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Your formula isn't wrong... but I'm not convinced it's easier.  You 
are essentially saying:

  deltaP = finalP - (((1-FO2) * finalP ) / .79)

which can be transmogrified in a few steps to the more common:

  deltaP = finalP * (1 - ((1 - FO2) / .79))
  deltaP = finalP * ((FO2 - 1 + .79) / .79)
  deltaP = finalP * ((FO2 - .21) / .79)

  deltaP = ((FO2 - .21) / .79) * finalP

I think this last formula is easier to type into the calculator, in 
particular because the final pressure only needs to be entered once
and because you don't have to mess around with either parenthesis or 
the +/- key.  (ignoring RPN to appease the non-mathgeek majority)

You could argue that your formula is easier to derive, but does 
anybody really derive it each time they fill?  Sure, you should 
be able to sit down and do it without racking your brain terribly, 
but I suspect it's better to just have the deltaP formula scrawled 
prominently on the shop/garage wall.

- Todd


>  From: CHK BOONE <CHKBOONE@ao*.co*>
>  Date: Thu, 12 Mar 1998 09:44:12 EST
>  To: techdiver@aquanaut.com
>  
>  All,
>  
>  I've always wondered why everyone teaches nitrox mixing by calculating how
>  much O2 you need to add to the tank.   Why not simply calculate it on the
>  basis of how much N2 you need ?     For example :
>  
>  For a 36% mix -->
>  
>  You need 64% N2
>  64% of 3000 is         1920 psi of N2
>  1920 = 79% of X
>  X = 2430 psi of air to add on top of . . . .
>  3000 - 2430 = 570 psi of O2 
>  
>  Much easier than trying to deal with two different percentages and much less
>  chance of an error.    
>  
>  This example is, of course, ideal and you might want to throw in a tiny fudge
>  factor where compressibility differences enter the picture with some mixes
>  and, of course, you may need to consider the residual gas components as well
>  if you don't flush the tank at the start.   
>  
>  Chuck Boone
>  --
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>  
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