Gas blending 996061 167879045 2007-10-29T17:27:30Z Cydebot 1215485 Robot - Moving category Diving (underwater) to Underwater diving per [[WP:CFD|CFD]] at [[Wikipedia:Categories for discussion/Log/2007 October 22]]. [[Image:Gas blending equipment.JPG|thumb|right|Air, oxygen and helium gas blending system]] '''Gas blending''' or '''gas mixing''' is the filling of [[diving cylinder]]s with non-[[air]] [[breathing gas]]es 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 [[valve]]s 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 analyser]]s 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 [[lubricant]]s, 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 [[fuel]]s and particles which could be sources of [[Combustion|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 valve]]s, 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 [[oscillation|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. [[Image:Gas blending oxygen and helium analyser.JPG|thumb|right|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. Generally the tolerance of each final component gas fraction should be within +/-1% of the required fraction. === Calculating composition=== When blending mixes with [[pressure]]s 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 cell]]s are used to measure the oxygen fraction. [[Helium gas analyser]]s 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. [[Pneumatic]]ally powered booster [[pump]]s, 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. [[Category:Underwater diving]]