Tank buoyancy numbers

[guruboy]

I was crunching some numbers and trying to figure out the buoyancy of a Luxfer AL80 tank when empty.

It listed as +4.4 lbs when empty here
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

But comes out to +3.4lbs if you do the math, such as here
Scuba tank size and buoyancy calculator

Not sure where the discrepancy is coming from.

Note - I am trying to figure out buoyancy of tank alone without valves or air.

 

[guruboy]

As a follow up, it seems that if you use 80 cuft (instead of 77.4) you will get +4.2 lbs when empty, which is closer to the listed number.

 

[decompression]

I think you nailed......also check to see if valves are or not included.

 

[guruboy]

the old LP72 numbers aren't adding up now

Typically quoted specs-
72 cu ft Standard
Service pressure: 2250 psi
Working pressure: 2475 psi (2250 psi +10%)
Actual air capacity: 71.2 ft3 (at a working pressure of 2475 psi)
Outer diameter: 6.9 in
Length without valve: 25.1 in
Empty weight: 26 lbs (w/o valve)
Buoyancy Empty: 0 lbs (w/valve)
Buoyancy Full: -5.4 lbs (w/valve)

Calculated -
Buoyancy Empty: +3.7 lbs (w/o valve)


A valve doesn't weigh +3.7 lbs...

 

[JamesBon92007]

What is your beginning reference? If the air weighs about 5.7 lbs and the buoyancy full is -5.4 I'm seeing that as about +.3 (3/10). Add the valve and you should be slightly negative. Add the real world (there still a little air in it) and you should be a pound or so negative. Perhaps they went further past the decimal point to get their air weight than I did.

 

[guruboy]

I don't know where the "typically quoted" specs originally came from. Just something another board member posted in another thread.

 

[JamesBon92007]

I don't know where the "typically quoted" specs originally came from. Just something another board member posted in another thread.

Those specs look right to me. A typical "72" cu/ft tank holds anywhere from about 71.1 to 71.4 cu/ft @ 2475 psi depending upon the manufacturer. And as you discovered a "typical" AL80 holds about 77.4 cu/ft @ 3000 psi. I think there is an "aluminum 80" that actually holds 80 cu/ft @ 3000 psi but it must be bigger than usual.

For practical purposes a typical steel 72 is usually somewhere around a pound negative at the end of a dive whereas a typical aluminum 80 is a few pounds positive. There are supposedly some AL80s that are neutral or slightly negative when empty but to me that would indicate that they weigh more to begin with. So, you either make the tank heavier or you add more weight to your belt.

Personally I like to fill my steel 72s to about 2650 which gives me about 76.25 cu/ft and the whole thing (including the weight belt) ends up weighing several pounds less and you get nearly the same amount of air as the aluminum 80.

 

[guruboy]

It sounds right but somehow the math doesn't work out.

Is the steel density from that online calculator wrong for this type of tank?

 

[Luis H]

You may find the data on this table useful.

Notice that the actual volume on my steel 72 will range from about 69.6 to 72.2 cubic feet.

The cylinders in water weight were measure in fresh water. The weight of the air inside the cylinder was accounted for using actual cylinder volumes (not published data). I measured the internal tank volume by accurately measuring the water capacity during the hydro test process.

The density of air at STP (Standard Temperature and Pressure) is 0.075 Lbs / cu ft.

Or 80 cu ft of air will weigh 6 pounds

 

[Luis H]

I suspect that the volume in aluminum 80 will also vary as much due to fabrication tolerances, but I have only measured two AL80 (I only own 3 AL80). One is a Luxfer and the air volume in that one came out to 80.6 cu ft. The other is a Catalina with an air volume of 80.7 cu ft.

I have always seen the published data of 77 cuft for an AL 80, so I suspected that I could be making an error in my readings or calculation. I checked my measurements and calculations several times.