DIR- Generic Why use GUE nitrox only

[Sebek]

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.

Unless at altitude...

 

[justinthedeeps]

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

Ok fair enough, indeed the slowest modeled Bühlmann compartment T_1/2 is assumed as 635 minutes, so at 1000 minutes, it would be ~66% saturated (assuming that there is indeed such a 'compartment'). So we get a GF of only 53% by 1000 minutes.

I am not really accustomed to hand-calculating M values, but here goes a try...
As for final M value (which if I am not mistaken with Bühlmann is given for pure nitrogen?)

M = M₀ + ∆M * depth [reference]
18.9 "metres" = 12.7 "metres" + 1.0395 * 6 metres
1.89 bar

Compare that to partial pressure of nitrogen at 6 metres, which is 1.264 bar (0.79 * 1.6)?

Ok then, 1.264 bar / 1.89 bar => 0.67, or 67%

Compare the partial pressure of air including oxygen instead, then the ambient pressure is 1.6 bar, giving 1.6 / 1.89 = 0.85, or 85%

So, are the Bühlmann M-value tables given for nitrogen pressures--as written--or for air including its oxygen component?

Or is this math still not the way to get GF?

 

[KevinNM]

I went to get my tank fill at Gue store and I ask for air and I got schooled for using air… but no real reason why Gue only use nitrox? Can someone educate me properly?
Thanks!

I’ve taken GUE classes using air. Not sure now, but at the time Catalina Island off LA only had air. So while 32% is a better gas than air, GUE isn’t obsessive about it, at least at the depths non-tech classes hit.

 

[Nick_Radov]

There are a few stories out there from U2 Pilots flying around Afghanistan that are crazy (in print). During the height of the conflict the pilots were doing way more sorties then they ever did before and having some serious consequences, some have no recollection of even landing. All associated with DCS or O2 toxicity, cant remember, been awhile since I've read the articles.

You might be thinking of this article. Some U-2 pilots have experienced severe DCS including type-2 neurological symptoms. It's worth reading for divers to help recognize if they experience similar symptoms. The Air Force has modified some equipment and procedures to reduce the risk but I don't know whether those have been effective.

Killer at 70,000 Feet

The occupational hazards of flying the U-2

www.smithsonianmag.com

 

[nadwidny]

Do astronauts on space rockets get the bends? I heard a rumor that they pre-breathe oxygen prior to launch to prevent this. That would be like breathing 100% oxygen at 6 metres before you surface. Kind of. (Not really though? lol)

Michael Collins in his autobiography mentioned that he got a bit bent after launch on his Gemini 10 mission because he missed some of the O2 pre-breathe before takeoff.

 

[inquis]

Or is this math still not the way to get GF

Still not it. The surfacing GF = (Ptissue - Psurface) / (M0 - Psurface). Your answer will come out a bit high because you're also ignoring water vapor.

 

[ChrisMBC]

Still not it. The surfacing GF = (Ptissue - Psurface) / (M0 - Psurface). Your answer will come out a bit high because you're also ignoring water vapor.

But unless I’m missing something, Ptissue @6 meters is 1.26, and M0 @ surface (for SurfGF) is 1.27, so surfGF should be ~100%?

 

[justinthedeeps]

While I think we have pretty much agreed (without agreeing?) that there is not much concern with air at 6 metres from a GF standpoint...

I will probably keep trying to get an exact understanding, because I need to know

@inquis equation above checks out here:

subsurface/core/profile.cpp at master · subsurface/subsurface

This is the official upstream of the Subsurface divelog program - subsurface/subsurface

github.com

Thanks to this thread

 

[inquis]

But unless I’m missing something

That'd be the water vapor pressure, about 0.51 msw.
Ptissue = (1.6 bar * 10 m/bar - 0.51 m) * 0.79 = 12.24 m
SGF = (12.24 - 10) / (12.7 - 10) = 0.83

 

[justinthedeeps]

Loving this textbook rn

subsurface/core/deco.cpp at master · subsurface/subsurface

This is the official upstream of the Subsurface divelog program - subsurface/subsurface

github.com

/* Inspired gas loading equations depend on the partial pressure of inert gas in the alveolar.
 * P_alv = (P_amb - P_H2O + (1 - Rq) / Rq * P_CO2) * f
 * where:
 * P_alv    alveolar partial pressure of inert gas
 * P_amb    ambient pressure
 * P_H2O    water vapour partial pressure = ~0.0627 bar
 * P_CO2    carbon dioxide partial pressure = ~0.0534 bar
 * Rq    respiratory quotient (O2 consumption / CO2 production)
 * f    fraction of inert gas
 *
 * In our calculations, we simplify this to use an effective water vapour pressure
 * WV = P_H20 - (1 - Rq) / Rq * P_CO2
 *
 * Buhlmann ignored the contribution of CO2 (i.e. Rq = 1.0), whereas Schreiner adopted Rq = 0.8.
 * WV_Buhlmann = PP_H2O = 0.0627 bar
 * WV_Schreiner = 0.0627 - (1 - 0.8) / Rq * 0.0534 = 0.0493 bar

 * Buhlmann calculations use the Buhlmann value, VPM-B calculations use the Schreiner value.
*/
#define WV_PRESSURE 0.0627         // water vapor pressure in bar, based on respiratory quotient Rq = 1.0 (Buhlmann value)
#define WV_PRESSURE_SCHREINER 0.0493    // water vapor pressure in bar, based on respiratory quotient Rq = 0.8 (Schreiner value)