From:
THE TWILIGHT ZONE: THE POTENTIAL, PROBLEMS, AND THEORY BEHIND USING MIXED GAS, SURFACE-BASED SCUBA FOR RESEARCH DIVING BETWEEN 200 AND 500 Feet. (Sharkey & Pyle, 1992)
USING TRIMIX TO OPTIMIZE DECOMPRESSION, NARCOSIS AND COST
The characteristics of the available mixes for deep diving can be illustrated with dive profiles prepared for the Wakulla Springs Project using the Hamilton Research DCAP computational program (Stone, William, 1991: "The Case for HELIOX, aquaCorps Journal, V3 N1, Sport Diver Publishing Group, Dania Beach, FL, USA). These profiles are 300 foot dives with either a 20 minute or 80 minute working phase for each of three different breathing mixes containing 14% oxygen. The mixes are NITROX-14, TRIMIX-14/34 (34% helium and the remaining 52% nitrogen) and HELIOX-14. Each profile planned for decompression inside a dry bell with air as a decompression mix from 60 to 40 feet and pure oxygen from 30 feet to the surface.
Runtime Comparisons for 300 foot dives
Bottom Mix.....20 minute.....80 minute
NITROX-14........174...........818
TRIMIX-14/34.....165...........698
HELIOX-14........163...........668
The table shows that for the 20 minute dives the runtime reduction in favor of HELIOX was only about 5%, but that for the 80 minute dives the reduction was a significant 18%. As illustrated in Figure 3, Helium mixes of up to 50% are increasingly advantageous for longer dives in the 300-foot range. This is because helium more rapidly approaches saturation level and out-gasses quickly. When helium is used for shorter dives, it yields little decompression advantage. However, the more helium in the mix the less narcosis. HELIOX-14 results in no narcosis, while for a 300-foot dive NITROX-14 has an unacceptable END of 325 feet and TRIMIX-14/34 has an END of about 200 feet that may be barely acceptable for some divers.
All things being equal HELIOX-14 is the mix of choice from a decompression and narcosis perspective. Still, helium is expensive. A K-bottles cost is $75 and that translates to $55 of helium for a 20 minute HELIOX-14 dive and $215 for an 80 minute HELIOX-14 dive contrasted with $25 and $85 for the TRIMIX-14/34 dives. The more helium the less narcosis and the higher the cost. It becomes a question of how much sobriety can you afford?
An additional advantage to TRIMIX is that preparing it in the field is easier than HELIOX. To fill a scuba cylinder with HELIOX, pure oxygen and helium must be combined. This requires a special booster pump to top off scuba cylinders to the desired pressure, and introduces the risk of exposing various components of the system to high pressure pure oxygen. TRIMIX, can be easily generated in the field. To brew TRIMIX-10/50 (10% oxygen, 50% helium and 40% nitrogen) all one need do is decant about 1500 PSI of helium into an empty aluminum cylinder and top it off with air from a standard air compressor. It should be noted that the ideal gas laws do not rigorously apply at the high pressures in a scuba cylinder, so slightly more than 1500 PSI of helium is actually required. Proper numbers, very accurate gauges, gas analysis equipment and slow, cool, fills are essential.
