Logging CCR dives

Davide DB dbdavide at gmail.com
Tue May 9 03:05:36 PDT 2017

On 8 May 2017 at 17:24, Robert Helling <helling at atdotde.de> wrote:
> as was recently discussed on github “OC gas” is a misnomer, the text should
> be changed to “gas used”.
> To be honest, Subsurface currently does not really do any reasonable gas
> calculations for CCR dives. As always, it only considers gas used if it is
> ever switched to and has the implicit assumption that at any moment in time,
> the diver is only breathing a single gas. Both is of course wrong for CCR
> dives. Apart from displaying the initial and final pressures of the
> cylinders anything more advanced is really hard to do (like interpolating
> pressures for example). For oxygen in CCR dives, I would only make a linear
> in time interpolation (independent of depth) while predicting diluent use as
> a function of the depth profile is really hard to do right.

I see. In my previous email I confused Info tab with Stats tab. I
meant "Info" tab.
For oxygen a simple "gas consumed" info would be enough. After all is
the most important parameter for a ccr dive. Basically diluent gas get
only "wasted" and its consumption depends on many factors.
Unfortunately right now we get only diluent data from Petrel.

IMHO (despite the programming effort needed to achieve this) an happy
day use case scenario for logging ccr dives from a ccr aware dive
computer would be:

Precondition: we have a "ccr dive type" info coming from the dive
computer. I automagically found my dives set as "ccr" after Petrel

#1 A default cylinder (taken from preferences) is automatically
populated with the correct gas got from the dive computer. The "Use"
field is set to "CC-gas". (*)
#2 A default cylinder (taken from preferences) is automatically
populated with oxygen. The use field is set to "CC-gas". (**)
#3 The diver compiles start/end pressure for the two cylinders.
#4 Anal retentive divers would add bailout cylinders marking them as "BO-gases".
#5 Info tab correctly display consumed gas for both cylinders. Right
now I get only consumed diluent. (***)
#6 Graph could happily skip time gas use for ccr dives.

(*)  "Use" field drop-down would have: OC-gas | CC-gas | BO-gas | Not
used (BO = bailout)

(**) In my case I have 3L as default cylinder so I would get the same
3L cylinder for oxygen. This would cover 99% of use cases: standard
ccr units use 2x2L; 2x3L or 2x4L configuration. There is small
dir-style ccr community which uses a configuration more similar to OC:
a 2x7L with diluent as back gas and a separate smaller cylinder for
oxygen. In this case the diver would use D7 as default cylinder
(diluent) and would simply change cylinder size for the default oxygen
added by Subsurface.

(***) Chatting with seasoned ccr divers I see that is very common
expressing own consumed oxygen as bar per hour. It's mainly a personal
(metabolic) data that divers learn after hours of ccr use. It would a
nice addition if Subsurface could express this value near the raw
liter data in the Info tab.

I'm a strong supporter of "defaults" in UX interactions. mainly
because for years I was involved in various betting terminal and
various point of sale terminals. Defaults are paramount to minimize
user interaction with keyboards and touch screens :-)
A lot of divers doesn't log dives of several reasons. Even me that I'm
fussy with my dive logs I find it a boring activity hence an help
doping this would be greatly appreciated.

Regarding BO-gas:

As you know, Petrel (as many other tech dc out there) has two menus to
define and activate CC-gases and BO-gases. I wonder if into downloaded
data we get the list of active gases for a dive. This would permit us
to automagically populate the cylinder list with these gas and mark
them as BO-gas. Hence avoiding the #4 anal-retentive step above ;-)



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