Abstract:
The invention relates to an assembly, in particular for measuring a process variable, which consists of a measuring instrument having a process connection and a housing, which is partially rotationally symmetrical at least in the region of the process connection, and of an adapter having a through-hole, which extends in the axial direction, and a sealing web in the through-hole. The measuring instrument and the adapter are connected to each other in a releasable and sealed manner via two sealing surfaces. According to the invention, at least one of the sealing surfaces then has a coating composed of parylene. The invention furthermore relates to an assembly for the releasable and sealed connection of two medium-carrying parts, in particular pipelines.

Description:
FIELD OF TECHNOLOGY 
     The present invention relates to an assembly for connecting a—measuring instrument to a container containing the medium to be measured and an assembly for the releasable and sealed connection of two medium-carrying parts, in particular pipelines, as well as a measuring instrument for the process measurement technology. 
     BACKGROUND 
     Measuring instruments are frequently used in automation engineering for monitoring a medium or, as the case may be the properties of a medium. In this connection, the filling level, the pressure or the temperature of the medium in a container are measured. Such measuring instruments mostly consist of a bottom part designated as process connection and a housing placed thereon as an upper part, which is mainly used as a protection for the sensor and the associated electronics. The process connection connects the measuring instrument to a container or, as the case may be to a pipeline or, as the case may be to a connecting piece and usually contains the sensor element as such. The sensor element is, for instance, configured as a piezoresistive or capacitive measuring cell in pressure measurement instruments. 
     In order to connect the measuring instrument to the equipment or, as the case may be, to the container containing the medium to be measured, sleeve-like adapters have proven advantageous, such as they are, for instance, described in the German specification DE 196 28 551 B4. These adapters have a through-hole that extends in the axial direction and a circumferential sealing web configured as a spring web. The measuring instrument is usually screwed into the adapter, the bottom part of the measuring instrument, i.e. the process connection, being in contact with the circumferential sealing web. The contact pressure between the process connection and the sealing web can be defined by screwing in the measuring instrument with a specific torque. 
     In some applications, it has proven to be advantageous to realize the connection between the process connection and the adapter without an additional sealing element because a separate seal is a losable component, on the one hand, which represents another cost factor as such, and on the other hand tolerances are consequently also added up. Without the additional sealing element, the contact surfaces of the process connection and the adapter form a so-called metal-metal seal. Connections with such a sealing concept are in fact not suitable for frequently releasing the connection, which can be disadvantageous for the inspection and maintenance work to be carried out. 
     SUMMARY 
     The underlying advantage of the invention is now to make the connection between the measuring instrument and the adapter or, as the case may be between both halves of the adapter of the assemblies mentioned above, or as the case may be the connection between the process connection and the measuring cell easier to use without an additional sealing element, and, in particular, further improve it with respect to repetitive releasing. 
     According to the present invention, this advantage is attained by means of an assembly, as well as by means of a measuring element. Advantageous embodiments of the invention are specified in the sub-claims. 
     According to the present invention, one of the sealing surfaces has a parylene coating. Sealing surfaces are understood as the contact surfaces respectively associated with the measuring instrument and with the adapter, or respectively to both halves of the adapter, where the measuring instrument and the adapter, or as the case may be both halves of the adapter contact one another in the region of the sealing web. Only one, or also both sealing, surfaces can be coated in each case. The durability was tested by means of a million pressure cycles at pressures up to 50 bar using the example of a pressure measuring instrument. 
     Two further aspects of the invention will be described below: the releasable and sealed connection of two medium-carrying parts along with a measuring instrument for static and dynamic pressure measurements, as well as the first aspect, the interaction of the process connection of a measuring instrument and an adapter; because the invention can also be used for the connection of two medium-carrying parts as well as being used as a pressure-measuring instrument. Medium-carrying parts are, in particular, understood to be pipelines or fittings, such as valves. If, for example, two pipelines are, in particular, to be connected in a releasable manner to equipment for the food industry, similar problems arise to some extent as when coupling measuring instruments to an industrial facility. 
     The advantage of the solution according to the present invention in particular is that the parylene coating prevents mutual abrasion of the sealing surfaces when joining the measuring instrument and the adapter, or as the case may be both halves of the adapter, or as the case may be the process connection and the measuring cell. This coating thus serves as a protection, but does not have a deteriorative effect on the good sealing properties of the metal-metal seal or, as the case may be when sealing material combinations without elastic properties. 
     Parylene is a hydrophobic, chemically resistant coating material with good barrier characteristic compared to inorganic and organic media, strong acids, leaches, gases and water vapor. As a biostable and biocompatible coating, it has FDA approval (Food and Drug Administration), is temperature resistant up to 220° C. and mechanically stable from −200° C. to +150° C. It is further advantageous that low mechanical tensions are generated during coating, and that it is in particular abrasion-proof, which is why such a coating is highly suitable for protecting the parts forming the metal-metal seal. This is also evident when the contact surfaces between both sealing surfaces are reduced to a quasilinear region in some applications. The parylene coating then yields to the increased compressive strength and does not tear. Furthermore, scratches and in general rough surfaces of the sealing surface can be smoothened by the parylene coating, which increases the sealing effect. A coating consisting of parylene is thus a highly precise barrier layer with good gliding properties. 
     The coating is usually applied by condensation from the gas phase, i.e. Parylene is vapor-deposited on the work piece. This has the advantage that a largely uniform coating is possible. The vapor-deposited layers can also be organically modified SiOX layers. On the other hand, there are also other coatings which are applied in the fluid or powder phase, such as sol-gel coatings or varnishes, where this uniformity is not, or not easily possible. This uniformity is, however, necessary with very thin coatings in order not to exceed the tolerance limits. 
     In this connection, similar materials, whose starting material is di-para-xylylene (or halogenated substituents), should also be encompassed in the term “parylene.” The generic term for these materials is called cyclophane. Parylene essentially is available in four different dimers with the following designations: parylene C, parylene N, parylene D and parylene F, parylene C being preferred. 
     Such coatings consisting of parylene are carried out by Plasma-Parylene Systems GmbH in Rosenheim, for example. 
     Apart from parylene, coatings consisting of polyetheretherketone (PEEK) or of polytetrafluoroethylene (PTFE) are also conceivable because, similarly to parylene, they are self-lubricating and accomplish a good dry lubrication effect, PEEK being preferentially still blended with PTFE and/or silicone portions to improve this property. PEEK is very stable chemically and does not have flow properties with typical load types and temperature ranges, e.g. in the food and pharmaceutical industry. As it is also approved for the food and pharmaceutical industry (FDA approval), in practice it stands out compared to other, similarly suitable high-quality plastic materials. It is likewise conceivable to add at least a filler or reinforcing material, i.e. a polymeric matrix with microscale and/or nanoscale particles consisting of e.g. ceramic, metal, glass, graphite or an elastomer or, as the case may be a combination of these materials. 
     In an advantageous embodiment the parylene layer has a thickness of between 0.1 pm and 50 pm. The thickness is in particular preferably between 5 pm and 30 pm, because there is an optimal relationship between technical effort and benefit in this range. 
     In another preferred embodiment, the sealing web is configured as a spring web. The spring web is prestressed when the measuring instrument is screwed into the adapter or, as the case may be when both halves of the adapter are joined. This prestressing of the sealing web now acting as a spring web generates a “force buffer” which can react dynamically to pressure and temperature fluctuations, i.e. the influences affecting the sealing effect. The prestressing is either further increased, or the prestressing decreases, as a result of which it is still ensured that the contact pressure originating at the spring web is strong enough to guarantee a durable sealing effect. The prestressing is preferably defined by a circumferential shoulder-like extension provided at the process connection. By means of this outward extension the process connection touches the adapter, as a result of which the insertion or, as the case may be screwing depth is limited depending on the type of the releasable connection. In the case of a screw connection, the screwing depth and thus the prestressing of the spring web must not be determined by a predefined tightening torque; it is rather defined by the constructive design of the assembly itself. 
     In an especially preferred embodiment, the releasable connection between the measuring instrument and the adapter, or as the case may be between both halves of the adapter, is realized by means of a screw and/or flange and/or clamping connection. The screw connection is established by means of an external thread provided on the process connection or, as the case may be on one half of the adapter and an internal thread provided on the respectively complementary half of the adapter. In the case of flange and clamping connections, snap-in, plug-in and clamping devices, in particular clamping connections, are conceivable with which the process connection inside the adapter or both halves of the adapter can be connected to one another in a releasable but firm manner. 
     Advantageous applications of the invention involve measuring instruments, in particular for measuring a process variable, which are connected to industrial plants ranging from dairies and breweries to pharmaceutical facilities, said measuring instruments thus being subject to specific demands with respect to temperature and pressure fluctuations, changes in the medium to be measured and automatic cleaning processes. All physical variables which are relevant to the process measuring technology, in particular the variables pressure, temperature, flow, or, as the case may be flow rate, filling level and analytical measuring technology are to be understood as process variables. 
     All devices in contact with media such as fluids or gases or media-carrying devices such as pipelines, containers, fitting, valves and the like are to be understood as media-carrying parts within in the sense of the invention. Accordingly, connections for media-carrying parts are connections between pipelines, containers and the like. Containers, pipelines, etc. are understood under the term container. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained below in connection with figures based on exemplary embodiments. 
       The figures show: 
         FIG. 1 : a partial sectional view of an assembly according to the present invention prior to assembling, 
         FIG. 2 : a longitudinal section view of an assembly according to the present invention consisting of an adapter and a screwed-in process connection of a measuring instrument, 
         FIG. 3 : an enlarged detail from  FIG. 2  showing the coating of the sealing spring web of the adapter, 
         FIG. 4 : a longitudinal section view across the wall of an adapter for the connection of two pipeline connections and 
         FIG. 5 : an enlarged detail from  FIG. 2  showing the coating of the spring web of the process connection. 
     
    
    
     Unless otherwise specified, like reference numerals designate like parts having the same meaning. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a measuring instrument  1  for the process measuring technology, in the present case a pressure measuring instrument, which can basically be divided into a process connection  100  and a housing  2  placed thereon. Even though a pressure measuring instrument is shown here by way of example, the invention is, however, not limited to such a measuring instrument, but can comprise any measuring instruments of the type mentioned above, in particular temperature and flow measuring instruments. The process connection  100  has a hexagon  101  on the outer side, a first outwardly extending circumferential stopping device  110  and an external thread  150 . Related details will be described in more detail in the figures below. If only one threaded connection between the measuring instrument  1  and the adapter  200  is mentioned, this merely represents a preferred embodiment. All types of known connection techniques, such as, for example, snap-in and coupling connections are obviously conceivable. 
     The process connection  100  is preferably made of stainless steel (V4A, V2A), high-quality steels or hard plastics. Stainless steel is highly suitable for applications in the food industry. The measuring cell  3  and the position of the measuring cell  3  on the sealing spring web  130  of the measuring instrument can be seen in the small section cut-out in the form of a longitudinal section view at the bottom on the left side. Detailed information in this regard is shown in the German specification DE 196 28 551 B4 mentioned above. Furthermore, the chamfered sealing border  160  can be seen in the cross section. 
       FIG. 1  further shows an adapter  200 , which as in the present case can be one half of a clamping connection. In this case, the invention is, however not limited to clamping connections but rather encompasses all comparable adaptors or flange devices. Owing to the applications in the food industry, the adapter  200  is preferably executed in stainless steel (V4A, V2A), very hard plastics being in principle also conceivable. There is a circumferential groove  220  on the bottom part of the adaptor  200 , into which a flat seal having a bilateral elevation can be inserted. These elevations then engage in the grooves  220  and thus prevent the flat seal from slipping if two similar adapters  200  are to be arranged opposite to one another. The circumferential sealing web  210  can also be seen. 
     The sealing web  210  will only be designated as “sealing spring web” below because the spring action of the sealing web  210  represents a preferred embodiment. Notwithstanding that, the invention can also be executed with a sealing web without spring-like properties, and is consequently not limited to a sealing spring web. The spring elasticity can likewise be accomplished by means of the intrinsic elasticity of the components used or, as the case may be, a by a rearward elastic position. 
     The sealing spring web  210 —viewed in diameter from the cross section—measures 2-8 mm in length in this region, and it can definitely be longer as well. The inner wall of the adapter has an internal thread  230 , with which the external thread  150  of the process connection  100  can engage when the measuring instrument  1  is screwed in. In order to illustrate the possibility of screwing one into another, the measuring instrument  1  and the adapter  200  are shown aligned, i.e. with the same central axis. It is therefore well conceivable how the measuring instrument  1  can be screwed into the adapter  200 . 
     Once the measuring instrument  1  has been screwed deeply enough into the adapter  200 , the sealing edge  160  of the process connection  100  ( FIG. 2 ) contacts the sealing spring web  210 . Owing to the spring elastic properties of the sealing web  210 , it can be prestressed in the axial direction in the range of a few tenths of a millimeter, preferably between 0.08 mm and 0.2 mm. The maximum screwing depth is limited by a circumferential stopping device  110  at the upper border of the process connection  100 . 
     The assembly according to the present invention consisting of the measuring instrument  1 —represented by its process connection  100  and the adapter  200 —is shown in  FIGS. 2 and 3 , where  FIG. 3  shows the detail displayed by means of a circle in  FIG. 2  in an enlarged manner. The measuring instrument  1  and the adapter  200  are connected to one another by means of a screw connection, in which the external thread  150  of the process connection  100  interacts with the internal thread  230  of the adapter  200 . During the screwing procedure, the region of the process connection  100 , designated as sealing edge  160 , contacts the adapter  200 , i.e. its sealing web or as the case may be the sealing spring web  210 , thus forming a metal-metal seal. These contact surfaces are designated as sealing surfaces. The deeper the measuring instrument  1  is screwed into the adapter  200 , the stronger the prestressing of the sealing spring web  210 . A limit stop  100  is provided to limit the maximum screwing depth. 
     In  FIG. 3 , the coating  300  is schematically shown by means of a thicker black line. It is irrelevant to the invention whether only the sealing surface of the sealing spring web  210  or the sealing surface of the process connection  100 , or both sealing surfaces are coated, or as the case may be the entire sealing spring web  210 , the entire adapter  200 , and/or the entire process connection  200 , etc. What is essential to the invention is that there is a coating  300  between the parts that contact one another: the sealing spring web  210  and the sealing edge  160  of the process connection  100 . 
       FIG. 4  shows the application of the inventive principle to the connection of two pipelines which are not shown in this document. The adapter  200   a , which establishes the connection, is in this case divided into a first part  201   a  and a second part  202   a . Both parts  201   a ,  202   a  are placed on top of the pipelines as an extension of them, or as the case may be are a part of them, the connection between the adapter parts  201   a ,  202   a  and the ends of the pipelines being preferentially welded. The adapter parts  201   a ,  202   a  are constructed differently, the first part  201   a  having the sealing spring web  210   a . The sealing spring web  210   a , in this case, is aligned in the axial direction. Ideally, the internal diameter of the adapter  200   a  matches the internal diameter of the pipeline. For this reason, the axial alignment of the sealing spring web  210   a  is a prime choice. Compared to the inwardly aligned embodiment for sealing a measuring instrument, the functionality is, however, identical. The second part of the adapter  202   a  has a sealing edge  160   a . The sealing spring web  201   a  contacts the sealing edge  160   a , as in the exemplary embodiment described above, as a result of which a metal-metal seal is formed. The advantages of the invention can also be applied to the connection of pipelines by means of the coating  300   a . The coating  300   a  is deliberately exaggerated by means of a black line in  FIG. 4  for reasons of clarification, therefore no conclusions can be drawn as regards the exact thickness or length. As it has already been presented in  FIG. 3 , it is irrelevant to the invention to what extent, i.e. what parts are coated, but the surface between the sealing spring web  210   a  and the sealing edge  160   a  that contact one another must be coated. 
       FIG. 5  shows a detail from  FIG. 2 , which illustrates the position of the pressure measuring cell  3  on the sealing spring webs  130  of the process connection  100 . As is known from the German specification DE 196 28 551 B4, the pressure-measuring cell, which is usually made of ceramic, can be placed on sealing spring webs  130  in order to utilize the already described advantages of the “force buffer”. Ultimately, the use of a separate sealing element has, however, always been assumed in this connection, e.g. by means of a flat sealing ring made of PTFE. The (quasi) metal-metal seal could, however, also be used in this case, the designation “metal-metal” no longer being fully appropriate. As it involves the type of seal, i.e. the seal without an additional sealing means, the designation metal-metal seal will be maintained in order to show the connection with the previously described embodiments. 
     As in  FIG. 4 , the coating  300  is deliberately exaggerated by means of a black line for reasons of clarification in this case as well, therefore no conclusions can be drawn as regards the exact thickness or length. As already presented in  FIG. 3 , it is irrelevant to the invention to what extent, i.e. what parts are coated, but the surface between the sealing spring web  210   a  and the sealing edge  160   a  that contact one another must be coated. The concept of the invention according to the previously described embodiments can basically be applied in a similar way to this embodiment as well. Reference is made to the corresponding passages in order to avoid repetition. 
     REFERENCE NUMERALS 
     
         
         
           
               1  measuring instrument 
               2  housing 
               3  measuring cell 
               5  connector socket 
               100  process connection 
               101  hexagon 
               110  stop 
               130  sealing spring web (of the measuring instrument) 
               150  external thread 
               160 ,  160   a  sealing edge 
               200 ,  200   a  adapter 
               201   a  first part of the adapter 
               202   a  second part of the adapter 
               210 ,  210   a  sealing web, sealing spring web (of the adapter) 
               220  groove 
               230  internal thread 
               300  coating 
               330  metal-metal seal