Tank pressure plots for CCRs

Dirk Hohndel dirk at hohndel.org
Mon Jan 6 13:44:04 UTC 2014

On Mon, 2014-01-06 at 19:27 -0200, Rodrigo Severo wrote:
> Hi,
> I'm trying to understand how to better show tank pressure plots for
> CCR dives.
> First let me present a few important facts about CCRs for the
> uninitiated:
>       * CCRs use 2 tanks simultaneously: one called the diluent
>         (usually air but can be some trimix) and the other with
>         oxygen;
>       * the usage pattern of these tanks differ greatly from the open
>         circuit (OC) usage pattern already supported by Subsurface and
>         also one from the other.
> With this in mind I would like to suggest that Subsurface be extended
> to:
>      1. support multiple tank plots at the same time;
>      2. support different usage patterns besides the current OC one.
> Item 2 is really simple as a switch inside pressure_time function and
> an optional extra step on "Add Gas change" add event menu does the
> trick.
> Item 1 isn't so simple as I believe any change to implement it will
> affect lots of code inside profile.c but it seems to me more as
> lots-of-work, not really-hard-work.

Tomaz is rewriting the profile system right now. This might be a great
time to make sure that the real requirements for this are well

One caveat - Subsurface has always been open to cater to specific
subsets of the user base, but we have a hard rule that we cater to the
"typical recreational diver" first. So we will not make changes that
make Subsurface less useful for recreational OC divers just because the
few suicidal people who dive rebreathers think that's what they need.

> Implementing item 1 could also have the positive side effect of fixing
> the gaps that consecutive tank pressure plots for the same tank
> present. The last pressure of a tank pressure line is always higher
> than the first pressure of the next line for the same tank. They
> should be exactly the same. These gaps exist now with the current code
> and also with my yet not approved nor refused patch "Discontinuity and
> stall on tank pressure interpolated lines". I would never suggest that
> a change so big as item 1 implies be done just because of these gaps
> but I think it would be great to fix this too.

Actually, there is no such thing as "it should be the same". Pressure
data can fluctuate for many reasons and it's entirely possible that
pressure has gone up because the diver crossed a thermocline. Or
pressure could have gone down because of leakage.

Since pressure data is NOT continuous but assembled from discrete
samples there can and will be sudden changes.


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