The open-circuit BIB mask exhaust leads to the chamber itself directly from
the mask, or to an overboard dump system, which discharges the unused gas
outside the chamber. The chamber exhaust or the overboard dump is preferred
to prevent an uncontrolled O2 buildup within the chamber which will present
a fire hazard. If the mask exhausts directly into the chamber, the
ventilation requirements specified by the U.S. Navy Diving Manual (12.5 cubic
foot per minute per man at rest, or 25 cubic foot per minute per man not at
rest) are excessive. The intent of the Diving Manual (Reference (3)) is to
maintain the oxygen level in the chamber at less than 22.5 percent. An
overboard dump system should have minimal exhalation resistance and fail-safe
provisions against depressurization in case of component malfunction. The
overboard dumps are in effect back pressure regulators which must pass a
large flow with only a very slight pressure rise.
Section 5.
ATMOSPHERE MONITORING
1.
ENVIRONMENT BREATHING GAS ANALYSIS. The quality of the environment and
the breathing gasses within the chamber complex must be continuously
monitored for the safety of the occupants. The monitoring must consist of a
combination of an on-line, full-time analysis of three important
constituents: oxygen, carbon dioxide, and total hydrocarbons. In addition to
the continuous analysis, there shall be a periodic (three times a week during
a saturation dive) gas sample taken and sent to a gas laboratory for a
complete gas analysis. This analysis will not only provide a record of the
complete constituency of gas within the chamber, but it will also verify the
accuracy of the on-line systems.
In the continuous on-line analysis shown in Figure 6-26, gas is drawn from
the chamber through a gas sample penetrator equipped inside with a suction
protection tee, designed to protect against injury due to a negative
pressure.
Outside the chamber, a hull stop globe valve followed by a pressure reducing
regulator is installed. The pressure regulator has been found to work best
when set at 15 psi output. Capillary tubing from the regulator to the
analysis equipment provides rapid transfer of gas from the chamber to the
instruments for rapid monitoring of chamber environmental changes.
Calibration gasses are necessary to verify and calibrate the monitoring
instruments during operational use.
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