Ascent Rate from Safety Stop

[kwinter]

According to your profile, Chesty, you have been diving for several years and have over 100 dives and deep certification. So you are not asking this question as a beginner wondering why pressure is less near the surface. And you are quoting GUE doctrine, so you are bright enough to question AJ's Kool-Ade ;-) Now let's see if we can address what I THINK you are really asking.

You should have no problem understanding that the pressure differential as you ascend gets bigger and bigger each meter. It's a 50% change from 30m (4ATA) to 10m (2ATA). It's another 50% change from there to the surface. So a 20 meter change at depth has the same physiologic impact as a 10 meter change near the surface.

You also must know that nitrogen (N2) and other "inert" gasses dissolve into your blood and tissues under pressure. These dissolved gasses have 2 ways of coming out of solution in your body as you ascend and reduce the pressure. When you ascend slowly so that the pressure gradient is small between the gas you are breathing and the level of that gas in your tissues, the dissolved gas gets exchanged at your lungs and gets exhaled. This is called "offgassing." If you ascend too quickly so that the pressure differential is too great for a slow transfer of the gas in its dissolved state, then the excess gas comes out of solution in the tissues and blood and forms bubbles. This is called "really bad ****."

The object of decompression or any ascent in diving is to maximize offgassing and avoid really bad ****. You should be able to see that this means slowing your ascent rate as you come up so that the pressure differential percent doesn't get too high. In simple terms, this would be a linear graph roughly analogous to gradient factors. In philosophical terms, it is exactly what TSandM mentioned about Zeno's Paradox. And I personally want to thank Lynne for raising the level of discussion.

As for the GUE recommendation, I think it should be clear that the ascent rate should really change constantly as you ascend. But since that is virtually impossible to model, then a step-change approach is taken with as fast as 9m/min for certain depths and then 6m/min, 3m/min and finally 1m/min. The object is to slow your ascent to maintain a constant percentage pN2 difference to enhance offgassing and avoid bubbles.

Did that help?



iPhone. iTypo. iApologize.

 

[dumpsterDiver]

Thanks for the feedback but still looking for the physiological answer. GUE standard operating procedures states "When feasible the last stop should be conducted at 6m/20ft followed by a slow ascent of 1m/3ft per minute to the surface". This is not a GUE only recommendation but why what is it about some level of pressure and off gassing that is aided by a slow ascent from 5 metre safety stop?

6 minutes to ascend from the last stop to the surface? Is this recommended for safety stops or just decompression dives? Since this is in the technical section, I am unsure.

I sometimes do 5-6 dives per day. If I did a safety stop for each one and then took 6 minutes to come up from the last stop, my dive buddies would begin to lose patience with that by the end of the day.

Depending on the dive and how I feel, I might do a minute or two extra at 15-20 feet for a deco dive and then stop at 8 or 10 feet for MAYBE 45 seconds, but I have never tried to do that kind of super slow ascent. I doubt it is even practical when the sea state is up.

 

[PfcAJ]

6 minutes to ascend from the last stop to the surface? Is this recommended for safety stops or just decompression dives? Since this is in the technical section, I am unsure.

I sometimes do 5-6 dives per day. If I did a safety stop for each one and then took 6 minutes to come up from the last stop, my dive buddies would begin to lose patience with that by the end of the day.

Depending on the dive and how I feel, I might do a minute or two extra at 15-20 feet for a deco dive and then stop at 8 or 10 feet for MAYBE 45 seconds, but I have never tried to do that kind of super slow ascent. I doubt it is even practical when the sea state is up.

6min ascent from 20-surface on deco dives is what GUE recommends these days.

 

[Chesty]

Now it makes perfect sense and a big thank you to Fabio and Imla particularly for detailed physiological answers which helped me understand it. It is not taught in OW that rising from safety stop to surface should be slower than previous ascent. Thank you all very very much. Paul

---------- Post added January 5th, 2015 at 06:22 PM ----------

Kwinter a HUGE thank you - missed your post and just reread it - this is excellent! Thanks Paul

 

[ajduplessis]

A (relatively) sudden change in ambient pressure can result in a bunch o' bubbles. That's bad.

A slower ascent allows any formed venous bubbles to collect and be dissipated gradually by the lungs.

Is this not what deep stop are doing? Oh wait Sorry!!!! I forgot that deepstop are not effective and everyone is moving away from them. Go shallow fast and dont worry about the venous bubbles then........

 

[Subcooled]

5m/15ft is a very very short distance. I bet you can do it in ten seconds if you inhale fully, inflate the BCD and kick a few times. And some day you will. Your lungs might not like it though, as some day you would forget to exhale during the ascent.

Then we have other reasons such as
- beeing safe if you miss the hole in the ice by one foot
- learning to fine tune your buoyancy (and to dive at shallow depth)

____________
"always open the coke as fast as possible, to let the bubbles out efficiently"

 

[lowviz]

...//... how does slowing the ascent in last 5 m help. I really need to understand this. Thanks Chesty

I had the same concerns. This may help: http://www.scubaboard.com/forums/ad...3-dive-computer-ascent-rates.html#post5403555

 

[PfcAJ]

Is this not what deep stop are doing? Oh wait Sorry!!!! I forgot that deepstop are not effective and everyone is moving away from them. Go shallow fast and dont worry about the venous bubbles then........

No, it's not.

20ft to the surface is a different ball game than 70ft and deeper.

 

[danvolker]

While I will never do deep stops for air or Nitrox dives, don't believe in them....and did too many fast decos from 280 foot dives with George Irvine and Bill Mee to count..."After" the 20 foot stop on pure O2, relaxing in the water, on the surface for another 5 minutes or so was also believed to offer a major offgassing value , prior to getting on the boat--which in and of itself, is quite bad for you if you are bubbling still ( the sudden high exertion)....Never had such a problem, but I always liked having plans available for unplanned scenarios.....

The moral here, could be that if you had been in a
"low on air" situation, and had to do less deco than you would have wanted, then floating on the surface for 5 to ten minutes, gently getting out of the gear, and ultimately having the boat crew get your gear and help you on the boat, might be just as mandatory as asking for the bottle of DAN O2 on the Boat.

 

[ajduplessis]

No, it's not.

20ft to the surface is a different ball game than 70ft and deeper.

Dont you dive deeper than 20ft or do you super powers eliminate venous bubbles?