The hyperbaric atmosphere within the DDC and PTC is controlled to maintain the
gaseous components as follows:
Oxygen Partial Pressure .44 – .48 ata
Carbon Dioxide Partial Pressure Less than 0.005 ppCO2 (.5% SEV) (3.8 millimeters of mercury)
Helium and Nitrogen Balance of total pressure
Oxygen levels and time limits are presented in Table 15-3.
These levels, particularly that of oxygen, are essential for safe decompression and
the use of the Unlimited Duration Excursion Tables. Increases in the oxygen
partial pressure above 0.6 ata for extended periods (greater than 24 hours) risk
pulmonary oxygen toxicity and should only be used in emergency situations. A
ppO2 below 0.42 ata may result in inadequate decompression, and a ppO2 below
0.16 ata will result in hypoxia. Once carbon dioxide concentration reaches 0.5
percent surface equivalent (3.8 millimeters of mercury) for 1 hour, the scrubber
canister should be changed, because carbon dioxide levels tend to rise rapidly
thereafter. An inspired carbon dioxide level of 2 percent surface equivalent (15.2
millimeters of mercury) can be tolerated for periods of up to 4 hours at depth.
Nitrogen concentration tends to decrease with time at depth, due to purging by
helium during service lock operation.
NOTE Discharging UBA gas into the PTC during diving operations may make it
difficult to control the oxygen level.
TABLE 15-3 Chamber Oxygen Exposure Time Limits.