Ascent Rate from Safety Stop

[Chesty]

I have been doing heaps of research to try and understand the increasing argument that the greatest pressure differential is from 5M to the surface as such the ascent rate should be around 2 minutes from 5M to surface. My question is why - we all know ambient pressure change toward surface decreases the inert gas pressure we are breathing and saturated tissues off gas accordingly so why not simply come to the surface for the greatest differential? I know that a quick ascent can mean bubbles form as gas comes out of solution to quickly but if we have had a perfect dive and we have done a safety stop (and even a deep stop) and had 5 mins at 5 M how does slowing the ascent in last 5 m help. I really need to understand this. Thanks Chesty

 

[NAM001]

99 ft to 33 ft is a cut in ambient pressure of 1/2 . 33 ft to surface is a cut of 1/2 again. when youstart from 100 and go to 33 you are starting the off gassing and it needs time for the off gassing to catch up to your shallower depth. So 99 to 33 you go from no off gassing to x amount of off gassing. 33- surface is going from off gassing to faster off gassing. very crude wording but there it is. I look at it as the last five foot of jumping off a ten foot roof is not the same as the last five feet jumping from a 20 foot high roof.

 

[TSandM]

If you have spent 5 minutes at 15 feet, and haven't pushed your NDLs deeper, is it likely that you will get DCS from just corking from 15 feet? No. Is it the best ascent strategy? Probably not.

If you look at the output from decompression programs, the proportional pressure changes result in a slower and slower ascent as you approach the surface. It makes sense in a way -- the pressure change from 15 feet to the surface is the same as going from 66 feet to 33, in terms of ratios. If you take this concept to its extreme, we should use Zeno's Paradox, and cut the distance to the surface in half, and in half again, and in half again . . . we'd never poke our heads up!

Your question points out the fallacy in that. In practice, if you have done a good safety stop, you are highly unlikely to be hurt by surfacing abruptly. But what is the problem with surfacing slowly? Not only is it probably better for you, but it is also an elegant exercise in buoyancy control, which can do nothing but help you in other situations at greater depths. Why not make it a game, and see just how much time you can spend between 15 feet and the surface? If you have the gas to do it (and if there is no boat traffic or other reason for not dawdling in the shallows) it can just be fun.

 

[Patoux01]

To me, the answer is "as slow as safety allows". By safety I mean: boats, swell (if you're surfacing in the swell next to your boat, you'll eventually get "sucked" against it, it somehow just happens), thunderstorm on its way, buddy needing assistance, out of air (though you probably need to more than 2bar to get out from 5m at a slow rate). If any of those apply, I'm out "fast".

+1 for TSandM, I personally make it a challenge to have "no more than 1 bar" (of ascent speed) shown on the computer during ascent. Which is basically around 3 meters/minute.
The reason for the slowing ascent at shallower depths is quite well explained in the GUE Rec 1 book. I'm no expert, others might think that's stupid. There are other books as well on the subject, but I haven't read any.

However, I have to say that bypassing the safety stop is quite unlikely to get you DCS, hence the name "safety stop", unlike a "mandatory stop" or "deco stop".
If you respect the ascent rates using your gauges, you're very unlikely to get a hit.

 

[Chesty]

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?

 

[PfcAJ]

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.

 

[NAM001]

Chesty
Perhaps the answer is in gradient factors

 

[Imla]

The reason for slowing your ascent is quite right because the change in pressure and hence the "transformation" from gas in solution to gas bubbles will happen faster the bigger the pressuredifference. The term no decompression diving is slightly disinformative. A dive will ALWAYS lead to inert gas in solution, and coming up from a dive will ALWAYS lead to gas coming out of solution. The difference between creating harm, and getting up scott free is dependent on the gas exchange in your lungs to be effective enough to eliminate the inert gas, and even more importantly, keeping the balance between gas in solution and "free gas" to make sure larger bubbles do not form.

A slow ascent will keep the balance between gas coming out of solution and the lungs ability to off gas. This is both dependent on different gradient between concentration in alveolar gas, and in the partial pressures of gas in solution. However... you are more than capable of ascending so fast that the sirkulatory system cannot bring blood fast enough to the lungs to off gas inert gas, and hence bubbles will form.... Bubbles... well... they are bad...

personally... I strive to do max 3m/min up to 6m, then 1m/min from there... (But it is so cold in the water, I might be happy with 2m/min )

 

[fsardone]

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?

While diving, gas in solution will stay in the tissues (and being asymptomatic) until you go over a critical, tissue related, value where a bubble forms and start giving troubles. When a bubble is already formed will follow Boyle's law which is (pressure)x(volume)=constant (constant temp and amount of gas).

This said at the end of almost any dive, DAN research has proven that there are bubbles in the blood stream. This bubbles if they stay small enough will go through our circulatory system without causing problems and eventually be filtered by the lungs releasing them into expired gas. If too big will block some part of the veins/arteries creating blood and oxygen starvation to tissues which might be subject to damage. So is not the off gassing that it is aided but the elimination of bubbles resulting from the off gassing. This is also the reason why if you cannot do the deco is better to go straight to surface and into a chamber rather than do part deco and surface in the middle of it (no liability assumed, better learn more before attempting and I am talking of surfacing to a boat having a chamber available on board to enter within 5 minutes).

Bottom line you NEED to ascend slowly (1m/minute) in the last (I would say 6 meters because I do my last deco there in pure o2) because ambient pressure goes from 1.6 bar to 1.0 bar hence making the bubbles 1.6 times bigger than they are and you need to give the time to your heart to pump all the blood to the lungs to purge them before they are too big to create troubles.

Very crude explanation but it should give the sense.
Keep in mind that a capillary blocked in the brain might bring down some important functions (capello is hair in Italian which comes from latin and indicate that the vase is as big as an hair ... a very small bubble might block it one 60% bigger might create additional damage).

Cheers

Fabio

 

[tbone1004]

personally I follow the 9m/min ascent to my last stop and depending on how long I've been done, how I conducted by deco stops, very rarely precautionary stops any more, and the conditions, depends on whether the rest of the ascent is 1m/min or 9m/min. Typically it's around 3-5 because I'll do an extra 2 minutes on deco, and since my ascent is on pure o2 I'm a little less concerned about bubble formation because I've gotten well below any NDL tissue loading. Sometimes a 9m/min ascent rate is unrealistic if there is a lot of boat traffic or a lot of swell, sometimes you have to jump up there to get picked up.

The reasoning for slowing down is due to the exponential pressure gradient instead of it being linear which is what your body cares about due to bubble size. Should have been covered in depth in your OW class