For measure level of an open tank, load cells can be used. While we use DP transmitter for the pressure measurement of an open tank, the LP line should vent to atmosphere. So that the atmosphere change causes the level measurement.
A load cell is fixed with strain gauges. A strain gauge change it's resistance when a pressure is exerted on the strain gauge. The strain gauge is connected as the part of a balanced Wheatstone bridge.
If there is any pressure applied at the stressed strain gauge then it unbalances the Wheatstone bridge and voltage corresponding will read at the voltmeter.
The level liquid in the tank is proportional to the weight of the tank
The water tank is placed one of the gauges. As the level increases the pressure exerted on the strain gauge increases, This makes the strain gauge shrink and to reduce resistance.
The weight of the tank is shown in the load.
Level of the fluid = volume of fluid / surface area
Here Volume of the fluid = weight of fluid / density of the fluid
The weight of the fluid = Weight of vessel with fluid - weight of the vessel.
Weight of the vessel is known and the weight of the vessel with fluid is what we measured using load cell.