repetative dives

[AaronR103]

So I was reading through some of GI's deco stuff on the internet instead of studying for a microbiology exam, much like I should be doing now, and I snagged on the concept of doing your shallower dive first, and that the shallower dive doesn't need to be taken into account when planning your ascent or deco for the deeper dive. I've run into this before but forgot to ask about it during my fundies class. I've got a pretty good grasp of physics and physiology, but I can't quite figure out why this would be better, and most tables work off of the opposite assumption. Can someone explain this to me?

A

 

[Soggy]

You know that game you used to play as a kid (ok, I still do it) where you'd put your fingers up and position some far away person's head inbetween your fingers and squish it whilst saying "Squish your head, squish your head"?

It's like that, only with bubbles.

 

[ArcticDiver]

So I was reading through some of GI's deco stuff on the internet instead of studying for a microbiology exam, much like I should be doing now, and I snagged on the concept of doing your shallower dive first, and that the shallower dive doesn't need to be taken into account when planning your ascent or deco for the deeper dive. I've run into this before but forgot to ask about it during my fundies class. I've got a pretty good grasp of physics and physiology, but I can't quite figure out why this would be better, and most tables work off of the opposite assumption. Can someone explain this to me?

A

go over to dr. decompression and dive medicine. there are lots and lots of posts on this. in fact there was a thread to which I, among others, contibuted that extensively discussed dive depth sequence. several links in that discussion.

i can be a compass to point you where you can get the information but for complete understanding it is best not to just jump on the bandwagon of "the expert says", not that i'm that expert, but for you to do the research and understand it yourself.

 

[AaronR103]

go over to dr. decompression and dive medicine. there are lots and lots of posts on this. in fact there was a thread to which I, among others, contibuted that extensively discussed dive depth sequence. several links in that discussion.

i can be a compass to point you where you can get the information but for complete understanding it is best not to just jump on the bandwagon of "the expert says", not that i'm that expert, but for you to do the research and understand it yourself.

Thanks. Found one of the threads. It covers pretty well why it's not BAD to do the deeper dive 2nd, but doesn't really go into why it is preferable. Since you mentioned research, do you know of any scientific papers on the topic?

A

 

[ArcticDiver]

Thanks. Found one of the threads. It covers pretty well why it's not BAD to do the deeper dive 2nd, but doesn't really go into why it is preferable. Since you mentioned research, do you know of any scientific papers on the topic?

A

Edited whe I realized I misread your post first time through:

No papers that I know of that say performing the deeper dive second is preferable. The thrust of everything I'm aware of is to show that from a DCS perspective dive depth sequence doesn't matter as long as proper dive planning is performed. The studies examined whether or not traditional dive depth sequencing was required in context of today's knowledge.

You can calculate the relative economic advantages by examining what dive depth sequence would put you on shore sooner. If a boat dive the lesser the time at sea presumably produces lower expenses. Whether boat or shore the less time spent actually diving the more that is available for other activities.

 

[Charlie99]

http://www.scubaboard.com/showthread.php?t=26685 is a thread where I asked a similar question, and also provided links to the George Irvine article where he says that what other agencies teach about doing the deeper dive first is "beyond the pale". He also says that one should ignore the first dive when planning a repetitive dive provided 1) it is not a bounce dive, and 2) the average depth of the 2nd dive is deeper.

It is interesting to see how the answers changed once the author of the article was disclosed.

 

[JeffG]

For one thing..you should not be using info based on and used for GI3's saturation deco dives and apply them to rec dives.

 

[lamont]

i think the points behind GI3s article are to:

1. wait for the free-phase bubble shower to calm down after you surface
2. dive deep enough to fully crush any residual bubbles
3. stay long enough to circulate any crushed bubbles so you don't expand them arterial-side
4. deco 'properly'

so, if you do a deep technical dive and then immediately go back down to collect a bottle, you need to stay at depth for 3 mins or greater, and then do another 1 fpm ascent from 20. better choice is to have some support divers who are clean and can go back down and do it for you.

its also going to be very hard to translate this to recreational diving. he's assuming that on the second dive of the series that you are doing proper deco. that doesn't mean doing another 30 min @ 90-100 fsw dive with a 30 fpm ascent to 15 and 3 min there and out. if you are treating your recreational diving like deco diving and doing stops from 1/2 your max depth, and lengthen at least the shallower stops to 2 mins for repetitive diving then you're more likely to be able to apply what GI3 is talking about in that article. if not, you're better off sticking with the PADI recommendations for SIs and NDLs.

 

[AaronR103]

For one thing..you should not be using info based on and used for GI3's saturation deco dives and apply them to rec dives.

Ok, this is where I was getting tripped up. Should have known better. I can see where in a sat dive the 1st shallower dive is not gonna make a bit of difference in planning the deco, but a non-sat would. Thanks all.

A

 

[*Floater*]

i think the points behind GI3s article are to:

1. wait for the free-phase bubble shower to calm down after you surface
2. dive deep enough to fully crush any residual bubbles
3. stay long enough to circulate any crushed bubbles so you don't expand them arterial-side
4. deco 'properly'

That's a good summary and explains why you should go deeper (or as deep) on the second dive, but it doesn't explain why GI3 says to ignore the shallower dive when "planning your ascent or deco for the deeper dive" (to quote the OP). Now I've posted before that I was also taught in DIRF to apply this concept to my rec diving, which was very contraversial here even among GUE trained divers, but all the same it seems that GI3 is not concerned with nitrogen retention from repetitive dives.

Also, if you accept his recommendation as valid, then it follows that you could do a third even deeper dive and throw throw out the first two dives when planning the ascent or deco for it. And same thing with the 4th dive and so on.

Here's some more GI3: http://www.baue.org/library/irvine_baue_talk.html#Decompression

You don't really have all this residual stuff during the surface interval. It doesn't really count. It doesn't really work that way. You can go back in the water and do it again as long as you're not bouncing. <snip>

We did a whole series of body recoveries in Palm Beach where we didn't have enough divers and I had to dive repeatedly. I did a lot of back to back to back 250s with not even an hour in between them, maybe 30 minutes. I did the same deco on each one and it didn't make any difference at all. I started with 50ft and then I do 50ft and oxygen, some of them we did 120ft and 50ft depending on what it was. We had to stay right on the bottom because that's the only way you can see &#8212; so it was all rectangular. You can't see down. and it's too dark, so you have to look sideways. On these offshore trips I'd always carry extra oxygen just in case there was a problem and I had to get back in the water.

Thoughts?