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 emmiting 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 as well as a circumferential portion to allow one or other of the source and detector to be located in a central chamber of the sensor housing.

Description:
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 utilising 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 focussed 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 important aspect of the design of optical absorption gas sensors is the path length between source and detector. In many known systems, long path lengths are used to increase the degree of absorption observed, typically by mounting the infra-red source and detector in separate housings for remote location from one another using long tubes or free space therebetween to define the optical path between source and detector. Such systems typically require pumping of gas through the detection chamber. 
     There is, however, a commercial requirement for highly compact, integral sensors, that can be simply plugged into, for example, portable gas detection units. This severely compromises the absorption path length available. In one compact sensor, as described in GB 2316172, a design of sensor attempts to increase path length between source and detector in a common housing and maximise signal to noise ratio characteristics by arranging the source and sensor at respective foci of an ellipsoidal chamber in which the light traveling between source and sensor is reflected at least three times. The sensor described requires high quality, polished focussing curved surfaces and careful positioning of the source and sensor elements. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a highly compact gas sensor that is easy and cost effective to manufacture and assemble that avoids the need for focussing surfaces and careful positioning of source or detector elements. 
     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; 
     a circumferential chamber, having optically reflective surfaces, extending between the source and the detector. 
     According to another 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; 
     a chamber, extending between the source and the detector, defined by a plurality of non-focussing, planar surfaces disposed to form a folded optical pathway that includes a plurality of segments substantially parallel to one another. 
    
    
     BRIEF 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 sensor according to one embodiment of the present invention 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 3-3 of FIG. 2; and 
     FIG. 4 is a schematic plan view of a sensor having a folded optical path, with the top cover removed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 the figures, a gas sensor  1  comprises an optical source  2  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 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  1  further comprises a detector  3  for detection of radiation emitted by the source  2 . The detector  3  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  3  is a pyroclectric detector. 
     The source  2  and detector  3  arc 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 . Preferably, the chamber base  7  provides a planar reflective surface, although a non-planar surface could be used. Although in the preferred embodiment the walls of the circumferential chamber  5  are formed from cylindrical walls  8 ,  9 , 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 could also be concave or convex along the axial direction. The radial end wall  10  is preferably planar, but could also be non-planar. 
     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. Preferably, the housing base  11  provides a planar reflective surface, in the central chamber  6 , although non-planar surfaces could be used. Although, in the preferred embodiment, the curved wall of the central chamber  6  is formed from the inner cylindrical wall  9 , it will he 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 could also be concave or convex along the axial direction. 
     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 reflecting surface  14  is preferably planar but may also be non-planar. The reflecting surface  14  is generally oblique to the tangent of the outer and inner circumferential walls  8 ,  9  at the position of the gap, but may also be radial. 
     The deflector  13  is preferably formed from a wedge shaped element which also forms the radial end wall  10 . The wedge shaped element can be fixed into position by screw  15  which may allow for some adjustment in the angle of the wedge shaped element. Alternatively, a reflector, fabricated from sheet metal and located in position by a pin or spot welding, may be used. 
     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 . Preferably, the gas permeable window  17  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. 
     Preferably, the disc element  17  has a radius that is greater than the radius of the inner cylindrical wall  9  and less than the radius of the outer cylindrical wall  8  so that the gas permeable window completely extends over the central chamber  6  and partially extends over the circumferential chamber  5 . The remaining portion  18  of the top  16  of the sensor housing provides a reflective inner surface  19  partially covering the circumferential chamber  5  to enhance the optical transmission characteristics of the circumferential chamber. 
     The detector  3  is mounted in the base  11  of the sensor housing and preferably comprises a dual element pyroelectric detector. The detector elements  3   a ,  3   b  are preferably 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 pathways 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. 
     Electrical leads  20  to both the source  2  and the sensor  3  pass through the housing base  11  and through an encapsulant layer  21  that holds the base  11  in position. The encapsulant layer  21  also seals the housing so that it is gas tight except for the controlled diffusion window  17 . 
     The housing of sensor  3  may be made to conform to an industry standard configuration in terms of external dimensions and positioning of a plurality of electrical leads  20 , 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  17  and encapsulant layer  21  each have a minimum thickness of 3 mm to meet safety regulations and are formed from injection moulded or machined plastics material, or metallic parts, as required. 
     In use of the preferred embodiment, 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.    
     The preferred planar geometry of the reflecting surface  14  is such that the radiation incident upon the detector elements  3   a ,  3   b  is principally normal to the elements&#39; surfaces which provides optimum temperature characteristics for the sensor  1  and ensures that a substantially equal amount of radiation falls on both elements. This provides for better matching conditions between the two detector element outputs. 
     The circumferential optical path  4   a  also utilises the space within the sensor housing in a highly efficient manner, and allows the chamber walls  8 ,  9  to be formed from cylindrical elements that are easy to manufacture and also easy to assemble. The completion of the optical path  4  with the radial portion  4   b  enables easy positioning of the detector within a large central chamber  6 . 
     A first detector element, eg.  3   a , incorporates an optical filter (not shown) that allows past only radiation in a bandwidth associated with the absorption spectra of the selected gas for detection, eg. carbon monoxide. The second detector element incorporates an optical filter that allows 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  4 . The ratio of the reference and selected gas signals will therefore be substantially unaffected by these other factors. 
     The gas permeable window  17  ensures that any changes in gas concentrations external to the sensor housing are rapidly communicated to the optical pathway  4  particularly in the circumferential chamber  5 , to be sensed by the detector elements  3   a ,  3   b , providing good real time output of sensed gas conditions. The preferred design of gas permeable window  17  as shown ensures that natural diffusion of gas into the circumferential chamber  5  is sufficient so that no pumping of gas through the chamber is required. 
     A number of variations to the embodiments described above are possible. 
     For example, although the preferred embodiment provides the source  2  located at the closed end of the circumferential chamber  5 , and the detector  3  located in the central chamber  6 , it will be understood that these positions may be reversed. Although the preferred embodiment provides a detector  3  in the central chamber  6 , the detector could alternatively be located at the end of the circumferential chamber, ie. in the place of the reflecting surface  14 , leaving the central chamber free for signal processing components or the like. 
     In another arrangement, both source and sensor might be located within a divided central portion of the sensor housing, there being a second gap in the inner cylindrical wall to allow radiation from the source to radiate outward and be reflected onto one end of the circumferential path as well as the first gap  12  to allow the radiation to be reflected inward to the detector  3  at the other end of the circumferential portion  4   a  of the optical path. In this embodiment, deflector element  13  may he appropriately shaped and positioned to provide two opposing oblique reflecting surfaces. 
     The circumferential chamber  5  need not be circular. For example, a square or generally rectangular package could incorporate a circumferential chamber extending around the four sides of the rectangle. Similarly, a polygonal package, eg. 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. 
     With reference to FIG. 4, an alternative configuration of detector uses a folded chamber that provides an optical pathway in the form of a snake. In this embodiment, a source  22  is located at one end of a folded chamber  25  providing an optical pathway  24  extending to a detector  23 . Multiple reflections along the chamber  25  are provided by angled mirrors  26 . Preferably, the chamber walls  27  are also polished mirror surfaces to maximise light transmission through the chamber. In this arrangement, the folded chamber  25  generally includes a plurality of non-focussing, planar surfaces that are disposed to form a folded optical pathway that includes a plurality of segments  28  substantially parallel to one another. 
     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  17  could also be formed from other materials, such as a gauze screen. The gas permeable window  17  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  3  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.