Depth measurement in a cave

[inquis]

if the entrance to the cave is at 10,000 feet altitude, and the diver goes to 20 feet of water, then the nitrogen loading would be 0.7ATM of air pressure plus 0.67 ATM of water pressure = roughly 16 feet for nitrogen loading.

The equivalent depth for nitrogen loading you calculated goes in the opposite direction as it should. The depth for nitrogen loading purposes is deeper when at altitude and in this case is the equivalent of 29 ft on a normal (non-altitude) dive table.

Think of it as multiples of surface pressure: 20 feet of water (which is actually 0.59 atm above surface pressure, not 0.67 atm) would still register as 20 feet of water on a DC because it would calibrate itself to 0 ft depth at the surface pressure of 0.7 atm (*). This makes the total pressure 0.7 + 0.59 = 1.29 atm. That pressure is a factor of 1.85x that of the surface pressure, which is what matters for nitrogen loading (and unloading).

You're correct that the inspired inert gas pressure is less than it would be at sea level, but absolute pressure doesn't matter for nitrogen off-gassing. You have to get it within a safe factor of the 0.7 atm surface pressure, which is further away than 1.0 atm.

(*) side note: if the DC does not sense surface pressure, it would display that 1.29 atm measurement as about 10 ft after erroneously subtracting 1.0 atm for "surface" pressure.

 

[huwporter]

We are basically in agreement, we are just talking about different things - which I covered in the comment "(But it's an altitude dive, so planning isn't the same as planning a sea level dive anyway.)" which was intended to indicate I was talking about physics, not dive planning.

The actual (physics) pressure you experience on this dive (to 20 feet linear depth of water at 10,000 feet altitude) is roughly equivalent to the pressure experienced from a dive to 10 feet of sea water (let's not start to get picky enough to start talking about the differences of fresh vs salt water), however for calculating your decompression you need to account for the fact that you need to safely surface not to 1ATM but to 0.7ATM - and one way of approximating the decompression this entails is, as you say, to use the more conservative calculations from a deeper depth than your local linear water depth from a non-altitude dive table, as the percentage pressure reduction you need to safely navigate surfacing from 1.3 to 0.7 ATM is relatively greater than the percentage pressure reduction of surfacing from 1.6 to 1.0 ATM..

You are right about the ~0.6ATM from 20 feet of sea water - that was a slip converting from metric, and it should be 10 feet not 16.

 

[Manatee Diver]

If someone is trapped in an air space inside a wreck, as has happened, it will be necessary to decompress before going to the surface.

Such as with Harrison Okene.

 

[mdwalter]

What do you mean by "the cave is 100 feet below sea level"?

If you mean the entrance to the cave is situated on land, in an area where the land is 100 feet below sea level - maybe somewhere around the shore of the Caspian Sea (List of places on land with elevations below sea level - Wikipedia), and then the cave is filled with 20 feet deep water, then the answer is 20 feet for nitrogen loading. (approx 1 ATM of air pressure, plus approx. 0.67ATM of water pressure.

If you mean the entrance to the cave is situated under sea water 100 feet deep, and then inside the cave it goes another 20 feet deeper past the entrance, then the answer is 120 feet for nitrogen loading (1 ATM of air pressure, plus 3 ATM of water pressure outside the cave, plus 0.67 ATM of water pressure inside the cave.)

If you mean the the entrance to the cave is situated in sea water 100 feet deep, and then inside the cave it goes back up 80 feet shallower so that the diver ends up 20 feet below sea level, then the answer is 20 feet for nitrogen loading. (approx 1 ATM of air pressure, plus approx. 0.67ATM of water pressure.)

I don't think you understand what a sump is. Your second two scenarios do not describe a sump, but rather an underwater cave. A sump is a section of passage in an otherwise dry(ish) cave that is filled from floor to ceiling with water over some horizontal distance. Also, the question is "If a diver in a sump is 20 feet underwater..." In your second scenario, the diver is not 20 ft under water, but 120 ft under water (and in an underwater cave, not in a sump).
Now, if the diver were 20 ft under water in a sump, inside a cave where the water level in the sump was 100 ft below the entrance to the cave, but the cave happened to be at 6000' above sea level, you would actually need to adjust for the 5880' altitude dive, co the effective depth would actually be LESS than 20' and when he surfaced, the air pressure is less than 1 ATA.