Snuba in Cozumel?

[ggunn]

Perhaps a source of confusion is the difference between PSIG (pounds per square inch gauge) and PSIA (pounds per square inch absolute). The pressure inside an open soda bottle at sea level is 0 PSIG and 14.7 PSIA. PSIG is differential, PSIA is absolute.

The fact that pressure is linearly related to depth is pertinent. For example, take a soda bottle to 99fsw and fill it with air, cap it, and then bring it up to 66fsw. The pressure inside the bottle is 14.7 PSIG, i.e., 1 atmosphere more than the pressure outside the bottle. Now fill the bottle at 33fsw and bring it to the surface; the pressure inside the bottle is the same: 14.7 PSIG. The pressure differential is what causes pulmonary embolism; filling your lungs to the point of inelasticity at 99 fsw and ascending to 66fsw will hurt you just as badly as doing so at 33fsw and ascending to the surface.

What is different is the size of the bubble that is the air in your lungs. Ascending from a shallower depth while holding your breath causes the bubble to increase in size more quickly as you ascend and therefore hit that point of inelasticity sooner if your lungs were not quite full when you started up - sooner than if you started deeper and ascended the same number of feet. The gas laws are only relevant until the lungs expand to the point of inelasticity; after that the internal absolute pressure is constant and it's all about the differential between internal and external pressure.

EDIT: I just wanted to add one more point; As your lungs expand when you inhale, the tissues inside them do not stretch, they unfold. Once they are completely inflated (what I somewhat inaccurately called the "point of inelasticity" above) they cannot stand more than a very small differential between inner and outer pressures. A doctor once told me that they have about the resiliency of a wet paper towel. That's why embolism is so dangerous and so easy to cause. Be careful out there.

 

[deepsea21]

You wonder how someone's life really is going when they feel the need to attack on Social Media, or chat rooms. As I was saying 33 feet is a full atmosphere. SNUBA is about getting people introduced to diving. I'm sure your boy will love it. I'll leave it to the Jac Cousteau's on this blog to criticize that too.

You know what I wonder? I wonder how someone who dubs himself "PadiPro" on social media or in a chat room can make the following statement:

"Probably for the fact that your not even at one atmosphere it's relatively safe. I fully recommend getting your little guy involved with SNUBA and get him comfortable with breathing compressed air under water so he can get his certs done when he's of age!"

Now that you hopefully understand how depth/atmospheres and changes in depth results in the compression/expansion volume of gases the next thing you should know is there nothing to "getting comfortable with breathing compressed air under water". Something tells me you have NEVER breathed compressed air under water. If you had, you would know that it is no different than breathing at sea level - there is nothing to get used to or "comfortable with" and that presents the danger... Espeically for an 8-year old who has been snorkeling and is probably very accustomed to and highly proficient at taking a deep breath at the surface and HOLDING it as he kicks down to the bottom to check out this or that before kicking back to the surface (probably holding his breath all the time until he surfaces again with that breath to clear his snorkle upon surfacing). I know I was at 8 years of age. What is the #1 rule of diving or breathing from your SNUBA hose? Do you know it? I'll tell you... NEVER HOLD YOUR BREATH.

In a panic situation or even not in a panic situation everyone has a tendancy to do exactly what they have done many times before and are accustomed to doing and that's fine... Until you change the rules and introduce breathing compressed air under water to the equasion. Without proper class training and in-water training that provides a full re-education of what someone has known and hopefully sticks with them to change prior behaviors it is dangerous at best and deadly at worst. Even with training there are plenty of certified divers who forgot their training in a panic situation and have ended up in the hospital or dead.

Next time - Don't tell a parent of an 8-year old you "fully recommend" anything about which you are not fully educated. If that 8-year old can pull on a rope would you "fully recommend" he strap on a parachute and try jumping out of a plane? Heck.. all he has to do is pull on that rip cord - seems simple enough.

You are dangerous. Get certified and get a clue.

 

[ggunn]

A few years ago a friend of mine who only snorkeled had this bright idea. He wanted to snorkel near me while I was diving and come down and get breaths from my octopus so he wouldn't have to go back to the surface to get air.

Um, no.

 

[deepsea21]

A few years ago a friend of mine who only snorkeled had this bright idea. He wanted to snorkel near me while I was diving and come down and get breaths from my octopus so he wouldn't have to go back to the surface to get air.

Um, no.

What a story! I guess the question is, "How many certified divers out there would see no problem with that?" I hate to say it but based on some of what I've seen I am beginning to wonder just what level of training some "certified" divers actually received. I guarantee you there are lots of certified divers out there who would see no problem whatsoever with fulfilling that request.

 

[mmmbelows]

If there are none or almost no reported expansion injuries on snuba and the number is correct of over 5,000,000 dives on it since inception that's less than a 1 in 5,000,000 chance of a problem. Contrast that with

The odds of becoming a lightning victim in the U.S. in any one year is 1 in 700,000, odds of being struck by lightning twice in your lifetime are 1 in 9 million

 

[DjDiverDan]

If there are none or almost no reported expansion injuries on snuba and the number is correct of over 5,000,000 dives on it since inception that's less than a 1 in 5,000,000 chance of a problem.

No doubt the odds are small, but they are not necessarily as small as 1:5,000,000 or even 1:1,000,000. You cannot simply calculate a probability function that way, and to assume that 5,000,000 events without an injury necessarily means that the odds are " less than 1 in 5,000,000" reflects a poor understanding of both probability and statistics. A fair coin has a 1:2 chance of coming up tails. Yes it is still possible, and not even that rare, to toss the coin 6, 8, even 10 times in a row and get a heads every time. But tossing a fair coin 10 times and getting 9 heads and only one tails does not establish that the chance of getting tails is only 1:10. The probability remains 1:2, but each toss is an independent event, unaffected by the results in previous tosses. Just because there have been 5,000,000 snuba dives without incident is no guaranty that we won't see a spate of expansion injuries in the next 5,000,000 dives. Further, it ignores the fact that tourists who try Snuba are not a homogenous group, and operators of Snuba concessions are not all equal in training and safety standards. We are not, after all, dealing with 5,000,000 "fair coins" - we are dealing with individuals, some relatively bright and mature, using Snuba operators who are careful in their training and customer supervision, others not so bright and not so mature, using Snuba operators who are less careful in training and supervision. I doubt very much that both groups have the same probability of suffering a Snuba-related injury. Without knowing a lot more about the relevant characteristics of both the customers and the Snuba operators who ran those 5,000,000 Snuba expeditions, it's impossible to try to even estimate a "true" probability of a Snuba injury for a hypothetical 8-year old who might be less inclined to listen to safety instructions or internalize warnings about risks.

 

[ggunn]

No doubt the odds are small, but they are not necessarily as small as 1:5,000,000 or even 1:1,000,000. You cannot simply calculate a probability function that way, and to assume that 5,000,000 events without an injury necessarily means that the odds are " less than 1 in 5,000,000" reflects a poor understanding of both probability and statistics. A fair coin has a 1:2 chance of coming up tails. Yes it is still possible, and not even that rare, to toss the coin 6, 8, even 10 times in a row and get a heads every time. But tossing a fair coin 10 times and getting 9 heads and only one tails does not establish that the chance of getting tails is only 1:10. The probability remains 1:2, but each toss is an independent event, unaffected by the results in previous tosses.

Although that is true, and I have no dog in the SNUBA safety hunt, I must point out that if you toss a fair coin 5 million times in a fair manner, there is an extremely high probability that total numbers of heads and tails will be very close to a 50:50 ratio. Large sample size tends to iron out statistical anomalies. Las Vegas exists because of the reliability of statistical data on a large sample size. If you leave Las Vegas with more money than you arrived with you are a statistical anomaly, and Las Vegas succeeds because there are more losers than winners, which the statistics predict.

Of course, your point about the nonhomogeneous nature of the SNUBA data, if such data exists, is also valid.

And I still hate those obnoxious things.

 

[mmmbelows]

You can try to quantify it any way you want but equal snuba concessions or not, if the numbers are accurate so far based on the past 30 years of the activity the risk of an injury looks to be less than the chance of being struck by lighten twice.

Like I said an actuary based on those numbers would rate the risk of an expansion injury while on snuba to be basically ZERO. Again, either the injuries reported aren't accurate or the risk some people associated with it is flawed. If certain people want to point out how risky it is based on science, how about explaining why the injuries simply aren't happening? I imagine there in lies the revelation to the flaw in the logic of how risky snuba is, cause the concrete evidence doesn't back up the theoretically hypothesis of the danger. There must be a huge cover up in the sport because the science people are throwing around says there should be dozens if not hundreds of expansion injuries in 30 years.

According to DAN the risk of death (not injury but death) for scuba divers is 820 per 5 million.

 

[ggunn]

You can try to quantify it any way you want but equal snuba concessions or not, if the numbers are accurate so far based on the past 30 years of the activity the risk of an injury looks to be less than the chance of being struck by lightning twice.

Aye, but there's the rub, isn't it? Who is collecting and reporting that data?

 

[DjDiverDan]

Although that is true, and I have no dog in the SNUBA safety hunt, I must point out that if you toss a fair coin 5 million times in a fair manner, there is an extremely high probability that total numbers of heads and tails will be very close to a 50:50 ratio. Large sample size tends to iron out statistical anomalies.

Your point about a large sample size is absolutely true. But just what constitutes a sufficiently large sample size depends upon the probabilities involved. For tossing a fair coin, where the odds are 50:50, a sample size of 1,000 is plenty large enough for the probabilities to emerge from any statistical anomolies. But for a much less likely event, like winning the Power Ball Lottery, where the odds of getting all 5 numbers plus the Power Ball are about 1 in 274 Million, a much, much larger sample size is required. The Power Ball Jackpot has gotten as high as $1.5 Billion, and with a payout of only about 50%, and chances costing $2, that means that more than 1.5 Billion tickets were sold, in the aggregate, over several drawings, before there was a winning combination sold. And in that case, there were 5 winners. In Lotto games, we can calculate the odds precisely because of the nature of the game - pure random number selection. But even a sample size which is about 6 times the calculated odds against (274 million x 6) may be necessary before a positive outcome appears.

With a sample size of 5,000,000 Snuba experiences, all we can say with any confidence is that the odds of an injury are probably (with about a 95% confidence) less than 1:1,000,000. And even with that big a sample size, there is still a non-zero chance (though very, very small) that the odds might be as high as 1:500,000.

I'm not claiming that the probability of an injury is high, only that you can't calculate the precise odds by simply looking at a sample size of 5,000,000 and say "Aha, probability is less than 1 in 5 Million!" It is much more complicated than that. But this has gotten off subject, and I'm getting much more pedantic than I like. It's just that innumeracy and people misunderstanding and misusing statistics is a pet peeve of mine.