Cave-diving and water pressure question

[Gombessa]

IMHO, it would read 10'. If you can walk into a cave, and there is standing water, then you know the top of the water column, which is at 60'.

What if you walked through an airlock to get into the cave? Then you could still have 70ft

Please explain why he'd die of lack of oxygen. I don't follow.

Pressure at 33ft depth is 2 ATA. Pressure at the surface is 1 ATA. If you use a siphon to raise the water column above the surface by 33ft, you're at somewhere between 0.1-0.5 ATA. So think reverse oxygen toxicity (instead of getting too much oxygen resulting in toxicity, you'll be getting too little to support life). At that "negative" pressure, the air coming out of the tank and into your regulator would have significantly less pressure than at the surface, so the volume of air you'd be inhaling would be half or less of what you would get at the surface.

You'd get the bends because you're going from 1 to 0 ATA, just as you'd potentially get the bends going from 2ATA to 1ATA while 100% saturated.

Does that sound right?

 

[ClayJar]

By the way, the partial pressure of water vapor in the gas space at the top of the column would be the vapor pressure of water at whatever water temperature is there. If you use 75°F/24°C, the partial pressure of water vapor in the gas phase would be 22.4 mmHg (i.e. about 0.0295 atmospheres).

If you started with the tube underwater with no air in it, the pressure in the gas phase (which is 100% water vapor in that case) would be about 3% of nominal air pressure at sea level. Assuming 1 ata ambient pressure outside the tube, the water column would be 32 feet tall above the surface of the water outside the tube (assuming seawater).

 

[perdidochas]

What if you walked through an airlock to get into the cave? Then you could still have 70ft

Depends on the pressure inside the airlock.

Pressure at 33ft depth is 2 ATA. Pressure at the surface is 1 ATA. If you use a siphon to raise the water column above the surface by 33ft, you're at somewhere between 0.1-0.5 ATA. So think reverse oxygen toxicity (instead of getting too much oxygen resulting in toxicity, you'll be getting too little to support life). At that "negative" pressure, the air coming out of the tank and into your regulator would have significantly less pressure than at the surface, so the volume of air you'd be inhaling would be half or less of what you would get at the surface.

Ok, makes sense (maybe).

You'd get the bends because you're going from 1 to 0 ATA, just as you'd potentially get the bends going from 2ATA to 1ATA while 100% saturated.

Does that sound right?

 

[Walter]

If you want another interesting mental challenge, think about a diver in a siphon.

Not a cave style siphon, but a true one ---- a tube 40' or 50' high, sealed at the top, the other end open and below the water level of a lake, with the water level in the siphon approx 33' feet above the surface of the lake. Kind of like an oversized, water-filled version of a barometer.

A diver entering the siphon from below the lake surface and swimming to the surface of the water at the top of the siphon would be in a heap of trouble.

He'd develop the bends, but before that happens he'd pass out and/or die from insufficient of oxygen, even if he was breathing from a tank of 100% O2.

Charlie Allen

I don't follow this one at all. Please explain.

 

[Gombessa]

Depends on the pressure inside the airlock.

Hence the word "could"

 

[Charlie99]

Please explain why he'd die of lack of oxygen. I don't follow.

The pressure at the top of the siphon/water barometer is very low. Not quite a vacuum --- as Clayjar points out it will be the vapor pressure of water at the temperature of the water in the tube. Even breathing 100% O2 this will be below the level needed to sustain life (and also, since your body is most likely warmer than the water in the siphon, the vapor pressure of you blood will be higher than ambient -- in other words, your blood will be boiling).

Another way to look at this is to try and figure out what the ambient pressure is about halfway up the tube, 17 feet above the freshwater lake. Clearly, just like horizontal distances have no effect in a cave, when you are inside the tube at the lake level you will be at atmospheric pressure of about 1ata. But as you ascend up the water column, pressure will get lower. Around 17 feet above sea level you will be 1/2 atmosphere less than the starting point. So the pressure halfway up the tube will be about 0.5ata.

Breathing air at 0.5ata you wouldn't have enough ppO2 to stay conscious. If breathing 100% O2 you'd be OK, and you would also be right at the limit where DCS becomes likely. The pressure inside the tube 17' above lake level will be about the same as at 18,000' altitude on a mountain.

--------------------

You can only suction water up a maximum of 34' (and practically even 30' is pretty hard). That's why deeper wells will have a pump submersed at the bottom --- there isn't any theoretical limit on how far you can pump upward like there is on how far you can suck/vacuum upward. When you have a pump above a lake or well taking a suction on the water, what is really happening is that you are creating a partial vacuum and atmospheric pressure forces the water up the suction intake. But since 1ata is 34ffw, once you have the pump 34' above lake level the atmosphere can't push the water up the suction intake that high, even if you were able to draw a perfect vacuum with the pump.

 

[Hank49]

If you want another interesting mental challenge, think about a diver in a siphon.

Not a cave style siphon, but a true one ---- a tube 40' or 50' high, sealed at the top, the other end open and below the water level of a lake, with the water level in the siphon approx 33' feet above the surface of the lake. Kind of like an oversized, water-filled version of a barometer.

A diver entering the siphon from below the lake surface and swimming to the surface of the water at the top of the siphon would be in a heap of trouble.

He'd develop the bends, but before that happens he'd pass out and/or die from insufficient of oxygen, even if he was breathing from a tank of 100% O2.

Charlie Allen

And this would be caused by....the water level dropping, expanding the gas (of which you would have had to start with...at least some) in the top of the siphon (assuming it didn't collapse) dropping the pressure of gas or air in the siphon far below 1 atm, which would in essence turn the dive into an extreme high altitude dive?

 

[jwelch_81]

Why do you see diagrams of tanks at depth where the air is being compressed in it at?

At 33 feet it shows the tank 1/3 empty and so on.

The air in the tanks dosn't compress from the water pressure. Does it?
If it did your pressure guage would go crazy.

 

[Charlie99]

Why do you see diagrams of tanks at depth where the air is being compressed in it at?

At 33 feet it shows the tank 1/3 empty and so on.

The air in the tanks dosn't compress from the water pressure. Does it?
If it did your pressure guage would go crazy.

The diagrams you are thinking of are probably for open mouthed containers, such as an upside down glass. In that case, the increased pressure at 33' would compress a full glass of air down to 1/2 full.

Your scuba tanks are sealed and rigid. Outside pressure doesn't affect the air in the tank.

 

[Charlie99]

And this would be caused by....the water level dropping, expanding the gas (of which you would have had to start with...at least some) in the top of the siphon (assuming it didn't collapse) dropping the pressure of gas or air in the siphon far below 1 atm, which would in essence turn the dive into an extreme high altitude dive?

That's pretty much it, except that you don't have to have any gas at the top of the siphon/barometer.

When making a barometer, either out of mercury or water, you work hard to make sure that there isn't any excess gas at the top of the tube. As you lift the barometer tube up out of the mercury pool, or in this case, lift the sealed tube up above the lake, the pressure at the top of the tube gets lower and lower. As the pressure at the top approaches a vacuum you eventually get to the point where the pressure is equal (or slightly less) than the vapor pressure of the liquid. At that point, the "empy" space at the top of the barometer is filled by the vapor.

Water boils at lower and lower temperatures as you go higher in altitude. At some very low pressure, it will boil (go from liquid to gaseous phase) even at room temperature.