Weight belt equation

[RIDIVER501]

If you are considering your suit a 14mm becuase it is two layers of 7mm then it is really a 7mm suit and then the rough equation is. 10% of your body weight plus 15 lbs.....more or less depending upon your body fat content and whether you are wearing hood gloves yadda yadda.....

 

[Groundhog246]

Whether you are using AL tanks or steel or anything else doesn't matter, as the weight of the tank stays the same, but you have to adjust for the air that you have used, as you will be carrying that much less weight at the end of the dive.

I've asked that same question, since the laws of physics are fairly stable. As far as I can see with a steel tank vs AL, you have either
A) reduced your total weight, if a steel tank holding 80CF weighs less than an AL holding 80CF and is still negative at 500PSI,
or
B) you've moved the weight if the steel tank is actually heavier empty than the equivalent AL.
Going with HP steels, which at their higher pressure put more air in a smaller space I can see where A is possible, but with LP steels, I'm not so sure. Since I wear a weight integrated BC, I can understand that moving the weight off a weight belt might be considered good. But to have someone wearing a s/s backplate and steel cylinder 'boast' about needing little or no weight on a belt, is mis-leading if his total equipment weight and belt equal my BC/Al tank and ditchable weight. I should bring a scale along to the dive site. We could weigh in without any gear and again ready to enter the water.

 

[fmw625]

I usually dive in FW using 4 lbs. Have no problems @ safety stops. Recently I dove in SW using (a calculated) 14 lbs. Was a little heavy but had no problems. Was really glad I had 14 lbs. since the current was a little ruff. Figure that next time, with no current I should be able to use only 12 lbs.

 

[scubatoys]

I usually dive in FW using 4 lbs. Have no problems @ safety stops. Recently I dove in SW using (a calculated) 14 lbs. Was a little heavy but had no problems. Was really glad I had 14 lbs. since the current was a little ruff. Figure that next time, with no current I should be able to use only 12 lbs.

I assume that was in a different suit than you use for fresh water, otherwise, the calculations are a bit off... For most people, with the same suit, add about 4-5 lbs for salt water. Salt water is 1.6 lbs heavier than fresh water (per cubic foot), so normally you take your body weight... lets do an example with a 170 lb guy, plus the weight you have to wear in fresh, 4 lbs to get 174. Then divide by 62.4 (the weight of a cubic foot of freshwater) so 174 / 64 = 2.718 That is how many cubic feet of space you take up. Then multiple that by 1.6 (the number of lbs extra that salt water weighs) and you get 2.718 x 1.6 = 4.34 lbs.


If you are only wearing 4 lbs normally, you obviously have a low body fat - and if you needed to add 10 pounds, that would put you in the 400 pound range... And if you weigh 400 pounds with a low body fat content... that should make you about 6 inches taller than Shaquille ONeal
... Please call Mark Cuban right away... The Mavericks have a Jersey waiting for you....

 

[fldivenut]

Going with HP steels, which at their higher pressure put more air in a smaller space I can see where A is possible, but with LP steels, I'm not so sure.

An ALU 80 and LP 80 are dry and empty roughly the same weight and dimensions (LP a pound or so lighter), and a HP 80 is smaller and lighter. I.e. any steel tank of the same size as an ALU tank will take weight of your total system in addition to provide much better overall bouyancy. (Steel is stronger than ALU and thus the tanks can be made thinner and thus lighter.)
One of many charts with tank data is here:
http://www.saudidiving.com/tanks.htm (They came up first in Google)
(I don't agree with their recommendations though, HP steel is IMO out.

The main reason for the popularity of ALU tanks is that people learn to dive with them and that is because that is what shops and operators has as rentals. And the reason for this is that they can handle the outside of an ALU tank much more careless than a steel and that they are somewhat cheaper. For an active diver owning your own tanks with a life of 20+ years for a steel tank it is a no brainer to chose, the cost issue becomes nil spread out over 20 years.

--A

 

[Don Burke]

I usually dive in FW using 4 lbs. Have no problems @ safety stops. Recently I dove in SW using (a calculated) 14 lbs. Was a little heavy but had no problems. Was really glad I had 14 lbs. since the current was a little ruff. Figure that next time, with no current I should be able to use only 12 lbs.

You would probably still be a little heavy. It is better to be a little heavy than a little light, so I would not worry about it while you are figuring out the proper amount.

The rough conversion from freshwater to saltwater is to add one pound for every forty pounds of overall weight.

Eight pounds of change would mean an overall weight of three hundred twenty pounds. That is possible for someone who is a tech diver or a pretty big person (or both).

 

[Don Burke]

I have a question. Surely the fact that AL tanks are positively buoyant at the end of the dive is a red herring.

No. The standard AL80 is indeed positive at 500psig.

The way that it is always stated has always implied to me that if you are diving with steel tanks then you don't need to worry about it, as they are still negatively buoyant at the end, but diving with AL tanks you have to adjust for.

If you are changing between type of tank, there is normally an adjustment involved.

Surely what matters is that at the end of the dive you are going to be carrying the weight of the air less weight than you were when you started the dive.

That is the buoyancy change you will see during the dive, along with whatever compression your gear goes through. The positive buoyancy of an aluminum tank is a static problem.

Whether you are using AL tanks or steel or anything else doesn't matter, as the weight of the tank stays the same, but you have to adjust for the air that you have used, as you will be carrying that much less weight at the end of the dive.

You will often find a person telling you that a given tank has no buoyancy shift as it empties. I have been told that by many very experienced instructors. They were wrong.

The bottom line is that tank buoyancy is part of the non-changing portion of your buoyancy. It may be different for each tank, but it will not change measureably during a dive. For instance, I need to take twelve pounds off if I go from double AL80s to double steel 72s.

The gas in the tank is part of the changing portion of your buoyancy. It is one of the things a BC needs to be able to deal with. I start the dive quite a bit heavier with a big tank than a small tank, but if I run the gas down to the pressure I weighted for in both cases, I will be neutral.

 

[Groundhog246]

An ALU 80 and LP 80 are dry and empty roughly the same weight and dimensions (LP a pound or so lighter), and a HP 80 is smaller and lighter. I.e. any steel tank of the same size as an ALU tank will take weight of your total system in addition to provide much better overall bouyancy. (Steel is stronger than ALU and thus the tanks can be made thinner and thus lighter.)

This doesn't make sense. if an Al80 and an ST80 (LP) are the same weight and dimensions, then their bouyancy would be the same, thus the steel LP will not take weight off your belt. If an HP80 is smaller and lighter, then it might (depends how much smaller in relation to how much lighter).
Yes steel is stronger (and important to tank life, more elastic), but is also much denser (2.9 times as dense).
Aluminum, 6061-T6 2.7g/cc
Soft Steel (0.06% C) 7.87g/cc

In my own situation, given that I would have to change regs (mine are yokes rated to 3000PSI) or have them changed over to DIN (not sure parts to do so are available) inorder to use HP tanks. Never mind the fact I have 2 ST 72's (LP), 1 AL30 and 5 Al80's that are all yoke type valves, I won't be switching anytime soon.
And since, the tank part of the bouyancy equation does not change (ie, if you add 4 lbs of lead to offset the bouyancy of the tank, they're both solid materials not affected by depth so the 4lbs at 0 feet is still 4 lbs at 60 feet), the only difference is carrying that 4 lbs to and from the water. I just can't get that excited about dropping that 4 lbs.

 

[miketsp]

This doesn't make sense. if an Al80 and an ST80 (LP) are the same weight and dimensions, then their bouyancy would be the same, thus the steel LP will not take weight off your belt. If an HP80 is smaller and lighter, then it might (depends how much smaller in relation to how much lighter).
Yes steel is stronger (and important to tank life, more elastic), but is also much denser (2.9 times as dense).
Aluminum, 6061-T6 2.7g/cc
Soft Steel (0.06% C) 7.87g/cc

Aluminium tanks have thicker walls so external dimensions are bigger for same weight of tank - so more displaced water volume - so more upward thrust - so lighter.

 

[Groundhog246]

Aluminium tanks have thicker walls so external dimensions are bigger for same weight of tank - so more displaced water volume - so more upward thrust - so lighter.

Yes, but an AL80 has a higher working pressure than an LP steel. 3000PSI vs 2640PSI for current LP steels. Thus an LP80 needs a larger internal volume than an AL80. (converesly an HP80 would need a smaller internal volume than an AL80)

This is an argument with no "right" answer. I will grant that using an HP steel will remove weight from your 'system' (be it weight belt, plate, whatever). But for a rec diver with a single tank, the difference in wearing an AL cylinder and an extra 4 lbs of lead, vs a steel cylinder will make little real difference in the water. I think people confuse the issue of Al vs ST with overweighting, which makes bouyancy a larger challenge. Whether you dive steel or aluminum the bouyancy of the cylinder and your lead will not change with depth, so as long as you have the correct weight and carry it in the correct position there will be no discernable difference. And if you consume 60 cu/ft of air the weight change will be the same whether it came out of a steel or aluminum tank. So the only place I see it matters is carrying the gear out of the water. And for that, I'm fit enough to carry the extra few pounds and if I wasn;t, I;d have much larger worries about diving.