Which pair correctly describes the basis of measurement for ultrasonic versus DP level sensing?

• A. Ultrasonic uses pressure differences; DP uses sound waves.
• B. Ultrasonic uses light reflection; DP uses capillary action.
• C. Ultrasonic uses magnetic fields; DP uses capacitance.
• D. Ultrasonic uses sound waves to measure level; DP sensors use the difference in pressures inside a tank.

Answer: (D)

The core idea is how each method determines the liquid level. Ultrasonic level sensing uses sound waves: a transducer emits a high‑frequency pulse, the wave reflects off the liquid surface, and the time it takes to return is used to calculate the distance to the surface with the speed of sound as the key factor. That distance directly translates to level.

Differential pressure (DP) level sensing relies on hydrostatic pressure. The liquid column creates pressure at the bottom proportional to its height (ΔP = ρ g h). A differential pressure transmitter measures the difference between bottom pressure and a reference pressure, and converts that into level using h = ΔP / (ρ g).

So the pair described is correct: ultrasonic measures level with sound waves, while DP measures level via the pressure difference inside the tank. The other ideas mix up these principles—for example, ultrasonic isn’t based on pressure differences, and DP isn’t about light reflection or capillary action.