neil:
I may not have an engineering degree, but I know back-pedaling when I see it!!
I'm trying to be all fun and jovial about this, but I do find it perhaps somewhat offensive that you refuse to believe me no matter how I try to phrase it. I'm not back pedaling. I'm just trying to explain the pedantry from which it came, as you do not seem to grasp how I truly *do* think. I don't care if you think it's stupid to be so attached to details, but it really *does* bother me when you treat me as if I don't care what I say other than to just try to cover it up later.
It may make me seem rather pathetic, but I *do* re-read what I'm about to post, and quite often, I am forced to better phrase something before posting. I hold myself to a higher standard, as I *should* know what I'm talking about.
neil:
At the same time we have Neil, who cant add 2 and 2 without a calculator, in the front row. Neil needs to understand the concept sans math, at least at first. Instructor ClayJar just told Neil that larger bubbles are harder to control than small ones. Neil is worried. Neil weighs 200 lbs. Neil IS a big bubble. Neil wishes he had chosen his parents more wisely. Golf sounds nice.
Neil wasn't in my class. If Neil was, he would have understood that for any given diver, if that diver carries more lead weight than he needs for proper end-of-dive buoyancy, he will have a larger bubble than he needs. He would have been shown that that extra lead weight is where the larger bubble comes from, and he'd have a desire to be properly weighted.
If Neil were like, say, Jose, who happens to a well-built (but not obese) young man, he may have commented that he's probably going to have a large bubble, as he's much more substantial than his girlfriend. I would have replied to that by saying that it doesn't really matter how big you are; anyone can dive. (I'd probably mention the *gargantuan* DM I dove with once, who had a great time.) He'd quickly grasp that the concept is simply to not make diving hard on yourself by carrying extra lead on your weight belt, as that means you need more air in your BC, so you have to control it more.
Strangely enough, I happen to have taught this little mini-lesson to a guy. Equally strange, he happened to be named Jose. Even more strangely, that's about how it went. Perhaps most strangely, he had a great grasp of the concept (i.e. overweighting makes diving harder; proper weighting makes diving relaxing and more fun) by the time we finished.
neil:
What was the point of telling the class that its easier to control the smaller bubble? Im not sure its pertinent, to be honest. In fact, its not really true. If we have a 100 lb. woman and a 200 lb. man learning to control buoyancy, it makes no difference. The rates of change are still the same, as we know. Theyre going to be adding or dumping air in about the same ratios, with about the same rate of difficulty. Dont nitpick about lungs and wetsuits.
You have missed the point, which isn't a big deal -- you didn't start at the beginning. Let me try a short TV Guide recap:
Take one diver. Load him down with lots of extra weight, like some places have been reported to do to their OW students. In order to be neutral, he has to add a lot of air to his BC. Now, take that same diver, and remove some of that weight so that he ends up with only the weight he needs to be neutral at the end of the dive when his tank's mostly empty and he's finishing his safety stop. Since he has much less weight on his weight belt, he doesn't need a bunch of air in his BC. (He still needs some, as the air in his tank at the beginning weighs something and his wetsuit will compress -- we covered pressure-vs-volume a previous day.)
The bathroom scale weight of the diver is irrelevant. The lungs and everything else matter greatly, but that was only mentioned later in the discussion once the whole "why you don't want to be overweighted" concepts were solidly understood.
neil:
In the hands-on experiment, the concept is not to "hold down" the cartesian divers. The goal is to hold the bottle such that the cartesian diver stays level with the line on the side of the bottle. If it starts sinking down, you have to loosen your grip slightly to reduce the pressure, allowing the bubble to expand and the buoyancy to increase. If it starts floating up, you have to tighten your grip slightly to increase the pressure, causing the bubble to contract and the buoyancy to decrease.
The two cartesian divers I used are two identical open-bottomed, top-capped plastic tubes. The "overweighted" one has four large paperclips slid on from the open bottom. The "properly weighted" one has two normal paperclips slid on from the open bottom. The paperclips maintain static stability, and their size and number are directly correlated with the size of the bubble needed to allow moderate squeezing on the bottle to yield neutral buoyancy. The only differences between the cartesian divers are the number and size of their paperclips and the size of their respective bubbles.
After I demonstrate the divers, I hand the large bubble diver to the student and ask him to squeeze it to hold the diver at the line marked mid-water. It generally takes a few seconds of trying before he starts to get the hang of it, but then he can usually do a decent job. Then I swap bottles with him, giving him the lightly-weighted small bubble diver, and I ask him to try the same "game" with that bottle. Quite directly, he finds that it's *much* easier to hold the small bubble diver at the marked mid-water line. Then, we proceed to the "Why?"
By themselves (or at least with just a little prodding for the shy), they can easily note that the bubble in the big bubble diver is much larger than the bubble in the small bubble diver. It's also readily observable that the large bubble changes size quite a bit more than the small bubble -- when you let go of the bottle, it takes almost 2/3 of the tube, and when you squeeze it hard, it gets quite a lot smaller. The small bubble was never that big to begin with, so while it gets squeezed proportionally (this was covered in the earlier ideal gas law stuff and is noted at this point of the demo for review and emphasis), it doesn't have nearly the same buoyancy change.
Then I ask why the bubble was larger and note this was because it had to be so in order for the diver to be neutral with all that extra weight from the paperclips. I say that it's the same way with scuba divers. If you add a lot of extra lead weight to your weight belt, you have to carry more air in your BC to offset it, and as they've just seen, the bigger the bubble, the more effort it takes to deal with it. At that point, they not only accept that they don't want to dive overweighted, but they understand *why*.
As for the other questions. You know they're a load of hooey; I know they're a load of hooey.
On the other hand, if you don't quite understand the demonstration as I've tried to explain it in this post, I'll write it up more completely and post a complete "lesson plan" for the experiment. (I'll certainly do that if it will help teachers and instructors who may want to consider using it.)
Anyway...
