Depth measurement in a cave

[JohnnyC]

One of the USAF Rescue guys that was on-site stated that they were pumping tanks with 80% to keep the boys O2 level up.

Makes sense considering the photos of all the O2 K-bottles stacked up and on the back of trucks. So in light of that, the media got it right, even if they didn't know it.

 

[Norwegian Cave Diver]

Pretty sure there was no 90ft sump dive in that exit. Media reports are contradictory but at least a couple say they were using FFMs in positive pressure mode fed by cylinders of 80% - which has a MOD of 30ft.

I agree. I was trying to answer the question of ATA, not if there was a sump with 90’ of water in it.

 

[seaseadee]

One of the USAF Rescue guys that was on-site stated that they were pumping tanks with 80% to keep the boys O2 level up.

Makes sense considering the photos of all the O2 K-bottles stacked up and on the back of trucks. So in light of that, the media got it right, even if they didn't know it.

That probably explains the article in the WSJ saying that they used 500(!) cylinders in the rescue. That is a boat load of gas for just diving, so they were just opening them up in the caves?

 

[KevinNM]

That probably explains the article in the WSJ saying that they used 500(!) cylinders in the rescue. That is a boat load of gas for just diving, so they were just opening them up in the caves?

They ran air lines from the outside into the cave. Not sure if they got all the way to the kid's spot or not, but you can see then in some of the pictures of the operations inside the outer part of the cave. The reduced air quality wasn't just occurring back where the kids were, it was apparently occurring where there were a lot of rescuers working too.

 

[rjack321]

They ran air lines from the outside into the cave. Not sure if they got all the way to the kid's spot or not, but you can see then in some of the pictures of the operations inside the outer part of the cave. The reduced air quality wasn't just occurring back where the kids were, it was apparently occurring where there were a lot of rescuers working too.

Cant tell from the photos I've seen which hoses are air and which are water. I'd assumed the ~5"/10cm sized ones were drawing down the water in the sump.

 

[dflaher]

I have a really basic question about cave diving that I hoped this thread would answer.

If a diver in a sump is 20 feet underwater, but the cave is 100 feet below sea level, what is the diver's depth for nitrogen loading purposes? 20 or 120? What depth would the diver's computer show?

 

[inquis]

If a diver in a sump is 20 feet underwater, but the cave is 100 feet below sea level, what is the diver's depth for nitrogen loading purposes? 20 or 120?

Close enough to 20 ft not to matter. 100 ft of air doesn't weigh much.

 

[huwporter]

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.)

 

[dflaher]

Thanks for the reply. I guess I thought (maybe incorrectly) that most caves start out with an entrance at sea level, then traverse below sea level.

 

[huwporter]

Thanks for the reply. I guess I thought (maybe incorrectly) that most caves start out with an entrance at sea level, then traverse below sea level.

Yeah that isn't true. There is a fourth scenario in addition to the three I listed above; where the entrance to the (water filled) cave is above sea level - many 'spring' type caves are like this. If the diver then goes to 20 feet depth of water in the cave then the nitrogen loading depth is still basically 20 feet irrespective of the altitude that they might be relative to sea level, unless the cave is at such a high altitude that it becomes an altitude dive, in which case the nitrogen loading can be less than 20 feet due to the reduced atmospheric pressure - example, 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 0.6 ATM of water pressure = roughly 1.3ATM or 10 feet for nitrogen loading. (But note it's - definitely! - an altitude dive, so planning isn't the same as planning a sea level dive anyway, and you shouldn't use that 10 feet for dive planning.)
Edit: flubbed the calculation converting from metric.