Measuring the pressure exerted by a column, or head, of liquid in a vessel is one of the oldest and most common methods of measuring liquid level. The advent of “smart” dp or dP cells, or transmitters, means renewed interest in this tried-and-tested technique of measuring differential pressure.

A typical dp cell works by applying differential pressure to either side of a metal diaphragm immersed in non-conducting oil. The movement of the diaphragm changes the electrical capacitance--the ratio of charge to potential difference--of the cell and, in turn, the electric output signal.

If the pressure in a closed vessel changes, the change applies equally to both sides of a dp cell. A dp cell responds only to changes in differential pressure--the difference in pressure between two points--so it remains unaffected by a change in static pressure. It therefore responds only to change in the liquid level.

In an open vessel--one that is not under pressure or a vacuum--the vessel is connected to a dp cell on the high-pressure side via a pipe. The low-pressure side is left open to the atmosphere.