Dives and dive time

[Gombessa]

PADI also recommends divers with only OW cert not diving deeper than 60 feet and how many new divers agree to let an operator take them below that recommendation?

PADI also recommends 130 feet as the limit for "recreational diving" but will my Suunto Viper calculate my NDL and SS times for a dive to 150'?

I've found that it's fairly hard to nail down the actual "recommendations" and what they mean, but from a recent call to PADI and one of their training directors, the specific 60ft recommendation only extends to "new divers," and not to OW divers generally or non-AOW divers (I called to ask because a dive op in HI told me that my OW cert was only good to 60ft, and that I was violating standards by diving deeper without AOW). Between that and the 130ft recreational floor, PADI's further recommendations/limits were described as the generic "do not dive beyond your level of experience or training." It's all kind of fuzzy from a standards perspective, e.g., what is a new diver and can you gain experience without formal training, etc.

As to your point, I was definitely offered a spot on a charter to a 100+ft wreck the day after getting certified, and on many resort dives I've encountered newly-certified divers going to 100ft and beyond.

 

[MauiScubaSteve]

I am interested in reading the analysis of data by an accredited scientific group that concludes...

Spinal DCS is far from uncommon (*) and usually hits after the second or third dive of the day (*) (but of course the previous dives are often triggering it). Especially if these dives get close to the (edit) NDL (end edit). All the more if the divers are dehydrated, aged 40 or more, etc ...

Edit : (*) Statistics from the French Federation (FFESSM). The majority of type II (neurological) DCS hits are spinal ones in France. Of course if one does four short (e.g. 30 minutes) and shallow (e.g. first dive 12 meters, second 10 meters, others 9 meters or less) dives a day, with proper ascent rates, DCS isn't a big concern, even with short SI's. That's typical of diving in some places. But on the other hand, if the dives stretch towards the 30 meters area (that was the case of the OP, BTW) or beyond, then surface interval duration becomes a very important factor. And in many places the 30 meters zone is very interesting.

I would be interested in the specifics of the study myself, since it directly contradicts what I have previously learned. In a presentation at our dive shop about two years ago, a DAN representative said that studies indicate that about 80% of DCS hits occur on the first day of a dive trip, and about 80% of those hits occur on the first dive of that first day. That goes far beyond any statistical probability, even when you factor in the fact that people don't continue to dive after they have had a noticeable DCS hit. DAN had no explanation for it.

In my previous post, I only underlined the pertinent statement that I was questioning. Since nobody has noticed the direction I was thinking, I will not only use bold and underline but then expand on the point as well...

I contend that DCS hits are very uncommon so even in the French data/analysis they never claim "Spinal DCS is far from uncommon."

The claims seem to only come from dives made in French waters ("The majority of type II (neurological) DCS hits are spinal ones in France") - How many dives are made in France annually? How many type II DCS hits happen in France annually (spinal/non-spinal)? What are the profiles of the type II spinal hits vs the profiles of the non-spinal hits? What are the profiles of the dives that did not result in hits?

Even regarding boulderjohn's DAN analysis the "80%" is of a very small percent of dives that result in a hit.

I know for a fact that a 90' max depth dive for 35 minutes with ~5 minute ascent/SS/surface (40 min total dive time), 45 minute SI and 60' max depth dive for 60 minutes can be made with a Suunto Vyper incurring no more than 3 minute SS time for dive 2, as long as you pay attention to the computer and never let the NDL time drop below 5 minutes.

There are times when I end dive 2 with a 4 minute stop, and I can not say that is because of the 3 dives the day before but I also can not rule that out.

Edit/revisit; Also, with regards to "Especially if these dives get close to the (edit) NDL (end edit)" - If we look at ndl's and speak of what percentage of the dive 5 minutes is, the deeper we go the more conservative the 5 minutes becomes. Just looking at a single dive on the RDP; the no decompression limit for a 60' max depth dive is 55 minutes, the limit for a 90' max depth dive is 25 minutes. Spending 50 minutes at 60 feet is ~91% of the allowable time (91% saturated?) and spending 20 minutes at 90 feet is only 80% of the allowable time (80% saturated?).

The first dive in this thread was likely planned as working their way shallower to be above 60 foot depth by no later than 30 minutes into the dive, and ~ 20' deep for the last 5 minutes, so there would be no noticeable SS.

 

[DaleC]

Thinking that NDL diving means that no SS's are ever required is somewhat simplictic IMO. I don't think it's logical to think that on a 100' dive at 19:59 you can directly ascend to the surface but at 20:01 you require manditory deco. The gray area, or manditory SS, covers this concept in the tables.

 

[Blackwood]

Dale - if you were responding to my post, I think you're reading into it something I did not intend.

 

[boulderjohn]

I contend that DCS hits are very uncommon so even in the French data/analysis they never claim "Spinal DCS is far from uncommon."

So how far is it from uncommon?

Here is one quote from DAN's PDE study:

The overall DCS incidence for PDE divers in our data was 4 DCS cases per 10,000 dives, but the population was not homogeneous, and the DCS incidence varied widely among subgroups (liveaboard, shore / day boat, Cozumel Dive Professionals, Scapa Flow Cold-Water Wreck). At 1 DCS case per 10,000 dives, liveaboard divers had the lowest DCS incidence in our data while Scapa Flow divers had the highest incidence at 30 cases per 10,000 dives. Cozumel dive professionals often made as many as 100 dives per series.

Here is another from a different article at a different time:

According to the PDE data collected between 1998 and 2004, the decompression sickness (DCS) incidence rate among warm-water dives fluctuated from 0 to 5 cases per 10,000 dives.

So, depending on where and how you dive, the rate of DCS of all types, not just spinal, varies between 0% to a high of 0.3% of dives. That means that in the most at risk population of divers (in the first citation only) in these studies completed 99.7% of their dives with no DCS whatsoever. In the second study of all reported dives from 1998-2004, the highest likelihood of DCS was 0.05%, with most being less than that.

That is for all types of DCS, not just spinal.

Thus, I guess the idea that "spinal DCS is far from uncommon" depends upon your definition of being far from uncommon. I suspect some people would use a definition that says it is pretty darn uncommon.

 

[DevonDiver]

I've found that it's fairly hard to nail down the actual "recommendations" ..... PADI's further recommendations/limits were described as the generic "do not dive beyond your level of experience or training."

I think that the phrase is quite self-explanatory and a good 'cover-all'. If you've not done the training, don't do the dive. At the very least, it should draw diver awareness to the need for training when undertaking new activities (inc deeper diving) or when using new kit.

Another reason I don't like PADI's addition of a "mandatory safety stop," since bottom time per their definition includes that stop.

Bottom time is defined as start of the descent until the beginning of the direct ascent to the surface. I wouldn't interpret that to mean the ascent to the surface from the safety stop....

I disagree. The concept of a mandatory stop is antithetical to no-stop (i.e. "NDL") diving.

As DaleC mentioned... deco/no-deco is a very grey line. Applying a set of tables or computer algorythm to individual human physiology is never accurate.

Your statement rings of a belief in a 'magic line' that instantaneously differentiates between two physical states.

A mandatory safety stop is not a deco stop. It doesn't serve the purpose of a deco stop in respect of calculated essential off-gassing. All it does is recognise the 'gray area'...and imposes a structured system of conservatism.

i.e. Mandatory conservatism.

 

[DaleC]

Dale - if you were responding to my post, I think you're reading into it something I did not intend.

I don't know who I'm responding to anymore

 

[MauiScubaSteve]

Bottom time is defined as start of the descent until the beginning of the direct ascent to the surface. I wouldn't interpret that to mean the ascent to the surface from the safety stop....

.....There are a number of boat dives in Hawaii where the safety stop (20-10 feet deep) is definitely part of the dive, and then you make a direct ascent to the surface.....

For many of our good shore dives if you lay down in 4 feet of water and count the first 20 minutes it took to "dive" 150 yards off shore and 20 feet deep (seeing a bunch of neat stuff the entire way) then why wouldn't you count the last 20 minutes coming in in (seeing just as much cool stuff or more).

A mandatory safety stop is not a deco stop. It doesn't serve the purpose of a deco stop in respect of calculated essential off-gassing. All it does is recognise the 'gray area'...and imposes a structured system of conservatism.

i.e. Mandatory conservatism.

Probably have to quote that part sometime!

 

[hg frogman]

I have to mend my previous posts somewhat, but in a frustrating manner, as part of what I wrote seems to be right, while part seems to remain an open subject instead of being proven completely wrong (which would be more enlightening for everyone, me included).

Here are diving statistics for the ten years 1995-2004 in France (a three pages report that you can google by searching "statistiques accidents de plongee", Alain Foret, Dr Grandjean).

A) French statistics. The average number of dive accidents was 315 per year (number of divers in France is less than 100,000). Dr Grandjean (hyperbaric MD) is an expert of the French Federation (FFESSM) who is in charge of supervising the analysis of these accidents. He analyzed annually in detail, not all cases, but a (quite random) sample of (average) 17% of them. All the percentages that follow are based on that sample (I admit that's not satisfying but that's what I got; see also my PS at the end of this post).

78% of the sample's cases were DCS, the rest being classified as lung over-expansion, ear barotraumas ,etc.

18% of the accidents happened to beginners or CMAS * divers (equivalent to OW); 82% to CMAS ** and CMAS *** divers, and instructors.

Only 1% of the accidents happened in shallow water (5 meters or less); 65% between 5 and 40 meters; 34% deeper than 40 meters.

Spinal DCS was 47% of DCS cases; cerebral DCS was 17%; vestibular 23%; bends 13%.

55% of the accidents happened while (seemingly) using proper deco procedures and ascent rates.

Amongst the main causes for the accidents were (most frequent ones in bold) sawtooth profiles, OOA, panic, poor use of equipment (e.g. inflator), fast ascent, stops omitted.

The main causes for "undeserved hits" were fatigue or lack of fitness and efforts during the dive.

I was disappointed to not find "successive dives" as a main factor for DCS because French National Instructors Medallin and Ricou mentioned in one of their books (a reference book for the French Federation) that "spinal hits represent the majority of neurological hits and often happen after a second dive, itself often close to the NDL". The statistics I found mention successive dives as a factor, only in a very limited set of cases. So it seems to not be a decisive factor from these stats BUT the stats are likely to be wrong on that specific subject (they contradict other reports; they show a weird result, as on-gassing obviously increases with repetitive dives; maybe this item was poorly documented in these stats) + French diving has some particularities (long SI's: 4 hours or more) that I will describe thereafter.

B) Some remarks about statistics and the French dive scene. Statistics describe a given population, and French divers behave differently from typical Caribbean resort divers. French diving is mostly air diving (also for deco) and mild deco diving (less than 15 minutes of total deco time), twice a day (morning and afternoon) with 4 hours or more of surface interval time. It's a long SI, and given that the total intake of Nitrogen after two or three shallow dives can be as much as the total intake during a mild deco dive (though the off-gassing differs) then if successive dives were a factor for DCS with French divers if would be all the more for resort divers diving 4 or 5 times a day with much shorter SI's. But, as I said, these French statistics don't validate this. That subject remains nevertheless open for repetitive dives deep enough and with short SI's, as shown by some British Navy experiments (described in the the PADI Diving Encyclopedia) which proved that "deep enough" dives in a "short enough" row led to DCS.

Another point to consider is that a higher (and unacceptable by most) risk of DCS doesn't mean one will actually be hit. In other words, nature often (not always) forgives big mistakes. There are also huge variations between divers, and between populations. To illustrate what I mean : three different dive tables (French Navy MN90, PADI RDP, French Navy GERS 65) give three VERY different NDL's for 30 meters/100 feet (respectively 10, 20 and 30 minutes) so that's three VERY different amounts of risk, yet all GERS 65 tables users weren't crippled.

Conclusion. Spinal DCS hits nearly 117 divers per year in France (if I extrapolate from the sample) so that's what I call "far from uncommon". On a more anecdotal note, two of my dive buddies (of which there is a fairly limited number, less than 12) have been hit by spinal DCS; one in Egypt and the other in the Maldives; both after several days of 2 to 4 no-deco dives a day (plus they were quite dehydrated).

To the OP I would say that, at least, French statistics confirm that the stops are of paramount importance (all the more at the end of repetitive dives) and should NOT be skipped. They should even be "padded", for example a safety stop should be at least 5 to 10 minutes at 4.5 meters/15 feet, followed by a very slow (3 meters/minute or less) ascent to the surface.

PS: total number of divers treated in French deco chambers per year is greater than 300. 40 to 45% of these are spinal DCS hits, according to Louge et al., Problématique des accidents de décompression médullaires (2010).

 

[Blackwood]

Bottom time is defined as start of the descent until the beginning of the direct ascent to the surface. I wouldn't interpret that to mean the ascent to the surface from the safety stop....

You and I must have a different concept of the 'direct ascent to the surface'. I seem to recall my PADI instructor defining a direct ascent as one in which one doesn't stop. But I could be wrong, and I suppose that's neither here nor there anyway.

As DaleC mentioned... deco/no-deco is a very grey line. Applying a set of tables or computer algorythm to individual human physiology is never accurate.

Agreed.

Your statement rings of a belief in a 'magic line' that instantaneously differentiates between two physical states.

Like Dale, you're reading into my post something I didn't intend. I don't believe in a magic line. And FWIW, this mandatory stop doesn't delete that magic line, it just moves it three ticks to the left.

I have no problem with mandatory stops. I personally make stops on all NDL dives. My complaint is with the mandatory stop within the context of PADI's dive system, a fundamental concept of which is the CESA.

CESA is emphasized in the opinion of many at the detriment of proper gas planning and/or buddy skills, but its viability is contradicted by a mandatory stop (plainly stated: if you have to stop, you can't CESA).

A mandatory safety stop is not a deco stop. It doesn't serve the purpose of a deco stop in respect of calculated essential off-gassing. All it does is recognise the 'gray area'...and imposes a structured system of conservatism.

i.e. Mandatory conservatism.

My suggestion previously was that there are better ways to install mandatory conservatism. Instructing students to ascend at no greater than 30fpm on tables which were developed for 60fpm is one way PADI does it. Peeling back NDLs from their algorithmic output is one way NAUI does it.

Arbitrary stops are a fine, but they do have consequences directly relating to how and what they teach new divers. My opinion is that it would be better to either take the NAUI approach and publish tables with NDLs are already shifted to the left, or to spend much more training time on the buddy system and air-sharing ascents.

(In my course, which admittedly is a single data point, we spent probably twice as much time practicing CESA as we did sharing gas).