Settling of mixed gases in tanks

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Instructor in mig-tig class brought up that mixed gases (Ar/CO2, etc) have a tendency to separate out in their tanks after several months if not used due to settling from different molecular weights.  He recommended rolling it around on the floor (after removing the valves and gauges) and even upending if tank is small enough, then let it sit for a while and repeating.Any comments, thoughts, and experiences with this?
Reply:No direct experience here, but, hey when does that stop the flow of opinions, right?  Well, let's see here.  Argon's atomic weight is 18. Co2 would be like 22 (8x2+6) right?  Since Argon never combines, you would eventually get some layering of the gasses, but I have no idea how long that would take.  I am also not sure if shaking things up would put things right in that short of a time period.  I am just imagining air in a car going down the road.  turning a sharp curve doesnt noticably increase the air movement in a closed car...can that be applied to a cylinder???  Maybe, maybe not.  I am just guessing here, but it sounds a bit fishy, but on the other hand what would it hurt?Smithboy...if it ain't broke, you ain't tryin'.
Reply:well i dont have that problemdoes anyone else use a gas mixer?smith equipment makes some awesome ones..they cost about a grande but well worth it! ...zap!
Reply:Would a hobby guy notice if the gas did seperate to a small extent? I buy what I think is C25, they could probably sell me 35% or 15% and I would associate any problems with myself or the wire, adjust and go on. However, the only thing that ever sets several months un-used around here is a weedeater. Back to the instructor tho, he is probably right. No reason to doubt, just never been and issue.
Reply:sounds like bull to me i have never heard of such maybe  if it sat for years and years and that is still maybeChuckASME Pressure Vessel welder