Weighting confusion or way over thinking

[spediver1]

Hello I searched but to no avail found the answer I was looking for so please help. I dive with a ss backplate(6lbs) a weighted sta(6lbs) and a hp100(6lbs) and 12lbs of actual weight I'm at 30lbs. Now my question is if I removed the steel tank I'm at 24lbs so if I put on a aluminum 80 do I just add 6 actual lbs of weight to get me back to 30lbs total or do I add 6lbs to get me to 30lbs and then add 6lbs more to compensate for the aluminum 80 to get me to a total of 36lbs? I know I'm way over thinking this but I can't help it. Please help. Thanks

 

[Storker]

You want the same net buoyancy after the switch as before. Net buoyancy is the difference between the negative buoyancy (i.e the weight) and the positive buoyancy of your rig.

What's the outer, physical volume of your steel tank? What's the outer, physical volume of the Al80? The difference between those is the difference in positive buoyancy for your rig. What's the weight difference between the two tanks? That's the difference in negative buoyancy for your rig. Net change is positive minus negative. Add or subtract lead to make the net change zero

The simplest method with hardly any math: add weight equal to the weight difference between the steel and the Al. Do a weight check. Adjust.

 

[spediver1]

Storker you instantly made my head hurt.lol

 

[Storker]

How much does the steel tank weigh?
How much does the Al tank weigh?
What's the difference? Add that amount of weight to your belt, go diving, do a weight check and adjust accordingly.

 

[KenGordon]

Do no maths. Do a weight check. Get in the water with a nearly empty cylinder and remove weight until you can only just stay down with empty BCD and as tight a suit as you are prepared to go in exceptional circumstances. This is so you can hold a stop if you need one having used pretty much all your gas.

If you are completely sure you are correct with one cylinder then adding the difference in empty buoyancy between the steel and the aluminium *ought* to work.

 

[Patoux01]

How much does the steel tank weigh?
How much does the Al tank weigh?
What's the difference? Add that amount of weight to your belt, go diving, do a weight check and adjust accordingly.

Eh. This post seems to forget about outer volume...

A 12l steel tank (faber, roth) is around 12kg, a 11l s80 (luxfer) is around 14kg. The first one sinks, the latter floats.
So if someone needs let's say 5kg with an aluminium tank and switches to a faber steel, he's not gonna need 7kg, but typically 3-4kg.


The mathematical way of doing it is looking at the manufacturer specs and buoyancy characteristics in the water. The practical way of doing it is simply hopping in the water and performing a check...

 

[Storker]

Eh. This post seems to forget about outer volume...

See post #2.

 

[victorzamora]

Hello I searched but to no avail found the answer I was looking for so please help. I dive with a ss backplate(6lbs) a weighted sta(6lbs) and a hp100(6lbs) and 12lbs of actual weight I'm at 30lbs. Now my question is if I removed the steel tank I'm at 24lbs so if I put on a aluminum 80 do I just add 6 actual lbs of weight to get me back to 30lbs total or do I add 6lbs to get me to 30lbs and then add 6lbs more to compensate for the aluminum 80 to get me to a total of 36lbs? I know I'm way over thinking this but I can't help it. Please help. Thanks

I think you've got it right, actually. A weight check would be best (and I recommend one at the end of the dive with an empty tank)...but math will get you a good starting point.

The only thing I'll "correct" you on is you need to use the EMPTY buoyancy of the tank. That steel HP100 is how negative EMPTY?

Per this chart, Luxfer Al80s are like 4.4# floaty when empty. I'll assume (for the math) that you're diving a PST HP100 at 1.3# negative empty. You'll need to add 5.7# of lead (4.4 - (-1.3)) to get you to the same buoyancy.