really appreciate everyone's feedback; been having trouble looking at the revision 6 in u.s.navy diving manual - very interested in that; we plan to use nitrox, air tables, and a conservative dive plan; after our surface interval of 5 hours, our repetitive group designation should be B (NAUI tables); even with that we can go even more conservative on our dive; as a side note: 35 years ago, when diving cozumel and cayman, we routinely flew after 12 hours and i can remember(and so can my local dive master) frequently flying after 6 hours; our hydration efforts consisted of a few beers between dives; certainly not promoting any of this - but time and additional research have shown us much; at this point, we plan to make the dive and to fly 5 hours later unless the info in the navy manual changes our minds; would still appreciate any other views, comments that anyone may care to post; thanks again
flying ( ceiling of 1000ft) after one no de comp dive
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I'm familiar with the type of flight you're describing. If you went up into the tower before takeoff you could see the destination airport, they're that close. Exit one airport's pattern, and right into the next.
Anyway, we often dive at Monterey, CA. Because we stay for the weekend, we'll usually do the last dive Monday morning. That dive often is recreational - say, 80'-90' for 45 minutes or so with 32% - or, occasionally, pretty deep and deco obligated.
About an hour after getting out of the water, we're on the way home. An hour after that, we usually cross the Pacheco Pass at 1,300' (nobody seems to ever bat an eye about this, but say "fly to 1,000' " and they get concerned); 3 hours later we cross the Donner Pass at 7,200' on I-80.
After 20 years, nobody we know has ever had an issue. Not a niggle.
Brought this up to bring some perspective to what you're considering. Low-level terrain induced altitude changes are almost never discussed; they happen all the time; nobody ever seems concerned by them after recreational dives.
...I'm the first to compare altitude changes against my dive profiles, especially if the dive is a significant exposure. That's why I return from Lake Tahoe, to Reno, via a route that gains no altitude if we've been deco obligated. Given what I'm reporting with Pacheco Pass, I know that seems contradictory, but you have to balance the type of exposure, pressure ratio change, and make a considered decision.
All the best, James
Anyway, we often dive at Monterey, CA. Because we stay for the weekend, we'll usually do the last dive Monday morning. That dive often is recreational - say, 80'-90' for 45 minutes or so with 32% - or, occasionally, pretty deep and deco obligated.
About an hour after getting out of the water, we're on the way home. An hour after that, we usually cross the Pacheco Pass at 1,300' (nobody seems to ever bat an eye about this, but say "fly to 1,000' " and they get concerned); 3 hours later we cross the Donner Pass at 7,200' on I-80.
After 20 years, nobody we know has ever had an issue. Not a niggle.
Brought this up to bring some perspective to what you're considering. Low-level terrain induced altitude changes are almost never discussed; they happen all the time; nobody ever seems concerned by them after recreational dives.
...I'm the first to compare altitude changes against my dive profiles, especially if the dive is a significant exposure. That's why I return from Lake Tahoe, to Reno, via a route that gains no altitude if we've been deco obligated. Given what I'm reporting with Pacheco Pass, I know that seems contradictory, but you have to balance the type of exposure, pressure ratio change, and make a considered decision.
All the best, James
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Hello yaml :
This Dive/Fly Question
It is always best to follow the recommendations of the tables, although this scenario seems benign. As posters said before, DCS is never completely an absent risk.
In General: Dive/Fly Tables
The problem, as I see it, with dive/fly tables is the absence of an understanding of divers of what exactly was tested. The tests are performed with the test subjects in a quiet, rested state post-dive. If this condition describes you, then you are following the tables as tested and as published.
However, if one hour [or less] post dive you are scurrying about gathering your belongings and scuba gear, then you might be generating nuclei and modifying the off gassing characteristics of your tissues. This is not the conditions of how the tables were tested! This is especially so with wet hair divers who follow nothing at all, I am told.
This is "know the test conditions" true with all tables. They must be used under the conditions they were tested. Excessive post dive lifting and such was not in the test conditions.
Dr Deco :doctor:
This Dive/Fly Question
It is always best to follow the recommendations of the tables, although this scenario seems benign. As posters said before, DCS is never completely an absent risk.
In General: Dive/Fly Tables
The problem, as I see it, with dive/fly tables is the absence of an understanding of divers of what exactly was tested. The tests are performed with the test subjects in a quiet, rested state post-dive. If this condition describes you, then you are following the tables as tested and as published.
However, if one hour [or less] post dive you are scurrying about gathering your belongings and scuba gear, then you might be generating nuclei and modifying the off gassing characteristics of your tissues. This is not the conditions of how the tables were tested! This is especially so with wet hair divers who follow nothing at all, I am told.
This is "know the test conditions" true with all tables. They must be used under the conditions they were tested. Excessive post dive lifting and such was not in the test conditions.
Dr Deco :doctor:
In an aircraft the rate of ascent would be fairly steep and rapid, in a car it would be a slower ascent and the difference is pressure would be pretty minimal. I am not questioning the tables, I have always used the 24 hr rule so I haven't had the need to figure it out.
So wouldn't the slower ascent of a road trip and the rapid ascent of a plane ride make a difference in the risk of DCS? Just curious.
The atm change at 1000' is -.5 psi. For a change of .5 psi in sea water would be the equivalent of <1 fsw (.445 psi/ft) while in the atmosphere it would be 1000'.
If my thinking isn't screwed up, The slower ascent in a car would influence the offgassing, just like a slow ascent <30fpm is recommended for a dive within NDL.
I hope I made sense. Basically just thinking out loud.
So wouldn't the slower ascent of a road trip and the rapid ascent of a plane ride make a difference in the risk of DCS? Just curious.
The atm change at 1000' is -.5 psi. For a change of .5 psi in sea water would be the equivalent of <1 fsw (.445 psi/ft) while in the atmosphere it would be 1000'.
If my thinking isn't screwed up, The slower ascent in a car would influence the offgassing, just like a slow ascent <30fpm is recommended for a dive within NDL.
I hope I made sense. Basically just thinking out loud.
I'm sure you are right. Unfortunately, there have been no studies on this that I know of, so all you have going for you is common sense.
When I was on the big island of Hawai'i, I went to the volcano the day after diving, despite the warnings of one of the people working for the operator I dived with. It was a long trip around the island. I did one ascent right out of Kona that was perfectly safe by both the NOAA and DAN recommendations. I stayed at that altitude for quite some time while driving. Then I made another ascent and drove for a while. Then I made the final ascent to the volcano. The way I saw it, I did two long deco stops on my ascent, after having a night's sleep at sea level. Of course, that's the way I saw it, without anything authoritative to back it up.
Actually I agree with you, that's why I posted:
John, quite a few years ago I was a test subject in the Bell-Borgwardt study (which showed that the Cross Corrections were excessivly conservative, generated altitude tables, and showed that atmospheric pressure based corrections fell apart with deco). We spent the summer doing wet and dry exposures in Lake Tahoe and validated tables at altitude. Part of that study included what was euphamisticly called the C.A.R. - or, the California Ascent Rate - which looked at nitrogen elimination while driving from sea level to Lake Tahoe. IIRC this was investigated (we did doppler and platelet studies) but not published as a formal study or a portion of one. Anyway, the outcome was that the time it took to get from Davis to Lake Tahoe (a 3 hour trip with a 1.5 hour ascent phase) would "preserve" your NDL pressure group leaving sea level, but, not incur additional penalty from the ascent.
This shows me that the current proceedure of adding pressure groups as one ascends is very conservative, and that's not always a bad thing.
All the best, James
OK, reading the above it seems to say that an increase in 2000' air elevation post no Deco diving is relatively safe. Does this only apply to a sea level to 2000' change or ANY 2000' increase.
Excellent question. DAN actually responded to this very question when I asked them something very similar. The answer is that no, not all 2,000 foot altitude gains are created equal.
A 2,000 foot gain from sea level is more significant in its impact than changes from higher elevations. I was specifically speaking of the altitude change that took place when one left Santa Rosa, NM (altitude 4,600 feet) and drove over Raton Pass (altitude 7,800 feet) three hours later. They told me that an altitude gain starting at 4,600 feet had less impact than an altitude gain starting at sea level.
The answer you received is debatable. Rounding your example to a 3,000’ ascent, the standard atmospheric pressure at 3,000 feet is 13.17 psi. So the pressure change from sea level to 3,000' is 1.53 psi (14.7 - 13.17). If this is compared to a 3,000' ascent from 6,000' (11.77 psi) to 9,000' (10.50 psi), the pressure change is only 1.27 psi. While this is a less overall pressure change, it results in a greater ratio change. The ratio change from sea level to 3,000' is 1.116 (14.70/13.17) but the ratio change from 6,000' to 9,000' is 1.121 and is greater . . . think back to an Open Water class and the balloons expanding two fold when they ascended from 33’. So if nuclei/bubble expansion is viewed in terms of a ratio change, an ascent from an elevation greater than sea level has a more pronounced effect foot for foot.
For comparison purposes, the change from sea level to 8’000 feet is a change of 3.79 psi or a ratio change of 1.35, so a 3000’ change (or even a 2'000' one) is not insignificant.
However as pointed out below by James, a ratio change is not the only way to view decompression.
Bell and Borgwardt’s study (in the 70’s) was based on the same principles of Buhlmann’s approach (of course using different criteria) and uses a linear (not ratio) extrapolation to calculate altitude time limits. This results in longer time limits than compared to a ratio extrapolation, but still less than sea level. More modern bubble mechanic theories approach the problem differently and Weinke disagrees with such a linear extrapolation model as it never extrapolates itself to a zero point.
So when deciding on an altitude ascent, many factors must be considered, especially how conservative the dive profile was and how long the exposure will be at altitude. The debate may still be open with small altitude changes, but the necessity for surface intervals needed to fly to a cabin pressure of 8,000' has clearly been shown.
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You can accelerate the no-fly time (of your choice) by breathing surface oxygen or rich nitrox mixes. If you don't want to go deep into details, the first half of the article titled "Accelerating no-fly time using surface oxygen" published in the first issue of Tech Diving Mag includes some interviews and expert opinions. The second half of the article is more into technical details. If you're even more interested, the article titled "Accelerating no-fly time: the final chapter" published in the fourth issue of Tech Diving Mag includes another approach for calculating the no-fly time acceleration. If you want a software tool to calculate this (embedded in a deco planning piece of software so primarily for deco dives), Ultimate Planner (not free) is your friend.
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