Stop Times and Run Times??

[keri.lewis]

New to the forum, but I haven't seen any information on planning.. we all know it's approximate and people get bent when the plan says they shouldn't and get minor issues from things that should really hurt...

I'd like to see how people are planning their decompression dives:

Votes for:

1) Ratio Deco
2) Plans based on stop time (Buhlmann or VPM/B)
3) Plans based on run time (Buhlmann or VPM/B)


It seems that Deco Planner has made the metamorphosis from stop time to run time planning for deep stops - which seems in line with controlled-ascent decompression.

However there seem to have been interesting research pieces on deep stops recently, and I'd be interested to understand what people are doing at the moment to see how developments change practice. I am certainly re-visiting my planning process, and will be re-evaluating practice for the new season.

[lamont]

Quote: (Originally Posted by keri.lewis)

However there seem to have been interesting research pieces on deep stops recently, and I'd be interested to understand what people are doing at the moment to see how developments change practice. I am certainly re-visiting my planning process, and will be re-evaluating practice for the new season.

If you're talking about the NEDU paper, I don't think its very relevant unless you're diving on air and significantly overstaying your stops:

The Deco Stop

[Clare Gledhill]

Not necessarily. David gave an very interesting talk at the GUE Conference on the problems associated with overstaying on the deep stops and he argued that shorter stops based on a haldanian model were proven to be beneficial over a deep stop bubble model profile.

The problem hightlighted appeared to be that the on gassing encountered during stops deep on the profile were effectively lengthening the bottom phase of the dive yet were unaccounted for when calculating deco - making the dive more - not less - aggressive.

For this reason, whilst I don't calculate my deco on any one of the methods you set out Keri, I would describe the way I approach my deco as more run time rather than stop time orientated. IE - if my ascent has been slower for any reason that I would have liked I'll keep going to get back on track.

Does that help?

[rjack]

For myself, I have confirmed that the notion of interchangible deep and shallow time at the T1 level is bunk. So if I languish on the deeper portions of the ascent (e.g. in the 30-18m depth range; deep stops and O2 window time on the 50%) I can't trim back on the shallow. If anything I need to add time to the shallow to compensate.

Deep (21m to 9m) and shallow (6 to 3m) I use stop to stop total runtime to track, but not across the whole profile.

[graham_hk]

Here is what I do:

I treat the times as way points - ie where I need to be at a certain time in the dive.

I have always been uncomfortable with 1min stops at 80% for short (20-30min) bottom times. 5 years ago Michael Waldbrener told me a neat trick - take your bottom time in mins and do that number in seconds as your deepstops. For example - 25min bottom time gives 25 sec stops at 80% ATA, given that 20s 3m is the ascent rate you get left with 5s pauses. 300s or 5min is 5h BT which is a saturation profile - that said I have never done more than 100min bottom time (but the method works for me).

I also don't like the square profile intermediate deco - for example 25 between 21 and 6 as 5 5 5 5 5. I would do 5 (o2 window) 2 3 4 5 6 - this leaves me feeling much better post dive.

YMMV

Graham

[Retardboy]

I guess there are good reasons why GUE changed from 1min to 30s deepstops (atleast in the T1 range).
When doing deeeeep stuff, like 90m, do you calculate deco based on actual BT or runtime? In decoplanner a dive to 90m increase with 20-30min ideco if you use actual BT 20 min compared to 20min run.(depending on settings)

[lamont]

Quote: (Originally Posted by Clare Gledhill)

Not necessarily. David gave an very interesting talk at the GUE Conference on the problems associated with overstaying on the deep stops and he argued that shorter stops based on a haldanian model were proven to be beneficial over a deep stop bubble model profile.

But are we talking about results based on that NEDU paper? If so, I don't think you can draw conclusions about the utility of deep stops in VPN/GF/Ratio/etc from that study. It does highlight the fact that if you overstay deeper stops that you can't take time off the shallower stops and may need to do your shallower stops in thier entireity plus additional padding.

In the study they are dramtically overstaying all their stops from 80 fsw to the surface. They've gone well over the point where doing the deep stops are beneficial to allow bubbles that form to offgas while small and prevent bubble growth, so its not a surprise that they find that their deeper stops result in significantly worse decompression. I don't think you can extrapolate that backwards linearly and conclude that deep stops require even more penalty to the shallower portions of the deco -- at some point the bubble models claim that the deep stops effect on offgassing from the free phase and reduction in bubble growth is going to be significant compared to the decrease in effciency of the dissolved-phase decompression.

[keri.lewis]

As I expected... a wide range of answers even amongst the (nominally) most "unified" community of divers in terms of skills, planning and techniques..

maybe worth a poll to see what people are doing..

Personally, I have always treated the deep part of the ascent as a run-time guidelien - if I get a little ahead I stay slow, if I get behind I try to catch up by going a little faster..

Clare's comments about the presentation at the conference are what I'm delving into (wish I'd gone now..). Is this the NEDU work that is being talked about? As that doesn't seem to fit the types of gas/profile I routinely dive.

[Alastair]

Quote: (Originally Posted by lamont)

But are we talking about results based on that NEDU paper? If so, I don't think you can draw conclusions about the utility of deep stops in VPN/GF/Ratio/etc from that study. It does highlight the fact that if you overstay deeper stops that you can't take time off the shallower stops and may need to do your shallower stops in thier entireity plus additional padding.

In the study they are dramtically overstaying all their stops from 80 fsw to the surface. They've gone well over the point where doing the deep stops are beneficial to allow bubbles that form to offgas while small and prevent bubble growth, so its not a surprise that they find that their deeper stops result in significantly worse decompression. I don't think you can extrapolate that backwards linearly and conclude that deep stops require even more penalty to the shallower portions of the deco -- at some point the bubble models claim that the deep stops effect on offgassing from the free phase and reduction in bubble growth is going to be significant compared to the decrease in effciency of the dissolved-phase decompression.

One of the reasons this was presented at the conference was it gave David a chance to discuss the research in some more detail. One of the points David discussed was that people on the internet have said the deep stops were too long. The way the study worked meant he felt it would not have made a material difference to the DCS incidence rate. It was his opinion that the bubbling caused by a 9m(30fsw)/min ascent rate was actually dealt with effectively by the shallow stops and did not need additional deep stops.

Some questions certainly arose in the Q&A specifically around whether the same study could be done with helium mixes and what effect it might have. Unfortunately it's unlikely they'd repeat the test with helium due to the cost of the study. David did go on to say that gas switching to higher PP02 mixes would have a substantial effect on the results. There were then some more questions around how this study should be interpretted for the sort of technical diving we do (bounce diving). The short answer was that we should ensure we don't do excessive deep stops and utilise gas switches to higher pp02 gases on the ascent.

Jarrod got and spoke afterwards and pointed out that GUE had already made a change around the practise around deep stops around a year ago. I believe he was specifically referring to the removal of the 1min at 80% ATA's in the T1 range. On that basis I'd say it was relevant to the kind of diving people are doing.

Cheers
Al

[limeyx]

Quote: (Originally Posted by Alastair)

Jarrod got and spoke afterwards and pointed out that GUE had already made a change around the practise around deep stops around a year ago. I believe he was specifically referring to the removal of the 1min at 80% ATA's in the T1 range. On that basis I'd say it was relevant to the kind of diving people are doing.

Cheers
Al

Pardon my ignorance, but I am a bit unclear on what "removal of the 1min at 80% ATA's in the T1 range" means.

GUE removed just the first stop at 80% and so your 1st stop is 10 feet shallower ? Surely it can't mean all the 80% stops are removed ?

Was this a change that was communicated to anyone? I don't seem to recall hearing about any changes like that.

Generally I stop 10 feet shallower than the 80% says anyway, and make the first stop less than a minute. So if 80% @ 150 comes out to 110, then I'd stop at 100 for whatever fraction of a minute was left on the timer, then do 1's