Neutrally buoyant tanks
- Thread starter DaveDog
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I'm not sure I understand the question as worded. Negative, positive and neutral buoyancy are not necessarily a function of tank size. Every tank, when empty, is going to be neutral or negative or positive and some of that has to do with the material the tank is made of. Steel tanks tend to be neutral to negative depending on the size, etc. Some aluminum tanks are quite buoyant (lighter metal) while others are not. A tank is heavier when it is full than it is when it is empty and it's that switch from negative buoyancy to positive buoyancy (with a standard AL80) as you use the air that causes you to need more lead to offset the natural buoyancy of that particular tank. Some tanks go from being negatively buoyant to neutrally buoyant as you use the air so you don't need to add extra lead to your weight belt to offset tank buoyancy.
Ber :lilbunny:
Ber :lilbunny:
DaveDog:
The "change in weight" as you breathe down the contents of a tank is not acylinder selection issue, it is a fact of life regardless of cylinder size or material. The phenomennon does enter into how and when you establish correct weighting to end your dive safely. The behavior of the cylinder is universal, who to properly configure the diver to the cylinder is the trick.
Depending on the cyilinder it could be considerably positive to considerably negative at the end of the dive. Remember that this is the "empty Buoyancy". To be precise you would add back the weight of you ending air but that's minor for this discussion.
Depending on the material, design and pressure range the cylinders have different densities, that is weight/displacement ratios. The different densities translate into different buoyancies in the water.
You might want to follow this old most of mine and see how a smaller, lighter cylinder can let you dive with less added weight and a much lighter overall weight.
http://www.scubaboard.com/showpost.php?p=1191538&postcount=16 There is a page referenced in the link that leads to a good specificaton chart for other cylinders.
You really need to understand the terms: Weight, Buoyancy, Density and Displacement Whether they teach it or not a little bit of physics goes along ways in understanding this sport.
Pete
donacheson
Contributor
Ber Rabbit:
This is probably as good a time as any to point out that the weight of a tank is a fairly large number of pounds and that the buoyancy of that tank, by Archimedes Law the weight of water displaced by that tank, is also a fairly large number of pounds. Both are significant forces. What we loosely, and I think incorrectly, refer to as "positive or negative buoyancy" is actually the difference between buoyancy and weight: buoyancy minus weight.
Without searching the board, I'm sure this distinction must have been discussed before, but the careless use of the term "buoyancy" seems to prevail. There isn't a problem with using the expressions "positive", negative", or "neutral"; it's using these words as "positive buoyancy" etc. that obscures the fact that it's the difference between buoyancy and weight we refer to, not buoyancy itself.
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