Is a long safety stop beneficial?

[buff]

The Suunto Cobra has an ascent indicator. My computer is attached right at heart level-I can see the indicator and still look around at all the beasts. It has a rate ascent alarm too which I am sad to say I have violated a few times chasing after critters to get a good photo.

 

[The Iceni]

Let me try and get my head around this.

DCI is not simply caused by an excessive pressure gradient from tissue, to blood, to lung. Since the ascending diver's body is a dynamic system the risk of DCI is primarily determined by the rate of change in pressure, which is what will determine whether the pressure gradient is ever exceeded. By its nature, as a liquid, the pressure gradients during an ascent in sea water are greatest when the diver approaches the surface.

I do not believe, therefore, that there is in fact a universal safe rate of ascent for all dive scenarios, from all depths.

For a short shallow dive a fairly rapid rate of ascent MAY be quite acceptable - even with no safety stop! - but for a longer deeper dive the rate of pressure change during the ascent may exceed the body's excretion rate of blood-born nitrogen from the lungs. The longer and deeper the depth the deeper this critical point is reached. Hence the need for progressively deeper initial deco stops and prolonged shallow stops after such dives.

For example. Consider a rate of ascent of 10 M per minute. From 30 M to 20 M the pressure change is (4.0-3.0)/4.0 or 25% per minute, from 20 M to 10 M it wil be 1/3 or 33% per minute and from 10 M to the surface it will be 1/2 or 50% per minute.

If the rate of change in pressure on such an ascent must be kept below or at 25% to prevent DCI the diver must increasingly slow his ascent after he passes the 20 M mark. From 20 M to 10 M he must ascent at 10 x (25%/33%) or 7.5 M per minute, and from 10 M he should ascend at 10 x 25%/50% or 5 M per minute and so on.

Were it practically possible for a diver to adjust his rate of ascent instantly and accurately I suppose such an ascent would descibe an hyperbola. Is this what decompression tables attempt to emultate, I wonder, zig-zagging around such an "ideal" line predetermined by tissue saturation at the end of bottom time?

If a safety stop at 20' is good practice, improving safety margins, I propose it would be far better to insert a shorter stop at 40' and a longer one at 10' rather than simply extending a single safety stop at 20'.

Indeed, unnecessarily prolonging a safety stop at 20', particularly on a "no-stop" dive simply encourages more ongassing of the slowest tissues and may even increase deco penalites for subsequnt dives.

Does this make sense?

:froggy:

 

[Lost Yooper]

A long safety stop is fine, just don't shoot to the surface afterwards. Personally, I place more weight on an extremely slow final ascent than simply a stop at 20' (for rec dives) followed by an immediate ascent to the surface. I use a 4'-5'/min final ascent for all my "substantial" dives (rec or tech, but especially tech). I've seen so many people, both rec and tech, finish their 20' stop then shoot to the surface at rates easily exceeding 100'/min. This is a big no no.

The bottom line is there is no rush to get to the surface on any dive, rec or tech (for the most part). Don't think of hang time as a penalty, but rather quality and healthy offgassing.

Mike

 

[Uncle Pug]

Originally posted by Dr Paul Thomas
Were it practically possible for a diver to adjust his rate of ascent instantly and accurately I suppose such an ascent would descibe an hyperbola. Is this what decompreession tables attempt to emultate, I wonder, zig-zagging around such an "ideal" line predetermined by tissue saturation at the end of bottom time?

Read Dr. Bennett's article in the June issue of Alert Diver for additional insight. The old contention between Hill & Haldane over a slow linear ascent vs. a staged ascent with stops was fairly settled by the death of of Hill's goats.

Slow ascent with stops is best.... slowing the ascent even more from 2 ATA.

 

[100days-a-year]

Uncle Pug,was that the article that mentioned the increase in DCS with a 10fpm straight ascent.There is an ongoing trend towards treating deco and ascents as a linear relationship between time and depth however this seems to indicate there is a point at which there are diminishing returns in too slow an ascent rate from longer dives.

 

[Rick Murchison]

As I've mentioned in several posts in the past, I have long held that a one minute stop at half your max depth, then half that and half that etc until intercepting the deco schedule is a wise thing. This was based on what *seemed* right to me...
This latest article lends credence to that feeling.
Rick

 

[Uncle Pug]

Originally posted by 100days-a-year
Uncle Pug,was that the article that mentioned the increase in DCS with a 10fpm straight ascent.

Editorial page - June 2002 issue...

 

[100days-a-year]

Yep ,that's the one....continued on page 5 last para.

 

[Uncle Pug]

Originally posted by 100days-a-year
Yep ,that's the one....continued on page 5 last para.

Ya know Tony, I went back and checked my first post and it does leave the impression that I am advocating slow ascent with no stop...

Thanks for pointing it out (sorta)....

Uncle Pug does not recommend nor endorse slow ascent rates as a substitute for stops with time spent at those stops.

I also do not recommend abitrary stops with additional but abitrary time spent as a substitute for a slow ascent.

The best course is to do minimum deco stops for all dives within the NDL of 1 minute at 30', 20', and 10' with very slow ascent rates between stops and especially the last stop to surface. This is not staged decompression diving... it is prudent NDL diving...

IMO

 

[100days-a-year]

UP,that was not a broadside at you.I know your methods of ascent as you're very clear about them.I was just making sure anyone who didn't get the mag didn't draw any false conclusions.The semantics almost everyone uses including Deco modelers seems to assume a prior knowledge of imposed stops without actually mentioning them .I'd hate to see some guy buy a BT and do 20min at 100' and then try a 10fpm without the stop .