Planning surface-supplied mixed-gas dives involves many of the same considerations
used when planning an air dive. Planning aspects that are unique to surfacesupplied
mixed-gas diving include the logistics of providing several different gas
mixtures to the diver and the limitations on the duration of carbon dioxide absorption
canisters in cold water.
The normal operational limit for surface-supplied
mixed-gas diving is 300 fsw. Within each decompression table (Table 14-7),
exceptional-exposure dives are enclosed in red boxes to separate them from
normal working dives. Exceptional-exposure dives require lengthy decompression
and are associated with an increased risk of decompression sickness and exposure
to the elements. Exceptional exposures should be undertaken only in emergency
circumstances. Planned exceptional-exposure dives require prior CNO approval.
Repetitive diving is not allowed in surface-supplied helium-oxygen diving.
table 14-7 Surface-Supplied Helium-Oxygen Decompression Table.
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table 14-7a Surface-Supplied Helium-Oxygen Decompression Table (Continued).
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table 14-7bSurface-Supplied Helium-Oxygen Decompression Table (Continued).
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table 14-7c Surface-Supplied Helium-Oxygen Decompression Table (Continued).
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table 14-7d Surface-Supplied Helium-Oxygen Decompression Table (Continued)
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table 14-7e Surface-Supplied Helium-Oxygen Decompression Table (Continued)
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table 14-7f Surface-Supplied Helium-Oxygen Decompression Table (Continued)
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Loss of body temperature (hypothermia) can be a major
problem during long, deep dives. Because the high thermal conductivity of helium
in a dry suit accelerates the loss of body heat, a hot water suit is preferred for
surface-supplied dives when using the MK 21 MOD 1 in very cold water.
Loss of body temperature (hypothermia) can be a major
problem during long, deep dives. Because the high thermal conductivity of helium
in a dry suit accelerates the loss of body heat, a hot water suit is preferred for
surface-supplied dives when using the MK 21 MOD 1 in very cold water.
Air, 100 percent oxygen, and several helium-oxygen mixtures will
be required to dive the surface-supplied mixed-gas tables over their full range. The
logistics of supplying these gases must be carefully planned. Analysis of the oxygen content of helium-oxygen mixtures shall be accurate to within ±0.5
percent.
For each depth in the decompression tables, the
allowable maximum and minimum oxygen percentage in the helium-oxygen
mixture used on the bottom is specified. For operations planning, the range of
possible depths should be established and a mixture selected that will meet the
maximum/minimum specification across the depth range. The maximum oxygen
concentration has been selected so that the diver never exceeds an oxygen partial
pressure of 1.3 ata while on the bottom. The minimum oxygen percentage allowed
in the mixture is 16 percent for depths to 200 fsw, 12 percent for depths from 200
fsw to 300 fsw, and 10 percent for depths in excess of 300 fsw. Diving with a
mixture near maximum oxygen percentage is encouraged as it offers a decompression
advantage to the diver.
On the surface, the diver’s gas mixture must contain a minimum
of 16 percent oxygen. When a bottom mix with less than 16 percent oxygen is to
be used, a shift to the bottom mix is made at 20 fsw during descent (see paragraph
14-3.2).
For dives deeper than 200 fsw in which the bottom mixture
contains less than 16 percent oxygen, a gas shift from the bottom mix to a 60
percent helium/40 percent oxygen mixture is required at the 100-fsw decompression
stop or the next shallower stop if there is no 100-fsw stop (see paragraph
14-3.3).
For dives to 200 fsw and shallower or for deeper dives in which
the bottom mixture contains more than 16 percent oxygen, a shift to 60 percent
helium/40 percent oxygen is not required but can be executed to increase decompression
safety if desired.
For exceptional-exposure dives, a shift to a 60
percent helium/40 percent oxygen mixture is required at the 100-fsw stop or the
next shallower stop if there is no 100-fsw stop.
On all dives, a shift to 100 percent oxygen is made at the 50-fsw or 40-fsw water
stop if there is no 50-fsw stop.
All divers are equipped with an emergency gas supply
(EGS). The EGS gas mixture will be the same as the bottom mixture unless the
bottom mixture contains less than 16 percent oxygen, in which case the EGS gas
mixture will be 16 + 0.5 percent oxygen and the balance will be helium. The EGS
bottle shall be a minimum of 64.7 (steel 72) cubic feet charged to 1,800 psi.
SURFACE-SUPPLIED HELIUM-OXYGEN DESCENT AND ASCENT PROCEDURES
The Surface-Supplied Helium-Oxygen Decompression Table (Table 14-7) is used
to decompress divers from surface-supplied helium-oxygen dives. The table is in a
depth-time format similar to the U.S. Navy Air Decompression Table and is used in a similar fashion. One additional table, the Emergency Procedures Decompression
Table (Table 14-1), is used under emergency conditions (see paragraph
14-4.4).
