Dive and Fly - - -

[lamont]

Now, we add to the problem of ascent [depressurization] the complication that some divers may be harboring bubbles in their tissues. One can see how this now is a second problem to simply determining residual tissue nitrogen. When DCS "hits" are found that occur 10 hours after diving, it is clear that this is most likely a residual bubble problem and not one of residual nitrogen.

Last time I looked into this, the kinds of profiles that were resulting in symptoms were something like 60 mins @ 60 feet, with a 60fpm ascent, with a 2h(?) surface interval and another dive just like it -- followed by ascent to 8,000 ft 12 hours later.

I'd argue that "harboring bubbles" was the primary cause there, and that those profiles exceeded all kinds of limits and that the divers in question were most likely subclinically bent before their ascent to altitude.

The justification for those kinds of profiles is that they do happen in the recreational diving community and any kind of unform rules about flying-after-diving needs to take into account the worst cases. Unfortunately, for divers who always do adequate decompression, that can just lead to them ignoring the results of these studies completely (I know of one technical agency with instructors who believe that these rules are 'bull****' and regularly fly after teaching technical courses).

PFO vs. non-PFO is also another factor that is typically ignored in these kinds of studies since they're designed to only address the average diving case.

So, what are the limits of non-PFO divers, who perform adequate decompression and stay within limits? Are there really cases where those divers, after 2 recreational dives and a 6 hour SI, ascend to 8,000 ft and get bent?

 

[gcbryan]

Actually, I wasn't looking at the dive tables, but rather was working through some math using Buehlmann's ZH-L16 equations. After a dive to 4 atmospheres (30m), the maximum ascent pressure was 0.54bar which indicated I should be able to fly. This got me wondering why the apparent "disconnect" between the math/physics and the physiology. No big deal, but being a pilot, a soon-to-be certified diver, and an engineer it was too much of a puzzle for me to not ask the question. Thanks for the reply.

I'm pretty sure I have the answer but it's not a satisfying one!

The reality is that the algorithm isn't really predicting anything so you can't use it in the way that you are.

The things that really keep us safe...NDL's and time-to-fly aren't based on the algorithm but rather on observed data. The algorithm is then modified to match that. It's not really much of an algorithm or theory in that sense.

For instance, there was a study that determined that with a 1% occurrence of DSC the NDL at 100 fsw on air would only be 8 minutes. The 20 minute NDL that we commonly use represents 1.7% DSC occurrence and if we were to accept a 5% DSC occurrence that NDL would be in the 45 minute range!

It's the same with the time-to-fly. A study was done among those showing no DSC symptoms before flying and a range of DSC cases of varying severity after flying.

For there to be no chance of DSC the time-to-fly was longer than people would accept so the current figures were agreed upon. It's similar to the NDL situation.

It's not that the algorithms are useless but they are continually fitted to known data and not the other way around.

The reality is that decompression is more complicated than the models.

 

[Mr Carcharodon]

The original thread was about if it was safe to fly after a 15 fsw dive. DAN was making a recommendation for all recreational diving. Clearly that recommendation is going to be more restrictive since they have to capture deeper and repetitive dives.

 

[mpgunner]

It is best to play this on the safe side. I was wrapping up a Rescue dive course (20ft max) and know I was going to fly the next day (SEA to LAX and back on the same day). At the last minute my buddies said "let's do a fun dive" and we dove to 70ft+. Well after my flight I wass useless on Tuesday. Couldn't think and very tired. It wasn't until a week later I relaized what had happened. Nevery doing that again.

 

[gcbryan]

The original thread was about if it was safe to fly after a 15 fsw dive. DAN was making a recommendation for all recreational diving. Clearly that recommendation is going to be more restrictive since they have to capture deeper and repetitive dives.

My point would be the same. It's safe if no one has ever gotten DSC under those circumstances and it's not it someone has regardless of what the algorithm predicted.

I agree that a 15 fsw dive is not likely to result in a problem. However, all of the rules are more conservative than what the algorithm would predict (as far as I can tell).

The rule (as I recall) for a single dive is for a 12 hour wait before flying. The controlling compartment (Buhlmann) for most recreational single dives is going to be one of the faster compartments...let's say the 12.5 min half-time compartment.

That compartment should be completely clear after 6 half-times so 6*12.5min or 75 minutes and yet the recommendation is for 12 hours.

It would be hard for a recreational single dive to have the controlling compartment be in the 2 hour range to result in a 12 hour no fly time.

Maybe, I'm missing something however.

 

[tlj3071]

I agree that the physics don't match up with reality. If it did, then you could dive to 120fsw for the PADI limit (13 minutes using a 1fps descent/ascent rate), a 3 minute safety stop, climb out onto the pontoon of a float plane, spend 3 minutes removing your equipment and immediately take off and climb to 10000 ft MSL and still be within the maximum ascent level of the controlling compartment (compartment 4 controls this with a max tolerated pressure of 0.677 bar = 10707 ft MSL - neglecting the time it takes to climb to 10000 ft).

Interestingly enough, if you spent 10 hours at 33 fsw (say in an underwater habitat), then you should probably not even take the elevator up to Lido deck on your cruise ship for a late lunch!

 

[Mr Carcharodon]

The physics just tells us the driving force behind decompression is pressure but not what the physiological limits are. For that one could use critical supersaturation ratios or m-values which were experimentally derived. Those approaches suggest that a depth can be determined where no-fly time drops to zero.

There does not appear to be a study of this in a recreational dive setting but NASA did study this to support flying after EVA practice (“NASA Flying After Diving Procedures”, Pollock and Fitzpatrick, in the DAN FAD Proceedings). The NASA no-fly time (during the 1980s) was zero for dives to 20 ffw for durations of less than 360 minutes. Later work (see table 2 in the referenced paper) suggests a 3 to 5 hour pre-flight surface interval after a 20 ffw dive, which appears to be largely based on pre-cordial Doppler bubble scores since symptomatic DCS was essentially absent for that exposure. But the original post was for a less severe exposure than NASA studied since the diving depth and subsequent altitude were both less. In that case we can assume that the no-fly time is less. M-values derived from Buhlmann’s tables suggest it is zero, the JSC table suggests it is less than 3 hours.

None of this is intended to suggest that the no-fly guidelines do not apply to deeper or repetitive dives. They do.

 

[Gene_Hobbs]

(“NASA Flying After Diving Procedures”, Pollock and Fitzpatrick, in the DAN FAD Proceedings).

and it is found here and is a part of our suggested reading list on this topic.

 

[tlj3071]

My thanks to all who took the time to contribute to my better understanding of this issue. After re-reading all responses, doing more research, cogitating for a few weeks, and after extensive conversations with my dive instructor (as an aside - I have now completed all coursework and all confined water dives!), I have come to these conclusions:

1) The physiology and physics of decompression are not necessarily the same.
2) Virtually all "constants" used in diving are generally conservative approximations of the actual physical constants (e.g. 10fsw only equals 1 bar at a specific constant temperature, 10m fsw is actually 32.808fsw - not 33fsw, surface pressure is only 1 ata on a "standard" day - 59 degF, ~29.92"Hg, etc, etc, etc)
3) The human body [apparently] either reacts differently to compression/decompression pressure gradients differently in water than in air, or due to the confined nature of air travel a much more conservative approach must be utilized.
4) No-fly limits have generally been determined by the most conservative estimates of groups of experts based on experimental data.

I want to stress that I find nothing amiss with any of the above. As an engineer with 30+ years of experience, I am trained to constantly question any answer that seems contrived - this is what I was doing when I made my first post in this series.

Hopefully, I'll complete my cert dives soon and I look forward joining the ranks of divers like yourselves. Perhaps in the future I can contribute in some small way to the diving community.

Tom

 

[Thalassamania]

Regardless of the number of micronuclei present it is not possible for more than 100% of all the dissolved gas present to transfer into bubbles. We know based on Henry’s law and Boyle’s law that the total volume of dissolved gas available for a 15 fsw dive directly followed by an ascent to 8000 feet above sea level is less than for a 30 fsw dive. Now the later dive has an infinite no stop time on every table out there. Why should a 15 fsw dive have a stop at the surface? No-fly time in plain language is a decompression stop.

."No-fly time in plain language is a decompression" That's a good observation.

There are a few things though that you need to consider.

There are no-D limits for 30 feet, the generally accepted saturation depth that permits return to the surface at 30 fpm is 24 fsw, not 30 fsw.

While there is a critical volume of gas, below which no damage is possible, the size a bubble that can (will? may?) create problems is much smaller than the amount of dissolved gas that would be needed to have a reasonable probability of having a bubble that size created AND lodged in a location that will present a problem.