Tank buoyancy numbers

[emoreira]

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.

A Luxfer S80 when empty is positive even with the valve threaded. This is even true in fresh water.
A S80 almost empty with the reg connected will float upside down in a pool.
Done this hundreds of times during class.

 

[JamesBon92007]

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.

That's very interesting. I've never actually tried to measure the capacity of a tank, I just believed the specs from the manufacturers. According to what you have posted the steel 72s are a little smaller than what I've read in the past and the aluminum 80 is bigger. You are causing me to question my faith in manufacturers specs.

 

[JamesBon92007]

A Luxfer S80 when empty is positive even with the valve threaded. This is even true in fresh water.
A S80 almost empty with the reg connected will float upside down in a pool.
Done this hundreds of times during class.

Oops, I was referring to a steel 72. I'd better go back and read what I responded to. I've never even used a steel 80 so I in no position to say one way or the other.

 

[emoreira]

Oops, I was referring to a steel 72. I'd better go back and read what I responded to. I've never even used a steel 80 so I in no position to say one way or the other.

Luxfer S80 is an Alu tank.
AFAIK Luxfer only manufactures Alu tanks.
http://www.luxfercylinders.com/products/scuba-cylinders/l6x-aluminium-cylinder

 

[Luis H]

That's very interesting. I've never actually tried to measure the capacity of a tank, I just believed the specs from the manufacturers. According to what you have posted the steel 72s are a little smaller than what I've read in the past and the aluminum 80 is bigger. You are causing me to question my faith in manufacturers specs.

If you take a look at the manufacturing process for pressure cylinders, you may be able to see why there is such a variation in tolerances. The metal is pressed and form into dies using very high forming pressures in order to yield the material into shape. They are not making Swiss watches.

In the case of steel 72, I measured 18 of them from two manufacturers, PST and Norris.
Steel 72 average volume: 70.5 cu ft
Max: 72.2 cu ft
Min: 69.6 cu ft
Standard deviation: 0.68

From the limited sampling size that I have, I could easily round the 70.5 cu ft and say that they average about 71 cu ft. That would be a reasonable round-up, that is just taking a rough average.

What I do find very puzzling, even annoying, is that they would say that a steel 72 has an actual volume of 71.2 cu ft. The use of 3 significant digits normally implies a higher precision than what is reasonable with these cylinders.

Manufacturing pressure vessels is not as precise as for example the machined parts that make a regulator first stage. The first stage parts have tight enough tolerances to fit together and be able to seal using tight O-ring tolerance.

My wife has two HP80 (made by PST), they both have a air volume capacity of about 85 cu ft. But, they were advertised as 80 cu ft.

 

[JamesBon92007]

From the limited sampling size that I have, I could easily round the 70.5 cu ft and say that they average about 71 cu ft. That would be a reasonable round-up, that is just taking a rough average.

What I do find very puzzling, even annoying, is that they would say that a steel 72 has an actual volume of 71.2 cu ft. The use of 3 significant digits normally implies a higher precision than what is reasonable with these cylinders.
.

I've been using 71.2 as the supposed "average" when calculating. It looks like it's time to change that to 70.5.

I have an old sportsways steel 72 with a 1/2" valve and the only way I know it is smaller than my other tanks is because it takes a couple of minutes less to fill. I'm guessing it is one of the 69.6 tanks.

 

[JamesBon92007]

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. .

A Luxfer S80 when empty is positive even with the valve threaded. This is even true in fresh water.
.

I think I missed something. I agree that it is positive.

When I said "practical purposes" I'm assuming that a valve is attached, a regulator is attached, and there is somewhere around 500 psi is the tank (but those of us who use steel 72s know it's probably goint to be less air than that ) I suspect that some of my molded-plastic harnesses might be slightly positive but I've never checked to see if they float without anything attached.

 

[Kharon]

Unlike most other LPs, 72s are rated 3AA2250 and have an authorized fill pressure of 2475 with the 10% + rating. They approach the rated capacity of AL80s when the LP72 is filled to around 2700 PSI.

Perhaps there are/were other steel 72s that had that rated pressure but all of mine clearly have "2250 stamped on them. Add 10% and that gives you 2475."

My PST steel 72's must be different from what ever you have. Mine say 2400 which gives 2640 with the 10%.

 

[guruboy]

What year were they made?

 

[2airishuman]

My PST steel 72's must be different from what ever you have. Mine say 2400 which gives 2640 with the 10%.

I have 11 LP72s manufactured between 1962 and 1978 by PST and Norris. I used to have more, including some made by Kidde which I have since sold due to local difficulties in getting the hydro shops to issue a + rating for them. They all bear a service rating of 3AA2250.

There are LP85s from several makers that differ only slightly in size from LP72s -- about 0.10 inch more in diameter and an inch higher. They bear a service rating of 3AA2400. I have a Faber one.