The term "sight glass" can refer to several types of units that allow observation of a process fluid or a feature inside of a pipe, tank or vessel. The following are the descriptions of units that are commonly referred to as sight glasses.
A liquid level gauge is an instrument that is used to monitor the fluid level in a tank or vessel. In old boiler related jargon, this unit had traditionally been referred to as sight glass. Currently, it is typically referred to as a liquid level gauge, a much more descriptive label.
Figure 1 (below) shows a typical see-through liquid level gauge. This is a common configuration, intended show the liquid level only. Since this level gauge has glass on two opposing sides, lights are sometimes mounted on the "back" side to help the viewer see the level. Obviously, this instrument is not intended for observing the interior of the vessel on which it is mounted. Therefore, it is possible to simplify the viewing arrangements by using a single sided unit with a slotted "reflex" gauge glass. This type of glass features slots that effectively enhance the differences in refractive indices between liquid and gas above it, making the identification of the gas/liquid boundary much easier in the absence of other lighting arrangements.
