A reflex level gauge, also known as a sight glass, is a device used to visually monitor the level of liquid in a tank or container. It consists of a transparent tube with a curved surface that is installed vertically into the side of the tank. The liquid level inside the tank can be seen through the curved surface, which reflects the liquid inside and allows the user to observe its height. The reflex level gauge is typically used in the industrial and process control industries to monitor the levels of liquids in tanks and containers, such as in storage tanks, boilers, and pipelines.

In addition to the design and construction requirements, IBR and NIBR also regulate the inspection and testing of reflex level gauges to ensure that they continue to operate safely and accurately throughout their service life. Any deviations from the prescribed standards must be promptly addressed to maintain safe and reliable operation of the equipment.