DIY CO2 Bell -- Science Question! - The Planted Tank Forum
 
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post #1 of 4 (permalink) Old 05-17-2005, 07:01 PM Thread Starter
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DIY CO2 Bell -- Science Question!

So I made a CO2 diffusion bell for my DIY system like the one shown here for my 29 gallon:

http://www.petfish.net/DIY_Diffusion_Bell.htm

So far it's working great. In fact, perhaps a little too well -- my KH is 11 and PH went from 8 with no CO2 to 7.1 this afternoon in less than 24 hours, so it may have gotten even lower overnight. One of my gouramis died and I think this PH swing was probably the cause. I was surprised by the good quality of diffusion given that I am running a Penguin 330 filter with not one but TWO BioWheels on it!

So I started thinking in more detail about this diffusion bell system and had a few questions about maintaining its efficiency over the life of the yeast mixture. If I can think out loud here, it seems to me that if I were letting each bubble dissolve in the water, either through direct injection through an airline or airstone, through a ladder, or sending it into the filter intake, then my CO2 levels would be highly dependent on the bubble rate and therefore highly dependent on the output of my yeast mixture.

In this system, however, the bubbles get trapped in the bell, making one very big bubble. Each individual bubble does not diffuse appreciably as it comes out of the tubing; rather the one large bubble with a big area of surface contact with the water is diffusing into the aquarium all the time. (When the bell fills up, it will occasionally "burp" out one very big bubble which, I assume, doesn't really diffuse in the water since it seems to rapidly fly up to the top of the tank and pop and so this overflow CO2 is probably irrelevant to the CO2 levels in my tank.) So I'm assuming that the big bubble of CO2 that sits in the bell is the primary source of CO2 in my tank, not the little bubbles that come out of the airline tubing (except to the extent that they eventually FORM the big bubble) or any CO2 bubbles that "burp" out from overflow. So my focus, in thinking about this system, should be on the big bubble, not on bubbles per second. In other words, this system seems less dependent on the strength of the yeast mixture at any given time, provided that said mixture is keeping a nice big bubble in the bell diffuser.

So if I'm thinking through this correctly...

(1) The optimum CO2 mixture will be one that, for the longest time possible between bottle changes, readily fills the bell but does not overload it so that the maximum amount of CO2 that is produced is diffused in the bell rather than bubbling up as wasted overflow. If I'm getting the results that I'm getting with one 2L bottle, then, it seems that going with 2 bottles doesn't make much sense, since the overflow would just bubble away to nothing. Am I right?

(2) Since the bubble in the bell itself is the CO2 that is of greatest concern, I should be able to watch the bubble in the bell, not the bubble rate coming out of the tubing beneath the bell, as a good indicator of when to change the mix in the bottle. I'm guessing that as long as the bubble continues to replenish itself thanks to the introduction of more little CO2 bubbles, I'm okay. But once the big bubble starts to diminish faster than the little bubbles coming from the turbing can replenish it, it's time to make a new mix. Does that seem reasonable?

(3) I'm sure that some gases other than CO2 will get into the bell, either as byproducts of the yeast generator process, or air trapped in the tubing/bottle initially (I emptied out the first two bells full of gas in an effort to minimize this possibility, since I figured the first bit of gas pushed through the tubing would be mostly air), or from the exchange of other gases between the water under the bell and the gases in the bell. Does anyone have any good ideas for how to account for this? Or do you think it would be so small a portion of gas in the bell that, as long as I'm seeing some bubbles of CO2 going from the tubing into the bell, other gases present would be more or less irrelevant?

I'm now really wishing I had taken organic chemistry in college... Thanks for any insights. Oh -- and with the numbers I'm getting, would you run an airstone at night with this system? I have to say I was pretty well STUNNED by the CO2 levels I seem to be getting from this!
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post #2 of 4 (permalink) Old 05-17-2005, 08:23 PM
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you seem to be right on in many respects... however, i disagree with a couple of issues you bring up here. first, the "big bubble" in the bell is not necessarily CO2, and quite a bit more active diffusion occurs when the small bubbles travel up into the bell than you realize. The "big bubble" is most likely a combination of CO2 and O2, as well as a few others. O2 is more difficult to dissolve, and more readily bubbles out of solution than CO2, so if you give it a surface (in this case the edges of the bubble trapped in your CO2 bell, or even small bubbles traveling up to the bell) it will gladly trade places with CO2, which dissolves readily. If you allow the bubble to get large, then your CO2 will partially diffuse, and partially stay in the large bubble. the diffusing gas is often replaced with O2, and this dilutes the gas in your big bubble, making diffusion of the remaing gas much slower. This is the reason that CO2 injection systems are most efficient when the bubble comes in contact with moving water as opposed to letting it passively diffuse in a bell, where oxygen can take up precious surface area that is in contact with the water. You are right, however, that bubbling at a rate that causes continous "burping", or loss of gas to the surface of the tank is wasteful. You might gain a negligible increase in CO2 ppm by doing this, but it's pretty much a wash, as the O2 will replace however much CO2 it can pretty quickly, and it's the CO2 diluted with O2 that's going to the surface. Your gain would be comparable to just moving your CO2 bell up from midway in your tank to an inch below the surface. the only gained diffusion is what is happening between the bell and the surface of the water, and while it's most likely measureable and would seem like a worthwhile effort, it falls so far short of an active diffusion / reactor system that it's hardly worth the trouble. As long as you're happy with your bell diffuser and get the ppm you're shooting for, no worries, but if you're looking to get a higher dosage of CO2 into your water, active diffusion is the only way to go. (ie bubbling into a gravel vac w/ powerhead or external reactor on a powerhead/pump/canister line)

very interesting post, I love this kind of stuff

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post #3 of 4 (permalink) Old 05-19-2005, 11:05 AM Thread Starter
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Thanks for the detailed reply, oqsy. A few follow up "thinking out loud"questions:

(1) I realized that what's in the bell is not pure CO2, and to counter the possibility of the small bubble traveling from hose to bell having too much gas exchange, I placed the bell very low in the aquarium (also for aesthetics). So would that mean that the CO2 trapped in a bell that is placed very low in the tank and very close to the output hose from the yeast system would be more pure, in theory, than the gas trapped in a bell higher up in the tank?

(2) With this placement, since the big "burp" of overflow CO2 (sometimes a big bubble, sometimes very small ones like the ones coming from the hose) has to travel quite far to the top of the tank, I'm guessing that the negligable benefit of moving the bell upward, if there is any, is probably offset to some degree by the fact that overflow bubbles (even though they would be less pure than the bubbles coming straight from the hose) have a long way to go and would diffuse along the way?

(3) If the gas trapped in the bell is a mixture of CO2 and other gases, would the other gases build up at a rate higher than the CO2? In other words, over time, does the bubble in the bell get less and less pure? And if so, would it make sense to "dump" the bubble every so often and let it start over, accumulating a new bubble that is more pure? Or do the big burps of overflow more or less accomplish this, and I shouldn't worry about it?

And finally...

(4) I'm getting a surprising rate of CO2 production from this system. KH is 11 or 12 and PH has dropped from a solid 8.0 to a stable 7.0. PH sometimes goes from 7.1 or 7.2 during the day back to 7.0 at night, but that range has stayed pretty much rock solid since the initial downswing from 8 when the system was added. How long would you continue checking PH levels daily?

If this system really is more stable and less dependent on the relative strength of the yeast mixture at any given time, as I surmised above, it seems that I could, at this point, just let it keep going until the bubble rate can't keep up with the diffusion of the big bubble in the bell (ie. until the bubble in the bell starts to shrink and ceases to overflow ever). Then when I do add a new bottle of mix (probably inline with the first one so the dying bottle #1 and new bottle #2 work together), I theoretically should not really have to worry about PH swings from any shift in the mixture, right?

Ultimately, I'll probably keep obsessively checking PH levels through the first couple of bottles of yeast mixture just to be on the safe side and test out this stability theory, but I thought I'd ask if my reasoning is correct and that I shouldn't have to worry too much about big swings with this kind of system.
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post #4 of 4 (permalink) Old 05-22-2005, 05:04 AM
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I started using a bell and a DIY set-up and moved on to a pressurized system. Nothing wrong with DIY. I still use it with a ladder in my small tank. I just got tired of the maintenace and inability set the dose rates whatever way I want.

If all of your fish died I would think about the CO2 being a problem, but that's probably not you problem. Were any of them gasping for air at the surface when you discovered your loss? I suspect something else is responsible.

I would make sure the fish stay out of the bell and if you are really concerned about pH, make it smaller.

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