Propane tank volume

Does anyone know the EXACT cubic capacity of air at ambient atmospheric pressure for a 20# propane tank? That is, a tank that has the valve opened, then left open until all the contents have gone out, and still has that empty space. I need to figure out the underwater lifting capacity of one of these tanks. And yes, I know I should shut the valve.
Steve

Since you want the lifting capacity, measure that directly, not round-about with volume, tank weight, & math. Jim was on the right track, if facetiously: water in a trash can & immerse the propane tank. Measure the force required to hold it under. Bathroom scale should be good enough.
KISS!
Bob

On 28/04/2012 05:18, Steve B wrote:
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Once you get your volume, don't forget you'll get a bit more upthrust if it is in seawater.

Less bouyancy if the tank is made of metal.
Christopher A. Young Learn more about Jesus www.lds.org .
On 28/04/2012 05:18, Steve B wrote:
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Once you get your volume, don't forget you'll get a bit more upthrust if it is in seawater.

Stormin Mormon submitted this idea :
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Please explain just a little. What has metal got to do with change in bouancy in sea water?
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If the tank is made of metal, it will tend to sink. Thus countering some of the air bouyancy.
Christopher A. Young Learn more about Jesus www.lds.org .
Stormin Mormon submitted this idea :
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Please explain just a little. What has metal got to do with change in bouancy in sea water?

Steve B wrote:
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http://norfolk.craigslist.org/rvs/2984196085.html

john wrote:
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New-Never Used- Propane gas cylinders steel 20 pounds capacity With 10 percent valve for overfill protection, Poly safety plug. Quality construction and a durable polyurethane finish. Specifications: Size 6-1/2 inch x 12-3/8 inch x 18-1/16 inch . LP-Gas capacity (approximate gallons) 5, water capacity 47.7 pounds, tare weight 18.5 pounds, cylinder volume 1,323 cubic inches, collar height 4 inch , footing diameter outside 7-13/16 inch . Manufactured in accordance with DOT-4BA-240.

47.7 pounds capacity (bouyancy) - 18.5 pounds of steel weight, = 29.2 pounds lift. Of course, there are some other factors, but they are minor.
Christopher A. Young Learn more about Jesus www.lds.org .
water capacity 47.7 pounds, tare weight 18.5 pounds,

Stormin Mormon wrote:
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In water the steel weights less. You have to figure the volume of the steel and then add the weight of water for that volume.
John

wrote in message
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Steel's density is 7.85, roughly 8, so the buoyancy is the weight of water the tank can hold minus ~7/8 of the empty weight.
jsw

That's about what I would have said.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote in message
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In water the steel weights less. You have to figure the volume of the steel and then add the weight of water for that volume.
John