19 cf pony equal to what PSI in AL80?

[dumpsterDiver]

Originally Posted by Kharon
I'm sure I have breathed tanks down to zero - at least that's what the guage showed on the surface and purging produced no air flow. Yeah, I know that there is error in what (at least some) guages show. However, if you couldn't breathe any of the last 500 psi the recommendation (requirement in some places) to board/exit wouldn't be 500 psi - it would probably be 1000psi. You certainly can breathe more than 8.9 cf of a 13cf pony and it doesn't take a $700 reg to do it. I've breathed one dry just to see what it felt like with a Cressi AC2/XS2 reg set (about $150).



Before this, I hadn't thought of tank volumes in near as much detail before and this is really interesting to me! I think what John and DoctorMike are referring to is the fact that there is measurable volume in the tank that simply can't be used. i.e. The 4.1 or so cubic feet of air over the 8.9 usable is simply the base volume of the tank at atmospheric pressure. Hence DoctorMike's inclusion of the phrase "Lab grade vacuum"

No that is not what they are thinking. They are saying that a tank can't deliver useful air as it drops down to low pressures of a few hundred lbs. This is NOT really true. Don't believe me!

Empty a tank down to 100 psi on the gauge and then (on land) try to slowly breath it... You will get a lot of useable air.

A tank delivers usable air down to essentially zero psi over ambient pressure, the problem with planning this is that many gauges are not so accurate at the bottom of the scale... so if the gauge says 200 psi it may actually be 50 psi. One GOOD reality check when you are getting a tank very low is to inhale as fast and as hard as you can. If you feel resistance at the end of the inhalation, the tank is getting low- regardless of what the gauge says.

 

[doctormike]

No that is not what they are thinking. They are saying that a tank can't deliver useful air as it drops down to low pressures of a few hundred lbs. This is NOT really true. Don't believe me!

Empty a tank down to 100 psi on the gauge and then (on land) try to slowly breath it... You will get a lot of useable air.

A tank delivers usable air down to essentially zero psi over ambient pressure, the problem with planning this is that many gauges are not so accurate at the bottom of the scale... so if the gauge says 200 psi it may actually be 50 psi. One GOOD reality check when you are getting a tank very low is to inhale as fast and as hard as you can. If you feel resistance at the end of the inhalation, the tank is getting low- regardless of what the gauge says.

Well, we are sort of getting beyond the scope of stuff that is useful when diving, but unless you are generating a vacuum, you can't draw gas out of a rigid container below ambient pressure, no matter what kind of regulator you have.

And yes, gauges are notoriously inaccurate at the low end of the scale, so that's a practical consideration which is relevant to diving.

And if you are at the surface, and if you have a precisely accurate SPG, and it says 100 PSI, then you should be able to breathe off that tank since the tank pressure is higher than the 14 PSI surrounding you.

I forget what we were talking about.