A sight glass is a filter accessory that allows you to see the water as it gets cleaned. They consist of a clear section of pipe that is normally installed on the backwash line coming out of the backwash valve. It can be installed anywhere though. When it's installed on the backwash line, the water is going directly into the drain and you can see the water become clean. Sight glasses come in a variety of sizes according to the pipe size. The Hayward Sight Glass, part # SP1074S is a 2" SKT that goes onto the 1.5" Multiport valve. For more information contact the pros at PoguePoolSpa.com.