4-20 mA Pressure Transmitters: Wiring, Scaling and Calibration Explained

Quick answer: a 4 to 20 mA pressure transmitter sends a current between 4 mA (bottom of the range) and 20 mA (top of the range) through a two-wire loop powered by a 24 V DC supply. To convert the current to pressure, use pressure = (current − 4) ÷ 16 × range. A reading below about 3.6 mA or above about 21 mA usually means a fault, and 0 mA means a broken wire or no power. This guide shows how to wire the loop, scale the signal in a controller, check that the supply is big enough, calibrate with a five-point test, and fix the most common problems, with worked examples throughout.

Why 4 to 20 mA is so widely used

Industry has used the 4 to 20 mA loop for decades because it solves several problems at once.

• Noise resistance: a current signal is less affected by electrical interference than a small voltage.
• Long cables: the current is the same at both ends of the cable, so cable resistance does not change the reading.
• Live zero: zero pressure is 4 mA, not 0 mA, so a broken wire (0 mA) is easy to tell from zero pressure.
• Two wires: the same two wires can both power the transmitter and carry the signal.
• Simple testing: you can check the loop with a basic milliamp meter.

If you are not sure how this differs from a voltage output, read our transducer vs transmitter vs sensor guide.

The conversion formula

Because 4 to 20 mA covers a span of 16 mA, the maths is:

Pressure = (I − 4) ÷ 16 × range, where I is the measured current in milliamps.

And the reverse: I = 4 + (pressure ÷ range) × 16.

Worked example 1: a 0 to 10 bar transmitter

Check a reading of 13.6 mA: (13.6 − 4) ÷ 16 = 0.6, and 0.6 × 10 bar = 6 bar.

Worked example 2: a 0 to 400 bar hydraulic transmitter

A reading of 10.4 mA gives (10.4 − 4) ÷ 16 = 0.4, and 0.4 × 400 = 160 bar. A relief valve set at 210 bar would show as 4 + (210 ÷ 400) × 16 = 12.4 mA.

Worked example 3: a compound range

A -1 to 5 bar transmitter has a span of 6 bar. 4 mA is -1 bar, 20 mA is 5 bar, and atmospheric pressure (0 bar gauge) is at 4 + (1 ÷ 6) × 16 = 6.67 mA.

Wiring a two-wire loop

A two-wire transmitter is “loop powered”. It connects in series with the supply and the receiver.

24 V DC (+) ---> Transmitter (+)
Transmitter (-) ---> Controller analogue input (+)
Controller analogue input (-) ---> 24 V DC (-)

Steps:

1. Isolate the power and check the datasheet for terminal markings and the supply voltage range.
2. Connect the positive of the supply to the positive terminal of the transmitter.
3. Connect the transmitter’s negative terminal to the positive of the controller’s current input.
4. Connect the controller input’s negative back to the supply negative.
5. Use shielded twisted-pair cable, and ground the shield at one end only, as your site standard says.
6. Keep the cable away from motor and mains cables.
7. Seal the cable entry so water cannot get in.

Two-wire, three-wire and four-wire devices

• Two-wire: loop-powered 4 to 20 mA. The most common industrial style.
• Three-wire: supply, ground and signal. Typical for 0 to 10 V outputs. The 0 to 10 V version of a sensor like the WIKA A-10 uses this style.
• Four-wire: separate power and signal pairs, used on some instruments.

Always follow the wiring diagram in the datasheet for your exact model.

Is the supply big enough? The loop voltage check

Every device in the loop uses some voltage. The transmitter needs a minimum voltage to work, and the receiver resistor drops voltage as current flows. A quick check:

Required supply = transmitter minimum voltage + (20 mA × total loop resistance)

Example (the numbers are illustrations; use your datasheet values):

• Transmitter minimum voltage: 10 V
• Controller input resistance: 250 ohms
• Cable resistance: 30 ohms
• Total resistance: 280 ohms. At 20 mA the voltage drop is 0.02 × 280 = 5.6 V.
• Required supply: 10 + 5.6 = 15.6 V. A 24 V supply passes with a comfortable margin.

If the margin disappears, for example with a long cable or an extra indicator in the loop, the transmitter may stop working when the pressure is high. That is a classic hard-to-find fault: the signal reads correctly at low pressure and flattens near the top of the range.

Scaling the signal in a controller

Most PLCs and controllers let you set the engineering range of an analogue input. The steps are:

1. Choose the input type: 4 to 20 mA.
2. Set the low scale value to the pressure at 4 mA (for example 0 bar).
3. Set the high scale value to the pressure at 20 mA (for example 10 bar).
4. Set alarms for under-range and over-range, for example below 3.6 mA and above 21 mA, to catch faults.
5. Test with a loop calibrator by injecting 4, 12 and 20 mA and checking the displayed values.

If your controller input is voltage, a precision resistor (commonly 250 ohms) converts 4 to 20 mA to 1 to 5 V. Check the controller’s manual first, because many have built-in current inputs.

Calibrating a pressure transmitter: the five-point check

Calibration checks that the output current matches the applied pressure. You need a pressure source, a reference gauge that is much more accurate than the transmitter, and a milliamp meter or a controller that shows the loop current. For reference gauge options, see pressure gauge accuracy compared.

1. Connect the transmitter to its loop supply and the milliamp meter in series.
2. Apply 0% of range and record the current. It should read 4.00 mA within tolerance.
3. Apply 25%, 50%, 75% and 100% and record each.
4. Come back down through 75%, 50% and 25% to check hysteresis.
5. If the zero or span is off, adjust as the manufacturer allows, then repeat the test.
6. Record the as-found and as-left results, the date and the reference used.

The allowed error comes from 0.5% of the 16 mA span, which is 0.08 mA. More on calibration practice is in pressure gauge calibration in Australia.

Troubleshooting a 4 to 20 mA loop

For pulsating hydraulic or pump lines, a snubber helps. See pressure sensors for hydraulic systems.

4 to 20 mA vs 0 to 10 V: which to choose?

Using the signal with load cell electronics

The same idea applies to weighing. A weight transmitter outputs 4 to 20 mA in just the same way, and the maths is identical. See load cell amplifiers and indicators explained.

Common wiring mistakes

• Reversed polarity: a loop-powered transmitter will not work, and some can be damaged. Check the plus and minus markings.
• Two supplies in one loop: only one device should provide loop power unless the instrument is designed for it.
• Shield grounded at both ends: this can create a ground loop and noise. Follow the maker and your site standard.
• Cable run beside motor leads: induced noise appears as a jumping signal.
• Unsealed entry: water in the connector causes drift or intermittent faults.

What is HART?

HART is a digital signal that can be carried on top of the 4 to 20 mA current without disturbing it. It lets a technician configure a transmitter, read diagnostics and change the range from a handheld communicator or software. It is optional. A plain 4 to 20 mA transmitter works perfectly well without it, and the analogue current is still the primary signal.

Frequently asked questions

Why does 4 mA mean zero?

So that a dead loop (0 mA) can be told apart from a genuine zero reading. It also lets the transmitter power itself from the loop.

What does a reading of 12 mA mean?

It means 50% of the range. On a 0 to 10 bar transmitter that is 5 bar.

Can I use any 24 V supply?

Use a stable DC supply that meets the datasheet. Check the voltage drop in the loop.

How long can the cable be?

Long, provided the loop voltage check passes and the cable is shielded. Hundreds of metres is common.

Do you supply transmitters?

Our WIKA Model A-10 is available with 4 to 20 mA output. Tell us your range and connection through the contact page.

Next steps

Read the transducer, transmitter and sensor guide, browse the pressure sensor category, and see the pressure gauge basics.