deep stops or not

[Qasar]

Good morning
I have a question that I can't solve related to deep stops.
During my training I have been tought about the rule for deep stops: (depth+6m)/2, meaning if you dive at 30m, your first deep stop is at 18m for 1mn, then (18+6)/2 second stop at 12m for another mn, then 3mn safety at 6m. All that is without decco.
Unfortunatly sometimes applying this rule led me to reach decco during the deep stops -as if starting the ascent with 1 or 2mn before decco- that I, may be, wouldn't had if I went directly to the safety 6m.
So my question is: what is the best? Deep stops as it allow more off-gasing but some tissues continue to saturate, or going directly to 6m?
The framework of this question is recreational dives around 25/30m (air or nitrox).

 

[GrimSleeper]

Or, leave the bottom earlier, rather than pushing the limits?

Actually, a more detailed response is fair.

I quite often carry out a dive to 28m on a plane wreck, where I go down first, send up a DSMB, and divers come down to the plane. This can mean I'm sat there for several minutes before anyone else arrives, so I do often push NDL on that dive. I generally find that ascending to 21m gives me enough of a bump in 'no-deco' time to stay there for a couple of minutes, then again on ascending to 15m.

If you're going into deco ascending from 25-30 to 18m, you're ascending too slowly. Try picking up the pace a little, so that you're coming up at 9m/minute, and you should find that solves your problem and allows you to both push the limits and do deep stops by your preferred method.

And now, stand by for all the people telling you that deep stops are of dubious benefit, etc etc.

 

[ajduplessis]

Good advise above.

Sounds like you ascent time is greater than 10m per min. Get off the bottom quicker and dont push NDL.

 

[Qasar]

Thanks for that and understand that coming up early solve the pb.... but this issue happens on repeated dives where your bottom time is going down very quickly so you need to pushh a little towards the limits. In fact my question was more towards the useflness of deep stops.

 

[dmainou]

Mate, first of all I would highly encourage you to read about decompression theory so you can make up your mind.

Amongst others please read erik baker's paper on gradient factors.

I really encourage you to use model derived stops as opposed to rule of thumb ones. At least there is some scientific basis fir adding them.

Then I would recommend you to read the NEDU paper on deep stops.

Finally, I would head to rebreather world and read the recent discussion around deep stops. Some high calibre people were kind enough to come into the discussion and spend a lot of time explaining things.

I know nothing about your diving but I hope this helps

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[ajduplessis]

I also noted the following in your calculations. Your deep stop calculation is wrong (partially).

Your calc: (depth+6m)/2, meaning if you dive at 30m, your first deep stop is at 18m for 1mn, then (18+6)/2 second stop at 12m

Yes, you take max depth+safety stop/2 to determine first deep stop. You then half the remaining depth, DO NOT ADD safety stop depth again. Just half the remaining depth, this is why you rack up deco. The first DS is @18, thus 18/2=9m. We (I) do not do deep stop above 10m within NDL.

Max depth of 30 m (100ft) will result in only 1 deep stop, you are doing a second deep stop that is not required.

 

[dmainou]

I would strongly advice you not to add stops that are not required by your deco model

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[Qasar]

thanks for all that dmainou. I found the NEDU paper very interesting, but I didn't found the rebreather topic.
but we come back to the basic question: if we can reach deco during a deep stop that means we are still loading gas and not off-gasing, so why doing them? In my case it happened at the deepest stop (18m). I'm trained for deco dives and it's not an issue, the pb is more theory behind. The NEDU paper seems to advocate clearly against deep stops...

 

[dmainou]

http://www.rebreatherworld.com/showthread.php?t=48083



http://www.rebreatherworld.com/showthread.php?t=47770

http://www.rebreatherworld.com/showthread.php?t=46994

The long one is all the discussion between Ross, who many argue has a commercial interest in preserving deep stops, and the scientists which are only expressing caution as deep stop models are largely untested and that the only hard data study doesn't fare well fir then.

The second one is a poll suggesting most people don't really trust Ross's claims after all that discussion.

The third one is a summary of the outcomes.

Above all, I'd say stay away from adding your own arbitrary stops. Feel free to exit the water as per your NDL model and if you wish to add stops upon decompression then use something that is substantially less aggressive in the deep side of the equation and more conservative in the shallow stop bit. That is something like 40/75 rather than 10/90 or any version of VPM.


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[Doppler]

thanks for all that dmainou. I found the NEDU paper very interesting, but I didn't found the rebreather topic.
but we come back to the basic question: if we can reach deco during a deep stop that means we are still loading gas and not off-gasing, so why doing them? In my case it happened at the deepest stop (18m). I'm trained for deco dives and it's not an issue, the pb is more theory behind. The NEDU paper seems to advocate clearly against deep stops...

You have to understand that arbitrary stops are potentially harmful. Throwing a stop into you ascent essentially means you are now testing a different algorithm to the one either in your PDC or your custom tables.

You might also want to research the term off-gassing ceiling... which is a theoretical point in the water column at which decompression (off-gassing) actually begins. Stops below this point add to one's bottomtime.

Read a book, speak with someone who understands decompression and ascent behavior.

STOP doing arbitrary STOPS.