Possible Bug with Stats and Info

[Ryan McLean]

@Linus,

Thank you for taking the time to educate me instead of leaving me in
ignorance, I honestly do appreciate it. While i figured things wouldn't be
exact, I thought it would be as near as no matter and i would never have
thought that the margin of error could get quite so large.

@Willem,

I realize they don't happen exactly as intended, i was just thinking given
the intention of the diver we could replicate that intention based on the
dive time & overall SAC rate for the purpose of visualization.



On Tue, May 9, 2017 at 12:35 AM, Willem Ferguson <
willemferguson at zoology.up.ac.za> wrote:

> The schedule for changing sidemount cylinders depends on your training.
> Many would agree that keeping them within 30 bar is a good policy to
> maintain redundancy of gas. Cylinder changes are almost never at exact
> multiples of pressure, so one needs to enter the cylinder changes by hand.
> If you do  not have air integration, that means writing down the *times* of
> cylinder changes on a slate or similar. A computer program cannot
> automatically recreate the schedule of changes.
>
> On 09 May 2017 02:30, "Linus Torvalds" <torvalds at linux-foundation.org>
> wrote:
>
>> On Mon, May 8, 2017 at 3:23 PM, Ryan McLean <pvtryan100 at gmail.com> wrote:
>> > Attached log.
>> >
>> > Would a semi-correct model be to allow a the user to specify the 1st
>> switch
>> > in bar so to take 35bar as an example on 2 cylinder charged to 200
>> > 1st switch is at -35bar (165bar remaining)
>> > 2nd cylinder then at 2x35bar (-70bar) from full (130bar remaining)
>> > 1st cylinder then at -70bar from last use (95bar remaining)
>> > ...
>> > continue until end of dive.
>>
>> I suspect we should aim to have a model where we can just mark both
>> cylinders as used at the same time (this is not just a sidemount thing
>> where people alternate regulators - it's also true of oxygen vs
>> diluent for example).
>>
>> But right now that's now how we track gas usage, and it would probably
>> be pretty painful to implement.
>>
>> Right now you are realistically forced to either consider it a single
>> dual cylinder manifold, or you'll have to manually add a fake gas
>> switch event somewhere in the middle of the dive or so.
>>
>> > I've probably not understood gas compressibility correctly but I took a
>> 10L
>> > cylinder to hold 2000L at 200bar. So if I had used 80 bar in each
>> cylinder
>> > then I had used 1600L total...
>>
>> Yeah, no.
>>
>> It's an approximation, but it's really not a particularly good one.
>>
>> First off, 200 bar is not 200 atm, although it's close. It's about 197.4
>> atm.
>>
>> So there's a 1.3% error there.
>>
>> But more noticeably, the compressibility factor of air at 200 bar is
>> 1.036 - it's not an ideal gas. So that's an additional error of the
>> magnitude 3.6%, and it's in the same direction as the bar-vs-atm one
>> (ie both errors are in the direction of less real air at STP).
>>
>> So 10L at 200 bar is actually only 10*197.4/1.036 L of air at STP. So
>> about 1906 L.
>>
>> And 120 bar is 118.4 atm, and at that point air still acts pretty much
>> like an ideal gas, so a compressibility factor of 1.0.
>>
>> So 10L at 120 bar is 10*118.4 L at STP, so 1185 L.
>>
>> So the amount of gas you used (in one cylinder) is 1906-1185=721L.
>>
>> Note that the small percentage errors all ended up being bigger due to
>> (a) being in the same direction, and (b) mainly being noticeable at
>> the higher pressure, so even though we're talking about low
>> single-digit percentages for the compressibility factor (and even less
>> for the bar-vs-atm factor), you ended up with about a 10% error
>> between the rough approximation (800 L used per cylinder) and the more
>> accurate one (721 L used per cylinder).
>>
>>                Linus
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>>
>
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