4-20 mA current loop systems became a de facto standard in the 1940s and 1950s. In current loop systems, a transducer monitoring some process variable — temperature, pressure, flow, etc.

The low output voltage level of the analog sensor and the long distance to the control room together constitute significant system design challenges. Here are clues on how to solve them. The 4-20 mA ...

If you were setting up a sensor to indicate the fluid level in a tank you would set the bottom output to 4 mA (meaning empty) and the top output to 20 mA (meaning full). In the case of a pressure ...