Which instrument is described as balancing hydrostatic pressure with a gas source to measure liquid level?

• A. Bubbler tube
• B. Ultrasonic level sensor
• C. Capacitance probe
• D. Float gauge

Answer: (A)

Balancing hydrostatic pressure with a gas source to determine how high the liquid column is, is what the bubbler tube does. In this setup, a vertical tube is submerged in the liquid and a controlled gas is pumped down into the tube. The pressure in the gas line builds as needed to push gas bubbles through the liquid; that gas pressure exactly counteracts the hydrostatic pressure of the liquid column above the tube opening. By measuring this gas pressure with a gauge or transducer, you know the liquid level because P = ρgh (the deeper the liquid, the greater the pressure needed to bubble through). The reading is then converted to height, giving the liquid level.

This method is particularly useful when the sensing element can be kept out of the liquid, which helps with corrosive or dirty liquids. It’s not how ultrasonic, capacitance, or float-type sensors determine level, which rely on sound waves, dielectric properties, or a moving float, respectively.