[PATCH] MOD of oxygen at pO2 of 1.6 is 20ft

[Rick Walsh]

Hi Robert,

On 5 July 2015 at 16:18, Robert C. Helling <helling at atdotde.de> wrote:

> Hi Rick,
>
> > On 05 Jul 2015, at 01:41, Rick Walsh <rickmwalsh at gmail.com> wrote:
> >
> > <0016-MOD-of-oxygen-at-pO2-of-1.6-is-20ft.patch>
>
> I am not really familiar with diving in imperial system but this patch
> seems quite specific. Could somebody please check that this problem really
> only affects the MOD of EAN100 and not other standard gases? I checked
> EAN50 (21m = 70ft) but I don’t know what people expect. For example what do
> imperial users list as MOD of EAN36 (being a quite common gas), 100ft or
> 110ft? According to our formula it is 108ft.
>
> I don’t know what to do is right. Somehow, for deco calculations it seems
> 1ft=30cm exactly (which would give 10ft/3m deco step intervals) or we could
> fudge a bit with the maximal pO2 in imperial mode so it comes out like this.
>

I've never dived using feet to measure depth either.  Perhaps we can ask
one of our friends from Liberia, Burma, or that other country that still
would rather divide by 3, 12 and 1760 than 10 and 1000.  But who are we to
judge?  There's nothing special about 3 m, we're the muppets using ~10ft
deco stop increments when we don't even measure in feet.

But, we should do something, calculating the default MOD of oxygen as 10ft
goes against expected behaviour.  At a minimum, we should fix it for this
gas, because it is commonly used.

One possible fudge would be to say that 1.60 bar = 1.62 bar.  I thought
about it, but don't really like it.  I'm unlikely to dive with anything
other than 100%, 80% and/or 50% nitrox for deco in the near future, but I
believe the following is a list of fraction O2 for standard/common deco
gasses, and calculated MOD based on pO2 = 1.6 bar, and usual/standard
adopted MOD for the gas.

  Mix MOD (m) @1.6 MOD (std) (m) MOD (ft) @1.6 MOD (std) (ft) pO2 at std ft pO2
at std m  100%* 6 6 19.7 20 *1.61* 1.60  80% 10 9 32.8 30 1.53 1.52  50%* 22
21 72.2 70 1.56 1.55  36% 34.4 33 113.0 110 1.56 1.55  35/25* 35.7 36 117.2
120 *1.62* *1.61*  32% 40.0 39 131.2 130 1.58 1.57  21/35* 66.2 57 217.2 190
*1.42* *1.41*  *GUE standard mix

My patch works for 100%, but not for 35/25 or 21/35.  Looking at that
table, it appears the best rounding method would actually be "round to
nearest 3m / 10ft", rather than rounding down as we currently do.  No
amount of rounding will give 21/35 an MOD of 57 m / 190 ft.

Another option would be to have special cases for gas with 100%, 35% and
21% oxygen.

Cheers,

Rick
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