Quote: (Originally Posted by Scubascooby)
Just for info, the Atmosphere is an obsolete unit in Britain and Europe, AFAIK it hasn't been used in diving circles for nearly 40 years.
I'm a fan of proper Imperial measure for day-to-day tasks (pint, miles, yards) but for physics and diving, metric is the tool of choice ;-)
Even Bar should be replaced by Pascals but that is taking things a bit too far.
Peter
No atmospheres IS STILL the standard UNIT for PO2.. All the exposure tables are based on ATAs of exposure... the units of measurement for depth doesnt matter, PO2 is still atas..
Most deco software handles this correctly when displaying the calculated po2.. When using metric measurements PO2 is still supposed to be calculated based on atas..
its not just depth times f02 as is commonly taught.. its close enough but it IS wrong.. depth must be converted to atas (/10 for m is not an ata, but /33 for fsw is)then multiplied by f02.. Its not a big difference but it is a difference..
If you don't believe me, just do a standard OC dive using something like vplanner and see the calculated PO2, when you work it out you will see its in Atas not bar.. This is also why it has an option for CCR to select the units for the controller since not all controllers use the correct units..
Here is a line cut from Vplanner
Level 100m 27:07 (50) Trimix 13/60 1.42 ppO2, 28m ead, 34m end
if it was actually in bar it should be 1.43, and the 1.42 is actually rounded up for conservatism in the overall exposure calculations, the calculated po2 is 1.41 atas
and if you really want to be anal, the standard surface pressure is 1.013bar not 1.0 bar as taught.. so if you really want to calculate it properly for sea level it should be ata absoulte = (1.013 + Depth in meters /10 ) / 1.013
the 1.013 can be substituted for any atmospheric pressure (eg altitude)