What 4-20mA actually means (and why your zero is at 4)

The 4-20mA standard is an analog signal used in industrial sensor data. It represents a physical process as an electrical current through a wire. The standard is old but still widely used. The zero point is at 4mA to indicate a healthy loop, and anything below 4mA is a failure signal. Your code needs to know the configuration of the transmitter to handle failure signals correctly.