Decompression sickness only occurs when a person is under elevated pressure for a period of time, and then the pressure is released. Pressurizing the sub could in fact cause the decompression sickness, not prevent it.

For this sub, during descent the pilot's chamber compresses and air is drawn out of that chamber to keep the pressure constant (this from JC's site). Rather unlike the airplane, into which the air is pumped from the outside, usually from the engine compressor's bleed, to keep the pressure from falling too low as it ascends.

Only a too rapid depressurization will cause decompression sickness. Unless the sub is pressurized to a much higher pressure than normal atmospheric pressure, decompression sickness won't be an issue.

According to National geographic (http://news.nationalgeographic.com/news/2012/03/120308-james...) the entire sub is only going to shrink by about 2.5 inches. If we assume that the shrinkage only occurs on the 43 inch pilot sphere, then the lost volume is only about 16% of the total original volume, which means pressure at the bottom is about 120% atmospheric pressure. Since most airplanes are pressurized to 10.9 psi, which is about 75% of atmospheric pressure at sea level (14.9 psi) I don't think decompression sickness will even be an issue.

> For this sub, during descent the pilot's chamber compresses and air is drawn out of that chamber to keep the pressure constant (this from JC's site). Rather unlike the airplane, into which the air is pumped from the outside, usually from the engine compressor's bleed, to keep the pressure from falling too low as it ascends.

I googled for James Cameron's website and couldn't find anything. Could you please provide a link?

Yes, obviously an airplane is going to pump air in from somewhere to keep the chamber at a higher pressure than the ambient environment and a submarine with a shrinking chamber is going to remove air to keep the chamber at a lower pressure than the ambient environment. However, both an airplane and a sub are designed to keep a chamber at a specific pressure that is relatively close to normal atmospheric pressure, which prevents humans in the chamber from being at risk for decompression sickness, short of some sort of catastrophic failure.

The info is scattered throughout the site.

http://deepseachallenge.com/the-sub/sub-facts/ states:"The pilot is descending about 36,000 feet (10,973 meters), but his ears won’t pop during the journey; the pressure inside the pilot’s sphere stays constant."

http://deepseachallenge.com/the-sub/systems-technology/ shows that the pressure remains at atmospheric even at full depth

http://deepseachallenge.com/the-sub/pilot-sphere/, about 60% down describes the atmosphere regulation system

If the air in a submarine starts to 'pressurize' the last thing you're worried about is the bends. In a pressurization event on board a submarine you're going to be far more worried about how to get the ocean out.

The other issue is that when the air pressurizes it heats up almost instantly, it's generally believed that a large leak in a sub would create a wall of flame.

What complete and utter rubbish. Please provide a single reference for this "general belief". Or better still, apologise for posting crap.

Air, and gases generally, do heat up when compressed adiabatically (without exchange of heat with surroundings). But filling half the sub with water - that's a large leak, right? - and hence doubling the pressure would only produce about a 60 degree increase in temperature - not quite enough for a wall of flame eh? - if no heat was lost from the air to such things as jets of cold water entering the sub in this scenario.

"When the hull of a sub fails, it normally fails ni one location, and water shoots in through that spot, relieving the stress on the rest of the structure. That's why all the wrecks you see in video from actual crash sites look so intact.

Of course, the inrush of water also has the effect of rapidly pressurizing the inside of the sub 'till it matches external pressure. This rapid compression of the air inside has the effect of drastically raising the temperature, so what the victems inside probably see is an explosion (big wall of flame rushing through the inside)."

"Interesting point. Since compression is quick, it is adiabatic, PV^\gamma = K applies. So if the pressure doubles, the volume of air reduces to .6 of its original volume the temperature will increase by a factor of 1.2 (eg from 300K to 360 K) which is pretty warm but not enough to ignite anything. When the wall of water has consumed 80% of the volume, the temperature is up to 570 K (300 C). So the sailors at the end of the sub watching the oncoming wall of water get fried before they drown."

"With the rapid compression of the air, there would be a simultaneous step rise in both pressure and temperature. The air pressure would rapidly approach, but not exceed, the water pressure. The temperature would, of course, be proportional to the air pressure, but what the actual numbers are I am not certain. I am not sure one way or the other whether they would raise to the point of combustion, but I agree that it’s possible. In any case, they would be high enough to cause severe burning of the exposed body surfaces. Of course, the body would also experience the effects of the sudden high pressurization, resulting in burst eardrums, a ruptured body, perhaps cracking of the skull (which under a pressure increase of the rapidity would be like it’s own pressure-tight space, just as would the main body), the eyes would probably burst (implode), and there would probably be bursting of many blood vessels. Of course, both any burning effect, already mentioned, and the result of a rapid pressure impulse on the body would both be in a fractional second. Loss of consciousness and death would be near instantaneous, as you state. I can see perhaps enough of a window for the body and conscious to experience perhaps a very brief moment of thought and sensation, but if there is any at all, it would be exceedingly brief.

Given what we know from actual hull crushes (and I’m sure that someone knows waaaaay more than I do since I’m just going by what we learned as part of being nuclear submariners), the entire submarine would not necessarily crush all at once. One example is the Golf-class submarine that we recovered from the Pacific floor as part of our espionage operations (Howard Hughes’ Glomar Explorer, etc.) Though we only recovered about the front third of that submariner, we obtained about two thirds of the interior works, since the submarine breached at some point in the rear and the pressure surge slammed the interior forward. A second example is the U.S.S. Scorpion, our nuclear SSN which was lost in the Atlantic in 1967. We believe that we captured on sound detection devices the sequential collapse of pressure-tight bulkheads as it crushed, indicating that the hull breached to sea pressure at some point (certainly the bow compartment based on the cause for sinking) and then as the boat sank and the rising sea pressure exceeded the strength of the bulkheads they collapsed in turn from forward to aft. Of course, in this case, each compartment is acting as its own pressure-tight space. In both submarine cases, of course, the indication is that the whole sub did not crush all at once, but experience hull failure at one point, and then had a pressure wave travel inside the boat. (While it’s horrifying to consider, there is a good likelihood that the men in the rear-most compartment of the Scorpion heard the collapsing of the forward bulkheads before the engine room bulkhead failed.) Given the experience of the Soviet sub, we also know that the forces are strong enough to rip machinery from it’s mounts and move it, so this adds the final event on the bodies inside the sub, as they are mutilated in varying ways as they are caught in both the movement and crunch of machinery and/or collapsing bulkheads or the hull."

" Theoretically, temperature rise during compression is given by:

T2/T1=(P2/P1)^((n-1)/n)) where n is about 1.3 for air.

If we start at 70F, T1= 530 R (degrees Rankine). P1 = 1 atmosphere P2 = 2000 ft water = 867 psi = 59.4 atm

We have:

T2/530 = (59.4)^(1.3-1)/1.3)

T2 = 530(59.4)^(0.231) T2 = 5302.57 = 1362 deg R = 901 deg F

Hot indeed!"

http://www.fleetsubmarine.com/phorum/read.php?f=5&i=20&#...

"I've heard speculation that the implosion would be similar to what happens when you set off a bomb calorimeter, in which the extreme pressure rise would heat up the internal atmosphere to the point that combustible materials would be incinerated as the water rushed in.

For illustration purposes, it can be calculated that a hole in the submarine's hull of just 1-foot diameter (at a depth of 800 feet) would fill the associated compartment in just a few seconds. Larger hull ruptures would flood the submarine essentially instantaneously."

http://boards.straightdope.com/sdmb/showthread.php?t=553491

"The myth that I heard a number of times when I was in the Navy was that the air in the sub will actually detonate due to the sudden increase in pressure which causes it to heat rapidly, and the crew will die from the ensuing firestorm long before they get a chance to die from drowning or the pressure."

"The "long before" comment was considering that all the things that are going to happen to you in this scenario are going to happen pretty quick. But the implosion will be ship-wide since the hull is likely to give way in more than one location and, LA class subs at least, effectively have only one compartment with people inside (there are actually 2 but the door is usually open). The pressure will be felt throughout the ship at the same time and therefore the fire will happen ship-wide as well."

http://community.discovery.com/eve/forums/a/tpc/f/9701967776...

Ignoring shrinkage of the chamber (not insignificant), yes the human chamber will be at the same pressure at the surface and at the bottom. Counting in shrinkage due to pressure, the absolute pressure change will be less than the absolute pressure change you experience when you get on an airplane.

> Because of course the problems of pressure are reversed in deep sea submersion as compared to high altitude flight.

Of course the pressure problems are reversed. But both the airplane and the submarine prevent humans from experiencing decompression sickness by keeping the absolute change in pressure small enough that decompression sickness isn't an issue.

It's a sealed unit, so presumably it's "pressurized" at ~1 atmosphere by the way of closing it up prior to launch.

Lets say a human can cope with 10 atmospheres. (I have no idea what a human can cope with.) That still leaves 990, plus extra for safety margin.

(http://www.extremetech.com/extreme/121183-the-tech-that-will...)

(http://news.ycombinator.com/item?id=3680367)

I agree with commentators here. This, and other deep sea stuff, is awesome. I'm fascinated that it's being done by private companies, and the competition between Cameron and Branson just adds to the awesomeness. (Branson appears to be losing badly.)

I don't believe there is any specific limit, as long as you give enough time to change the pressure gradually. However at higher pressures you need to reduce the oxygen and co2 concentrations (since at higher pressures they dissolve too easily). (You want to keep the partial pressure constant, even though the absolute pressure goes up.)