As I recall from another thread somewhere, Vertex uses a restriction orifice in their diffusers with the idea that if you have multiple diffusers hooked to the same pump the restriction will help equalize flow between diffusers. That may be one reason you are seeing the pressure rise when you have all the air going to the diffuser. Vertex might also do this to help protect the membrane from being over expanded to to excessively high air flow compared to what the membrane was designed to pass.

Just speculation on my part. I have no experience with Vertex diffusers. Maybe someone that does can comment to help us all understand the dynamics of pressure vs depth vs back pressure to the pump.

John

I subscribe to Pond Boss Magazine