Order of dives - deep first versus last

[DrSteve]

Hi,

I am sure this has been discussed before, but I cannot find the thread. Here's the question...
In OW classes you are always taught to dive deep on the first dive and get progressively shallower on subsequent dives. This appears to be true whether taught by SSI, PADI or BSAC.
It makes conceptual sense to me.

I have heard annecdotal evidence that making the second dive the deep dive does not affect the probability for getting hit and has negligable effect on your nitrogen loading. This also makes sense to me if nitrogen loading is a function of p(N2) and time.

The difference comes in my mind when you look at the speed of tissue off gassing at the surface for moderately saturated tissues. From a deep dive you will degass to a nominal level during the surface interval and then do a shallow dive. From a shallow dive you will absorb less nitrogen, but also degass less during the surface interval.

Can anyone comment on if this is true or not?

Oh finally...I have my DM deco and physiology lecture coming up. I am 99% certain that I already know more than the person giving the class...are there any nice questions people can think of to seperate the person who knows what they read (ie the instructor) and someone who understands at a deeper level?

 

[Cave Diver]

Making your deeper dive second may have a negligble effect on the nitrogen loading (although there is some disagreement regarding this) but there is another factor to consider.

Previously dissolved nitrogen bubbles may get shrunk when doing the second deeper dive and move to areas that they can cause more harm during your ascent.

Maybe they will, maybe they won't. The real question is, "is it worth the chance?"

 

[Walter]

Making your deeper dive first increases your available bottom time, but doesn't change your chances of DCS.

 

[drbill]

As Walter says, reverse profile dives are not necessarily less safe as long as you keep track of your bottom time which will be diminished if you dive such profiles. I do them quite frequently, but also dive conservative profiles when doing so.

 

[Packhorse]

Hi,


Oh finally...I have my DM deco and physiology lecture coming up. I am 99% certain that I already know more than the person giving the class...are there any nice questions people can think of to seperate the person who knows what they read (ie the instructor) and someone who understands at a deeper level?

If you know more than the instuctor shouldnt you already know what questions to ask?

"Otherwise, repetitive diving is a good thing, and you should do your shallower dive first and then your deeper one. The stupidity taught in that regard is beyond the pale."

quoted from

http://gue.com/Projects/WKPP/Decompression/repetitive.html

 

[PenguinAdLitem]

I've read online (so it MUST be true!) that some animal studies have brought up bent guinea pigs more often on so-called reverse profiles. As I recall, there were three dead guinea pigs on reverse profiles and not even a single case of DCS on the forward profiles. I think the theory is that the deeper dives shrink but don't eliminate bubbles, which then became arterial instead of venous and cause all manner of hurt. Also, if there are more bubbles at the beginning of the dive (as there will be on the second in series) then (unless bubbles get "crushed" at depth) there will be more at the end of that dive, and/or larger bubbles at the end (any way you cut it, more gas in the blood to mess with your head or organs).

Of course, in 2000 a DAN-sponsored workshop at the Smithsonian concluded, among other things, that there was no convincing evidence that reverse profiles got more people bent than forward profiles. I believe the dead guinea pigs came after that workshop.

 

[Scuba]

Here is a good summary of the Smithsonian Workshop. Worth a read.
http://www.undercurrent.org/UCnow/articles/DeepDive200005.shtml

Findings and Conclusions

Neither the U.S. Navy nor the commercial sector has prohibited reverse dive profiles, and they are performed in recreational, scientific, commercial, and military diving. Since the prohibition of reverse dive profiles cannot be traced to any definite diving experience that shows an increased risk of DCS and no convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS, the workshop participants found no reason to prohibit reverse dive profiles for no-decompression dives less than 130 fsw and depth differentials less than 40 fsw.

Here is the reference for the summary, at pubmed.gov, for the multi level reverse dive profile diving guinea pigs research study. (non-human guinea pigs)

This abstract doesn't state what the profiles and exposures were. At least some appear to be greater than the rec ones above. The results were not good for the reverse dive profile guinea pigs

McInnes S, Edmonds C, Bennett M.
The relative safety of forward and reverse diving profiles.
Undersea Hyperb Med. 2005 Nov-Dec;32(6):421-7.
PMID: 16509284 [PubMed - indexed for MEDLINE]

Yes, fortunately, it's all online. Does anyone else have experience with numerous reverse dive profiles? Please do tell the tale.

 

[SDAnderson]

Oh finally...I have my DM deco and physiology lecture coming up. I am 99% certain that I already know more than the person giving the class...are there any nice questions people can think of to seperate the person who knows what they read (ie the instructor) and someone who understands at a deeper level?

It sounds to me like you're setting yourself up for a couple of long days lugging gear in circles for the check-out divers.

At the risk of screwing up your "deeper level" of understanding by actually, you know, reading something, you might find of interest:

http://tenfootstop.blogspot.com/2006/02/debate-relative-safety-of-forward-and.html
http://www.dmscuba.com/Understanding_M-values.pdf
http://www.dmscuba.com/Modern_Deco.pdf

Oops, sorry, another student just dropped their weightbelt in 30 ft of water - you'll have to read these later, 'cause right now, you gotta go...

 

[DrSteve]

It sounds to me like you're setting yourself up for a couple of long days lugging gear in circles for the check-out divers.

At the risk of screwing up your "deeper level" of understanding by actually, you know, reading something, you might find of interest:

Well the deeper level understanding comes from a PhD in chemistry, having had to live and breath thermo for way too long and having already read those articles (and very good they are). I am simply trying to seperate the instructor who knows what they have read and can only read what is in the PADI instructor manual, from one who actually understands it. To my mind anyone can teach about how to assist instructors based on experience alone, but only someone with true practical understanding should be considering teaching about deco theory. I am not looking to argue some minutiae about whether a certain correction factor is valid, more get into a healthy discussion to see how they deal with it. The deep first/last seems to be a good one.

 

[DrSteve]

Here is a good summary of the Smithsonian Workshop. Worth a read.
http://www.undercurrent.org/UCnow/articles/DeepDive200005.shtml

Now that's a nice story - who would have thought the Smithsonian was involved with Diving theory and practices? I thought all they did was display and preserve. Cool!