Re-using air from BCD in dire emergency?

[Rick Murchison]

...I believe scuba gauges show the apparent pressure in a tank. (inside tank pressure - outside depth pressure= gauge pressure) at 220fsw the gauge pressure of a tank (not breathed on) should read 100psi less than it did at the surface...

That is correct.
Rick

 

[DareDevil]

...
Since there was not a significant difference between a 60 fpm ascent rate and a 30 fpm ascent rate, and since you are obviously not going to do a safety stop (which is what you would need to do to ake a real difference), there is no real point in worrying about the ascent rate difference when your real problem is that you are out of air and are primarily interested in coming up with some more.
...

I am confused.

So based on the ground there is no safety stop are you stating that a CESA at 60 fpm is safer than ascend at 30 fpm getting one-two breaths from regs/BCD/whatever?

Do you routinely ascend at 60 fpm? If that's the case, then by ascending at 60 fpm you aren't really doing anything different, except for avoiding a safety stop (assuming no-deco).

And if you do not routinely ascend at 60 fpm, that implies you do not consider that speed so safe, right?

DareDevil

 

[DCBC]

I believe scuba gauges show the apparent pressure in a tank. (inside tank pressure - outside depth pressure= gauge pressure) at 220fsw the gauge pressure of a tank (not breathed on) should read 100psi less than it did at the surface. to get a true pressure reading the gauge needs to be in a vacuum.

For the majority of pressure guages, the pressure being measured is the only source of energy required to provide a visual indication of static pressure. Some form of an elastic chamber inside the gauge case converts the pressure to motion, which is translated through suitable links, levers, and gearing into movement of a pointer across an indicating scale. External pressure is not a mitigating factor in this process. Its simply a function of a piston movement on a graded scale. The pressure guage does not measure usable pressure, rather cylinder pressure. If this were not the case a full scuba tank would read less than 0 psi at 1400 FSW. The 3000 psi may not be usable at that depth, but this doesn't take away from the amount of pressure that's actually inside the cylinder. The pressure guage measures pressure within the tank only.

This leaves "100psi" of (unusable) air in an "empty" tank at 220fsw. If you ascend to the surface that 100psi is now usable, and 100psi is plenty of air for about 2min of breathing.

The usable amount of mixture within the tank is a function of the first stage pressure setting to the 2nd stage. Depending upon the regulator, this will vary but for example I'll use 140 psi over ambient. If this was the case, at 220 feet the regulator would stop air delivery when the internal cylinder pressure dropped to approximately 650psi. As an aside, at 1000 FSW a 80 cu.ft. cylinder with 3000 psi will be unbreathable after 600 psi is reduced from the cylinder (enter the world of the saturation diver). This is why umbilicals and rebreathers are used.

If you had 100 psi of usable air i.e. ambient pressure + 140 psi (regulator setting) + 100 psi, the usable 100 psi would equate to a surface volume of approx. 2.7 cu. ft. or approx .7 cu. ft. ambient volume at 100 FSW. As a surface consumption rate in excess of 4 cu.ft. per minute is common for divers in an emergency (16 cu.ft. per minute at 100 ft) you will not have much breathable air available at this depth.

 

[Rick Murchison]

... External pressure is not a mitigating factor in this process. Its simply a function of a piston movement on a graded scale. The pressure guage does not measure usable pressure, rather cylinder pressure...

Ambient pressure outside the mechanism does affect the reading, and gauge pressure is the difference in the pressure being measured (internal pressure) and the pressure surrounding the gauge (ambient pressure)

If this were not the case a full scuba tank would read less than 0 psi at 1400 FSW...

No, at 1400 FSW it'll read about 2375 psi.; at 6800 FSW it'll read zero.
But don't take my word for it.
Take a tank with a SPG on it down to 140 FSW or so and purge the reg 'til the gauge reads (as precise as you can get it) zero. Close the valve and bring it back to the surface... Voilà... it'll read between 50 and 100 psi.
Rick

 

[boulderjohn]

I am confused.

So based on the ground there is no safety stop are you stating that a CESA at 60 fpm is safer than ascend at 30 fpm getting one-two breaths from regs/BCD/whatever?

I never said that. What I meant was that if I am OOA, I will ascend at 60 fpm so that I can be sure to reach the surface. The fact that it might be marginally safer from a DCS point of view (an improvement from say 99.92% safe to 99.96% safe) is not enough of a benefit for me to worry about in comparison to not reaching the surface when I am out of air. In this situation, the risk of DCS is very, very remote, but the risk associated with OOA is very, very real. I will not sacrifice the real threat for the benefit of the remote threat.

Similarly, if I am driving someone who is suffering from a heart attack to the hospital, I will likely risk driving a few miles per hour over the speed limit, deciding that the risk of a speeding ticked is less significant than the risk of the person dying. I will not, however speed to the point of creating a significant danger of crashing. (Just as I will not sprint to the surface with a CESA.)

As I said earlier, I will happily breathe the air from my regulator as I ascend should I need it. I will not, however, discard that valuable regulator to suck on my BCD inflator.

Do you routinely ascend at 60 fpm? If that's the case, then by ascending at 60 fpm you aren't really doing anything different, except for avoiding a safety stop (assuming no-deco).

I do not normally ascend at 60 fpm, but if I were to do so it would not concern me. Many people do ascend at 60 fpm, and, yes, if they were to do so on a CESA, it would be exactly the same as a normal ascent. In fact, PADI considers 60 fpm to be a normal ascent rate, and it advocates maintaining that ascent rate as an ideal for the CESA.

And if you do not routinely ascend at 60 fpm, that implies you do not consider that speed so safe, right?

I prefer vanilla ice cream. That does not mean I don't like chocolate.

More details on the DAN study:

They compared three ascent rates: 10 fpm, 30 fpm, and 60 fpm. 30 fpm had the best results in terms of bubble detection, and 10 fpm had the worst. For that reason, I prefer a 30 fpm ascent rate. The 60 fpm was not, however, determined to be unsafe. It is still an acceptable, safe ascent rate. The probability of getting DCS with either rate rounds off to 0%.

 

[DCBC]

Ambient pressure outside the mechanism does affect the reading, and gauge pressure is the difference in the pressure being measured (internal pressure) and the pressure surrounding the gauge (ambient pressure)No, at 1400 FSW it'll read about 2375 psi.; at 6800 FSW it'll read zero
Rick

Thanks Rick, your right about the depth (I made the same mental miscalculation as Dave). I'm not so sure about the 0 though. I thought that most spgs used a dual tube which provides the cylinder pressure regardless of absolute P2. I'll give your scenario a try.

 

[Teamcasa]

Ambient pressure outside the mechanism does affect the reading, and gauge pressure is the difference in the pressure being measured (internal pressure) and the pressure surrounding the gauge ...

Sure the SPG is affected by ambient pressure but any change to the actual pressure inside a Scuba tank would be hard to determine without extremely precise instruments (at recreational depths).

 

[Cave Diver]

(at recreational depths).

Man, if I've told you guys once, I've told you three times now. You're not going deep enough!

 

[pittyyofool]

For the majority of pressure guages, the pressure being measured is the only source of energy required to provide a visual indication of static pressure. Some form of an elastic chamber inside the gauge case converts the pressure to motion, which is translated through suitable links, levers, and gearing into movement of a pointer across an indicating scale. External pressure is not a mitigating factor in this process. Its simply a function of a piston movement on a graded scale. The pressure gauge does not measure usable pressure, rather cylinder pressure. If this were not the case a full scuba tank would read less than 0 psi at 1400 FSW. The 3000 psi may not be usable at that depth, but this doesn't take away from the amount of pressure that's actually inside the cylinder. The pressure gauge measures pressure within the tank only.

Only nice quality spg's have a Bourdon Tube in them, like what you are talking about. the everyday plastic oceanic/sheerwood gauge have a coil spring that flexes when pressure is applied. both types are subject to atmospheric changes.

does your gauge read 14.7psi when stowed in the bag?

The usable amount of mixture within the tank is a function of the first stage pressure setting to the 2nd stage. Depending upon the regulator, this will vary but for example I'll use 140 psi over ambient. If this was the case, at 220 feet the regulator would stop air delivery when the internal cylinder pressure dropped to approximately 650psi.

I have to disagree with your theory here, 140 psi over ambient is where the piston/diaphragm in your regulator closes and will not let more air threw. your regulator i is perfectly capable of operating below 140 psi over ambient, as demonstrated every time you take a breath. When the pressure on the LP side drops below 140 the piston/diaphragm opens. then closes when 140psi is reached. If "apparent" tank pressure drops below 140psi, then 140psi is not reached on the LP side your piston/diaphragm will just stay "open" and not close.

"open" technically closes when pressure is equal on HP/LP side.

If your theory is true, if you breath a tank "empty" on the surface until you can not pull another breath out of your regs. there will still be 140psi in the tank when you attach a tank gauge.

go in your garage ant test it and let me know the answer. I know that when my tanks are "empty" at the surface they are EMPTY. but not really they still have 14.7 psi in them...

I just tested it and actually just sucked one on my tanks in to a slight vacuum.

 

[Rick Murchison]

... I have to disagree with your theory here, 140 psi over ambient is where the piston/diaphragm in your regulator closes...

This is correct. IP is a valve closing, not a valve opening pressure. Once tank pressure drops below the IP setting, the first stage is wide open and will allow whatever pressure remains in the tank (over ambient) into the hoses leading to the second stages and the BC. At that point the keen observer will notice a decrease in air delivery efficiency no matter how finely tuned or high performance the second stage until it gets very hard to breathe, then impossible. With my old Conshelf the decrease is attention getting just as soon as tank pressure drops below IP; with my Legend it's barely noticeable until the final few breaths. (Translation: If I'm at 100 FSW I still have about two minutes usable gas when I first notice it with the Conshelf; with the Legend I'll be lucky to notice before I'm down to about 40 seconds) (DO NOT TRY TO VERIFY THIS ONE ON YOUR OWN at depth! Trust me. If you must breathe a tank dry, make sure it isn't the one on your back, and that you have plenty of air in that one.)
Rick