Cylinder type and proper weighting (SPLIT FROM 'Pull Dumps - Lose them')

[BRT]

Yes. Thank you. I was going to say the same thing without the details. Regardless of type of tank, you need the same amount of total (body and kit) weight to dive properly (be "properly weighted). With a steel tank there is simply more of that weight in the tank as opposed to lead being worn.
It has been pointed out that some switch to larger steel tanks than the standard AL80 to get more gas. And that those tanks are quite heavy for long walks (I once owned a steel 120). I also owned a steel 72, which of course meant carrying less lead than with my AL80s, but it also had less air.

I don't think this is true. An Al80 and an HP100 have similar weights but I can take off 6 lb of lead with the HP80 so my total weight is less, because the buoyancy of the HP100 is less.

 

[TMHeimer]

I don't know what to say. When you do a weight check your total body weight (body plus kit) has to be the same regardless of what tank and amount of weight you use (otherwise you are not doing a proper weight check). I think this is true when comparing any two objects for buoyancy. For example, a big rock will sink faster than a little one of the same shape--unless you have enough little ones to match the weight (and shape of course) of the big one. Then they will sink at the same speed. Your body buoyancy is the same whether you use an AL or a steel tank. Assuming all your other gear is exactly the same in both situations, you will have to add those 5-7 pounds with the AL tank. Then your total weight and buoyancy is the same. I don't think it's an exact analogy because of size, but you know the old question "which weighs more, a ton of bricks or a ton of feathers?" I think my physics is correct, but perhaps someone else can elaborate.

 

[BRT]

I don't know what to say. When you do a weight check your total body weight (body plus kit) has to be the same regardless of what tank and amount of weight you use. Otherwise you are not doing a proper weight check. I think my physics is correct, but perhaps someone else can elaborate.

No your body plus kit does not have to remain the same. Your total buoyancy in the water (different between fresh and salt) including the air in your lungs and the BCD has to be zero to just float in the same position. Changing to a tank with less buoyancy, a suit with less buoyancy, a BCD with less buoyancy allows you to dive with less total weight.

 

[TMHeimer]

No your body plus kit does not have to remain the same. Your total buoyancy in the water (different between fresh and salt) including the air in your lungs and the BCD has to be zero to just float in the same position. Changing to a tank with less buoyancy, a suit with less buoyancy, a BCD with less buoyancy allows you to dive with less total weight.

Agree. The steel tank is less buoyant than the AL, so you can use less lead. But you're saying your AL and steel tanks are similar in weight (you didn't say exactly the same). They are also similar in size. That would mean you would have to use about the same amount of lead. I could be missing something here regarding the properties of AL vs. steel. But I don't think so if they are similar in size. An AL tank is less buoyant even though the metal is thicker than that of a steel tank. But again, you said they weigh the same.
Or as boulderjohn puts it regarding total package weight "It all comes out the same".

 

[BRT]

Agree. The steel tank is less buoyant than the AL, so you can use less lead. But you're saying your AL and steel tanks are similar in weight (you didn't say exactly the same). They are also similar in size. That would mean you would have to use about the same amount of lead. I could be missing something here regarding the properties of AL vs. steel. But I don't think so if they are similar in size. An AL tank is less buoyant even though the metal is thicker than that of a steel tank. But again, you said they weigh the same.
Or as boulderjohn puts it regarding total package weight "It all comes out the same".

They are similar, but smaller. I used the example of HP100's because I have a bunch of them, along with 120's and 130's. An HP80 has similar buoyancy, similar capacity and less weight than an AL80. You can remove the same 5-6 lb of lead in addition to less tank weight. You do not have to dive the same total weight of body and equipment.

You said "When you do a weight check your total body weight (body plus kit) has to be the same regardless of what tank and amount of weight you use"

The simple difference between total weight in the tropics and total weight in Canada shows this is not true.

 

[woodcarver]

 

[TMHeimer]

They are similar, but smaller. I used the example of HP100's because I have a bunch of them, along with 120's and 130's. An HP80 has similar buoyancy, similar capacity and less weight than an AL80. You can remove the same 5-6 lb of lead in addition to less tank weight. You do not have to dive the same total weight of body and equipment.

You said "When you do a weight check your total body weight (body plus kit) has to be the same regardless of what tank and amount of weight you use"

The simple difference between total weight in the tropics and total weight in Canada shows this is not true.

Well, I admit I'm not an expert on the various weights and buoyancies of the tanks you mention.
Don't know what tropics vs. Canada has to do with it. The same comparisons can be made regardless of where you dive, assuming you're making those comparisons in the same place. You obviously were less lead in the tropics because of different wetsuit (and slightly saltier water).
I think we need input from others.

 

[BRT]

Well, I admit I'm not an expert on the various weights and buoyancies of the tanks you mention.
Don't know what tropics vs. Canada has to do with it. The same comparisons can be made regardless of where you dive, assuming you're making those comparisons in the same place. You obviously were less lead in the tropics because of different wetsuit (and slightly saltier water).
I think we need input from others.

My only exception was with your statement "When you do a weight check your total body weight (body plus kit) has to be the same regardless of what tank and amount of weight you use" and I think you agree that is not true? Changing the tank can change the total weight of your body plus kit to be neutrally buoyant?

Saltier water will cause you to need more lead, not less. Google diving the dead sea.

 

[TMHeimer]

My only exception was with your statement "When you do a weight check your total body weight (body plus kit) has to be the same regardless of what tank and amount of weight you use" and I think you agree that is not true? Changing the tank can change the total weight of your body plus kit to be neutrally buoyant?

Saltier water will cause you to need more lead, not less. Google diving the dead sea.

Yes, I didn't describe that right. The saltier water, the more weight you need. But even though the closer to the equator you go the water is generally saltier, the use of a body suit instead of a thick wetsuit or drysuit causes you to be far less buoyant than in the Arctic, so you need far less lead. You float like a cork in the Great Salt Lake while swimming.
Yes, changing the tank can change your total body & kit weight, so if you dive an AL vs. a steel tank means you are more buoyant and and need more lead. You're balancing out the "lighter" AL tank by adding 5-7 pounds. So your total body & kit (including weights for whichever tank type) must come out the same or your buoyancy will be off.
Again, using the Dead Sea as an example means nothing. You have to have the appropriate total weight to be able to "sink" in the Dead Sea. You're talking about changing total body & kit/tank/lead according to where you dive (salinity of water, etc.) This has nothing to do with the question we discuss.
Let's continue this tomorrow when more may chime in.

 

[Magnus Lundstedt]

When i dive in warm water, i use aluminum tanks.

I pack about 2-2,5 kg of lead.

If i change one AL80 to an Steel 12l i need to drop about 4,5kg of weight.

Where can i get the extra 2-2,5 kg i need to drop??