deep stops

[markg]

thanks for the replies thought i was right but didn't want to argue with regional coach until he has signed up my master diver and instructor logs.
then i will tell him he's a tw*t
cheers
markg

[Mark]

Originally Posted by Mark Chase

Using my normal 20/80 profile my first stop would be 27m. Using Pyle stops it would be 31m. Using DIR's deco on the fly it would (I beleave) be 40m.

Not quite right. To work out your from where the deepstops begin you can do one of two calculations:

• 80% of Max ATAs (IE 6 x .8 for a 50m dive)
  or
  
• 75% of Max depth in Metres (ie (50m/2)+(25/2)) The easier way IMO!

Our dives are planned and executed in increments of 3m so I would count a 50m dive as 51m. I would then round up my deepstop to 39m.

When you have reached the start depth you would do a 3m per minute ascent up to your gas switch or 1st extended deco stop.

HTH

Regards,

Mark.

[wreckweasel]

Originally Posted by MartinS

Zak may be able to help here - does anyone know if the cochran will penalise me for deep stops? I guess I could try it and see, but just wondered how it coped with e.g. Pyle Stops.
Regards and dive safe
Martin

Martin,

Given that cochranes are pretty damn fast to get you out of the water, I cant say I've ever noticed. It tends to clear before my tables, so I dont worry about it.

I suspect it does give you a small penalty, which it then makes up for with its fast clearing in the shallows. Not exactly its original design intention, but it seems to work.

I hasten to add, I always over-deco the cochrane.......

what works, works



/Zak

[MartinS]

Thanks for that Zak

Yes, as you say, it does get you out quicker anyway (compared to the same dive/depth/duration, I got out 15 minutes before an Aladin!!!) Have upped the conservation to 50% to reduce this difference, but am interested to seeing if I can help reduce post-dive fatigue with deep stops, especially as I'm not racking up the same deco as some!

Regards and dive safe

Martin

[wreckweasel]

Originally Posted by MartinS

Thanks for that Zak

Yes, as you say, it does get you out quicker anyway (compared to the same dive/depth/duration, I got out 15 minutes before an Aladin!!!) Have upped the conservation to 50% to reduce this difference, but am interested to seeing if I can help reduce post-dive fatigue with deep stops, especially as I'm not racking up the same deco as some!

Regards and dive safe

Martin

leave the conservatism alone... it only seems to effect dives in a series (not that common over here). Just pad it out with some manual deep stopping.... then when it gets shallow, pull all your stops 1 depth deeper than it recommends with a final at 6m. you'll find you end up with quite a good deco curve.....


/Z

[Brian Donaghy]

Originally Posted by chrisch

As we nearly all use a software package to cut our own table these days I don't see any "new" tables emerging any time soon. I believe NAUI have some recent RGBM tables out, whether these have deep stops or not I don't know - they are not on the website.

NAUI has recreational air/EAN32/EAN36 RGBM tables and "technical" trimix RGBM tables. They both use real RGBM rather than the BS-RGBM that many computers use.

The recreational tables are very simple and so long as you play along with the "instructions" are way less conservative than you would expect. They only implement deep stops in the sense that NAUI recomend a one minute stop at half max depth for recreational dives > 12m.

The technical tables are the sort of 3m incremental stops thing you would get from GAP or Abyss. They are however usless past 50m to my mind because they only use one deco gas (O2) even if you spend 30 mins at 100m!

[rossh]

Originally Posted by Mark

Not quite right. To work out your from where the deepstops begin you can do one of two calculations:

• 80% of Max ATAs (IE 6 x .8 for a 50m dive)
  or
• 75% of Max depth in Metres (ie (50m/2)+(25/2)) The easier way IMO!

Our dives are planned and executed in increments of 3m so I would count a 50m dive as 51m. I would then round up my deepstop to 39m.
When you have reached the start depth you would do a 3m per minute ascent up to your gas switch or 1st extended deco stop.
HTHRegards,
Mark.

The calculation mentioned above is in common use for the Ratio Deco method, but it's not very accurate, and will over estimate the first stop in all but the deepest dives.

The first stop placement is a function of pressure variation from your bottom depth. The pressure variation experienced from bottom depth is what starts the bubbling and limits the deco. i.e. a supersaturation pressure gradient.

A more accurate quick value for first stop placement would be

Bottom depth - 1.5 to 2 ATA. (Bottom - 15 to 20 m)

Regards

[Mark]

A more accurate quick value for first stop placement would be

Bottom depth - 1.5 to 2 ATA. (Bottom - 15 to 20 m)

That appears to be an oxymoron. I know that deco is not an exact science but 15 to 20m is not exactly accurate either.

I'll stick with my GUE training - 80% of ATA's and ratio deco if it's all the same.
Regards,

Mark.

PS. I just ran a 51m profile on V-Planner and it said that off gassing starts at 37m. So by starting a 3m/min ascent from 39m (doing it the GUE way) it is so similar that it will not make any odds at all. The rest of the profile was quite unlike a GUE deco profile but that is an entirely seperate subject me thinks.

Here is the profile:

V-Planner 3.43 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM-B
DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0m
Conservatism = + 2
Dec to 51m (3) on Trimix 21.0/35.0, 15m/min descent.
Level 51m 26:36 (30) on Trimix 21.0/35.0, 1.27 ppO2, 24m ead, 30m end
Asc to 30m (32) on Trimix 21.0/35.0, -9m/min ascent.
Stop at 30m 0:40 (33) on Trimix 21.0/35.0, 0.84 ppO2, 12m ead, 16m end
Stop at 27m 1:00 (34) on Trimix 21.0/35.0, 0.77 ppO2, 11m ead, 14m end
Stop at 24m 1:00 (35) on Trimix 21.0/35.0, 0.71 ppO2, 9m ead, 12m end
Stop at 21m 5:00 (40) on Nitrox 50.0, 1.54 ppO2, 10m ead
Stop at 12m 3:20 (44) on Nitrox 50.0, 1.10 ppO2, 4m ead
Stop at 9m 5:00 (49) on Nitrox 50.0, 0.95 ppO2, 2m ead
Stop at 6m 8:00 (57) on Nitrox 50.0, 0.80 ppO2, 0m ead
Stop at 3m 15:00 (72) on Nitrox 50.0, 0.65 ppO2, 0m ead
Asc to sfc. (72) on Nitrox 50.0, -9m/min ascent.
Off gassing starts at 37m
OTU's this dive: 74
CNS Total: 28.8%
3854.0 ltr Trimix 21.0/35.0
1119.0 ltr Nitrox 50.0
4973 ltr TOTAL

[rossh]

Originally Posted by Mark

That appears to be an oxymoron. I know that deco is not an exact science but 15 to 20m is not exactly accurate either.

Hello Mark,

Correct, the 15 - 20m is not an exact value, and is not intended to be so either. I have provided this in the same context in your RD method - quick, nasty, and approximate. I think it is however, more valid and has a wider range of use than the 80%. The 80% works well down deep, but is out of proportion for lesser dives.

This 15 to 20m value for a first stop can be seen in other aspects of diving too. eg. look at the NDL's times for dives of 15 through 25m. Notice how the times rapidly change in this area - showing also this 2 ATA pressure difference is a common limiting factor.

Originally Posted by Mark

PS. I just ran a 51m profile on V-Planner and it said that off gassing starts at 37m.

The "Off Gassing Starts At" value, is the computed depth at which gas reversal occurs (on gassing stops). The first stop will be some distance again from this point, in order to create the gradient to drive off gassing. The total of these two is typically 15 to 25m, and the sample dive shown is 21m.

This applies to the dive in ascent off the bottom. We continue to accumulate gas loading from bottom up to the "Off Gassing Starts At" point. We should move through this area without delay (as planned, or faster), as extra time in this deepest section will only add to our planned gas loadings.

Regards

[bang-on]

Originally Posted by rossh

The "Off Gassing Starts At" value, is the computed depth at which gas reversal occurs (on gassing stops). The first stop will be some distance again from this point, in order to create the gradient to drive off gassing. The total of these two is typically 15 to 25m, and the sample dive shown is 21m.

This applies to the dive in ascent off the bottom. We continue to accumulate gas loading from bottom up to the "Off Gassing Starts At" point. We should move through this area without delay (as planned, or faster), as extra time in this deepest section will only add to our planned gas loadings.

Regards

ross,
in terms of reaching the "off gassing starts point" it is correct to assume a slowing in "on gassing" as soon as you leave the max depth and commence an ascent?
if so,is it not then wiser to make that initial ascent at a slower rate to ease through the on to off gassing process particularly on deeper dives with higher % helium mixes?


cheers
barrie