How much is really left in tank?

[treidm]

Thanks Bubbletrubble....Yes will account for the at depth rate, again sorry I don't include more info, explaining everything in greater detail.

The numbers were just arbitrary rounded numbers, my RMV is lower than 1, but I do know some new divers can have quite high RMV's, until they learn to be comfortable.

I really appreciate all the input provided!

....Reid

 

[TSandM]

I have been truly astonished at the rate at which new or stressed divers can go through gas. I would seriously hope that, once the initial problem was controlled, MY gas consumption would fall back toward normal . . . but I know it's quite possible that 2 cfm is not nearly conservative enough for a certain kind of buddy under stress. But you have to come up with something, because you otherwise could just start setting aside more and more and more gas. I think using 2 cfm for each of the divers is a HUGE safety margin, since I'd have to triple mine to get to 1 . . . so there's a lot of "my" 2 cfm for the other person to use.

 

[RJP]

I use a spreadsheet with precision as high as 10 decimal places...

 

[Akimbo]

…I use a spreadsheet with precision as high as 10 decimal places...

Why wouldn’t you use a the highest precision you can copy and paste? It isn’t like I am using a slide rule or punching it into a calculator for every answer.

Seriously, the spreadsheet has a single set of conversion factors and constants used by all formulas. It covers everything from calculating usable gas (in a bail-out or onboard), Nitrox MOD, partial pressure mixing cylinders from 15-35,000 Ft³, the weight change of a helium tube trailer, to the buoyancy of a camera housing or a manned submersible. Rather than round down for each situation, I just round the bottom line value to an appropriate precision for the application. I am [-]a lazy SOB[/-] an efficient guy!

 

[farsidefan1]

Next question: How far can you lean out the window before you fall? That is What I thought when I first saw the question but after more reading I'd like to thank the op. Some good food for thought in here. I've always been a 500 psi kind of guy. It's good to calculate emergency bailout air supply, Of course my wife/buddy doesn't use any air so 2X0 = 0, guess I can just plan for myself

 

[Rick Murchison]

...when the pressure in the tank is less than 140 psi above ambient, the first stage will fail to open...

I'm surprised no one has jumped on this basic misconception... when tank pressure drops to intermediate pressure the first stage valve is wide open and stays that way. If you'd like to see how hard it is to get that very last drop of gas from the tank just take the dust-cover off your first stage and inhale through the second stage without attaching the reg to the tank. This will show you the total breathing resistance of the system (well, almost... you still have to suck the air out of the tank through that little hole in the valve and get the last back through that tiny little hole in the HP hose fitting).
As for the OP's question, assuming you have the patience and the regulator to breathe the tank dry (see above), zero gas is lost in the hoses, for they are filled to ambient pressure before being hooked up to the tank and will finish at ambient pressure when the tank is empty. You do lose usable gas due to depth, however, at the rate of a tankful (+ hoses) per atmosphere of pressure... about 45 psi (or 1.2CF for an AL80) at 100 FSW. To take it to the extreme, an AL80 that is full on the surface is empty at ~6600 FSW. Open the valve at that depth and you get nothing out of the tank. You get that gas back as you return to the surface and ambient pressure drops to one ATA.

Rick

 

[Akimbo]

I have been truly astonished at the rate at which new or stressed divers can go through gas. I would seriously hope that, once the initial problem was controlled, MY gas consumption would fall back toward normal . . . but I know it's quite possible that 2 cfm is not nearly conservative enough for a certain kind of buddy under stress. ...

The convention for estimating gas supplies in commercial diving “on demand” is 1.5 CFM (x working depth/33+1 or equal) for working divers. Systems are designed for peak flow rates to support freeflow at 6 CFM. Easy jobs end with extra gas on deck, but it is a pretty close average on most jobs. The point is peak consumption is waay above 1.5 CFM for significant periods on demand since their gas consumption at rest is similar to Scuba SAC rates when they are relaxed and waiting for something… a condition project managers work very hard to avoid.

For those unfamiliar with commercial gear, freeflow is the normal intentional continuous flow of air (rarely mixed gas for more than a few seconds) into a helmet or mask through the air control valve. It does not refer to a failed second stage regulator. “Air Hats” don’t even have demand regulators.

 

[Akimbo]

..; when the pressure in the tank is less than 140 psi above ambient, the first stage will fail to open...

I'm surprised no one has jumped on this basic misconception... when tank pressure drops to intermediate pressure the first stage valve is wide open and stays that way…

“Fail open” may have been a poor choice of words, but aren’t you both saying the first stage valve is open when supply pressure drops to the IP? The first stage is “normally open” when supply pressure is equal to or less than the Intermediate Pressure. Like your test suggests, sucking on a regulator with dust cap removed demonstrates it.

 

[TSandM]

Rick, thank you, that was my misstatement. The first stage doesn't fail to open; gas simply has no impetus to flow out of the tank, as the pressure at the other end of the system is the same as the pressure inside.

Thank you for correcting me.

 

[Akimbo]

Rick, thank you, that was my misstatement. The first stage doesn't fail to open; gas simply has no impetus to flow out of the tank, as the pressure at the other end of the system is the same as the pressure inside.

Thank you for correcting me.

Actually, gas will flow until the cylinder pressure matches the ambient pressure. It is just so darn slow after reaching the IP that you have to suck your brains out to get gas out fast enough to breathe. More accurately, a balanced diaphragm or flow through piston first stage performs that way starting very close to the IP. Unbalanced piston first stages gradually increase noticeable breathing resistance starting 300-500 PSI over bottom pressure. This is a condition where excellent performance actually reduces safety in most recreational diving situations.