atmospheric air is 300-400 ppm CO2, Why inject CO2, instead of air?
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Old 12-30-2012, 07:05 PM   #1
spidangular
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atmospheric air is 300-400 ppm CO2, Why inject CO2, instead of air?


I think I got most of my question out in the subject. If I just put an airstone on a big enough pump, why wont the CO2 stay in the water column with the 02? or will it?

Also, it seems like all surface agitation assists in the loss of injected CO2 to the atmosphere, so why do people use any other method of filtration other than canister filters since sponge and HOB filters let all the CO2 out? Does the loss of CO2 via diffusion just slow enough? Of course, that depends on the type of filter and the contact that that filter puts atmospheric gas against the tank water.

Now that I think about it, if I just took a working drop checker (which I don't have) out of a tank with, say 30pppm CO2, then the it should indicate that the concentration of CO2 increases. . and would increase with just a big air stone as I mentioned earlier.

The atmospheric ppm of CO2 is just throwing me off. .. Just google "atmospheric co2 ppm".

Now I realize it probably has everything to do with pH and kH..
thanks for your thoughts.
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Old 12-30-2012, 07:34 PM   #2
mistergreen
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There are 2 issues.
#1. I think it's about the density of water that makes it harder for plants to absorb CO2 like that would in air so we in fact inject many folds more CO2 by volume into the water than there is in the air.

#2. CO2 concentrations are measure differently in air vs in water. In air it is ppm(v) by volume. In water it is ppm(w) by weight. So 300ppmv is only 1.3ppmw of CO2 in water.

ps. If you set up a low tech tank, CO2 is generated by the bacteria in your soil/substrate. They'll produce something like 20ppmw of CO2 which is way more than what's available from the air. You can search for my research in a DIY CO2 sensor to see the fun.

Last edited by mistergreen; 12-30-2012 at 07:47 PM.. Reason: ....
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Old 12-31-2012, 01:56 AM   #3
HD Blazingwolf
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Co2 concnetrations are higher in air. Much higher. And the lethal limit for a human at sea level is about 20,000 ppm for one hour

The problem is water resists the uptake of co2 becuase of surface tension. And whats knwon to the science and hobby community as partial pressure.

An airstone would indeed inject tons of co2 and oxygen, but its also riding the 70% nitrogen gas bubble right back out before any of it really dissolves.

Also breaking the surface of water doesnt release as much co2 as an airstone becuase the gas bubbles break the suspension of the co2 molecule easily and absorb co2 and transport it to the surface. Very wastefull
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Old 12-31-2012, 04:39 AM   #4
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Quote:
Originally Posted by HD Blazingwolf View Post
Co2 concnetrations are higher in air. Much higher.

The problem is water resists the uptake of co2 becuase of surface tension. And whats knwon to the science and hobby community as partial pressure.
Not true on these points. The level of co2 in the air will reach an equilibrium through time. Partial pressure is only .80 or around 80% meaning if the air reads 100ppmv, the water will read 80ppmv; not Much higher

There isn't a Ton of co2 in the air as you mentioned. There is only 300-400 ppmv of co2 in fresh air. It'll jump to 1500-2000 ppmv with people in the room. We all know that it takes 30 ppmw -60 ppmw of co2 in the water for good plant growth in a high light environment. That's 10,000 - 20,000 ppmv of co2 if it was in the air. Yeah, that could kill us. Good thing fishes are used to it in the water.


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