4–20 mA: the live zero

Every circuit so far spoke in bits — a contact is open or closed, a motor runs or doesn't. But the tank doesn't have a level switch at every inch. It has ONE transmitter that says how full, and it says it in the plant's universal dialect: a 4–20 mA current loop.

Why current, and why 4?

Two wires run from the transmitter to the PLC's analog input card, and the transmitter regulates the current flowing in that loop. Current — not voltage — because current is the same at every point of a loop no matter how long the wires are or how many terminals it crosses. Five hundred feet of cable can't water it down.

And the scale starts at 4, not 0:

• 4 mA means 0% — tank empty, but the loop is alive and talking.
• 20 mA means 100%.
• 0 mA means the loop is DEAD — broken wire, dead transmitter, blown fuse. That's the live zero: the signal's own health check, built into the numbering. A plant where empty reads 0 mA can't tell "empty" from "unplugged." A 4–20 plant always can.

The scaling, one line

The PLC's input card reads milliamps; your logic wants engineering units. The conversion is a straight line:

Subtract the live zero, divide by the 16 mA span, multiply by the range. The same line works for 0–300 psi or 0–150 °C — only the range changes. The PLC signal scaling tool runs this in both directions when you need it on a job.

In the builder: you are the transmitter

The builder's AI device is the far side of this lesson: an analog input already scaled to 0–100%. Open the tank circuit and drag the AI1 slider — that slider is the transmitter, and the number it feeds is what every rung downstream will reason from. The next lesson is about how a rung, which only understands true and false, reads it.