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Your chances are slim, indeed ... but not "non-existent". The reality is that it does happen ... even at times to experienced divers ... and if you don't have a source of breathing gas handy, then your life is measured in seconds.
Things that can cause even an experienced diver to run out of air ...
- debris clogging a dip tube (ask Dr. Bill about that one)
- an SPG that is either stuck or malfunctioning
- task-loading and/or narcosis
The primary reason to dive with a buddy is to have a redundant air source nearby. If you're going to adopt the philosophy that "it can't happen to me", then I highly recommend a pony bottle ...
... Bob (Grateful Diver)
At the risc of sounding cocky, here goes...
What are the chances that your next breath is the last you get from your tank ? I would say next to nothing.
When diving a diver should have a sense of time and gas consumption. Checking time and spg on a regular basis will eventually lead to being able to predict your tank pressure by just looking at the dive time. If in the first 15 minutes of your dive your tankpressure drops from 200 to 150 bar ( or 3000 - 2250 psi ) it will not be very likely that you will hit 100 bar or 1500 psi within the next 10 minutes if cirumstances do not change drastically.
Checking your spg every 5 - 10 minutes will prevent a situation where your gas supply suddenly stops causing you to start sprinting to your buddy. This will increase your comfort and will enable you to comfortably be more than arms length away from you budy.
A diver that unknowingly empties his tank is a diver that should take up knitting or checkers. Running out of gas is a situation that a no-brainer should be able prevent and should thus be non existent.
Equipment failure will cause freeflow and will deplete your gas supply really quick but gives you enough time to get to your buddy or reach the surface. A failure like this is more likely to happen with higher tank pressures than lower tank pressures so an occurence like this in the second half of your dive is quite low risk.
Knowing your gas consumption and current tankpressure is a much bigger safety measure than staying frantically close to you buddy. Knowing your buddies gas consumption more than doubles that. And if your buddy knows yours, well... need i say more ?
p.s. - is that what/how you teach???????