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# Re: "WW", O2, etc.

Jim Kostich replied:
>Recent articles claim that the first two events involving reactions at the
>water/bubble interface are now thought to be of very little significence -
>at least as far as the addition of oxygen is concerned. Does this also hold
>true for CO2 exchange? And if so, doesn't this mean that the circulation
>caused by water pumps will be just as effective at "allowing diffusion to
>occur at a more rapid rate"? I've always been mystified as to why plant
>books, including _The Optimum Aquarium_, denounce bubble-type aeration, yet
>recommend "good circulation". At first, I thought that perhaps the surface
>tension of the water partialy blocked the exchange of gases, but then noted
>such books also recommend surface agitation and even wet/dry filters.
Agitation is agitation and whether it is caused by bubbles or a power head,
the diffusion of gasses in and out of the water is a function of the
surface area exposed to the gas and the agitation in that surface. Aeration
adds the additional area of the bubble surface to the diffusion area.
Estimate the size of the bubbles and the number of bubbles that exist in
the bubble column at one time and figure out the surface area.
The area of sphere is 4 * PI * r^2. For a 1/4 inch diameter (1/8 inch
radius) bubble the area is 0.196 square inches. So if you have 50 of these
bubbles moving to the surface as any one time, you have an addition of 9.8
square inches of area (albeit, highly agitated area).
The volume of a sphere is 4 * PI * r^3. Therefore you would get 8 times as
many bubbles of 1/8 inch diameter for the same air volume. 400 1/8 inch
diameter bubbles have a surface area of 19.625 inches. So smaller bubbles
give you more surface area.
This example also shows how ineffective bubbling CO2 directly into the tank
is. One 1/8 inch diameter bubble every 5 seconds that takes 5 seconds to
get to the surface adds 0.049 square inches of CO2 diffusion area.
Paul
"Figures don't lie, but liars can figure!" Mark Twain