joethesci wrote:I was speaking to a diver who referred to tank weight as negative 10lbs and belt weight as negative 10lbs. What was she talking about? I did not want to ask a silly question so I figured I could ask here.
The term typically refers to the weight of an item in water. And the question is not all that silly. To really understand its impact can be quiet involved...
When a diver mentions it, it typically refers to how much a scuba tank weighs in the water, when it is empty (or close to empty). Negative 10lbs means it weighs 10 pounds when it is empty underwater.
Due to buoyancy, the weight of an item in water is not the same as the weight of an item in the air.
The weight of salt water can change with temperature. But generally speaking salt water weighs about 64 lbs per cubic foot (trivia, in similar conditions, fresh water is 62.4 pounds per cubic foot, this is important to those of us, whose only dives tend to be in our pools checking out our new gear and dreaming of someday when we will use it...). If you put a 1 cubic foot object in salt water, that (in the air) weighs less than 64 lbs, it will float. If it weighs more than 64lbs, it will sink. If it weighs 65 lbs then it is said to be 1 lb negatively buoyant.
I like to have several hours of air with me when I dive, so I dive with 120 cubic foot steel tanks. They weigh
When the term is used for tanks, it is important to remember that tanks weigh more when they are full of compressed air than they do when they are close to empty. But the tanks remain pretty much the same size when they are empty as when they are full. So they always displace the same amount of water, but their weight changes. So as a tank is emptied, it gets lighter and lighter, but it is still displacing the same amount of water... So a Scuba tank's buoyancy changes as the tank is used.
As you start to dial in your perfect amount of diving weight, one of the factors will be the change in the tank's buoyancy.
Steel Tanks, start off negatively buoyant and most of them stay negatively buoyant. Aluminum tanks start off negatively buoyant but some of them can actually change to positively buoyant as they are emptied (a tank that is positively buoyant, will float).
If a tank becomes positively buoyant at the end of the dive, then you will need more weight on your weight belt to counteract the positive buoyancy of the tank.
On this chart of tanks, the last 2 columns show the tanks buoyancy when it is empty and when it is full.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.htmlSome of these tanks will become 3 or more pounds buoyant. This means you will need to carry an extra 3 pounds on your weight belt. The overall weight of the tank, may make that acceptable.
In the chart, the empty weight plus the difference in the full and empty bouyancies is the weight of the tank when it is full... If you have your weight dialed in right for a specific tank, and you know the values in the last 3 columns, for your tank, and for another tank. You can easily figure out the change in weight required to switch to the other tank, and keep your weight dialed in right.
Tank weight and buoyancy is one of the factors to consider when choosing a tank and when adjusting your diving weight.
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