All tanks have the same buoyancy change per cubic foot of air.
It doesn't matter if your tank is made from steel, aluminum or kryptonite. If you have used up 80 cubic feet of air, it (and you) will be 6.45 pounds more buoyant than when you started.
flots.
Yes, all tanks become
MORE buoyant during the dive. But the tank does make a difference in how much weight you need at the end of the dive.
We agree, as you use air the tank will become lighter due to the loss of the weight of the air. The difference is that at the end of the dive, which as you point out you must weight for at the beginning and plan for the loos of the weight of the air, AL tanks will float and Steel, tend to sink. See the chart for your particular tank. ALUMIUM 80 tanks will be about 4 pounds positive when empty (it will float) and a steel 80 tank will be 2-4 pounds negative (it will sink) when totally empty (note some tanks AL and STEEL are NEUTRAL when empty). See
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan for the numbers on this. EXAMPLE Catalina S80 AL tank is -1.8 pounds FULL and a +4 empty. STEEL PST E7 80 is -8.5 FULL and -2.5 EMPTY. So with a Steel tank of the same size you need 4-6 pounds LESS weight at the END of the dive. And the end of the dive is the most important part because that is where you are lightest due to the loss of the weight of the air consumed.
Definition: Buoyant - object that floats in the present medium (air, fresh water, salt water, salad oil etc). Buoyancy is obtained when an object displaces more weight in the medium than the object weighs. Example a cubic foot object weighing 64 pounds or less floats in salt water. In the true strict definition of the word, an object is either buoyant or it is not. (It either floats or it doesn't). In SCUBA we talk about POSITIVE, NEUTRAL and NEGATIVE Buoyancy. Technically these are not correct terms but are generally accepted to make talking about Buoyancy easier. POSITIVE: Floats, Neutral - neither floats or sinks, it's buoyancy characteristics are equal to the medium in which it rest. Negative - sinks.
BACK TO THE OPs TOPIC: The issue here is the OP was improperly weighted at the beginning of the dive by an estimated 2 pounds of weight. Towards the end of the dive, having consumed 3-4 pounds of air weight his tank moved from being heavier (negative 1.8 pounds) than the weight of the water it displaced to lighter (positive 4 pounds) than the weight of the water it displaced (it floats). In my limited experience, an AL80 will start to get noticeably "light" at about 1000 PSI and if you are not properly weighted to account for the tanks positive buoyancy characteristics at the end of the dive, it will start to pull you to the surface.
OP: Review the below Videos and your books to get the weight right. Remember in scuba when you change anything, your weighting needs to be re-checked for any changes (something I forgot when on dive 3 of the day I took a buddies empty camera case down on a dive to check for leaks. That empty air filled case made me underweighted). Define Anything: You gain or lose weight or change dimensions (change fat to mussel or the other way around), change wetsuit (or it ages), new regulator, BCD, carrying a camera etc.
These videos break it down into small steps of you, tank, bcd etc. You can save time by just going to the last video and doing a check with everything.
AL80 Cylinder Buoyancy Full Vs Empty Illustration:
http://www.youtube.com/watch?v=XvHJEuOXszc&feature=player_embedded#t=8
AL80 Cylinder Buoyancy weighting:
http://www.youtube.com/watch?v=PG_fhMMV5Kw
BCD GEAR Buoyancy Check:
http://www.youtube.com/watch?v=_8wiRKbdIXw
How to do a Buoyancy Check for you and your wetsuit only:
http://www.youtube.com/watch?v=2oWMAPbqrGY
Put it all together - Buoyancy check for You and your gear with tank:
http://www.youtube.com/watch?v=sd0cHmOXb9U Best to do the test with 500 PSI, but if you do it with a full tank, you will need to add weight to compensate to the air you will consume during the dive and the empty buoyancy characteristics of the tank. Don't forget that in Salt water you need more weight than in freshwater. Note, salt water swimming pools typically do NOT replicate the buoyancy characteristics of the ocean.
Also consider your body position and where the air bubble is located. Use the air dump closest to the surface at the time.