Shallow before deep?

[iainwilliams]

Hi;

Personally I do not condone this type of dive, but after watching several photographers recently at a dive resort, I was curious to the perceived or real increase in risk with regard to a DCS hit.

These guys were diving from the beach and making there way to a dropoff. The depth of water out from the beach was 3-5 meters and they would take 20-30 minutes to reach the dropoff as they were taking photographs. They would then decend to 20-30 meters followed by a long safety stop at 5 meters to 3 meters as they made their way back to the shore. They did this day in day out for three weeks with a day off here and there. They also had 3 hours between subsequent dives and kept within the limits of their computer. None of them received an type of DCS and they could not see any hassles with diving this way.

Is the DCS risk greater doing this type of dive than the usual deeper first then shallow rule/methodology?

Thanks........Iain

 

[Wildcard]

The deep before shallow rule is generaly for mutpl dives. It is better to start deep and come back up, if you can as it allows for a little more off gasing at the end of the dive. I don't see how it would increase any DCS problems if your still with in limits.
Im curious to see if any one thinks different and why?

 

[Diver0001]

Hi;

Is the risk greater doing this type of dive?

Thanks........Iain

In theory I'd say the risk is probably slightly higher. Typically a "textbook" profile will put you deep at the beginning of the dive and progressively shallower where your body will have more time to offgas. Therefore you'd conclude that the nitrogen loading you're left with on a dive where you do the deepest part of the dive directly before ascending to the surface is slightly higher.

On the other hand, such things as the speed of the ascent and how hard they worked on the bottom probably have as much if not more impact on their risk.

R..

 

[wedivebc]

I frequently dive a similar profile. One of my favorite shore dives requires a 10-15 min swim to get to the wall. I usually use a 50% nitrox mix to get to the wall, switch to bottom mix for a 15-20 deep excursion then swim back after switching to 50% at the shallow depth again. If I limit my depth to 50ft before I get to the wall my equivalent air depth will be less than 20ft due to the nitrox and using V-planner I don't see any penalty for the time spent before the wall.

 

[Dive-aholic]

There's probably no harm. I don't think there are any incidents of DCS occurring after diving like this. The reason it's not recommended is because noone's done studies on that profile so better to err on the safe side. I imagine if anyone ever does get around to studying the effects of this profile they will probably conclude that the risk is minimal if any at all, as long as you stay within limits. Realize though that the only way to dive this profile is with a computer or by planning 2 separate dives and combining the PGs somehow. RDP and the wheel aren't set up to plan a shallow to deep dive. I don't have mine with me right now, but I'll have to play with it and see how that would work. Anyone ever try it?

 

[scubadobadoo]

I would imagine this profile is common while beach diving in SO Cal, Grand Cayman, Bonaire etc... I am very interested in more info here as well due to the fact that I love shore diving and frequently prefer to swim out to deeper water beneath the surface as apossed to doing a surface swim. Especially in a place like GC where you get life as soon as you enter the water.

 

[iainwilliams]

Hi; Thread starter here. Some interesting and useful answers. It seems that there is a lack of hard data to actually prove that this type of profile is completely safe, but it seems that it is not too risky provided you use a computer and stay within limits.

My call on the subject is that the shallow portion of the dive at 3-5 meters will accumulate nitrogen in the slow tissues (controlling tissues) whilst the deeper portion of the dive will have the faster tissues controlling the dive. I do not think that the acculumation of nitrogen in the slow tissues during the shallow portion will exceed the N2 limits as long as you do not do it for days on end. Nitrox was a good idea as mentioned in an earlier reply for the shallow section.

Does Dr. decompression have any thoughts on the issue?

Thanks everybody...............Iain

 

[Dr Deco]

Hello Readers:

I do not see anything particularly bad with the dives that these fellows are making.

As far as “tissues” and gas loading goes, the algorithm will load all of the compartments from fast to slow. These compartments should not be thought of as actually, anatomically defined, “tissues.” At times in the past, there were believed to be anatomical correlates for these halftimes, but that is not accord with current concepts of the etiology (cause) of DCS.

Dr Deco :doctor: