Studies on CO2 diffusion varying volume and surface area

jmhart

jmhart

Revolutionary
Sep 8, 2007
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Atlanta,GA
Has anyone come across stuides examining the diffusion rate of CO2 out of water? I'm curious to examine what effect the surface area to volume ratio has on the diffusion rate.

Since our closed system of the Earth is constantly trying to reach a state of equilibrium, and our aquarium that we try so hard to keep 30 ppb of CO2, is an open system, we are constantly fighthing this drive. Constantly, CO2 escapes our system because Mother Earth wants her fair share of CO2 too.

Many factors play into the diffusion rate of CO2 across the water air boundary, but for the moment I am more concerned with concentration of CO2, surface area, and volume. Clearly, we can all understand what impact surface area plays. Larger surface area, greater rate of diffusion(or loss). However, I ask, if the concentration(30ppb) and surface area stay the same, shouldn't an increase in volume not affect the diffusion rate?

The reason I ask is that I will soon be upping one of my tanks from a 30g to a 45g. Everyone says that DIY CO2 doesn't work for tanks over 30g, but if you're surface area remains the same, and you simply add an additional 2 liter, why shouldn't DIY CO2 work for this tank? The two tanks have the exact same dimensions, except the 45g is taller.


So, I'm curious if anybody has anything objective to say. I recognize that pressurized co2 would eliminate this question, but I like the challange. Additioinally, comments like "DIY CO2 doesn't work for larger tanks" without supporting information isn't what I'm looking for.
 
your hypothesis sounds reasonable though I assume you will be upgrading your filter too which means more current which may increase CO2 loss but not significantly I would think. give it a go, what's the worst that can happen?
 
My understanding is that the natural surface tension of a smooth surfaced water body prevents the exchange of gases between the water and the air. This is why most folks use a submerged spraybar pointing away from the surface in a tank where co2 is being added.
 
TwoTankAmin said:
My understanding is that the natural surface tension of a smooth surfaced water body prevents the exchange of gases between the water and the air. This is why most folks use a submerged spraybar pointing away from the surface in a tank where co2 is being added.
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This is true. Reduced flow, keeping the surface as still as possible, does decrease the loss of CO2 and other dissolved gases. I can't say for sure, but I think it's less a matter of surface tension and more a matter of reducing surface area and reducing the force driving equilibrium. By reducing the flow across the surface, you affect how quickly that infinitesimal last layer of water increases it's CO2 concentration forcing the exchange between the air and water.

It could also be surface tension, what I just said was mostly talking about of my @*$.
 
As far as diffusion, your theory makes sense but I have another consideration that may make DIY less effective, that nobody seems to have mentioned. A deeper tank will likely contain plants that are taller (even if the footprint is the same), this results in a higher plant mass and therefore increases the amount of CO2 consumed. This of course can be overcome by adding more CO2 as you suggested, it just becomes problematical on how much work and money you want to invest in yeast sugar and bottles. I don't doubt that DIY CO2 can be effective in any size tank, its just a trade off where cost and convenience falls on the side of a pressurized system.
 
Canuck said:
Now that I can understand :P
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Yep, and this list just keeps growing. Eheim Ecco cannister, Jebo Inline UV Sterilizer, 45g tank, build new stand, soilmaster substrate...

I check one thing off the list, two more go on it. I did get 3 Visi-Therm Stealth heater's yesterday, so that was nice.
 
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