Return pump micro bubbles

[NanoSF]

I can't for the life of me figure out how my return pump can be spitting microbubbles when there are no bubbles in the return chamber. It is not a constant spitting, it's at random intervals of every 5-15 seconds a little blast of microbubbles. Nothing too large, but it is aggregating.

I tried running the return pump with a large sponge filter over the inlet, and I tried covering that with some micro filter bags. Still have the same problem.

My return chamber has a solid bubble trap before it. One of the sections of the trap even is packed with filter floss. My skimmer outlet is sent way back to the beginning of the sump so that any micro bubbles can dissipate.

I shine a 400w light into the return chamber and it is crystal clear. NO BUBBLES. How the heck it the return pump still spitting bubbles? I clean the pump every two weeks in an attempt to rid this problem, but it comes back the very same day I clean it. I don't get how bubbles can come from nothing. I mean there has to be air to be bubbles right? Where is it coming from?

Any suggestions would be great. Thanks

[JetJumper]

I have the same issue when I open the pump up full throttle into my tank.  I think where mine are coming from is that the last bit of plumbing is loc line with a threaded fitting.  I think the air is actually being "venturi"d into the plumbing and causing it to do that.

Thats all I can narrow it down to so far.

[NanoSF]

Interesting. I would figure though that  anywhere that air can get sucked into the plumbing water would be leaking out instead wouldn't it. I mean you are forcing water through the plumbing at relatively high pressure. Anywhere that air can get sucked in would at least show salt creep I would have thought. I don't know the science of it though, you might be totally right. My return plumbing is threaded into a bulkhead and then there is loc line once in the tank. So is the suggestion that air is being sucked in where the return line is threaded into the bulkhead?

[JetJumper]

I see salt creep where I am thinking this occurs on my system.

The idea behind a venturi is that slow flow the water will travel through the tube, however when there is a fast flow, it actually "SUCKS" the air instead of pushing water out of the small hole.  This is why I figured it was my threaded sections as it doesn't happen when there is slow flow and only fast.  My pump is a 1500GPH pump for a return, so when its open fully there is a decent amount of flow. 

I might be drawing conclusions based on nothing, but its the only thing I could think of for my setup.

I guess the only way to tell would be to remove the loc line or to submerge the loc line sections under water completely and see if it still does it.

[NanoSF]

Okay I see. I figured the science was something to do with the speed of the water rushing past the area with access to open air. In my case though the only area that is not under water is that one joint going into the bulkhead. It has nylon tape on it and there is no signs of slat creep. I could run a bead of silicone around the joint to prevent any possibility of air sucking in there and see what that does.

[Bob P]

Could be sucking air as stated.
Could also be cavitation.
Check that the pump intake and impeller chamber
is nice and clean. The spinning impeller can cause
the oxygen to separate from the water. Much like a boat propeller.
Can be worse if the intake port has junk growing on it, the flow is turbulent.

[NanoSF]

I do clean well all the time, but I also notice that there is a bit of a manufacturing flaw on the impeller. It has a bit of a burr. Maybe this is what is agitating the water enough to create the cavitation (which I had never heard of, but makes sense). Maybe sanding down that burr will help. Thanks for the help.