DAN and De-Co recommendations

[nick kay]

Apologies if I missed a thread referring to this subject...

I just read an article in the December issue of Diver concerning DAN's recommendation re De-Co, i.e. that deep stops are advisable.

(1) BSAC teach deep stops at ERD
(2) Other agencies teach deep stops for ER and Trimix

Therefore:
(a) Should we (BSAC) recommend/teach deep stops "in general"?
(b) Should we (BSAC) recommend/teach deep stops on the CNx SDC?

Thoughts/optinions please...

[John Thirlwell]

Apologies if I missed a thread referring to this subject...

I just read an article in the December issue of Diver concerning DAN's recommendation re De-Co, i.e. that deep stops are advisable.

(1) BSAC teach deep stops at ERD
(2) Other agencies teach deep stops for ER and Trimix

Therefore:
(a) Should we (BSAC) recommend/teach deep stops "in general"?
(b) Should we (BSAC) recommend/teach deep stops on the CNx SDC?

Thoughts/optinions please...

It seems to me that we should be teaching the safest method of achieving decompression.

Do other agencies teach deep stops following a 50m dive (BSAC depth for DL)? if so then surely BSAC should follow suit if this is acknowledged as the safest and most effective method for decompression.

If however, deep stops are only an issue having dived beyond 50m than it would make sense to include the teaching of deep stops on the ER SDC and not before, as the BSAC recommended limit of 50m wouldn't need them.

Just my rambling thoughts!

[nick kay]

DAN's findings were based on a 25min dive to 25m, followed (3hrs 30mins later) by a 20min dive to 25m

The article also says that DANs findings "appear to support the recent recommendation by US trainer NAUI to alter its recommendations for safety stops, intriducing a deep stop aalied with a shorter shallow-water stop"

According to BSAC-88 tables this would give 1min at 6m for the 1st dive, 1min at 9m and 6mins at 6m for the 2nd dive

The recommendation rsulting from this, was stops at 15m and 6m with an ascent rate of 10m/min

I'm presuming the 15m stop was based on (25m+6m)/2 = 15m (rounded), i.e. as per tech diving principles of halving the absolute pressure

i.e. for depth/time we're in Sports Diver territory here...

Interestingly, the reserach states/shows that divers ascending at a slower rate (3m/min) were more disposed to DCI

[Will Swift]

DAN's findings were based on a 25min dive to 25m, followed (3hrs 30mins later) by a 20min dive to 25m

The article also says that DANs findings "appear to support the recent recommendation by US trainer NAUI to alter its recommendations for safety stops, intriducing a deep stop aalied with a shorter shallow-water stop"

According to BSAC-88 tables this would give 1min at 6m for the 1st dive, 1min at 9m and 6mins at 6m for the 2nd dive

Does anyone doing deco, dive BSAC-88 tables anymore?


The recommendation rsulting from this, was stops at 15m and 6m with an ascent rate of 10m/min

I'm presuming the 15m stop was based on (25m+6m)/2 = 15m (rounded), i.e. as per tech diving principles of halving the absolute pressure

i.e. for depth/time we're in Sports Diver territory here...

Interestingly, the reserach states/shows that divers ascending at a slower rate (3m/min) were more disposed to DCI

I assume the slower ascent means longer on-gas time at depth and then once the off-gas starts in the lower pressure range there was no stop before the surface - sounds like it should be bad.

From memory the article also said that Haldane suggested the 2:1 pressure ratio for deep stops and demo'd to Hall, who was the slow ascent-no stop convert, that deep stops were preferable.

Personally once the stops start (GF 20/85) I stop every 3M. - It would be interesting to see if this is a better profile?

Does NAUI advocate the use of deep stops to reduce shallow? - isn't this the VPM model - which has some curious characteristics if you use EANx to accelerate

AIUI, GF or Pyle extends the shallow stops because of the deep.

[nick kay]

Does anyone doing deco, dive BSAC-88 tables anymore?
Was just pointing out that even if you're a Sports Diver using BSAC-88 tables, these were hardly "technical", "deep" or "aggressive" dives...

Personally once the stops start (GF 20/85) I stop every 3M. -Agreed - and throw in an extra "safety" stop at 3m if its safe/practical?

[Will Swift]

:=Does anyone doing deco, dive BSAC-88 tables anymore?
Was just pointing out that even if you're a Sports Diver using BSAC-88 tables, these were hardly "technical", "deep" or "aggressive" dives...


Aye - point taken.

I think this all feeds back into a thread a while back about the validity of 88 tables these days, personally I'd like to see them as non-stop dive tables and remove the deco - but I accept that there are many recreational divers using them with minimal deco very safely - its just that when you see 21 minutes deco on them they're not very 'recreational'.

Wills.

[Matt]

:=According to BSAC-88 tables this would give 1min at 6m for the 1st dive, 1min at 9m and 6mins at 6m for the 2nd dive

Does anyone doing deco, dive BSAC-88 tables anymore?

I won't go near them for deco dives. Even a limited decompression dive on 88s looks very aggressive to me.

:=Interestingly, the research states/shows that divers ascending at a slower rate (3m/min) were more disposed to DCI

I assume the slower ascent means longer on-gas time at depth and then once the off-gas starts in the lower pressure range there was no stop before the surface - sounds like it should be bad.

There are two lines of thought;
1. A deep stop is simply a slowing of the average ascent rate, hence by doing an ultra slow ascent you don't need to make a deep stop. This explanation appears to assume that the Haldanian model is a correct representation of decompression.

2. There is more to decompression than is reflected in a dissolved gas algorithm. A significant pressure differential is required to 'push' nitrogen out within the boundaries of the Haldanian model. This ascent causes bubbles to form and the greater the density of bubbles in the blood stream the more likely a DCI event inside the dissolved gas limits. A pause in the ascent discourages new bubbles from forming and gives ones that have form time to reach the lungs. Hence the density of bubbles in the blood stream is reduced.

I tend to go with the second explanation. DANs results appear to indicate that the ascent rate can limit the speed at which bubbles are produced and grow but you need to stop completely to actually reduce the number of bubbles.

Let me attempt to make that a but easier to understand. You have to ascend to off gas. If you don't ascend far enough, fast enough, you will not off gas as quickly as your traditional table predicts. Bubbles were 'discovered' after the traditional tables were developed. An ascent causes bubbles. The faster you ascend the more bubbles form. The further you ascend the more bubbles grow. The deep stop removes bubbles from the blood stream. The traditional stop removes dissolved gas.

From memory the article also said that Haldane suggested the 2:1 pressure ratio for deep stops and demo'd to Hall, who was the slow ascent-no stop convert, that deep stops were preferable.

Interesting.

Personally once the stops start (GF 20/85) I stop every 3M. - It would be interesting to see if this is a better profile?

I prefer Pyle. GFs sound plausible but I really am not sure of the science, Baker is not a physiologist. The Lo GF needs to be set very low to make the deep stop sufficiently deep. The Hi GF then has to be set very high to avoid a long shallow stop. You end up with people running some very questionable numbers, like 10/120 trying to make the profile look the way they want. It's all a bit Emperor's new clothes to me.
I am not at all convinced that alteration to the Haldanian (disolved) model is an appropriate measure to deal with a completely separate process (bubbles). YMMV

Does NAUI advocate the use of deep stops to reduce shallow? - isn't this the VPM model - which has some curious characteristics if you use EANx to accelerate

The original VPM-A looked pretty startling. Unfortunately it lead to more bends than predicted. The VPM-B model produces similar times to Buhlmann. I think what we are learning is, it is always going to take about 5 minutes to boil an egg however good your egg timer happens to be.

AIUI, GF or Pyle extends the shallow stops because of the deep.

For the sort of Air/Nitrox profiles we are talking about Pyle stops makes little difference to Buhlmann shallow stops. GFs can make a very significant difference depending on the Lo and Hi numbers you use.

Getting back to the original question. I don't know how much longer BSAC can justify sticking with 88s for deco diving. Every bit of new research shows up a new limitation.

If we speak frankly; most of us agree the ascent rate is too fast, the stops too short and they don't fit the model in the computers every one is using. I think this may be a case of few people understanding the science (I am not claiming to) hence no one wants to be responsible for the decision.

[Nigel Hewitt]

I prefer Pyle.

Ditto.
Much as the physicist in me loves models and modeling that guy did masses of diving and made something empirical from it that works. The DAN numbers that imply you need two minutes for the significant part of the blood to circulate through the avioli (a very efficient micro bubble stripper) seems to put the reason to why Pyle seems to work.

I always worry about stuff that is just number or that is tested on brave young heros half my age and a zillion time fitter.

nigelH

[iainmsmith]

:=Personally once the stops start (GF 20/85) I stop every 3M. - It would be interesting to see if this is a better profile?

I prefer Pyle. GFs sound plausible but I really am not sure of the science, Baker is not a physiologist. The Lo GF needs to be set very low to make the deep stop sufficiently deep. The Hi GF then has to be set very high to avoid a long shallow stop. You end up with people running some very questionable numbers, like 10/120 trying to make the profile look the way they want. It's all a bit Emperor's new clothes to me.

On the other hand, that's no different from Pyle Stops. Pyle is an ichthiologist (sp?), not a physiologist and the "serial halving" of his method was a description of how one can easily put together a means of generating stops "on the fly". IIRC, he wasn't expecting it to be published. There's certainly no more science to it than there is to Gradient Factors which, as you suggest, are simply a mathematical fudge to produce results which "feel" right.

Iain