DIR- Generic Why use GUE nitrox only

[justinthedeeps]

Thanks for details @inquis & @steinbil
Subsurface is wonderful, agreed.

So my quick ambient pressure ratio approximation (more Haldanean w.r.t. 100% oversaturation) is actually approximately close--not surprisingly--and probably sufficient for this matter about DCS/DCI from air saturation shallower than 10 metres.

For precise M values and GFs, there is newer math...

Dive-Tech: Decompression theory - Robert Workman and Prof A Buhlmann

Decompression theory, Trimix, decompression, theory, deco, procedures, Paul Bert, Haldane, Haldanian, Workman, Buhlmann, ZHL, RGBM, VPM, bubble, trimix, nitrox, accelerated decompression

www.dive-tech.co.uk

Robert Workman introduced the term "M-value" to describe how much overpressurization each tissue compartment can tolerate at any depth. He revised Haldane's model to account for the fact that different tissue compartments can tolerate different levels of overpressurization, and that this level changes with depth.

This .pdf pretty nice and succinct

http://www.dive-tech.co.uk/resources/mvalues.pdf

Since the Bülhmann Surfacing M₀ is given as "12.7 metres" for the slowest compartment ("#16"), for [pure?] nitrogen saturation, how are we seeing a surface GF as high as 83% (@inquis was this a typo?) at only 6 metres on 79% N₂? That seems surprising. But maybe it's still the math.

EDIT:
Ok I did open up Subsurface and "plan dive" on air to 6 metres for 1000 minutes... the surface GF is given as 53%.

So my 60% Haldanean-style approximation was SUPER WRONG, was it?

 

[inquis]

Since the Bülhmann Surfacing M0 is given as "12.7 metres" for the slowest compartment ("#16"), for [pure?] nitrogen saturation, how are we seeing a surface GF as high as 83% at only 6 metres on 79% N2? That seems surprising. But maybe it's still the math.

Yes, it's in the math. You seem to be forgetting the surface pressure (~10 msw). Water vapor plays a role as well.

 

[justinthedeeps]

Back to a more realistic case about whether we should be using air or EAN32 for "shallow dives" let's look at 15 metres / 50 feet.

[According to Subsurface...]
On air, 80% surface GF occurs in about an hour at 15 metres.

Extended duration dives like this can certainly occur up to and beyond an hour, particularly for tec-rec style diving in twinset configs etc, including GUE training dives. Bump up to EAN32, and it's only 60% surface GF after an hour.

Does that difference matter? I don't know. Maybe. For slow tissues, if you don't want to surface on a 80% GF, then use EAN32. Might also matter if it is higher altitude diving, or if you plan to fly within 24 hours.

I have gotten deco on a long dive to 15 metres before, on GF settings 65/80. It was a rebreather dive on a "low" setpoint of 0.7 atm ppO₂, which equates to approximately EAN28. That deco prescription began at around 2 hours into the dive. Theoretically, surfacing immediately/quickly at that time might have been risky from a DCS/DCI standpoint. (Not to mention a potential drop in ppO2 to 0.28 atm if the solenoid or diver aren't adding oxygen on ascent.)

 

[justinthedeeps]

Yes, it's in the math. You seem to be forgetting the surface pressure (~10 msw). Water vapor plays a role as well.

Ok... Subsurface does not agree with your 83% figure though--which you suggested using.

 

[inquis]

Subsurface does not agree with your 83% figure though--which you suggested using.

I'm not going to worry about a 2% difference. Just trying to be helpful.

 

[crofrog]

Research on saturation divers indicates that any tissue fully saturated at 20 feet or less can go directly to the surface. You essentially cannot get DCS at that depth, no matter how long you stay there. Extra oxygen does you no good, so I don't see a benefit to nitrox on a 20 foot dive, especially if you have to pay extra for it.

Even on 20ft dives pDSC > 0

 

[justinthedeeps]

I'm not going to worry about a 2% difference. Just trying to be helpful.

Subsurface gave me 53%, not 83%. Where did 83% come from? Typo? Or another way of simulating the 6m saturation?

53% is pretty close to my original quick estimation of 60%.

 

[justinthedeeps]

Even on 20ft dives pDSC > 0

Let's get even further out there... Is an AGE upon from surfacing from 6 metres with trapped gas (e.g. closed airway) more survivable if it happened on 100% oxygen, versus compressed air? Does that matter at all for 32% vs air? Probably also not.

 

[inquis]

Subsurface gave me 53%, not 83%.

You said "saturated", and your choice of 1000 minutes is not long enough to do that.

 

[crofrog]

Let's get even further out there... Is an AGE upon from surfacing from 6 metres with trapped gas (e.g. closed airway) more survivable if it happened on 100% oxygen, versus compressed air? Does that matter at all for 32% vs air? Probably also not.

Absolute risk is likely lower, but practical risk probably not.