Planner: minimum Gas on CCR dives

[Robert Helling]

Davide,

> On 30. Mar 2018, at 00:02, Davide DB <dbdavide at gmail.com> wrote:
> 
> Hi all,
> 
> Is there a way to display minimum gas even on CCR dives?
> I'm thinking to DIR style CCR units with diluent as backgas.
> 
> I was trying to overcome it planning in OC but tank size drops
> immediately below the minimum gas and I get a "Warning: this is more
> gas than available in the specified cylinder!" So it's impossible to
> get MG for deep dives: the only way to get a real 70m dive is
> extending bottom time to 50 minutes and again subsurface gives me a
> Warning: this is more gas than available in the specified cylinder!
> 
> Maybe the "Warning: this is more gas than available in the specified
> cylinder!" alert should not eliminate minimum gas calculation. I mean:
> my cylinder could not be enough for BT but enough for bailout leg. My
> thought may appear absurd but I'm just trying to separate BT gas
> calculation from MG gas calculation, in other words MG should be
> calculated regardless of the BT gas calculation
> 

As you noticed, Subsurface does not attempt to calculate any gas usage in CCR mode. But what you could do is to add an extra waypoint at the end of your bottom time (with a duration of 1 minute, say) and in the waypoint enter a 0 in the set point column. This signifies bailout to OC. That should turn on the gas calculation for the ascent including minimum gas.

> PS
> How exactly Subsurface calculates the minimum gas reserve?
> 
> Does it take in account gas switch time at the first deco stop?

Subsurface calculates minimum gas only for the last bottom gas. Thus it does not have to take into account the switch time. It takes into account the SAC factor (multiplying the SAC for higher gas usage plus the problem solving time as entered in the corresponding field).

Best
Robert
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