How risky are bounce dives?

[ChrisF]

I'm specifically thinking about what happens when tying into a wreck. If you're out on a commercial dive charter, typically the DM will jump in before everyone else and tie you into the wreck. The DM surfaces after ensuring that the line is secure and then everyone else goes diving. I don't think the DM typically returns to the wreck to dive with the rest of the group.

But what if you're diving from a private boat? If you send someone down to tie into a wreck, is it safe for that diver to surface and then return to the wreck after a surface interval of only a few minutes?

Let's assume we're talking about a wreck that's in the 80 to 100 foot (deep) range.

 

[Pez de Diablo]

Not specific to your question, but here is a thread with different opinions on doing bounce dives

Bounce dive

 

[Charlie99]

That other thread pretty well covers it, but a comment specific to your question is that the first bounce to tie into the wreck isn't really a problem since you don't have bubbles in your system at that time (assuming it is the first dive of the day).

The 2nd dive that the DM does with the rest of the group is NOT a bounce dive and any bubble from the 1st dive are properly handled.

What the DMs should avoid is another bounce dive to bring up the anchor or to untie from the wreck after surfacing from the other dives.

Charlie

 

[ChrisF]

Charlie99 once bubbled...
The 2nd dive that the DM does with the rest of the group is NOT a bounce dive and any bubble from the 1st dive are properly handled.

Charlie

So you're saying in order to be a bounce, you've got to stay at depth for a period of time to on gas enough nitrogen to form bubbles and that's typically longer than it would take to set the anchor. Hmmmm.....

After speaking about this with another friend of mine, I think he's got an even better solution (at least with regard to the wreck scenario I described). His advice was to avoid the protential for problems all together by just sending a team down to hook into the wreck, shooting a marker once the hook is secure to let the boat captain know, and then continuing with the dive.

I like that approach becuase: 1) It avoids the "solo" thing, 2) It avoids the potential for bubble formation if you're down longer than you planned to set the anchor

 

[Charlie99]

ChrisF once bubbled...
So you're saying in order to be a bounce, you've got to stay at depth for a period of time to on gas enough nitrogen to form bubbles and that's typically longer than it would take to set the anchor.

That's not really what I said. My point was that the initial bounce dive of the day is not as dangerous because you don't have any bubbles in your venous system from a prior dive.

The second dive the DM does is just like any other repetitive dive --- you may very well be bubbling from an earlier dive, but they don't cause any problems unless you do yo-yo profiles or rapid ascents. About 2/3 of the repetitive dives that DAN Europe studied had significant venous gas emboli (aka doppler bubbles) -- so bubbles on the venous side aren't necessarily a problem.

As posted in the previous thread, this is a good review of the problem with bounce dives: http://www.wkpp.org/articles/Decompression/why_we_do_not_bounce_dive_after_diving.htm

The big problem with a bounce dive is when you do one while still bubbling from a _previous_ dive. The bubbles compress and get passed over to the arterial side, and increase the risk for Type II (nervous system) DCS if you then make a rapid ascent before those bubbles have been eliminated.

Charlie

 

[Dr Deco]

Dear Readers:

Short, Bounce Dives

This question appears somewhat frequently on SCUBA BOARD. In laboratory trials with animals as subjects, transpulmonary passage ( = across the lung capillary barrier) of gas bubbles did not occur with recompression. However, these studies were all performed with quadrupeds and humans, in contrast, are bipeds. The blood flow characteristics to the heart are different.

In cases where there are numerous gas bubbles in the venous return, the possibility may well exist that arterialization could occur with short bounce dives since this problem has been reported in human divers.

I do not believe that there will be any laboratory tests performed to elucidate the cause and effect of bubbles and repetitive dives such as these are. The study would be deemed too risky to the subjects. We will need to rely on field data.

Dr Deco :doctor: