The pressure developed will depend on the depth. The compressor has to overcome head pressure of the water, which is approximately a half psi per foot depth. One atmosphere in water is 34'. One atmosphere in air or water is just under 15 psi (the weight of air at sea level at the depth of outer space down to earth). So 34 feet depth of water is equivalent in pressure to the pressure exerted by the weight of air in our atmosphere. So at 9' water depth your pressure gauge will read about 4.5 psi.

You can make the manifold with ball valves, or another option is to use a water hydrant three or four outlet manifold. Some of the water manifolds made for hydrants to expand the number of outlets have full flow ball valves, some don't. The down side of using the water hydrant valves is that they have water hose threads on the inlet and outlet. This requires the proper hose barb adapters which are available at most hardware stores. Some are pretty cheesy though, the brass ones are the best. Either way, with a pre-made water hydrant manifold or individual ball valves will work fine. Just different plumbing solutions.

Pressure gauge is a good idea. If you record what the pressure is when the diffuser is new, you might be able to tell a slight difference in pressure as the diffuser starts to become plugged allowing you to know when to clean it. Pressure change will likely only be slight though.

John

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