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Some thoughts and questions about CO2

Discussion in 'CO2 Enrichment' started by VaughnH, Dec 2, 2008.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    My mind does crazy things when I am laying in bed waiting for sleep. Here are a couple of thought sequences I ran through a few nights ago:

    1. CO2 is made up largely of oxygen, 73% oxygen, 27% carbon.
    2. When we see pearling, we are seeing about how much CO2 is being consumed by the plants. The bubbles are O2, with a volume approximately the same as that of the CO2 being consumed by the plants to generate the oxygen.
    3. When a bubble rate is set, with the lights on, which gives us about 30 ppm of dissolved CO2 in the water, as much as a third of that CO2 is being consumed by the plants, and the remainder is lost from the water surface.
    4. If we turn off the lights that is about the same as increasing the bubble rate by as much as a third, because the plants are no longer consuming the CO2.
    5. That is a strong argument against leaving the bubble rate the same 24 hours a day - if it is high enough to give us a maximum of CO2 during the day, it becomes too much for the fish at night.

    A. In natural bodies of water, not including springs with high CO2 content in the water, plants get some/much CO2 from the “substrate”, which must be bubbling out under the plant leaves.
    B. Silicone tubing has at least 10 times the permeability for CO2 as other plastic tubing, but our CO2 setups never have more than about 2 psi pressure downstream of the needle valve, so the permeability is not a problem.
    C. If we increase the pressure in the tubing to 40 psi, the loss of CO2 could be measurable.
    D. What if we install a ten foot long loop of silicone tubing in the substrate, with one end sealed and the other end attached to the CO2 line from the needle valve/bubble counter? Once we bleed the air out, would we see a bubble rate on the bubble counter when we increase the output pressure from the regulator to 40 psi? Would that bubble rate be comparable to what we use for our aquariums? Would that CO2 be optimally placed for growing plants in the aquarium?
    E. Assuming our usual bubble rate is 5 bbs or less, using a silicone tubing "diffuser" would not likely give visible bubbles of CO2 in the water. The typical bubble counter has bubbles about 1/8 inch in diameter or a bit smaller. That same flow of CO2, spread over ten feet of silicone tubing, with bubbles of CO2 coming out every tenth of an inch of tubing, would give about 1000 bubbles per second of about 1/30 the diameter, or .005 inch in diameter. But, there is no reason to expect that there wouldn't be a lot more bubbles than that, reducing their size below what can be seen.
    F. Who wants to do some testing?
     
  2. VaughnH

    VaughnH Lifetime Charter Member
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    More about permeability:
    From Wiley InterScience :: Session Cookies, "Gas permeability constants of polymers are generally expressed by composite units which contain (volume of gas per unit time) times (thickness of the material) divided by [(area of material through which the gas goes) times (the pressure differential across the material)]. So, volume flow per unit time equals the permeability constant times pressure differential times area divided by thickness. For silicon tubing the permeability constant is about 20,000 times 10 to the minus 5 power, with all units in cm, seconds, and atmospheres. (Cole-Parmer Technical Library)

    For silicone air tubing, the size we generally use, this equation gives about 1.5 bubbles per second of 1/8 inch diameter bubbles for 3 atmosphere pressure differential and 10 feet of tubing. With 25 feet of tubing ($10 worth), it gives about 4 bubbles per second.

    Based on that, the basic idea of using a long length of silicon tubing, with much higher than normal pressure CO2 in it, will work as a CO2 diffuser. The higher pressure also makes it much less likely that biofilm will plug up the pores.
     
  3. Tom Barr

    Tom Barr Founder
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    And plants are about 35-45% carbon.

    Well, there is a strong correlation between them, but the O2 is from water being split, not from the O2 from CO2, this is just the first part of growth, the light reaction for "tools" to fix CO2 into carbon sugars in the next stage(dark reactions). Growth and O2 are strongly linked still.

    So light starts the ball rolling, then CO2 comes into things, then later, the nutrients...................

    Just like modeling how to reduce growth, you reduce the light, you reduce the CO2 demand and the O2 production................and thus reduce nutrient demand as well.

    If that much..........most is lost.........that's the whole delivery of the dosed product to the plant idea. But waste is relative...........is it waste if our goal is really to provide non limiting CO2?

    Can we improve the delivery and thus use less CO2?
    But simply using less light will do that also............

    Okay, I buy this basically.........

    So does the algae films on their surface...........some comes in from above, but it, like the source from the sediments....is limited in the rate of production............too much CO2, and you must have plenty of O2, otherwise no fish/dead fish, low O2.

    That's the trade off using organic carbon reduced sources for CO2 additions.
    Folks tend to not address that part, but it's important.

    It will work but the rate is too slow.
    I used RFUG filters, CPVC and bubbled CO2, it works pretty well, but do you want that much gas pockets burping up through the sediment pulling up muck?

    At high rates, eg RFUG, this works fine and no OM builds up, at no flow rates, the bubbles are less frequent, not much or an issue, most of CO2 gets dissolved.
    With lots of plant roots(O2 source) and active bacteria with reduced carbon, you have a lot of CO2 via bacteria respiration.

    I think this limit is fine.

    However, using a spray bar along the back of the tank and shooting mist will do the same thing. Better than this and at a much higher easier to control rate.
    Likewise, the CPVC grid will as well.


    Regards,
    'Tom Barr
     
  4. VaughnH

    VaughnH Lifetime Charter Member
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    That is probably the reason this idea isn't workable - I neglected to consider that the very small bubbles will linger in the substrate, combining with other very small bubbles, to make very big bubbles and a massive burp! If the silicone tubing isn't under the substrate, but laying on top of it, it would work, but look like (@#%$^&) But, I will be back in bed again tonight, and who knows, I may think of a good solution to that.

    The slow rate of diffusion isn't a problem, since it is easy to solve by using more tubing or higher pressure.
     
  5. Tom Barr

    Tom Barr Founder
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    Well, this gets to that membrane method of CO2 diffusion really that AM sells, not particularly useful since it cannot handle a high rate of CO2 influx into water.............and given the loss rates to the surface of your tank, this stuff and bacteria etc offer little respite,

    You might be interested to note that many folks used tight fitting lids to hold the CO2 in in the past.

    However, this means less O2 as well:cool:
    So there's always a bad trade off there somewhere.
    But..if you have no fish, like many of the Dutch tanks, may as well include the painful low stocking levels in the ADA tanks, this might be okay for you.

    Regards,
    Tom Barr
     
  6. VaughnH

    VaughnH Lifetime Charter Member
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    One more problem - releasing CO2 throughout the substrate would lower the pH in the substrate, possibly too much. How about black silicone tubing that looks like black Flourite, on top of the substrate? The idea isn't dead yet, just terminal.
     
  7. Tom Barr

    Tom Barr Founder
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    I think you might find better quanatification if you try sealing the lid of your tank(without fish etc).

    This would give a decent estimation of rates of loss for CO2.
    No issues with pH however, all sediment has buffering and some O2 is still there......
    Sediments are pretty tough when it comes to adding CO2.

    Try using the Reverse flow UG filter idea. Faster rates and easier to use/control etc.



    Regards,
    Tom Barr
     
  8. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    I just noticed this thread and I had to comment. I have been thinking about pretty much the same thing as what you mentioned ever since the thread about the permeability of silicone tubing where we did some calcs. I have been thinking about it from a different angle though. I was more intrigued by trying to design a way to diffuse CO2 more evenly through a large volume of water without needing huge amounts of circulation to do the mixing for me.

    I was thinking of using the silicone tubing and making it into a large 3-D maze or grid inside of a sump filter, and then letting the water in the sump slowly flow through the maze (like air through a porous air filter) and the CO2 would become very evenly dissolved in a large volume of the sump water without needing to be stirred up much. Then you could just pump out the water from the other side of the sump and split that return flow to several different jets within the tank and get a nice even concentration of CO2 infused water (in high volume, with low pressure and low water velocity for a more even and gentle current for the plants) coming out of all of the jets in the tank.

    I was thinking that the sump could be sealed on top as well to trap any CO2 that bubbled to the surface before dissolving if it was deemed necessary.

    You could easily put 100+ feet of silicone tubing into a reasonable sized sump. Then you could run the CO2 at a lower pressure since you would have a lot more tubing to diffuse through. That way all you would need is the silicone tubing fed off of a bubble counter coming off of the CO2 pressure regulator and that's it. You just adjust the bubble rate by adjusting the output pressure from the CO2 tank using the main regulator, the same thing you were saying.

    A few things that I didn't know how to quantify in advance without actually trying this were:

    A- The bio-film that would grow on the silicone tubing in the sump. Would it have any effect at all or would the pressure of the CO2 coming through the tubing be enough to make the bio-film a non-factor as long as the tubing was occasionally cleaned off? You might just need to use a higher pressure than the calcs would indicate for the bubble rate you want and that's it, but how much higher?

    B- The permeability is directly related to the thickness of the silicone tubing, so can the pressure be raised enough to give a good bubble rate without the tubing rupturing, expanding unevenly due to slight variations in tubing wall thickness over the length of the tubing, etc.?

    Just a side note for anyone that is interested- The pressure factor of the diffusion rate would be based not only on the pressure inside the tubing, but on the difference between the pressure in the tubing and the water pressure outside the tubing. If you have a 2' deep tank with the silicone tubing at the bottom, the water itself would be canceling out almost 1psi worth of diffusive pressure, so the tubing pressure would need to be about 1psi higher than you would expect for a certain CO2 bubble rate. The direct calcs for diffusion that were done in the aforementioned thread on tubing permeability only work in air.

    That also means that if you have a tubing grid that has CO2 tubing placed at varying water depths (for instance in a sump grid of some sort) the diffusion rate will be slightly higher in the tubing that is closer to the water surface even though it is all the same pressure within the tubing, because there will be less water pressure counteracting the force of diffusion closer to the water surface. That extra CO2 output at the surface might very well counteract some of the losses from the CO2 escaping into the air at the water surface in something like a sump.

    Have a good one, Jeremy
     
  9. VaughnH

    VaughnH Lifetime Charter Member
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    Isn't it funny how seeing an obviously significant difference in materials immediately leaves you trying to figure out a way to take advantage of that difference? I have usually been joking when I have said that if silicone tubing loses so much CO2 it must be a good diffuser for CO2, knowing that at the normal 1-2 psi pressure in that tubing very little CO2 will diffuse out. Then I started wondering why not raise the pressure - because it would pop the bubble counter (no, a carbonated beverage bottle withstands over a 100 psi.) and it would explode the tubing (possible I guess, but I suspect the tubing burst pressure is way above 30-40 psi) and you couldn't control the flow (no, just use the needle valve as usual.). Then it occurred to me that I could easily get a whole 25 foot roll of silicone air tubing laid flat on the tank bottom. I still haven't given up the idea.

    Tomorrow I may make a bubble counter, buy a roll of tubing, and do some testing.
     
  10. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    Your idea for trying this has me intrigued. I have been thinking about setting up a small RUGF tank in order to be able to have more control over the nutrients (by only having water column nutrients) so I can observe changes in the tank balance and plants with different nutrient levels, etc. more accurately in hopes of being able to better read plants and the overall tank needs using only my eyes. I might try something along the lines of yours, my, or a combination of the two proposed CO2 diffusion methods in that tank (at a separate time from the plant nutrient testing of course, haha) if I ever get around to it.

    The following are just some questions that I would be looking for answers to if I tried this experiment myself using your proposed method of diffusion and/or they are things I would be keeping a close eye on to see if anything drastically changed within the tank-

    1- It would be a much larger area for diffusion having the whole bottom of the tank as your CO2 source, so wouldn't it naturally make the CO2 levels in the tank more consistent throughout?

    2- The CO2 coming up from the gravel would hit the bottom of the plant leaves where the stomata are that absorb the CO2, possibly making uptake more efficient, or at least the CO2 more easily obtained for the plants?

    (Side note- Tom told me a while ago as an answer to one of my other questions, that not all plants even have stomata, so if the previous statement is in error I apologize. I have not spent the time as of yet to look into that further in order to better understand that part of plant growth and fully understand Tom's comments.)

    3- If bubbles do actually form and rise to the surface instead of the CO2 directly dissolving into the water, they will catch and stay on the bottoms of many of the plant leaves if the water currents allow that to happen if water currents allow it.

    If the bubbles stick on the bottoms of the leaves, does that mean you could possibly get the benefits in growth that many people have now been saying occur when venturi's/ mazzei's are used for making CO2 mist without the fine tuning, etc. that mazzei's seem to require in many instances?

    4- Could water circulation be reduced some without hindering plant growth or health just due to better CO2 penetration through the plant beds?

    5- I could see this helping densely planted "carpet" type plants? They would be getting a nice and even flow of CO2 rising from the substrate instead of only the tips of all the leaves grabbing it from the water moving horizontally above them?

    I always think of water as if it was air, and if you think of a box fan circulating air within a room in your house, even with high levels of airflow and circulation throughout the room there is still very little air flow in the nooks and corners of the room due to tiny eddy currents, localized minor low/high pressure zones, etc. A bedroom would have much better total air circulation in all the more boxed or hard to get to areas if fresh air just slowly rose up evenly from the entire floor and then vented out the ceiling. That's pretty much what you would be doing with CO2 if it was done in a RUGF type of a setup with some water also flowing upwards through the substrate.

    6- If the bubbles of CO2 that were rising knocked off some of the pearling O2 bubbles (before they would have left the plant leaf on their own) wouldn't that help the plant reduce the negative affects of photorespiration when light levels are high and thus allow the plant to grow faster under high light?

    I just thought I would write down my thoughts and throw it all out there and see where it leads, haha.

    Have a good one, Jeremy
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    When I was first wondering about this, my first thought was a perforated plastic plate, like an UGF, under the substrate with CO2 introduced there. Then I realized that this would be like a ceramic disc diffuser, where most of the CO2 comes from just a few pores, so I gave up that idea, and thought of a tank bottom plate made of a material like ceramic disc diffusers. That got me back to the problem of only a few pores emitting CO2. So, my mind wandered to a membrane made of silicone tubing material, but that would be a balloon under the substrate - very unpleasant. Only then did the idea of using silicone tubing itself ressurect itself.

    The advantages you mention are the ones I thought about too.

    But, would a 25 foot loop of silicone tubing under at least some of the substrate layers quickly become messed up by biofilm, or would plant rootlets grow into the tubing or around the tubing, or would the tubing deteriorate with time, etc.? It would be somewhat easier to remove and clean a little ceramic disc than to remove and clean 25 feet of silicon tubing under the substrate - somewhat.
     
  12. Tom Barr

    Tom Barr Founder
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    I think the real issue is controlling the rate of CO2 inflow.

    Very hard to do here. And also, the rates of CO@ transfer are pitifully slow.
    And with more than a few psi..............leaks, and other mechanical issues.

    You also might consider micro bubble tubing and directing flow along the bottom surface of the sediment, the bubbles will rise and hit the bottom of the leaves.

    Regards,
    Tom Barr
     
  13. VaughnH

    VaughnH Lifetime Charter Member
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    The needle valve will still work well to control the rate of CO2 flow, as long as the downstream conditions remain constant, meaning there are no ruptures in the tube or temporary opening of the pores, etc. It will be like having a ceramic disc that needs 20 psi to force CO2 through it instead of 2 psi. The whole system downstream of the needle valve will just be at a higher pressure, but the rate of flow will still be determined by the "orifice" geometry in the needle valve, and the outlet pressure of the regulator.

    I haven't yet found a pressure limit for silicon tubing, so that could be a problem. My gut feeling is that the tubing will work fine with 20-40 psi, unless it tends to creep under that load.

    It is the pitiful slow diffusion of CO2 through the tubing that would make this work. With 25 feet of tubing, the area that the CO2 diffuses across is large enough for that minute flow to add up to a significant amount.

    I'm not interested in a diffuser that I have to remove weekly and clean, so the micro bubble tubing wouldn't work for me. My hope is that the high pressure and huge (relatively) area for diffusing will eliminate the need for routine maintenance for a long time.

    The odds against this working well are pretty large, but not so large that I don't want to do some more studying and some testing in a container of water.
     
  14. Tom Barr

    Tom Barr Founder
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    Odds, risk, likeyhood, all things you might consider before wasting any time or $ on something.

    Sometimes such things are cheap and do not take long, so the risk is low or we note some observations that suggest something other than an expectation etc.
    I go for the low picked fruit first, then risk more after those are all gone.

    As I have less issues over time with my CO2 methods as they are, having played long enough and done various ideas, I do not think this is going to be that fruitful.
    Removal is also not pretty for this method either.

    Recall those gas permable membranes for pH/KH ref solution?
    You might look at those etc in tube form.
    You also cannot clean it down there either.

    These are the signs of doom for any decent viable method.

    Regards,
    Tom Barr
     
  15. VaughnH

    VaughnH Lifetime Charter Member
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    Today I tested some silicon tubing. It is Top Fin, 25 foot silicone air tubing, the cheapest one I could find. First I used an empty club soda bottle to make a bubble counter that will withstand 40 psi. Then, I set this up to see if I could diffuse CO2 into a container of water as shown below:

    [​IMG]

    [​IMG]

    I left the end of the tubing open and turned on the CO2 for several seconds to purge most of the air out of the tubing. It took folding the tubing over, tieing it with wire, and clamping the fold with a bag clamp to stop CO2 from leaking out the end. Then I measured the pH of the water in the container.

    With the needle valve open more than usual, I set the regulator to about 5 psi. I got a heavy stream of bubbles in the bubble counter as the pressure built up in the tubing, but the stream dropped off to about 20 bubbles per minute in 5 minutes. A leak check showed a leak at the check valve I used as a tubing connector, so I replaced that with a piece of rigid tube, stopping the leak. Opening the needle valve further didn't increase the bubble rate.

    After 20 minutes I increased the pressure to about 10 psi, and the bubble rate went way up, but only for about a minute. It settled down to about 30 bubbles per minute, but a very small leak began at the bubble counter where one of the tubes was pulled through the lid.

    I raised the pressure to 15 psi, again getting a temporary increase in bubble rate as the tubing came up to pressure. In 2 minutes the bubble rate was back to 30 bubbles per minute, and the leak at the bubble counter remained the same.

    Next was 20 psi, with the same results, except that the final bubble rate was only 20 bubbles per minute.

    At 25 psi, the same result, but the bubble rate was settled at 12 bubbles per minute in only 2 minutes.

    At 30 psi, the same result. The bubble counter leak remained the same. All of this took about 45 minutes. A check of the pH of the water in the container gave the same color as the original one. The container held about a gallon of water.

    So, I called in a rocket scientist to.......nope....I evaluated my results and concluded that:
    1. Silicon tubing does not leak CO2 even at 10X the normal pressure at which we use that tubing, running from the needle valve to the aquarium.
    2. You can't use this tubing as a CO2 diffuser unless you start out with at least 100 feet of it, and probably not even then. The higher pressure needed to have any chance at all of getting CO2 through the tubing in usable amounts leads to big leak problems.
    3. The CO2 filled tubing floats, which shouldn't be a surprise. I had to weight it down to keep it under water.
    4. I suspect many of our DIY devices, where we drill an undersized hole and pull a tube through the hole, after cutting the end at a sharp angle, using needlenose pliers, leak at that connection. But, the leak is very small, generating a very fine bubble foam with soap solution, that takes minutes to build up to be visible.
     
  16. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    I have a few questions for you related to your results. You talked about bubble rates and line pressure changes and at the same time you were also talking about leaks, etc. and maybe I just got confused. Are you saying that with your setup the bubble rate through the tubing was actually going down as you increased the pressure, or was it just the leaks causing the higher bubble rates with the lower pressures?

    One thing that I forgot to mention that I was pondering related to possibly trying something like this myself was that there are always some other gases mixed in with the CO2 (in varying trace amounts) and there is not really any way to get around that.

    I was thinking that since the permeability of silicone tubing changes depending on what gas you are trying to permeate through it, that maybe the CO2 would work fine at the beginning and then slowly dwindle to nothing, because the small amounts (of something like oxygen for instance) mixed in with the CO2 wouldn't permeate through the silicone very well at all. That would cause the trace gases that are not very permeable to build up in the tubing over time and eventually block the CO2 from being able to diffuse through the tubing at all.

    If you were finding that (once leaks were factored out of the results) the bubble rates were going down as you increased the line pressure it would be interesting to try the experiment again starting first with the higher pressures and then working your way down to the lower pressures to see if you get the same results as your first data set.

    I would think the bubble rate would increase (even faster than the permeability equation would predict if all you change is the pressure when doing the calcs) as line pressure increases not only from the increase in pressure, but from the tubing swelling a little bit and that swelling in turn causing the surface area to increase to some small degree, and also the tubing wall thickness decreasing a little bit as well for the same reasons.

    Very interesting stuff, thanks for taking the time to try this out and test this stuff Vaughn.

    Have a good one, Jeremy
     
  17. VaughnH

    VaughnH Lifetime Charter Member
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    The bubble rates, which are the rate at which CO2 is entering the silicone tubing, went down as the pressure went up. That is a strange situation. It is complicated a bit because the flow rate through the needle valve is proportional to the pressure drop across that valve, so as the silicone tube is filled with CO2 the pressure drop decreases, and the flow rate decreases. So, I had the needle valve open much too far to be the main restriction in the flow, but as the pressure drop decreased it is possible that the needle valve began to be the primary restrictor.

    I tried to wait until the bubble rate stopped slowing down, and was consistent before going to the next pressure. But, measuring one bubble per 5 seconds is never going to be an accurate measurement. The only leak I had appeared to be that at the bubble counter, and it was extremely small, so I doubt that the flow was all due to that leakage.

    I stopped testing when it was apparent that I could never get as much flow going through the tubing walls as I needed. It is a bit scary to be working with your face close to confined pressurized gas, even an the low pressure of 30 psi.

    One possible problem with my test: I did try to purge out the air in the silicone tubing, but I can't be sure I got it all - highly doubtful. As I understand it, if the tubing started full of air, once I added enough CO2 to get 15 psi, I had a 15 psia partial pressure of CO2, not enough for a pressure differential to force any through the silicone. But, at 30 psi, I should have had 30 psia partial pressure of CO2 (45 psia total pressure), and that should have been enough to get significant flow out through the silicone. But, it didn't do so.

    I don't think the contaminants in CO2 are significant. Unless they are gases that liquify at 800 psi or so, they will remain gases, and will largely be removed with the first usage of the CO2 - being constantly diluted as CO2 is used.

    I'm too old to go through this test again!:eek:
     
  18. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    I don't know if I agree with that but I don't have any proof one way or the other right now either, haha. I would think that contaminants could just dissolve in the liquid CO2 just like how Oxygen, Nitrogen, and carbon Dioxide (for instance) easily and readily dissolve into liquid water. I might be wrong on that though.

    I realize that you are pretty much done looking into this now, but I just thought I would mention the following in case anyone else is like me and still interested in possibly pursuing some DIY projects in this area of largely unexplored aquarium technology, haha.

    I was watching a show on scuba diving and I think I might now know what was going on related to your CO2 bubble counter results. I was watching the bubbles that some scuba divers were exhaling increase in volume as they rose to the water surface and it dawned on me that pressure would be affecting your bubble counter in your test setup as well, and that the pressure change (and its' effect on the CO2 bubbles) wasn't being accounted for in just the visual counting of bubbles.

    The bubble counter itself is designed to keep the size (volume) of the bubbles consistent no matter what the pressure is, not for keeping the mass of the bubbles consistent, and I think that is the key. As you increase the CO2 line pressure the "Ideal Gas Law" would cause there to be more actual mass of CO2 in each CO2 bubble even though the bubble remains the same volume, just like there would be more air in an air compressor tank at 30psi than there would be in that same tank at 0psi. A bubble counter would no longer work taken just at face value when line pressure was added to the system, the visible bubble rate would have to first be compensated for according to the "Ideal Gas Law". That's what was missing, and that could be why your bubble rates were going down as pressure increased. In actuality you were still diffusing more CO2 through the silicone tubing at higher pressures than you were at lower pressures, it just didn't look like it.

    I did a rough calc using the "Ideal Gas Law" and also checked my work quickly with an online calculator using a constant temperature, constant volume, the mass for carbon dioxide per mole, 0psi as the initial pressure, and 30psi as the highest pressure in the CO2 line. I found that at 30psi each CO2 bubble going through the bubble counter (since the bubble counter keeps the bubbles at a constant volume) would actually carry within it the same mass of actual CO2 as about exactly 3 CO2 bubbles (each of the same volume) at 0psi.

    That means that at 30psi you would have to multiply the visible bubble rate by 3 before you could make a direct comparison with the bubble rates at 0psi pressure. The difference between volumes at 5psi and 30psi was around 2.3x as well (that was the only other pressure comparison I did calcs for), so all the conversion factors would need to be factored in to truly compare bubble rates. Whether or not that correction factor is enough to make this technique for diffusion possibly beneficial again still remains to be seen. I just thought I would throw it out there for anyone interested.

    Have a good one, Jeremy
     
  19. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    I realize that you are pretty much done looking into this now, but I just thought I would mention the following in case anyone else is like me and still interested in possibly pursuing some DIY projects in this area of largely unexplored aquarium technology, haha.

    I was watching a show on scuba diving and I think I might now know what was going on related to your CO2 bubble counter results. I was watching the bubbles that some scuba divers were exhaling increase in volume as they rose to the water surface and it dawned on me that pressure would be affecting your bubble counter in your test setup as well, and that the pressure change (and its' effect on the CO2 bubbles) wasn't being accounted for by just visually counting the bubble rate.

    The bubble counter itself is designed to keep the size (volume) of the bubbles consistent no matter what the line pressure is, not for keeping the mass of the bubbles consistent, and I think that is the key. As you increase the CO2 line pressure the "Ideal Gas Law" would cause there to be more actual mass of CO2 in each CO2 bubble even though the bubble remains the same volume, just like there would be more air mass in a workshop air compressor tank at 30psi than there would be in that same tank at 0psi. A bubble counter would no longer be accurate taken just at face value when line pressure was added to the system, the visible bubble rate would have to first be compensated for according to the "Ideal Gas Law". That's what was missing, and that could be why your bubble rates were going down as pressure increased. In actuality you were still diffusing more CO2 through the silicone tubing at higher pressures than you were at lower pressures, it just didn't look like it.

    I did a rough calc using the "Ideal Gas Law" and also checked my work quickly with an online calculator using a constant temperature, constant volume, the mass for carbon dioxide of 44.0 grams per mole, 0psi as the initial pressure, and 30psi as the highest pressure in the CO2 line. I found that at 30psi each CO2 bubble going through the bubble counter (since the bubble counter keeps the bubbles at a constant volume) would actually carry within it the same mass of actual CO2 as about exactly 3 CO2 bubbles (each of the same volume) at 0psi.

    That means that at 30psi you would have to multiply the visible bubble rate by 3 before you could make a direct comparison with the bubble rates at 0psi pressure. The difference between 5psi and 30psi was around 2.3x as well (that was the only other pressure comparison I did calcs for), so all the conversion factors would need to be factored in to truly compare bubble rates. Whether or not that correction factor is enough to make this technique for diffusion possibly beneficial again still remains to be seen. I just thought I would throw it out there for anyone interested.

    Have a good one, Jeremy

    P.S.- For the greatest accuracy in real situations the "Non-Ideal Gas Law" should be used for this instead of the "Ideal Gas Law" which I used. The "Non-Ideal Gas Law" adds in an extra "compressibility factor" to take into account the individual and unique properties of the specific gas being calculated for. It would change the result a little bit, but I couldn't find a compressibility factor chart for CO2 under different pressures in order to do that more accurate calculation.
     
  20. VaughnH

    VaughnH Lifetime Charter Member
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    You are right! PV=RT, true for all gases which are compressible. So, P1V1=P2V2, and V2=V1P1/P2. That does explain a lot about why the "bubble rate" decreased with increasing pressure. Since my data isn't accurate, this doesn't eliminate all of the decrease in bubble rate, but I'm sure that what I saw was, as you said, the effect of pressure increasing the mass of CO2 in each bubble.

    I still got far too little CO2 flow through the tubing walls to make it interesting for me, but at least this clears up why the bubble rate went down.
     
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