Deep Stops

[scubahiro]

Hi,

I just read a fascinating article in the recent Dive Training about the recent research into deep stops, that is taking a safety stop for one minute at half your deepest depth before slowly ascending for a 3 minute safety stop. The theory behind this seems to make sense. Basically, if you dive to 100 ft, make a one minute stop at 50 ft before the 3 minute safety stop at 15-20 ft.

Here is my question: How does one factor in multilevel diving? Often during multilevel dives, the dive will reach, say 100 ft, and then spend most of the dive time at 60 to 70. For deep stops, would one still make the 50 foot stop for make the deep stop at half the depth most time was spent?

Any thoughts?

Scubahiro

 

[MechDiver]

Hi,

Here is my question: How does one factor in multilevel diving? Often during multilevel dives, the dive will reach, say 100 ft, and then spend most of the dive time at 60 to 70. For deep stops, would one still make the 50 foot stop for make the deep stop at half the depth most time was spent?

Any thoughts?

Scubahiro

This is not a deep stop per se, but simply a pause in your ascent rate. There is another thread here about it, and its not really a new thing. If you ascend slowly, you will accomplish the same thing and not have to worry about average depth.

That being said, making the stop for only a minute will teach you good buoyancy control without increasing your ongassing to any degree.

MD

 

[MyDiveLog]

I've read a number of articles on this and they haven't covered multi-level diving, since the empirical evidence is just beginning to be collected on the simple theory. My sense is that the time you spend at 60 - 70 feet basically counts as your "half depth" stop for the 100 feet dive but, to be perfectly safe, you might also want to repeat the process, considering the 60-70 feet as the 'bottom.'

One other thing: While it probably doesn't make much difference physiologically, I was taught that the "half depth" stop is at a depth which is an average of your deepest and your safety stop. That means, for a 100' dive, your 1-minute would be at (100+15)/2 ~ 60' anyway (not 50'). The half-depth stop for 60' would be 35 feet.

 

[DA Aquamaster]

An ideal dive profile would be a decent to max depth followed by a slow gradual ascent during the dive to depths above 33 ft. before starting the "ascent" to the surface. In this situation off gassing occurs through most of the dive with only small pressure gradients and little chance for bubble formation.

A deep stop on a "square" profile helps to approximate some of the benefits of the ideal dive profile by slowing your ascent and allowing you a minute or so to off gas at depth before bubbles actually start to form.

If you look at many dives within recreational limits with software using a bubble gradient model, they will tend to be very conservative and show a couple deep stops and a 10' stop on dives where a conventional dive table or computer would approve a direct ascent all the way to the surface.

 

[MikeFerrara]

I've read a number of articles on this and they haven't covered multi-level diving, since the empirical evidence is just beginning to be collected on the simple theory.

No, Imperical evidence has been being collected for years. The recreational community is just starting to catch on though. Check out some of BRW's work or the v-planner data base.

My sense is that the time you spend at 60 - 70 feet basically counts as your "half depth" stop for the 100 feet dive but, to be perfectly safe, you might also want to repeat the process, considering the 60-70 feet as the 'bottom.'

Don't fret too much. The "half depth" thing is just a rule of thumb over simplification.

One other thing: While it probably doesn't make much difference physiologically, I was taught that the "half depth" stop is at a depth which is an average of your deepest and your safety stop.

Well I'd like to see how that relates to any decompression model. Take some decompression software and run the real numbers and learn what the model is trying to do for you.

Check out..."Technical Diving in Depth"By Bruce W, check out the vplanner web page and read the articles on the GAP web page.

 

[DepartureDiver]

If it is a multi-level dive where at lest several minutes are spent at that depth, then use the new multi-level depth with the rule ... so you divide the shallower multi-level depth in half. I would recommend a modification though. Once you divide your depth in half, then do a one minute stop at every 10' up to a final safety stop of 10 to 15 feet for a few minutes.

 

[Saturation]

You may find a more clarifying and detailed analysis of deep SAFETY stops in the May/June 2004 Alert Diver, where DAN gives a sneak peak at a yet-to-be published paper describing bubbling and deep stops .. it covers different stop times and ascent rates.

In summary, the least post-dive bubbling occurs with a 30 fpm ascent rate to a mid-depth stop at 1/2 the max bottom depth, a stop for 5 minutes here, then ended with a 5 minute safety stop at 15'. The study suggests they will still optimize the time for the mid-depth stop to see if this can be shortened. Other issues were tried: direct to 15' safety stop at 10fpm, 30fpm, etc., and they were not as optimal as the 30fpm with 5 min at mid-depth. The Pyle stop is still available, but its simpler to remember the 1/2 max depth stop for little penalty in offgassing mechanics.

For a multilevel dive, an accurate way to approach it would be the average depth just prior to ascent, then take half that depth for the deep safety stop. If your computer can readout average depth, just halved that prior to ascent. Alas, most computers do not read average depth.

Its not easy to calculate this on-the fly. This method, similar to one used by the GUE, is an estimate of the non-linear parameters for on-off gassing using conventional halftime compartments.

For example, 100' for 10 min gives 1100 ft-min.
Next level at 60' for 10min gives 600 ft-min.
Average depth = 1700 ft-min/20min = 85' ft, average depth. Your deep safety stop is 43'.

 

[rjchandler]

an estimate of the non-linear parameters for on-off gassing using conventional halftime compartments.

.

huh???

 

[BigJetDriver]

---non-linear parameters for on-off gassing using conventional halftime compartments.

By way of explanation, if I may, until the Doc gets back:

Originally, the half-time compartments, (which you will remember, are not DIRECTLY related to specific tissues but are a mathematical representation of tissure types or groups), were thought to on-gas and off-gas at EQUAL rates (i.e. their relationship is LINEAR).

Later research has shown that on-gassing occurs at one rate, and off-gassing occurs at a different rate (usually slower) for each half-time compartment. The relationship of the rate for each compartment, and over-all, is then described as NON-LINEAR.

Just as a note of information, in planned decompression work, especially with helium in the mix, the latest "best" practice seems to be including a short (typically two minute) deep stop halfway between your actual maximum bottom depth and your first required (by your deco planner) decompression stop.

Also, as the good Doc noted, slow ascent rates are ALWAYS a healthy part of the program.

Hope that helps! Cheers!

 

[BigJetDriver]

Check out..."Technical Diving in Depth"By Bruce W, check out the vplanner web page and read the articles on the GAP web page.

Further to my post above:

Mike makes some excellent recommendations here! I heartily second them!

Cheers!