Re: [PSUBS-MAILIST] Scuttle valve

[Peter McKellar <mckellar@earthlink.net>]

Hi All,

I had wanted to make a comment regarding floatation devices etc that has been bypassed in all the posts of late.  Fly deep does make a very valid point regarding flooding his craft if it is ABS rated to 300 metres.  One assumes that it would in fact be engineered to a crush depth around 600m and actually been tested to 450m.

with the free ascent record around 300 FEET(!) any attempt to leave the sub at a worse case of 599m depth would be immediate suicide.  Its a simple case of pick your poison.

could someone check some decomp charts and advise what is the maximum depth you could free ascend from (bounce dive?) and not suffer the bends?  The charts in this case assume that the diver has descended to this depth, absorbing nitrogen as they go.  They don't calculate rapid pressurisation from 1ATM to ambient without ensuing travel time, so the value would likely be VERY conservative.  Maximum bailout depth without surface support (hyperbaric chamber) may well be fatal from well above the 300 foot record.

The real issue is when you find yourself grounded in 10m of water a mere 200m from shore and no-one knows you are there.

I would also like to throw my bit in about free ascent and bouyancy.  Below 30m (90 ft) a human body is negatively bouyant (you sink), above that you float.  The change is slight and linear as the gases in your body get compressed and change density.  Because of this a compressed lungful of (ambient pressure) air makes little difference to your bouyancy (unless they could harmlessly expand like a ballon to allow the overall density to be correspondingly lower as you rose).

The quantity of air would also be far greater than the two lungsful as mentioned.  Add one more lungful for every 10m of depth.  SCUBA bouyancy vests frequently permit the diver to breath into (or in emergency from) their vests, but I would not recommend a direct airway coupling as suggested.  For the pressure to equalise properly in your lungs you don't want to risk any back pressure or impeding the flow in any way.  Maybe the suggested 'parachute' would work ok.

For emergency gear (per person), I would be stowing a small air bottle (maybe just the one on many commercial bouyancy vests) and a bouyancy vest.  I figure if I have to exit and I don't have a support boat - what then?  Ability to float is one thing but longer term survival becomes an important issue.  I suspect in these cases I should plan for significantly more emergency gear (inflatable liferaft, radio, rations etc?)

cya
peter