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Re: Area for CO2 absorption

> From: George Booth <booth at frii_com>
> Subject: Re: CO2 Diffusion area
> >Date: Wed, 26 Apr 2000 22:58:33 -0500
> >From: "Dustin Swanson" <dustins at greatlakeseng_com>
> >I was wondering if there's a general formula or chart for the surface area
> >of the CO2 in the "bell" that should come in contact with the water based on
> >the size of the tank.  
> There is such a table in "The Complete Book of Aquarium Plants" by Allgayer
> and Teton. It shows surface area in cm^2 needed based on KH and tank volume
> in liters. 
> For a typical value of 5 dKH, surface areas are as follows:
> Volume:  1,000   800   600   400   200   100 
> Area:      180   140    80    50    30     *
> For a value of 3 dKH, 
> Volume:  1,000   800   600   400   200   100 
> Area:      140    90    40    40     *     *
> * CO2 produced by fish is sufficient under normal circumstances
	I had a go at this a few years ago.  I reckoned (and still do) that
the important thing wasn't the volume of the tank but the surface area.
If the normal equilibrium value of CO2 in water (in air) is about 0.5 ppm,
let's suppose we want about 40 times that, or 20 ppm.  We originally
had the full area of the tank at 330 ppm CO2 in the gas, and we are going
instead to use a smaller area at 1000000 ppm.  The gas concentration
increase is 1000000/330 or about 3030, and we actually wanted forty
times as much in the water, so we can use 40/3030 or about 1/75 of the
surface of the tank.  My 160 L has a surface of about 3600 cm^2,
so I need about 3600/75 = 48 cm^2 of bell area.  This is in the same
ballpark as the numbers above.  I actually use a smaller area of highly
agitated water (faster transfer), and it seems to work.  Someday I'll try
to model this a bit more rigorously, but surface transfer effects are
not the easiest of things to model! 

Paul Sears        Ottawa, Canada