Ascent Rates on Deep Air Deco Dives

[Wreckaddict]

If you are an experienced deep air deco diver, do you ascend from depth at a constant rate eg 10m/33ft per minute or do you initially ascend at a faster rate from the deep water eg 20m/66ft per minute?

Initially I ascended at a constant 10m per minute but recently I have been ascending at 20m per minute from deep water and slowing this rate from about 30m.

I have not experienced any problems but have been a bit worried about the potential long term effects of microbubble formation.

Any comments?

 

[blackice]

what do you clasify as deep?

 

[Doc Intrepid]

You may have had a chance to review these articles, but perhaps not. They provide some background:

http://www.abysmal.com/web/library/articles/the_elusive_bubble.html

http://www.abysmal.com/web/library/articles/abys1.pdf

http://www.abysmal.com/web/library/theory.html

The 'rate of ascent to benefit' ratio (ascent rate allowing most efficient gas transfer/tissue offgassing) ascribes a curve, so eventually you'll reach a point of diminishing returns, however, in general a slower ascent rate during offgassing is often preferable.

Before you hit the point in your ascent where you are 'off-gassing', however, you are still 'on-gassing', or put another way you are continuing to experience tissue loading in those tissue groups that absorb gas more rapidly.

Therefore during that portion of your ascent while you are still 'ongassing' or continuing to incur decompression obligation, you may choose to ascend at a more rapid rate. The rationale is that for every minute you remain below that point where you begin offgassing you will continue to incur additional decompression obligation.

So the simple answer is that your initial ascent rate to that point where you begin offgassing can be more rapid, and your rate of ascent thereafter should be significantly slower.

But it isn't a simple process. Different tissue groups ongas and offgas at varying rates, therefore during your ascent from depth (depending on how long you remained at what depth) some tissue groups will be ongassing while others are offgassing. Identifying 'the point where you begin off-gassing' may depend on which tissue group is controlling your ascent - offgassing fastest.

Other divers will have their own approaches, but my buddy and I ascend at a rate somewhat faster than 30fpm to our deep stop, 30fpm from deep stop to 70' stop, and more slowly from 70' during the deco curve thereafter. But we dive trimix. The 'most efficient' ascent rate from 70' on can possibly be best described by thinking of your rate of ascent in terms of ever more frequent pauses (e.g. if every 10 feet, if every 5 feet - theoretically if the diver could hold a momentary pause every 1 foot - the deco curve would really smooth out). Most divers I know pause every 10 feet or so to slow their ascent rates until they hold their stops.

BTW - While you'll likely hear more in posts to follow, I'll point out that the foundation of your post - 'diving to sufficiently deep depths for long enough that you need decompression profiles' - ON AIR - is unwise.

A case might be made that you have more to be concerned about from the effects of nitrogen narcosis impacting your decision-making over time at depth than you do from the theoretical long-term effects of microbubbles.

FWIW,

Doc

 

[divermasterB]

Doc Intrepid once bubbled...


Before you hit the point in your ascent where you are 'off-gassing', however, you are still 'on-gassing', or put another way you are continuing to experience tissue loading in those tissue groups that absorb gas more rapidly.

I may be wrong here, but I think that you ment to say that the slower tissues are the ones still taking on more loading.

The faster compartments will be the fastest ones to reach saturation. So before the ascent begins on a dive, the tension in the faster compartments will be higher than the slower compartments. As the ascent begins, the faster compartments will be the first to reach supersaturation and start offgassing.

The slower ones are the ones that might (depending on exposure time) still be ongassing well into the decompression series of the dive.

 

[DeepTechScuba]

TDI's ascent rates (yah yah yah I know a lot of you hate TDI) are as follows:

from below 100 ft ascend at 100 ft/min

from 100 ft to 60 ft ascend at 60 ft/min

from 60 ft to the surface ascend at 30 ft/min

Those rates are taught specific to any gas mix used in TDI, whether deep air or trimix.

You need to remember that somewhere along the way you need to make a 1 min deep stop. How you calculate that point depends on your doctrinal orientation and your software subscription.

Then after that first deep stop, there are at least 2 schools of thought about what to do next. Some believe that you continue to ascend at the above rates to your next stops, etc.

Others believe that from your first deep stop, you should ascend very slowly after that.

In terms of philosophy, I do not believe the answer is know-able, so its more a matter of faith, like going to church, than like true science and math. Thats my personal view.

You definitely should NOT be experimenting with your own body, without a support team and a deco chamber available. You should instead follow some written guidelines, and see how you feel after applying their guidelines, then make it more conservative as needed to make you feel better.

If you are diving air, then the best dive tables I have found for air are the Canadian DCIEM. I have played with GUE's decoplanner software as well, and it tends to be very conservative when you plug in air data, like DCIEM.

Bruce Wenke recommends Abyss software over DecoPlanner, FYI. NAUI considers Bruce to be one of the foremost technical authorities in scuba diving and publishes his articles in NAUI's instructor magazines.

 

[Doc Intrepid]

I stand corrected. DivemasterB is correct.

It IS indeed the tissue compartments which absorb gas more slowly that may be continuing to ongas during the ascent phase of your dive.

Sorry for the confusion, hadn't had my latte yet.

Doc

 

[MechDiver]

DeepTechScuba once bubbled...
TDI's ascent rates (yah yah yah I know a lot of you hate TDI) are as follows:

from below 100 ft ascend at 100 ft/min

from 100 ft to 60 ft ascend at 60 ft/min

from 60 ft to the surface ascend at 30 ft/min

Those rates are taught specific to any gas mix used in TDI, whether deep air or trimix.

Those ascent rates are way too fast, and way way too fast for mix.

MD

 

[cnidae]

DeepTechScuba once bubbled...


from below 100 ft ascend at 100 ft/min

from 100 ft to 60 ft ascend at 60 ft/min

from 60 ft to the surface ascend at 30 ft/min

Those rates are taught specific to any gas mix used in TDI, whether deep air or trimix.

These are old ways of thinking and I believe that most TDI instructor stopped teaching this way some time ago.

DeepTechScuba once bubbled...
You need to remember that somewhere along the way you need to make a 1 min deep stop. How you calculate that point depends on your doctrinal orientation and your software subscription.

You cannot just assign a 1 min deep stop. It’s dependent on how long you’ve been down at depth. It may range from no deep stop up to around 5 minutes or so (saturation dive).

DeepTechScuba once bubbled...
Then after that first deep stop, there are at least 2 schools of thought about what to do next. Some believe that you continue to ascend at the above rates to your next stops, etc.

Others believe that from your first deep stop, you should ascend very slowly after that.

In terms of philosophy, I do not believe the answer is know-able, so its more a matter of faith, like going to church, than like true science and math. Thats my personal view. [/B]

Your right, there are two ways of thinking. One is to get DCS by traveling at the rates above and treat it in water (the old way of thought). The other is to use the gradient factor and optimize your oxygen window to accelerate off gassing and come out of the water as clean as possible. The second method is preferred and I believe Bruce Wenke will agree with me.

 

[Wreckaddict]

blackice once bubbled...
what do you clasify as deep?

I do most of my deco dives on one wreck in 57m/187ft with a number of other wrecks in 40m/131ft.

 

[blackice]

as part of the next steps of my training I'm doing advanced nitrox and deco procedures. with this we go to 45m.

is this classified as "deep"?

I realise that in an ideal world we would only dive with nitrox to 30m and trimix beyond 30m.

But training is a step by step effort and it seems that 45m on air in next step.