Depth measurement in a cave

[seaseadee]

I've been reading the thread of the rescue of the soccer team in Thailand, it has left me with a question. One of the posts mentions that a portion of the dive was at 90'. That got me thinking about what that even means:

1. As an Open Water diver, I instinctively understand the pressure exerted by the water above me. How does this translate in a sump or a cave?
2. What happens in an air space like the one the boys were trapped in? Is that air space at 1ATA or is it at the depth that the water level is at? What does that mean for a diver's deco obligation when getting out of the water and into that airspace?

Inquiring minds want to know.

 

[Superlyte27]

Look at the caisson workers nearly 100 years ago. You can google it. They were dry while working, but would go home and get DCS symptoms. Back then it was called Caisson’s disease. They’d come back to work the next day, and immediately start feeling better.

 

[DA Aquamaster]

Caissons were pressurized to keep water out. Water in a sump is just that, in a sump. The air on either side of the sump will be at close to ambient pressure.

Worst case, if the cave is completely sealed at the far end, and the water in a "V" shaped passage starts to rise, there would be some compression of the air on the far side of the sump as the volume past the sump is decreased. But, the small change in actual air volume in the cave behind sump would equate to almost no increase in pressure.

 

[Nirvana]

1. As an Open Water diver, I instinctively understand the pressure exerted by the water above me. How does this translate in a sump or a cave?

In a closed system, every section of water at the same level is at the same pressure, independently of the geometry of the system. If you looked at a vertical cross section of a cave, any point at the same horizontal would be at the same pressure.

2. What happens in an air space like the one the boys were trapped in? Is that air space at 1ATA or is it at the depth that the water level is at? What does that mean for a diver's deco obligation when getting out of the water and into that airspace?

Any air space will be at the same pressure as the water surface it is in contact with. This is easy to understand: if the water was at greater pressure, it would compress or dislodge the air mass until the pressure equalized.

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.

In the case of the boys in Thailand, the room they were in, to my knowledge, was at 1 atm.

 

[rjack321]

In the case of the boys in Thailand, the room they were in, to my knowledge, was at 1 atm.

It would be an exceptionally rare cave where an air dome was not at 1 atm. Limestone is quite porous.

 

[Nirvana]

It would be an exceptionally rare cave where an air dome was not at 1 atm. Limestone is quite porous.

Well, there are flooded mines (I often dive a deactivated gold mine), which were made by excavation and, I suspect, are composed of less porous rock.

 

[rjack321]

Well, there are flooded mines (I often dive a deactivated gold mine), which were made by excavation and, I suspect, are composed of less porous rock.

Yes there are other geologic formations that are more "air tight" where an air dome wont be at atmospheric pressure. In the Thai cave the dome was not pressurized but the sumps above.

 

[Norwegian Cave Diver]

Would depend on the elevation of the cave. If the cave was at 2000 feet then subtract 90 feet it would be a dive at elevation. So surface pressure would be less than 1 ata.

 

[rjack321]

Would depend on the elevation of the cave. If the cave was at 2000 feet then subtract 90 feet it would be a dive at elevation. So surface pressure would be less than 1 ata.

90ft? where did that number come from?

In this case, it depends entirely on the elevation of the dry patch in the air dome, not the cave entrance. Atmospheric pressure at 2000ft is 0.93 atm plus or minus a bit depending on the weather. Botttom line: Thai kids were not exposed to elevated pressures requiring decompression.

 

[Norwegian Cave Diver]

90 Feet comes from the very first post.
I stated it would depend on the elevation of the cave.