Pressure Transducer 4-20 mA Output: How the Loop Works, Scaling and Wiring

A pressure transducer with 4-20 mA output is the most common pressure instrument in industry: two wires, a 24 VDC supply, and a current that varies linearly with pressure. This guide explains what the signal actually means, how the loop is wired, how to convert milliamps into pressure readings, and when 4-20 mA beats (or loses to) 0-10 V.

What 4-20 mA Means for Pressure Measurement

In a 4-20 mA system, the transducer's output current represents pressure across its calibrated range:

• 4 mA = 0% of span — the lower range value (for example 0 bar on a 0-10 bar unit).
• 20 mA = 100% of span — the upper range value (10 bar in the same example).
• 12 mA = 50% of span — mid-scale (5 bar).

The 4 mA "live zero" is deliberate: because the minimum signal is above 0 mA, a broken wire or failed transducer drops the loop to 0 mA and is immediately visible as a fault. A healthy loop stays at 4 mA or above, so 0 mA clearly signals a problem. A 0-10 V output has no equivalent safety margin — 0 V could mean zero pressure or a dead sensor.

The relationship between current and pressure is linear, which keeps scaling simple: a 0-16 bar transducer reads 8 bar at 12 mA, and a -1 to 3 bar compound unit reads 0 bar at 12 mA (4 mA = -1 bar, 20 mA = 3 bar).

Why Two-Wire 4-20 mA Is the Industrial Standard

Four properties make the current loop the default for industrial pressure measurement:

• Immunity to voltage drop — the signal is a current, so cable resistance does not attenuate it. A 4-20 mA loop can run hundreds of meters without loss of accuracy, where a voltage output would sag.
• Loop power — two-wire transducers draw their operating power from the same loop, so no separate supply wire is needed.
• Wiring simplicity — two conductors, one shield. No polarity confusion beyond plus and minus at each end.
• Fault visibility — 0 mA, a current below 4 mA, or a saturated 20+ mA signal all indicate a problem that a controller can alarm on.

These properties are why every PLC analog input card accepts 4-20 mA as standard, and why most OEM pressure transducers — industrial pressure transducers included — offer it as the first output option.

How the 4-20 mA Loop Is Wired

A two-wire loop has three elements in series: the transducer, the supply, and the controller input. A simplified schematic:

24 VDC supply (+) ──► transducer (+) terminal
                           │
transducer (−) terminal ──► PLC current input (+)   [often a 250 Ω resistor inside]
                                │
PLC input (−) ────────────────► 24 VDC supply (−)

Current path: supply → transducer → PLC input → back to supply

The PLC current input is usually a precision shunt resistor — 250 Ω is common — across which the controller measures a 1-5 VDC drop at 4-20 mA. The transducer must be able to drive the total loop resistance (cable plus shunt plus any series devices) at 20 mA. A practical check: at a 24 VDC supply, most two-wire transmitters drive a total loop resistance of roughly 500-650 Ω, which comfortably covers typical cable runs and a 250 Ω input. If you add barriers, isolators or long cable, confirm the loop resistance stays within the datasheet limit.

Scaling 4-20 mA to Pressure: The Conversion Formula

To convert a current reading into pressure, use the linear mapping:

Pressure = Lower range value + ((I − 4) / 16) × (Upper range value − Lower range value)

Example, 0-10 bar transducer:
  I = 4 mA  → 0 + (0/16) × 10 = 0 bar
  I = 12 mA → 0 + (8/16) × 10 = 5 bar
  I = 20 mA → 0 + (16/16) × 10 = 10 bar

Most controllers do this internally — you enter the range into the analog input card and it displays engineering units directly. When checking values by hand, remember the span is 16 mA (20 − 4), so each 1 mA step equals 6.25% of full scale.

Unit conversions come up constantly on site: 1 bar = 100 kPa = 14.5 psi. A 0-10 bar unit at 12 mA is therefore 5 bar, 500 kPa, or roughly 72.5 psi. Choose the unit of the scale you are comparing against before converting, and keep the transducer and the display set to the same unit.

4-20 mA vs 0-10 V: Which Output to Choose

There is no universal winner — the choice depends on the controller and the distance. If your PLC has current inputs (most do), 4-20 mA is the safer default. If the device feeds a local meter or a controller with voltage inputs only, and the cable run is short, 0-10 V works well and simplifies the power supply. For digital plants, RS485/Modbus adds multi-drop capability on top of the same physical installation.

Common Wiring and Installation Mistakes

• Loop resistance too high — long cable plus multiple series devices can push the loop beyond the transducer's drive capability; the output saturates below 20 mA. Measure total loop resistance and keep it inside the datasheet limit.
• Supply voltage too low — a 24 VDC nominal supply that sags under load may starve the loop. Check voltage at the transducer terminals under operating conditions.
• Reversed polarity — a reversed two-wire loop simply does not work; verify plus-to-plus and minus-to-minus before applying power.
• Ungrounded or incorrectly grounded shield — leave the shield grounded at one end only, typically at the controller side, to avoid ground loops and induced noise.
• Wrong range assumption — confirm whether the transducer is 0-10 bar or 0-16 bar before trusting a reading; the same 12 mA means 5 bar on one and 8 bar on the other.

Frequently Asked Questions

Why does a 4-20 mA transducer output 4 mA at zero pressure instead of 0 mA?

The 4 mA live zero powers the electronics and provides fault detection. A healthy loop never drops below 4 mA, so 0 mA clearly signals a broken wire, a dead supply or a failed instrument.

Can I run a 4-20 mA loop over 500 m of cable?

In most installations, yes — current signals are not attenuated by cable resistance the way voltage signals are. The limiting factors are total loop resistance and supply voltage; check the transducer datasheet for the maximum loop resistance at your supply voltage.

How do I convert 12 mA on a 0-10 bar transducer to psi?

12 mA equals 50% of span, so the reading is 5 bar. Using 1 bar = 14.5 psi, that is approximately 72.5 psi. Apply the conversion formula to the pressure value, not to the current.

Is 4-20 mA compatible with all PLCs?

Virtually all industrial PLCs, DCSs and process controllers accept 4-20 mA on analog input cards. It is the most universally supported analog signal in industrial automation. Confirm whether your card expects two-wire (loop-powered) or three-wire transmitters before wiring.

Conclusion

4-20 mA is the backbone of industrial pressure measurement because it is simple, noise-immune, long-distance capable and universally supported. Wire the loop correctly, keep total loop resistance within limits, and scale with the linear formula — and a 4-20 mA pressure transducer will give you dependable readings for years. For the underlying sensing technology, see our working principle guide.

PRESSUREPOINT supplies factory-direct pressure transducers with 4-20 mA two-wire output, 316L wetted parts, ranges from -1 bar vacuum to 600 bar, and CE marking as standard. Tell us your range, media and connection, and we will confirm a model with datasheet and pricing — usually within one working day.