What device is best for accurately measuring liquid level in a closed tank under pressure?

For accurately measuring liquid level in a closed tank under pressure, a differential pressure (DP) transmitter is the best choice. The reason for this is that DP transmitters are specifically designed to measure the pressure difference between two points, which allows them to provide accurate readings of liquid levels, especially in situations where the tank is pressurized.

In a closed tank, the liquid level can affect the pressure at the bottom of the tank. The DP transmitter captures this difference in pressure between the liquid column and the reference pressure, which is often atmospheric pressure or a known reference in the tank. By converting this differential pressure into a level measurement, DP transmitters can accurately reflect the liquid level even when external pressures are present.

Other devices like a bubble tube, diaphragm box, or float cable have specific applications and limitations that may not make them suitable for closed, pressurized tank scenarios. For example, bubble tubes are better suited for applications where lower levels of accuracy are acceptable, and they can be affected by changes in temperature and pressure. Diaphragm boxes may not be designed for continuous level measurement in pressurized environments, and float cables can encounter issues with maintaining accuracy if the liquid's density varies or if there are obstructions in the tank.

Overall, the DP