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4-20 mA Signal Converter

Most plant transmitters send a 4 to 20 mA signal where 4 mA is the bottom of the range (LRV) and 20 mA is the top (URV). Use this to check what the PLC or SCADA should show for a current you measure, or what current to expect for a process value. It also flags currents that NAMUR NE43 treats as a failure.

mA to Engineering Units

Formula

% of span = (mA - 4) / 16 x 100
Process value = LRV + (% of span / 100) x (URV - LRV)
Square root (DP flow): Flow fraction = square root of (mA - 4) / 16
NAMUR NE43: valid 3.8 to 20.5 mA; failure at or below 3.6 mA or at or above 21.0 mA
Inputs used
  • Lower range value (LRV, at 4 mA) (EU)
  • Upper range value (URV, at 20 mA) (EU)
  • Measured loop current (mA)
  • Transmitter output

Worked example (fictional plant data)

  1. Level transmitter ranged 0 to 20 ft reads 13.6 mA
  2. % of span = (13.6 - 4) / 16 x 100 = 60.00%
  3. Level = 0 + 0.60 x (20 - 0) = 12.000 ft
  4. Voltage across a 250 Ω input resistor = 13.6 x 0.25 = 3.400 V

Engineering Units to mA

Formula

mA = 4 + 16 x (Value - LRV) / (URV - LRV)
Square root (DP flow): mA = 4 + 16 x (flow fraction)²
Inputs used
  • Lower range value (LRV, at 4 mA) (EU)
  • Upper range value (URV, at 20 mA) (EU)
  • Process value (EU)
  • Transmitter output

Worked example (fictional plant data)

  1. % of span = (12 - 0) / (20 - 0) x 100 = 60.00%
  2. mA = 4 + 16 x 0.60 = 13.600 mA

Cautions

  • Measure loop current with a meter in series, or across a known resistor. Do not break a loop that controls equipment without coordinating with operations.
  • Not every transmitter follows NE43. Check the manual for its own failure currents (some use 3.75 mA and 21.75 mA) and set SCADA alarms to match.
  • Square root scaling must be done once only: in the transmitter or in the PLC, never both.
  • With square root scaling, very low DP signals are noisy. Most systems use a low flow cutoff (often 1 to 6% of flow).

References

  • NAMUR Recommendation NE 43, Standardization of the signal level for the failure information of digital transmitters.
  • ANSI/ISA-50.00.01, Compatibility of analog signals for electronic industrial process instruments (4-20 mA).

Results are estimates for operator and technician use. Equations follow the references above; always check against the instrument manual, your calibration procedure and plant data.

This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.