If this was your first trip to Cozumel, what would you do.....?

[DandyDon]

I didn't know about Shallow Water Blackout when I used to free dive. Hope you do. There also cautions about not free diving after scuba diving same day.

Have a good trip...! ilot:

 

[nodakdive]

I didn't know about Shallow Water Blackout when I used to free dive. Hope you do. There also cautions about not free diving after scuba diving same day.

Have a good trip...! ilot:

I'm not starting a new thread, though that would probably be a better way to address this, but I saw the comment above and since I just finished a several day email discussion about this topic with some professionals in the field, I thought I'd reply.

"Cautions" about free diving after scuba seems to be the appropriate word.

Though one would think that free diving to, say, 33 feet, and then ascending at the average free diver ascent rate of 6 feet per second would be strictly contraindicated after a scuba dive, the experts say that it's not a good idea based on theory. An excerpt from one of my email correspondences with the experts. "...but it would require even more hand waving to take a hard position against a fast ascent during free diving post compressed gas dive since the free dive is not adding significant new gas to cause a problem during the ascent..."

The example I used in our email discussion was a 1 hour multi-level dive to 75' max followed by a one hour surface interval, followed by a free dive to 30 fsw and a rapid ascent to the surface. Along the way of our discussion I suggested that I was a "D Diver" (tables) after the surface interval and before the free dive. Earlier in our conversation it was suggested that "...Regarding your dive to 75fsw and possible free dive to 30 fsw one hour later, we agree that there is a certain hazard there but the risk is probable rather small since we do not here about divers being injured..."

In any event, the professionals did NOT say "You won't get hurt.", They said (and I'm paraphrasing) "We don't have much data. It's probably a good idea to avoid the practice of free diving after scuba diving."

The reason I sought more info about this subject is that I often find myself in the water with a snorkel in my mouth after scuba diving. I don't free dive in those cases. An article appeared in Alert Diver magazine which addressed this issue, and I was surprised that the article didn't strictly contraindicate free diving after scuba diving based on residual nitrogen in the body system after a compressed gas dive and the rapid ascent rates involved in free diving. I thought, "Of course you shouldn't!" Well, it's not that simple. (though I still find myself a little perplexed about the issue).

-Blair

 

[DandyDon]

Hi Blair. One of the reasons we don't hear about problems from free diving after scuba is few do it. If it were done very soon after hard diving, it'd be like shaking a can of beer or pop, then opening it. A D-diver from the Padi tables wouldn't have anything to worry about, but that's light diving; much more comes in play as a W-diver, including a chance of bubble pumping to the arterial side.

A 90-120 minute surface interval tho, and you are in a much safer area.

 

[nodakdive]

Hi Blair. One of the reasons we don't hear about problems from free diving after scuba is few do it. If it were done very soon after hard diving, it'd be like shaking a can of beer or pop, then opening it. A D-diver from the Padi tables wouldn't have anything to worry about, but that's light diving; much more comes in play as a W-diver, including a chance of bubble pumping to the arterial side.

A 90-120 minute surface interval tho, and you are in a much safer area.

Thank you Don, for the clearest acknowledgment that I've gotten. After several exchanges, it still wasn't clear to me that being a "D diver" means that the residual nitrogen in the body system is negligible. Actually, when I presented that idea to the pros, ths was the response: (which, I think, is basically saying what you've said, and what I had come up with during my talk with a dive instructor.) (i.e. "...acute gas load...", "...from an absolute gas content point of view.")

"Your position to avoid rapid ascent rates across the board is certainly a reasonable rule but I think that your justification is a little confused. The slow ascent rate during compressed gas diving is appropriate to manage the acute gas load. Since significant gas is not added during the subsequent shallow breath-hold dive, a slow ascent is not important from an absolute gas content point of view. Your ascent from the original dive already controlled the decompression risk. The compression phase of a subsequent freedive could enable some migration of bubbles - theoretically a hazard if sensitive tissues were to be effected - and the physical exercise could certainly promote additional passage of existing bubbles across the lung - definitely a bad thing. A fast ascent rate during this subsequent shallow freedive, however, is not likely to create an independent safety risk. I do not write this to encourage you to change your plan, merely to clarify that the justification is not quite so simple."

I started diving and was certified "late in the game" so to speak, that being 2004. What I/we were taught was that any excess nitrogen is bad nitrogen, so don't shake the can and pop the top. To me, that means it doesn't matter if I get to depth with a regulator in my mouth or a snorkel. It took talking to a dive instructor whose been diving since the late seventies, and certifying since then or the early 80's to understand that, as you say, "A D-diver from the Padi tables wouldn't have anything to worry about..." You probably read the Alert Diver article, as did I, which stated that they don't have much data on "free diving after scuba diving". For me, that didn't make much difference. You have excess nitrogen in your body. Dear Lord, don't shoot to the surface, hour interval, D-diver or not. Heck, they say we have to wait 12 to 24 hours to get on a plane. Now then, I realize that the body factors for being in a "less than 1 ATA" environment probably have different considerations than going from a 2 ATA environment to a 1 ATA environment, but still..... It just didn't make sense.

I've tried to copy and paste one of my replies, post talk with the dive instructor, but all I can get to paste is one line. In short, my discussion with the instructor included our differing definitions of a "bounce dive". To him, a bounce dive is a dive that is made to depth followed by an almost immediate ascent without regard to safe ascent speed consideration. Back "in the day", they'd dive down to free the anchor, for instance, and shoot right back up. To me, a bounce dive is one wherein the diver descends to depth, and then immediately begins ascent, *but heeding the recomendations for safe ascent speed.* Well, this shed some light. What is considered "safe practice" has changed considerably throughout the years. He also stated that "...we'd dive, and then within an hour or two be on an airplane...." As to his definition of a "bounce dive", he reported that it is no longer considered a condoned practice. Back then, it was no big deal to be a D-diver and make a rapid (more than 1 foot per second) ascent. I did not know that. Again, to me, and from what I was taught, excess nitrogen means don't shake the can and pop the top with an ascent faster than 1 foot per second. Case closed.

Anyhow, thanks for pointing to the idea that being a D-diver means there isn't a ton of excess nitrogen to be concerned with. I'm still not going to free dive after scuba. Heh. (Ok, not for 6 hours after a dive, anyhow.) Maybe that makes me a worry wart, but I can live with not adding to a short data set.

-Blair