Filling a 20 cf cylinder

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Cheap as I am, thought about purchasing a small 20 cf cylinder and filling it from my 80 cf.  Cannot think of any reason why this would not work.  Would need to connect with fittings rated same as the tank.  Fill for the 80 is $24 so 1/4 that volume would be $6, but they probably charge $20.  Its the principle, not the dollars.  No, got that wrong, its the dollars .  Fill slowly until equalized; would probably squeal until that happened.  So, any reason not to do this?
Reply:Um, why? mobility?
Reply:As the pressure goes down in your 80cf cylinder, the 20cf cylinder the pressures are eventually going to have to equalize, so you will be refilling more and more. yes it would sort of make sense, but not really, seems like more work than it is worth unless you need the mobility like sillypower said.Miller Dynasty 300dxSpeedglas 9002XMiller Spectrum 375 XtremeLincoln PowerMig 255xtLincoln PowerMig 140cMilwuakee Portaband
Reply:Originally Posted by Chris BarCheap as I am, thought about purchasing a small 20 cf cylinder and filling it from my 80 cf.  Cannot think of any reason why this would not work.  Would need to connect with fittings rated same as the tank.  Fill for the 80 is $24 so 1/4 that volume would be $6, but they probably charge $20.  Its the principle, not the dollars.  No, got that wrong, its the dollars .  Fill slowly until equalized; would probably squeal until that happened.  So, any reason not to do this?
Reply:Yes it will work, but there are a few cautions. For O2 it's just a bad idea. You have to keep the flow rate super low and the whole assembly needs to be O2 clean and stay that way. That means basically a dedicated whip for just O2. Take a look at the post on the tank transport to see what can happen if you don't take O2 seriously.Theres a bit of complicated math that will let you figure out how much will go from tank A to tank B. It's not quite as simple as you just take 20 CF from the 80. I crunched the numbers and IF the 80 is 100% full and the 20 is 100% empty and the cylinders are rated at the same pressure, you would end up with about 15 cf in the 20 cf bottle. If you try and fill off the remaining 60 CF and the 20 is empty, figure you will end up with about 10 cf in the 20. After that it's basically useless. The next fill is like 5psi. All this assumes you fill very slow to keep the heat down to the minimum, and have no loss thru leaks. If you fill fast and the 20cf tank heats up a bit you can drop your final fill pressure by up to half that of a cold fill.Now if you wanted to fill from say a 280 cf tank instead, then it has some value as you would get more fills.Just a thought for you, I build whips designed to do exactly this for transferring dive gas. ALL the fittings have to be rated for HP gas. That means no fitting from say Home Depot, nipples couplings etc. To do a very basic argon to argon whip, my basic cost for the special HP hoses, HP connections and the argon fittings is probably close to $100+. Thats quite a bit of money to recoup getting a 3/4 fill and then a 1/2 fill on a 20 cf tank. Add in the cost of a 20 cf bottle and you will need to do a ton of fills to ever get even close to breaking even. If you are interested in one you can PM me and we can talk..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Portability is the reason, and the only reason.  On trips I take air compressors, numerous tools etc, and now the opportunity to take a welder as well, just in the off chance I might need it.  Bought the MM211 since only 110 v. was available.  Yes, I understand the capacity issue, and your evaluation is correct.  So, there are no other problems which have not been considered; its just a matter of cost.  Shortly I will be buying a 20 cf cylinder for stainless only and thought until that was needed, I would just fill from the larger bottle.  Both bottles would be privately owned and not exchanged, so that concern is valid but not applicable.The smaller bottle would later be filled with Tri-mix, so I assume (perhaps in error) that the same cylinder can be used once it is emptied, that the LWS would not have an issue to fill with different mix.  For the infrequent occasions that I would need the tank, filling at the gas farm would be the best option and forget filling at home.  For a closure, how is Tri-mix for plain steel MIG, other than $$$...i.e. if I carried the small cylinder stainless mixture instead of 75/25?And thanks for your thoughts and suggestions.
Reply:Hey ChrisWell if portability is the main concern.... Get a 5 or 10 POUND Co2 tank. When I need MIG on the road thats all I use. Yes you'll have a tiny bit more spatter but you may not even notice it. I forget what the conversion rate is liquid to gas but you'll be impressed.