CO2 tanks

there are different size tanks you can use for co2: 2.5, 5, 10, 15, 20, 50 lb, etc.
it might make sense to think that the cost to fill a 5 lb tank would be twice as much as a 2.5 lb tank, and to fill a 10 lb tank would be twice as much as filling a 5 lb tank, and so forth
but it turns out that at welding supply shops, the cost to fill a tank that is larger is usually only a few dollars more than for a tank half its size.
these costs are off the top of my head, but at a welding supply shop, the prices were like this:

2.5# - 8 dollars
5# - 10 dollars
10# - 12 dollars
20# - 13 dollars
50# - 15 dollars

so to fill a 50 lb tank, was less than twice the cost of a 2.5 lb tank, yet you get 20 times as much co2.
the '#' sign and lb are interchangable, and the '#' is stamped on the tanks.

when i say 'fill' i actually mean swap for most places. what this means is you take in your empty tank, and swap it for a full tank for the cost of the fill. you will lose your pretty new tank if you do this, but i bought all my tanks used anyway.

also, if you didn't know, the tanks need to be hydrostatically tested at least every five years. this means the tank is filled with water to a certain pressure, and the integrity of the tank is measured (i believe they measure the expansion of the tank and contraction of the tank during and after the water to see the durability of the tank) plus they look for micro cracks that can develop around the valve.

what this means to you though, is if you find a tank used (and i recommend you do! such as on craigslist or ebay - but check for shipping costs), always ask for the most current test date. if it is longer than five years, or close to five years, expect to pay for a hydrotest, and deduct that from what you are willing to pay for the tank. just so you know i bought almost all of my tanks for 20 dollars, and they were all out of test (i have a 50 lb, 3 x 20 lb, 2.5 lb, and used to have a 5 lb). so to get a full tank within test, cost me about 50-60 dollars a tank.
hope this helps!