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A 4-20 mA signal is an analog current loop in which 4 mA represents the zero (or minimum) value of a measured range and 20 mA represents the full-scale (maximum) value. Pressure transmitters use it because current stays constant around a series loop even when cable resistance, connection corrosion, or electrical noise try to change it, so the reading arrives at the controller far more reliably than a voltage signal over long runs. The 4 mA "live zero" also gives you a built-in way to tell a legitimate zero reading apart from a broken wire or dead transmitter.

The basic idea: a loop, not a wire

A 4-20 mA circuit is a closed series loop. Current leaves the power supply, passes through the transmitter, travels down the field wiring, and returns through the receiver (a PLC analog input, panel meter, or controller) back to the supply. Because every device sits in the same series path, the same current flows through all of them. The transmitter's job is to regulate that current to a value proportional to the pressure it senses.

Two practical consequences follow:

• The receiver measures current, not voltage. A typical analog input converts the loop current into a voltage across a sense resistor (commonly 100 Ω to 250 Ω) and reads that. The transmitter does not care what the receiver's resistance is, within its rated compliance range.
• The loop must be complete. Break any connection and the current falls to zero, which is outside the valid 4-20 mA band — that is exactly how a fault is detected.

Why current instead of voltage?

Voltage signals such as 1-5 VDC, 0-5 VDC, or 0-10 VDC are perfectly usable over short cable runs inside a panel. The trouble starts when the cable gets long or the environment gets electrically noisy.

The key point is not that current is "more accurate" in a laboratory sense. It is that current is more robust in the field, where the signal has to survive distance, temperature swings, vibration, and nearby motors and drives.

The live-zero advantage of 4 mA

If zero pressure produced 0 mA, you could never distinguish "the process is genuinely at zero" from "the wire fell off." By shifting the bottom of the scale to 4 mA, the standard reserves 0-4 mA as an invalid or fault region.

• 4 mA = minimum of the configured range (often zero pressure, but not always — see below)
• 20 mA = maximum of the configured range
• Below ~3.6 mA or at 0 mA = fault, broken loop, or unpowered transmitter
• Above ~21 mA = over-range or a transmitter fault, depending on the device

Many transmitters also drive the current slightly outside the 4-20 mA band on internal failure so the controller can flag it. Check the specific transmitter's datasheet for its fault behavior rather than assuming.

How 4-20 mA maps to a pressure range

The current does not represent pressure directly — it represents a percentage of a configured span. That span is set at the transmitter, and it is why the same 4-20 mA output can mean very different pressures on different units.

Suppose a transmitter is ranged for 0 to 100 PSI:

The conversion is linear:

Pressure = Range minimum + (Current − 4 mA) ÷ 16 mA × (Range maximum − Range minimum)

For the example above at 12 mA: 0 + (12 − 4) ÷ 16 × 100 = 50 PSI.

Two things to watch:

• Elevated zero. Some ranges do not start at zero. A transmitter ranged −10 to 40 PSI still uses 4 mA for −10 PSI and 20 mA for 40 PSI. Always confirm the actual configured range before scaling your controller.
• Reverse or custom spans. Transmitters can be configured so that increasing pressure decreases current. This is uncommon but real, and it will invert every reading if you assume the standard direction.

Practical wiring and loop power considerations

Most industrial pressure transmitters are two-wire, loop-powered: the same two conductors supply power and carry the signal. Others are three-wire or four-wire, with separate power and signal connections. The distinction matters when you are choosing a receiver.

• Loop-powered (two-wire): The transmitter draws its operating power from the loop. The supply voltage must be high enough to cover the transmitter's minimum voltage plus the voltage dropped across the receiver and the cable at 20 mA. If the supply is too low, the transmitter cannot reach full-scale current and the reading will clip.
• Separately powered (three- or four-wire): The transmitter has its own supply and sources current into the receiver. Wiring is more forgiving of supply voltage but requires more conductors.
• Receiver input type: Confirm whether your PLC or meter expects a sourcing (current into the input) or sinking configuration, and whether it provides loop power or needs an external supply.
• Polarity: Current loops are polarity-sensitive. Reversed wiring typically produces 0 mA or an erratic reading, not a negative value.
• Shielding and grounding: Use twisted-pair wiring and ground the shield at one end only, following the receiver manufacturer's guidance, to avoid ground loops.

If you are unsure of the supply voltage requirement, the transmitter datasheet will state a minimum loop voltage or a load limit curve. Do not guess — an underpowered loop is one of the most common causes of a transmitter that "works" at low pressure but cannot reach 20 mA.

When a voltage output makes more sense

4-20 mA is not automatically the right choice for every application. Voltage outputs such as 1-5 VDC, 0-5 VDC, 0-10 VDC, or 0-100 mV are common and often preferable when:

• The transmitter and receiver are in the same enclosure or on the same board.
• Cable runs are short and the electrical environment is clean.
• The receiver only accepts voltage inputs.
• You need a very low-cost or very compact signal path.

The trade-off is the one described earlier: voltage signals are more susceptible to drop and noise over distance, and a 0 V reading is ambiguous. If your run is long, your environment is noisy, or you need fault detection, 4-20 mA is usually the safer default.

A quick selection checklist

When specifying a pressure transmitter with a 4-20 mA output, confirm each of these before ordering:

1. Pressure range and type — gauge, differential, or vacuum; minimum and maximum values, including any elevated zero.
2. Output — 4-20 mA, and whether the transmitter is two-wire loop-powered or separately powered.
3. Supply voltage and loop load — verify the supply can drive the full 20 mA through your receiver and cable.
4. Receiver input — current input range, sense resistor value, and sourcing/sinking compatibility.
5. Environment — temperature range, media compatibility, and whether the process media needs isolation from the instrument.
6. Fault behavior — what the transmitter outputs on internal failure, so your controller can detect it.
7. Connections and housing — conduit entry, connector type, and mounting.

Because 4-20 mA is a standard rather than a single product, transmitters from different manufacturers will interoperate as long as the range, wiring, and power requirements line up. The customization that matters is usually in the pressure range, the process connection, and the wetted materials — not in the signal itself.