Measuring Level By Measuring Pressure

Viatran uses a variety of pressure measurement techniques to determine liquid levels in different tank setups and applications. By measuring the hydrostatic head pressure at the bottom of a tank and the specific gravity of the liquid, the level in the tank can be determined.

Hydrostatic head pressure at a certain point in a tank, is caused by the weight of the liquid above pushing down. Specific gravity is the ratio of the mass of the tank liquid to the mass of an equal volume of water at 4C. Viatran's pressure measurement instrumentation can be used to determine both of these quantities.

In order to understand how pressure is used to measure liquid level, consider the following equation:

h—level of the liquid in a tank
P—hydrostatic head pressure at the bottom of the tank
SG—liquid's specific gravity

The specific gravity of the liquid in a tank is an important factor in the level measurement process. There are three basic situations where specific gravity factors into tank level measurement.

The simplest case is when a tank is used for water only. The specific gravity of water is 1.0, making the above equation elementary. No adjustments are necessary in order to compensate for specific gravity. Tank level is determined directly from the hydrostatic head pressure.

The second situation relates to a tank that contains a liquid other than water, but it always contains the same liquid. This situation is only slightly more complicated. The level measuring transmitter, or level indicator, must be scaled to compensate for the difference in specific gravity of the liquid. Once this step is complete, tank level can be determined directly from the hydrostatic head pressure measurement.

The third, and most complicated case, is when a single tank is used for more than one type of liquid. The measurement system must determine the specific gravity of the tank's liquid and the hydrostatic head pressure at the base of the tank simultaneously, in order to establish the level of liquid.

Viatran provides liquid level measurement for five basic tank setups: vented/open tanks; insulated vented/open tanks; sealed tanks; vented/open tanks with changing media; and sealed tanks with changing media. This technical note discusses how Viatran has successfully engineered systems that measure the liquid levels in all five of these setups.

Click here to download the entire technical note in pdf format.