It's been more than 30 years since I took physics classes...

[Elfguy]

I'm setting up an underwater entrance to a dungeon, and I want to set up the following trap:

Airlock Trap

However, I haven't done any physics in a long, long, time. Can anyone crunch some numbers and figure out how many atmospheres of pressure would be required to displace that 25,000 cubic feet of water at sea level?

Also, if that much pressure was suddenly released, how severe would the decompression sickness ("The Bends") Be? Actual damage, or more of a "Fort save or sickened for 1D4 minutes?"

And yes, this is a high level dungeon...this trap would kill anyone less than level 10 outright most likely.

[Thaago]

Elfguy wrote:

I'm setting up an underwater entrance to a dungeon, and I want to set up the following trap:

Airlock Trap

However, I haven't done any physics in a long, long, time. Can anyone crunch some numbers and figure out how many atmospheres of pressure would be required to displace that 25,000 cubic feet of water at sea level?

Also, if that much pressure was suddenly released, how severe would the decompression sickness ("The Bends") Be? Actual damage, or more of a "Fort save or sickened for 1D4 minutes?"

And yes, this is a high level dungeon...this trap would kill anyone less than level 10 outright most likely.

Its a magical air supply, so feel free to disregard, but:

The pressure required is the same pressure as 25 feet of water: about .75 atmospheres. http://www.wolframalpha.com/input/?i=pressure+of+25+feet+of+water

If the spike room is at normal atmosphere pressure, than the change is only .75 atm. I'm not a diver, but I do not think that is enough to cause the bends after a short exposure.

Looking at: https://www.naui.org/tables.aspx I don't see any entries for under 40 feet of dive.

[Edit] Depending on how fast the pressure change is though, it could be enough to blow out the characters ear drums.

[BigDTBone]

Well, at low tide the airlock chamber is pulling a vacuum so an exceptionally small amount of air would displace much of the water and the water is trying to escape anyway. (Like when you put your finger on a straw in a glass and lift the straw the liquid remains in the straw. In this example the tide is essentially lowering the "glass" away from the "straw.")

At sea level, at high tide, 1ATM would be sufficient to displace that much water. Much more than 1ATM of pressure and the air will push back the water in the tunnel and then bubble to the surface. If you were deeper it would be more. I don't see why anyone would need to worry about the bends.

[Elfguy]

Odd...I seem to remember caissons caused the bends, and those were pretty much at sea level or very close to it...I forgot to draw it in, but there is actually going to be a small vent to the surface of the cliff above from the room where the magical air supply is generated...when the door mechanism is triggered, that vent closes and then pressurization begins. So no vacuum at low tide. (IIRC, 30 feet is about the vacuum threshold at sea level for water)

[Orfamay Quest]

There is an element of time as well. I can stay at 40 feet for about two hours with no ill effects, but I can't do a full days work in there. At 90 feet, half an hour will be enough to cause dangerous levels of nitrogen buildup.

So the caissons of the Brooklyn Bridge, at about 60 feet, would be fine if you were only in there for an hour or so. I wouldn't want to stay down there and work though.