Level transmitters are sensors which produce an electronic output, normally 4-20mA, which represents a continuous level measurement in a tank or vessel as the level changes. The level output can be displayed locally at the tank or vessel with an LCD display and remotely in a control room.
The level can be displayed in engineering units, for example, feet, gallons, inches, percent full, and many other units. Some displacer type level devises can provide a pneumatic output, usually 3-15psig or 6-30psig. These are often mounted on or close to a pneumatic control valve. The output is used to move the valve so that a level setpoint for a tank or vessel is maintained.
There are many level sensor options which include acoustic, caged displacers, capacitive, float, guided wave radar, hydrostatic pressure, Laser, magnetorestrictive, non contact radar and ultrasonic.
There are many special level measurement applications. For one example, facilities measure the oil-water interface in sump pits or vessels which collect oily water. The oil and water separates, forming an oil level on the top with water on the bottom. Knowing oil/water the interface point can help prevent the unplanned removal of oil as the sump or vessel is drained, and to prevent oil overflow as the sump or vessel is filled.