Elevation after diving

[inquis]

I also did some math looking at the comparative rate of ascent/pressure change of ascending from two context-relevant ascent scenarios

Yes, there's definitely more to it than the pressure change or rate of change. Even flying (equivalent to 8000 ft) is only a reduction of 26 kPa, which is also less than the ascent from 15 ft.

If you're familiar with gradient factors used by many dive computers, we regard a surfacing GF of 75% (a proxy for tolerated tissue loading above ambient pressure) to be very conservative. (By comparison, the Navy tables are somewhere above 90%.) However, Duke Univ. did a study where they waited for a bit after a 60 ft/55 min chamber dive, then took subjects to 8000 ft, resulting in a SGF of a mere 38% based on the reduced ambient pressure. You'd think that would be extremely safe, right? Nope -- one in 12 subjects suffered DCS.

I don't know the additional variables factored into the Navy ascent guidelines, but clearly there is something substantial.

 

[dmaziuk]

A) Ascent from a 15 ft safety stop (delta=~46 kPa; >60 ft/min= >184kPa/min), compared to:

B) 15 minute car ride to 2000 ft (delta= ~7 kPa, 133 ft/min= 0.47kPa/min). And this is ignoring the built in minimum 45 min surface interval getting back to the docks and unloading, etc.

All of this makes me question what the additional variables are that factored into the creation of these Navy table values?

You don't have to dive, just drive up Mauna Kea without stopping at the welcome center, and see how you feel up there after 5-10 minutes (give bubbles time to develop).

You could breathe O2 or nitrox while getting back to the docks and off the boat, to off-gas more before the drive up.

 

[MonkeyBubbles]

Oh I'm sure there's more to it, I'm just trying to figure it out. I'm familiar with and use GF99/SurGF. Monitoring these numbers in addition to standard ascent/stop recommendations definitely reduces post-dive fatigue. Still trying to wrap my head around how the actual algorithms work.

I think a failing in my previous attempt at math was that I used atmospheric pressure changes, when looking at the pO2/pN2 pressure gradient rate changes is probably more relevant.

I’m factoring in at least 24 hrs surface interval before heading up to Mauna Kea.

 

[inquis]

You may like to read this thread for a related discussion:

Ascent To Altitude From Santa Rosa

Divers traveling north to Colorado from Santa Rosa, which is a high percentage of the people who dive there, know they have to ascend to altitude, and they have to make a decision about how and when to do that. I think everyone knows that if we go by the rules about flying after diving, we can't...

www.scubaboard.com

 

[dmaziuk]

Oh I'm sure there's more to it, I'm just trying to figure it out. I'm familiar with and use GF/SurGF. Monitoring these numbers in addition to standard ascent/stop recommendations definitely reduces post-dive fatigue. Still trying to wrap my head around how the actual algorithms work.

I think a failing in my previous attempt at math was that I used atmospheric pressure changes, when looking at the pO2/pN2 pressure gradient rate changes is probably more relevant.

I’m factoring in at least 24 hrs surface interval before heading up to Mauna Kea.

Well, spend at least half an hour at the visitor center on the way up, is my advice.

Gases are compressible, liquids are not (for our purposes). Ambient pressure in the air column does not change the same way as in the water column, so however your dive computer algorithms work, they only work in the water. Once you're out, they don't.

Here's an old post from @atdotde showing the math: Can we calculate no-fly-times? – The Theoretical Diver for your amusement.

 

[Jai Bar]

Gases are compressible, liquids are not (for our purposes). Ambient pressure in the air column does not change the same way as in the water column, so however your dive computer algorithms work, they only work in the water. Once you're out, they don't

Correct, water pressure increases linearly by 1 atmosphere for each 10 meters whereas air pressure drops exponentially, approximately half every 5000 meters.