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Weight & Buoyancy of Scuba Tanks March 14, 2009

Posted by Chris Sullivan in Technical Diving.
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I’ve seen some interesting misconceptions around the web and even in books about the weight of a tank and would like to take this opportunity to clear it up (hopefully!). Today I’m going to use Aluminum (or Aluminium if you prefer) tanks as examples, and save the discussion on steel tanks to a later date.

I have a couple of Catalina AL80s. I’ve heard from the owner of my local dive shop that the Catalinas are more top heavy than the Luxfers of the same capacity. I haven’t tested this myself, although when I dropped (this is a figure of speech denoting placing a tank somewhere underwater for retrieval later, not an accident) one my Catalinas on the deck of a shipwreck in sat in an almost inverted position. I’d like to try the same thing with a Luxfer sometime, but I suspect given the weight of the regulator it would do exactly the same thing.

These charts show the specs of Catalina Aluminum tanks (they don’t make steel ones) of various capacities, with the top chart listing tanks from an AL50 to an AL100. The actual cubic footage is slightly less than the cylinder names would suggest, with the AL50 having 48.4 cubic feet, the AL80 having 77.4, and the AL100 at 98.8 cubic feet. Take note of the various in-water buoyancies. In the AL80, a full tank is -1.6 pounds but at 500 PSI it’s +2.8 pounds, a difference of 4.4 pounds. The ALN80, runs from -5.9 to -1.4 pounds, a difference of 4.5 pounds.

Notice that the half-full pressure for the ALN80 is listed as the same as the AL80 at +0.2 pounds. Do you find that curious? You should because the chart is incorrect and showing an impossible situation! By the time you read this it may have been corrected as I let the web site owner know about the mistake.

Why is there a slight difference between the two tanks? You could put it down to rounding error, which might be a factor, as the difference is only .1, which is at the limit of the precision of the information. There is another factor as well. On the ALN80, more air is consumed by the time the tank is down to 500 PSI, as the working pressure is 200PSI higher at 3200PSI, vs. the 3000PSI of the standard AL80. With a capacity of 77.4 cubic feet the AL80 has 77.4 x 2500/3000 PSI left while the ALN80 has 77.4 x 2500/3200 PSI left, which is 64.5 and 60.5 cubic feet respectively. So 4 extra cubic feet have been consumed in the ALN80. You’ll see later that this indicates that there is another error in the chart.

The difference will also depend on the temperature of the tank when the pressure was measured. Tank temperatures become important with technical diving because gas planning is that much more critical. Without getting into that now, what is the air density?

Let’s use the AL80 and a difference of  4.4 pounds between 3000 and 500 PSI. To get the density of air we need to divide the change in buoyancy by the change in cubic feet of air which is 64.5 cubic feet. So 4.4 pounds divided by 64.5 cubic feet is .068 pounds (1.09oz)  per cubic foot of air. So the problem I have with the ALN80 specs is that 4 cubic feet less at 500PSI should result in almost .3 pounds more change in buoyancy, while only .1 change is listed on the chart. This discrepancy can’t be due to rounding error alone. Of course, the ALN80 may be right in this regard and the AL80 wrong.

The chart isn’t clear on the specification of the empty weight of the tank as to whether it includes the valve or not. Let’s assume it does. The AL80 has an empty weight of 31.3 pounds. Underwater, the empty buoyancy is the 500 PSI buoyancy of +2.8 pounds plus the weight of 500PSI worth of air.  500 PSI of air weighs .7 pounds so the empty tank is 3.5 pounds buoyant (in sea water!). The tank displaces 31.3 – 3.5 pounds of sea water (27.8 pounds), which divided by 64 pounds per square foot means that the  volume/displacement of the tank is .43 cubic feet.

This same tank displaces .43 x 62.4 pounds of fresh water, or 26.8 pounds, about a pound less than in sea water. So it is still positively buoyant at 500 PSI, as anyone who has slung one on his or her BC would know.

The important thing to remember, and something that the book got wrong, is that a cubic foot of air weighs the same in a steel tank as it does in an aluminum one. A cubic foot weighs about 1.1 oz in any vessel. You need to plan for this weight difference accordingly so you are not overly buoyant on your safety or decompression stops. When you’re carrying multiple high-capacity tanks, this difference can be 20 pounds or more.

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Comments»

deepstop - February 27, 2010

Hi Nick,

Couldn’t really tell you off hand. My shop sells the 80′ for a bit more than $200, depending on the type of valve. Steel 95s are more like $400. Depends a lot on the exchange rates. As it’s not worth shipping such heavy items across the ocean, you might as well look locally. Dive shop would be happy to help, I’m sure, and there’s lots of info you can find with Google.

Cheers,
Chris

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1. scambuster - May 26, 2010

Warning: Nick Brown is a fraudster from Ghana. IP address 41.210.10.27.

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deepstop - May 28, 2010

The comment did seem a bit fishy, but if there was bait in his comment I didn’t take it. Thanks for the warning though. I’ve deleted it as it didn’t contribute to the discussion anyway.

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