So much to respond to...
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.
Ron Brandt had an interesting reply in that it appears (the way I read it) at least one Cozumel operator is offering simple gas planning:
PhinDiver states:
Now, the trick is, without 1000+ dives, how do you teach an OW student when to turn the dive?
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 concept is amazingly simple for cavers -- we know when to turn the dive, we'd have a lot of deaths if our gas planning was "500psi back at the entrance" -- when do we turn around? Do we have enough gas to get our buddy out alive? To answer those questions we come up with a pressure of when to *turn* the dive. At that point the dive's over and we come out and I personally don't give a hoot what my pressure is when I surface outside the cave -- the only important number is when we turned.
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. De-emphasize what number you should have "back on the boat" -- that's completely unimportant (just make sure the "begin ascent" number is well chosen to assure the "back on the boat number is sufficient).
In addition, drill into the beginning OW student that that "begin ascent" number (such as 850psi) is sufficient for them to share air with an OOA diver and that both they and the OOA diver can perform a safe, unhurried ascent to the surface including a safety stop. Will they end up with less than 500 psi back on the boat? You bet -- but if you eliminate that number completely form your curriculum as a goal, they won't care and they'll be safe and less stressed.
Roak
Nope. It's a horrible idea. Note that I'm not saying that being back on the boat with 500psi is a horrible idea, but considering that as "gas planning" is a horrible idea. Horrible in that it ISN'T gas planning.Rick Murchison:Bottom line... for the average tropical sightseer diver, "500 psi on the boat" is not a "horrible idea" at all, but rather a reasonable rule of thumb that is supported - not negated - by gas planning.
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.
Ron Brandt had an interesting reply in that it appears (the way I read it) at least one Cozumel operator is offering simple gas planning:
Note the use of the word "end" -- I take that to mean "you start you ascent when you hit this pressure" -- you may not agree with it, but *thats* gas planning!Ron Brandt:There are dive opps in Cozumel that either want you to end at 700 and one I dove with wanted us to end at 1000!
PhinDiver states:
Though PhinDiver *thinks* their gas planning is "500psi on the boat" it isn't -- "Usually, i start going up on shallow dives at about 900 to 800 psi." is their gas planning -- based on when to end the dive, not pressure back on the boat. With his 1000+ dives he knows when to end the dive to have 500 psi back on the boat. But his gas planning is not "500 psi back on the boat" -- it's really the 800-900 psi ON SHALLOW DIVES (which implies he knows how to tweak this on deeper dives).PhinDiver:...For me it is 500 psi ON THE BOAT, not start ascending with 500 psi. Which means, I start ascending with more psi depending on the depth of my dive. Usually, i start going up on shallow dives at about 900 to 800 psi.
Now, the trick is, without 1000+ dives, how do you teach an OW student when to turn the dive?
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 -- heck, tell them that youll teach them that in AOW if you want to!Wristshot:1) If you don't give 'em a number, they will return with the tank empty.
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 has the same problem -- it's an "after the fact" number. It's turning right 1/2 mile before the big red barn.Charlie99:I've been on several dive charters where the "500psi rule" is expressed as "Be at safety stop no later than 500psi."
This concept is amazingly simple for cavers -- we know when to turn the dive, we'd have a lot of deaths if our gas planning was "500psi back at the entrance" -- when do we turn around? Do we have enough gas to get our buddy out alive? To answer those questions we come up with a pressure of when to *turn* the dive. At that point the dive's over and we come out and I personally don't give a hoot what my pressure is when I surface outside the cave -- the only important number is when we turned.
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. De-emphasize what number you should have "back on the boat" -- that's completely unimportant (just make sure the "begin ascent" number is well chosen to assure the "back on the boat number is sufficient).
In addition, drill into the beginning OW student that that "begin ascent" number (such as 850psi) is sufficient for them to share air with an OOA diver and that both they and the OOA diver can perform a safe, unhurried ascent to the surface including a safety stop. Will they end up with less than 500 psi back on the boat? You bet -- but if you eliminate that number completely form your curriculum as a goal, they won't care and they'll be safe and less stressed.
Roak