Bouyancy and Lead Weighting Fact Check

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carlk3
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Bouyancy and Lead Weighting Fact Check

Post by carlk3 »

I'm trying to understand buoyancy and played with this calculator, http://www.divebuddy.com/calculator/weight.aspx. Here is what I observe with respect to Northwest drysuit diving for a 175 lbs diver. Does this ring true?
  • Going from Steel 100 to Steel 80 would lightens weights by 2 lbs (+ the weight of the tank)
  • Going from novice to proficient is worth 6 lbs of lead!
  • A body weight change of 10 lbs is worth 2 lbs
  • question: How much difference is there among cold-water, drysuit undergarments with respect to buoyancy?
  • Various other sources say that lung size changes buoyancy by 8 to 10 lbs. I assume this is the mechanism by which divers with more experience need less lead, right?
  • According to http://scuba.about.com/od/scuba101/p/Bu ... Divers.htm (see quote below), overweighting makes it harder to control buoyancy.
Thanks for any info that will help me understand this better.

- Carl
Divers with an excessive amount of weight will have a more difficult time controlling their buoyancy. The more weight a diver uses, the more air he will need to add to his BCD to compensate for the negative buoyancy from his weights. As air in a diver's BCD expands and compresses with any small change in depth, the more air he has in his BCD, and the greater volume air that is expanding and compressing. This makes it more difficult for the diver to control his buoyancy as he changes depth.
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Desert Diver
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Re: Bouyancy and Lead Weighting Fact Check

Post by Desert Diver »

It depends on the steel tanks. The Worthington 3442 cylinders are pretty close on buoyancy when empty. What will make a big difference is when they are full. The 130's just have a lot more weight of air in them than the 80's and you are going to sink fast.

Different bodies just float different and it isn't all just "fat floats". Ask TSM, picture that I saw didn't show any excess weight and she says she uses a bunch of lead.

Drysuit undergarments make a big difference but even bigger is whether you dive them tight or not. I like some air in mine so I stay warm. You have to sink the air.

Usually we use less lead as we get better at diving. I've dropped about the 6 pounds you mention as I got better control of my drysuit.

Most experts will tell you you should use less lead. This is fine until you use more air than you planned and built up more nitrogen than you planned, you are trying to hold a safety stop that you really need to do, and you are swimming straight down to do it. You can't vent any more air for 2 reasons. One is you already vented most of it as you came up because you realized you were light. The other reason is because if you don't swim down you are going to bob to the surface so you can't get your shoulder higher than the rest of your body. I'd rather have enough lead to keep me down until my tank is empty.
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fmerkel
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Re: Bouyancy and Lead Weighting Fact Check

Post by fmerkel »

In order (with a bit of tongue in cheek):
1. I don't change anything between my 100 and 80. Both work fine, not worth the bother. There can be far more variation between different brands of 100, than between some 100 and some 80's. IMO 2# difference is close to the ignore point. I'd rather be a tad heavy than a tad light for warmth and safety.
2. Depends on the novice, but in general more experience allows you to drop some lead.
3. No idea. I weight the same as I did in high school, a long time ago.
4. There is so much variation and options for suits and undergarments, along with body size and distribution, that it is essentially impossible to answer that question. Learn to dive what you got.
5. Yes, breath control makes a big difference, but it's not THE answer, it's just part of the answer. You need to breathe. That requirement is incorporated into the requirements of the dive. To do so smoothly takes some experience.
6. An experienced diver can deal with a little underweighting and a whole lot of overweighting.....IF NEEDED. Mostly it should not be needed. An inexperienced diver generally has more problems with buoyancy. The more they are off optimal, the more problem they are likely to have.

There's a reason new students tend to be overweighted.
a. Many OW1 classes (AND all the way throught DM in some cases....yes I've seen DM's with lousy buoyancy) there simply is no time to deal with buoyancy issues, let alone trim. There SHOULD be but there ain't. You can thank the major teaching organizations and shops for that. It's an unfortunate reality.
b. New students can dive over weighted, not optimally but then how do you tell :stir: Too underweighted and they won't be ....diving.....at all, just bobbing on the surface. (See a.)
To Air is Human,
To Respire, Divine.
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BillZ
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Re: Bouyancy and Lead Weighting Fact Check

Post by BillZ »

So here's a simple way to get properly weighted

After a dive in your typical dive gear set-up,drain your tank to 500 PSI

Descend to 10 feet and lay on the bottom. Make sure all of the air is out of your wing. Inflate your drysuit enough to tale the squeeze off but not so much that you have a bubble.

Relax. slowly inhale and exhale (yoga breaths). If you rise off the bottom when you inhale and drop when you exhale, you are properly weighted. If you're stuck to the bottom have your buddy remove weight, if you can't stay on the bottom have him add weight.

Once you've done this you now have a baseline. if you change anything with your gear you can make small incremental changes in how much lead you carry.

So to answer your original questions....

Going from Steel 100 to Steel 80 would lightens weights by 2 lbs (+ the weight of the tank)
http://www.huronscuba.com/equipment/scu ... ation.html
Look up the buoyancy characteristics of your tank (when empty) that you used in your baseline vs the one you are switching to. Adjust for the delta.
Going from novice to proficient is worth 6 lbs of lead!
Not true. If you took a good open water class from a quality instructor you should be properly weighted on your first open water dive. If you took a crappy $199 class they probably strapped the weight on you until you sunk to the bottom so they could run you through your skills and issue you your c-card.
A body weight change of 10 lbs is worth 2 lbs
Nah - I lost 60 lbs and it only made about a 2 lb difference in my weight belt.
question: How much difference is there among cold-water, drysuit undergarments with respect to buoyancy?
A big difference. I have 2 undergarments; A Wheezle, which is a puffy sleeping-bag-like suit and a 4th Element Arctic polypropylene undergarment. If I use the Wheezle I need to add about 4 lbs of lead.
Various other sources say that lung size changes buoyancy by 8 to 10 lbs. I assume this is the mechanism by which divers with more experience need less lead, right?
No. Your lungs (breathing) are used more to adjust vertical position in the water and fine tune your buoyancy. It has nothing to do with how much lead is on your belt.
According to http://scuba.about.com/od/scuba101/p/Bu ... Divers.htm (see quote below), overweighting makes it harder to control buoyancy.
Yep - That pretty much explains it.
Last edited by BillZ on Mon Feb 16, 2015 10:51 pm, edited 1 time in total.
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mpenders
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Re: Bouyancy and Lead Weighting Fact Check

Post by mpenders »

It looks like Bill beat me to it. What the hell, it took awhile to type it up - here it is anyway:

I think it's important to realize that the calculator mentioned in the original post gives an estimation of how much lead a diver will need, based on certain criteria. The criteria that they use are valid, but the figures they provide are not hard and fast numbers that anyone should rely on. Consider any amount it recommends to be a ballpark figure that will need to be dialed in for you.

My thoughts on the questions/points you posed:

• Going from Steel 100 to Steel 80 would lightens weights by 2 lbs (+ the weight of the tank)

Not necessarily. Tanks have different buoyancy characteristics. Check out this tank specification chart, and take note of the Buoyancy Empty column. These are the numbers that you need to understand when changing from one tank to another. Notice that they can vary quite a bit, depending on the manufacturer of the tank.

• Going from novice to proficient is worth 6 lbs of lead!

A more experienced diver will generally be more in tune with his/her position in the water column, and will have a better "feel" as to if air is needing to be added to or released from their bcd. Less experienced divers may use more lead to help arrest runaway ascents. 6lbs seems to be an arbitrary amount, not sure who/what that number is based on.

• A body weight change of 10 lbs is worth 2 lbs

I can't agree with this. All other things being equal, a 200lb couch potato with a sizeable layer of "bioprene" should need quite a bit more lead than a 220lb weightlifter that has 5% bodyfat. Body densities vary, some divers sink like rocks, others float like corks.

• question: How much difference is there among cold-water, drysuit undergarments with respect to buoyancy?

Lots. There are those that do not compress as much as others, trap more air, and are therefore more inherently buoyant.

• Various other sources say that lung size changes buoyancy by 8 to 10 lbs. I assume this is the mechanism by which divers with more experience need less lead, right?

Wrong. Larger divers tend to have larger lungs than smaller divers. With the larger lungs, a diver may have an easier time adjusting their position in the water column (inhale deeply...ascend, exhale deeply...descend). It would be more accurate to say that the larger lung volume allows buoyancy change to be more responsive when compared to a smaller lung volume.

I don't see how divers would be able to gain lung volume with experience.

• According to http://scuba.about.com/od/scuba101/p/Bu ... Divers.htm (see quote below), overweighting makes it harder to control buoyancy.

True.

Your primary goal when calculating the amount of lead you need to carry is to be able to stay underwater until you choose to come up. As the air in your tank is depleted, it becomes lighter, right? So, ultimately you need to be able to stay underwater when your tank is "empty" at 500psi. And you need to be able to hold a safety stop at 15ft, with that same "empty" tank.

A very precise method of determining how much lead you need is to conduct a weight check at the end of a dive. With your tank at 500psi, dump all the air from your bcd and settle on the bottom in 15ft of water. Slowing start removing weight, one or two pounds at a time (this is easier with multiple soft weights instead of one big weight belt), then take a deep inhalation and see if you rise off the bottom. If you don't, remove another one or two pounds and take another inhalation. Repeat this until you rise off the bottom. Congrats, you've just determined your buoyancy for that suit, undergarment, bcd, tank, hood, etc. If you change your gear configuration, be aware that your buoyancy may also change.
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