Tweet
I was told a few days ago that for Tek diving PADI are not in favour of deep stops anymore. Is this true? If it is what research is this based upon please?
-
-
Deep stops became fashionable when bubble models gained traction as preferable decompression models a fair few years ago. The theory was that allowing absorbed nitrogen to disperse slowly would prevent micro bubble formation at depth and thus larger damaging bendy bubbles would not form in the shallows.
The problem was that deep stops allowed continued on-gassing of the more absorbent ("faster compartment") bits of the body like the blood, and, more significantly, the slower absorbing bits ("slow compartments") like your bones which then take longer to decompress, so bends became more, rather than less, likely.
The only empirical evidence for this was in a US Navy experimental diving unit study that was conducted by the US Navy who compared deep stop strategies against "traditional" gradient models of which Bühlman 16c is the most common. The results suggested the bubble model resulted in more bends, hence the US Navy rejected a bubble model as a basis for future dive tables.
That probably sounds like gobbledygook so listen to a guy who can actually explain this stuff:
Last edited by Tewdric; 21-11-2017, 09:11 PM. -
The deep stops enthusiasts had 'get you out faster' profiles and the USN tested them properly and they failed dismally.
I think they pulled the test as not safe before they finished the planned dives.
Then a pretty good hatch job on the theory was done by Simon Mitchell, my favourite deco guru.
So we dialled up 40/75 GFs on our computers, did Bühlmann plus a bit and all lived happily ever after.
It's probably 100 times more complicated but that's my reading of it.Last edited by nigel hewitt; 23-11-2017, 10:00 PM.Helium, because I'm worth it.
Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.Comment
-
I'm guessing that link goes to Prof simon Mitchell. He's the guy to watch/listen to. But yes deep,stops have generally fallen out of fashion. You'll prob find comments around where divers talk about changing their gradient factors to mive away from deep stops , preferring longer shallow stops.Comment
-
Deep stops/bubble models always were deco woo - there was no actual evidence that they did what they were supposed to do (which is something to do with the assumed maths of bubble growth) but they got you out of the water faster - the NEDU trials simply proved it by bending a bunch of divers!Comment
-
To be exact, they did 20 profiles across the test group on Buhlmann with no bends.
They got to 18 profiles with bubble model and stopped after the 2nd bend.
*edit - "exact" appears to be a poor choice of word. Those results are from a different presentation.gif "(happy)")
This study was pretty brutal and roundly rubbishes bubble model theory (at least for the bubble model in question):
On Thalmann (gas content) - 192 dives resulting in 2 pain-only bends and one with spinal symptoms
On BVM (bubble model) - 198 dives resulting in 11 bends, including pain-only, skin & neurological.
Ultrasound scans also observed a higher instance of large bubbles in the BVM dives.
Would have been nice (for us) if they'd done Buhlmann vs. RGBMLast edited by PhilPage; 22-11-2017, 11:31 AM.Comment
-
Yes, and not limited to PADI - everyone has been forced to re-evaluate their deco approaches (unless they'd already binned deepstops). The only thing that's remarkable is that it's taken this long to get decent traction.
2011 NEDU study: http://archive.rubicon-foundation.or...pdf?sequence=1Comment
-
at a risk of going slightly off topic - what's the thinking on how this extends to the "deeper" stops being added to no-deco dives? a fair number of computers stick a stop in at 10/11m when you get close to NDL's, and I've found adding a stop in at 9m seems to reduce the feelings of fatigue I get on longer 20/25m dives... is this thinking still valid, or debunked like "proper" deep stops?Comment
-
I was alway a little puzzled that the studies were of just deep stops against just shallow stops and (when I looked at the ages ago) not deep stops with extra shallow stops as well.
Sent from my iPhone using TapatalkComment
-
The study didn't address them in any way if I recall correctly.
If you think it makes you feel better, why change it? I tend to do them if I remember and the profile lends itself to doing them, but then again I also feel less tired diving nitrox and that's a myth too apparently, so what do I know?!The views expressed are my own, worth what you've paid for them, are not on behalf of anyone else and not those of any company I worked for etc.Comment
-
In a gas content model at a given point of the dive, there's a depth at which you start (in theory) offgassing slow tissues. Any stop below that depth is a deep stop. Anything above it is shallow.
On a 25m dive, 9m is probably shallow and therefore disarable (although not for long, I'd imagine). On a 20m dive, you'll probably be better off spending the time you would have spent at 9 at 6 instead.
The latest stuff from NEDU (about the helium penalty) implies that nitrogen management is underemphasised (i.e. we're not doing enough deco) in air/nitrox tables, as the "penalty" applied when adding helium appears to be producing the right amount of deco, but for the wrong reason.
i.e. there's no need to penalise helium, but the nitrogen penalty should be higher.
or to put it another way, can the deep stops, but add that time to shallow stopsLast edited by PhilPage; 22-11-2017, 03:56 PM.Comment
-
I think that is effectively just slowing your ascent - adding time relatively shallow when you are offgassing (like a safety stop but a bit deeper) and not "proper" deepstops which are supposed to shorten your overall time for the same risk (or allow you to reduce your risk with the same in-water time) so unsurprising you feel better as you are diving more conservatively. You could get the same effect by doing more time at 6m instead. "Deepstops" TM are deeper and you are still ongassing in medium and slow tissues which seem to give the problemsLast edited by Energy58; 23-11-2017, 04:17 AM.Comment
Comment