Dives and dive time

[boulderjohn]

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.

Halemano,

Thanks to you, I found a brain fart of mine in my previous post and I corrected it.

I put in your edit so that others don't have to search for it.

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.

This study was discussed on SB about that time, and I recall a lot of speculation about the reason for it.

I apologize, but I was unable to find a link to either that information or the SB thread.

 

[Elvistheking]

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.

I wonder if it's due to "holiday" divers who only dive once a year and get rusty between dive trips, and jump straight into a 30m have a minor problem (mask flood, free flow, OOG, whatever) and bolt for the surface.

Related to this is the figures that I have seen from a Coastguard station in the UK which looked at the "type" of divers who were getting choppered of the local dive boats (the person responsible was the diver liaison officer who was a "PADI" diver and who got fed up being told it was only PADI trained divers who got bent rather than BSAC trained divers so wanted to look into it further).

She concluded that it wasn't PADI or BSAC but down to where most of the divers recent experience had been, whether PADI warm water stuff or BSAC quarry stuff they were both at risk in the UK sea until they got experienced in diving it.

 

[TSandM]

John, I remember seeing a discussion of that, and the author was considering the possibility that microbubbles get crushed with the first day of diving, and that the reason they saw fewer and fewer cases of DCS on multi-day trips was because of diminished nucleation. It was an intriguing idea.

 

[snturner]

Ok so I have to jump back in here. The dive boat is Mermaid Divers in Aruba.

A few of you have mentioned the schedule. You may find it interesting that I dove with this boat three mornings in a row. Same as my buddy. It is also important to know that every morning the boat was 45 mins late from leaving, according to their scedule. Several divers sitting on the boat took note of this every morning. Does this clear the picture up for any of you?

My assigned buddy also did the dives in the afternoon and one night dive. In my opinnion he was in serouis risk of deco sickness / bends. I did not do any of those dives. Only 2 morning dives for me.

I believe the reason I did not have issues was because I spent about 1 to 2 mins at 72 feet and then back up to 60. This is why I think a short saftey stop would be sufficent for me. I spotted a pair of lobsters below me and went down to get a better look. I pointed the lobster out to everyone else, except dive leader, he was off on his own. So it wasn't that I completely disregarded the dive limits. I knew exactly what I was doing. I would bet that most, if not all of you, have done something very simular.

 

[DevonDiver]

PADI RDP - General Rules.

Safety stops -- the optional 3-minute safety stop at 15 feet becomes mandatory if you get within three pressure groups of a no-decompression limit, or for any dive deeper than a hundred feet.

It's a shame that dive computers don't apply a mandatory safety stop and/or differentiate when a dive comes close enough to an NDL that such a stop is more critical.

It's been said many times in this thread that "a safety stop wasn't needed". Several anonomous DMs have also been quoted as saying that.

If in doubt... consult a table. Plan the dive/s on a table and see what you get. Apply the safety stop rule, even if you're diving with a computer.

For me, as a rule of thumb, any dive within 10 minutes of an NDL equates to a 'tables scenario' where you are in a high-end pressure group... thus the safety stop is needed.

No dive computer applies a mandatory safety stop if you haven't exceeded an NDL or exceeded an ascent rate. No dive computer applies a mandatory safety stop if you exceed 100ft/30m. That doesn't mean the safety stop isn't needed.... or wouldn't be mandatory if you were using tables.

The same is true for Surface Intervals... divers should apply common sense in equating a table 'pressure group' against a comparible time remaining from their computers' NDL...

Multiple dive rules -- Anytime the ending pressure group on the PADI table is W or X, all following surface intervals must be at least an hour. Anytime an ending pressure group is Y or Z, all following surface intervals must be three hours.

 

[boulderjohn]

Ok so I have to jump back in here. The dive boat is Mermaid Divers in Aruba.

For some reason I was unable to access the official PADI list of expelled dive organizations, but perhaps you will be able to get into it by going to the PADI web site and following the site map through Quality Management to that list.

When I went to Aruba, I chose not to use Mermaid because of the presence on that list. PADI does not routinely expel dive shops from their organization.

 

[BoltSnap]

For some reason I was unable to access the official PADI list of expelled dive organizations, but perhaps you will be able to get into it by going to the PADI web site and following the site map through Quality Management to that list.

When I went to Aruba, I chose not to use Mermaid because of the presence on that list. PADI does not routinely expel dive shops from their organization.

This gets better and better. At beginning I didn't think that this thread would last long.

 

[Blackwood]

It's a shame that dive computers don't apply a mandatory safety stop and/or differentiate when a dive comes close enough to an NDL that such a stop is more critical.

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

Given the paradigm under which most new divers learn ("provided you neither exceed this time/depth combination nor this ascent rate, you can always make a CESA"), it would be better IMO to simply reduce NDL further.

 

[wve]

It's a shame that dive computers don't apply a mandatory safety stop and/or differentiate when a dive comes close enough to an NDL that such a stop is more critical... divers should apply common sense in equating a table 'pressure group' against a comparible time remaining from their computers' NDL...

I think you make a good point, which is, I think, don't push the limits when using a computer. Just because it is "high tech" and can eliminate rounding, it is not proven that computers keep you any safer than diving with a table.

When trying to equate tables to computers, new divers should be aware that there are important differences in the way they work.

(1) For example, tables assume a square dive profile where as computers do not. Most recreational dive profiles are not square. So a table that put you within three pressure groups of the NDL would be assuming that you spent all your bottom time at that depth. But if you didn't, and if your average depth is actually more shallow, then maybe that safety stop is not as "mandatory."

(2) Tables require rounding in a conservative (safer) direction, whereas computers eliminate rounding. By eliminating rounding, computers allow more bottom time, but at the same time eliminating the rounding also eliminates the extra safety factor. (Not necessarily making it more risky, but not making it safer, either.)

(3) Bottom time (BT) is calculated differently on tables than with computers. I don't know about all computers, but mine calculate bottom time differently than is done with a table. The RDP defines bottom time as from the beginning of the descent to the beginning of the direct ascent to the surface. My computers calculate bottom time from the beginning of the descent, but also include any ascent (direct or indirect) to within 4 ft of the surface. This difference in BT definition can make equilibrating a dive with a computer to one with a table impossible.

In any case, I agree that it makes good sense to dive well within the limits of what has been proven to be reasonably safe whether using tables or a computer.

 

[Blackwood]

The RDP defines bottom time as from the beginning of the descent to the beginning of the direct ascent to the surface.

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