Gas sensing system

Abstract

Described is a method for determining a concentration of a specific gas within an environment using multiple sensors that are exposed to the same conditions. One of the sensors may be protected from a flow of gas from the environment by a barrier material. The flow of gas from the environment may treated by one or more conditioning features before it reaches the sensors. The gas sensing system may also comprise a flow generator configured to generate the flow of gas and a controller configured to be in electrical communication with the gas composition sensors and the flow generator.

Description

GAS SENSING SYSTEMTECHNICAL FIELD

[0001] The present invention relates to a gas sensing system for determining a concentration of one or more gases in an environment.BACKGROUND

[0002] Gas sensing systems can be used to determine a concentration of one or more gases (e.g. methane) within an environment. Gas sensing systems can be used in environments surrounding, for example oil and gas drilling or refining sites, waste sites, or any other sites where the concentration of one or more specific may present a health risk or may be required for regulatory compliance.

[0003] One important gas for detection is methane. Methane emissions are currently the second largest cause of global warming. Strategies for its measurement are typically based on optical absorption at one of methane’s absorption lines. However, these are weak and need a long pathlength to achieve the required sensitivity, hence instruments based on this approach have sophisticated but expensive optical designs.

[0004] It is desired to address or ameliorate one or more disadvantages or limitations associated with the prior art, provide a gas sensing system, a method for determining a methane concentration within an environment, or to at least provide the public with a useful alternative.SUMMARY

[0005] According to a first aspect, the present disclosure describes a method for determining a concentration of a gas within an environment using multiple sensors that are exposed to the same conditions, wherein one of the sensors is protected from a flow of gas, that comprises the gas to be sensed, from the environment by a barrier material and the flow of gas from the environment is treated by one or more conditioning features before it reaches the sensors.

[0006] According to another aspect, the present disclosure describes a gas sensing system for determining a concentration of a gas within an environment, comprising: a flow path for a flow of gas from the environment to be sensed, an inlet for receiving the flow of gas into the flow path, one or more gas conditioning features within the flow path,two gas composition sensors in proximity to one another and both configured to measure the concentration of a gas within the flow of gas downstream of the one or more gas conditioning features, a flow generator configured to generate the flow of gas, and a controller configured to be in electrical communication with the gas composition sensors and the flow generator, wherein one of the two gas composition sensors is separated from the flow of gas by a barrier material that prevents molecules of the specific gas to be sensed from passing through.

[0007] According to another aspect, the present disclosure describes a method for determining a methane concentration within an environment, comprising: generating a flow of gas from the environment into a flow path of a sensing system using a flow generator, reducing the concentration of non-methane substances within the flow of gas by passing the flow of gas through one or more scrubbers positioned within the flow path, sensing the concentration of methane within the flow of gas with two gas composition sensors in proximity to one another and both configured to measure the concentration of methane within the flow of gas downstream of the one or more scrubbers, wherein one of the two gas composition sensors is separated from the flow of gas by a barrier material that allows water molecules to pass through but prevents methane from passing through, and determining the methane concentration within the flow of gas with a controller that is in electrical communication with the gas composition sensors, wherein the controller determines the methane concentration by inputting a sensor resistance parameter, calculated as a function of the ratio of a resistance reading from one of the gas composition sensors to a resistance reading from the other gas composition sensor, into a calibrated polynomial function which relates resistance to methane concentration.

[0008] The following configurations may relate to any of the above aspects.

[0009] In one configuration, the barrier material has a thickness less than 10, 20, 30, 40, 50, 100, 200, 300, 400, or 500 pm, and suitable ranges may be selected from between any of these values.

[0010] In one configuration, the barrier material allows water molecules to pass through.

[0011] In one configuration, the barrier material prevents water molecules from passing through.

[0012] In one configuration, the one or more gas conditioning features comprises a dehumidifier.

[0013] In one configuration, the one or more gas conditioning features comprises a length of tubing forming a section of the flow path, wherein the length of tubing is formed from a material that allows water molecules to pass through a wall of the length of tubing.

[0014] In one configuration, the length of tubing has a length less than 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 cm, and suitable ranges may be selected from between any of these values.

[0015] In one configuration, the one or more gas conditioning features comprises one or more scrubbers.

[0016] In one configuration, the one or more scrubbers comprises an activated carbon scrubber configured to reduce the amount of volatile organic compounds within the flow of gas.

[0017] In one configuration, the one or more scrubbers comprises a palladium-doped activated carbon scrubber configured to reduce the concentration of hydrogen within the flow of gas.

[0018] In one configuration, the palladium-doped activated carbon scrubber has a palladium weight percentage less than 20, 15, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1%, and suitable ranges may be selected from between any of these values.

[0019] In one configuration, the one or more scrubbers are housed in a scrubber subhousing.

[0020] In one configuration, the two gas composition sensors are metal oxide sensors.

[0021] In one configuration, at least one of the gas composition sensors is a Tin(IV) oxide sensor.

[0022] In one configuration, at least one of the gas composition sensors is a Zinc oxide sensor.

[0023] In one configuration, at least one of the gas composition sensors is a Titanium dioxide sensor.

[0024] In one configuration, the two gas composition sensors are housed in a sensor sub-housing comprising: a sensor support block comprising: a base surface, a through hole passing through the sensor support block and configured to form a section of the flow path, and two sensor holes beginning at the base surface and extending into the through hole, wherein each sensor hole is configured to position the measurement end of one of the gas composition sensors within the flow path, and a circuit board configured to be attached to the base surface of the sensor support block and to be in electrical communication with the two gas composition sensors and the controller.

[0025] In one configuration, the sensor support block is formed from a single piece of polymer.

[0026] In one configuration, the sensor sub-housing further comprises a main seal configured to form a seal between the sensor support block and the circuit board, and wherein the sensor support block defines a main seal channel configured to locate the main seal.

[0027] On one configuration, each of the two gas composition sensors comprises an internal heater configured to be in electrical communication with the controller, and wherein the controller is configured to control each internal heater to maintain each gas composition sensor at a temperature above 250 °C.

[0028] In one configuration, the controller is configured to use a Wheatstone Bridge circuit to control the current supplied to each internal heater to maintain each gas composition sensor at a constant temperature independent of a surrounding temperature.

[0029] In one configuration, the gas sensing system further comprises a flow orifice within the flow path and configured to limit the flow rate of the flow of gas, wherein the flow orifice is configured to be positioned downstream of the gas composition sensors and upstream of the flow generator.

[0030] In one configuration, the maximum flow rate may be limited by the transport rate of molecules through the barrier material.

[0031] In one configuration the barrier material is provided as a single piece of material.

[0032] In one configuration, the flow orifice is configured to control the flow rate of the flow of gas to be less than 0.5, 0.4, 0.3, 0.2, 0.1 , 0.09, 0.08, 0.07, 0.06, or 0.05 LPM, and suitable ranges may be selected from between any of these values.

[0033] In one configuration, the controller is configured to determine the concentration of the specific gas to be sensed by inputting a sensor resistance parameter, calculated as a function of the ratio of a resistance reading from one of the gas composition sensors to a resistance reading from the other gas composition sensor, into a calibrated polynomial function which relates resistance to methane concentration.

[0034] In one configuration, the controller is configured to determine the concentration of the gas to be sensed within the flow of gas by:■ calculating a sensor resistance parameter as a function of the ratio of a resistance reading from one of the gas composition sensors to a resistance reading from the other gas composition sensor,■ subtracting a heater resistance parameter from the sensor resistance parameter, wherein the heater resistance parameter is a function of a rolling average of the sum of resistances of the internal heaters, and■ inputting the result into a calibrated polynomial function which relates resistance to methane concentration.

[0035] In one configuration, the method further comprises reducing fluctuations in the humidity of the flow of gas by passing the flow of gas through a length of tubing upstream of the two gas composition sensors, wherein the length of tubing is formed from a material that allows water molecules to pass through a wall of the length of tubing.

[0036] The term “comprising” as used in the specification and claims means “consisting at least in part of”. When interpreting each statement in this specification that includes the term “comprising”, features other than that or those prefaced by the term may also be present. Related terms “comprise” and “comprises” are to be interpreted in the same manner.

[0037] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0038] As used herein “(s)” following a noun means the plural and / or singular forms of the noun.

[0039] As used herein the term “and / or” means “and” or “or” or both.

[0040] It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[0041] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 shows a schematic representation of a gas sensing system as described.

[0043] Figure 2 shows a perspective view of a sensor support block as described.

[0044] Figure 3 shows a cross-section view of a sensor support block along A-A of Fig.2.

[0045] Figure 4 is a graph showing the effect of the presence of Nation™ tubing that increases the stability of the baseline with respect to changes in ambient humidity.

[0046] Figure 5 is a graph showing the effect of hydrogen scrubbers in reducing the concentration of hydrogen in ambient air.

[0047] Figure 6 is a graph showing the impact of correcting for temperature on the detection of methane.

[0048] Figure 7 is a graph showing the small variation in resistance from methane sensor measurement relative to baseline variation.

[0049] Figure 8 is a graph showing the effect of the use of a Nation™ sheet covering sensor “SR0” while sensor “SRG” is exposed to methane.DETAILED DESCRIPTION

[0050] Described is a method for determining a concentration of a gas within an environment using multiple sensors that are exposed to the same conditions. One of the sensors may be protected from a flow of gas from the environment by a barrier material. The flow of gas from the environment may treated by one or more conditioning features before it reaches the sensors.

[0051] The gas sensing system comprises a flow path for a flow of gas from the environment to be sensed and an inlet for receiving the flow of gas into the flow path. One or more gas conditioning features are positioned within the flow path. The gas sensing system also comprises two gas composition sensors in proximity to one another and both configured to measure the concentration of a gas within the flow of gas downstream of the one or more gas conditioning features. The gas sensing system also comprises a flow generator configured to generate the flow of gas and a controller configured to be in electrical communication with the gas composition sensors and the flow generator. One of the two gas composition sensors is separated from the flow of gas by a piece of barrier material that prevents molecules of the specific gas to be sensed from passing through.

[0052] Shown in Figure 1 is a gas sensing system 1 for determining a concentration of one or more gases (e.g. a specific gas such as methane) within an environment 3. For example, within the air in an environment 3.

[0053] As shown in Figure 1, the gas sensing system 1 comprises a flow path 10 for a flow of gas from the environment 3 to be sensed. The flow of gas comprises the specific gas or gases to be sensed.

[0054] The gas sensing system 1 compises a system casing 2.

[0055] The gas sensing system 1 comprises an inlet 11 for receiving the flow gas into the flow path 10. The gas sensing system 1 also comprises an outlet 12 by which the flow of gas can reenter the environment 3 after passing throught the flow path 10.

[0056] The gas sensing system 1 may comprise one or more gas conditioning features within the flow path 10. The one or more gas conditioning features are configured to condition the flow of gas by removing certain substances and / or correcting any fluctuations of humidity within the flow of gas.

[0057] Conditioning the flow of gas may improve sensing accuracy and precision. Conditioining the flow of gas may increase the life of the sensing system 1 by removing substances that could damage or obstruct parts of the sensing system 1.

[0058] The gas sensing system 1 comprises two gas composition sensors 40. The two gas composition sensors 40 are positioned within the flow path 10 and are configured to measure the concentration of the same one or more specific gases within the flow of gas.

[0059] The two gas composition sensors 40 are positioned downstream from the one or more gas conditioning features (relative to the direction of gas flow indicated by the arrows in Figure 1).

[0060] The two gas composition sensors 40 are positioned in proximity to one another. Used in this context, “proximity” means that the two gas composition sensors 40 are configured to measure the concentration of the one or more specific gases within the flow of gas under the same or similar conditions.

[0061] The gas sensing system 1 comprises a barrier material 41. One of the two gas composition sensors 40 is separated from the flow of gas by the barrier material 41. In other words, the barrier material 41 covers a measurement end of the gas composition sensor 40.

[0062] The barrier material 41 is configured to prevent molecules of the one or more gases to be sensed from passing through.

[0063] The gas sensing system comprises a flow generator 70 to generate the flow of gas. The flow generator 70 may be positioned downstream of the two gas composition sensors 40.

[0064] The flow generator 70 may comprise a pump. The flow generator 70 may comprise a fan. The flow generator 70 may comprise other kinds of known mechanisms for generating a steady flow of gas.

[0065] The gas sensing system comprises a controller 80. The controller is configured to be in electrical communication with the gas composition sensors 40. The controller receives data from the gas composition sensors 40. The controller is configured to be in electrical communication with the flow generator 70. The controller may control the flow rate of the flow of gas by controlling the flow generator 70.

[0066] As mentioned, the gas sensing system 1 comprises a barrier material 41 that separates one of the two gas composition sensors 40 from the flow of gas.

[0067] The barrier material 41 may have a thickness less than 10, 20, 30, 40, 50, 100, 200, 300, 400, or 500 pm pm, and useful ranges may be selected from any of these values. For example, the barrier material 41 may have a thickness between 10 pm and 500 pm. The barrier material 41 may have a thickness between 20 pm and 400 pm. The barrier material 41 may have a thickness between 30 pm and 300 pm. The barrier material 41 may have a thickness between 40 pm and 200 pm. The barrier material 41 may have a thickness between 35 pm and 100 pm. The barrier material 41 may have a thickness less than 10, 20, 30, 40, or 50 pm. Suitable ranges may be selected from between any of these values. For example, the barrier material 41 may have a thickness of 50 pm.

[0068] The barrier material may comprise Nation™. Nation™ is a perfluorinated polymer that consists of a polytetrafluoroethylene (PTFE) backbone and regularly spaced long perfluorovinyl ether pendant side chains terminated by sulfonic or carboxylic ionic functional groups. Nation™ allows molecular diffusion of water molecules through it but not methane or other atmospheric gases. It will be appreciated that Nation™ is one example of a suitable barrier material, and that any barrier material can be used provided that it is selectively impermeable for the gas or gases to be sensed.

[0069] The minimum practical thickness of the barrier material 41 may be limited by the strength of the material. If the material is too thin, it may be damaged during manufacturing.If the material is too thin, it may comprise holes that allow unwanted transport of gases across the barrier material.

[0070] The maximum practical thickness of the barrier material 41 may be limited by the transport rate of molecules through the material. If the material is too thick, the response time of the two gas composition sensors 40 may be significantly different.

[0071] The barrier material 41 may be configured to allow water molecules to pass through (from the flow of gas to the measurement end of the gas composition sensor 40).

[0072] If the barrier material 41 is configured to allow water molecules to pass through, one of the two gas composition sensors 40 will respond to the one or more gases to be sensed and the humidity of the flow of gas, and the other gas composition sensor 40 will only respond to the humidity of the flow of gas. In this way, the gas sensing system 1 may (at least partially) isolate the effect of humidity from the determination of the concentration of the specific gas.

[0073] Similarly, if the barrier material 41 is configured to allow water molecules to pass through, and is configured to provide only a negligible thermal barrier (by being sufficiently thin for example), one of the two gas composition sensors 40 will respond to the one or more specific to be sensed, and the humidity and temperature of the flow of gas, and the other gas composition sensor 40 will only respond to the humidity and temperature of the flow of gas. In this way, the gas sensing system 1 may (at least partially) isolate the effects of humidity and temperature from the determination of the concentration of the specific gas.

[0074] The barrier material 41 may be configured to prevent water molecules from passing through (from the flow of gas to the measurement end of the gas composition sensor 40).

[0075] If the barrier material 41 is configured to prevent water molecules from passing through, and is configured to provide only a negligible thermal barrier (by being sufficiently thin for example), one of the two gas composition sensors 40 will respond to the one or more specific gases to be sensed, and the humidity and temperature of the flow of gas, and the other gas composition sensor 40 will only respond to the temperature of the flow of gas. In this way, the gas sensing system 1 may (at least partially) isolate the effect of temperature from the determination of the concentration of the specific gas.

[0076] The one or more gas conditioning features may comprise a dehumidifier (e.g. a chemical dehumidifier or a mechanical dehumidifier). The dehumidifier may be particularly useful if the barrier material 41 is configured to allow water molecules to pass through, because, in such a case, the barrier material 40 will not enable the gas sensing system 1 to isolate the effect of humidity from the determination of the concentration of the specific gas. Therefore, in such a case, dehumidifying the flow of gas before it reaches the gas composition sensors 40 will benefit the accuracy of the sensing system 1.

[0077] In particularly humid environments, it may not be practical because it may quickly become saturated or may require a significant amount of energy. In such environments, it may be more practical and / or economical to isolate the effect of humidityfrom the determination of the concentration of the specific gas by using a barrier material 41 is configured to allow water molecules to pass through (as described above).

[0078] As mentioned, the gas sensing system 1 may comprise one or more gas conditioning features configured to condition the flow of gas before it reaches the gas composition sensors 40.

[0079] The one or more gas conditioning features may comprise a length of tubing 30 that forms a section of the flow path 10. The length of tubing 30 may be formed from a material that allows water molecules to pass through a wall of the length of tubing 30 (to or from the flow path).

[0080] The length of tubing 30 may flatten out fluctuations in the humidity of the flow of gas over time. In this way, the gas sensing system 1 may (at least partially) isolate the effect of variations in humidity from the determination of the concentration of the specific gas. This effect is demonstrated in Figure 4 which shows the effect of the presence of Nation™ tubing which increases the stability of the baseline with respect to changes in ambient humidity. Figure 2 shows this effect comparing the first half of the test in which air passed from activated carbon directly to the sensor housing, to the second half of the test in which a 20 cm length of Nation™ tubing was inserted between the activated carbon and the sensor housing.

[0081] The length of tubing 30 may have a length in the range of 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 cm, and suitable ranges may be selected from between any of these values cm. For example, the length of tubing 30 may have a length in the range of 5 cm to 50 cm The length of tubing 30 may have a length in the range of 10 cm to 45 cm. The length of tubing 30 may have a length in the range of 15 cm to 50 cm. The length of tubing 30 may have a length in the range of 20 cm to 35 cm. The length of tubing 30 may have a length in the range of 20 cm to 30 cm. The length of tubing 30 may have a length of 25 cm.

[0082] The minimum practical length may be limited by the transport rate of water molecules across the wall of the length of tubing 30. If the length is too short, the length of tubing 30 may not facilitate transport of a useful amount of water molecules. Therefore, the length of tubing 30 may not effectively flatten out fluctuations in the humidity of the flow of gas over time.

[0083] The maximum practical length may be limited by the space within the system casing 2.

[0084] The one or more gas conditioning features may comprise one or more filters.

[0085] The one or more gas conditioning features may comprise one or more scrubbers 20.

[0086] The one or more scrubbers 20 may be used to reduce the concentration of substances within the flow of gas that are not the specific gas to be sensed. This effect is demonstrated by Figure 5 which shows the response to 20 ppm hydrogen. Without the scrubber the system provides a reading of 14 ppm. When a Pd doped activated carbon scrubber is added the hydrogen concentration reduces to 9 ppm, showing the scrubber can effectively remove 5 ppm hydrogen from the sampled air. Ambient hydrogen concentration is approximately 0.6 ppm.

[0087] In this way, the one or more scrubbers 20 may improve the sensing accuracy of the sensing system 1 as they may reduce the amount by which the gas composition sensors 40 respond to molecules that do not belong to the specific gas to be sensed. That is, the one or more scrubbers 20 may reduce false positive sensor readings.

[0088] The one or more scrubbers 20 may comprise an activated carbon scrubber. The activated carbon scrubber may be configured to reduce the amount of volatile organic compounds within the flow of gas.

[0089] The one or more scrubbers 20 may comprise a palladium-doped scrubber.

[0090] The one or more scrubbers 20 may comprise a palladium-doped alumina scrubber.

[0091] The one or more scrubbers 20 may comprise a palladium-doped silica.

[0092] The one or more scrubbers 20 may comprise a palladium-doped activated carbon scrubber. The palladium-doped activated carbon scrubber may be configured to reduce the concentration of hydrogen within the flow of gas. The palladium-doped activated carbon scrubber may have a palladium weight percentage in the range of 1% to 10%. The palladium-doped activated carbon scrubber may have a palladium weight percentage in the range of 1% to 9%. The palladium-doped activated carbon scrubber may have a palladium weight percentage in the range of 2% to 8%. The palladium-doped activated carbon scrubber may have a palladium weight percentage in the range of 3% to 7%. The palladium- doped activated carbon scrubber may have a palladium weight percentage in the range of 4% to 6%. The palladium-doped activated carbon scrubber may have a palladium weight percentage of 5%. The palladium-doped activated carbon scrubber may have a palladiumweight percentage less than 20, 15, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1%, and suitable ranges may be selected from between any of these values.

[0093] The one or more scrubbers may be configured to reduce the concentration of any other substances. For example, substances that may be prevalent at common gas sensing environments, for example oil and gas drilling or refining sites, or waste / refuse sites. Other such substances may include butane, ethanol, and hydrogen sulfide.

[0094] The one or more scrubbers 20 and / or filters may be housed in a scrubber subhousing 21. The scrubber sub-housing 21 may keep the one or more scrubbers 20 and / or filters in an effective position. The scrubber sub-housing 21 may help to prevent any materials captured by the one or more scrubbers 20 and / or filters from entering other parts of the gas sensing system 1.

[0095] As mentioned, the gas sensing system 1 comprises two gas composition sensors 40 positioned within the flow path 10. One of the two gas composition sensors 40 is separated from the flow of gas by the barrier material 41

[0096] One or both of the two gas composition sensors 40 may be a metal oxide sensor.

[0097] Without wishing to be bound by theory, the metal oxide may produce a current in the presence of target gas, enabling the detection of the target gas based on the amount of current produced.

[0098] When the metal oxide sensor is heated in air at high temperature, oxygen is adsorbed on the particle surface by capturing free electrons. The adsorbed oxygen has a high electron affinity and so pulls electrons towards the surface layer of absorbed oxygen. This forms a potential barrier at the grain boundary, preventing the flow of electric current.

[0099] The target gas can react with the absorbed oxygen. This reaction causes the density of the absorbed oxygen to decrease which in turn causes the release of electrons. As oxygen surface density decreases and electrons are released, current can flow between the grain boundary of tin dioxide crystals. Thus, the amount of current flowing through the sensor can be used to determine the amount of methane present in the air.

[0100] One or both of the gas composition sensors 40 may be a Tin(IV) oxide (SnO2) sensor. It may be practical to use one or more Tin(IV) oxide sensors because, relative to some other metal oxide sensors, Tin(IV) oxide sensors are simple to fabricate.

[0101] One or both of the gas composition sensors 40 may be a Zinc oxide sensor.

[0102] One or both of the gas composition sensors 40 may be a Titanium dioxide sensor.

[0103] One or both of the gas composition sensors 40 may be a heated metal oxide sensor containing a metal oxide material with an operating resistance less than 10, 100, 1,000, 10,000, 100,000, 1,000,000, or 10,000,000 ohms, and suitable ranges may be selected from between any of these values.

[0104] The two gas composition sensors 40 may be housed in a sensor sub-housing42. As shown in Figure 2 and Figure 3, the sensor sub-housing 42 may comprise a sensor support block 43.

[0105] The sensor support block 43 may comprise a base surface 44. The sensor support block 43 may comprise a through hole 45 passing through the sensor support block43. The through hole 45 may be configured to form a section of the flow path 10 from an upstream through hole inlet 46 to a downstream through hole outlet 47. The sensor support block 43 may comprise two sensor holes 50 configured to receive the two gas composition sensors 40.

[0106] The sensor support block 43 may comprise a through hole inlet thread 48 at the through hole inlet 46. The through hole inlet thread 48 may be configured to mate with a corresponding thread on an upstream section of the flow path 10. The sensor support block 43 may comprise a through hole outlet thread 49 at the through hole outlet 47. The through hole outlet thread 49 may be configured to mate with a corresponding thread on a downstream section of the flow path 10. These threaded connections may help to prevent leaks between the sensor support block 43 and the upstream and downstream sections of the flow path 10.

[0107] Each of the two sensor holes 50 may be a blind hole that begins at the base surface 44. Each of the two sensor holes 50 may extend from the base surface 44 to the through hole 45. Each of the two sensor holes 50 may comprise a sensor hole shoulder 51 part way between the base surface 44 and the through hole 45.

[0108] Each sensor hole 50 is configured to position the measurement end of one of the gas composition sensors 40 within the flow path. Each sensor hole shoulder 51 is configured so that, when the corresponding gas composition sensor 40 is inserted from the base surface end, the sensor hole shoulder 51 mechanically prevents the gas compositionsensor 40 from moving any closer to the through hole 45. Therefore, the sensor hole shoulders 51 may contribute to the accurate and consistent positioning of the gas composition sensors 40 relative to the flow path 10.

[0109] The gas composition sensor 40 that is not separated from the flow of gas by the barrier material 41 may directly contact the corresponding sensor hole shoulder 51.

[0110] The gas composition sensor 40 that is separated from the flow of gas by the barrier material 41 may be separated from the corresponding sensor hole shoulder 51 by the barrier material 41. The gas composition sensor 40 that is separated from the flow of gas by the barrier material 41 may be separated from the corresponding sensor hole shoulder 51 by the barrier material 41 and a sensor seal that is configured to be positioned between the gas composition sensor 40 and the barrier material 41.

[0111] The sensor seal may comprise, for example, a sensor O-ring.

[0112] The distance between the gas composition sensor 40, that is separated from the flow of gas by the barrier material 41, and the barrier material 41 may be between 5 mm and 15 mm. This distance may be between 6 mm 14 mm. This distance may be between 7 mm 13 mm. This distance may be between 8 mm 12 mm. This distance may be between 9 mm 11 mm. This distance may be 10 mm. This distance may be less than 25, 20, 15, 10, or5 mm, and suitable ranges may be selected from between any of these values.

[0113] The distance between the gas composition sensor 40, that is separated from the flow of gas by the barrier material 41, and the barrier material 41 may help to prevent any heat generated within the gas composition sensor 40 from affecting the properties of the barrier material 41.

[0114] The sensor sub-housing 42 may comprise a circuit board 52. The circuit board 52 may be configured to be attached to the base surface 44. The circuit board may be configured to be in electrical communication with the two gas composition sensors 40 and the controller 80.

[0115] The circuit board 52 may be configured to mechanically prevent the gas composition sensors 40 from moving any further from the through hole 45.

[0116] The circuit board 52 may be attached to the sensor support block 43 with one or more fixtures. The one or more fixtures may interact with the sensor support block 43 via one or more fixture holes 55 in the sensor support block 43.

[0117] The sensor support block 43 may be formed from a single piece of polymer. Using a single piece of polymer may help to prevent any leaks between the sensor subhousing 42 and the rest of the gas sensing system 1.

[0118] The sensor sub-housing 42 may further comprise a main seal 53. The main seal 53 may be positioned between sensor support block 43 and the circuit board 52. The main seal 53 may form a seal between the sensor support block 43 and the circuit board 52.

[0119] The main seal 53 may help to prevent any leaks between the sensor subhousing 42 and the rest of the gas sensing system 1.

[0120] The sensor support block 43 may define a main seal channel 54 configured to locate the main seal 53. The main seal channel 54 may be formed in the base surface 44.

[0121] The main seal may be, for example, a main O-ring.

[0122] It may be useful for each of the gas composition sensors 40 to include a heater.

[0123] Each of the gas composition sensors 40 may comprise an internal heater 60. Each internal heater 60 is configured to be in electrical communication with the controller 80. The controller is configured to control each internal heater 60 to maintain each gas composition sensor 40 at a temperature above a temperature of the environment 3.

[0124] The controller may control each internal heater 60 so that the temperature of its gas composition sensor 40 is maintained at a steady temperature between 250 °C and 600 °C. The controller may control each internal heater 60 so that the temperature of its gas composition sensor 40 is maintained at a steady temperature between 250 °C and 500 °C. The controller may control each internal heater 60 so that the temperature of its gas composition sensor 40 is maintained at a steady temperature between 250 °C and 400 °C. The controller may control each internal heater 60 so that the temperature of its gas composition sensor 40 is maintained at a steady temperature between 250 °C and 350 °C. The controller may control each internal heater 60 so that the temperature of its gas composition sensor 40 is maintained at a steady temperature of approximately 300 °C. The controller may control each internal heater 60 so that the temperature of its gas composition sensor 40 is maintained at a steady temperature above 250 °C. Suitable ranges may be selected from between any of these values.

[0125] The gas sensing system 1 may comprise a Wheatstone Bridge circuit. The controller 80 may be configured to use the Wheatstone Bridge circuit to control the current supplied to each of the internal heaters 60. In this way, the controller 80 may maintain eachgas composition sensor 40 at a constant temperature independent of a surrounding temperature (e.g. independent of the temperature of the environment 3 or a temperature within the remainder of the system casing 2).

[0126] Maintaining each gas composition sensor 40 at an elevated temperature may improve sensing performance by reducing the resistivity of the metal oxide within each of the gas composition sensors 40. Reducing the resistivity may in turn allow changes in the resistivity to be more accurately quantified. This is demonstrated by Figure 6 which shows the impact of correcting for temperature.

[0127] Maintaining each gas composition sensor 40 at an elevated temperature may reduce the sensor response time by speeding up the reactions between the one or more specific gases to be measured and the metal oxide within each of the gas composition sensors 40.

[0128] The gas sensing system 1 may comprise a flow orifice 75 within the flow path 10. The flow orifice 75 may be configured to limit (and thereby control) the flow rate of the flow of gas.

[0129] The flow orifice 75 may be configured to be positioned downstream of the gas composition sensors 40. The flow orifice may be configured to be positioned upstream of the flow generator 70.

[0130] The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to between 0.05 and 0.5 LPM. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to between 0.05 and 0.4 LPM. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to between 0.05 and 0.3 LPM. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to between 0.05 and 0.2 LPM. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to between 0.05 and 0.1 LPM. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to between 0.065 LPM and 0.075 LPM. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas to be less than 0.5, 0.4, 0.3, 0.2, 0.1, 0.09, 0.08, 0.07, 0.06, or 0.05 LPM. Suitable ranges may be selected from between any of these values. The flow orifice 75 and / or the flow generator 70 may be configured to control the flow rate of the flow of gas at 0.07 LPM.

[0131] The minimum practical flow rate may be limited by a desired response time of the gas composition sensors 40. If the flow rate is too low, it may take an impractical amount of time for molecules of gas entering the inlet 11 to reach the gas composition sensors 40. Therefore, it may take an overly long time for the gas composition sensors 40 to respond to changes in the molecules of gas entering the inlet 11.

[0132] The maximum practical flow rate may be limited by the speed at which the one or more conditioning features can effectively condition the flow of gas. For example, if the flow rate is too high, the one or more scrubbers 20 may not be able to effectively reduce the concentration of substances within the flow of gas that are not the specific gas to be sensed. Alternatively or additionally, if the flow rate is too high, the length of tubing 30 may not be able to effectively flatten out fluctuations in the humidity of the flow of gas over time.

[0133] The maximum practical flow rate may be limited by the transport rate of molecules through the barrier material 41. For example, if the flow rate is too high, it may take an impractical amount of time for the gas sensing system 1 to respond to changes in humidity within the flow of gas.

[0134] As mentioned, the gas sensing system 1 can determine a concentration of one or more specific gases.

[0135] The controller 80 may be configured to determine the concentration of the specific gas to be sensed by first calculating a raw sensor resistance parameter, Rraw. This raw sensor resistance parameter may be calculated as a function of the ratio of a resistance reading from one of the gas composition sensors to a resistance reading from the other gas composition sensor. For example, this raw sensor resistance parameter may be calculated with the equation,„ _ RREF“raw ~KDGAS where RREFis the sensor resistance of the gas composition sensor 40 that is separated from the flow of gas by a barrier material 41 and RGASis the resistance of the other gas composition sensor 40.

[0136] The controller 80 may be configured to then calculate a corrected sensor resistance parameter, R, that considers the resistance of the internal heaters 60. This resistance parameter may be calculated with the equationwhere Rheater 's arolling average of the sum of the resistances of the internal heaters 60, Ravgis a time average of the parallel heater resistance of the two internal heaters 60 at 30 °C, and Sheateris constant representing the sensitivity of the resistance of the internal heaters 60 to the changes in the temperature of the gas composition sensors 40.

[0137] The controller may then be configured to calculate the concentration of the specific gas to be sensed by inputting the resistance parameter, R, into a polynomial. The polynomial may be a cubic polynomial.EXAMPLE

[0138] This example describes the effects of a gas sensing system, as shown in Figure 1, for determining the concentration of methane.

[0139] The gas sensing system comprised the following features.■ Scrubber comrpising activated carbon and Pd doped activated carbon■ Nation tubng downstream of the scrubber and upstream of the sensors■ A sensor housing comrpsing Figaro CH4 sensors and an adaptor board■ Downstreat tubing and a flow offifice with a flow rate of 0.07 LPM■ Pump

[0140] The accuracy of gas sensing systems can be detrimentally affected by humidity and / or the presence of other molecules or compounds within the environment to be measured. The instrument features compensation for temperature and humidity and selective scrubbers for cross-interferences such as atmospheric hydrogen. This is important since there can be large sensor drift due to temperature and humidity. Thus, sub-ppm methane sensitivity can be lost in baseline variation as shown in Figure 7. The enlarged view of the resistance curve in Figure 7 shows the small peaks in sensor resistance (required to be detected) relative to the baseline variation. A benefit of the gas sensing system as described is that it allows for modulation of the gas path to measure baseline and gas concetration simultaneously.

[0141] The methane sensor uses a dual palladium doped SnC>2 sensor design to meature methane. A pump pulls air into the module through the inlet and out the exhaust . A flow orifice controls the flow rate at 0.07 ± 0.005 LPM. The air stream first passes through a combined activated carbon and 5 wt% Pd doped activated carbon scrubber. These scrubbers remove important interfering compounds. The carbon scrubber removes volatileorganic compounds and the Pd doped activated carbon scrubber removes hydrogen but not methane.

[0142] The gas sensing system provides high sensitivity (<1 ppm detection limit) and a fast response time.

[0143] After the scrubber the sample air passes through a short length of nation tubing, which buffers fluctuations in air humidity. Then the air enters the sensor housing which holds two SnC>2 sensors in proximity to each other. The SnC>2 sensors operate at elevated temperature (about 300 °C) and have internal heaters. The two sensor heaters are controlled by a Wheatstone Bridge circuit to maintain a constant operating temperature in each sensor, independent of the surrounding temperature.

[0144] The sensor housing is designed so that one SnC>2 sensor (SRG) is exposed to the air stream, while the other sensor (SR0) has a barrier membrane (in the form of a Nation™ membrane) between it and the air steam, which isolates it from methane, but allows the sensor to be exposed to the same humidity as sensor SRG.

[0145] Figure 7 demonstrates the improved sensitivity achieved by the present apparatus. As shown, the resistance of the reference sensor SRO remains constant and is thus used as a baseline reference for SRG and enables a stable baseline and lower detection limit for the instrument.

[0146] Although embodiments have been described with reference to a number of illustrative embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as herein described with reference to the accompanying drawings.

Claims

CLAIMS:

1. A gas sensing system for determining a concentration of a specific gas within an environment, comprising: a flow path for a flow of gas from the environment to be sensed, an inlet for receiving the flow of gas into the flow path, one or more conditioning features for the flow of gas within the flow path, two gas composition sensors in proximity to one another and both configured to measure the concentration of a specific gas within the flow of gas downstream of the one or more gas conditioning features, a flow generator configured to generate the flow of gas, a controller configured to be in electrical communication with the gas composition sensors and the flow generator, and wherein one of the two gas composition sensors is separated from the flow of gas by a barrier material that prevents molecules of the specific gas to be sensed from passing through.

2. The gas sensing system of claim 1, wherein the barrier material has a thickness less than 500 pm.

3. The gas sensing system of claim 1 or claim 2, wherein the barrier material allows water molecules to pass through.

4. The gas sensing system of claim 1 or claim 2, wherein the barrier material prevents water molecules from passing through.

5. The gas sensing system of claim 4, wherein the one or more gas conditioning features comprises a dehumidifier.

6. The gas sensing system of any one of claims 1 to 5, wherein the one or more gas conditioning features comprises a length of tubing forming a section of the flow path, wherein the length of tubing is formed from a material that allows water molecules to pass through a wall of the length of tubing.

7. The gas sensing system of claim 6, wherein the length of tubing has a length in the range of 5 cm to 50 cm.

8. The gas sensing system of any one of claims 1 to 7, wherein the one or more gas conditioning features comprises one or more scrubbers.

9. The gas sensing system of claim 8, wherein the one or more scrubbers comprises an activated carbon scrubber configured to reduce the amount of volatile organic compounds within the flow of gas.

10. The gas sensing system of claim 8 or claim 9, wherein the one or more scrubbers comprises a palladium-doped activated carbon scrubber configured to reduce the concentration of hydrogen within the flow of gas.

11. The gas sensing system of claim 10, wherein the palladium-doped activated carbon scrubber has a palladium weight percentage in the range of 3% to 7%.

12. The gas sensing system of any one of claims 8 to 11 , wherein the one or more scrubbers are housed in a scrubber sub-housing.

13. The gas sensing system of any one of claims 1 to 12, wherein the two gas composition sensors are metal oxide sensors.

14. The gas sensing system of any one of claims 1 to 13, wherein the two gas composition sensors are housed in a sensor sub-housing comprising: a sensor support block comprising: a base surface, a through hole passing through the sensor support block and configured to form a section of the flow path, and two sensor holes beginning at the base surface and extending into the through hole, wherein each sensor hole is configured to position the measurement end of one of the gas composition sensors within the flow path, and a circuit board configured to be attached to the base surface of the sensor support block and to be in electrical communication with the two gas composition sensors and the controller.

15. The gas sensing system of claim 14, wherein the sensor support block is formed from a single piece of polymer.

16. The gas sensing system of claim 14 or claim 15, wherein the sensor sub-housing further comprises a main seal configured to form a seal between the sensor support block and the circuit board, and wherein the sensor support block defines a main seal channel configured to locate the main seal.

17. The gas sensing system of any one of claims 1 to 16, wherein each of the two gas composition sensors comprises an internal heater configured to be in electrical communication with the controller, and wherein the controller is configured to control each internal heater to maintain each gas composition sensor at a temperature above 250 °C.

18. The gas sensing system of any one of claims 1 to 17, wherein the controller is configured to determine the concentration of the specific gas to be sensed by inputting a sensor resistance parameter, calculated as a function of the ratio of a resistance reading from one of the gas composition sensors to a resistance reading from the other gas composition sensor, into a calibrated polynomial function which relates resistance to methane concentration.

19. The gas sensing system of claim 17, wherein the controller is configured to determine the concentration of the specific gas to be sensed within the flow of gas by:• calculating a sensor resistance parameter as a function of the ratio of a resistance reading from one of the gas composition sensors to a resistance reading from the other gas composition sensor,• subtracting a heater resistance parameter from the sensor resistance parameter, wherein the heater resistance parameter is a function of a rolling average of the sum of resistances of the internal heaters, and• inputting the result into a calibrated polynomial function which relates resistance to methane concentration.

20. A method for determining a methane concentration within an environment, comprising: generating a flow of gas from the environment into a flow path of a sensing system using a flow generator, reducing the concentration of non-methane substances within the flow of gas by passing the flow of gas through one or more scrubbers positioned within the flow path, sensing the concentration of methane within the flow of gas with two gas composition sensors in proximity to one another and both configured to measure the concentration of methane within the flow of gas downstream of the one or more scrubbers, wherein one of the two gas composition sensors is separated from the flow of gas by a barrier material that allows water molecules to pass through but prevents methane from passing through, and determining the methane concentration within the flow of gas with a controller that is in electrical communication with the gas composition sensors, wherein the controller determines the methane concentration by inputting a sensor resistance parameter, calculated as a function of the ratio of a resistance reading from one of the gas compositionsensors to a resistance reading from the other gas composition sensor, into a calibrated polynomial function which relates resistance to methane concentration.

21. The method of claim 20, further comprising reducing fluctuations in the humidity of the flow of gas by passing the flow of gas through a length of tubing upstream of the two gas composition sensors, wherein the length of tubing is formed from a material that allows water molecules to pass through a wall of the length of tubing.

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