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Old 02-12-2013, 02:15 AM   #1 (permalink)
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Using Big CO2 tank to refill smaller one

Hi everyone

If I want to use a big CO2 Tank and connect it to a smaller Co2 tank via 2 asa and a stainless steel line, can I use that big CO2 tank to refill the smaller one?

Reason being my local field Camp Pendleton charge $5 per fill regardless of size and a lot of my gun use the smaller 3.5 oz tank.

Thanks
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Old 02-12-2013, 02:21 AM   #2 (permalink)
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If you weighed the smaller co2 tank I don't see why you couldn't. Oh and you would have to hold the big one upside down. If I'm worng someone correct me.
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Old 02-12-2013, 02:32 AM   #3 (permalink)
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Is there a risk of overfilling the smaller tank by not weighting it? I figure that if it get full the gas would just balance it self out?
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Old 02-12-2013, 03:48 AM   #4 (permalink)
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Like any other co2 tank you would have to weigh it to make sure it is filed properly. There's a reason they are called 20oz, 3.5oz etc. that's how much co2 by weight is inside. So weigh the tank before filling then just add that much co2 to it. If not it will fill too much.
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Old 02-12-2013, 09:02 AM   #5 (permalink)
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I would use 2 on/off ASA's, A Palmers 1/8" on/off valve, T-Fitting, and your braided steel hoses.

The idea is to simply put a valve to vent the pressure in the line after turning off the ASA's.

At $5 a fill, I'd build the same and use my 48oz tanks to refill the others lol.
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Old 02-12-2013, 09:33 AM   #6 (permalink)
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Anyone remember the average filled weight of a 3.5? I'm thinking it was 200something grams, so, around 8 oz?

Pk, I'll try to remember to check mine. It's best if you know what size of scale to get. I have some old copier toner scale in grams. Cheap and small and still works without batteries. I write the empty weight and fill weight on the bottom with a paint pen. I haven't filled my own in a long time, but I still check CO2 fills that I get at events, always when they don't use a scale. My home field does great fills under the "all day air" fee.

Too little puts you on the field with little to no gas. Half a fill on a 3.5 is pretty weak. Too much can blow your rupture disk and then you really have no gas.
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Old 02-12-2013, 12:30 PM   #7 (permalink)
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You could just go the even cheaper route of getting a bulk tank and filling it at your house. I think it works out to $1 to fill my 3.5 oz each time.
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Old 02-12-2013, 12:57 PM   #8 (permalink)
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A 20 ounce will not over-pressurize a 3.5 ounce tank. If that was the case, both my dual tank VM-68 and my Tippmann Mini Lite would blow out the 3.5 tank when I run my 20 ounce on back and 3.5 ounce up front.

In fact, both tanks having on-offs valves, I used to turn off the 20 when I played, then shoot through the 3.5 till the gun burped. Once people heard my gun burp usually a few people rush me thinking I was out of air, but I would turn on my 20 and blast them. Worked well for a few games till people caught on.

The pressure simply balances itself out.

That being said, bulk CO2 tanks that are meant to fill smaller ones have much higher pressure and usually have siphons to force the liquid into the smaller tank to fill. That is why you must always weigh tanks when you use a fill station.
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Old 02-12-2013, 01:40 PM   #9 (permalink)
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Quote:
Originally Posted by jason_finnie View Post
A 20 ounce will not over-pressurize a 3.5 ounce tank. If that was the case, both my dual tank VM-68 and my Tippmann Mini Lite would blow out the 3.5 tank when I run my 20 ounce on back and 3.5 ounce up front.

In fact, both tanks having on-offs valves, I used to turn off the 20 when I played, then shoot through the 3.5 till the gun burped. Once people heard my gun burp usually a few people rush me thinking I was out of air, but I would turn on my 20 and blast them. Worked well for a few games till people caught on.

The pressure simply balances itself out.

That being said, bulk CO2 tanks that are meant to fill smaller ones have much higher pressure and usually have siphons to force the liquid into the smaller tank to fill. That is why you must always weigh tanks when you use a fill station.
Over Pressurize and over fill are not the same when dealing with Co2. A 20oz tank can easily overfill a 3.5oz tank. You fill a Co2 tank with liquid Co2, and to get it to accept a full fill you usually have to chill it somewhat. If the tank being filled is chilled properly you can easily over fill it, and when the tank warms up it can then over pressurize and blow the burst disc.
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Old 02-12-2013, 02:01 PM   #10 (permalink)
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Quote:
Originally Posted by jason_finnie View Post
A 20 ounce will not over-pressurize a 3.5 ounce tank.
Yes, of course it can !!!
All co2 tanks need 20% headspace in the vessel in order for the liquid co2 to reach pressure equilibrium. (ie 800psi at 70 degrees). Without that 20% "empty space", the liquid will go supercritical, and you will likely blow a burst disc.

If you use the simply method of hooking up the 20oz to the 3.5, and letting gravity fill the 3.5oz, it will likely get overfilled. THIS IS FINE as long as you weighed the 3.5oz empty, then weighed it full, then emptied it to the proper weight right away.

I once worked at a field that filled tanks via the "gravity method", which was similar. Every tank would get overfilled if we were not carefull.

Quote:
If that was the case, both my dual tank VM-68 and my Tippmann Mini Lite would blow out the 3.5 tank when I run my 20 ounce on back and 3.5 ounce up front.
What?
CO2 is stored as a liquid, and needs at least 20% headspace. The location of the tanks is irrelevant. Though, if you pointed the gun down, and let all the liquid flow into the 3.5oz, then shut off the 3.5oz, it would likely go supercritical and blow the burst disc.

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That being said, bulk CO2 tanks that are meant to fill smaller ones have much higher pressure and usually have siphons to force the liquid into the smaller tank to fill.
No.
The pressure in any vessel that contains liquid co2 is the same based on temp. At 50lb tank has the same pressure as a 3.5oz tank.

You fill tanks via thermal variance. Basically, you chill the target tank because the lower temp will lower the pressure, and siphon in liquid from the "warmer source tank" which has higher pressure.

This is the reason why you need to chill the tanks. Ever tried topping off a nearly empty co2 tank? you can't since the pressure has equalized.
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