DECOMPRESSION FROM He-N2-O2 (TRIMIX) BOUNCE DIVES IS NOT MORE EFFICIENT THAN FROM He-O2 (HELIOX) BOUNCE DIVES
Technical Report,01 Feb 2013,01 Jan 2015
NAVY EXPERIMENTAL DIVING UNIT PANAMA CITY FL Panama City United States
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Heliox He-O2 enables diving deeper than limits imposed by breathing N2-O2, but heliox has some costs, and several navies have pursued a trimix He-N2-O2 diving capability as an alternative to heliox. It is widely believed that trimix bounce dives can be conducted with substantially reduced decompression times than corresponding heliox dives. If this were true, trimix would be an attractive alternative to heliox for U. S. Navy MK 16 MOD 1 underwater breathing apparatus UBA diving. However, there is no direct evidence of greater decompression efficiency of trimix than of heliox. Decompression efficiency was assessed by comparing the incidence of decompression sickness DCS following decompression dives using MK 16 MOD 1 UBAs 1.3 atm PO2 set point with either heliox 88 He 12 O2 or trimix 44 He 44 N2 12 O2 diluent. Both trimix and heliox dives followed the identical depthtime schedule 200 fsw for 40 minutes bottom time followed by 119 minutes of decompression stops. This schedule was selected for having the largest difference in estimated probabilities of DCS between trimix and heliox among a range of candidate schedules that were practicable to man-test and operationally relevant. The trial ended at an interim stopping criterion with fifty man-dives completed on the heliox schedule with no diagnosed incidents of DCS, and forty-six man dives completed on the trimix schedule with two diagnosed incidents of DCS. The null hypothesis was retained decompression from trimix bounce dives is not more efficient than decompression from heliox bounce dives. Potential disadvantages of heliox with respect to cost, thermal balance, and voice communications are of limited relevance to MK 16 MOD 1 diving. In the absence of any decompression advantage, trimix is not an attractive alternative to heliox for U. S. Navy MK16 MOD 1 or other closed-circuit, self-contained diving.