Level measurement is to monitor and measure quantitatively the liquid content in vessels , reservoirs and tanks. The determination of the location of the interface between two fluids, separable by gravity, with respect to a fixed horizontal datum plane.
The respective fluids may be any fluids, liquid or gaseous, which do not mix and have specific gravities significantly different from one another. Fluids include granular or particulate solids which are fluidized or handled like fluids. The most common level measurements are, however, between a liquid and a gas or vapour.
Level measurement may be classified in the main categories: direct visual indication of interface location; remotely transmitted indication of interface location; interface location inferred from hydrostatic pressure; interface location inferred from fluid properties. Level measurements may be of an analog or on-off nature.