Txfirefighter4305
Guest
Side note...didn't a guy named Numo Gomes make it to 1000' on open circuit?
80cu Tank at 800 Feet ????
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I remember when they asked Gomes about his ascent rates, he chuckled and said "well over 100 ft per minute." I think he was hauling arse to get back up to a more sensible depth. IIRC, he also had the shakes pretty bad at depth, too. HPNS.
UCFKnightDiver
Contributor
9 hr Deco says VPlanner rofl. Using a bunch of different gasses Heliox, Trimixes, 50%, 100%etc
That's about what I came up with too, with some off the cuff settings.
Sorry, the .445 PSI/Ft in the above formula is the pressure per foot of sea water. You probably know that sea water weighs 64.1 Lbs/Ft³. If you take 64.1 Lbs divided by 144 Square inches/Ft² = .445 Lbs/Ft of sea water.
Example:
800' x .445 PSI = 356 PSI above sea level or
356 PSI / 14.7 PSI (per atmosphere) = 24.22 Atmospheres or ATM.
Add 1 atmosphere and you get Atomospheres Absolute or ATA
356 PSI / 14.7 PSI (per atmosphere) = 24.22 Atmospheres or ATM.
Add 1 atmosphere and you get Atomospheres Absolute or ATA
If you could get 80 Ft³ out of an 80 Ft³ bottle at 800', you would have a negative pressure of 356 PSI. That's some pretty hard sucking.
With a good quality balanced first stage, you need about the intermediate pressure in your tank, 135 PSI average, for the first stage to operate. You can suck it down to 100 PSI over bottom, but you will notice the resistance. By 80 PSI over bottom you are in serious distress. So at 800' you are effectively starting with a 491 PSI (356 + 135) lower pressure, and correspondingly lower effective volume, than at the surface.
This is true at any depth but is more in the margin of error at 100', or 44.5 PSI bottom pressure. Hopefully I typed all these numbers right and you understand.
Deep South Divers
Contributor
Edit: Oops, looks like I was viewing an old, cached version of this thread, and the question's been long answered, with new ones asked and answered. My fault, sorry. 
I was answering the question, "How to figure (ambient) pressure at that depth."
1 ATA = 14.7 psi, so...
25.24 ATAs = 371.06 psi. At 372 psi, you'd no longer be able to draw a breath at that depth. 'Course, your pressure gauge wouldn't be reading 372 psi, since it's also relative - it'd be reading zero.
The bottom line is that you'd have about 2628 psi of usable gas at that depth. Given that your average AL80 tank is actually 77.7 cubic feet of air, you're looking at about 68 cubic feet of usable gas, given a starting pressure of 3000 psi.
At a SAC (breathing) rate of .5 (average, skilled, and comfortable diver), one would use that in about 136 minutes at the surface.
Assuming nothing changes - stress level, exertion level, etc. - a diver with a SAC rate of .5 would burn through an AL80 in about 5.39 minutes (1/25.24*136=5.39), or about five minutes, 23 seconds.
This also assumes that the tank was not used to GET to 800 feet, and is thus full when you start breathing on it at 800 feet.
Interestingly, at the surface, the full AL80 would read 3000 psi. At depth, the full tank would read 2628 psi. The empty tank at depth would read zero psi, and the empty tank at the surface would read 372 psi.
Breathing air at 800 feet would almost certainly be lethal, by the way, so don't try it.
I was answering the question, "How to figure (ambient) pressure at that depth."
1 ATA = 14.7 psi, so...
25.24 ATAs = 371.06 psi. At 372 psi, you'd no longer be able to draw a breath at that depth. 'Course, your pressure gauge wouldn't be reading 372 psi, since it's also relative - it'd be reading zero.
The bottom line is that you'd have about 2628 psi of usable gas at that depth. Given that your average AL80 tank is actually 77.7 cubic feet of air, you're looking at about 68 cubic feet of usable gas, given a starting pressure of 3000 psi.
At a SAC (breathing) rate of .5 (average, skilled, and comfortable diver), one would use that in about 136 minutes at the surface.
Assuming nothing changes - stress level, exertion level, etc. - a diver with a SAC rate of .5 would burn through an AL80 in about 5.39 minutes (1/25.24*136=5.39), or about five minutes, 23 seconds.
This also assumes that the tank was not used to GET to 800 feet, and is thus full when you start breathing on it at 800 feet.
Interestingly, at the surface, the full AL80 would read 3000 psi. At depth, the full tank would read 2628 psi. The empty tank at depth would read zero psi, and the empty tank at the surface would read 372 psi.
Breathing air at 800 feet would almost certainly be lethal, by the way, so don't try it.
Sorry, the .445 PSI/Ft in the above formula is the pressure per foot of sea water. You probably know that sea water weighs 64.1 Lbs/Ft³. If you take 64.1 Lbs divided by 144 Square inches/Ft² = .445 Lbs/Ft of sea water.
Example:
800' x .445 PSI = 356 PSI above sea level or
356 PSI / 14.7 PSI (per atmosphere) = 24.22 Atmospheres or ATM.
Add 1 atmosphere and you get Atomospheres Absolute or ATA
If you could get 80 Ft³ out of an 80 Ft³ bottle at 800', you would have a negative pressure of 356 PSI. That's some pretty hard sucking.
With a good quality balanced first stage, you need about the intermediate pressure in your tank, 135 PSI average, for the first stage to operate. You can suck it down to 100 PSI over bottom, but you will notice the resistance. By 80 PSI over bottom you are in serious distress. So at 800' you are effectively starting with a 491 PSI (356 + 135) lower pressure, and correspondingly lower effective volume, than at the surface.
This is true at any depth but is more in the margin of error at 100', or 44.5 PSI bottom pressure. Hopefully I typed all these numbers right and you understand.
Whew, I'm tired. I had to read that 3 times before I caught your meaning.
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