Why do people add a few minutes to their last deco stop?

[cerich]

Several years ago I published an article on the latest research and the current thinking for decompression procedures on decompression dives, and I wanted to do the same thing for NDL dives. I discovered that there is not a whole lot of recent research on that topic. I realized that one of the reasons for that is that after more than 100 years, the science is considered pretty much settled. We have reached the point that people who dive within accepted guidelines are pretty darn safe, with only a very tiny percentage getting DCS.

Ford Pinto Actuary

They calculated they were safe as well, just a few had to pay the price. Funny how math and stats can be characterized when people are so inclined

 

[rob.mwpropane]

On a Great Lakes charter a few years ago a bunch of us are dekitting our CCRs on the back deck when "Diver B" comes up the ladder. He's still standing thigh to knee deep in the lake. Its July, calm and the surface is relatively warm in Lake Huron.

Diver B: Hey guys could you help me with my fins? My arm starting hurting on the way up.
Us: Dude what are you flooded? No? What are you doing here? You still have O2 and scrubber? Drop back down if you can.
Diver B: Oh ok I'll drop in to 20ft
Us: One of us should watch him, yeah
Extra diver splashes

10-15 mins later, Diver B is on the ladder again, extra diver behind them

Diver B: Better than it was last time
Us: Just better?
Diver B: Yeah better but still started aching at 10ft
Us: Why did you keep ascending??!?
Diver B: I dunno
Us: Go back down silly your watcher buddy is still there

15-20 mins later, Diver B is on the ladder again

Us: (preemptively) Still hurt?
Diver B: No not this time
Us: Phewww

If it *hurts* stop watching to your computer and stay where you are longer! Going up is almost for sure going to make it worse. Or repeat the previous stop (barring OOA). It was a ~45min deco kind of dive and Diver B ended up doing about 75-80mins of deco in multiple pieces.

Did the diver ever figure out what the cause was? PFO or something?

That's scary to me.. if it's a cautious diver doing everything right, crawling to the surface and still get a hit.. that's scary.

 

[dmaziuk]

That's scary to me.. if it's a cautious diver doing everything right, crawling to the surface and still get a hit.. that's scary.

Then maybe you shouldn't be diving because that's what the decompression models actually say: you do everything to the letter, you get an "acceptably" small chance of a DCS hit. Don't take my word for it, read the first couple of chapters of the DSAT report (PDF should be google-findable) for a more in-depth discussion straight from the horse's mouth.

 

[Nick_Radov]

That doesn't make it "not science."

There has been a limited amount of real scientific research into decompression. This has been enormously valuable, but if you dig into the primary sources then you'll find it covers only a very small set of profiles. Once you get into technical and repetitive diving the quality research is really thin. And we still have major gaps in theoretical knowledge around how bubble mechanics and individual human physiology impact deco.

Divers and deco algorithm implementers then frequently use pseudo-science to interpolate, extrapolate, and adjust from limited scientific guidance in order to get plans that we can use for real-world diving. A lot of stuff that most of us do is based on hearsay and anecdotes. Hence, at various times over the past few decades we have seen practices like deep stops, helium "penalty", lowered gradient factors, back gas breaks, arbitrarily extended stop times, oxygen "window", etc. Not to say that some of those practices are necessarily wrong but just unscientific.

In the end it doesn't matter much. As long as we can get out of the water safely the scientific basis or lack thereof for our practices is irrelevant. If someone wants to pad their shallow stops by a few minutes then go for it, we don't have any scientific basis to say that it's a bad idea (assuming no hypothermia, gas management issues, or environmental hazards).

 

[SpeedyJ]

No one has called decompression modeling pseudoscience - that term which seems to be taking things off the rails was introduced as a rebuke. The statement that was made is:

Unfortunately we can't do better. The deco algorithms that most of us use have only undergone limited scientific validation, especially when it comes to deep or repetitive diving. For most tech diving it's a mix of science, guesswork, and experience. Seems to work well enough, and if we want to do these dives at all then we just have to accept that our approach isn't truly scientific.

Every single talk I've watched on decompression seems to end with the expert saying something along the lines of "We just don't know"
As a diver, when I choose my gradient factors I am looking at the limited body science, my experience, and ultimately taking a guess - If I dive 50/70 is that better than 15/70? I guess... Both should work though according to the model. And both do work, until they don't and someone gets an "undeserved hit." Then people come back and make a bunch of guesses as to what their gradient factors should have been instead.

One scientist alone with limited budget is still doing science.

I didn't dismiss them as "not doing science." If that's your takeaway you've missed the point - the amount of study on decompression, especially controlled human experiments, is absolutely tiny. The body of science behind it is tiny. Its easy to understand why this isn't a focus - those of us conducting dives that include meaningful decompression probably represent 0.00001% (<100K people?) of the worlds population.

 

[boulderjohn]

Ford Pinto Actuary

They calculated they were safe as well, just a few had to pay the price. Funny how math and stats can be characterized when people are so inclined

Yep. That is consistent with my last post. A company like Ford benefits financially from this sort of thing.

So who is getting the big bucks from the research (or lack of it) in decompression theory?

 

[Lorenzoid]

I like these semantics debates. I'd say decompression is an inexact science that is more accurate along certain portions of the spectrum of decompression scenarios than others.

 

[rob.mwpropane]

Then maybe you shouldn't be diving because that's what the decompression models actually say: you do everything to the letter, you get an "acceptably" small chance of a DCS hit. Don't take my word for it, read the first couple of chapters of the DSAT report (PDF should be google-findable) for a more in-depth discussion straight from the horse's mouth.

I disagree. Having a healthy fear of something that you like doing is good, doesn't mean you should stop. I see it in my kids. One is fearless, and he'll push the envelope. I have another that is way more cautious, has some fear, and knows his limitations. Which one do you think is more likely to get hurt?

What's that saying, there are old divers and there are bold divers, but there are no old bold divers.

 

[dmaziuk]

Right, we ain't bold, we're just various degrees of bolding.

 

[tursiops]

Several years ago I published an article on the latest research and the current thinking for decompression procedures on decompression dives, and I wanted to do the same thing for NDL dives. I discovered that there is not a whole lot of recent research on that topic. I realized that one of the reasons for that is that after more than 100 years, the science is considered pretty much settled. We have reached the point that people who dive within accepted guidelines are pretty darn safe, with only a very tiny percentage getting DCS.

Pretty damn good for pseudo-science....