Dumpsterdiver; You state that two divers can easily share one dive computer... I guess my next question would have to be, what certifying agency teaches this practice? I personally can not think of one. My understanding of dive tables (square profiles-time and depth-) and dive computers(math based pressure and time related variable profile) is that they do not mix,due to safety concerns involving no decompression limits. If your computer should malfunction you should call the dive. Not swim to your buddy and use their safety instrument which has been calculating his or her dive up to that point, not yours, and signal now I'm just going to stay exactly where you are for the rest of the dive!!! Both you and I know thats not going to happen. To many factors come into place, buoyancy, assent rate, distractions and heaven forbid other equipment failures are just a few... lets face it we shouldn't mix dive tables and computers because we need to dive safely Do you as a instructor want to go home after the dive buddy you have allowed to share your computer takes a hit and say well... I didn't see any thing wrong with it at the time.... and we were having so much fun!! Food for thought..... Not only do I teach my classes under the premise that you should never share a dive computer, I also strongly advise if they have already are into their dive and the computer craps out they should call the dive and refrain from diving for the rest of the day. Safety first. Fun second!
Chris SSI instructor