Reflex Sight Glass Level Gauge

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 simplest type of sight glass consists of a vertical glass connected to a vessel by piping, referred to as a level gauge bridle, or just simply a bridle. 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.

However, the simple level 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 level measurement instrument is required, for example the Reflex Sight Glass.

How does a reflex level gauge work?
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. Indeed, it is for this reason that the reflex glass is one of the more popular water level measurement methods.

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 look darker.


Sight glasses are commonly connected to the level gauge 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.

Reflex Sight Glass Limitations
Reflex level gauges cannot be used in all applications. They are not suitable for:
- interface level measurement,
- when the process fluid could corrode glass e.g. high temperature alkaline solutions or hydrofluoric acid,
- when the operator is required to view the colour of the liquid in addition to its level.

Gauge Glass Lights
For most applications the reflex level gauge does not need a specific illuminator or light; normal plant light levles should be enough. If however the liquid level gauge is located in a dark area of the plant, or if the lighting at night is insufficient then an artificial light source would need to be provided. Thought should be given to ambient light levels in an effort to avoid having to provide additional artificial light sources as these may have to be certified suitable for use in a hazardous area and will also need maintenance.

guided wave radar

interface level measurement

magnetic float level gauges


Further Reading

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