The 4-20 mA signal in a 2-wire loop is used to represent the process variable.

• A. Yes, it represents the process variable
• B. No, it represents current draw only
• C. No, it represents voltage level
• D. No, it represents digital code

Answer: (A)

In a 4-20 mA loop, the current is the signal that carries the measurement. The transmitter reads the process variable and modulates the loop current to reflect that value, with 4 mA representing the lower end and 20 mA the upper end of the range. The two wires supply power to the transmitter and carry this current back to the receiver, which converts the current back into a readable process value. The voltage across the loop can vary depending on the load, but the actual signal is the current level, not the voltage.

So the 4-20 mA signal represents the process variable because the current level directly encodes the measured value, making the system robust to long cable runs and load variations. The other ideas aren’t the signal: the loop current is not merely current draw, nor a fixed voltage level, nor a digital code in the basic analog signal (although digital data can be overlaid with HART, the fundamental analog signal is the current).