Log import of Trimix dive from Shearwater Teric

[Attilla de Groot]

On October 8, 2021 19:46:03 Linus Torvalds <torvalds at linux-foundation.org> 
wrote:

> On Fri, Oct 8, 2021 at 2:24 PM Attilla de Groot <attilla at attilla.nl> wrote:
>>
>> (have to use less air on my travel gas though, my SAC rate was a 1L higher 
>> than a dive to 70m yesterday).
>
> Hmm. You have "AL40" as the type of all your cylinders except for the
> main one. But then you've set the sizes to something else by hand.
> Which looks a bit odd (so your air cylinder is 4.1l, while your deco
> cylinders are 5.5l).
>
> I wonder how much of your SAC rate difference might be due to possibly
> incorrect cylinder sizes?
>
> I assume your main back gas is a dual AL80 or something like that?
> That's kind of what the 22.2 L size implies.
>
> Normally I'd assume that the 5.5L is a AL40, and the 4.1L is an AL30.

On this dive I indeed used an a double AL80 as back gas, 2x AL40 as deco 
and an AL40 for travel gas.

> But the exact wet size ends up being very manufacturer-dependent, the
> "40" in AL40 is really just approximate (and who knows what working
> pressure it is at, and whether it took gas compressibility into
> account).
>
> A Catalina AL40 ("S40") has a wet size of 5.8 liter, for example,
> while the AL30 ("S30") is 4.3 liter. But the work pressure on those is
> 3000 psi (207 bar), and you seem to have specified yours as 232 bar
> cylinders, so maybe yours are steel?

I'm sure these are all aluminum and I thought they had a max working press 
of 232,but I would have to double check that tomorrow. I am always confused 
about the non-metric system, so this can be on me.
>
> NOTE! The tank sizes that are built into subsurface are a bit random.
> I know I took _some_ of the sizes from actual technical documents (I
> think mostly Catalina, but maybe Luxfer?), but there's no real logic
> to it all, and different manufacturers use different actual sizes.

Would it be possible to add an AL40 and double AL80? They seem to be 
standard available in tropical areas at least.
>
> If you care deeply about SAC rates, you should check what your actual
> cylinder wet size is in liters, and set that explicitly. The imperial
> "cuft of gas at surface pressure" is completely misdesigned and
> horribly unreliable.

I'll check that in more detail tomorrow. I put it in quickly, perhaps that 
is the culprit.

>
> But another thing to look out for is that usually the temperature of
> the cylinder has a huge impact on the pressure inside of it, and
> subsurface has absolutely no clue. But a small cylinder that has been
> sitting out in the sun, and that you start breathing as you descent at
> the beginning of the dive could easily have the pressure in it change
> a lot due to the temperature going down.
>
> Looking at your ssrf file, for example, it looks like you have your
> air integrated pressure gauge on your back gas, and even though you
> aren't breathing that gas at all for the first three and a half
> minutes, the pressure drops from ~198 bar to ~194 bar just from the
> temperature, before you take your first breath. Maybe not a huge drop,
> but I suspect it would have been even more noticeable in a smaller
> cylinder.
>
> On your small air cylinder, you have used manual pressures, and claim
> it started at 210 bar and ended at 140 bar. That 210 bar was
> presumably when it was hot before you jumped in the water.
>
> Of course, the error bars there are likely enormous even if you ignore
> the temperature effect on pressure (particularly if you use one of
> those tiny pony bottle pressure gauges). The air integrated numbers
> are likely a lot more accurate.
>
> There are also error bars from exactly when you told your dive
> computer that you switched gases, and when you actually did the
> switch. Again, on a big 22.2l dual cylinder, a few breaths won't make
> much of a difference, but the errors will be more noticeable on a
> small AL30.
>
> Anyway, that was just a long rambling "I'm not sure how much you can
> trust the SAC rates when your cylinder sizes are a bit odd, the
> pressures are mostly eye-balled, and it is all a bit inexact anyway"

Temperature could be the cause of this. As you can see from the temperature 
statistics, we went through two thermoclines and my Teric is always off by 
2c. Since they we in the sun at buildup, in the back of a pickup for 
30min... This would be quite likely. Tomorrow I'll have my 90m dive and 
I'll note the starting pressure when I'm in the water.


-- Attilla

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