Abstract:
The capacitive measuring probe has the following: two electrodes, a plastic casing which encapsulates the two electrodes, the plastic casing having at least one section made of a conductive plastic, which is electrically connected to one of the two electrodes.

Description:
BACKGROUND INFORMATION 
       [0001]    Capacitive measuring probes are described in German Patent Nos. DE 195 11 556 and DE 198 50 291. Capacitive measuring probes are suitable for determining the level of a liquid. The measuring probes have an outer cylindrical, metallic electrode and an inner metallic counter-electrode. The liquid is able to flow in between the two electrodes. Based on the different permittivities of air and the liquid, the capacitance of the measuring probe changes as a function of the liquid level. A suitable evaluation unit estimates the liquid level with the aid of a capacitance measurement. 
         [0002]    The use of such capacitive measuring probes is limited to non-corrosive liquids. Liquids such as uric acid change the metallic electrodes, and consequently, their liquid level cannot be determined in a stable manner. 
       SUMMARY OF THE INVENTION 
       [0003]    The capacitive measuring probe according to the present invention has the following: two electrodes, one plastic casing that encapsulates the two electrodes, the plastic casing having at least one section made of a conductive plastic that is electrically connected to one of the two electrodes. 
         [0004]    The method according to the present invention for producing a capacitive measuring probe takes place using the following steps: The forming of two electrodes, the forming of at least one ring made of polyamide on at least one of the two electrodes, the spraying on of a conductive plastic onto one of the two electrodes, adjacent to the polyamide ring, and the spraying on of an insulating plastic for the complete encasing of the areas of the two electrodes that are not covered by the conductive plastic. 
         [0005]    The capacitive measuring probe utilizes a coating of the metallic electrodes with a plastic. Improved corrosion resistance is achieved thereby. The plastic may be selected according to the liquid that is to be measured. If suitable plastics are used, one may consequently produce measuring probes even for uric acid, among other things. 
         [0006]    The section(s) having conductive plastic increase(s) the sensitivity of the measuring probe. 
         [0007]    The production of the capacitive measuring probe, using a closed plastic casing, ensures that no liquid is able to penetrate through joints to a metallic core. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a capacitive measuring probe. 
           [0009]      FIG. 2  shows a sectional view of the capacitive measuring probe of  FIG. 1 . 
           [0010]      FIG. 3  shows the capacitive measuring probe of  FIG. 1  in a different perspective. 
           [0011]      FIG. 4  shows the capacitive measuring probe of  FIG. 1  without the plastic casing. 
           [0012]      FIG. 5  shows a detailed view of  FIG. 4 . 
           [0013]      FIG. 6  shows an additional measuring probe. 
           [0014]      FIG. 7  shows a detailed view of the measuring probe of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a specific embodiment of a capacitive measuring probe  1 . Measuring probe  1  has two electrodes  2 ,  3 . The two electrodes  2 ,  3  are preferably made of a metal. The two electrodes  2 ,  3  are surrounded by a closed plastic casing  4 ,  5 . A sectional plane A-A through the two electrodes  2 ,  3  and their respective plastic casing  4 ,  5  are shown in  FIG. 2 . Plastic casing  4  of first electrode  2  is made of a conductive plastic. Plastic casing  5  of second electrode  3  is made of a simple insulating plastic. 
         [0016]    The conductive plastic may be produced by the admixture of metal particles or carbon fibers to the plastic granulate during the spraying on. The organic component of the conductive plastic and of the insulating plastic may be the same. This is able to improve the adhesive properties of the two casings to each other at their common joint  6 . Consequently, no liquid penetrates the joints. An organic component of the two plastics may be based on polyamides or may be made of polyamides. 
         [0017]    The two electrodes  2 ,  3  are connected mechanically via a crosspiece  7 . Crosspiece  7  has a wall thickness d. Diameter D of the two encapsulated electrodes  2 ,  3  is greater than the wall thickness d. This creates a groove  8  between the two encapsulated electrodes  2 ,  3 . 
         [0018]    A liquid is able to flow into groove  8  which, based on its dielectric properties, changes the capacitance between the two electrodes  2 ,  3 . The change in capacitance is recorded quantitatively by an evaluation circuit, and from this the liquid level is ascertained. 
         [0019]    Because of its dielectric property, plastic casing  4 ,  5  has an influence on the measured capacitance. Furthermore, it turns out that the plastic is able to soak up the liquid and gives off the liquid again during drying. This reversible process goes hand-in-hand with a change in the dielectric properties of the plastic. 
         [0020]    In the specific embodiment described, the absolute contribution to the capacitance by plastic casing  4 ,  5  is decreased by the fact that the one plastic casing is developed to be conductive. The dielectric constant is reduced by the conductive inclusions of metal particles or carbon fibers. Capacitive measuring sensor  1  is consequently less sensitive to the changes in the dielectric properties of plastic casing  4 ,  5 . 
         [0021]    Beside a liquid level range  9 , capacitive measuring probe  1  may have an additional capacitive measuring area  10  for a permittivity measurement of the liquid. This measuring area  10  also has two electrodes  11 ,  12 . The two electrodes  11 ,  12  in turn are encapsulated in a plastic material. The design corresponds to the design of the electrodes at liquid level range  9 .  FIG. 3  shows measuring probe  1  in a position rotated about its longitudinal axis compared to the one shown in  FIG. 1 . A supply line  13  to electrode  12  may now be seen. 
         [0022]      FIG. 4  shows the measuring sensor before insulating plastic  5  is sprayed on. Sprayed-on conductive plastic  4  may have a plurality of ribs  15 . These ribs  15  lie at joints  6 , at which the two plastics border on each other in finished measuring sensor  1 . Ribs  15  increase the surface and are able to improve the adhesion of the insulating plastic to the conductive plastic. 
         [0023]      FIG. 5  shows the detail of an additional specific embodiment. A ring  20  is first applied onto electrode  2  or supply line  13  made of metal. Ring  20  may be made of a polyamide or a polypropylene which has particularly high adhesive properties to metal. Ring  20  may be produced from a polyamide or polypropylene which has a greater coefficient of thermal expansion than the plastics that are subsequently sprayed on. Because of this, ring  20  contracts more than the other plastics, and closes joints between conductive plastic  4  and insulating plastic  5 . 
         [0024]    Conductive plastic  4  is sprayed onto electrode  2  or supply line  13  adjacently and partially onto ring  20 . A partial overlapping of conductive plastic  4  with ring  20  ensures a mechanically stable connection. 
         [0025]    In the specific embodiment shown, conductive plastic  4  is not sprayed over the entire length of electrodes  2 , but only over one section. This section may be in a lower liquid level measuring range  9  by which an even lower liquid level is recorded. The use of conductive plastic  4  in lower liquid level measuring range  9  makes it possible quantitatively to determine a low liquid level using an increased resolution. Thus, it may be reliably ascertained whether liquid has to be replenished or other actions have to be taken. 
         [0026]    The other sections of liquid level measuring range  9  may be extrusion-coated with the insulating plastic. The admixture of metal particles or carbon fibers may thereby be omitted. Insulating plastic  5  may be sprayed, in a partially overlapping manner, onto ring  20 , adjacent to conductive plastic  4 . 
         [0027]      FIGS. 6 and 7  show a completely produced measuring probe  1 , in which only a section  30  is formed of conductive plastic  4 . As shown, section  30  may be applied onto only one electrode  2 . On the other hand, a conductive section  30  of conductive plastic may also be provided on both electrodes  2 ,  3 . Conductive section  30  is preferably applied at an area of the liquid level sensor which records a low liquid level. 
         [0028]    A ring (not shown) may be sprayed onto the metallic core of electrodes  2 ,  3 . Just as in  FIG. 5 , the ring may be produced of a polyamide that adheres particularly well to metal. The ring preferably has a plurality of ribs which are later extrusion-coated with the conductive plastic.