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John,

> But who made the standards ?  The only way I can see the rolling
> method work is because the gas layer right next to the wall adheres
> to the wall weakly but is this effect even significant relative
> the thermally driven random motion(mixing)  ?
> 
The standards are set by the US Compressed Gas Association.  I dunno the 
physics behind their recommendation.  However, for passive (thermodynamic)
mixing, they recommend gases sit for 12 hours, or be rolled for 1 hour.  So
it seems to me that their standards assume rolling is better by 12-1.

Disclaimer - the 1 hour and 12 hour numbers are being pulled out of rusty
file drawers in my head, the actual numbers may not be exactly that.  I'll
look 'em up if anyone is really interested.

BTW, I'd recommend the handbook of the CGA for anyone interested in doing
their own gases, its fascinating reading. (Not really, but hey...)
--
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