Planning The Operation

Planning surface-supplied mixed-gas dives involves many of the same considerations used when planning an air dive. Planning aspects that are unique to surface-supplied 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.

14-2.1 Depth and Exposure Time. 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.

14-2.2 Water Temperature. 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.

Refer to Chapter 3 for more information on thermal problems and the signs and symptoms of hypothermia. Refer to Chapter 11 for information on ice and cold water diving operations.

14-2.3 Gas Mixtures. 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.

14-2.3.1 Maximum/Minimum Mixtures. 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.

14-2.3.1.1 On the Surface. 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).

14-2.3.1.2 Deeper than 200 fsw. 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).

14-2.3.1.3 Up to 200 fsw. 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.

14-2.3.1.4 Exceptional Exposure Dives. 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.

14-2.3.2 Emergency Gas Supply. 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.

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