Scuba Diving Class – Day 1
So last night was the first day of scuba glass. We started off filling in some forms and then moved on to some discussion of key points from the instruction manual. Before getting into the pool, we did a quiz which asked very similar questions to what was asked in the instruction manual. Yours truly got a perfect score 🙂
I wasn't able to resist questioning the explanation that air in a scuba tank takes up less and less volume as you descend. I'm of the impression that since the scuba tank is sealed and rigid, changes in pressure do not affect the contents. My instructor, the dive master, and the rest of the class are more accepting of what they read the PADI instruction manual. I'm hoping a Google session will clear things up one way or the other.
The part of the night that was the most fun was getting into the pool with scuba tank and regulator. Almost immediately I was underwater and breathing. Awesome! But of course the instructor wanted to actually teach us some stuff so I had to surface and pay attention. As an introduction, the first confined water dive wasn't particularly tricky, but we did go through breathing, recovering the regulator, clearing the regulator, and emptying a flooded mask while underwater. All good things to know when you're 60 feet below the surface, no?
Speaking of breathing, a couple of the students had a little trouble getting comfortable with the whole underwater breathing thing. No outright panicking or anything like that. Just a strong reluctance to stay underwater and breath in and out through the regulator. Fortunately, by the end of the class these folks seemed to have conquered their fears.
That's it for now. Today's confined water dive starts in just over an hour so I've got to go and get ready!
Note: One night isn't long enough for wetsuits to completely dry. Bummer.
Re lesser volume. Nomenclature is not a diving specialty. And at the professional level of Divemaster or higher there really is only 1 module on physics.
The correct explanation and nomenclature should be that you are carrying some 2 kilograms of compressed air (insufficiently compressed to reach liquid form) when you dive with a 80 cuft/12L aluminium tank.
As you correctly noted, the internal volume of a sealed and uncompressable container does not change. And cannot change. Perceptibly or otherwise because if it does, that container is doomed to catastrophic failure.
What actually happens as you dive on open circuit (your bubbles escape into the atmosphere and surrounding environment as opposed to going back into your breathing system aka rebreather systems), the air pressure in your tank drops.
Correlating higher density with weight (1m2 of steel vs 1m2 of water yields different weights though they are the same exact volume) - your air density stored in the tank declines and towards the 'end' of your dive as your air is depleted you can roughly assume that you get 'lighter' by 1 kilogram/2lbs per 100 bar / 1470 psi of air that you consume out of your 80cuft/12L tank. The volume inside your tank never changes.
Its just amazing there are not more deaths in the recreational scuba industry. Its already hard to explain and understand using the metric system of round numbers. Using US imperial units of measure (cuft/psi/lbs) throws off any prospective student even further out with all the definitely confusticating numbers. A listing follows:
Metric New (filled 12L tank): 2kg of air, 200 bar pressure, 12 L volume.
Metric Used (depleted 12L tank): 0kg of air, 0 bar pressure, 12 L volume.
Thus you actually wear scuba lead weights to keep you DOWN at the end of your dive so you can do your requisite 3 minute 5 meter safety stop.
Imperial New (filled 80 cuft tank): 4lbs of air, 3000 psi pressure, 80 cuft volume.
Imperial Used (depleted 12L tank): 0lbs of air, 0 psi pressure, 80 cuft volume.
Thus you actually wear scuba lead weights to keep you DOWN at the end of your dive so you can do your requisite 3 minute 15 feet safety stop.
Yes, the above notations contains simplified numbers that do have rounding errors but this is done for 'teaching' and simplification purposes.
Weights assume no suits and neutrally bouyant person (while holding a half breath of air) diving who will sink upon exhalation and rise upon inhalation of air.
Which is why I no longer teach the PADI system. Its rigorously dead ended. From the agency's regulatory perspective.
The dive industry is also about selling you shit you mostly don't need - but it lines their pockets nicely (just like that XL soda you REALLY NEED with EVERY MEAL you eat). You're not supposed to be touching stuff underwater - no real need for gloves.