The gas mixture for scuba diving (or Gas mixing) is the filling of a diving cylinder with non-air respiratory gas such as nitrox, trimix and heliox. The use of these gases is generally intended to improve the overall safety of planned dives, by reducing the risk of decompression and/or nitrogen narcosis, and can improve breathing ease.
Filling the cylinder with a gas mixture has a danger for both the filler and the diver. During charging there is a risk of fire due to the use of oxygen and the risk of explosion due to the use of high pressure gas. The composition of the mixture should be safe for the depth and duration of planned dives. If the oxygen concentration is too lean, the diver may lose consciousness due to hypoxia and if too rich, the diver may suffer oxygen toxicity. Inert gas concentrations, such as nitrogen and helium, are planned and examined to avoid nitrogen narcosis and decompression diseases.
The methods used include mixing batches with partial pressures or mass fractions, and a continuous mixing process. A complete mixture is analyzed for composition for user safety. Gas blenders may be required by legislation to prove competence if filling for others.
Video Gas blending for scuba diving
Apps
For some dives, a mixture of gases other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gas) can be used for profit, as long as the diver is competent in its use. The most commonly used mixture is nitrox, also called Enriched Air Nitrox (EAN), which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reduces the risk of decompression or allows longer exposure. to the same pressure for the same risk. Decreased nitrogen also allows no stops or shorter stop decompression times or shorter surface intervals between dives. A common misconception is that nitrox may reduce anesthesia, but studies have shown that oxygen is also narcotic.
Increased oxygen partial pressure due to higher nitrox oxygen content increases the risk of oxygen toxicity, which becomes unacceptable under the maximum operating depth of the mixture. To replace nitrogen without increasing oxygen concentration, other diluent gases may be used, usually helium, when the resulting gas mixture is called a trimix, and when nitrogen is completely replaced by helium, heliox.
For dives requiring long decompression stops, divers can carry cylinders containing different gas mixtures for different dive phases, commonly referred to as Travel, Down, and Decompression gas. These different gas mixtures can be used to extend down time, reduce the effects of inert gas narcotics, and reduce decompression time.
Maps Gas blending for scuba diving
Dangers
There are several hazards associated with mixing gases:
- The cylinder is filled with high pressure gas. If there is damage or corrosion to pressure vessels or cylinder valves, this is an opportunity when they are most likely to fail structurally.
- oxygen supports combustion; if in contact with fuel and heating the three materials for the fire there. Fires in the presence of high concentrations of oxygen burn more strongly than those in the air. Fire in the presence of high pressure gas can cause the cylinder to fail.
- other high-pressure equipment such as whips, compressors, gas banks and valves used, which can cause injury if pressure is released or there is mechanical damage under pressure
- There is a fire hazard of fuel and power supply from the compressor
- there is danger of injury from the moving parts of the compressor
- there is the possibility of asphyxia due to, in limited space, large oxygen-free gas concentrations, such as helium
It is possible for a gas blender to create a toxic gas mixture and harmful to divers. Too much or too little oxygen in the mix can be fatal to the diver. The oxygen analyzer is used to measure the oxygen content of the mixture after mixing. Inadequate mixing can lead to inaccurate analysis. To ensure that the gas composition is known to the end user, its contents are analyzed by the presence of a diver who acknowledges the content by signing the log.
It is possible that toxic additives, such as carbon monoxide or hydrocarbon lubricants, will enter the cylinder from a dive air compressor. This is generally a problem with the maintenance of the compressor or the location of the air input to the compressor although it can be from other sources.
Toxic additives may also enter the respiratory mixture if there is material in the blending valves or burning pipe, for example when adiabatic heating occurs when decanting or increasing oxygen.
oxygen precautions
In the presence of large volumes of high pressure oxygen, one of the corners of the Fire Triangle is in good measure. Very important two other angles are not allowed.
Internally, mixing equipment and diving cylinders must be clean oxygen; all fuels and particles that could be ignition sources should be eliminated. Materials selected for use in valves, joints and compressors should be compatible with oxygen: they should not be combustible or degraded easily in high oxygen environments.
In gas mixing, high temperatures are easily produced, with adiabatic heating, simply by decomposing the high pressure gas into the pipe or low pressure cylinder. The pressure falls when the gas leaves the valve open but then increases when the gas encounters a barrier such as a cylinder or a bend, narrowing or particles in a pipe work.
One simple way to reduce decanting heat is to open the valve slowly. With a sensitive valve, such as a needle valve, the gas can slowly be allowed through the valve so that pressure rises slowly on the low pressure side. Pipe work, connections and valves in the blending system should be designed to minimize sharp turns and splicing. Sometimes a 360 degree loop is present in the pipe work to reduce vibration.
The space in which the gas is mixed or stored oxygen must be well ventilated to avoid high oxygen concentrations and the risk of fire.
Mixing nitrox
With nitrox there are several methods of mixing gases:
- Mixing with partial pressure: the measured oxygen pressure is poured into the cylinder and the cylinder "filled" with air from the dive air compressor. To mix with an oxygen fraction of 40% or more, the quality of air delivered should be appropriate for oxygen service. This is usually achieved by using appropriate oil levels and additional in-line filters to reduce residual oil contamination in compressed air to more stringent requirements for mixing with high oxygen partial pressure gases. Cylinders used for mixed partial pressures and for mixtures with oxygen fractions of more than 40% are required by law in some countries to be cleaned for oxygen service.
- Mixture of pre-mixing: gas suppliers provide large cylinders with popular mixtures such as 32% and 36%.
- Mixing with continuous mixing: the quantity of measured oxygen introduced to the compressor inlet. Compressors and especially compressor oil, shall be appropriate for this service. If the oxygen fraction is less than 40%, some countries do not require cylinders and valves to be cleaned for oxygen service.
- Mixing with a mass fraction: oxygen is added to a portion of a full cylinder that is accurately weighed until the desired mixture is reached.
- Mixing with gas separation: nitrogen permeable membranes are used to remove some of the smaller nitrogen molecules from low pressure air until the required mixture is reached. The resulting low pressure nitrox is then pumped to the cylinder by the compressor.
Blending the helium mixture
The helium mixture can be prepared by mixing partial pressures, mixing the mass fraction or compressing the mixture mixture at atmospheric pressure (continuous mixing).
Partial pressure mix
The gas is mixed by pouring or compressing the gas components into a high pressure cylinder, measured by a partial pressure, added sequentially, and corrected for temperature.
With trimix, the measured oxygen and helium pressure is poured into the cylinder, which is "closed" with air from a dive gas compressor, producing three mixtures of oxygen, helium and nitrogen gas. The alternative is to first remove the helium into the cylinder and then add it to the working pressure with the known nitrox mixture. Both NAUI and TDI offer courses using trimix they call "helitrox", mixed with the latter method, which limits the helium fraction to about 17-20%. The mixture prepared by combining helium with nitrox containing about one-third of oxygen such as EAN32 (common mixed nitrox) has the desired property which is at its maximum operating depth for a 1.4 bar oxygen partial pressure, equivalent narcotic depth is always about 32 meters (105Ã, ft) , safety limit.
With heliox, the measured oxygen and helium pressure is poured or pumped into the cylinder, producing two mixtures of oxygen and helium gas.
With the heliir, the measured helium pressure is poured into the cylinder, which is "inflated" with air from the dive gas compressor, producing three mixtures of oxygen gas, helium and nitrogen, with a nitrogen: oxygen ratio fixed at 4: 1.
Mixed bulk fractions
Mixed mass fractions require an accurate scale that should be set to zero with an empty cylinder connected to a whip filling that stands on a scale.
The gas mass to be mixed should be calculated based on the ratio of the final partial pressure and total pressure, and the cylinder is charged with the weight corresponding to the additional weight of each component. The advantage of this system is that the temperature does not affect the accuracy, because the pressure is not measured during the process. The disadvantage is that helium has a much lower density than any other component, and a small error in measurable helium mass will result in a relatively large composition error.
Continuous mixing and compression
Principles
Continuous mixing is the process of adding mixed gas components together as a continuous process and then compressing the mixture into a storage cylinder. The objective is to supply the gas component to the compressor intake in a continuous flow at a pressure corresponding to the compressor design, which is already mixed with the correct specifications. This generally requires equipment to monitor and control the flow of input gas, which is usually supplied from high pressure storage cylinders, except for air normally taken from the surrounding environment.
Mixing gas
Most high-pressure gas pressure compressors are designed to receive gas intake at normal atmospheric pressure. and one of the components commonly used to inhale a gas mixture for diving is atmospheric air, so it is convenient to mix the gas at atmospheric pressure in an accessory on a compressor called a mixing tube or mixing rod > The mixing tube can be constructed in various ways, provided that it does not limit the flow too much, and adequately mixes the gas prior to the analysis and before entering the compressor. A variety of commercially produced and homemade blending tubes have been successfully used.
One popular configuration for mixing tubes is a large drill tube with a series of internal baffles that create turbulence in the mixture after the injection point, which causes rapid mixing into the homogeneous mixture, which can then be continuously analyzed by monitoring instruments before further processing, or can be directly processed and then analyzed from storage cylinders. Continuous analysis allows adjustment of the gas flow rate added to improve the mixture if it deviates from the specification. Post-analysis makes corrections more difficult. The addition of components can be done sequentially or together. Adding them together means that mixing is done once, and this reduces the pressure loss in the intake system. It is important that the gases are mixed thoroughly before analysis because the analysis will be more reliable. It is also desirable to ensure that the intake gases do not vary significantly in the oxygen content over time for safety reasons, since the compressor may only be safe for a limited oxygen fraction.
The gas flow rate is usually controlled by the industrial gas regulator on the cylinder, and can be measured by an industrial flow meter. Flow rate measurements may be a substitute for mixed gas analysis, but are generally less accurate in predicting the mixture sent due to temperature variation and gas delivery efficiency of the compressor, which may vary as the shipping pressure changes.
The mixed gas at the intake to the compressor will be at a pressure slightly below the ambient, due to a loss in the blending tube. This may make it impractical to use several types of analytical instruments, which depend on the gas flow through the instrument being driven by the measured gas pressure. Oxygen cells are also sensitive to pressure drops, since they directly measure the partial pressure, and this may cause the mixture to be richer than intended, since the oxygen flow can be adjusted to the partial pressure corresponding to atmospheric pressure, while the measured mixture is at a pressure lower. This can be compensated by using a sample gas sampling gas sample from the blending tupe and sending it to the instrument, or by allowing it to reduce the inlet pressure for oxygen analysis with in-line sensor cells. This will require a vacuum gauge to measure the pressure drop or abbsolute pressure on the sensor. The partial pressure of oxygen must be true as the absolute pressure fraction at the point of measurement.
Compression
Many high-pressure compressors used for respiratory gas are suitable for compressing respiratory gas mixtures containing moderate fractions of oxygen and helium, but manufacturers should be consulted on the boundaries of both gases. Compression mixtures with high oxygen fractions are an increased fire hazard, and compressor lubricants must be compatible to minimize these risks. Helium presents a very different problem, because it is completely inert, and does not create a direct fire hazard, but it rises more than oxygen and nitrogen when compressed, which can cause the compressor to be designed for air to overheat. This can eventually cause problems with compressor and bearing lubrication, and if the oxygen fraction is also high, this will increase the fire hazard. Fortunately most of the Trimix mixture has an oxygen fraction inversely proportional to the helium fraction, which reduces the likelihood of this problem.
Mixed analysis
The gas mixture should be analyzed before use, since inaccurate composition assumptions may cause hypoxia or oxygen toxicity problems in case of oxygen analysis, and decompression disease if the inert gas component is different from the planned composition. The oxygen fraction analysis is usually performed using an electro-galvanic oxygen sensor, whereas the helium fraction is usually performed by the ratio of heat transfer between the analyzed gases and the standard samples.
Amount and accuracy
To avoid the toxicity of oxygen and narcosis, divers need to plan the mixture needed for mixing and to check the proportion of oxygen and inert gas in mixed mixture before diving. Generally the tolerance of each fraction of the final gas component must be within/- 1% of the required fraction. The usual analysis tools used by recreational/technical gas mixers are usually capable of 0.1% resolution for oxygen and helium.
Calculate the composition
When the mixture mixes with pressure up to about 230 bar (3,300 psi), Ideal gas law provides a reasonable approach and simple equations can be used to calculate the pressure of each gas component required to make the mixture. At this pressure and normal temperature, air travels from a linearity of about 5%, ie. a 10-liter cylinder filled to 230 bar with only about 95% air of the estimated 2300 liters of free air. Above this pressure, the composition of the final mixture is difficult to predict using simple equations but requires a more complex Van der Waals equation.
The ideal gas calculation
The mixture of partial pressure with ideal gas calculations is quite easy. The required mixture is selected, either as the best mixture optimizing the decompression advantage for acceptable oxygen exposure based on the planned dive profile, or selected from a range of appropriate mixtures of standards suitable for different depths and times. , or optimized to match the available gas stock or other constraints. This mixture is determined in the gas fraction of the component gases, and the convention is to determine the type, (nitrox, trimix or heliox) and composition as a percentage of the volume of oxygen, helium if present, and nitrogen. The remaining nitrogen is not always specifically stated, and is assumed to be a balance.
- Example:
- "Tx 20/40" (or Tx 20/40/40) will be a trimix mixture with 20% oxygen, 40% helium and 40% nitrogen remaining. This will be suitable for depths up to 60 meters (200 feet) if the partial pressure of oxygen will be limited to 1.4 bars. It is a mixture of normoxic and safe to use on the surface.
- "He/O 2 12/88" will be a mixture of heliox with 12% oxygen and 88% helium. This gas will be used in commercial dives to a depth of up to about 100 meters (330 feet), depending on duration, but can not be used more shallower than about 7 meters (23 feet) without the risk of hypoxia.
- "Nitrox 32", or EAN 32, is a mixture of nitrox with 32% oxygen and 68% nitrogen. This is a popular recreation blend for diving to a depth of up to 33 meters (108 feet).
Nitrogen in the mix is ​​almost always provided by breaking the cylinder with air to the filling pressure. All helium, and some oxygen are provided by means of casting or booster of bulk cylinder.
The amount of helium to pour is very easy to calculate: Duplicate the desired helium gas fraction (F He ) with total charging pressure (P tot ) to obtain the partial pressure of helium (P He ). In the case of Tx 20/40, in a 230 bar cylinder, it will be 230 bar x 40% = 92 bar (or for 3,000 psi fill, it will need 3,000 x 40% = 1,200 psi helium).
The amount of oxygen is harder to calculate, because it comes from two sources, supplemental oxygen and top-up air. However, all nitrogen is provided by top-up air, so the nitrogen partial pressure is calculated in the same way as for helium, which allows air pressure to be calculated, assuming nitrogen to be 79% of air. In the example of Tx 20/40, the nitrogen fraction is 100% - (20% 40%) = 40%. The required partial pressure of nitrogen is 230 bar x 40% = 92 bar, so the top-up air pressure is 92 bar/79% = 116 bar (for 3,000 psi this will be 3,000 x 40%/79% = 1,500 psi air). The remaining pressure of 230 bar - 92 bar - 116 bar = 22 bar is the additional oxygen pressure required for the mixture (for 3,000 psi this fill would be 3,000 - 1,200 - 1,500 = 300 psi oxygen).
Adiabatic heating effect
Increasing the temperature during charging makes it difficult to accurately pour or pump gas quantities measured based on pressure measurements. When the cylinder is filled with gas quickly, usually within 10 to 60 minutes at the dive filling station, the gas inside becomes hot, which increases the gas pressure relative to its mass. When the cylinder cools, the gas pressure drops resulting in a decrease in the volume of breathing 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 reached. The required cooling interval depends on the ambient temperature. This step should be followed for each mixed component.
- the contents of the cylinder in a water bath. Higher thermal conductivity of water compared to air means the heat in the cylinder is removed faster than the cylinder while filling. In order for this to produce accurate results, charging should be slow enough to avoid significant temperature rise. It's very slow.
- the contents of the cylinder with 5 to 20% more gas (as pressure reading) than required. If overcharging (in hot pressures) is rated properly, when the cylinder cools the last pressure will be within the required pressure tolerance. This is relatively fast, but requires good judgment based on experience, or measurement of gas temperature in the cylinder after each mixing stage, and corrections should be made to allow for the influence of temperature.
Gas analysis
Before the gas mixture leaves the mixing station and before the diver breathes, the oxygen fraction in the mixture should be checked. Usually the electro-galvanic oxygen sensor is used to measure the oxygen fraction. Helium analysis also exists, although it is expensive today, which allows Trimix divers to know the proportion of helium in the mix.
It is important that the gas mixture in the cylinder is completely mixed before analyzing or the results are not accurate. When the partial pressure or mass mixing is carried out at low flow rates, the gas entering the cylinder does not move fast enough to ensure good mixing, and in particular when the mixture contains Helium, they may tend to remain plated due to the difference in density. This is called stratification, and if left long enough, diffusion will ensure complete mixing. However, if the gas will be analyzed immediately after mixing, mechanical agitation is recommended. This may be by placing a single cylinder on a flat surface and rolling it for a short time, but twins are usually more often reversed several times. Stratification is more pronounced with helium-containing mixtures, but may also lead to inaccurate analysis of the Nitrox mixture.
Reliable specifications for the amount of agitation required for complete mixing are not available, but if the analysis remains the same before and after agitation, the gas may be completely mixed. Once mixed, the gas will not rise with time.
Gas supply
In the UK and South Africa, oxygen and helium are purchased from industrial gas and commercial gas suppliers and are usually delivered in 50 liter "J" cylinders at a maximum of 200 bar. In addition to gas costs, charges can be made for cylinder rental and shipping.
The "cascade system" is used to describe economically from the cylinder bank of storage so that the maximum gas may be removed from the bank. This involves filling a diver's cylinder with a decanting of a bank cylinder with a lower pressure higher than the pressure of the diving cylinder and then from the next high-pressure bank tube to the full dive cylinder. This system maximizes the use of low pressure gas banks and minimizes the use of high pressure gas banks.
Booster pumps, such as Haskel pumps, can be used to scavenge the remnants of expensive gas in an almost empty cylinder allowing low pressure gas to be pumped safely into an already gas-containing cylinder at higher pressures.
Training and competence
Training and certification for the mixing of scuba gases is provided by some diver training institutes, and may be required in terms of national laws or standards. ISO 13293 provides a minimum training standard for gas blenders for recreational diving services at two levels.
See also
References
Source of the article : Wikipedia
0 Comments