Reconciling deco models with no-fly recommendations.

[kr2y5]

I'd like to better understand the relationship between deco models and no-fly recommendations, and I'd be grateful for any insights people might be willing to share. I'm not trying to question DAN's (or anyone else's) guidelines, or to imply that they are overly conservative, or to otherwise promote any sort of unsafe practices, I just like to understand things for the sake of understanding. For example, imagining a purely hypothetical situation, would you be willing to rely on the tissue loading chart in your Petrel to decide, in the last minute, whether you can board the plane or not? If not, why not? If yes... what would the tissue bar need to look like at the time you board the plane, assuming the typical 8000 ft cabin pressure? Do you think the model, say the Buhlmann/GF in the Petrel, is even applicable in this setting? If you were to drive over a mountain range at altitude to get back home, would you be willing to rely on the computer? Would you trust one algorithm over the other? How do you think that would compare against DAN's guidelines?

 

[Dr. Lecter]

I doubt this is a terribly useful question, because the only honest answer is: it depends. How badly do you need to fly/drive to altitude? What's stopping you from applying SI O2 and how long can you stay on it before flying/ascending (great article on that in the new tech mag issue)? How closely did you follow your dive plan on BT/ascent? Are we talking about after a 200' OC air dive with 45 min of deco or after a 500' tmx CCR dive with several hours of deco?

As a general rule, no, I don't trust my tissue graph to serve as a go/no go indicator in a context for which neither it nor the model underlying it was designed to work. That said, if I had to triage my DCS risk against an actual need to get somewhere using lower than sea level air pressure travel...I'd certainly consider what it said in making my totally by the seat of my ing pants plan.

Saving a few bucks or a few days to catch a flight that you might have to divert anyway due to a DCS hit seems like piss-poor prioritizing, though, so the list of reasons to ever need to integrate this information into a choice rather short.

 

[kr2y5]

I doubt this is a terribly useful question, because the only honest answer is: it depends. How badly do you need to fly/drive to altitude? What's stopping you from applying SI O2 and how long can you stay on it before flying/ascending (great article on that in the new tech mag issue)? How closely did you follow your dive plan on BT/ascent? Are we talking about after a 200' OC air dive with 45 min of deco or after a 500' tmx CCR dive with several hours of deco?

I am perfectly happy following DAN's guidelines, with a good margin of safety, and absolutely nothing stops me from applying surface O2, but I would still love to better understand under what circumstances that O2 may be needed, for how long, or how much margin of safety I am really applying by boarding the airplane in the given situation, etc. Between the two example dives you have just mentioned, how would your no-fly interval and precautionary steps taken differ?

As a general rule, no, I don't trust my tissue graph to serve as a go/no go indicator in a context for which neither it nor the model underlying it was designed to work. That said, if I had to triage my DCS risk against an actual need to get somewhere using lower than sea level air pressure travel...I'd certainly consider what it said in making my totally by the seat of my ing pants plan.

I may be wrong, but I thought Buhlmann's model was designed for altitude diving... why do you not trust it? ( I'm not saying you should, just trying to understand what your reasons are. )

Saving a few bucks or a few days to catch a flight that you might have to divert anyway due to a DCS hit seems like piss-poor prioritizing, though, so the list of reasons to ever need to integrate this information into a choice rather short.

I agree!

 

[Dr. Lecter]

Q1: It. Depends. Either you don't fly within 24-48 hours after any deco dive, or, you decide there's something more important than an approximately 0% risk of DCS (and that list does exist, it's just short and very subjective) and you do the best you can and live (or not) with the consequences of your actions. Obviously the longer, deeper, helium loading dive is going to present you with a starker choice, all else being equal.

Read the article on SI O2 I referenced, and it should be pretty clear that there's not even a solid theory for doing what you're proposing within some kind of theoretical safety margin. You makes your guesses and you takes your chances.

Q2: Altitude diving and post-dive pressure changes are not the same thing.

 

[rjack321]

If you do enough deco you can go into outer space for all your body cares.

Ps helium is the least of my worries, its not soluble and the half life is silly short. I'd worry a lot more about N2.

 

[Agility]

of course the Bühlmann Model is suitable for altitude diving, but if you calculate your dive for an elevation of 2500m you will incur a lot of deco (if you are at sea level). And I doubt you could cheat your Petrel to believe you are diving at a higher altitude than the Petrel measures with its pressure sensor.

 

[Dr. Lecter]

If you do enough deco you can go into outer space for all your body cares.

Unfortunately there's no model of which I'm aware that will tell you what's "enough" deco for ascending to X thousand feet after Y hours of SI. After any given dive, you really have no clue whether you're miles away from being bent or right on the edge, though your deco model's level of theoretical conservation should give some insight.

Ps helium is the least of my worries, its not soluble and the half life is silly short. I'd worry a lot more about N2.

Yes and no. The half life is a theoretical matter, not what actually happens 100% of the time across each and every part of your body, and unlike N2, SI O2 isn't going to accelerate your He offgassing since you're not breathing any of it at the surface. And He comes out faster, making a sudden ascent like takeoff that much more dangerous than N2.

 

[rjack321]

Unfortunately there's no model of which I'm aware that will tell you what's "enough" deco for ascending to X thousand feet after Y hours of SI. After any given dive, you really have no clue whether you're miles away from being bent or right on the edge, though your deco model's level of theoretical conservation should give some insight.



Yes and no. The half life is a theoretical matter, not what actually happens 100% of the time across each and every part of your body, and unlike N2, SI O2 isn't going to accelerate your He offgassing since you're not breathing any of it at the surface. And He comes out faster, making a sudden ascent like takeoff that much more dangerous than N2.

Is it that difficult to assume your dive is already at 8,000ft and do enough deco to surface at 8,000ft? (even though you are on the ocean at about 4ft...) That would be the absolutely easiest way to calculate "enough", even a monkey could do that. Of course you still might get bent, the only sure thing is not to dive.

If you think you have helium in your body even 6 hrs after your deco is done you are either: 1) bent already with trapped bubbles 2) an idiot.

 

[decidedlyodd]

Unfortunately there's no model of which I'm aware that will tell you what's "enough" deco for ascending to X thousand feet after Y hours of SI. After any given dive, you really have no clue whether you're miles away from being bent or right on the edge, though your deco model's level of theoretical conservation should give some insight.

US Navy tables contain a specific table for a required SI before ascending to altitude. See the slide with a heading of "Flying after diving-2" about half way through the following:

http://www.ndc.noaa.gov/pdfs/USNDeco3.pdf

 

[Dr. Lecter]

US Navy tables contain a specific table for a required SI before ascending to altitude. See the slide with a heading of "Flying after diving-2" about half way through the following:

http://www.ndc.noaa.gov/pdfs/USNDeco3.pdf

That table appears to be for NDL diving tables with repetitive dive pressure groups. Is it somehow applicable to decompression diving? I'll admit it at least suggests the Navy has a theory for post-dive off gassing vs. ascending to altitude, and I'd be interested to see a table for deco dives based on that theory.

---------- Post added June 4th, 2014 at 10:08 PM ----------

Is it that difficult to assume your dive is already at 8,000ft and do enough deco to surface at 8,000ft? (even though you are on the ocean at about 4ft...) That would be the absolutely easiest way to calculate "enough", even a monkey could do that.

I wouldn't be surprised to find a monkey doing that. A rational human being might stop to consider that a dive commenced at 8,000' assumes the diver's body is at an 8,000' elevation at the start of the dive in terms of N2 loading, and wonder how the diver actually having sea level N2 loading would affect things. If I thought I had to fly sooner than 24-48hrs after deco diving while planning the ing dive I could certainly try your approach nonetheless, and hope the added conservatism was enough despite that discrepancy. I'd probably not do the dive if the flight was that important, however.

If you think you have helium in your body even 6 hrs after your deco is done you are either: 1) bent already with trapped bubbles 2) an idiot.

Cool, this is where you show me your degree in hyperbaric medicine, right? Because if so, awesome, I love educated certainty. On the other hand, there are few things more dangerous than uneducated certainty. Am I good to fly 6 hours after doing a major deco dive because I was on rich He mixes for all deep phases? Probably, but I'm unlikely to spout off about how it's a sure thing.