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