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Old 07-28-2018, 03:38 PM   #41 (permalink)
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Default Re: Passive CO2 diffuser

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Originally Posted by Gerald View Post
True, for complete CO2 dissolution in a sealed tank with no other CO2 inputs or losses. But that's just a snapshot quantity, not a rate of CO2 input. Now let's estimate the CO2 loss into the air, plant uptake, CO2 production by animals and bacteria ... might need research grade lab paper towels for those calculations ... have at it!
I had a CO2 sensor that logged data every hour. I put it in a dirt tank. It was neat to see the cycle of falling and rising CO2 throughout the day.
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Old 07-28-2018, 04:15 PM   #42 (permalink)
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Default Re: Passive CO2 diffuser

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I had a CO2 sensor that logged data every hour. I put it in a dirt tank. It was neat to see the cycle of falling and rising CO2 throughout the day.
Can you post a chart of that rising and falling CO2? It would be very educational.
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Old 07-28-2018, 05:23 PM   #43 (permalink)
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Default Re: Passive CO2 diffuser

Here it is... The cycle starts everyday.



The chart also maybe indicates that the plants don't consume CO2 at the same rate in the photoperiod.

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Old 07-29-2018, 06:50 AM   #44 (permalink)
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Default Re: Passive CO2 diffuser

I assume this data is from a tank with no source of CO2 except the atmosphere and the substrate? Doesn't this also show that a drop in tank water pH of 1.0, as measured in the tank, would mean the tank had 10 times 7 to 8 ppm of CO2 (assuming you were adding CO2)? This also demonstrates why you need to use that 1.0 pH drop method only with a tank water sample that sits out in the air for a day or more.

This also suggests that the substrate can supply more than 4-5 ppm of CO2 minimum, since CO2 would constantly be escaping from the tank water to air interface. That makes substrate CO2 a more significant source of CO2 than the atmosphere is. And, it verifies Ms Walstad's theory that the natural substrate works so well partly because it is such a good CO2 source. With medium or low light, that is enough CO2 to let many plants grow very well.

Thanks for posting that! (When I was working as an engineer there was a "joke" always repeated - "no engineer ever has too much data!")
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Old 07-29-2018, 07:02 AM   #45 (permalink)
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Default Re: Passive CO2 diffuser

Yes, this is a dirt tank with no extra CO2. I didn't get a chance to measure the pH with this dataset. This was measure with a CO2 sensor, no guessing CO2 based on pH & kH.

The 10G tank was a jungle so all that plant mass only consumed ~1.5~2 ppm of CO2 (while CO2 is being generated and lost). I'm wondering why do you even need CO2 levels to be as high as 30ppm maybe unless it's a huge tank with huge plant mass? The smaller your tank, the less CO2 you need I think. So a target number for all tank sizes isn't needed. You're just wasting CO2 in a 10G if you're targeting 30ppm.

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Old 07-29-2018, 08:01 AM   #46 (permalink)
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Default Re: Passive CO2 diffuser

Ole Pedersen's paper, http://www.bio-web.dk/ole_pedersen/p...02_477_163.pdf shows that the concentration of CO2 determines how fast at least some plants can grow, and the higher the light intensity, the higher the CO2 concentration needs to be to get maximum growth rate of the plants. Nothing in that paper suggests that tank size is a factor, except that it takes more grams of CO2 to achieve a given ppm of CO2 in a large tank vs a small tank. With 25 to 90 PAR lighting, on a tank with a soil substrate, you get near maximum growth rate (for the plant species Pedersen used) at about 8 ppm, so your data suggests that adding CO2 to such a tank gains almost nothing for you. That soil substrate is a very potent CO2 generator!!
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Old 07-29-2018, 08:33 AM   #47 (permalink)
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Default Re: Passive CO2 diffuser

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Originally Posted by mistergreen View Post
I had a CO2 sensor that logged data every hour. I put it in a dirt tank. It was neat to see the cycle of falling and rising CO2 throughout the day.
What co2 sensor do you use? I much prefer direct measurement than kH pH guessing.
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Old 07-29-2018, 08:56 AM   #48 (permalink)
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Default Re: Passive CO2 diffuser

I think plant mass has a huge influence of CO2 absorption rate. It makes sense. Bigger machines need more fuel.
I've noticed that dirt tanks doesn't work so well in large tanks like a 75G. I guess I needed much more dirt but I fear it'll go anaerobic. It's the balance we have to play with.
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Old 07-29-2018, 09:06 AM   #49 (permalink)
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Default Re: Passive CO2 diffuser

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What co2 sensor do you use? I much prefer direct measurement than kH pH guessing.
It depends on your needs and price points.
https://www.co2meter.com/collections/co2-sensors

You'll have to build a waterproof casing for it and allow for a CO2 gas membrane. I used a thin silicone membrane. It works ok for the price. There are better membranes like PTFE, Polytetrafluoroethylene but they're very expensive. Water molecule can enter as well so if it gets too humid, you have to open it up and let the humidity out.

Be aware these measure in ppm by volume. In water, it is by weight (smaller number to deal with).
10,000 ppmv is 17.99 ppm(weight)
You'd want 30,000 ppmv or 100% sensor to measure higher values.

I use this formula to convert
Quote:
ppmw = ppmv×44.01÷(0.08205×298.15)÷1000
44.01 is mol weight of CO2
0.08205 is a gas constant
289.15 is 77F in Kelvin
1000 is density of water

They have a Windows software for you to collect the data or you can hook it to an Arduino for remote data logging.

Last edited by mistergreen; 07-29-2018 at 10:36 AM..
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Old 07-29-2018, 10:48 AM   #50 (permalink)
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Default Re: Passive CO2 diffuser

Also in my previous post,
Quote:
To raise a 29G tank to 30ppm
12"x18"x30"x0.0165

I need 106.92 cubic inch of CO2.
More cubic inch of CO2 is need for a larger tank volume to maintain a 30ppm. Volume does matter.
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