that the inhabitant becomes helpless at an oxygen partial pressure of 0.12
atm. In the region of high oxygen concentrations, it may be seen that oxygen
toxicity limits the maximum partial pressure for long-term exposure to about
1.2 atm, but that higher concentrations can be tolerated for short periods.
These toxicity limits are imposed by effects on the central nervous system
that result in unconsciousness and convulsions. A different limit is
encountered in exposures over extended periods, which is imposed by lung
irritation and eventual lung damage. This occurs with 10 percent incidence
in a 14-day period with an oxygen partial pressure of 0.6 atm. In order to
avoid pulmonary irritation, present practice in exposures for extended
periods is to limit oxygen partial pressure to about 0.3 atm. (See Reference
(1), NAVSHIPS 0994-003-7010, U.S. Navy Diving-Gas Manual and Reference (2),
Lambertson, Limitations and Breakthroughs in Manned Underseas Activity.)
c.
Oxygen versus Depth Ranges. From inspection of Figure 6-1 it is
evident that, at any fixed pressure, it is feasible to breathe a wide range
of mixtures without ill effects. For example, at a pressure depth of 200
feet, the mixture could be as lean as 3 percent oxygen (0.21 atm) or as rich
as 17 percent oxygen (1.2 atm) without encountering any short-term
physiological limits. Likewise, for a given oxygen concentration in the
breathing mixture, the chamber inhabitant can make excursions over a fairly
wide depth range. A mixture containing 10 percent oxygen, for example, would
permit operations between 36 and 360 feet, with the possibility of short
excursions as deep as 600 feet.
CAUTION:
It is important to note that dangerous anoxia can
occur if lean mixtures, intended for deep operations,
are breathed during ascent and decompression. It is
necessary to provide higher oxygen concentrations at
shallower depths to stay within the working range
indicated in Figure 6-1.
d.
Diluent Gasses. The oxygen percentages shown in Figure 6-1 are
applicable with any diluent gas or mixture of diluent gasses.
Oxygen-nitrogen mixtures, including air, can be used freely if the nitrogen
partial pressure is limited to 3 atm, the pressure at which the first
symptoms of nitrogen narcosis appear. However, nitrogen partial pressures to
5.5 atm can be used with care by experienced divers for short periods (see
Reference (3), U.S. Navy Diving Manual). (This is equivalent to diving at
200 feet with air.) For depth greater than 200 feet, helium is the diluent
gas now generally used because it is nontoxic at any practical depth and
because its low density reduces breathing effort. When helium is used, it
introduces two serious problems which must be solved: Occupant body heat loss
due to extremely high heat transfer coefficient of helium and occupant
high-pitched speech due to the sonic velocity of helium. The primary body
heat loss problem can be solved by raising the chamber temperature under
pressure at least 15 deg. F above normal for diver comfort. Because the
occupant cannot be returned to normal pressure and temperature immediately,
the PV must have a good heating system with precise temperature control and
at least one backup heat source to keep the occupant warm for a mission of
many days. The high-pitched speech problem can be eliminated by the use of
the helium speech unscrambler for communications.
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