4-20mA Signal Scaling Calculator
Convert a 4–20 mA current loop signal to a scaled engineering value (e.g. pressure, temperature, flow) or reverse-calculate the current from a known process value.
Signal Range
Engineering (Process) Range
Conversion Direction
Formulas
Current → Process Value:
PV = PVmin + [ (I − Imin) / (Imax − Imin) ] × (PVmax − PVmin)
Process Value → Current (reverse):
I = Imin + [ (PV − PVmin) / (PVmax − PVmin) ] × (Imax − Imin)
Where the ratio in brackets is the normalised span fraction (0.0 = 0 %, 1.0 = 100 %).
Assumptions & References
- The standard 4–20 mA current loop is defined in IEC 60381-1 and is the dominant analogue signal standard in process instrumentation.
- The relationship between current and process value is assumed to be linear across the full span.
- 4 mA represents 0 % of span (live-zero); 20 mA represents 100 % of span. The live-zero allows wire-break detection (0 mA = fault).
- A 250 Ω burden resistor is the most common PLC/DCS input impedance, producing 1–5 V across it (used for voltage-input cards).
- ADC count equivalents assume ideal, full-scale conversion with no offset error.
- Values outside the defined range are extrapolated linearly; physical transmitters will typically clamp at their hardware limits (often 3.8 mA and 20.5 mA per NAMUR NE 43).
- References: IEC 60381-1, NAMUR NE 43, ISA-50.1.