4–20 mA Current Loop

The industrial standard: 4 mA = zero, 20 mA = full scale. Current is immune to voltage drop down long cables, and a broken wire reads 0 mA — distinguishable from a genuine zero, which is why it survives in plants.

Loop resistance is the sense resistor plus cable plus every other device in series. 250 Ω is conventional because it turns 4–20 mA into a 1–5 V signal.