If you are weighted properly you should be able to maintain 10 ft with near empty tanks and no air in the bc or suit(if diving dry). This goes for dry or wet.
Correct so far.
If diving wet you need enough weight to over come the boyency of the tanks and your suit. At the end of the dive this should be around 5-10 pounds for the suit and 4 pounds if diving double steel tanks.
Not unless you're diving helium.
The weight of the air (or Nitrox) in an AL80 is approximately 5lbs. For a pair of double 100s, the weight of the AIR is 10lbs.
It weighs 10lbs no matter if you have it in aluminum tanks, fiberglass tanks, or STEEL tanks! It makes ABSOLUTELY ZERO DIFFERENCE WHAT THE CONTAINER IS MADE OUT OF!
The weight required to sink the SUIT is also an invarient.
The type of tank you dive has no bearing on the suit's buoyancy! The ONLY bearing on the suit's buoyancy shift is the suit's construction (not just its thickness, but the specifics of how much gas is in the neoprene, etc)
So is the weight required to sink YOU (which is typically a few lbs; if you're extraordinarily muscular, you may even be negative, but this is uncommon. If you're obese you will require more weight - possibly significant weight. Most "average" people need anywhere from 2-6lbs to sink THEM.)
OK so far no problem, now lets check the start of the dive. You hop in with 15 pounds on your waist and 20 on your back due to the weight of your tanks. At 100 feet your suit has compressed and has no redundent lift. So you add air to your wing and find out there is a hole and it wont hold air. Now you have to swim up 20 pounds because your weight belt is off(ditched because no one is swiming up 35 pounds from 100ft). Back at 40 feet you cant slow down because you sucked down half you tank because it was so hard to swim up that weight and your CO2 retention is at its peek. So your too light to maintain depth and you pop to the surface. Sounds like a sucky day to me.
The material the tank is made of is IRRELAVENT to this discussion.
Let's take the hypothetical diver with an AL80 on his back. EMPTY, that tank is +5. With a couple hundred lbs of air its +4 (which is what we do our weight check with, since if its COMPLETELY empty you want to be on the surface, for obvious reasons)
The diver is inherently +2, and he wears a full 3 mil wetsuit that is +9.
He thus requires 9 + 2 + 5 or 15lbs of total negative ballast to be able to stay below the surface at the end of the dive.
This diver puts on a steel backplate and STA, which is -9. He then puts on a weight belt with 6lbs of lead, and is now -15. This is a balanced rig and he now can go diving.
Now, said diver gets to the bottom. His wetsuit, which started out as +9, is now +1. He also has 5lbs of gas in the tank (he breathed one on the way down), so he is -13 overall. He goes to add 13lbs of buoyancy to the wing and finds that the elbow has detached, making it useless. He is still -13, and now must swim up a -13 lb kit. As he begins to ascend the suit regains 8lbs, and when he reaches the surface he is -4 (he breathed another lb of air on the way up.) He ditches his belt ON THE SURFACE and is now +2, and safe.
Now let's run the numbers for HP steel 80s.
The diver takes an Aluminum BP + STA. It is -6. He has on the same wetsuit, which is +9, and he's +2. The tank is -1 empty, and -7 full (same amount of mass for the gas).
The diver therefore needs 4lbs of lead to sink. He goes diving, and at the bottom the same thing happens. He is -6 for the tank (he breathed one on the way down), +1 for the suit, -6 for his BP + STA, and -4 for the lead. He is therefore -15 and has to swim that up, which is awfully close to -13, isn't it? When he reaches the surface, he is +3 (BP+STA+suit) - 5 for the tank (breathed one more on the way up) or -2 - he ditches and is +2, which is perfectly fine.
Now let's say you wear DOUBLES - either AL80s OR HP Steels.
With Double AL80s, you have +10 for the tanks empty, +9 for the suit, -9 for the plate and STA, and +2 for you. That's 11lbs of lead that you must carry.
You get to the bottom, and are -1 for the tanks, +1 for the suit, +2 for you, -9 for the plate and -11 for the lead. He's therefore -18, which is pretty un-good! If he gets back to the surface, and ditches, he can still be very close to positive or neutral though.
With Double HP80s, you have -2 for the tanks empty, +9 for the suit, -6 for the BP + sta, and +2 for you. You thus must carry 3lbs of lead.
You get to the bottom, and are -12 for the tanks, +1 for the suit, +2 for you, -6 for the plate and -3 for the lead. You are therefore still -18 at the bottom! Heh, wait a second! Wasn't the argument that HP steels are MORE negative? No they're not!
Now if you swim them up, you could have trouble though. You get back to the surface and are -11 for the tanks, +9 for the suit, +2 for you, -6 for the plate and you ditch the lead. You're still -6! Oh oh! But the good news is that -6 isn't all that bad, really, and further, you DO have 10lbs of gas in there - so as you breathe it while finning upward to stay at or near the surface, you will (before the gas runs out) become +4. Whew!
What chance? You are almost exactly as negative with steels as aluminums, all things considered. You are taking no additional risk, EXCEPT possibly at the surface after you swim up the kit. In an extreme case you could DITCH the kit and float like a cork, or simply breathe it down until it becomes positive on-balance.
The diver who uses a steel plate with the HP steelies doesn't need a weight belt at all (or if he does, its only a pound or two.) Yet he is NO MORE NEGATIVE AT DEPTH than the diver WITH the belt (the balance works out the same!) The only possible risk is that he cannot ditch the belt at the surface to insure positive buoyancy - the upside is that he can't ACCIDENTALLY drop the belt at 100'!
That is why it is a bad idea to wear steel with a wet suit.
Only if you can't do the math.
The mass of the air in the tanks is not dependant on what contains them. If you cannot swim the kit up from depth with full tanks, due to the weight of the gas contained in them, then you should carry a redundant means of buoyancy, whether its a drysuit or something else.
The reason for the claim that you should not dive steelies when wet is based upon
low pressure steel tanks, which are commonly -6 or -7 EMPTY! With a -6 BP+STA, that's -13 or so with NO gas, which exceeds the buoyancy of the suit plus the diver at the surface.
THAT is a dangerous combination. Put TWO of those on a diver and he's a stone in the water.
The issue is NOT the material the tank is made of - it is the EMPTY buoyancy of the tank, and thus what must be compensated for (either negative or positive) by the rest of the kit.
With a drysuit having tanks that are -14 and a -9 STA+BP is kinda nice, as you might easily be +20 with the suit and underwear! Thus, you wind up needing a REASONABLE amount of weight to sink you, whereas otherwise you end up walking around looking like a lead foundary and might even need TWO weight belts to hold it all
