Title of Invention

AN IMPROVED POISON RESISTANT PELLISTOR

Abstract

The invention relates to an improved poison resistant pellistor which is particularly useful in resisting the common poisons like sulphur dioxide and chlorine. The invention mainly consists of creating an effective barrier, of filter material like the glass wool impregnated with barium chloride and O-toludine around the active (sensitive) element of the pellistor - to prevent undesirable and harmful sensor poisons like sulphur dioxide and chlorine from reaching the active element. The invention offers a cost effective .and a simple method for tackling the common problem of some of the sensor poisons.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) AND THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10; rule 13) "AN IMPROVED POISON RESISTANT PELLISTOR" We, UNITED PHOSPHORUS LIMITED, a company incorporated under the Companies Act, 1956 and having its corporate office Uniphos House, 11th Road, C. D Marg, Khar (West), Mumbai - 400 052, State of Maharashtra INDIAN. The following specification particularly describes the nature of this invention and the manner in which it is to be performed:- An improved poison resistant Pellistor Field of Invention: The invention relates to pellistors. More particularly, the present invention relates to poison resistant pellistors that are capable of detecting and measuring common gases or poisons such as chlorine and sulfur dioxide. Background of the Invention: Instruments for detecting and/or measuring ignitable gases use catalytic (hot bead) sensors - usually referred to as pellistors. The pellistor is a miniature calorimeter that contains two coils of fine platinum wire which are coated with a ceramic or porous alumina material to form refractory beads - typically referred to as a detector and a compensator (reference element). The 'detector' bead is additionally treated with platinum or palladium based material that allows catalyzed combustion to occur - on the treated surface of the bead. The beads are wired into opposing arms of a balanced Wheatstone bridge electrical circuit. The bridge is supplied with a constant D.C. voltage that heats the elements to 500 - 550°C. Combustible gases are oxidized only on the detector element, where the heat generated increases its resistance, producing a signal proportional to the concentration of combustible gases. The compensator helps to compensate for changes in the ambient temperature, pressure and humidity which affect both elements equally. The pellistors have been widely used in the industry to detect the presence of combustible gases and vapors for safety purposes and to provide a warning of potentially hazardous conditions before these gases or vapors reach explosive levels. Chemical poisoning is one of the fundamental problems encountered with the effective use of these pellistors. The sensitivity of catalytic bead elements can be severely reduced by the presence of contaminants in the atmosphere, which can poison the catalyst. There are various ways in which the poison can affect the performance of the catalytic bead sensors by reducing catalytic activity resulting in loss of Sensitivity of the gas sensor. The effect of poison may be temporary or permanent. If the effect of poison is reversible it is referred to as inhibitor. The combustible sensor poisons include lead containing compounds, sulfur containing compounds, like H2S, S02 etc., silicones and other chlorine compounds. Some compounds may form a solid barrier over the catalyst surface. Other substances are absorbed or form compound which are temporarily absorbed by the catalyst inhibiting the normal reaction. In case of these substances the inhibition is usually temporary and the sensor may substantially recover after a period of operation in fresh air. Substances like hydrogen sulphide may function in both ways to degrade performance of the sensor. With respect to tolerance / resistance of the gas sensor to poison and particularly poisoning by substances like silicone compounds -the chemical and the physical nature of the surface material plays a significant role. In general poison tolerance depends upon the interaction between the poison and the solid surface, e.g., the poison may retain its chemical identity while being loosely or moderately bound to the solid surface. A poison may also chemically react at a 'site' on the surface. The improved poison resistance of pellistors can be obtained by selecting the catalyst having high intrinsic resistance to poisoning. However, sitch a catalyst may have reduced sensitivity to the flammable gas to be detected. Other way of doing this is by increasing the effective surface area available for reaction, which will lower the effect of poisoning of the sensor but may not completely prevent it frorn getting poisoned. Chemical compounds can also be used to scavenge poisons frorn a gas sample before the gas reaches the active element of the gas sensor. Thus, for example, silver containing compounds can be used as a dopant upon the surface of a porous material to transform inactive sites into active sites. European patent EP0062466 describes a detector pellet having greater resistance to catalyst poisoning, and the detector pellet comprises of helical platinum wire embedded in a pellet formed overall of an oxidation catalyst and a porous non -catalytitfrnert carrier wherein the pellet has a laminated, onion like structure and consists of a multiplicity of concentric layers in which layers of carrier preferably alternate with layers of catalyst.. US Patent No 4246228 describes a combustible gas detector with better tolerance to poisoning, in which heatable wire filament is embedded in a pellet consisting of a homogenous mixture of an oxidation catalyst and Zeolite material of X,Y,L with large pore size. Additional layers of catalytically active material and/or inactive non-catalytic porous material is provided around the outside of the pellet. GB 2121180 describes a combustible gas detector with greater resistance to catalytic poisoning especially by silicones which incorporates within the pellet a significant proportion of colloidal silica carrier material US Patent No. 6344174 describes the use of copper containing compounds preferably copper sulfate to improve the tolerance /resistance bof the gas sensors to a number of poisons including sulfur containing compound. As explained above, efforts are being made to overcome the difficult problem of sensor poisoning by different methods but the results are still far from what is desired and feasible. And the methods have the limitations of being useful, in most cases against a particular poison only. There is also a need to have pellistors particularly resistant to inhibitors such as sulphur dioxide (SO2) and Chlorine (CI). Objects of the invention : Thus, an object of the present invention is to provide a pellistor that is resistant to poisons. Another object of the present invention is to provide a pellistor that is resistant to poisoning, particularly against common poisons such as chlorine and sulfur dioxide. Yet another object of the present invention is to provide a poison resistant pellistor that is capable of detecting and measuring common poisons such as chlorine and sulfur containing compounds. Yet another object of the present invention is to provide a poison resistant pellistor that possesses a high intrinsic resistance to poison without having a reduced sensitivity to the flammable gas to be detected. These and other objects of the present invention are achieved by way of the invention described hereinafter. Summary of the invention An improved poison resistant pellistor comprising at least one filter component impregnated with a resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof. An improved poison resistant pellistor comprising: (a) a compensator element comprising a platinum wire coil embedded within a bead; (b) a detector element comprising a platinum wire coil embedded within a bead and being partially or completely covered with at least one catalyst layer; and (c) at least one filter component disposed over said compensator and detector elements, said filter component being impregnated with a resistance enhancing material selected from (i) an alkali or alkaline earth halide, or an alkali or alkaline earth hydroxide; or (ii) an ortho, meta or para toluidine; or combinations thereof. A process for making an improved filter component for use in a poison resistant pellistor, said process comprising: (a) dissolving predetermined quantities of at least one resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth metal hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof in a protic solvent to form a clear solution; (b) soaking a provided filter component having predetermined dimensions into said clear solution for a predetermined time; and (c) drying said soaked filter component at a predetermined temperature for a predetermined time. A process for making an improved poison resistant pellistor comprising: (a) bending provided pure platinum wires to a plurality of platinum wire coils; (b) spot welding at least two of said platinum wire coils on a provided sensor base plate support pins; (c) repetitively applying a slurry of an aluminum salt over said spot welded platinum wire coils and heating said applied aluminum salt, said step being repeated until the formation of a porous alumina bead having a desired bead size; (d) embedding at least one catalyst layer over one said spot welded platinum wire coil; (e) dissolving predetermined quantities of at least one resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth metal hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof in a protic solvent to form a clear solution; (f) soaking a provided filter component having predetermined dimensions into said clear solution for a predetermined time; (g) drying said soaked filter component at a predetermined temperature for a predetermined time; (h) positioning said impregnated filter component above said porous alumina beads such that a combustible gas reaching the beads permeates through said filter component; and (i) optionally calibrating said pellistor obtained thereby for a plurality of predefined parameters. Brief description of the drawings Fig 1. labels 1 - SS 316 meta! cup housing 2 - SS 316 sintered metal disc 3 - Filter (Glass wool / Ceramic Fiber paper) 4 - Sensor bead holding PCB. 5- Sensor pins 6 - Potting compound 7 - Top PCB .Fig.l is a schematic representation of a sensor of the present invention with the different components illustrated including the position of the impregnated glass wool (3) above the sensor beads (4).The sintered metal disc (2) below the aperture (inlet )allows the passage of the target gas to the sensor beads. Sensor pins (5) enable the attachment of the sensor to the electronic circuit on printed circuit board to form other arm of Wheatstone Bridge. Detailed description of the invention: It has been found by the present inventors that positioning a filter component over the compensator and detector elements of a pellistor, wherein the filter component is impregnated with a poison resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof surprisingly enhances the resistance of the pellistor to poisoning, especially induced by compounds containing sulfur dioxide and chlorine. Accordingly, in one aspect, the present invention provides an improved poison resistant pellistor comprising at least one filter component irnpregnated with a resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof. The term "resistance enhancing material" according to the present invention denotes a substance that surprisingly enhances the resistance of the pellistor to poisoning. In a preferred embodiment, the resistance enhancing material of the present invention enhances the poison resistance of the pellistor sensors especially to the poison induced by compounds containing sulfur and chlorine. In another aspect according to the present invention, the flammable gas sensor 'pellistor' consists of two matched elements viz. detector and compensator. These two elements form the two arms of a Wheatstone bridge circuit. They consist of porous alumina beads surrounding a thin platinum wire, which acts both as a heater and as a resistance thermometer. The detector bead is impregnated with a suitable catalyst and can cause combustion of flammable gas at much lower temperature of the order of 400 to 500°C. But the compensator bead is poisoned and is incapable of producing any combustion. The amount of heat generated and the consequent rise in temperature of the detector bead results in the change in resistance of the platinum coil. This change of resistance of the detector bead, is measured using a bridge circuit and is related to the gas concentration. The compensator bead serves to compensate for the change in resistance of the platinum thermometer due to causes other than the changes in flammable gas concentration. Pellistors are suitable for the measurement of many of the combustible gases where the decomposition products are volatile and do not deposit on the bead. Therefore, in another aspect, the present invention provides an improved poison resistant pellistor comprising: (a) a compensator element comprising a platinum wire coil embedded within a bead; (b) a detector element comprising a platinum wire coil embedded within a bead and being partially or completely covered with least one catalyst layer; and (c) at least one filter component disposed over said compensator and detector elements, said filter component being impregnated with a resistance enhancing material selected from (i) an alkali or an alkaline earth halide, or an alkali or an alkali or an alkaline earth hydroxide; or (ii) an ortho, meta or para toluidine; or combinations thereof. The filter component according to the present invention is a gas permeable filter that is capable of withstanding high temperature. Such filters are well known to persons skilled in the art and may be conveniently selected to meet the specific requirements. In an embodiment of the present invention, the filter material may be preferably glass wool or ceramic fiber. In another embodiment, the alkali or alkaline earth metal halide poison resistance enhancing material is a fluoride, chloride, bromide or iodide of a metal selected from the group comprising lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. More preferably, said resistance enhancing material is barium chloride. - * " In another embodiment, the resistance enhancing material may be an alkali or an alkaline earth metal hydroxide, which may be hydroxide of a metal selected from lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. Preferably, the poison resistance material according to this embodiment may be sodium or potassium hydroxide. In an embodiment, the preferred toluidine is ortho-toluidine though meta- and para -toluidine may also be used. In a preferred embodiment, the poison resistance enhancing material according to the present invention is a combination comprising barium chloride and ortho-toluidine or alkali metal hydroxide and ortho-toluidine or barium chloride, alkali metal hydroxide and ortho-toluidine. More particularly, the preferred concentration of barium chloride is 5% and the preferred concentration of ortho-toluidine is 1%. In another embodiment, the catalyst layer provided on the detector element comprises a platinum catalyst or a palladium catalyst. Preferably, the bead present in the poison resistant pellistors according to the present invention comprises alumina. The platinum wire is preferably from about 25 (am to about 40 urn in diameter. In an embodiment, the detector element of the poison resistant pellistor according to the present invention is further impregnated with thorium nitrate. In another embodiment, the filter component according to any aspect of the present invention is further impregnated with lead acetate. In yet another aspect, the present invention provides a process for making an improved filter component for use in a poison resistant pellistor, said process comprising: (a) dissolving predetermined quantities of at least one resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth metal hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof in a protic solvent to form a clear solution; (b) soaking a provided filter component having predetermined dimensions into said clear solution for a predetermined time; and (c) drying said soaked filter component at a predetermined temperature for a predetermined time. The improved filter component according to the above process is incorporated with a pellistor to impart poison resistance to the pellistor obtained thereby, particularly to poisoning induced by sulfur dioxide and chlorine. In an embodiment, the protic solvent according to the present invention is selected from water, methanol, ethanol, formic acid, hydrogen fluoride and ammonia. Preferably, the protic solvent is methanol. Thus, in another aspect, the present invention provides a process for making an improved poison resistant pellistor, said process comprising: (a) bending provided pure platinum wires to a plurality of platinum wire coils; (b) spot welding at least two of said platinum wire coils on a provided sensor base plate support pins; (c) repetitively applying a slurry of an aluminum salt over said spot welded platinum wire coils and heating said applied aluminum salt, said step being repeated until the formation of a porous alumina bead having a desired bead size; (d) embedding at least one catalyst layer over one said spot welded platinum wire coil; t (e) dissolving predetermined quantities of at least one resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth metal hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof in a protic solvent to form a clear solution; (f) soaking a provided filter component having predetermined dimensions into said clear solution for a predetermined time; (g) drying said soaked filter component at a predetermined temperature for a predetermined time; (h) positioning said impregnated filter component above said porous alumina beads such that a combustible gas reaching the beads permeates through said filter component; and (i) optionally calibrating said pellistor obtained thereby for a plurality of predefined parameters. The pure platinum wire of 25 to 40 micron diameter preferable 30 micron is used for making coils using coil winding machine. The number of turns are 10 to 11 preferably 10 and a half turns. The former used on which the wire is wound is of 0.5 mm in diameter. The coils prepared as above are first washed in acid using a mixture of H2S04 and KMn04 and then with a mixture of HN03 and H202. Finally they are thoroughly washed with distilled water and dried in air over. The coils thus cleaned are spot welded on the sensor base plate using a specially designed spot welding machine for the purpose. The sensor base plates containing bare coils are transferred on to the zig specially designed for holding and processing multiple sensors at a time. Thus, according do a preferred embodiment, the step of spot welding at least two of said platinum wire coils on sensor^base plate support pins comprises washing the bent platinum wire coils in an acidic mixture of sulfuric acid and potassium permanganate; thereafter washing the bent platinum wire coils in a mixture of nitric acid and hydrogen peroxide; washing thus washed platinum wire coils with distilled water; drying the washed platinum wire coils in air and spot welding the washed platinum wire coils using a spot welding machine. For the formation of porous alumina bead , the coils are applied with slurry of an aluminum salt, which is preferably aluminium nitrate, which gets decomposed to form alumina on heating the coils, electrically by passing current. The process of heating can be in a controlled way using a programmable power supply wherein starting from 50 mA, the current is increased to 165 to 200 mA at the rate of 1 mAJmin. Thus, in a preferred embodiment, the step of repetitively applying a slurry of an aluminum salt and heating said aluminum salt comprises applying a slurry of aluminum nitrate on said platinum wire coils and heating the applied slurry in a controlled manner using a programmable power supply gradually increasing from about 50 mA to about 200 mA at the rate 1 mA per minute. When the bead is dried completely second or third layer of aluminium nitrate is applied on it to achieve the desired size of the bead. The heating process can be repeated as mentioned above. After successfully forming the white porous alumina bead, they are separately used for making sensing and compensating sensor elements. The detector element (or the sensing element) is formed by applying a catalyst layer over the porous alumina bead, wherein either platinum or palladium catalyst may be preferably used. A saturated solution of palladium chloride is convenient for this purpose. The bead is dried in the same way as mentioned above after applying palladium chloride solution. Several such layers are applied to deposit sufficient amount of catalyst over the porous alumina bead. Thus, in an embodiment, the step of embedding at least one catalyst layer over one said spot welded platinum wire coil comprises applying a saturated solution of a platinum or palladium catalyst, preferably palladium chloride to a plurality of alumina beads, heating said applied saturated solution in a controlled manner using a programmable power supply gradually increasing from about 50 mA to about 200 mA at the rate 1 mA per minute and optionally impregnating the bead with saturated thorium nitrate solution in a likewise manner. The flammable gas sensor prepared according to this invention is subjected to conditioning for stabilizing its physical and electrical characteristics. For this purpose the flammable gas sensor is connected in a bridge circuit. Suitable sensor operating voltage is applied to the bridge so as to maintain the bead at a temperature where combustion of methane readily occurs. The sensor is exposed to 12% methane in air. When the combustion of methane occurs the sensing bead glows bright red. The sensor is kept exposed for 10 minutes to methane. Then it is removed from the gas and exposed to clean air for 10 minutes in powered condition. Thus, in another preferred embodiment, the step of calibrating the pellistor comprises connecting the pellistor to a Wheatstone bridge circuit; applying a sensor operating voltage to said bridge so as to maintain a temperature at which combustion of a test gas, preferably methane, readily occurs; exposing the pellistor to air containing a predefined concentration of the test gas; keeping the pellistor exposed to air containing a predefined concentration of the test gas for a pre-defined amount of time; removing the pellistor from said air containing a predefined concentration of the test gas and exposing to ambient air for a pre-defined amount of time. It is believed that this process of gas conditioning of flammable gas sensor helps in reducing palladium chloride to palladium metal. The flammable gas sensor prepared according to this invention is then packed in a suitable sensor housing depending upon its area of application viz. Domestic or Industrial. Example 1: Method of preparing the glasswool impregnated with appropriate substances for inhibiting the poisons: The process of making filter for S02 is as follows. 1. BaC12 qty 5.0 grams is dissolved in 100 ml distilled water to form a clear solution. 2. Filters of required size are cut and soaked in to this solution and kept overnight or say 24 hrs. 3. At the end of 24 hrs the solution is decanted and the filters are dried in air oven at 125 deg centigrade for 2 hrs. 4. The filters are then used in sensors during assembly. Example 2: The process of making filter for CI2 is as follows: .1. O- Toludine qty 1.0 gram is dissolved in 100 ml Methanol to form a clear solution. 2. Filters of required size are cut and soaked in to this solution and kept overnight or say 24 hrs. 3. At the end of 24 hrs the solution is decanted and the filters are dried in air oven at 125 deg centigrade for 2 hrs. 4. The filters are then used in sensors during assembly. Example 3: The actual process of making combined filter for S02 and CI2 is as follows: 1. BaC12 qty 5.0 grams and O- Toludine qty 1.0 grams is dissolved in 100 ml Methanol to form a clear solution. 2. Filters of required size are cut and soaked in to this solution and kept overnight or say 24 hrs. 3. At the end of 24 hrs the solution is decanted and the filters are dried in air oven at 125 deg centigrade for 2 hrs. In another'embodiment of the invention, to eliminate SO2, the alkali hydroxide such as NaOH or KOH is successfully used in place of BaC12 to impregnate the filters. The filters are then used in sensors during assembly placing them above the pellistor beads in such way that combustible gas reaches to the beads only after passing through the filters. The filter material preceding the beads is further impregnated, if desired, with lead acetate solution for making the Pellistor resistant to poisoning by hydrogen Sulphide (H2S). The preferred concentration of the lead acetate is 20% w/v in water. Wherein the aforegoing reference has been made to integers or components having known equivalents, then such equivalents are herein incorporated as if individually set forth. Accordingly, it will be appreciated that changes may be made to the above described embodiments of the invention without departing from the principles taught herein. Thus, it will be understood that the invention is not limited to the particular embodiments described or illustrated, but is intended to cover all alterations or modifications which are within the scope of the present invention. WE CLAIM: 1. An improved poison resistant pellistor comprising at least one filter component impregnated with a resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof. 2. An improved poison resistant pellistor comprising: (a) a compensator element comprising a platinum wire coil embedded within a bead; (b) a detector element comprising a platinum wire coil embedded within a bead and being partially or completely covered with least one catalyst layer; and (c) at least one filter component disposed over said compensator an'd detector elements, said filter component being impregnated with a resistance enhancing material selected from (i) an alkali or an alkaline earth halide, or an alkali or an alkali or an alkaline earth hydroxide; or (ii) an ortho, meta or para toluidine; or combinations thereof. 3.. An improved poison resistant pellistor as claimed in claim 1 or claim 2, wherein said filter component is a gas permeable filter material capable of withstanding high temperature. 4. An improved poison resistant pellistor as claimed in claim 3, wherein said filter component is selected from glass wool or ceramic fiber. 5. An improved poison resistant pellistor as claimed in claim 4, wherein said filter component is glass wool. 6. An improved poison resistant pellistor as claimed in any preceding claim, wherein said resistance enhancing material is a.fluoride, chloride, bromide or iodide of a metal selected from the group comprising lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. 7. An improved poison resistant pellistor as claimed in claim 6, wherein said resistant enhancing material is barium chloride. 8. An improved poison resistant pellistor as claimed in any preceding claim, wherein said resistance enhancing material is a hydroxide of a metal selected from the group comprising lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. 9. An improved poison resistant pellistor as claimed in any preceding claim, wherein said resistance enhancing material is selected from the group comprising sodium hydroxide and potassium hydroxide. * 10. An improved poison resistant pellistor as claimed in any preceding claim, wherein preferred toluidine is the ortho-toluidine. 11. An improved poison resistant pellistor as claimed in any preceding claim, wherein said resistance enhancing material comprises either barium chloride or an alkali metal hydroxide or a combination of any or both of these with ortho-toluidine. 12. An improved poison resistant pellistor as claimed in any preceding claim, wherein the catalyst layer comprises a platinum or palladium catalyst. 13. An improved poison resistant pellistor as claimed in any preceding claim, wherein said bead is an alumina bead. 14. An improved poison resistant pellistor as claimed in any preceding claim, wherein said platinum wire is of from about 25 urn to about 40 urn in diameter. 15. An improved poison resistant pellistor as claimed in any preceding claim, wherein said detector element is further impregnated with thorium nitrate. 16. An improved poison resistant pellistor as claimed in any preceding claim, wherein said filter component is further impregnated with lead acetate. 17. An improved poison resistant pellistor as claimed in any preceding claim, wherein the preferred concentration of barium chloride is 5% and the preferred concentration of ortho-toluidine is 1%. 18. An improved poison resistant pellistor as claimed in any preceding claim, wherein at least two of said platinum wire coils are wired to opposing arms of a balanced Wheatstone bridge circuit. 19. An improved poison resistant pellistor as claimed in any preceding claim, wherein said pellistor is resistant to poisoning induced by compounds containing sulfur dioxide and chlorine. 20. A process for making an improved filter component for use in a poison resistant pellistor, said process comprising: (a) dissolving predetermined quantities of at least one resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth metal hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof in a protic solvent to form a clear solution; (b) soaking a provided filter component having predetermined dimensions into said clear solution for a predetermined time; and (c) drying said soaked filter component at a predetermined temperature for a predetermined time. 21. A process for making an improved filter component as claimed in claim 20, wherein said filter component is glass wool. 22. A process for making an improved filter component as claimed in claim 20 or claim 21, wherein said resistance enhancing material comprises either barium chloride or an alkali metal hydroxide or a combination of any or both of these with ortho-toluidine. 23. A process for making an improved filter component as claimed in claim 22, wherein said filter component is further impregnated with lead acetate. 24. A process for making an improved poison resistant pellistor comprising: (a) bending provided pure platinum wires to a plurality of platinum wire coils; (b) spot welding at least two of said platinum wire coils on a provided sensor base plate support pins; (c) repetitively applying a slurry of an aluminum salt over said spot welded platinum wire coils and heating said applied aluminum salt, said step being repeated until the formation of a porous alumina bead having a desired bead size; (d) embedding at least one catalyst layer over one said spot welded platinum wire coil; * (e) dissolving predetermined quantities of at least one resistance enhancing material selected from (a) an alkali or alkaline earth halide, or an alkali or alkaline earth metal hydroxide; or (b) an ortho, meta or para toluidine; or combinations thereof in a protic solvent to form a clear solution; (f) soaking a provided filter component having predetermined dimensions into said clear solution for a predetermined time; (g) drying said soaked filter component at a predetermined temperature for a predetermined time; (h) positioning said impregnated filter component above said porous alumina beads such that a combustible gas reaching the beads permeates through said filter component; and (i) optionally calibrating said pellistor obtained thereby for a plurality of predefined parameters. 25. A process for making an improved poison resistant pellistor claimed in claim 24, wherein said protic solvent is selected from the group comprising water, methanol, ethanol, formic acid, hydrogen fluoride and ammonia. 26. A process for making an improved poison resistant pellistor claimed in claim 25, wherein said protic solvent is methanol. 27. A process for making an improved poison resistant pellistor claimed in claim 24, wherein said slurry of aluminum salt comprises aluminum nitrate. 28. A process for making an improved poison resistant pellistor as claimed in claim 24, wherein said step of spot welding at least two of said platinum wire coils on sensor base plate support pins comprises washing the bent platinum wire coils in an acidic mixture of sulfuric acid and potassium permanganate; thereafter washing the bent platinum wire coils in a mixture of nitric acid and hydrogen peroxide; washing thus washed platinum wire coils with distilled water; drying the noshed pkttNtam ww-ecvwk AT airjtud spot needing the washed platinum wire coils using a spot welding machine. 29. A process for making an improved poison resistant pellistor as claimed in claim 24, wherein the step of repetitively applying a slurry of an aluminum salt and heating said aluminum salt comprises applying a slurry of aluminum nitrate on said platinum wire coils and heating the applied slurry in a controlled manner using a programmable power supply gradually increasing from about 50 mA to about 200 mA at the rate 1 mA per minute. 30. A process for making an improved poison resistant pellistor as claimed in claim 24, wherein said step of embedding at least one Catalyst layer over one said spot welded platinum wire coil comprises applying a Saturated solution of a platinum or palladium catalyst, preferably palladium chloride to a .plurality of alumina beads, heating said applied saturated solution in. a controlled manner using a programmable power supply gradually increasing from about 50 mA to about 200 mA at the rate 1 mA per minute and optionally impregnating the bead with saturated thorium nitrate solution in a likewise mariner. 31. A process for making an improved poison resistant pellistor as claimed in claim 24, wherein the step of calibrating the pellistor comprises connecting the pellistor to a Wheatstone bridge circuit; applying a sensor operating voltage to said bridge so as to maintain a temperature at which combustion of a test gas, preferably methane, readily occurs; exposing the pellistor to air containing a predefined concentration of the test gas; keeping the pellistor exposed to air containing a predefined concentration of the test gas for a pre-defined amount of time; removing the pellistor from said air containing a predefined concentration of the test gas and exposing to ambient air for a pre-defined amount of time. 32. An improved poison resistant pellistor substantially as described herein with reference to the accompanying drawings and the detailed description of the drawings. 33. A process for making an improved filter component for use in a poison resistant pellistor substantially as described herein with reference to the detailed description and the accompanying drawings. 34. A process for making an improved poison resistant pellistor substantially as described herein with reference to the detailed description and the accompanying drawings. Dated this 19th day of January 2009

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121-mum-2009-abstract.doc

121-mum-2009-abstract.pdf

121-MUM-2009-CLAIMS(AMENDED)-(13-6-2012).pdf

121-MUM-2009-CLAIMS(AMENDED)-(30-12-2011).pdf

121-MUM-2009-CLAIMS(MARKED COPY)-(13-6-2012).pdf

121-MUM-2009-CLAIMS(MARKED COPY)-(30-12-2011).pdf

121-mum-2009-claims.doc

121-mum-2009-claims.pdf

121-MUM-2009-CORRESPONDENCE(20-9-2011).pdf

121-MUM-2009-CORRESPONDENCE(27-3-2012).pdf

121-MUM-2009-CORRESPONDENCE(3-2-2009).pdf

121-mum-2009-correspondence(3-4-2009).pdf

121-mum-2009-correspondence.pdf

121-mum-2009-description(complete).doc

121-mum-2009-description(complete).pdf

121-mum-2009-drawing.pdf

121-MUM-2009-FORM 1(13-6-2012).pdf

121-MUM-2009-FORM 1(3-2-2009).pdf

121-mum-2009-form 1.pdf

121-mum-2009-form 13(20-9-2011).pdf

121-MUM-2009-FORM 18(3-2-2009).pdf

121-mum-2009-form 2(title page).pdf

121-mum-2009-form 2.doc

121-mum-2009-form 2.pdf

121-mum-2009-form 3.pdf

121-mum-2009-form 5.pdf

121-mum-2009-form 9(31-3-2009).pdf

121-MUM-2009-GENERAL POWER OF ATTORNEY(20-9-2011).pdf

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121-MUM-2009-REPLY TO HEARING(13-6-2012).pdf

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