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CO2 dissolves easily?

Discussion in 'CO2 Enrichment' started by scottward, Sep 3, 2009.

  1. scottward

    scottward Guru Class Expert

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    Hi.

    When I think of something dissolving easily, I think of something along the lines of salt dissolving in a glass of water; you put the salt in, give it a stir and it dissapears within seconds.

    CO2 is always said to dissolve easily, but, if this is the case, why do bubbles of CO2 float around in my tank for so long??

    I'm sure this has been asked before. Sorry!

    Even the tiniest bubble of CO2 will drift about in my tank, which is hardly at saturation point (my fish are still alive!), for a very long time. If CO2 dissolved easily in water, why don't the bubbles dissapear very quickly?

    Scott.
     
  2. Philosophos

    Philosophos Lifetime Charter Member
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    "Dissolves easily" is a relative term. We're aiming for something like 25-50ppm here. Lets say a 5lb CO2 system lasts 5 months on a 20-30 gal tank, and achieves 40ppm for 10 hours a day.

    To achieve 40ppm CO2 with 3ppm coming from air, we'd need 37ppm of CO2. CO2 has a density of 1.562mg/L in air, and I'm not going to bother compensating for the 3-4% increase in pressure under a foot of water.

    So then:

    40 - 3 * 1.562 = 57.794mg of CO2 needs to be maintained.

    57.794mg of CO2 needs to be maintained for 10 hours a day, 7 days a week, for roughly 152 days.

    A 5lb CO2 container contains roughly 2268000mg of CO2. How much is that per hour going in to the tank?

    2268000/(7*10*152) = ~213mg

    So then, only about a quarter (57.794/2268000=0.2713) of the CO2 you pump in to your tank actually hangs around while your system is on. The other 75% just gasses right off. Even accounting for plant growth, I'd imagine a sizable amount of carbon never dissolves; 45% dry weight means something more like 4.5% wet, trying to suck up 2 grams of carbon a day means something in around 44 grams of plant growth in that 10 hour period. I doubt it could even push 10 grams in our little 20-30 gal thought experiment tank.

    Naturally there's a lot of give; this is just a general example. People go through CO2 at different rates for different reasons.

    -Philosophos
     
  3. Tom Barr

    Tom Barr Founder
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    Of that CO2 that does hang around, even much less actually makes it to the plant.
    Most is degassed and wasted.

    Regards,
    Tom Barr
     
  4. Biollante

    Biollante Lifetime Charter Member
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    Hi Scott,

    What Philosophos and the Tom Barr said.;)

    From a purely practical point of view just about all of the CO2 you see in the form of bubbles is wasted. :(

    This leads me to one of those things that bother me.

    Why do folks spend so much on CO2 equipment, filling and refilling, then ‘cheap-out’ on the diffusers, on actually getting the CO2 into solution?

    More time, effort and money spent on efficiently getting the maximum CO2 into solute the greater the control over the CO2 in the aquaria.;)

    I suspect (can’t prove, can’t cite) that we planted aquaria sorts are going to end up stealing an idea or two from them poor saps what keeps reef systems.:eek:

    Biollante
     
  5. SuperColey1

    SuperColey1 Guru Class Expert

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    Maybe just the language barrier here BUT the 'cheaping out' here is a little off the mark. Getting the CO2 into solution isn't the aim. Getting it INTO the plant is. Therefore from the many suspicions about bubbles performing better in terms ofdelivering the CO2 through the plants 'outer barrier' it would mean those with reactors are the ones that are worse off as the 'in solution' CO2 would struggle a little harder to get through this barrier.

    One thing that I continually read from 'reactor users' is the misconception that using a mister loses more. They believe this because they can see thebubbles eventually reaching the surface. They seem to forget that with any method the water is trying to return to equilibrium and that means with reactors too. The 'in solution' CO2 is still gassing off but you can't see it.

    Do misters/bubblers gass off quicker? I don't think so. Bubbles are much more bouyant I suspect and therefore may reach the surface slightly quicker but then what do we see when our flow isn't so good? Bubbles covering the surface. They stay there for ages.

    I would suspect that the bubbles gas off slightly quicker than 'in solution' BUT I suspect it isn't a huge difference. Then if bubbles/mist break through the plant's outer barrier easier it would more than likely reverse which method is wasting more!!!

    Another consideration is the difference in method in terms of the source of delivering the CO2.

    With a reactor it is inline with the filter and is therefore delivered pretty close to the surface and pushed at high velocity across the tank.

    With a disc you can place it much lower in the tank away from the water surface. You can then position it so that either the filter outflow or a circulation pump pushes it back down.

    The reactor Vs mist debate has raged on for years and I suspect always will. Its more a case of 1 person's view against anothers and if you are succesful with 1 method then there's no argument really. Most reactor people go from assumtions that they can see the CO2 rising from misters and forget that just because the in solution CO2 is not visible it is still doing the same!!!

    I personally use a disc diffuser and have it half way up the tank, then use a Koralia to blow the bubbles down from the diffuser and around the tank.

    Do I think this method is better than reactor? All I can say is that I use less CO2 on my tanks with similar dimensions and parameters that use reactors.

    AC
     
  6. Philosophos

    Philosophos Lifetime Charter Member
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    I think the question of reactor vs. diffuser (esp. needlewheel) is efficiency vs. aesthetic. In a higher light tank, you'd want higher efficiency because of higher CO2 demands. Meanwhile in a lower light tank, bubbles may not be required to easily hit non-limiting CO2. In this case, less junk in the tank and not having bubbles everywhere may be worth more to the hobbyist than efficiency for efficiencies sake.

    -Philosophos
     
  7. Tom Barr

    Tom Barr Founder
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    A simple way to test the difference between one method of diffusion vs another, at least in a meaningful way as plant growers.........

    Use a mass flow controller to accurately sample and adjust the flow rates of CO2(none of this bubbles per second business), use plants of equal cutting length and several sub plot replicates in pots with similar sediments. Several small 10-20 Gal tanks for plot reps.
    Current/pattern needs to be similar.
    Use a non CO2 enriched control.
    Water column dosing the same.

    Measure the total dry weight of the stems.

    You can also measure CO2 [aq] in both, and sample a small amount of water to degas and check with pH shifts. even if the measure is relative, the pH change from degassing should be a relative good indication.

    Still, plant growth is the goal.

    Mist[gas] vs [aq] is not a direct measure, it is best tested using plant growth.
    To account for boundary layers, you need to add N2 gas mist along with dissolved CO2.

    Any differences between the N2 gas mist+ CO2[aq] and the CO2 mist are due to the gas vs dissolve states.

    There's some issues with this, but it would be fairly reasonable still I think.

    Regards,
    Tom Barr
     
  8. scottward

    scottward Guru Class Expert

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    Thanks for the info guys. I guess because I have my AM1000 hooked up using Tom's suggestion, the dual venturi method, I have a reactor and a mister all in one. ;-)
     
  9. Biollante

    Biollante Lifetime Charter Member
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    Cheaping Out?

    Actually, I think all of you are right. :)

    For one, ‘cheaping out’ doesn’t convey quite what I meant and SuperCooley1 is certainly correct when he so eloquently and correctly points out the object of our CO2 use.

    Though I will openly state I do come down on the side of the reactor; in the reactor versus diffuser debate at least as far as efficiency of CO2 usage. I have seen beautiful and magnificent planted aquaria keeping marvelously high and constant levels of CO2 in solute, using diffusers and I begrudge no one there use.

    Philosophos’ point is well made; aesthetics and expectations certainly play an important role, ultimately perhaps the only real role.

    Tom Barr is as usual correct and even gives us a method for testing in the meaningful way that would satisfy (and has satisfied me) SuperCooley1.

    SuperCooley1’s well made point that ultimately what should concern us is the CO2 that we get past the Prandtl boundary layer along the plant leaf.

    In effect the good Professor Prandtl, has defined a number of problems we face in getting our, plant friendly carbon disbursed. SuperCooley1 in truth makes an excellent point with respect to the movement through the water, the aforementioned Prandtl also has a number that adds to our problems in affecting that perfect circulation for CO2 as well as nutrient distribution we all seek.

    One point on which the ever-erudite SuperCooley1 and I part company is over the issue of weather seeing the bubbles make any difference to the ‘out-gassing’. In a perfect system, indeed any CO2, or other gas entering solution would indeed ‘disappear.’ That gas in solute would be relatively difficult to remove from solution. Just as CO2, dissolves relatively easily into water (the thing that started the topic), it in fact leaves solute with some difficulty.

    I will stipulate as obvious that while a bubble of CO2 (or any water-soluble gas) remains in contact with water some gas is dissolved and that the smaller the bubbles the greater the total surface area the better the gas will dissolve.

    Hence, from my view a diffuser bubbling into a ‘dome’ or other container holding the gas in contact with the water longer would increase markedly the efficiency of the diffuser.

    The subject that I was addressing was somewhat more a practical matter. I stand by my statement as a practical matter those bubbles are wasted CO2.

    I fear, I allowed myself to be influenced by, and rather seeing this thread as a continuation of a couple of other of Scott’s threads. As well as other discussions of volume of CO2 used. I have done quite a bit of reading and found tremendous, I might even say irreconcilable differences in claims.

    I will note that I was rebuked for suggesting that the expensive-Japanese-guy may have efficiently diffused CO2 into his tanks.

    While an illness prevented me from completing the experiment, some years ago I did the experiment that the Tom Barr has suggested here, to my satisfaction, convinced me of the inherent efficiency of reactors over diffusion in supplying aquatic plants with plant friendly carbon. :)

    I see nothing wrong with using diffusers or any other method so long as it pleases the user.

    After saying all of this, I believe that we may well all have the wrong end of the proverbial stick. I think it may very well have been staring me in the face, at times like these, I wish I had a brain.:eek:

    Biollante
     
  10. scottward

    scottward Guru Class Expert

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    Biollante, nothing wrong with your brain at all!

    Thanks guys for putting together all that info for me.

    My mind is at ease now, I just do what I can to get as much CO2 in the water but understand that there will always be bubbles that just float about and don't dissolve.

    Just the way it is.

    Scott.
     
  11. abcemorse

    abcemorse Prolific Poster

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    Personally, the constant presence of bubbles makes the water look a little cloudy, especially from a distance. So here again it is personal preference. I run degassing line from one reactor into the intake of a powerhead where it is delivered to a secondary reactor and returned virtually bubble free. It used to just go to the powerhead and into the tank, producing a mild misting effect. Actually I just made that mod over the weekend and have noticed a substantial increase in pearling. Works well, like I said it's all about preference, personally I like the crystal clear bubble free (almost) look.:cool:
    I think too that some of the bubbles are not necessarily CO2 but some other gases that do not dissolve so readily....
     
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