Can someone analyze this incident please?

[aquaregia]

I will do the same as you and surface with her together.
Never leave anyone alone even at 5m.

How about when it is in conflict with "a thumb's a thumb" or "one victim is better than two"? (serious question)

Although I was not taught to use the rule of thirds in this way, it can be used for the kind of dive you did. If you had said, "I will use one third of the gas I have going out, one third coming back, and reserve the final third for emergencies," it would have worked out okay. You would have turned back at 2000 psi, and had 1000 psi more gas for the ascent. You would have been okay.

I wouldn't have. If I leave 100'+ with 1000psi, I'll be struggling to have 500psi on the boat, even without a panic and air share.

 

[doctormike]

Interesting thread. If the OP is really pushing up against NDLs with an 80 on a 100 foot dive, he or she much have pretty good air consumption, especially considering less than 100 dives. In my experience, it takes a while for this to happen, and is even a badge of honor the first time you end a dive based on NDL and surface with gas to spare...

I think that TSandM's use of the cave diver's rule of thirds is appropriate in this situation. It was developed for a type of diving in which ascent is ONLY possible if you return to a specific point. Ocean diving in current and/or poor visibility means that ascent up the anchor line is preferred. While obviously the consequences of failing to return to your entry point "only" means a free ascent at sea in current (as opposed to death in a cave), it is still something that should be avoided if at all possible.

The "let the divemaster know when you reach 1500 PSI and surface with at least 500" rule is more appropriate for Caribbean dives in clear water, good surface visualization conditions, and no current.

 

[TSandM]

Well, if you really want to do rule of thirds right on such a dive, you first calculate your minimum gas for an air-sharing ascent from 100 feet. Then you subtract that from your total gas, and divide the remaining gas in thirds, because you might need to get a buddy back from your farthest point from the upline. GUE minimum gas from 100 feet is 40 cf; if you calculate a 60 fpm ascent and a 3 minute 15 foot stop, I think it comes out more like 25. That's 1000 psi out of an Al80, leaving you with 2000 psi for your bottom time. Divided in thirds, that's 600 psi for the outgoing leg. That's 15 cf of gas -- at a fairly decent SAC rate of .5 cfm, that's 7 1/2 minutes of outbound time, or a total of 15 minutes of bottom time. I don't know about anybody else, but paying for a boat charter in order to do 15 minutes on the wreck doesn't pencil out very well. That's why Al80s are not good tanks for deep diving, ESPECIALLY wreck dives from an anchored boat.

 

[CAPTAIN SINBAD]

All this has been very helpful. Thanks so much folks.