Dive computers and depth measurements

[kombiguy]

OK, forgive my ignorance, but here goes. I know almost nothing about cave diving, except that it exists.
An OW student of mine asked me a question about the incident at Eagle's Nest, and I had no idea how to answer his question. So I thought I'd ask here.
Suppose you are in a cave 300 feet below the surface, and some distance beyond the entrance. How does your depth gauge/computer reflect an accurate depth? There may only be 30 feet of water above you, the rest being earth. How exactly odes that work, or does it?

 

[gfaith]

Interesting question.. IMHO, the weight of the entire continuous water column would determine the vertical pressure.

 

[tbone1004]

correct, there is still that amount of water pressure pressing on you even though it isn't directly above you. That water pressure is helping to hold the rock up above your head. The depth sensors are measuring absolute pressure and then convert that pressure reading to depth.

 

[kombiguy]

Thanks! Now I won't look quite so stupid.

 

[gfaith]

I am thinking you and the OW student are pretty sharp.. Asking for answers when you don't know is smart.

 

[Nirvana]

By Pascal's Law, the difference in pressure of a fluid inside a static system is proportional to the difference in elevation of the points of measure. This means that the pressure of water inside a cave will be the same as if the cave was a lake, with its surface being that of the water level at the entrance to the cave. You can calculate the pressure at a given point by measuring the difference in elevation from that point to the water surface, when you look at a map of a cave system.

 

[Lorenzoid]

The classic "u-shaped tube" or manometer physics problem that is used to explain pressure measurement to students bears some resemblance to a cave. It doesn't matter whether the other side of the tube is at the end of a "U" in a tube or down a long passage at the entrance to the cave. Google "u-shaped tube pressure problem" for all kinds of explanations of how pressure works in this sort of scenario.

 

[kombiguy]

All of these answers make me realize that I vaguely learned the answer when I learned to repair brakes. But, I appreciate all the answers, and now I have a grasp of the principles involved!