skimmer gas exchange myth?
- Thread starter jmaneyapanda
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Those are some good numbers, platypus.
However, there's more to the puzzle than just the amount of surface area. Surface area is inconsequential compared to the volume of gas (and thus O2) available in each bubble compared to an open source (such as the top of a tank). And we also can't calculate partial pressure of O2 in the skimmer bubble vs. the ppO2 within the skimmer water, at least not directly or quantitatively while it's running. If the O2 in the water surrounding the bubble has a higher or equal partial pressure than the ppO2 of the bubbles, it isn't going to absorb anything; in fact, it'll go the other way.
To state another way - it's not enough that there's a bzillion little bubbles in the skimmer body. A skimmer is designed to maximize surface area for the absorption of bipolar organic molecules, specifically so that the hydrophillic portion is attracted to air/surface barrier. However, this is not necessarily optimal for O2 absorption, nor does it mean that O2 absorption doesn't happen. What we're hoping for is the O2 from within the skimmer bubbles to transfer into the surrounding water.
Using platypus's numbers of 0.5mm bubble size (probably too big, but just makes my point), that's 4/3pi(0.25^2) = 0.261 cu mm = 0.000261mL of air in each bubble. Using the same theoretical numbers for skimmer performance, that means that the water in the skimmer was exposed to 88.9 liters of air over one hour. A standard size fan blowing over a sump measures about 15 cu ft / min (I've posted this long ago on these forums when I borrowed an anenometer.), then that's 424 liters of air each minute over the surface of the tank, compared to 88.9/60 = 1.48 liters per minute.
So, by doing the math based on volume of air exposed, there's 286 times as much air being exposed to the surface by that fan than by the skimmer in the same amount of time. Granted, there's inefficiencies in the fan over the water, but my point is still this - numbers aren't everything.
The only true way to test this would be to use a dissolved O2 meter and check the effluent of the skimmer vs. a tank with only circulation. Or, better yet, the same system without external factors (livestock), with and without the skimmer for a 24+ hours.
Jeremy isn't questioning that you may have read something. Heck, I might have read something. What he wants to see (and I would too, for that matter), is a scientific study, paper, or other substantial research that provides fact about skimmer gas exchange. Same as any other research paper, he wants to reference the actual study; He was very clear about this in his first post.
In God we trust. Everyone else must bring data...
However, there's more to the puzzle than just the amount of surface area. Surface area is inconsequential compared to the volume of gas (and thus O2) available in each bubble compared to an open source (such as the top of a tank). And we also can't calculate partial pressure of O2 in the skimmer bubble vs. the ppO2 within the skimmer water, at least not directly or quantitatively while it's running. If the O2 in the water surrounding the bubble has a higher or equal partial pressure than the ppO2 of the bubbles, it isn't going to absorb anything; in fact, it'll go the other way.
To state another way - it's not enough that there's a bzillion little bubbles in the skimmer body. A skimmer is designed to maximize surface area for the absorption of bipolar organic molecules, specifically so that the hydrophillic portion is attracted to air/surface barrier. However, this is not necessarily optimal for O2 absorption, nor does it mean that O2 absorption doesn't happen. What we're hoping for is the O2 from within the skimmer bubbles to transfer into the surrounding water.
Using platypus's numbers of 0.5mm bubble size (probably too big, but just makes my point), that's 4/3pi(0.25^2) = 0.261 cu mm = 0.000261mL of air in each bubble. Using the same theoretical numbers for skimmer performance, that means that the water in the skimmer was exposed to 88.9 liters of air over one hour. A standard size fan blowing over a sump measures about 15 cu ft / min (I've posted this long ago on these forums when I borrowed an anenometer.), then that's 424 liters of air each minute over the surface of the tank, compared to 88.9/60 = 1.48 liters per minute.
So, by doing the math based on volume of air exposed, there's 286 times as much air being exposed to the surface by that fan than by the skimmer in the same amount of time. Granted, there's inefficiencies in the fan over the water, but my point is still this - numbers aren't everything.
The only true way to test this would be to use a dissolved O2 meter and check the effluent of the skimmer vs. a tank with only circulation. Or, better yet, the same system without external factors (livestock), with and without the skimmer for a 24+ hours.
Jeremy isn't questioning that you may have read something. Heck, I might have read something. What he wants to see (and I would too, for that matter), is a scientific study, paper, or other substantial research that provides fact about skimmer gas exchange. Same as any other research paper, he wants to reference the actual study; He was very clear about this in his first post.
In God we trust. Everyone else must bring data...
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Do you have a meter or test kits? It sounds to me as if Panda won't accept anything short of a peer reviewed paper on this. Feel free to set up a controlled experiment and report the results. I emailed Wet Web Media in hopes that they can point me in the direction of any studies on this topic.
I have access to a DO meter, I'll calibrate it and test the following:
-effluent of skimmer (running of course)
-main tank at 4 hr intervals with skimmer on
-main tank at 4 hr intervals at the same time of day as above with skimmer off
I dont have any macro algae in the main tank and I'll shut off my refugium, the micro algae in the main tank should not skew the results significantly.
what else would you guys want? I dont mind "wasting" my time on this, not that I really see the point in it other then me now being curious about it because even if a skimmer doesn't exchange gases efficiently (or even at all) I'm still going to use one to remove wastes. Most experienced "reefers" have open tops anyway to control heat and allow maximum light penetration, so yeah...
-effluent of skimmer (running of course)
-main tank at 4 hr intervals with skimmer on
-main tank at 4 hr intervals at the same time of day as above with skimmer off
I dont have any macro algae in the main tank and I'll shut off my refugium, the micro algae in the main tank should not skew the results significantly.
what else would you guys want? I dont mind "wasting" my time on this, not that I really see the point in it other then me now being curious about it because even if a skimmer doesn't exchange gases efficiently (or even at all) I'm still going to use one to remove wastes. Most experienced "reefers" have open tops anyway to control heat and allow maximum light penetration, so yeah...
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I got a response from the WetWebMedia crew.
I didn't ask for him to look it up for me, just to point me in the right direction. Which he did. I might try to telnet into the library and see what I can come up with from Stephen Spotte. Anyway here is his response.
<No time to look this up for you... Maybe some of Stephen Spotte's in-print
works would reference, off to the library: http://wetwebmedia.com/litsrchart.htm">
</a>
The DO is best increased at/within the skimmer though am very sure. Try
it/experiment and see. Bob Fenner>
I didn't ask for him to look it up for me, just to point me in the right direction. Which he did. I might try to telnet into the library and see what I can come up with from Stephen Spotte. Anyway here is his response.
<No time to look this up for you... Maybe some of Stephen Spotte's in-print
works would reference, off to the library: http://wetwebmedia.com/litsrchart.htm">
</a>
The DO is best increased at/within the skimmer though am very sure. Try
it/experiment and see. Bob Fenner>
I don't have the data so I am sure this won't "solve" the issue, but I did uncover this information when I was doing skimmer research from a considered expert on oxygen levels in salt water (not a reef tank expert though). Ultimately it works like this as I understand it. Skimmer bubbles do saturate the column of water with oxygen as each bubble is surface area and those millions of bubbles create a huge total surface area. However, total oxygen in the water is ultimately a form of flow as getting water depleted of oxygen to the skimmer is the difficult part. If the flow isn't there the water in the skimmer body is high, but sucks everywhere else. Even compared to the main tank with say 10x flow, the water at the bottom (likely the lowest saturation) is turned over so much more quickly so gas exchange occurs more readily in the main tank at the surface than the comparative little water that is pumped to the sump, through the skimmer and back up. I am convinced the water coming out of the skimmer is some highly saturated oxygenated water, but I think it pales compared to how much gas exchange exists in the main tank simply because less oxygen rich water is circulated more frequently in the main tank. In the end it seems to not be the fact a skimmer can or can't oxygenate the water, but how fast can you turn the water over in your system to maximize the gas exchange process. Unplug the skimmer and the fish will swim along quite happy because the flow is turning over the water. Unplug the flow and you will have some great oxygenated water at the surface of your tank and in the skimmer, but nowhere else. There are also a lot of other variables such as how much plant life is in a reef tank which also accounts for some oxygenation of the water column but in the end the flow is what matters.
Hope that helps.
Hope that helps.
