so at the end of a dive where you've had to donate air to an out of air buddy who's regulator malfunctioned and your tank is less than half full, instead of making a nice controlled swim, you find yourself positively buoyant and struggling to stay together on an E-ticked ride?
Sorry to be contrary, but if you are going to do a buoyancy check I like to recommend my students do it with low tank (I'd give a specific PSI, but there are so many tanks in use these days that it can mean many things), and be able to hold a controlled position at 5' with all the air out of BCD and/or drysuit.
You never want to be so heavy that at the deepest point of our dive (I'm taking into account suit compression) with all your gear that you can't "swim" yourself to shallower depth (where suit buoyancy will start being helpful again) if you drop your weight belt.
You never want to be so light that you are a cork halfway through your dive and putting rocks and cinder blocks in your pockets on the way up the slope.
Many instructors overweight so they don't have over excited BCD happy bubble babies bouncing all over, but also so that when they say "let all the air out of your BCD and STAY PUT" the student does so. This is not my philosopy. The problem is teaching proper neutral buoyancy and control of 'self' while weightless can be hard to teach if you have only 2 pool sessions. better to park the students on the bottom on their knees and use that time teaching air sharing and safety drills if that's all you've got. they sometimes overweight because they are starting with "less full' tanks (student tanks with hot fills) also expect that some of you to may breath the tank down lower than others as it can be hard to get a feel for your air consumption (unless you are a super hoover) in the pool sessions (even harder if you only have one or two sessions) so it's just easier to start with a "few extra pounds".
4 lbs can also be the difference in the tank they may have given you (steel vs aluminum).
I always tell students to re-check their weight needs periodically in the first dive or so out of class, and then the next 10-15 dives, especially if they are using new/different gear. You may find that you can drop even more than 4 lbs if you were overweighted during class. As you get more comfortable, your breathing pattern settles down and you have less of a tendancy to breath off the "top" of your lungs (and/or take big breaths) as your breathing becomes closer in tidal volume to your topside breathing, your weight needs may decrease as well.
rawls:
Jon-boi...you won't be able to determine whether you are under or overweighted until you actually get into the water and do a buoyancy check. You should be at eye level with the surface with an empty BC, holding a normal breath and ideally a half empty tank. But You will still be ok with a full tank. You may have to add or delete weights as necessary which would ideally be done in shallower water...or a pool. Just remember fresh water weighs 1 kg/liter and salt water 1.03kg/liter so if you do a buoyancy check in a pool you will have to adjust for the weight of salt water.
