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Old 02-17-2006, 03:14 PM   #1 (permalink)
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Default CO2 fluctuations in a non-CO2 tank

The following images and info was gathered by Deyan (Dido on www.aquariumbg.com, contest pages are here) a Bulgarian guy using his DIY microprocessor. The graphs don't show anything revolutionary different from what we already know but it's very nice to see everything graphed like that. Also on the second graph there is proof for gradual exhaustion of the CO2 - something that I personally thought happened in less than one day.

The first graph shows the moment of the CO2 bottle running out on Jan. 14 about 4:00 PM. The controller was set to maintain pH=6.2 and as we see up to that point the pH was properly maintained. The lights come on at 12:00 and are turned off at 21:45. It's interesting to see how the CO2 drops (pH goes up) in a straight line between midnight of the day the CO2-supply was cut off and noon the next day. I suppose that's the time when the plants were releasing CO2 and I'd expect that the pH increase should be slower due to the acidic nature of the CO2 coming from the plants. But that is not so.

The portion of the graph between noon (lights come on at noon) and 10:00 PM shows an even faster pH increase. As expected the light stimulates the plants to use CO2 and as a result the pH increases sharply.

After peaking at about 7.3 the pH drops abruptly once the lights go off at 9:45 PM. Note how at 10:02 PM there is a sudden drop of pH due to the release of CO2 by the plants. It happens FAST!



The second graph shows the pH fluctuations in the span of 3 days. As expected when the lights are on and the plants consume CO2 the pH increases. And when the lights are off the pH drops because the plants release CO2 at night. What's interesting is to note how pH increases over the 3 day period. The first day the highest pH was 7.26, the second day - 7.48, and the third day it's about 7.5. It appears that the tank is gradually exhausting the amount of CO2 over time - on Jan 16 we had more CO2 in the whole system than we have on Jan 18. I personally thought that the pH hits a peak and stays there the first day after the CO2-supply has being stopped:



The third graph is a magnification of one of the pH peaks. It shows that the pH increase (the CO2 usage) is dependent on the light. Since the lights come on at noon and are shut at 9:45 we can see that the connection is straightforward - when the lights are on CO2 is being consumed and the pH increases. The opposite happens during the period of darkness between 9:45 PM and noon the next day. What's interesting to note is how fast the CO2 is being consumed (pH increasing fast when the lights are on) in comparison to the CO2 being accumulated (duing the period of darkness). Maybe someone will explain that difference, I can't:



Thanks Dido!

--Nikolay
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Old 02-17-2006, 05:30 PM   #2 (permalink)
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Wow, this is really neat. Any idea of what kind of microcontroller he used (PIC?) - robotics is another of my hobbies :icon_fU: ? This curve looks almost exactly like the resistor-capacitor charge/discharge curve. Thanks!
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Old 02-18-2006, 10:49 AM   #3 (permalink)
 
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Niko, why do you assume that the plant release CO2?
They are always producing CO2, every aerobe does 24/7.

Compare this with the O2 levels.
You'll find a correlation.

Quote:
What's interesting to note is how fast the CO2 is being consumed (pH increasing fast when the lights are on) in comparison to the CO2 being accumulated (duing the period of darkness). Maybe someone will explain that difference, I can't:
Sure, the plants remove the CO2 very fast, at night, the flux rate from the air into the the tank is much slower.

Regards,
Tom Barr
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Old 02-19-2006, 11:56 AM   #4 (permalink)
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Quote:
Originally Posted by plantbrain
Sure, the plants remove the CO2 very fast at night, the flux rate from the air into the the tank is much slower.
During the day, plants photosynthesize and consume CO2; during the night, they perform aerobic respiration and release CO2 (they don't remove CO2 at night). Also, I'm not sure I get the connection to the flux of CO2 from air to tank. Do you mean that the flux is insignificant, eliminating that as a factor?

With regards to Niko's comment about the difference in rates - I don't really see that there is much of a difference in the rates (maybe I'm just not understanding the comment fully). As the concentration of CO2 drops, plants have a harder time utilizing what is available, and the rate of photosynthesis decreases (which explains plateau on the daytime part of the curve). Then, at night, the plants have a harder time doing aerobic respiration because of the increase in the ratio of CO2 to O2 (the plateau on the night time curve).
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Old 02-19-2006, 01:12 PM   #5 (permalink)
 
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Thanks a lot Niko, I was quite surprised to see these graphs here. They didn't gather much attention at my forum

The actual samples are taken every round hour - 12:00, 13:00, etc. So the graph slopes aren't quite accurate - especially around the lamp on/off time. I'm currently developing another firmware revision with settable log periods for accurate data acquisition - you will be able to collect data every 3 minutes for example.

The graphs itself actually don't show something extraordinary. It's just interesting to see how the theory works 'live'. I haven't seen such graphs on the net yet so I decided it will be useful to publish the data.
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Old 02-20-2006, 02:05 PM   #6 (permalink)
 
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The graphs are a little misleading because the ph at bottom right is different for each one. This makes the ph swing seem much bigger than it actually is.

I wonder what the KH is?
If we assume that the ph controller being set at 6.2 is trying to achieve around 30ppm CO2, then we assume the KH to be 1.5. Therefore the PH swing at night is only from 1.5ppm to 3.5ppm.

I have read that around 3ppm is what you would get from the atmosphere without any CO2 injection.
So during the day the plants use up 2ppm CO2 in photosynthesis and at night the atmospheric CO2 slowly goes back into the tank.

BTW When I let my KH 12 water sit, ph is 8.5. Does that mean high KH water will absorb less CO2 directly from the atmosphere?
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Old 02-20-2006, 04:10 PM   #7 (permalink)
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At night the atmospheric CO2 isn't the only source - plants produce CO2 at night.
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Old 02-21-2006, 01:35 AM   #8 (permalink)
 
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Quote:
Originally Posted by redFishblueFish
Wow, this is really neat. Any idea of what kind of microcontroller he used (PIC?)
I used TI's MSP430F149 here. I don't like PICs at all

Quote:
Originally Posted by R0bert
The graphs are a little misleading because the ph at bottom right is different for each one. This makes the ph swing seem much bigger than it actually is.

I wonder what the KH is?
As niko wrote above, that's probably an evidence of gradual CO2 exhaustion. Maybe after a couple more days without CO2 injection the low/high levels would not differ daily.

Tank's kH is about 6-7 deg - a good carbonate buffering ability. I suppose the swings would be much greater in softer water.
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Old 02-21-2006, 02:01 AM   #9 (permalink)
 
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Quote:
Originally Posted by dido9

Tank's kH is about 6-7 deg - a good carbonate buffering ability. I suppose the swings would be much greater in softer water.
Wow that's over 100ppm CO2 at ph6.2!
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