Gas blending for scuba diving


Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.
Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion due to the use of high-pressure gases. The composition of the mix must be safe for the depth and duration of the planned dive. If the concentration of oxygen is too lean the diver may lose consciousness due to hypoxia and if it is too rich the diver may suffer oxygen toxicity. The concentration of inert gases, such as nitrogen and helium, are planned and checked to avoid nitrogen narcosis and decompression sickness.
Methods used include batch mixing by partial pressure or by mass fraction, and continuous blending processes. Completed blends are analysed for composition for the safety of the user. Gas blenders may be required by legislation to prove competence if filling for other persons.

Application

For some diving, gas mixtures other than normal atmospheric air can be used to advantage, so long as the diver is competent in their use. The most commonly used mixture is nitrox, also referred to as Enriched Air Nitrox, which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the risk of decompression sickness or allowing longer exposure to the same pressure for equal risk. The reduced nitrogen may also allow for no stops or shorter decompression stop times or a shorter surface interval between dives. The hypothesis that nitrox can reduce narcosis may have some validity, as research has shown that if oxygen is also narcotic, it works differently to nitrogen, and apparently to a lesser effect.
The increased partial pressure of oxygen due to the higher oxygen content of nitrox increases the risk of oxygen toxicity, which becomes unacceptable below the maximum operating depth of the mixture. To displace nitrogen without the increased oxygen concentration, other diluent gases can be used, usually helium, when the resultant three gas mixture is called trimix, and when the nitrogen is fully substituted by helium, heliox.
For dives requiring long decompression stops, divers may carry cylinders containing different gas mixtures for the various phases of the dive, typically designated as Travel, Bottom, and Decompression gases. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.

Hazards

There are several hazards associated with gas mixing:
  • cylinders are filled with high pressure gas. 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 structurally.
  • oxygen supports combustion; if it comes into contact with fuel and heat the three ingredients for a fire exist. Fires in the presence of high concentrations of oxygen burn more vigorously than those in air. A fire in the presence of high-pressure gas may cause cylinders to fail.
  • other high-pressure equipment such as whips, compressors, gas banks and valves are used, which can cause injury if the pressure is released or there is a mechanical failure while under pressure
  • there are dangers of fire from the fuel and electric power supplies of the compressor
  • there are dangers of injury from the moving parts of the compressor
  • there is the possibility of asphyxiation due to the presence, in a confined space, of large concentrations of gases that contain no oxygen, such as helium
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 after blending. Inadequate mixing may cause inaccurate analysis. To ensure that the composition of the gas is known by the end user, the contents are analysed in the presence of the diver who acknowledges the contents by signing a log.
It is possible that toxic contaminants, such as carbon monoxide or hydrocarbon lubricants, will enter the cylinders from the diving air compressor. This is generally a problem with the compressor maintenance or location of the air input to the compressor though it can be from other sources.
Toxic contaminants can also get into the breathing mix if any material inside the blending valves or pipes burns, for instance when adiabatic heating occurs when decanting or boosting oxygen.

Oxygen precautions

In the presence of large volumes of high-pressure oxygen, one corner of the fire triangle exists in good measure. It is vital the other two corners are not allowed to exist.
Internally, the blending equipment and diving cylinders must be oxygen clean; all fuels and particles which could be sources of ignition must be removed. The materials chosen for use in the valves, joints and compressors must be oxygen compatible: they must not burn or degrade readily in high oxygen environments.
In gas blending, high temperatures are easily produced, by adiabatic heating, simply by decanting high-pressure gas into lower pressure pipes or cylinders. The pressure falls as the gas leaves the opened valve but then increases when the gas encounters obstructions such as a cylinder or a bend, constriction or particle in the pipe-work.
One simple way to reduce the heat of decanting is to open valves slowly. With sensitive valves, such as needle valves, the gas can slowly be allowed through the valve so that the pressure increase is slow on the low-pressure side. The pipe-work, joints and valves in the blending system should be designed to minimize sharp bends and sudden constrictions. Sometimes 360-degree loops are present in the pipe-work to reduce vibration.
Spaces where gas is blended or oxygen is stored should be well ventilated to avoid high concentrations of oxygen and the risk of fire.

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. In South Africa, a cylinder to be used for mixes with a high partial pressure of oxygen must be cleaned before being put into that service.
  • Pre-mix decanting: the gas supplier provides large cylinders with popular mixes such as 32% and 36%.
  • Mixing by continuous blending: regulateed flow of oxygen is fed into a static mixer with air, analysed, and fed 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.

    Blending helium mixes

Helium mixes may be made by partial pressure blending, mass fraction blending or compressing a premix blended at atmospheric pressure.

Partial pressure blending

Gas is mixed by decanting or compressing the component gases into a high-pressure cylinder, measured by partial pressure, added in sequence, and corrected for temperature.
With trimix, measured pressures of oxygen and helium are decanted into a cylinder, which is "topped up" with air from the diving gas compressor, resulting in a three gas mix of oxygen, helium and nitrogen. An alternative is to first decant helium into a cylinder and then top it up to the working pressure with a known nitrox mix. Both NAUI and TDI offer courses using a trimix that they call "helitrox", blended by the latter method, which limit the fraction of helium to about 17–20%. Mixtures made by blending helium with nitrox containing around one-third oxygen such as EAN32 have the desirable property that at their maximum operating depth for a partial pressure of oxygen of 1.4 bar, their equivalent narcotic depth is always approximately, a safe limit.
With heliox, measured pressures of oxygen and helium are decanted or pumped into a cylinder, resulting in a two gas mix of oxygen and helium.
With heliair, a measured pressure of helium is decanted into a cylinder, which is "topped up" with air from the diving gas compressor, resulting in a three gas mix of oxygen, helium and nitrogen, with the nitrogen:oxygen ratio fixed at 4:1.

Mass fraction blending

Mass fraction blending requires an accurate scale which should preferably be capable of being set to zero with the empty cylinder connected to the filling whip standing on the scale.
The masses of the gases to be mixed must be calculated based on the final partial pressure ratio and total pressure, and the cylinder is filled to the appropriate weight corresponding to the added weight of each component. The advantage of this system is that temperature does not affect the accuracy, as pressure is not measured during the process. The disadvantage is that helium has a much lower density than the other components, and a small error in measured mass of helium will result in a relatively large error in composition.