Deepest dive first?

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racerx_

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Just wondering what the general concensus is here regarding the "deepest dive first" rule. Was taught in OW to always do the deepest dive first, however at the time my instructor said that DAN's research seemed to indicate that there wasn't a benefit to this. Now i dunno whether he was right or just blowing smoke, but i got to thinking about the nature of multilevel dives and am wondering if there is advantage to doing the deepest dive first. Since no matter what, in repetitive diving i'll end up at depths potentially deeper than some of the later parts of my previous multilevel dive isn't it to a large degree the same as not doing the deepest dive first?
 
I read in another thread as well that it wasn't a rule that was completely set in stone.

However, I still usually follow it, especially if I'm going to 100ft on the deeper dive. Why? Better to be safe, and also allows for more bottom time on the deeper dive (unless you have a 3+ hour surface interval).
 
Most of my boat diving has been straight forward--ei. 1st dive 80', 2nd dive 60'. Square profile. Mostly because this is your typical 2 tank dive where I've dived. As mentioned, you get the most total bottom time this way. The dangers (or lack of such) of doing the deeper dive second has been discussed a few times. Consensus seems to be that if you follow your computer/tables properly you should be OK with this. PADI also points out that it is wise to do the deeper one first as there has been little or no reverse profile test dives. One may wonder why not? Of course, multi level dives kind of blur the picture.
 
PADI changed their policy on this the other year. They now permit deeper dives later, but recommend that deeper subsequent dives aren't substantially deeper (something like 10m).

The reason that 'deeper dives first' was always recommended was due to a lack of studies/data on the implications, rather than definitive evidence that showed it is dangerous. Basically, it was a liability driven policy. As time progresses, it is becoming clearer that it is not that much of a problem.
 
interesting stuff, thanks for the responses..

Devon- sent you a *rather long* pm ;o)
 
The most recent conference on reverse profiling run (I think) by DAN, came to the conclusion that for diving within accepted recreational limits (i.e. 40m/130' maximum depth, no mandatory decompression) there is no problem with reverse profiles. Some of the bubble-model decompression theorists insisted on the rider that no dive should be more than 12m/40' deeper than the previous dive, since apparently some of the models - especially RGBM - go a bit wobbly outside those parameters.

Off the top of my head it's still recommended that 3rd and subsequent dives should not be deeper than 13m/45', so really you're only looking at whether your second dive can be much deeper than your first - and the answer is yes, as long as it isn't more than 12m deeper.
 
Some of the bubble-model decompression theorists insisted on the rider that no dive should be more than 12m/40' deeper than the previous dive, since apparently some of the models - especially RGBM - go a bit wobbly outside those parameters.

My understanding of the bubble theory proponents issue it is that by doing the deeper dive second, you risk a much higher rate of bubble "seeds" which bypass the lungs and can turn into gas bubbles on the ascent. So aside from just the higher level of Nitrogen loading, you are also adding additional gas seeds that can increase the number and size of bubbles formed in your blood stream.
 
I thought it was more of a practical and less of a safety reason. In other words, you lose more bottom time by doing the deeper dive second. First off, your bottom time on a deeper dive is shorter top begin with, but then you subtract time from the nitrogen left in solution in your tissues.

Finally there's the issue of tissue compartments. I definitely won't be explaining this clearly, but try to bear with me. Basically, the deeper you go, the more fast tissue compartments fill up and thus offgas quicker when you come up. Thus, you have less nitrogen left in solution at the end of your , for example, hour surface interval than if you had done a shallower, longer dive first.

In conclusion, you aren't in any more danger of DCS, but you won't be getting the most bang for your buck.
 
My understanding of the bubble theory proponents issue it is that by doing the deeper dive second, you risk a much higher rate of bubble "seeds" which bypass the lungs and can turn into gas bubbles on the ascent. So aside from just the higher level of Nitrogen loading, you are also adding additional gas seeds that can increase the number and size of bubbles formed in your blood stream.

That makes sense to me, and since I don't want to stroke out with a cerebral blockage, I'll stick with "The Gospel" until proven otherwise: Deepest Dive First.

With that said: Has anybody run a algorithm in Divepal? It would be interesting to see if there's a difference in +N loading, and if it does indeed, cut into your 2nd dive bottom time.
 
https://www.shearwater.com/products/perdix-ai/

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