Reading a 4–20 mA Loop

Most of the analog signals moving around your plant ride on a 4-20 milliamp current loop. Level in a digester, pressure across a filter, the position of a modulating valve, the temperature of a bearing housing. If you can read one of these loops with a meter and turn the number into something real, you can settle a lot of arguments before anybody calls a vendor. This is a teaching primer, written the way I'd walk a maintenance tech through it standing at the panel. My goal is that by the end you understand why the signal works the way it does, not just which jack to plug the leads into.

Why this matters on the floor Picture a level transmitter on a tank two hundred feet from the control room, wired back through a cable tray that also carries motor leads and VFD (variable frequency drive) output. The operator says the tank reads low. Is the tank actually low, is the sensor lying, or is the wire the problem? A 4-20 mA reading, taken right, tells you which world you're in. It's the single most useful measurement a controls person can teach a mechanical person to take, and it takes about five minutes once you understand it.

What an analog current loop actually is An analog signal is one that varies smoothly to represent a real-world quantity, as opposed to a digital on/off. A current loop carries that analog signal as a flow of electrical current, measured in milliamps (mA, thousandths of an amp), around a single series circuit. The transmitter out at the equipment adjusts how much current it lets flow based on what it's measuring, and everything wired in series with it — the wire, the PLC (programmable logic controller) analog input card, any indicators — sees that exact same current. That series arrangement is the whole trick. In a series circuit the current is identical at every point. So the number the transmitter sends is the number the input card receives, no matter what happens to the voltage along the way.