Taco CX partial acceptance Captive Air diaphragm expansion tanks are applied in commercial, institutional and industrial applications for the control of pressure in hydronic systems. Diaphragm tanks use a diaphragm to permanently separate the air and water. In a diaphragm tank the air is held captive by the use of a diaphragm with the expanded water being held on one side of the diaphragm and air on the other. This permanent separation allows the tank to be precharged on the air side to the minimum operating or fill pressure. This eliminates many gallons of water to compress atmospheric pressure air in an air cushion or plain steel tank to the fill pressure. This allows the reduction in Captive Air expansion tank sizes of up to 80% compared to air cushion or plain steel tanks. In a diaphragm tank the diaphragm is attached to the tank wall and cannot move inside the tank. As a result the tank has a limited acceptance volume. In addition, there is some water in contact with the tank wall providing an opportunity for corrosion