A gauge located inside a storage tank that is capable of constantly monitoring the level of liquid inside the tank.

Two basic types of guages are available, mechanical and electronic.

Mechanical gauges consist of a float connected to a metal tape, similar to a carpenter's measuring tape. This tape is threaded through a little window at the top of the tank and the operator, by observing the figure visible on the tape in the window, can determine the depth of liquid in the tank. 

A variation of the mechanical liquid-level gauge is an "alarm clock" gauge. The up-and-down movement of a float, connected to the "clock," causes hands on the clock face to move, thus providing a reading of the liquid level in the tank.

Another variation of the mechanical level gauges are the swing arm style.  Inside the tank, the gauge consists of a floating arm that swings from empty to full in an arcing motion.  This arm operates an indicator located on top of the tank.  This indicator shows the level of the tank in eighths of a tank increments.

Electronic guages involve the use of probes installed inside the tanks. The probes operate on either a capacitance, sonic, or magnetostrictive principle.

See also Automatic tank gauge and Magnetostrictive probe.