Altitude and diving

[boulderjohn]

I was coming at it the other way. What's the difference in NDL for a diver who lives at sea level, vs a diver who lives at 2000 ft. and descends to sea level for a dive? The comment that a trip to altitude prior to a dive might not matter (briefly, one day earlier in this case) got me wondering about the importance of baseline altitude acclimation for NDL estimations. Surely you can't just assume the same baseline at sea level for everyone, independent of what altitude they are acclimated to?

The kind of question the OP asked - going to Hawaii then traveling to 1000 or 2000 ft elevation at some short interval after diving - comes up a lot, often with real concern expressed. If one can look at two options for a 30 minute dive - 50 ft vs 52 ft and make the reasoned assessment that the DCS risk is not much different between the two, does it not similarly follow that a 30 minute dive to 50 ft, followed by a fairly prompt drive to 2000 ft elevation is closely in the same ballpark of indifference?

If you read the altitude articles in the link I posted earlier, you will see the issues explained.

Going from one altitude to another prior to diving certainly makes a difference, but it is not possible with current research to quantify it with any real accuracy. Some software programs will make an effort--for example, Multi-deco will ask you what altitude you came from and how long you have been there, but there is no good research to support whatever conclusions it draws. The same program will plan a decompression dive for you at 16,000 feet, even though that has never been done successfully and so has no real basis for its conclusions. (The only dive I know at that altitude resulted in one fatality and one 3-month paralysis.) The conclusions of such software are little more than guesses, which I can say with impunity here because the author has been banned from ScubaBoard and he will instead have to attack me (as usual) via private email.

If you are ascending to altitude in a car ride, you are off-gassing in a way similar to doing decompression stops, and it should be better than ascending in an aircraft, but there is no research to help you on that. I do that kind of off-gassing myself regularly after serious decompression dives, and when I do, I breathe oxygen to help with nitrogen off-gassing, but there is no research to help me with that, either. It's guesswork.

As for your example, DAN and the US Navy have no issues with you ascending 2,000 feet after a dive (despite what Hawai'ian snooba operators may tell you). According to the US Navy tables, your 50 foot dive for 30 minutes puts you in Pressure Group E, meaning you can immediately ascend to 6,000 feet. Wait 4:39 and you can fly.

I recently wrote an article on current thinking on decompression theory regarding deep stops (Evolving Thought on Deep Decompression Stops), and in writing it I got extensive help from Dr. Simon Mitchell. I talked with Dr. Michell about two other articles I wanted to write, hopefully with his help, one of which was ascending to altitude after diving. He politely declined both requests, and with good reason. The research on those two topics is not strong, and an academician of his standing cannot have his name on an article that does not have strong research support. I therefore wrote my ascent to altitude article without help, and if you read it, you will see that I have several strongly worded "doing my best--don't blame me if I'm wrong" statements.

 

[markmud]

I recently wrote an article on current thinking on decompression theory regarding deep stops (Evolving Thought on Deep Decompression Stops), and in writing it I got extensive help from Dr. Simon Mitchell.

HI John,

You failed to mention in your post that your article was listed as an additional resource for Dr. Doolette's article on deep stops.
Which can be found here: Gradient Factors in a Post-Deep Stops World

When you write, there are many of us who pay attention.

Keep up the good work and thanks bud,
m

 

[boulderjohn]

When you write, there are many of us who pay attention.

...and many who pay attention hate every word and let me know it. That is partially why this is my first true thread participation in nearly 10 months.

 

[spoolin01]

You've left a durable impression of thoughtful and informative posts. Maybe I'm forgetting all your bad acting.

 

[markmud]

...and many who pay attention hate every word and let me know it. That is partially why this is my first true thread participation in nearly 10 months.

Hi John,

A wise man stated that a person who waste's his life straddling the yellow line on the road in an attempt to avoid controversy, ends up getting run over by one side or the other. Paraphrased; author shall not be cited.

Thanks for taking a stand. Thanks for choosing one side of the road to be on. Life is too short to be a fence-sitter; you have the guts to state your opinion and stand behind your beliefs.

The spear guns, the sharks, and the keyboard commandos are still out to get you, but you will have an army of us behind you.

m²v2

 

[broncobowsher]

1,000 feet of elevation is 977 mBar of air pressure (sea level is 1013)
Big storms can have lower air pressure that the elevation change. Really big storms (Katrina?) have pushed as low as almost 900 mBar. That's about 3,000 feet elevation air pressure at sea level.

 

[spoolin01]

1,000 feet of elevation is 977 mBar of air pressure (sea level is 1013)
Big storms can have lower air pressure that the elevation change. Really big storms (Katrina?) have pushed as low as almost 900 mBar. That's about 3,000 feet elevation air pressure at sea level.

I remember starting there when trying to place elevation changes in perspective...