Nitrox and Trimix mixes

[Laurie_the_Knot]

I'm not a trimix qualified diver but have been diving for many years and 15 of those with nitrox and Draeger Dolphin SCR. I have been examining the setup menus and acceptable ranges for settings in the OSTC2 and encountered this, I consider, an anomaly.

The question I have is this:

Is it reasonable for a gas mix to have more than 79% nitrogen?

With an O2/He/N2 mix is it reasonable for the ratio of N2/O2 to be more than 79/21? If not should the OSTC2 validate for these conditions or should it be left to the diver to ensure the mix being configured is reasonable?

Currently the OSTC2 will reject any O2/N2 mix with more than 79% N2 but no validation appears to be performed once any amount of He is included, for example a mix of 4% O2, 1% He and therefore 95% N2% can be configured in the Gas Setup list.

[jb2cool]

I guess validation should be completed to create a sum of 100%. I can't really see when this wouldn't be the case.

[Laurie_the_Knot]

The problem arises when you try to dive with nitrogen making up more than 79% of the gas mix. The purpose of nitrox and trimix mixes is to reduce the nitrogen in the mix not allow it increase.

[jb2cool]

Ok, I see what you are saying now.

So how about limiting the amount of N2 allowed in any given mix to 79% (as well as ensuring that the combined gas percentages comes to a maximum of 100%)? This would allow the weakest nitrox mix (air) as well as any trimix that I can think of.

[Laurie_the_Knot]

The question is: Do you think the OSTC2 should validate for this or should it be left up to the diver to ensure he hasn't made a mistake?

[Bardass]

What this OSTC 2 dive computer is:
An experimental platform for all experienced divers who are interested in the theory of diving and who value maximum transparency and individual adaptabily .

I don't think the software need to have more controls

[Laurie_the_Knot]

It would seem Matthias and his software team have already thought of this.

Referring to the OSTC2 Owner's Manual, page 18, column 3, it states: If the sum of oxygen and helium fractions is less than 21% the gas list shows ERR indicating an unusual gas mix.

Unfortunately a bug must have crept into the code and this condition does not report an error any more. Maybe we can get Matthias' attention and he can look into fixing this bug. The software currently accepts a gas mix with 4% oxygen, 1% helium and 95% nitrogen. Clearly a very dangerous mix though not one any sensible diver would ever attempt to dive with.