Freediving after a dive or deco dive physics question

[declan long]

Hi Everyone

Obviously most of us know that you should not free dive after a dive but not alot of people understand the physics/physiology behind it muself included. So, i have a coupld og questions,, as far as i know the reason is becuase as you free dive you increase the ambient presure so the gas you breathe in at the surface increases in pressure as you descend and then diffuses into the blood, on the ascent this causes bubbles due to a fast ascent, is ths correct, if not can some one explain this to me please? also does it make a difference between a normal dive, deoth, time, deco dive etc?

thanks guys
Declan Long

 

[Makhno]

OK, physiology folks please check my explanation and correct it where it's wrong:

During the first hour or so after a dive, divers are off-gassing in a way that often generates small gas bubbles. ("Silent bubbles" = bubbles which can usually be detected by Doppler ultrasound monitoring of the circulatory system, but don't cause classic symptoms of DCI (such as joint pain) -- that's why they're called "silent" bubbles; they're asymptomatic).

Suppose a diver wants to free dive while he or she is still offgassing in a way that causes gas bubbles in the veins. The risk is that by diving, he or she will then compress those bubbles to the point that they'll be small enough to pass through the filter of the lungs, which usually do a fine job of filtering out bubbles so that they get exhaled. After those compressed gas bubbles pass through the lungs they enter the arterial system, causing a risk of arterial gas embolism. Sort of like a PFO (patent foramen ovale).

Right?

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[LiteHedded]

can we just have a declan long thread where we teach him how to dive?
one long thread for me to avoid would be nice

 

[Michael Guerrero]

And yet you keep clicking on them when you see his name. It's like you just can't help but see what he's going to ask next .

---------- Post added August 5th, 2014 at 07:39 PM ----------

Declan,

Here's an article from DAN.

Alert Diver | Could Breath-Hold Diving after Scuba Cause Decompression Sickness?

Enjoy!

 

[GrimSleeper]

Declan,

Here's an article from DAN.

Alert Diver | Could Breath-Hold Diving after Scuba Cause Decompression Sickness?

Enjoy!

Although the article references scuba in the intro, it's actually entirely about the (un)likelihood of DCS being caused solely by breath-hold diving, so it doesn't address the question of free diving after scuba diving.

I can't be bothered to search for references or research, but my own barely informed opinion is that, within the range most of us are likely to breath-hold dive, the pressure increase isn't enough to cause significant decrease in the radii of existing 'silent' bubbles, so it seems unlikely to me that bubbles are really going to pass through the capillary beds in the lungs. On the other hand, all sorts of other physiological weirdness is going on, especially after a formal decompression dive, so why take the risk? Possibly the mere effort of swimming down is enough to churn out micro-nuclei in the bloodstream, and away you go.

My own experience, caused by a dropped VR3, is that free-diving to 23m immediately (as in, had just climbed back onto the boat) after a 50m deco dive caused me no ill effects. Not that one anecdote = data. Mind you, if I'd realised what a POS the VR was going to turn out to be, I'd probably have stuck to my normal post-dive policy of taking it easy...

Edit: Must not skim read. Article does address scuba/breath-hold in the middle. So ignore me. It's insanely hot and my brain has melted.


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[Makhno]

There's an old article titled something like "why the wkpp divers don't bounce after we dive" that reports divers getting bent going to 20 feet after a big dive to get deco bottles or to retrieve the anchor or something, but as mentioned, anecdotes are not solid cause-effect data (maybe they would have gotten bent anyway).

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[LiteHedded]

There's an old article titled something like "why the wkpp divers don't bounce after we dive" that reports divers getting bent going to 20 feet after a big dive to get deco bottles or to retrieve the anchor or something, but as mentioned, anecdotes are not solid cause-effect data (maybe they would have gotten bent anyway).

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i know at least one guy this happened to

 

[mathauck0814]

Something you should probably discuss further and in detail with your instructor.

 

[Michael Guerrero]

i know at least one guy this happened to

As was stated, demonstrating causality is the problem. Given the uncertainty, I'd personally refrain.

 

[Nasser]

And yet you keep clicking on them when you see his name. It's like you just can't help but see what he's going to ask next .

That's hilarious !