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It all boils down to pre-dive planning. Take in account the type of dive, shore, boat or surf. Add to that the conditions, depths, diver skill levels and purpose of the dive (Just looking around, training photo ect.).
Rule of thumb, plan your dive.
There are numerous thread on gas and dive planning, many are very good reads.
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Dave
The plural of anecdote is not data.
Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you're a mile away and you have their shoes.
What numbers should I keep track of so if at any point my buddy has a loss of air we can share without having to compromise ascent rates?
Great question.
Answer is: it depends.
Variables include depth and gas consumption rate (which is itself a function of depth and environmental variables).
If you know your gas consumption rates (something you can learn by tracking usage, time and average depth), you can make some conservative estimates about how you will react during an emergency (i.e. a stressful situation) and do a simple calculation to determine how much gas you should be reserving, doubling it to account for two divers sharing from one source).
Note that your gauges likely display PSI whereas your tank is rated for volume, so going between different tank configurations will require some slight tweaks to your numbers.
For open water ascent only: take your SAC rate and your buddy's sac rate and add them together (0.6 + 0.5 = 1.1). Take the depth that you're going to start your ascent from, cut that in half and translate that into ATA (66 FSW/2 = 33 fsw = 2.0 ata). Take the same depth and translate it into minutes at the ascent rate you use (66 feet @ 30 FPM = 2.2 min). How much air will you need for your ascent? 1.1 cfm * 2.2 min * 2.0 ata = 4.84 cubic feet). That's bare bones, no room for excitment (double it for that later) no room for a safety stop (which at 30 fpm is really not necesscary and which in an emergency you're going to blow off anyway). So I'd round the 4.84 to 5.0 which translates to a bit less than 200 PSI. So multiply in by the excitement factor of two, and you're at 400 PSI. Now that's just your ascent based on those SAC rates, figure that you never want to drop your tank below 200 psi ... esp. if two people are trying to breathe off it, and now you're at 600 psi for these two divers starting their ascent from 66 fsw.
Bingo air (rock bottom) or turn point is a completely different discussion.
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"Too often ... people enjoy the comfort of opinion without the discomfort of thought" - Leapfrog
"... what good would it have done? People like this are always right and no one can ever prove otherwise." - Paladin954
"It truly does boil down to motivation ... if you believe something is hard, or unnecessary to learn, you won't learn it ... even if it's completely within your capability" - Bob (Grateful Diver)
Bingo air (rock bottom) or turn point is a completely different discussion.
Not trying to be pedantic, just curious: what you described:
Average depth * time * consumption rate
...is fairly close to the rock-bottom concept. Or are you saying rock bottom is bingo air is turn point pressure, which would indeed be a completely different discussion?
Bingo air = rock bottom, they are both terms for the point at which you must turn the dive based on the idea that one of you has exhausted his or her gas supply to the point that he or she is packing just barely enough gas to get both of you back to the surface safely. Of course the concept is the same but the factors you must consider are real different for caves, wrecks, decompression and just plain "I want to exit at this point" diving.
__________________
"Too often ... people enjoy the comfort of opinion without the discomfort of thought" - Leapfrog
"... what good would it have done? People like this are always right and no one can ever prove otherwise." - Paladin954
"It truly does boil down to motivation ... if you believe something is hard, or unnecessary to learn, you won't learn it ... even if it's completely within your capability" - Bob (Grateful Diver)
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