CFM (Cubic Feet per Minute) is a flow rate. I other words...the volume of air that the pump pushes per unit of time.
PSI (Pounds per Square Inch) is pressure or, in our case, back pressure which should be considered the resistance to air flow created by the pump.
Pumps will have a curve chart that relates the CFM to the PSI, but is rarely displayed for general public purchases to avoid confusion (or hide the real details where the pump falls short)
As the back pressure (PSI) increases, the flow (CFM) is reduced. For example, the deeper you put your diffuser in your pond, the more back pressure your pump realizes and the less air volume your pump can push to overcome the back pressure.
You need to determine the pressure (PSI) that your pond depth puts on the system. You said 12 ft deep equals 6 psi. That's right. Now, determine if your diffusers create any back pressure. Vertex diffusers do at about 1 psi. I would add 6+1 to get 7psi. The pump curve chart should allow you to take the calculated 7 psi and determine what CFM the pump can put out at that pressure. This is a little hard to explain without visual aids, but hopefully it sheds some light on what your are trying to determine.
Last edited by Quarter Acre; 06/01/18 02:07 PM.
