Tank buoyancy numbers

[Rred]

Does anyone know if the scuba specs tables online are giving salt water buoyancy numbers?

If the web page you are looking at says "buoyancy in salt water" it is. If it doesn't, assume nothing. There are also tables with typos, tables that list salt and fresh, tables that don't say which brand of tank they are referring to--and some vary by maker.

 

[guruboy]

Dove my LP85 today. According to the tank buoyancy calculator (linked in the first post), the LP85 should be +2.5# buoyant when empty (no valve, fresh). However, if my weighting for an AL80 is correct, the LP85 is actually 0# buoyant when empty (no valve, fresh).

Will take the tank to get weighed tomorrow to see if the weight differs from listed specs.

If it does, it seems that 1-2# (possibly more) deviation from quoted specs is possible for tanks. This fact is corroborated by Luis' LP72 tank data as well.

Seems that tank manufacturing has rather loose tolerances..

 

[Rred]

Note that the calculator is just using tank sizes. It does not let you input who the manufacturer is, and tanks of the same size, from different manufacturers, are known to vary in comparative buoyancy and weight.

 

[guruboy]

Note that the calculator is just using tank sizes. It does not let you input who the manufacturer is, and tanks of the same size, from different manufacturers, are known to vary in comparative buoyancy and weight.

That's why I plan to weigh it, but I don't have any convenient way of measuring actual capacity.

 

[Rred]

What, you don't have a handy calibrated burette tube, so you can remove the valve and water-fill the tank? (G)
Actually, a hydro shop *ought* to be able to tell you that during the next hydro, if you tape a big note on the tank and drop off a box of donuts or a case of beer before they get around to it. Since THEY do have the equipment, and they're required to use it.

 

[guruboy]

I wonder if it's recorded in their test data log. I'll ask next time I'm there.

 

[Rred]

I think that's part of the data that has to be logged. After all, you can't tell how far the tank has stretched unless you run the numbers. As to where they have "old" log books and how motivated they would be to look it up...

Bring pizza and beer.(G)

 

[guruboy]

I don't think they record it, as the guy fills them with water prior to testing.

However, I did just realize I can calculate capacity by measuring the weight of the tank it at pressure and when empty.

 

[Rred]

I may have it backwards but in order to record the expansion and contraction, I thought they filled the tank with a calibrated amount of water (since all tanks aren't the same) and then measured the amount that had to be added after bringing them up to pressure. In which case, you'd have to log to CC's of water before and after, in order to show that tank passed.

Dunno, I never purchased tickets to that show.(G)

 

[Bob DBF]

At hydro they fill the tank and hydro system to a level, then they pressurize and depressurize and measure the new level. If it is inside the normal range for the tank it passes.


Bob