Special Design Considerations of Gas Cell - Essay Example

Gas Cell: As its is self explaining (Gas Cell means Cell related to Gas or Gases) the gas cell is used for detection of gases in environment. It is a very useful device in analyzing the air pollution. It can detect the % of a particular gas into the air. For example it can detect various harmful gases in an air sample like nitric oxide, sulphur dioxide, HCl, HBr, Cl2, WF6, SiH4, SiH2Cl2, NH3, PH3, etc. Construction of Gas Cell The gas cell has a number of special design considerations that have beenintegrated to handle the special nature of the very active gases that will bepassed through the gas cell and the problem of measuring moisture (H2O) down to very low levels. This heated cell can be configured as a gas cell or for vaporizing samples that are normally solids or liquids at room temperature. Configured for gas sampling, the cell incorporates 2 valves, an inlet and an outlet, that enable flowing gas samples through the cell. In its solid and liquid vaporization configuration, the cell has an outlet needle valve from which a vacuum can be pulled and two (2) sample inlets. One inlet is a needle septa injection port on the top of the cell which is similar to the type used in gas chromatography. There is a second inlet in the form of a side port for inserting solid samples, which also doubles as the thermocouple port. Using the side port eliminates the need to recheck vacuum seals whenever the cell is opened to insert solid samples. The 250w heating jacket extends over the end of the cell so that both the cell body and the optics are heated, and allows operation at temperatures up to 1000K. Condensation on the optics is minimized by heating them to the same temperature as the sample chamber. A type J iron constantan thermocouple is provided with plugs which are compatible with the optional temperature controller. The cell body is type 304 stainless steel and an assortment of seals is provided - silicon rubber, viton and PTFE. Metals & Metal Preparation for reducing the likelihood of failure of the cell due to creep: Stainless Steel 304: SS 304 is the most versatile and the most widely used of all stainless steels. Its chemical composition, mechanical properties, weld ability and corrosion/oxidation resistance provide the best all-round performance stainless steel at relatively low cost. It also has excellent low temperature properties and responds well to hardening by cold working. SS 304 is used in all industrial, commercial and domestic fields because of its good corrosion and heat resisting properties. Using SS304 is reducing the chances of failure of cell due to heating & creep. Creep Data Stress for a creep rate of 1% in 10 000 h. Temperature, oC 550 600 650 700 800 Temperature, K 823 873 923 973 1073 Stress, MPa 120 80 50 30 10 Environment Data/ Corrosion Rate The performance of SX 304 compared with other metals in various environments is shown in the following table. The corrosion rates are based on a 10 year exposure. Environment Corrosion Rate (m/year) SS 304 Mild Steel Rural 0.0025 5.8 Marine 0.0076 34.0 Marine Industrial 0.0076 46.2 So as compared to other metals stainless steel 304 is having very less corrosion rate, so this metal is best for the Gas cell. Stainless steel is also a preferred substrate because of its lower cost to fabricate into a mirror it requires multi-layer coatings, including gold as the primary reflective surface, which is expensive. Copper: Copper is a chemical element. Copper is malleable, ductile, excellent electrical conductor, non magnetic, resistant to corrosion, excellent heat transfer capacity, durable, & recyclable element. It is a ductile metal with excellent electrical conductivity and is rather soft in its pure state and has a pinkish luster which is. It finds extensive use as an electrical conductor, heat conductor, as a building material, and as a component of various alloys. Though copper has ductility property hence it can be drawn into thin wire. The ductility property of copper can reduce the chance of failure of cell due to creep. Nickel: Nickel is a silvery-white metal that takes on a high polish. It belongs to the transition metals, and is hard and ductile. It occurs most usually in combination with sulphur and iron. Nickel is a very reactive element, but is slow to react in air at normal temperatures and pressures. Nickel is also a naturally magnetostrictive material, meaning that in the presence of a magnetic field, the material undergoes a small change in length. Nickel-plated brass and electro polished stainless steel exhibit good resistance to acid attack, but once again the retention of adsorbed moisture molecules by the oxygen sites of stainless steel diminishes its utility in applications where moisture measurements are critical. Nickel-plated stainless steel and the high nickel-content alloys are the best materials for construction, because of their high resistance to acid attack and their low retention of moisture molecules. This property of copper can reduce the chance of failure of cell due to heating & creep. The mirrors used in gas cell have a very important role & is a very critical component. The mirrors are mixture of such chemical components which enables it to be resistance to attack by electronic gases and the ultimate energy throughput of the cells. Mirrors made of pure nickel are sufficiently reflective in the infrared to use in multi-path cells. Special Design Considerations of Gas Cell that can operate up to temperature of 1000K and a maintained pressure of 20 bar. For making a gas cell that can operate up to temperature of 1000k and a maintained pressure of 20 Bar, we should make the body part by Stainless Steel 304 because it can tolerate high temperature. Also the VCR fittings are to made by Stainless steel 304. Mirror used in gas cell should comply with the information given in nickel subheading. Its protective surface should be over coated with nickel for high chemical resistance after that it is coated with gold or MgF2 for high reflection efficiency, The important point taken into consideration while designing/ making a gas cell are as follows: 1) Stinger fill tube should be of good quality and heat & pressure resistance. 2) The base path should be of 6 Inches. 3) The length of gas path should be from 1.2 to 6.1 meters. 4) The volume of the cylinder should be 500 milliliter. 5) The pressure of the vessel should be 20 bars. 6) The metal conflate seals are made of copper or nickel only. 7) The following window options are to be used in Gas cell KBr, KCl, ZnSe, CaF2, BaF2, Quartz, other. References: 1) G. Kaindl, M. Domke, (1992) Rev. Sci. Instrum., 63 p1234. 2) J. B. West, A. G. Beckett, (1987). A Photodiode Calibration Facility for the VUV and Soft X-ray Region 3) G. C. King, M. Trone, F. H. Read, R. C. Bradford, (1977) J. Phys. 2479 4) P. Hitchcock and C. E. Brion, (1980) J. Phys. B, 13 Read More