Specification:Item Type: Water Check ValveProduct Material: PVCProduct Package: BagProduct Color: RedProduct Size: Single length about 5.5cm/2.2inApplication: Air inlet/outlet pressure gauge tube: Approx. 4mm/0.16inPackage List:20 x Water Check ValvesNote:Tip: Each air outlet needs to be connected a check valve; the double-head air pump requires two check valves; and so on. It is strongly recommended to purchase, because it can prevent the water in the hose of the aeration pump from flowing back to the machine and damage the front-end air pump or other equipment.1. The use of a check valve will slightly reduce the amount of air delivered because of resistance.2. If you receive the check valve and find that it does not vent after the connection, please check if the installation sequence is correct. If it is correct, please disassemble the stop valve, and gently squeeze the vent by hand. Sometimes it is because that the rubber is slightly stuck due to high temperature.