Those 2 gases sound mysterious to most divers. Let’s try to explain.
Enriched Air Nitrox is a breathing mixture of two components: nitrogen and oxygen, with less nitrogen and more oxygen than air. Thus with the same dive profile as for an air dive, the diver’s body is less saturated with nitrogen, and decompression stops are shorter, with a smaller risk of decompression sickness. It is as if we were diving at shallower depths.
Diving with Enriched Air Nitrox is very simple: a half-day course is enough to get the Nitrox Diver certification, which allows the use of any nitrox mixture containing 21 to 40% oxygen. With regular scuba gear, this is the perfect mix for recreational diving down to 40-45 meters.
Advanced Nitrox Certification allows the use of any nitrox from 21 to 99% oxygen and also pure oxygen. The course teaches the use of 2 different mixes during the same dive, e.g. a 15 litre single cylinder full of Nitrox 30 to explore the shipwreck of the Rubis at 40 meters and a 5 litre oxygen cylinder for decompression stops. This is the first step towards technical diving; and allows 30 minutes bottom time on the Rubis with only a 10 minute decompression stop.
Trimix is a mix containing 3 gases: nitrogen, oxygen and helium. Helium is an inert gas, like nitrogen and is not metabolized. Unlike nitrogen, helium is not narcotic, and does not induce narcosis. It can be used to fully enjoy deep dives with a clear mind.
Hypoxic Trimix (also called Advanced Trimix) is intended for highly motivated divers. It allows to dive deeper than 100 meters after completion of the course. The equipment used is complex, heavy, and bulky: dry-suit, twin cylinders for the bottom gas, two to three cylinders for decompression, and generally a small extra cylinder to inflate the suit. All safety equipment is in duplicate, since very deep diving requires maximum safety.