In a Bubbler level gauge, a bubbler tube is used to measure and indicate level.
Air is forced to send through the bubbler tube. The bubbles come out of the bottom of the liquid level. The greater the height of the liquid in the tank, the more pressure it needs to push the air out. The bubbler system starts to operate when a gas is passed through the bubbler tube.
The back pressure from the bubbles is measured and converted into level value.
There is just enough gas pressure for the bubbles to appear when the liquid is at the maximum level in the vessel. For maximum level of liquid, there will a be a maximum back pressure, and the pressure gauge reads it.
At zero level the back pressure will be zero and the gauge or transmitter will read zero. As the level increases the pressure reading on the pressure gauge or transmitter increases.