Pond Boss Forums AERATION Systems questions CFM VS PSI

I'm researching aerator pumps for my pond. When looking at the specs, in most cases the flow is rated in CFMs instead of PSI.

If I understand how this works, if I'm going to run the diffuser from a depth of 12 feet, then I need a pump capable of pushing 6 PSI to counteract the pressure of 1/2# per foot of drop.

How do I convert the CFM into PSI so I know whether the pump is capable of that depth? Or easier still, is there a certain CFM rating I should be concentrating on as a minimum flow rate?

I intend to elevate the diffuser off the bottom by about 2 feet, which will put it at around 10, so if the pump is rated for 12 feet, it should be more efficient and last longer between rebuilds.

Find out if the pump was rated at depth.

Also look for the rating in MPa(megapascal). If it has that rating, it can be converted to PSI. You pump should have a chart indicating flow at depth as well.

1 MPa =145.038 PSI

So 0.01 would be 1.45038 PSI

I use an aerator was rated at 0.12 MPa and 100 L per min and it can bubble the air stone with good flow at 15+ feet deep.

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.

Here's a visual for a very popular pump that is used commonly here by PB menbers...

Look at the flow chart that has "Free Air Flow (CFM)" and Pressure (psig)". It's the top right grid with all the dots in it.

Our example system has 7 psi of back pressure. Find 7 psi on the graph and go straight up from there until you hit the solid line (this line is for 60hz applications which is the US standard), now shoot straight to the left and you hit the CFM line about at the 4 mark. This means your pump will put out about 4 cfm with a back pressure of 7 psi. That's an appropriate flow for a single vertex diffuser or even a double or triple. (edit: 4 CFM is likely too much for your average diffuser - 1 CFM per diffuser is a good rule of thumb)

You really need this chart to know exactly what's going on. A pump can say it's rated to 5 psi, but does that mean that it will pump air up to 5 psi back pressure (10 foot pond depth) AND then no more air comes out. OR, does it mean that an ample amount of air will still be coming out at 10 for depth...they (the advertisers) don't tell you that, but the chart does. When a pump is advertised at 5 CFM...that usually means that it will pump 5 CFM without any back pressure.

So if I'm understanding everything that's been said, the real key is in the diffuser. If it's equipped with a check valve that prevents back pressure the pump can operate sufficiently even with lower CFM?

The unit/kit I'm leaning toward says it's rated to operate up to 30 feet deep at .8cfm. It comes with two rubber membrane diffusers (The tube shaped kind) equipped with a check valve, mounted on a plate and 50ft of sinking 3/8 hose. The pump itself is a 1/8 hp rotary vain/diaphragm design.

Notes - comments: a check valve does not prevent back pressure. A check valve stops or prevents water from leaking into the airline past the valve. Water in the airline does cause excess, unnecessary, back pressure on the pump when the pump restarts - this shortens compressor life span. Be aware that 3/8" ID hose is fairly small diameter for common large pond aeration. Smaller diameter hose can cause reduced air flow and some increased back pressure due to the narrow diameter hose tending to resist flow of air inside the hose. The amount of back pressure in these cases is highly determined by the length of hose, longer hose = increased back pressure as hose or pipe diameter is reduced/smaller. There are charts of air flow for this. If you are using only 50 ft of 3/8"ID hose, IMO little back pressure will be measured with good air gauge. At 100+ft consider 1/2" ID hose especially when the pump is near its rated peak for normal running pressure.

Most single diffusers require around at least 0.5cfm for normal efficient operation. The less cfm a diffuser receives the fewer bubbles it makes and the less water it moves. Excess cfm to a diffuser causes the slits to over expand, makes bubbles larger and less air lift efficiency occurs.

Hey Bill, thanks for clarifying that for me. That's exactly the information I was hoping to gather.

My thoughts are to run 1/2" PEX, buried to the full pool line, that run may be 60 feet from the power source, then reduce to the 3/8 sinking line to the center of the pond. The total run of 3/8 should be in the neighborhood of 30 feet, so the relative back pressure should be nill.

The diffuser will be fixed to a platform of milk crates to elevate it from the bottom to a depth of 11 feet. I may go two high on the crates to get it up to 10 feet, and still be light enough to retrieve when need arises.

I'm thinking this should help the pump operate well under it's maximum limits. If 10 feet of depth causes the diffuser to over expand I can take it lower and still be ok.