Adjusting weights when changing the type of tanks.

[Ducpilot]

I give up….

 

[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.

An aluminium Luxfer AL80 weights 31.38 lbs, a steel Faber HP80 weighs only 28.3 lbs, and actually holds 2.4 cft more air. Meaning the steel tank is actually 3 lbs lighter out of the water. However, since it is more dense, the empty steel is 1.74 lbs negative buoyant while the empty AL80 is 4.1 lbs positive buoyant. So the steel tank is 5.84 lb more negatively buoyant in water.

I have no idea where you got the idea that steel tanks weigh 12-13 lbs more. Were you comparing a steel 120 to an AL80?

 

[Edward3c]

An aluminium Luxfer AL80 weights 31.38 lbs, a steel Faber HP80 weighs only 28.3 lbs, and actually holds 2.4 cft more air. Meaning the steel tank is actually 3 lbs lighter out of the water. However, since it is more dense, the empty steel is 1.74 lbs negative buoyant while the empty AL80 is 4.1 lbs positive buoyant. So the steel tank is 5.84 lb more negatively buoyant in water.

I have no idea where you got the idea that steel tanks weigh 12-13 lbs more. Were you comparing a steel 120 to an AL80?

You’ve made the classic mistake of comparing the full capacity of cylinders. It’s the mass and water displacement that determines whether a cylinder is buoyant or not when empty.

I’ve used 12Lt cylinders, all at 232bar, with tare weights ranging from 8kg to 17kg.

 

[L13]

You’ve made the classic mistake of comparing the full capacity of cylinders. It’s the mass and water displacement that determines whether a cylinder is buoyant or not when empty.

I’ve used 12Lt cylinders, all at 232bar, with tare weights ranging from 8kg to 17kg.

You have make the classic mistake of not looking at the actual numbers, using water displacement rather than capacity, you are even more wrong:

A 12Lt steel is 32.5 lbs empty, a 12.2Lt aluminum is 41.1 lbs empty. So the steel is 8.6 lbs lighter(again)! However, due to buoyancy difference, that same steel is -1.0 lbs empty, while the aluminium is +1.9 lbs buoyant, making the steel only 2.9 lbs different, again no where near the 12 - 13 lbs you mentioned. If we compare at the same water displacement, the steel is always lighter than the aluminium.

I would love to have some of your 8kg empty 12Lt 232 bar tanks! Where can I get such an awesome thing? Do some 12Lt in Alexandria actually weight twice as much as others?

 

[jborg]

I would love to have some of your 8kg empty 12Lt 232 bar tanks! Where can I get such an awesome thing?

I don’t think that exists, unfortunately (which is obviously your point). Maybe some carbon fibre monstrosity? However, there are 12x200 tanks at 12.4 kg, which ought to be about neutral at the end of the dive. Sidemount specials.

 

[RIHappyDiver]

Thanks for all of the input. I adjusted based on empty buoyancy, which @Jcp2 had me right in the ballpark, and it worked wonderfully! The dive, on the other hand, was not so wonderful. Vis was about 3-4 ft, which would be good for my solo dives, but not with my newly certified wife. Amazingly, she was not clinging to my arm, lol. So we called it early. I would have liked to gotten the tanks down to 500 psi, just to dial it in even better, but next time.

Thanks,

Erik

 

[Mystical Dragon]

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.

It actually makes complete sense. Weight of a cylinder on land is irrelevant as the only thing that matters is the buoyancy characteristics when empty. This is why weight checks when empty, aka ideally 500psi, are so important. The LAST thing you want is to weight yourself when a cylinder is full, then find yourself to be positive buoyant at your safety stop and unable to stop the ascent.

I ran LP85's on my ocean dive but need more gas. I'm renting HP120's. I was pleasantly surprised that the only thing I need to change is dropping about 2lbs since I'm going from +2.32lbs empty buoyancy to +.65lbs empty buoyancy with the HP120's. Yes I will be more negative buoyant at the start of the dive due to the extra weight of the increased gas. However I will breathe that off and will need to adjust my bcd during the dive accordingly.