Abstract:
A pressure measurement device usable for monitoring pressure of fluids such as blood, waste, and replacement fluid in a blood treatment system provides a reliable signal and other benefits by virtue of a number of features of the various embodiments disclosed. The pressure of fluid carried by a vessel or tube is measured by measuring a change in shape of the vessel or tube via a sensor element contacting it. Materials, shape, and mechanical support cooperatively ensure that the little inelastic strain occurs and pressure measurements are repeatable. The embodiments are compatible with the use of disposable vessels and tubes.

Description:
BACKGROUND 
     Pressure transducers are used widely for pressure measurement. An example prior art device is described in U.S. Pat. No. 4,576,181 and illustrated in  FIG. 1A . Such devices require connection to a flow channel or chamber to provide fluid communication with a sensor portion. For example, a flow channel  32  of a prior art device provides fluid communication between a diaphragm  45  and a vessel or conduit  30  containing a fluid whose pressure is to be measured, from some flow or containment system  47 . An intermediate fluid in a space  35  on an opposite side of the diaphragm  45  communicates with a pressure transducer  40 . The fluid whose pressure is to be measured exerts a pressure on the diaphragm  45  in turn exerting a pressure on the intermediate fluid in space  35 . A pressure transducer  40  generates a signal corresponding to the pressure of the intermediate fluid in the space  35  by any of various mechanisms, typically involving a strain gage or load cell. 
     Another known device for measuring pressure is illustrated in  FIG. 1B . In this device, a thin plate  30  has a strain gage  10  on a back surface  31  thereof. A pliant thin-walled vessel  20  rests against a front surface  32  of the thin plate  30 . When fluid  25  inside the vessel  20  pressurizes the vessel, which is bounded by walls  15  and  22 , thin plate  30  flexes, stretching a strain gauge  10  attached to it, thereby causing a signal from which pressure can be correlated by calibration. 
     The pressure sensor of  FIG. 1B  may be employed in medical systems and devices that transport biological fluids. In such systems, the use of certain plastics is very common, due to its durability, flexibility, low cost, and low chemical and biological reactivity. Such plastics, however, when strained, are susceptible to change in terms of their elastic response. 
     For example, if substantially deformed, thicker walled plastic vessels such as  20  in  FIG. 1B  will exhibit a condition known as “creep”, causing the displacement-versus-pressure response to change over time. Creep is caused by changes in the conformation of polymer molecules over time. Creep may lead to errors in measurement of pressure changes in a configuration such as that of  FIG. 1B . 
     Referring to  FIG. 1C , another type of prior art pressure sensor in which a pressure transducer  50  is in pressure communication with an interior  70  of a drip chamber  60 . Blood flows through an inlet tube  65  and out an outlet tube  75  while a trapped volume of air  62  communications pressure to the pressure transducer  50  through a coupling tube  57 . An isolator  55  protects the pressure transducer  50  by preventing any flow through it via a flexible membrane within it (not shown). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is one type of pressure sensor according to the prior art. 
         FIG. 1B  is another type of pressure sensor according to the prior art. 
         FIG. 1C  is yet another type of pressure sensor according to the prior art. 
         FIG. 2A  is a diagonal projection of two opposing portions of an inventive pressure transducer that detects changes in the shape of a flattened portion of a tube to measure pressure inside the tube. 
         FIG. 2B  is an exploded view of the components of the transducer of  FIG. 2A . 
         FIG. 3A  is a cross-sectional view of the transducer of  FIGS. 2A and 2B , showing the opposing parts separated prior to clamping around a portion of a plastic tube. 
         FIG. 3B  is a cross-sectional view of the transducer of  FIG. 3A , showing the opposing parts in a clamped position suitable for measurement of pressure changes in the plastic tube. 
         FIG. 4A  is a diagonal projection of the opposing halves of a pressure transducer suitable for measuring pressure changes in a thin-walled flexible vessel or conduit having large dimensions such that a confining spacing is provided by some external mechanism. 
         FIG. 4B  is a diagonal projection of the opposing halves of a pressure transducer suitable for measuring pressure changes in a hanging fluid bag commonly used for biological fluids and having large dimensions such that a confining spacing is provided by some external mechanism. 
         FIGS. 5A and 5B  are cross-sectional views of circular and elliptical tubes or vessels for purposes of discussing the effect of hoop-strength on pressure measurement. 
         FIG. 5C  is a cross-sectional view of a tube or vessel for purposes of discussing the effect of features that increase material strain and thereby impact pressure measurement. 
         FIG. 6A  is a cross-sectional view of an example of a tube or vessel configuration for purposes of discussing features that ameliorate pressure measurement even for thick material. 
         FIGS. 6B-6E  are cross-sectional views of tubes or vessels for discussing the effect of using thin walls and other features to ameliorate creep effects. 
         FIG. 6F  is a cross-sectional view of a tube or vessel for discussing the effect of aspect ratio and other features to ameliorate creep effects. 
         FIG. 7  is a cross-sectional view of a pressure transducer according to another aspect of the present invention, in which the transducer is shaped so as match a tube or vessel having a non-flat-shaped portion. 
         FIG. 8  is a cross-sectional view of a pressure transducer that uses a cantilever to transmit pressure changes to a load sensor. 
         FIG. 9A  is a cross-sectional view of a pressure transducer that uses a cantilever to transmit pressure changes to a tension transducer or extension displacement transducer. 
         FIG. 9B  is a side view of the pressure transducer design of  FIG. 9A . 
         FIG. 10  is a cross-sectional view of a pressure transducer providing a direct contact between a metal plate bearing a strain gage and a flattened portion of a tube or vessel. 
         FIG. 11  is a side view of a curved strain gage mounted on a flexible pillar transducer. 
         FIG. 12  is a cross-sectional view of an alternative mounting mechanism for the anvil employed in the embodiment of  FIGS. 2A and 2B  and an alternative location for a strain gage. 
         FIG. 13A  is a cross-sectional view of a pressure transducer, in which a flexible plate with a strain gage is positioned against a cross-sectional view of a pressure transducer. The pressure transducer is shown in an open position. 
         FIG. 13B  is a cross-sectional view of the pressure transducer of  FIG. 13A , shown in a closed operating position that enables measurement of pressure changes within the flattened portion. 
         FIG. 13C  is another aspect of the pressure transducer of  FIG. 13A , in which the pressure transducer is mounted on the base wall. The pressure transducer is shown in an open position. 
         FIG. 13D  is a cross-sectional view of the pressure transducer of  FIG. 13C , shown in a closed operating position that enables measurement of pressure changes within the flattened portion. 
         FIG. 14A  is a cross-sectional view of a pressure transducer, in which a flexible plate with a curved strain gage is positioned to wrap partly around a tube or vessel. The pressure transducer is shown in an open position. 
         FIG. 14B  is a cross-sectional view of the pressure transducer of  FIG. 14A , shown in a closed operating position that enables measurement of pressure changes within tube or vessel. 
         FIG. 15  illustrates an automatic calibration configuration for use with various aspects of the present invention. 
         FIG. 16  is a schematic diagram of a blood-processing machine, which may incorporate one or more of the various aspects of the present invention. 
         FIG. 17A  is a diagonal view of a blood treatment machine suitable for use with the cartridge of  FIG. 17A . 
         FIG. 17B  is an illustration of a cartridge and tubing set which is suitable for use in a blood treatment machine. 
         FIG. 17C  is an illustration of a pressure transducer suitable for use in the cartridge of  FIG. 17B , as mounted in the blood treatment machine of  FIG. 17A . 
         FIG. 18  is a flow chart showing a method for implementing a pressure transducer to make measurements according to one or more aspects of the present invention. 
         FIGS. 19A and 19B  illustrate certain principles and design features that may be provided to ensure that pressures of fluids at negative gage pressures can be measured. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring now to  FIGS. 2A ,  2 B,  3 A, and  3 B, a pressure sensor  100  includes backing portion  101  that holds a flattened portion  115  of a tube  150  against a sensor portion  102  when the pressure sensor  100  is in a closed operational configuration as shown in  FIG. 3B . The backing portion  101  has springs  105  in a cavity  104  to urge a backing plate  112  against the flattened portion  115  of the tube  150 . Standoffs  110  provide repeatable spacing in a receiving gap  111  that is defined when the pressure sensor  100  is in the closed operational configuration. A surface of the tube  150  flattened portion  115  is held against a tip  126  of an anvil  122  held slidably within a guide  127 . A backing retainer  103  limits a range-of displacement of the backing plate  112  by means of a guide/catch mechanism  170 , which may permit vertical movement of the backing plate  112  relative to the backing retainer  103 . 
     When the backing portion  101  is brought together with the sensor portion  102 , the standoffs  110  rest against a housing stage  120  as shown in  FIG. 3B . The springs  150  are compressed such that the receiving gap  111  is reliably defined. The tube  150  flattened portion  115  is shaped such that it is only minimally compressed in the receiving gap  111 . This helps to ensure that while the flattened portion  115  rests in the receiving gap  111  it is minimally strained. In addition the flattened portion  115  of the tube  150  is supported by the top  123  of the housing stage  120  so that when pressure increase in the flattened portion  115  of the tube  150 , there is minimal strain of material of which the tube  150  is made. The benefit of this is that in configurations in which the material, of which the tube  150  is made, is prone to creep, little change in the shape and elastic response of the material may occur due to the flattened portion  115  being held in the receiving gap  111  and pressurized. These features translate to a reduced susceptibility of the apparatus to respond variably over time to pressure in the tube  150  due to the creep, to a smoother monotonic relationship between pressure and strain. 
     When fluid in the tube  150  is pressurized, the flattened portion  115  presses against the tip  126  of the anvil  122  forcing the anvil  122  toward a flexible plate  140  with an attached strain gage  145 . A pin  135  presses against the flexible plate  140  when the anvil  122  is forced toward it by pressure in the tube  150 . The amount of strain to which the strain gage  145  is subjected (due to pressure inside the tube  150  and transmitted through the flattened region  115 ) can be altered by changing the shape and or size of the pin  135 , due to the differences in the bending moment to which the flexible plate  140  is subjected by displacement of the anvil  122 . The pressure may be measured by means of a curve fitted to a pressure-versus-strain gage signal curve generated by means of a calibration procedure. Calibration is discussed further below. 
     The tip  126  of the anvil  122  and the top  123  of the housing stage  120  may form a nearly continuous flat surface, with the top  126  jutting only slightly above the top  123  of the housing stage  120 . In this way, the deformation of the flattened portion  115  of the tube  150  may be minimal. This may be a benefit where the material of the tube  150  is subject to creep. Also, the overall configuration may be such that the displacement of the anvil  122  may have a low magnitude to help reduce the potential creep problem. 
     Referring now also to  FIGS. 4A and 4B , the pressure sensor  100  may be used detect pressure in a variety of vessels other than a tube  150 . For example, a flexible chamber  215  connected to, or connectable to, a flow conduit (not shown) by flow lines  225  and  220 , may have very flexible walls  205  reducing the magnitude of the potential creep problem. Backing  101  and sensor  102  portions without standoffs  110  may be brought into a desired relationship by a suitable structure such that when pressure is applied to fluid in the flexible chamber  215 , the anvil  122  is forced toward the flexible plate  140  thereby permitting measurement of pressure by means of the strain indicated by the strain gage  145 . Although not shown, portions of the flexible chamber  215  outside that subtended by the backing  101  and sensor  102  portions of the pressure sensor  100  may be confined in a recess defined by walls of a machine (e. g., a renal therapy machine as described in U.S. Pat. Ser. Nos. 09/513,564, 09/512,927, and 09/513,773 hereby incorporated by reference in their entirety as fully set forth herein). Such walls may be substantially coplanar with the backing plate  112  and the top  123  of the housing stage  120 . Pressure may also be measured in a fixed vessel such as shown at  235 , which defines, flow-wise, a dead-end. 
     A number of configurations are preferred for use with the pressure sensor  100  as well as others discussed in the instant specification. The preferred configuration may depend on various features, including the material from which the tube or vessel is made, the thickness of the tube or vessel wall relative to the top of the anvil  126 , the shape of the wall, the length of time during which the tube or vessel is subjected to pressure, the amount of pressure to which the tube or vessel is subjected, the conformity of the surface defined by the top of the anvil  126  and the top  123  of the housing stage  120 .  FIGS. 5A through 5C  and  6 A through  6 F illustrate vessels or tubes of a variety of configurations for purposes of illustrating various features that may influence the design of a pressure sensor according to the embodiments disclosed and variations thereof. 
     In  FIG. 5A , a vessel or tube  250  with a substantially circular cross-section has significant hoop strength requiring a great deal of material strain to displace a contact sensor such as the anvil  122  described with reference to the foregoing figures. The same is true for a tube or vessel  255  having an elliptical shape ( FIG. 5B ), and for plastic tubing  260  of a generally oval shape with rigidity-enhancing ridges  256  as shown in  FIG. 5C . In addition, the thickness of the walls affects the degree of strain to which the material of the tube or vessel must be subjected to generate a displacement for actuating the foregoing embodiments and others described elsewhere in the instant specification. Note that the tubes or vessels  250 , 255 , and  260  shown above are illustrated in a relaxed state. To be used in a pressure sensor device as described in the current specification, the tubes or vessels  250 , 255 , and  260  may be compressed to force an outer surface against the tip  253  of an anvil and housing stage surface  252  to preload the tube or vessel  250 , 255 , and  260  or not. In either case, whether the tube or vessel  250 , 255 , and  260  is preloaded or not loaded in advance of calibration and pressure sensing, the creep may play a significant role in the deformation of the tube or vessel  250 ,  255 , and  260 . When preloaded, the tube or vessel  250 , 255 , and  260  may gradually deform thereby generating a lower elastic rebound over time making the pressure signal from calibration less related to the pressure signal after calibration. If not preloaded, the variation of shape due to pressure change would tend to cause the same effect, namely, a time-varying response due to gradual accommodation to a current shape. 
     The above problems relating to creep may be overcome by suitable choice of materials. For example, a material which is not subject to creep may be used. Alternatively, or in combination with such a material selection, the wall thickness of any of the foregoing shapes or similar may be reduced. For example, see  FIGS. 6C-6E .  FIGS. 6C and 6D  show the circular cross-section shapes of  FIG. 5A  with thinner walls that the embodiment of  FIG. 5A .  FIGS. 6D and 6E  show the elliptical and complex cross-section shapes of  FIGS. 5B and 5C  with thinner walls.  FIG. 6C  illustrates preloading of the circular cross-section tube or vessel  250 A by compressing the latter between a backing surface  254  and an anvil  253  and stage  252  combination forming an opposing surface. If the tube or vessel  250 A has substantial strength and elasticity, negative gage pressures may be measured and preloading may be used to select the response characteristic. 
     In contrast, by providing a tube (or vessel) having a flattened portion that contacts the pressure sensor (see  FIGS. 6A through 6F , described in more detail below), the contact area for the pressure sensor is increased. In addition, by using a relatively thin-walled and/or flattened portion of a tube or vessel, preload strain becomes less of a problem and any pressure changes within the it are transmitted more quickly and reliably to the pressure sensor. 
     In one aspect, a thick-walled tube  270  having a flattened portion  272  provides an enhanced area that contacts not only the central portion  253  of pressure sensor  251 , but also the outer portions  252  as well ( FIG. 6A ). In a second aspect, illustrated in  FIG. 6B , a thin-walled tube  265  having the same conformation as that of  FIG. 6A  not only provides the enhanced contact area for pressure sensor  253 , but also further produces little strain, resulting in a greatly reduced amount of creep. In another aspect, a thin walled circular tube  250 A illustrated in  FIG. 6C , is flattened between backing surface  254  and pressure sensor  251  causing the thin-walled circular tube  250 A to form a more oval shape. Similarly, in another embodiment, a thin walled elliptical tube  255 A illustrated in  FIG. 6D , is flattened by backing surface  254  and the pressure sensor  251 . In yet another aspect, shown in  FIG. 6E , the strain at the site of contact of pressure sensor  251  may be reduced even in the presence of rigidity-enhancing ridges  256 A. In another aspect, a portion of a circular or elliptical piece of tubing may be flattened at the point of contact with pressure sensor  253  to provide the enhanced contact area. The flattened portion may be created by physical alteration of the tubing, or by incorporation of a different piece of tubing that is flatter than the rest of the tubing. One method of creating the flattened portion of a tube, such as the flattened portion  272  of the tube  270  shown in  FIG. 6A  is to thermoform a cylindrical tube by heating and compressing it. In another aspect, the flattened portion may also be thinner than the rest of the tubing. Referring to  FIG. 6F , the walls  280  of a non-tubular vessel enclosing a volume  281  with a pressure inside can be sensed by means of a pressure sensor  251  in a manner similar to the foregoing embodiments. 
     In another aspect, as shown in  FIG. 7 , creep in a flattened portion of a tube  815  may be reduced by arranging the tube in a housing so that the tubing adopts a concave shape where the tubing rests on top of the anvil  810 . Tubing  815  is held in place atop anvil  810  between backing retainer  820  and housing stage  805  in a formed inner face  830 , and contact anvil  810  at anvil tip  825 . The formed inner face  830  may have other shapes and may be convex, saddle-shaped, or asymmetrical or three dimensional curves in them. 
     Other methods and devices may be used in place of the anvil shown in  FIGS. 3A-3B  to transmit detected pressure changes to a measuring device. As an example, a cantilever mechanism may be used to transmit pressure changes to a pressure transducer. 
     Referring now to  FIG. 8 , a flattened portion  330  of a tube  320  is held in place between a wall  315  and a fixed base  370 . An anvil  340  contacts flattened portion  330 . The distal end of anvil  340  is affixed to an arm  327  of cantilever  325 . At the other end of cantilever  325  is a knee  335  in contact with a pressure transducer  345  mounted on a fixed base  371 . Cantilever  325  terminates at and is fixed to a fixed base  310 . Movement of anvil  340  is translated through cantilever arm  327  and is sensed by pressure transducer  345 . 
     Referring now to  FIGS. 9A and 9B , in the alternative embodiment, pressure changes may be transmitted to an extendible type pressure transducer such as a displacement-type strain gage  385 . An arm  355  pivots from a fixed hinge  360 . An end portion  353  of the arm  355  is in contact with a tube  351  such that pressure changes within the tube  351  cause the arm  355  to move. 
     The arm  355  movements are transmitted to a displacement-type strain gage  385  connected between the arm  355  and a fixed base  380 . The end portion  353  of the arm  355  is surrounded by a fixed base  352  which supports the tube  351 . 
     In another aspect of the present invention, the flattened portion of tubing may directly contact a strain gauge and flexible plate. In other words, the device functions without an anvil, cantilever, etc., to transmit the detected pressure changes to a pressure transducer. Referring to  FIG. 10 , flattened portion  460  is held in place between a wall  465  and flexible plate  455  atop a strain gage  450  mounted on a fixed base  475 . Movement in flexible plate  455  is transmitted directly to strain gage  450 . 
     In another aspect of the invention, the pressure change is sensed by a curved strain gauge mounted on a flexible pillar transducer, as shown in  FIG. 11 . Tubing flattened portion  485  is mounted between a wall  486  and a flat terminus  483  of a flexible pillar transducer  482 . Flexible pillar transducer  482  is held in contact with tubing flattened portion  485  by the flexible pillar, which urges the terminus  483  against the tubing flat portion  485 . The other end of flexible pillar transducer  482  is attached to a fixed base  488 . Attached to the outside surface of flexible pillar transducer  482 , between the arm attached to pressure sensor  484  and the arm attached to wall  488 , is a strain gauge  480 . Pressure changes in tubing flattened portion  485  produce a change in the shape of flexible pillar transducer  482 , which is detectable by the strain gauge  480 . 
     In the embodiment illustrated in  FIG. 3B , anvil  122  may be subject to frictional resistance to movement within housing stage  120 . Such resistance to movement may result in loss of sensitivity or an apparent “spike” in pressure as the anvil suddenly overcomes the resistance. In another aspect of the present invention, the device may be constructed to reduce frictional contact between anvil  122  and housing stage  120 . 
     Referring now to  FIG. 12 , anvil  522  is held within housing stage  505  with one or more rings  530 . Although in  FIG. 12  a total of two internal rings are shown, it is intended that any number of rings  530  could be employed consistent with the goal of reducing frictional contact with anvil  522  while maintaining anvil  522  in a fixed position at rest. As in  FIG. 3B , when fluid in the tube  555  is pressurized, the flattened portion presses against the anvil  522 , forcing plunger  520  toward a flexible plate  535  with attached strain gauge  540 . Alternatively or in combination, one or more strain gages  541  may be mounted on rings  525  to detect pressurization of tube  555 . 
     In another aspect, the pressure transducer is in the form of a strain gage mounted on a flexible plate that contacts the flattened portion of a tube  555  mounted on a wall  565 . 
     Referring now to  FIGS. 13A and 13B , a flexible plate  550  is mounted between one or more standoffs  585 . A strain gauge  570  is mounted on flexible plate  550 , at a position where flexible plate  550  contacts the flattened portion of tube  555  when standoffs  585  are lowered to contact wall  565 , as shown in  FIG. 13B . Another aspect of the invention is shown in  FIGS. 13C and 13D , in which tube  555  is mounted between standoffs  585 , and a flexible plate  550  is mounted on wall  565 . A strain gage  570  is mounted on flexible plate  550 , and contacts the flattened portion of tube  555  when tube  555  is lowered so that standoff  585  contacts wall  565 . 
     Although  FIGS. 13A and 13B  show two standoffs in opposing position, and  FIGS. 13C and 13D  show a single standoff, it is intended that the present invention not be limited to these embodiments. For example, more than two standoffs may be used, and they may be regularly or irregularly spaced. In addition, although strain gauge  570  is shown in FIGS.  13 A through  13 D as being located in the center of flexible plate  550 , it may be mounted at any point along flexible plate  550  where flexible plate  550  contacts the flattened portion of tube  555 . 
       FIGS. 14A and 14B  illustrate yet another aspect of the present invention, in which a transducer having a flexible plate and bearing a curved strain gauge is positioned to wrap partly around a tube or vessel. As shown in  FIGS. 14A and 14B , flexible plate  620  is mounted between a pair of opposing side supports  610 , such that it forms a curved shape between side supports  610 . Flexible plate  620  may be mounted on a wall  630 , as shown. The curved shape of flexible plate  620  matches the outside curve of a tube  605 . A curved strain gauge  625  is mounted on flexible plate  620 , such that tube  605  may fit inside and in contact with flexible plate  620 . Strain gauge  625  is located on the outside portion of flexible plate  620 . 
     Calibration of the pressure sensor may be achieved by a standard curve-fitting procedure using a calibration system. 
     Referring to  FIG. 15 , a calibration system includes a pump  660  connected via a tube  645  to a pressure sensor  650 . The rate of fluid flow through the tubing  645  and the pressure sensor  650  is controlled by either or both of the pump  660  or a clamp  665  downstream from the pressure sensor  650 . The pump  660  and the clamp  665  are controlled by a controller  655 . A precalibrated pressure sensor  640  is located in the tubing line between the pump  660  and the clamp  665 . To calibrate pressure sensor  650 , the controller  655  establishes a predetermined internal pressure from precalibrated pressure sensor  640  by altering the flow rate of pump  660  or the degree of closure of clamp  665 , or both. The controller  655  then measures the output from pressure sensor  650 , and associates the output with the predetermined pressure. 
     The process is performed for at least two different pressure settings, and the results are used to establish a calibration curve for the pressure sensor  650  (i. e., output of pressure sensor  650  vs. pressure). 
     A number of configurations are preferred for use with the pressure sensor as well as others discussed in the instant specification. The preferred configuration may depend on various features, including the material from which the tube or vessel is made, the thickness of the tube or vessel wall relative to the top of the anvil, the shape of the wall, the length of time during which the tube or vessel is subjected to pressure, the amount of pressure to which the tube or vessel is subjected, the conformity of the surface defined by the top of the anvil and the top of the housing stage. 
     The use of the pressure sensor of the present invention in a blood treatment machine is illustrated schematically in  FIG. 16 . The operation of the blood treatment machine is described in detail in copending U.S. patent application Ser. No. 09/513,911, filed Feb. 25, 2000, hereby incorporated by reference in its entirety. Controller  655  regulates the flow rate of pumps  710 ,  744 , 746 , and  747  to flow blood from the patient, through a hemofilter  715 , and then back to the patient. The machine includes a blood handling unit, a fluid management unit, and a ultrafiltration unit. The blood-handling unit circulates the patient&#39;s blood in a controlled manner through the hemofilter  715  and back to the patient after treatment. Note that the hemofilter  715  may be a dialyzer as well. The hemofilter  715  removes waste fluid, containing urea and other toxins, from the blood. The fluid management unit replaces the waste fluid with a sterile replacement fluid for return with the treated blood to the patient&#39;s blood supply. The replacement fluid also acts to maintain the patient&#39;s electrolytic balance and acid/base balance. The ultrafiltration unit removes waste fluid from the patient without the need for addition of replacement fluid. 
     Referring now to  FIG. 16 , blood from the patient  725  is pumped by pump  710  through hemofilter  715  via arterial blood supply line  727 , and then returned to the patient  725  via venous return line  729 . Wastes, including liquid and uremic toxins, are separated by the hemofilter  715  from the rest of the blood. 
     The waste material exits the hemofilter  715  and is separated into an ultrafiltration path and a balancing path. Waste material in the ultrafiltration path is moved by pump  744  to a waste fluid container  742 . Waste material in the balancing path is pumped by pump  746  through an inline balancing mechanism  749  that displaces replacement fluid, pumped by another pump  747 , drawn from a replacement fluid chamber  740 . Various valves, pumps and sensors are employed to determine and deliver the appropriate amount of replacement fluid required to insert into the venous return line to maintain the patient&#39;s blood pressure. 
     The pressure sensor  705  of the present invention is placed in the venous return line to measure venous pressure in the patient&#39;s return blood line. 
       FIG. 17A  illustrates a blood treatment machine  800  that may incorporate the pressure sensor of the present invention.  FIG. 17B  illustrates a cartridge  805  that is insertable in a space  915  in the blood treatment machine  900 . The blood treatment machine  800  has a first portion  920  that closes in a clamshell fashion onto a second portion  925  causing various actuators and sensors  940  to engage tubing and other components  945  held by the cartridge  805 . Among these components is a sensor, a portion of which is visible at  905 , which contacts a flattened tube portion  910  supported by the cartridge  805 . The cartridge  805  may be disposable.  FIG. 17C  shows an enlarged view of the flattened tube portion  910 , which may be in a venous return line of the cartridge of  FIG. 17A . 
     Referring now to  FIG. 17C , a portion of the cartridge  805  supports a tube  815  with the flattened tube portion  910  that engages the sensor partly visible at  905 . Note that in an alternative embodiment, the portion  910  may be cylindrical rather than flattened as indicated or its flattened dimension may be oriented differently depending on the configuration of the sensor with which it is configured to mate. An opening  810  may be provided to give the sensor (partly shown at  905 ) access to the portion flattened portion  910 . The cartridge may or may not have a panel  825 , that is, it may be an open support structure (not illustrated). 
       FIG. 18  provides a flowchart that illustrates a procedure for standardizing the pressure sensor described above. The circuit is first primed (S 10 ), then the lines are closed off or pinched and the pump maintains a first flow rate (S 15 ). The controller then takes a first pressure reading from  640  of  FIG. 15  and a P signal (S 20 ) which may be a voltage signal from a pressure transducer of any of the foregoing embodiments. The pump flow rate then is changed to a second flow rate different from the first flow rate (S 25 ), and after the system re-equilibrates the controller takes a second pressure reading and a second P signal (S 30 ). The system then checks to determine if the required number of data points has been collected (S 32 ). If not, steps S 25 , S 30  and S 32  are repeated until the required number of data points is collected. At that point, the data points collected are used to generate a standard curve (S 35 ) by conventional statistical methods. When a treatment is run, the signals obtained during the run are converted to pressure values by fitting the P signals to the standard curve (S 40 ). 
     Note that in any of the foregoing embodiments, it is possible to provide for some preload of the vessel/tube or support thereof such that when there is a negative pressure in the vessel/tube, it does not collapse. Under such circumstances, any of the above embodiments may allow for the measurement of negative gage pressures in the same manner as positive gage pressure is measured. Certain kinds of vessels/tubes would not provide a substantial preload without an external support, however, for example thin-walled vessels/tubes or those with walls with large area relative to thickness. 
     Referring to  FIG. 19A , a vessel or tube  845  has molded portions  830  that engage with arms  835  of a spring  850 . The spring  850  has attached to a base portion  860  thereof, a strain sensor  840  to detect changes in shape of the spring  850 . The engagement between the spring  850  and molded portions  830  is such that if a negative pressure develops within an interior  855  of the vessel or tube  845 , the tendency of the vessel or tube  845  to collapse may be resisted by the spring  850 . In the configuration of  FIG. 19A , it may be that the resilience of the spring  850  ensures against collapse of the vessel or tube  845  if the vessel or tube  845  lacks sufficient integrity to avoid collapse with the help of the spring  850 . In such a case the spring  850  would relax when there is zero gage pressure in the interior  855  and would experience reverse tension when the interior pressure dropped below gage pressure. But the same result may be achieved even if the spring were always under tension in the same direction as positive if the structure of the vessel or tube is such that a positive preloading is presented to the spring. Referring to  FIG. 19B , a thick-walled vessel or tube  860  is squeezed by a spring  870 , the force of the spring being resisted by the vessel or tube  860  even when its interior  857  is under negative pressure. Thus, it should be clear that the pressure sensors of at least some of the earlier embodiments may be altered as illustrated by the examples of  FIGS. 19A and 19B  to provide for measurement of negative pressure. Note that another alternative to provide for positive loading when the vessel or tube is under negative pressure is a third spring, for example, one mounted inside the vessel or tube. 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. For example, although load cells and strain gages are disclosed as a preferred mechanism for detecting shape change of fluid vessels or tubes, it is possible to detect such shape change by other means. For example, any kind of displacement transducer such as a mechanical, resistance, or optical encoder could be used to measure the change in shape of the tube or vessel due to pressure variation. In fact, even non-contact detectors could be used, for example, an interferometric displacement encoder. 
     The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.