Gas blending

Gas blending or gas mixing is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox.

Hazards

There are several hazards with gas mixing:
* cylinders are being 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 being used, which can cause injury if the pressure is released
* 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 volumes of gases that contain no oxygen

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, the contents are analysed 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 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.

Poisonous additives 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 oxygen.

Oxygen Precautions

In the presence of large volumes of high-pressure oxygen, one corner of 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 minimise 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
* 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

Blending helium mixes

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.

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.

Quantities and accuracy

To avoid oxygen toxicity and narcosis, the diver needs to plan the required mix to be blended and to check the proportions of oxygen and inert gases in the blended mix before diving. Generally the tolerance of each final component gas fraction should be within +/-1% of the required fraction.

Calculating composition

When blending mixes with pressures less than 250 bar / 3600 psi, the Ideal gas law holds and simple equations can be used to calculate the pressures of each component gas needed to create the mix. Above this pressure, the composition of the final mix is difficult to predict using simple equations but needs the more complex Van der Waals equation.

Effects of adiabatic heating

Increases in temperature when filling make it difficult to accurately decant or pump a measured quantity of gas. When cylinders are filled with gas quickly, typically in 10 to 60 minutes at a dive filling station, the gas inside gets hot, which increases the pressure of the gas. But, when the cylinder cools an hour or two later, the gas pressure falls reducing the volume of breathable gas available to the diver.

There are several solutions to this problem:
* fill the cylinder to the required pressure, let the cylinder cool and measure the gas pressure and then repeat the process until the correct pressure is achieved. The cooling interval needed depends on the ambient temperature.
* fill the cylinders in a water bath. The higher thermal conductivity of water compared to air means that heat in the cylinder is more quickly removed from the cylinder as it fills.
* fill the cylinders with 5 to 20% more gas than required. If the overfill is well judged, when the cylinder cools the final pressure will be within the tolerance of the required pressure.

Gas analysis

Before a gas mix leaves the blending station and before the diver breathes from it, the fraction of oxygen in the mix should be checked. Usually electro-galvanic fuel cells are used to measure the oxygen fraction. Helium gas analysers also exist, although they are expensive at present, which allow the Trimix diver to find out the proportion of helium in the mix.

Gas supplies

In the United Kingdom, oxygen and helium is bought from commercial industrial and medical gas suppliers and typically delivered in 50 litre "J" cylinders at a maximum of 200 bar. In addition to the cost of the gas, charges may be made for cylinder rental and delivery.

The "cascade system" is used to decant economically from banks of storage cylinders so that the maximum possible gas is removed from the bank. This involves filling a diving cylinder by decanting from the bank cylinder with the lowest pressure that is higher than the diving cylinder's pressure and then from the next higher pressure bank cylinder in succession until the diving cylinder is full. The system maximises the use of low pressure bank gas and minimises the use of high pressure bank gas.

Pneumatically powered booster pumps, such as the Haskel pump, are used to scavenge the remnants of expensive gases in nearly empty cylinders allowing low pressure gases to be pumped safely into cylinders already containing gas at higher pressure.