Abstract:
The present invention lessens the amount of air entering between mating membranes of a pressure sensor. The pressure sensor of the present invention includes a transducer portion and separate patient or medical fluid transfer portion or dome. The transducer portion is reusable and the dome is disposable. The dome defines a fluid flow chamber that is bounded on one side by a dome membrane. Likewise, the transducer is mounted inside a housing, wherein the housing defines a surface that holds a transducer membrane. The two membranes mate when the dome is fitted onto the transducer housing. The pressure sensor enhances the seal between the mated membranes by creating higher localized contact stresses. The pressure sensor also reduces the amount of gas that permeates from the fluid chamber across the dome membrane and between the interface by making the dome membrane from a material having a low vapor transmission.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to medical treatments. More specifically, the present invention relates to pressure sensing devices for medical fluids. 
   Due to disease, insult or injury, a person may require the infusion of a medical fluid. It is known to infuse blood, medicaments, nutrients, replacement solutions, dialysis fluids and other liquids into a patient. It is also known to remove fluid from a patient, for example, during dialysis. Dialysis is used to treat renal system failure, including kidney failure and reduced kidney function. 
   Renal failure causes several physiological effects. The balance of water, minerals and the excretion of daily metabolic load is no longer possible in renal failure. During renal failure, toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissues. Dialysis removes waste, toxins and excess water from the body that would otherwise have been removed by normal functioning kidneys. 
   Hemodialysis and peritoneal dialysis are two types of dialysis therapies commonly used to treat loss of kidney function. Hemodialysis (“HD”) treatment utilizes the patient&#39;s blood to remove waste, toxins and excess water from the patient. The patient is connected to an HD machine and the patient&#39;s blood is pumped through the machine. Catheters are inserted into the patient&#39;s veins and arteries to connect the blood flow to and from the HD machine. As blood passes through a dialyzer in the HD machine, the dialyzer removes the waste, toxins and excess water from the patient&#39;s blood and returns the blood to infuse back into the patient. 
   Peritoneal dialysis (“PD”) utilizes a dialysis solution or “dialysate”, which is infused into a patient&#39;s peritoneal cavity. The dialysate contacts the patient&#39;s peritoneal membrane in the peritoneal cavity. Waste, toxins and excess water pass from the patient&#39;s bloodstream through the peritoneal membrane and into the dialysate. The transfer of waste, toxins, and water from the bloodstream into the dialysate occurs due to diffusion and osmosis, i.e., an osmotic gradient occurs across the membrane. The spent dialysate drains from the patient&#39;s peritoneal cavity and removes the waste, toxins and excess water from the patient. This cycle is repeated on a semi-continuous or continuous basis. There are manual PD techniques, known as Continuous Ambulatory Peritoneal Dialysis (“CAPD”). There are also Automated Peritoneal Dialysis techniques (“APD”). 
   In each type of dialysis treatment, it is critical to know the pressure of the fluid that is being transported to or from the patient. Moreover, in any type of blood transfusion, saline transfusion, or any other type of fluid infusion or flow to or from a patient&#39;s body, it is important to know and control the pressure of fluid entering and leaving a patient&#39;s body. 
   Fluid pressure, generally, is sensed using a transducer or strain gauge. Medical fluid transducers have included strain gauges made from a silicon chip. Some medical fluid pressure transducers employ a mechanical linkage to transmit pressure from the fluid to the strain gauge. Many medical transducers, however, have abandoned the mechanical linkage in favor of a hydraulic pressure coupling medium comprised of a silicone elastomer, or “silicone gels”. In use, the gel is positioned between the medical fluid (that is sensed for pressure) and the transducer chip, wherein the gel conveys a hydraulic pressure signal to the integral sensing diaphragm of the transducer chip. At the same time, the gel isolates the chip electrically from the medical fluid. 
   In one type of medical transducer, the entire transducer assembly, including the chip, is discarded after a single use, since the internal components cannot be adequately cleaned for resterilization or reuse. Disposable transducer designs employing semiconductor strain gauge sensors and gel coupling media are desirable because they are rugged and accurate. Further, the disposable transducers do not require attachment of a separate disposable dome as do reusable types of medical pressure transducers. 
   Regardless of the advantages of the completely disposable medical pressure transducers, manufacturing costs for the pre-calibrated semiconductor chip and associated wiring of these types of transducers remain high. Moreover, the electronics, which could otherwise be reused, are thrown away with the rest of the unit. This is wasteful and costly. Indeed, because the waste contains electronics, it is more costly to dispose. 
   Accordingly, a pressure sensor that enables the valuable electronics of the transducer to be reused and allows the inexpensive sterile portion for the transfer of the medical fluid to be discarded is desirable. Such pressure sensors exist and typically have a dome portion, which defines a fluid lumen for the medical fluid, and a transducer portion, housing the electronics. The hurdle presented by these types of sensors is in trying to accurately transfer pressure fluctuations in the dome to like fluctuations in the transducer. 
   In many systems, the medical fluid carrying dome employs a first membrane and the transducer employs a second membrane. The two membranes abut one another and attempt to transmit medical fluid pressure fluctuations through to the strain gauge. One problem with these sensors that employ a membrane to membrane seal is in attempting to maintain the seal along the length of the membranes. A slight amount of air entering even a small part of the interface between the two membranes can falsify readings. 
   A similar problem exists with materials that have been used for the membranes. In particular, dome membranes can be susceptible to gas diffusion. Certain materials, such as ethylene propylene diene methylene (“EPDM”), have relatively high vapor transmission properties, enabling gas to diffuse from the dome, through the dome membrane, and into the interface between the membranes. 
   A need therefore exists for a medical fluid pressure sensor having a reusable transducer, a disposable medical fluid dome and an improved and repeatable seal between abutting membranes. 
   SUMMARY OF THE INVENTION 
   The present invention relates to medical fluid pressure sensors. More specifically, the present invention provides an apparatus that reduces the amount of air that enters between mating membranes of a pressure sensor. The pressure sensor of the present invention includes a transducer portion and separate patient or medical fluid transfer portion (referred to herein as a “dome” or a “body”). The transducer portion is reusable and the dome is disposable. The dome defines a fluid flow chamber that is bounded on one side by a dome membrane. Likewise, the transducer is mounted inside a housing, wherein the housing defines a surface that holds a transducer membrane. 
   The transducer can be any type of strain gauge known to those of skill in the art. In an embodiment, the sensor includes a silicone force sensing chip. The transducer membrane in an embodiment is silicone. The dome can hold and allow the transportation of many types of medical fluids such as blood, saline, dialysate (spent or clean), infiltrate, etc. The pressure sensor can likewise be used with many medical treatments, including but not limited to HD, PD, hemofiltration, and any other type of blood transfusion, intravenous transfusion, etc. Accordingly, the pressure sensor can be used with many types of medical devices including dialysis devices. In an embodiment, the reusable transducer housing mounts to the medical or dialysis device, wherein the dome or body removably couples to the housing. 
   The two membranes mate when the dome is fitted onto the transducer housing. The dome body and transducer housing include mating devices that enable the dome to removably couple to the housing. The pressure sensor enhances the seal between the mated membranes by creating higher localized contact forces or stresses. The pressure sensor also reduces the amount of gas that permeates from the fluid chamber across the dome membrane by making the dome membrane from a material having a low vapor transmission property. 
   In an embodiment the increased contact forces or stresses are provided by a sealing member or O-ring integral to the dome membrane. The integral sealing member or O-ring of the dome membrane compresses to help prevent air from leaking between the dome and transducer membranes, which mate when the housing and dome are mated. The integral O-ring can have various cross-sectional shapes and in an embodiment is at least partly circular in cross-section. The dome membrane in an embodiment also includes an integral mounting ring that pressure fits into the dome. 
   In another embodiment, the increased contact forces or stresses are provided by a sealing member or O-ring integral to the dome membrane in combination with a groove defined by the surface of the transducer housing. The surface of the transducer housing surrounds the transducer membrane. In an embodiment, this surface is metal, for example, stainless steel. The integral O-ring of the dome membrane compresses into the groove of the transducer housing when the housing and dome are mated. At the same time, the dome and transducer membranes are mated. 
   In a further embodiment, the increased contact forces or stresses are provided by a separate O-ring. Here, the O-ring compresses between the dome membrane and the surface of the transducer housing. Like the above embodiment, the surface of the transducer housing surrounds the transducer membrane and defines a groove into which the separate O-ring seats. The separate O-ring compresses into the groove of the transducer housing when the housing and dome are mated. At the same time, the dome and transducer membranes become mated. 
   In another embodiment, the O-ring compresses between the surface of the transducer housing and a surface of the dome. Here, either one of the surfaces of the transducer housing or the dome defines a groove into which the separate O-ring seats. The separate O-ring compresses into the groove of the transducer or dome surfaces when the housing and dome are mated. At the same time, the dome and transducer membranes become mated. 
   In yet another embodiment, the increased contact forces or stresses are provided by a raised portion of the surface of the transducer housing, which surrounds the transducer membrane. In an embodiment, this raised portion is metal, for example, stainless steel. The raised portion of the transducer housing compresses into the dome membrane when the housing and dome are mated. At the same time, the dome and transducer membranes become mated. 
   In any of the above-described embodiments for the increased contact forces, the dome membrane, in one preferred embodiment, is made of a material having a low gas permeability. That is, the dome membrane material has low vapor transmission properties. In an embodiment, the dome membrane includes butyl rubber, which is generally understood to have one of the lowest gas (especially air) permeabilities of all similar materials and is consequently one of the best rubber sealants. In another embodiment, the dome membrane includes a plurality of members or layers. One of the layers is of a material having a low gas permeability, such as a metal foil, a sputter coating of metal or a layer of saran or mylar. The other layer or layers include a flexible and expandable material, such as EPDM, silicone, polyurethane and any combination thereof. 
   It is therefore an advantage of the present invention to provide a pressure sensor having a reusable transducer. 
   Another advantage of the present invention is to provide a pressure sensor having a disposable medical or patient fluid transfer portion. 
   Moreover, an advantage of the present invention is to provide an accurate pressure sensor. 
   Still another advantage of the present invention is to provide a low cost pressure sensor. 
   A further advantage of the present invention is to provide a pressure sensor having a relatively gas impermeable membrane. 
   Yet another advantage of the present invention is to provide a pressure sensor having an additional relatively gas impermeable membrane layer. 
   Yet a further advantage of the present invention is to provide a pressure sensor having a localized area of high contact force. 
   Still further, an advantage of the present invention is to provide a pressure sensor having an integral O-ring. 
   Additionally, it is an advantage of the present invention to provide a pressure sensor having a separate O-ring. 
   Further still, it is an advantage of the present invention to provide an improved medical infusion device that employs the pressure sensor of the present invention. 
   Still another advantage of the present invention is to provide an improved dialysis device that employs the pressure sensor of the present invention. 
   Yet another advantage of the present invention is to provide an improved method of sealing membranes in a medical fluid infusion device. 
   Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1A  is a sectioned elevation view of one embodiment of the pressure sensor of the present invention having an integral O-ring that is just about to be compressed. 
       FIG. 1B  is the sectioned elevation view of  FIG. 1A , wherein the O-ring has been compressed and the pressure sensor is fully sealed. 
       FIG. 2  is a sectioned elevation view of one embodiment of a dome membrane of the present invention having an additional low gas permeability layer. 
       FIG. 3  is a sectioned elevation view of another embodiment of the pressure sensor of the present invention having an integral O-ring and a mating groove. 
       FIG. 4  is a sectioned elevation view of another embodiment of the pressure sensor of the present invention having a separate O-ring and a mating groove. 
       FIG. 5  is a sectioned elevation view of a further embodiment of the pressure sensor of the present invention having a separate O-ring and a mating groove. 
       FIG. 6  is a sectioned elevation view of yet another embodiment of the pressure sensor of the present invention having a raised contact force increasing portion. 
       FIG. 7  is a sectioned view of still another embodiment of the pressure sensor of the present invention, wherein the dome body includes a localized contact extension. 
       FIG. 8  illustrates various different cross-sectional shapes that the sealing member of the present invention can assume. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a pressure sensor and a membrane therefore that helps to prevent air from entering between the membrane and a second membrane when the two membranes are mated. The membranes each belong to a separate component of the pressure sensor, namely, a fluid transfer portion (referred to herein as a “dome” or “body”) and a pressure sensing portion (referred to herein as the “transducer housing”). The pressure sensor of the present invention can be used with a variety of fluid transfusion treatments. The pressure sensor is adaptable for use with patient fluids, such as blood, urine, etc. The pressure sensor is adaptable for use with medical fluids, such as saline, dialysate (spent or clean), infiltrate, etc. The pressure sensor can likewise be used with many medical treatments, including but not limited to HD, PD (including CAPD and APD), hemofiltration, and any other type of blood transfusion, intravenous transfusion, etc. 
   Referring now to the figures, and in particular to  FIG. 1A , one embodiment of a pressure sensor  10  is illustrated. Pressure sensor  10  includes a reusable portion or housing  12 . The housing  12  can be a separate housing that mounts to a panel or enclosure of a medical device, for example, a dialysis device or machine. The housing  12  is alternatively integral to the housing or enclosure of the medical device or dialysis machine. 
   The housing  12  holds or supports a transducer  14 . In the illustrated embodiment, the transducer  14  threads to the housing  12 . The transducer  14  alternatively removably mounts to the housing via fasteners, etc., or permanently mounts to the housing, for example, via a weld. 
   The transducer  14  includes a number of electrical conductors  16 , for example two, three or four conductors, which convey electrical signals to and from a transducer chip  18 . The electrical conductors  16  are insulated so that the electrical signals can convey away from the transducer housing  14  to a pressure monitor (elsewhere on the medical or dialysis machine or to a remote device) without risk of shocks, shorts or signal distortion. The chip  18  in an embodiment is a silicone force sensing chip. The housing  12 , into which the transducer  14  and chip  18  mount is, in an embodiment, stainless steel. 
   The transducer housing  12  defines a chamber  20 , which in an embodiment holds a pressure transmitting and an electrically and biologically isolating gel, hydraulic fluid or other type of pressure transmission material  22 . In an embodiment the pressure transmission material  22  includes silicone. Regardless of the type of pressure transmission material  22  used, the material  22  is responsive to negative or positive pressure signals from the medical fluid flowing through the dome or body. The material  22  transmits the positive or negative pressure signals to the transducer chip  18 . In an embodiment, the transducer chip  18  includes a pressure sensing surface, which is exposed to the pressure transmission material  22 . Also, in an embodiment, the chip  18  includes on-chip circuitry for predetermined gain and temperature compensation. 
   A disposable body or dome  30  removably mounts to the transducer housing  12 . The disposable body or dome  30  is detached from the reusable transducer housing  12  usually after a single use. The dome  30  defines an inlet fluid port  32 , an outlet fluid port  34  and a fluid chamber  36 . The illustrated embodiment defines a generally “T” shaped inlet/outlet, wherein the chamber  36  forms the leg of the “T”. The dome  30  or body can otherwise define angled or “V” shaped inlets and outlets and/or a contoured chamber. One such dome is disclosed in published PCT application WO 99/37983, entitled, “Connecting Element for Connecting a Transducer With a Sealed Fluid System”, the teachings of which are incorporated herein by reference. PCT application WO 99/37983 discloses a dome ceiling, similar to the ceiling  38  of the present invention, which is curved and has a central portion that slopes downward towards the chamber  36  and the membranes. 
   The body  30  can be constructed from any inert, biologically safe material, such as an inert plastic, for example, a polycarbonate. In an embodiment, the body  30  is clear or transparent. The inlet port  32  and outlet port  34  can include any suitable medical industry interface for connecting to a tube connector or directly to medical fluid tubes. The ports can individually or collectively include a conical packing seat. 
   The dome or body  30  releasably engages the transducer housing  12 . In an embodiment, the body  30  includes a series of tabs  40  that frictionally engage a mating ring  42  defined by the housing  12 . When a user presses the body  30  onto the housing  12 , the tabs  40  bend slightly outward so that tips  44  of the tabs  40  slide over a rib  46  partially defining the ring  42 . Eventually the tips  44  extend far enough over the housing  12 , wherein the tips  44  snap into the ring  42 . Each of  FIGS. 1 and 3  to  6  show the body  30  as it is just about to fully engage the housing  12  (with the tips  44  shown overlapping the rib  46 ). The body  30  disengages from the housing  12  in the opposite manner, wherein the tabs  40  again bend outwardly, so that the tips  44  slide back over the rib  46  and away from the ring  42 . 
   Both the housing  12  and the body  30  of the pressure sensor  10  include a flexible membrane. The housing  12  includes a membrane  50  disposed over and defining a bounding surface of the chamber  20 . The membrane  50  is positioned substantially flush along the top surface (e.g., stainless steel surface)  52  of the housing  12 . The transducer membrane  50  is, in an embodiment, silicone of approximately 0.1 to 0.5 mm thickness. Other materials and thicknesses may be used for the transducer membrane  50 . 
   The transducer membrane  50  contacts the dome membrane  60  when the dome  30  and the housing  12  have been mated together. The contacting membranes  50  and  60  enable positive and negative pressure fluctuations of medical or patient fluid in the chamber  36  of the body  30  to be transmitted to the transmission material  22  and to the chip  18 . In past pressure sensors, the interface between the contacting membranes  50  and  60  has become corrupted with gas leaking into the interface through the sides of the membranes  50  and  60  and from the medical or patient fluid though a relatively gas permeable dome membrane. The present invention seeks to address both these problems. 
   First, the dome membrane  60  is made from a substantially gas impermeable material. In a preferred embodiment, the dome membrane  60  is made from butyl rubber or from a blended rubber using butyl, such as halobutyl rubber. Butyl is generally known to have very good sealing properties and to have a very low gas permeability rate. Butyl also has relatively good tear strength, chemical resistance, environmental resistance (including resistance to ozone attack) and is relatively easy to manufacture. The membrane  60  material can be made using a high state of cure (i.e., crosslinic density), wherein the crosslinking reduces the rate of permeation. 
   Butyl rubber, with respect to air at standard temperature and pressure, is approximately thirty-five times less permeable than ethylene propylene diene methylene (“EPDM”), a known membrane material. Butyl rubber is approximately eighteen times less permeable than natural rubber. Other materials, besides butyl, which have low vapor permeability or transmission rates, and which alone or in combination with butyl rubber or with each other, can be used in the present invention, include neoprene (about 7.5 times less permeable than EPDM), polyurethane (about 6.7 times less permeable than EPDM), Buna-N (Nitrile) (about 7.5 times less permeable than EPDM), Alcryn® (about 25 times less permeable than EPDM), Hypalon® (about 13.5 times less permeable than EPDM), Vamac® (about 19 times less permeable than EPDM), and Viton® (about 19 times less permeable than EPDM). 
   The membrane  60  also defines a sealing rib  62  that press fits inside of an annular ring  64  defined by the body  30 . In an embodiment, sealing rib  62  has an inner radius slightly less than the inner radius of the annular ring  64 , so that the membrane  60  has to stretch to fit the rib  62  inside of the ring  64 . The sealing rib  62  and the thin portion of the membrane (that engages at least a portion of the membrane  50 ) are made of the same material in an embodiment, but may be of different materials in other embodiments. The thin, sealing portion of the membrane  60  is, in an embodiment, approximately 0.4 mm thick. 
     FIG. 1B  illustrates the pressure sensor  10  of  FIG. 1A , which is now fully sealed. The dome or body  30  is now ready to receive a medical fluid. The dome membrane  60  is flush against the transducer membrane  50 . That is, the dome membrane  60  sealingly engages the transducer membrane  50 . When the dome membrane  60  moves due to either a positive or negative pressure fluctuation of medical fluid in chamber  36 , the transducer membrane  50  follows or moves along with the dome membrane  60 . The transducer membrane  50  in turn imparts a positive or negative force on the transmission material  22 , which activates the chip  18  of the transducer  14 . 
   Referring now to  FIG. 2 , another embodiment for making a low vapor permeable dome membrane  70  is illustrated. The dome membrane  70  includes the sealing rib  62  described above. The dome membrane  70  also includes a low vapor transmission layer  72 . The low vapor transmission layer  72  can be a layer of metal foil, a sputter coating of metal, saran, mylar and any combination thereof. In another embodiment, the low vapor transmission layer  72  includes butyl rubber, one of the other low vapor transmission materials described above or a film such as SiO2 glass film and EvOH barrier film. In a further embodiment, a low vapor transmission filler is used, such as a reinforcing or lamellar type, which has a plate-like structure that lengthens the diffusion pathway and reduces the rate of permeation. 
   The low vapor transmission layer  72  in an embodiment is co-extruded with the rest of the membrane  70 , so that the layer  72  resides within outer layers  74  of a flexible material, which may also have a low or high vapor transmission rate. The outer layers  74  can include any type of flexible material, for example, EPDM, silicone, polyurethane or any combination of these. In another embodiment, the low permeability layer  72  is bonded to the flexible layer  74  via a suitable adhesive or heat sealing technique. 
   The low permeability membranes  60  and  70  tend to prevent gas entrained in the medical or patient fluid in the chamber  36  of the dome  30 , or present when no medical/patient fluid resides in the chamber  36 , from permeating across the dome membrane  60  or  70 . Either of the dome membranes  60  and  70  can be used in the embodiments for creating local areas of high contact force, which are about to be presented in  FIGS. 1 and 3  to  6 . The increased contact forces act to keep gas from entering between the sides of the dome membrane  60  or  70  and the transducer membrane  50 . 
     FIGS. 1A and 1B  illustrate one embodiment, wherein the increased contact forces or stresses are provided by an O-ring or sealing member  80 , which is formed integrally to the dome membrane  60  or  70 . The integral O-ring  80  of the dome membrane  60  or  70  compresses to the top surface (e.g., stainless steel surface)  52  of the housing  12  to help prevent air from leaking between the sides of the dome membrane  60  or  70  and the transducer membrane  50 . The integral O-ring  80  compresses enough so that the dome membrane  60  or  70  contacts and seals to the transducer membrane  50 . The integral O-ring  80  is co-extruded or co-molded with the remainder of the dome membrane  60  and with at least part of the dome membrane  70 . 
   Referring now to  FIG. 3 , in another embodiment, the increased contact forces or stresses are provided by the integral O-ring  80  in combination with a groove  82  defined by the surface  52  of the transducer housing  12 . The groove  82  is formed to fit the cross-sectional shape of the O-ring  80 . The surface  52  of the transducer housing  12  surrounds the transducer membrane  50  and is metal, for example, stainless steel. The integral O-ring  80  of the dome membrane compresses into the groove  82  of the transducer housing  12  when the housing and dome are mated, so as to allow the dome membrane  60  or  70  and transducer membrane  50  to contact and seal to each other. 
   Referring now to  FIG. 4 , in a further embodiment, the increased contact forces or stresses are provided by a separate O-ring or sealing member  90 . In an embodiment, the O-ring  90  compresses between the dome membrane  60  or  70  and the surface  52  of the transducer housing  12 . Here, like the above embodiment, the surface  52  of the transducer housing  12  surrounds the transducer membrane  50  and defines a groove  92  into which the separate O-ring  90  seats. The separate O-ring  90  compresses into the groove  92  of the transducer housing  12  when the housing and dome are mated, so as to allow the dome membrane  60  or  70  and transducer membrane  50  to contact and seal to each other. The separate O-ring  90  can have any of the cross-sectional shapes described below, wherein the groove  92  has a similar shape. The groove  92  in an embodiment also serves to provide a storage place for the separate O-ring  90 , during packaging, shipping and set-up. The O-ring  90  therefore slightly pressure fits into the groove  92 . 
   Referring now to  FIG. 5 , in another embodiment, the O-ring or sealing member  90  compresses between the surface  52  of the transducer housing  12  and a surface  94  of the body  30 . Here, either one of the surfaces  52  or  94  of the transducer housing  12  or the dome  30 , respectively, defines a groove  92  (in surface  52  shown previously in  FIG. 4 ) or  96  (in surface  94 ) into which the separate O-ring  90  seats and is stored during packaging, shipping and set-up. The separate O-ring  90  compresses into the groove  92  or  96  of the transducer or dome surfaces  52  or  94 , respectively, when the housing and dome are mated, so as to allow the dome membrane  60  or  70  and transducer membrane  50  to contact and seal to each other. The separate O-ring  90  can have any of the cross-sectional shapes described below, wherein the groove  92  or  96  has a similar shape. 
   Referring now to  FIG. 6 , in yet another embodiment, the increased contact forces or stresses are provided by a raised portion  98  of the surface  52  of the transducer housing  52 , which surrounds the transducer membrane  50 . In an embodiment, the raised portion  98  is metal, for example, stainless steel. The raised portion  98  of the transducer housing  12  compresses into the dome membrane  60  or  70  at a point where the membrane  60  or  70  is backed up by the sealing rib  62 , i.e., where the membrane  60  or  70  has enough material to accept the raised portion  98 . The raised portion  98 , like the O-rings, can have a variety of cross-sectional shapes, such as rectangular, trapezoidal, circular, etc. The raised portion  98  compresses into the dome membrane  60  or  70  when the housing  12  and dome  30  are mated, so as to allow the dome membrane  60  or  70  and transducer membrane  50  to contact and seal to each other. 
   Referring now to  FIG. 7 , yet another embodiment places a raised portion on the dome or body  30  rather than the transducer housing  12  as in FIG.  6 . Here, the increased contact forces or stresses are provided by an extension  99  of the surface  101  of the dome  30 . In an embodiment, the extension  99  is made of the same material as the dome  30 , for example, plastic. The extension  99  of the transducer housing  12  compresses into the dome membrane  60  or  70  at a point where the membrane  60  or  70  thickened as seen in  Fig. 7  i.e., where the membrane  60  or  70  has enough material to accept the extension  99 . 
   The extension  99 , like the O-rings, can have a variety of cross-sectional shapes, such as rectangular, trapezoidal, circular, etc. As illustrated, the extension  99  compresses into the dome membrane  60  or  70  when the housing  12  and dome  30  are mated, so as to allow the dome membrane  60  or  70  and transducer membrane  50  to contact and seal to each other. Further, the annular ring  64  presses on the sealing rib  62  so that the membrane  60  or  70  also seals generally to the surface  52  of the transducer housing  12 . 
   Referring now to  FIG. 8 , any of the sealing members disclosed herein, such as the integral O-ring  80  or the separate O-ring  90 , can have at least a partially circular cross-sectional shape as illustrated in  FIGS. 1 and 3  to  6 . Alternatively, the sealing members can have various partial or full cross-sectional shapes, such as those shapes commonly associated with a delta-ring  102 , D-ring  104 , T-ring  106 , square-ring  108 , lobed-ring  110 , cored-ring  112 , hollow-ring  114  and K-ring  116 . 
   It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.