Difference in lead AL80 vs. Steel 120

I pretty much always dive aluminum 80's and have the lead I add to my BC down to 6lbs.
This is diving in a rash guard, or a 2MM shorty.
Later this summer I will have an opportunity to dive a few boat dives with steel 120's.
Short of a proper weight check, which will not be practical, is there an amount of lead to to take along as a guideline? I will probably be in the 2MM shorty for these dives as well, down to 120ft.
Thanks in advance

You need to find out which steel120 you will be using, since the different 120's have different buoyancy.

The key number is the empty buoyancy. You need to remove lead equal to the difference in the empty buoyancy of the two tanks (assuming that you use the 2mm shorty in both cases).

The most common AL80 tank has about +4 pounds buoyancy when empty.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan is one of many tables you can find on the internet that list buoyancy specs for various tanks. If the "steel 120" is a PST E7 120 with 0 pounds buoyancy when empty, then you would remove 4 pounds from your belt. If the tank is a 3500psi PST120, which has -1.3 pounds buoyancy when empty, then you would remove about 5 pounds.

Charlie

p.s. If the boat is supplying the steel 120, then they have probably been asked this question dozens of times and can tell you how much lead to remove. I've even run into cases where the boat knew the difference in buoyancy between their tanks and an AL80, but couldn't tell me the make and model of the tank they supplied since they weren't at the boat when I called.

From personal experience the al 80 with 6 # lead is the same to dive as a HP120.

Thanks for the info guys. That gives me a ballpark. I'll ask the captain.

An al80 is +4lb empty a worthington 120 is -2lb empty. that is a 6lb difference, you should be good with no weight if the tank is a Worthington.

I don't think you'll need any lead as long as it's one of the HP (3442) 120s. I think the LP 120s are a bit more buoyant. At the beginning of the dive you'll have no problem sinking, that's for sure. It's a lot more gas, which means more swing weight.

That also means you'll be at least 11 lbs negative with a full tank at the start of the dive before allowing for wet suit compression (minor with a 2mm), and maybe a little extra weight for the safety stop. Since you probably won't have any weights to ditch make sure you are able to swim from depth to the surface with that rig if your BC fails. I can do it with my hard rubber paddle style Turtle fins but not with my softer blade SP Twin Jets.

I'm diving Jet Fins. Should do the trick. Thanks for the info.

I just got a Worthington steel 100 tryed it last saturday. I was using an al 80, I took off 4 lbs. to use the steel 100 I need to loose 2 more lbs. The steel tanks are awsome ,I never want to use al 80 again. Just need to save 360.00 to buy another one.