Put simply, a sight glass is a transparent glass tube or window installed in a tank or boiler system that allows an observer to view what is happening inside the container, ensuring optimal quality and safety. Sight glasses can also serve a number of functions aside from basic viewing, but this is the most common use.

While “sight glass” is a general, overarching term, there are also several more specific terms that are used to refer to different types of sight glasses. Some of the most common types of sight glasses are outlined below.

Sight glasses and sight glass windows, as mentioned earlier, are commonly used in industries that use pressurized systems; steam boilers and steam generators are found in a wide variety of industries, such as utilities and power generation, cement production, food and beverage, and dry cleaning. All processes that require pressurized systems or vaporization use sight glass windows in some manner for monitoring liquids and ensuring optimal safety. Boiler systems also use a more specialized type of sight glass, called boiler sight glass, to monitor the pressure within.

Industries that utilize heat or heat transfer make use of sight glasses as well; many of the heating systems in these applications contain a viewport or sight port to ensure the system is functioning properly. In these scenarios, the operators certainly can’t look directly inside the heating system to make sure the pilot lights or heating elements are still working, which is why the viewport is such a critical component in the overall system.