I grew up with a Swim Club, and our pool went to 12' deep - and just getting to the drain at the center of the deep-end hurt my ears so bad, that it was an effort.
I can pop my ears at will in Air, but underwater, at 12' the pain is intense, and I cant normalize. I couldnt imagine say, 30' deep or anything more.
My first, brief, dive to 135 feet (41m) was in 1976 off the coast of Cozumel. I was in a group led by a dive master and we quickly descended, swam through a large ring shaped coral structure at around 135ft, and started our ascent. More accurately, all but one of use started our ascent.
Back then, of course, we had to rely on the Navy dive tables for maximum safe bottom time (just 10 min from entry at the surface to start of final assent), and we still added about 5 minutes at 15 feet as a safety stop before exiting the water. These limits, determined by the Navy for Navy divers are intended to prevent decompression sickness (the "bends").
Unrelated to the bends is another hazard, nitrogen narcosis that can interfere with clear thinking. It usually starts to affect people at around 150 feet and I barely noticed it at 135ft. However one diver in the group was affected and after passing through the coral archway continued to swim down the wall of coral toward the bottom at least 100ft deeper. The dive leader had to chase this diver, catch him, and then go though a real decompression stop or two with him on the way up.
Now days, sport divers can explore these depths more safely by using a mix of nitrogen and oxygen called Nitrox which is a 65/35 percent mix of Nitrogen/Oxygen vs the atmospheric proportions of 78/21 percent. This lowers the partial pressure of Nitrogen while diving and thus allows safe diving at greater depths and duration for sport divers.
If the research described by the featured article is true, this suggest a simple performance enhancer. Perhaps in the future, SAT exams will have to outlaw the use of Nitrox SCUBA tanks in the exam room.
I wonder what prevents a fish that goes down to 100 meters from going down to 300 meters or 3000 meters. Does it feel an internal pressure that tells it to not dive further, or is it the amount of light or availability of food it seeks? Since fish wouldn't suffer from decompression sickness, I wonder if they could dive much deeper if they wanted to.
Now that I think about it, I've scuba dived to 130 feet and it didn't feel any different to me than being at 10 feet. The only reason I didn't go deeper is because the depth gauge, divermaster, and training told me not to go deeper, and not because I was feeling the pressure.
> How dangerous is scuba diving in shallow waters, say less than 15ft/5meters?
Bear in mind that the rate of change in pressure is higher at shallow depths. At the surface pressure is 1 atmosphere, it will be 2atm at ~33ft, 3atm at ~66ft and so on. This means that if you have a momentary loss of buoyance control and shoot up, say, 10ft, it is a much larger sudden change in pressure (i.e. more dangerous) to go from 20ft to 10ft than from 100ft to 90ft. So having expert buoyancy control is actually more important at shallower depths.
It just takes practice, but it is a risk for new divers who go in thinking they'll just stay shallow but do so without having mastered boyancy yet.
For shallow dives I use nitrox at 36% oxygen to reduce nitrogen absorption. It's easy to get nitrox certified after completing basic & advanced training so I'd recommend it if shallow diving is a primary goal.
Although at 15ft or less I generally just snorkel if the visibility is any good. At such depths you can see the same by snorkeling, with less risk and hassle.
You can experienced this in pools! 12ft of water adds an extra 1/3 atmosphere of pressure, compressing the gas in your body by 25%. (That's why your ears pop!)
By varying the amount of air in your lungs, you can choose which depth to be neutrally buoyant at. Weirdly fun!
The beginning certification level is 60 feet. You unlock the deeper limit after additional training, and it includes performing a three-minute "safety stop" at 5 meters. 60 feet is about as deep as you want to shoot to the surface from (and if memory serves the limit they give in training is 30 feet, or 2 atmospheres).
A few hundred meters? Even 10 meters under the water feels like an inhospitable alien world, likely to induce a minor sense of panic if you look up and contemplate the amount of water between you and the surface.
The issue with depth is 1) you can't do a direct ascent to the surface past...18m or so?...with a high level of safety, for a large number of users, and from 30m or so (the recreational limit for most programs) it's even harder, plus that's the upper bound where nitrogen narcosis starts to set in. I've done down to 40m on air which is the next limit and the point where a lot of people have more serious nitrogen narcosis. Beyond that you really should be doing decompression diving and start to be looking at different gas mixes which makes it all a lot more complex. At depths beyond >50m oxygen toxicity on air would be bad too. The current normal limit is ~56m on air, previously people have done 60-65m pretty consistently on air, and the record is probably 155m (but that is insane and almost certainly caused harm, plus had high risk).
Ascent from depth without having cavities open to equalize pressure will be a really bad deal even from 18m (and really from comparatively shallow depths like 3-6m), but that really just means breathing out at slow rate during ascent. For that it'd be explode vs. implode though :)
30 meters is around the commonly accepted limit for recreational scuba diving.
If you are diving you can take a breath of air from your regulator at 30 meters and then, keeping your airway open, ascend to the surface on a single breath. Because you are ascending, the air in your lungs expands as you go up, and you get to the surface with a full breath of air, even though you've been blowing bubbles all the way up.
At 30 meters you limit your dive to 20 minutes... beyond that you risk decompression sickness.
At 10 meters the limit is around 3 hours.
In both cases, if you were down for a couple of days, you'd need decompression. The deeper you go, of course, the more dangerous it is as there is more dissolved gas.
http://www.adventuresunderthesea.com/scuba-class-information...
The deeper you go, the less time you can stay there before you have to control your ascent.
Even a mild case of the bends can be excruciating, not something you can tough out: http://en.wikipedia.org/wiki/Decompression_sickness
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