A sight glass instrument consists of a graduated tube of toughened glass which is connected to the interior of the tank at the bottom whose water level gauge is to be measured.
The image below shows a simple sight glass for an open tank in which the liquid level gauge in the sight glass matches the level of liquid in the tank. As the level of liquid in the tank rises and falls, the level gauge in the sight glass also rises and falls accordingly. Thus by measuring the level gauge in the sight glass, the level of liquid in the tank is measured. When it is desired to measure a liquid level with the liquid under pressure or vacuum, the sight glass must be connected to the tank at the top as well as at the bottom, otherwise the pressure difference between the tank and the sight glass would cause false reading.
In this case, the glass tube is enclosed in protective housing, and the two valves are provided for isolating the level gauge from the tank in case of breakage of the sight glass. The smaller valve at the bottom is provided for blowing out the level gauge for cleaning purpose.