Log import of Trimix dive from Shearwater Teric

[Linus Torvalds]

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.

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?

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.

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.

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"

                 Linus