Post by SeaRat on May 20, 2015 21:51:06 GMT -8
Eban,
You bring up some interesting points. But remember that you cannot get from 100 feet to 10 feet instantaneously. I suspect that if you stayed at 10 feet, and breathed down to the reserve, then pulled the reserve rod, you would have approximately the same amount of air pressure in the tanks that you would have if you breathed the tanks down to the reserve, pulled the reserve rod at 100 feet, then ascended at <60 feet per minute to ten feet and looked at your gauge. (I know, it's a long sentence.) This is because of the depth compensation of the reserve. The "extra" 44 psig at 100 feet actually is not available to the diver, as the ambient pressure is exactly that pressure. That is why in the ol' days we talked about "surface air consumption rate," then made a conversion to the depth that we were at. I have a paper titled "Computing Bottom Time Air Consumption Rate" that I put together many years ago (1970s) that I'll scan and include here, which explains it a bit better than I did above.
In the same way, using the universal gas laws, it doesn't make any difference whether we are calculating using 38 cubic feet at 1800 psig or 42.2 cubic feet at 2000 psig as the internal volume of the tank does not change. (Note that your calculation of 41.9 cubic feet in the tank at 2000 psig is not quite correct, and it should be 42.2 cubic feet, not that you'd notice the difference in the calculation when it the water.)
The calculation I did above came out to 12.66 cubic feet of air for a 300 psig reserve, center mounted (off both cylinders) of a 38 cubic foot tank at 1800 psig. This would use a Calypso-J, or a Scubair 300 (sonic) reserve center-mounted starting at 300 psig. These are surface calculations.
Your reserve, with a double manifold at 500 psig on your 38 cubic foot tank (single tank) at 1800 psig would amount to 10.55 cubic feet of air. Again, this is a surface calculation.
To do the math on the depth, you need to use the absolute pressure. So that would be 100 feet / 33 feet per atm = 3 atm x 14.7 psi/atm = 44.54 psi + 14.7 psi (atmospheric, to convert to absolute) = 59.25 psi. (Note also that there is a difference between sea water and fresh water for depth per atmosphere.)
Now, I'm pretty sure we have mucked up the waters on these calculations. My apologies.
One very interesting result from these calculations is that there is more reserve air using your 38 cubic foot doubles than is in the 50 cubic foot, 3000 psig twins I used to use. I think this is because the internal volume of the 38 cubic foot tank is actually greater than the internal volume of the AL 3000 psig 50 cubic foot tank.
John
You bring up some interesting points. But remember that you cannot get from 100 feet to 10 feet instantaneously. I suspect that if you stayed at 10 feet, and breathed down to the reserve, then pulled the reserve rod, you would have approximately the same amount of air pressure in the tanks that you would have if you breathed the tanks down to the reserve, pulled the reserve rod at 100 feet, then ascended at <60 feet per minute to ten feet and looked at your gauge. (I know, it's a long sentence.) This is because of the depth compensation of the reserve. The "extra" 44 psig at 100 feet actually is not available to the diver, as the ambient pressure is exactly that pressure. That is why in the ol' days we talked about "surface air consumption rate," then made a conversion to the depth that we were at. I have a paper titled "Computing Bottom Time Air Consumption Rate" that I put together many years ago (1970s) that I'll scan and include here, which explains it a bit better than I did above.
In the same way, using the universal gas laws, it doesn't make any difference whether we are calculating using 38 cubic feet at 1800 psig or 42.2 cubic feet at 2000 psig as the internal volume of the tank does not change. (Note that your calculation of 41.9 cubic feet in the tank at 2000 psig is not quite correct, and it should be 42.2 cubic feet, not that you'd notice the difference in the calculation when it the water.)
The calculation I did above came out to 12.66 cubic feet of air for a 300 psig reserve, center mounted (off both cylinders) of a 38 cubic foot tank at 1800 psig. This would use a Calypso-J, or a Scubair 300 (sonic) reserve center-mounted starting at 300 psig. These are surface calculations.
Your reserve, with a double manifold at 500 psig on your 38 cubic foot tank (single tank) at 1800 psig would amount to 10.55 cubic feet of air. Again, this is a surface calculation.
To do the math on the depth, you need to use the absolute pressure. So that would be 100 feet / 33 feet per atm = 3 atm x 14.7 psi/atm = 44.54 psi + 14.7 psi (atmospheric, to convert to absolute) = 59.25 psi. (Note also that there is a difference between sea water and fresh water for depth per atmosphere.)
Now, I'm pretty sure we have mucked up the waters on these calculations. My apologies.
One very interesting result from these calculations is that there is more reserve air using your 38 cubic foot doubles than is in the 50 cubic foot, 3000 psig twins I used to use. I think this is because the internal volume of the 38 cubic foot tank is actually greater than the internal volume of the AL 3000 psig 50 cubic foot tank.
John