Abstract:
A fluid processing cassette and sensor coupling system is disclosed, comprising a cassette comprising a cap having an opening formed by an inner cylindrical wall having a first diameter, an outer cylindrical wall having a second diameter, and a contact surface connecting the inner and outer cylindrical walls. The contact surface includes a varying diameter that decreases from the second diameter to the first diameter. A sensor post comprises a ring disposed around a cylindrical body and is positioned to engage with the contact surface of the cap to form a seal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent App. No. 62/320,160 filed Apr. 8, 2016, which is expressly incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to coupling systems. More specifically, the present disclosure relates to systems and methods for coupling fluid processing disposable cassettes to pressure sensing hardware. 
       BACKGROUND 
       [0003]    Whole blood may routinely be separated into its various components, such as red blood cells, platelets, and plasma. Conventional blood processing methods may use durable separator equipment (e.g., centrifuge, spinning membrane) in association with single use, sterile processing sets, typically made of plastic. The configuration of the single use processing sets used in combination with different separator equipment may vary widely, but some sets may include a molded plastic piece commonly referred to as a cassette. As used herein, the term “cassette” refers to a component of a blood processing system which includes a number of defined fluid passageways and valve stations. The cassette is commonly secured to a cassette holder of the durable equipment via motor-powered grippers or mechanical latches. The cassette holder may include actuators for opening and closing the valve stations, which determine which of the fluid passageways are connected to each other, thereby directing fluid between a number of sources and destinations. 
       SUMMARY 
       [0004]    According to an exemplary embodiment, the present disclosure is directed to a fluid processing system comprising a cassette comprising a molded body having a plurality of valve stations and at least one pressure sensing station, wherein the pressure sensing station comprises a cap having an opening extending beyond the molded body. The fluid processing system also comprises a loading area configured to receive and hold the cassette, and configured to position the cassette onto a valve and sensor assembly, wherein the valve and sensor assembly comprises valve actuators and at least one pressure sensing transducer comprising a sensor post, the valve and sensor assembly configured to align respectively with the plurality of valve stations and the pressure sensing station. The opening of the cap is formed by an inner wall having a first diameter, an outer wall having a second diameter, and a contact surface connecting the inner and outer wall. The contact surface includes a varying diameter that decreases from the second diameter to the first diameter, and the sensor post comprises a ring disposed around a cylindrical body and positioned to engage with the contact surface of the cap to form a seal. 
         [0005]    According to an exemplary embodiment, the present disclosure is directed to a fluid processing cassette and sensor coupling system comprising a cassette comprising a cap having an opening formed by an inner cylindrical wall having a first diameter, an outer cylindrical wall having a second diameter, and a contact surface connecting the inner and outer cylindrical walls. The contact surface includes a varying diameter that decreases from the second diameter to the first diameter. A sensor post comprises a ring disposed around a cylindrical body and is positioned to engage with the contact surface of the cap to form a seal. 
         [0006]    According to an exemplary embodiment, the present disclosure is directed to a fluid processing cassette and sensor coupling system comprising a cassette comprising a cap having an opening formed by an inner cylindrical wall having a first diameter, an outer cylindrical wall having a second diameter, and a contact surface connecting the inner and outer cylindrical walls. The contact surface includes a varying diameter that decreases from the second diameter to the first diameter. The system also comprises a sensor post comprising a ring disposed around a cylindrical body and positioned to engage with the contact surface of the cap to form a seal, a bottom lip configured to contact and support the ring, a top lip, and a shim disposed between the ring and the bottom lip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Features, aspects, and advantages of the present embodiments will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
           [0008]      FIG. 1  is a front view of a fluid processing system for processing various fluids, according to an exemplary embodiment; 
           [0009]      FIG. 2  is a perspective view of a loading area of the system of  FIG. 1 , according to an exemplary embodiment; 
           [0010]      FIG. 3  is a perspective view of a cassette prior to being loaded onto the loading area of  FIG. 2 , according to an exemplary embodiment; 
           [0011]      FIG. 3A  is a diagrammatic view of the defined pathways of a cassette, according to an exemplary embodiment; 
           [0012]      FIG. 4  is a perspective view of the bottom of a cassette, according to an exemplary embodiment; 
           [0013]      FIG. 5  is a top view of a valve and sensor assembly of a cassette, according to an exemplary embodiment; 
           [0014]      FIG. 6  is a cross-section elevational view of a cassette prior to engagement with a valve and sensor assembly, according to an exemplary embodiment; 
           [0015]      FIG. 7A  is a perspective view of a sensor post known in the art, according to an exemplary embodiment; 
           [0016]      FIG. 7B  is a cross-sectional diagram showing a sensor post of  FIG. 7A  forming a bore seal with a cap of a cassette sensing station, according to an exemplary embodiment; 
           [0017]      FIG. 7C  a cross-sectional diagram showing a sensor post forming a face seal with a cap of a cassette sensing station, according to an exemplary embodiment; 
           [0018]      FIG. 8A  is a cross-sectional diagram of a cap of a cassette sensing station, according to an exemplary embodiment; 
           [0019]      FIG. 8B  is a perspective view of a sensor post, according to an exemplary embodiment; 
           [0020]      FIG. 8C  is a cross-sectional diagram showing a sensor post forming a crush seal with a cap of a cassette sensing station, according to an exemplary embodiment; 
           [0021]      FIG. 9  is a cross-sectional diagram showing one side of a sensor post fitted against a cap of a cassette sensing station at different compression levels, according to an exemplary embodiment; and 
           [0022]      FIG. 10  is a cross-sectional diagram showing one side of a sensor post and shim fitted against a cap of a cassette sensing station at different compression levels, according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    There are several aspects of the present subject matter which may be embodied separately or together in the devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto. 
         [0024]    Some embodiments may facilitate measuring draw and return pressures from a donor/patient without blood being in direct contact with a pressure sensor. 
         [0025]    Some embodiments may enable measuring pressure in multiple locations within a fluid transport system, e.g., a cassette. 
         [0026]    Some embodiments may enable alignment of multiple couplings, thereby accommodating larger manufacturing variations. 
         [0027]    An exemplary blood processing system as well as its cassette and cassette holder are described in greater detail in U.S. Pat. No. 5,868,696, which is hereby incorporated herein by reference in its entirety. In blood processing systems, fluid flow may be controlled by a disposable cassette with preformed fluid passages, which may interface with an array of actuators and sensors located on a panel of a durable reusable hardware. The cassette may have a flexible membrane on the side facing the actuators and sensors. A vacuum may be applied by the hardware through small spaced-apart apertures to draw the membrane into contact with the surface of the array and with the sensors therein for more reliable and accurate sensing. Specifically, a thin, elastomeric membrane may be associated with the cassette holder and cover all of the actuators, The gasket may protect the actuators from liquids, dust, and other debris that could otherwise interfere with the performance of the actuators. 
         [0028]      FIG. 1  shows a fluid processing system  10  that can be used for processing various fluids, but may be particularly well suited for processing whole blood and other suspensions of biological cellular materials. The system  10  may include a centrifuge or spinning membrane (not visible) suitable for separating a fluid into its components based on the density and/or size of such components. 
         [0029]    Referring to  FIG. 2 , a sloped front panel  12  of the system  10  may include at least one loading area, such as cassette holder  14 , which may be configured to receive and grip a cassette  16  ( FIGS. 3 and 4 ) of a disposable, single-use processing set. 
         [0030]    The cassette  16 , as shown in  FIGS. 3, 3A, and 4 , may include an injection molded body  18  that may be compartmentalized to form defined pathways, e.g., pathways  34  in  FIG. 3A . The body  18  may be covered by a top cover  22  ( FIG. 3 ) and a bottom cover  24  ( FIG. 4 ), enabling the cassette and its fluid contents to be closed from the surrounding environment. For the purposes of description, the top cover  22  is disposed on the side of the cassette  16  that, in use, faces away from the system  10 , while the bottom cover  24  faces towards the system  10 . A flexible diaphragm  26  (not shown) may be disposed between the bottom cover  24  and the body  18 , thereby sealing the underside of the body  18  from the system  10  and cassette holder  14 . In one embodiment, the cassette  16 , the cassette body  18 , the top cover  22 , and/or the bottom cover  24  may be made of a rigid medical grade plastic material, while the diaphragm  26  may be made of a flexible sheet of medical grade plastic. 
         [0031]    As shown in  FIG. 4 , the bottom cover  24  may include an array of valve stations  30  disposed under select locations of the various defined pathways of the body  18 . The bottom cover  24  may also include pressure sensing stations  32   a  and  32   b.  The valve stations  30  and the pressure sensing stations  32   a  and  32   b  may communicate with the various defined pathways in a predetermined manner. The number and arrangement of the valve stations  30 , and the sensing stations  32   a  and  32   b  may vary. 
         [0032]    Turning to  FIGS. 2 and 3 , the cassette holder  14  may be configured to receive and grip the cassette  16  in a desired operating position. The cassette holder  14  may include any number of peristaltic pump stations  50 . When the cassette  16  is loaded, tubing loops  52  extending from the cassette  16  may make operative engagement with the pump stations  50 . The pump stations  50  may be operated to cause fluid flow through the cassette  16 . 
         [0033]    Turning to  FIGS. 2 and 3 , the cassette holder  14  may be configured to receive and grip the cassette  16  in a desired operating position. The cassette holder  14  may include any number of peristaltic pump stations  50 . When the cassette  16  is loaded, tubing loops  52  extending from the cassette  16  may make operative engagement with the pump stations  50 . The pump stations  50  may be operated to cause fluid flow through the cassette  16 . 
         [0034]    When the cassette  16  is loaded onto the cassette holder  14 , the blood processing system  10  may be configured to lower the cassette holder into contact with a valve and sensor assembly  54 , illustrated in  FIG. 5 , located underneath the cassette holder  14  within the blood processing system  10 . The cassette holder  14  may include a plurality of openings  42  to allow components of the valve and sensor assembly  54  to access the valve stations  30  and sensing stations  32  of the cassette  16  when the cassette  16  and cassette holder  14  are lowered into contact with the valve and sensor assembly  54 .  FIG. 5  is a top view of one embodiment of a valve and sensor assembly  54 . The valve and sensor assembly  54  may act in concert with the valve stations  30  and sensing stations  32   a  and  32   b  of the cassette  16  to control and monitor fluid flow within the cassette  16 . The valve and sensor assembly  54  may include valve actuators  56  and pressure sensing transducers  58   a  and  58   b.  The valve actuators  56  and the pressure sensing transducers  58   a  and  58   b  may be mutually arranged in the same layout as the valve stations  30  and sensing stations  32   a  and  32   b  on the underside  24  of the cassette  16 . 
         [0035]    When the cassette  16  is gripped by the cassette holder  14 , the valve actuators  56  may align with the cassette valve stations  30 . At the same time, the pressure sensing transducers  58   a  and  58   b  may mutually align with the pressure sensing stations  32   a  and  32   b,  respectively. The pressure sensing transducers  58   a  and  58   b  may utilize a piezoelectric mechanism and comprise a sensor post, although any suitable pressure sensing mechanism may be used. Other pressure sensing mechanisms include bridge-based, capacitive, and/or optical mechanisms. An exemplary valve/sensor assembly, cassette, and cassette holder are described in greater detail in U.S. Pat. No. 8,758,288, which is hereby incorporated herein by reference in its entirety. 
         [0036]      FIG. 6  shows an elevational view of the cassette  16  prior to engagement with sensor posts  59  that form a part of the pressure sensing transducers  58   a,    58   b,  of the valve and sensor assembly  54 .  FIG. 6  depicts the cassette  16  seated onto the cassette holder  14 . The pressure sensing stations  32   a  and  32   b  of the cassette  16  may include caps  41  extending from the cassette&#39;s underside  24 . When the cassette  16  is seated onto the cassette holder  14 , the caps  41  may extend beyond the plane of the cassette holder  14  via the openings  42  ( FIG. 2 ) of the cassette holder  14 . Cap  41  may be configured to engage sensor post  59 , ensuring that an effective seal is created for proper pressure sensing at each pressure sensing station  32   a  or  32   b.    
         [0037]      FIG. 7A  depicts a sensor post  59   a  known in the art. Sensor post  59   a  comprises a main cylindrical body  61   a  and a flexible ring  60   a.  Ring  60   a  is configured to tightly engage the circumference of the cylindrical body  61   a  to provide a tight seal. 
         [0038]      FIG. 7B  is a cross-sectional diagram showing sensor post  59   a  fitted within a cap  41   a  such that ring  60   a  is entirely within the cap  41   a  and the contact surface  47   a  is parallel to the axis Za of the seal. A seal is created by the ring  60   a  being tightly fitted within the cap  41   a  such that the ring  60   a  provides an effective barrier to fluid flow. The ring  60   a  exerts a force Fa against the cap  41   a  primarily in a radially outward direction perpendicular to the vertical wall of the cap  41   a  and perpendicular to the vertical axis Za of the seal. A seal such as that depicted in  FIG. 7B  is generally known in the art and is sometimes referred to as a bore seal. 
         [0039]      FIG. 7C  is a cross-sectional diagram showing sensor post  59   b  engaged with a cap  41   b  such that the contact surface  47   b  between the ring  60   b  and edge  43  is perpendicular to the axis Zb of the seal. A seal is created by the ring  60   b  making contact at a contact surface  47   b  having a horizontal component, disposed along the bottom edge  43  of the cap  41   b.  An effective seal is created by the ring  60   b  making contact with the contact surface  47   b  along a circumference of the bottom edge  43 . The ring  60   b  exerts a force Fb against the bottom edge  43  primarily in a z-direction perpendicular to the horizontal contact surface  47   b  of the bottom edge  43  but parallel to the vertical axis Zb of the seal. A seal such as that depicted in  FIG. 7C  in which the contact surface between the ring  60   b  and edge  43  is perpendicular to the axis Zb is sometimes referred to as a face seal. 
         [0040]    Referring to  FIG. 6 , in order for an effective seal to form between a sensor post  59  of and a cap  41  of the cassette  16 , precise alignment of the sensor post  59  and cap  41  is oftentimes important. In the case of a bore seal, an effective seal may be more sensitive to precise alignment, as a bore seal requires a sensor post ring to fit entirely within the cap  41 . A face seal may also be sensitive to precise alignment, as a shift in alignment in any x, y, z-direction may cause a sensor post ring to lose contact at a point along a circumference of the cap&#39;s bottom edge  43 . Even after an effective seal is created, maintaining the seal is also important for accurate pressure sensing. Due to an exertion of force by the sensor post  59  onto the cap  41  as well as continuous impact forces exerted by the valve actuators  56  upwards onto the cassette  16  during a fluid processing procedure, the cassette  16  may incur deflection or distortion during the fluid processing procedure. Such deflection and/or distortion of the cassette  16  may lead to decoupling of the sensor post  59  to the cap  41 , leading to a potential leak in the seal. 
         [0041]    Turning to  FIG. 8A , a cross-sectional diagram of cap  41   c  according to an exemplary embodiment is shown. Cap  41   c  may comprise a cylindrical body  44  having an inner wall  44   a  having a diameter A and an outer wall  44   b  having a diameter B. A sloped wall  45  may connect the inner wall  44   a  to the outer wall  44   b.  The sloped wall  45  may form an opening  46  having a diameter that gradually decreases from B to A moving inwards further into the opening  46  of cap  41   c.  The sloped wall  45  may be disposed at an acute angle α relative to the vertical axis Zc of the seal. Angle α may have a value in the range of 30 to 60 degrees, preferably in the range of 40 to 50 degrees, and more preferably approximately 45 degrees. Angle α should preferably be large enough to maintain an effective seal even with minor shifts in the x-y (horizontal) direction between a sensor post and the cap  41   c.  The sloped wall  45  may comprise a surface finish, including, for example, SPI B-1. A higher (smoother) surface finish may be conducive to an effective seal, as a lower (rougher) surface finish may require a higher seal force to compress a ring into void areas of the cap  41   c  to prevent leakage. 
         [0042]      FIG. 8B  shows a sensor post  59   c  according to an exemplary embodiment. Sensor post  59   c  comprises a main cylindrical body  61   c  and a flexible ring  60   c.  Ring  60   c  is configured to tightly engage the circumference of the cylindrical body  61   c  to provide a tight seal. A bottom lip  62   c  may support the ring  60   c  and ensure that the ring  60   c  stays in place when the ring  60   c  is compressed against a cap of a cassette. A top lip  63   c  having a diameter T greater than the diameter C of the cylindrical body  61   c  may retain the ring  60   c  so that the ring  60   c  does not slide upwards off the sensor post  59   c.  Ring  60   c,  when engaged with the cylindrical body  61   c,  may have an outer diameter R. According to an exemplary embodiment, ring  60   c  may have a hardness value on the durometer scale in the range of 20 to 100 durometers, preferably in the range of 35 to 60 durometers, and more preferably approximately 40 durometers. The ring material may comprise one or more suitable materials, such as silicone, nitrile rubber, and/or a fluoropolymer elastomer, such as Viton. 
         [0043]      FIG. 8C  is a cross-sectional diagram showing a sensor post  59   c  fitted against the cap  41   c.  A seal is created by the ring  60   c  being fitted against the cap  41   c  such that the ring  60   c  provides an effective barrier to fluid flow. The ring  60   c  exerts a force Fc against the cap  41   c  perpendicular to the sloped wall  45  of the cap  41   c  and at angle α to the vertical axis Zc of the seal, the same angle as that which is formed between the sloped wall  45  and vertical axis Zc of the seal. A seal such as that depicted in  FIG. 8C  in which the ring  60   c  exerts a force Fc at an acute angle to the vertical axis Zc of the seal may be referred to herein as a crush seal. The crush seal described herein may be maintained without breaking due at least in part to its larger tolerance to movement and/or shifting between the cap  41   c  and sensor post  59   c  in any x, y, or z direction before an effective seal is lost. 
         [0044]    In one embodiment, the inner wall  44   a  of the cap  41   c  may have a diameter A of at least 0.010 inches, preferably at least 0.250 inches, and more preferably approximately 0.281 inches. The outer wall  44   b  may have a diameter B greater than diameter A, preferably at least 0.030 inches greater than diameter A, and more preferably approximately 0.060 inches greater than diameter A. The difference in value of diameters A and B should be large enough to maintain robustness of the cap  41   c  to maintain its shape while in contact with the sensor post  59   c.  The top lip  63   c  of the sensor post  59   c  may have a diameter T having a value in the range of 1-25% less than diameter A and more preferably 5-10% less than diameter A. Diameter T of the top lip  63   c  should be a value low enough to prevent the top lip  63   c  from interfering with an effective seal. The outer diameter R of ring  60   c  may be a value greater than the value of diameter A and less than the value of diameter B. A ring having an R-value less than that of diameter A may not make contact with the cap  41   c  and not achieve an effective seal. A ring having an R-value greater than diameter B of the cap  41   c  may create a face seal instead of a crush seal. 
         [0045]      FIG. 9  is a cross-sectional diagram showing one side of a sensor post  59   c  fitted against the cap  41   c  at different compression levels, according to an exemplary embodiment. When the sensor post  59   c  is engaged with the cap  41   c,  ring  60   c  may be lodged between the cap  41   c  and the cylindrical body  61   c  of the sensor post  59   c.  The ring  60   c  may be positioned on the bottom lip  62   c,  making contact with a top surface  62   d  of the bottom lip  62   c.  The ring  60   c,  being tightly engaged with the cylindrical body  61   c,  may be compressed against a vertical surface  61   d,  along the length of the cylindrical body  61   c,  between the bottom lip  62   c  and the top lip  63   c.  The ring  60   c  may also make contact with the top lip  63   c  at a bottom surface  63   d  of the top lip  63   c.  An outer portion of the ring  60   c  may make contact with and/or be compressed by a contact surface  45   d  along the sloped wall  45  of the cap  41   c.  The sloped wall  45  may connect the inner wall  44   a  of the cap  41   c  to the outer wall  44   b.  The sloped wall  45  of the cap  41   c  may taper off its angle α to a horizontal or near-horizontal surface  45   e  as the sloped wall  45  meets the outer vertical wall  44   b.  The sloped wall  45  may also taper off its angle α to a vertical or near-vertical surface as the sloped wall  45  meets the inner wall  44   a.    
         [0046]    In one embodiment, the inner wall  44   a  has a diameter A of 0.281 inches, the outer wall  44   b  has a diameter B of 0.341 inches, the top lip  63   c  of the sensor post  59   c  has a diameter T of 0.265 inches, and the ring  60   c  has a diameter R in the range of 0.281 to 0.341 inches. In  FIG. 9 , P 0  indicates a position at which the sloped wall  45  of the cap  41   c  makes initial contact with the ring  60   c.  P 1  refers to a minimum position at which wall  45  is in contact with ring  60   c  and is still able to maintain a seal. Compared to P 0 , at position P 1 , the bottom-most point  45   a  of wall  45  is 0.003 inches below that of position P 0 . Position P 1  may result when all valve actuators  56  of the valve and sensor assembly  54  ( FIG. 5 ) are making impact onto the cassette  16 , resulting in cassette deflection. Position P 2  may result when fewer than all valve actuators  56  are making impact onto the cassette  16 , resulting in less cassette deflection than when all valve actuators are making impact. Compared to P 1 , at position P 2 , the bottom-most point  45   a  of wall  45  is approximately 0.020 inches below that of position P 1 . Position P 3  may be described as the nominal position of the cassette  16  when cassette deflection is at zero. Compared to P 2 , at position P 3 , the bottom-most point  45   a  of wall  45  is approximately 0.030 inches below that of position P 2 , and 0.053 inches below that of position P 0 . 
         [0047]      FIG. 10  is a cross-sectional diagram showing one side of a sensor post  59   c  fitted against the cap  41   c  at different compression levels, according to an exemplary embodiment. In this embodiment, a shim  64  is inserted between the lower lip  62   c  and the ring  60   c.  The inner wall  44   a  may have a diameter A of 0.281 inches, the outer wall  44   b  may have a diameter B of 0.341 inches, the top lip  63   c  of the sensor post  59   c  may have a diameter T of 0.265 inches, and the ring  60   c  may have a diameter R of between 0.281 and 0.341 inches. The shim  64  may be inserted between the ring  60   c  and the bottom lip  62   c  to increase tolerance in the z-direction to account for cassette deflection during a fluid processing procedure. The shim  64  may have a height S in the range of 0.001 to 0.100 inches, preferably in the range of 0.010 to 0.030 inches, and more preferably approximately 0.020 inches, although any suitable shim height may be used. According to various embodiments, the shim may be compressible or non-compressible. 
         [0048]    In  FIG. 10 , in one embodiment in which a shim  64  having a height S of 0.020 inches is implemented, position Q 0  may comprise bottom-most point  45   a  of wall  45  resting 0.020 inches below position P 0  of  FIG. 9 . Unlike at position P 0 , at position Q 0 , wall  45  may still sufficiently compress ring  60   c  to maintain an effective seal, due to the added height of 0.020 inches provided by the shim  64 . Q 1  refers to a new minimum position at which wall  45  is in contact with ring  60   c  and able to maintain a seal. Compared to Q 0 , at position Q 1 , the bottom-most point  45   a  of wall  45  is 0.003 inches below that of position Q 0 . Compared to P 1 , at position Q 1 , the bottom-most point  45   a  of wall  45  is 0.020 inches below that of position P 1 , due to the shim  64 . Position Q 1  may result when all valve actuators  56  of the valve and sensor assembly  54  are making impact onto the cassette  16 , resulting in cassette deflection, Position Q 2  may result when fewer than all valve actuators  56  are making impact onto the cassette  16 , resulting in less cassette deflection than when all valve actuators are making impact. Compared to Q 1 , at position Q 2 , the bottom-most point  45   a  of wall  45  is approximately 0.020 inches below that of position Q 1 . Compared to P 2 , at position Q 2 , the bottom-most point  45   a  of wall  45  is 0.020 inches below that of position P 2 , due to the shim  64 . Position Q 3  may be described as the new nominal position of the cassette  16  when cassette deflection is at zero. At position Q 3 , the bottom-most point  45   a  of wall  45  is approximately 0.030 inches below that of position Q 2 , 0.053 inches below that of position Q 0 , 0.073 inches below that of P 0 , and 0.020 inches below that of P 3 . 
         [0049]    In the embodiment in which a shim  64  of 0.020 inches is implemented, a compression of at least 0.005 inches may consistently be maintained at all times even when all valve actuators are making impact simultaneously, preferably a compression of greater than 0.020 inches, and more preferably a minimum compression of approximately 0.023 inches. In an embodiment in which inner wall  44   a  has a diameter A of 0.281 inches, top lip  63   c  of the sensor post  59   c  has a diameter T of 0.265 inches, and shim  64  has a height S of 0.020 inches, an effective seal may still be maintained when the cap  41   c  and sensor post  59   c  have shifted away from axis Zc ( FIG. 8C ) from each other in an x/y direction of up to 0.030 inches from their original concentric positions about axis Zc. 
         [0050]    The embodiments disclosed herein are for the purpose of providing a description of the present subject matter, and it is understood that the subject matter may be embodied in various other forms and combinations not shown in detail. Therefore, specific embodiments and features disclosed herein are not to be interpreted as limiting the subject matter as defined in the accompanying claims.