In a 4-20 mA control loop, which statement about the current is true?

• A. It is solely a power supply value.
• B. It represents the process variable within the 4-20 mA range.
• C. It has no relation to the process measurement.
• D. It is used only for alarms.

Answer: (B)

In a 4-20 mA loop, the current is the signal that conveys the measured process variable. A transmitter converts the process variable (like temperature, pressure, level) into a loop current that ranges from 4 mA up to 20 mA. The 4 mA end typically represents the minimum value of the process variable, and the 20 mA end represents the maximum. This current is read by controllers, indicators, and recorders to know the current process state, and it also powers the loop devices. While the loop must be powered, the essential purpose of the current is to carry the process variable value, not just to supply power. Alarms or fault conditions can influence the current level, but the main idea is that the current represents the process measurement within that 4–20 mA span.