Air, oxygen and helium gas blending system

Gas blending or gas mixing is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. A diving cylinder, scuba tank or diving tank is used to store and transport high Pressure Breathing gas as a component of SCUBA (Self-Contained Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Air is the most common and only natural breathing gas. Other artificial gases either pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such Nitrox refers to any gas mixture composed (excluding trace gases of nitrogen and oxygen this includes normal Air which is approximately 78% Nitrogen See Heliox (cryogenic equipment for another use of the word Heliox Heliox is a Breathing gas composed of a mixture of

Hazards

There are several hazards with gas mixing:

• cylinders are being filled with high pressure gas. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface 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. Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings A pressure vessel is a closed container designed to hold gases or liquids at a Pressure different from the ambient Pressure. For other uses see Valve (disambiguation. For the electronic component see Thermionic valve.
• oxygen supports combustion; if it comes into contact with fuel and heat the three ingredients for a fire exist. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of Fuel is any material that is burned or altered in order to obtain energy In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature Fire is the heat and light energy released during a Chemical reaction, in particular a combustion reaction. 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. An electro-galvanic fuel cell is an electrical device used to measure the concentration of Oxygen gas in Scuba diving and Medical equipment 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. Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. A lubricant (sometimes referred to as a "Lube" is a substance (often a liquid introduced between two moving surfaces to reduce the Friction between them improving A Diving Air Compressor is a Gas compressor that can fill Diving cylinders with high- Pressure Air pure enough to be used as a Breathing gas 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. The fire triangle or combustion triangle is a simple model from the science of firefighting, for understanding the ingredients necessary for most Fires 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. Fuel is any material that is burned or altered in order to obtain energy Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of 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. A needle valve is a type of valve having a small orifice and a threaded needle-like plunger 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. Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states

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. Nitrox refers to any gas mixture composed (excluding trace gases of nitrogen and oxygen this includes normal Air which is approximately 78% Nitrogen Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the A Diving Air Compressor is a Gas compressor that can fill Diving cylinders with high- Pressure Air pure enough to be used as a Breathing gas Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical

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. See Heliox (cryogenic equipment for another use of the word Heliox Heliox is a Breathing gas composed of a mixture of

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. Heliair is a Breathing gas consisting of mixture of Oxygen, Nitrogen and Helium and is often used during the deep phase of dives carried out

A combined oxygen and helium gas analyser measuring a trimix fill

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. Oxygen toxicity or oxygen toxicity syndrome (also known as the " Paul Bert effect" or the "Lorrain Smith effect" describes harmful effects caused 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. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface The ideal gas law is the Equation of state of a hypothetical Ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834 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. The van der Waals equation is an Equation of state for a Fluid composed of particles that have a non-zero size and a pairwise attractive inter-particle Force

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. An electro-galvanic fuel cell is an electrical device used to measure the concentration of Oxygen gas in Scuba diving and Medical equipment 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. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located The litre or liter (see spelling differences) is a unit of Volume. 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. Pneumatics, Pressurized gas to affect mechanical motion Pneumatic power is used in Industry, where it is common to have factory units plumbed for Compressed For information on Wikipedia project-related discussions see WikipediaVillage pump.