The phenomenon behind level measurement using differential pressure transmitter is hydrostatic paradox. Regardless the shape and volume of a vessel, the hydrostatic pressure at a measuring point of a vessel or tank is proportional only to the filling height.
A differential pressure transmitter is always placed below the bottom level of the tank, for which level is to be measured.
The differential pressure transmitter’s both end is connected to the tank, if the tank is closed.
The pressure line connected the bottom of the tank is HP and the other one is LP. HP get filled as the tank starts filling liquid. LP is store impulse liquid.
When the tank is empty, the pressure difference between LP and HP is zero and the differential pressure transmitter shows no measuring variable.
As the liquid level increases in the tank the HP line start pressurized. This pressure makes a constructive movement in the diaphragm, which is placed between two capacitive plates. This movement creates a change in the capacitance between the plates. This capacitance is measured and processed to convert the signal to 0 -4mA standard signal format .
Level measurement using differential pressure transmitter is will not effected by liquid pressure or temperature.
If the tank is not closed the LP line will be opened and the LP pressure will be always atmospheric pressure. Environment can effect in level measurement in such cases, where humidity, atmospheric temperature cause the change in atmospheric pressure.