Driving home to altitude

[Shearwater]

Soooo....
If I finish my dive and turn off my computer, quickly drive up 2000 feet and turn on my computer, will my tissue graph look different than it would have if I turned it off and turned it back on after a similar interval, but stayed at sea level? (I'm trying to not box you in to a place you don't want to go. I'm just trying to understand my Perdix)

Yes, keeping in mind that even while turned off, Shearwater computers are still keeping track of barometric pressure.

 

[broncobowsher]

Although not accurate, but close enough for normal altitudes (up to about 15,000').
1000' of altitude = 1' of water.
Altitude has a compressibility factor, but it still gets you fairly decent numbers that are good for guessing and mental thought games.

 

[rsingler]

The math guys tell us that Buhlmann GF wasn't designed to give us meaningful numbers with changing surface pressure.
And the Schreiner equation doesn't allow it at all, apparently.
But thinking simplistically, if exceeding a tissue pressure ratio of 2:1 is a bad thing (GF>100), and you reduce the denominator by driving to altitude, you should be able to quantify the change, if you know current pressure at altitude, and calculated leading compartment partial pressure from the previous dive.
I'm talking with one of our folks to see if there's a way to at least look at some numbers. Yes, driving to 2000' may be only like ascending another 2' in water (and I'm not picking on you, @broncobowsher), but when comparing all your tissue loading with a "surface" that is now ~5% lower in pressure means you're maybe looking at a >5% bump in gradient (not accounting for offgassing during the drive). After many repetitive dives and full slow tissues, you may be happy with surfacing at GF75, but maybe not happy with that rising to 80 during your drive. It would be nice to examine this concept in detail, with guys that understand gradients.
I think everyone can buy into not popping to 7,500' cabin altitude, and instead using some blanket wait time to account for those divers with full slow tissues. But the drive to altitude problem is one that us Tahoe divers confront regularly. You're already gas stressed by surfacing at 6,000'. You were conservative and are happy with your status. But it's Sunday afternoon, and you have to crest 8,000' very briefly on the drive home.
Realistically, how long should you wait? Why can't we use a mathematicsal model to help us understand this, based on all the data we've acquired in a very similar situation (diving)?

Thank you to @Shearwater for telling us how our toy calculates part of its data!

 

[Jay]

The math guys tell us that Buhlmann GF wasn't designed to give us meaningful numbers with changing surface pressure.
And the Schreiner equation doesn't allow it at all, apparently.
But thinking simplistically, if exceeding a tissue pressure ratio of 2:1 is a bad thing (GF>100), and you reduce the denominator by driving to altitude, you should be able to quantify the change, if you know current pressure at altitude, and calculated leading compartment partial pressure from the previous dive.
I'm talking with one of our folks to see if there's a way to at least look at some numbers. Yes, driving to 2000' may be only like ascending another 2' in water (and I'm not picking on you, @broncobowsher), but when comparing all your tissue loading with a "surface" that is now ~5% lower in pressure means you're maybe looking at a >5% bump in gradient (not accounting for offgassing during the drive). After many repetitive dives and full slow tissues, you may be happy with surfacing at GF75, but maybe not happy with that rising to 80 during your drive. It would be nice to examine this concept in detail, with guys that understand gradients.
I think everyone can buy into not popping to 7,500' cabin altitude, and instead using some blanket wait time to account for those divers with full slow tissues. But the drive to altitude problem is one that us Tahoe divers confront regularly. You're already gas stressed by surfacing at 6,000'. You were conservative and are happy with your status. But it's Sunday afternoon, and you have to crest 8,000' very briefly on the drive home.
Realistically, how long should you wait? Why can't we use a mathematicsal model to help us understand this, based on all the data we've acquired in a very similar situation (diving)?

Couldn't one use an altitude table, and take the difference as an estimation?

 

[rsingler]

Couldn't one use an altitude table, and take the difference as an estimation?

Exactly! Estimates are out there. And just as we moved from tables to computers, how can we use our current tools to give us a better sense of our risk than, "Wait 24 hours!"

I think we're on the edge of a new degree of precision in our estimates. We might be wrong, but that's okay! We've been experimenting on ourselves as the tec community has expanded our dive horizons. It carries risk, but that doesn't make it wrong to try. Should we not have ever tried deep stops? The theory was intriguing!

 

[Jay]

True ... and I don''t like starting a point with a but, but, remember that DAN European study and some of the divers were super/bubble-prone, with DAN suggesting a 24+/36 hour no-fly time for those. And none of them had had DCI previously. Flying After Diving: Finally, the Facts (Not Just Theory)

I would guess that until individual physiologies can be adequately (and inexpensively, perhaps frequently) assessed we'll be stuck with the broad-brush one size fits most approach?

 

[adrian_diving]

The math guys tell us that Buhlmann GF wasn't designed to give us meaningful numbers with changing surface pressure.
And the Schreiner equation doesn't allow it at all, apparently.
But thinking simplistically, if exceeding a tissue pressure ratio of 2:1 is a bad thing (GF>100), and you reduce the denominator by driving to altitude, you should be able to quantify the change, if you know current pressure at altitude, and calculated leading compartment partial pressure from the previous dive.
I'm talking with one of our folks to see if there's a way to at least look at some numbers. Yes, driving to 2000' may be only like ascending another 2' in water (and I'm not picking on you, @broncobowsher), but when comparing all your tissue loading with a "surface" that is now ~5% lower in pressure means you're maybe looking at a >5% bump in gradient (not accounting for offgassing during the drive). After many repetitive dives and full slow tissues, you may be happy with surfacing at GF75, but maybe not happy with that rising to 80 during your drive. It would be nice to examine this concept in detail, with guys that understand gradients.
I think everyone can buy into not popping to 7,500' cabin altitude, and instead using some blanket wait time to account for those divers with full slow tissues. But the drive to altitude problem is one that us Tahoe divers confront regularly. You're already gas stressed by surfacing at 6,000'. You were conservative and are happy with your status. But it's Sunday afternoon, and you have to crest 8,000' very briefly on the drive home.
Realistically, how long should you wait? Why can't we use a mathematicsal model to help us understand this, based on all the data we've acquired in a very similar situation (diving)?

Thank you to @Shearwater for telling us how our toy calculates part of its data!

My Shearwater Teric does adjust both Tissues and GF99 as I drive up to altitude after the end of a dive. It keeps track of the tissues depending on the altitude I offgas at. If I stay at sea level, over time the tissues will converge to sea level partial pressures. If I drive to altitude and stay there, it will eventually converge to the partial pressures at that altitude. This computation is important during the surface interval, otherwise the computer would not know the correct tissue loading for the next dive after the surface interval if the surface interval was carried out at altitude.

Are you saying that this computation using Buhlmann GF is not correct?