Feature Request: Set Peregrine Altitude to Adjust NDLs for Post-Dive Travel

[g2]

subclinical bubbles (at altitude) vs subclinical bubbles followed by an ascent.

Naturally there will be subclinical bubbles no matter what you do; the intent is to reduce the subclinical bubbles (at sea level or thereabouts) so that when you ascend to altitude any additional bubbles don't exceed what you would get had the dive actually been at altitude, i.e., no additional risk vs an altitude dive. That's the NDL adjustment I'm seeking.

If we're following the US Navy dive manual guidelines for ascent to altitude (sect. 9-14), they also provide exactly the kind of scenario @boulderjohn is talking about and what I'd like my computer to know:

Example: Upon completion of a dive at an altitude of 4000 feet, the diver plans to ascend to 7500 feet in order to cross a mountain pass. The diver’s repetitive group upon surfacing is Group G. What is the required surface interval before crossing the pass? The planned increase in altitude is 3500 feet. Enter Table 9-6 at 4000 feet and read down to Repetitive Group G. The diver does not require a surface interval before crossing the pass.

Physiologically, pressure is pressure, regardless of whether it's due to water or air. I don't see how your body would know whether you did an altitude dive, or a dive at sea level (with altitude-adjusted NDLs) and then ascended to altitude. If anything, driving to altitude should have a much less adverse effect since the ascent rate is so much slower. Am I missing something?

I contacted Shearwater about features they could add to assist with this. The reply was a pretty terse comment that I should refer to the standard flying after diving guidelines.

No surprise there.

 

[inquis]

For the no fly time, a cabin pressure of 0.6 bar is used

Seems overly conservative to me. 0.6 bar maps to about 13,800 ft / 4200 m. Commercial planes are pressurized to between 0.8 and 0.75 bar (equivalent to 6000-8000 ft / 1830-2440 m).

 

[g2]

Besides the no fly time, that indicates how long you should not fly after diving, the OSTC can also indicate the time you should wait to do an ascent up to 1000, 2000 or 3000 meters after diving.

On further consideration, this seems like a better approach.

Rather than spoofing the altitude, keep everything the same but have the computer present time-to-ascend for various altitudes and flying, in both the planning and post-dive screens. That way you can either plan ahead or know the necessary surface interval afterwards.

 

[inquis]

the intent is to reduce the subclinical bubbles (at sea level or thereabouts) so that when you ascend to altitude any additional bubbles don't exceed what you would get had the dive actually been at altitude

I understand your intent, but it doesn't work that way in the testing that's been performed. In the study Duke University did in 2002 (the basis for DAN's 12 hr guideline), they did a chamber dive to 60 fsw for 55 minutes. After 3 hours, they decreased the pressure to that of 8000 ft (commercial flight pressurized altitude). At that point, their Gradient Factor (one measure of their tissue supersaturation) was 38% (at 8000 ft), and three of 36 people got DCS.

No one has any issue exiting the water with a surfacing GF well above 38%; in fact, 85% is the default GF (high) setting on recent Shearwater and Garmin computers. So by your logic they should be extremely safe with a "mere" 38% after ascending to 8000 ft. And yet, a significant portion of those tested were bent. That is what the hard evidence shows.

Moreover, one out of six got bent after waiting 6 hours before the ascent. That was a GF of 26%.

They then ascended 9 people to 8k ft after 9 hours, and one of them get bent. That was a GF of a mere 20%. Surely that would be safe, right? Nope.

The Navy ascent tables also agree with this trend. I've looked at the 8000 ft column, and it has an average GF of about 13.5% relative to the 8000 ft ambient pressure if you wait the specified times. (That is what I personally use for my TTF indication after importing into Subsurface. I make a new dive in the planner for 3 ft/1 min and just change the starting time until the surfacing GF is 13% or less.)

Again, your supposition does not generally hold up in empirical testing. You may get lucky in isolated cases like the 3500 ft elevation gain from Group G.

 

[g2]

You may get lucky in isolated cases like the 3500 ft elevation gain from Group G.

That's what the Navy says, not me. Presumably they did empirical testing too, not luck.

 

[inquis]

By "lucky" I meant that your particular elevations and surfacing pressure groups may be such that they require a delay before ascent that fits your desires or schedule.

 

[L13]

I understand your intent, but it doesn't work that way in the testing that's been performed. In the study Duke University did in 2002 (the basis for DAN's 12 hr guideline), they did a chamber dive to 60 fsw for 55 minutes. After 3 hours, they decreased the pressure to that of 8000 ft (commercial flight pressurized altitude). At that point, their Gradient Factor (one measure of their tissue supersaturation) was 38% (at 8000 ft), and three of 36 people got DCS.

No one has any issue exiting the water with a surfacing GF well above 38%; in fact, 85% is the default GF (high) setting on recent Shearwater and Garmin computers. So by your logic they should be extremely safe with a "mere" 38% after ascending to 8000 ft. And yet, a significant portion of those tested were bent. That is what the hard evidence shows.

Moreover, one out of six got bent after waiting 6 hours before the ascent. That was a GF of 26%.

They then ascended 9 people to 8k ft after 9 hours, and one of them get bent. That was a GF of a mere 20%. Surely that would be safe, right? Nope.

The Navy ascent tables also agree with this trend. I've looked at the 8000 ft column, and it has an average GF of about 13.5% relative to the 8000 ft ambient pressure if you wait the specified times. (That is what I personally use for my TTF indication after importing into Subsurface. I make a new dive in the planner for 3 ft/1 min and just change the starting time until the surfacing GF is 13% or less.)

Again, your supposition does not generally hold up in empirical testing. You may get lucky in isolated cases like the 3500 ft elevation gain from Group G.

I think these case of DCS can be explained by the further expansion of the bubbles previously formed on initial surfacing GF.

Basically, this is the phenomenon that the Deepstop/Bubble algorithms are based on. There is no question that the phenomenon they were concerned with is real, I appears (based on the limited actuall research data we have from NEDU, etc.) that they were just slightly off on the relative balance between that phenomenon and the adverse effect of further ongasing of slow tissues due to the deeper stops.