What size C02 tank?

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I bought this lincoln 180 wleder and plan on using it for sheetmetal repair.  I want to get a bottle for it but have no clue what size would be good.  This welder will be mainly weekend use.  Assuming i find a place that will refill used bottles would a 20lb be ok?  I notice everyone measers in cuft but all the bottles on craigslist are listed in lbs.  Any help please?I.B.E.W. Electrician Local 213
Reply:"Normal" Industrial CO2 is 50 lb. Which will give about 8 hrs of arc time. i would thinka 20lb would be adequate, but if you are going to do sheet metal work, I would advise a Ar/CO2 mix. 75/25 is common but a 90/10 does an excellent job with short circuit transfer on sheet (reduces burn thru) and if your machine has enough amps, will allow you to go into spray transfer for heavier work 3/16 and above. Down side is you will pay more for either than CO2.
Reply:20# of Co2 will take you through about 80# of .035" E70S6 wire on steel.Co2 is the only gas you need for steel unless you don't know how to weld, or like to make and use excuses for your lack of ability.  Co2 is also a hell of alot less expensive than the unnecessary 75% argon mixed gas, yields more heat for the electrons expended in the process, and you can claim you're helping in the battle against greenhouse gasses by consuming Co2.  Now lets see how many weekend weldebators that pissed off!
Reply:Gases are measured in cubic feet (volume based measurements).CO2 by itself, compressed in a cylinder, is stored in liquid form (under pressure), so it is measured in either weight, or liquid volume (we normally express CO2 in weight, but propane, which is also a liquid under pressure, is sometimes measured in gallons).Now, at standard temperature and pressure, an ideal gas takes up around 22.4 liters, per mol.  CO2 has a molecular weight of 44, so 50lbs of CO2 is 516 mol, which would be just under 410 cubic feet.At 20CFH, that should give you over 20 hours of welding.If you're using only straight CO2, a 20lb should last a while.  BUT, when you switch to a mix, you get about one fourth the capacity in a similar sized cylinder, so with a mix, you'll want a bigger tank.
Reply:So for sheetmetal only i should be using Co2 correct?  I don't plan on welding anything else than steel.  I don't know how to weld so i will be doing lots of practice.I.B.E.W. Electrician Local 213
Reply:For sheet metal you will probably be better off with a 75/25 mix. You should be able to also weld sheet metal with straight co2, just a little trickier. Co2 will be cheaper in the long run if you do any good amount of welding. I am from the fraser valley also; what company are you planning to get your gas from?
Reply:I have no idea.  Kms sells tank for almost $270 and fills for about $50.  Haven't looked around yet...I.B.E.W. Electrician Local 213