Freediving after scuba diving

[TMHeimer]

My response take with a BIG grain of salt. I have never free dived or done any snorkeling after a scuba dive. I can'r figure a reason to do this (after all, I took scuba so i could stay down). I've read that people disagree on how dangerous this may be.
My layman's feeling is pretty safe if you free dive to 10+ feet. Maybe no so much if you free dive to 50'.
Would one free dive during a S.I. out of boredom?

 

[JoshNZ]

No. Strictly to harass people on deco.

Hahaaa.. Excellent. Nothing so boring as a deco in mid water with nothing to look at.

Interesting reading, anyway... Thanks for the comments and links.

I suspect I have been affected doing this haha there's my confession and partly why I'm interested. A year or two ago after a normal dive a bunch of kingfish schooled in on us during our ascent so when I got to the surface I shed the bottle and grabbed the gun and went after one. I don't remember diving particularly deep but I ended up with a really sore foot, left me hobbling around for several days. It looked like bruising on the sole, red/blueish and darkened veins visible through the skin. The guy who filled our bottles was a dive master he said it looked like skin bends but he didn't know - gave the same ambiguous answer to this question.

So that was the last time I've done it. Before that, Dad and I particularly when we were young would do it all the time, between our two bottles, I guess because we were wet and had suits on. I don't remember ever being told not to =/ but maybe I forgot. Maybe I just followed his lead and he was taught old school, maybe he forgot...

I was wearing a new pair of fins for the first time which were wickedly tight, and I kind of scratched it up to that, hoping my idea of blood nitrogen meant it couldn't have been bends. I'd prefer it to be that, this conversation alone makes my joints hurt hah. It would be nice to know the exact science of it

 

[O-ring]

Yeah..it would be nice to know the exact science of anything related to decompression. I suspect you are a more accomplished freediver than I am so maybe depth had something to do with it. It could also be that I have gotten lucky or that I am in such ridiculous shape and so well perfused that I can get away with things that no other diver can. Yep, definitely lucky.

 

[jadairiii]

...that I am in such ridiculous shape and so well perfused that I can get away with things that no other diver can....

For a moment there I thought I was back in 1999 on Techdiver/Cavers lists......

 

[Bob DBF]

...if your gas tension is already at a safe level - at the surface.

Assuming your gas tension is at a safe level at the surface, if you submerge it changes. Now if you SCUBA dive you have tables or a computer that can keep you “safe”. There is no similar way to stay safe if freediving after scuba.

These days I don’t freedive after tank diving because of what I’ve learned, and because I’m getting old and don’t have the resilience I used to have.

I don’t recommend anyone freedive after scuba, as it is a bigger DCS crap shoot than one or the other alone in my opinion.

On the other hand, in my youth, I’d do two scuba dives, then freedive for Abalone. Now, because of where I was diving, the scuba dives were around 60’ and freedives were 40’ max. Surface Intervals were well over an hour between tanks and before the freedives. Not a big N2 load, and I did the same 60 fpm ascent for both.


For what it’s worth

Bob

 

[JoshNZ]

You would ascend at 60fpm freediving? That's almost a whole minute you'd lose gathering abalone haha.

I suppose a reasonable answer to this question is you shouldn't do it for the same reason you shouldn't go back down on a tank for 2 minutes then rapidly ascend. It's significant enough.

Thanks for all the responses, interesting reading. I had no idea it wasn't an exact science, pretty amazing it is such common practice considering. I read about the guys diving beyond limits before dcs was discovered, some time ago, what a job that would have been =/.

 

[dmaziuk]

I'm aware the rapid ascent is the reason it is advised not to freedive but my question is, why is this rapid ascent harmful if your gas tension is already at a safe level - at the surface. Surely a rapid ascent to the surface where your level of dissolved nitrogen is already proven safe, is harmless (without adding more of course).

It's not "proven safe", that's the problem. It is only proven to give you statistically "acceptably low" risk of clinical DCS upon surfacing.

You go freediving, mountain climbing, or hot-tubbing, chances are you'll be fine, especially if your surfacing tissue loading wasn't too close to M-values. But since there's neither any statistics for this sort of thing, nor a real full understanding of how DCS works, all bets are off. So nobody's going to tell you it's OK.

 

[Lostdiver71]

Unfortunately there is more that we do not know about DCS than we actually know. The theory is that because your bloodstream already has an increased level of saturation you will on gas more nitrogen during the free dive after scuba than you would if you were only free diving. Also because your tissues already have nitrogen loading you may have an increased risk of bubbles forming during or after the ascent causing DCS. I have never personally seen it happen but I know dive operators that have and I knew 1 person that had it happen to them when free diving after several scuba dives on the same day. I did this for many years while working on dive boats during surface intervals and while other divers where at depth without any issues. I only stopped when I read an article about the possibility of DCS while free diving after scuba. Many of the boats back in the 80's and 90's would take us to a 20-30 foot reef on surface intervals to snorkel and free dive, that stopped in the late 90's. Knowing what I know now I can't believe that I never ended up in a chamber with DCS!

 

[JackD342]

Aside from all of the questions about the risk of DCS from the activity itself, it occurs to me that free diving mixed with scuba will affect and invalidate the off-gassing calculations from the scuba dives, unless a suitable computer is used to switch back and forth between the two modes. Along those lines, here is the page from such a computer, the Aqualung i300C.


FREE DIVE MODE DETAILS


• Although breathing apparatus is not utilized for free dive activities, nitrogen tissue loading remains a factor. Nitrogen

loading is calculated based upon a fixed FO2 of Air.


• Since a user has the option of alternating between SCUBA and free dive activities within a 24 hour period, nitrogen

calculations and the displayed value of No Deco Dive Time Remaining are carried over from one operating

mode to the other, which permits the user to maintain awareness of nitrogen absorption and off-gassing status.

• The mathematical models currently used in the i300C are based on no decompression/decompression multilevel

repetitive dive schedules.

• These algorithms do not take into account the physiological changes associated with the high pressures that

competitive type free diving can expose a diver to.

! WARNINGS:

• Ensure that you know which Operating Mode is selected (DIVE, GAUGE, or FREE) prior to

commencing any dive.

• Conducting free dives within a 24 hour period after conducting SCUBA dives, combined with the

effects of multiple rapid free dive ascents, increases your risk of decompression sickness. Such

activities may result in accelerated entry into decompression which could cause serious injury or

death.

• Combining competitive type free dive activities that involve multiple descents/ascents with activities

utilizing SCUBA during the same 24 hour period is not recommended. Presently, there is no data

relating to such activities.

• It is highly recommended that anyone planning to become involved in competitive type free dive

activities obtain proper instruction and training from a recognized free diving trainer. It is imperative

that the physiological affects be understood and the diver is physically prepared.

 

[Bob DBF]

You would ascend at 60fpm freediving? That's almost a whole minute you'd lose gathering abalone haha.

It isn’t like they run fast, and where I went was thick with them. Also the majority of recreational abalone divers we would characterize as 15’ or 15 second divers.

I tried to ascend about the same rate surfacing on both to avoid an error when tired or narked.



Bob