Resetting computer

[cellowithgills]

I understand the idea of freediving after scuba diving interfering with desaturation, but how would free diving before scuba diving effect N2 load?

I won't reset the computer, BTW.

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---------- Post added August 19th, 2014 at 07:18 PM ----------

I read the article, I'm not even close to those depths. I hunt in 20-30ft of water and stay down for 30-60 seconds.

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[BRT]

So how many minutes did you have at 20-30 feet? You are absorbing nitrogen when free diving. Why would you think that nitrogen absorbed while freediving before scuba diving would be different than nitrogen absorbed while freediving after scuba diving?

 

[victorzamora]

You inhale air at the surface, right? Then you go to depth, with air in your lungs. The pressure you're at and the type of air in your lungs matters, not where the air came from or what pressure it was at originally. You staying down means you're ongassing. Think of all of your freediving as a dive. That "dive" gives you residual nitrogen buildup, which you must account during your next dive.

Having said all of that, Suuntos are super goofy when it comes to repetitive dives because of their algorithm. Maybe you're suffering from that a bit with your Mares, also using an RGBM algo?

 

[Colliam7]

How do you figure that???? Alert Diver | Could Breath-Hold Diving after Scuba Cause Decompression Sickness?

Easy. Simple pulmonary physiology ( and some basic mathematics).

While it is theoretically possible to on-gas some nitrogen in circumstances where a free diver repeatedly descends to depth, after replenishing the gas supply in his / her lungs after surfacing between each dive, in free diving you are not - during a dive - replacing the gas in the lungs with air containing nitrogen. (I certainly didn't get the impression from the OPs comments that such was the case.). At worst, even if you on-gassed every molecule of nitrogen contained in the air in your lungs at the start of the dive (which, of course, does not happen), that amount of nitrogen is trivial - on average, total lung capacity is ~5-6 L (, e.g. only 0.18 to 0.21 CF). Even adjusting for the fact that experienced free divers expand their TLC through breathing techniques, the amount of nitrogen contained in that volume of air is, again, trivial. However, I will concede the theoretical point - if the OP made 30 descents to 150 feet over the course of an hour, then there is certainly the possibility of on-gassing a minimal amount of nitrogen. But, compared to breathing compressed air, at depth, where there is 'new' nitrogen being introduced with each breath (in fact a lot of new nitrogen at depth, because of the compression of the gas and the consequent higher partial pressure of nitrogen), which can be on-gassed, that is not the case in free diving. All that happens during a free dive - all that can happen - is that you on-gas a portion of what is in your lungs at the surface - a very (very, very) small amount consistent with the limited lung capacity of the average adult (or even the average free diver).

And, since you cite it, read the article.

"There are anecdotal and retrospective data in the literature that are consistent with a diagnosis of DCS. A recent attempt to model the risk found that it was negligible for dives to depths of less than 330 feet, then rising as a function of exposure depth until the depth was sufficient for airway collapse to limit gas uptake from the lung, possibly in the range of 755 feet.2 The magnitude of the hazard is unclear, but the absolute risk is probably very low for most freedivers, particularly when conservative surface times between dives are maintained."

 

[redacted]

Easy. Simple pulmonary physiology ( and some basic mathematics).

While it is theoretically possible to on-gas some nitrogen in circumstances where a free diver repeatedly descends to depth, after replenishing the gas supply in his / her lungs after surfacing between each dive, in free diving you are not - during a dive - replacing the gas in the lungs with air containing nitrogen. (I certainly didn't get the impression from the OPs comments that such was the case.). At worst, even if you on-gassed every molecule of nitrogen contained in the air in your lungs at the start of the dive (which, of course, does not happen), that amount of nitrogen is trivial - on average, total lung capacity is ~5-6 L (, e.g. only 0.18 to 0.21 CF). Even adjusting for the fact that experienced free divers expand their TLC through breathing techniques, the amount of nitrogen contained in that volume of air is, again, trivial. However, I will concede the theoretical point - if the OP made 30 descents to 150 feet over the course of an hour, then there is certainly the possibility of on-gassing a minimal amount of nitrogen. But, compared to breathing compressed air, at depth, where there is 'new' nitrogen being introduced with each breath (in fact a lot of new nitrogen at depth, because of the compression of the gas and the consequent higher partial pressure of nitrogen), which can be on-gassed, that is not the case in free diving. All that happens during a free dive - all that can happen - is that you on-gas a portion of what is in your lungs at the surface - a very (very, very) small amount consistent with the limited lung capacity of the average adult (or even the average free diver).

And, since you cite it, read the article.

"There are anecdotal and retrospective data in the literature that are consistent with a diagnosis of DCS. A recent attempt to model the risk found that it was negligible for dives to depths of less than 330 feet, then rising as a function of exposure depth until the depth was sufficient for airway collapse to limit gas uptake from the lung, possibly in the range of 755 feet.2 The magnitude of the hazard is unclear, but the absolute risk is probably very low for most freedivers, particularly when conservative surface times between dives are maintained."

On-gassing of N2 while free diving is not just "theoretically possible". I does occur. Sure, on-gassing so much as to cause DCS is very unlikely but it has also occurred in extreme cases. The point is that free diving may load N2 and those effects can and should be considered as it interacts with scuba diving when a diver switches between the 2 activities. Clearing the computer's residual N2 is not the solution.

Another reference that addresses mixing the 2 activities: Breath-hold Diving:Taravana

 

[boulderjohn]

Here is another factor to consider. You already know that the computer thinks you are breathing compressed air. Acting on that assumption, it also thinks you are significantly violating the safe ascend speed when you go to the surface. Not only that, it thinks you are doing it again and again and again and again. Many computers will penalize you for violating the ascent rate. Do it multiple times, and you will really get dinged.

 

[cellowithgills]

Fortunately the puck has a way to shut off the fast ascent rate alarm, which is one of the reasons I bought it.

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[redacted]

Here is another factor to consider. You already know that the computer thinks you are breathing compressed air. Acting on that assumption, it also thinks you are significantly violating the safe ascend speed when you go to the surface. Not only that, it thinks you are doing it again and again and again and again. Many computers will penalize you for violating the ascent rate. Do it multiple times, and you will really get dinged.

I keep forgetting about those computers that think they are the "morality police". My Oceanics don't do things like that.

It is not so much a question of the alarm (another feature thankfully absent on older Oceanics) as much as does it penalize you for such "violations".

 

[victorzamora]

Fortunately the puck has a way to shut off the fast ascent rate alarm, which is one of the reasons I bought it.

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Alarm? Yes. Penalties? Nope. Those are permanent and you can't disable those punishments.

 

[boulderjohn]

I keep forgetting about those computers that think they are the "morality police". My Oceanics don't do things like that.

Are you sure that a rapid ascent does not factor into the calculated bottom time for your next dive? It isn't obvious on the Suuntos that do it.

It is not so much that they are the police--they figure it is one of the factors that need to be considered in dive safety. They believe there is a biological reason for that penalty.