Studies on CO2 diffusion varying volume and surface area

[Octavarium]

I think your original theory makes sense. If you had a goofy shaped aquarium that was like a tapered triangle, say only a small 50 sq. in surface, and the rest was 120 gallons, then most CO2 in the system would stay. The hydrogen bonding properties of water make it excellent for keeping gasses trapped in, and it is very efficient if the turbulence at the surface is kept to a minimum. Just don't drop any Mentos in your tank

 

[Volenti]

one reason why they say diy doesnt work is its not worth it. yes you can set up a diy system for a 120 gal but the time and money will far out weigh the pressurized co2 costs.

This depends on where you live, in Australia sugar is dirt cheap while CO2 equipment is very expensive and hard to get, you can't 'buy' large gas cylinders here, only 'rent' for $100 a year (refills are also pricey). I'm running 3 tanks totaling over 200gal with ~6gal of brew, it costs me less to in consumables to run it for a year than the rental on a gas cylinder.

 

[Mgamer20o0]

also a time factor you have to look at.... with my co2 tank i just fill and forget. no worries. diy every week or two having to mix and switch and everything.

 

[Ketso]

This wont be of use (maybe) to th OP, but ....

Yup, for me, time was more of an issue with DIY than the cost. I was doing DIY on a 55g. I started out changing one of the 6 2-liter bottles every other day. This meant I needed to be home every other day. Not feasible with the job I was doing at the time. I then started changing 3 bottles every weekend. Worked for a time until I had family issues that kept me 200 miles from home on the weekends for a month straight.

So, in my opinion, time is definately the main reason NOT to do a DIY system. If you know you'll be home all the time, it's no problem really. After you get a system figured out, it can go pretty smoothly. I was changing 3 bottles out in 20 minutes while fixing dinner. LOL.

 

[plantbrain]

Jeffery,

Cornelius reg's on ebay: 20$ or so
10-20lb tank: 40-60$ filled, River city Fire
Needle valve: 10-20$ on line.

Cost: 70-100$ tops.
Peace of mind?
At least that.

Mrs?

Sell some weeds and you can have the hobby pay for itself.
I sold some java ferns for 50$ ea for several, bought 2 CO2 systems inside 2 month;s worth of growth.

And paid for the extra electric.

Regards,
Tom Barr

 

[jmhart]

I appreciate all of the suggestions about switching to pressurized, and in the long run I am going to, however I was more interested in any scientific data regarding the diffusion rate of co2 out of water.

 

[plantbrain]

Well, the diffusion rate out of a water would depend on:

1.The upper atmosphereic layer concentration of CO2, how well stirred this layer is.

2. The concentration differences between the water's CO2 and the air's.

3. The volume of the water relative to surface area available for exchange.

4. The influx of CO2 coming.

If you add plants/bacteria etc, then the effects will be greater between additions and uptake, basically you'll get higher high's and lower lows in the day and night times.

5. Diurnal fluctuations with dense plants

You can measure the CO2 loss yourself specifically for your tank.
I think most tanks with good plant growth, decent light run out in about 1 hour.

Studies that look solely at loss without plants involved really are non applicable here.
However, you can measure that easy enough.
Add CO2 to the tank etc and measure the lost under normal flow condition etc without plants.

A large lake might take several hours whereas your tank might take 30 minutes to draw down, at night, the CO2 lost will go down and the tank and lake will become a source of CO2 due to respiration.

So it depends on when you want to know how the CO2 is moving through the system as well.

Regards,
Tom Barr

 

[jmhart]

Well, the diffusion rate out of a water would depend on:

1.The upper atmosphereic layer concentration of CO2, how well stirred this layer is.

2. The concentration differences between the water's CO2 and the air's.

3. The volume of the water relative to surface area available for exchange.

4. The influx of CO2 coming.

If you add plants/bacteria etc, then the effects will be greater between additions and uptake, basically you'll get higher high's and lower lows in the day and night times.

5. Diurnal fluctuations with dense plants

You can measure the CO2 loss yourself specifically for your tank.
I think most tanks with good plant growth, decent light run out in about 1 hour.

Studies that look solely at loss without plants involved really are non applicable here.
However, you can measure that easy enough.
Add CO2 to the tank etc and measure the lost under normal flow condition etc without plants.

A large lake might take several hours whereas your tank might take 30 minutes to draw down, at night, the CO2 lost will go down and the tank and lake will become a source of CO2 due to respiration.

So it depends on when you want to know how the CO2 is moving through the system as well.

Regards,
Tom Barr

I was considering conducting experiments myself, but for any degree of reliability and ease I'd have to use pressurized co2, which as mentioned, will have to wait. I believe the data that I would collect from this study is, not only out there, but also easily calculated. However, despite a chemistry degree, I'm lazy and would rather conduct the experiment than perform the calculations.

I'm not convinced volume plays an important role. I believe surface area and co2 concentration are the determining factors here.

That is an important point regarding the presence of plants in the aquarium. While an experiment with non-planted tanks would not be as accurate, it would be easily applied to the planted aquarium. Certain generalizations could be made. Additionally, the presence of fish would also affect the diffusion rate, due to surface agitation, currents, and their own(minimal) co2.

Unless somebody beats me to it, whenever I do go pressurized, I probably will conduct this experiment.

 

[plantbrain]

If you question to the volume, it also relates the distance that is required as well as mixing.

Diffusion must take place over a distance.

You can have the same Surface to Area ratios for 1 gal tank as a 1600 gallon tank, which do you think will degas FASTER, SLOWER, THE SAME?

You are claiming the same.

You are going to have a rough time measuring anything with DIY CO2.
Flux is what you are asking, see Fick's 1st law:
http://en.wikipedia.org/wiki/Fick's_law_of_diffusion

I think in situ measurement of CO2 is more applied and is wiser than Chem and math modeling.

And likely easier.........I've done both.

No two systems, let alone one's with various degrees of plant density and fish and other sources of CO2 are going to be the same.

So the system you are talking about really depends on that system's sources, flow rates/patterns, temps, presence of oil on the surface, plant health status etc.

Regards,
Tom Barr