Evidence based guidelines on gas density planning

[Dive Africa]

You are using something other than 10/50 heliair? I have never heard of anyone using 6/70 before. Seems like you are just replacing O2 that you need with N2 that you don't need, so in order to get up to a 1.3 setpoint you end up adding more O2 which is then displacing a bit of the helium. Why not just use the 10/70 and start off closer to your desired setpoint?

We use 6/70 (5/75) heliar all the time - this is my 100m choice of dil - 0.6 X 11 = .66 not that far off from 1.3 (need very little O2 at that depth to get mix to 1.3)

[rjack]

We use 6/70 (5/75) heliar all the time - this is my 100m choice of dil - 0.6 X 11 = .66 not that far off from 1.3 (need very little O2 at that depth to get mix to 1.3)

what is your target ppO2 for dil? ppO2 of 1.0 would be 9% O2 at 11ata


Either way you're too dense with the helium %age.


These guidelines would have you using 80% or more. Which for heliair drops your fO2 to 4%. And at 4% you have a ppO2 of 0.44 which is pretty far from 1.3


If you top the 80% He with EAN32 instead you end up with roughly 8.5 to 9/80. More O2 in your dil gets you closer to the target dil ppO2 and displaces N2 - both positives.

[matthewoutram]

If you top the 80% He with EAN32 instead you end up with roughly 8.5 to 9/80. More O2 in your dil gets you closer to the target dil ppO2 and displaces N2 - both positives.

Perhaps I am tired, but isn't a EANx32 top on 80% He 6.4%?

Matt.

[Igor P]

Yes...it is 6.4 using ideal gas

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[Igor P]

Real gass would be 5.6 using Gerg2004

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[rjack]

Perhaps I am tired, but isn't a EANx32 top on 80% He 6.4%?

Matt.

Yes I was thinking of too many things at once. The point was that with heliair you don't have enough helium for your target density and target dil ppO2 simultaneously and you (always?) want more O2 in your dil. 32% top gets you closer but a little more O2 and a little less N2 is better still.

[matthewoutram]

Yes I was thinking of too many things at once. The point was that with heliair you don't have enough helium for your target density and target dil ppO2 simultaneously and you (always?) want more O2 in your dil. 32% top gets you closer but a little more O2 and a little less N2 is better still.

I'm not overly fussed myself with the O2 content. I like the flush at depth to be ~1.0. Hypoxic is hypoxic.

What the chart shows is that heliair is used too deep when density is considered.

Matt.

[rjack]

I'm not overly fussed myself with the O2 content. I like the flush at depth to be ~1.0. Hypoxic is hypoxic.

What the chart shows is that heliair is used too deep when density is considered.

Matt.

Yes because there's too much N2 in there for density calcs compared to just evaluating narcosis. If there were more O2 there would be proportionally less N2 and that would drive down the total O2 and N2 which is what is raising the density.

[Nubium]

Hello,


The Rebreathers in Scientific Diving Workshop Proceedings (a workshop hosted by the AAUS, DAN, NOAA, and the US National Parks Service at Catalina Is in Feb 2015) were released today.


You can download it here:


https://dl.dropboxusercontent.com/u/...ngs%202016.pdf


In this volume (which contains lots of interesting papers) there is (on page 66) a paper proposing the first published evidence-based guidelines for planning gas density in rebreather dives (or any dives for that matter) in order to avoid dangerous CO2 retention. The paper contains simple examples of how to calculate gas density for any gas mix at any depth.


I hope you find it interesting. I (and I am sure Claudia also) would be happy to answer any related questions.


Simon M

Hi Simon,

That link is now dead, it was last year that you posted it. Would you be able to post it again please ?

Thanks

[rjack]

Hi Simon,

That link is now dead, it was last year that you posted it. Would you be able to post it again please ?

Thanks

https://www.omao.noaa.gov/sites/defa...ngs%202016.pdf