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Discussion Starter · #41 ·
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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Discussion Starter · #43 · (Edited)
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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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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Discussion Starter · #45 · (Edited)
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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Ole Pedersen's paper, http://www.bio-web.dk/ole_pedersen/pdf/Hydrobiologia_2202_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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Discussion Starter · #48 ·
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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Discussion Starter · #49 · (Edited)
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
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.
 

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Discussion Starter · #50 ·
Also in my previous post,
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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Also in my previous post, More cubic inch of CO2 is need for a larger tank volume to maintain a 30ppm. Volume does matter.
Yes, it certainly does. The way your previous post (#45) was worded, it sounded like you were saying that a lower CO2 "level" (concentration) in a small tank would have the same benefit as a higher concentration in a larger tank. I (and apparently Hoppy) misunderstood your meaning.

This is great discussion BTW!
 

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What is the CO2 concentration (either mg/L or mole/L) of plain water in an open container at equilibrium with air (400 ppm-volume) with no animals, plants, or algae? Let's assume pH 7.0, low KH (20-40 mg/L), temp 25 C, and sea level pressure (14.7 psi) if those things matter significantly. Trying to find this via Google search I get pages and pages of equations, solubility graphs, gas physics, etc, but can't find an answer to this "simple" ??? question.
 

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Discussion Starter · #53 ·
it sounded like you were saying that a lower CO2 "level" (concentration) in a small tank would have the same benefit as a higher concentration in a larger tank.
That's what I'm postulating using my 10G dirt tank at 8ppm as an example probably because you can only have so much plant mass in a 10G.
 

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Discussion Starter · #54 · (Edited)
What is the CO2 concentration (either mg/L or mole/L) of plain water in an open container at equilibrium with air (400 ppm-volume) with no animals, plants, or algae? Let's assume pH 7.0, low KH (20-40 mg/L), temp 25 C, and sea level pressure (14.7 psi) if those things matter significantly. Trying to find this via Google search I get pages and pages of equations, solubility graphs, gas physics, etc, but can't find an answer to this "simple" ??? question.
CO2 concentration in water should be the same in the air. CO2 enters and leaves water easily. I did measure with a CO2 sensor and can confirm... I tested straight tap water too. You always hear of tap water have extra CO2 in it but where I'm from, it doesn't. It's possible for ground water.

Temperature and pressure play a huge role. The ocean depth is a huge CO2 sink. Sadly as the temperature rises even by only a degree or two, the CO2 is released and creates a cycle.
 

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CO2 concentration in water should be the same in the air. CO2 enters and leaves water easily. Although temperature and pressure play a huge role. The ocean depth is a huge CO2 sink. Sadly as the temperature rises even by only a degree or two, the CO2 is released and creates a cycle.
No, it's not a direct one to one conversion. You need to do a Henry coefficient calculation which is defined as the ratio of aqueous phase concentration to air phase concentration. Henry coefficient is gas specific and temperature dependent.

https://en.m.wikipedia.org/wiki/Henry's_law

The co2 meter you referenced to is air quality measurement device, and you still need to convert air concentration to aqueous concentration using Henry law equilibrium calculation. It's not direct measurement, and the device is pricy for hobbyists use, and no wonder it is not commonly used in the hobby.
 

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Discussion Starter · #56 ·
CO2 is one to one proportion or close to it given normal conditions from what I can tell. It's the reason how we can inject CO2 into water so easily.

There is no difference in an air CO2 sensor and a aqueous CO2 sensor. They both sense the % of CO2 in a medium. I've read research on CO2 sensors made by a science college. They made one using an 'air' CO2 sensor. Electronics don't work in water so we're stuck with using dry air sensors.
 

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Discussion Starter · #58 · (Edited)
wrong value for calculation.
Gerald in post 60 is right about the calculation assumes pure co2, not partial gas...
I'll have to find the math.
 

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Discussion Starter · #59 · (Edited)
I just remembered, As for why not using Henry's Law in a CO2 sensor enclosure is because there is no partial pressure in a container containing the sensor. If the sensor is freely open to the atmosphere above the water, yes, there would be partial pressure. You can use Henry's law then.
 

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CO2 concentration in water should be the same in the air.
Air contains .04% CO2 by volume (400 ppm-vol) and .06% by mass (since it's heavier than the other major gases in air). A liter of air at 25 C at sea level pressure is about 1.2 grams, so the mass of CO2 in 1 liter of air is about .0007 grams (0.7 mg) per liter.

>>> So, is that also the air-equilibrium concentration of CO2 in water? The equilibrium concentration in water is determined by each gas's solubility and its partial pressure in the air above the water. That's the part I'm confused on; CO2 has greater solubility than the other gases in air, but lower partial pressure.

Also: "... It says, there's 1.84 grams of CO2 in a liter of air."

That can't be right; it's more than the mass of 1 liter of air, which is 1.2 grams (at sea level pressure). I think 1.8 grams is the mass of 1 liter of pure CO2 gas, not the CO2 fraction in air.
 
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