Refilling CO2 tank from another tank

If I have a 20lb tank and a 5lb tank, can I use the 20lb tank to fill the 5lb tank when it is running low?

I think you'd need a dip tube in the 20lb tank in order to transfer Co2 as liquid. Or hold it upside down, which seems a little dicey!

Probably more trouble then it's worth. IMO. Have never tried it.

I think with the correct high pressure fittings you could connect them, gas dynamics are such that the pressure between the two would equalize, but I don't think you could get a full 5lbs into the tank and with each progressive fill the amount going into the 5 lber would decrease: Hypothetically, no idea if it will work, if you have a full 20 lb tank and connect a 5 lb tank without additional pressure they should equalize out with about 16 lbs in the 20 lb tank and 4 in the 5 lb tank, then next time it will be 3 and 13 and so on as the pressure is equalizing between the two, and you are not filling the tanks.

Just be aware there has to be a lot of pressure going from one tank to the other to fill them and there may be restrictors on the valves preventing you from doing this, also whatever you use to connect the two canisters needs to be able to hold a LOT of pressure, and don't think normal pipe fittings will do it, they probably won't. but I wish you the best of luck in your endevor, let me know how it works out, as I have contemplated doing this several times.

I have to agree that it's probably more trouble than it's worth, and probably somewhat dangerous as well. I'd rather just head down to the local tavern services shop and plop down my $10 for a 5lb refill. I've got three tanks, so there is always enough Co2 on hand to carbonate and dispense.

yes it can be done. I have a portable 4.5 lb tank and the machinist where I work has told me to bring it in when I want free refills. I mentioned the liquid/gas argument and he told me its no problem to lay both tanks on their sides to do the transfer. Since his is a 60 lber the valve on his will be higher when both tanks are on their sides. I'm going to do it, but plan on being in another building for the first transfer.

Donasay said:

if you have a full 20 lb tank and connect a 5 lb tank without additional pressure they should equalize out with about 16 lbs in the 20 lb tank and 4 in the 5 lb tank, then next time it will be 3 and 13 and so on as the pressure is equalizing between the two, and you are not filling the tanks.

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I think you're missing one of the more basic principles of liquefied gas.
a c02 tank will always be around 800 PSI (at 70 degrees) if there is liquid in it...right up until it spits out the last few puffs of co2. That is why your pressure gauge is a piss poor indicator of how much gas you have left. If you make sure the valve is below the fluid level...ie inverting a standard tank...you will fill the other bottle with LIQUID C02. It will be forced into the small bottle at 800 PSI Every time. Also be aware that you can get that 50# tank with a siphon tube. It virtually works like a corny. It has a tube that goes to the bottom of the tank so you don't have to invert the bottle to fill another. This is how nearly every brew store fill your 5# bottle, and also how every paint ball supply on the planet fills the little c02 tanks for paint ball guns.
It is also worth mentioning that if you screw around and run liquid co2 through your regulator, it may be the last thing you do with that regulator.

I think you all are missing the fact that the cylinder is designed to hold a weight of fluid and not a volume. If you fill it by direct transfer you run the risk of overfill which at the very least will cause pressure release to vent, worse if your release is faulty (big if probably) burst bottle. This is not to say it can't be done you just need to weight the bottle while you transfer so you know how much goes in. Also the only way you will probably get a full charge is to prechill the bottle with a quick purge of co2, put in a pound and let it out quickly. That being said, direct gas cylinder to cylinder is probably ok you just won't get much. So don't transfer liquid without a scale.