Abstract:
A gas sensor of the type that detects the presence of a specific gas by monitoring the absorption of optical radiation transmitted through a chamber containing a sample of gas under test comprises an optical source for emitting radiation therefrom and a detector sensitive to radiation emitted from the source at opposing ends of a circumferential chamber, having optically reflective surfaces, extending around the periphery of a sensor housing. The optical pathway between the source and detector may include a radial portion, a circumferential portion and an axial portion to allow a compact optical path. The gas sensor includes, within a single housing, electronic circuitry for conditioning the electrical output of the detector to provide an output that is a function of at least one selected gas concentration and which is automatically compensated for at least one of temperature, pressure, humidity, and range normalization.

Description:
RELATED APPLICATIONS 
   This application claims the benefit, under 35 U.S.C. §119, of GB 0328684.6, filed Dec. 9, 2003, and GB 0414127.1, filed Jun. 24, 2004. 
   FIELD OF THE INVENTION 
   The present invention relates to gas sensors, and in particular to gas sensing devices that detect the presence of a specific gas by monitoring the absorption of optical radiation transmitted through a chamber containing a sample of gas under test. 
   BACKGROUND OF THE INVENTION 
   Gas sensors utilizing an infra-red source and a corresponding infra-red detector are well known, in particular in the design of, for example, carbon dioxide and hydrocarbon gas detectors. Infra-red radiation emitted by the source is focused onto the detector, having passed through a chamber containing the gas under test, where some of the infra-red radiation will be absorbed by the gas. The absorption by a specific gas is a function of the wavelength of the infra-red radiation, and by careful selection of an appropriate optical band-pass filter at the detector, it is possible to determine the presence of a specific gas. 
   A particularly compact form of optical gas sensor has been described in GB 2372099 B to Dynament Limited, and is shown in  FIGS. 1 to 3 . A gas sensor  1  comprises an optical source  2  for emitting radiation in the optical spectrum and a detector  3  for detection of radiation emitted by the source  2 . The source  2  and detector  3  are respectively located at opposite ends of an optical pathway  4  ( FIG. 2 ) which pathway is defined by a circumferential chamber  5  and a central chamber  6  respectively defining a generally circumferential portion  4   a  of the optical pathway  4  and a generally radial portion  4   b  of the optical pathway. 
   As best seen in  FIG. 3 , the circumferential chamber  5  is defined by: a chamber base  7 ; an internal surface of an outer cylindrical wall  8  of the sensor housing; an external surface of an inner cylindrical wall  9  of the sensor housing; and a radial end wall  10 . The central chamber  6  is defined by an internal surface of the housing base  11  and an internal surface of the inner cylindrical wall  9  of the sensor housing. The housing base  11  provides a planar reflective surface, in the central chamber  6 . 
   Optical communication between the circumferential chamber  5  and the central chamber  6  is by way of a gap  12  in the inner cylindrical wall  9 . To enhance reflection of radiation from the circumferential chamber  5  to the central chamber  6 , a deflector element  13  provides a reflecting surface  14  which generally extends from the outer cylindrical wall  8  to the inner cylindrical wall  9 . 
   The top  16  of the sensor housing includes a gas permeable window  17  to allow controlled diffusion of gas under test from the external ambient of the sensor housing to the optical pathway  4  in the chambers  5  and  6 . The gas permeable window  17  typically comprises a disc shaped element of sintered flame arresting material that allows diffusion of gas but forms a combustion barrier so that the source  2  cannot accidentally act as an ignition source when the sensor is operating in a hazardous and combustible gaseous environment. 
   The detector  3  is mounted in the base  11  of the sensor housing and comprises a dual element pyroelectric detector. The detector elements  3   a ,  3   b  are arranged in a spaced relationship along a vertical axis V of the sensor housing, i.e. an axis parallel to the central axis defined by the inner and outer cylindrical walls  8 ,  9 . This axial spacing of the detector elements  3   a ,  3   b  ensures that the characteristics of the optical pathway leading to each of the elements are substantially similar. Each element  3   a ,  3   b  includes a filter (not shown) to allow the transmission of optical radiation at selected frequencies or frequency ranges. This dual element configuration enables the sensor to operate with one reference or compensation detector to increase accuracy of the measurements, as will be described hereinafter. 
   In use, the incandescent source  2  emits infra-red radiation over a broad spectrum of frequencies. The reflective surfaces formed by the inner and outer cylindrical walls  8 ,  9  and the radial end wall  10  guide the infra-red radiation around the circumferential chamber  5 . The non-focussing nature of the reflector surfaces means that positioning of the source  2  within the circumferential chamber  5  is not critical. Once the radiation reaches the other end of the circumferential chamber  5 , via optical pathway  4   a , radiation is reflected off the reflecting surface  14  of deflector  13  onto the radial inward optical path  4   b , towards the detector elements  3   a ,  3   b.    
   A potential disadvantage with optical gas sensors, as opposed to other types of gas sensor, is that the detector output is not directly related to gas concentration. Therefore, complex signal processing must ordinarily be performed on the detector output in order for it to provide a reliable and accurate signal indicating the gas concentration. 
   In conventional gas detection equipment, this complex signal processing is generally carried out by electronics that is external to the sensor housing. The reason for this is typically an issue of space. It is desirable to adhere to industry standard dimensions in the construction of sensor housings to ensure backward compatibility with installed gas detection equipment. 
   Even though some limited signal processing may be carried out within the sensor housing, this is generally limited to relatively simple and straightforward functions such as zero adjustment, rectification and filtering to remove noise from the output. These functions do not require extensive signal processing capacity. Typically, these functions are carried out in the analogue domain. More complex processing such as the derivation of the gas concentration and linearization of the output signal taking into account temperature compensation, pressure compensation and other functions have hitherto been performed remote from the sensor housing. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a highly compact optical path arrangement for gas sensor. 
   It is a further object of the present invention to provide an integrated gas sensor which provides both optical gas sensing components as well as signal processing electronics within a single sensor housing. 
   According to one aspect, the present invention provides a gas sensor comprising: 
   an optical source for emitting radiation therefrom; 
   a detector sensitive to radiation emitted from the source; 
   an optical pathway extending between the source and the detector; 
   a chamber having optically reflective surfaces defining a substantially circular portion of the optical pathway, a substantially radial portion of the optical pathway, and a substantially axial portion of the optical pathway; 
   at least a first reflector oriented generally at an oblique angle to the circular portion to separate the circular portion and the radial portion, and 
   at least a second reflector oriented generally at an oblique angle to the plane of the circular portion of the optical pathway to separate the radial portion from the axial portion. 
   According to another aspect, the present invention provides an integrated gas sensor comprising, within a single housing: 
   an optical source for emitting radiation therefrom; 
   a detector sensitive to radiation emitted from the source; 
   an optical pathway extending between the source and the detector; and 
   electronic circuitry for conditioning the electrical output of the detector to provide a sensor output that is a function of at least one selected gas concentration and which is automatically compensated for at least one of temperature, pressure, humidity, and range normalization. 
   According to another aspect, the present invention provides an integrated gas sensor comprising, within a single housing: 
   an optical source for emitting radiation therefrom; 
   a detector sensitive to radiation emitted from the source; 
   an optical pathway extending between the source and the detector; and 
   electronic circuitry for conditioning the electrical output of the detector to provide a sensor output signal that has been mathematically processed to take into account at least one of temperature, pressure, humidity and range normalization, and that is directly proportional to at least one selected gas concentration. 

   
     DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic perspective view of a prior art optical gas sensor showing internal detail; 
       FIG. 2  is a plan view of the sensor of  FIG. 1 , with the top cover removed; 
       FIG. 3  is a cross-sectional side view taken on line A—A; 
       FIG. 4  is a cross-sectional side view of a modified optical gas sensor having substantially reduced headroom for the optical components, according to one embodiment of the present invention; 
       FIG. 5  is a plan view of the sensor of  FIG. 4 , with the top cover removed; 
       FIG. 6  is a perspective side and top view of the sensor of  FIG. 4 , with the top and side walls of the sensor housing removed; 
       FIG. 7  is a partial cross-sectional side view of the sensor of  FIG. 4 , on a section that is orthogonal to the section of  FIG. 4 ; and 
       FIG. 8  is a schematic diagram of internal components of the optical gas sensor of  FIG. 4 . 
   

   DESCRIPTION OF THE INVENTION 
   The optical gas sensor of  FIGS. 1 to 3  has been described in detail above. As will now be described in connection with  FIGS. 4 to 7 , a significant modification to the layout of the optical components of  FIGS. 1 to 3  has been made which substantially decreases the headroom required for these optical components. This, in turn, allows for a substantial increase in the space available for electronic components, so as to enable the inclusion of signal processing circuitry, for a given sensor housing size. Alternatively, if no additional electronic components are required other than those already provided within the sensor housing of  FIG. 1 , then the arrangement of  FIGS. 4 to 7  allows construction of a very low profile sensor housing. 
   Throughout the present specification, expressions of relative position such as “top”, “bottom”, “cap”, “base”, “up”, “down” etc, are used solely for convenience and clarity in relation to the sensor as oriented in the drawings. They are in no way intended to be limiting as to the orientation of use of the sensors described. 
   With reference to  FIGS. 4 to 7 , a gas sensor  41  comprises a non-focused optical source  42  for emitting radiation in the optical spectrum. The expression “optical” is intended to cover all parts of the electromagnetic spectrum that are useful for the function of gas detection by absorption and includes the infra-red, visible, and ultra-violet ranges of the electromagnetic spectrum. The source  42  is preferably of the incandescent variety, producing a broad range of frequencies with which to measure absorption characteristics, but may also be of the solid state variety such as diodes producing limited frequencies or frequency bands. 
   The gas sensor  41  further comprises a detector  43  for detection of radiation emitted by the source  42 . The detector  43  may be of any suitable type for sensing variations in intensity of radiation received from the source and providing as output a voltage or current as a function thereof. In a preferred embodiment, operating in the infra-red spectrum, the detector  43  is a pyroelectric detector. 
   The source  42  and detector  43  are respectively located at opposite ends of an optical pathway  44  ( FIGS. 5 and 6 ) which pathway is defined by a circumferential chamber  45  and a central chamber  46  respectively defining a generally circumferential portion  44   a  of the optical pathway  44  and a generally radial portion  44   b  of the optical pathway. 
   The circumferential chamber  45  is defined by: a chamber base  47 ; an internal surface of an outer cylindrical wall  48  of the sensor housing; an external surface of an inner cylindrical wall  49  of the sensor housing; and a radial end wall  50 . Preferably, the chamber base  47  provides a planar reflective surface. Preferably, the chamber base  47  provides a gas-tight fit against the sensor housing outer cylindrical wall  48  by way of an o-ring seal  74 . 
   Although in the preferred embodiment the walls of the circumferential chamber  45  are formed from cylindrical walls  48 ,  49 , it will be understood that some departure from smooth convex and concave surfaces is possible, for example using a multifaceted configuration to form generally circumferential walls. The circumferential walls  48 ,  49  could also be concave or convex along the axial direction. The radial end wall  50  is preferably planar. 
   The central chamber  46  is defined by an internal surface of the housing base  51  and an internal surface of the inner cylindrical wall  49  of the sensor housing. The housing base  51  provides a planar reflective surface, in the central chamber  46 . Although, in the preferred embodiment, the curved wall of the central chamber  46  is formed from the inner cylindrical wall  49 , it will be understood that some departure from a smooth concave surface is possible, for example using a multifaceted configuration to form the internal surface. The internal surface of the inner cylindrical wall  49  could also be concave or convex along the axial direction. 
   Optical communication between the circumferential chamber  45  and the central chamber  46  is by way of a gap  52  in the inner cylindrical wall  49 . To enhance reflection of radiation from the circumferential chamber  45  to the central chamber  46 , a deflector element  53  provides a reflecting surface  54  which generally extends from the outer cylindrical wall  48  to the inner cylindrical wall  49 . The reflecting surface  54  is preferably planar. The reflecting surface  54  is generally oblique to the tangent of the outer and inner circumferential walls  48 ,  49  at the position of the gap  52 , but may also be radial. 
   The deflector  53  is preferably formed from a wedge shaped element which also forms the radial end wall  50 . The wedge shaped element may also be oblique (rather than orthogonal) to the chamber base  47  so that light from the circumferential optical path  44   a  is directed at least partially upwards. Alternatively, the deflector  53  on the wedge shaped element may be comprised of multiple facets each at an oblique angle to the chamber base  47  so that light from the circumferential optical path  44   a  is directed at least partially upwards. 
   The top  56  of the sensor housing includes a gas permeable window  57  to allow controlled diffusion of gas under test from the external ambient of the sensor housing to the optical pathway  44  in the chambers  45  and  46 . Preferably, the gas permeable window  57  comprises a disc shaped element of wire mesh forming a flame arrester. The flame arrester could alternatively use any flame arresting material that allows diffusion of gas but forms a combustion barrier so that the source  42  cannot accidentally act as an ignition source when the sensor is operating in a hazardous and combustible gaseous environment. 
   Preferably, the disc element  57  has a radius that is greater than the radius of the inner cylindrical wall  49  and less than the radius of the outer cylindrical wall  48  so that the gas permeable window completely extends over the central chamber  46  and partially extends over the circumferential chamber  45 . The remaining portion  58  of the top  56  of the sensor housing provides a reflective inner surface  59  partially covering the circumferential chamber  45  to enhance the optical transmission characteristics of the circumferential chamber. 
   The detector  43  is mounted to the base  51  in the sensor housing and preferably comprises a dual element pyroelectric detector. The detector elements  43   a ,  43   b  are preferably arranged in a spaced relationship along a horizontal axis H of the sensor housing, i.e. a diametric axis orthogonal to the central axis defined by the inner and outer cylindrical walls  48 ,  49 . This axial spacing of the detector elements  43   a ,  43   b  ensures that the characteristics of the optical pathways leading to each of the elements are substantially similar. Each element  43   a ,  43   b  includes a filter (not shown) to allow the transmission of optical radiation at selected frequencies or frequency ranges. This dual element configuration enables the sensor to operate with one reference or compensation detector to increase accuracy of the measurements, as in known in the art. 
   Electrical leads  60  to both the source  42  and the detector  43  pass through the housing base  51  and through an encapsulant layer  61  that holds the base  51  in position. The encapsulant layer  61  also seals the housing so that it is gas tight except for the controlled diffusion window  57 . The encapsulant may be protected by way of a closing plate  62 . 
   The housing of sensor  41  may be made to conform to an industry standard configuration in terms of external dimensions and positioning of a plurality of electrical leads  60 , not all of which are shown in the drawings. Preferably, the overall outside casing diameter is approximately 2 cm, and the casing height is approximately 2 cm. Preferably, the diffusion window  57  and encapsulant layer  61  each have a thickness suitable to meet any applicable safety requirements or regulations. More preferably, the overall outside casing diameter is 20 mm±0.15 mm and the casing height is 16.6 mm±0.15 mm (excluding projecting leads), as shown in  FIG. 4 , so as to provide backward compatibility with prior art sensor casing styles. 
   A particular feature of the sensor  41  is that the detector  43  is mounted with its detector elements  43   a ,  43   b  disposed on the horizontal axis H, and preferably facing upwards. An inclined reflector  75  is provided with its reflective surface at an oblique angle to the horizontal axis H and facing the gap  52  so that light from the circumferential path  44   a  and deflector  53  is directed downwards onto the detector elements  43   a ,  43   b  along an at least partially axial optical path  44   c . Preferably, the inclined reflector presents its reflective surface at an angle of 45 degrees to the horizontal axis H. 
   In use of the preferred embodiment, the incandescent source  42  emits infra-red radiation over a broad spectrum of frequencies. The reflective surfaces formed by the inner and outer cylindrical walls  48 ,  49  and the radial end wall  50  guide the infra-red radiation around the circumferential chamber  45 . The non-focussing nature of the reflector surfaces means that positioning of the source  42  within the circumferential chamber  45  is not critical. Once the radiation reaches the other end of the circumferential chamber  45 , via optical pathway  44   a , radiation is reflected off the reflecting surface  54  of deflector  53  onto a primarily radially inward (and possibly slightly axially upward) optical path  44   b , towards the inclined reflector  75 . From the inclined reflector  75 , the radiation is reflected in an axially downward optical path  44   c  towards the detector elements  43   a ,  43   b . Depending upon the precise configuration 1  the optical path  44   c  may also include a radially inward component. 
   The preferred planar geometry of the reflecting surface  54  and reflector  75  is such that the radiation incident upon the detector elements  43   a ,  43   b  is principally normal to the elements&#39; surfaces which provides optimum temperature characteristics for the sensor  41  and ensures that a substantially equal amount of radiation falls on both elements  43   a ,  43   b . This provides for better matching conditions between the two detector element outputs. 
   The circumferential optical path  44   a  also utilizes the space within the sensor housing in a highly efficient manner, and allows the chamber walls  48 ,  49  to be formed from cylindrical elements that are easy to manufacture and also easy to assemble. The completion of the optical path  44  with the radial portion  44   b  and axial portion  44   c  enables the detector to be mounted centrally and with its electrical contact pins directed straight into a printed circuit board  76  with its major dimension lying flat to the printed circuit board. The arrangement also facilitates easy positioning of the detector within the central chamber  46 . 
   As indicated above, the layout of the optical components releases a significant proportion of the sensor housing for signal processing electronics. Below the chamber base  47  and sealed within the encapsulant  61  is the printed circuit board  76  which includes various electronic components  77 . 
   With reference to  FIG. 8 , in a preferred embodiment, these electronic components  77  include a driver circuit  100  for the optical source  42 . The detector  43  provides two electrical outputs  101 ,  102  corresponding respectively to the first detector element  43   a  and the second detector element  43   b . The first detector element provides a first output signal on line  101  associated with the absorption spectra of the selected gas for detection, e.g. carbon monoxide. The second detector element provides a second output signal on line  102  associated with a broader spread of frequencies, or preferably a selected bandwidth different from that of the first filter and relatively immune from undesirable attenuation from other common gases, to provide a reference signal. The reference signal is used to provide compensation of the attenuation measured by the first sensor that arises from temperature, humidity, degradation of the source intensity and other obscuration factors, rather than from the presence of the selected gas in the optical pathway  44 . The ratio of the reference and selected gas signals will therefore be substantially unaffected by these other factors. 
   The two output signal lines  101 ,  102  are connected to a pre-amplification circuit  103 , the output from which is connected to an analogue-to-digital converter  104  (hereinafter ‘ADC’). The digitised output of the ADC  104  is supplied to a microprocessor  105 . The microprocessor  105  is provided with a memory module  106  that is used for program storage and for storing look-up tables  107  (hereinafter ‘LUT&#39;s’) required for computing a gas concentration value from the inputs to the microprocessor. The microprocessor  105  is also preferably provided with an external programming interface  108  which may be implemented by way of programming contacts  78  as shown in  FIG. 4 . The microprocessor also includes an output path or driver  109  coupled to the electrical leads  60 . Where an analogue output is required (e.g. a voltage that is a function of gas concentration), the output driver  109  includes a digital-to-analogue converter (hereinafter ‘DAC’) for converting the digital output of the microprocessor  105  to a proportional analogue voltage. Where a digital output is required, the output driver  109  may simply comprise an electrical bus. 
   For ensuring a fully compensated gas concentration output, the gas sensor  41  is preferably also provided with one or more of suitable temperature, pressure and humidity sensors  110  so that any necessary corrections to the detector  43  readings can be made. For sensors that provide an analogue output, the ADC  104 , or a further ADC, provide digitized readings to the microprocessor  105 . 
   In a preferred arrangement, the microprocessor  105  uses the LUT&#39;s in order to determine corrections for temperature, pressure and/or humidity, according to the accuracy and application required of the specific gas sensor. Alternatively, these corrections could be made by computation according appropriate formulae, e.g. stored in the microprocessor memory  106 . In an alternative configuration, analogue outputs from the temperature, pressure and/or humidity sensors could be used directly to compensate the detector output signals in the analogue domain. 
   The various components described in connection with  FIG. 8  are preferably connected to a common power supply (e.g. 5V DC) by way of the external electrical leads  60 . The output driver  109  is similarly connected to the external electrical leads  60 . 
   All of the above components are preferably provided within the sensor  41  housing to result in a fully integrated gas sensor that provides a direct, compensated gas concentration output that requires no further signal processing. Preferably the compensated output is temperature compensated. Preferably, the compensated output is pressure compensated. Preferably the compensated output is humidity compensated. 
   Preferably, the microprocessor  105  is also configured to calibrate the sensor output so that the sensor output lies within predetermined maximum and minimum end values. This is referred to herein as ‘normalization’ of the output. Such normalization may be useful to offset any gradual degradation in the optical source performance over time. 
   Preferably, the microprocessor  105  is also configured to determine the gas concentration versus voltage output slope to ensure that the gas sensor  41  has a desired output characteristic. This is referred to herein as the ‘transfer characteristic’. This is particularly useful where it is desired to use the gas sensors as replacement units for other existing types of gas sensor. 
   For example, pellistor-type gas sensors have particular output characteristics and it may be useful to configure the optical gas sensor  41  such that the output characteristic behaves exactly the same as a pellistor output. 
   In a preferred arrangement, the programming contacts  78  may be used to configure the output characteristics of the sensor  41 . For example, the same optical sensor  41  may be used in a variety of different applications, as a direct replacement for different types of gas sensor (e.g. non-optical), by first switching the characteristics of the sensor  41  using the programming contacts  60 . This configuration operation may comprise up-loading predetermined LUT&#39;s  107 , or selecting an installed LUT for use. The configuration operation may be a one-off event, at the point of manufacture, or may be provided as a user function to be done once on installation, or multiple times during operation of the sensor. 
   The configuration operation need not be restricted to programming LUT&#39;s. For example, variables may be altered for use in formulae used to calculate gas concentrations, and programs or sub-routines may be updated. 
   In use, the gas permeable window  57  ensures that any changes in gas concentrations external to the sensor housing are rapidly communicated to the optical pathway  44  particularly in the circumferential chamber  45 , to be sensed by the detector elements  43   a ,  43   b , providing good real time output of sensed gas conditions. The preferred design of gas permeable window  57  as shown ensures that natural diffusion of gas into the circumferential chamber  45  is sufficient so that no pumping of gas through the chamber is required. In addition, having temperature, pressure and/or humidity sensor located within the housing very close to, or within, the optical chamber housing, ensures that changes in environmental conditions are rapidly communicated to the sensor output. 
   A number of variations to the embodiments described above are possible. 
   For example, although the preferred embodiment provides the source  42  located at the closed end of the circumferential chamber  45 , and the detector  43  located in the central chamber  46 , it will be understood that these positions may be reversed. 
   The illustrated embodiment shows a gas sensor  41  having an optical path which includes the circumferential portion  44   a , the radial portion  44   b  and an axial downward portion  44   c . However, it will be understood that the detector could be inverted and positioned adjacent to the top  56  of the sensor in which case an axial upward portion of the optical pathway would be required. This arrangement would be less preferred as the coupling of the detector  43  to the associated electronic components  77  would be more complex and at least part of the gas permeable window  57  may be occluded. 
   The circumferential chamber  45  need not be exactly circular to provide a substantially circular portion of the optical pathway. For example, a polygonal package, e.g. hexagonal, could include a circumferential chamber extending around the sides of the hexagon. The circumferential chamber may also be of a spiral configuration, in which the chamber wraps over itself after one complete circuit of the periphery of the chamber. 
   In all of the above described embodiments, some or all of the reflective surfaces may be gold coated (or coated with other suitable reflective material) to enhance signal amplitude, and/or coated with a passivation layer to provide protection against corrosive gases. 
   The gas permeable window  57  could also be formed from other materials, such as a gauze screen. The gas permeable window  57  need not be disc shaped, but could comprise a series of discrete openings in the top of the housing or be annular in shape. 
   The detector  43  need not be of the dual element type if a reference detector is not required. Multiple detector elements, each with an appropriate filter, may be provided for simultaneous detection of more than one selected gas. 
   Other embodiments are intentionally within the scope of the accompanying claims.