Diving and Flying

[got4boyz]

Thanks scubadoc, that article was very informative!

However, I'm a little confused. Why does flying matter if you are in a pressurized cabin as all commercial flights have? Just curious!

Also, it sounds like you shouldn't fly for awhile even if you did non-deco dives. That surprised me!

Thanks again for the info!

 

[BillP]

Hi got4boyz:

As Scubadoc briefly mentioned in his post above, the ambient pressure in commercial airliners at altitude is not the same as at sea level. At their 30,000ft or so cruising altitude they are only pressurized to the equivalent of approximately 8000ft. The pressure at 8000ft is only about 3/4 what it is at sea level.

And you are correct. You should allow adequate time before flying even if you have just done a single no decompression dive.

Bill

 

[DocVikingo]

Hi got4boyz,

If I might chime in with a couple of additional pieces of info, it is important to appreciate that ~8,000' is the target cabin pressure on commercial flights. This figure varies due to a number of factors, and may transiently depart from the target by as much as several thousand feet. Also, although a very rare event, cabins can depressurize. I wouldn't want to be at 30,000'+ with a dive-to-fly time of 5 hours after 30 live-aboard dives in 6 days when it did.

Many dive venues have some neat topside activities. If you can save some for your last day before flying, it makes being out of your gear less painful.

Dive & fly safely.

DocVikingo

 

[got4boyz]

Thanks everyone for answering my questions.

a. A minimum surface interval of 12 hours is required before ascent in a commercial aircraft (8000 foot (2438 m.) cabin).

When I read the above excerpt from scubadoc's post I didn't understand that the (8000 foot (2438 m.) cabin) meant cabin pressue. Thanks for clarifying that for me!

And yes, DocVikingo, I wouldn't want to be flying soon after diving should a cabin depressurize.

I'm leaving Thursday morning for Cozumel, and rest assured I will wait 24 hours before flying!

Thanks again for all your comments, advice, etc. I LOVE THIS BOARD! :1st:

 

[Dr Deco]

Dear got4boyz:

As mentioned by DocVikingo, the cabin pressure can depart from 8,000 equivalent. I have heard that captains have been known to reduce the cabin pressure, and hence the oxygen content, if the passengers get a bit rowdy, e.g., to many preflight parties. This is denied, but I would still be concerned.

“I trust my mother, but I cut the cards”

Dr Deco

 

[turnerjd]

Originally posted by Mario S Caner
got4boyz,
Your question can't be answered without some more information. What was your dive profile like, how many dives over how many days... all this matters. It could be 12-24 hours. A computer is great in getting you the info you need accurately.

From John Lipmans book 'Deeper into diving', I read the chapter on flying after diving, and got the distinct impression that all the computers used different methods of calculating no fly time, and the vast majority were WAG's (wild assed guesses). Some wait to complete desaturation, some use long tissue campartments, others mid range tissue compartments, they even use different definitions as to when its safe to fly.

If I've said it once, I've said it 1000 times. Don't blindly follow a computer.

I have copied from here some more technical information. for those interested.

On a personal note, I tend to wait 24 hours after diving to fly if at all possible, to allow as much N2 to disperse, any longer and it starts getting unfeasable.

Jon T


5. Flying after diving

In flying we confront a situation where the ambient pressure drops rapidly after the airplane takes off. If the allowable partial pressure in any of the tissues is overstepped by this drop in pressure, symptoms of decompression sickness are likely to arise.

To avoid this situation, the "do not fly" time must be observed after diving. Most dive computers calculate and display this waiting period, which may correspond to the period required for complete desaturation. However, there is considerable variation in the "do not fly" period calculated by different dive computers for the same dive profile. This raises the question of how this waiting period is calculated, and why significant differences are found between the calculations of different dive computers.

According to Bühlmann, the "do not fly" period is the time span after which even the slowest tissue has decompressed sufficiently that even an instantaneous drop in ambient pressure to a specific level will not exceed the partial pressure tolerance of this tissue
- quite a mouthful ! -

This "specific level" in question is the usual cabin pressure of a commercial airliner, equivalent to atmospheric pressure at an altitude of 6000 feet. However, in an emergency, this pressure can drop to ambient levels, and therefore an altitude of 13000 feet may be used to provide a further safety factor. The Aladin dive computers go even further and use an altitude of 15000 feet. But . . .

To further confuse you (or make you think !) here's a small example: after a given dive, the Aladin Air X showed a "do not fly" time of 9 hours, while my buddy's Scubapro DC12 showed 24 hours. TAUSIM, using the same dive profile, calculated a "do not fly" period of 17 hours. (But when a cabin pressure of 6000 feet was stipulated, this dropped to only 13 minutes !) What's the right answer ?

To deepen the confusion even more, it must be noted that the 17 hours "do not fly" time generated by TAUSIM is not accurate either - not even within the framework of the ZH-L16 model ! This is due to the fact that with longer and/or deeper dive simulations the slower tissues show negative values for the argument of the logarithm due to mathematical limitations of the Bühlmann model. Since negative arguments for the algorithm are not allowed, these tissues can't be used in the calculation of the "do not fly" time, and so it is determined using the slowest tissue that still shows a positive value for the argument of the logarithm. This means that the real "do not fly" time is longer than that shown by TAUSIM.

Because of this, one must take as a given, on longer dives, that the actual "do not fly" time is longer than that shown (because the slower tissues have saturated further on such a dive). And this is why TAUSIM also reports, when giving the "do not fly" time, which tissues were included in the calculation. Whenever less than 16 tissues produce a positive argument for the logarithm, then the real "do not fly" time is significantly greater than shown.

Yet we previously mentioned that the Aladin dive computer uses a cabin pressure of 15000 feet for extra safety. How can its "do not fly" time be so much shorter at 9 hours, when one would expect it to be longer compared to times calculated on a cabin pressure of 13000 feet ? It's due to the fact that Aladin dive computers use a mid-range tissue instead of the slowest tissue for the calculation.

Experience has shown that the symptoms of decompression sickness produced by flying to soon occur (predominantly or only ???) in the mid-range (for half-time) tissues.

Thus Uwatec can advertise seemingly more safety with its "do not fly" calculations based on a 15000 feet cabin pressure, while keeping the "do not fly" time quite short. Since the ZH-L8ADT model is safer (more conservative, especially for repetitive dives) than the ZH-L16 model according to Uwatec, one would hardly suspect that the "do not fly" time calculated by the Aladin dive computers might be a bit skimpy! Unfortunately, there is no indication in the user's manual of the Aladin dive computer of the actual compartments used in this calculation. :-(

How does the Scubapro DC12 deal with this situation ? It simply gives the time to complete decompression as the "do not fly" time. Since in using the ZH-L16 model practically every dive profile results in unuseable data for the 4 slowest tissues, the actual "do not fly" time is always longer than that produced by TAUSIM. Simply using the time to complete desaturation as the "do not fly" time assures that one is always safe ! However, it also means that the "do not fly" time resulting from two, three - or sometimes even more ! - dives per day over a period of several days can significantly exceed 24 hours. How many divers would choose to give up diving during the last two days of a week-long dive vacation ?

How do other dive computer manufacturers handle this problem ? On this subject they either plead "trade secrecy" or offer such useless descriptions as "modified Haldane algorithm", without providing any details of the "modification" (in principle the Bühlmann model is a haldanian model, too. - May be, all the different dive computers use the Bühlmann model . . . ??? ). This is the dark side of dive computers, which constitutes a significant risk for divers. :-(

Max Hahn has stated that the "do not fly" times of some dive computers are more a product of the influence of strict US product liability considerations informed by the position of the US-dominated Undersea and Hyperbaric Medical Society (UHMS) than of experimental data. According to Hahn, the earlier "do not fly" periods of Bühlmann are too short. A reliable rule of thumb for safe "do not fly" times would be to take 60% of the "do not fly" time shown by the DC12/TRAC.

According to Hahn, the "total decompression" time (ie. all tissues clear of N2 excess pressure) calculated by various dive computers is equally unreliable, because it is dependent on the number of bits used to store a value by each computer. Careful review of all the experimental data led Max Hahn to come up with the rule : To fly you must be nitrogen "clean".

 

[got4boyz]

Originally posted by Dr Deco
Dear got4boyz:I have heard that captains have been known to reduce the cabin pressure, and hence the oxygen content, if the passengers get a bit rowdy

Dr Deco: I wish I had the means of doing that in my house. I'd be reducing the cabin pressure a lot around here! LOL


turner: I'm beginning to wish I had never posted this subject! Just kidding!

That was actually very interesting and informative info. So to keep it simple, I just will wait 24 hours before flying.

Which brings up another question! When should I wait longer than 24 hours to fly

It would be much simpler if they devised a simple spit test or something that would tell you how much N2 you had in you, and then tables telling how long you had to wait.

 

[Mario S Caner]

Yep, it's all one big guessing game right now. That's why computers tend to give you different values for time to fly time. Just as I extend my computers recommendations for deco stop time, I extend this required 'down time' before I hop on a plane as well. So far I like Rick's plan for waiting 24 hrs regardless and doing some shopping. It's better to be safe than sorry.

My previous post however was inteded to say, listen to you computer as apposed to not having a clue yourself and saying "ah, it's been long enough... I think... let's just catch the early flight instead".

Many of us tend to forget that the average "weekend warrior" diver whom travels once a year, is not the honed, avid, experienced diver that most of the board members here are. Most divers sadly do not put that much effort forward, to keep informed. Most have no clue, but hopefully as we get more and more divers to know about this site, we will help to bring the standard up.

 

[Dr Deco]

Dear Readers:

With regard to what is inside a dive computer, i.e., the actual algorithm, they are virtually a “black box.” There are only a few decompression systems for which extensive documentation exists. The UN Navy tables is one example and the DSAT tables (“Recreational Dive Planner” RDP) is the other [Hamilton, RW, RE Rodgers, MR Powell, and RD Vann. Development And Validation Of No-stop Decompression Procedures For Recreational Diving. Diving Science and Technology. (pp. 78 + appendix). February 28, 1994].

It would be nice if there was information available regard deco devices and possibly even some degree of “approval” by an outside organization (sort of an Underwriter's Laboratories for diving). The Divers Alert Network formed a committee to review the RDP, and regarding this, Dr. R W. Hamilton wrote:

• “Overall we conclude that this program succeeded in developing and validating a new mode of decompression management for recreational diving. The Recreational Dive Planner was designed with repetitive and multilevel diving as its main features; as such it is the first decompression planner to do this. Further, no other recreational decompression device, whether tables or computer has had this level of validation testing for the specific implementation. We know of no prior laboratory testing of multilevel procedures.”

This was written in 1994, and I am not aware of any change in the situation. [sp]

Regarding dive computers, in the same report, Dr Hamilton wrote:

• “It has not been general practice for dive computers to be tested experimentally before being issued, on the theory that that they are based on established models. We do not address that issue. One might wonder why the DAN committee has ignored this phenomenon.”

Again, I do not note any change in the situation with regard to decompression devices. As we say on the Forums, “just my two cents.”

Dr. Deco :doctor: [This little fellow, by the way, looks nothing like me.....]

 

[gozumutti]

heavens to murgatroyd, all these erudite people!! can someone translate please? does this mean that the 12 hours given in the PADI OW manual is pi times my wristwatch, or shaken out of a sleeve? Everything seems to be questionable at this point.....