The following gases shall be available for use in a UBA, for emergency supply, and for the treatment of decompression sickness.

NOTE Discharging UBA gas into the PTC during diving operations may make it difficult to control the oxygen level.


An adequate quantity of gas within an oxygen partial pressure range of 0.44–1.25 ata shall be available for use.

Emergency Gas

Emergency gas is used as a backup breathing supply in the event of DDC or PTC atmosphere contamination. An emergency gas with an oxygen partial pressure of 0.16 to 1.25 ata shall be immediately available to the built-in breathing system (BIBS). The volume of emergency breathing gas shall be sufficient to supply the divers for the time needed to correct the DDC atmosphere.

Upward excursions of the PTC or DDC or decompression shall not be started during emergency gas breathing unless the oxygen partial pressure of the diver’s inspired gas is 0.42 ata or above.

Example. An emergency gas schedule for a dive beyond 850 fsw is:

Bank Mix #1 84/16 HeO2 #2 96/4 HeO2 Allowable Depth Range (fsw) 0–224 99–998 Shift Depth (fsw) 200 99

Treatment Gases

Treatment gases having an oxygen partial pressure range of 1.5 to 2.8 shall be available in the event of decompression sickness. The premixed gases shown in Table 15-4 may be used over the depth range of 0 – 1,600 fsw. A source of treatment gas shall be available as soon as treatment depth is reached. The source shall be able to supply a sufficient volume of breathing gas to treat each chamber occupant.

TABLE 15-4

TABLE 15-4 Treatment Gases.

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