[PATCH 00/11] Use cylinder index in divedatapoint structure and best mix patches

[Rick Walsh]

On 10 Jul 2016 07:19, "Robert Helling" <helling at atdotde.de> wrote:
>
> I only have two questions or remarks: What exactly does the MND column
display/mean? It seems, for air it’s always 30m (which, as I understand is
the MND that GUE divers aim for). It seems to me, the meaning is something
like „what would be the MND of this dive if the depth were 30m?“. Is that
impression correct? Shouldn’t it rather show the MND at the max depth? So
it would always be max depth if the gas is air?
>
The MND as I understand and intended is the depth at which the total
partial pressure of narcotic gasses (pN2 + pO2 = 1 - pHe) is equivalent to
that of air at 30m, or whatever the END (equivalent narcotic depth)
preference is.  Like MOD at a particular pO2, it depends on the gasmix, not
what the actual depth of the dive is. For air or any nitrox mix, the MND
will equal the END as there is no helium so all gas in the mix is
considered narcotic.

> My other remark is about the renaming of the „Switch at“ column to „Deco
MOD“. I am not sure it is the best possible name. It suggests (at least to
me) that it is the same as the „Bottom MOD“ column, just for the other max
ppO2 value. But it is not. Its meaning is that the planner changes to that
gas at the given depth. It is initialized with the MOD but when the user
changes it it does not mean that this implicitly changes the gas to adopt
the new MOD, it only tells the planner to switch to that gas at a different
depth. Thus I kind of liked the old name better. But please convince me
otherwise.

You are correct - I don't think I need to convince you otherwise. The
difference is unimportant in a typical case and most efficient deco (and
what I was considering when I renamed the Switch at column), when the diver
will want to switch at the MOD. But if there's a reason to switch
shallower, and that's what a diver chooses, the columns behave differently,
as you point out. One will adjust the fO2 while the other doesn't.

You've convinced me that Deco MOD isn't the best name and the column should
be re-renamed. But the problem with "Switch at" is that it doesn't make
clear that it only applies to deco gas. It should be not too confusing and
not too long. How about "Deco switch depth"?

I might have a chance to send a little patch next week but probably not
before. A tooltip or two would be helpful too. By all means beat me too it,
or suggest an alternative.

> Maybe my problem is also that I am not sure what the most common use case
of a user entering a MOD (for subsurface to figure out the best mix) would
be. Would she expect that to be a bottom gas or a deco gas? Should we try
to be clever about this (e.g. only treat the gas with the lest O2 content
as bottom gas or maybe something else)? Of course, when a software is too
clever it is confusing for the user what it actually does. I don’t know.
>
I only imagined entering the MOD to determine fO2 would be useful for a
bottom mix. I can see the idea could be used in some situations for a deco
gas, but I think this would be less commonly helpful. Making it work would
make things too complicated (coding and ui) in my opinion.

Cheers,

Rick
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