Quote:
Originally Posted by irish1958
I have always wondered: why can't the sub exhaust the stale air with the compressed air?
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Because then you'd be compressing the air inside the sub, requiring decompression stops before you could surface.
For every 33ft (so call it 10 meters?) of depth, 1 atmosphere of pressure (14 psi) is added to the pressure outside the hull. In order to vent the bad air with the good air, you'd have to make the pressure inside the hull greater than the outside pressure. at 100m, that's 10 atm of pressure. At that pressure, gaseous nitrogen liquefies into the bloodstream and lodges in various locations in your bodies, primarily the joints, lung and brain tissue.
The pressure would have to be very slowly bled off to allow the crew back onto the surface. If you were at 150m, and vented the air, and then had to surface, You couldn't just pop the hatch. The rapid depressurization would cause every body to rapidly, and very painfully, die.
US Navy dive charts for recreational divers (and I'm doing this from memory), state that at about 110ft (30m +/-). humans have about 5 minutes of bottom time before they are required to make decompression stops on their ascent back to the surface. I am not familiar with deco charts so I cannot speak for times, but Google should present you with some answers on that one. But if at 110 ft, you have 5 min max, imagine how long it would take to deco from a 450 ft (150m) dive for 10 hours. In comparison, at 10m, humans can stay something like 120 minutes, just shows the logarithmic scale of the chart.
Also, rapid pressurization can cause Nitrogen narcosis. Imagine going from sober to 10 beers in 5 seconds. That's what it feels like, been there done that. Some people do it for fun, but a whole crew full of instantly drunk guys is a bad idea IMO.
They could (in theory) slowly add some air to the sub, forcing to CO2 to a lower level closer to the floor, allowing the crew to breathe better when standing. But they could only do a little of this, for reasons I outlined above.
Maybe having some sort of pump force the air out, while slowly letting good air in would work, but any discrepancy in pressures would cause great discomfort to the crew, possibly death.