FORM 2
THE PATENTS ACT, 1970
(39 of 1970) AND
THE PATENT RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
"A DRIFT STABILIZED 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:-


A DRIFT STABILIZED PELLISTOR
Field of the invention
The present invention relates to a pellistor. More particularly, the present invention relates to a drift stabilized pellistor comprising individual resistance-synchronized beads.
Background of the invention
A pellistor is a device for detecting and/or measuring combustible gas concentration. A 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 (sensing element) and a compensator (reference element). The 'detector' bead is additionally treated with platinum or palladium based catalytic material that allows catalyzed combustion to occur - on the treated surface of the bead. The beads are wired into opposing arms of a Wheatstone bridge electrical circuit. The bridge is supplied with a constant D.C. voltage that heats the elements to 450 - 550°C. Combustible gases are oxidized only on the detector element, where the heat generated increases its resistance and upsets the balanced bridge (which is balanced in zero air) and produces an out of balance voltage signal proportional to the concentration of combustible gases. The compensator helps to compensate for changes in the ambient temperature, pressure and humidity and such other factors 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. These catalytic bead sensors are based on a low cost technology and such sensors are able to detect a wide range of combustible gases and vapors. Another advantage of these catalytic bead sensors is that these are simple devices requiring no special equipment for commissioning or maintenance apart from the calibration gas. However, these devices also suffer from various known drawbacks.
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A major problem associated with the use of the presently known pellistors when used on instruments is the baseline shift which requires frequent corrections or setting. This problem does not pose a serious impediment when these sensors are used on hand held and portable instruments used for occasional measurements of combustible gases. This is because before making measurement it is possible to adjust the baseline to zero in clean air. However when these pellistors are used on fixed systems for continuous measurement it is not practicable to adjust the zero in clean air. Measurements made with a drifted baseline lead to erroneous readings. In such applications a highly stable pellistor against zero drift is required.
Moreover, since the detector and the compensator beads are wired into the opposing arms of a Wheatstone bridge, it is important to ensure that the detector and the compensator resistances remains balanced and ratio of their resistances should not change in an ideal case during the entire life span of the pellistor. However, it is often seen that the individual resistances of the detector and compensator beads vary unpredictably with the passage of time thereby disturbing the Wheatstone bridge balance. It is further seen that the individual resistances of the detector and the compensator do not always change in the same direction i.e. while the detector resistance may increase, the compensator resistance may actually decrease with time and vice versa. In a prefabricated pellistor the above parameter and the trend of their change with time is beyond the control of the user.
Practically, in all prefabricated pellistor bead pairs, the ratio of resistances referred to hereinabove changes to different degree and zero drift is inevitable during prolonged operation of the pellistor. The zero drifts as described above can be as high as or even cross the alarm set point leading to false alarms. In such a scenario, the zero-drifted pellistor sensors remain electrically operational and display a reading even when pellistor has not encountered any gas. It is also seen that when combustible gas is present, the reading which it shows is higher or lower by the amount of zero drift. The conventional prefabricated pellistor bead pairs further display a tendency because of which their resistance ratio changes quite erratically and also by a large amount with the passage of time. Thus, these catalytic bead sensors require frequent adjustment of zero particularly
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during the initial stage of their life and even later the drift can continue although at a slower rate.
Numerous attempts have been made in the art to provide improved pellistors having improved sensitivity and poison resistance. However, these previous efforts do not address the need for a pellistor having stabilized zero drift with the passage of time. The present inventors believed that pellistor beads having improved stability to zero drift would solve the aforesaid problems existing in the art. However, the drift stabilization had been an unfulfilled requirement in the art.
Another problem of the catalytic bead sensors is their susceptibility to poisoning. 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 poison resistance of pellistors can be achieved by selecting a catalyst having an inherent property of high resistance to poisoning. Platinum is one such catalyst which has very high resistance to poison like H2S, Silicon etc. However, the sensitivity of platinum catalyst expressed in terms of mV output per % LEL is considerably lower as compared to some of the other catalysts. The platinum catalyst also requires a temperature of above 1000°C for optimum performance. On the other hand, palladium catalyst requires only about 500°C for a catalytic reaction of methane over it. It also has a high sensitivity expressed in terms of mV output per % LEL as compared to platinum catalyst. But it has an inherent property of being poisoned by the above mentioned poison gases.
There is a demand in the industry to have a pellistor having an improved sensitivity of the order of 20 mV per % LEL and yet having the property of poison resistance to the aforesaid poisons.
European patent EP0062466 describes a detector pellet having greater resistance to catalyst poisoning. The detector pellet comprises a helical platinum wire embedded in a

pellet formed overall of an oxidation catalyst and a porous non -catalytic inert 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 of the gas sensors to a number of poisons including sulfur containing compound.
However, a need continues to remain in the art for a catalyst displaying optimum poison resistance as well as improved sensitivity.
Objects of the invention:
Thus, it is an object of the present invention to provide a drift stabilized pellistor comprising individual resistance-synchronized catalytic beads.
Yet another object of the present invention is to provide a pellistor that is less prone to zero drifts and which gives reliable readings consistently over a long time without having to adjust the zero.
Another object of the present invention is to provide a pellistor wherein the detector and compensator bead resistances maintain the same ratio during the entire life span of the pellistor.

Another object of the present invention is to provide a pellistor wherein the individual detector and compensator bead resistances shift in the same direction.
Yet another object of the present invention is to provide a pellistor that reduces the incidences of false alarms and missing or failing to detect dangerous levels of gas.
Another object of the present invention is to provide a pellistor wherein the fall in the catalytic bead sensitivity is minimized and its reliability is improved during the life-span of the pellistor.
Yet another object of the present invention is to provide a pellistor wherein individual catalytic sensor beads exhibit improved poison resistance without compromising the sensitivity of the sensors.
Yet another object of the present invention is to provide an improved individual catalytic bead that is less prone to drifts over its life span.
Another object of the present invention is to provide a method for manufacturing an improved individual catalytic bead that is less prone to drifts over its life span.
Yet another object of the present invention is to provide a method for manufacturing an improved pellistor.
These and other objects of the present invention are achieved by way of the invention described hereinafter.
Summary of the invention
A method for preparing a drift stabilized sensor bead comprising: (a) maintaining a provided bead at a predetermined temperature and thereafter exposing said bead to a predetermined concentration of a combustible gas in air at said temperature for a first predetermined length of time; and (b) subsequently maintaining said sensor bead at its operating temperature for a second predetermined length of time by passing electric current of a predefined magnitude through said bead.
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A drift stabilized sensor bead obtained by a conditioning method for drift-stabilizing said sensor bead, said conditioning method comprising: (a) maintaining a provided bead at a predetermined temperature and thereafter exposing said bead to a predetermined concentration of a combustible gas in air at said temperature for a first predetermined length of time; and (b) subsequently maintaining said sensor bead at its operating temperature for a second predetermined length of time by passing electric current of a predefined magnitude through said bead.
An improved drift stabilized pellistor comprising:
(a) a drift stabilized compensator bead comprising a platinum wire coil embedded within a bead;
(b) a drift stabilized detector bead comprising a platinum wire coil embedded within a bead and being covered with a predetermined catalyst; and
(c) a balanced Wheatstone bridge circuit comprising said stabilized compensator and detector beads being two arms of said circuit, wherein said stabilized compensator and detector beads are resistance synchronized.
A method for fabricating an improved drift stabilized pellistor, said method comprising:
(a) drift stabilizing a plurality of compensator beads by conditioning said compensator beads;
(b) drift stabilizing a plurality of detector beads by conditioning said detector beads; and
(c) selecting at least one drift stabilized compensator bead and at least one drift stabilized detector bead such that said selected compensator and detector beads are resistance synchronized.
A method for fabricating an improved drift stabilized pellistor comprising:
(a) Winding provided pure platinum wires to a plurality of platinum wire coils;
(b) spot welding 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 a first group of said spot welded platinum wire coils to form a plurality of detector beads;
(e) embedding at least one alkali hydroxide layer over the remaining second group of said spot welded platinum wire coils to form a plurality of compensator beads;
(f) drift stabilizing said plurality of compensator beads by conditioning said compensator beads;
(g) drift stabilizing a plurality of detector beads by conditioning said detector beads;
(h) selecting at least one drift stabilized compensator bead and at least one drift
stabilized detector bead such that said selected compensator and detector beads
are resistance synchronized; (i) forming a Wheatstone bridge circuit comprising said selected compensator and
detector bead being two arms of said circuit; and (j) optionally calibrating said pellistor obtained thereby by applying a known
concentration of a flammable gas. to the sensor.
A method for detecting and measuring the level of a combustible gas in an environment, said method comprising: (a) fabricating a drift stabilized pellistor comprising a drift stabilized detector bead and a drift stabilized compensator bead wherein said detector and compensator beads are resistance synchronized; and (b) utilizing said drift stabilized pellistor to detect and measure the presence of a predefined gas in an environment.
Brief description of the drawings
Fig.l is a schematic representation of a pellistor and its essential components.
Fig. 2 represents a catalytic bead sensor Wheatstone bridge circuit.
Figure 3 is a flowchart depicting a method for preparing a drift stabilized sensor bead.
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Figure 4 is a flowchart depicting a method for fabricating an improved drift stabilized pellistor.
Figure 5 is a graph depicting the variation of the percentage LEL baseline curve for four conventional pellistor beads.
Figure 6 is a graph depicting the variation of the percentage LEL baseline curve for five conditioned pellistor beads according to the present invention.
Detailed description of the invention:
The present inventors have found that conditioning a provided compensator and detector bead at a predetermined temperature and exposing the bead to a known concentration of a combustible gas in air for a length of time and thereafter maintaining the sensor bead at its operating temperature by the passage of electric current stabilizes the resistance of said bead throughout the life span of the bead. A sensor bead stabilized in this manner has surprisingly less susceptibility to zero drift when used on a balanced bridge circuit of a pellistor thereby stabilizing the drift characteristics of the pellistor as well.
The predetermined temperature depends upon the nature of the catalyst for example methane requires a temperature of about 1000 ° C for it to combust, whereas the palladium catalyst requires only 450 C to 550 C for methane to combust on it. A lower combustion temperature for catalyst is preferable as pellistors with such catalyst consume less power and give pellistors with lower operating temperatures.
The origin of zero drift of a pellistor fabricated using conventional sensor is the small variation in the ratio of the internal resistances of the detector and compensator beads induced as a result of the continuous operation of a pellistor. It has been found that the resistances of the compensator and detector beads do not always change in a common direction (i.e. either increase or decrease), thereby disturbing the balance of the Wheatstone bridge circuit. Any variation in the ratio of the compensator bead resistance to the detector bead resistance has been shown to introduce zero drifts in the pellistor. During the operation of the pellistor, the present inventors observed that the individual
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resistance variations occur at a faster rate initially and tend to taper off with the passage of time.
Thus, in one aspect, the present invention provides a method for preparing a drift stabilized sensor bead comprising: (a) maintaining a provided bead at a predetermined temperature and thereafter exposing said bead to a predetermined concentration of a combustible gas in air at said temperature for a first predetermined length of time; and (b) subsequently maintaining said sensor bead at its operating temperature for a second predetermined length of time by passing electric current of a predefined magnitude through said bead.
In an embodiment, the combustible gas is preferably methane. The provided bead is initially maintained at the combustion temperature of methane, preferably from about 450°C to about 550 C. The predetermined concentration at which the bead is exposed to methane is preferably 12% methane v/v in air. The preferred first length of time is at least about 10 minutes. Thus, in a preferred embodiment, the first step of conditioning the bead comprises maintaining or heating the bead at its operating temperature in presence of 12% methane v/v in air for about 10 minutes..
Subsequently, the bead is maintained at its operating temperature for a second length of time by passing an electric current through the bead. Preferably, the second length of time is at least about 1 week to about three months and is preferably at least about 2 months though prolonged or reduced maintenance lengths of time are not excluded. Thus, in an embodiment, the second step of said conditioning method involves maintaining the bead for at least about 1 month and preferably about 2 months for continuous heating at sensor operating temperatures by passing electric current through the bead.
In a more preferred embodiment of this method, the provided detector or compensator bead is conditioned in order to stabilize its physical and electrical characteristics by connecting the bead in a bridge circuit. Thereafter, a suitable sensor operating voltage is applied to the bridge in order to maintain the bead at a temperature wherein the combustion of methane occurs spontaneously. The sensor is exposed to 12% methane in
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air. It has been observed that the sensor bead glows bright red during the combustion of methane. The sensor is kept exposed to methane in this manner for about 10 minutes.
Preferably, the sensor is thereafter taken out of the combustible gas exposure and thereafter exposed to clean air for about 10 minutes in powered mode.
Thus, in this embodiment, the present invention provides a method for preparing a drift stabilized sensor bead wherein the sensor is additionally exposed to clean air for about ten minutes in powered mode prior to being maintained at said second length of time.
Without wishing to be bound by theory, it is believed that the described conditioning method reduces the catalyst in the form of platinum or palladium chlorides to the respective metallic form.
In a second step of said conditioning method, said bead is preferably maintained for at least about 1 week to about 3 months, preferably at least about 2 months for continuous heating at the sensor operating temperature by passing electric current through it. The second length of time is not particularly limiting and varies from about 1 week to about 3 months in order to confer acceptable stabilization of the beads.
It was found that the compensator and detector beads conditioned according to the method described hereinbefore possessed extraordinary stability with respect to their zero drifts over their entire life-span.
Accordingly, in another aspect, the present invention provides a drift stabilized bead obtained by the conditioning method described hereinabove.
The drift stabilized detector and compensator beads thereafter form the two arms of a balanced wheatstone circuit in a drift resistant pellistor according to the present invention.
Thus, in another aspect, the present invention provides an improved drift stabilized pellistor comprising:
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(a) a drift stabilized compensator bead comprising a platinum wire coil embedded within a bead;
(b) a drift stabilized detector bead comprising a platinum wire coil embedded within a bead and being covered with a predetermined catalyst; and
(c) a balanced Wheatstone bridge circuit comprising said stabilized compensator and detector beads being two arms of said circuit, wherein said stabilized compensator and detector beads are resistance synchronized.
The term "drift stabilized" as appearing herein shall denote an individual compensator bead or an individual reference bead that is conditioned according to the conditioning method described in any aspect or embodiment thereof hereinabove. These drift-stabilized beads possess extraordinary stability with respect to their zero drifts over their entire life-span.
The predetermined catalyst coated or embedded onto said detector bead is either a platinum based catalyst such as platinum chlorides or a palladium based catalyst such as palladium chlorides or combinations thereof. In an embodiment, the preferred catalytic material comprises a 1:1 ratio mixture of platinum and palladium based catalysts.
It has been further found that a 1:1 ratio mixture of platinum and palladium based catalysts confers an improved poison resistance property to the pellistor yet having adequate sensitivity and more than a pellistor having only the platinum catalyst.
The drift stabilized detector and compensator beads are thereafter incorporated as two arms of a balanced wheatstone bridge circuit. The bridge is supplied with a constant DC voltage that heats the beads to about 450 - 550°C. The combustible gases that are present in the ambient air are oxidized only on the detector bead. The heat generated during the combustion raises the resistance of the detector bead thereby disturbing the balance of the bridge circuit. The out of balance signal generated by the bridge circuit is thereafter correlated to the concentration of the combustible gas. The compensator bead helps to compensate for external changes in the ambient temperature, pressure and humidity which affects both the detector and compensator beads equally.
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It has been seen that during the operating life of the conventional pellistor, the resistance of the two beads varies and the change is erratic, with the resistance ratio varying widely with passage of time leading to zero drift. Therefore, in a preferred embodiment, the detector and compensator beads are selected such that the resistances of both the beads display a tendency to shift or vary in the same direction from their original resistances maintaining a constant ratio.
The term "resistance synchronized" as appearing herein in respect of the detector and compensator beads shall denote a pair or pairs of "drift stabilized" detector or compensator beads which are selected out of a plurality of such drift stabilized detector and compensator beads such that said selected detector and compensator beads display a tendency where their resistance shift in the same direction from their original resistances. A resistance synchronized pair of detector and compensator beads tends to drift along the same direction and also retaining the resistance ratio between them. Under this condition the bridge balance is not disturbed and hence no zero drift is observed.
In yet another aspect, the present invention provides a method for fabricating an improved drift stabilized pellistor comprising:
(a) Winding provided pure platinum wires to a plurality of platinum wire coils;
(b) spot welding 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 a first group of said spot welded platinum wire coils to form a plurality of detector beads;
(e) embedding at least one alkali hydroxide layer over the remaining second group of said spot welded platinum wire coils to form a plurality of compensator beads;
(f) drift stabilizing said plurality of compensator beads by conditioning said compensator beads;
(g) drift stabilizing a plurality of detector beads by conditioning said detector beads;
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(h) selecting at least one drift stabilized compensator bead and at least one drift
stabilized detector bead such that said selected compensator and detector beads
are resistance synchronized; (i) forming a Wheatstone bridge circuit comprising said selected compensator and
detector bead being two arms of said circuit; and (j) optionally calibrating said pellistor obtained thereby by applying a known
concentration of a flammable gas. to the sensor.
In an embodiment, the platinum wire coils are prepared by winding the provided pure platinum wires using a special purpose coil winding machine. These coils are thereafter cleaned with prescribed dilute acid solutions followed by washing with distilled water.
In another preferred embodiment, the step of spot welding at least two of said platinum wire coils on sensor base plate support pins comprises washing the wound platinum wire coils in an acidic mixture of sulfuric acid and potassium permanganate; thereafter washing the wound 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.
In another embodiment, the porous alumina beads are formed by applying water based slurry of aluminum oxide over the coil. This process is repeated once or twice to get desired size of the bead. The beads are thereafter preferably dried by passing electric current through the coils using a programmable DC power supply for heating.
Preferably, the beads present in the drift stabilized pellistors according to the present invention comprise alumina. The platinum wire is preferably from about 25 um to about 40 um in diameter.
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Detailed Description of a preferred embodiment
A pure platinum wire of 25 to 40 micron diameter preferably 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 H2SO4 and KMnO4 and then with a mixture of HNO3 and H2O2. Finally they are thoroughly washed with distilled water and dried in air oven.
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 jig specially designed for holding and processing multiple sensors at a time.
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 mA/min.
Thus, in a preferred embodiment, the step of repetitively applying a slurry of an aluminum salt and heating the said aluminum salt comprises applying a slurry of aluminum nitrate on the said platinum wire coils and heating the applied slurry in a controlled manner using a programmable power supply gradually increasing the current from about 50 mA to about 200 mA at the rate 1 mA per minute.
When the bead is dried completely a second and 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 detector and compensator beads.
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The detector bead is formed by applying a catalyst layer over the porous alumina bead, wherein a 1:1 mixture of platinum and palladium catalyst may be preferably used. A slurry comprising 1:1 mixture of palladium chloride and platinum chloride in hydrochloric acid is convenient for this purpose. The bead is dried in the same way as mentioned above after applying the catalyst mixture. 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 slurry comprising a 1:1 mixture of platinum and palladium catalyst to a plurality of alumina beads, and heating said 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.
The sensor beads prepared according to this invention are subjected to conditioning method described above in any aspect or embodiments thereof for stabilizing its physical and electrical characteristics. For this purpose the sensor beads are 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.
The detector and compensator beads are then kept for preferably two months for continuous heating at the operating temperature by passing electric current through the beads. Their individual resistances are measured and recorded. At the end of the two months, a perfectly compensated pair of compensator and detector beads are selected from the recorded data such that both the selected beads are resistance synchronized i.e. both the beads demonstrate a tendency to shift in the same direction with respect to their original resistances. The drift stabilized pellistor fabricated utilizing such resistance synchronized detector and compensator beads were found to possess extra-ordinary stability with respect to the zero drifts.
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The drift stabilized compensator and detectors beads prepared according to the present invention are thereafter placed in suitable pellistor housing and utilized for fabricating drift stabilized pellistors. The pellistors were observed for their baseline drift over a period of time and were found to be excellent when compared with the conventionally prepared pellistors.
Thus, in another aspect, the present invention provides a method for detecting and measuring the level of a combustible gas in an environment, said method comprising: (a) fabricating a drift stabilized pellistor comprising a drift stabilized detector bead and a drift stabilized compensator bead wherein said detector and compensator beads are resistance synchronized; and (b) utilizing said drift stabilized pellistor to detect and measure the presence of a predefined gas in an environment.
The fabricated pellistor may be utilized to detect and measure the level of a combustible gas in a conventional manner that is known as such to a person skilled in the art.
Detailed description of the drawings
Figure 1 represents a pellistor, which is used for detecting and/or measuring the level of combustible gases. The pellistor is like a miniature calorimeter that contains two coils (1) of fine platinum wire which are coated with a ceramic or porous alumina material (2) to form refractory beads - typically referred to as a detector (sensing element) and a compensator (reference element). The 'detector' bead is additionally treated with platinum or palladium based catalytic material or their mixtures that allows catalyzed combustion to occur on the treated surface of the bead.
Figure 2 exemplifies a wheatstone bridge circuit used to measure the output of catalytic bead sensors. There are four circuit branches arranged in a square. The source of the electrical current (3) is connected between the two points shown in the figure and output measurement is made (4) across the two points shown in the figure. In the circuit shown herein, (5) is the trim resistance that is used to keep the bridge circuit balanced under clean air. A balanced bridge has no output signal in clean air with no combustible gas
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present. When combustible gas is present in the environment and it burns on the active sensor surface, the heat of combustion causes the temperature of the platinum coil to rise, which in turn changes its resistance leading to unbalance bridge and an out-of-balance voltage output across the bridge output points. This offset voltage is measured as the signal whose magnitude depends upon the concentration of the combustible gas and is a measure of its concentration.
Figure 3 is a flowchart depicting a preferred method for drift stabilizing a sensor bead. The method comprises maintaining the provided bead at the combustion temperature of methane (SI). Thereafter, the bead is exposed to about 12% methane for about 10-30 minutes (S2). The bead is then maintained at its operating temperature for from about 1 week to about 3 months by passing an electric current through it in order to stabilize the physical and electrical characteristics of the bead (S3).
Figure 4 is a flowchart depicting a method for fabricating an improved drift stabilized pellistor according to the present invention. The provided pure platinum wire is wound to a plurality of platinum wire coils (S4). The platinum wire coils are spot welded on provided sensor base plate support pins (S5). A slurry of an aluminum salt is applied over the spot welded platinum wire coil and heated (S6). This step is repeated until a desired bead size is achieved. At least one catalytic layer is embedded over a first group of spot welded platinum wire coils to obtain a plurality of detector beads (S7). At least one alkali hydroxide layer is embedded over the remaining platinum wire coils to obtain a plurality of compensator beads (S8). The plurality of compensator and detector beads are drift stabilized by the conditioning method described in any aspect or embodiment thereof hereinabove (S9). At least one drift stabilized compensator bead and at least one drift stabilized detector bead is selected such that the selected compensator and detector beads are resistance synchronized (S10). A wheatstone bridge balanced circuit is formed comprising the selected resistance synchronized and drift stabilized compensator and detector beads as the two arms. Preferably, the pellistor obtained thereby is calibrated for detecting pre-defined gases (S11).
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Figure 5 is a graph depicting the variation of the percentage LEL baseline curve for four conventional pellistors. It was observed that for the conventional pellistors, the zero drifts were found to be as high as + 15 (baseline % LEL) or -15 (baseline % LEL), which was higher than many of the generally set low alarm set points.
Figure 6 is a graph depicting the variation of the percentage LEL baseline curve for five conditioned pellistors according to the present invention. It was observed that in case of the improved pellistors fabricated following the method described in the present invention, the zero drifts were found to be within + 5 (baseline % LEL) or -5 (baseline % LEL), which was within usual alarm set points. Thus, the pellistors according to the present invention caused no false alarms during the test period.
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.

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CLAIM:
1. A method for preparing a drift stabilized sensor bead comprising: (a) maintaining a provided bead at a predetermined temperature and thereafter exposing said bead to a predetermined concentration of a combustible gas in air at said temperature for a first predetermined length of time; and (b) subsequently maintaining said sensor bead at its operating temperature for a second predetermined length of time by passing electric current of a predefined magnitude through said bead.
2. The method as claimed in claim 1, wherein said predetermined temperature is equal or higher than the temperature at which the combustible gas burns or oxidizes over the catalytic bead.
3. The method as claimed in claim 1, wherein said combustible gas is methane.
4. The method as claimed in claim 1, 2 or 3, wherein said predetermined temperature is from about 500°C to about 550°C.
5. The method as claimed in claims 1, 2, 3 or 4 wherein said predetermined concentration at which the bead is exposed to methane is preferably 12% methane v/v in air.
6. The method as claimed in any preceding claim, wherein said preferred first length of time is at least about 10 minutes.
7. The method as claimed in any preceding claim, wherein said second length of time varies from about one week to about three months.
8. The method as claimed in claim 7, wherein said second length of time is at least about two months.
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9. The method as claimed in any preceding claim, wherein said bead is additionally exposed to clean air for about ten minutes in powered mode prior to being maintained at said second length of time.
10. A drift stabilized sensor bead obtained by a conditioning method for drift-stabilizing said sensor bead, said conditioning method comprising: (a) maintaining a provided bead at a predetermined temperature and thereafter exposing said bead to a predetermined concentration of a combustible gas in air at said maintained temperature for a first predetermined length of time; and (b) subsequently maintaining said sensor bead at its operating temperature for a second predetermined length of time by passing electric current of a predefined magnitude through said bead.
11. The drift stabilized sensor bead obtained by a conditioning method as claimed in claims 2-9.
12. An improved drift stabilized pellistor comprising:
a. a drift stabilized compensator bead comprising a platinum wire coil
embedded within a bead;
b. a drift stabilized detector bead comprising a platinum wire coil embedded
within a bead and being covered with a predetermined catalyst; and
c. a balanced Wheatstone bridge circuit comprising said stabilized
compensator and detector beads being two arms of said circuit, wherein
said stabilized compensator and detector beads are resistance
synchronized.
13. The drift stabilized pellistor as claimed in claim 12, wherein said catalytic material comprises a 1:1 ratio mixture of platinum and palladium based catalysts.
14. The drift stabilized pellistor bead as claimed in claim 12, wherein said detector and compensator beads are selected such that both the detector and compensator beads display a tendency to shift or vary in the same direction from their original resistances.

15. A method for fabricating an improved drift stabilized pellistor, said method
comprising:
a. drift stabilizing a plurality of compensator beads by conditioning said
compensator beads;
b. drift stabilizing a plurality of detector beads by conditioning said detector
beads; and
c. selecting at least one drift stabilized compensator bead and at least one
drift stabilized detector bead such that said selected compensator and
detector beads form a pair which are resistance synchronized
16. The method as claimed in claim 15, wherein said conditioning is carried out using a method for preparing a drift stabilized sensor bead as claimed in claims 1-9.
17. A method for fabricating an improved drift stabilized pellistor comprising:
a. winding provided pure platinum wires to a plurality of platinum wire
coils;
b. spot welding 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 a first group of said spot welded
platinum wire coils to form a plurality of detector beads;
e. embedding at least one alkali hydroxide layer over the remaining second
group of said spot welded platinum wire coils to form a plurality of
compensator beads;
f. drift stabilizing said plurality of compensator beads by conditioning said
compensator beads;
d. drift stabilizing a plurality of detector beads by conditioning said detector beads;

e. selecting at least one drift stabilized compensator bead and at least one
drift stabilized detector bead from the recorded data on their individual
resistances such that said selected compensator and detector beads are
resistance synchronized;
f. forming a Wheatstone bridge circuit comprising said selected compensator
and detector bead being two arms of said circuit; and
g. optionally calibrating said pellistor obtained thereby by applying a known
concentration of a flammable gas to the sensor..
18. The method for fabricating an improved drift stabilized pellistor as claimed in claim 17, wherein said step of spot welding at least two of said platinum wire coils on sensor base plate support pins comprises washing the wound platinum wire coils in an acidic mixture of sulfuric acid and potassium permanganate; washing the wound 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.
19. The method for fabricating an improved drift stabilized pellistor as claimed in claim 17, wherein said 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 the current from about 50 mA to about 200 mA at the rate 1 mA per minute.
20. A method for detecting and measuring the level of a combustible gas in an environment, said method comprising; (a) fabricating a drift stabilized pellistor comprising a drift stabilized detector bead and a drift stabilized compensator bead wherein said detector and compensator beads are resistance synchronized; and (b) utilizing said drift stabilized pellistor to detect and measure the presence of a predefined gas in an environment.

21. A method for detecting and measuring the level of a combustible gas in an environment as claimed in claim 20, wherein said fabrication of said drift stabilized pellistor is carried out using the method for fabricating an improved drift stabilized pellistor as claimed in claims 15-19.
22. A method for preparing a drift stabilized sensor bead substantially as described herein with reference to the accompanying drawings.
23. A drift stabilized bead substantially as described herein with reference to the accompanying drawings.
24. An improved drift stabilized pellistor substantially as described herein with reference to the accompanying drawings.
25. A method for fabricating an improved drift stabilized pellistor substantially as described herein with reference to the accompanying drawings.
26. A method for detecting and measuring the level of a combustible gas in an environment substantially as described herein with reference to the accompanying drawings.
Ashwini Sandu
GM - IPR United Phosphorus Limited