# [PULL REQUEST] VPM-B Attempt 2.

Rick Walsh rickmwalsh at gmail.com
Sat Jul 4 02:23:12 PDT 2015

```Hi,

On 4 July 2015 at 18:29, Jan Darowski <jan.darowski at gmail.com> wrote:

>
> eh... from deepocean.net: "7500 fsw min = 250 bar min"
> It's not the first mistake I found there. And it seems that the author
> of existing
> c code based his implementation on this site also.
>
>
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.

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.

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

Cheers,
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