4-20 mA Loop Scaling Calculator

Convert between a 4-20 mA current signal and an engineering unit value (pressure, temperature, level, etc.). Bidirectional — leave either side empty to compute it.

Scaled value —

Signal (mA) (mA) —

NAMUR NE 43 status —

Formule

SIGNAL → PROCESS VALUE

  PV  =  PV_min + (I − 4) / 16 × (PV_max − PV_min)

  Variables :
    I        current signal (mA), from 4 to 20
    PV_min   process value at 4 mA (0 % of span)
    PV_max   process value at 20 mA (100 % of span)
    PV       computed process value

  Rationale : linear conversion — each additional mA of current
  matches 1/16 of the process span. Example : 0–10 bar pressure
  sensor with a 12 mA signal → PV = 0 + (12−4)/16 × 10 = 5 bar.


PROCESS VALUE → SIGNAL

  I  =  4 + (PV − PV_min) / (PV_max − PV_min) × 16

  Rationale : reverse conversion, useful to configure a PLC that must
  generate a 4-20 mA signal on an analog output (valve positioner,
  motor setpoint).


WHY 4-20 mA AND NOT 0-20 mA

  4 mA       →  "live zero" — distinguishes 0 % from a wire break
  0 mA       →  detectable fault (vs. 0 mA = true minimum value in 0-20)
  Span       =  16 mA usable
  2-wire loop  →  the minimum 4 mA powers the transmitter

  Rationale : the 4-20 mA standard became the norm in the 1950s for
  its robustness over long cable runs (up to several kilometres) and
  its immunity to noise. The "live zero" at 4 mA became THE diagnostic
  convention — basis of the NAMUR NE 43 standard.

Reference: NAMUR NE 43

The 4-20 mA current loop is the de-facto industrial standard for analog signal transmission since the 1970s. This calculator handles the bidirectional conversion between the current value (mA) and the process variable (PV) for any range.

NAMUR NE 43 defines the diagnostic bands :

< 3.6 mA : sensor failure or wire break
3.6 – 4 mA : underrange (saturation low)
4 – 20 mA : normal operating range
20 – 21 mA : overrange (saturation high)
> 21 mA : sensor failure (overcurrent)

A modern transmitter signals diagnostic faults explicitly via these out-of-range values rather than failing silently inside the normal band.

4-20 mA Loop Scaling Calculator

4-20 mA Loop Scaling Calculator

Standards

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