Bubbler Level Measurement Animation shows the working principle of bubbler level sensor.
A fixed constant air supply will be given at the inlet (min 10psi). When there is no level in the tank then total air passed to atmosphere through pipe in the tank. When the level in tank increases means the proportional back pressure increases and the increased back pressure is proportional to level in tank.
When tank level is more then back pressure will increases. With the use a electronic transmitters (like pressure transmitters) we can convert the back pressure signal into 4-20mA level signal.