Steel VS Aluminum Backplate
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Perhaps I'm misunderstanding something. Help me out here.RTodd:
Say I'm in an Al tank that's say 2 pounds negative at the start of the dive, wearing a 6 lb weight belt. When the tank is empty it is 4 pounds positive, I'm neutrally buoyant at the surface if properly weighted. Therefore, at the beginning of the dive I have to be 6 pounds overweight.. due to the weight of the air in the tank.
How does this change with a steel tank? You're merely moving the weight from the lead on the weight belt into the tank. If I were diving an 80cf steel tank that started out -6 and went to 0, I would need 2 pounds on the belt, but still be exactly 6 lbs overweighted at the start because of the air in the tank.
How is it possible that if you're properly weighted, the *material* of the tank makes you more or less overweighted at the start of the dive?
Charlie99
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I'm not following your logic. Air is air, and weighs the same whether it is in aluminum or steel. If the steel tank is 3 pounds negative at the end of the dive, and you need to add one more pound of lead to be neutral, then you have the correct weighting. Similarly, if you need 6 pounds of lead to be neutral at the end of the dive with an AL80,then you are correctly weighted with 6 pounds of lead. In BOTH cases, at the beginning of the dive you will be "overweighted" by the weight of the air in the tank --- roughly 6 pounds whether its an AL80 or a steel 80. I don't see have you come up with different "overweighted" numbers in the example you gave.RTodd:
Now if you are saying that it's not DIR to dive overweighted, yes, of course I agree. If the negative buoyancy of the steel is such that you are overweighted at the end of the dive, then clearly it's not a good way to dive.
If you are saying that it's not good to have all of your negative buoyancy as non-ditchable, then someone else will have to answer whether or not that is DIR in all envirionments.
Spectre
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jonnythan:
Point #1: In a wetsuit, your #6 negative on the surface becomes more when you get to the bottom.
Point #2: You can ditch your weight belt. You ain't gonna ditch your tank.
Charlie99
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Nor are you going to ditch a 6# backplate, nor a V weight.Spectre:
Spectre
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Didja follow the link I posted earlier?Charlie99:
#1: That's absolutely correct, and has absolutely nothing to do with the question I asked.Spectre:
#2: See #1.
Are you implying that RTodd was talking about buoyancy after the weight belt has been ditched?
Charlie99
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Yep. It's an excellent post on why heavy steel backplates aren't a good idea with wetsuits.Spectre:
Seriously, though, it makes more sense to me to talk about 1) being properly weighted, and 2) having the appropriate amount of ditchable weight.
Many seem to be very concerned about "steel tanks and wetsuits", but feel that diving with zero ditchable weight is fine.
Spectre:
Evidently, I am not very good at explaining the obvious stuff and need Spectre to fill in the blanks for me. Thanks.
Note, I used simplified math for the tank weights and change of air but still screwed up the math. Yes, assuming you started out in the same size tanks you would start out the same amount overweighted. The point is, AL tanks give you more ditcable weight and many steel tanks are so heavy they prevent you from carrying a useful amount of ditchable weight. Also (my intended point that missted badly) steel tanks typically carry more gas so you do start out more overweighted by the volume of this additional gas.
lamont
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the problem with steel tanks and wetsuits are that they are inherantly unditchable weight.
to simulate failure of wing along with wetsuit compression at depth, you could take all your gear to a pool, jump in without a wetsuit on and without your ditchable weight and deflate your wing completely. you should be negative by the weight of your tanks, BP/W, can light, etc. if you've got a steel tank (or particularly two) or a steel backplate you will be much more negative than with Al.
LP104s are going to be particularly bad for this excersize because they swing from -15# to -5# or so, and an LP104 and a steel backplate in a wetsuit is probably a bad idea.
to simulate failure of wing along with wetsuit compression at depth, you could take all your gear to a pool, jump in without a wetsuit on and without your ditchable weight and deflate your wing completely. you should be negative by the weight of your tanks, BP/W, can light, etc. if you've got a steel tank (or particularly two) or a steel backplate you will be much more negative than with Al.
LP104s are going to be particularly bad for this excersize because they swing from -15# to -5# or so, and an LP104 and a steel backplate in a wetsuit is probably a bad idea.
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