Deco for Divers by Mark Powell

Author:Mark Powell
Language: eng
Format: epub, mobi
Publisher: Aqua Press

2400–0600

Rest Stop

0600–1400

Travel

1400–1600

Rest Stop

1600–2400

Travel

TABLE 24: Daily decompression schedule

This type of schedule minimises travel when the divers are normally sleeping. However, this specific daily routine is not mandatory. Other 16-hour periods of travel per 24- hour routines are acceptable, although they usually include at least two stop periods dispersed throughout the 24-hour period and may continue decompression while the divers are asleep.

After surfacing from a saturation dive, the divers are still at risk from decompression illness as the slowest tissue is still close to critical supersaturation. As a result saturation divers will remain in the immediate vicinity of a chamber for 2 hours after completion of decompression and stay within 30 minutes travel of a chamber for a further 48 hours. Saturation divers should not fly for 72 hours after the completion of a dive.

Other than adjusting the ascent rate the only other factor that can be used to affect saturation decompression is the partial pressure of oxygen breathed in the DDC. A higher partial pressure of oxygen means a lower fraction of inert gas than would otherwise be present. This allows the inert gas in the body to off-gas faster.

The maximum ascent rate is directly related to the oxygen partial pressure. However the long duration of saturation dives and decompression means that a partial pressure of 0.5 bar or above is likely to lead to problems with oxygen toxicity. To balance out the benefit of faster off-gassing with the risk of pulmonary oxygen toxicity a partial pressure of between 0.4 and 0.5 bar is often used. Some commercial companies use slower ascent rates coupled with a higher partial pressure to try and increase the safety of decompression. In a series of experimental dives carried out to determine the benefits of higher partial pressures of oxygen it was shown that raising the inspired oxygen partial pressure from 0.22 atm to 0.4 atm reduced the incidence of DCS from 52% (14 incidents in 27 dives) to zero (no incidents in 42 dives).

Despite significant research into saturation decompression, most saturation decompression rates were based on trial and error rather than any specific decompression theory. A simple model relating the rate of ascent from a saturation dive to the inspired oxygen partial pressure assumes that ascent rate is a linear function of oxygen partial pressure. This can be expressed as

rate = k ppO2

where rate is ascent rate, ppO2 is oxygen partial pressure and k is an empirically determined constant. To develop a saturation decompression procedure, the value of k and the resulting ascent rate is adjusted in successive dives until the incidence of DCS becomes acceptably low.

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