Figure 11 Closed Tank, Wet Reference Leg
The filled reference leg applies a hydrostatic pressure to the high pressure side of the transmitter,
which is equal to the maximum level to be measured. The DP transmitter is exposed to equal
pressure on the high and low pressure sides when the liquid level is at its maximum; therefore,
the differential pressure is zero. As the tank level goes down, the pressure applied to the low
pressure side goes down also, and the differential pressure increases. As a result, the differential
pressure and the transmitter output are inversely proportional to the tank level.