What does a 2-wire transmitter regulate to represent the process variable?

• A. Loop current (4-20 mA)
• B. Loop resistance
• C. Voltage across the sensor
• D. Temperature

Answer: (A)

A two-wire transmitter regulates the loop current to represent the process variable. It both powers itself from the loop supply and modulates the current flowing through the loop so that the current level encodes the measured value. In practice, this is the 4–20 mA signaling standard, where 4 mA corresponds to the minimum process value and 20 mA to the maximum. A receiving instrument measures the loop current to determine the PV. Using current is advantageous because it stays consistent over long distances and through varying load resistances, making the signal robust to wiring losses. The loop resistance or the voltage across the sensor aren’t how the PV is conveyed—these are determined by the current and the load. The actual process variable could be temperature or another quantity, but the method of transmission is by regulating the loop current.