Cave diving depth question.

[diver_doug]

I'm not a cave diver, but this is something I'm curious about and I figured people here would know.

How does depth work (as far as gas management, decompression limits, etc. are concerned)? What I mean is this: Let's say you enter, for example, a passageway near the surface that is, 10ft (i.e. a passageway near the surface that measures 10ft from top to bottom). And for several hundred feet you traverse through this downward-slanting 10ft "tall" passageway until you are, say, 100ft below the surface. At this point you are 100ft below the surface, BUT you are in a water-filled passage that is only 10ft "tall". So at this point would it be "as if" you are in about 10ft of water, or "as if" you are in about 100ft of water? Or neither?

 

[Brewone0to]

That would be a wonderful thing if it worked that way.
But, unfortunately it is not so.
At said point you would find yourself at that depth in the water column, dealing with the pressure from the weight of all the water above you in the column.
The truth is you then are at 100' depth but further from the surface than one would be if in OW.
Hope this helps.

 

[TSandM]

Remember, your depth gauge is reading pressure -- so if it is registering 100 ffw, you are experiencing an ambient pressure equivalent to that.

In fact, if you think about it -- take a 100 foot tall plastic tube. Pressure at the bottom of it is 100 ffw. Now bend it so that the bottom is no longer quite below the top -- the pressure at the bottom will still be close to 100 ffw (not quite equal, because you used up some of the length in the bend).

 

[Dive-aholic]

Wrap your head around this.

You are at 100' depth and fins an air pocket on the ceiling. You ascend to the air pocket and stick your head up into it (keeping your reg in your mouth because you don't know what the air in there is). You look at your depth gauge that is just below the water line. It still reads 100'. Raise your depth gauge into the air pocket. It still read 100' (okay, maybe 99').

Hydrostatic pressure is based on the pressure of all the water in that space, including the water "above" even if it's above through a passage. Even the air pockets remain at pressure.

 

[dmoore19]

Maybe this will help. There is measured depth, how far back in the cave you are. There is also True Vertical depth, if you could go straight to the surface, not having to return through the passage, that would be True Vertical depth. True vertical depth is what your computer's depth will be displaying. Regardless of how much lateral displacement there is, the true vertical depth is the only thing that matters when dealing with hydrostatic pressure.

The hydrostatic pressure can be calculated using this formula:

PSI hydrostatic pressure= water weight ppg X .052 X True vertical depth

WW=8.34 (weight of freshwater in ppg)
.052= constant
TVD=measured from the surface straight down to your actual depth 100' in this case

SO:

8.34 X .052 X 100=43.368 PSI

Convert the PSI to bar by dividing by 14.69

43.368/14.69=2.95 Bar or atmospheres

you could use the same formula for saltwater. just use 8.55 for the weight of seawater (this is an average as density does change slightly with temperature and from place to place)


So in the ocean the Bar would be 3.05.



hope it helps.