80cu Tank at 800 Feet ????

[speedyal]

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.

Ok so when you use the term sac at .5 your saying the average is .5 sac per minute or breath ?

 

[Deep South Divers]

Seal the end and stick it on the end of a long fishing line.

I'd have to be fair - I'll fill one of my argon bottles with air, put a full reg set on it, pressurize the system and drop it to 1000 feet and see what happens. I have a couple of gauges sitting around anyway... Wet inside, reading wrong, or whatever. Could still use them to test the theory, though. It'd be an interesting experiment...

Now I've gotta think of a way to film it and put it on YouTube. It'd be cool to watch something like that implode, if it actually would.

 

[Deep South Divers]

Ok so when you use the term sac at .5 your saying the average is .5 sac per minute or breath ?

Sorry! SAC = "surface air consumption." It's measured in cubic feet of gas used. A SAC rate of "1" means that a person uses one cubic foot of air per minute at the surface. A SAC rate of ".5" means that a person uses half a cubic foot of air per minute at the surface. Most divers are actually higher than .5, but I've seen some as low as .3. A newbie usually has a SAC rate of at least 1.0 - sometimes as high as 3.0.

 

[Akimbo]

... You sure that an SPG would crush? Pressurized internals wouldn't, 'cause they would be pressurized. Most of the rest of gauges are wet and pressure-neutral. Are you just talking about the actual gauge face itself? You could simply use a common oil-filled... Assuming that a standard brass & glass wouldn't handle 375 psi of pressure. I don't know if it would or not....

Every SPG I have ever seen:
http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/340231-operating-depth-spg.html

The space between the housing and the bourdon tube would be at 1 atmosphere -- unless you had it in the chamber. Then the Helium would have leaked in through the glass. But that is never done since the glass face or the blowout plug would go on decompression.

You know the story how Rolex patented the helium relief valve on watches? Allow me to bore you anyway.

During some early saturation experiments at the old Navy Experimental Diving Unit in Washington a couple of guys were decompressing. The chamber was small so they were sitting on a bench with their hands folded between their knees. All of a sudden the face of a Rolex watch blew out and hit a guy's inside thigh hard enough to make a serious edema, besides hurting like hell.

They did some research and found that Helium molecules will leak through all known transparent materials. The gas slowly leaked into the watch, which could easily take the external pressure but was never designed for internal pressure. They called Rolex, they put a tiny relief valve in a new watch with a deeper rating, named it the Sea Dweller, and called their Patent lawyers. The 17 year patent is long expired now.

The guy with a 1½" round scar on his leg was one of the Diving Officers when I was getting qualified, several years latter.

 

[speedyal]

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.

how are you getting the .445 again ? and how long will a tank last based on your math ?

 

[Deep South Divers]

During some early saturation experiments at the old Navy Experimental Diving Unit in Washington a couple of guys were decompressing. The chamber was small so they were sitting on a bench with their hands folded between their knees. All of a sudden the face of a Rolex watch blew out and hit a guy's inside thigh hard enough to make a serious edema, besides hurting like hell.

They did some research and found that Helium molecules will leak through all known transparent materials. The gas slowly leaked into the watch, which could easily take the external pressure but was never designed for internal pressure. They called Rolex, they put a tiny relief valve in a new watch with a deeper rating, named it the Sea Dweller, and called their Patent lawyers. The 17 year patent is long expired now.

The guy with a 1½" round scar on his leg was one of the Diving Officers when I was getting qualified, several years latter.

Cool story! Neat how small the world really is.

I can tell you that brass & glass SPGs made by Thermo (DIR Zone, Halcyon, OMS, HOG, Dive Rite, etc.) are regularly taken to the 475' range without anything weird happening with them.

Dunno about 800' though...

Your super-cool pressure gauge rated to "Mariana Trench" is way badass, though... Definitely Rule 6 compliant... Gorgeous machining...

 

[Akimbo]

how are you getting the .445 again ? and how long will a tank last based on your math ?

Imagine a 12" x 12" x 12" container full of sea water. That water weighs 64.1 Lbs. Therefore, it would exert a pressure of 64 Lbs Per Square Foot on the bottom of the container. Since there are 144 square inches in a square foot, it exerts 0.445138888 Pounds Per Square Inch (PSI) (64.1 divided by 144) on the bottom of the container -- I figure .445 is close enough. This container scales up the same as the Pacific Ocean.

I used .445 PSI/Ft of sea water for months before back calculating it like this. It is a little more accurate than using 33' of sea water for one atmosphere, but both are close enough for calculating partial pressures.

We can get really precise and adjust for salinity and temperature, use a more precise weight of sea water, and a lot more decimal points.

 

[Deep South Divers]

Since I can't find where anyone's actually asked... Why the questions, speedyal? Just a mental exercise? Have a buddy that claimed to have used a single AL80 at 800' for 20 minutes? Planning on doing an 800' dive sometime in the near future?

 

[RJP]

how are you getting the .445 again ? and how long will a tank last based on your math ?

Wouldn't you just rather see a tank explode instead?

:cool2:

 

[speedyal]

Since I can't find where anyone's actually asked... Why the questions, speedyal? Just a mental exercise? Have a buddy that claimed to have used a single AL80 at 800' for 20 minutes? Planning on doing an 800' dive sometime in the near future?

Just interested in the science of diving think i may start working on a dive calculator shortly :cool2: thanks for all the help guys and i do have a buddy who claims 480 feet on a single lol