4–20 mA Loop Scaling
Convert loop current to process value and back — with the burden check.
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The engineering
The live-zero is the point: 4 mA means 'zero, and the wire is intact.' 0 mA means broken loop — a distinction 0–20 mA scaling throws away, and the NAMUR fault bands on this card exploit.
The 250 Ω default sense resistor turns 4–20 mA into the 1–5 V your DAQ actually reads; check the transmitter's compliance voltage budget before stacking burdens.
Where this math comes from
Before wires it was air: 3–15 psi pneumatic signaling ran process plants from the 1940s, live-zero included. Electronic transmitters of the late 1950s copied the idea into current, which laughs at wire resistance over plant-scale distances, and 4–20 mA won the standards fight (ISA S50.1, 1966) to become industrial automation's most durable interface.
HART (1980s) then taught the same two wires to carry digital data on top of the analog value — which is why a 1966 standard still ships on new instruments.
- 1942Pneumatic era3–15 psi live-zero signaling standardizes process control.
- 1957Electronic transmitter makersCurrent-loop transmitters replace air lines.
- 1966ISA (S50.1)4–20 mA standardized.
- 1986RosemountHART overlays digital comms on the analog loop.
See the full timeline of the math behind every calculator →
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