# Re: Area for CO2 adsorption

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On Mon, 1 May 2000, Paul Sears quoted G. Booth as saying:

> > 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!

It makes my head spin.

I did a similar thing a while back, trying also to account for water vapor
in yeast-generated CO2 + the tendency for dissolved gasses to enter the
bell from the water.  I don't remember exactly what the area was but I do
remember it was more than I wanted to use in my tank and certainly more
than the 3/4" diameter(?) area that Dustin Swanson reported in the letter
that George replied to.

Roger Miller

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