14-2 Change A
U.S. Navy Diving ManualVolume 3
Water Temperature. Loss of body temperature (hypothermia) can be a major
problem during long, deep dives. A hot water suit is preferred for surface supplied
dives in cold water.
Gas Mixtures. Four gas mixtures are required to dive the surface supplied mixed
gas tables over their full range:
Bottom Mixture - The bottom mixture may vary from 90% helium 10% oxy-
gen to 60% helium 40% oxygen depending on the diver's depth. The allowable
range of bottom mixtures for each depth is shown in Table 14-3.
50% Helium 50% Oxygen - This mixture is used from 90 fsw to 40 fsw during
decompression. Oxygen concentration in the mixture may range from 49 to 51
100% Oxygen - Oxygen is used at the 30 and 20 fsw water stops during
inwater decompression and at 50, 40 and 30 fsw in the chamber during surface
Air - Air is used as an emergency backup gas throughout the dive and to
provide air breaks during oxygen breathing.
Helium oxygen mixtures must be analyzed for oxygen content with an instrument
having an accuracy of ±0.5 percent.
Emergency Gas Supply. All divers are equipped with an emergency gas supply
(EGS). The EGS gas mixture shall be the same as the bottom mixture unless the
bottom mixture contains less than 16 percent oxygen, in which case the EGS gas
mixture may range from 15 to 17 percent oxygen. The EGS shall be an adequately
charged ANU approved scuba cylinder. An adequately charged scuba cylinder is
defined as: the pressure that provides sufficient gas to bring the diver to his first
decompression stop or the surface for no-decompression dives. It is assumed that
this will give topside personnel enough time to perform required emergency
procedures to restore surface supplied air.
SURFACE SUPPLIED HELIUM OXYGEN DESCENT AND ASCENT PROCEDURES
The surface supplied Helium Oxygen Decompression Table (Table 14-3) 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
Selecting the Bottom Mix. The Surface Supplied Helium Oxygen Decompression
Table (Table 14-3) specifies maximum and minimum concentrations of oxygen
allowable in the helium oxygen mixture at depth. The maximum oxygen concen-
tration 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 14 percent for depths to 200 fsw and 10 percent for depths in excess of
200 fsw. Diving with a mixture near maximum oxygen percentage is encouraged