[PULL REQUEST] VPM-B Attempt 2.

Robert C. Helling helling at atdotde.de
Sat Jul 4 02:28:27 PDT 2015


Hi,

> On 04 Jul 2015, at 11:23, Rick Walsh <rickmwalsh at gmail.com> wrote:
> 
> I multiplied 7500 fsw by 0.304 (feet to metres) divided by 10 (metres salt water to ata) and multipled by 1.01325 (ata to bar).
> 7500 * 0.304 / 10 * 1.01353 = 231.021 bar
> 
> It's close, but I think my conversion was very slightly out, and assumes a certain density of salt water, which varies slightly around the world.  If I recall correctly, fsw is a "special" American unit of pressure rather than depth.  I think it is actually defined as 33 fsw = 1 ata.  In which case, to convert to bar should be:
> 7500 / 33 * 1.01325 = 230.284 bar
> 
> We should use that number if we take 7500 fsw as the correct value.  2301.021 vs 230.284 makes no perceivable difference to the calculated plan.  But ~230 vs 250 does.  I have updated the patches anyway.

speaking of units: For total clearity, we could use types indicating units as those defined in dive.h for all kinds of things (although for these calculations we might invent new ones to handle doubles).

Best
Robert
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