Which level measurement device uses gas injection to balance hydrostatic head?

• A. DP Flow Meter
• B. pH Probe
• C. Thermocouple
• D. Bubbler Tube

Answer: (D)

Balancing hydrostatic head with a gas pressure is how a bubbler level system works. A tube is immersed in the liquid, and gas (often air) is bubbled into the tube. The gas pressure in the line must overcome the liquid’s hydrostatic pressure to push bubbles out at the open end, so the pressure in the line equals the pressure exerted by the liquid column above the tube. A pressure sensor reads that gas pressure, and since liquid height is directly related to hydrostatic pressure (h = P/(ρg)), the level can be determined from the sensed pressure. This setup isolates the sensor from the process liquid, which is advantageous in dirty or corrosive environments.

The other options measure things unrelated to level by gas balancing: a differential pressure flow meter detects pressure drop across a flow element to infer flow rate, a pH probe measures hydrogen ion activity to determine acidity, and a thermocouple measures temperature.