General Discussions
This is the place to discuss general issues related to the U-boat war or the war at sea in WWII.
Re: modern escape gear mk9
Posted by:
Rob Hoole
()
Date: February 12, 2008 12:20PM
As most people in this forum already know, decompression sickness (the bends) is caused by nitrogen stored in the body manifesting as bubbles in the blood stream during rapid decompression (as when unsealing a bottle of fizzy lemonade). Breathing nitrogen under pressure can also cause nitrogen narcosis or 'rapture of the deep'.
Apart from difficulties associated with storage and handling, substituting helium for nitrogen as an oxygen diluent for submarine escape apparatus would bring its own problems. Apart from causing the distinctive 'Micky Mouse' voice, helium is an efficient conductor of cold and breathing it would accelerate the onset of hypothermia, already a major hazard for submariners prior to escape. That is why oxy-helium and trimix (oxygen, helium and nitrogen) gas mixtures supplied to deep divers are heated.
A 'diluent' is required when breathing oxygen under pressure because, according to accepted civilian standards, oxygen (O2) becomes toxic at a partial pressure greater than 1.6 Bar 'absolute' (i.e. 1 Bar atmospheric pressure plus 6m water depth (0.6 Bar)). However, several navies have adopted 1.8 Bar 'absolute' as the maximum allowable. Italian WWII human-torpedo divers breathed pure O2 to depths of 30m and then wondered why they had unexplained casualties. The onset of acute oxygen poisoning or CNS (Central Nervous System toxicity) varies between individuals and their exertions at the time. However, for most naval purposes, pure O2 is accepted as becoming toxic beyond water depths of 8m. On this basis, even the 21% proportion of O2 in normal air would cause oxygen poisoning at around 9 Bar 'absolute' (i.e. approx 80m depth) although some SCUBA air divers have experienced acute oxygen poisoning at depths as shallow as 50m, especially when working hard. Of course, in a stranded submarine, the partial pressure of oxygen would reduce as crew members replaced it with CO2 during respiration. However, this introduces a new set of problems as CO2 causes drowsiness above 1% of volume at atmospheric pressure (partial pressure of 0.01 Bar 'absolute') and becomes toxic (hypercapnic) at levels above 5% at atmospheric pressure (0.05 Bar 'absolute'). Dalton's Law of Partial Pressures means that even smaller percentages will have these effects at depth.
There is little point in providing a gas supply for submariners during the short time spent in ascent.
For a video showing the British MK-10 Submarine Escape and Immersion Equipment (SEIE) being used in the Submarine Escape Training Tank (SETT) at Gosport, see [submariners.co.uk].
Rob Hoole
[www.mcdoa.org.uk]
Apart from difficulties associated with storage and handling, substituting helium for nitrogen as an oxygen diluent for submarine escape apparatus would bring its own problems. Apart from causing the distinctive 'Micky Mouse' voice, helium is an efficient conductor of cold and breathing it would accelerate the onset of hypothermia, already a major hazard for submariners prior to escape. That is why oxy-helium and trimix (oxygen, helium and nitrogen) gas mixtures supplied to deep divers are heated.
A 'diluent' is required when breathing oxygen under pressure because, according to accepted civilian standards, oxygen (O2) becomes toxic at a partial pressure greater than 1.6 Bar 'absolute' (i.e. 1 Bar atmospheric pressure plus 6m water depth (0.6 Bar)). However, several navies have adopted 1.8 Bar 'absolute' as the maximum allowable. Italian WWII human-torpedo divers breathed pure O2 to depths of 30m and then wondered why they had unexplained casualties. The onset of acute oxygen poisoning or CNS (Central Nervous System toxicity) varies between individuals and their exertions at the time. However, for most naval purposes, pure O2 is accepted as becoming toxic beyond water depths of 8m. On this basis, even the 21% proportion of O2 in normal air would cause oxygen poisoning at around 9 Bar 'absolute' (i.e. approx 80m depth) although some SCUBA air divers have experienced acute oxygen poisoning at depths as shallow as 50m, especially when working hard. Of course, in a stranded submarine, the partial pressure of oxygen would reduce as crew members replaced it with CO2 during respiration. However, this introduces a new set of problems as CO2 causes drowsiness above 1% of volume at atmospheric pressure (partial pressure of 0.01 Bar 'absolute') and becomes toxic (hypercapnic) at levels above 5% at atmospheric pressure (0.05 Bar 'absolute'). Dalton's Law of Partial Pressures means that even smaller percentages will have these effects at depth.
There is little point in providing a gas supply for submariners during the short time spent in ascent.
For a video showing the British MK-10 Submarine Escape and Immersion Equipment (SEIE) being used in the Submarine Escape Training Tank (SETT) at Gosport, see [submariners.co.uk].
Rob Hoole
[www.mcdoa.org.uk]
