# Combining CO2 cylinders?



## NeonFlux (May 15, 2008)

Have you guys ever attempted it? Is it possible? Say if you want a 40 lb, but don't have the space for it, would it work to combine two 20lb cylinders somehow? Say if you want 20 PSI, you would turn both cylinders to 10 PSI, etc.. any thoughts into this?


----------



## jmhart (Nov 13, 2007)

pressure isn't additive like that, 10 psi + 10 psi doesn't equal 20 psi, it equals 10 psi.

So......if you have all the right parts this can be done, but really shouldn't be done by an amateur. 

First off, there's really no point. A 20# tank will last a long time, so trying to double it to 40# is overkill. If that's not a good enough reason, I understand, sometimes you want to do things just for the sake of doing them even if they don't really benefit you. 

Secondly, IF you connected both tanks on the high pressure side, they would equilize in pressure. This can be a problem for two reasons. A)you can get blow back into the lower pressured cylinder which can contaminate it, with the potential to later break your regulator. B)if they cylinders are not both the same, for example one 5# and one 10#, you can damage the smaller cylinder and/or put yourself in a dangerous situation because 5# cyilinders cannot hold as much pressure as 10# cylinders.


You can connect them downstream(low pressure side), but that would require two separat regulators, and 10 psi + 10 psi would only equal 10 psi. Point is, why do it? Both cylinders will last the same amount of time as if they were hooked up completely independently.


Back in the day, SCUBA divers used to dive with two tanks all the time, to increase the amount of air they took down with them. But that helped them double their dive time from say 20 min to 40 min. We're talking about a 20# cylinder lasting 18 months or longer. What's the advantage?

I remember a while back you were trying to split your one cylinder. If now you have mulitple cylinders and regulators, I'd aim to just use each reg/cylinder independent of the other(s)


----------



## helgymatt (Sep 12, 2007)

Isn't it easy enough just to change out one empty 20# tank with the _extra_?


----------



## m-jackson (Dec 23, 2008)

if you are worried about run out I guess with two cylinders and regulators you could set one just a little lower and it probably should not be used until the other one empties.


----------



## josh1604 (May 7, 2008)

what i do is have my main tank and then a much smaller 5 lb tank that i will replace the big tank with when it needs to be refilled


----------



## NeonFlux (May 15, 2008)

Sorry guys, I completely forgotten about this thread...


My 20 lb is going to last me only about 3 months believe it or not lol... I have it on two tanks. 1 is on a 60 gallon and one is on a 10 gallon.


----------



## Seattle_Aquarist (Mar 7, 2008)

Hi NeonFlux,
You may want to check for leaks, I provide CO2 to two tanks (a 45 and a 30) and my 5# tank lasts 3 months.


----------



## jmhart (Nov 13, 2007)

yeah, leak or wicked crazy surface agitation/poor diffusion method. A 5# tank last 2-3 months feeding 2 45g tanks for me where I feed into the intake of my eheim 2128.


----------



## NeonFlux (May 15, 2008)

Man are you guys serious? What if I only get a refill gas of only 500 PSI, which is half..the darn paintball shop is only weighing or something


----------



## Seattle_Aquarist (Mar 7, 2008)

Hi NeonFlux,
Three months per 5# fill for my two tanks has been constant for about a year and four fills, definitely serious. Weighing is certainly acceptable. If you weigh your 20# tank when empty, then have it filled, the filled weight should be 20 pounds greater than the empty weight. If your current filling location can't do that, find a new place to fill.


----------



## BryceM (Nov 6, 2005)

For comparison, a 20# tank lasts me between 4 and 6 months on a 180g tank. Those small leaks can really be a pain to track down. Also, look at getting CO2 specific tubing. You can loose quite a bit through regular airline.


----------



## Bonsai_Swamp (Oct 14, 2004)

NeonFlux said:


> Man are you guys serious? What if I only get a refill gas of only 500 PSI, which is half..the darn paintball shop is only weighing or something


At room temperature, 500 psi is pretty close to empty, not half full - there's no liquid left in the cylinder, just gas. Either the shop screwed up the fill or the valve leaked before you put the regulator on.

You can't use pressure to gauge the amount of CO2 in a cylinder - most of the CO2 is stored as a liquid at high pressure and that makes pressure gauges a bit misleading. A full cylinder will start out at around 900 psi at room temperature with almost all its CO2 sloshing around the bottom as liquid. During use the CO2 liquid boils off and keeps the gas pressure nearly constant, dropping very slowly towards 700 psi. Once all the liquid has been converted to gas the pressure drops very rapidly. My main tank would take several months to drop to 650-700 psi, then drop to 0 within a week.

Manifolding two CO2 cylinders together is simple - you need a CGA-320 pigtail and a CGA-320 manifold T. The centre of the T screws onto one of your cylinders, the pigtail connects one side of the T to the other cylinder and your regulator screws onto the remaining side of the T. A welding supplier would probably have this stuff in stock or be able to order it. Putting it together is like attaching your regulator to the cylinder - same type of connectors.

There's a couple of problems with this approach:

1) You now have three more CGA-320 connectors in your system with those $#!#%##!! fibre seals looking for an excuse to leak.

2) Both cylinders will empty at the same time. If you get busy and don't get in to refill them for a while, will the algae take over your tank in the meantime? I prefer to keep my second cylinder disconnected from the system and swap it in once I notice a pressure drop on the main cylinder. If I don't get in to the fire extinguisher shop for a couple of weeks no big deal, the CO2 is still flowing uninterrupted.


----------



## jmhart (Nov 13, 2007)

I concun with Bonsai on this. 

As far as tubing in concerned, I wish I could find a good permeability table(Cole Parmer used to have one, but I can't find it now). If you look at the permeability of co2 through your commonly available types of tubing, the permeability is certainly lowest with something like Tygon FEP. However, when you actual do the calculation, where permeation is a function of differential pressure across the membrane, or tube in this case, and factor in that our tubing is at very low pressure 10-20 psi upstream of the needle valve, most like ~1.1 psi downstream of the needle valve, the loss of gas through the tubing is extremely minimal. Over the course of a year, it's way less than $1's worth of gas. 

Point is, no need to spend a whole lot of money on tubing because even co2 safe tubing degrades eventually, and by the time it degrades, it wouldn't have paid for itself.


----------

