Doubles and weight requirements

[stas]

Hi,
My first set of doubles comprising 2 PST e7 120's is being assembled right now. I plan to use the doubles right now in a recreational setting to become comfortable with them for future technical trainning. All of my diving is boat diving and I plan to use the same set for both dives of the day. According to the specs on the tanks, each tank has a 10.5 lb change in bouncy from full to empty. Once I know how much weight I need to be neutral with full tanks, do I have to add 21 lbs to my rig compensate for air consumption? I know I will be burning only one of the tanks (gas equivalent to one tank) on the first dive but I dont want to reconfigure my belt metween dives? How do peole handle this issue?
Thanks, Stas

 

[TSandM]

I asked this question, too, and the answer I got was that you have to weight yourself for the entire swing on all dives. This is because if you only weight for part of the swing, and for some reason (entanglement or whatever) end up using more than you planned for, you wouldn't be able to control your ascent at the end of that dive properly. Made sense to me.

 

[CIBDiving]

... According to the specs on the tanks, each tank has a 10.5 lb change in bouncy from full to empty. Once I know how much weight I need to be neutral with full tanks, do I have to add 21 lbs to my rig compensate for air consumption? ....

You don't ADD weight to be Neutral with FULL tanks, you add lead to be neutral with empty tanks!! oh god, and another teck diver is born.

 

[TSandM]

CIBDiving, one way to calculate weighting is to figure out what neutral with full tanks is, and then add the weight of the gas to be consumed. It's a perfectly legitimate approach, and sometimes a necessary one, if you are diving a new tank setup and don't have a chance to do a weight check at 500 psi before the dive.

 

[stas]

CIBDiving,
Thanks for your reply but wouln't figuring out how much weight one needs to be neutral with full tanks and then adding the weight neccessary to compensate for the bouyancy swing be the same as weighting your self for neutral bouyancy with empty tanks?
Stas

 

[BabyDuck]

it may end up being a moot point. i'm so heavy in mine that i don't need any additional weight.

 

[Dive-aholic]

I don't add any weight to my doubles set up either. I'm overweighted in both 85s and 95s in my dry suit with undergarment in fresh water. In the Sea of Cortez, which his denser than the Atlantic or Pacific, I need about 4 lbs if I let my pressure get below 1000psi.

 

[mirrocraft]

the first thing i want to say is you wont have to shift weight from one side to the other when you are diving with doubles and you have a manifold you keep it open you will have the same weight in both tanks if you are diving 120s you shouldnt need much weight as is i dive 100s high pressure steels and only wear weight in salt water emty or with dry suit if your wanting to learn them i think its great that your going to learn first and then pogress your skills but i would at least get some reading about the set up and how to configure to at least get the basics

 

[Blitz]

With double E7 120s you probably won't require weight. How much weight do you dive with now?

 

[dsteding]

I see this in reference to singles rigs, which is where I know my weighting is for me.

Seems to me you can get an accurate estimate of what you'll need in terms of weighting with doubles by doing the following (assuming all you are changing is the tanks).

1) Figure out your weighting with one of the tanks in question.

2) Subtract from that weighting (a) the manifold's buoyancy (minus what the old valve was); (b) the band's buoyancy, and maybe a bit for your extra regs and then (c) the second tank's buoyancy at 500 PSI.

(a) and (b) can probably be estimated, maybe what, 2-3 pounds total? (c) can be figured out by reference to a tank spec chart and the weight of 500 psi of air in the tank, probably also ~3-4 pounds for a typical HP 119 or 130.

So, I'm thinking about 6 pounds less weight than my singles rig, which requires 24 pounds total between plate and weights . . . but we'll see on Wednesday.