Abstract:
A photoacoustic detector includes a sensing region for receiving atmospheric samples of a gas. A permeable membrane overlays a gas input port of the sensing region. The membrane is mechanically clamped to the sensing region by a compression force.

Description:
FIELD 
     This application pertains to photoacoustic detectors. More particularly, the application pertains to such detectors which include a mechanical clamping structure to attach a gas permeable membrane to a sensing chamber. 
     BACKGROUND 
     Various types of photoacoustic sensors are known to detect gases. These include, Fritz et al., US Patent Application No. 2009/0320561, published Dec. 31, 2009 and entitled “Photoacoustic Cell”; Fritz et al., US Patent Application No. 2010/0027012, published Feb. 4, 2010 and entitled, “Photoacoustic Spectroscopy System”; Fritz et al., US Patent Application No. 2010/0045998, published Feb. 25, 2010 and entitled “Photoacoustic Sensor”; and Tobias, US Patent Application No. 2010/0147051, published Jun. 17, 2010 and entitled, “Apparatus and Method for Using the Speed of Sound in Photoacoustic Gas Sensor Measurements. The above noted published applications have been assigned to the assignee hereof, and are incorporated herein by reference. 
     Some known types of photoacoustic sensors incorporate resonant sensors. Others include gas valves. Members of another class of photoacoustic sensors incorporate diffusion membranes. 
     Diffusion membranes in photoacoustic sensors provide controlled ambient gas permeation into a sensing region. They also contribute to photoacoustic pressure confinement and bound a working volume of the photoacoustic chamber or sensing region. 
     In known sensors or detectors, this membrane is is attached with a layer of adhesive material. The adhesive material exhibits inherent problems which can impact functional performance of the membrane thus produce a strong impact on the functional performance of the photoacoustic sensor. These problems include: strong susceptibility to delaminate due to ambient conditions (temperature, humidity), and susceptibility to delaminate due to dimensional changes of a substrate as a function of ambient temperature variations (expansion and contraction). Membrane degradation, as described above, results in photoacoustic pressure variance or loss of the photoacoustic signal. Proper functioning of the diffusion membrane for these types of photoacoustic sensors is important for successful construction and functioning of the photoacoustic sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B are over-all diagrams, partly broke away of a detector in accordance herewith; 
         FIG. 2  is an exploded view of the detector of  FIG. 1A ,  1 B; 
         FIG. 3  is a top plan view of the detector of  FIG. 2 ; 
         FIG. 4  is a sectional view of the detector of  FIG. 3  taken along plane  4 - 4 ; and 
         FIG. 5  is an enlarged view of Detail A of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Embodiments disclosed herein utilize a controlled mechanical force which ensures consistent pressure and retention of a diffusion membrane with respect to the photoacoustic chamber. A mechanical feature provides a capability of self-adjustment of a pressure value due to dimensional variation of the photoacoustic chamber. Unlike the prior art, the disclosed embodiment advantageously does not rely on or use any form of adhesive material. Instead, a compression force, which could be annular, is applied to the membrane. 
     The implementation is accomplished by utilization of wave spring material with the compression properties selected for application. As an alternative to the wave spring, other metallic or plastic spring arrangements or elastomers can be utilized. The chosen spring component applies uniform pressure distribution to the membrane through the use of a shaft, pressure plate and retaining ring. 
       FIGS. 1-5  illustrate various details of an embodiment of the present disclosure. A photoacoustic detector  10  includes a housing  12  which carries an upper structure  16  which is coupled to a sensing chamber or cell  18 . Structure  16  defines a recess  22  and carries therein a gas permeable membrane which is held in place by a mechanical clamp  22   a  which applies a compression force. 
     The clamp  22   a  has an enlarged head  26   a  which is attached to an elongated shaft  26   b . The head  26   a  abuts a pressure plate  28  which applies an annular compression force on the membrane  30  which overlays a gasket  32 . If desired, the compression force could be applied to only portions of the membrane  30 . 
     Clamp  22   a  is held in place in element  16  by a spring element  34  and a retaining ring  36  carried at a free end  26   c  of the shaft  26   b . Plate  28  is compressed against the membrane  30  by the head  26   a  and the retaining ring  36  which locks to shaft  26   b  with a snap fit, as best shown in  FIG. 5 . Openings  28   a, b  in plate  28  and  32   a, b  in gasket  32  provide a path to/from the membrane  30  through which gas can permeate into the sensing chamber  18 . 
     Detector  10  can also include control circuits  40  carried by housing  12 . Control circuits  40  can be implemented with a programmable processor  40   b  which executes pre-stored control programs  40   a . A radiant energy source  40   c  can also be coupled to the processor  40   b.    
     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.