Adjusting weights when changing the type of tanks.

[Divin'Papaw]

Absolutely!!! Factoring in the specific tank & exposure protection are the most important variables I use to determine my ballast requirements for a given dive. I always strive to be properly weighted … not too negative but certainly not positive. The specific brand matters sometimes. For example, Faber, Worthington & PST HP100s all vary in terms of buoyancy characteristics.

 

[rjack321]

This makes no sense to me. You need to adjust lead based on different tanks, the weight difference associated with the different volumes is relatively small.

minus1 to plus2 is a 3lbs difference in EMPTY buoyancy. Whatever the tanks are when full is irrelevant.

 

[Edward3c]

My weight is set up for a 300bar 12Lt. When I change to a 232bar 12Lt I add 3.5kg. This is also the weight I ware when using a 232bar 10Lt cylinder.

For the 12s, it’s only the tare weight that changes.
For the 10s, it’s both there weight and displacement.

 

[Ducpilot]

Went from AL 80 to 119 Steel and dropped all but 2 lbs trim weight. Obviously the big difference is the weight of the tank itself. Because it is heavier, buoyancy delta from full to empty is much less than the AL Tank.

 

[Jcp2]

It’s not about which one is heavier, it’s about which one is denser.

 

[gmerick]

When I started doing calculations with the forums Buoyancy Calculator, things at first didn't seem to make sense to me, either. Then I tried this practical demonstration. After a weekend of diving, I had empty tanks -- I had breathed the tanks down, I selected a 12l and a AL80 that both had 500psi left. I rigged them both up and took an extra (full) tank with out to the dive training platform at 15 ft depth. I tied the two empty tanks to the platform, so they didn't float away. I was already wearing correct weight for diving with an AL80. The empty AL80 was upside down, and ever so slightly positive buoyant (the valve weight and rigging straps just about balanced the positive buoyancy of the tank. I dumped all the air from my wing, and stood my full tank on the platform. I, too, was neutrally buoyant. I could clip that empty AL80 on with no change in buoyancy.
Next I checked out that 12l steel. It was laying sideways on the platform. The slight current could move the base of the tank in an arc around the point where the shoulder rested on the platform. I swapped empty tanks, and now I was slightly negative. I could compensate by breathing off the top of my lungs, but dropping 3 lbs fixed things up.
I proved to myself that I really did only need to change carried weight by that 3 lbs, even though the big tanks weighed 6 lbs more empty, and contained 2 lbs more air full.

 

[J-Vo]

How I do it,

I do a weight check with all my gear except tank/s and regulator/s. With my drysuit and Halo AR, I need 13lbs in my local lakes.

I use a luggage scale to weigh each type of tank I own with regulator submerged at 500psi.

In fresh water:

Luxfer AL80s, neutral, use all led from weight check.
Faber LP85, -5lbs, remove 5lbs lead per tank carried.
Faber LP108, -4lbs, ....

Note, not all water is made equal. Items have noticably less buoyancy in my pool than in my local lakes. The FL springs are somewhere in between the two. To be super accurate you need to do this in the water you are diving.

 

[Ducpilot]

It’s not about which one is heavier, it’s about which one is denser.

Well there is obviously a material density difference between steel and aluminum. But the steel tank weighs about 12-13 lbs more. So buoyant force is equal to mass of water displaced. If the tanks are roughly equal dimensions the it largely comes down to weight of the tank.

 

[L13]

Well there is obviously a material density difference between steel and aluminum. But the steel tank weighs about 12-13 lbs more. So buoyant force is equal to mass of water displaced. If the tanks are roughly equal dimensions the it largely comes down to weight of the tank.

But they aren't even close to equal dimensions. Changes in Volume (buoyancy) mater as much as changes in weight.

 

[johndiver999]

But they aren't even close to equal dimensions. Changes in Volume (buoyancy) mater as much as changes in weight.

Yes that is why there is much more important factors than simply adjusting for the different tank volume capacities. Some tanks are so dense, I can wear no lead and be fine in a thin suit in saltwater, and this works ok for several different (steel) tank sizes between 85 and 149 cuft. If the current is strong, I will normally wear an extra 6-7 lbs of lead on a belt simply for stability on the bottom, but I don't need any of it for actual buoyancy control.