ORIGINAL: cgroen
It looks like the bubbles comes from the ''bottom'' of the UAT, whether that is because the feedline from the tank actually ''splashes'' hard into the fuel, I don't know, but that could be the source of the bubbles that eventually will go thru the sock ?
Cavitation through the filter bag.
I explain it on the first post:
"This will restrict the flow around the air trap bag and might induce cavitation downstream it !
So in that case the air trap system becomes a bubble generator..."
This type of cavitation can occur very badly in case of UAT collapsing ( your first test runs ) or be more subtle ( subsequent tests with the appropriate tubing ).
The fine bubbles are generated through the bag mesh due to the pressure differential and local speed velocity increase in the mesh.