Free diving during the surface interval

[FPDocMatt]

A few people today were swimming during the surface interval. One person dropped something and swam to the bottom to get it. It was probably 15 feet. He wanted to know how deep it was, so swam back to the boat and picked up his computer.

Someone on the boat said that's not a good idea, because the computer will think he's done another dive. That is, the computer won't realize he wasn't breathing from a high-pressure tank.

I thought this made sense. But then someone said it's also not a good idea to free dive during the surface interval, because you will take on more nitrogen.

Since you're not breathing air at a higher pressure than one atmosphere, I wondered how you would take on more nitrogen. He replied that the higher pressure at that depth will cause you to absorb more nitrogen from that breath than you would if you had held your breath for the same amount of time at the surface.

This also makes sense, but it's not as clear to me.

So I asked, "Do you mean a free diver can get the bends?" He said no, because you're only down there for 5 minutes or less.

Anyway, it sort of makes sense, but I'm still confused. Any clarifying insights would be appreciated.

 

[Hank49]

Free divers can get bent.
The computer will freak out because it does think you're scuba diving and did a very rapid ascent.

Free diving isn't about taking on more nitrogen. It's what it does to the existing residual nitrogen in your system. I believe it can cause larger bubbles to form from recompressing on the free dive descent and not decompressing on a rapid ascent.

 

[mathauck0814]

Some computers have a "gauge mode" that will track time and depth but not calculate NDL. If you're going to free dive on a surface interval it's best to change your computer to that mode so that it won't mess with your next dive's calculation.

There's no compelling reason I've ever heard not to free dive on your SI. You're not on-boarding any additional nitrogen and you're only interrupting normal gas exchange for a few seconds at a time so it shouldn't have any measurable impact on your next dive's profile.

 

[Doc Harry]

Someone on the boat said that's not a good idea, because the computer will think he's done another dive.

Irrelevant

That is, the computer won't realize he wasn't breathing from a high-pressure tank.

All non-air integrated computers have this same "problem." Again, irrelevant.

I thought this made sense. But then someone said it's also not a good idea to free dive during the surface interval, because you will take on more nitrogen.

False

So I asked, "Do you mean a free diver can get the bends?" He said no, because you're only down there for 5 minutes or less.

False

....


IF you have a residual nitrogen load from your previous dive on compressed gas, and then go free diving, you can get bent.

The rapid ascents from depth associated with free diving can cause the residual nitrogen load to form bubbles and thus DCI.

 

[dumpsterDiver]

Freediving after scuba diving can cause the bends, but a drop or two to 15 ft is probably not "dangerous".

DCI and freediving - DeeperBlue.com Forums

 

[TMHeimer]

Free divers can get bent.
The computer will freak out because it does think you're scuba diving and did a very rapid ascent.

Free diving isn't about taking on more nitrogen. It's what it does to the existing residual nitrogen in your system. I believe it can cause larger bubbles to form from recompressing on the free dive descent and not decompressing on a rapid ascent.

That's what I thought. Obviously you solve the confused computer problem by simply not wearing it on the free dive, but I wouldn't do it (unless there may be $1M lying in 15' of water-then I'd probably come back tomorrow anyway).

 

[-hh]

Free divers can get bent.

IIRC, there's been some documented reports of hits on some of the Japanese pearl divers. The basic reality is that it is possible, but it also quite difficult to free dive enough to build up your residual nitrogen levels. However, when free diving after a scuba dive, the scuba part provided a "head start" on the N2 loads, so a couple of free dives could be enough to push one over - - however, it is more likely that the rapid ascents are going to be more of the problem, as there's going to be asymptomatic micro bubbles that get ... call it "manipulated" ... by the up/down/up/down of free diving.

The computer will freak out because it does think you're scuba diving and did a very rapid ascent.

Well, yes, that too. And if one doesn't take your computer, you're also not at the surface really doing your SI and off gassing like you "told" the computer you were doing.

Free diving isn't about taking on more nitrogen. It's what it does to the existing residual nitrogen in your system. I believe it can cause larger bubbles to form from recompressing on the free dive descent and not decompressing on a rapid ascent.

There's been several mechanisms that have been mentioned in DCS research. One that IMO has pretty strong credence is that the formation of subclinical micro bubbles probably occurs fairly random/uniform throughout the body and they travel through the bloodstream - - until many of them get caught by the "filter" of the capillaries of the lungs, which results in the rest of the body effectively being protected. A free dive which occurs during off-gassing will recompress these bubbles "caught in the filter", thereby allowing them to pass through the filter (lungs)...but particularly since a free dive is relatively brief, when the pressure is released, the bubbles reform and now they're downstream of the lungs - - but now also traveling as a big cluster together through the bloodstream, en masse. YMMV if this is a good thing...I'd suspect not, as such close proximity would logically provide greater opportunities for bubble combining (size growth) as well as simple "congestion" leading to flow restrictions or obstructions.


-hh

 

[Eric Sedletzky]

If somebody is freediving during a surface interval, then it isn't really a surface interval, is it?

 

[BoltSnap]

Some computers have a "gauge mode" that will track time and depth but not calculate NDL. If you're going to free dive on a surface interval it's best to change your computer to that mode so that it won't mess with your next dive's calculation.

You to be careful about changing your computer mode into "gauge mode" since you won't be able to changing it back into dive mode until after a very long time on the surface (several hours). I know that this happens to Suunto computers.

 

[supergaijin]

I think the saving grace is that most people aren't really freediving- more like snorkeling with their head underwater

If you're honestly capable of getting down over 30ft, then you should really think about not doing so between dives.