Best way to approach being overweighted?

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Makes sense. So the difference in weight of the two tanks is a difference in the weight of the actual metal? ie aluminum is positively buoyant, so as you breathe more you're working against the positive buoyancy of the tank? Sorry if that's not right, just trying to understand the mechanics.
There's a really good blog post on different tank types and their effect on weighting at Rich asks…Why are you diving with that Tank?
 
That blog post reminded me there's variation even among steel cylinders. You can get one holding about 80 cuft of gas with empty buoyancy of 0 lb or another at -7.2 lb. (By comparison, an AL80 is +4.4 lb). There are 120 cf steels that are even positively buoyant at +1 lb all the way down to -18 lb.

In other words, keep good notes on your equipment configuration (incl. cylinder model) and weight used.
 
I was thinking about another analogy this morning. Take a 2L soda bottle full of air. Quite buoyant, right? If you push it underwater, it's displacing 2L / 2kg of water. You'd have to strap 2kg of lead to it to make it neutral. Now imagine you have a 2L steel water bottle. Same amount of air, but obviously you wouldn't need as much lead to get it to neutral because it's starting out significantly heaver than the plastic bottle.

(And yes, I do realize you need a bit more than 2kg of lead to make it neutral because the lead itself is displacing some water, but to a first approximation it's reasonable.)
 
Makes sense. So the difference in weight of the two tanks is a difference in the weight of the actual metal? ie aluminum is positively buoyant, so as you breathe more you're working against the positive buoyancy of the tank? Sorry if that's not right, just trying to understand the mechanics.

Your buoyancy while diving is a product of the weight and volume of your total package--you, your tanks, your weights, your thermal protection, etc. The buoyancy swing during the dive is caused by lost weight--specifically the weight of the air that was breathed during the dive. If you breathed 6 pounds of air during a dive, you will be 6 pounds more buoyant at the end of the dive. PERIOD. It does not matter whether those 6 pounds came out of an aluminum tank or a steel tank.

Now, if the aluminum tank is the same size (external volume) as a steel tank, and the steel tank is 5 pounds heavier, then you are 5 pounds less buoyant with the steel tank throughout the dive, from beginning to end. That does not change. That means that you will need to carry 5 more pounds of lead with the aluminum tank than the steel tank.

The supposed "floatiness" of the aluminum tank does not matter. If you lose 6 pounds of air during a dive, you will be 6 pounds more buoyant at the end of the dive, even if those 6 pounds came out of a steel tank.
 
As boulderjohn states, the changing buoyancy of an aluminum tank doesnt matter re the weight you need to carry, but it may matter when it comes to trim. At the end of a dive, as an aluminum tank becomes buoyant, depending on its location and your own trim characteristics, it may cause you to become top-floaty.

For me, with aluminum tanks, I need about 4 lbs up high - in a "trim weight" location (on the tank strap for me, or in trim pockets back when I had a vest style BCD). Otherwise I become top-floaty, especially towards the end of a dive

With steel tanks, I need no trim weights, as the tank never becomes buoyant and I am otherwise trimmed out.
 
Boulderjohn nailed the buoyancy.

On trim shift, there should also be no difference. If all the parts of the diver stay firmly attached to each other.

Say we have:
- a diver (plus some diver lead)
- a steel tank
- an AL tank plus some tank lead.
So that the steel buoyancy equals the aluminum plus tank lead buoyancy. Say both tanks have the same shape, and let's ignore the lead volume.

Trim out the steel tank and diver using the diver lead. Now clone them and swap tanks.

Restoring the trim with the aluminum tank's lead should put the lead in the spot that makes the two tank packages have identical mass centers. Otherwise, you would not have restored the trim. If you want them trimmed out in 3D, the tank lead needs to go on the tank sides. If you just care about head/toe and left/right trim, the lead can go on the BC or diver. And by trim, I mean a balanced weight distribution such that the diver with all their gear has mass and volume centers are in the same spot.

Now we breathe down the gas. Mass leaves each diver from the center of the two tanks. Which have the same shape, so the trim shift is the same. Both tanks will be getting lighter. The steel tank will press less strongly downward on the body. As will the aluminum tank, in equal amounts. From the same spots.

If the aluminum lead was attached to the aluminum tank, there should be no differences.

If the aluminum lead was attached to the diver's front weight belt, and the tank is only attached to the diver at the shoulders, then the aluminum bottom will eventually start floating up above the diver, changing the shape and mass distribution differences between the two divers. But that would not be the case if the tank was firmly attached to the diver, or at least to the tank's lead, at top and bottom.

That is how I read the physics of the trim.

Steel and aluminum tank walls have different thicknesses, and the tanks often have different pressures. This means equal displacement and equal gas volume tanks are unlikely. Steel lets us build different tanks, so we do that. But the trim shift is due to the amount of gas, its location, and attachment to the diver, not the metal that holds it.

I know empty aluminum tanks float up if not tied down, I'm a side-mount diver. Our tanks are attached with line, bolt snaps and bungee cord, not 2" webbing.
 
That's a good point.

The real life check is a trim check. Get neutral and chill. Legs together and arms together going straight out away from your body.

You should have zero rotation in the axis of your shoulders. You may have a slow tilt either arms down or legs down. Shifting your weight location or adjusting your tank band location can help shift the weight the other way (Do this above water and retest next time). If it tilt fast, you probably need to change fins if nothing else fixes it.
 
Sounds like the details of what exactly buoyancy is, what exactly weight is, the relationships between tanks and lead etc are all well-covered. What this thread needs is some specificity and numbers! I would like to refer you to a great thread from @tmassey: Updated SCUBA tank specifications list -- in PDF and spreadsheet

It is a table of the buoyancy characteristics of many different tanks. Very useful to add some numbers to a conversation like this one. So you can see, for example, that a typical empty AL80 is around +4lbs when empty, versus a typical steel HP80 is around -2lbs when empty. Same amount of air, same amount of bottom time, same amount of change in buoyancy throughout the dive, but you will need approximately 6lbs less of lead. And they are a little lighter to lug around on land as well. Problem with the HP80's for some people is that they are rather short, so might end up being awkward to trim out, especially for taller divers.
 
the trim shift is due to the amount of gas, its location, and attachment to the diver, not the metal that holds it.
I would add that it's also due to the location of the buoyancy compensating mechanism. In order to remain neutral (maintain depth), every kilo of gas consumed must be accompanied by 1 liter less air in the wing. The center of mass of the gas (in the cylinder) and the center of volume of the wing are rarely aligned. The equal change in both result in an unequal change in the overall mass and overall volume. With the overall center of mass no longer aligned with the overall center of volume, a rotation is induced. Fortunately, one can make slight positional changes of the feet or hands to restore alignment of the mass/volume centers.
 

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