atmospheric CO2???

3446 Views 38 Replies 15 Participants Last post by flanders, Aug 29, 2009

Shadowcat3

Discussion Starter · Aug 17, 2009

OK, just did some quick checking on the levels of CO2 in the atmosphere. Apparently they are somewhere above 300ppm or higher depending on location, weather, etc.. Now I don't use CO2 injection on my tanks, so I don't claim to know much about that particular subject. However, it would seem to me,... and obviously I must be missing something,... that we could eliminate all that fancy CO2 injection stuff, simply run airstones into our tanks and get tons of free CO2 as a result. What am I missing here?? If you typically shoot for a few ppm's of CO2 in a tank for good plant growth, and the air outside the tank is loaded with the stuff, relatively speaking, why spend money to BUY it??? Clearly I don't understand the process.

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KevinC

· #2 · Aug 17, 2009

Shadowcat3 said:
OK, just did some quick checking on the levels of CO2 in the atmosphere. Apparently they are somewhere above 300ppm or higher depending on location, weather, etc.. Now I don't use CO2 injection on my tanks, so I don't claim to know much about that particular subject. However, it would seem to me,... and obviously I must be missing something,... that we could eliminate all that fancy CO2 injection stuff, simply run airstones into our tanks and get tons of free CO2 as a result. What am I missing here?? If you typically shoot for a few ppm's of CO2 in a tank for good plant growth, and the air outside the tank is loaded with the stuff, relatively speaking, why spend money to BUY it??? Clearly I don't understand the process.

Distribution constant and extraction efficiency. You are assuming you can get all the CO2 out of the air and into your water - not true. The ratio of CO2(air) to CO2(dissolved) is constant for a given temperature and amount of CO2 in the air. You have to change something (amount of CO2 in the air for example) to change the amount of CO2 in the water. By bubbling in pure CO2 you drive the system out of equilibrium and temporarily add more CO2 to the water.

Shadowcat3

Discussion Starter · #3 · Aug 17, 2009

OK, I get what you're saying...sorta, that by pumping straight CO2 into the water you're increasing absorbtion....and yet, as I understand it, atmospheric CO2 is much higher than we'd ever have or want in a tank, so if that is true why wouldn't a simple airstone tend to increase CO2 levels somewhat and not flush it out as is the common perception??...or so I have read.......If a given gas (CO2) in solution (water) is at a lower concentration than in the gas being injected into it (air) than it would seem natural (to me) that it would tend to equalize and therefore the CO2 levels in the water should increase, although to what degree I have no idea.....I'm not a scientist, just a curious guy. I'm not disputing what you're saying and clearly thousands of beautiful CO2 injected tanks prove it's value......I'm just not clear on why simple air injection won't achieve similar results if the point is only to achieve a few ppm's of CO2?!...Sounds like a physics thing to me or very clever marketing

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kid creole

· #4 · Aug 17, 2009

I've thought the same thing before, but I don't remember my chemistry well enough to explain why it doesn't work.

Shadowcat3

Discussion Starter · #5 · Aug 17, 2009

Nor do I KC. It simply seems to me that if a few bubbles per minute of pure CO2 will desolve in water, then you should get a similar effect when pumping a steady stream of CO2 laden air into the tank. ....We all know that some percentage of the air does in fact get aborbed into the water, so it's reasonable to assume that the CO2 in that air is also absorbed to a similar degree.... Lots of things in lots of hobbies are useful....but not necessarily to the degree those that sell and promote them would have you believe, nor are they always the least expensive method of achieving the same end result....so I'm always a bit skepticle

.....Ain't looking to start a war with all you CO2 folks out there, just thinking out loud and trying to understand what I may (or may not) be missing in the equation.

Postal Penguin

· #6 · Aug 17, 2009

Here is the thing, while the CO2 in the air is absorbed with similar efficiency as the pure CO2, there is a lot less CO2 in the air bubble. So in a high tech tank, you will likely not be able to dissolve enough air into the tank to keep the CO2 levels up. Since atmospheric air is about 0.03% CO2 you'd have to use 3000 times the amount of air compared to pure CO2 used in pressurized set ups.

Shadowcat3

Discussion Starter · #7 · Aug 17, 2009

OK, so it comes down to an efficiency thing....I can buy that..........so all I need is a HUGE air pump blasting the tank and I could forego the whole buying CO2 thing..........LOL

....Obviously kidding there, but it's still an interesting question, at least for me. Somebody with a better understanding of chemistry than I have could probably explain it....As for the relative volume of air needed to supply an equivalent amount of CO2, if you think about it, you probably ARE pumping thousands of times the volume of air from a typical pump/airstone setup as compared to the tiny amounts that are injected from a pressurised CO2 tank....I'm still not clear as to why A should work better than B...or if it even does. Only way this makes any sense to me is if it's a saturation thing. A given volume of liquid can only carry a given percentage of dissolved gases, so by only injecting pure CO2 vs a CO2/air mixture you can raise the ratio of dissolved CO2 in the water column.............But even then the ratio of CO2 dissolved in air is much higher, so it should still increase accordingly..............I'm gettin a headache!!

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Shadowcat3

Discussion Starter · #8 · Aug 17, 2009

I'd think an interesting experiment for someone with both pressurised CO2 and an airpump/airstone would be too measure your CO2 and Dissolved Oxygen with the CO2 system operating. Then turn off the CO2 system for a day and run an airpump full on with a couple of stones in the tank for 24 hours and retest. It would be easy to do and wouldn't bother your plants for one day. I'd try it, but I don't use CO2 . Might get some interesting results?!

KevinC

· #9 · Aug 17, 2009

Water in equilibrium with air that contains 300ppm CO2 will reach only about 5ppm CO2 (at room temperature). Adding more air doesn't change that. Add more CO2 to the air and the water will take up more as well. Add more CO2 to the water and the air will take up more. Remove CO2 from the air and the concentration in the water will go down too. In a simplified manner, the distribution constant K equals 5/300.

jjp2

· #10 · Aug 17, 2009

To add air to water, you create surface aggitation which de-gasses the CO2 you are adding (or more than you are adding) so it defeats the purpose.

kid creole

· #11 · Aug 17, 2009

Postal Penguin said:
Here is the thing, while the CO2 in the air is absorbed with similar efficiency as the pure CO2, there is a lot less CO2 in the air bubble. So in a high tech tank, you will likely not be able to dissolve enough air into the tank to keep the CO2 levels up. Since atmospheric air is about 0.03% CO2 you'd have to use 3000 times the amount of air compared to pure CO2 used in pressurized set ups.

This makes perfect sense.

toughtimes

· #12 · Aug 18, 2009

Why not run an air pump through an in line reactor instead of using the air stone. Hmmm I think I may try this. The biggest problem I could see if it worked would be that there is no control to a system like that it becomes an all or nothing game then. With a pressurized system you can control the amount of co2 you add right down to the bubble.

kid creole

· #13 · Aug 18, 2009

Postal Penguin said:
Here is the thing, while the CO2 in the air is absorbed with similar efficiency as the pure CO2, there is a lot less CO2 in the air bubble. So in a high tech tank, you will likely not be able to dissolve enough air into the tank to keep the CO2 levels up. Since atmospheric air is about 0.03% CO2 you'd have to use 3000 times the amount of air compared to pure CO2 used in pressurized set ups.

toughtimes said:
Why not run an air pump through an in line reactor instead of using the air stone. Hmmm I think I may try this. The biggest problem I could see if it worked would be that there is no control to a system like that it becomes an all or nothing game then. With a pressurized system you can control the amount of co2 you add right down to the bubble.

See above. To get 10 bubbles of CO2 equivalent with air, you would need ~30,000 bubbles of air per second. It's going to be a big air pump.

A Hill

· #14 · Aug 18, 2009

KevinC said:
Water in equilibrium with air that contains 300ppm CO2 will reach only about 5ppm CO2 (at room temperature). Adding more air doesn't change that. Add more CO2 to the air and the water will take up more as well. Add more CO2 to the water and the air will take up more. Remove CO2 from the air and the concentration in the water will go down too. In a simplified manner, the distribution constant K equals 5/300.

I think you guys should reread this post

With that being said, I just use atmospheric CO2 and some excel here and there... It wasn't worth the argument with the parents. Crazy I know, but I gave up a while back.

-Andrew

plantbrain

· #15 · Aug 18, 2009

Let me ask you this:

Air has 21% O2, so that's 210,000ppm.
210,000/1,000,000 right?

So why does water have only 6-7ppm of O2 at 100% equilibrium at a temp of 28C?

Not 210,000ppm?

What is different in water?
Space for one........solubility is another.
O2 is not very soluble, CO2 is comparatively.
Rate of replacement to get back to equilibrium is another.
Viscosity/scale is another.

Plants take up a lot more than can be replaced by diffusion alone in water.
Particularly when there are dense stands of weeds and high light.

Like our tanks pretty much.
Diffusion rates are 10,000 X slower in water than in gas.

So they are greatly limited by CO2.
Air alone is not going to increase it much above a couple of ppm's.

To drive growth at a non limiting rate for CO2, you need more than 2-3ppm, you need about 15-40ppm depending on light intensity(yes, non limiting levels vary proportionally with light intensity).

Regards,
Tom Barr

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kid creole

· #16 · Aug 18, 2009

A Hill said:
I think you guys should reread this post

With that being said, I just use atmospheric CO2 and some excel here and there... It wasn't worth the argument with the parents. Crazy I know, but I gave up a while back.

-Andrew

Are you saying that bubbling air through water wouldn't add more of the air's components to the water?

KevinC

· #17 · Aug 19, 2009

kid creole said:
Are you saying that bubbling air through water wouldn't add more of the air's components to the water?

Yes - we all are.

As long as the water is already in equilibrium with the air (which it is if you have normal flow in your tank), increasing surface area/contact/bubbles won't change anything.

kid creole

· #18 · Aug 19, 2009

KevinC said:
Yes - we all are.

As long as the water is already in equilibrium with the air (which it is if you have normal flow in your tank), increasing surface area/contact/bubbles won't change anything.

I may be wrong, but this doesn't make sense to me. You can add more CO2 to the water by diffusing CO2 into it, but if it is a mix of gasses they won't diffuse into the water? That doesn't make sense.

Edit: I thought about it some more. I'm not wrong.

Hoppy

· #19 · Aug 19, 2009

The mistake in this line of reasoning is to assume that parts per million is a measurement similar to grams or ml or other absolute quantities. Parts per million is just a ratio. If you took a flask and pumped most of the air out of it with a vacuum pump, what would remain in the flask would still be air with 300 ppm of CO2. But, the actual mass of CO2 in the flask would be only a small fraction of what it was before you pumped out most of the air. Similarly, if you pumped that flask up to 100 atmospheres of pressure, it would still contain 300 ppm of CO2, but there would be 100 times more mass of CO2 in the flask.

One part per million of anything in water is much more mass than one part per million of anything in air, just because the water is so much more dense than air. One ppm of a liter of water is is one thousandth of a gram of mass. But, one ppm of a liter of air is about one millionth of a gram, which is one thousandth as much. There is very, very little CO2 in air, so bubbling air through water can't add much CO2 to the water, and if the water already is in equilibrium with the air, as far as CO2 is concerned, which it always is after the water sits out in air for a few hours, bubbling more air through the water does nothing at all.

CO2, unlike the other components of air, dissolves very easily into water. You can't eliminate the CO2 in water by stirring it, shaking it, etc. Water always has CO2 in it, unless you take great pains to remove it. It just doesn't have nearly as much as we want it to have.

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kid creole

· #20 · Aug 19, 2009

That's not a flaw in anyone's line of thinking, that's a flaw in reality. If you diffuse CO2, you get results commensurate with a gas that 1 million ppm CO2. If you diffuse air, which is 300 ppm CO2, you get commensurate results as well. You get thousands of times less CO2 per bubble, but not zero.

The OP's question has already been answered by postal penguin yesterday.

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