Abstract:
A membrane unit configured for use in a housing of a pressure measuring unit, preferably for measuring pressure in an extra-corporal blood circuit, has a flexible membrane and a fixing ring which is integral with the membrane so that the membrane and the fixing ring form a one-piece element. The fixing ring has a lower flexibility than the membrane, and has fixing elements for fixing the fixing ring to the pressure measuring housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 11/667,944 filed May 17, 2007, now issued as U.S. Pat. No. 7,603,907, the disclosure of which is incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention refers to a membrane unit adapted for use in a housing of a pressure measuring unit which unit is preferably adapted to measure the pressure in an extra-corporeal blood circuit. Such pressure measuring units are usually referred to as “dome” or “pressure dome”. These units comprise a membrane forming one surface of a pressure measuring chamber which membrane in use is in contact with a fluid and with a pressure transducer for converting pressures and pressure changes of the fluid into electrical signals. 
     2. Description of the Prior Art 
     A pressure measuring unit comprising a membrane and a housing is known from U.S. Pat. No. 5,614,677. The pressure measuring unit disclosed in this document comprises a housing comprising a pressure measuring chamber which is in fluid communication with an inlet and outlet of the housing. One surface of the pressure measuring chamber is formed by a membrane which is fixed to the housing by means of a tension ring inserted in a groove on the upper surface of the housing. The tension ring is fixed in its position by aid of a guide of the transducer which is abutting the upper surface of the housing. 
     Prior art document WO99/37983 also discloses a pressure measuring unit comprising a housing with a pressure measuring chamber one surface of which is formed by a membrane. The membrane comprises a circumferentially extending bead which is located on the side of the membrane facing the measuring chamber and which is pressed into a groove of the housing. A similar arrangement is known from U.S. patent application US 2004/0050168 A1. 
     The pressure measuring units described in the prior art are disadvantageous as the mounting of the membrane to the housing is cumbersome or has to be established by the membrane itself which is made of soft material. 
     Summary of the Invention 
     Accordingly, it is the object of the present invention to provide a membrane which can easily be fixed in a pressure measuring housing and at the same time exhibits good pressure transmitting and sealing properties. 
     This object is solved by a membrane unit as described herein. According to the invention a membrane unit is provided adapted to be inserted into a housing of a pressure measuring unit preferably used for measuring pressure in an extra-corporeal blood circuit which membrane unit comprises a flexible membrane and a fixing ring which is integral with the membrane so that the membrane and the fixing ring form a one-piece element, wherein the fixing ring has a lower flexibility than the membrane and wherein the fixing ring comprises fixing elements for fixing the fixing ring to the housing. Accordingly, the membrane is part of a one-piece element composed of the membrane and a fixing ring having a lower flexibility than the membrane. The invention ensures that the membrane is fixed reliably in the housing and thus reveals a good pressure transmitting property to a transducer facing the membrane on the other side than the fluid. Further a good sealing property of the membrane is obtained. A further advantage of the membrane unit according to the invention is that it can be easily mounted into the housing by engaging the fixing elements of the ring with the housing. 
     The fixing ring as well as the membrane can be connected to each other by means of a suitable connection technique for example by means of glueing or welding. 
     Alternatively, the fixing ring and the membrane can form a joint element which is manufactured in a common process of manufacture. One example of such a process is two-component injection moulding. Accordingly, the membrane unit can be manufactured as one part or can also be manufactured as two elements which are linked to each other in a second step. 
     In a further embodiment of the invention the fixing elements of the fixing ring are formed as at least one projection, groove or thread. Alternative fixing elements are also conceivable. 
     The fixing elements of the fixing ring are located in a further embodiment of the invention at the outside portion of the fixing ring. Further the fixing ring may comprise a U-shaped portion and the fixing elements are located at one or both facing sides of the legs of the U-shaped portion. 
     The fixing ring may be arranged in a standing upright position on the membrane, preferably on the circumferential portion of the membrane. Preferably the ring extends perpendicularly relative to the membrane. 
     Furthermore, the circumferential portion of the membrane may have a larger thickness than the central portion of the membrane to improve the sealing properties. 
     In a further embodiment of the invention the membrane has a circumferential extending sealing lip for sealing the contact surface between the membrane and the housing. The sealing lip is fixed in the correct position by means of the fixing ring which itself is fixed at the housing. 
     In a further embodiment the sealing lip, in particular together with the possible larger thickness portion in the circumferential portion of the membrane, extends into a first direction perpendicular relative to the planar area defined by the central portion of the membrane whereas the ring extends relative to this area into the opposite direction of said first direction. In this embodiment, during use the fluid whose pressure is to be determined contacts the central portion of the membrane from the side of the first direction whereas the pressure transducer contacts the membrane from the opposite site. 
     In another embodiment of the invention the membrane has a groove on the surface facing the fixing ring for receiving a portion of the fixing ring. 
     The contact surface between the membrane and the fixing ring may be provided with one or more projections extending from one of the membrane or the fixing ring which engage with a groove in the other of the membrane and the fixing ring. The contact surface for example can be formed by a groove in the surface of the membrane facing the fixing ring with the fixing ring being accommodated in this groove with its portion abutting the membrane. 
     In a further embodiment of the invention the shape of the sealing lip of the membrane equals the shape of the contact surface between the membrane and the fixing ring. 
     The present invention further refers to a housing of a pressure measuring unit which housing is adapted to receive a membrane unit, the housing comprising an inlet and an outlet for a fluid, the pressure of which is to be measured as well as a pressure measuring chamber being in fluid communication with the inlet and the outlet, wherein the pressure measuring chamber has an opening which is to be closed by a membrane which is to be inserted into the housing, wherein the housing further comprises fixing elements for fixing a membrane unit, preferably a membrane unit as described herein, to the housing wherein the fixing elements are located at one or more projections extending around said opening of the pressure measuring chamber. 
     The projection may extend perpendicularly relative to that surface of the pressure measuring chamber which is formed by a membrane, i.e. perpendicularly relative to the opening of the pressure measuring chamber which is to be closed by the membrane. 
     The projection may extend circumferentially around the longitudinal axis of the pressure measuring chamber which axis is perpendicular to said opening of the pressure measuring chamber. In a preferred embodiment the projection is located concentrically with the longitudinal axis of the pressure measuring chamber. 
     The fixing elements of the projection of the housing may be formed as groove, jut or thread. 
     In a further embodiment of the invention the fixing elements are located on that side of the projection which is oriented to said opening of the pressure measuring chamber. The fixing elements may extend perpendicularly to the projection. 
     The invention is further directed to a pressure measuring unit comprising a membrane unit and a housing as described herein, wherein the fixing elements of the membrane unit engage with the fixing elements of the housing and wherein the membrane of the membrane unit forms one surface of the pressure measuring chamber of the housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the invention become apparent from an embodiment which is now described as an example by aid of the drawings: 
         FIG. 1  shows the pressure measuring unit according to the invention with a pressure transducer inserted therein, 
         FIG. 2 , 
         FIG. 3  show in detail the fixing ring and the membrane in different embodiments and 
         FIG. 4  shows the fixing ring and the membrane after and before assembly according to  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       FIG. 1  shows the housing  10  comprising an inlet  12  and an outlet  14  for connection to an extra-corporeal blood circuit of a dialysis apparatus. Between the inlet  12  and the outlet  14  is located the pressure measuring chamber  16  which is in fluid communication with the inlet  12  and the outlet  14 . In this embodiment the pressure chamber  16  has a circular cross section when viewed from the top. The pressure measuring chamber  16  is terminated at the top by the lower surface of a circular membrane  20  which closes the upper opening  17  of the pressure measuring chamber  16 . At the edge of the membrane  20  there is a sealing lip  22  at the lower surface of the membrane  20  facing toward the housing  10  and a groove  24  on the upper surface of the membrane  20  facing a fixing ring  30 . The interior walls of the pressure measuring chamber  16  below the membrane  20  may have any shape suitable for the required purpose (e.g. rounded edges for blood compatibility, not shown in detail). 
     The membrane  20  and the fixing ring  30  form a one-piece element which is connected after manufacture of both parts or which is manufactured in a single process, for example by injection moulding. 
     The fixing ring  30  is made from a rigid material for guaranteeing the position of the soft sealing membrane  20  after assembling. The membrane  20  is made from elastic flexible material (for example thermoplastic elastomer, rubber) for sealing purposes as well as for transmitting the pressure/pressure changes to the transducer  100  abutting the upper surface of the membrane  20 . 
     The membrane  20  and the fixing ring  30  are in permanent contact with each other and the contact surface  40  between both parts is not flat for providing more surface for adhesion between the rigid material of the fixing ring  30  and the soft material of the membrane  20 . In the present embodiment the contact surface  40  comprises a projection  42  of the fixing ring  30  which engages with a groove  24  on the upper surface of the membrane  20  facing the fixing ring  30 . 
     As further shown in  FIG. 1  the fixing ring  30  at its outer surface comprises a radial projection  32  which is inserted into a corresponding groove  19  on the inside of a cylindrical projection  18  of the housing  10 . The projection  18  extends perpendicularly to and circumferentially around the membrane  20 . At its inner surface the projection  18  comprises the circumferentially extending groove  19  in which as mentioned above the also circumferentially extending projection  32  of the fixing ring  30  is inserted. The projection  18  and the fixing ring  30  are arranged vertically, i. e. perpendicularly to the membrane  20 . 
     The membrane unit consisting of membrane  20  and fixing ring  30  can easily be inserted into the housing  10 . Due to the presence of a rigid material in form of the fixing ring  30  the desired position of the fixing ring  30  and of the membrane  20  is reliably established so that a good sealing property as well as a good pressure transmitting property of the membrane  20  are obtained. The membrane unit is easily connected to the housing  10  by pushing the membrane unit from the top until the projection  32  snaps into the groove  19 . As both the fixing ring  30  and the projection  18  are made of sufficiently rigid materials a firm connection is achieved. At the same time the membrane  20  touches the rim  25  of an opening  17  in the housing  10  that is to be closed by the membrane  20 . Using appropriate sizes this contact can be used to apply a certain amount of tension to the membrane  20  upon fixing the ring  30  to the housing  10 . This tension may be advantageous for the pressure transmitting properties for fluid pressures both below and above ambient pressure. 
     As the circumferential portion of the membrane  20  comprises a larger thickness portion  26  that is positioned in a groove  27  surrounding the rim  25  of the housing  10  the membrane  20  is well sealed by this circumferential portion against the housing  10 . The sealing is further supported by the sealing lip  22  that is pressed by the fixing ring  30  into a complementary structure in the groove  27 . 
     As shown in  FIG. 1  the fixing ring  30  is joined perpendicularly to the circumferential portion of the membrane  20 . 
     In the shown embodiment the two parts are joined in a direction being opposite to the direction the larger thickness portion  26  extends relative to the planar area defined by the central portion of the membrane  20 . 
       FIGS. 2 and 3  show two different embodiments of the invention in which the contact surface  40  between the membrane  20  and the fixing ring  30  differs. According to  FIG. 2  the contact surface  40  differs in its shape from the shape of the sealing lip  22  and in the embodiment of  FIG. 3  the contact surface  40  as well as the sealing lip  22  have a similar or identical shape. 
     As an alternative embodiment the fixing ring  30  may also have an inverted U-shaped upper portion so that it can be pushed over the projection  18  of the housing  10 . In this case fixing elements may be provided on one or both of the legs of the U-shaped portion of the fixing ring  30 . 
     In a further embodiment the fixing ring  30  and the projection  18  may have a thread or other suitable fixing elements. 
       FIG. 4  shows different views of the membrane unit composed of membrane  20  and fixing ring  30 . The embodiment of  FIG. 4  corresponds to that shown in  FIG. 3 . As may be gathered from  FIG. 4  the membrane unit is composed of two parts which are linked to each other after manufacture thereof for example by glueing or welding. After the assembly the complete unit consisting of membrane  20  and fixing ring  30  is inserted into the housing  10  shown in  FIG. 1 . 
     The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims.