It's pretty much moot now since what I was describing originally seems to no
longer be available from aquarium supply stores... and from lab supply houses
will run close to $300.

That said, I wasn't describing a fritted glass filter. Picture a piece of
glass tubing 1/4" diameter and long enough to reach the bottom of a
container. Picture the last 1/2" or so being about 1/2" in diameter and made
entirely of fritted (porous) glass, no hole in the end. So all the air pumped
down the tube comes out thru the fritted glass as very small bubbles. These
things are all designed with a pore size that will produce bubbles using very
low pressure, like from an aquarium pump.

You're right that fritted glass funnels have porous plates with a variety of
pore sizes... the thing about that being that some pore sizes can be so small
that the surface tension of the etchant solution may not allow gas to pass at
such a low pressure. They're used with a vacuum flask and maybe an 80% or
more vacuum to draw solution thru them. And heaven help you if you plug one
up by using it in a solution that can generate a precipitate that won't
readily dissolve in a fresh, rejuvenated solution. There are special
techniques for cleaning these filters if they become stopped up and most of
them are seriously dangerous in the hands of casual PCB makers. That and
they're all hugely expensive. Not recommended even if you just won the
lottery. ;-)

But if you can beg one from a local college or lab that's being tossed out
for maybe having a chipped rim... because they don't have anyone who knows
you can just fire polish out the chip (or just ignore it)... go for it!

Jim H



Received from designer_craig at 3/24/2019 01:17 AM UTC:

>Actually in "chemistry lab terminology"? they are called fretted glass
>filters.? Generally, they are 1-3" dia funnel sort of like devices with a
>fretted glass disk down near the bottom.? For filtering I assume they are
>available with various pore sizes, they will also work for generating very
>small bubbles.