Mixed gas is generally classified as a gas mixture that contains helium in addition to other gasses, as apposed to air, which contains 21% oxygen and 79% nitrogen and nitrox, which are oxygen mixtures of greater than 21% and the balance being nitrogen. There are two forms of mixed gas that divers use primarily today: trimix and heliox. Trimix is a combination of helium, nitrogen, and oxygen, and heliox is a combination of helium and oxygen only. Mixed gas is used to eliminate nitrogen narcosis, to reduce the chances of getting oxygen toxicity, and to reduce decompression requirements.
Trimix and heliox are generally used for dives below 200 feet (60 m). Another variation of trimix is called normoxic trimix, which is used between 130 and 200 feet (40 to 60 M). Normoxic trimix is still a combination of helium, oxygen, and nitrogen, but generally the oxygen percentage remains at 21%, like standard air. Normoxic trimix is for divers who want to lessen the effects of nitrogen narcosis and who want to reduce decompression requirements.
Mixed gas diving is becoming the standard for deep diving applications. Not so many years ago, diving deep on air was being done far to extensively. The practice of diving deep on air is extremely dangerous, especially below 220 feet and can be deadly. In addition, the narcosis levels at those depths could render your thinking ability useless, which in itself can be deadly. If helium expenses are too high for your budget, then deep diving should not be an option. Learning to dive mixed gas and diving the right mixtures at the depths you’re diving are key to being safe and enjoying the new levels of your diving.