Difference in buoyancy between one steel tank and two.

[ScubaSense]

Heya,

I'm a bit fuzzy on the difference in buoyancy between one steel tank and two.

What confuses me is that it's been mentioned to me that you actually need more weight with twins to account for water displacement but as I'm diving steels I don't really see how that works. I also can't imagine climbing in and out with anything extra.

Come to think of it, this was also mentioned when weighting a child with a single steel 10ltr....

Can anyone explain this to me please or post a link to the elusive previous threads? How does one compensate for the difference in buoyancy between one steel tank and two?

 

[aquaregia]

If one tank is +4, two are +8. If one is -2, two are -4. If one regulator is -2, two regulators are -4. If one tank has 8lbs of gas, two have 16lbs of gas. Etc.

End result is I agree with you, even before you look into the bands and manifold weights. You'd start dramatically more negative, so I wouldn't trust the "eye level with empty bc" trick to determine weighting.

 

[clifftondiver]

Are we to assume you will be getting into technical diving? Not that it matters ( I dove double 119's) for 1.5 years recreationally. I just liked knowing I had MORE than enough gas for any dives I did and I knew eventually I'd get into technical diving, which I am now learning. Anyway. When you're weighting yourself, you want to be weighted so you can hold a deco hang with nearly empty deco bottles and nearly empty tanks at 20'

If you will be doing recreational diving with doubles, you still want to be neutrally bouyant with nearly empty tanks at 20'. One thing I did notice, though. I could dive without a weight belt for my first dive (using double 119's). But since they each hold about 10 pounds of gas, I needed a 10 pound weight belt for the 2nd dive. I once found myself up-side down at the end of a dive, holding onto a bit of kelp at 20' for 3 minutes, not fun.

Aqua, you're right, you will be heavy, very heavy. My rig weighs about 115 pounds on my back. When getting into technical diving, if you think like a recreational diver, you'll kill yourself. The best thing to do is to find a mentor. Find a technical instructor and take some classes. Or if you're not ready for classes, take that instructor our for beers and pick their brains. Also, do NOT buy anything with consulting him/her first. Getting into technical diving gear is quite expensive.

 

[TSandM]

The net buoyancy of a tank is a balance between its volume (displacement) and its density. If a tank is -1 when it is empty, two of them will be -2. Yes, you have doubled the displacement, but you have also doubled the dense mass. And THEN you add the weight of the bands, and the second regulator . . . in general, people can take weight OFF their rigs when they switch to doubles. This is even true with Al80s, which are probably the most problematic tanks, because they are positive when empty. When you add a second Al80, you have added 4 lbs of positive force at the end of the dive. But with the bands and the second regulator, you bring the whole rig down to about neutral when empty, which is still less weight than the +4 of the single Al80 would require.

However, as aquaregia very properly points out, you are very negative at the beginning of the dive (massively so with big doubles) because of all the gas you are carrying. "Eye level with empty wing" works . . . but only if you do it with tanks as empty as you ever intend to get them. A lot of people (including, to my continuing amazement, cave divers) don't weight themselves for empty tanks. All that means, to me, is that, should you ever end up in a situation where you have to use the spare gas you brought, you're going to compound the problem with difficulty maintaining proper buoyancy as the tanks empty -- not good!

 

[evil_xander]

How about this, play with the calculator on this url and draw conclusions for yourself.
www.subaqua.co.uk Calculations Cylinder Buoyancy

 

[ScubaSense]

Thanks guys,

I spose the concept quoted to me must have been in very vague reference to the increased buoyancy at the end of the dive.

I'm part way through TDI adv nit/deco I got sick though and couldnt complete the dives, must wait till that nagging plastic card in my pocket gives the go ahead again. For that I dived twin 120's (about 100lbs I'd say, not sure of the make) and certainly did remove quite a bit of lead. At least 10 lbs. Difficult to compare though because I also made the switch from neoprene to trilam in the same period.

Thanks evil_xander for that site, have certainly found some good tools on here!

SS