A bubbler tube is used for level measurement by introducing gas until the pressure matches what?

• A. The atmospheric pressure
• B. The vapor pressure of the liquid
• C. The hydrostatic head of the liquid in the tank
• D. The external gas pressure of the room

Answer: (A)

Bubbler level measurement works by using the pressure created by the liquid column in the tank. A tube is submerged in the liquid and gas is forced down through it. The gas pressure inside the tube must balance the pressure exerted by the liquid at the depth of the tube tip. Since the gas supply is connected to ambient air, its reference pressure is atmospheric pressure. The deeper the tube is, the more pressure above atmospheric is needed to push the gas through the liquid. By measuring how much pressure sits above atmospheric, you determine the liquid’s height. So the pressure is referenced to atmospheric pressure, which is why this option is the best answer.