chamber atmosphere. Even when the treatment masks are provided with an
overboard dump system, oxygen leakage from the mask face seal can add to the
oxygen in the chamber.
Atmosphere. When a chamber is at surface pressure, the absolute
pressure in the chamber is 14.7 psia, this is 0.0 psig or one atmosphere (1.0
ATA). At one atmosphere, the partial pressure of oxygen is 3.09 psia and the
concentration is 21 percent, When a chamber is pressurized with air to 33
feet of seawater, the pressure in the chamber will be 29.4 psia or 14.7 psig
(2.0 ATA). At this pressure the partial pressure of oxygen will have
increased to 6.18 psia and the fire hazard will be much greater, although the
concentration is still only 21 percent. As a chamber is pressurized with
air, the partial pressure of oxygen and the fire hazard increase together,
Fire Zone. When the fire hazards in recompression chambers are
discussed, a condition referred to as the Fire Zone is frequently mentioned.
This Fire Zone, in which combustion can take place, is considered to exist
down to about 200 feet of seawater, after which the atmosphere is supposed to
be too dense to support combustion.
THIS IS A SERIOUS MISCONCEPTION.
When a chamber is pressurized with air, the fire hazard increases as the
pressure increases, apparently indefinitely.
The Fire Zone is a condition which is obtained when a chamber is pressurized
with air to 33 feet, at which time the oxygen pressure is 6.18 psia. After
reaching this depth, if the pressurization of the chamber is now continued
using helium, the ability of the chamber atmosphere to support combustion
will decrease from a maximum at 33 feet to essentially zero at 200 feet. At
this pressure, the thermal capacity and other effects of the helium makes
normal combustion essentially impossible.
THIS EFFECT IS THE FIRE ZONE.
Generally, the oxygen concentration in the chamber atmosphere for deep dives
is maintained at low levels to avoid oxygen toxicity. This decrease in the
oxygen percentage may also render the atmosphere incapable of supporting
combustion under certain carefully-defined conditions. Three zones
representing complete combustion, incomplete combustion, and noncombustion
are shown in Figure 8-2. From this figure it is possible to compute the
depth at which combustion will be supported for a given oxygen content in the
environment. Figure 8-3 compares the oxygen absolute pressure with the depth
in feet of seawater (FSW) showing the Fire Zone, which is defined to exist in
the range of oxygen content above the zone of noncombustion in Figure 8-2.
It should be noted that for chamber operation where the oxygen pressure is
maintained at 6 psia, the Fire Zone will range from the surface to 200 FSW.
When different concentrations of oxygen exist in the chamber, Figure 8-3
should be consulted to determine the appropriate depth of the Fire Zone.