Usage: for corrosive liquids, liquids that contain suspended objects, high viscosity liquids, and underground tanks.
Principle of operation: A bubble tube is inserted into a tank, and a fixed flow of air is forced through the tube such that bubbles emerge from the end of the tube. If the air flow rate is not extremely large, the air pressure in the bubble tube can be considered to be the same as the pressure of the liquid at the end of the tube. The liquid level can be measured by measuring the air pressure.