Weight and doubles

[McHugh]

Hi guys, I recently bought my first set of doubles (2x12l). I have been diving on a single tank 12l and carried 6 kg of lead for my drysuit. By how many kg can i reduce the weight approximately when diving doubles? I mean for sure I will do proper buoyancy checks however as a starting point I would really appreciate your experiences (with a 7mm wet suit I only use 2kg in saltwater)

 

[Boarderguy]

Stick with the 6kg and reduce a couple kg per dive. I dove 28 lbs with a single steel 100 and drysuit because I keep a bit extra air for warmth. With double 100s I dive 16 lbs lead. You may find 3-4kg is good but you'll need to adjust where you wear it to balance out. 2 1st stages and the manifold will put more weight at your head.

 

[elmo]

Assuming you’re using the same backplate and drysuit undergarments you probably won’t need to carry any lead to achieve the same buoyancy. But, and this is a big “but”, you will probably be head-heavy in twin 12s.
Once in the water, get horizontal and stop moving. Really stop moving, no little mini-kicks, wriggles or hand flaps. See which way you tip. If you are head-heavy a tail weight (hung below the lower bolt) will help. So will heavier fins. If you end up being a bit negative to achieve trim. I use a 1.5 kg tail weight and jet fins with twin 12s and a steel backplate in salt water.

 

[McHugh]

Thanks in advance, to clarify I already use 2 first stages in cold water, so that is not that much of an addition, however the manifold comes extra.

 

[grantmac]

This will depend a lot on the specific cylinders.

 

[DiveLikeAMuppet]

As a ballpark, if you are diving Faber or Eurocylinders (*, depending on your undersuit, drysuit and your build, you might need anywhere between nothing and 10 kg. Hope that was helpful.

OK now for real: I used to carry about the same amount of lead (6-7kg) as you carry in a steel single 12l with a "medium" undersuit - think Arctics.

With the thickest winter undersuit and an extra vest in a trilam drysuit and my current amount of blubber bioprene , I need about 4 kg to sink with a steel backplate and a bunch of crap attached to me torches etc.) in sea water. 3 kg in fresh water with ali backplate. With less warm but still pretty thick undersuit I can get away with 2 kg in salt and no weight in fresh water. In warm fresh water, I need 1 kg in 2xali80s with ali backplate and a fleece undersuit.

So my (un)educated guess would be that you could get away with half the weight you are using now.

(* The difference is a joke. In my opinion, Eurocylinders are slightly more positive, otherwise there is no difference. People claim that Faber/Eurocylinder/flat bottom/convex/whatever impacts their trim and buoyancy, it's a skill issue with 0.5 kg difference in buoyancy ...

 

[MarkA]

This will depend a lot on the specific cylinders.

As an example that is not common. The other things being equal. I added 4kg when going from a single 15l to 2x10l. All because the buoyancy characteristics of the cylinders were different. The 10l cylinders are fabers and without valves etc are +0.6kg buoyant.

 

[jborg]

An ECS 12 L 232 bar with 40 bars of gas in it is about -1.5 kg boyancy wise. All other things being equal, adding another one means you can drop 1.5 kg from your belt; with some bolts and the manifold etc call it 2 kg.

You will of course be a lot more negative at the start of the dive because you’ll be carrying an additional 3 kg of gas as well, but that doesn’t matter for your lead calculation.

 

[Subcooled]

Hi guys, I recently bought my first set of doubles (2x12l). I have been diving on a single tank 12l and carried 6 kg of lead for my drysuit. By how many kg can i reduce the weight approximately when diving doubles? I mean for sure I will do proper buoyancy checks however as a starting point I would really appreciate your experiences (with a 7mm wet suit I only use 2kg in saltwater)

I assume you have 12L 232bar steel Eurocylinders.

Negativity in water when empty (1 bar):
12L: -1,0 kg
2x12L: -4,2 kg

The latter inludes a steel tank band to keep the cylinders together, another cylinder valve and the manifold connecting the two cylinders. Hence, you might only need 3 kg of lead, but I'd start with 4kg. As a sidemount drysuit diver with 2x12/232 steels need 10 lbs = 4,5 kg of weight in sweet water, but as a backmount doubles diver you'll have some more steel to carry.

So, you should start with 4kg or 3kg and reduce weight as needed. Going with 3kg at first will not be a problem unless you go ice diving. If 3kg is too little, you'll float to the surface prematurely at the end of the dive, but in open water diving that's not a huge problem. And please take heed of the double set trim advice given in another post. I only dove once with BM doubles but was head down with leg cramps because of trim issues.

 

[Subcooled]

But, and this is a big “but”, you will probably be head-heavy in twin 12s.

Just like I was on my first and last BM doubles dive