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What is the gas that builds up inside the Reactor during the day? This.

Discussion in 'CO2 Enrichment' started by Tom Barr, Feb 27, 2015.

  1. Tom Barr

    Tom Barr Founder
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    Note, the gas build up is not N2, nor O2. It is CO2. N2 and O2 are very tough to dissolve.


    CO2 gas builds up, not the others.


    Why? CO2 dissolves easily right?


    Does the CO2 dissolve FASTER when you have aquarium water with say 2 ppm of CO2?


    Or say 40 ppm of CO2?


    We assume that the CO2 dissolving rate is the SAME. But is this correct?


    What types of test can we do, as aquarist, to see and get a good idea?


    1. You can add O2 gas and see how long it takes for a chamber full to dissolve, it takes a long time(measure this, say 10 minutes).


    But you need O2 gas, but that's available from gas places for torches etc.


    2. You can also run a CO2 reactor on a small powerhead loop inside the sump. While doing a water change, the tank water STOPS flowing into the sump water. the sump water is a relatively small volume, but has a high rate of CO2 gas being added, not the entire tank. So the reactor quickly fills up. But wait....I thought CO2 dissolves really fast?


    No, not as NEAR as fast when the water is say 1-2 ppm of CO2.


    When you start the tank pump back up, the gas bubble disappears in a less than 1 minute(say 45 seconds) with a tiny bubble head at the top. You can add CO2 to the chamber and fill it with pure CO2 and repeat this process. You get the same time for dissolving.


    You can also stop the CO2 late in the day when gas bubble forms(since we assume we are not sure what the gas actually is).


    Then do a large water change(removed the CO2 gas from the main tank) and the gas bubble quickly dissolves in 45 seconds again.


    There are no other gases that dissolve this quickly in the aquarium. If it was a hard to dissolve gas like Ar, N2, O2, the bubble would remain, but it does not.


    Try it and see.


    I'm trying to think of a problem with this deduction. But I do not know of any other potential gases that would dissolve so rapidly once the level of CO2 and O2 ppm are dropped suddenly.


    This means as concentration of CO2 builds up, the ability for the system to dissolve the gas into solution changes. This makes common sense also. It's harder to add something when there's already a bunch of it in there.


    The other issue is that as the concentration increases in the tank, the degassing RATE also increases.


    Fick's 1st law predicts this.


    This is a decent reference for some of what is going on.


    However, few models include degassing rates. Which is what we have going on also. A sump has a much small area for degassing and much less water to add gas to.


    Tokumura, M., Baba, M., Kawase, Y., Dynamic modeling and simulation of absorption of carbon dioxide into sea water. Chemical Engineering Science, 62, pp. 7305-7311 (2007).


    [​IMG]


    What do these graphs suggest at high CO2 dosign rates?


    Now if you increase the volume of water suddenly(say goign from the just thr sump water, to the entire tank and sump volume of water), does the rate remain the same?


    No! It does not.


    Note, this does not include the degassing rate increase either.


    As we add more CO2 into the aquarium over **time***, the reactor has more and more trouble dissolving the CO2 since the concentration of CO2 increases.


    So simply testing which gas stays in the gas phase(O2, N2 etc) and which dissolves rapidly, we have our gas of interest................ pretty well identified.


    The graph above also looks a lot like this graph:


    [​IMG]


    If this gas was O2, we should see higher O2 ppm than if the gas was CO2 alone.


    But I have an O2 meter. The O2 does not change.


    My next test is adding a probe holder into the CO2 reactor top. That should pretty much resolve any questions.


    I have a pH probe and then an O2 LDO. So I can assume O2 to have minimal pH effect, CO2, rapid. O2, well, if it's all O2, then.......that should be measured as well.


    I can get small bottles of N2 and O2 at the CO2 gas supply place also. This has been a nagging question for me for some time. While I'm convinced, I want to be more certain.
     
  2. Christophe

    Christophe Subscriber
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    I run a 4"x20" Rex Grigg-style reactor, fed by a Sicce pump at ~200 GPH. The pump runs 24/7, the CO2 solenoid is on 2 hrs before photoperiod, off 90 min before the end.


    This system also builds up quite a bit of residual gas over the course of the day. It doesn't seem to cause a problem other than being slightly noisy. I often invert the reactor after the CO2 has shut off for the day to clear the buildup. If I let it go for 4-5 days, the reactor is more than half full of residual gas, even the next day.


    Here's what I find though...


    When the CO2 solenoid shuts off for the day, my pH meter pretty quickly climbs back up from its low point, whether or not I have emptied the residual gas from the reactor. By 6AM the next morning, pH meter indicates nearly complete desaturation of CO2 -- even if I've left big residual bubble rumbling away in the reactor. My pH curve is about the same, whether or not I left the gas bubble in the reactor,
     
    #2 Christophe, Feb 27, 2015
    Last edited by a moderator: Feb 27, 2015
  3. Tom Barr

    Tom Barr Founder
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    Have you tried doing a water change late in the day when the gas bubble is large........and then watch the gas bubble after you turn things back on?
     
  4. Christophe

    Christophe Subscriber
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    Generally as part of a water change/maintenance, I clear the reactor, so I haven't observed that particular sequence. I'll make a point of it next time I do a mid-week evening change.


    For me, the telling thing is that my reactor still has a good-sized gas bubble in the morning, even though my pH has come back up to fully desaturated level. If it were CO2, it would be gone in 30 minutes.


    I tend to think it's N2 -- Here's why:

    • Nitrogen concentration in the reactor during CO2 admin is lower than ambient air, i.e., the percentage of N2 in the gases in the reactor is lower than in ambient air due to injected CO2; admittedly, partial pressure of N2 is the same, so I dunno...
    • Agitation of inflowing water/CO2 mix in the reactor brings some of the (admittedly small) amount of dissolved N2 out of solution from the water under these conditions
    • CO2 in the mix in this residual bubble is quick to dissolve into the tank water for distribution once the solenoid shuts off, evident by the pH rising again, reaching just about full desat value by the time CO2 turns on again the next day even though the residual bubble is still there and still quite large sometimes, if I haven't emptied it in a few days.
    • Not much difference at all in pH level at 6AM, whether I emptied the residual gas or not the previous night after CO2 shutoff.


    I've been way curious about the gas content since I've set up this system, I'm psyched to hear you're applying some more equipment to checking it out!
     
  5. oliverpool

    oliverpool Lifetime Charter Member
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    this is interesting. Other then the water change method to see if the gas does dissolve quickly like in mins and the PH drops rapidly again, I always thought it was CO2 as well. I mean, thats the only additional thing we are putting into the system via the regulator right? Of course the co2 tank is not 100% co2, but its close.


    In my reactor, the bubble more or less is gone in around 2-3 hours after the regulator switches off and my highest PH is around another 1-2 hours after that which was also the reason i thought it was co2.
     
  6. Dennis Singh

    Dennis Singh SynKing!

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    When I pump co2 into my reactor it does seem to be dissolved very fast, at the end of my photoperiod, there is build up. But I blast so much no matter what the reactor never seems to build up until time goes on and on and then you see a build up...What does this explain, i do not know.
     
  7. Donald Cutler

    Donald Cutler Junior Poster

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    I have 2 Rex Grigg and 1 Cerges. The Rex Grigg reactors are fed about 5 BPS and gurgle all day...The Cerges does not. Within an hour of the CO2 being shut off the gurgling stops in the Grigg style. Weird. If I build another reactor, I think I am going with a Cerges.
     
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