When are 4-wire transmitters typically used?

• A. When sensors require higher power such as analyzers or ultrasonic devices.
• B. When the loop has very long runs.
• C. Only in non-hazardous areas.
• D. When minimal power is needed.

Answer: (A)

Four-wire transmitters have their own separate power supply for the transmitter electronics, so they can deliver more power to the sensor or processing circuitry than the current loop alone would allow. This is essential when the sensors or devices being monitored require higher excitation or more processing power—situations like analyzers or ultrasonic devices. Hence, the best choice describes a need for higher power.

Long loop runs can introduce voltage drop and noise, but they don’t by themselves mandate a four-wire setup; designers often address that with proper loop sizing or supply voltage. Four-wire configurations aren’t restricted to non-hazardous areas, and they aren’t used simply because minimal power is needed—the opposite is true.