Ascent Rate - Deep Dives

[mccabejc]

I guess the question arose because of a belief that you didn't want to exceed 30fpm (again, I've always seen 30fpm as a "not to exceed" limit) because of the increased possibility of bubble formation. Therefore you wanted to ascend at a slower rate. Also, since my computer throws a mandatory decomp stop at me if I exceed 33fpm continuously (and I doubt if even the best ascender could thread that needle between 30fpm and 33fpm), I'm assuming that you want to maintain a bit of a margin below 30fpm for your ascent.

It never occurred to me that someone would decide to "ascend", then continue along the bottom, unless of course the bottom is a sloping beach and you are effectively ascending by following it.

What I like to do to ascend (if following the bottom doesn't give me the desired ascent rate) is, instead of going totally vertical, swim gently on a "glide slope" toward the surface, and toward the exit point. Since I usually shore dive, without a float, I'd otherwise have to do a freewater ascent with no reference, and those are a bit more of a challenge. A swimming ascent gives a bit more control, and brings you closer to the exit point.

 

[Charlie99]

If you are at 100ft with 1min of NDL left, and don't want to do mandatory decomp, you have no choice but to head to the surface fast enough so that the dropping NDL doesn't catch up with you.

1 min left of NDL vs 1 minute into decompression is NOT a material change of state like you assume. 1 minute short of NDL is very heavily loaded. 1 minute past NDL is still heavily loaded, but just a bit more.

This is an area where the saying "every dive is a decompression dive" is more that just a catch phrase.

At this point in the dive, you should really be looking into what is the best ascent to the surface rather than trying to stay on the NDL side of a rather arbitrary and fuzzy NDL/deco line.

 

[Charlie99]

A very interesting article is https://www.daneurope.org/eng/whatascent2.pdf

It reports doppler bubble measurements for various types of ascents after a 25 minute dive to 25m / 83'.

The linear 3m/min or 10fpm ascent was the very worst profile. In fact it was worse than either a 10m/min or 33fpm ascent straight to the surface without a stop, and worse than an 18m/min (60fpm) ascent with a 5 min stop at 6m (20').

The 2nd worst ascent profile was a 3m/min (10fpm) ascent with a 5 min 6m(20') stop.

The best profile from bubble generation point of view was 10m/min (33fpm) with 5 minute stops at 15 and 6 meters (50' and 20'). The 2nd best was the same, but with 18m/min (60fpm) ascent rate.

 

[Diver0001]

What I like to do to ascend (if following the bottom doesn't give me the desired ascent rate) is, instead of going totally vertical, swim gently on a "glide slope" toward the surface, and toward the exit point.

Well...I think this may be an example of what I was talking about. There's nothing inherently wrong with ascending by following the bottom or swimming "gently on a glide slope" but you'll probably find if you examine it carefully that you're achieving much slower than 10 metre per min ascent rates. Once again, nothing inherently wrong with this if you have the time but this is what I was describing as "continuing over the bottom" and it probably explains why the NDL is creeping up on you.

An alternative would be to sort your navigation plan out such that you find yourself along a steeper part of the bottom and/or closer to the exit at the end of the dive or alternatively, that you use a deco buoy to create a reference for ascent.

R..

 

[serambin]

I've always figured when it comes to ascents, slower is better. Well, except for deep dives, in which you may still be ongassing even while ascending, and your remaining NDL time may keep ticking down.

What I'm trying to figure is this: is there a goal, an ideal ascent rate? Of course, too fast (over 30ft/min) is bad, and my computer will throw in a mandatory stop if I exceed 33 fpm continuously or 39 fpm momentarily. But too slow is also bad, cause you may suddenly run out of NDL time.

So if you are deep, and need to start offgassing, is the goal to ascend at around 30fpm, or is slower really better?

After going to a deeper level, all tissues were exposed to a higher gradient of inflowing gases. Why wouldn't both the total amount of gasses and the gas in each tissue compartment decrease at a shallower dive level?

Dive charts clearly show a non-linear infusion of on-gassing as depth increases. This should assure off-gassing of all tissues at shallower depths until an equilibrium point is reached for that tissue compartment at that specific depth.

Stan

 

[Doc Intrepid]

Why wouldn't both the total amount of gasses and the gas in each tissue compartment decrease at a shallower dive level?

Dive charts clearly show a non-linear infusion of on-gassing as depth increases...

Stan,

They do. The total amount of gasses, in their respective tissue compartments, decreases as the diver's depth in the water column decreases and off-gassing begins to occur.

This decrease, however, like the increase on descent, is nonlinear. The various tissue groups (which are fundamentally theoretical constructs - there are no precise "tissue groups") off-gas at varying rates.

There are many factors that influence the process, not all of which are well-understood, however there are two factors that significantly influence "the gradient", or the relative rate at which diffusion occurs across a semipermiable membrane (alveoli):
1. relative pressure (which the diver controls by controlling depth, or conversely, rate of ascent) which determines the rate at which N2 comes out of solution; and
2. the amount of N2 in the lungs relative to the amount of N2 in (coming out of) solution in the bloodstream (which the diver controls by breathing gas mixes containing progressively less and less N2).

All else being equal, the steeper the gradient, the more rapid the offgassing process.

As Charlie99 pointed out in his reference to the DAN-Europe doppler study on seed bubble formation, however, it isn't always intuitive - "cleanest decompression" appears to occur with a fairly rapid ascent rate, but with stops at 50' and 20' to allow offgassing to occur - which would fit a profile seeking to provide for a steep enough gradient that N2 diffuses out of the body as rapidly as possible - but not fast enough that seed bubble development can occur. Slow ascents are definitely not the simple answer.

Doc

 

[Charlie99]

As Charlie99 pointed out in his reference to the DAN-Europe doppler study on seed bubble formation,

Interestingly, that Dan Europe article don't talk about seed bubble formation, but instead such haldanean concepts such as ppN2 of the leading compartment. Lots of phrases like "The DAN Europe Safe Dive program showed that post dive High Bubble Grades are directly related to Fast to Medium half time tissues and Nitrogen Venous Partial Pressure greater than 80% of the allowed M-value for that tissue." Very Haldanean sort of analysis.

The Erik Baker article on Deep Stops and gradient factors very directly lead to the same sort of optimization of ascent profiles also. Erik Baker's Deep Stop article clearly shows how gradient factor adjustment of profiles leads to deep stops while limiting the overpresure gradients in the fastest tissues.

No matter how you get to that conclusion -- VPM, RGBM, gradient factors, experimental observation of doppler bubbles --- the conclusion always comes back to a variable ascent rate, which gets slower and slower as you get shallow, or the more chunky equivalent ascent created by short stops at various depths.

 

[kidspot]

I see people that say the 60fpm is 20 years ago. My table is the PADI dive table updated 2003 and my dive book is the 2004 revision and there is no mention of 30fpm. Is the 30fpm maybe from another of the dive organizations? What is the date on your tables and books? My dive computer is also a Uwatec Pro and is a newer model and states the 60fpm in the instruction manual as where the computer will warn you if you exceed it.

Again I am new to diving and not trying to tell anybody they're wrong. I also know that slower IS better, to a point. I just want to know which numbers to follow. Thanks.

Jay

PADI still says no more than 60fpm, most other agencies are now teaching 30fpm, which PADI allows, they just don't allow faster than 60fpm

You may also find that you just feel better at the end of a dive if you do a slower ascent I used to get headaches when I did direct ascents (at 30fpm) so I started slowing my ascent down by adding in stops at 10 ft intervals (usually starting at 30ft) and doing the last 10ft of ascent at 10ft/min... the headaches went away, along with the tired feeling I used to get after a dive...

ymmv

Have fun diving though - so much to learn and only one lifetime to learn it in

Aloha, Tim

 

[Diver0001]

Interestingly, that Dan Europe article don't talk about seed bubble formation, but instead such haldanean concepts such as ppN2 of the leading compartment. Lots of phrases like "The DAN Europe Safe Dive program showed that post dive High Bubble Grades are directly related to Fast to Medium half time tissues and Nitrogen Venous Partial Pressure greater than 80% of the allowed M-value for that tissue." Very Haldanean sort of analysis.

The Erik Baker article on Deep Stops and gradient factors very directly lead to the same sort of optimization of ascent profiles also. Erik Baker's Deep Stop article clearly shows how gradient factor adjustment of profiles leads to deep stops while limiting the overpresure gradients in the fastest tissues.

No matter how you get to that conclusion -- VPM, RGBM, gradient factors, experimental observation of doppler bubbles --- the conclusion always comes back to a variable ascent rate, which gets slower and slower as you get shallow, or the more chunky equivalent ascent created by short stops at various depths.

I've seen this somewhere else before too and frankly I was quite surprised that the 3m/min ascent faired so badly. We're drilled in the blanket statement that slow is better but the evidence suggests quite a different picture. It had me looking for the logic.

R..

 

[mccabejc]

We're drilled in the blanket statement that slow is better but the evidence suggests quite a different picture.

Exactly the point of my original question. Would be nice if there developed a concensus on the best ascent profile, described in terms a bit easier to understand than Erik Baker's papers on M values and deep stops. Gotta confess I've tried a few times to get thru those papers, and both times I got stuck in the mud.

My PADI manual says less than 60fpm, my NAUI manual says less than 30fpm, and the Navy Dive Manual says "Always ascend at a rate of 30fpm (20 seconds per 10fsw). Minor variations in the rate of travel between 20 and 40 fsw/min are acceptable". My computer wants me to stay away from 30fpm, although apparently a lot of divers shoot for 30fpm when they need to minimize ongassing. And the latest research seems to favor some type of deep stops.

Sounds like there isn't a clear concensus in the dive industry on ascents...