The MK 16 UBA is fabricated of Acrylonitile Butadiene Styrene (ABS) or polycarbonate,
nylon, brass, neoprene and other nonmagnetic materials. By necessity,
however, certain components such as oxygen and diluent bottles (high-pressure
components) are fabricated of Inconel 718 which may have a magnetic signature
imparted to them. The components and materials used in the MK 16 UBA have
been specifically selected and assembled to exhibit a minimum magnetic
signature.
Diving Safety. Closed-circuit mixed-gas UBAs are mechanically more complex
than open-circuit scuba. Diving safety is achieved only when:
-
The diver has been thoroughly trained and qualified in the proper use of the
UBA.
-
All equipment has been prepared for the specific diving conditions expected.
-
The dive is conducted within specified depth and duration limits.
-
The diver strictly adheres to and immediately implements all operational and
emergency procedures.
MK 16 UBA Basic Systems. The MK 16 UBA is broken down into four basic
systems (housing, recirculation, pneumatics, and electronics) and their subassemblies
as described in the following paragraphs. These systems provide a contolled
ppO2 breathing gas to the diver.
Housing System. Major components of the MK 16 UBA are housed in a reinforced
ABS or fiberglass, molded case. The equipment case is a contoured
backpack assembly designed for minimum interference while swimming, and is
equipped with an integral harness assembly. A streamlined, readily-detachable
outer cover minimizes the danger of underwater entanglement. External to the housing are components such as the mouthpiece, pressure indicators, hoses, and
primary and secondary displays.
Recirculation System. The recirculation system consists of a closed loop incorporating
inhalation and exhalation hoses, a mouthpiece or FFM, a carbon dioxideabsorbent
canister, and a flexible breathing diaphragm. The diver’s breathing
gases are recirculated to remove carbon dioxide and permit reuse of the inert
component of the diluent and residual oxygen in the breathing mixture. Inhalation
and exhalation check valves in the mouthpiece assembly (or manifold of the FFM)
ensure the unidirectional flow of gas through the system.
Closed-Circuit Subassembly. The closed-circuit subassembly has a removable
cover, a center section attached to the fiberglass equipment case, a flexible rubber
breathing diaphragm, and a CO2 scrubber assembly. Moisture-absorbent pads
inside the scrubber assembly absorb any condensation formed on the cover walls.
The space between the scrubber canister and the cover serves as a gas plenum,
insulating the canister from the ambient cold water.
Scrubber Functions. The scrubber has two functions:
-
Carbon Dioxide Removal. Before the diver’s exhaled breath reaches the
breathing diaphragm, it passes through the scrubber canister. The scrubber
canister is filled with an approved, high efficiency, granular carbon dioxideabsorbent
material. Two filter discs in the scrubber canister serve as gas distributors
to minimize effects of any channeling in the absorbent. After passing
through the filters, the exhaled gas passes through the carbon dioxide-absorbent
bed, chemically combining with the carbon dioxide created by metabolic
use of the diver’s breathing oxygen but allowing the diluent and unused oxygen
to pass through it.
-
Water Removal. Moisture produced by diver exhalation and the reaction
between carbon dioxide and carbon dioxide-absorbent is assimilated by moisture-
absorbent pads located outside the canister.
Pneumatics System. The pneumatics system comprises:
-
High-pressure bottles for storing oxygen and diluent gases.
-
Indicators to permit monitoring of the
remaining gas supply.
-
Regulators, fittings, tubing, filters and valves regulate and deliver oxygen and
diluent gases to the recirculation system.
Electronics System. The electronics system maintains a constant partial pressure
of oxygen in the closed-circuit UBA by processing and conditioning signal
outputs from the oxygen sensors located in the breathing loop, stimulating the
oxygen-addition valve, and controlling the output of the primary display.
Oxygen Sensing. The partial pressure of oxygen within the recirculation system
is monitored by three sensors. Each sensor’s output is evaluated by the primary
electronics package through a voting logic circuit negating the output from a
faulty sensor. Sensor averages are shown by the primary display. Backup reading
of each individual sensor can be read on the secondary display which requires no
outside power source.
Oxygen Control. Oxygen concentration in the recirculation system is measured
by sensors. The sensors send signals to the primary electronics assembly and the
secondary display. The primary electronics assembly compares these sensor
signals with the setpoint value, providing output to the primary display and
controlling the oxygen-addition valve. An actual ppO2 value less than the setpoint
automatically actuates the oxygen-addition valve to admit oxygen to the breathing
loop.
Oxygen control involves several factors:
-
System Redundancy. The primary electronics assembly in the MK 16 UBA
treats each of the sensor signals as a vote. The sensor vote is either above or
below the predetermined setpoint. If a simple majority of the sensors is below
the predetermined setpoint, a drive signal is sent to the oxygen-addition valve;
when a majority of the sensors is above the predetermined setpoint, the signal
is terminated. In effect, the electronics circuit ignores the highest and lowest
sensor signals and controls the oxygen-addition valve with the middle sensor.
Similarly, the electronics circuit displays a high-oxygen alarm (flashing green)
if a majority of the sensors’ signals indicates a high oxygen level and displays
a low-oxygen alarm (flashing red) if a majority of the sensors’ signals indicates
a low oxygen level. If only one sensor indicates a high oxygen level
and/or only one sensor indicates a low oxygen level, the electronics circuit
output alternates between the two alarm states (alternating red/green)
-
Setpoint Calibration. The normal operational ppO2 setpoint for the MK 16
UBA is 0.75 ata. Appropriate calibration procedures are used to preset the specific
ppO2 setting.
-
Oxygen Addition. In response to the sensor outputs, the oxygen-addition valve
admits oxygen to the breathing loop in the recirculation system. The control
circuits continuously monitor the average ppO2 level. If the oxygen partial
pressure in the recirculation system is lower than the setpoint level, the oxygen-
addition valve is energized to admit oxygen. When the ppO2 reaches the
required level, the automatic control system maintains the oxygen-addition
valve in the SHUT position. Should the oxygen-addition valve fail in an
OPEN position, the resulting free flow of oxygen in the MK 16 is restricted by
the tubing diameter and the orifice size of the piezoelectric oxygen-addition
valve.
Displays. The MK 16 UBA has two displays that
provide continuous information to the diver about ppO2, battery condition,
and oxygen sensor malfunction.
Primary Display. The primary display consists of two light-emitting diodes
(LEDs) that are contained within the primary display housing. This display is
normally mounted on the face mask, within the peripheral vision of the diver
(Figure 17-4). The two LEDs (one red and one green) powered by the primary
electronics assembly battery indicate the general overall condition of various electronic
components and the ppO2 in the breathing loop as follows:
-
Steady green: Normal oxygen range, 0.60 to 0.90 ata ppO2 (using a set point
of 0.75 ata)
-
Steady red or simultaneously illuminated steady red and green: Primary
electronics failure
-
Flashing green: High oxygen content, greater than 0.90 ata ppO2
-
Flashing red: Low oxygen content, less than 0.60 ata ppO2
-
Alternating red/green: Normal transition period (ppO2 is transitioning from
normal to low, from low to normal, from normal to high, or from high to normal),
one sensor out of limits, low primary battery power (displayed on
secondary display) or primary electronics failure.
-
No display (display blanked): Electronics assembly or primary battery
failure.
Secondary Display. The MK 16 secondary display is designed to provide quantitative
information to the diver on the condition of the breathing medium, the
primary battery voltage and the condition of the secondary batteries. It also serves
as a backup for the primary display in the event of a failure or malfunction to the
primary electronics assembly, the primary display, or the primary battery. The
secondary display functions concurrently with, but independently of, the primary
display and displays the O2 sensor readings and primary battery information in
digital form. The secondary display is powered by four 1.5-volt batteries for illumination
of the LED display only. It does not rely on the primary electronics
subassembly, but receives signals directly from the oxygen sensors and the
primary battery. It will continue to function in the event of a primary electronics
assembly failure. See Figure 17-4.
