The sight glass is an important method for visually determining level. The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of liquid in the process tank.
In process systems that contain a liquid under high pressure a reflex sight glass is used. This device is armored, to permit it to tolerate higher temperatures and higher pressures. Gage glasses are typically glass covered ports in a vessel that make it possible to observe the level of the substance in the vessel. Many gage glasses will have a scale mounted on the tank that allows the level to be read.