Abstract:
A method of sensing fluid pressure in which a deformable tube provides an accessible and substantially flat section. Deflection of the flat section reflects line pressure. In one embodiment, a fluid pressure sensing apparatus comprises a deformable tube for carrying fluid and at least one constraint member comprising an inner surface and a pressure sensing opening. The inner surface comprises a substantially flat portion and a constraint portion. The tube is positioned against the inner surface and constrained by the constraint portion such that a portion of the tube is deformed against the substantially flat portion of the inner surface. This produces a substantially flat section of the tube adjacent to, and accessible through, the pressure sensing opening. Similarly, a method of manufacturing a fluid pressure sensing apparatus comprises the steps of: providing a deformable tube, and constraining the tube to provide an accessible and substantially flat section of the tube. The method and apparatus are suitable for cardioplegia safety systems and other systems involving a shear sensitive fluid such as blood.

Description:
FIELD OF THE INVENTION 
     This invention relates to sensing fluid pressure. 
     BACKGROUND 
     Pressure measurements of fluid flowing within a tube can be made in a variety of ways. For example, a strain gage, may be placed on the outside of the tube. However, as internal pressure of a tube changes, the tube “balloons,” i.e., the tube wall stretches, varying the tube wall thickness, in the area where the strain gage is located. Forces due to the internal pressure of the tube, and forces due to variations in thickness along the tube wall, hinder accurate measurement of the internal pressure alone. 
     Another method uses a “T” fitting to divert a portion of the fluid to a pressure transducer. This method adds cost, complexity, and increases the probability of leakage, and may produce trauma to the fluid (if the fluid is blood, trauma can result in hemolysis). 
     SUMMARY OF THE INVENTION 
     The invention is a method of sensing fluid pressure in which a deformable tube provides an accessible and substantially flat section. Deflection of the flat section reflects line pressure. In one embodiment, the invention is a fluid pressure sensing apparatus, comprising a deformable tube for carrying fluid and at least one constraint member comprising an inner surface and a pressure sensing opening. The inner surface comprises a substantially flat portion and a constraint portion. The tube is positioned against the inner surface and constrained by the constraint portion such that a portion of the tube is deformed against the substantially flat portion of the inner surface. This produces a substantially flat section of the tube adjacent to, and accessible through, the pressure sensing opening. Similarly, in another embodiment, a method of manufacturing a fluid pressure sensing apparatus comprises the steps of: providing a deformable tube, and constraining the tube to provide an accessible and substantially flat section of the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1A to  1 E are cross-sectional views of embodiments of the invention. 
     FIG. 2A is a perspective view of one embodiment of the invention. 
     FIG. 2B is an exploded view of the embodiment of FIG.  2 A. 
     FIG. 2C is a top view of the embodiment of FIG.  2 A. 
     FIG. 3A is a cross-sectional view of the embodiment of FIG. 2A taken line  3 A— 3 A. 
     FIG. 3B is a cross-sectional view of a further embodiment of FIG. 2A taken along line  3 A— 3 A. 
     FIG. 4A is a perspective view of assembler tool. 
     FIG. 4B is a perspective view of the combination of an assembler tool and a tube constraint apparatus. 
     FIG. 5 is a flow diagram illustrating a method of assembly for a tube constraint apparatus. 
     FIG. 6A is a perspective view of a pressure sensor housing. 
     FIG. 6B is an exploded view of the pressure sensor housing of FIG.  6 A. 
     FIG. 7 is a cross-sectional view of the embodiment of FIG. 3B positioned in the pressure sensor housing of FIG.  6 A. 
     FIG. 8 is a schematic view of a cardioplegia safety system using a tube constraint apparatus, such as the tube constraint apparatus shown in FIGS.  2 A- 2 C. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1A, a tube  20  is constrained as to facilitate ease and accuracy of fluid pressure measurements. Fluid pressure in the tube  20  is measured by constraining the tube  20  such that fluid pressure measurements are taken at an accessible, substantially flat section  26  of the tube  20 . Reliable fluid pressure measurements can easily be taken through a pressure sensing opening  30  defined in a constraint member  18 . The substantially flat shape of the tube  20  at the pressure sensing opening  30  minimizes the influence of tube variables, such as the tube wall thickness, which can prevent accurate fluid pressure measurement. In effect, the substantially flat section  26  of the tube  20  acts like a pressure diaphragm, measuring only linear deflection of the tube wall in response to fluid pressure on the tube wall. 
     Fluid flow in the tube  20  should not be substantially impeded. For example, impeding fluid flow may create a pressure drop in the area of constraint, undesirably altering the pressure measurements. Furthermore, it is undesirable to impede fluid flow through the tube  20  such that damaging shear forces act on the fluid. When shear-sensitive fluids, such as blood, flow through the tube  20 , damaging shear forces can break down various components of the fluid. For example, excessive shear force can cause hemolysis of blood. 
     Accurate fluid pressure readings within a fluid flow system are important. Fluid pressure readings can be used to control the fluid flow. 
     FIG. 1A, shows one embodiment of the constraint member  18 . Tube  20  is constrained in a tube opening  21  such that stresses from spring forces and other variations in the tube wall thickness are localized at two areas  22  and  24  on either side of the substantially flat section  26  of the tube  20 . A change in curvature of the tube  20  creates localized stress at the areas of curvature  22  and  24 , substantially fixing these areas  22  and  24  in the tube opening  21  defined by the constraint member  18  and forming the substantially flat section  26 . 
     The constraint member  18  has an inner surface  32  that defines the tube opening  21 . The inner surface  32  includes a substantially flat portion  28  and a pressure sensing opening  30 . The inner surface  32  also includes a constraint portion  33  for use in constraining the tube  20  in the tube opening  21   
     The tube  20  is constrained in the tube opening  21  such that a substantially flat section  26  of the tube  20  is formed adjacent the substantially flat portion  28  of the inner surface  32 . The tube  20  is further constrained in the tube opening  21  such that a first constrained section  22  of the tube  20  adjacent or along a first side of the substantially flat section  26  of the tube  20  is substantially fixed within the tube opening  21 . A second constrained section  24  of the tube  20  adjacent or along a second side of the substantially flat section  26  of the tube  20  is also substantially fixed within the tube opening  21 . 
     The first and second constrained sections  22  and  24 , respectively, of the tube  20  correspond to the areas of localized stress at areas of curvature described previously. The constrained sections  22  and  24  maintain the substantially flat section  26  of the tube  20 . A void  23  may be created between each constrained section  22  and  24  of the tube  20  and the inner surface  32 . The voids  23  are located at about the intersection of the substantially flat portion  28  and the constraint portion  33  of the inner surface  32 . Such voids  23  accommodate tolerances in defining the tube opening  21  by the constraint member  18 , and are created when the tube  20  is constrained in the tube opening  21 . For example, as shown in FIG. 1A, the voids  23  are created such that the substantially flat section  26  is not forced into the sensing opening  30  or away from the substantially flat portion  28  of the inner surface  32 . The voids  23  help accommodate typical tolerances in the dimensions and properties of the tube  20 , but they are not essential to the invention. 
     The change in curvature of the tube  20  substantially fixes the constrained sections  22  and  24  within the tube opening  21 . Substantially fixing the tube  20  by localizing the stresses at constrained sections  22  and  24  provides the flat diaphragm-like section  26  accessible at the pressure sensing opening  30 . The tube  20  is substantially fixed when it remains stationary despite substantial variations in the fluid system pressure, and does not resume its pre-constrained configuration. In particular, the tubing must have an elasticity which is essentially constant over the anticipated temperature range of the fluid within the tubing. In preferred embodiments of the invention, the tubing is made of a silicone-based material suitable for medical grade transport of blood and/or cardioplegia applications, for which the relevant temperature range is about 2-40 degrees Celsius. The preferred tubing for such applications is standard medical grade tubing manufactured by conventional techniques from the general purpose elastomers available under the tradename SILPLUS models SE6035 and SE6075 from the General Electric Corporation (http://www.ge.com). The materials are blended together by conventional techniques to achieve durometer of 55 to 65 Shore A, most preferably 55 Shore A. In other applications of the invention, the tubing material would be chosen to fit the particular circumstances present. 
     In any embodiment of the invention, the constraint member  18  can be a single integral component or comprise any number of component parts. In the example of FIG. 1A, the constraint member  18  includes a first constraint component  34  and a second constraint component  35 . This embodiment provides ease of assembly when the assembler tool and method of assembly described below. Similarly, in any embodiment of the invention any component can itself be formed of more than one component or it can be one integral component. 
     When the constraint member  18  includes more than one component, as illustrated in FIG. 1A, the first constraint component  34  may include the substantially flat portion  28  of the inner surface  32 , and pressure sensing opening  30 . 
     The first constraint component  34  includes a channel having a bottom wall  36  and two side walls  37 . The channel aids the assembly of the constraint member  18  by providing a guide for positioning and formation of the substantially flat section  26  of the tube  20 . The first constraint component  34  substantially fixes the first and second constrained sections  22  and  24  of the tube  20  at intersections of the bottom wall  36  and the two side walls  37  within the channel. 
     The second constraint component  35  is shaped suitable to engage the first constraint component  34 , and thus modify the shape of the tube  20  in the tube opening  21 . The shape of the tube  20  is modified to substantially fix the first and second constrained sections  22  and  24  as described above. 
     The constraint member  18 , whether it is one integral component or more than one component, defines the tube opening  21  to comprise at least two bends of about 90 degrees or less, thus substantially fixing sections  22  and  24  of the tube  20  adjacent respective sides of the substantially flat section  26  within the tube opening  21 . A first bend is located along one side of the substantially flat section  26  of the tube  20  and adjacent the first constrained portion  22 . A second bend is located along an opposing side of the substantially flat section  26  of the tube  20  and adjacent the second constrained portion  24 . In this embodiment, the tube  20  (apart from the substantially flat section  26 ) is a semi-circular, or semi-elliptical, shape within the tube opening  21 . 
     However, the shape of the tube opening  21  can include one or more additional bends as shown in the illustrative embodiments of FIGS. 1B-1E. For example, as illustrated in FIGS. 1B and 1C, when the inner surface  132 ,  232  of the constraint member  118 ,  218 , respectively, has three bends, the tube  20  is constrained in a substantially triangular shape within a similarly shaped tube opening  121 ,  221 . The substantially flat portion  128 ,  228  of the constraint member  118 ,  218  lies between two of the bends. 
     The preferred bend angle is approximately 90 degrees, as this has been found to provide an optimum amount of capture of the tube in the apparatus in the axial direction, i.e., the tube does not slip out of the apparatus along its length in either direction. 
     As further illustrated in FIG. 1B, an optional escape portion  138  enables a void to be created between the tube  20  and the constraint member  118  is defined in the second constraint component  135  of the constraint member  118  (which includes first and second constraint components  134  and  135 ). Escape portion  138  is defined in the second constraint component  135  of the constraint member  118  (which includes first and second constraint components  134  and  135 .) Even with escape portion  138  defined in the second constraint component  135  of the constraint member  118 , constrained sections  22  and  24  of the tube  20  remain substantially fixed on opposite sides of the substantially flat section  26  of the tube  20 . The escape portion  138  and associated void are located adjacent a region of the tube  20  outside of the substantially flat section  26 . For example, when the tube opening  121  is substantially triangular-shaped, an escape portion  138  can be defined at a bend opposite the substantially flat portion  128  of the inner surface  32 , i.e., at an apex of the triangular tube opening opposite the flat portion  128 . The escape portion  138  defines a void between the tube  20  and the constraint member  18 . 
     In the embodiment of FIG. 1B, the escape portion  138  defined in the inner surface  132  is symmetrically located with symmetry relative to the substantially flat section  26  of the tube  20 . The escape portion  138  is located about equidistant from each end of the substantially flat portion  128  of the inner surface  132 , i.e. symmetrically at the apex opposite the substantially flat section  26 . 
     In the embodiment of FIG. 1C, two escape portions  238  (creating voids between the tube  20  and the constraint member  218 ) are defined in the second constraint component  235  of the constraint member  218 . Constraint member  218  includes first and second constraint components  234  and  235 . The voids are symmetrically located relative to the of the substantially flat portion  228  of the inner surface  232 . The escape portions  238  are located about equidistant from each end of the substantially flat portion  228  of the inner surface  232 , i.e., at the same position adjacent respective legs of the triangular shaped tube. 
     Thus, in both embodiments the escape portion  138  or multiple escape portions  238  are symmetrically located in the constraint components  135  and  235  with respect to of the substantially flat portion  128 ,  228  of the inner surface  132 ,  232  of the respective constraint members  118 ,  218 . This relationship is generally true; that any number of escape portions may be utilized and that any position of such escape portions, including symmetric positioning with relative to the substantially flat portion of the constraint member, is possible. 
     Constraint of the tubes illustrated in FIGS. 1B and 1C is performed without the use of a channel within the first constraint component  134 ,  234  of the respective constraint member  118 ,  218 . Constraint section  22 ,  24  of the tube  20  are formed substantially at the intersection of the first and second constraint components of the constraint members  118  and  218 . 
     As described previously with reference to FIG. 1A, the tube  20  is constrained in a substantially semi-circular or semi-elliptical shape. As described previously with reference to FIGS. 1B and 1C, the tube  20  is constrained in a substantially triangular shape. FIG. 1B, the escape portion  138  is located at an apex or bend of the substantially triangular shape between the tube  20  and the constraint member  118 . In FIG. 1C, the escape portions  238  are located along legs of the substantially triangular shape between the tube  20  and the constraint member  218 . In these embodiments, the pressure sensing opening  30  is located approximately at a center of a base of the substantially triangular shape. 
     The tube  20  need not be constrained in a Triangular shape or semi-circular or semi-elliptical shape. The tube  20  may take any shape as long as the substantially flat section  26  of the tube  20  is formed by substantially fixing respective sections adjacent or along opposing sides of the substantially flat section  26  within the tube opening, formed by the various constraint members. Such other configurations may also include one or more escape portions in the various configurations of the constraint members defining the tube opening as described above. 
     For example, as illustrated in FIG. 1D, the lube  20  can be constrained in a substantially trapezoidal shape. Respective ends of the substantially flat section  26  of the tube  20  are substantially fixed at constrained sections  22  and  24  of the tube  20  at the intersection of the constraint component  335  and constraint component  334 . Voids  323  are located between the constrained sections  22  and  24  and the constraint member  318  at such intersections and at the various other bends of the constraint member  318 , The voids perform the same functions as described above with respect to the voids  23  and may also function like the escape portions previously described. The tube opening  321  of this configuration is defined by the constraint portion  333  and substantially flat portion  328  form the substantially flat section  26 , accessible through opening  30 . 
     Another example is illustrated in FIG.  1 E. In this embodiment, the tube  20  is constrained in a substantially rectangular shape in a similarly shaped tube opening  421  defined by constraint surface  433  and flat surface  428  of inner surface  432 . Respective sections adjacent the substantially flat section  26  are substantially fixed at the intersection of the walls  437  and bottom  436  of a channel defined in the first constraint component  434 . Voids  423  between the tube and constraint member  418  function similar of FIGS. 1B and 1C. The substantially flat section  26  of the tube  20  is accessible through the pressure sensing opening  30  defined in the first constraint component  434 . 
     Pressure measurements are taken on the substantially flat section  26  of the tube  20  using any suitable pressure sensing mechanism  39 , as illustrated in FIG.  1 A. For example, a commercially available strain beam, such a model 800 planar beam sensor available from Revere Transducers, Inc. of Cerritos, Calif. (http://reveretransducers.com) can be used. The strain beam is coupled to a pin  65  in contact with the tube  20 , as generally illustrated in FIG.  1 A and illustrated in one particular embodiment in FIG.  7 . The diameter of the pin  65  can be, for example, about 0.060 to about 0.1875 inches. The larger the pin diameter, the larger the force that can be read per pressure reading. By reading a larger force, a more accurate pressure reading can be taken. Thus, the diameter of the pin is at least one element that sets the sensitivity of the pressure measurement. 
     More than one tube  20  can be constrained, as illustrated in FIGS. 2A-3A; this embodiment is substantially a dual representation of FIG.  1 A. Further detail of a dual constraint tube apparatus  13 , including a dual tube constraint member  19 , FIGS. 2A to  2 C. The dual tube constraint member  19  includes a first constraint component  534  and second constraint component  535 . The first constraint component  534  includes two pressure sensing openings  30  each having a diameter large enough to allow for measurement of linear deflection of tubes  20 . The larger the diameter of the pressure sensing openings  30 , the larger the pressure sensing area can be for sensing deflection of the tube  20  (i.e., a larger pin diameter in pressure sensing mechanism  36 ). As described above, a larger pressure sensing area typically results in less error in the measurement. For example, the diameter of the pressure sensing openings  30  may be about 0.25 inches. 
     As shown in FIG. 3A, the first constraint component  534  includes channels  539 . Each channel  539  includes a bottom wall  536  and two side walls  537 . The channels  539  aid the assembly process of the constraint member  19  by providing a guide for formation of the substantially flat section  26 . The first constraint component  534  substantially fixes the first and second constrained sections  22  and  24  of each tube  20  at intersections of the bottom wall  536  and the two side walls  537  within each channel  539 . 
     The second constraint component  535  is engaged with the first constraint component  534  encasing and constraining the tubes  20 . The first and second constraint components  534  and  535  can be manufactured such that they interlock. For example, as illustrated in FIGS. 2B and 3A, posts  541  on the second constraint component  535  fit into channels  543  in the first constraint component  534 . Any suitable other locking mechanism may be used. 
     The bottom walls  536  of the channels  539  of first constraint component  534  are substantially flat. This facilitates forming of the substantially flat sections  26  of the tubes  20 . As shown in FIG. 3A the shape of the second constraint component  535  is semi-elliptical or semi-circular. This fixes the constrained sections  22 ,  24  of the tube  20  which are adjacent the substantially flat section  26 . The shape in which the tube is constrained can vary widely, i.e., trapezoidal, rectangular, etc. The second constraint component  535  may include escape portions or voids. The constraint components forming the member  19  can be integral components or multiple components. 
     FIG. 3A is a cross-sectional view of the dual tube constraint member  19  without escape portions which constrain two tubes  20 . For many applications, it is desirable to obtain pressure measurements on more than one tube  20 , each pressure measurement having the same sensitivity and extraneous forces present. Thus, all tubes  20  within the multiple tube constraint member  19  should have identical number, shape, and location of escape portions, if utilized. As shown in FIG. 3A, the tubes  20  are constrained in the tube openings  521  such that both tubes include a substantially flat section  26  accessible through a respective sensing openings  30 . 
     As further shown in FIGS. 2A-2C, the dual constraint tube apparatus  13  may include couplings  50 ,  150  at each end of the tubes  20 . The couplings  50 ,  150  permit connection of the tubes  20  to other parts of a system in which the apparatus  13  is used. Each of the couplings  50  are sized to fit within a channel  48  defined at one end of the first and second constraint components  534 ,  535 . 
     In FIG. 3B, the constraint member  219  includes a first constraint component  554  and second constraint component  555 . This embodiment is substantially the same as the embodiment shown in FIG. 3A, with the addition of two escape portions  538  defined in the second constraint component  555  in substantially the same manner and for the same purpose as described with reference to FIG.  1 B. Even with the escape portions  538  defined in the second constraint component  555 , constrained sections  22  and  24  of the tube remain substantially fixed on opposite sides of the substantially flat section  26  or of each tube  20 . The escape portions  538  and voids are located adjacent regions of the tubes  20  outside of the substantially flat sections  26 . 
     Apparatus  13  should be assembled such that multiple tubes  20  can be constrained in substantially the same way, or consistently and reproducibly constrained from one apparatus to the next, or both. An assembler tool  40 , as that illustrated in FIGS. 4A-4B, may be used. In general, assembly of at least one tube  20 , may follow the steps described in the flow diagram of FIG. 5 but other assembly methods resulting in a constrained tube or tubes may also be followed. 
     As an example, dual tube constraint apparatus  13  as shown in FIGS. 2A-3A may be assembled using the assembler tool  40  of FIGS. 4A and 4B and the method shown in FIG.  5 . The assembly method includes positioning the first constraint component  534  in a depression  57  defined in an assembler tool body  52  of the assembler tool  40 . The depression  57  is sized to receive the first constraint component  534  securely i.e., (does not move from side to side). When placing the second constraint component  535  over the tube  20  and first constraint component  534 , tubes  20  should not be pushed through the pressure reading openings  30 . Thus, filler elements  53 , formed in a depression  57  of the assembler tool  40 , provide for filling of the pressure reading openings  30  during the positioning of the second constraint component  535  relative to the first constraint component  534 . The assembler tool has filler elements  53  corresponding to each tube  20 . Filler elements  53  extend through the pressure reading opening  30  of the first constraint component  534  when each filler element is about planar with an inner surface  532  of the first constraint component  534 . Filler elements  53  prevent the substantially flat sections  26  from moving into the pressure sensing openings  30 . 
     If necessary, an optional bonding material can be applied to the first constraint component  534 , or the second constraint component  535  or both. The inner substantially surface  532  and the inner constraint surface  533  are preferred locations for the bonding material. Prior to positioning the tube  20  relative to the first constraint component  534 , UV-curable silicone adhesive is preferred for ease of application, curing and bonding qualities. The cured bonding material, prevents movement of the tube  20  within the constraint member  19 . The assembly may be completed prior to exposure of the assembly to UV radiation. It is preferred to avoid use of the bonding material, if possible, to avoid additional effort and cost. 
     After the first constraint component  534  is positioned in the depression  57 , tubes  20  are located in channels  56 . The channels  56  provide for precise positioning of the tubes  20  over and across the first constraint component  534 . Each tube  20  is positioned in a stretched configuration across the first constraint component  534  by engaging each end of the tube  20  with an appropriate tool. For example, the mechanism as shown in FIGS. 4A-4B includes tube couplings  50 ,  150  each fitted in the ends of the tube  20 , as shown in FIGS. 2A-2C. Each coupling  50 ,  150  fits into an engagement aperture  48 ,  148  integrally formed in the first constraint component  534 , such as illustrated in FIG.  2 B. When the tube couplings  50  are engaged with the apertures or channels  48  in the first constraint component, and when the first constraint component is positioned in the depression  57 , the tube is secured at one end of the assembler tool. The other engagement apertures  148  are defined in laterally movable element  149  and sized for receiving tube couplings  150 . Thus, when movable element  149  is fixed to body  52  and the couplings  150  in position, the tube is secured at the other end of the assembler tool  40 . The positioning of the movable element  149  allows the tube to be stretched, across the constraint component  534 . 
     To assemble each tube  20  within constraint member  19 , tension resulting in about 5% to about 10% elongation of the tube  20  is preferred. This stretches each tube  20  across the first constraint component  534  so that a substantially linear tube section is positioned across the first constraint component  534 . 
     Next, the second constraint component  535  is loosely positioned over the tubes  20  and the first constraint component  534 . The second constraint component  535  is positioned such that inner surfaces  533  contact the tubes  20 . As described previously, a bonding material is applied prior to positioning. 
     In another optional step, prior to actually constraining the tubes  20  between the first and second constraint components  534  and  535 , tubes  20  are pressurized with any suitable fluid (gas of liquid), and then, if necessary, again as the second constraint component  535  is interlocked with the first constraint component  534 . Alternatively, the second constraint component  535  can be placed over the tubes  20  before the tubes  20  are pressurized. To pressurize the tubes  20 , one end of each tube  20  is plugged using any suitable method. A tube stop (not shown) could be inserted at either end of each tube. And fluid introduced in the other end. Pressures on the order of 800 mmHg are suitable. 
     Once the tube  20  is pressurized and the second constraint component  535  is positioned over the first constraint component  534 , a positioning element  54  of the assembler tool  40  applies a force on the second constraint component  535 . The positioning element  54  is rotatably mounted along its rear edge  55  on the assembler tool body  52 , such that a pin  59  contacts the second constraint component  535  aligning the components  534  and  535  together. After assembly of the apparatus  13  is completed, it is removed from the assembler tool  40 . 
     Using an assembler tool  40  as described above is only one way of constraining the tubes  20 . For example, a tube could be inserted into a tube opening having a shape to form the substantially flat section. 
     Once the tubes  20  are constrained within the dual constraint member  19 , as illustrated in FIGS. 2A-2C, the apparatus  13  is positioned within a pressure sensor housing, such as the pressure sensor housing  60  illustrated in FIGS. 6A-7. The pressure sensor housing  60  contains components for positioning a sensing mechanism relative to the substantially flat section  26  of the tube  20 , as illustrated in the cross-sectional view of FIG.  7 . In FIG. 7, the dual constraint apparatus  13  is positioned in the sensing housing  60  with pins  65  positioned adjacent the substantially flat section  26  of the tube  20 . 
     The pressure sensor housing  60 , as shown in the exploded view of FIG. 6B, includes a main pressure assembly block  62  having an opening  63  for receiving pressure sensing pins  65 . S-shaped members  64  are coupled to the pins  65  for translating linear deflection of the tube  20  to strain beams  66  fixed to the S-shaped members  64  with movable clamps  68  and fixed clamps  70 . A pressure seal plate  72  is coupled to the assembly block  62 , opposite the strain beams  66 . Openings  73  in the pressure seal plate  72  allow the pins  65  contact the tube  20  through the pressure sensing openings  30  in the constraint member  19 . A pressure fitting latch  74  and pins  76  complete the pressure sensor housing  60  A conventional arrangement of spring loaded pins and detents is suitable. The dual constraint member  19  and tubes  20  are placed in the housing as illustrated in the cross-sectional view of FIG.  7 . The pressure sensor housing and method of assembly should provide precise and repeatable positioning. Conventional techniques, such as the use of close tolerances and spring loaded assembly, are preferred. 
     Pressure measurements taken from the substantially flat sections  26  of tubes  20  have an error of +/−10 mmHg when taking pressure readings of about 0 to about 100 mmHg. Pressure measurements have an error of +/−10% when taking pressure readings of about 100 to about 500 mmHg. In general, the design requirements for the apparatus are those found in conventional cardioplegia delivery systems and components. 
     A preferred application of the invention is measurement of fluid pressure in a cardioplegia safety system (CSS). As illustrated in FIG. 8, two tubes  20  are constrained in a dual constraint member  19 . Fluid pressure measurements are taken at the substantially flat sections of the tubes  20  The two tubes  20  are constrained in substantially the same shape, so as to be able to provide uniform, accurate readings of fluid pressure within both tubes  20 . One tube  20  carries oxygenated blood pumped from an external blood oxygenator  80 . The other tube  20  carries drugs, such as a candioplegia solution, pumped from an external drug supply  82 . The dual constraint apparatus  13  is connected to the oxygenator  80  and drug supply  82  using the tube couplings  150 . Roller pumps, linear pumps, rotary peristaltic pumps, or any other suitable pumps can be used. 
     When using a CSS, accurate fluid pressure within each tube  20  helps ensure that correct drug dosages are be delivered to a patient  84 . 
     If more than two tubes  20  are used, the constraint member  19  is modified in accordance with the number of tubes  20  used, so as to obtain uniform, accurate readings of fluid pressure within each tube  20 . 
     Another application of the method and apparatus of the invention is measurement of input blood pressure in a blood collection system. In this application, a single tube constraint member  18 , such as that illustrated in FIG. 1A, is positioned around a tube  20 . The tube  20  extends from a patient to a blood collector. Reliable pressure readings are taken using a pressure sensor mechanism through a pressure sensing opening  30  in the constraint member  18 .