System and method for multi-gas detection at a single operating temperature

Abstract

Systems and methods for multi-gas detection using dielectric excitation of a single sensing material operating at a single operating temperature. The metal oxide semiconductor sensing material is maintained at a constant temperature, and the dielectric excitation response of the sensing material is measured at this constant temperature while the sensing material is exposed to a fluid sample. The disclosed gas sensors and gas detection methods unexpectedly provide desirable characteristics such as improved multi-gas discrimination while operating at a constant operating temperature. For example, by measuring the dielectric excitation response using at least one gas sensing element at a single operating temperature, the disclosed gas sensors and gas detection methods rely on multiple resistance measurements performed at several different operating temperatures compared to other gas sensors and other gas detection methods. Demonstrate excellent multi-gas discrimination.

Description

Technical Field

[0001] Statement Regarding Research and Development Funded by the Federal Government This invention was made with government support under Contract No. W15QKN-18-9-1004 awarded by ACC-NJ to the CWMD Consortium. The government has certain rights in this invention.

Background Art

[0002] The subject matter disclosed herein generally relates to gas detection, and more specifically, to gas detection using metal oxide semiconductor (MOS) sensors.

[0003] Metal oxide semiconductor (MOS) sensors can operate as chemical resistors and are popular due to their ability to detect a number of gases by appropriately selecting a base semiconductor material and a doping material. In such gas-responsive chemical resistors, a change in the resistance of the MOS sensing element is measured, and this change in resistance is proportional to the gas concentration in the fluid sample. However, due to the limited selectivity of MOS gas-responsive chemical resistors, the use of such sensors in certain multi-gas detection applications is hindered. Furthermore, in order to achieve at least some degree of gas selectivity, traditional MOS-based gas sensors are required to perform resistance measurements at two or more operating temperatures, particularly at least two different operating temperatures, which can shorten the lifespan of the sensor and have undesirable effects on electrical measurements.

Summary of the Invention

Means for Solving the Problems

[0004] With the above in mind, the present embodiment is directed to a system and method for multi-gas detection using dielectric excitation of a single sensing material at a single constant operating temperature. Conventional gas sensor designs typically implement metal oxide semiconductor sensing materials that operate at several different operating temperatures while collecting resistance responses to distinguish different gases in a fluid sample. In contrast, in the case of the present embodiment, the dielectric excitation response of the sensing material is measured at this constant temperature while the sensing material is exposed to the fluid sample. The present embodiment demonstrates that when the dielectric excitation response of a gas sensing material is measured at a constant operating temperature, superior multi-gas discrimination is achieved compared to the discrimination achievable based on measurement of the resistance response of the same gas sensing material at a number of different operating temperatures. For example, by measuring the dielectric excitation response using at least one gas sensing element at a single operating temperature, the disclosed gas sensor and gas sensing method demonstrate superior multi-gas selectivity compared to other gas sensors and other gas sensing methods that rely on a number of resistance measurements performed at several different operating temperatures.

[0005] For example, in an embodiment, a gas sensor system for multi-gas analysis of a fluid sample includes a gas sensing element configured to contact the fluid sample, a heating element coupled to the gas sensing element and configured to heat the gas sensing element, and a heating element controller operably coupled to the heating element and configured to control the heating element to heat the gas sensing element to a constant temperature while the gas sensing element is in contact with the fluid sample. The gas sensor system includes a measurement circuit operably coupled to the gas sensing element and configured to provide dielectric excitation to the gas sensing element and measure the dielectric excitation response of the gas sensing element while the gas sensing element is heated to a constant temperature and in contact with the fluid sample, the dielectric excitation response providing an improvement in discrimination between at least two gases in the fluid sample compared to the resistance response of the gas sensing element when in contact with the fluid sample at a number of operating temperatures.

[0006] In an embodiment, a method of operating a gas sensor for multi-gas analysis of a fluid sample includes exposing a gas sensing material of the gas sensor to the fluid sample, controlling a heating element of the gas sensor via at least one heating element controller of the gas sensor to heat the gas sensing material to a constant temperature, and measuring a dielectric excitation response of the gas sensing material while the gas sensing material is heated to the constant temperature and exposed to the fluid sample via a measurement circuit of the gas sensor. The method includes receiving, via an on-board data processor of the gas sensor, the dielectric excitation response of the gas sensing material at the constant temperature, and distinguishing at least two gases in the fluid sample based on at least a portion of the received dielectric excitation response via the on-board data processor.

[0007] In an embodiment, a method of manufacturing a gas sensor for multi-gas analysis includes determining a plurality of analyte gases to be distinguished by the gas sensor. The method may include determining a type of base semiconductor metal oxide and one or more dopants used for detection of the analyte gases to be distinguished by the gas sensor. The method includes determining an operating temperature for operating a gas sensing material of the gas sensor at which the gas sensing material interacts reversibly with each of the plurality of analyte gases, and configuring a first heating element controller and a second heating element controller of the gas sensor to provide a combined voltage to a heating element of the gas sensor to heat the gas sensing material to the constant operating temperature. The method includes determining a plurality of frequencies for dielectric excitation of the sensor applied to the gas sensing material to measure a dielectric excitation response of the gas sensing material when the gas sensing material interacts with the plurality of analyte gases, and configuring a measurement circuit of the gas sensor to apply each of the plurality of frequencies for dielectric excitation of the gas sensing material when the gas sensing material interacts with the plurality of analyte gases at the operating temperature to measure the dielectric excitation response of the gas sensing material.

Brief Description of the Drawings

[0008] These and other features, aspects, and advantages of the present invention will be better understood when the following detailed description is read in conjunction with the accompanying drawings, which are referred to herein by like reference numerals throughout.

[0009]

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[0010] This embodiment is directed to a system and method for multi-gas detection using dielectric excitation of a single metal oxide semiconductor sensing material disposed as a single gas sensing element. It should be noted that metal oxide semiconductor sensing materials are often abbreviated in the industry as metal oxide semiconductor (MOS) materials or semiconductor metal oxide (SMOX) or semiconductor metal oxide (MOX) materials. Traditional MOS-based gas sensors measure only the direct current (DC) resistance response. Measuring the single response for each sensor is also known as a single output readout. Traditional MOS-based gas sensors measure the DC resistance response at several different temperatures to distinguish multiple gases in a fluid sample. In the present disclosure, measuring the dielectric excitation response of a MOS-based gas sensor at a constant operating temperature can provide distinct advantages such as improved sensor selectivity and improved operating life. That is, at a constant operating temperature, the impedance spectrum of a MOS-based sensing element is affected differently by different gases, and such desired differences are now recognized to be more prominent compared to the resistance response of the same sensing element when multiple operating temperatures are used. Thus, this embodiment unexpectedly demonstrates a MOS-based gas sensor that can distinguish different gases at a single operating temperature, and this distinction is superior in differentiating between different gases and baseline stability compared to the resistance response of the same sensing element operating at two or more operating temperatures.

[0011] As described above, traditional MOS-based gas sensors typically measure the DC resistance response of MOS-based sensing elements at two or more operating temperatures when performing multi-gas analysis of a fluid sample. In contrast, this embodiment of the gas sensor includes a MOS-based sensing element that interacts with a gas in a fluid sample at a constant operating temperature and provides an impedance response to a specific dielectric excitation frequency, and these dielectric excitation responses of the MOS-based sensing element are measured and analyzed to distinguish two or more gases in the fluid sample. As used herein, the terms "analyte", "analyte gas", or "analyte fluid" refer to the component of interest in the measured fluid. As used herein, the terms "interferent", "interfering gas", "interfering fluid" refer to any component in the measurement fluid that may have an undesirable effect on the accuracy and precision of the measurement of the analyte using the sensor. A gas sensing element that provides an impedance response at two or more dielectric excitation frequencies, and these impedance responses of the gas sensing element are measured and analyzed to distinguish two or more analyte gases in a fluid sample, can be referred to as a multivariate gas sensing element. A multivariate gas sensing element has two or more responses or outputs. A multivariate gas sensing element has a multi-output readout.

[0012] With the above in mind, FIG. 1 is a schematic diagram of an embodiment of a gas sensor 10 for multi-gas analysis of a fluid sample according to the present technology. In different embodiments, the gas sensor 10 may be a wearable multi-gas sensor for personal (e.g., patient) monitoring, an ingestible gas sensor, or a tattooed gas sensor. In a particular embodiment, the gas sensor 10 may be an industrial environment sensor, an asset monitoring sensor, an industrial process monitoring gas sensor, a consumer sensor, a transportation sensor, a security sensor, or any combination thereof.

[0013] In the embodiment illustrated in FIG. 1, the gas sensor 10 generally includes at least one gas sensing element 12, a control circuit 14, and one or more output devices 16. Each gas sensing element 12 of the gas sensor 10 includes a substrate 18 having a sensing electrode 20 disposed thereon, and a gas sensing material 22 (e.g., a suitably formulated metal oxide semiconductor material (MOS) applied to form a gas sensing film) disposed on the substrate 18 between the sensing electrodes 20. In certain embodiments, the gas sensor 10 may include a number of gas sensing elements 12 (e.g., an array of gas sensing elements 12), such as gas sensing elements having different MOS-based gas sensing materials 22. In some embodiments, a single gas sensing element 12 may include a number of gas sensing materials 22 (e.g., an array of gas sensing materials 22), each having a respective set of sensing electrodes 20 for performing electrical measurements. In certain embodiments, there may be three or more sensing electrodes 20, and the sensing electrodes 20 may include a plurality of intermeshing sensing electrodes. It will be appreciated that the gas sensing material 22 is generally applied onto the electrode 20 to form a gas sensing film, whereby dielectric excitation of the gas sensing material 22 and measurement of the excitation response of the gas sensing material 22 are performed via the electrode 20.

[0014] Furthermore, a resistive heating element 24 is disposed on the surface of the substrate 18 opposite the gas sensing material 22 and is designed to heat the gas sensing material 22 to a suitable operating temperature during multi-gas analysis of the fluid sample 26. In certain embodiments, the heating element 24 may be disposed on the surface of the substrate 18 opposite the gas sensing material 22, and in other embodiments, the heating element 24 may be disposed on the same surface of the substrate 18 as the gas sensing material 22. In the case of embodiments having a number of gas sensing elements 12, in certain cases, two or more gas sensing materials 22 may be applied to a common substrate to form a number of gas sensing elements 12 on a common substrate 18 that can be heated by a single heating element, and in other cases, each gas sensing material 22 may be disposed on a respective substrate 18 having a respective heating element 24. Furthermore, in certain embodiments, the heating element 24 may be integrated with the substrate 18 as a monolithic structure.

[0015] During operation of the gas sensor 10, the gas sensing material 22 of the gas sensing element 12 is heated to a constant operating temperature when the gas sensing material 22 is exposed to the fluid sample 26, and the fluid sample may include one or more analyte gases or at least one analyte gas and at least one interfering gas. Accordingly, the control circuit 14 of the illustrated gas sensor 10 includes one or more heating element controllers 28 that are electrically connected to the heating element 24 and control the heating element 24 to achieve a constant operating temperature. In different embodiments, this electrical connection may be independent or may be electrically connected in parallel or in series. In embodiments where a number of gas sensing elements 12 have separate heating elements 24, each of the heating elements 24 may be controlled by one or more heating element controllers 28 to achieve a constant operating temperature.

[0016] In the illustrated embodiment, the first controller 28A of the heating element 24 is calibrated during manufacture to provide a predetermined constant or static voltage to the heating element that is sufficient to achieve the desired operating temperature under calibration conditions. However, it is now recognized that this predetermined constant voltage provided by the first controller 28A may not be able to heat the gas sensing material 22 to the desired operating temperature in all situations. For example, the heating element 24 may degrade over time during the operating life of the gas sensor 10, whereby the heating element 24 may generate less heat in response to the predetermined constant voltage provided by the first controller 28A. Further, a particular measurement environment may be substantially cooler than the measurement environment in which the gas sensor 10 was calibrated, whereby the gas sensing material 22 may not be able to reach the desired operating temperature in response to the predetermined constant voltage provided by the first controller 28A.

[0017] With this in mind, in the illustrated embodiment, the second controller 28B of the heating element 24 is a feedback controller designed to provide an additional adjustable or variable voltage to the heating element 24, whereby, regardless of the aging deterioration of the heating element or the ambient conditions (e.g., temperature, humidity) of the analyzed fluid sample 26, a combination of a predetermined constant voltage and an additional adjustable voltage heats the gas sensing material 22 to a desired constant operating temperature. As used herein, "operating temperature" is the temperature of each gas sensing element 12 (e.g., gas sensing material 22, substrate 18) of the gas sensor 10 while electrical measurements are being performed. For example, in certain embodiments, the second controller 28B determines feedback by measuring the current flowing through the heating element 24 as a result of the predetermined constant voltage provided by the first controller 28A, and in response to determining that the current is below a predetermined threshold, provides an additional voltage to the heating element 24 until the desired current is reached. In some embodiments, the second controller 28B is communicatively coupled to a temperature sensor 30 that is operably in contact with the gas sensing element 12 to receive feedback regarding the actual temperature of the gas sensing material 22, and in response to determining that the temperature of the gas sensing material 22 is below a predetermined operating temperature, may provide an additional voltage to the heating element 24 until the desired operating temperature is reached. In some embodiments, the second controller 28B may be communicatively coupled to a current sensor or an electrical resistance sensor of the gas sensing element 12 to monitor the performance of the heating element 24. Thus, by cooperating to control the heating element 24, the controller 28 of the heating element 24 ensures that the gas sensing material 22 is maintained at a desired constant operating temperature regardless of the age of operation of the gas sensor 10 or the ambient conditions of the fluid sample 26. For example, in certain embodiments, the constant operating temperature of the gas sensing material 22 may be 30°C to 1000°C, 50°C to 900°C, or 80°C to 600°C. Further, in certain embodiments, the operating temperature may vary by less than 50%, more preferably by less than 30%, and most preferably by less than 20% while the gas sensing material 22 is in contact with the fluid sample 26.The operating temperature may vary undesirably over time due to different modes such as the aging deterioration of the heating element and the variations in the ambient conditions around the heating element. The disclosed design is intended to address this variation that would otherwise exist.

[0018] In the case of the illustrated embodiment, the sensing electrode 20 of the gas sensing element 12 is electrically coupled to the measurement circuit 32 of the control circuit 14 of the gas sensor 10. The measurement circuit 32 is designed to provide at least dielectric excitation to the gas sensing material 22 at a preselected frequency and measure the excitation response of the gas sensing material 22 (e.g., impedance response, DC response) at these excitation frequencies. In a particular embodiment, the measurement circuit 32 may further be capable of (or designed to) provide direct current (DC) excitation to the gas sensing material 22 and measure the DC response (e.g., resistance response) of the gas sensing material 22 to this DC excitation. In a particular embodiment, the measurement circuit 32 may measure both the alternating current (AC) and DC responses of the gas sensing material 22. However, in a particular embodiment, the measurement circuit 32 may be designed to provide only dielectric excitation to the gas sensing material 22 and measure only the dielectric response of the gas sensing material 22.

[0019] As used herein, "dielectric excitation" of a MOS sensing material refers to AC excitation of the MOS sensing material at the shoulder of its dielectric relaxation region. As used herein, "impedance" is a non-limiting term for any electrical response of the sensing system to an alternating current (AC) applied to the gas sensing material 22. It will be understood that such responses may be measured as different electrical properties in different embodiments. Non-limiting examples of these electrical responses of the gas sensing material 22 to an alternating current include impedance, the real part of the impedance, the imaginary part of the impedance, admittance, reactance, susceptance, and the like. In this specification, an example of the response is shown as impedance; however, other electrical responses of the gas sensing material 22 to AC excitation may be similarly generated. In one embodiment, the electrical response of the gas sensing material 22 may be monitored at the gas-modulated high-frequency shoulder of the dielectric relaxation peak of the sensing material. In one embodiment, the electrical response of the sensing system may be monitored at the gas-modulated low-frequency shoulder of the dielectric relaxation peak of the sensing material.

[0020] The gas sensor 10 may represent one or more different versions of the multi-gas sensing system described herein. In one or more embodiments, the measurement circuit 32 may include a resistor-capacitor (RC) electrical circuit including one or more resistor (R) and capacitor (C) components, and these components may be electronically varied by the controller circuit 14 by the presence of one or more target analyte gases. In one or more embodiments, the measurement circuit 32 may perform dielectric excitation and impedance measurements at one or more different frequencies or with one or more different RC configurations of the measurement circuit 32. For example, the measurement circuit 32 of the gas sensor 10 may measure the impedance response of the gas sensing material 22 at different frequencies, with different resistances of the RC electrical circuit of the measurement circuit 32, with different capacitances of the RC electrical circuit of the measurement circuit 32, or with any combination of two or more thereof. The measurement circuit 32 provides excitation and measured values of the response of the gas sensing element 12 to the gas. The measurement circuit 32 is not designed to be affected by the measured gas concentration. Rather, only the gas sensing element 12 is designed to be predictably affected by the measured gas concentration.

[0021] The control circuit 14 of the illustrated gas sensor 10 includes a data processing unit 34 (also referred to herein as a data processing circuit) communicatively coupled to the measurement circuit 32 to receive the excitation response measured by the measurement circuit 32. The data processing unit 34 includes an on-board data processor 36 and a memory 38 that stores a gas analysis model 40 including an analyte gas classification model 42, an analyte gas quantification model 44, or any combination thereof. These gas analysis models 40 are generally mathematical models that store the relationship between an excitation response (e.g., impedance response, DC response) and a specific classification or concentration of an analyte gas in a fluid sample. For example, the gas classification model 42 may store the relationship between the excitation response of the gas sensing material 22 and a specific classification of the analyte gas, and the gas quantification model 44 may store the relationship between the excitation response of the gas sensing material 22 and a specific concentration of the analyte gas. In certain embodiments, the gas analysis model 40 may include one or more coefficients having values determined experimentally and stored in the memory 38. In some embodiments, the number of analyte gases determined by the analyte gas classification model 42 or the analyte gas quantification model 44, or any combination thereof, for the illustrated gas sensor 10 can range from one analyte gas to ten analyte gases, fifty analyte gases, and hundreds of analyte gases. A gas sensing element 12 having two or more responses or outputs is referred to as a multivariate gas sensing element. Multivariate data processing principles are applied to analyze the output from the multivariate gas sensing element. Multivariate data processing principles can be applied to quantify the diversity of responses of a multivariate sensor to different gases. A multivariate transfer function can be constructed to quantify different gases. The constructed multivariate transfer function can be implemented to quantify different gases in new measurement data from this multivariate gas sensing element.

[0022] Non-limiting examples of multivariate data processing principles include ways to perform gas classification / cluster analysis and quantification. Classification / cluster analysis can be performed to accurately determine the type of analyte gas. Quantification can be performed to accurately determine the concentration of the analyte gas. Examples of classification / cluster analysis algorithms include, but are not limited to, principal component analysis (PCA), hierarchical cluster analysis (HCA), independent component analysis (ICA), linear discriminant analysis (LDA), and support vector machine (SVM) algorithms. Non-limiting examples of methods for performing analyte quantification to determine the concentration of a specific analyte gas include principal component regression (PCR), independent component regression (ICR), non-linear regression analysis (NRA), discriminant function analysis (DFA), or artificial neural network analysis (ANN). In certain aspects of the subject matter of the present invention described herein, a quantification algorithm can follow a classification algorithm.

[0023] As discussed below, the on-board data processor 36 receives the excitation responses measured by the measurement circuit 32, selects specific excitation responses (e.g., impedance response, DC response) for analysis, provides these excitation responses as inputs to one or more of the stored gas analysis models 40, and the gas analysis models 40 return an output that distinguishes two or more analyte gases in the fluid sample 26. As used herein, "resolving" two or more gases in a fluid sample, or "providing resolution" between two or more gases in a fluid sample, or "distinguishing" between two or more gases in a fluid sample, refers to determining a respective classification for each of the gases in the fluid sample, determining the respective concentration of each of the gases in the fluid sample, or determining both the respective classification and the respective concentration of each of the gases in the fluid sample. As used herein, "classifying" or "determining a classification" of a gas refers to determining the exact chemical identity of the gas (e.g., acetylene, chlorine, ethylene oxide, nitrogen dioxide, sulfur dioxide, hydrogen, hydrogen chloride), or determining the chemical class to which each gas belongs (e.g., hydrocarbon, alcohol, phenol, ether, aldehyde, ketone, carboxylic acid, ester, etc.). As used herein, "on-board" is used to describe the data processor 36 as part of the control circuit 14 integrated with the gas sensor 10, and is also used to describe "on-board" data processing, where the on-board data processor 36 processes data locally without transmitting the data to an external computing device for processing.

[0024] In certain embodiments, the memory 38 may be integrated with the on-board data processor 36. In certain embodiments, the on-board data processor 36 is a multi-core processor. For example, in some embodiments, the on-board data processor 36 is a multi-core processor on a single integrated circuit having two or more separate processing units (also referred to as cores) each of which reads and executes program instructions. In certain embodiments, the multi-core processor may include only a single central processing unit (CPU) and a number of additional cores. In embodiments where the on-board data processor 36 is a multi-core processor, different gas analysis models and / or different signal processing algorithms may be independently executed by different cores in order to reduce the power consumption of the data processing unit 34 and / or the gas sensor 10.

[0025] In the illustrated embodiment, the gas sensor 10 includes one or more output devices 16. In certain embodiments, the output device 16 includes one or more display devices 46 configured to present information regarding multi-gas analysis, such as the classification and / or concentration of two or more gases in the fluid sample 26. In some embodiments, the other output device 16 may include an alarm 49 such as a visual alarm (e.g., a light-emitting diode (LED)), an auditory alarm (e.g., a speaker), and / or a tactile alarm (e.g., a tactile feedback device). In certain embodiments, the output device 16 includes one or more communication devices 48 (e.g., a wired communication interface, a wireless communication interface) that enable the gas sensor 10 to communicate with other computing systems such as a desktop computer, a mobile computing device (e.g., a laptop, a smartphone), a remote server (e.g., an Internet server, a cloud server), or other sensors of a multi-sensor monitoring system (e.g., a gas sensor, a temperature sensor, a vibration sensor, a health monitor). For example, in certain embodiments, information determined by the on-board data processor 36 regarding the discrimination of two or more gases in the fluid sample 26 can be provided to an external computing system that functions as a controller for a mesh of sensors including the gas sensor 10. In some embodiments, the gas sensor 10 may additionally or alternatively use the communication device 48 to provide excitation response measurements to an external computing system, such that the external computing system can use these measurements to calculate one or more coefficient values for one or more of the gas analysis models and return these coefficient values to the gas sensor 10 for storage in the memory 38.

[0026] Furthermore, the illustrated gas sensor 10 includes a battery 50 that is electrically coupled to provide power to various components of the gas sensor 10, including a control circuit 14 and an output device 16. It will be appreciated that the battery 50 should have a capacity suitable for providing power to all components of the gas sensor 10. For example, this may include heating the gas sensing material 22, inducing dielectric excitation in the gas sensing material 22, measuring the dielectric excitation response of the gas sensing material 22, analyzing the measured dielectric excitation response to distinguish two or more gases in a fluid sample, and presenting the results of the analysis via a suitable output device 16. In certain embodiments, the battery 50 may have a capacity sufficient to operate the gas sensor 10 for at least 10 hours. In some embodiments, the battery 50 may have a battery capacity of 1 milliampere-hour (mAh) to 500 mAh, or 1 mAh to 200 mAh, or 1 mAh to 100 mAh. In certain embodiments, such as those where the gas sensor 10 is designed to be particularly thin (e.g., in ingestible or tattooed embodiments of the gas sensor 10), the battery 50 may have a thickness of less than about 5 millimeters (mm). In some embodiments, all of the components of the gas sensor 10 may be coupled to or at least partially disposed within a package or housing suitable for a particular gas sensing application. For example, in the case of personal monitoring applications, the packaging of the gas sensor 10 may be made of a biocompatible polymer that can be worn externally, injected subcutaneously, or ingested to perform multi-gas analysis on an individual or patient.

[0027] The gas sensor 10 may be a wearable device that can be worn or moved by an operator from one location to another. The gas sensor 10 may be positioned within a helmet, hat, glove, or other clothing item, or may be an integrated part thereof. For example, the gas sensor 10 may be held within a wearable or non-wearable transportable object such as the frame of military or industrial eyeglasses, a wearable pulse oximeter, a safety vest or harness, clothing items, mobile devices (e.g., mobile phones, tablets, etc.), and the like. The wearable device may be integrated into the fabric of clothing, positioned on clothing such as on a pocket, and may be in the form of being worn on an armband, wrist, or other limb. The wearable device may be worn by a subject such as a human, animal, or robot, may be removably coupled or integrated with an item worn by the subject (e.g., shirt, pants, safety vest, safety protective clothing, glasses, hat, helmet, hearing aid, etc.), or may be movable so that the sensor can be moved between different positions, and may be stationary or substantially stationary, and may be any alternative device. The wearable device may be worn or carried by different subjects or individuals such as, but not limited to, soldiers, medical professionals, athletes, system operators, students, and other active or inactive individuals. Optionally, the wearable sensing system may be coupled to, integrated with, and disposed on an asset such as a mobile system like a drone, a fixed system, and the like. The wearable system may be positioned on an item worn by a subject such as a helmet, pocket (e.g., of a shirt, pants, bag, etc.), glove, armband, earphone, etc., or may be attached to or directly coupled to a subject or asset such as around the wrist, ankle, etc. The wearable device can be fabricated using manufacturing techniques based on complementary metal oxide semiconductor electronic circuits, flexible electronic circuits, flexible hybrid electronic circuits, and other known techniques for providing conformal and flexible designs, implementations, and uses.Optionally, the gas sensor 10 may be a fixed device, may be independently movable (e.g., removable from an operator and capable of moving independently of the operator), or may be floating in the air or the like.

[0028] The gas sensor 10 may be in the form of a fluid container that can be in the form of a container having a controlled volume, or in the form of an open area such as indoor equipment (e.g., a room, a hall, a house, a school, a hospital, a confined space, etc.), or outdoor equipment (e.g., a stadium, a gas production site, a refueling station, a gasoline filling station, a hydrogen filling station, a compressed natural gas filling station, a liquefied natural gas filling station, a gas distribution site, a fuel distribution site, a coast, a forest, a city, an urban environment, a marine environment, etc.) and may contact the fluid 26. In one embodiment, the gas sensor 10 may provide continuous monitoring of the fluid 26 in a reservoir or a flow path. In one or more embodiments, the gas sensor 10 may be an impedance gas sensor, an electromagnetic sensor, an electronic sensor, a hybrid sensor, or another type of sensor. Optionally, the gas sensor 10 may be part of a sensor array.

[0029] The fluid 26 may be, for example, a gas, a liquid, a gas-liquid mixture, a solid, a particle, or a particulate matter, and may contain one or more analyte gases therein. In another embodiment, the fluid 26 may be a gas or a fuel such as a hydrocarbon fuel. An example of the fluid 26 is natural gas or hydrogen gas supplied to a power system (e.g., a vehicle, an aircraft engine, or a stationary generator set) for consumption. Other examples of such fluid 26 can include gasoline, diesel fuel, jet fuel or kerosene, biofuels, petroleum diesel-biodiesel fuel blends, natural gas (liquefied or compressed), and fuel oil. Another example of the fluid 26 is indoor or outdoor ambient air. Another example of the fluid 26 is air in industrial, residential, military, construction, urban, and any other known locations. Another example of the fluid 26 is ambient air containing relatively low concentrations of benzene, naphthalene, carbon monoxide, ozone, formaldehyde, nitrogen dioxide, sulfur dioxide, ammonia, hydrofluoric acid, hydrochloric acid, phosphine, ethylene oxide, carbon dioxide, hydrogen sulfide, chemical weapons, such as nerve agents, vesicants, blood agents, and choking agents, hydrocarbons and / or other contaminants. Another example of the fluid 26 is a disinfectant such as alcohol, aldehyde, chlorine dioxide, hydrogen peroxide. Another example of the fluid 26 is ambient air with relatively low, medium, and high concentrations of flammable or combustible gases such as methane, ethane, propane, butane, hydrogen, and / or other gases. Another example of the fluid 26 is at least one gas dissolved in an industrial liquid such as transformer oil, bioprocess media, fermentation media, wastewater, etc. Another example of the fluid 26 is at least one gas dissolved in a consumer liquid such as milk, non-alcoholic beverages, alcoholic beverages, cosmetics, etc. Another example of the fluid 26 is at least one gas (e.g., a biomarker) dissolved in a body fluid such as blood, sweat, tears, saliva, urine, etc.

[0030] In certain embodiments, fluid 26 may include an analyte gas that is a toxic industrial material or a toxic industrial chemical. A non-limiting list of exemplary toxic industrial materials and chemicals includes, but is not limited to, ammonia, arsine, boron trichloride, boron trifluoride, carbon disulfide, chlorine, diborane, ethylene oxide, fluorine, formaldehyde, hydrogen bromide, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen sulfide, nitric acid (fuming), phosgene, phosphorus trichloride, sulfur dioxide, sulfuric acid, and tungsten hexafluoride. In certain embodiments, fluid 26 may include an analyte gas that is a toxic substance with a moderate hazard index. A non-limiting list of exemplary toxic substances with an intermediate hazard index includes, but is not limited to, acetone cyanohydrin, acrolein, acrylonitrile, allyl alcohol, allylamine, allyl chloroformate, boron tribromide, carbon monoxide, carbonyl sulfide, chloroacetone, chloroacetonitrile, chlorosulfonic acid, diketene, 1,2-dimethylhydrazine, ethylene dibromide, hydrogen selenide, methanesulfonyl chloride, methyl bromide, methyl chloroformate, methyl chlorosilane, methylhydrazine, methyl isocyanate, methyl mercaptan, nitrogen dioxide, phosphine, phosphorus oxychloride, phosphorus pentafluoride, selenium hexafluoride, silicon tetrafluoride, stibine, sulfur trioxide, sulfuryl chloride, sulfuryl fluoride, tellurium hexafluoride, n-octyl mercaptan, titanium tetrachloride, trichloroacetyl chloride, and trifluoroacetyl chloride.

[0031] In certain embodiments, fluid 26 may include an analyte gas that is a low-hazard-index toxic substance. A non-limiting list of exemplary low-hazard-index toxic substances includes, but is not limited to, allyl isothiocyanate, arsenic trichloride, bromine, bromine chloride, bromine pentafluoride, bromine trifluoride, carbonyl fluoride, chlorine pentafluoride, chlorine trifluoride, chloroacetaldehyde, chloroacetyl chloride, crotonaldehyde, cyanogen chloride, dimethyl sulfate, diphenylmethane-4,4'-diisocyanate, ethyl chloroformate, ethyl chlorothioformate, ethyl phosphonous dichloride, ethyl phosphonous dichloride, ethyleneimine, hexachlorocyclopentadiene, hydrogen iodide, pentacarbonyliron, isobutyl chloroformate, isopropyl chloroformate, isopropyl isocyanate, n-butyl chloroformate, n-butyl isocyanate, nitric oxide, n-propyl chloroformate, parathion, perchloromethyl mercaptan, sec-butyl chloroformate, tert-butyl isocyanate, tetraethyl lead, tetraethyl pyrophosphate, tetramethyl lead, toluene 2,4-diisocyanate, and toluene 2,6-diisocyanate.

[0032] In certain embodiments, fluid 26 may include an analyte gas that is an indoor pollutant. A non-limiting list of exemplary indoor pollutants includes, but is not limited to, acetaldehyde, formaldehyde, 1,3-butadiene, benzene, chloroform, methylene chloride, 1,4-dichlorobenzene, perchloroethylene, trichloroethylene, naphthalene, and polycyclic aromatic compounds. In certain embodiments, fluid 26 may include an analyte gas that is an outdoor pollutant. A non-limiting list of exemplary outdoor pollutants includes, but is not limited to, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide.

[0033] Embodiments of the gas sensor 10 have the ability to distinguish different concentrations of gas in the fluid 26. For example, the gas sensor 10 can distinguish an analyte gas at regulated vapor exposure limits established by different organizations. In certain embodiments, the gas sensor 10 can distinguish an analyte gas below the permissible exposure limit (PEL). In some embodiments, the gas sensor 10 can distinguish an analyte gas below the threshold limit value short-term exposure limit (TLV-STEL). In some embodiments, the gas sensor 10 can distinguish an analyte gas below the threshold limit value time-weighted average (TLV-TWA). In some embodiments, the gas sensor 10 can distinguish an analyte gas below immediately dangerous to life or health (IDLH). In certain embodiments, the gas sensor 10 can distinguish an analyte gas below and above the lower explosive limit (LEL). In certain embodiments, the gas sensor 10 may be capable of distinguishing an analyte gas having a concentration of less than 5%, less than 100 parts per million (ppm), less than 100 parts per billion (ppb), less than 100 parts per trillion (ppt).

[0034] Figure 2 is a flow diagram illustrating an embodiment of a process 70 in which a gas sensor 10 performs a multi-gas analysis of a fluid sample 26. Process 70 begins with exposing the gas sensing material 22 of the gas sensing element 12 to a fluid sample having at least two gases (e.g., at least two analyte gases, at least one analyte gas and one interfering gas) (block 72). For example, the entire gas sensor 10, or only the gas sensing element 12 of the gas sensor 10, may be exposed to the fluid sample. Process 70 includes heating the gas sensing material 22 to a constant operating temperature using one or more heating element controllers 28 (block 74). As discussed above, in certain embodiments, a first controller 28A of the heating element 24 provides a predetermined constant voltage to the heating element 24, and a second controller 28B of the heating element 24 provides an additional adjustable voltage to the heating element 24, such that the combined heating element voltage causes the gas sensing material 22 to maintain a desired constant operating temperature. Typically, the gas sensing material 22 is heated before, during, and after being exposed to the fluid sample 26.

[0035] Once the gas sensing material 22 is exposed to the fluid sample 26 and heated to a constant operating temperature, process 70 causes a measurement circuit 32 to effect dielectric excitation of the gas sensing material 22 operating at a constant operating temperature using at least two preselected frequencies (block 76), and then proceeds to measure the dielectric excitation response (e.g., impedance response) of the gas sensing material 22. In certain embodiments, the measurement circuit 32 may further apply DC excitation to the gas sensing material 22 and measure the DC excitation response (e.g., resistance response) of the gas sensing material 22 at a constant temperature. However, in some embodiments, the measurement circuit 32 may measure only the impedance response of the gas sensing material 22 when in contact with the fluid sample 26 at a constant operating temperature.

[0036] Traditionally, the MOS gas sensor 10 measures the DC resistance response of the MOS-based sensing element 12 and uses the power-law relationship between the measured resistance and the gas concentration to associate the measured DC resistance response with the concentration of the gas. Such a DC resistance response from the MOS gas sensor 10 can be provided as a signal output in the form of an analog signal (e.g., to a user). Depending on the design of the analog circuit, the analog signal from the MOS gas sensor 10 can represent linear resistance, logarithmic resistance, or conductivity. Alternatively, the DC resistance response from the traditional MOS-based gas sensor 10 may be provided as a signal output in the form of a digitized DC resistance response signal. Depending on the design of the analog / digital circuit, the digital signal from the MOS gas sensor 10 can be correlated with linear resistance, logarithmic resistance, or conductivity. The digital signal from the MOS gas sensor 10 that correlates with the DC resistance response can be provided (e.g., to a user) by any of the digital communication protocols, such as I2C (Inter-Integrated Circuit), alternatively known as IIC, and any other communication protocol.

[0037] In the case of the illustrated embodiment, the process 70 continues by the on-board data processor 36 of the gas sensor 10 performing on-board data analysis of the measured dielectric excitation response (block 78) based on at least one of the stored gas analysis models 40 to provide real-time resolution (e.g., real-time discrimination) of the gas in the fluid sample. In some embodiments, the real-time resolution or discrimination of the gas in the fluid sample is not affected by ambient conditions (e.g., ambient temperature, ambient humidity, ambient pressure). In certain embodiments where the DC excitation response is also measured by the measurement circuit 32, the on-board data processor 36 may also provide this DC excitation response as an input to at least one of the stored gas analysis models 40 when discriminating the gas in the fluid sample. In this context, "real-time" refers to the on-board data processor 36 of the gas sensor 10 that can locally and rapidly discriminate the gas in the fluid sample without the need to provide the measured excitation response to an external computing system for processing.

[0038] In the case of the embodiment of process 70 illustrated in FIG. 2, after distinguishing the gases in fluid sample 26, gas sensor 10 may provide an output based on each classification 80 of the gases in the fluid sample, each concentration 82 of the gases in the fluid sample, or both, using one or more output devices 16 (block 84). For example, one or more output devices 16 of gas sensor 10 may present or display each classification 80 and / or each concentration 82 of the gases in fluid sample 26. In certain embodiments, gas sensor 10 may provide each classification 80 and / or each concentration 82 of the gases to an external computing system via one or more suitable communication devices 48 (e.g., a wireless communication interface) of gas sensor 10. In certain embodiments, gas sensor 10 may use one or more output devices 16 to output each alert 49 of the presence of a gas in the fluid sample that exceeds a particular predetermined threshold level stored in memory 38 of gas sensor 10.

[0039] FIG. 3 is a graph illustrating an exemplary impedance spectrum 100. In impedance spectroscopy, measurements of the real part Z’ and the imaginary part Z” of the impedance are performed over a wide range of frequencies to determine the shape of the impedance spectrum 100 of gas sensor 10. As illustrated, the impedance spectrum includes two curves, each representing a portion of the impedance response of gas sensor 10 over a wide range of frequencies to determine the shape of the impedance spectrum. In particular, the first curve 102 represents the real part (Z’) of the impedance of gas sensor 10, and the second curve 104 represents the imaginary part (Z”) of the impedance of gas sensor 10 measured over a wide range of frequencies. Different from broadband impedance spectroscopy measurements, dielectric excitation measurements are performed over a particular frequency range by following the front (high frequency or low frequency) shoulders of the dielectric relaxation regions obtained from those impedance measurements when (n-type or p-type respectively) MOS materials are exposed to various gas concentrations.

[0040] In the case of the present embodiment, the measurement circuit 32 is an impedance detector that measures the dielectric excitation response of the gas sensor 10 in two or more frequency ranges 106, 108 (which may or may not be arranged in the "dielectric relaxation region" of the gas sensor 10), or includes an impedance detector. For example, in a particular embodiment, each dielectric excitation response measured by the measurement circuit 32 may include a combination (e.g., sum, difference, any other mathematical expression) of values (e.g., actual impedance values) from the first curve 102 and values (e.g., virtual impedance values) from the second curve 104, both selected from the frequency ranges 106, 108. Alternatively, in some embodiments, each dielectric excitation response measured by the measurement circuit 32 may include a combination (e.g., sum, difference, any other mathematical expression) of values (e.g., actual impedance value Z') from the first curve 102 and values (e.g., virtual impedance value Z") from the second curve 104, both selected from the frequency ranges 106, 108 or other frequency ranges. The selection of the frequency ranges 106, 108 may depend on the type of the gas sensing element 12 of the gas sensor 10. For example, in relation to the gas sensing element 12, the selection of the frequency ranges 106, 108 may depend on the type of MOS sensing material, e.g., n-type or p-type, or a combination of n-type and p-type MOS sensing materials, as well as the type of gas for measurement such as a reducing gas or an oxidizing gas. As a result, either the high-frequency shoulder region or the low-frequency shoulder region can be selected for measurement.

[0041] In the case of this embodiment, it will be appreciated that the dielectric excitation measurement represented in FIG. 3 is performed at a constant operating temperature in order to achieve multi-gas discrimination. This is in contrast to traditional MOS-based gas sensors that collect resistance measurements (e.g., DC excitation response) to achieve multi-gas discrimination. That is, in the case of traditional MOS-based gas sensors, to achieve multi-gas discrimination, the resistance response of the MOS-based gas sensor in contact with the fluid sample is measured at two or more different temperatures. It will be appreciated that there are many advantages to operating the disclosed gas sensor 10 at a constant operating temperature to achieve multi-gas discrimination. For example, temperature regulation involves a significant effort to synchronize the heating cycle with the data analysis interpretation.

[0042] Furthermore, it is now recognized that continuous temperature cycling of traditional MOS-based gas sensors undesirably degrades the heating elements and limits the operating life of these sensors. Additionally, it is recognized that temperature switching in traditional MOS-based gas sensors can negatively affect the quality of electrical measurements by introducing undesirable instabilities, noise, and hysteresis. Furthermore, although unexpected, in the present disclosure, for the disclosed gas sensor 10 operating at a constant operating temperature, the dielectric excitation response at a specific frequency of the sensor dielectric relaxation spectrum provides excellent multi-gas discrimination and baseline stability compared to the resistance responses of the same gas sensing material 22 over several different operating temperatures.

[0043] Experimental Example 1

[0044] To further demonstrate the excellent performance of the disclosed technology, experiments were conducted to compare the ability of conventional DC / resistance measurements versus dielectric excitation measurements for distinguishing between different analyte gases in a fluid sample. For these experiments, all measurements were performed using tin oxide (SnO2) as the gas sensing material 22. The measurements were performed by the measurement circuit 32 as either the DC excitation response (e.g., resistance measurement) or the dielectric excitation response (e.g., impedance measurement) selected from the high frequency shoulder region 106 of the corresponding dielectric relaxation spectrum 100 after dielectric excitation. Figures 4-10 correspond to a first example of the operation of the gas sensor 10 when measured under dielectric excitation using the resistance response.

[0045] As shown below, certain measurements were performed using the total heating element voltage or the combined heating element voltage from one or more heating element controllers 28 of either 4.5 volts (V) or 5.0 volts (V). For the embodiments of the gas sensor 10 used in these experiments, a total heating element voltage of 4.5 V corresponds to an operating temperature of approximately 250 degrees Celsius (°C), and a total heating element voltage of 5.0 V corresponds to an operating temperature of approximately 300 °C. For the experiments, the gas sensing element 12 of the gas sensor 10 was exposed to each of four different concentrations of each analyte gas. Three analyte gases were utilized, including acetylene (the first analyte gas, gas 1), hydrogen (the second analyte gas, gas 2), and water vapor (the third analyte gas, gas 3). The four different concentrations of the first analyte gas were 2300 parts per million (ppm), 4600 ppm, 6900 ppm, and 9200 ppm. The four different concentrations of the second analyte gas were 1.88 ppm, 3.75 ppm, 5.63 ppm, and 7.50 ppm. The four different concentrations of the third analyte gas were 10% relative humidity (RH), 20% RH, 30% RH, and 40% RH. Further, for the experiments represented by Figures 4-7, the gas sensing material 22 of the gas sensor 10 was (A) heated to a first operating temperature, (B) exposed to each of the four concentrations of the first analyte gas, (C) exposed to each of the four concentrations of the second analyte gas, and (D) exposed to each of the four concentrations of the third analyte gas. Then, the gas sensor 10 was then heated to a second operating temperature, and steps B-D were repeated at the second operating temperature.

[0046] Figures 4 and 5 are a set of graphs 120A and 120B showing examples of the temperature-dependent chemiresistor response patterns of the gas sensing material 22 for three analyte gases at two different operating temperatures. More specifically, plot 120A in FIG. 4 illustrates the raw resistance measurements (e.g., DC excitation response) collected by the measurement circuit 32 for each of the three analyte gases at a first heater element voltage (4.5 V) on a logarithmic resistance response scale. Plot 120B in FIG. 5 illustrates the raw resistance measurements (e.g., DC excitation response) collected by the measurement circuit 32 for each of the three analyte gases at a second heater element voltage (5.0 V) on a logarithmic resistance response scale.

[0047] Based on the data illustrated in FIG. 4, the gas sensing material 22 of the gas sensor 10 preferably demonstrates a response to each of the analyte gases. However, currently, the resistance response patterns between the second and third analyte gases were similar at both operating temperatures, but it has been recognized that the response patterns between the first and second analyte gases and between the first and third analyte gases were different at different operating temperatures. The horizontal arrows 122 and 124 in FIG. 4 and the horizontal arrows 126 and 128 in FIG. 5 show the response patterns on the logarithmic resistance response scale between different gases. As illustrated, the response intensity to the highest concentration of gas 2 was higher than the response to the highest concentration of gas 1 at a heater element voltage of 4.5 V and a heater element voltage of 5.0 V. However, the resistance response patterns for gas 2 and gas 3 were similar at the two heater element voltages of 4.5 V and 5.0 V.

[0048] In contrast to FIGS. 4 and 5, FIG. 6 is a set of graphs 140A showing the dielectric excitation response patterns of the gas sensing material 22 for each of three analyte gases measured across the real part of the impedance (Z’) and the imaginary part of the impedance (Z”) at a first operating temperature over seven operating frequencies. FIG. 7 is a set of graphs 140B showing the dielectric excitation response patterns of the gas sensing material 22 for each of three analyte gases measured across the real part of the impedance (Z’) and the imaginary part of the impedance (Z”) at a second operating temperature over seven operating frequencies. The seven operating frequencies for the Z’ and Z” measurements were Z’ at 1.2 megahertz (MHz), Z’ at 10 MHz, Z” at 32 kHz, Z” at 100 kHz, Z” at 230 kHz, Z” at 0.9 MHz, and Z” at 2.3 MHz.

[0049] Arrow 142 in FIG. 6 indicates that the response to the highest concentration of gas 1 is either similar to the response to the third concentration of gas 2 or less than the response to the first concentration of gas 2. Arrow 144 in FIG. 6 indicates that the response to the minimum concentration of gas 3 is either greater than or less than the response to the first concentration of gas 2. Thus, at a single operating temperature, the different frequencies of sensor operation in the real and imaginary parts of the impedance provide the desired diversity in the responses to all three gases.

[0050] Arrow 146 in FIG. 7 indicates that the response to the highest concentration of gas 1 is either greater than the response to the fourth concentration of gas 2 or less than the response to the second concentration of gas 2. Arrow 148 in FIG. 7 indicates that the response to the minimum concentration of gas 3 is either greater than or less than the response to the first concentration of gas 2. Thus, at each individual operating temperature, the different frequencies of sensor operation in the real and imaginary parts of the impedance provide the desired diversity in the responses to all three analyte gases.

[0051] PCA was applied to analyze the resistance responses (e.g., DC excitation responses) in FIGS. 4 and 5, and the ability of the gas sensor 10 to distinguish three analyte gases at two operating temperatures was determined. For each sensor state (three analyte gases at different concentrations and a blank), two data points were extracted from the raw dynamic responses at the steady state of the sensor response. FIG. 8 shows the results of PCA as a score plot 160 of the first two principal components of the logarithmic resistance responses from FIGS. 4 and 5. In the case of the analysis of the resistance response, a logarithmic scale is typically used due to the highly non-linear resistance response that follows a power law between the MOS resistance and the concentration of the analyte gas.

[0052] As shown in FIG. 8, the analysis of the logarithmic resistance responses from the plots 120 in FIGS. 4 and 5 by PCA demonstrates that the responses to the three analyte gases are correlated with the concentration of the gas. However, in the score plot 160 of FIG. 8, gas 2 and gas 3 were not sufficiently distinguished or separated from each other. Therefore, the analysis of the logarithmic resistance response did not provide the desired multi-gas discrimination even when measured at two or more temperatures.

[0053] For comparison with the resistance measurement and analysis, PCA was applied to analyze the dielectric excitation response (e.g., impedance response) in FIG. 6, and the ability of the gas sensor 10 to distinguish three analyte gases at a single operating temperature was determined. FIG. 9 shows the PCA results as a score plot 170 of the first three principal components (PC1, PC2, PC3) of the dielectric excitation response of the gas sensing material 22 at seven frequencies when operating at a single operating temperature. The single operating temperature in FIG. 9 corresponds to a heating element voltage of 4.5V. More specifically, the score plot 170 in FIG. 9 demonstrates a clear distinction between the three analyte gases. Therefore, FIG. 9 unexpectedly demonstrates the distinction between the three gases under the dielectric excitation response of the gas sensing material 22 and the respective concentrations of the three analyte gases even though only a single operating temperature is used.

[0054] PCA was also applied to analyze the dielectric excitation response (e.g., impedance response) in FIG. 7 and determine the ability of the gas sensor 10 to distinguish three analyte gases at different single operating temperatures. FIG. 10 shows the PCA results as a score plot 180 of the first three principal components (PC1 vs. PC2 vs. PC3) of the dielectric excitation response of the gas sensing material 22 for seven preselected dielectric excitation frequencies when operating at a single operating temperature. The single operating temperature in FIG. 8 corresponded to a heating element voltage of 5.0 V. More specifically, the score plot 180 in FIG. 10 demonstrates the distinction between all three analyte gases. Thus, FIG. 10 also unexpectedly demonstrates a strong correlation or proportional relationship between the dielectric excitation response of the gas sensing material 22 and the respective concentrations of the three analyte gases, even though only a single operating temperature was used.

[0055] Thus, in the first experimental example, the dielectric excitation response of the sensor at a single operating temperature provided a distinction between all three test gases, but even when multiple operating temperatures were used, the resistance response of the sensor at two operating temperatures did not distinguish between two of the three gases. Here too, the performance of the gas sensor 10 under dielectric excitation resulted in a higher response dispersion (also known as the sensor response dimension) compared to the response dispersion of the sensor with its resistance readout and operation at two temperatures, even at a single operating temperature. As shown in FIGS. 9 and 10, the gas sensor 10 under dielectric excitation and a single operating temperature demonstrated a three-dimensional response (3D dispersion) compared to the two-dimensional response (2D dispersion) of the gas sensor 10 with the resistance response of the sensor at two operating temperatures.

[0056] Experimental Example 2

[0057] To further demonstrate the excellent performance of the disclosed technology, experiments were performed comparing the ability of conventional resistance measurements to distinguish different analyte gases in a fluid sample for dielectric excitation measurements. For these experiments, an embodiment of a gas sensor 10 with a dual gas detection element 12 was used to perform all measurements, each having different MOS-based gas sensing materials 22 operating at a constant temperature. The measurements were performed by a measurement circuit 32 as either a DC excitation response (e.g., resistance measurement) or a dielectric excitation response (e.g., impedance measurement) selected from the high-frequency shoulder region 106 of the corresponding dielectric relaxation spectrum 100 after dielectric excitation. FIGS. 11-14 correspond to a second example of the operation of the gas sensor 10 when measured under dielectric excitation using the resistance response.

[0058] The second experimental example demonstrates the advantages of dielectric excitation versus resistance readout for the MOS-based gas sensing material 22. More specifically, the second example demonstrates that dielectric excitation readout provides advantages in improved multi-gas discrimination compared to resistance measurements using the same gas sensing material in the detection of various inorganic gases. For this experiment, the fluid sample 26 contained several gases including chlorine (Cl2, gas 1), ethylene oxide (EOx, gas 2), nitrogen dioxide (NO2, gas 3), sulfur dioxide (SO2, gas 4), and hydrogen cyanide (HCN, gas 5). Each gas was presented to both gas sensing materials 22 of the gas sensor 10 at nine equally spaced concentrations up to 75% of the maximum level within their respective gas tanks. The concentrations within each gas tank were 5 ppm for Cl2, 10 ppm for EOx, 20 ppm for NO2, 20 ppm for SO2, and 25 ppm for HCN, respectively. FIG. 11 is a set of graphs 190 illustrating the responses of a gas sensor 10 having two different gas sensing materials 22 operating at a constant temperature using resistance (log scale) readout utilized for multi-gas discrimination. Plot 192A in FIG. 11 illustrates that the resistance response of the first gas sensing material 22 was strongest for gas 3, and the responses for gases 1, 5, 2, and 4 decreased gradually. Further, the resistance responses for gases 1, 3, and 4 were in one direction, and the responses for gases 2 and 5 were in the opposite direction. Plot 192B in FIG. 11 illustrates that the resistance response of the second gas sensing material 22 was strongest for gas 3, and the responses for gases 1, 2, 4, and 5 decreased gradually. Further, in FIG. 11, the responses for gases 1, 3, and 4 were in one direction, and the responses for gases 2 and 5 were in the opposite direction.

[0059] FIG. 12 is a set of graphs 200 illustrating the responses of a gas sensor 10 having two different gas sensing materials 22 operating at a constant temperature using dielectric-excitation readout (linear scale) utilized for multi-gas discrimination. Compared to FIG. 11, FIG. 12 shows a substantial diversity in the responses of the two gas sensing materials 22 of the gas sensor 10 under their dielectric excitation. For example, depending on the operating frequency, the strongest response can be either for gas 3 (plot 202A), gas 5 (plots 202B and 202C), or gas 2 (plot 202G). The dielectric excitation responses for gases 1-5 may be similar to the resistance responses in the opposite direction (plots 202A, 202C, 202G), or in the same direction (plots 202B and 202D). Such a desired diversity in the responses at different frequencies enables improved discrimination of gases compared to the resistance response.

[0060] The results of the PCA analysis of the resistance response of FIG. 11 and the dielectric excitation response of FIG. 12 are presented in FIGS. 13 and 14, respectively. For this analysis, three data points for each concentration of the five gases and three points for the blank were extracted. FIG. 13 is a PCA score plot 210 of the resistance response of FIG. 11, which illustrates the overlap between the responses for gases 2 and 5, as well as some overlap between the responses for gases 1 and 4. A clear distinction is shown only between gas 3 and the other gases 1, 2, 4, and 5. In contrast, FIGS. 14A and 14B are a set of PCA score plots 220A and 220B of the dielectric excitation response of FIG. 12, which illustrate a clear distinction between gases 2, 3, and 5, and the overlap in the responses for gases 1 and 4 is limited. Thus, the dielectric excitation of the same number of gas sensing materials 22 enables an improvement in the discrimination between a larger number of gases compared to classical resistance readout. Such an improvement in discrimination enables substantial advantages with respect to rejection of interferents and reduction of false alarms.

[0061] Experimental Example 3

[0062] To further demonstrate the excellent performance of the disclosed technology, experiments were conducted to compare the ability of conventional resistance measurements to distinguish different analyte gases in a fluid sample with respect to the dielectric excitation response. For these experiments, all measurements were performed using an embodiment of the gas sensor 10 having the dual gas detection element 12, each having a different MOS-based gas detection material 22 that operates at a constant temperature. The measurements were performed by the measurement circuit 32 as either a DC excitation response (e.g., resistance measurement), or a dielectric excitation response (e.g., impedance measurement) selected from the high frequency shoulder region 106 of the corresponding dielectric relaxation spectrum 100 after dielectric excitation. FIGS. 15-20 correspond to a third example of the operation of the gas sensor 10 when measured under dielectric excitation using its resistance response.

[0063] In a third experimental example, two gas detection materials 22 of the gas sensor 10 operating at a constant temperature were used to study the effect of increasing levels of interferents on the ability to distinguish the target analyte gas. The analyte gases included chlorine (Cl2, gas 1), ethylene oxide (EOx, gas 2), and hydrogen cyanide (HCN, gas 4), and the interferents included diesel exhaust (gas 3), isopropyl alcohol (IPA, gas 5), toluene (gas 6), and humidity (gas 7). These gases were presented to the gas detection material 22 of the gas sensor 10 while increasing their concentrations. Note that the analyte gases were selected to evaluate the effect of interferents on analytes that tend to provide a relatively strong sensor response (e.g., for Cl2), as well as analytes that tend to provide a relatively weak sensor response (e.g., for EOx and HCN). The responses of the two gas detection materials 22 were collected over 3 cycles using the same two levels of analyte (gases 1, 2, and 4) and increasing levels of interferents (gases 3, 5, 6, and 7). The graph 230 of FIG. 15 shows the resistance responses of the two gas detection materials 22, and the graph 240 of FIG. 16 shows the dielectric excitation response.

[0064] To evaluate the ability of a limited number of gas sensing materials 22 (e.g., two gas sensing materials 22) in an embodiment of the gas sensor 10 to distinguish between many gases, the resistance responses of these two gas sensing materials 22 were analyzed using principal component analysis (PCA) and hierarchical cluster analysis (HCA) algorithms. As shown in FIG. 17, the PCA score plot 250 includes both principal components (PCs) corresponding to the responses of the two gas sensing materials 22, which accounts for all (i.e., 100%) of the variation in the dataset from the two gas sensing materials 22, describing its single output readout as its 2D dispersion. The 2D dispersion is, mathematically, the highest dispersion that can be achieved by using two gas sensors with a single output readout. As shown in FIG. 17, the PCA score plot 250 from the resistance responses of the two gas sensing materials 22 shows that IPA, toluene, RH, and Cl2 are well-distinguished, but HCN, EOx, and diesel demonstrate an undesirable strong overlap. As illustrated by FIG. 18, the HCA plot 260 from the resistance responses of the two gas sensing materials 22 demonstrates the inability to distinguish some diesel responses from all HCN and EOx responses, and the inability to distinguish between HCN and EOx responses. Thus, an embodiment of the gas sensor 10 having two gas sensing materials 22 with a single output resistance response did not distinguish between HCN, EOx, and diesel.

[0065] In comparison, the results of PCA and HCA analyses of the dielectric excitation responses of the gas sensor 10 having two gas sensing materials 22 are presented in FIGS. 19 and 20, respectively. As shown in FIG. 19, the PCA score plot 270 is PC2 vs. PC3 vs. PC4, which emphasizes the 4D dispersion of the sensor responses under dielectric excitation. However, as shown in FIG. 17, at most, only the 2D dispersion of the sensor responses can be achieved with a single output resistance readout. The PCA score plot 270 from the dielectric excitation responses of the two gas sensing materials 22 shows that all gases, including diesel exhaust gas, are well-distinguished. As shown in FIG. 20, the HCA plot 280 from the dielectric excitation responses of the two gas sensing materials 22 demonstrates two clusters of interferents, including distinct clusters of responses to the analyte gases HCN, Cl2, and EOx, and IPA and toluene (cluster 1) and diesel exhaust and humidity (cluster 2). Thus, the embodiment of the gas sensor 10 having two gas sensing materials 22 with their multi-output dielectric excitation responses adequately distinguishes all test gases, including the discrimination from other gases in diesel exhaust.

[0066] With the above in mind, FIG. 21 is a flow diagram illustrating an embodiment of a process 300 that can be configured to fabricate the gas sensor 10 and distinguish a particular gas analyte during multi-gas analysis. In the illustrated embodiment, process 300 begins with the designer determining a plurality of analyte gases to be distinguished by the gas sensor 10 (block 302). Based on the analyte gases to be distinguished by the gas sensor 10, the designer determines a base semiconductor metal oxide material 22 to be used for detection of the analyte gases by the gas sensor and one or more dopants to be used (block 304). The designer also determines an operating temperature for operating the gas sensing material 22 of the gas sensor 10 (block 305) and configures a first controller 28A of the heating element 24 and a second controller 28B of the heating element 24 to provide a synthetic heating element voltage suitable for heating the gas sensing material 22 to the operating temperature (block 306). Based on the analyte gases to be distinguished by the gas sensor 10, the designer also determines a dielectric excitation frequency to be applied to the gas sensing material 22 for measuring the dielectric excitation response of the gas sensing material 22 when the gas sensing material 22 interacts with the plurality of analyte gases (block 308) and configures a measurement circuit 32 to apply each of the dielectric excitation frequencies to the gas sensing material for measuring the dielectric excitation response of the gas sensing material when the gas sensing material interacts with the analyte gas at the operating temperature (block 310). In embodiments where the gas sensor 10 includes an on-board data processor 36, process 300 can include the designer determining one or more gas analysis models 40 to be used by the on-board data processor 36 for distinguishing the analyte gases (block 312) and storing the one or more gas analysis models 40 in a memory 38 associated with the on-board data processor 36 (block 314). In certain embodiments, determining the gas analysis model 40 can include generating at least one equation (and / or determining corresponding coefficient values of the equation) that associates an experimentally measured dielectric excitation response of the gas sensing material 22 at a constant operating temperature with each of the analyte gases and a particular classification of the analyte gases, or a particular concentration of the analyte gases, or any combination thereof.In certain embodiments, the plurality of gases can include at least one analyte gas and at least one interfering gas. In certain embodiments, multivariate data processing principles can be applied to perform gas classification / cluster analysis and quantification.

[0067] In certain embodiments, at block 304, the desired operating temperature of gas sensor 10 can be determined experimentally. For example, the gas sensing material 22 of gas sensor 10 can be exposed to a sensing environment (e.g., a controlled variable fluid sample) to collect excitation responses while heating the gas sensing material 22 to different operating temperatures. At each of the different operating temperatures, a first sequence in which the concentration of a first analyte gas increases is introduced into the sensing environment, and measurement circuit 32 applies at least one of a plurality of dielectric excitation frequencies to gas sensing material 22 to measure a first set of dielectric excitation responses of gas sensing material 22 to the first sequence in which the concentration of the first analyte gas increases at a particular temperature. Further, at each of the different temperatures, a second sequence in which the concentration of a second analyte gas is increased is introduced into the sensing environment while applying at least one of a plurality of dielectric excitation frequencies to gas sensing material 22, and a second set of dielectric excitation responses of gas sensing material 22 to the second sequence in which the concentration of the second analyte gas is increased at a particular temperature is measured. Subsequently, the first set of dielectric excitation responses at each of the different temperatures, and the second set of dielectric excitation responses at each of the different temperatures are analyzed (e.g., using PCA as discussed above) to determine at least one operating temperature for operating gas sensing material 22 of gas sensor 10 when detecting the first and second analytes in the fluid sample. For example, based on PCA or any other multivariate analysis algorithm, a desired constant operating temperature can be identified at which the first set of dielectric excitation responses correlates or is proportional to the concentration of the first analyte gas and the second set of dielectric excitation responses of gas sensing material 22 correlates or is proportional to the concentration of the second analyte gas. It will be appreciated that this experimental determination of the operating temperature can be extended to include three or more analyte gases, such as three, four, or five or more analyte gases.

[0068] In certain embodiments, at block 308, the number of dielectric excitation frequencies used by measurement circuit 32 can be determined experimentally. For example, the gas sensing material 22 of gas sensor 10 may be exposed to a sensing environment while heating the gas sensing material to a desired operating temperature. Then, a first sequence that increases the concentration of a first analyte gas is introduced into the sensing environment. For each concentration of the first analyte gas in the first sequence, measurement circuit 32 applies a first set of dielectric excitation frequencies to gas sensing material 22 and measures a first set of dielectric excitation responses of gas sensing material 22 to the first sequence in which the concentration of the first analyte gas increases. A second sequence that increases the concentration of a second analyte gas is introduced into the sensing environment. For each concentration of the second analyte gas in the second sequence, measurement circuit 32 applies a second set of dielectric excitation frequencies to gas sensing material 22 and measures a second set of dielectric excitation responses of gas sensing material 22 to the second sequence in which the concentration of the second analyte gas increases. Then, the first set of dielectric excitation responses at each of the different concentrations of the first analyte gas and the second set of dielectric excitation responses at each of the different concentrations of the second analyte gas are analyzed (e.g., using PCA as discussed above) to determine a plurality of dielectric excitation frequencies for operating gas sensing material 22 during multi-gas sensing in a fluid sample from the first set of dielectric excitation frequencies and the second set of dielectric excitation frequencies. For example, based on PCA, at least two dielectric excitation frequencies can be identified whose corresponding dielectric excitation responses are correlated or proportional to the concentration of the first analyte gas, the concentration of the second analyte gas, or a combination thereof.

[0069] The technical effects of the present disclosure include enabling multi-gas detection at a certain operating temperature. Using the disclosed technology, MOS-based gas sensors can be designed, manufactured, and used to distinguish multiple gases in a fluid sample at a single operating temperature. Maintaining the gas sensing element at a certain operating temperature provides many advantages with respect to improved efficiency, improved electrical measurements, and improved operating life of the gas sensor. By measuring and analyzing the dielectric excitation response of the gas sensing material at a certain operating temperature, the gas sensor unexpectedly provides excellent multi-gas selectivity compared to other gas sensors and other gas sensing methods that rely on a number of resistance measurements performed at several different operating temperatures.

[0070] This written description uses examples to disclose the invention, including the best mode, and to enable any person skilled in the art to make and use the invention, including making and using any device or system and performing any incorporated method. The patentable scope of the invention is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements that do not differ substantially from the literal language of the claims.

Claims

Claim 1 A gas sensor system for multi-gas analysis of a fluid sample, comprising: a gas sensing element configured to contact the fluid sample; a heating element coupled to the gas sensing element and configured to heat the gas sensing element; a heating element controller operably coupled to the heating element and configured to control the heating element to heat the gas sensing element to a constant temperature while the gas sensing element is in contact with the fluid sample; a measurement circuit operably coupled to the gas sensing element and configured to provide dielectric excitation to the gas sensing element and measure a dielectric excitation response of the gas sensing element while the gas sensing element is heated to the constant temperature and in contact with the fluid sample, wherein the dielectric excitation response provides an improvement in discrimination between at least two gases in the fluid sample as compared to a resistance response of the gas sensing element when in contact with the fluid sample at a number of operating temperatures; A gas sensor system comprising the above components. Claim 2 a second gas sensing element configured to contact the fluid sample; a second heating element coupled to the second gas sensing element and configured to heat the second gas sensing element; a second heating element controller operably coupled to the second heating element and configured to control the second heating element to heat the second gas sensing element to the constant temperature while the second gas sensing element is in contact with the fluid sample, and the measurement circuit is operably coupled to the second gas sensing element and configured to provide a second dielectric excitation to the second gas sensing element and measure a second dielectric excitation response of the second gas sensing element while the second gas sensing element is heated to the constant temperature and in contact with the fluid sample, wherein the second dielectric excitation response provides an improvement in discrimination between the at least two gases in the fluid sample as compared to a resistance response of the second gas sensing element when in contact with the fluid sample at the number of operating temperatures; The gas sensor system according to claim 1. Claim 3 The heating element controller is electrically coupled to the heating element and configured to provide a predetermined constant voltage to the heating element. The gas sensor system further comprises a second heating element controller electrically coupled to the heating element, the second heating element controller being configured to provide an additional adjustable voltage to the heating element, and a combination of the predetermined constant voltage and the additional adjustable voltage heating the gas sensing element to the constant temperature while the gas sensing element is in contact with the fluid sample. The gas sensor system according to claim 1.

4. An on-board data processor communicatively coupled to the measurement circuit, receiving from the measurement circuit the dielectric excitation response of the gas sensing element at the constant temperature while the gas sensing element is in contact with the fluid sample, and selecting at least two of the dielectric excitation responses of the gas sensing element at the constant temperature to distinguish at least two gases in the fluid sample. The gas sensor system according to claim 1, comprising.

5. The gas sensor system according to claim 4, wherein the at least two selected dielectric excitation responses are impedance responses of the gas sensing element at the constant temperature.

6. The gas sensor system according to claim 4, wherein the on-board data processor is configured to distinguish the at least two gases by determining a respective classification, respective concentration, or combination thereof of the at least two gases in the fluid sample based on the at least two selected dielectric excitation responses of the gas sensing element at the constant temperature.

7. The gas sensor system according to claim 1, wherein the constant temperature is between 80°C and 600°C.

8. The gas sensor system according to claim 1, wherein the constant temperature varies by less than 20% while the gas sensing element is in contact with the fluid sample.

9. A method of operating a gas sensor for multi-gas analysis of a fluid sample, comprising: exposing a gas sensing material of the gas sensor to the fluid sample; and controlling a heating element of the gas sensor via at least one heating element controller of the gas sensor to heat the gas sensing material to a constant temperature. Measuring a dielectric excitation response of the gas sensing material while the gas sensing material is heated to the constant temperature and exposed to the fluid sample via a measurement circuit of the gas sensor; Receiving, via an on-board data processor of the gas sensor, the dielectric excitation response of the gas sensing material measured at the constant temperature; Distinguishing at least two gases in the fluid sample based on at least a part of the received dielectric excitation response via the on-board data processor. A method comprising:

10. The method according to claim 9, wherein the at least two gases in the fluid sample cannot be distinguished using a resistance response of the gas sensing element in contact with the fluid sample at a plurality of operating temperatures.

11. Exposing a second gas sensing material of the gas sensor to the fluid sample; Controlling a second heating element of the gas sensor via the at least one heating element controller of the gas sensor to heat the second gas sensing material to the constant temperature; Measuring a second dielectric excitation response of the second gas sensing material while the second gas sensing material is heated to the constant temperature and exposed to the fluid sample via the measurement circuit of the gas sensor; Receiving, via the on-board data processor of the gas sensor, the second dielectric excitation response of the second gas sensing material at the constant temperature; Distinguishing the at least two gases in the fluid sample based on at least a part of the received dielectric excitation response via the on-board data processor. The method according to claim 9, comprising:

12. Controlling the heating element is Supplying a predetermined constant voltage to the heating element via a first heating element controller of the gas sensor; Providing an additional adjustable voltage to the heating element via a second heating element controller of the gas sensor, wherein a combination of the predetermined constant voltage and the additional adjustable voltage heats the gas sensing material to the constant temperature while exposing it to the fluid sample, whereby the dielectric excitation response of the gas sensing material is not affected by ambient temperature and whereby the gas sensing material maintains the constant temperature while in contact with the fluid sample even if the heating element undergoes aging changes over the operating life of the gas sensor. The method according to claim 9.

13. providing the additional adjustable voltage to the heating element, measuring an initial temperature of the gas sensing material in response to the predetermined constant voltage provided to the heating element while being exposed to the fluid sample via a temperature sensor coupled to the gas sensing material and communicatively coupled to the second heating element controller; determining, via the second heating element controller, a difference between the initial temperature and the constant temperature; determining, via the second heating element controller, the additional adjustable voltage provided by the second heating element controller to heat the gas sensing material to the constant temperature, the method of claim 12.

14. providing the additional adjustable voltage to the heating element, measuring a resistance of the heating element that correlates to the temperature of the heating element; determining, via the first heating element controller and / or the second heating element controller of the gas sensor, a difference between an initial temperature and the constant temperature; determining, via the second heating element controller, the additional adjustable voltage provided by the second heating element controller to heat the gas sensing material to the constant temperature; the method of claim 12.

15. measuring is measuring two or more dielectric excitation responses of the gas sensing material at two or more excitation frequencies that correlate to respective concentrations and respective classes of two or more gases in the fluid sample via the measurement circuit the method of claim 9.

16. distinguishing the at least two gases in the fluid sample is determining, via the on-board data processor, respective classes and respective concentrations of each gas in the fluid sample based on the measured dielectric excitation responses at two or more excitation frequencies and a stored model that correlates the dielectric excitation responses at the measured excitation frequencies to the respective classes and the respective concentrations of different gases the method of claim 15.

17. distinguishing the at least two gases in the fluid sample is Determining each respective class of the gases in the fluid sample based on the measured dielectric excitation responses at two or more excitation frequencies and a stored model that correlates the measured dielectric excitation responses at the measured excitation frequencies to the respective classes of the gas in the fluid sample, via the on-board data processor, the method of claim 16.

18. A method of manufacturing a gas sensor for multi-gas analysis, comprising: determining a plurality of analyte gases distinguishable by the gas sensor; determining a semiconductor gas sensing material composition; determining an operating temperature for operating the gas sensing material of the gas sensor at which the gas sensing material interacts reversibly with each of the plurality of analyte gases; configuring a first heating element controller and a second heating element controller of the gas sensor to provide a combined voltage to a heating element of the gas sensor to heat the gas sensing material to the operating temperature; determining a plurality of dielectric excitation frequencies applied to the gas sensing material to measure a dielectric excitation response of the gas sensing material when the gas sensing material interacts with the plurality of analyte gases; configuring a measurement circuit of the gas sensor to apply each of the plurality of dielectric excitation frequencies to the gas sensing material to measure the dielectric excitation response of the gas sensing material when the gas sensing material interacts with the plurality of analyte gases at the operating temperature.

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