Open tank (unpressurized) measurement
Regarding differential pressure level measurement, there are two main types of storage tanks: open (unpressurized) and closed (pressurized). Open, non-pressurized storage tanks are connected to the atmosphere; there is a clear path from the tank to the surrounding environment, and the air pressure acting on the liquid is equal to atmospheric pressure.
Therefore, the reference pressure is equal to atmospheric pressure. The pressure measured at the transmitter mounting point depends only on the weight of the liquid, not in a closed pressurized tank, where the pressure measured at the transmitter mounting point is equal to the liquid pressure plus the gas pressure. .
Because the reference end of the pressure transmitter is atmospheric pressure, we can use the pressure transmitter to measure the liquid level of the open tank, because the pressure transmitter is essentially a differential pressure transmitter, and its negative pressure side is atmospheric pressure.
Closed tank (pressurized) measurement
A closed pressurized tank is not connected to the atmosphere, so reference measurements are more complicated. The gas phase usually exists on top of the liquid. This pressure can be higher or lower than atmospheric pressure. Therefore, this air pressure must be compensated to produce an accurate level measurement. Without this compensation, changes in air pressure will cause changes in the level transmitter output and introduce errors.
Differential pressure transmitter with remotely sealed capillary system mounted on closed tank
For these reasons, pressurized closed tank applications require precise reference connections. The reference connection is located on the top of the tank and connects to the low pressure side of the differential pressure transmitter. Its purpose is to compensate for the air pressure above the liquid so that level measurements can be made accurately. Without a reference connection, the transmitter measures the total pressure of liquid level and air pressure.
The following equation illustrates how to use reference conditions to eliminate the effects of air pressure
Pressure difference = P(high) – P(low) = H*ρ (density) G (gravitational acceleration)