roakey:
You need to know when to END the dive, when to start your ascent or it's not gas planning. Taking wild guess at depth to see how close you can come to "500psi back on the boat" is NOT gas planning -- that's my point.
No argument here. No disagreement. My post was answering a question, not defending anything.
roakey:
Now, the trick is, without 1000+ dives, how do you teach an OW student when to turn the dive?
Good question. For the average beginning OW student, they may have trouble with this. However, I agree that it should be taught.
roakey:
Well, if you really only want to teach them one number (I'm assuming an AL80 here) and assuming they stick to the 60 feet or less depth for an OW diver, tell them to begin their ascent with no less than 850 psi in their cylinder. From 60 feet with a safety stop this will give them about 500 psi "back on the boat" at a SAC of 1cf/min (if I did my calculations right). Now tell them that if they need to swim back underwater to an entry point, they need to add to that 850psi number --
The point is, the number they should have burned in their brains is when to start their ascent. Not having that, they really have no idea when to start their ascent whatsoever, and who knows what they'll come up with?
As an added bonus, they'll no longer be goal driven for 500 psi on the boat -- embarrassed if they have 100, and annoyed if they have 1000 -- they will have simply turned the dive when they should have (say, 850psi) and whatever they have when they get out is what they have -- who cares what the number is!?
This thinking scares me. Not so much the "thinking" but the application. If instructors used this method there would be more problems. Why? Because newbies would remember only select parts. In your response you used the example of 850 three times. Some would remember the "start your ascent at 850" and they would forget the 60 foot etc. Just my opinion.
roakey:
This has the same problem -- it's an "after the fact" number. It's turning right 1/2 mile before the big red barn.?
Good analogy. The only way for them to know to turn 1/2 mile early is to overshoot or to undershoot.
roakey:
This concept is amazingly simple for cavers --
Why would you even raise this issue? The "rule of thumb" (so accurately defined here by multiple people) would not apply to cavers in any way shape or form. If I ever heard a caver mention the 500psi rule (other than in ridicule) I would run away screaming.
roakey:
In conclusion, by all means give the beginning diver a single number if need be, but make it the number at which the ascent begins.
I do not agree with this at all. I agree with you that they SHOULD teach how to figure out when to begin the ascent, but that should NOT be a single number.
roakey:
In addition, drill into the beginning OW student that that "begin ascent" number (such as 850psi)
There you go again, reinforcing that dangerous idea.
If the thread were about what should be taught instead of what is taught we could cover this in more detail. (although we probably already beat this horse to death).
Just remember the capabilities and capacity of the average OW student.
I agree that I wish they had done a better job of teaching us how to know when to begin our ascent so as to end with sufficient air, or more correctly to ALWAYS have sufficient air for whatever arose. The bottom line is that they did not, and I had to learn it bass-ackwards. I wish that they taught or at least introduced the "rule of thirds" (another rule of thumb) or "Rock Bottom" calculations. At least if I had known that those things existed I could have chosen to explore them on my own instead of trying to "reverse engineer" my dive profile to achieve the elusive 500psi at the boat.
Thank goodness that there are some excellent instructors out there, and such a great resource as ScubaBoard to help educate people beyond the ultra-simplified rules of thumb.
Wristshot
