# Passive CO2 diffuser



## mistergreen (Mar 3, 2007)

I saw where a man fills CO2 into a bottle in his tanks every couple days. It passively diffuses into the water supplying what the plants need. It gave me the idea for this design.


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## DutchMuch (Apr 12, 2017)

im not a big Co2 nut- even though I have it. 
But wouldn't that basically just be a horizontal reactor?  appreciate the diagram it looks great.


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## mistergreen (Mar 3, 2007)

A reactor forces water into the CO2, diffusing it and reducing the flow.


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## DutchMuch (Apr 12, 2017)

mistergreen said:


> A reactor forces water into the CO2, diffusing it and reducing the flow.


See shows how much I know right?
Here is another thing similar to your idea which has been around for ages:
Floating a bottle upside down in your tank and filling it with co2.


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## mistergreen (Mar 3, 2007)

DutchMuch said:


> Floating a bottle upside down in your tank and filling it with co2.


Yeah, that's where I got the idea.


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## DutchMuch (Apr 12, 2017)

Nice! if you end up making it ill be interested to see it + how it goes!


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## hoppycalif (Apr 7, 2005)

The rate at which CO2 dissolves into water depends on the area of the CO2/water contact. This is a very small area. Consider that CO2 from the atmosphere has the total footprint of the tank as the contact area. This is more like drop checker! It would be interesting to find out how much CO2 gets into the water.


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## mistergreen (Mar 3, 2007)

Yeah, about the surface area. Maybe an oversized Tee, like 2inches diameter then reduce down to 1/2 inch for the hoses. I'll have to find some transparent PVC for the vertical to see where the CO2 and water meet.


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## hoppycalif (Apr 7, 2005)

A larger Tee fitting would seem to work the best. I wonder about the turbulence of the water flowing through, caused by the change in diameter, and the lack of flow through the side of the tee with CO2. I can imagine that the water in that leg would be whirling around, up and down, and let big CO2 blobs into the water flow. But, a lot of things I can imagine are nothing more than my imagination, too.


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## mistergreen (Mar 3, 2007)

I'm curious too. I want to see what's going on. Anybody know where I can get cheap 2" diameter clear plastic rigid tubing? Clear 2" PVC is quite expensive.


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## hoppycalif (Apr 7, 2005)

Acrylic pipe is probably the cheapest and easiest to get. Amazon has it: https://www.amazon.com/11-7-Polycar...531517310&sr=8-12&keywords=acrylic+pipe+clear To make a "tee" out of it requires some skill. And, shipping charges for things like this are pretty high. This is how I would make such a device though.


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## mistergreen (Mar 3, 2007)

I can use a regular PVC Tee and extent the vertical with the clear tube.


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## hoppycalif (Apr 7, 2005)

I have had trouble finding acrylic pipe that has the same diameter as PVC pipe. I finally gave up trying.


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## mistergreen (Mar 3, 2007)

I found some acrylic at USplastics. It's a little cheaper than PVC.
For prototyping, I might use a tube from a python water changer I don't use.


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## mistergreen (Mar 3, 2007)

Oh! I found some clear 2" pvc tubing off eBay for $10 with free shipping. The walls are thin but it should work.


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## hoppycalif (Apr 7, 2005)

If I remember correctly, the cement that works well with acrylic, also works well enough with PVC. But, the 2" PVC tubing is the easiest way to go. This should be interesting!!


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## mistergreen (Mar 3, 2007)

I've used these check valves from clippard, http://www.clippard.com/part/MCV-1BB
previously in my CO2 setup but they both failed. I guess when in contact with water (dirty aquarium water), it fails. I have a spare one but would a cheap check valve from the fish store be better?


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## hoppycalif (Apr 7, 2005)

I have been using these check valves, https://www.usplastic.com/catalog/item.aspx?itemid=23369&catid=489 in my CO2 system for about 3 years, with no failures yet. I have had, one in my tank, in the water, with CO2 going through it for all of that time, and it still works fine. They are made of plastics that are highly chemical resistant. Shipping costs for buying them drive up the cost substantially, but I still like them. I mistakenly bought the 3/16 inch size the last time, so it can be difficult to slip tubing over them, but they still work. The 1/8 inch size works much better.


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## vvdo (Mar 7, 2006)

Sent from my iPhone using Tapatalk


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## mistergreen (Mar 3, 2007)

vvdo said:


> Sent from my iPhone using Tapatalk


yup, saw that. pretty cool.


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## mistergreen (Mar 3, 2007)

hoppycalif said:


> I have been using these check valves, https://www.usplastic.com/catalog/item.aspx?itemid=23369&catid=489 in my CO2 system for about 3 years, with no failures yet. I have had, one in my tank, in the water, with CO2 going through it for all of that time, and it still works fine. They are made of plastics that are highly chemical resistant. Shipping costs for buying them drive up the cost substantially, but I still like them. I mistakenly bought the 3/16 inch size the last time, so it can be difficult to slip tubing over them, but they still work. The 1/8 inch size works much better.


cool, order a couple from eBay with free shipping. I think I bought the 1/4" though. I learn a cool trick recently to slip small hoses over an oversized barb. Heat the hose in boiling/hot water. It'll slip right through and when cooled, it's an extremely tight seal.


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## mistergreen (Mar 3, 2007)

So here it is.










I hooked it up and this is the result









The pressure in the tube determines the water level. I released some pressure by pulling out the air tube and the water level rose into the clear tube part. I also hooked the CO2 line to a DIY yeast generator. Let's see where this goes.


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## mistergreen (Mar 3, 2007)

Ok, the yeast CO2 is enough to push the water level down.

I've stopped the CO2 coming through by opening the CO2 bottle. I want to see if the water level rises back up as the CO2 dissolves into the water.


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## hoppycalif (Apr 7, 2005)

One reason the water level might not go back up is that DIY CO2 has a lot of air mixed in, until many days later, and probably always has a significant amount of air. If it is pure CO2 the water level has to rise, since CO2 does dissolve in water very easily.


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## mistergreen (Mar 3, 2007)

It looks like you're right. The water level did move up a little as expected when most of the CO2 dissolved in the water, leaving behind most of the other gasses.

I guess I'll have to wait till I get a refill on my CO2 tank.

***
What I can do now is let all of the Non CO2 gasses purge out of the yeast bottle and the reactor chambers. It should be all CO2 by tomorrow.


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## mistergreen (Mar 3, 2007)

I just read that temperature and pressure influences the rate of gass diffusion into water. The gass chamber is naturally high in pressure so CO2 diffusion should be greater than 1 atmospheric pressure. How much more specifically I don't know. I'd need to find a pressure gauge and a formula to calculate.

Once the chamber is filled with pure CO2, I guess I can time how long it takes for the volume of CO2 to dissolve into water.


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## Gerald (Mar 24, 2008)

Your CO2 diffuser device has its own built-in pressure gauge; just measure the depth of the CO2/water interface in the pipe below the surrounding water. Each inch of water depth displaced is equal to 0.036 psi greater than ambient pressure.

Edit: I'm assuming the device is inside the aquarium, so the aquarium water level is pushing back against the CO2-enriched gas pressure in the tube. If this is all outside the tank, then info above may not apply.


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## mistergreen (Mar 3, 2007)

yeah, the device is outside the tank, attached to a canister filter.

I like your thinking though. I'll look into it.

Here are some preliminary numbers
*Cylinder volume*
2" diameter x 7" height
21.99 cubic inch

*in 2.5 hours*
3.93 cubic inch of CO2 dissolved into the water.


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## Gerald (Mar 24, 2008)

Keep in mind that gases are diffusing BOTH WAYS at the interface: CO2 from your diffuser gas to the aquarium, and N2 and O2 from aquarium to diffuser gas (assuming the diffuser gas is lower in those components). This is similar to how diving beetles and spiders use an air bubble as a "lung" to extract oxygen out of the surrounding water. The change in gas volume is not really a good measure of how much CO2 has dissolved.


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## mistergreen (Mar 3, 2007)

yeah, I hadn't thought about the gas exchange coming from the other end.

I'm seeing the a change in rate of pressure as it goes on. I'll post a graph later. I have a drop checker in a 29G tank to make sure CO2 is dissolved in the water. It does go from blue to green.

I was just thinking, this system is impossible to gas/kill your fish. The rate of CO2 is somewhat constant so this is another 'Pro'. Any excess CO2 would leave the chamber and out the tank without dissolving in the water and the rate of dissolving drops over time.


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## mistergreen (Mar 3, 2007)

Here it is









At 9.5 hour CO2 stopped dissolving into the water. This is perfect for a planted tank. Fill the chamber with CO2 every morning and just let it go for the day.


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## hoppycalif (Apr 7, 2005)

A 5 pound bottle of CO2 at 800 psi contains about 3.7 million cubic inches of CO2 gas at 15 psi. Your reactor put about 10 cubic inches of CO2 into the tank in a single photoperiod. So, a 5 pound tank would last about 370,000 photo periods - lets call that 1000 years! (Did I misplace a decimal point somewhere???) I think the problem is that in order to keep 30 ppm of CO2 in a tank of water you have to add many times more cubic inches of CO2 spread out over the photoperiod, because CO2 leaves the water almost as fast as you add it. And, this would mean that your ppm of CO2 in the water is extremely low. I find this hard to believe, so perhaps someone with better decimal point control should do this calculation.


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## mistergreen (Mar 3, 2007)

Lol, no more refilling CO2 tanks!

Yeah, as expected there's a limit to how much CO2 can inject into the tank. If you want more CO2, you'd have add more CO2, maybe 2 x a day instead of once. This is good for low to medium light tanks.

I'll have to fill up the CO2 tank again get this running in a real planted tank.


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## mistergreen (Mar 3, 2007)

Keeping notes
Density of CO2 is 0.0000636574	lbs/cubic inch

or 1762.030766 ppm


***
If you want more CO2, could be as easy as make the vertical chamber longer. It's at 7" now with a 6" clear PVC tube.

***

2,000ppm(air) CO2 becomes toxic to plants


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## mistergreen (Mar 3, 2007)

Does anybody know of a formula to calculate 'x' volume of CO2 needed to raise 'x' volume of water to a certain ppm of CO2?

never mind, found it



> Width x length x height x desired CO2 level in parts per million.


For example, if you have a 8x8x7 foot grow room, you would need to cover 448 cubic feet ( 8x8x7=448 ). To compute the amount of CO2 required to raise the level to 1,000ppm use this equation.
Width x length x height x desired CO2 level in parts per million.
So in our example, we have 8x8x7x0.001 = 0.448 cubic feet of CO2 needed to raise the level inside the grow room to 1,000ppm CO2.

Keep in mind this is ppm by volume (in air)... PPM in water is by weight. There's another formula to convert that.... It's easier to convert everthing to volume for the above formula.

16,500 ppmv = 30 ppmw

yikes, we're pumping a lot of CO2 into the tank. Anybody have health issues from this?


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## mistergreen (Mar 3, 2007)

hoppycalif said:


> A 5 pound bottle of CO2 at 800 psi contains about 3.7 million cubic inches of CO2 gas at 15 psi.


From what I'm reading 


> A pound of liquid CO2 in a tank contains 8.741 cubic feet of gas


So a 5 pound is 75522.24 cubic inch of CO2, not 3.7 mil


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> From what I'm reading
> 
> So a 5 pound is 75522.24 cubic inch of CO2, not 3.7 mil


That is the volume of CO2 at about 700 psi. Multiply it by 700/15 to get the volume at 15 psi. (It is still 5 pounds of CO2).

EDIT:
I'm wrong! Wikipedia says CO2 density at one atmosphere and about 70F temperature is 1.98 kilograms per cubic meter. That means 5 pounds of CO2 at one atmosphere (15 psi) and 70F is about 70,000 cubic inches. This time I used the back of a piece of scrap paper instead of an envelope, so it is more accurate.


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## mistergreen (Mar 3, 2007)

So according to the formula above,



> Width x length x height x desired CO2 level in parts per million.


To raise a 29G tank to 30ppm
12"x18"x30"x0.0165

I need 106.92 cubic inch of CO2.


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## Gerald (Mar 24, 2008)

mistergreen said:


> So according to the formula above, To raise a 29G tank to 30ppm
> 12"x18"x30"x0.0165 I need 106.92 cubic inch of CO2.


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!


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## hoppycalif (Apr 7, 2005)

Gerald said:


> 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 checked my desk - no paper towels of any kind. Sorry.

Actually we have enough information to come up with a crude answer, but my mind isn't in it yet.


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## mistergreen (Mar 3, 2007)

Gerald said:


> 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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## hoppycalif (Apr 7, 2005)

mistergreen said:


> 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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## mistergreen (Mar 3, 2007)

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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## hoppycalif (Apr 7, 2005)

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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## mistergreen (Mar 3, 2007)

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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## hoppycalif (Apr 7, 2005)

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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## tiger15 (Apr 9, 2017)

mistergreen said:


> 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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## mistergreen (Mar 3, 2007)

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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## mistergreen (Mar 3, 2007)

tiger15 said:


> 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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## mistergreen (Mar 3, 2007)

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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## Gerald (Mar 24, 2008)

mistergreen said:


> 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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## Gerald (Mar 24, 2008)

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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## mistergreen (Mar 3, 2007)

Gerald said:


> 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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## mistergreen (Mar 3, 2007)

Gerald said:


> 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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## tiger15 (Apr 9, 2017)

mistergreen said:


> 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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## mistergreen (Mar 3, 2007)

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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## mistergreen (Mar 3, 2007)

Here this video is smarter than I explaining CO2 and Henry's Law.

https://www.khanacademy.org/science...ystem/gas-exchange-jv/v/o2-and-co2-solubility


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## mistergreen (Mar 3, 2007)

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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## mistergreen (Mar 3, 2007)

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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## Gerald (Mar 24, 2008)

mistergreen said:


> 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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## mistergreen (Mar 3, 2007)

All this Henry's law talk gave me a new idea for the CO2 sensor design. I don't have to put it in water and fear there would be a leak. I can just place it above the water level and use Henry's law to calculate... My only fear would be water splashing into the electronics.


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## mistergreen (Mar 3, 2007)

Gerald said:


> That can't be right. CO2 concentration in air is around 400 mg/L. Fish would all be dead at 1/10 of that concentration in water.


You're forgetting the different units. In air it is ppm by volume. In water it's measured by weight.. like 400ppmv = 0.7ppmw something like that.


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## Gerald (Mar 24, 2008)

mistergreen said:


> You're forgetting the different units. In air it is ppm by volume. In water it's measured by weight.. like 400ppmv = 0.7ppmw something like that.


Yup I did. I noticed that error about a minute after I posted, and went back to fix it, but you caught me before I got it fixed! [smilie=b:


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## mistergreen (Mar 3, 2007)

Gerald said:


> Yup I did. I noticed that error about a minute after I posted, and went back to fix it, but you caught me before I got it fixed! [smilie=b:[/QUOTE]
> 
> If you want to know the Co2 of a glass of water, that ph&kh chart works great.no need for a sensor.


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## mistergreen (Mar 3, 2007)

Ok, found the right calculator
https://www.lenntech.com/calculators/ppm/converter-parts-per-million.htm

400ppmv of co2 is 775mg/m3 or 0.775mg/L

I'll find the value for in the water when I'm awake


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## tiger15 (Apr 9, 2017)

mistergreen said:


> All this Henry's law talk gave me a new idea for the CO2 sensor design. I don't have to put it in water and fear there would be a leak. I can just place it above the water level and use Henry's law to calculate... My only fear would be water splashing into the electronics.


It's a common environmental sampling technique called head space analysis. It proceeds with collection of a water sample in a capped jar, let it sit and reach equilibrium, then punch through the cap with a Syringe to draw an air sample that feed into an air monitoring device. It's analogous to a drop checker where the test liquid is separated from the tank water with an air space in which co2 reach equilibrium by Henry Law in all three media.

Unless your tank top air is sealed, your measurement won't be accurate. You need to collect the tank water sample in a jar and sealed it with a plastic wrap, wait for equilibrium, and find a way to insert the sensor.


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## mistergreen (Mar 3, 2007)

Ok, using Henry's Law to find the C of CO2 in water

P is Partial pressure of 400ppm of CO2 in ATM
C=P/K
C = 0.0004/ 29.4
C = 0.0000136M (m/L)
grams = 0.0000136 * 44.01 g/m
grams = 0.0006g/L or .6mg/L or 0.6ppmw

We found earlier the air has .775mg/L 

So it's pretty close theoretically... I'll turn on the sensor and measure CO2 on top of water to verify.... I have to do it outside since CO2 in the house is significantly higher.


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## Gerald (Mar 24, 2008)

mistergreen said:


> Ok, using Henry's Law to find the C of CO2 in water
> 
> P is Partial pressure of 400ppm of CO2 in ATM
> C=P/K
> ...


Yup those look like numbers I found too, LATE last night. You're right that the mass concentration of CO2 gas dissolved in water (0.6 mg/L @ equilibrium with 20°C air) is very close to the mass concentration of CO2 in air (0.7 mg/L). It turns out that's just a coincidence due to CO2's high solubility (low K value), not a general principal of gas physics. N2 and O2 mass concentrations in air are MUCH higher than their mass conc's dissolved in water:

N2 gas in air: 75% (by mass) x 1.2 g/L = 900 mg/L in air (20°C)
N2 gas in water: .78 atm / 1639 (K) = .00048 mol/L = 13.4 mg/L in water
---------------------------------------------------------------
O2 gas in air: 23% (by mass) x 1.2 g/L = 280 mg/L in air (20°C)
O2 gas in water: .21 atm / 789 (K) = .00027 mol/L = 8.7 mg/L in water

And of course the other weird & important feature of CO2 is that it REACTS chemically with water, which N2 and O2 don't do. The dissolved CO2 gas molecules may be only 0.6 mg/L (at air equilib), but lots more CO2 can be held in water as carbonic acid and bicarbonate, so the total amount of CO2 that will dissolve in water (much of it converted to carbonic acid and bicarbonate after dissolving) is a good bit more than the Henry equation shows us.

I'm beginning to understand why my simple question doesn't have a simple answer. If I've botched any of this analysis (again) please correct me.


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## mistergreen (Mar 3, 2007)

yup, it's because CO2's high solubility that it's close to water & air.
The Henry's Law constant for it is LOW, 29.4 L*atm/mol. Meaning it really doesn't want to leave water.

Yeah, it's pretty complicated. The concentration is influenced by temperature too.


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## tiger15 (Apr 9, 2017)

Gerald said:


> Yup
> 
> And of course the other weird & important feature of CO2 is that it REACTS chemically with water, which N2 and O2 don't do. The dissolved CO2 gas molecules may be only 0.6 mg/L (at air equilib), but lots more CO2 can be held in water as carbonic acid and bicarbonate, so the total amount of CO2 that will dissolve in water (much of it converted to carbonic acid and bicarbonate after dissolving) is a good bit more than the Henry equation shows us.
> .


I don't know if your undertanding is correct. I think Hernry coefficient is derived experimentally by comparing air concentration of co2 to total aqueous concentration of co2 that is sum of ionized (carbonic acid) and free co2. This is similar to total ammonia determination which is the sum of free and ionized NH3.


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## mistergreen (Mar 3, 2007)

The CO2 in carbonic acid will eventually break loose, in addition to the free CO2, as we all know the pH in CO2 filled water will eventually rise left alone. It's the carbonic acid that affect the pH.


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## Gerald (Mar 24, 2008)

tiger15 said:


> I don't know if your undertanding is correct. I think Hernry coefficient is derived experimentally by comparing air concentration of co2 to total aqueous concentration of co2 that is sum of ionized (carbonic acid) and free co2. This is similar to total ammonia determination which is the sum of free and ionized NH3.


OK thanks tiger - so you're saying the K value for CO2 *includes* all the dissolved forms: CO2 gas, carbonic acid, and bicarbonate, right? I guess that explains why its K is so much lower than other gases.


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## tiger15 (Apr 9, 2017)

Gerald said:


> OK thanks tiger - so you're saying the K value for CO2 *includes* all the dissolved forms: CO2 gas, carbonic acid, and bicarbonate, right? I guess that explains why its K is so much lower than other gases.


Actually, I am not certain, but can only assume that K is derived from total dissolved CO2 that by definition includes free and ionized CO2. The two exist simultaneously and interchangeably. Free dissolved CO2 in equilibrium is not the same as CO2 mist created by artificially injection.

Ionized CO2 is carbonic acid. Carbonate and bicarbonate are dissolved salt of cations, not CO2. The only way to ascertain what is included in K is to review the lab procedure that derived K.

The reason CO2 has much lower K than other atmospheric gases is that it is a polar molecule that has great affinity for polar H2O, and can ionize. O2 and N2 are non-polar and dissolve in free phase only, no comparison.


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## mistergreen (Mar 3, 2007)

I'm finally back in the CO2 game after these many years. I plugged in this contraption into my 75G.
It doesn't look good. The bubble rate is less that 1/sec for the CO2 to dissolve or the the CO2 will overflow out the chamber. The amount of CO2 in the 75G is only around 3ppm. Going back to the regular PVC reactor.


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