Weighting Myself

[divinh]

In a pool...

3/2mm full wetsuit and fins - no weights, full lungs head above water, exhale to drop to eye level; almost neutral?

empty Sopras Tek/Sub Compact Lite BC with added pockets from Dive Rite and Dive Gear Express - with 2 lbs in trim pockets, floats; with 4 lbs in trim pockets, sinks; guessing 3 lbs neutral?

With a steel tank, I wore no weight and was able to comfortably reach neutral buoyancy with a bit of air in the BC. It was easy to cruise inches from the bottom.

Previous test with aluminum, 4 lbs in cam trim and 6 lbs in weight belt, I had difficulty reaching neutral buoyancy easily. I felt like I was bouncing up and down with the slightest BC adjustments.

My understanding is the steel is 13 lbs negative full, while the aluminum is 1.5 lbs negative full. With the amount of weight I had, I would have been 11.5 lbs negative with the aluminum full and would want an additional 4 lbs for the aluminum to compensate for the end of the dive, whereas I don't need to add more weight for the steel.

I was happy with the way the steel felt. What should I do for the aluminum to get a similar feel?

Then for salt water, add 0.025 * X?

 

[kmarks]

My understanding is the steel is 13 lbs negative full, while the aluminum is 1.5 lbs negative full. With the amount of weight I had, I would have been 11.5 lbs negative with the aluminum full and would want an additional 4 lbs for the aluminum to compensate for the end of the dive, whereas I don't need to add more weight for the steel.

I'm confused, if you are 11.5 lb negative with a full aluminum tank, you'll be ~5.5 lb negative with an empty one (an AL80's gas weighs about 6lbs). Why do you need to compensate for that loss? You are still negative overall. You should just have less air in your BC as the dive goes and your tank empties. Your steel tank should lose the same amount of negative buoyancy, or more if it has a larger capacity, over the course of emptying.

I think you forgot the buoyancy of your BCD: +3 for BCD + -1.5 for AL80 (full) + -4 for trim + -6 for belt = -8.5 lb. Lose your six lbs of gas, you are still -2.5 lbs. With the steel tank, and no weights, you started your dive -10 (+3 BCD + -13 SS80) and end at about -4 lbs. You are maybe just barely overweighted, but with the steel tank there isn't much you can do about it.

Maybe switch your belt and your trim pockets? Putting more weight around your Al tank might help mimic the SS tank better, and dropping 4 lbs will still be enough to get your buoyant at the end of your dive.

 

[divinh]

I'm confused, if you are 11.5 lb negative with a full aluminum tank, you'll be ~5.5 lb negative with an empty one (an AL80's gas weighs about 6lbs). Why do you need to compensate for that loss? You are still negative overall. You should just have less air in your BC as the dive goes and your tank empties. Your steel tank should lose the same amount of negative buoyancy, or more if it has a larger capacity, over the course of emptying.

I think you forgot the buoyancy of your BCD: +3 for BCD + -1.5 for AL80 (full) + -4 for trim + -6 for belt = -8.5 lb. Lose your six lbs of gas, you are still -2.5 lbs. With the steel tank, and no weights, you started your dive -10 (+3 BCD + -13 SS80) and end at about -4 lbs. You are maybe just barely overweighted, but with the steel tank there isn't much you can do about it.

Maybe switch your belt and your trim pockets? Putting more weight around your Al tank might help mimic the SS tank better, and dropping 4 lbs will still be enough to get your buoyant at the end of your dive.

Oh, you're right! (I got fixated on the +4 of the aluminum at the end of the dive and not thinking it was part of the air usage, thus the -4 compensation.)

I'll see about putting more in the trim pockets so I can do without a weight belt or have very few weights on the belt. It certainly feels nicer to be without the weight belt. I think the only thing I need to figure out now is where the tank should be on the cam strap so that I pivot like I do with the steel tank.

 

[mi000ke]

As Tom Petty so eloquently put it, "The weighting is the hardest part."

 

[Lobzilla]

This really does not need to be so hard and confusing.

Rather than me writing a long "dissertation" let's try to get to the bottom of this (no pun intended) interactively.

QUESTION ONE:
What is the point of correct weighing, or asked differently, why don't we just want to strap a bunch of extra lead on and inflate the BC or wing accordingly?

 

[Lobzilla]

There will be two more questions after which you will wonder why correct weighting was ever so confusing. Promise.

 

[divinh]

This really does not be so hard and confusing.

Rather than me writing a long "dissertation" let's try to get to the bottom of this (no pun intended) interactively.

FIRST QUESTION:
What is the point of correct weighing, or asked differently, why don't we just want to strap a bunch of extra lead on and inflate the BC or wing accordingly?

Well, to start with, my BC provides 30 lbs of lift, so if I have more weight than that, I will pretty much sink unless I actively do something to compensate (kick downward), which uses up more air. Along with that, if overweight beyond the lift capacity of the BC, floating on the surface would be a struggle. If the BC were to lose all of its air, I would have a better chance of kicking my way up when more properly weighted. If I didn't know how to reach neutral buoyancy with the BC quickly, I might experience equalization problems due to dropping too fast and maybe even have a mask squeeze. Those are the things that come to mind first. It's been a while since I have read the text, so I'm happy to hear the reasons.

 

[Lobzilla]

Well, to start with, my BC provides 30 lbs of lift, so if I have more weight than that, I will pretty much sink unless I actively do something to compensate (kick downward), which uses up more air. Along with that, if overweight beyond the lift capacity of the BC, floating on the surface would be a struggle. If the BC were to lose all of its air, I would have a better chance of kicking my way up when more properly weighted. If I didn't know how to reach neutral buoyancy with the BC quickly, I might experience equalization problems due to dropping too fast and maybe even have a mask squeeze. Those are the things that come to mind first. It's been a while since I have read the text, so I'm happy to hear the reasons.

Most of these reasons are valid but they mean that we are operating on the edge of the physically possible, which we don't want to do.

The first reason for us wanting to weight ourselves correctly long before reaching the limits you cited is

ANSWER TO QUESTION ONE:
Any compressible volume (like unneeded air in the BC to compensate for unneeded weight) will destabilize buoyancy control, will make it harder to remain at a desired depth.

You experienced this when you dove over weighted with the AL tank. If that bold faced statement above makes sense, let's move on. If not, let's discuss this point further. We need to understand the "why" before we can move on to the "how"

 

[Lobzilla]

Lets recap before moving on.

Examples of compressible volume (that changes with depth/pressure):

• Air in BC
• The many little bubbles in our neoprene suit or the air inside of our drysuit

Examples of incompressible volume (that stays constant with depth/pressure)

• The outside volume of our tank
• The volume of our body (discounting small gas pockets in our sinuses and intestines)
• The volume of our fins and most of the BC shell

Why is compressible volume bad for buoyancy control? Because it expands as we ascend and the pressure decreases, which makes us more buoyant, which makes us ascend even faster, which makes us even more buoyant, and so on...

Descending we get the same "run away" behavior with ever decreasing buoyancy. This type of behavior is characteristic for an unstable equilibrium. Once we deviate from where the forces (weight and buoyancy) are balanced, things get worse and worse unless there is corrective action. This is why buoyancy control is so hard for the novice diver. In the presence of compressible volume, the weight vs. buoyancy equilibrium is as unstable as our upright walk that we only managed after having learned to constantly make subtle, subconscious corrections to our balance. Standing "still" is the result of tiny corrective actions - on land and in the water.

Diving with too much air in our BC - the result of carrying too much weight - is like learning to walk on stilts. Much harder than it needs to be.

 

[Lobzilla]

QUESTION TWO:

If our goal is to minimize compressible volume then why don't we just find a combination of tank type and lead weight that makes us neutrally buoyant at the beginning of the dive with ZERO air in our BC and call it done?