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August 6, 2007
First Scuba certification anniversary and diving Tahoe again
Time flies. Exactly a year ago I received my PADI Open Water certification at Folsom Lake. It seems like a lifetime ago. I look at the picture of my instructor, Chuck Odell, with his arms outstretched, proclaiming, "You are now certified!" Back then I didn't know what to expect. Everyone had told me about the camaraderie among dive class graduates, and how we should keep in touch and go dive together. And, of course, how our local dive shop, the place where we learned and got certified, would have all those trips and opportunities. Well, my class, small to begin with, showed no interest in keeping in touch. I tried, sending everyone emails, but that all quickly fizzled. And my dive shop went under a few days after I got certified. Sure, some of the folks who got stranded when the dive shop tanked got together and formed their own loosely knit group. I even did their website and attended the initial meeting. But they were mostly couples who had known, and vacationed with, each other for decades. I probably could have become part of that group, but for whatever reason, I didn't.
So it's a year later, and as with everything else in my life, I don't know whether I succeeded or bombed. I certainly didn't drop out of diving like so many newly certified divers do. I do have 27 dives in my log book, and have spent a total of 806 minutes underwater, 13-1/2 hours. I got my advanced NAUI certification, and have seen some very interesting places. And the scubadiverinfo.com website certainly has grown. I've written some 65,000 words of blog entries alone, and site traffic keeps growing. On the negative side, it's a year later and I have just 27 dives under my belt (or integrated weight system), and four of them were the certification dives. That's not exactly a whole lot in a year, and I still haven't even tasted salt water. All my dives have been in lakes and rivers and sinkholes. I certainly read and learned a lot, and it's been a thoroughly interesting experience. But not having a local dive buddy certainly meant fewer dives than I had anticipated.
But now the next trip is planned. It's Lake Tahoe again. SeaLife is sending me three of their latest underwater cameras for review, and the big Olympus Evolt rig certainly needs to get back in the water. I also have four other new cameras that need to be reviewed, and what better place than Tahoe?
So that means I have to think about high altitude diving again. Sadly, I never did get the PADI high altitude diver card Chuck had said I had earned. I reminded him a few times, but life got in the way, and I think he moved elsewhere. Oh well. In any case, I've been thinking about high altitude diving and what it means.
It's actually an interesting topic with several aspects. I had given it a lot of thought last year before I dove Tahoe with Chuck. So much that his curt reply to my long email was "You're overthinking!" Well, as far as I am concerned, that's better than underthinking. Anyway, the issue with high altitude diving is that the sea level dive tables don't apply. Why is that?
Well, Lake Tahoe is at 6,230 feet, and at that altitude, the air pressure is only about 80% that of sea level. So if you dive down to, say, 66 feet, the 66 feet of water add the usual two atmospheres of pressure. But when you ascend again, you don't go from the usual three atmospheres to one, absolute. Instead, you emerge to an air pressure of only 0.8 atmospheres. So as far as nitrogen absorption goes, it's as if you had done a dive not down to 66 feet, but to 1.2 times 66 feet, or 80 feet. But, you may say, what if prior to the dive you had stayed at high altitude long enough to have off-gassed enough nitrogen to be at an equilibrium? Admittedly, to my way of thinking, the 66 foot dive should then be just a 66 foot dive. But that's not the way high altitude dive tables see it. In fact, they're even based on not only the water level of where you are diving, but the highest elevation you'll be going 24 hours after a dive. So if you drive back over an 8,000 foot mountain pass where air pressure is just over 75% of that at sea level, your 66 foot dive is now considered 1.25 x 66 feet, equals 88 feet. Now, that is just my way of reckoning. The altitude dive tables I've seen are even a bit more conservative. It's also recommended to only do two dives a day, and that safety stops are mandatory.
But there's more. I'll be driving up from Folsom, which is at just 300 feet or so. So by the time I arrive the summit, the pressure will be much lower, and my body will be off-gassing. Using the PADI pressure group system, each 1,000 feet of elevation is equivalent of two pressure groups, so at the 8000 foot summit, that'll be 16 pressure groups. So up there you're all of a sudden a PADI "P" diver and it'll take your body so and so much time to off-gas and reach equilibrium. But wait, I'll quickly be descending again to Tahoe lake level, and then it'll be perhaps an hour and a half until I actually hit the water. The PADI table would indicate that I am now a "C" diver, which is why every altitude dive should be considered a repetitive dive, even the first. It's easy to see how this can quickly become complicated, so better safe than sorry.
There's actually an additional interesting aspect: In terms of ACTUAL pressure, at our depth of 66 feet, it's really less. Instead of the weight of 66 feet of water and then a whole atmosphere of air pushing down on us, it's just the water and then 80% of what the air normally contributes, so from that perspective, the 66 foot dive is actually more like a 66 divided by 3 times 2.8 dive, or 61.6 feet. But since it is the nitrogen we're concerned about, this means nothing.
Posted by conradb212 at August 6, 2007 11:16 PM
