Physics - surface interval discussion

[Kaltim Diver]

Hi guys. During a recent surface interval between dives, and following some deeper trimix dives, the following question came up:

"If a diver was at 100m, and the gas in his/her tanks got as low as 10 Bar - would the diver be able to breath that gas or would the ATA be too high?"

KTD

 

[Kingpatzer]

Not for long . . .

 

[fppf]

No, once the absolute pressure outside the tank equals the absolute pressure inside the tank, no more gas will flow out of the tank. The diver would need to ascend to lower pressure to get anymore gas. If they descend, water could get forced into the tank.

 

[Crowley]

Also you wouldn't be able to get to 10 bar anyway - there would not be enough pressure in the tank to get your average piston regulators to function at that depth - even balanced. I'm sure somebody clever in here could do the calculations but I reckon your second stage would stop functioning well before that - 30 bar maybe? Please feel free to correct!

As fppf points out - once the outside pressure is greater than the tank, water will flow into the tank. Gauge pressure at 100m is 10 atm so even if you could get the tank to 10 bar, any deeper and water will be filling your tank, via your regulators...

Cheers,

C.

 

[DevonDiver]

....and the corresponding pressure in your lungs would mean trouble

 

[Kaltim Diver]

Guys, thanks for the interest raised, and it was my understanding that as the outside pressure (11 bar at 100m) was greater than the pressure in the tank, you wouldn't be able to get more air. To simplify and ignore the regulator first stage step down pressure and the needs of the second stage, if you opened a tank with 10 bar at 100m, I would assume that water would actually flow into the tank.

The author wishes to point out that the main risk from doing this would be a severe beating from your buddy or instructor, or your dive shop from letting water into the tank. I am in no way advocating breathing your tank down to 10 bar at any depth.

KTD

 

[Blackwood]

The author wishes to point out that the main risk from doing this would be a severe beating from your buddy or instructor, or your dive shop from letting water into the tank. I am in no way advocating breathing your tank down to 10 bar at any depth.

KTD

Somehow I think if I was down to 11 bar at 100 meters, I'd welcome the beating since it would mean I didn't drown.

 

[fppf]

Yeah, the beating would be much better than death.
Actually, I think I would paint the tank gold, put it on the mantle, and surrender all my C cards.

 

[halocline]

Also you wouldn't be able to get to 10 bar anyway - there would not be enough pressure in the tank to get your average piston regulators to function at that depth - even balanced. I'm sure somebody clever in here could do the calculations but I reckon your second stage would stop functioning well before that - 30 bar maybe? Please feel free to correct!

Pressure does not open the first stage, it shuts it (that's what IP is) so the regulator would function right down to ambient pressure. The 2nd stage would be much harder breathing, that's for sure. But you can get air with less than IP.

Not that it matters, but 10 bar is just a teeny bit higher than most regulators' IP. Of course, if ambient was 10 bar then the IP would be closer to 20, because IP is always X above ambient pressure. Otherwise your reg would be very difficult to breathe at any depth.

 

[elan]

I can breath through the reg when it is not connected to the tank and the cap is removed. So when the pressure is equal (in this case at 90m) you can definitely breath, so if you can create a pressure of 1 atm with your lungs you can definitely breath at 100m with 11atm ambient. To estimate I think the following rough model can be used, if you can hold a bar with a side of inch by inch with a weight of 14lbs approx with your lips using the suction power of your lungs you will definitely be able to breath in those conditions