Abstract:
A test waveguide ( 33 ) for evaluating the performance of microwave probe assemblies ( 1, 13 ) and their associated analysis equipment is mounted on a stand ( 56 ). The test waveguide ( 33 ) includes geometry that is similar to that found on the test cell assembly ( 2 ) used during commercial production activities. The test waveguide ( 33 ) includes an unsealed interior space ( 41 ) that remains accessible while the probe assemblies ( 1, 13 ) are fastened to the test waveguide. One or more reference blocks ( 59 ) are formed having known characteristics that permit calibration and evaluation of the probe assemblies and their associated analysis equipment. Each reference block ( 59 ) is manually inserted into the unsealed interior space ( 41 ) within the test waveguide ( 33 ) and the probe assemblies ( 1, 13 ) are activated to permit immediate evaluation of the accuracy of the probes and associated equipment

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of Guided Microwave Spectroscopy, and more particularly to calibration and testing assemblies. 
       DESCRIPTION OF RELATED TECHNOLOGY 
       [0002]    The use of a microwave waveguide cutoff frequency to characterize properties of materials is commonly referred to as Guided Microwave Spectroscopy (GMS) and is described, for example, in U.S. Pat. No. 5,331,284 (METER AND METHOD FOR IN SITU MEASUREMENT OF THE ELECTROMAGNETIC PROPERTIES OF VARIOUS PROCESS MATERIALS USING CUTOFF FREQUENCY CHARACTERIZATION AND ANALYSIS). In typical GMS implementations a flowing fluid or slurry material is continuously introduced into a chamber that is subject to microwave radiation. A microwave signal that has passed through the flowing material has altered characteristics when compared to the originally transmitted radio frequency energy, and a comparison of the transmitted and received signals permits certain properties of the material to be determined including most notably dielectric properties. 
         [0003]    A typical GMS installation often exists in a food processing facility where the GMS equipment is installed more or less permanently as part of a relatively high speed production line in continuous operation. In order to verify that all GMS components are operating properly, the microwave components that actually irradiate the material under test should periodically test for correct operation. Since the material under test is typically a slurry or fluid that flows through a sealed conduit or pipeline, a calibrating material that would serve to verify proper operation would necessarily need to be in a liquid state and also flow through the food processing pipeline. This is inherently impractical for several reasons, including problems such as introducing a nonfood substance into food processing machinery, identifying an appropriate point in the system at which such calibrating fluids could be introduced, determining exactly when the calibration slurry enters and exits the measurement chamber, removing the calibrating slurry from the system, and creating, storing and transporting calibration slurries that would have truly homogeneous and known characteristics at the moment the slurry resides within the chamber. A need therefore exists for a convenient empirical method of verifying the proper functioning of the microwave exciting and receiving components in a GMS system with minimal disruption of the food processing operation. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is a reference block system using a fixed, storable and easily transportable mass having known, constant dielectric properties. The reference block is suitably dimensioned to fit within a test waveguide that is substantially identical to the waveguide used in the production equipment that is being tested or calibrated. The use of a test waveguide eliminates the need to remove the actual production waveguide from the food processing line. Actual production equipment such as the microwave emitting probe and the microwave receiving probe are removed from the production waveguide and fastened to opposite sides of the test waveguide. The test waveguide is formed as a rectangular channel that forms a slot or opening into which a reference block may be manually inserted and removed. A set of reference blocks having differing dielectric properties may be inserted and removed from the test waveguide slot. This arrangement permits rapid testing of a production GMS device using all of the actual production components except for the production waveguide itself. The production waveguide is a relatively inert, rugged and massive structure which is unlikely to alter its characteristics even after prolonged use in an operating environment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is an isometric view of a guided wave spectroscopy measurement cell as used in a typical production environment 
           [0006]      FIG. 2  is an isometric view of the measurement cell as shown in  FIG. 1  with some of the components depicted in a spaced apart relationship; 
           [0007]      FIG. 3  is an exploded view of the microwave probe assembly depicted in  FIG. 2 ; 
           [0008]      FIG. 4  is a top plan view of a reference stand and test waveguide constructed according to the principles of the present invention; 
           [0009]      FIG. 5  is a front elevation view of the reference stand and test waveguide as depicted  FIG. 4 ; 
           [0010]      FIG. 6  is a side elevation view of the reference stand and test waveguide as depicted in  FIG. 4 ; 
           [0011]      FIG. 7  is an isometric view of the test waveguide depicted in  FIG. 6 ; 
           [0012]      FIG. 8  is an exploded isometric view of an enclosure that forms part of the present invention; 
           [0013]      FIG. 9  is a detail drawing of the region  9  depicted in  FIG. 8 ; and 
           [0014]      FIG. 10  is an isometric view of a reference block that forms part of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  depicts two examples  1  and  13  of a microwave probe assembly as they are typically affixed to a measurement cell assembly  2 . The measurement cell assembly includes a generally rectangular test chamber or waveguide  5 . A flowable material under test flows generally in the direction of arrow  4  through the test chamber  5 . The material under test enters the measurement cell  2  at inlet  3  and exits at cell outlet  6 . Referring also to  FIG. 2 , a transitional section  7  resides between the test chamber  5  and the outlet  6  and includes an orifice  10  formed to accept and retain a resistance temperature detector (RTD) assembly  8  which measures a temperature value within the material under test based on the current or voltage variation through an electrical conductor such as a platinum coil. 
         [0016]    The chamber or waveguide  5  includes a generally rectangular opening  11  which permits access to material flowing through the chamber. The probe assembly  1  is mounted onto the generally planar surface  12  of the chamber or waveguide  5  by means of four captive bolts  23 ,  14 ,  15  and  16  which are retained by mating orifices, such as orifices  17  and  18 , formed within the planar surface  12 . Referring also to  FIG. 3 , the probe assembly  1  includes an antenna  19  which is interconnected to a microwave emitter that is accessed by a coaxial cable  21  which enters the probe assembly  1  via a conduit assembly  28  which passes through orifice  20 . The antenna  19  emits a microwave signal into the interior region  22  of the chamber  5  through a microwave transparent process seal  24  and gasket  26 . The location of the antenna  19  causes any material flowing through the chamber or waveguide  5  to be irradiated by the emitted microwave radiation. The substantially identical probe assembly  13  is mounted in an opposed relationship to the probe assembly  1 . The antenna within the probe assembly  13  receives the emitted signal originating from the probe assembly  1 . Ideally the material under test flowing through the chamber  5  alters the emitted signal in a manner that permits at least some characteristics of the flowing material under test to be discerned from subsequent analysis of the signal received by probe assembly  13 . 
         [0017]    In order to verify proper operation of the foregoing apparatus, a reference assembly  25  constructed in accordance with the principles of the present invention is shown in  FIG. 4 . The reference assembly  25  includes a planar horizontally oriented base  27  which supports a substantially orthogonal plate or stand  56 . As seen in  FIG. 5 , attached to the base  27  are bolts  31  which secure four feet such as feet  29  and  30 , for example, to the bottom surface  32  of the base. The plate or stand  56  serves as the support for a test waveguide  33  as best seen in  FIGS. 6 ,  7  and  9 . The test waveguide  33  is formed to include a left sidewall  34  and a right sidewall  35  which are held in a parallel, spaced apart relationship by front plate  36  and rear plate  37 . The structural combination of the left sidewall  34 , the right sidewall  35 , the front plate  36  and the rear plate  37  define the electromagnetic boundary of the test waveguide  33 . The left and right sidewalls  34  and  35  are affixed to the stand  29  by means of bolts passing through the stand, such as, for example, bolts  71 ,  70  and  38 , that are threaded into mounting bores formed within the sidewalls. 
         [0018]    The left and right sidewalls  34  and  35  are each formed with a generally rectangular orifice  39  and  40 , respectively, so as to create a horizontally aligned, unsealed access path to the interior space  41  residing between the two sidewalls. Each sidewall  34  and  35  also includes mounting bores  42 ,  43 ,  44  and  45  which are suitably oriented and dimensions so as to align with the position of bolts  14 ,  15 ,  16  and  23  of the probe assembly  1 . 
         [0019]    In an alternate embodiment of the present invention illustrated in  FIG. 8 , the test waveguide  33  is mounted within an enclosure  46  which includes a hinged door  47  and a generally rectangular housing  48 . Formed within the substantially planar rear wall  49  of the housing  48  are a plurality of mounting holes, such as mounting hole  50 , for example, which are aligned with the mounting bores of the sidewalls  34  and  35 . In this manner the test waveguide  33  is rigidly affixed to the rear wall  49  and the sidewalls  34  and  35  assume a substantially vertical orientation. 
         [0020]    Additional mounting holes, such as mounting holes  51 ,  52  and  53 , for example, are also formed within the rear wall  49  to permit mounting of various support pegs or rods, such as, for example, rods  54 ,  55 ,  56  and  73 . When mounted on the rear wall  49  the rods, such as rods  54 ,  55 ,  56  and  73 , assume a rigid, substantially orthogonal relationship to the planar rear wall  49 . The rods, such as rods  54  and  55 , are typically mounted as a spaced apart pair, separated by a distance  58  which is selected to permit a probe assembly, such as probe assembly  1 , to rest on the rods  54  and  55 . The enclosure  46  is preferably mounted so that the rear wall  49  is substantially vertical, thereby causing the rods  54  and  55  to assume a substantially horizontal orientation. 
         [0021]    Regardless of whether the test waveguide  33  is mounted within the enclosure  46  or to the plate or stand  56 , the test waveguide is rigidly supported so that the left sidewall  34  and the right sidewall  35  reside in a substantially vertical plane. The sidewalls  34  and  35  are separated by a distance  57  to form a test waveguide  33  which has electromagnetic characteristics that are substantially identical to the actual waveguide  5  used on a measurement cell assembly  2  as found on a typical production line. Unlike the measurement cell assembly  2 , the test waveguide  33  forms an open vertical channel  72  into which a calibrated reference mass, such as the reference block  59  depicted in  FIG. 10 , may be inserted and removed. The reference block is composed of a rigid epoxy based magnetic microwave absorbing material with the addition of a secondary material to achieve the desired dielectric or attenuation characteristics. Such materials may be obtained from Resin Systems Corporation located in Amherst, N.H. The reference block  59  is formed a substantially rectangular solid having radiused edges  60 . The width  61  and depth  65  is approximately 1.875 inches and somewhat less than the spacing  57  of the test waveguide sidewalls  34  and  35 , thereby permitting insertion of the reference block  59  into the test waveguide  33 . The height  62  of the reference block is approximately 3.25 inches, thereby permitting the block  59  to fit entirely within the test waveguide  33  and be substantially aligned or flush with the top edge  63  and the bottom edge  64  of the waveguide. An orientation arrow  67  is placed on the top surface  66 . 
         [0022]    In operation, a user of the present invention detaches each of the probe assemblies  1  and  13  from the test cell assembly  2  with the conduit assemblies  28  of each probe assembly still attached to any instrumentation that is normally used during actual commercial production. Each of the probe assemblies  1  and  13  are then rigidly mounted to either sidewall  34  and  35  of the test waveguide  33  by inserting the captive bolts  14 ,  15 ,  16  and  23  into the mounting holes  42 ,  43 ,  44  and  45  on each sidewall. By affixing the probe assemblies  1  and  13  to the sidewalls  34  and  35 , the horizontally aligned unsealed access path becomes an electromagnetically sealed transmission and reception path between the probe assemblies  1  and  13 . 
         [0023]    The user then chooses a desired reference block  59  based on the dielectric properties of that particular block. Typically a reference block  59  is chosen that has properties similar to those of the proposed material under test flowing through the actual test cell assembly  2 . The user orients the reference block  59  above the top edge  63  of the test waveguide  33  so that the arrow  67  on the reference block is aligned with the arrow  68  marked on the top surface  69  of the front plate  36 . The reference block is then momentarily lowered into the space  41  within the test waveguide  33  and the probe assemblies  1  and  13  are activated. The instrumentation normally used during actual commercial production is then consulted to determine if the analysis matches the characteristics of the reference blocks. When complete, the probe assemblies  1  and  13  may then be removed from the test waveguide  33  and promptly reattached to the test cell assembly  2  in order that production operations may be resumed. 
         [0024]    While the invention has been described with reference to the preferred embodiments, various modifications to the foregoing concept of an easily installable and removable clean in place probe assembly may be readily envisioned. For example, the specific structure used to mount the waveguide may be altered as is convenient in any particular commercial setting. In some cases the vertical orientation of the sidewalls  34  and  35  may be abandoned to accommodate the convenience of the user. Further, the test waveguide  33  may have differing physical dimensions based on variations in operational equipment, whereas the physical dimensions of the reference block  59  will typically remain the same since the reference block is still able to fit within the test waveguide. Other modifications may be practiced by those skilled in this field without departing from the scope of the claims.