4-20 mA Current-Loop Signal Scaling
The engineering value a 4-20 mA instrument signal represents, read off a loop meter. 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.
Formula and source
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.
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