I as because I recently came across a screaming good deal on 4 20lb aluminum nitrogen cylinders. Screaming good as in free. I can have the valves changed and fill them with CO2 (already talked to a welding supply company about it) for $35.00 each. I was wondering why not just use N2? Maybe the refill price for N2 is much higher, I haven't checked that, but I can't think of any reason N2 wouldn't work as well as CO2. Any ideas?