The initial phase of the dive is the compression of the dive team to the selected storage depth. This phase includes establishing the chamber oxygen partial pressure at a value between 0.44 and 0.48 ata, instrument and systems checkouts, and the actual compression of the divers to storage depth.

Establishing Chamber Oxygen Partial Pressure

Prior to compression to storage depth, the chamber oxygen partial pressure shall be raised from 0.21 ata to 0.44– 0.48 ata. There are two methods of raising the oxygen partial pressure to the desired level.

  • Air Method. Compress the chamber with air at a moderate rate to 36 fsw. This will raise the chamber ppO2 to 0.44 ata. If desired, further elevation of the chamber ppO2 to 0.48 ata can be undertaken by using the oxygen makeup system.
  • Helium-Oxygen Method.Compress the chamber at a moderate rate with a helium-oxygen mixture containing less than 21 percent oxygen. The depth of the required compression can be calculated using the following formula:

            PPo2  - 0.21
Compression Depth (fsw)   =   33 * ------------ * 100

Example. If a 20 percent mixture of helium-oxygen is used and the desired ppO2 is 0.44 ata, calculate the compression depth.

            0.44   -  0.21
Compression depth   =   33 * ------------ * 100

Compression to Storage Depth

Rapid compression to saturation storage depth may provoke symptoms of High-Pressure Nervous Syndrome (HPNS) and may intensify compression joint pains. To avoid these complications, the slowest rate of compression consistent with operational requirements should be used. Table 15-6 shows the range of allowable compression rates.

If operational necessity dictates, compression to storage depth of 400 fsw or shallower can be made at the maximum rates indicated in Table 15-6 with little risk of HPNS. Direct compression at maximum rates to deeper storage depths, however, may produce symptoms of HPNS in some divers. These divers may be unable to perform effectively for a period of 24 to 48 hours. Experience has shown that the appearance of such symptoms can be minimized by slowing compression rates or introducing holds during compression.

The depth and time duration of holds, if used, may be adjusted to suit operational requirements and diver comfort.

TABLE 15-6

TABLE 15-6 Saturation Diving Compression Rates.

Precautions During Compression

During compression the chamber atmosphere shall be monitored carefully. The chamber atmosphere may not mix well during rapid compression, resulting in areas of low oxygen concentration.

Abort Procedures During Compression

The following abort procedure is authorized if a casualty occurs during compression. Consult with a Saturation Diving Medical Officer prior to committing to this procedure. This procedure is normally used for shallow aborts where the maximum depth and bottom time do not exceed the limits of the table.

Using the Surface Supplied HeO2 Tables, the following procedure applies:

  • Depth. Use the actual chamber depth.
  • Bottom Time. If the initial compression uses air, time spent shallower than 40 fsw, up to a maximum of 60 minutes, is not counted as bottom time. If the initial compression uses helium, time starts when leaving the surface.
  • BIBS Gas.Maintain BIBS between 1.5 – 2.8 ppO2.
  • Stops. Follow the scheduled stops of the Surface Supplied HeO2 Tables.
  • O2 Breaks. For every 25 minutes of breathing BIBS gas, take a 5-minute break breathing a gas between 0.16 to 1.25 ata ppO2. The 5-minute break counts as a stop time. The lower oxygen percentage shall not be less than 0.16 ata ppO2.

Upon completing abort decompression, all divers shall be closely monitored and observed for a minimum of 24 hours. For deeper emergency aborts beyond the limits of the Surface-supplied HeO2 Tables, refer to paragraph 15-23.7.2.

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