cave depths

Can somone please explain in simple form how a computer calculates your depth in a cave? for example the other day my computer told me i was at 20 metres, but there was only 3 metres of water above me to the ceiling? thanks in advance

By way of introduction, and possibly help the course of discussion, would you mind stating your current certification level?

About grassgreen

Location:mexico
Gender:Not specified
Certification AgenciesADI
Certification Historyadi and IANTD
Certification Level:cave diver
# of Logged Dives:1,000 - 2,499
Dive Classification:Instructor / Assistant Instructor / Dive Master / Dive ConYears
Certified:Six - Ten Years

Iam a padi staff instructor and tec 45 diver and IANTD full cave diver

Cool, then you should know that:

View attachment 133654

Yes i know that, and definatly felt the difference, but how does the computer? in the sea the computer can "feel" the weight of 3ata/ 20 metres, but not the case in the cave??

Because you still have 20 meters of water above you somewhere in the system. The pressure at a given depth is consistent across that depth as long as there's continuous water to convey the hydrostatic pressure.

Sent from my Nexus 7 using Tapatalk 2

grassgreen:

Yes i know that, and definatly felt the difference, but how does the computer? in the sea the computer can "feel" the weight of 3ata/ 20 metres, but not the case in the cave??

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Why not? Nothing has changed, the rules of physics that allow the depth guage to "feel" the pressure in the open ocean have not changed in the cave. The only difference is the path to the surface is twisty, not straight up verticle.

As lowviz beautifully illistrated, the depth is the depth below the level of the true surface, not the roof of the cave above you. The explainations just don't get any clearer than his drawing. IF I were to do the drawing the only thing I would add would be small down arrows showing the direction of the pressure of the water. This will show you that in the verticle or sloped areas of the tube (cave) the pressues is pushing down, in the horizontal sections the pressure is pushing down (but agaist the bottom of the cave) which explains why when you are 30 feet below the true surface and 100 feet into the cave you pressure guage shows 30 feet and not the weight of all the water in the cave. If you were in a cave that was straight with a slope of 1 foot of drop for every 2 feet of run and you were 200 feet into the cave your depth would be 100 feet (distance from where you are straight up). Just because there is 3 feet of clearance above your head when you are 90 below the surface of the water (note if you are in an air pocket, the pressure is still the pressure of that depth and does not count as the "surface"), then you are at 90 feet. Just becaue the path to the surface is a tube and twisty and slanted here and there till it gets to the surface, as long as their is a continous connection from your current depth to the true surface (open air) does not mean the pressure is any less that it would be in the ocean. Remember you swam to where you are, therefore the water is that is above you (including that which is behind you and angling up to the surface) is still above you (even though it is apparently benind you because a rock roof is 3 feet above you) and its weight is bearing down.

To make it simple, take you finger and put it on Lowviz drawing at the point where it says YOU SEE THIS, now trace the water back to the surface. This path allows the weight of all the water above you (distance below the true surface) to be transmitted to you and your dive computer. Lets try this, IF the cave were EMPTY of water, and you poored 1 gallon of water into the cave from the top, what would happen? The water would flow into the cave and go to the lowest depth of the cave. The next gallon of water will do the same and so on until the cave is full. This is true of a straight or bendy tube (cave) or a bowl (ocean). So the verticle distance you are below the surface of the water (where you stoped breathing air in through your nose and switched to SCUBA) is you depth in either situation. The pressure is transmited though the water because water is essentially non-compressibe. You have to look at the total verticle depth from surface to where you are in the water for your depth and yes the computer/depth guage can "feel" that with their pressure sensor.

Another way to look at it...
If you are 150 ft down in a wreck, enter a small chamber, and create a 2 ft air pocket above you from a spare air cylinder, would you be confused about your equivalent depth whilst in the air pocket, 150 ft down, or on the surface?

Kilili:

...//...If you are 150 ft down in a wreck, enter a small chamber, and create a 2 ft air pocket above you from a spare air cylinder, ...//...

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Way too funny, but no, I'm trying to behave...