4-20 mA Current-Loop Signal Scaling

The engineering value a 4-20 mA instrument signal represents, read off a loop meter.

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Details, formula, and sources

The loop is a LIVE ZERO: 4 mA = the low end of the range (0% of span), 20 mA = the high end (100%), so percent of span = (mA - 4) / 16 x 100 and value = range_low + percent/100 x (range_high - range_low). A transmitter ranged 0-100 psi reads 50 psi at 12 mA and 75 psi at 16 mA; a -40 to 120 F range reads 40 F at 12 mA. The live zero lets the loop tell a real zero from a dead wire (0 mA): per NAMUR NE43, 3.8-20.5 mA is the valid band, while <=3.6 mA or >=21 mA is a driven sensor/loop fault, and below 4 / above 20 is under/overrange (flagged). Linear scaling only -- a DP (square-root-extracted) flow transmitter is different; the transmitter's configured range and calibration govern.

percent_of_span = (signal_ma - 4) / 16 x 100; engineering_value = range_low + (percent_of_span / 100) x (range_high - range_low). NAMUR NE43: <=3.6 or >=21 mA = fault; 3.8-20.5 mA valid.

4-20 mA current-loop (live-zero) analog signal scaling by name (ANSI/ISA-50.00.01 analog signal ranges; NAMUR NE43 fault-signal levels); the transmitter's range and calibration govern.

The linear live-zero scaling is public instrumentation practice; the 4-20 mA range and the NAMUR NE43 fault thresholds are open industry standards, and the transmitter's engineering-unit range comes from its configuration.

Estimate. AHJ and licensed professional govern.

Field names used by the API: signal_ma, range_high, percent_of_span, engineering_value

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