The level gauge, or sightglass is to liquid level measurement as manometers are to pressure measurement: a very simple and effective technology for direct visual indication of process level. In its simplest form, a level gauge is nothing more than a clear tube through which process liquid may be seen.

Level gauges are sensing and measuring devices used to monitor the level of a fluid in a tank or other storage container. These instruments are commonly used in industrial process applications to monitor fluid levels in drums, tanks, pressure vessels, and other similar applications.