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Just not enough dive time.
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I have been using a 12 plus pony but am thinking of moving to twin 7's (232) anybody know what that's going to do to my weight requirements. Assuming the 12 is neutral when empty (50bar), what is the bouyancy of a 7 when at 50bar, pos or neg, so am I going to be adding or shedding weight to compensate. The 12 plus pony weighs more than twin 7's so it would be nice if they were negative when empty allowing me to drop even more weight from the belt.

Cheers peeps
Matt
 

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<font color='#000080'>The difference isn't a lot, if at all. Personally, I'd be on the same weight for both. There's a cylinder buoyancy calculator on Dom's site I think, I've not used it, but it might have the answers you need.

If you put them on an S/S plate, you'd move the weight from a belt to you back, which might be preferable, I like it, I dive 300 7s a lot and I like havng it all there without extra weight at all.

Steel 7s will be negative when empty, but how negative you'd have to check. It's also affected by what 7s you get. Fabers are pretty lightweight, other brands vary.
 

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"hardly ever here"
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<font color='#000080'>did the same thing last saturday and after a buoyancy check found that i needed about the same weight. i'd say your normal weightbelt should be fine for the first dive, then you can fine-tune.
 

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Faber 7l cylinder can be anything from 7.6 to 9.1 kilos each.

Faber 12s can be 12.8-15.2 kilos.

According to you-know-who's calculator, the buoyancy figures (in sea water) are:

Full light 7: 1.35 -ve
Empty light 7: 0.6 +ve

Full heavy 7: 2.66 -ve
Empty heavy 7: 0.69 -ve

Full light 12: 2.12 -ve
Empty light 12: 1.125 +ve

Full heavy 12:  4.21 -ve
Empty heavy 12: 0.83 -ve

So if you bear in mind that you're talking twin 7s, you're looking at a light set being about a kilo buoyant, with a heavy set being a little over a kilo negative.

That's compared to 12s being between a bit over a kilo positive and nearly a kilo negative, depending on your weights. So the simple answer is, they're about the same, empty-buoyancy-wise.

On the other hand, 7s are heavier out of water than a single 12. But when you throw in a pony as well, which is a good 3 kilos, you're looking at the same weight out of water, and a more complicated buoyancy issue..
 

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Just not enough dive time.
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Cheers for the info, it looks as though the weight belt stays the same, the cylinders weigh a bit less than 12 & pony and I have the advantage of being able to lug them about individualy if I go indy which makes out of water a bit easier than the 12.

Matt
 

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Just not enough dive time.
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apparently if you click on the www icon on any of his replies it will direct you to the site.
He told me  
 so I'm not being a smart ar5e.

HTH
Matt
 

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These are up for sale on the SSAC site;

TWIN SET
2x 7LTR 300BAR CYLINDERS
MDE MANIFOLD WITH ISOLATION TAP
TANK BANDS BOOTS CYINDERS IN GOOD CONDITION SPOTLESS INSIDE
COMES WITH A 2.5 YEAR TEST
£250.00

Peter
 

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Just not enough dive time.
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Discussion Starter #11
Justin
in the end I went for twin 10's. I now use about 4Kg on the belt in Seawater and nothing really in Fresh. I sometimes add weight in fresh as I find the trim is pants without it, foot light. I use the tanks inverted and a plastic b/p, they are virtually neutral in fresh water with 30bar in them, they are 232's.

HTH
Matt
 

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The Artist formerly known as 'John Duncan'
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Digger said:
Steel 7s will be negative when empty, but how negative you'd have to check. It's also affected by what 7s you get. Fabers are pretty lightweight, other brands vary.
I have ROTH 7.5 litre indies and they are VERY floaty, they weigh 7.56Kg and when 1/2 full they float nicely under and behind my armpits when used as sidemounts.
 

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Weights of cylinders

Dominic said:
Faber 7l cylinder can be anything from 7.6 to 9.1 kilos each.

Faber 12s can be 12.8-15.2 kilos.
Is this difference in weight by design or are they production variations? If it's the latter I'd be shocked.

If it's the former, can you specify at purchase different weights of cylinder?

I ask as I'm toying with the idea of changing from my Faber twin 12's to either twin 7 or 10's.
 

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You can use them as side mounts, when you move to twin 12s
so its money well spent,

buoyancy-wise get in the water and play,
hold 3m with 25bar X 2 =50bar in set , i used to dive with twin 7s,
had a 3l pony in the back as well with a deco gas 100%
good for 40m and thats your lot ,

Steve G
 
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