Why can't I dive twice on the same tank?

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Originally Posted by Papa Bear

When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds!

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Of course I carry enough air for my buddy! & yes, we follow the rule of 3rds & dive our plan. Whoever first uses 1/3 of their starting supply & we turn back regardless. I do agree with you about the pitfalls of H&W, B&G buddies, so that is why planning is important. In a perfect world, we would be buddied-up with those who have the same dive-style, SAC rates, air supply & gear. [I think we're starting to get into DIR territory here....]

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Originally Posted by Papa Bear

I am not putting my life on the line for a maybe! Never go in an Overhead environment unless planned and with the amount of air you planned on! So its not about her or her sac rate, its about the dive, and where your going and what your doing!

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& for an overhead environment, I would not buddy with a known hog, or potential hog (aka he/she-who-becomes-a-hog-when-conditions-are-less-than-ideal). In my limited experience, I've seen such individuals not follow basic rules/common sense much less the rule of 3rds b/c they are so focused on trying to entend their time/depth/range with little thought of the consequences. These are the same people that routinely ask for air at the bottom of the line & surface at barely 500psi. These are the people I'm kvetching about.

& as for diving in an overhead environment with a 60cuft remaining air supply & a buddy with 120cuft - now what is the point in that?

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Originally Posted by littleleemur

These are the same people that routinely ask for air at the bottom of the line & surface at barely 500psi. These are the people I'm kvetching about.

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Now there is nothing wrong with surfacing with less than 500psi if the site and conditions allow.

Two points, one overhead and the other open water! In open water my feeling as long as your breathing when you hit the surface you have enough air! Overheads a lot different! Requires planning and understanding of all the factors!
If a woman has a sixty and her buddy (male) goes in on 120 if stuff hits the fan with her no problem, but with him big problem because she has to have enough to get both out! She won't! In overhead you have to carry enough for your buddy otherwise go solo! If she has 120 and he has a problem then there would be plenty of air to get both out! A miss match in overhead does spell potential disaster! If you share usable are on the bottom I see no problem at all and if they end with 500psi its 500psi they didn't need!

Note the OP is from Canada. I'm not sure how litigious that country is compared to the US.

Unless I planned a very short dive to retrieve something, I always prefer descending with a full tank. Over the 47 years I've been diving, I've had enough situations where a partially emptied tank might have made the dive less safe. Having said that, I used to do two, sometimes three dives off one HP120 tank.