Ascent rate below off gassing ceiling?

[CephBirk]

I have been reading (and enjoying) "The Six Skills and Other Discussions" by Steve Lewis. He introduces the concept of off-gassing ceilings which I had not come across before. He recommends when ascending from your max depth to your off gassing ceiling to ascend no slower than 9 mps (don't dawdle around because you're continuing to on gas your tissues). This got me thinking, as long as you're below your off gassing ceiling are there decompression risks to rapid ascents? Although I never plan to ascend rapidly, I'm curious what physiological implications there are for DCS. Obviously lung-overexpansion can still be an issue but is DCS a risk since there's no off-gassing (in theory)?

 

[w3dge]

Yes. While ascending you are still decompressing on the move. If you ascend to fast to a given depth, you won't have had enough to time off-gas sufficiently to avoid exceeding critical tissue tensions, resulting in bubble formation. Its important to remember that while some tissues may indeed still be on-gassing (and thus be under-saturated) whilst you dawdle at depth, others will be doing the opposite, and so will reach super-saturation much more quickly.

I guess the real answer is that it depends on your tissue tensions before you begin the ascent.

 

[Coztick]

9mps?

 

[TrimixToo]

9mps?

Meters per second, I'm sure. 9mps is about 30fpm.

 

[Bobby]

Yes faster ascent rates when all compartments are on gassing is something some of us have been using for years. Unless extended run times, such as caves, are involved this is something generally used on 90 meters and deeper dives.

 

[Trace Malinowski]

Years ago, when the sport ascent rate was changed from 60 ft/min to a 30 ft/min ascent rate, many divers employed a 60 ft/min ascent until they reached 100 feet. Above that, they'd use 30 ft/min.

 

[Duke Dive Medicine]

I have been reading (and enjoying) "The Six Skills and Other Discussions" by Steve Lewis. He introduces the concept of off-gassing ceilings which I had not come across before. He recommends when ascending from your max depth to your off gassing ceiling to ascend no slower than 9 mps (don't dawdle around because you're continuing to on gas your tissues). This got me thinking, as long as you're below your off gassing ceiling are there decompression risks to rapid ascents? Although I never plan to ascend rapidly, I'm curious what physiological implications there are for DCS. Obviously lung-overexpansion can still be an issue but is DCS a risk since there's no off-gassing (in theory)?

That depends on a number of things, e.g. individual physiology, tissue inert gas load, rate of ascent, and how far the diver travels during the ascent. For example, if you're doing a deep technical dive and you ascend rapidly from your max depth to 10-15 feet above that depth, it's likely that there will not be enough of a pressure gradient change to cause any adverse effects. Longer ascents and faster ascent rates would increase the likelihood of bubble formation and possible DCS. In practice, you have to try pretty hard to bend yourself underwater, but in the right circumstances it's still possible. Does this help?

Best regards,
DDM

 

[elbig]

Meters per second, I'm sure. 9mps is about 30fpm.

?? 9mps is about 30fps. 9 meters per second is *very* rapid ascent. I think 9mpm is what the op meant.

 

[cathal]

In the context of the post from Dive Duke Medicine - As fast as possible to your first stop, as your only on gassing otherwise,

 

[arew+4]

9 meters per second is is nearly Olympic sprinter fast.
100/9=11.111
That would be an 11 second 100 meters