# Sponge Filter Physics Test - Discussion

## which creates more lift?

• ### many small bubbles

• Total voters
1
Kody Grieve
ok so ive seen a number of videos on youtube and people just in general using sponge filters without airstones. now I'm annoyed when I see these because in my mind, a lot of small bubbles creates a million times more suction then fewer very large bubbles. yet time after time I see people that have just stuck an airline into the pvc lifter and I can't help but wonder, is that spongefilter that's supposedly "running" the tank actually doing anything at all?

because if no/little suction is actually occurring from having big bubbles, are millions of people essentially just putting sponges in their tanks and calling it a day?

david1978
to my understanding bubble sizes work different ways. Small bubbles work by reducing the pressure inside a tube and give a more constant flow. Larger bubbles have more of a mechanical effect similar to a piston pump so you get water then air bubble then water. So both work just in different ways.

Kody Grieve
so do we know which creates the larger flow rate?

david1978
I found a formula to calculate it but I don't know what all the symbols stand for. Lol.

bitseriously
Well, since were testing the laws of sponge filter physics…
Is there a threshold of increasing airflow at which uplift of water diminishes and possibly even stops? For example, imagine you were able to blow enough air into a sponge filter that air bubbles came out the uplift tube and through the sponges. If we accept that that is in fact possible (don’t bother me with what’s realistic, I’m on a roll here) then as we diminish the rate of air from that very high level, there must come a point where there is zero suction through the sponges, but air bubbles still coming out the uplift tube, and as we further reduce air pressure, the sponge filter starts functioning normally. And, if all that is true, there must be a point somewhere between zero airflow and massive airflow that is optimal for fish tank filtration, i.e. the point at which the amount of water passing through the sponge(s) is maximized. Who wants to take a crack at figuring that out?
Sorry OP, I get a bit carried away sometimes.

Swampgorilla
Not all sponge filters are created equally and not all of them have the capability to add an airstone. For instance ... the Bacto-Surge, which is an awesome sponge filter ... has no way you can add an air stone.

Swiss Tropicals manufactures a "jet lifter" at the base of the tube for their sponges and I give that design a big thumbs up.

Which brings up another variable. A Hydro IV sponge has the air terminating at the top of the sponge ... the air doesn't go down into the sponge body unless you build a small connection for an air stone. The Bacto-Surge carries the air all the way down to the bottom of the sponge body - where it then connects to a "ring manifold" that sprays out bubbles along the diameter of the ring to travel up the uplift tube.

Which provides better flow? I dunno.

And ... if YOU DO put an air stone in something like the Hydro IV ... would the physical presence of the air stone limit flow and negate any advantage you might get from smaller bubbles?

So it's a complicated problem that involves more than bubble size ... but also involves physical components.

Kody Grieve
bitseriously, I'm totally vibing to your thought process however, you missed a spot. you went so far as to say air was coming out the sponge. while that is correct, I think it stops completely BEFORE then. see if you look at it from a process engineering POV, and think "what goes in must come out", then having enough air bubbles to mean that ONLY air is coming out of the lifter must also mean that no water is going through the sponge. at this point you probably wouldnt have any air coming out of the sponge but I see what you mean. I wonder if maximum flow rate can be found experimentally by having a small prop just above the lifter outlet and then test different air flow rates vs lifter diameter. and from what Swampgorilla said, very good point as to having an airstone in the lifter. you're undoubtedly going to increase the resistance by adding an airstone to the lifter, however in the Hydro IV you mentioned, while it doesn't have an airstone it does have a ring manifold which essentially acts in the same way. which is fantastic because you have the bubble size of an airstone (ish) without the resistance of putting one in your lifter. David1978, what was the equation? I should be able to guestimate my way through it.

Kody Grieve
we should also take into consideration that larger bubbles will have a higher terminal velocity than smaller bubbles. (just in regard to the mechanical "piston" style lifting of larger bubbles).

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