Deeper Second Dive

[ChillyWaters]

Through courses we were taught to do repetitive dives shallower than the first dive. Dive tables have been designed under this assumption.

Now, what if we wanted to do a repetitive dive deeper than the first? If we are using tables (say the standard PADI RDP), can we simply calculate our first dive with a depth equal to the depth of our repetitive dive? Is this proper and safe?

For example. First dive to 40'. Second dive to 70'. To calculate bottom time for second dive, calculate residual nitrogen time of first dive considering a depth of 70' instead of 40'.

Thoughts?

Yes, I always dive shallower for repetitive dives, but if we are, by circumstance, limited to a shallow first dive, do we have to limit ourselves on subsequent dives?

- ChillyWaters

 

[wedivebc]

Here is food for thought. You do the 1st dive to 50' and do the second dive to 70' but use a 40% nitrox mix. Your equivalent air depth is 45ft for dive 2 so you're not violating any deeper dive 1st rule.

 

[Mako Mark]

read this if you want you head messed with..

reverse profiles..

 

[neil]

There has been a major workshop on this topic.

http://www.soc.soton.ac.uk/OTHERS/SDSC/dn1_00a.html
www.oseh.umich.edu/reverse.pdf
http://www.scubadiving.com/index2.php?option=content&task=view&id=3502&pop=1&page=0

Just for starters, look at the above (I picked them at random), then google "reverse profiles". You can read articles by real experts without getting your head "messed with" .

 

[ChillyWaters]

Okay, so after doing some searching, it seems to be the concensus that reverse profile diving is okay. However, the PADI RDPs were not designed to deal with reverse profiles... so...

As mentioned in my original post, is it safe to calculate residiual nitrogen for your first dive, based on the max depth of your second dive? This question posed to anybody who knows the theory behind the PADI tables.

- ChillyWaters

 

[Diver0001]

Through courses we were taught to do repetitive dives shallower than the first dive. Dive tables have been designed under this assumption.

Now, what if we wanted to do a repetitive dive deeper than the first? If we are using tables (say the standard PADI RDP), can we simply calculate our first dive with a depth equal to the depth of our repetitive dive? Is this proper and safe?

For example. First dive to 40'. Second dive to 70'. To calculate bottom time for second dive, calculate residual nitrogen time of first dive considering a depth of 70' instead of 40'.

Thoughts?

Yes, I always dive shallower for repetitive dives, but if we are, by circumstance, limited to a shallow first dive, do we have to limit ourselves on subsequent dives?

- ChillyWaters

I'll save you a lot of reading. The workshop basically concludes that if the difference in depth isn't more than 40ft that it's not a big deal. So your dive to 50 and then 70 would be ok according to the workshop people. However, they do note that the phenomenon is real for more extreme profiles so it's still something to take into account in your planning.

I like Dave's suggestion.

R..

 

[Diver0001]

Okay, so after doing some searching, it seems to be the concensus that reverse profile diving is okay. However, the PADI RDPs were not designed to deal with reverse profiles... so...

As mentioned in my original post, is it safe to calculate residiual nitrogen for your first dive, based on the max depth of your second dive? This question posed to anybody who knows the theory behind the PADI tables.

- ChillyWaters

fudging the RNT is a good way to build in a little conservatism in your planning but you need to take the RNT from the next *shallower* depth, not the next deeper. Take a good look at the table and you'll see why.

R..

 

[ChillyWaters]

... but you need to take the RNT from the next *shallower* depth, not the next deeper.

Okay, at first I didn't get you here... but I got 'ya now.

I wasn't going to take RNT from the shallower depth, but rather...

If I dove 30 minutes at 40', but was going to dive at 70' for my next dive, then I would consider a group of O instead of F. This actually builds in a lot more conservatism than you suggests.


I'm still interested if this conservatism has scientific merit under the assumptions of the PADI RDP design.

- ChillyWaters

 

[Mako Mark]

the assumption that the traditional (deepest dive first) profile is safer has no basis that I have ever heard except other than that the math was too complex to do back then in the absence of number crunching computers.

The theory that a shallower dive could in effect allow microbubbles to pass through the bubble scrubber action of the pulmonary capiliaries has some merit and could explain the "undeserved hits" that are so common. There is no such thing as an undeserved hit, only an unexplained hit or a hit that is not explained by present theorys.

The theory that a deeper dive will in fact force micro bubbles back into solution therefore eliminating the need to calculate residual nitrogen also has merit.

After all, virtually all dive tables are based on disolved nitrogen, and I dont know a single case of a diver getting bent from disolved nitrogen.... PERIOD.

Bubbles cause DCS not disolved gas, so what the heck are we calculating disolved gas for...

 

[paolov]

Bubbles cause DCS not disolved gas, so what the heck are we calculating disolved gas for...

Nice one.