Gas blending or gas mixing is the filling of diving cylinders with non-air breathing gases.

There are several hazards with gas mixing:

• cylinders are being filled with high pressure gas. So, if there is any damage or corrosion in the pressure vessel or valves of the cylinder, this is the occasion when they are most likely to fail.
• oxygen supports combustion. So, it comes into contact with fuel and heat the three ingredients for a fire exist. A fire in the presence of high pressure gas may cause cylinder failures.
• other high pressure equipment such as whips, compressors, gas banks and valves are being used, which can cause injury
• there are dangers from the fuel and electric power supplies of the compressor
• there are dangers from the moving parts of the compressor

It is possible for gas blenders to create toxic and dangerous gas mixes for divers. Too much or too little oxygen in the mix can be fatal for the diver. Oxygen analysers are used to measure the oxygen content of the mix. In good blending sites, this is done in the presence of the diver who acknowledges the contents by signing a log.

It is possible that poisonous additives, such as carbon monoxide or hydrocardon lubricants, will enter the cylinders from the air from the compressor. This is generally a problem with the compressor maintenance or siting of the air input to the compressor.

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
• 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
• Mixing by density (weight) : 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 smaller nitrogen molecules from the mix until the required mix is achieved

With trimix, measured pressures of oxygen and helium are decanted into a cylinder which is "topped up" with air from the diving gas compressor.

With heliair, a measured pressure of helium is decanted into a cylinder, which is "topped up" with air from the diving gas compressor.