Abstract:
An economically producible pressure-resistant housing with a display element and/or operating element for use in potentially explosive areas, comprising an inner space which serves to accommodate electronics, and a recess on the outside, inside of which a display and/or operating element is placed whose connecting lines extend into the inner space via a pressure-resistant leadthrough, wherein the display and/or operating element is surrounded on all sides by a pressure-resistant, crystal-clear encapsulation, which fills remaining cavities in the recess and outwardly closes the recess.

Description:
TECHNICAL FIELD 
     The invention relates to a housing with a display- and/or operating-element for use in explosion-endangered areas. 
     BACKGROUND DISCUSSION 
     In industrial measurements technology, electronic devices, especially measuring devices, are regularly used in locations where a danger of explosion exists. Examples include flour silos, gas, i.e. fueling, stations, and chemical plants, where explosive gases can exist. 
     Commonly used measuring devices include e.g. flow, fill-level and pressure measuring devices. 
     Special safety requirements are placed on devices usable in explosion-endangered areas. These requirements have the goal of avoiding spark formation, which could, in the right circumstances, lead to an explosion, or of preventing that a spark occurring in a sealed chamber has any effects on the environment of the chamber. Such a goal is achievable in different ways, which are referred to in corresponding European standards as ignition protection types. 
     Thus e.g. according to European Standard EN 50 020 of the year 1994, explosion protection is given, when devices are built according to the protection class defined therein named ‘Intrinsic Safety’ (Ex-i). According to this protection class, the values for the electrical variables, current, voltage and power, in a device have to lie at all times each below a predetermined limit value. The three limit values are so selected that, in the case of malfunction, e.g. due to a short circuit, the maximum occurring release of energy is not sufficient to produce an ignition spark. The variables are maintained below their respective predetermined limit values as follows: Electrical current e.g. by resistors, voltage e.g. by Zener diodes, and power by appropriate combinations of current and voltage limiting elements. 
     Another protection class is described in the European Standard EN 50 018 of the year 1994 under the title ‘Pressure-Resistant Encapsulation’ (Ex-d). Devices built according to this protection class have a pressure-resistant housing, by which is assured that an explosion occurring in the interior of the housing can not be transmitted outside. Pressure-resistant housings are, in order that they have a sufficient mechanical strength, thick walled and, thus, heavy and expensive. 
     USA, Canada, Japan and other countries have standards comparable with these European standards. 
     The requirements of explosion protection hold true, naturally, not only for the actual measuring device, but also for display- and/or operating-elements used in explosion-endangered areas. Display elements serve e.g. for displaying measured values on-site. Operating elements are used e.g. for adjusting device settings or inputting operating parameters on-site. 
     Display- and/or operating-elements are, for reasons of explosion protection, today frequently located in inner spaces of the housing of the measuring device or in separate housing parts, which are sealed e.g. by means of a pressure-resistant lid. Such lids include, as a rule, a viewing window, for enabling the display to be observed. It is possible to equip the operating element with optical buttons, or keys, which can be operated through the viewing window. More economical, mechanical buttons cannot be operated through the lid. The lid represents an additional component, with which increased production costs are associated. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a housing having a display- and/or operating-element. The housing is to be usable in explosion-endangered areas and economical to manufacture. 
     To this end, the invention resides in a housing having
         an inner space for accommodating an electronics, and   an external recess,
           in which a display- and/or operating-element is located,
               connection lines of which extend via a pressure-resistant feedthrough into the inner space,   
               
           wherein the display- and/or operating-element is surrounded on all sides by a pressure-resistant, transparent pottant,
           which fills vacant spaces left in the recess and   environmentally seals the recess.   
               

     In a further development, the feedthrough comprises a metal insert, which has a first section screwed into a threaded bore connecting the inner space with the recess. 
     In a further development of the last-mentioned further development, the insert has, adjoining the first section, a second section, which lines the recess and which has, open in a direction facing away from the first section, an inner space, in which the display- and/or operating-element is located. 
     In a further development, the first section has, at an end facing the inner space of the housing, and open to the inner space of the housing, a cavity, in which a pressure-resistant sealant, especially a casting resin, is provided, which surrounds the connection lines. 
     In a further development, the display- and/or operating-element includes optical keys, which are operable through the pottant. 
     In another further development, the display- and/or operating-element includes mechanical keys, which are operable through passageways in the pottant sealed by means of a key cover, and is fed via current- and voltage-limiting elements located in the housing. 
     In another further development, the display- and/or operating-element is connected with the insert and the insert is rotatable into a desired position and securable by means of a securement. 
     In another further development, a retaining ring is situated in an outer groove of the first section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and further advantages will now be explained in greater detail on the basis of the figures of the drawing, in which two examples of embodiments are presented; equal parts are provided with equal reference characters in the figures. The figures show as follows: 
         FIG. 1   a  section through a housing constructed according to the protection class Ex-d, including a display- and/or operating-element with optical keys; 
         FIG. 2   a  section through a pressure-resistant housing including a display- and/or operating-element with mechanical keys; and 
         FIG. 3   a  view of the display- and/or operating-element shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a section through a first example of an embodiment of a housing  1  of the invention, with a display- and/or operating-element  3 . Housing  1  is, for example, a housing of a measuring device, e.g. a flow-, pressure- or fill-level-measuring device. The housing includes an inner space  5  for accommodating an electronics  7 , e.g. a measuring device electronics, shown only schematically in  FIG. 1 . 
     Housing  1  is made of metal, e.g. aluminum, and is constructed to be pressure-resistant. In this way, it is assured that an explosion occurring in the interior of the housing cannot be transmitted to the exterior. Because of this, housing  1  can be used in explosion-endangered areas. 
     Housing  1  includes an external recess  9 , in which the display- and/or operating-element  3  is located. In the illustrated example of an embodiment, recess  9  is formed by a hollow, cylindrical element  11  formed on housing  1  and open outwards. 
     The display- and/or operating-element  2  includes a circuit board  13 , on which both keys  15  as well as also a display are situated in the illustrated example of an embodiment. Keys  15  and display  17  are mounted directly on the circuit board  13  and are surrounded by a plastic holder  18  likewise secured on the circuit board  13 . In such case, both conventional circuit boards as well as also three dimensional, injected circuits, so-called 3-D MID&#39;s (3-Dimensional Molded Interconnection Devices) can be used. The display- and/or operating-element  3  includes connecting lines  19 , which extend via a pressure-resistant feedthrough into the inner space  5  of the housing  1 . 
     The pressure-resistant feedthrough comprises a metal insert  21 , e.g. of steel. Insert  21  has a first section  23 , which is screwed into a threaded bore  25  connecting the inner space  5  with the recess  9 . By this screwed engagement, insert  21  is connected externally pressure-resistantly with the housing  1 . 
     Adjoining first section  23  is a second section  27 , which lines the recess  9 . Second section  27  has, open in a direction facing away from the first section  23 , an inner space, in which the display- and/or operating-element  3  is located. In the illustrated example of an embodiment, the second section  27  has the form of a hollow cylinder closed on one end by a floor  29 . The outer, lateral surface of the hollow cylinder lies flushly against the corresponding inner, lateral surface of recess  9 . 
     First section  23  is a solid cylinder, which has, on its end facing the inner space  5 , and open to the inner space  5 , a cavity  31 , through which the connection lines  19  extend. 
     Connection lines  19  are assembled in the illustrated example of an embodiment into a flat tape cable. A pressure-resistant sealant  33 , especially a castable resin, such as e.g. polyurethane, is charged into the cavity  31  and surrounds the connection lines  19 . Preferably, a material with a Shore hardness of 70 to 80 is used. The pressure-resistance of sealant  33  assures the pressure-resistance of the feedthrough. 
     Display- and/or operating-element  3  is surrounded on all sides by a pressure-resistant, crystal-clear, or transparent, pottant  35 . Suitable for this purpose is e.g. an epoxy resin or a polyurethane. Also here preferably a material with a Shoren hardness of 70 to 80 is used. The pottant  35  fills the vacant space remaining in the recess  9  and seals the recess  9  to the exterior, i.e. externally. 
     For reasons of electromagnetic compatibility, display- and/or operating-element  3  is preferably shielded with an electrically conductive layer  37  electrically connected with the insert  21  and with the housing  1 . Conductive layer  37  is e.g. a conductive film or a sheet-metal stamping, and has openings to expose the keys  15  and the display  17 . 
     In the illustrated example of an embodiment, conductive layer  37  extends across the front of recess  9  and is electrically connected with an, in this case, annular end surface of the insert  21 . Through the direct contact between the insert  21  and the housing  1 , there is, at the same time, an electrically conductive connection to the housing  1 . Outside of the conductive layer  37 , there is an, in this case, circular, disk-shaped layer of the pottant  35 . Its outer diameter is equal to the outer diameter of the insert  21 . This offers the advantage that the display- and/or operating-element  3  can be pre-mounted in the insert  21  and surrounded with pottant  35 , and the thus-formed unit can then be installed into the housing  1 . 
     In manufacture, insert  21  is populated with the display- and operating-element  3  and the connection lines  19  thereof fed through. Then, the conductive layer  37  is mounted, and the unit formed in this way is placed in an appropriate mold and pottant is cast in place. Following hardening of the pottant  35 , it provides a pressure-resistant seal. 
     Further structural elements, such as e.g. the pressure-resistant glass lids regularly used in the prior art, built according to specifications of explosion protection, are not required for sealing housing  1  and/or the display- and/or operating-element  3 . In this way, manufacturing costs are markedly reduced as compared with the prior art. 
     In the case of the example of an embodiment illustrated in  FIG. 1 , the keys  15  are optical keys, which are operable through the transparent pottant  35  located thereover. 
     Alternatively, more cost-favorable, mechanical keys  39  can be used.  FIG. 2  shows, by way of a section through a pressure-resistant housing  1  with a display- and/or operating-element  3  equipped with mechanical keys  39 , an example of another embodiment.  FIG. 3  is a view of the display- and/or operating-element illustrated in  FIG. 2 . Due to the great similarity of the embodiment of  FIG. 2  to the embodiment of  FIG. 1 , in the following, only differences are explained in detail. These arise from the fact that, instead of the optical keys  15  shown in  FIG. 1 , here mechanical keys  39  are provided. For each mechanical key  39 , there is a passageway  41  in the pottant, for exposing the associated key  39 . The keys  39  terminate flushly with the pottant  35 . The passageways  41  and a surrounding area of the pottant  35  are sealed with a key cover  43 . The key cover  43  is e.g. a layer of elastomer glued to the pottant  35 . The passageways  41  represent interruptions of the pressure-resistant encapsulation of the display- and/or operating-element  3  in this region. However, on the side of the circuit board  13  facing away from the keys  39 , potting compound  35  located between the floor  29  of the insert  21  and the circuit board  13  still provides a pressure-resistant seal, exactly as in the case of the example of an embodiment illustrated in  FIG. 1 . 
     Supply of power to the display- and/or operating-element  3  occurs via elements  45  located inside the housing  1  for limiting current and voltage according to the above-described protection class Ex-i. If the inner space of housing  1  is pressure-resistantly encapsulated, then it is sufficient to connect the current and voltage limiting elements  45  between the electronics  7  and the connection lines  19  for this purpose. If the entire electronics  7  located in the interior of housing  1  is already constructed according to protection class Ex-i, then the current and voltage limiting elements  45  can be omitted. 
     Insert  21  is screwed into the threaded bore  25 . This screwed connection offers the advantage that the insert  21  and, consequently, the display- and/or operating-element  3  connected therewith, can be rotated into a desired position. When insert  21  is in the desired position, it is preferably secured by means of a securement  47  for preventing further rotation. Suitable for this purpose are e.g. the locking pins  49  shown in  FIGS. 1 and 2 , which, in each case, screw in a threaded bore in the hollow, cylindrical element  11  into locking contact against the insert  21 . 
     Preferably, additionally provided is a safety element, for assuring that insert  21  cannot be screwed too far out, in the process of adjusting to the desired position. In most applications, a thread length of about 15 mm is adequate for assuring a sufficient pressure resistance. These thread lengths, which are, as a rule, predetermined by national specifications, must not be subceeded. In the illustrated examples of embodiments, the safety element is provided in the form of a retaining ring  51 , which is placed in an outer groove in the first section  21 . Due to the retaining ring  51 , the insert  11  can only be screwed out so far, until the retaining ring  51  comes into contact with an abutment surface  53  bordering the threaded bore  25 . 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  housing 
           3  display- and/or operating-element 
           5  inner space 
           7  electronics 
           9  recess 
           11  hollow, cylindrical element 
           13  circuit board 
           15  keys 
           17  display 
           18  plastic holder 
           19  connection lines 
           21  metal insert 
           23  first section 
           25  threaded bore 
           27  second section 
           29  floor 
           31  cavity 
           33  sealant 
           35  pottant 
           37  conductive layer 
           39  mechanical keys 
           41  passageway 
           43  key cover 
           45  current and voltage limiting element 
           47  securement 
           49  locking pin 
           51  retaining ring 
           53  abutment surface