Abstract:
The invention relates to a level meter for metering the level of a filling substance in a container by means of electromagnetic signals that are guided into and out of the container along a waveguide. The inventive level meter guarantees a high tightness even at large temperature variations at the point of measurement or under great pressure or tensile forces acting upon the waveguide. The inventive level meter comprises at least one waveguide that projects into the container, guides the signals into the container and guides the signals reflected by the surface of the filing substance out of the container and is fixed in a housing mounted on the container. An elastic molded element that is clamped parallel to a longitudinal axis of the waveguide sealingly rests against the housing and the waveguide and adjoins a recess that encloses the waveguide.

Description:
TECHNICAL FIELD 
   The invention relates to a level meter for measuring a level of a product in a container by means of electromagnetic signals. The signals are transmitted to at least one waveguide protruding into the container. This waveguide carries the signals into the container, and carries signals reflected from a product surface out again. A transit time of the electromagnetic signals can be determined, for instance, and from that the level can be ascertained. 
   BACKGROUND OF THE INVENTION 
   As the waveguide, either a single waveguide or two or more parallel waveguides can be used, which extend downward into the container from a point above the highest level to be measured. Bare metal wires, also known as Sommerfeld waveguides, are suitable for instance as the waveguides, or else metal wires provided with an insulation. The latter are also known as Goubau waveguides. 
   An electronic circuit for generating electromagnetic signals, and a reception and evaluation circuit for determining a fill level, are described for instance in European Patent Disclosure EP-A 780 665. 
   Level meters that employ electromagnetic signals can be used in many applications, both in the field of storage and in the processing industry, such as in chemistry, the foods industry, and the oil industry. 
   The level meter is exposed to often great temperature fluctuations, and strong tension or compression forces can act on the waveguide. There are many applications in which despite these adverse conditions, the product absolutely must be prevented from being able to escape through the meter. 
   In European Patent Disclosure EP-A 928 955, a level meter for measuring a level of a product in a container by means of electromagnetic signals is described which includes:
         at least one waveguide protruding into the container,
           which carries the signals into the container and carries signals reflected from a product surface back out, and   which is secured in a housing that can be mounted on the container.   
               

   In the housing, inserts of a dielectric, such as a thermoplastic, a pressure-setting plastic, an elastomer, a ceramic, polyetherimide (PEI), polytetrafluoroethylene (PTFE), polyphenyl sulfide (PPS), or polycarbonates are provided, by which the waveguide is secured in the container. The housing, waveguide and inserts have conical jacket faces, which are disposed relative to one another such that they prevent a motion of the inserts and of the waveguide into the container. On a side remote from the container, the housing is closed off by a metal insert, by which a motion of the waveguide and inserts in the direction remote from the container is prevented. Spring elements are provided, by which the metal insert is pressed in the direction toward the container, and the inserts with the waveguide wedged into them are pressed in the direction away from the container. 
   For sealing purposes, in one exemplary embodiment, O-rings are provided, which are disposed in grooves between the housing and one of the inserts and between that insert and the waveguide. 
   Because of the fastening by means of springs, the insert, waveguide and housing can move slightly relative to one another. This can adversely affect the sealing action. 
   The waveguide, insert and housing form a coaxial cable in which the electromagnetic signals are carried. The grooves and O-ring represent discontinuities at which the impedance of the coaxial cable changes abruptly. Such abrupt changes in impedance cause reflection of a portion of the electromagnetic signals. That portion is subsequently no longer available for level measurement and instead forms interference signals, resulting in a markedly worse signal-to-noise ratio. 
   In another exemplary embodiment, in the side toward the container, a conical face is provided both between the waveguide and one insert and between that insert and the housing, and with this face the waveguide rests closely against an insert and that insert rests closely against the housing. Once again, a sealing action is attained thereby. 
   It is a problem in general that all the load-bearing parts must comprise rigid materials, if they are to perform their supporting function. If the level meter is exposed to major temperature changes, then these parts will shrink or expand in accordance with their coefficients of thermal expansion. While an expansion upon heating can be well compensated for by the spring construction, sealing problems can occur at low temperatures. Because of the temperature-dictated smaller size of the components, the spring prestressing is less, and tension and compression forces can cause gaps to form between sealing faces that in the normal state rest closely against one another. 
   SUMMARY OF THE INVENTION 
   It is the object of the invention to provide a level meter for measuring a level of a product in a container, by means of electromagnetic signals carried into and out of the container along a waveguide, in which especially in the event of major temperature fluctuations at the measurement site and with major pressure or tension forces acting on the waveguide, high tightness is assured. 
   To that end, the invention comprises a level meter for measuring a level of a product in a container by means of electromagnetic signals, which includes:
         at least one waveguide protruding into the container,
           which carries the signals into the container and carries signals reflected from a product surface back out, and   which is secured in a housing that can be mounted on the container, and   
           an elastic molded element, fastened parallel to a longitudinal axis of the waveguide,
           which rests sealingly on the housing and the waveguide, and   is adjoined by a recess surrounding the waveguide.   
               

   In one feature, the recess extends within the molded element. 
   In a further feature, the molded element has solely conical or cylindrical jacket faces. 
   In one feature,
         the waveguide is guided into the container through a first insert secured in the housing;   the waveguide has a first conical jacket face, with which it rests on an inside face of the same shape, remote from the container, of the insert;   the waveguide has a second conical jacket face, on which the molded element rests with an inner face of the same shape oriented toward the container; and   a second insert is provided, which with a conical jacket face oriented toward the container rests on a jacket face of the same shape, remote from the container, of the molded element;   and the molded element is fastened in place by the first and the second insert.       

   In a further feature, the recess is defined by the waveguide, the molded element, and the second insert. 
   In a further feature, the molded element has a region in which a diameter of the molded element changes abruptly. 
   In a further feature, the molded element is an annular cylinder. 

   
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention and its advantages will now be described in further detail in conjunction with the drawing figures, in which three exemplary embodiments are shown; identical elements are identified by the same reference numerals in the drawings. 
       FIG. 1  is a schematic illustration of a level meter disposed on a container; 
       FIG. 2  is a schematic illustration of a level meter in which the recess outside the molded element adjoins the molded element; 
       FIG. 3  is a schematic illustration of a level meter in which the recess extends inside the molded element; and 
       FIG. 4  is a schematic illustration of a level meter in which the molded element has a diameter that changes abruptly. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a schematic illustration of a level meter  3  disposed on a container  1 . It is used to measure a level of a product  5  in the container  1  and has an electronic circuit  7  for generating electromagnetic signals S. 
   The level meter includes a waveguide  9 , protruding into the container  1 , that carries the signals S into the container  1  and carries signals R reflected from a product surface out again. 
   The waveguide  9  is for instance a mechanically rigid rod or a mechanically rigid wire. However, a taut cable can equally well be used, one end of which is secured to a bottom of the container  1 . Instead of being fastened to the container bottom, a weight can also be secured to the other end, by which the cable is tensed. Both bare rods, wires or cables of metal, such as a special steel, or metal wires, rods or cables provided with an insulation can be used. Polytetrafluorethylene (PTFE), for instance, is suitable as the insulator. 
   In operation, the reflected signals R are carried to a reception and evaluation circuit  10 , which from a transit time of the signals S to the product surface, for instance, and from the signals R reflected back from the product surface determines the fill level in the container  1 . The propagation speed of the electromagnetic signals S, R and the distances between the electronic circuit  7  and the container bottom and between the reception and evaluation circuit  10  and the container bottom are either already known or can be obtained by simple reference measurements. With these data, the height of the fill level is obtained from the measured transit time. An outcome of measurement is accessible to further processing, display and/or evaluation via connection lines  11 . 
   The waveguide  9  is secured in a housing  13  that can be mounted on the container  1 . The housing  13  comprises an electrically conductive material, such as a metal, and preferably a special steel.  FIG. 2  shows a section through the housing  13  and the waveguide  9  secured in it. 
   The housing  13  essentially has the form of a hollow cylinder. On a lower end of the housing  13 , toward the container, a thread  15  is formed onto the outside, by means of which the housing  13  can be screwed into an opening  17  in the container  1 . 
   In the housing  13 , there is a first insert  19  comprising a dielectric, through which the waveguide  9  is guided into the container  1 . The insert  19  has a conical outer jacket face, toward the container, with which it rests sealingly on a conical inner jacket face  21 , of the same shape, of the housing  13 . An inside diameter of the housing  13  decreases along the jacket face  21  in the direction toward the container, so that a motion of the first insert  19  in the direction toward the container is prevented. The portion of the housing  13  that has the jacket face  21  is adjoined in the direction toward the container by a cylindrical housing portion  23 . In the interior of this housing portion  23 , the first insert  19  tapers in the direction toward the container, until it ends at the waveguide  9 . 
   The waveguide  9  has a head  25 , disposed in the interior of the housing  13 , with a conical first jacket face  27  toward the container, whose diameter decreases in the direction toward the container, and a conical second jacket face  29 , remote from the container, whose outside diameter decreases in the direction remote from the container. With the conical jacket face  27  toward the container, the waveguide rests on an inside face of the same shape of the first insert  19  in a sealing manner, so that a motion of the waveguide  9  in the direction toward the container is prevented. 
   According to the invention, an elastic molded element  31  is provided, fastened parallel to a longitudinal axis L of the waveguide  9  and resting sealingly on the housing  13  of the waveguide  9 . 
   In the exemplary embodiment shown in  FIG. 2 , the molded element  31  surrounds the waveguide  9  coaxially and has an inside face of the same shape as the second conical jacket face  29  of the waveguide  9 , with which inside face it rests on the second conical jacket face  29 . 
   The molded element  31  also has a conical outer jacket face  33 , toward the container, with which it rests on an inner jacket face of the same shape of the first insert  19 . With a cylindrical outer jacket face  35 , the molded element  31  rests on an inner wall of the same shape of the housing  13 . 
   Between a portion  39  of the waveguide  9  remote from the container and tapering in the direction remote from the container, there is a recess  41  adjoining the molded element  31  and surrounding the waveguide  9 . 
   An inside face  43  of the molded element  41 , defining the recess  41  in the direction away from the container, is cylindrical, and an outer jacket face  45 , remote from the container, of the molded element  41  is conical, and its outside diameter decreases in the direction away from the container. 
   Thus the molded element  31  has solely conical or cylindrical jacket faces. This offers the advantage that the coaxial cable formed by the waveguide  9 , insert  19  and housing  13  has no abrupt changes in impedance in the propagation direction of the electromagnetic signals S, R, where some of the power is reflected and is thus lost for the purposes of fill level measurement. It is even possible to a certain extent to adapt the impedance by means of suitable shaping of the molded element  31 . 
   A second insert  47  is provided in the housing  13 ; it closes off the housing  13  at the end in the direction away from the container. The second insert  47  is of metal and is cylindrical on the outside. It has a central axial bore  49 , into which an extension  51  of the waveguide  9  protrudes. The extension  51  and bore  49  are preferably shaped such that from the side remote from the container, a standard plug, such as a BNC plug, for connecting commercially available coaxial cables can be plugged into the bore  49 , in order to connect the waveguide  9 , via its extension  51  and the coaxial cable, to the electronic circuit  7 . The metal second insert  47  and the electrically conductive housing  13  form an extension of an outer conductor of the coaxial cable. 
   The second insert  47  has a conical jacket face  53 , toward the container, whose inside diameter increases in the direction toward the container, until at the end it is equal to the inside diameter of the housing  13 . 
   The second insert  47  rests, with an outer, rotationally symmetrical portion of this jacket face  53 , on the jacket face  45 , of the same shape, remote from the container, of the molded element  31 . 
   The molded element  31  is fastened in place by the first and second inserts  19 ,  47 . To that end, the second insert  47 , in the exemplary embodiment shown in  FIG. 2 , has a male thread  55 , with which it is screwed into a female thread of the housing  13  in the direction toward the container. On an upper end, away from the container, of the second insert  47 , a stop  57  is provided, up to which the insert  47  is meant to be screwed, so that it exerts sufficient pressure on the molded element  31 . 
   The molded element  31  is compressed in the axial direction, parallel to the longitudinal axis L of the waveguide  9 , and as a result develops its sealing action in the radial direction, perpendicular to the longitudinal axis L of the waveguide  9 . Excess material of the molded element  31  is pressed into the adjacent recess  41  in the process. The recess  41  is defined by the waveguide  9 , the molded element  31 , and the second insert  47 . The recess  41  serves figuratively as an overflow. If at high temperatures the individual components expand because of their coefficients of thermal expansion, then the pressure on the molded element  31  rises, and still more material from the molded element  31  is received in the recess  41 . Conversely, if the temperature drops, the process takes place in reverse. Material passes from the recess  41  back again and assures that there is always enough material at adequate pressure for the molded element  31  to develop its sealing action. Additionally, this assures that the components in the interior of the housing  13 , which in this case means the waveguide  9  and the first and second inserts  19 ,  47 , are supported without play at all times in the housing  13 . 
     FIG. 3  shows a further exemplary embodiment of a level meter of the invention. Because of the extensive agreement with the exemplary embodiment described above, only the differences will be described in detail below. 
   For instance, in the exemplary embodiment shown in  FIG. 3 , the level meter has a molded element  59 , disposed in the interior of the housing  13 , in whose interior an recess  61  of oval cross section extends annularly all the way around. This recess  61  is again suitable for compensating for a thermal expansion of the surrounding components of the level meter and thus of guaranteeing adequate tightness and a play-free disposition of these components, even in the presence of major temperature fluctuations. 
   Also in the exemplary embodiment shown in  FIG. 3 , a further recess  63  is provided. It is essentially the same shape as the recess  41  shown in  FIG. 2  and is also located at the same place, but is markedly smaller. 
   The shape and size of the recesses are not limited to the two examples shown in  FIGS. 2 and 3 . Recesses of completely different shape and size and also a different number of them may be provided. For instance, a plurality of slits may be provided. In designing the recesses, it must merely be assured that the total size of the recesses is enough to compensate for a thermal expansion of the components that surround it and of assuring adequate tightness and a play-free disposition of the components. 
     FIG. 4  shows a further exemplary embodiment. In this level meter, a molded element  65  is provided, which has a region in which a diameter of the molded element  65  changes abruptly. This creates an intentional abrupt change in impedance at the place where in operation a portion of the electromagnetic signals is reflected. This reflected portion or its transit time can serve as a reference time or reference point for the fill level measurement, for instance. 
   In  FIG. 4 , an extreme example of this kind of abruptly changing diameter, namely an annular cylinder, is shown. 
   In this exemplary embodiment as well, just as in the exemplary embodiments described above, a housing  13  is provided, in which a first insert  67  of a dielectric, a waveguide  69 , and a second insert  71  of metal are disposed. The first insert  67 , the waveguide  69 , and the second insert  71  differ from the corresponding components in the previous exemplary embodiments only in that they are adapted in terms of shape to the annular-cylindrical form of the molded element  65 . Thus instead of a conical jacket face remote from the container, the waveguide  69  has an annular-disklike face  73 , remote from the container, that rests on the molded element  65 ; the first insert  67 , instead of a conical jacket face remote from the container, has an annular-disklike face  75 , remote from the container, that rests on the molded element  65 ; and the second insert  71 , instead of a conical jacket face toward the container, has an annular-disklike face  77 , toward the container, that rests on the molded element  65 . 
   The second insert  71 , in this exemplary embodiment as well, has a central axial bore  49 , into which a cylindrical extension  79  of the waveguide  69  that is passed through the molded element  65  protrudes. Via this bore  49  and the extension  79 , the electrical connection is made. 
   Between the extension  79  and the second insert  71 , in a region adjoining the molded element  65 , there is an annular-cylindrical recess  81 , which has the same functions as the recesses described above. 
   The molded element  31 ,  59 ,  65  is all the exemplary embodiments preferably comprises a material with the lowest possible coefficient of thermal expansion, which ideally is even a negative coefficient. Dielectrics and metals, of the kind used for the housing  13 , waveguides  9 ,  69 , first insert  19 ,  67  and second insert  47 ,  71 , have a positive coefficient of thermal expansion. Correspondingly, by using a molded element  31 ,  59 ,  65  of a material with the least possible and ideally even negative coefficient of thermal expansion, the recesses  41 ,  61 ,  63 ,  81  can be embodied smaller, since the molded element  31 ,  59 ,  65 , in the event of rising temperatures, or in other words whenever the recesses  41 ,  61 ,  63 ,  81  act as an overflow, itself has only a slight increase in volume and ideally even a reduced volume. 
   Fundamentally suitable materials for the molded elements  31 ,  59 ,  65  are also, however, elastomers, such as silicone rubber, natural rubber, or elastic foams, such as polyurethane foams. The use of a foam offers the fundamental advantage that foam has pores, which also act as recesses.