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Instrumentation

How 4–20 mA Scaling Works

The 4–20 mA current loop is the dominant analogue signal standard in process instrumentation. Learn the scaling formula and how to convert between signal and process value.

TypeEngineering guide — concept explainer

Definition

The 4–20 mA current loop is an analogue signalling standard used in process instrumentation. A transmitter outputs a current between 4 mA and 20 mA that represents a process variable (pressure, temperature, flow, level, etc.) scaled linearly between a lower range value (LRV) and an upper range value (URV). 4 mA corresponds to LRV, 20 mA corresponds to URV, and intermediate values map linearly across the 16 mA span.

Why it matters

The 4–20 mA signal is the backbone of analogue process control. Engineers need to convert between the mA signal and the engineering-unit process value constantly — during commissioning, troubleshooting, and calibration. The offset zero at 4 mA (rather than 0 mA) allows detection of a broken wire: if the signal reads 0 mA, the loop is faulted. Understanding the scaling formula prevents incorrect range configuration and measurement errors.

Formula

Process value from signal
PV = LRV + ((mA − 4) / 16) × (URV − LRV)
Signal from process value
mA = 4 + ((PV − LRV) / (URV − LRV)) × 16
Span
Span = URV − LRV

Units involved

  • •mA — milliamperes (the signal current)
  • •PV — process value in engineering units (°C, bar, L/s, etc.)
  • •LRV — lower range value (the PV at 4 mA)
  • •URV — upper range value (the PV at 20 mA)
  • •Span — the difference URV − LRV

Concept diagram

Signal (mA)Process Value041220LRV50%URVdead zerospan4 mA20 mA

Worked example

A pressure transmitter is ranged 0–100 bar (LRV = 0, URV = 100). The DCS reads 12 mA. What is the pressure?

  1. 01LRV = 0 bar, URV = 100 bar
  2. 02mA = 12
  3. 03PV = 0 + ((12 − 4) / 16) × (100 − 0)
  4. 04PV = (8 / 16) × 100
  5. 05PV = 50 bar
Result

12 mA on a 0–100 bar range = 50 bar (midpoint)

Common mistakes

  • •Using 0–20 mA instead of 4–20 mA — the live zero at 4 mA is essential for fault detection and correct scaling.
  • •Confusing LRV with zero — LRV can be any value, not necessarily zero. A range of 50–250 °C has LRV = 50.
  • •Applying square-root extraction to a linear signal — square-root compensation is used for differential-pressure flow transmitters but is separate from basic scaling.
  • •Forgetting that the formula assumes linear scaling — some transmitters can be configured for non-linear output, but this guide covers linear only.

When to use the calculator

Use the 4–20 mA Scaling calculator when you need to convert between a mA signal and a process value. Enter any two of three values (mA, LRV, URV) and the calculator returns the process value — or enter the PV and range to find the mA signal.

4–20 mA Scaling Calculator →

FAQ

Why does the signal start at 4 mA instead of 0 mA?
The live zero at 4 mA allows the control system to distinguish between "process value at the bottom of range" (4 mA) and "broken wire or power failure" (0 mA). This is a fundamental safety feature of the 4–20 mA standard.
What is the difference between LRV, URV, and span?
LRV (lower range value) is the process value at 4 mA. URV (upper range value) is the process value at 20 mA. Span is URV − LRV. For example, a range of 10–50 bar has LRV = 10, URV = 50, span = 40 bar.
Can a 4–20 mA signal go above 20 mA or below 4 mA?
Some transmitters allow a small over-range (e.g., up to 20.5 mA) or under-range (down to 3.8 mA) to indicate the process value is slightly beyond the configured range. NAMUR NE 43 defines specific fault current levels. However, the standard linear scaling applies only between 4 and 20 mA.
Does this apply to 4-wire transmitters?
The scaling formula applies regardless of wiring configuration. 2-wire (loop-powered) and 4-wire (separately powered) transmitters both output a 4–20 mA signal that scales linearly. The wiring affects power supply, not the scaling math.