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Discussion Starter · #1 ·
Sometimes you can learn a lot by failing at a project!

I have been thinking that the reported high CO2 diffusion constant published for silicone tubing offers the possibility of using a long piece of that tubing, coiled in the substrate as a diffuser for CO2 that would release the CO2 where it would be most effective. So, I bought a 25 foot roll of Top Fin Silicone Airline Tubing, about $6 worth, and tested it today.

I made a bubble counter out of a club soda bottle to be sure it would be strong enough to withstand up to 40 psi. That was a very conventional bubble counter, with a couple of pieces of silicone tubing pulled through undersize holes in the cap, one deep into the bottle, the other ending near the top of the bottle.

Then I set up for a test, as shown:


I started with a little plastic check valve as a connector, but that leaked CO2 at the connections, so I replaced that with a short piece of rigid tubing:


I put the coil of tubing in a container with about a gallon of water in it, having to weight it down so it wouldn't float up. I purged the air out of the tubing by running CO2 through it for a minute or so, and clamped it shut by folding it over, tieing it with wire and adding a "bag clamp" to squeeze it even more.

Using the regulator, with the needle valve open more than normal, I raised the CO2 pressure to 5 psi, and got a surge of bubbles through the bubble counter, until the tube filled up to the 5 psi pressure. The final bubble rate was about 30 bubbles per minute.

Raising the pressure to 10 psi, 15 psi, 20 psi, 25 psi and 30 psi did nothing but reveal a tiny leak at the tubing connection going through the bubble counter cap - a very small leak, taking minutes to generate a fine foam of soap solution. The bubble rate decreased as the pressure went up until it was about 12 bubbles per minute at 30 psi.

Clearly, little or no CO2 was diffusing into the water. Certainly not enough was diffusing to be able to use the tubing for a CO2 diffuser. So, the idea isn't practical, if it would work at all.

This did prove that we can use cheap silicone air tubing for our CO2 systems, and not lose any CO2 as a result. The normal pressure our CO2 runs at, downstream of the needle valve is about 2 psi, so there should be no measurable leak at that pressure.
 

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I had a big "discussion" with someone who used to sell CO2 equipment (and tubing) on this forum (and APC). Can't (cough-cough) recall who ;) - it was right after I highlighted the CO2/tubing loss being a MYTH.

I was pretty (what's the word) scolded that dare I say that vinyl or silicone tubing DID NOT leak CO2... We debated, and found a web-site that discussed CO2 diffusion between different membrane types - and it provided a formula to indicate what that loss through the membrane would be, based on the difference in pressure inside the tubing vs. outside the tubing.

After doing (and posting) the math, I figured I'm losing less than a nickle's worth (yes, $0.05) of CO2 out of a 20lb tank.

Then the "discussion" got some "clarification" - (cough, cough, cough) - "Well, that's because you don't have any pressure in your tubing..."

Don't most of us run fairly low-pressure (mine's actually slightly negative - as I feed a mini-venturi).

I didn't want to do the math again...

Looks like your experiment is THE Myth-buster ;)

Thanks!

- Jeff

p.s., we should start a "Common Aquarium Myths" series of threads...
p.p.s, LOL - I got a ghetto counter just like that one ;)
 

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Seriously though - the tubing "co-efficient" for osmotic transfer for the "CO2-proof" tubing is like a factor of 1000 over that of Vinyl (which most people keep confusing with Silicone tubing). But again, it's all based on the differential pressure.

Maybe later I'll look up the thread (Can't remember if it was here or APC) and post the table/math.

- Jeff
 

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Discussion Starter · #7 ·
Unlike those who use silicone tubing for their CO2 systems I was hoping for the tubing to be very leaky. I was hoping, based on a calculation I made, that the tubing would leak CO2 at about a 10 bubbles per second rate, with 25 feet of it, and 30-40 psi pressure. So, for my purposes this was an utter failure. All I can conclude about the tables of "leakiness" for various material tubing for CO2 is that the units used for the coefficient are a hodgepodge of inconsistent units - like mm for thickness, meters for length, cm for diameter, inches of mercury for pressure, etc. That could cause several orders of magnitude errors in calculating the "leakage" unless you guessed the correct units. I must not have guessed correctly. Now I'm simmering the idea of using a RFUG of some sort - so far it hasn't cooked.:redface:
 

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So, for my purposes this was an utter failure.
Funny, Columbus was disappointed with his discovery of the Americas...

As long as we can dispel an Aquarium myth that people waste lots of money on, then success be damned!

And as Edison replied if he was discouraged with the number of failures he experienced when he tried 1000 times to make a lightbulb, he replied, "I didn't fail 1000 times, I learned 1000 ways how NOT to make a lightbulb." (or something like that).

- Jeff
 

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isn't regular airline "supposed" to break down over the long run. i thought that over time it became brittle leading to small leaks, i don't believe it would happen immediately if it was going to happen at all.
 

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Hi Hoppy

Would it be possible that the CO2 bubbles may be dissolving in the water so fast that you can't see the bubbles form?

If you could accurately weigh the container of water before and after the addition of CO2, would you possibly see an increase in the weight? CO2 is ~ 37 times more soluble in water than O2 is?

Solubility of CO2 and O2 in H2O

- CO2

0.1449g in 100g H2O @ 25° C or 77° F

0.001449 parts or 1449 ppm

- O2

0.003931g in 100g H2O @ 25° C or 77° F

0.00003931 parts or 39.31 ppm



1449 ppm / 39.31 ppm = 36.86084966

CO2 is 36.86084966 or ~ 37 times more soluble in H2O than O2 is.



Left C
 

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Discussion Starter · #11 ·
LeftC, I wasn't looking for bubbles in the one gallon of water the tubing was in, just measuring the CO2 flow rate in bubbles per second in the bubble counter. I doubt ever seeing bubbles from the CO2 leakage out of silicone tubing, because that CO2 would almost certainly just dissolve as it comes out.

Fishyface, regular vinyl airline tubing does get brittle with even air use, but more so with CO2 use. But, the silicone tubing I have used doesn't do anything but get slightly more stiff, and that takes quite a few months to notice at all. The table of diffusion constants for various tubing materials shows silicone tubing to be by far the worst material to use for CO2. That is what I was testing
 

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Cool test Hoppy :)

Sure you can use tubing X to get co2 from a-b. But in due time your tubing will harden then split or crack. By the time this happens you will probably have long forgotten that you used tubing X. And your new leak has been born.
Is it worth the chance of having your tubing harden and crack? Not for me, but maybe for some folks.

Cheers, Orlando
 

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Discussion Starter · #13 ·
Cool test Hoppy :)

Sure you can use tubing X to get co2 from a-b. But in due time your tubing will harden then split or crack. By the time this happens you will probably have long forgotten that you used tubing X. And your new leak has been born.
Is it worth the chance of having your tubing harden and crack? Not for me, but maybe for some folks.

Cheers, Orlando
I would never claim that my test shows that this silicone tubing is as good as the CO2 proof tubing made for that purpose. What it does is show that the worries about losing half the CO2 before it even gets to the tank are very much over done. I continue to use CO2 resistant tubing on my CO2 system, except for short pieces, usually in the tank.

Has anyone actually experienced silicone tubing hardening and cracking as you predicted?
 

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The tubing will usually harden at the connection points only, not the length of the tubing.
I would have to cut the ends off to make for fresh malleable tubing every time I cleaned a diffuser. The tubing would turn into a hard plastic almost.
But it still did the job no doubt, until I started using Tygon tubing it was smooth sailing.:)

I wonder what it would take to diffuse co2 in the substrate and if it would be beneficial? Have you thought of trying anything else for this Hoppy?
It sure sounds interesting.

Regards, O
 

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The tubing will usually harden at the connection points only, not the length of the tubing.
I would have to cut the ends off to make for fresh malleable tubing every time I cleaned a diffuser. The tubing would turn into a hard plastic almost.
But it still did the job no doubt, until I started using Tygon tubing it was smooth sailing.:)

I wonder what it would take to diffuse co2 in the substrate and if it would be beneficial? Have you thought of trying anything else for this Hoppy?
It sure sounds interesting.

Regards, O
I concur with this observation. It is not unusual for me to have to clip off the end, maybe 1/2" of silicon tubing every now and then when I used it with my DIY CO2 set up. I also never noticed the hardening in the middle portions of the tube.

If so much CO2 was lost in silicon tubing then the water you have the tubing in should have a lower ph than the tap where you took the water from. Great experiement:thumbsup:
 

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the calculations for diffusion rates are usually done using air at standard atmosphere and pressure. recall that the rate of diffusion depends on the difference in partial pressures of CO2. i suspect immersing the tubing in water changes things a bit.
 

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I concur with this observation. It is not unusual for me to have to clip off the end, maybe 1/2" of silicon tubing every now and then when I used it with my DIY CO2 set up. I also never noticed the hardening in the middle portions of the tube.

If so much CO2 was lost in silicon tubing then the water you have the tubing in should have a lower ph than the tap where you took the water from. Great experiement:thumbsup:

Are we sure we are talking about SILICONE tubing - the clear stuff we get at the store is really a clear vinyl tubing (most people call it silicone because it is clear), and gets hard after a long period of use - this stuff here:

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=6124

Which is almost as good as Tygon (or, according to this, as good or better)...

Real silicone tubing stays flexible for just about forever.
 

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Are we sure we are talking about SILICONE tubing
--snip--
Real silicone tubing stays flexible for just about forever.

I can verify/confirm what others are saying here. I have had blue silicone tubing stiffen from exposure to CO2 and water. The tubing outside of the water was fine, but anything in the water hardened.
 
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