I know Ted pretty well and I sent him an email with a link to this thread about our back pressure problem.
It has always been my experience and preference to have no or very little back pressure (0-0.5psi) on the pump when a diffuser is connected to a compressor. This is very important when building your own diffuser. You ideally want all the air to escape out the diffuser with no pressure on the gauge until you start putting the diffuser in the water. If there is back pressure on a home made diffuser, you don't have enough holes in the diffuser or the holes are too tight/small or both, i.e too much air trying to get out too few or too small of holes. My thinking is why create unnecessary back pressure for the pump??. Use all pressure to operate the diffuser.
For commercial small slit diffusers there will be a small amount of psi needed to inflate or balloon a membrane, but this I think should be low enough to not register on the standard 30psi gauge.
When in the water, then 0.5psi is created for every foot of depth the diffuser descends into the pond.
snrub says: ""but I would expect more back pressure from the diffuser at shallow or no depth than from deeper depths. For example, at 10 feet depth, the compressor already has to overcome the static water pressure at that depth. The diffuser might only add an additional tenth psi or something very small to open the slits for the bubble to escape.""
From actual practice there is no backpressure from the diffuser at no depth. If there is any back pressure out of the water it should not come from the diffuser. What would create the backpressure? Generally it is due to too few holes or too small of holes, or too much air volume trying to get through too few holes. You say ""The diffuser might only add an additional tenth psi or something very small to open the slits..."" This is correct. Very little psi is needed to open the slits, similar to a check valve. Low pressure should be needed to open the valve. At 10 ft the static or water head pressure needed to release air is 5psi. IMO very little extra pressure of force should be then be needed to open the check valve and slits on the diffuser. PSI to open slits plus head pressure, plus tubing back pressure/resistance equals total pressure on the air gauge. Back pressures are additive. Is all this clear as mud???
Perfectly clear, and you are absolutely correct. I was incorrect in my thinking that head pressure would make a difference. It does not.
