3.5 bar

Loop current

Output logic
Inject a field fault

Channel healthy

Signal is inside the expected operating range

Terminals
+24 V4–20 mA loop +AI− / 0 V

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.

PLC wiring reference

Trace the complete electrical path instead of treating the PLC tag as magic. Confirm the device datasheet before wiring real hardware.

  1. 1+24 V
  2. 24–20 mA loop +
  3. 3AI− / 0 V

Commissioning checkpoint

Confirm the transmitter range and PLC scaling use the same engineering units.

PLC address
%IW64
Expected signal
Loop current

Field questions

Frequently asked questions

What signal does a Pressure Transmitter (4-20 mA) send to a PLC?

Loop current is read at %IW64. The exact electrical connection is +24 V, 4–20 mA loop +, AI− / 0 V.

How do you commission a Pressure Transmitter (4-20 mA)?

Confirm the transmitter range and PLC scaling use the same engineering units.

Next skill

Connect it to PLC logic

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Free first success

Use the pressure transmitter (4-20 ma) signal in PLC logic

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Technical reference and worked-example guide

Pressure transmitter PLC guide: field reference

Direct answer

The reader can specify pressure reference and units, trace the electrical loop, scale raw values and distinguish process, impulse-line, transmitter, wiring and input faults.

Written for instrumentation and PLC learners connecting gauge, absolute or differential pressure transmitters to analog inputs.

an instrumentation calibration bench connecting pressure, temperature, load, level and smart sensors to PLC input channels and reference measurements while studying pressure transmitter range, loop, scaling and diagnostic evidence
System map / 02

NODE 01observable

Definition

Pressure type, reference, range, span, units, overpressure, wetted materials, process connection, impulse line, damping, output standard, loop supply, accuracy and failure indication.

NODE 02observable

Signal path

Process pressure through connection and sensing element to transmitter output, loop, PLC input, raw conversion, scaled value, quality, alarm and control decision.

NODE 03observable

Worked example

Zero, midpoint and high reference conditions produce stable engineering values within the declared system tolerance.

NODE 04observable

Limits

Blocked or leaking impulse line, overrange, reversed manifold, lost loop power, open circuit, wrong input mode, offset, drift, damping and restart.

NODE 05observable

Common mistake

A process, reference, connection, transmitter, supply, wiring, input, conversion, scale or unit mismatch.

NODE 06observable

Verification

The installation inspected and calibrated with approved pressure references and procedures.

Answer surface / 07

How is a pressure transmitter connected to a PLC?

Connect its compatible current, voltage or digital output through the complete supply and return path to the configured input, then scale the declared range and units.

What is the difference between gauge and absolute pressure?

Gauge pressure is referenced to local atmospheric pressure, while absolute pressure is referenced to a vacuum; the selected reference changes the engineering meaning.

Real 4-20ma pressure transmitter footage

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4–20 mA Pressure Transmitter — Signal, Scaling and Faults