Abstract:
A photoacoustic detector includes a sensing region for receiving atmospheric samples. Radiant energy from a source is directed toward the sensing region. A thermal isolator and a displaced optical filter are positioned between the incoming radiant energy and the sensing region so that the radiant energy passes, at least in part, through both elements.

Description:
FIELD 
       [0001]    This application pertains to photoacoustic detectors. More particularly, the application pertains to such detectors which include circuitry to remove background noise. 
       BACKGROUND 
       [0002]    Various types of photoacoustic sensors are known to detect gases. These include, Fritz et al., U.S. Patent Application No. 2009/0320561, published Dec. 31, 2009 and entitled “Photoacoustic Cell”; Fritz et al., U.S. Patent Application No. 2010/0027012, published Feb. 4, 2010 and entitled, “Photoacoustic Spectroscopy System”; Fritz et al., U.S. Patent Application No. 2010/0045998, published Feb. 25, 2010 and entitled “Photoacoustic Sensor”. The above noted published applications have been assigned to the assignee hereof, and are incorporated herein by reference. 
         [0003]    Such sensors, while useful, can be affected by thermally generated noise. Such noise can create errors in output signals indicative of ambient sensed gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a block diagram of a detector in accordance herewith; 
           [0005]      FIG. 2  is a diagram of another detector in accordance herewith; and 
           [0006]      FIG. 3  is a set of graphs which illustrate operational aspects of the detector of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    While embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof, as well as the best mode of practicing same. No limitation to the specific embodiment illustrated is intended. 
         [0008]    In a disclosed embodiment, thermally induced noise can be substantially reduced or eliminated in a photoacoustic detector by using two spaced apart, radiant energy transmissive elements. One element can be an optical window with high transmissivity at a pre-determined wavelength. The second element, displaced from the window, can be implemented as an optical filter. 
         [0009]    As radiant energy from a local source falls on the optical window it, in part, passes through the window. A larger portion of the incident energy results in heating the optical window. The radiant energy which has passed through the window is incident on the optical filter. It, in part passes though the filter into a gas sensing cell or chamber. 
         [0010]    The presence of the displaced optical window thermally isolates the filter, which could be an infra-red filter for example, which is in contact with the test gas in the cell or chamber. This structure substantially eliminates any error signal produced by heating of the filter. 
         [0011]      FIG. 1  illustrates an embodiment  10  of a photoacoustic detector in accordance herewith. Detector  10  can include a housing  12  suitable for portable or fixed use such as by attachment to a wall, ceiling or other mounting structure as desired. Detector  10  can monitor gas concentration in a region R. 
         [0012]    Detector  10  includes a sensing chamber, or gas cell  20 . The cell  20  can have a variety of shapes as would be understood by those of skill in the art. The shape of the cell  20  is exemplary only. 
         [0013]    Cell  20  defines an internal region indicated generally at  22  with an atmospheric/environmental input port  24   a.  Port  24   a  is covered by a gas permeable membrane  28 . 
         [0014]    Cell  20  defines a light, or radiant energy input port  24   b  which can receive infra-red radiant energy from a source  30 . Radiant energy E from the source  30  can be focused by a reflector  32 . Radiant energy E incidient on a thermal isolator  34   a  passes in part therethrough. The remaining incident energy E heats the isolator  34   a  thereby thermally isolating optical filter  34   b  therefrom. 
         [0015]    A portion of the remaining radiant energy E is incident on the filter  34   b , carried by the cell  20  and sealed to the port  24   b.  The portion of the radiant energy that is transmitted through the filter  34   b  enters the cell  20  and heats a subject gas therein as would be understood by those of skill in the art. 
         [0016]    As illustrated in  FIG. 1 , source  30 , isolator  34   a,  filter  34   b  and optical input port  24   b  lie on a common line L. While other configurations come within the spirit and scope hereof, locating the isolator  34   a  between the radiant energy source, such as infra-red source  30  with the isolator  34   a  displaced from filter  34   b  substantially eliminates heating of the filter  34   b.    
         [0017]    Cell  20  also defines an acoustic port  24   c  to which is coupled a microphone  40 . The microphone  40  has an audio input port  40 - 1   
         [0018]    Control circuitry  50  can be coupled to source  30  so as to modulate same at a selected frequency, as would be understood by those of skill in the art. An output signal  40 - 2  from the microphone  40  provides an indicium to control circuits  50  as to a level of gas concentration in the region  22 . 
         [0019]    Also as would be understood by those of skill in the art the control circuitry  50  can include wired or wireless interface circuitry  52  so that the detector  10  can communicate with an associated monitoring system, or diagnostic and test equipment via a wired or wireless medium  54 . 
         [0020]      FIG. 2  is an exploded diagram of another embodiment  60 . Embodiment  60  includes an external housing  22 - 1  and a diffusion membrane  62  which overlies a gas cell  64 . Membrane  62  and cell  64  are carried adjacent to a cell-to-PCB gasket  68 . 
         [0021]    A first combination of optical filter  74   a  and gasket  74   b  is positioned adjacent to cell portion  64   a.  A second, thermal isolating, optical filter  76  is carried adjacent to gasket  74  and reflector  88 . Combination  76 ,  74   a  provides thermal isolation and filtering of incident radiant energy as explained above relative to elements  34   a,    34   b  of  FIG. 1 . 
         [0022]    A printed circuit board  82   a  can carry the cell  64 . A source of radiant energy  86 , such as a lamp or a laser diode or the like along with a reflector  88  complete the cell portion  64   b  on the printed circuit board  82   a.  Microphones  40 - 1  can be carried on a printed circuit board  82   b.    
         [0023]      FIG. 3  is a set of graphs indicating performance of the unit  60  in response to concentration of CO2. 
         [0024]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope hereof. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.