as "19/30" is used in the depth range; a hypoxic mix such as "10/50" is used for deeper diving, as a bottom gas only, and cannot safely be breathed at shallow depths where the PO2 is less than 0.18 bar. In fully closed-circuit rebreathers that use trimix diluents, the mix in the breathing loop can be hyperoxic (meaning more oxygen than in air, as in enriched air nitrox) in shallow water, because the rebreather automatically adds oxygen to maintain a specific partial pressure of oxygen. Hyperoxic trimix is also sometimes used on open circuit scuba, to reduce decompression obligations. Blending Gas blending of trimix generally involves mixing helium and oxygen with air to the desired proportions and pressure. Two methods are in common use: Partial pressure blending is done by decanting oxygen and helium into the diving cylinder and then topping up the mix with air from a diving air compressor. To ensure an accurate mix, after each helium and oxygen transfer, the mix is allowed to cool, its pressure is measured and further gas is decanted until the correct pressure is achieved. This process often takes hours and is sometimes spread over days at busy blending stations. Corrections can be made for temperature effect, but this requires accurate monitoring of the temperature of the mixture inside the cylinder, which is generally not available. A second method called 'continuous blending' is done by mixing oxygen and helium into the intake air of a compressor. The oxygen and helium are fed into mixing tubes in the intake air stream using flow meters or analysis of the oxygen content after oxygen addition and before and after the helium addition, and the oxygen and helium flows adjusted accordingly. On the high pressure side of the compressor a regulator or bleed orifice is used to reduce pressure of a sample flow and the trimix is analyzed (preferably for both helium and oxygen) so that the fine adjustment to the intake gas flows can be made. The benefit of such a system is that the helium delivery tank pressure need not be as high as that used in the partial pressure method of blending and residual gas can be 'topped up' to best mix after the dive. This is important mainly because of the high cost of helium. Drawbacks may be that the high heat of compression of helium results in the compressor overheating, especially