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

Dirk Hohndel dirk at hohndel.org
Sun Jul 5 06:09:04 PDT 2015

> On Jul 5, 2015, at 4:29 AM, Rick Walsh <rickmwalsh at gmail.com> wrote:
> 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.

EAN36 has a MOD of 114ft and that's what people would write on their tank / regulator.
1.6 / 0.36 = 4.44
3.44 * 33 = 113.52 -> 114ft MOD 

> 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.

I learned diving in one of those retarded countries and while I grew up with the metric system I still tend to use and think imperial when diving.

> 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.

Special cases seems wrong. That's what I didn't like about your patch.
But what I've seen in diver training and in blender training (I'm a certified trimix diver and trimix blender) is that you round to the nearest foot. Not truncate.
So this gives you 20ft for 100%
I don't know of a blender who would write down 120ft as MOD 35/25, nor of someone who'd suggest 190ft for 21/35. So these I would classify as GUE oddities.
I'll happily take a patch that rounds to the nearest foot in imperial and that should solve the one annoyance we currently have.


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