The Planted Tank Forum - View Single Post - Pressurized CO2...Just thought I'd share.
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post #8 of (permalink) Old 04-28-2010, 02:00 AM Thread Starter
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8) Miscellaneous Equipment (Check valve, airline tubing)

Here, we will cover miscellaneous equipment (this maybe considered essential!)

a) Check valve
Placing a check valve is important to prevent a back siphon from occurring. If water were to back siphon, it could go back through the needle valve, destroying your regulator diaphragm. To protect your investment, a check valve provides good protection for a few dollars.

Plastic check valves work fine, but will harden with time (the CO2 makes the plastic brittle), rendering the check valve useless. It is worthwhile to invest a bit more in a brass check valve, as these will not become brittle like their plastic counterparts.

b) Airline tubing
Of course, without airline tubing, you would not be able to get the CO2 from your CO2 tank into your aquarium, so it goes without saying that you will require airline tubing.

The type of tubing does not really matter. I have used your standard vinyl airline tubing as well as your standard silicone airline tubing. Both work fine for our purposes. Some people will point out that silicone tubing is thousands of times more permeable to CO2 than other types of material (there is a website out there with a table showing permeabilization of the various materials). However, given the low pressures at which we work, and the relatively short distances of tubing (say 10-20 feet, at most), the amount of CO2 gas that is loss through silicone tubing is negligible.

While you can purchase CO2 resistant tubing, I find this to be an unnecessary expense.

We can actually do some calculations to see how much CO2 is loss when different types of tubing are used.

Let's crunch some numbers. All data is taken from the following site

Silicone has a permeability of 20,132 (cc-mmsec-cm2-cmHg) according to that table. Note that the units are as follows:

cc-mmsec-cm2-cmHg - the flow of a gas in cubic centimeters per second per area in cm2 through a thickness in mm which has a pressure difference in cmHg

Also, that the magnitude is to the -10 magnitude (important!)

Typical OD of silicone tubing is 5 mm (3/16") with an ID of 4 mm, so a wall thickness of 0.5 mm.

Let's assume the length of the tubing is 100 cm (1 metre). We can calculate the surface area (top and bottom of the tubing's surface area (the ends) is essentially negligible in this calculation). Using the above length, we get a surface area of 157.47 square cm.

Typical pressure is 30 PSI, or 206.84 kPa. Standard atmospheric pressure is 101.3 kPa, meaning the pressure difference is 105.54 kPa or 76.16 cmHg.

Let's assume this is over 1 second.

So now, we have all the necessary values to substitute into the equation.

20,132 cubic centimetres of CO2 (to the -10th magnitude) is lost over a surface area of 1 square centimetres, with a wall thickness of 1 mm, over 1 second, with a pressure differential of 1 cmHg.

Thus, over 157.47 square cm, with a wall thickness of 0.5 mm, over 1 second, with a pressure differential of 76.16 cmHg works out to be:

120,720,684 cubic centimetres (to the -10th magnitude). This works out to be 0.012 (rounded) cubic centimetres (mL) of gas per second.

Now, to put this in real world terms. A 10 pound cylinder of gas, coming out at 30 PSI, at 25C works out to be 1235.51L of gas (10 pounds works out to be 103.09 moles).

Assuming you keep your CO2 on for 8 hours a day, you are losing 345.6 mL of gas a day, or about 0.028% of your gas per day (over 1 metre of tubing).

Using similar calculations for vinyl tubing, you get a loss only 6.18 mL of gas per 8 hour day, over 1 metre of tubing. Tygon (for food/beverage, according to the Cole Parmer site) yields slightly better results at 4.63 mL of gas per day, over 1 metre of tubing.

So, even in the worst case scenario with silicone, you are losing a negligible amount of CO2.


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Last edited by Darkblade48; 02-18-2014 at 03:30 AM. Reason: Added new information
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