Flying After Diving

[chrisch]

This could be interpreted to mean, "if the answer is not exact then it is useless. You may as well just guess."
I hope you did not mean that, because it is a silly thing to say.

I agree - the idea that an approximation is useless is wrong. It is however useful (well IMHO) to understand that the mathematical model of decompression is not based on hard science but an hypothesis. Buhlmann tested the model enough that it has real value for diving and is universally accepted as the best way to combat DCI.

Poor old Albert passed away before he could do the same for helium mixes and didn't do much in the "flying after diving" debate. Hence the personal opinion versus consensus that dominates that (and this) debate.

Belief that because the mathematical model works is proof that it is perfect is what is wrong.

 

[LFMarm]

I agree - the idea that an approximation is useless is wrong. It is however useful (well IMHO) to understand that the mathematical model of decompression is not based on hard science but an hypothesis. Buhlmann tested the model enough that it has real value for diving and is universally accepted as the best way to combat DCI.

Poor old Albert passed away before he could do the same for helium mixes and didn't do much in the "flying after diving" debate. Hence the personal opinion versus consensus that dominates that (and this) debate.

Belief that because the mathematical model works is proof that it is perfect is what is wrong.

I think we are confusing topics here. The Navy tables are based on experimental data on Navy divers, they are not theoretical.

 

[boulderjohn]

It is however useful (well IMHO) to understand that the mathematical model of decompression is not based on hard science but an hypothesis. Buhlmann tested the model enough that it has real value for diving and is universally accepted as the best way to combat DCI.

There has been a lot of hard science done regarding decompression over more than 100 years, and Buhlmann was only one of many researchers.

 

[chrisch]

I think we are confusing topics here. The Navy tables are based on experimental data on Navy divers, they are not theoretical.

No the confusion is between the data collected by experiment and the theoretical mathematical model that is used to explain it. You can establish a fit active Navy diver can be exposed to a PPN2 for a given time and then taken to surface at a given speed and no visible sign of DCI is present. One can only make a theory of why this is so. The industry (and the Navy) use a marhematical model to explain it. The model is hypothetical.

 

[chrisch]

There has been a lot of hard science done regarding decompression over more than 100 years, and Buhlmann was only one of many researchers.

True and he lacked things like modern ultrasound. However he had less health and safety to worry about so in many ways his experimental data was more coprehensive.

Nonetheless the modelling to explain the observable phenomena is hypothetical and as such limited. In my experience the greatest limitation is the "flying after diving" part which really was only done by the military. The recommendations to civilian divers is therefore limited by what the military are prepared to share.

The US Navy tables are in the public domain but it is the habit of the recreational dive industry to add conservatism to them to account for the difference in fitness and BMI of civilians, Hence the 12 or 24 hour times given. In truth these can be shortened but in so doing we are moving from established profiles that have been tested to either military sources or hypothetical times based on a model that has well understood limitations.

Modern mathematics and the ability to manipulate numbers with a computer have a propensity to encourage people to work with the mathematical model to come up with my hypothesis and quite a large proportion of that is untested. In my experience the main bit lacking is generally the time to fly simply because it is very hard to generate data. It would require experimentation with subjects in less than atmospheric pressure. This requires a specialist chamber and more time and money than you are likely to get a return on. Therefore the model is extrapolated which is mathematically unsound.

 

[Mobulai]

I personally am not complaining (my inner nerd is actually enjoying this), but is the thread now about model over-fitting

Also are we forgetting about how the Thalmann statistical model comes into play?

 

[chrisch]

Nerd away my friend - I too enjoy a good nerd fest....

For anyone bored enough and willing to waste a few hours of their lives the declassified military documentation can be found here:

https://apps.dtic.mil/sti/tr/pdf/ADA132039.pdf

As this shows the model is extrapolated. If anyone has a friend in the US Navy please ask them how much data is available (and unclassified).

I just wanted (and probably failed) to give the OP a quick understanding of why there are no real hard fast rules. I am sure they are bored and have given up so its just us deco maths nerds left

 

[boulderjohn]

. In my experience the greatest limitation is the "flying after diving" part which really was only done by the military.

The Diver Alert Network (DAN) studied flying after diving from 1992-1999. The military was not involved. The process and results are described here.

A few years ago DAN Europe did a maddening flying after diving test to determine if their 24 hour recommendation was sufficient. They determined it was sufficient by testing for bubbles immediately after diving and then 24 hours later, showing that the bubbles were pretty much gone by then. The maddening part is that it did not show 24 hours was necessary. The bubbles could have been gone after the DAN America's suggested 18 hours--or even earlier. Why they did not test earlier than 24 hours was not explained.

 

[LFMarm]

No the confusion is between the data collected by experiment and the theoretical mathematical model that is used to explain it. You can establish a fit active Navy diver can be exposed to a PPN2 for a given time and then taken to surface at a given speed and no visible sign of DCI is present. One can only make a theory of why this is so. The industry (and the Navy) use a marhematical model to explain it. The model is hypothetical.

You don't necessarily need to build a model to explain a phenomenon. You can just look at the statistics based on experiments. Navy tables show fly after dive profile that statistically do not cause DCI. No explanation required.

 

[dmaziuk]

They don't. You are doing a 45 minute deco stop.

That's the whole point: it's a deco stop that will let you come up to your starting ambient pressure unbent; it does not claim to let you come up to whatever the airplane is pressurized to unbent.