Reflex Sight Glasses

Reflex Sight Glasses, also known as gauge glasses, provide a continuous visual indication of liquid level in a vessel.

The simplest type of sight glass consists of a vertical glass connected to a vessel by piping, referred to as a bridle. The liquid level in the gauge glass will be the same as that in the vessel.

The simple glass is only suitable for very low pressure applications. For the pressures usually encountered in the oil and gas industry, a much more robust form of instrument is required, for example the Reflex Sight Glass.


The Reflex Sight Glass consists of a metal column with a recess machined in one side. A tempered glass window fits over the recess and is held in place by "U" bolts and a housing. The glass window forms a pressure tight seal with the column. The column is connected via the bridle to the vessel.

The glass slab of the window is smooth on the outside but has triangular grooves cut into its inside face. This helps give a clear indication of the interface between liquid and gas. This is particularly useful when the liquid is transparent, for example water.

The clear indication is achieved due to the behaviour of light. The light rays entering the glass above the liquid level strike the grooves which are in contact with the gas and are reflected back to the outside which makes the outside of the glass look light. And, the light that enters the glass where the grooves are in contact with the liquid is only partially reflected with some being absorbed. This makes this portion of the glass to look darker.




Sight glasses are commonly connected to the bridle using special valves called "Ball Check Valves". These are designed to block off the flow of liquid or gas should the sight glass be ruptured. Under normal operating conditions, the ball within the valve remains stationary allowing the flow of liquid or gas as the level moves up and down. If the glass breaks there would be a rush of fluid through the valve forcing the ball tight against its seat stopping the further flow of fluid, thereby arresting the leak.

Further Reading

For those who want to delve further into the techniques employed in level measurement then the following may be of interest: