Weighting question... slightly too heavy or light?

[stuartv]

An AL80 starts NEG and ends POS as it empties. An AA80 starts Neg and ends Neg as it empties. So there's a little more physics involved than just throwing on 6 # and calling it all even.

Actually, there is not. The buoyancy of the tank when it's empty determines how much total weight you need. So, you would need a different amount for an AL80 than for a steel 80 in order to be neutral with an empty tank.

But, with either tank, once you are neutral with an empty tank, all that matters is how much the gas weighs that you are going to put into. If you put in ~80 cu ft of gas, then you will be ~6# negative. That does not change for aluminum vs steel. It IS simply physics. Thus:

An AL80 holds 6# of gas. If you start the dive 6# heavy, you will be perfectly neutral when it's empty. It's the same for a steel 80.

If you are diving a tank with 80 cu-ft of gas in it and you are starting any lighter than 6 # negative, then you WILL be positively buoyant if you breathe it down to near empty, and it doesn't matter if the tank is steel or aluminum.

 

[stuartv]

I dive with enough weight to be comfortable, but I don't try to shave every ounce, either. Using the lowest amount of weight has become a competition much like having the lowest SAC or the smallest bladder on your BCD. Everybody wants to be a special snowflake. If you want to see how much weight you really need, try Dr Bob's weight titration:

At the beginning of the dive, on a normal breath with no air in your BC, no weight in your pockets, legs crossed (to prevent kicking) and arms crossed (to prevent sculling), have someone estimate how far the top of your head is above water. For every inch (25mm) add 2 punds (1 kg). Repeat until the top of your head is just awash (barely under water). When you exhale, you should begin to sink. Need to sink faster? Kick or pull on a down line. Using this method, you should comfortable at your safety stop.

That really only works for tanks around 80 cu-ft, right? If you do that and then dive a 130 and run it down to nearly empty, you'll probably still be too light at the end, right? Because you should start a dive with a full 130 about 63% more negative than what you would be with an 80, right?

 

[eelnoraa]

stuartv is right in post #31, it is how much gas you are carrying, not the tank. If you can't do a weight check with empty tank, just do one with full tank, then add the weight of the gas. If you need to error on one side, go a bit heavy. A few pounds heavier is not a big deal.

 

[The Chairman]

That really only works for tanks around 80 cu-ft, right?

Dang! I wish you told me this earlier. I only dive LP 120s, LP90s and HP 130s. It seems to work fine for me and I rarely wear neoprene! BTW, the more neoprene you wear, the less likely you'll be light at your safety stop. Quite a bit of compression happens in that first 15 feet.

No matter what weight you come up with, if you come to the end of your dive and you feel a bit light, simply pause on the exhale rather than the inhale. We all pause in our breathing, but most of us go with our default: lungs full. Sure, this uses a bit more air, but you shouldn't need it until your safety stop. In any event, if you're a bit light add a couple of pounds (1kg) for your next dive.

Remember: your head needs to be just awash. Forget that eye level stuff: that won't work here. Your head has to be just under the water with a normal breath and both arms & legs crossed. I promise: you'll be fine!

 

[AdivingBel]

or the smallest bladder...

Glad you finished that sentence...I was starting to think "dive competition" was being raised to a whole new level...

BTW, how's the achilles, Pete? We all felt bad you couldn't dive at the Surge. Iknow, Iknow, you've dived all over the world, but geez on your b-day. Hope you're feeling better. Sorry for the hijack...carry on.

 

[ls1dreams]

I like to travel light, so probably will just err on slightly overweights (+1.5kg or 3.3lbs). Will ask dive shops in the future though if they have smaller increment weights that I can bring on board if they aren't on the boat.

I never felt dangerously overweighted, though without BCD assistance I was definitely sinking without finning up.

 

[The Chairman]

The buoyancy of the tank when it's empty determines how much total weight you need. So, you would need a different amount for an AL80 than for a steel 80 in order to be neutral with an empty tank.

Almost... sorry I didn't see this earlier. The weight you need to descend is dependent on your total buoyancy, and your total weight. That's a little over a kg/l in order to be neutral. The weight you need to stay at your safety stop is a bit less due to the compression of you, your exposure protection and your gear that increases your over density. Steel tanks are just naturally heavier full or empty and since they are smaller (thinner walls) they are inherently denser.

 

[stuartv]

Dang! I wish you told me this earlier. I only dive LP 120s, LP90s and HP 130s.

That's why I put it as a question. "..., right?" 'cause I realize I might be missing something here.

Are you saying that the right amount of negative buoyancy (at the start, with a full tank) is the same number, no matter whether you're diving an 80 or a 130? Or are you saying that this method produces a different amount of negative buoyancy depending on what size tank you're wearing? Because I *think* the method you posted would result in you being the same amount negative, no matter what size tank you're wearing. And if that amount is enough for when you're using a 130, then you must surely be about 4# more negative than you need to be when diving an 80, right? Or what am I missing?

Almost... sorry I didn't see this earlier. The weight you need to descend is dependent on your total buoyancy, and your total weight. That's a little over a kg/l in order to be neutral. The weight you need to stay at your safety stop is a bit less due to the compression of you, your exposure protection and your gear that increases your over density. Steel tanks are just naturally heavier full or empty and since they are smaller (thinner walls) they are inherently denser.

I'm unclear on exactly what you are saying is incorrect about my statement that you quoted.

 

[dmaziuk]

Graig's picture claims buoyancy swing of 10.5 lbs for HP133. It's almost double of an Al80, but in absolute terms it's only 4-5 pounds. Controlling that with the lungs really shouldn't be much of a problem, i.e. the method is close enough for practical dolphins.

 

[The Chairman]

That's why I put it as a question.

I was screwing with you. It works fine because of the compression at fifteen feet, It's probably close to being at eye level at the end of the dive.