CHAPTER 17 Closed-Circuit Mixed-Gas UBA Diving
Change A 17-21
Primary Display. Check the primary display frequently as outlined in the MK
16 O&M manual (paragraph 3-4.6.1) to ensure that the oxygen level remains
at the setpoint during normal activity at a constant depth (the oxygen-addition
valve operation on the MK 16 cannot be heard).
Secondary Display. Check the secondary display frequently (every 2-3 min-
utes) as outlined in the MK 16 O&M manual (paragraph 3-4.6.2) to ensure that
all sensors are consistent with the primary display and that plus and minus bat-
tery voltages are properly indicating.
High-Pressure Indicators. Check the oxygen- and diluent-pressure indicators
frequently as outlined in the MK 16 O&M manual (paragraph 3-4.6.3) to
ensure that the gas supply is adequate to complete the dive.
The maximum ascent rate for the MK 16 is 30 feet per minute. During ascent,
when water pressure decreases, the diaphragm dump valve compensates for
increased gas volume by discharging the excess gas into the water. As a result,
oxygen in the breathing gas mixture may be vented faster than O2 is replaced by
the addition valve. In this case, the primary display may alternate red/green before
the low-ppO2 signal (blinking red) appears. This is a normal transition period and
shall not cause concern. Monitor the secondary display frequently on ascent and
add oxygen by depressing the bypass valve during this instance.
Postdive procedures shall be completed in accordance with the appropriate post-
dive checklists in the MK 16 UBA O&M manual.
When diving with an open-circuit UBA, ppO2 increases with depth. With a
closed-circuit UBA, ppO2 remains constant at a preset level regardless of depth.
Therefore, standard U.S. Navy decompression tables cannot be used.
Use of Constant ppO2 Decompression Tables. Closed-circuit UBA users must
use constant ppO2 decompression tables Oxygen in Nitrogen (air diluent), and
Oxygen in Helium (Helium-Oxygen diluent). Closed-circuit, mixed-gas UBA
decompression tables (Table 17-14 and Table 17-15) are included at the end of
Monitoring ppO2. During decompression, it is very important to frequently
monitor the secondary display and ensure a 0.7 ppO2 is maintained as closely as
possible. Always use the appropriate decompression table when surfacing, even if
UBA malfunction has significantly altered the ppO2.