4–20 mA conversions are one of those “small” skills that quietly drive everything:
Calibration. Loop checks. PLC/DCS scaling. Troubleshooting weird readings.
If you can convert between PV, % span and mA in your sleep, everything else in a control loop gets easier.
4-20 mA Conversion Formulas
In this carousel, I used one simple example:
Pressure transmitter
LRV = 0 bar, URV = 10 bar → Span = 10 bar
Then ran it through 7 common formula types using the same numbers:
PV = 7.5 bar and mA = 12.0 mA
-
Span
Span = URV − LRV = 10 − 0 = 10 bar -
%Span from PV
% = (PV − LRV)/Span × 100 = (7.5 − 0)/10 × 100 = 75% -
mA from %Span
mA = 4 + (%/100) × 16 = 4 + 0.75 × 16 = 16 mA -
%Span from mA
% = (mA − 4)/16 × 100 = (12 − 4)/16 × 100 = 50% -
PV from mA
PV = LRV + (mA − 4)/16 × Span = 0 + 8/16 × 10 = 5 bar -
PV from %Span
PV = LRV + (%/100) × Span = 0 + 0.5 × 10 = 5 bar -
mA from PV (direct)
mA = 4 + (PV − LRV)/Span × 16 = 4 + 7.5/10 × 16 = 16 mA
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