"no fly" time

[Teamcasa]

Airlines and pressurized cabins. Sure most cabin pressures are set for 7000’-8000’ but sometimes the differential in pressure changes pretty fast and is not very consistent. This is not a big problem for someone who is not loaded up with nitrogen. However, for someone with a fresh load of micro nitrogen bubbles floating around inside them, a rapid change in cabin pressure could cause real serious problems.

One reason I choose a single island for diving is so I may dive more and fly less. Why risk it?

 

[Teamcasa]

Simply stated. This practice is very dangerous.

Just because you suffered no ill effects does not mean no one else will. The tables, research and warnings from virtually every reputable diving resource says to wait x hours before flying. There is a real reason and real science behind stating it.

Yep, and this science is based on protecting the 400 pound, obese, alcoholic, smoking, dehydrated, holiday diver who does bounce dives once a year with no real idea of deep stops, tissue loadings, ascent rates or anything other than pressing the "up" button to go up and the "down" button to go down. I don't believe I fall into any of those categories, do you?

No I don’t but I also don’t advocate dangerous activities simply because it worked out OK for someone, once.

On the flip side, the Darwin Awards are given to people where the dangerous practice did not turn out so well for them.

 

[Bismark]

No I don’t but I also don’t advocate dangerous activities simply because it worked out OK for someone, once.

On the flip side, the Darwin Awards are given to people where the dangerous practice did not turn out so well for them.

Lots of assumptions in there Teamcasa. How many examples would you like? 10? 100? 1000? Do you disagree with my about how the level of conservatism is arrived at?

Seems to me Darwin was not about protecting the weak last time I checked. My suggestion is that proper education would remove the need for scare mongering and very restrictive limitations. But then I approach Darwin from the other perspective, that of furthering the strong rather than protecting the weak.

 

[bleeb]

If they fly above 8000 then sure, the cabin would be pressurized to 8000. However if they fly lower, then the exercise would be different, as I said.

Not to be too nit picky or anything, but depending on what you're flying on, it may be unpressurized even when above 8000'. Depends in part on the rules for the particular country you're in, how long you're up there, max altitude and whether the pilot has supplemental oxygen.

 

[Walter]

That would represent cabin pressure in a jet liner.

He's diving in Seattle and lives in Los Angeles, I didn't think he would be on an island hopper.

what is 8000' in FSW? maybe 6-10 feet?

It doesn't matter. What matters is the pressure difference between where you are saturated and the altitude to which you are going. After a dive, you are not saturated at 1 atm.

Again, levels of conservativism added on top of levels of conservatism to cover the very lowest denominator

If that were true, the recommendation would be 48 hours.

Seems to me Darwin was not about protecting the weak last time I checked.

Darwin merely described a process which describes how natural selection works. Darwin neither protects the weak nor furthers the strong. Natural selection works when those with particular traits live long enough to breed while those without those traits don't. In this case (assuming death is the result) not getting bent could be selecting for a natural trait that resists DCS or for those who are smart enough not to push the flying after diving rules. I'd bet on intelligence over being "strong."

 

[Thalassamania]

First let's address groups, U.S. Navy repetitive dive groups are what I mean when I talk about groups. There's no such thing as NOAA groups, they use the Navy tables. Some training agencies (NAUI and maybe YMCA) have published tables that were based on U.S. Navy tables but that had everything shifted one column to make them more conservative. PADI uses group designators that have nothing to do with U.S. Navy group designators and which must not be mixed and matched.

That said, in the old days it was said that you could fly once you were in U.S. Navy Group D or lower. Careful examination of this concept showed that if you were a D diver, but very close to the E diver line you could not ascend to 8,000 feet, so the rule was moved back to group C, which is, actually, rather conservative. If you look at Table 9.5 Required Surface Interval Before Ascent to Altitude After Flying, USN Dive Manual, Revision 4, Jan. 20, 1999, you will notice that as you scan down the 8,000 foot column you are safe to fly until you get to group D, which requires a 9 minute wait.

Many of us, for many years, operated on the group C rule and flew with no problem. Frankly, the new rules do not make much sense to me. First of all, repetitive group designators are a letters that describe the amount of nitrogen remaining in a diver’s body during a 12-hour period following a dive. What the justification is (aside from conservative fudge factors) to extend the designator concept back 24 hours is beyond me. Lets consider two dive profiles:

1. A dive to 130 feet for 10 minutes (group E) followed 23 hours later by a dive to 60 feet for 60 minutes (group J).
2. A dive to 60 feet for 60 minutes (group J) followed 23 hours later by a dive to 130 feet for 10 minutes (group E).

For the both series of dives Table 9-5 requires that you go back to group J and read across to 8,000 feet. That requires that you wait 17:35 before ascending to 8,000 feet. Now, does that make any sense?

In the old days the first series of dives (note that since the dives are more than 12 hours apart they are not repetitive dives) would have required a wait of 4:04 (group J to group C) before flying, whilst the second series of dives would have required a wait of 0:56 (group E to group C) before flying.

Clearly the two dive series are different, yet but current rules they both require the same wait before flying. Application of the old rules (which I can say empirically worked fine for me for decades) shortens the wait by more than half a day, even in the worst case.

Of course there are other things to consider here, hydration is a BIG deal. Refraining from alcohol and other diuretics is important, drinking water or fruit juice before and during the flight is important, possibly wearing a surgical mask during flight and even hedging your bet with a tank of of oxygen before departure might be prudent moves.

I can't tell you what to do. I would not presume to do so. All I can say is what I, knowing the risks and a little bit of the math, choose to do. If it does not matter I wait 24 hours to fly after my last dive. If I have to push it I will use the old U.S. Navy group C protocol combined with careful hydration and oxygen washout (at 20 feet if possible).

So the only question that remains is, "how do I get back onto U.S. Navy tables when I've been diving a computer?" That can be problematic. When I know I'm going to face this sort of situation I dive a U.S. Navy model computer (an old Edge, Skinny Dipper, or Phoenix). At the end of my last dive I take the scrolling no-D limits and compare them, depth/time pair by depth/time pair to the U.S. Navy No-Decompression Limits and Repetitive Group Designation Table for No-Decompression Air Dives. I take the highest repetitive group designator that this results in and use that to determine how long a must wait until I am in group C.

I have not done a careful analysis of using this technique with non-U.S. Navy model computers. I can say, anecdotally, that when I dove other computers along side the Orca products they have invariably been more conservative and the technique above yielded more conservative U.S. Navy repetitive designators, but I would not count on this working with whatever computer you are currently using, some testing or at least serious calculation would be in order.

Anyway ... DCS is not to be taken lightly, especially DCS whilst on an aircraft. I've been involved in dive computer development and decompression theory since the early 1980s and these are conclusions that I reached for myself. I DO NOT RECOMMEND THEM TO YOU AS ANYTHING MORE THAN A STARTING PLACE FOR YOU TO BEGIN TO ASK QUESTIONS.

 

[Bismark]

He's diving in Seattle and lives in Los Angeles, I didn't think he would be on an island hopper.



It doesn't matter. What matters is the pressure difference between where you are saturated and the altitude to which you are going. After a dive, you are not saturated at 1 atm.



If that were true, the recommendation would be 48 hours.



Darwin merely described a process which describes how natural selection works. Darwin neither protects the weak nor furthers the strong. Natural selection works when those with particular traits live long enough to breed while those without those traits don't. In this case (assuming death is the result) not getting bent could be selecting for a natural trait that resists DCS or for those who are smart enough not to push the flying after diving rules. I'd bet on intelligence over being "strong."

(Sorry I don't know how to use this multiple quote thingy)

So Walter, how saturated do you believe you are after 2 x 60' for 30 minute dives with 1 hour between them, particularly if you follow an ascent profile that is designed to reduce bubbles and also a pause to help with the slow tissues? For that matter, a 30' deepstop isn't really much of a deep stop at that. I understand that we are talking about the pressure differential, so how much pressure differential do you think we are talking about? 8000' is somwhere in the 6-10ft range in FSW. Again, if that much bends you, you probably have other issues.


Second, why did you chose 48 hours? Why not really, really be safe and chose 60 or even 72 hours? My point is that the levels of conservatism we have already are designed to make it a statistical improbability for the diver I described above to get bent. I actually don't know any divers like that myself but "making sure" the lowest common denominator is safe seems to me just silly.

If you want to talk natural selection, what are we doing when we make it possible for those type of divers to continue on and pass those predispositions on to the next generation? My argument here is that we have created a situation (fill in the blank as to what situation you want) where those less adapted to a situation are not penalized. In fact, in this case, we have put in place procedures to make sure they don't get penalized. Do you know of any other place in the animal or plant kingdom where this happens? I don't want to get into a natural selection argument here but don't expect me to apply the same conservative standards to myself as the diver mentioned above. You stated that "Natural selection works when those with particular traits live long enough to breed while those without those traits don't". Do you not think that we are artifically influencing this process when we create something like a 18 or 24 hour waiting before flight time so those divers I mentioned above have a nearly 99.9 percent chance of not getting bent? I don't see it any other way but if you can explain it to me, I am all ears.

BTW, Thalassamania, thanks for the always informative and detailed posts.

 

[Riger]

As a Scuub Nuub with a recent PADI AOW cert, I find myself pondering this;

When I use my Computer as the yardstick for my bottom times, it gives me credit for an ascending profile, this technically violates my NDL Table limits. The reason that this is OK, is because my Comp is using a safe algorithm .... yes?

By the same logic, is it acceptable to follow the "No Fly" recommendation of my comp after single or multiple dives, even if they are lower than the DAN and/or PADI recommendations?

 

[diversteve]

That would represent cabin pressure in a jet liner.

Not particularly applicable to an island hopper though.

You would need to call the air service and ask them at what altitude their island hoppers fly between the various islands. Then look this altitude up in the NOAA diving manual, referenced above by Doc Vikingo.

If they fly above 8000 then sure, the cabin would be pressurized to 8000. However if they fly lower, then the exercise would be different, as I said.

Check the 1st page of this thread. The OP will be on flights of around 20,000-23,000' altitude.

It's actually about a 50min. flight between Oahu and Hilo (big island) 1/2 that to Molokai...

 

[Walter]

(Sorry I don't know how to use this multiple quote thingy)

I cut and paste.

So Walter, how saturated do you believe you are after 2 x 60' for 30 minute dives with 1 hour between them, particularly if you follow an ascent profile that is designed to reduce bubbles and also a pause to help with the slow tissues? For that matter, a 30' deepstop isn't really much of a deep stop at that.

OK, 60 feet for 30 minutes puts you in group F. After 1 hour, you're now in group E. As a group E diver to 60 feet, you have 30 minutes RNT. Since the NDL at 60 ft is 60 minutes (straight US Navy, NAUI's limit is 55, YMCA's is 50), with a 30 minute dive, you will have pushed your NDL to the edge with no magrin of safety (not recommended), leaving you in group J at the end of the second dive (NAUI's tables would require a 5 minute stop at 15 ft). The NOAA tables would require 17 hours 35 minutes before ascending to 8000 feet.

I understand that we are talking about the pressure differential, so how much pressure differential do you think we are talking about? 8000' is somwhere in the 6-10ft range in FSW. Again, if that much bends you, you probably have other issues.

If I'm not mistaken, the difference in pressure between sea level and 8000 ft is the same as about 8½ fsw. It seems that you are missing the point. You are on a decompression dive. On a decompression dive, 8 or 9 feet can bend you. You are supersaturated and need quite a long staged decompression stop at the surface before you can continue to ascend. In this case, a 17 hour, 35 minute decompression stop. Skipping a long decompression stop like that is not a minor thing. Other issues is not the factor here.

Second, why did you chose 48 hours? Why not really, really be safe and chose 60 or even 72 hours? My point is that the levels of conservatism we have already are designed to make it a statistical improbability for the diver I described above to get bent. I actually don't know any divers like that myself but "making sure" the lowest common denominator is safe seems to me just silly.

48 hours is the time recommended before flying after exceptional exposure. It's also the length of time required to clear the 480 minute compartment.

If you want to talk natural selection, what are we doing when we make it possible for those type of divers to continue on and pass those predispositions on to the next generation? My argument here is that we have created a situation (fill in the blank as to what situation you want) where those less adapted to a situation are not penalized. In fact, in this case, we have put in place procedures to make sure they don't get penalized. Do you know of any other place in the animal or plant kingdom where this happens? I don't want to get into a natural selection argument here but don't expect me to apply the same conservative standards to myself as the diver mentioned above. You stated that "Natural selection works when those with particular traits live long enough to breed while those without those traits don't". Do you not think that we are artifically influencing this process when we create something like a 18 or 24 hour waiting before flight time so those divers I mentioned above have a nearly 99.9 percent chance of not getting bent? I don't see it any other way but if you can explain it to me, I am all ears.

By creating a waiting period after diving before we fly, we are selecting for intelligence, not an inherent ability (which probably does not exist) to not get bent. I like the idea of selecting for intelligence.