A bubbler tube is a level measurement instrument where gas is slowly fed into the bubbler system until the pressure equals what?

• A. The atmospheric pressure
• B. The hydrostatic head of the liquid
• C. The vapor pressure
• D. The instrument's reference pressure

Answer: (B)

In a bubbler level measurement, gas is pushed through a tube that ends below the liquid surface. The gas pressure must overcome the liquid’s weight above the tube tip, so the pressure in the line rises until it balances the hydrostatic pressure of the liquid column. That hydrostatic pressure is the product of the liquid’s density, gravity, and the height of the liquid above the tube, P = rho g h. Because gauges read relative to atmospheric pressure, the measurement you read corresponds directly to this hydrostatic head, linking pressure to level. Vapor pressure of the liquid and any fixed reference pressure aren’t what set the balance in this setup.