tank and boyancy logic, am i correct with this?

[Codyjp]

I am shopping for a tank and have some questions with weighting and boyancy.

I plan to buy an HP tank (one of the new 3442 jobbers). I am trying to decide between an 80, 100 or even thinking of going for the gusto and getting a 119 or 130.

The diving that i will be doing with this tank will be all in cold water with a 7mm/hooded wetsuit. I currently use about 24lbs of lead.

Currently i am using rental AL80's. so here is my question. an AL 80 weighs ~33lbs and is about +3lbs when empty. so that would mean if i consider the extra lead required to sink this tank it really weighs 33+3=36? am i right? this isn't considering the weight of the actual air (doesn't matter for this).

Then, if i were to get an X-80 which weights 28lbs but is -3lbs empty. so this would mean that the tank really weighs 28-3=25. That would mean this tank would allow me to shed about 11lbs off my legs. (36-25).

are my figures correct?

me with AL80=

36+24=60lbs ughhh!


X80=

25+21=46=√HD

HD=happy diver!

if this is all correct maybe i could get the gusto and get a bigger tank

 

[BKP]

I don't quite understand your figures, Cody, but I believe you're going about it wrong. The weight of the tank is completely inconsequential (except for hauling it back and forth to your car, or climbing into the dive boat). The relative buoyancy at approx. 500lbs. psi (or empty for comparative purposes), is much more relavent.

I'm assuming you're diving singles, not doubles (based on your statements).
I'm also assuming you currently use 24lbs. lead with a single standard AL80.

A standard Al80 is about 4 lbs. positive empty. You're dealing with that now, with 24lbs. lead.
A steel HP80 is about 3 lbs. negative empty. 7 lb. difference. You can drop 7 lbs. of lead. Now you're down to 17 lbs. needed
A steel HP100 is about 2.5 lbs. negative empty. 6.5 lb difference. You can drop 6-7 lbs. of lead. Now you're at 17 to 18 lbs. needed.
A steel HP120 is about 2 lbs. negative empty. 6 lb. difference. You can drop 6 lbs. of lead. Now you're at 18 lbs. needed.

Hope that makes sense ('cause I'm sipping Courvoisier, and a little "surface narc'd" myself).

 

[SparticleBrane]

The only thing that matters (other than hauling the tanks around) are the int-water buoyancy characteristics.
If the tank is 28lbs and is 3lbs negative, that doesn't mean it "weighs" 25 pounds. It means it weighs 3 pounds, in the water, when it's completely empty.

 

[Jake]

BKP is correct: you are doing things a little incorrect. You don't really have to "sink" the 33lb or the 28lb tank per se. In the case of the aluminum tank, you do have to sink the 3 pounds positive bouyancy at the end of the dive (according to your figures). In the case of the steel, the tank will be sinking you by about 3 pounds. That's a combined weighting difference of 6lbs.

Of course, these figures will depend on the exact specifications of the tank, but you get the picture.

 

[TSandM]

You're on the right track. When you stand up and walk in your gear, you have to carry the weight of the tank and the weight of the lead required to sink it. The lead you can get rid of in your case is 6 lbs. It's the difference between the -3 and the +3 buoyancy. You're carrying 3 lbs to get the current tank to neutral; you can shed that, and the 3 negative lbs for the new tank.

But then you have the weight difference between the tanks, too. So the NET savings is 11 lbs that you don't have to carry from the car to the water. This is the huge advantage of steel tanks.

You're much better off with the more negative tank, as long as you can trim it out.

 

[Codyjp]

excelent help guys. I think i just worded it incorrectly but you understood me anyway. All my math was in reference to how much my rig will weigh out of the water, in the water it doesn't make a lot of difference.

when i said 'weigh' that was in reference to me having to schlub it down that #$*&ing hill to the dive spot! It would be nice to lose some weight when i am traveling to and from the water!

 

[skynscuba]

I just brought home a pair of Worthington XS-80s, and they seem a bit heavier than advertised. After accounting for differences in air (new ones empty, AL80 1100psi), they only weight 1.5 to 2 pounds less than my Catalina AL80s. I don't know what the valve and boot weigh, but I'm guessing these XS-80 tanks weigh about 30 pounds without them. I hope they make up for it with being more than 3 pounds negative
when empty. I bought them as a Christmas present for my girlfriend. She is just getting into cold water diving, and I wanted to make her walk out after the dive as light as possible.

 

[NJMike]

Codyjp, I am going through the exact same decision making process now. Here's another thing I read somewhere on this site - if you get an HP 80 @ 3442 psi, you could run into scenarios where you can't get it filled over 3000 psi. In that case, you would end up with less than 80 cu ft of air...you can do the math but I think it was around 67 cu ft of air.

I definitely would not want this scenario so I think I am looking at an HP 100. Even when filled at 3000 psi, ... let's do the math...

100 cu ft @ 3442 psi = .029 tank baseline (psi/cu ft)
tank filled to 3000 psi = .029 * 3000 = 87 cu ft

This means if you can only get a fill to 3000 psi, you still get 9.6 cu ft more than the AL 80 (which has 77.4 cu ft of air)

For the HP 80:

80 cu ft @ 3442 psi = .023 tank baseline (psi/cu ft)
tank filled to 3000 psi = .023 * 3000 = 69 cu ft

So with the AL 80, you would have (77.4-69) approx. 9.4 cu ft less air.

If anyone wants to correct my numbers, please do!

 

[Leadking]

I just brought home a pair of Worthington XS-80s, and they seem a bit heavier than advertised. After accounting for differences in air (new ones empty, AL80 1100psi), they only weight 1.5 to 2 pounds less than my Catalina AL80s. I don't know what the valve and boot weigh, but I'm guessing these XS-80 tanks weigh about 30 pounds without them. I hope they make up for it with being more than 3 pounds negative
when empty. I bought them as a Christmas present for my girlfriend. She is just getting into cold water diving, and I wanted to make her walk out after the dive as light as possible.

You need to go back to Archimede's law an "an object is buoyed by the weight of what it displaces" a Worthington 80 displaces less than an aluminum 80. If for example the Worthington displaces 35 lbs of water 30(tank) -35(water) the answer is -5 lbs.
If an aluminim displaces 40 lbs of water (it is bigger than a steel HP80) and weighs 35 lbs it is 5 lbs positive 40 (water) -35 (tank)=+5.

Even though both tank weigh the same the buoyancy characteristics are different.

 

[TSandM]

The HP 100s are really nice tanks to carry around. They're lighter than my 95s, smaller on your back, and take lead off your waist compared with an Al80. But they're expensive.