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Discussion Starter #1
I was reading on another thread about some divers at hodge surfacing into an air pocket in one of the chambers. Now I few assumptions:

Assuming that
1) This air gap was created by exhaust gas
2) The air gap is below the surface

If you look at the attached profile what depth would a computer read? Would a computer turn off since the wet contacts are now dry? Would this vary depending on the type of materials above you've head? sandstone, granite etc?
 

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The smell of freshly turned delrin is more powerfu
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just a thought..
dont breath from the air gap keep your reg in...
unless you know its safe could be all sorts of problems :) otherwise
 

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MIFLEX DIR = "Did It Rupture"??
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The computer would still read the correct depth as it measures the pressure. Therefore, the pressure of the water compressing the air space would give the same reading as the water itself. The only way the computer would give any other reading wouldbe if there was a direct air passage to the surface for the pressure to escape, but if that were the case the water would fill the void completely also.

If your computer uses wet contacts then yes, it would turn off. Hence when divers go for chamber dry dives to experience recompression therapy they take a bucket in to drop their computers into.

The pressure would not vary dependant upon the geology. It's the pressure that matters.

G
 

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The computer, provided it was still on, would read the same depth as you are at. If the pressure was not equal between the water and the air, the water level would rise or fall in order to make up for this.

The material above your head should not affect this, unless the air was allowed to escape, in which case the water level would go up.

Digs.
 

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Discussion Starter #6
The computer would still read the correct depth as it measures the pressure. Therefore, the pressure of the water compressing the air space would give the same reading as the water itself. The only way the computer would give any other reading wouldbe if there was a direct air passage to the surface for the pressure to escape, but if that were the case the water would fill the void completely also.

If your computer uses wet contacts then yes, it would turn off. Hence when divers go for chamber dry dives to experience recompression therapy they take a bucket in to drop their computers into.

The pressure would not vary dependant upon the geology. It's the pressure that matters.

G
But at hodge for an example the chamber wouldn't be compressed by water. There would only be rock above. Certainly this air pocket must be below the water table but since it exists we know that the rock above isn't that permiable to allow it to fill back up with water.
 

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PADI Internet Specialty Diver
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Gas pockets are formed by many things not just exhaust gasses. You should never breathe from them unless you know they are OK. The gasses in a pocket will diffuse into the water over time as well so the contents can be dynamic. Oxygen and CO2 move into and out of water for example. If they did not most fish in small ponds would die.
 

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PADI Internet Specialty Diver
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But at hodge for an example the chamber wouldn't be compressed by water. There would only be rock above. Certainly this air pocket must be below the water table but since it exists we know that the rock above isn't that permiable to allow it to fill back up with water.
If there is no escape for the gas above the water the gas pressure will be the same as the water pressure therefore the "depth" will be the same in the water or in the gas. If there is a route out for the gas the space will fill up with water.
 

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For some bizarre reason....
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But at hodge for an example the chamber wouldn't be compressed by water. There would only be rock above. Certainly this air pocket must be below the water table but since it exists we know that the rock above isn't that permiable to allow it to fill back up with water.
Imagine a diving bell below the surface of the water with an open bottom. The gas inside the bell will be compressed to the pressure of the water (if no gas is added, then the water will enter the bell as the remaining gas volume decreases).

With a cave the same will happen if the air pocket is below the 'surface' of the main body of water - the gas in the pocket will be compressed.

With a cave where the air pocket is at or above the 'surface', then IME the pocket is at atmospheric pressure.

-Paul
 

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MIFLEX DIR = "Did It Rupture"??
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But at hodge for an example the chamber wouldn't be compressed by water. There would only be rock above. Certainly this air pocket must be below the water table but since it exists we know that the rock above isn't that permiable to allow it to fill back up with water.
You forget that the force of the water pressure will be applied to all areas. The water will naturally try and flow up into the void due to the weight of the water sourrounding the void. The only thing stopping the water from filling the void is the air that the void contains. This will compress to the same pressure as the weight of the water sourrounding it.

Think of it this way - Take a drinks bottle or bucket, anything that can trap air. Invert it and then sink it while inverted. The water will compress the air space and go into the bottle or bucket slightly (as air is compressable). If you were to then measure the pressure with a depth gauge (depth gauge is simply a pressure gauge) it would equal the depth of the bucket as the air was compressed equal to the water depth by the force of the water trying to enter the bottle / bucket.

G
 
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Discussion Starter #11
Imagine a diving bell below the surface of the water with an open bottom. The gas inside the bell will be compressed to the pressure of the water (if no gas is added, then the water will enter the bell as the remaining gas volume decreases).

With a cave the same will happen if the air pocket is below the 'surface' of the main body of water - the gas in the pocket will be compressed.

With a cave where the air pocket is at or above the 'surface', then IME the pocket is at atmospheric pressure.

-Paul
Thanks, that was what I thought would happen but thought I'd check. For some reason it was just the thought that there was no water overhead that made me think something would change. I.e. going into the hydrobox air gap in Stoney makes perfect sense that the depth would stay the same since above you is another 30m of water. However with the hodge scenario I thought it could be a little different.

Thanks all.
 

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Nigel Hewitt
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Air pockets in recent wrecks normally, or maybe I'm just unlucky, have a nice thick layer of gungy oil in them.
 

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Jesus don't want me for a sunbeam
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Air pockets in recent wrecks normally, or maybe I'm just unlucky, have a nice thick layer of gungy oil in them.
Makes sense, oil like air is going to try to find the highest point. Must admit, most air pockets I've seen on wrecks are usually oily as well now that I think about it.
 

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Team Peanut Butter
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DO NOT breath from any gas space under water unless you have taken the gas in with you and created the air gap your self. 1988 or 1989 two Essex Police divers died when they came up in a submerged barge and breathed in the gas that had collected there.

Graham
 

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Who f#cked with your edit button?
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There are two kinds of air spaces found in caves.

There are compressed gas spaces, formed by exhaled gases or some other process. These are normally quite small and may disappear over time through the permeability of the rock or solubility in the water. Hodge Close is slate and not very permeable so the air is trapped. This is similar to hydrobox example. Depth guage reads the pressure of the water surface in the space.

There are atmospheric pressure spaces that are generally connected to the outside somehow. These can be massive and often extend into dry cave systems. Depth guage reads 0.0m in the space. These surfaces are at the same level (i.e height above sea level) as the entry point.


As an aside, the 'water table' is a confusing term. Water tables are only level in very permeable ground with no groundwater flow. The level of ground water often follows the surface level. e.g. You can dig a shallow hole on a hill and find water. It's just slowly making it's way downhill to the river. You can also have several 'water tables' at the same spot. This happens when the rock has an impermeable layer. The extreme example being a waterfall.

Breathing anything from either type of space needs some caution.

Steve
 

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Many years ago, when I was young and foolish, we filled up all the holes in a bath and took it into the tunnel in Dorothea. We filled it full of air and put it in the depression in the roof, half way along. You could stick your head up and have a conversation at about 22m!
One of our group, being old and foolisher, had a cigarette in the bath and polluted our clean air....but he said it tasted like sucking on 40 all at once!

Don't remember what our depth gauges said as we were otherwise distracted with the project :)
 

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loop14
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Visited the Stig in Chamber 3 yesterday; he still has a dildo up his arse! Went up into the air bell and my computer showed 20M. I was videoing on my GoPro but the chambers are too dark for any decent footage.
 

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The King Of The Divan
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tongue in cheek, I know...... but I always thought it would be a good idea to take a snorkel into hodge close in case of total gas failure and buddy separation as you could always snorkel your way out to safety from air pocket to air pocket down the tunnels :)

I didnt raise it on my cave course in Florida but it always makes me chuckle to think about it.

Snash
 
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