[PULL REQUEST] VPM-B Attempt 2.

[Dirk Hohndel]

On Sat, Jul 04, 2015 at 07:23:12PM +1000, Rick Walsh 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

Yes, I believe that's the right formula.

BTW, Jan: if you need conversions look at what we have in units.h - and if
you need something that isn't there yet, please add it there as a helper
function. That makes things so much more readable, and if you do make a
mistake it's easy to fix it in one spot.

> 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.
> 
> You may be interested to see that HHS Software (V-Planner/MultiDeco) say
> they use critical N2 and He radii of 0.8 and 0.7 with VPM, and 0.55 and
> 0.45 with VPM-B.
> https://www.hhssoftware.com/images/critrad.gif
> 
> They have defined conservatism levels of +1, +2, +3 and +4 as increasing
> the critical radii by 5%, 12%, 22%, 35%, respectively.
> https://www.hhssoftware.com/v-planner-cde/faqhelp.html#config

Rick, thanks for finding this information. That's extremely helpful.

/D