Two cylinders same gas, no SAC on Information pane

Linus Torvalds torvalds at
Tue Sep 11 10:52:41 PDT 2018

On Tue, Sep 11, 2018 at 4:45 AM Crawford Currie <cc at> wrote:
> The Statistics pane then shows a calculated SAC which is consistent with my rule-of-thumb calculation

Right. So the statistics side is the "trivial" SAC calculation, which
just does "how much gas used, what was the average depth, calculate
SAC from that".


> However the SAC box on the Information pane is empty (no SAC shown).

The information pane is actually *per cylinder*. So if you have two
cylinders, and you have gas change events, subsurface will look at the
dive data and look at when you used a particular cylinder, and then
calculate how the SAC rate  differed for each cylinder.

If you have a sidemount setup where you switch from one cylinder to
another every once in a while (whch is what it sounds like you are
doing), and just breathe the cylinders down without bothering with
telling subsurface about the gas changes, then the per-cylinder SAC
rate obviously is not something we can calculate.

We could still do the "what was the overall SAC rate", of course, but
that's literally what the statistics pane was designed for.

I guess we could make the whole "ok, same gas, treat them as one
cylinder", but we just don't have that kind of special-casing.

> I have the same gas in both cans, it ought to be able to make sense of that (Subsurface-mobile seems to manage it OK)

I think subsurface-mobile only shows that single over-all SAC rate,
and doesn't do the per-cylinder ones at all. So what you're seeing is
basically what you see in the desktop version in the statistics pane.

And the thing about "same gas" is that some have "same gas" but for
different things. You might have two tanks with air, but the second
one is a much smaller emergency bottle.

So then you could have a "ok, but gas pressure doesn't change for the
emergency bottle, so you might as well take it into account and nobody
would care". And you'd probably be right. But it's one of those things
where we just haven't had anybody who cared enough...

> (Also: I just noticed that when a gas change event is deleted, it doesn't update the information pane)

Yeah, we have a few "oh, you changed some dive detail, but we don't
recalculate anything until you switch to another dive and back" cases.


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