Level measuring devices are used to accurately measure the volume of fluid in containers on a continuous basis. Common level measuring instruments include level gauges, float devices, those that measure hydrostatic pressure, displacement type, echo type, and capacitive type. Level gauges provide a direct visual indicator by using a sealed cavity with a transparent wall. Float devices use floats that move up and down with the liquid level. Hydrostatic pressure instruments measure pressure at the bottom of a fluid column to indicate level. Displacement instruments use Archimedes' principle to measure the weight of displaced fluid. Echo type instruments measure the time of flight of waves reflected off the liquid surface. Capacitive instruments measure changes in capacitance between a probe and vessel walls