Alum vs. Steel Weighting difference

[Black Dog]

I know this has been discussed before but searched and can't seem to find it.

I always though that an Alum. tank was heavier than a steel at the start and lighter then a steel at the end of the dive, but my experience last weekend were just the opposite. I usually dive with 8 lb with a steel 80 and had to take on 15 lbs with an Al 80 just to get me down. What is the rule of thumb here?

 

[Charlie99]

80 cubic feet of air weighs the same whether in a steel tank or an aluminum tank -- about 6 pounds. So both the steel tank and the aluminum tank will get lighter by about 6 pounds at the end of the dive.

The key spec to look at is "buoyancy when empty". For the typical AL80 is is about +4pounds -- positively buoyant. Most steel tanks are negatively buoyant when empty. Compare the specs on a site like http://www.huronscuba.com/equipment/scubaCylinderSpecification.html .

The difference in lead when changing tanks will be the difference between the numbers in the "empty buoyancy" column for the respective tanks. Since the AL80 is +4 and the various types of steel 80's run from -1 to -7, you should expect to add between 5 and 11 pounds, depending upon which particular type of steel 80 you were using.

 

[Black Dog]

Thanks C99, Great explanation.