Co2 Tanks

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I'm new to welding and want to hook up the Hobart 140 to gas. My local Craig's List has Co2 tanks for paintball guns and beer, others with no description at all, some are steel and others aluminum. Are there tanks that can and can't be used with a welder? Does the valve threading vary by intended use? What do you look for when purchasing a used tank ... or just go to the local welding supply store and bite the bullet?

Would a tank like this be appropriate for welding ... http://philadelphia.craigslist.org/tls/5244429865.html