CO2 Tubing

[zelmo]

Do most of you use special CO2 tubing, or just regular airline tube? Where do you get CO2 tubing?

[brad]

co2 tubing. Most LFS will have some.

[Hypancistrus]

I recommend vinyl if you want flexible tubing, or either polyethylene (good), polypropylene (better), or FEP (best) if you want semi-rigid.

Avoid silicone if possible, it has the worst permeability rating of most materials.

"Standard airline tubing" is 1/8" inner diameter 1/4" outer diameter, but if you go with semi-rigid, you may need 3/16" inner diameter in order to get the tubing to fit your hose barbs & check valves.

[Rex Grigg]

I use the tubing I sell. Great stuff.

And I actually doubt that most LFS will have CO2 tubing. They will have vinyl and silicone. Vinyl does work but it will get brittle and crack.

[zelmo]

Hey Rex - I have about 500 feet of PolyFlo (polyethylene) tubing gathering dust. Any idea if that will work with your suggestion of drilling an undersized hole in a DIY reactor and having it not leak?

[plantbrain]

Tygon tubing from www.aquaticeco.com is cheap and super for longer gas transport distances, higher pressures(disc users), but for most things, silicone(100%) works very well.

Regards,
Tom Barr

www.BarrReport.com

[adamjh]

You can find tubing labeled "CO2 resistant" on ebay. It goes for ~50 cents per foot.

[Rex Grigg]

Originally Posted by adamjh

You can find tubing labeled "CO2 resistant" on ebay. It goes for ~50 cents per foot.

One thing you need to keep in mind. "resistant" doesn't mean proof. When tubing is labeled "resistant" that means it doesn't break down in the presence of of the substance. Doesn't mean it doesn't leak like all get out.

A lot of the tubing I have seen on eBay is silicone.

Polyethylene will work fine.

Book mark this chart. It helps a lot. Now if we could get one of the big brains to explain what the measurements are.

[zelmo]

Originally Posted by Rex Grigg

Book mark this chart. It helps a lot. Now if we could get one of the big brains to explain what the measurements are.

Well, that chart sure ends any speculation about the comparison of different materials. The question is now what permeability numbers are significant. How much CO2 are we actually going to lose in a short run of any of the commonly available materials? Or, in a more practical sense, how much will it shorten the life of a CO2 bottle, especially since we are on the low pressure side of the regulator?

[Georgiadawgger]

Just to be on the safe side myself, I got some of the PVC tubing from US Plastics...I believe that's what Troy used to sell with Glass Gardens...rated pretty well supposedly and my cylinder seems to last 5+ months (5 lbs) with a relatively high co2 rate.

[snafu]

here's my interpretation of it... what the permeability coefficient tells you, is that net flux (loss) is as follows (from fick's law):
- proportional to the area of the membrane
- proportional to the difference in partial pressures inside and outside of the membrane
- inversely proportional to the thickness of the membrane

or net flux = permeability constant x area x deltaP / thickness

essentially... for a given tubing thickness, ID, length of tube, and operating pressures, the silicone will lose about 20132/90 times more CO2 than polypropylene (224x) tubing.

Example calculation:
1) area = PI x diameter x length of tubing
= PI x 0.47625cm x 30.28cm
= 45.60 cm2

if we assumed the following
ID = 3/16 inch = 0.47625 cm
length = 1 foot = 30.48 cm

2) deltaP = pCO2_inside - pCO2_outside
= 77.572 mm-Hg - 2.386 mm-Hg

if we assumed the following
0.0314 = CO2 mole percent in dry air
76 cm-Hg for std atmospheric pressure
pCO2_outide (Dalton's law) = 76 cm-Hg x 0.0314

pCO2_inside = 15 psi = 77.572 cm-Hg (pure CO2)

3) thickness = 1/32 inch = 0.79375 mm

4) permeability constant for silicone = 20132e-10 cc-mm/(sec-cm2-cmHg)

Therefore,

net flux_silicone = 20132e-10 x 45.60cm2 x 75.2cm-Hg / 0.79375mm
= 0.0087 cc/sec (Loss)

Assuming you are bubbling at 2 bubbles per second:
Let each bubble be around 0.5cm in diameter (SWAG)
volume (sphere) = 4/3 PI R^3
= 0.065 cc

Bubble_rate = 2 bubbles per sec x 0.065 cc
= 0.13 cc/sec

Net CO2 loss would be around 0.0087 / 0.13 = 6.6% per ft of tubing. so, a short run of silicone tubing (i.e., several feet could mean a non-trival amount of CO2 loss).

-snafu

[zelmo]

Originally Posted by snafu

Net CO2 loss would be around 0.0087 / 0.13 = 6.6% per ft of tubing. so, a short run of silicone tubing (i.e., several feet could mean a non-trival amount of CO2 loss).

-snafu

My current system uses a combination of polyethylene and vinyl and total run is about 12 feet. If it was silicone this means CO2 loss from the tubing would be 79%!!? Is there something wrong with my math?

If not, it seems you really do want to avoid silicone. Amazingly, Foster & Smith has a semi-automatic CO2 injection system http://www.drsfostersmith.com/Produc...&N=2004+113924 that comes with 20 feet of silicone tubing.

[snafu]

well, there are many variables at play in the notional calculation, that i wouldn't necessarily take it as an absolute value. what is important to note is that

a) silicone is generally a poor choice for high pressure CO2 transport since it's widely known to have high permeability,
b) keep the runs of tubing as short as possible, and
c) thicker walled tubing helps reduce losses

also, i imagine the CO2 setup has a lot to do with it, since it affects the partial pressure gradient. a setup using a ceramic disc / glass diffuser with a very low bubble rate at a fairly high constant pressure is going to have a much different net flux than someone feeding the inlet of a CO2 reactor at low pressure where the CO2 is being 'sucked' in from venturi effects. in the later case, silicone might work great.

note that the silicone tubing from drs fosters and smith is 1/8" ID versus 3/16" ID, with possibly greater wall thickness too?. so, the net flux for the 1/8" ID should be 2/3 that of the 3/16" ID at the very least.

lastly, i'm not sure 'silicone is silicone is ...', so the properties of the one offered at drs fosters and smith may be different than the ones from cole-parmer.

like you said, there are LOTS of other choices of tubing which are an order of magnitude or more better for CO2 permeability than silicone.

-snafu

[ShortFin]

I just stop by Petco today and saw some black air tubing. Anyone know what kind of material it is made of? It felt thicker than the clear vinyl tubing.