Sight Glass Sight glass, also known as a level gauge or sight tube, is a device used to observe the level of a liquid or gas in a container without opening the container. Installed between the condenser and the flow restrictor to observe the condition of the refrigerant, the sight glasses used in the ducts measure the level of refrigerant or lubricant flowing within the HVAC or refrigeration system, as well as the amount of liquid contained in the refrigerant at any given moment. By looking at the flow in the sight glass, the technician can tell if the refrigerant is circulating properly in the system, whether there is a blockage or other problems such as running low on the refrigerant, and it can help the technician quickly identify and solve the problem. The sight glass is installed in the liquid line of the refrigerant to observe the flow of the refrigerant and the gas-liquid phase transition, and to detect the water vapor content in the refrigerant. Installed on the return line t…