The typical process plant contains many tanks, vessels, and reservoirs. Their function is to store or process materials. Accurate measurement of the contents of these containers is vital. The material in the tanks is usually liquid, but occasionally it may consist of solids. A float-and-cable or float-and-tape instrument measures liquid level by transmitting to a mechanism the rise and fall of a float that rides on the surface of the liquid. Float-and-cable devices are used primarily in open tanks, whereas float level switches may be designed to operate in a pressurized tank. Float devices have the advantage of simplicity and are insensitive to density changes. Their major disadvantage is their limitation to reasonably clean liquids. The displacement, or buoyancy, technique is a type of force-balance transmitter. It may be used to measure liquid level, interface, or density by sensing the buoyant force exerted on a displacer by the liquid in which it is immersed.