Gas Blending - Blending Nitrox

Blending Nitrox

With nitrox there are several methods of gas mixing:

• Mixing by partial pressure: a measured pressure of oxygen is decanted into the cylinder and cylinder is "topped up" with air from the diving air compressor. For mixtures with oxygen fraction of 40% or more, the delivered air quality must be suited for oxygen service. This is usually achieved by using a suitable grade of oil and an extra in-line filter to reduce the residual oil contamination in the compressed air to the more stringent requirements for blending with high oxygen partial pressure gases. Cylinders used for partial pressure blending and for mixtures with oxygen fraction greater than 40% are required by law in some countries to be cleaned for oxygen service.
• Pre-mix decanting: the gas supplier provides large cylinders with popular mixes such as 32% and 36%.
• Mixing by continuous blending: measured quantities of oxygen are introduced to the compressor inlet. The compressor and particularly the compressor oil, must be suitable for this service. If the oxygen fraction is less than 40%, some countries do not require the cylinder and valve to be cleaned for oxygen service.
• Mixing by mass fraction: oxygen is added to a partially full cylinder that is accurately weighed until the required mix is achieved.
• Mixing by gas separation: a nitrogen permeable membrane is used to remove some of the smaller nitrogen molecules from low pressure air until the required mix is achieved. The resulting low pressure nitrox is then pumped into cylinders by a compressor.