Weight of the air in the tank.

[Alan Browne]

Just a math check, really.

If I'm sporting a 100 cu. tank, I assume that means 3000 PSI with 100 cu. ft. sea level air.

Per the great font of all knowledge temporal and otherwise (Wikipedia) air has a sea level density of

0.0765 Lbs/ft^3.

So, the air in there has a weight of 7.65 Lbs. (Is this correct)?

So, if I climb up on the boat at 500 PSI, then I've blown off 6.375 Lbs (is this correct)?

Thanks.

 

[Jiminy]

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

 

[michael-fisch]

Air weighs around 800g per 1000 liters..
Converting that to cubic feet works out to around 22.7g a cubic foot

Michael

 

[tursiops]

Just a math check, really.

If I'm sporting a 100 cu. tank, I assume that means 3000 PSI with 100 cu. ft. sea level air.

Per the great font of all knowledge temporal and otherwise (Wikipedia) air has a sea level density of

0.0765 Lbs/ft^3.

So, the air in there has a weight of 7.65 Lbs. (Is this correct)?

So, if I climb up on the boat at 500 PSI, then I've blown off 6.375 Lbs (is this correct)?

Thanks.

Yes...if your assumption is correct that it is 100 cuft at 3000 PSI. Most AL 100's, however are full at 3300 psi, so if you get back on the boat with 500 psi you've used 85% of your 100 cuft, and thus used 6.5 pounds of gas.

 

[JackD342]

Just a math check, really.

If I'm sporting a 100 cu. tank, I assume that means 3000 PSI with 100 cu. ft. sea level air.

Per the great font of all knowledge temporal and otherwise (Wikipedia) air has a sea level density of

0.0765 Lbs/ft^3.

So, the air in there has a weight of 7.65 Lbs. (Is this correct)?

So, if I climb up on the boat at 500 PSI, then I've blown off 6.375 Lbs (is this correct)?

Thanks.

The current 100cf aluminums I am familiar with have a 3300 psi service pressure.

 

[Basking Ridge Diver]

So, the air in there has a weight of 7.65 Lbs. (Is this correct)?

So, if I climb up on the boat at 500 PSI, then I've blown off 6.375 Lbs (is this correct)?

Yes both are correct.

 

[Alan Browne]

Thanks everyone.

As to the 3300 PSI, as much as that's true, at two resorts I've been to they've delivered them to the boat at 3000. Perhaps to save wear and tear on the regs? (Even though I brought my own...).

 

[JackD342]

Yes, very likely. But then it does not contain 100cf at 3000 psi, so account for that in your math if you require that level of precision.
And their fill station may have a regulator set at 3000 max which they don’t ever re-adjust as a matter of course.

 

[Alan Browne]

Yes, very likely. But then it does not contain 100cf at 3000 psi, so account for that in your math if you require that level of precision.
And their fill station may have a regulator set at 3000 max which they don’t ever re-adjust as a matter of course.

Doh! I really walked by that little bit of implication. . Thanks.

I'm trying to fine tune my weights down as much as possible on my next trip and want to understand the air weight as best possible. That 10% will have to figure in. I also expect I can ask for another 300 PSI on the fills. Maybe.

 

[tursiops]

Don't try too hard for precision. Remember you can control several pounds with just your lungs, breathing off nearly empty or nearly full lungs.
In round numbers, carry 5-6 extra pounds for the AL80, to account for the weight lost from it during the dive, and 6-7 pounds for the AL100. That means 6 pounds extra will probably be very close no matter your AL tank.
For steel tanks, it is the same, because the air weights the same.....what changes is not the extra weight you need at the beginning, but rather the base weight you are carrying in additional to the extra.