Instrumentation

4–20 mA Signal Scaling Explained

How a 4–20 mA instrument signal is scaled to a process value and back, why 4 mA is a live zero, and how span, percent of span, over-range, and under-range fit together.

TypeEngineering guide — concept explainer

Definition

A 4–20 mA current loop is the dominant analogue signal in process instrumentation. The transmitter drives a current between 4 and 20 mA that represents the process variable linearly across the calibrated range: 4 mA is the lower range value (LRV, 0% of span) and 20 mA is the upper range value (URV, 100% of span). Scaling is the conversion between this milliamp signal and the process value it represents.

Why it matters

Almost every loop in a plant — pressure, level, flow, temperature — carries its measurement as a 4–20 mA signal. Technicians and engineers constantly move between the signal and the engineering value: checking a reading at a marshalling cabinet, verifying an alarm setpoint, commissioning a new transmitter, or troubleshooting a loop. Getting the scaling right is the difference between a correct reading and a silent error. The 4 mA live zero is also a diagnostic feature: because 0% of span is 4 mA and not 0 mA, a reading of 0 mA is unambiguous evidence of a fault rather than a legitimate low measurement.

Formula

Span

span = URV − LRV

Percent of span

% span = (mA − 4) / 16 × 100

Process value from mA

PV = LRV + ((mA − 4) / 16) × span

mA from process value

mA = 4 + 16 × ((PV − LRV) / span)

Units involved

•mA — loop current, the analogue signal (4 mA to 20 mA in the normal range)
•LRV — lower range value, the process value at 4 mA (0% of span)
•URV — upper range value, the process value at 20 mA (100% of span)
•span — URV − LRV, the width of the range in engineering units
•% span — position of the process value within the range, 0% to 100%
•PV — process value in engineering units (kPa, °C, L/s, etc.)

Concept diagram

Worked example

A pressure transmitter is ranged 0–100 kPa (LRV = 0, URV = 100). The loop reads 12 mA. What process value does that represent?

01span = URV − LRV = 100 − 0 = 100 kPa
02% span = (12 − 4) / 16 × 100 = 50%
03PV = 0 + ((12 − 4) / 16) × 100
04PV = 0 + 0.5 × 100 = 50 kPa

Result

A 12 mA signal on a 0–100 kPa range represents 50 kPa (50% of span).

Common mistakes

•Treating the signal as 0–20 mA. The live zero is 4 mA, so the span sits over 16 mA, not 20. Using (mA / 20) instead of (mA − 4) / 16 puts every reading wrong.
•Forgetting that LRV need not be zero. On a 50–250 °C range, 4 mA is 50 °C, not 0 °C. The span is 200 °C, and percent of span is measured from the LRV.
•Applying linear scaling to a square-root (DP flow) signal. Differential-pressure flow transmitters in linear mode output a signal proportional to ΔP, which is proportional to flow squared. Recovering flow needs square-root extraction — a separate step this scaling does not perform.
•Confusing percent of span with the milliamp value. 50% of span is 12 mA, not 50 mA; they describe the same position but on different scales.

When to use the calculator

Use the mA to Process Value calculator to turn a measured loop current into an engineering value, and the Process Value to mA calculator to find the signal a given process value should produce. Both take the LRV and URV and offer an optional clamp to the 4–20 mA window. For the position-within-range view, the Sensor Percent Span calculator converts between a process value and its percent of span.

mA to Process Value Calculator →Process Value to mA Calculator →4–20 mA Scaling Calculator →Sensor Percent Span Calculator →

FAQ

Why 4–20 mA and not 0–20 mA?

The 4 mA live zero means a healthy loop never sits at 0 mA. If the current falls to 0, the wire is broken or the loop has lost power — a fault, not a low reading. It also leaves headroom: a transmitter can drive slightly below 4 mA or above 20 mA to signal under-range, over-range, or a diagnostic state.

What does a reading below 4 mA or above 20 mA mean?

It is outside the calibrated 0–100% span. Below 4 mA usually means an under-range process value, a wiring fault, or loss of power; above 20 mA usually means an over-range value or a fault. Many transmitters use defined fault currents (for example 3.6 mA or 21 mA) to signal a specific failure.

How does percent of span relate to the signal?

They are two views of the same position. 0% of span is 4 mA, 100% of span is 20 mA, and the relationship is linear in between. Percent of span is dimensionless; the milliamp value is the electrical signal that carries it.

Does this work for DP flow transmitters?

Only if the transmitter is in linear mode. In square-root mode the output represents flow, which is proportional to the square root of differential pressure. That requires square-root extraction, which is a separate function not covered by linear 4–20 mA scaling.