What is the key difference between ultrasonic and DP level sensors?

• A. Ultrasonic sensors use ultrasonic sounds to measure level. DP sensors use the difference in pressures inside a tank.
• B. Ultrasonic sensors rely on magnetic fields; DP sensors rely on electric resistance.
• C. Ultrasonic sensors measure flow; DP sensors measure level by buoyancy.
• D. Both sensors use radioactive sources to determine level.

Answer: (A)

The key difference is how the level is measured. Ultrasonic level sensing uses a sound pulse: the sensor emits an ultrasonic wave, it travels to the liquid surface and reflects back. By measuring the time it takes for the echo to return and knowing the speed of sound in the liquid, it directly computes the distance to the surface, giving the level. It’s a non-contact method, sitting above the liquid, but its accuracy can be affected by foam, vapor, and temperature changes that alter the sound speed.

Differential pressure level sensing measures the pressure created by the liquid column. A transmitter compares the pressure at the bottom tap with a reference pressure, and the difference equals ρ g h (density times gravity times height). From that pressure difference, the level is inferred, which is common in pressurized or dirty tanks and involves a contact pathway through taps. This method requires knowledge of the liquid density and can be influenced by density variations.

So, the practical distinction is that one uses acoustic distance measurement, while the other uses hydrostatic pressure head to determine level. The other statements describe principles not used for these level sensing methods (magnetic fields or resistance, flow or buoyancy, radioactive sources).