double tank weighting issue?

[cool_hardware52]

a bad situation to be in is when you are positively buoyant with bingo air, in an air sharing situation, and have some deco obligation. how are you going to stay down?

I have actually been struggling with this weighting issue for about 9 months now. with steel doubles, al backplate and 7mm suit (summer wear) and the same doubles, steel backplate, drysuit (winter wear). I have come up with a design for a modular set of lead weights that sit behind my plate and let me adjust weight in 5 lb increments to allow for a variety of situations, trim requirements, and which are firmly mounted and easy to adjust. I have the plaster of paris male moulds hanging in the garage right now, will be making my female moulds this week and hope to cast my prototypes as soon as this weekend.

A simple, inexpensive, and easily adjustable means to add ballast to a doubles rig is to place soft weight pouches between the wing and the backplate.

The upper and lower band bolts will prevent the pouches from falling out the top or bottom, the wing keeps the weights from passing through between the tanks and the back plate will clamp them in place.

Depending on the tanks used (7.00", 7.25" or 8.00" dia) and the exact back plate used, it is possible to fit 4-8 lbs in this space.

No hot lead required.

Tobin

 

[Flightlead]

A simple, inexpensive, and easily adjustable means to add ballast to a doubles rig is to place soft weight pouches between the wing and the backplate.

The upper and lower band bolts will prevent the pouches from falling out the top or bottom, the wing keeps the weights from passing through between the tanks and the back plate will clamp them in place.

Depending on the tanks used (7.00", 7.25" or 8.00" dia) and the exact back plate used, it is possible to fit 4-8 lbs in this space.

No hot lead required.

Tobin

yeah, I tried that. I didn't like the fact that I couldn't really get my plate and wing quite positioned right with the weight in there, plus I couldn't get the 10lbs I need in there, and I'll need 5lbs more when I'm carrying two empty stages.

 

[Meng_Tze]

yeah, I tried that. I didn't like the fact that I couldn't really get my plate and wing quite positioned right with the weight in there, plus I couldn't get the 10lbs I need in there, and I'll need 5lbs more when I'm carrying two empty stages.

weight the stages, not yourself. If you have to leave them, you are still properly weighted rather than too heavy.

 

[Flightlead]

weight the stages, not yourself. If you have to leave them, you are still properly weighted rather than too heavy.

you've done this before haven't you?

 

[silverfish88]

Pool practice is ideal. You can get out add weight and get back in and and see the results. Just remember that if you are going to then go dive in salt water, things will be different. You will be a tad more positive or lighter. (Take my word for it. I did this. I had to swim around with a 5lbs rock for an entire dive. Funny pictures, hard to explain to a non-diver) Seawater is more dense and since your boyant force is equal to the weight of the water you displace you are pushed up more. So if you displace heavier water, you will need to add a little more lead. Prepare for this when you set up. Hope that helps.

 

[Flightlead]

...you may not realize if you are underweighted until you get in a situation where you are down to 300-500 psi and disvoer you are no longer neutral but maybe 5 pounds positive. That can be a real problem when you are in deco and do not have an anchor line available to help hold you down.

Ideally you should be neutral at 10-15 feet with no gas in the wing and 300-500 psi in the tanks. ....

just a thought but...

why 300-500 psi? I personally shoot for neutral at 10-15' with 0 psi for exactly the problem you identify. if I have 500 psi in my steel doubles I have over 34 cu ft of air, at a SAC of .5 that's 46 minutes of gas. That gas weighs just under 3 lbs, which I can easily compensate for by adding air to wing, drysuit or heck, taking deep breaths if they are both holed, which I complete my deco obligation. however if I am neutral at 500 psi I'm positively buoyant at 400 psi and almost 3 lbs underweighted at 0 psi. If I'm air sharing without an anchor line while completing deco I have to swim downwards the entire time - difficult, increases gas consumption, increases deco requirements.

Am I missing something?

 

[Thalassamania]

A few thoughts:

1. If I take my tank off I want to be as close to neutral as possible, this is easy in a dry suit, with a wet suit you have to ask yourself, "at what depth?" If I'm diving a we suit I want to be neutral, with no tank at 10 feet.
2. I use a BC or BC/w to buoy up an gas that I'm carrying, again with a dry suit this is easy: keep your suit squish pretty constant and then trim out with your BC or BC/w (there are several approaches to accomplishing this). With a we suit you have the added problem of your suit, you get more negative as you descend and you only have one gas bag to compensate with. This is not a big deal for someone with big lungs and a thin suit, but can be a daunting task for someone with small lungs and a thick suit.
3. If I'm diving a rig where the tanks themselves are significantly negative I'll give serious consideration to adding hard positive buoyancy to the rig to trim it neutral when empty.
4. I concur that it is better to aim for neutral at 10 feet with empty rather than when at reserve, and you certainly don't want that at Bingo Air (Rock Bottom for you non-fliers).

 

[DA Aquamaster]

just a thought but...

why 300-500 psi? I personally shoot for neutral at 10-15' with 0 psi for exactly the problem you identify. if I have 500 psi in my steel doubles I have over 34 cu ft of air, at a SAC of .5 that's 46 minutes of gas. That gas weighs just under 3 lbs, which I can easily compensate for by adding air to wing, drysuit or heck, taking deep breaths if they are both holed, which I complete my deco obligation. however if I am neutral at 500 psi I'm positively buoyant at 400 psi and almost 3 lbs underweighted at 0 psi. If I'm air sharing without an anchor line while completing deco I have to swim downwards the entire time - difficult, increases gas consumption, increases deco requirements.

Am I missing something?

Yes and no. You are technically right, and you can do the math. If you know how much the gas you have left when you are neutral at 10-15' and you klnow how much that gas actually weighs, you can add that amount to make yourself neutral at 0 psi.

On the other hand what is the advantage of actually testing the theory by breathing everything dry at 10 ft? 300 psi in double X7-100's is 17 cu feet of gas and that gas weighs about a pound, so do I need to breathe my rig down to nothing to prove it? No - I just need to establish that I am neutral at 300, 500 or even 3500 psi and add the weight required to be neutral at near 0 psi.

And 34 cu ft is a lot of gas at 500 psi - a good argument for going with 300. Feel free to deviate but what matters is being near neutral when you are nearly out of gas, not actually breathing it down to zero.

 

[Flightlead]

Yes and no. You are technically right, and you can do the math. If you know how much the gas you have left when you are neutral at 10-15' and you klnow how much that gas actually weighs, you can add that amount to make yourself neutral at 0 psi.

On the other hand what is the advantage of actually testing the theory by breathing everything dry at 10 ft? 300 psi in double X7-100's is 17 cu feet of gas and that gas weighs about a pound, so do I need to breathe my rig down to nothing to prove it? No - I just need to establish that I am neutral at 300, 500 or even 3500 psi and add the weight required to be neutral at near 0 psi.

And 34 cu ft is a lot of gas at 500 psi - a good argument for going with 300. Feel free to deviate but what matters is being near neutral when you are nearly out of gas, not actually breathing it down to zero.

ah, perhaps I misunderstood you. I thought you were suggesting configuring the gear to be neutral at 500 psi. am I now to understand you were talking about taking the measurement at 500 psi and then doing the math to figure out how to be neutral at empty? sorry if there was confusion. I did my measurement at 200 psi and backed out the difference.

 

[Flightlead]

A few thoughts:

1. If I take my tank off I want to be as close to neutral as possible, this is easy in a dry suit, with a wet suit you have to ask yourself, "at what depth?" If I'm diving a we suit I want to be neutral, with no tank at 10 feet.

This I'm struggling with. I am currently diving double X8's with a steel backplate and 10lbs to be neutral at empty in my drysuit and thick undergarments. Quick swag that's 51 lbs (11 lbs ea tank, 5 lbs manifold, 3 lbs each reg, 3 lbs bands, 8lbs plate, 10lbs weights). to be neutral with my rig off in my drysuit I'd need to put that on my belt, which would then make me waaaay overweighted even after taking off 10lbs of lead and going to an AL backplate. I'm missing something...

If I'm diving a rig where the tanks themselves are significantly negative I'll give serious consideration to adding hard positive buoyancy to the rig to trim it neutral when empty.

like what? Styrofoam outriggers?

my rig sinks like a rock with full tanks and fully inflated wing when I'm not in it.