LUSITANIA 99 EXPEDITION GAS MENU|
The initial travel gas and first deco gas, nitrox 35, was chosen as it gave a good deco profile. Thirty-five per cent oxygen also roughly balanced the quantity of gas required for the two side-mounted cylinders (the other being nitrox 70) balanced in 10-12 litre cylinders on each side. It also avoided a large helium-to-nitrogen swing, which is typical from deeper switches to air.
The bottom gas of heliair 11/47 was chosen as it gave an END (equivalent narcotic depth) of 35m and a ppO2 of 0.94 bar at the absolute top of the Lusitania, and 45 m and ppO2 of 1.13 bar on the seabed in the debris field (93m), where most of the dives took place.
The low bottom ppO2 (nearer 1 bar than 1.4 bar) enabled higher oxygen percentages to be used during decompression, while keeping the overall CNS (central nervous system) clock below 100 per cent for bottom times up to 25 minutes. The deco schedules also included times that would get the divers out of the water using only 70 per cent in the absence of O2.
Oxygen (that is, zero inert gas) on the final stop was important, especially on repeated deep dives. A 5m break on nitrox 35 was inserted following every 25 minutes on oxygen.