steel-better buoyancy???

[docmartin]

i am really confused. i keep reading about the better buoancy characteristics of steel because the tank stays negative even when empty. why does this matter? the weight of air in an 80cf steel vs alu is exactly the same i.e. ~6lb. therefore, the change in buoyancy between the beginning and the end of the dive is the same for both types of tanks. i will have to add the exact same amount of swing weight to stay neutral at my 15ft stop at the end of the dive. why would the buoyancy characteristics of steel be any better then?
the total weight i carry is based on the whole package i.e. diver plus equipment incl. tank. a heavier steel tank may help me to carry less weight on my belt but not less total weight. so this is merely an issue of weight distribution and has nothing to do with buoyancy.
can someone set me straight and explain what i am missing?

 

[crispos]

The initial buoyancy of the steel is going to be more negative than the equivalent aluminum, even though the change from air depletion is the same. So -6 lbs for a full steel 80 versus say -2 lbs for a full Al 80 means 4 pounds off the belt for the whole dive. Exact numbers depend not only on metal and size but also tank model, fresh versus salt water, overfill, valve system, etc.

The second issue is where those weights/buoyancy are positioned on your body. With the Luxfer Al 80 with K valve in salt water and 500 psi it is 2.7 lbs positive. This means you have the tank butt end wanting to rise up. Hence the familiar "eagle swooping down" position of novice divers at the end of a dive on the line. The steels are generally neutral in salt water and negative in fresh, if they are not totally breathed empty.

 

[jonnythan]

Easy.

Having a tank that weighs 6 pounds more in the water means that's 6 pounds you can take off your belt when going from an Al 80 to a steel 95.

 

[Walter]

Doc,

You are mostly correct. What you are missing is buoyancy is related to density, not weight. One 5 lb object may float while another 5 lb object may sink.

Steel is stronger than aluminum, because of this the walls in a steel tank are thinner than the walls in an aluminum tank. In general, a steel tank will be lighter than an aluminum tank of the same volume.

By using a more negatively buoyant tank, you've eliminated weight from your belt. By using a lighter tank you've eliminated total weight. By using a lighter, more negatively buoyant tank; your entire rig weighs less and you have less weight on your belt.

 

[crispos]

Rereading your post, to reiterate why weight in the tank may be preferable to weight on the belt. If you have a lower back problem like I do, it is much more comfortable to have less weight on the belt. In fact, I prefer to have softy weights in my integrated BCD, steel tank, and little or no belt weight. Contrary to this is the position that if you had to take your tank off underwater, you would float up if you didn't have any belt weights. And if you had to ditch, you can ditch your belt but not your tank. I am sure others have opinions on this weight position issue.

 

[Dyno Bill]

At point I had 4 alum 80's and 3 steel 72's and a steel 95. I ended up giving the aluminum tanks away because I quit using them, cant stand having to carry all the extra led.
Bill