heat insulation and high pressure resistant mica gasket

A reflex sight glass, also known variously as a gauge glass, a level gauge and a reflex glass, 

provides a continuous visual indication of liquid level in a vessel or tank.

Being a relatively cheap level measurement technique they are found throughout the process, and oil & gas industries.

The liquid level in the gauge glass will be the same as that in the vessel. This is one of the simpler liquid level measurement methods often used by the instrument control engineer.