The sterile sight glasses has an internal diameter (22mm) that is consistent all the way through ensuring there is zero dead space. They are great for putting inline to see fluid as it passes through, such as when transferring, recirculating or racking beer from a fermenter to kegs. They connect to a 1.5" TC sanitary ferrule and are 98mm/3.8" in length.
The union sight glasses are great for keeping an eye on what's happening in your tank. They connect to any 1.5" TC ferrule and have a full 34mm/1.3" line of sight into the tank.