What is Sight Glass Level Gauge?

The sight glass consists of a strengthened glass tube, attached to the container as shown in Figure, through which the fluid level in the container can easily be observed by the operator.

Sight Glass Level Gauge

Monitoring the level from a distance, is facilitated when a magnetic float inside the tube, is allowed to rise and fall with the liquid level, causing metallic flags (or louvers) to flip and expose a different colour, indicating the level.

Sight Glass Level Gauge

These devices are available with continuous monitoring equipment, allowing both a local visible indication as well as an external signal for remote monitoring.


Glass Level Gauges can be divided into the below (most common) 3 types:

  • Reflex Glass Type
  • Transparent Glass Type
  • Bi-color Type



Gas or steam phase: Light reflects by the prismatic grooves of the glass, which therefore gives a clear appearance.

Liquid phase: the light is absorbed, thus providing a dark indication of the level.



In this type of level gauges, the liquid is contained between two transparent glasses. This permits a through vision of the fluid(s), thus providing a perfect indication of the level.



Bi-colour gauge is in fact a transparent gauge with its liquid chamber in a wedge shape section and fitted on a rear side with an illuminator.

Two color filters (red and blue) are positioned directly facing the light source in the illuminator, hence the name bi-color gauge.

In cases where red color light radiations hit upon the water, they gets averted in one side and then absorbed whereas when they encounter the steam region, they tends to pass through without any constrictions and emerges red in color.

On the other hand, reverse action takes place for light radiations passing via blue filters.