Weight changes to twin tanks

[Denis Sydney]

I am setting up a twin 12.2l (100cuf) steel tanks on my bcd. I usually dive on a single of these.

I am planning to do a test dive and have picked some of the issues about this but one thing I have not worked out is the change in weight i should carry.

Can someone help me with this please

Thanks
Denis

 

[Sas]

Do a weight check. This is done by having your tanks near empty, emptying your wing (and not having any air in your drysuit if applicable), and add weight until you get to a point where your eyes are at the water level and if you breath in you rise above eye level and if you breathe out you drop slightly below the surface.

If you have to do this check at the start of a dive with full tanks, you can just do the same but add weight according to how much air in your tanks (I think about 2-3kg for that tank size but I could be wrong).

 

[Denis Sydney]

Thanks for that. I understand that as the fine check but i am more concerned about a rough how much to drop off by adding the extra tanks.

Thanks

 

[Sas]

Maybe start with dropping about 5-6kg for the extra tank. That is what I was advised to do when switching to twin 12s. Left me pretty close.

 

[JBRES1]

I would think that taking 2 or 3 kgs would be my start point for the doubles.
The added cylinder will be about neutral when empty and -3kgs when full.
When diving doubles I also place the weight up front to help off set the added weight on my back.
It is much easier to dive a little heavy rather than light .

Jim breslin

 

[Sas]

The added cylinder will be about neutral when empty and -3kgs when full.
When diving doubles I also place the weight up front to help off set the added weight on

I thought Faber 12s were -7lb empty, -14lb full? Could be wrong though as there are lots of different types and the ones in Australia seem different to what is overseas.

Edit: Ok I found the standard 12L ones and you're pretty much right: ~1kg negative when empty, 4kg negative when full. So about dropping 3kg seems good as a starting point but there are other weights such as bands + manifold as well which I think mean a bit extra than that is needed so I'd probably go 5kg less to start with if those are applicable.

 

[Blackwood]

I add six pounds of lead (+ reg + manifold) when switching from double to
single HP100s.

 

[Wreckdiver George]

Hay Denis,

I can't say pacifically what your weight change is going to be with out some more info. (The weight you are using now, and the types of tanks you have been using.) Here was my experience. I went from using a 80cf alum w/30cf pony bottle. Using about 28lbs going to twin steel 72s I only need 9lbs. I started out with a weight harness with about 25lbs in it and simply went out in the shallows and just dropped 2.5 lbs at a time till I found the right weight. After some time I would simply adjust it and after a few dives I found the right weight and trim.

Hope this helps, George

 

[Colliam7]

I am setting up a twin 12.2l (100cuf) steel tanks on my bcd. I usually dive on a single of these. I am planning to do a test dive and have picked some of the issues about this but one thing I have not worked out is the change in weight i should carry.

Denis,

Assuming a) everything else stays the same (BP, wing, exposure suit, extra gear) after you double them up, b) you are referring to manifolded, backmounted doubles, and c) the buoyancy characteristics of a PST HP100 can be used just for an initial calculation:

1. A 3500psi PST 100 is -1.3 lbs when empty and -8.8 pounds when full, so it would be ~ -2.4 lbs at 500 psi
2. Add the estimated weight of the bands and manifold, ~4 lbs. Divers Supply lists the shipping weight of a set of Highland Bands as 4 lbs. Don't have a value for the manifold. But, assuming it is 2 more pounds, then taking a lower (conservative) figure to account for the difference between absilute weight and buoyancy, I would say 6 lbs of weight, but use 4 lbs for the negative buoyancy
3. Add the weight of the second regulator, ~2 lbs

So a rough starting point would be to assume you will add ~ 8.4 lbs of negative bouyancy when you double them up, and can therefore drop 8 lbs of lead as a beginning step. Could be more, this was just a starting point.

 

[vinegarbiscuit]

My $0.02: in a 3mm suit with a single AL80, I wear 7lb of weight (5lb SS plate and 1 lb apiece for my STAs - no weightbelt required). When I made the switch to 100 cf steel manifolded doubles, I didn't need to add any weight whatsoever, even though I needed a thicker exposure suit (my training was in the Florida caves). Those suckers sure can sink!