Abstract:
A blood analyzer sensor cartridge comprises a housing having chamber and a sensor assembly within the chamber, a first fluid port having an articulated inlet aspiration tube for direct introduction of a sample, a first fluid path in the housing communicating the first fluid port with the sensor assembly, a second fluid port in the housing adapted for connection to an analyzer, and a second fluid path in the housing communicating the sensor assembly with the second fluid port, the articulated tube is pivotally mounted to the housing for selective orientation within a range of up to ninety degrees, the tube is moveable from a protective recess in the housing to a position normal to a face of the housing.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation-in-part of PCT application Ser. No. PCT/US/97/0773 filed on May 6, 1997, and application Ser. No. 08/648,692 filed on May 16,1996, now U.S. Pat. No. 5,718,816. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to systems for analyzing fluids, and more particularly to an system for mechanical, electrical, and fluid interconnection of sensors to a blood analyzer. 
     2. Description of Related Art 
     In a variety of instances it is desirable to measure the partial pressure of blood gasses in a whole blood sample, concentrations of electrolytes in the blood sample, and the hematocrit value of the blood sample. For example, measuring pCO 2 , pO 2 , pH, Na + , K + , Ca 2+  and hematocrit value are primary clinical indications in assessing the condition of a medical patient. A number of different devices currently exist for making such measurements. Such devices are preferably very accurate in order to provide the most meaningful diagnostic information. In addition, in an attempt to perform these analyses in close proximity to the patent, the devices which are employed to analyze a blood sample are preferably relatively small. Furthermore, it is important to reduce the size of the cavities and pathways through which the analyte must flow in order to reduce the amount of analyte required. For example, performing blood analysis using a small blood sample is important when a relatively large number of samples must be taken in a relatively short amount of time. More particularly, patients in intensive care require a sampling frequency of 15-20 per day for blood gas and clinical chemistry measurements, leading to a potentially large loss of blood during patient assessment. Furthermore, the amount of blood available may be limited, such as in the case of samples taken from a neonate. In addition, by reducing the size of the analyzer sufficiently to make the unit portable, analysis can be performed at the point of care. Also, reduced size typically means reduced turnaround time. Furthermore, in order to limit the number of tests which must be performed it is desirable to gather as much information as possible upon completion of each test. 
     In one blood analyzer currently in use, a sensor/calibrant package comprises a sensor assembly mounted within a housing. The sensor/calibrant package also comprises a plurality of fluid pouches mounted within the housing. These pouches hold calibrants and flush fluids necessary for the operation of the blood analyzer. A series of tubes and valves within the housing interconnect the sensors within the sensor assembly to each of the fluid pouches. Since the tubes which transport a sample to the sensor assembly are within the housing, the operator of the blood analyzer can not see the sample as it flows into and out from the sensor assembly. Accordingly, the operator cannot determine visually whether the sample has entered the sensor assembly. This can be a significant problem, since the operator may not visually see that a blockage has occurred in the fluid flow path. 
     A heater assembly is mounted to the housing in order to raise the temperature of the fluids, the sensor assembly, and the sample to be measured. Raising the temperature allows the analysis of the sample to be carried out at a predetermined temperature. Due to the thermal mass of the components and fluids that must be heated, such blood analyzers may not be used for one or more hours after a new sensor/calibrant package has been installed. Furthermore, the need for such a heater substantially increases the cost of the sensor/calibrant package. 
     In addition to requiring that the sensor/calibrant package be heated, it is necessary to hydrate the sensors within the sensor assembly. Such hydration of the sensors takes one or more hours. Accordingly, the blood analyzer is not operational for one or more hours after installation of a new sensor assembly. In many cases analysis must be performed at regular and closely spaced intervals. Accordingly, if the heating and temperature stabilization time and the hydration time are relatively long, the number of times such analysis can be performed within a particular amount of time (i.e., turn around time) can be limited to a number less than would otherwise be desirable. 
     The fluid interface between the fluid pouches and the sensor assembly must be controlled to prevent fluid from pouches from flowing to the sensor assembly prior to installation of the sensor/calibrant package be installed in the blood analyzer. This requirement adds a measure of complexity to the mechanical design of the sensor/calibrant package, thus increasing the cost for fabricating the sensor/calibrant package. Furthermore, the complex interface between the sensor/calibrant package and the blood analyzer makes installation of the sensor/calibrant package more difficult, increases the chance that fluid will leak from the sensor, and can potentially increase the length of the fluid path (thus increasing the chance that a clot will occur and increasing the required volume of the sample). A portion of elastomeric tubing which interfaces the sensor assembly to the fluid pouches and a refuse pouch (into which exhausted samples and other fluids are pumped) is stretched over a concave surface. When the sensor/calibrant package is placed within the blood analyzer, a pump arm strokes the tubes in order to create a peristaltic pump, thus increasing the complexity of the mechanical interface between the sensor/calibrant package and the blood analyzer. Further complicating the mechanical interface is the need to provide a mechanism by which the blood analyzer can control the valves within the sensor/calibrant package. A first valve must be rotated to allow a controller within the blood analyzer to configure the fluid path. A set of additional slide valves must be actuated upon installation of the assembly into the blood analyzer in order to open the flow path from each of the fluid pouches. 
     The sensor assembly has a plurality of sensors formed on a front side of a polymeric substrate along a flow path between an inlet and outlet port. The fluid flow path is formed as a groove in a polymeric substrate. Electrodes are formed in the substrate. The electrodes communicate with a measurement flow channel formed in the substrate. The electrodes also communicate with a measurement flow channel which is formed by the combination of substrate and a cover plate. 
     The electrical interface between the sensor assembly and electronics external to the sensor assembly is provided through an plurality of contacts fabricated on the rear surface of the substrate. These contacts slide against a spring loaded mating contact in the blood analyzer. As the contacts of the sensor assembly slide against the mating contacts within the blood analyzer, the contacts of the sensor assembly and analyzer are worn down. Therefore, after inserting and removing the cartridge from the blood analyzer a number of times, the electrical connection between the external circuits within the blood analyzer and the sensors within the sensor assembly will be degraded. 
     Due to the use of electrical slide contacts, the structure of the interface between the elastomeric tubes and the pump, and the configuration of the valve controls, the sensor/calibrant package must first be inserted into the blood analyzer, and then slide generally at a right angle to the insertion angle. This process makes installation of the sensor/calibrant package awkward and increases the risk that either the electrical, mechanical, or fluid interface between the sensor/calibrant package and the blood analyzer will be faulty. 
     Furthermore, since the sensor is an integral part of the sensor/calibrant package, when a sensor fails (i.e., can no longer perform in accordance with specified parameters) the entire sensor/calibrant package must be replaced. 
     Accordingly, in as much as installation and fabrication of sensors within a blood analyzer are both cumbersome and susceptible to leaks, and long delays result after installation, it would be desirable to provide an assembly which allows the operator of a blood analyzer to replace merely the sensor assembly with a fast turn around time, no special training, and with highly reliable electrical, mechanical and fluid interface. 
     The aforementioned parent application solved many of the above enumerated problems of the prior art. However, a number of further improvements are desirable. For example, in the aforementioned system, the sample is introduced into the system through a port in the analyzer and passes through plumbing therein to the sensor cartridge. This has a number of disadvantages such as a longer fluid passage requiring a larger sample. The greater distance of travel of the sample also introduces a greater chance for contamination from gases and other materials. 
     Another problem is that the fluid passage in the analyzer becomes contaminated, not just from the blood but from calibrant materials which have salts in them. 
     Furthermore, it would be desirable to provide such an assembly which further allows the user to see a blood sample as it enters, flows through, and exits the sensor assembly. 
     SUMMARY OF THE INVENTION 
     The present invention is a sensor cartridge into which sensors are installed. The sensor cartridge allows the sensors to be easily and reliably installed into a blood analyzer. The cartridge includes six basic components: (1) a housing; (2) a housing cover; (3) a sensor assembly; (4) a “pump tube” assembly; (5) a right angle fluid coupling; and (6) a capture/release arm. 
     In accordance with the preferred embodiment present invention, the sensor assembly has an electrical connector mounted on the rear side of the assembly. The body of the connector protrudes through a first opening in the housing. The walls of the first opening conform generally to the profile of the protruding body of the connector. Thus, the mechanical interface between the body of the connector and the walls of the first opening in the housing retain the sensor assembly in a predetermined position within the housing. 
     A plurality of inner walls within the housing locate the pump tube assembly and the right angle fluid coupling within the housing. One end of the pump tube assembly is formed as a straight end fluid coupling and is coupled to the sensor assembly. The other end of the pump tube assembly is formed as a right angle end fluid coupling. A portion of the right angle end fluid coupling protrudes through a second opening in the housing. The walls which define the second opening conform to that portion of the right angle end fluid coupling which protrudes through the housing. The right angle fluid coupling is essentially similar to the right angle end fluid coupling of the pump tube assembly. A portion of the right angle fluid coupling protrudes through a third opening in the housing in a manner similar to the protrusion of the right angle end fluid coupling of the pump tube assembly. Another portion of the right angle fluid coupling is coupled directly to the sensor assembly. 
     In accordance with one embodiment of the present invention, a fourth opening in the housing receives a first boss which extends from the blood analyzer. The first boss is generally cylindrical and solid with a “ring-like” groove machined near the distal end of the boss. Alternatively, the boss may be formed as an elongated structure having a rectangular, oval, or other cross-section. In accordance with this embodiment, a second boss is formed in the housing as a hollowed cylinder having an inner diameter which is nearly equal, but slightly larger than the outer diameter of the first boss. The outer diameter of the second boss is greater than the inner diameter by an amount which is essentially equal to the thickness of the housing walls. 
     The capture/release arm has an opening through which the first boss protrudes. The arm is resiliently held in place such that an inner edge of the opening is captured within the ring-like groove in the boss that extends from the blood analyzer when the cartridge is installed in the blood analyzer. A portion of the arm extends beyond the housing to allow an operator to press the arm and thus release the edge of the arm from the groove in the boss. 
     Electrical contacts of the connector on the rear side of the sensor assembly are aligned to mating electrical contacts of the blood analyzer as the sensor assembly is being installed by alignment of the boss which extends from the blood analyzer to mate with the boss which extends from the housing, and alignment of two male fluid connectors, one of which mates with the right angle fluid coupling and the other of which mates with the right angle end fluid coupling of the pump tube assembly. Each of these will engage with the mating member prior to the electrical contacts of the sensor assembly engaging the electrical contacts of the blood analyzer. Accordingly, the electrical contacts of the sensor assembly will be in close alignment with the electrical contacts of the blood analyzer as the contacts approach one another. 
     A resilient portion of the pump tube assembly exits the housing at one end and re-enters the housing at the same end, forming a “U” shaped loop. The loop formed by the pump tube is sufficiently flexible and resilient to allow the loop to be stretched over and into engagement with a roller pump located on the blood analyzer. The roller pump rotates to massage the loop of the pump tube with which the roller pump is engaged in order to form a peristaltic pump. 
     In accordance with the preferred embodiment of the present invention, a heater is disposed within the substrate. The heater is capable of heating a blood sample and the array of sensors to a known stable temperature and maintaining that temperature as the sample is being analyzed. Accordingly, fluids that enter the sensor assembly are rapidly heated due to the small volume and low thermal mass of such fluids. 
     The sensors of the present invention have very good signal to noise ratio due to a short electrical path length between the sensors and external detecting and analyzing electronics within the blood analyzer. Thus, unamplified, low level sensor outputs from the sensors can be used directly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, advantages, and features of this invention will become readily apparent in view of the following description, when read in conjunction with the accompanying drawings, in which: 
     FIGS. 1 a  and  1   b  are perspective views of a disassembled sensor cartridge in accordance with one embodiment of the present invention. 
     FIG. 1 c  illustrates one embodiment of the housing of the present invention with a pump tube assembly and a right angle fluid coupling installed within the housing. 
     FIG. 1 d  illustrates a cartridge in accordance with one embodiment of the present invention in which a boss protruding from a blood analyzer mates with a hollow boss in the cartridge. 
     FIG. 1 e  is an illustration of the cartridge cover having an opening through which a sensor assembly can be viewed in accordance with one embodiment of the present invention. 
     FIG. 2 a  is an illustration of a blood analyzer in accordance with one embodiment of the present invention. 
     FIG. 2 b  is an illustration of another embodiment of a blood analyzer in accordance with the present invention. 
     FIGS. 3 a   1 - 3   a   2  is an illustration of a latch used to mechanically secure a cartridge to a blood analyzer in accordance with one embodiment of the present invention. 
     FIGS. 3 b   1 - 3   b   2  is an illustration of a protective cover in accordance with one embodiment of the present invention. 
     FIG. 3 c  is an illustration of one embodiment of the present invention in which barbs extending from a blood analyzer latch a sensor cartridge into place. 
     FIG. 4 is a front plan view of the sensor assembly of the present invention. 
     FIG. 5 is a back plan view of the sensor assembly of the present invention shown in FIG.  4 . 
     FIG. 6 a  is an illustration of one pattern to which a heater conforms when deposited on a substrate in accordance with the present invention. 
     FIG. 6 b  is an illustration of the back side of a substrate after each of the dielectric layers have been deposited in accordance with one embodiment of the present invention. 
     FIG. 7 is an illustration of the art work used to generate a screen, which in turn is used in the preferred embodiment of the present invention to deposit the second layer of conductors and connector pads. 
     FIG. 8 is an illustration of an oxygen sensor in accordance with the preferred embodiment of the present invention. 
     FIG. 9 is a cross-sectional view of a portion of a substrate through which a sensor through hole is formed and on which metal layers of an electrolyte sensor electrode have been deposited in accordance with one embodiment of the present invention. 
     FIG. 10 is a cross-sectional view of one of the hematocrit sensor electrodes in accordance with one embodiment of the present invention. 
     FIG. 11 is a cross-sectional view of a sensor showing the first layer of encapsulant in accordance with one embodiment of the present invention. 
     FIG. 12 is a cross-sectional view of one of the hematocrit sensors showing the first layer of encapsulant in accordance with one embodiment of the present invention. 
     FIG. 13 is a top plan view of the sensor assembly installed within a plastic encasement. 
     FIG. 14 is a cross-sectional view of the sensor assembly installed in the plastic encasement. 
     FIGS. 15 a - 15   c  illustrate alternative embodiments of the present invention in which the relative positions of the sensors differ from those shown in FIG.  4 . 
     FIG. 16 a  is an exploded assembly view of another embodiment. 
     FIG. 16 b  is a front elevation view of the cartridge of FIG. 16 a  assembled X. 
     FIG. 16 c  is an end view of the cartridge of FIG. 16 b  showing the aspiration port in the setracted position X. 
     FIG. 16 d  is a view like FIG. 16 c  showing the aspiration prot in the extended position X. 
     FIG. 17 is a perpective view of a blood analyzer in accordance with a further embodiment of the invention. 
     Like reference numbers and designations in the various drawings refer to like elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than limitations on the present invention. 
     Sensor Cartridge 
     FIGS. 1 a  and  1   b  are perspective views of a disassembled sensor cartridge  100  in accordance with one embodiment of the present invention. The sensor cartridge  100  shown in FIGS. 1 a  and  1   b  has five component parts; (1) a housing  102 ; (2) a housing cover  104 ; (3) a pump tube assembly  106 ; (4) a fluid coupling  108 ; and (5) a sensor assembly  400 . 
     The housing  102  shown in FIGS. 1 a  and  1   b  has a floor  101 , four walls  103 ,  105 ,  107 ,  109 , and an opening  110 . Male electrical contact pins  1207  of an electrical connector  1205  of the sensor assembly  400  protrude through the opening  110 . In accordance with one embodiment, the connector  1205  has a body  116  which also protrudes through the opening  110 . The walls  118  of the opening  110  generally conform to the shape and size of the body  116  of the connector  1205 . Thus, the sensor assembly  400  is constrained from movement in the plane of the floor  101  of the housing  102 . Preferably, the connector body  116  fits loosely within the opening  110 . However, in one alternative embodiment of the present invention, the body  116  may be friction fit within the opening  116  to more securely hold the sensor assembly in place during assembly of the cartridge  100 . Alternatively, the sensor assembly may be held in place merely by the forces exerted by the fluid coupling of the sensor assembly  400  to the pump tube assembly  106  and the fluid coupling  108 . In yet another alternative embodiment, walls which extend up from the floor  101  of the housing  102  may be formed to constrain any motion of the sensor assembly  400 . FIG. 1 a  shows one such wall  120 . 
     The pump tube assembly  106  preferably comprises a right angle end fluid coupling  126 , a straight end fluid coupling  124 , and a pump tube  136 . In accordance with one embodiment of the present invention, the end fluid couplings  124 ,  126  are formed (such as by a conventional molding process) from an elastomer. The fluid coupling  108  may also be formed from an elastomer. The fluid coupling  108  is preferably formed as a right angle coupling. That is, the coupling provides a means by which a fluid flow path through a first mating fluid coupling may be placed in fluid connection with a fluid flow path through a second mating fluid coupling when the fluid flow paths of the first and second coupling are at right angles to one another. The pump tube  136  is preferably very resilient in order to allow the pump tube  136  to properly interface with a roller to form a peristaltic roller pump, as is described below in greater detail. A fluid path is formed through the pump tube assembly  106  such that fluid enters at one end of the pump tube assembly and exits from the other end. 
     Walls  122  may be provided to retain the pump tube assembly  106  and fluid coupling  108  in position within the housing  102 . FIG. 1 c  illustrates one embodiment of the housing  102  of the present invention with a pump tube assembly  106 ′ and a fluid coupling  108 ′ installed within the housing  102 . It can be seen that FIG. 1 c  shows an alternative to the embodiment shown in FIG. 1 a  and  1   b , in that the end fluid coupling  124 ′, the right angle end fluid coupling  126 ′, and the fluid coupling  108 ′ shown in FIG. 1 c  are generally rectangular (in contrast with the generally cylindrical shapes shown for the end fluid coupling  124 , the right angle end fluid coupling  126 , and the fluid coupling  108  shown in FIG. 1 a  and  1   b ). Hollow cylindrical protrusions from the body of the couplings  108 ,  108 ′,  126 ,  126 ′ have fluid channels therethrough. The fluid channel in each coupling  108 ,  108 ′,  126 ,  126 ′ is at a right angle to a fluid channel along the longitudinal axis of the each coupling  108 ,  108 ′,  126 ,  126 ′. Regardless of the shape of the couplings, the protrusions  130 ,  128  are seated in two openings  132 ,  134  in the floor  101  of the housing  102  (best seen in FIGS. 1 a  and  1   b ). Preferably, the openings are shaped and sized such that the cylindrical protrusions  128 ,  130  fit snugly within the openings  132 ,  134  and extend just beyond the outer surface of the floor  101 . In either case, a pump tube  136  of the pump tube assembly  106 ,  106 ′ passes through openings  138  in the housing wall  109 . 
     In accordance with one embodiment of the present invention, ports  1202 ,  1204  of the sensor assembly are directly coupled to the pump tube assembly  106  and the fluid coupling  108 . However, in an alternative embodiment, an extension tube (not shown) with a fluid channel therethrough may be provided between the inlet port  1202  and the fluid coupling  124  or between the outlet port  1204  and the fluid coupling  108 . The fluid channel through the extension tube is preferably relatively narrow to reduce the volume of the sample being analyzed and the amount of calibrant and other fluids used during analysis. 
     The cover  104  is preferably translucent or clear and has five protrusions  140 ,  142 ,  144 ,  146 ,  148  which extend upward from the surface of the cover  104 . Furthermore, as will be described in greater detail below, a plastic encasement  1200  (see FIG. 14) is also preferably either translucent or clear. Since the cover and the plastic encasement are either translucent or clear, the user can view the movement of analytes gas bubbles, and reagents through the sensor assembly within the cartridge. In accordance with one embodiment of the present invention, illustrated in FIG. 1 e , the cover  104 ′ has an opening  170  which allows the user of a blood analyzer into which the cartridge is to be installed to view the sensor assembly directly. Accordingly, the user may directly observe an analyte gas bubbles and reagents flowing through the sensor assembly. 
     In one embodiment of the present invention, the protrusions  140 ,  142 ,  144 ,  146  align the cover  104  to the housing  102 . The protrusion  146  also applies pressure to the top of the sensor assembly  400 , together with the protrusion  148 , in order to retain the sensor assembly in position after the cover  104  is applied. It will be understood by those skilled in the art that the protrusions may be formed in a wide variety of shapes in order to align the cover and retain the sensor assembly  400  in place. Furthermore, in one embodiment of the present invention, no such protrusions are provided. 
     Two reinforced holes  150 ,  152  are provided through the cover  104 . The holes  150 ,  152  align with two hollow generally cylindrical bosses  154  which extend up from the floor  101  of the housing  102  to accept retaining devices, such as screws, which secure the cover  104  to the housing  102 . In an alternative embodiment of the present invention, studs extend from the cover in alignment with the bosses  154 . Each stud fits tightly within the opening in one of the bosses  154  in order to secure the cover  104  to the floor  101  of housing  102 . 
     In accordance with one embodiment of the present invention, the cartridge of the present invention is assembled by coupling the fluid coupling  108  to a first port  1204  of the sensor assembly  400 . The fluid coupling  124  is coupled to the other port  1202  of the sensor assembly  400 . The combination of fluid coupling  108 , sensor assembly  400 , and pump tube assembly  106  are then lowered into the housing  102  and the protrusions  128 ,  130  are inserted into the openings  132 ,  134 . The pump tube  136  is inserted into openings  138  in the wall  109  of the housing  102 . The cover  104  is then placed over, and secured to, the housing  102 . 
     Once the cartridge  100  is assembled, it may be installed in a blood analyzer, such as the blood analyzer  200  illustrated in FIG. 2 a . The blood analyzer of the present invention has a first and second male fluid connector  202 ,  204  respectively. The first and second male fluid connectors mate with the cylindrical protrusions  128  and  130  to complete a fluid flow path from the first male fluid connector  202 , through the right angle end fluid coupling  126  of the pump tube assembly  106 , into the sensor assembly  400 , through the inlet port  1202 , out the outlet port  1204 , through the fluid coupling  108 , and into the second male fluid connector  204 . 
     Fluids are pumped along the fluid flow path by a peristaltic roller pump which includes a roller  206  that massages the pump tube  136 . That is, the pump tube  136  is preferably resilient enough to be stretched over the roller  206 . The roller  206  applies areas of alternating greater and lesser pressure to the pump tube  136 , causing those portions of the pump tube  136  that lie over an area of greater pressure to be internally constricted and those areas of the pump tube  136  that lie over an area of lesser pressure to be relaxed to essentially the full unstressed diameter of the channel through the interior of the pump tube  136 . As the roller  206  rotates, the areas of alternating greater and lesser pressure traverse the pump tube  136  to generate a peristaltic action in the pump tube  136 . 
     In addition to the first and second male fluid connectors  202 ,  204 , a female electrical connector having a plurality of female electrical contact receptacles are provided on the blood analyzer. The female receptacles mate with the male electrical contact pins  1207  of the sensor assembly  400 . The first and second male fluid connectors  202 ,  204  preferably extend out further from the blood analyzer than do male electrical contact pins from the sensor assembly. Accordingly, the mating of the fluid connectors causes the electrical connectors to align for mating. In one embodiment of the present invention shown in FIG. 2 b , a generally cylindrical boss  208  extends outward from the blood analyzer  200 ′. The boss  208  preferably has a generally ring-shaped groove  210  disposed near the distal end of the boss  208 . 
     In accordance with one embodiment of the present invention, a blood analyzer  200 FIG. 2D is provided with a boss  208 . A cartridge  100 ′ such as shown in FIG. 1 d  is provided. The cartridge  100 ′ has an hollow boss  156  located in alignment with the boss  208 . The hollow boss  156  of the cartridge has an inner diameter which is slightly larger than the outer diameter of the boss  208 . Four support projections are provided around the periphery of the boss  156 . Two of the support projections form generally “L-shaped” latch supports  158 . The other two supports  160  merely provide additional strength to support the boss  156 . FIG. 3 a  is an illustration of a latch  300  which rests on the horizontal edge  162  of each latch support  158  and between the upright portions  164  of each latch support  158 . 
     A first smaller opening  301  is formed near a proximal end of the latch  300 . A second larger opening  302  in the latch  300  is sized such that the boss  208  may pass though the second opening  302 . At one end of the second opening a step  304  is formed. The first opening is sized to accept a “tooth”  166  which projects upward from a depressed portion  168  of the wall  105 ′, as shown in FIG. 1 d . The wall  105 ′ is cut away from the floor  101  of the housing  102  in order to allow that portion of the wall  105 ′ which is under the tooth  166  to flex inward. Thus, when the latch  300  is in position between the upright portions  164  of the latch supports  158  with the tooth  166  engaged with the opening  301 , the latch may be urged inward by applying an inward pressure to the edge  306  of the latch  300  which will protrude from the wall  105 ′. When the cartridge  100 ′ is completely assembled, the cover  104  retains the latch  300  in position. 
     When the cartridge  100 ′ is installed in the blood analyzer  200 ′, the groove  210  in the boss  208  engages the step  304  in the latch  300 . That is, the distance between the edge of the first opening  301  in the latch and the edge of the step  304  is equal to the distance between the inner edge of the tooth  166  and the furthest point of the inner wall of the boss  156  minus the depth of the groove  210  in the boss  208 . The width “w” of the step  304  is preferably at least equal to the depth of the groove  210 . Furthermore, the thickness of the step “t” is slightly less than the width of the groove  210 . Thus, the cartridge  100 ′ is captured in the blood analyzer by the engagement of the step  304  in the groove  210 . By applying inward pressure to the edge  306  of the latch  300 , the latch will move slightly inward as the wall  105 ′ flexes, thus releasing the step  304  from the groove and allowing the cartridge  100 ′ to be removed from the blood analyzer  200 ′. It can be seen that all of the connections between the blood analyzer and the cartridge are preferably made by moving the cartridge in one direction along a straight line toward the blood analyzer. Upon proper engagement between the blood analyzer and the cartridge, the latch  300  snaps into position, providing a positive audible response to indicate that proper engagement has been achieved. 
     In accordance with one embodiment of the present invention, a protective cover is provided which generally conforms to the shape of the cartridge  100 . FIG. 3 b   1 - 3   b   2  is an illustration of one such cover. Plugs  350  protrude from the cover  352 . The plugs  350  are sized to engage the protrusions  128 ,  130  in the couplings  108 ,  126  in order to seal the couplings when the cartridge is not installed in a blood analyzer. Preferably, each plug  350  fits snugly within the channel through one of the protrusions  128 ,  130 . A portion  354  of the cover is extends outward from the cover  354  to support the pump tube  136 . A pair of walls  356  prevent the cartridge from seating too deeply into the cover  352  and thus prevent the contacts of the electrical connector  1205  from contacting the bottom of the cover  352 . The cover  352  thus seals the fluid path through the sensor cartridge and covers and protects the electrical contacts of the sensor assembly  400 . 
     It can be seen from the above description of the cartridge that the present invention provides a cartridge that: (1) is very easy to install, and thus may be installed with virtually no training; (2) establishes both electrical and fluid connections in one installation process with little or no risk of misaligning the electrical or fluid connections of the cartridge with the corresponding connections of the blood analyzer; (3) includes an integral inexpensive and reliable pump tube assembly; (4) allows the user of the blood analyzer to see the movement of an analyte, gas bubbles, or reagent during analysis; (5) is inexpensive and thus may be disposed of without concern for excessive cost; (6) facilitates rapid, reliable replacement of the sensors of the blood analyzer; (7) reduces contact between blood elements and the analyzer; (8) is compact in size; (9) can be used for sensors with different analyte panels; and (10) allows one type of analyzer to accept many different types of sensors. 
     It should be understood that the cartridge of the present invention may be provided in numerous alternative configurations. For example, a plurality of sensor assemblies may be coupled in series to provide redundancy or to increase the number or type of sensors that are provided within the cartridge. Furthermore, straight fluid couplings may replace the right angle fluid couplings, and flexible tubing may be used to alter the direction of the flow path. Furthermore, the pump tubing may be directly coupled to the sensor assembly without the need for a fluid coupling between the pump tubing and the sensor assembly. Furthermore, a wide variety of latching mechanisms may be used to securely latch the cartridge to a blood analyzer. For example, the analyzer may have resilient barbs. FIG. 3 c  is an illustration of one embodiment in which barbs  212  spread apart as each edge of a cartridge  100  engages one of the barbs  212 . Upon completely installing the cartridge  100 , the barbs  212  then return to essentially the same position as they maintain without the cartridge with the barbed ends latching the outer surface of the cover of the cartridge. Furthermore, a resilient strap may be stretched across the cartridge to retain the cartridge in engagement with the analyzer  200 . Still further, a hole through the cartridge may be provided to allow a threaded member to engage a tapped hole in the analyzer, thus securing the cartridge to the analyzer. It will be clear that numerous other alternatives are possible. 
     Sensor Assembly 
     FIG. 4 is a front plan view of one embodiment of the sensor assembly  400  of the present invention. FIG. 5 is a back plan view of the sensor assembly  400  of the present invention shown in FIG.  4 . The present invention is a sensor assembly  400  having a plurality of sensors  403 , including highly pure, planar circular silver potentiometric and amperometric electrode sensors disposed on an inorganic substrate  405 . The sensor assembly  400  is preferably enclosed within a housing which defines a flow cell into which an analyte is transferred for analysis by the sensors  403 . Each sensor  403  is fabricated over a subminiature through hole through the substrate  405 . In accordance with the preferred embodiment of the present invention, each subminiature through hole is preferably laser drilled through the substrate. These through holes reduce the amount of area required on the front side of the substrate by each of the sensors  403 . That is, the present design geometry permits a number of sensors to be arrayed in a plane with fewer restrictions, since the layers of the conductors do not interfere with the placement of the sensor electrodes. Reducing the required area on the front side of the substrate allows a relatively large number of sensors  403  to be located in a relatively small area on the sensor assembly  400 , and thus allows the volume of the flow cell to be reduced. Reducing the volume of the flow cell reduces the sample size, which is important, since in some situations many samples are required from the same patient. Furthermore, as a consequence of the small sample size, the low thermal mass of the sensor assembly  400 , and the placement of a heater on the back side of the substrate, the present invention rapidly reaches a stable temperature at which analysis can be performed. Accordingly, the present invention can be installed into a blood analyzer (not shown) to provide rapid results (i.e., approximately 60 seconds in the case of one embodiment). 
     In addition to reducing the area required for each sensor  403 , the use of subminiature through holes through the substrate under each sensor  403  allows the sample and reference solution to be physically isolated by the substrate  405  from the electrical conductors  410  which transfer electrical charge or current from each sensor electrode to an associated connector pad  411  (see FIG.  5 ). Only the sensor electrodes and a thermistor  409  are located on the front side of the substrate. The predominant use of the back side of the substrate to route conductors allows the front side of the substrate (i.e., where space is at a much greater premium) to be reserved for those elements which must reside on the front side (such as the sensor electrodes). It should be noted that the conductors  410  and pads  411  are shown using broken lines in FIG. 5 to illustrate that an encapsulant  415  is applied over the conductors  410  and a portion of the pads  411 . As will be discussed in greater detail below, solder is deposited over the pads  411  to provide an appropriate electrical and physical interface to a surface mount connector (not shown in FIG.  5 ). As will also be described in more detail below, the thermistor  409  (see FIG. 4) is also encapsulated after being deposited on the front of the substrate  405 . While the term “deposited” is used throughout this document, the meaning is intended to be inclusive of all means for forming a structure in a layered device, including screening, plating, thick film techniques, thin film techniques, pressurized laminating, photolithographic etching, etc. 
     In accordance with one embodiment of the present invention, all of the connections which couple the sensors  403  to external devices are deposited on the back side of the substrate. These connections are spaced apart to provide the greatest possible insulation resistance. In one embodiment of the present invention, electrical conductors are deposited on a plurality of different fabrication layers deposited on the back side of the substrate  405 . No sample or reference solution contacts the back side of the substrate, as will be clear from the description provided below. A conventional surface mount electrical connector is preferably mounted on the connector pads to provide an electrical conduction path through a mechanical interface from the sensors  403  to external devices which detect and process the electrical signals generated by the sensors  403 . 
     The substrate  405  of the preferred embodiment of the present invention is essentially impervious to aqueous electrolytes and blood over relatively long periods of time (i.e., more than six months in the case of one embodiment of the present invention). In accordance with the preferred embodiment of the present invention, the inorganic substrate  405  is a sheet of approximately 0.025 inch thick commercial grade 96% alumina (Al 2 O 3 ). The substrate  405  is preferably stabilized by a heat treatment prior to purchase. One such substrate is part number 4S200 available from Coors Ceramic Company, Grand Junction, Colo. Alternatively, the substrate may be any non-conductive essentially flat surface upon which the sensors may be deposited, as will be described in further detail below. For example, the substrate may be any silicon, glass, ceramic, wood product, non-conducting polymer or commercially available frit that can be used as a substantially smooth flat surface. However, the substrate preferably should be capable of withstanding the presence of an electrolyte having a pH of more than 6 to 9 and remaining essentially unaffected for an extended period of time (i.e., in the order of weeks). 
     Use of an alumina substrate provides the following advantages: (1) low thermal mass; (2) dimensional stability when subjected to aqueous electrolytes and blood for extended periods time; (3) establishes a mechanically and chemically stable substrate for use with thick film deposition techniques; (4) can be accurately laser drilled to high precision with very small diameter holes; (5) does not react with any of the materials which are used to fabricate sensors; and (6) very high electrical resistance. As a consequence of the fact that the assembly, including the inorganic substrate  405  and each deposited layer, is very stable and does not breakdown when subjected to aqueous electrolytes and blood, the sensor assembly  400  maintains very high isolation between (1) each of the sensors  403 ; (2) each of the sensors  403  and each electrical conductor; and (3) each of the electrical conductors. 
     Because the substrate  405  and each of the layers deposited thereon are stable and resists breakdown in the presence of aqueous electrolytes and blood, extremely high electrical resistance is maintained through the substrate. Accordingly, the present invention provides very high electrical isolation between each of the sensors  403 , even after exposure to a corrosive environment over a relatively long period of time. This is advantageous for the following reasons. In accordance with one embodiment of the present invention, an isotonic reference medium (e.g., a gel or other a viscous solution having a known ion concentration) is placed over a reference electrode to provide a reference for potentiometric sensors which are fabricated on the substrate  405 . The present sensor assembly  400  can be stored in a sealed pouch (not shown) having a humidity that reduces any evaporation of the isotonic reference medium. Storing the present invention in a sealed pouch having a controlled humidity also ensures that the sensors  403  remain partially hydrated during storage. Since the sensors  403  remain partially hydrated during storage of the sensor assembly  400 , the sensors  403  of the present invention require minimal conditioning after installation. Therefore, having the sensors  403  stored in partially hydrated state greatly reduces the amount of time the user must wait before results can be attained from the sensors  403  of the present invention. This differs from prior art sensors which are stored in an essentially dry environment. Such prior art sensors must be assembled or preconditioned many hours prior to use. It is advantageous to provide a sensor assembly  400  which is available for use shortly after installation. For example, blood laboratories which use prior art blood analyzers must maintain at least two such prior art blood analyzers or risk being out of service for many hours after replacement of a sensor assembly (i.e., the time required to assemble, condition, calibrate, and rehydrate the sensors). The sensor assembly of the present invention can output results in as little as 10 minutes from the time the sensor assembly is installed, thus reducing the need for a second blood analyzer which would otherwise be required as a backup. 
     In accordance with the sensor assembly  400  shown in FIG. 4 and 5 the following sensors are provided: (1) sodium sensor  403   h ; (2) potassium sensor  403   g ; (3) calcium sensor  403   f ; (4) pH sensor  403   e ; (5) carbon dioxide sensor  403   a ; (6) oxygen sensor  403   b ; and (7) hematocrit value sensor  403   c ,  403   d . A reference electrode  407  is also provided. The reference electrode is common to each of the potentiometric sensors (i.e., the sodium sensor  403   h , potassium sensor  403   g , calcium sensor  403   f  and carbon dioxide sensor  403   a  sensors) and provides a voltage reference with respect to each such sensor. It will be understood by those skilled in the art that these sensors, or any subset of these sensors, may be provided in combination with other types of sensors. 
     Fabrication of the Sensor Assembly of the Present Invention 
     The following is the procedure by which one embodiment of the present invention is fabricated. It will be understood by those of ordinary skill in the art, that there are many alternative methods for fabricating the present invention. Accordingly, the description of the preferred method is merely provided as an exemplar of the present invention. 
     Initially, a series of through holes are drilled through the substrate  405 . Preferably, each through hole is laser drilled using a CO 2  laser to a diameter in the range of approximately 0.002-0.006 inches as measured on the front side of the substrate  405 . By maintaining the small diameter of each through hole, the planar characteristic of an electrode which is deposited over the through hole is not distorted by the presence of the through holes. In the preferred embodiment of the present invention, thirteen holes are required, such that one hole is provided for each sensor, except for the hematocrit sensor  403   c ,  403   d  and the oxygen sensor  403   b , each of which require two holes. The hematocrit sensor requires two holes in light of the two electrodes  403   c ,  403   d . The oxygen sensor  403   b  preferably has one through hole for connection to the cathode of the sensor and one through hole for connection to the anode of the sensor. In addition, two through holes are preferably used for the connections to the thermistor  409 . Also, two through holes are preferably used for the reference electrode  407  to reduce the risk of a defective through hole creating an open circuit. In the preferred embodiment of the present invention, each through hole that is associated with a sensor electrode is located under the location at which the associated sensor electrode to be deposited. Each such through hole is preferably located essentially at the center of the sensor electrode with the exception of the oxygen sensor  403   b . However, in an alternative embodiment of the present invention, each through hole may be located anywhere underneath an electrode. 
     When the substrate  405  is a ceramic material, such as alumina, the substrate is preferably annealed after drilling all of the through holes at a temperature in the range of approximately 1000-1400° C., and more preferably in the range of approximately 1100-1200° C. Annealing the substrate after drilling ensures re-oxidation of a nonstoichiometric residue that attaches to the holes after the laser drilling. Without annealing, the residue (which is very reactive) contaminates the sensor electrodes, resulting in less pure electrode surfaces, which can lead to poor sensor performance. In the preferred embodiment of the present invention, the substrate is scribed after annealing. However, in an alternative embodiment of the present invention, the substrate may be scribed either before annealing, or not at all. Scribing the substrate allows several individual sensor assemblies formed in the same deposition processes on one substrate to be separated after all of the assemblies have been completed. 
     Once the through holes have been drilled and annealed, a thermistor paste is deposited in a predetermined pattern on the front side of the substrate  405  to form a thermistor  409  as shown in FIG.  4 . In an alternative embodiment of the present invention, the particular geometry of the thermistor may vary from that shown in FIG.  4 . In an alternative embodiment, the thermistor  409  is a discrete component which is not formed directly on the substrate. In the preferred embodiment of the present invention, the thermistor paste is part number ESL  2414 , available from Electro-Science Laboratories, Inc. The thermistor paste  501  is preferably deposited to a thickness of approximately 15-29 μM when dried (10-22 μM when fired). The thermistor paste is oven dried and fired at a temperature of approximately 800-1000° C. for approximately 1-20 minutes. It will be understood by those skilled in the art that the thermistor  409  may be fabricated with any material that will provide information to an external control device by which the temperature of the sensor assembly  400  can be controlled. The thermistor is preferably be placed adjacent to any sensor that is particularly temperature sensitive or appropriately when measuring a temperature sensitive analyte. In an alternative embodiment of the present invention, a number of sensors and independently controllable heaters may be used to regulate the temperature of each sensor and the local temperature of the analyte at different locations along the flow path. 
     Once the thermistor paste has been deposited, dried, and fired, the substrate  405  is preferably placed in a vacuum fixture. The vacuum fixture (not shown) has a plurality of vacuum ports, each placed in contact with the opening of a through hole on the front side of the substrate. Preferably, each vacuum port is concurrently aligned with one of the through holes to create a relative low pressure within each through hole of the substrate with respect to the ambient pressure outside the through holes. A metallic paste, which is preferably compatible with the metal to be used to form the metallic layer of the electrodes of the electrolyte sensors  403   h ,  403   g ,  403   f , as will be described in more detail below, is deposited over the through holes on the back side of the substrate  405 . The deposited metal forms a conductive pad over the through hole. However, due to the vacuum applied to the front side of the substrate  405 , a portion of the metal is drawn through the through holes. In accordance with the present invention, the metallic paste is preferably a silver paste, such as part number ESL 9912F, available from Electro-Science Laboratories, Inc. In accordance with the preferred embodiment of the present invention, the metallic paste is applied through a screen having a mesh density of  250  wires per inch (each wire having a diameter of approximately 0.0016 inches and a spacing of 0.0025 inches) and an emulsion thickness of approximately 0.0007 inches. The emulsion is developed to form a mask which allows the metal paste to pass through the screen only at the locations of the through holes on the back side of the substrate  405 . The metallic paste is formed by the screen into columns above each through hole. Those columns of metal paste are then drawn down into the through holes by the reduction in pressure caused by the vacuum fixture. This procedure is preferably performed twice to ensure that each through hole is filled with the silver paste. 
     The substrate is then rotated to place the back side of the substrate  405  in contact with vacuum ports. The ports are aligned with the through holes over which the hematocrit electrodes  403   c ,  403   d  are to be deposited. The metal with which the front side of the through holes are filled is preferably selected to be compatible with the particular metal from which the electrode to be formed over the through hole is to be formed. In the preferred embodiment of the present invention, the hematocrit electrodes are formed using platinum. Therefore, the metallic material which fills the front side of these through holes and forms conductive pads on the front side of the substrate is preferably a silver/platinum paste, such as a mixture of silver paste, part number QS175, available from DuPont Electronics, and platinum paste, part number ESL 5545, available from Electro-Science Laboratories, Inc. The use of a silver/platinum paste presents a compatible interface between the platinum hematocrit sensor electrodes and the silver conductive material which fills the back side of the through holes which will underlie the hematocrit sensor electrodes. The mixture preferably has 50 parts silver, and 50 parts platinum. However, in an alternative embodiment, other alloys of silver and platinum may be used. Furthermore, any alloy which is compatible with platinum (i.e., with which platinum forms a solid solution), may be used. In a next screening process, each of the other eleven through holes (i.e., each of the through holes except the two over which the hematocrit electrodes  403   b ,  403   c  are to be deposited) are preferably filled from the front side of the substrate  405  using the same metallic paste that was previously used to fill the through holes from the back side of the substrate. Conductive pads, similar to the conductive pads formed on the back side of the substrate  405 , are formed on the front side of the substrate  405 . Filling the through holes from both the front and the back side of the substrate ensures that the entire through hole will be filled, and that a low resistance electrical contact will be made between the front and back side of the substrate through each through hole. 
     FIG. 6 a  is an illustration of one pattern to which a heater  601  conforms when deposited on the substrate  405  in accordance with the present invention. In the embodiment shown, the heater  601  conforms generally to a complex serpentine pattern. FIG. 6 a  also shows a number of electrically conductive traces  603  which provide electrical conduction paths for current and/or electrical potential to be communicated from the electrodes of the sensors  403  to the pins of a connector to be affixed to the substrate, as will be described in greater detail below. The heater  601  is preferably deposited on the back side of the substrate  405 . In accordance with one embodiment of the present invention, a heater paste blend including 10 parts of part number 9635-B, available from Heraeus Cermalloy, and 90 parts of part number 7484 available from DuPont Electronics is deposited to a thickness of 15-33 μM dried (7-20 μM fired). In accordance with one embodiment, a through hole vacuum may be applied to seal any through holes that remain open. It will be appreciated by those skilled in the art that the heater may be any heater device that provides a source of heat which can be readily controlled by a control device that receives information regarding temperature from the thermistor  409 . It will also be appreciated that the particular routes taken by the conductors  603  may vary in alternative embodiments of the invention. 
     Once the heater  601  and conductors  603  have been deposited, a series of dielectric layers  419  are deposited on the back side of the substrate  405  which electrically insulate the heater  601  and the conductors  603  from additional layers which are to be later deposited over the heater  601  and the conductors  603 . The dielectric includes openings through which “vias” can be formed to provide electrical contact paths to the conductors  603  through the dielectric layers. A dielectric paste (such as part number 5704, available from E.I DuPont) is applied to the back side of the substrate  405 , preferably using a conventional thick film screening technique. The screen used to apply the dielectric paste masks all locations except those at which a via is to be formed. FIG. 6 b  is an illustration of the back side of the substrate  405  after each of the dielectric layers  419  have been deposited. It should be noted that the heater  601  and conductors  603  are shown in broken lines to indicate the presence of the dielectric layer  419  over the heater  601  and conductors  603 . After two layers of the dielectric paste have been deposited, dried, and fired at a temperature of approximately 800°-950° C., a metallic paste, such as a palladium/silver composite, which in the preferred embodiment is part number 7484, available from E.I. DuPont, is deposited over those locations  750  at which vias are to be formed. In an alternative embodiment of the present invention, other noble metal mixtures can be used to achieve the desired resistance value within the available surface area. The metallic paste is then fired at 800°-950° C. for approximately 1 to 20 minutes. Two more layers of dielectric paste and metallic paste are deposited, each such layer being fired at 800°-950° C. for approximately 1 to 20 minutes directly after being deposited. It will be clear to those skilled in the art that other methods for depositing the dielectric layer and the vias may not require multiple layers of dielectric and metal. However, due to limitations on the thickness of layers which are deposited through a screen, more than one layer of both dielectric paste and metallic paste are preferably deposited. The dielectric layers between the conductive lines of the heater  601  build to a height which is nearly equal to the height of the dielectric layer over the heater  601 , thus providing a relatively smooth surface at the back side of the sensor assembly  400 . 
     After the last dielectric layer  419  is deposited, a second layer of conductors is deposited. FIG. 7 is an illustration of a second conductive layer, including the second layer of conductors  410 , a plurality of connector pads  411 , and connections  803  to the resistor  412  (see FIG.  5 ). In one embodiment of the present invention, the second conductive layer is formed from a metallic paste, such as palladium/silver, which in the preferred embodiment of the present invention is part number 7484 available from E.I DuPont. The second conductive layer is then oven dried and fired at a temperature in the range of approximately 800°-950° C. for approximately 1 to 20 minutes. The conductors  410  and conductive connector pads  411  complete the connection between the sensor electrodes and external devices (not shown) coupled to the connector fixed to the connector pads  411 . The second layer of conductors is oven dried and fired at a temperature in the range of approximately 800°-950° C. for approximately 1 to 20 minutes. 
     In accordance with the present invention, conductors  603 ,  410  are deposited on only two layers (i.e., the heater layer and the connector pad layer). However, in an alternative embodiment of the present invention in which the geometry of the sensor assembly  400  makes it difficult to route the conductors from each sensor to an appropriate electrical contact pad to which a connector is to be electrically coupled, more than two layers having conductors may be used. In such an embodiment, each such conductor layer is preferably separated by at least one layer of insulating dielectric material. 
     After the second layer of conductors has been deposited on the back side of the substrate  405 , each of the layers which form the electrodes of the sensors  403  are deposited on the front side of the substrate  405 . Concurrent with the deposition of the first metal layer of each electrode, contacts  414  to the thermistor  409  are deposited to couple the thermistor to the through holes that are adjacent the thermistor  409  (see FIG.  4 ). FIG. 8 is an illustration of an oxygen sensor  403   b ′ in accordance with an alternative embodiment of the present invention. Both the oxygen sensor  403   b  and  403   b ′ are essentially conventional amperometric cells. The only difference between the oxygen sensor  403   b  shown in FIG.  4  and the oxygen sensor  403   b ′ shown in FIG. 8 is the shape of the anodes  701 ,  701 ′. In accordance with the preferred embodiment of the present invention, the anodes  701 ,  701 ′ are essentially straight conductors which deflect from straight at the distal end  703 ,  703 ′. Preferably, the area of the anode is a minimum of 50 times greater than the area of the cathode to ensure the most stable operation. In addition, the distance between the anode and the cathode is preferably approximately 0.020-0.030 inches to ensure that the potential developed across the anode to cathode is not too great. It should be noted that the anode of the oxygen sensor may be configured to conform to any number of alternative shapes. These two shapes are provided merely as exemplars of the shape of the anode in accordance with two particular embodiments of the present invention. In one embodiment of the present invention, a metal, such as silver paste, part number QS 175, available from DuPont Electronics, is deposited to form the anode  701 ,  701 ′ of the oxygen sensor  403   b ′. Alternatively, any metal suitable for use in forming the anode of an amperometric cell may be used, such as platinum, ruthenium, palladium, rhodium, iridium, gold, or silver. A distal end  703 ,  703 ′ of the anode  701 ,  701 ′ is deposited over one of the above described through holes  705  through the substrate  403 . 
     The cathode conductor  707  is then deposited. A distal end  709  of the cathode conductor  707  is deposited over another of the through holes  711  through the substrate  403 . The cathode conductor  707  and the anode  701 ,  701 ′ are oven dried and fired at a temperature of approximately 800° C. to 950° C. for approximately 1 to 20 minutes. 
     FIG. 9 is a cross-sectional view of a portion of the substrate  405  through which a sensor through hole  702  is formed and on which metal layers of an ion sensitive sensor electrode have been deposited. Concurrent with the deposition of the oxygen sensor  403   b , and by deposition of the same type of material (preferably silver) deposited to form the metallic layer of the anode  701 ,  701 ′ of the oxygen sensor  403   b , a first metallic layer  704  of each of the electrodes associated with each of the other sensors  403   a ,  403   e - 403   h  and the reference electrode  407  are deposited on the substrate over a through hole  702 . In the case of sensors  403   a ,  403   e - 403   h  which are to have a polymeric membrane disposed over the metallic layer, a second metallic layer  706 , preferably of the same material as the first metallic layer  704 , is deposited over the first metallic layer  704  in order to reduce any distortion in the flatness of the surface due to the presence of the through hole  702  located beneath the first metallic layer  704 . That is, electrodes formed over a through hole  702  with only one layer of metallic material tend to develop a depression over the through hole  702 . Such a depression is generally of no consequence if the electrode is not to be coated with a polymeric membrane. 
     However, in sensors which have polymeric membranes, such a depression can cause the membrane to become embedded in the electrode  704 . As a result of this distortion, optimal performance would not be achieved. That is, very uniform membrane geometry is important to achieving optimal sensor function and performance. This can be understood in light of the fact that in the preferred embodiment of the present invention, the thickness of a polymeric membrane that is applied over the metallic layers  704 ,  706  is determined by pouring a controlled volumetric quantity of a membrane solution into a sensor cavity having well defined dimensions (as will be discussed further below). The membrane formed over the metallic layer  706  is very thin (i.e., approximately 5-250 μM). Any variation in the thickness of the membrane at one point, effects the thickness of the membrane at each other point. Such variations in the thickness of the membrane adversely effect the performance of the sensor  403 . Therefore, if a depression exists in the metallic layer which underlies the polymeric membrane, the membrane will be thicker over the depression, and thus thinner over the remainder of the electrode. Depositing a second metallic layer  706  smooths any such depression which might otherwise exist. The second metallic layer  706  preferably has a different diameter than the first layer  704  in order to reduce the chances that the metallic layers will puncture the polymeric membrane due to the abrupt edge that would be formed at the perimeter if both the first and second metallic layers  704 ,  706  were to have the same diameter. Since the presence of a depression is insignificant in electrodes of sensors which do not require a thin membrane, these sensors are preferably formed having only one metallic layer  704 . 
     The preferred dimensions for the metallic layers  704 ,  706  of each sensor in accordance with one embodiment of the present invention are provided below. It will be understood by those skilled in the art that other dimensions may be quite suitable for fabricating sensors. However, the dimensions presented reflect a tradeoff between reduced impedance and reduced size. A tradeoff is required because of the desire to form the sensor in as small an area as possible, and the competing desire to form a sensor which has a relatively low impedance. These two goals are incompatible because of the inverse relationship between size and impedance. That is, in general, size is inversely proportional to impedance. Therefore, the greater the size of the sensor electrode, the smaller the impedance of that electrode. 
     The diameter of the first metallic layer  704  of the CO 2  sensor  403   a , the pH sensor  403   e , and each of the electrolyte sensors  403   f ,  403   g ,  403   h  is 0.054 inches. The diameter of the second electrode layer  706  of each of these sensors is 0.046 inches. The second layer  706  is deposited over the first layer  704 . The metallic layer  704  of the reference electrode is generally rectangular, having rounded comers with radius equal to one half the width of the electrode. The width of the electrode is preferably 0.01 inches, and the length is preferably 0.08 inches. It will be understood by those skilled in the art that the reference electrode  407  may be formed in numerous other shapes. After the first metallic layer  704  is deposited, the substrate  405  is oven dried and fired at approximately 800°-950° C. for approximately 1-20 minutes. After deposition, the second metallic layer  706  is similarly dried and fired. Each of the metallic layers  704 ,  706  is preferably 16-36 μM thick after drying, and 7-25 μM thick after firing. 
     FIG. 10 is a cross-sectional view of one of the hematocrit sensor electrodes  403   c . Only one of the two electrodes  403   c ,  403   d  are shown, since each are essentially identical. In accordance with the preferred embodiment of the present invention, the metal used to form the electrodes of the hematocrit sensor  403   c ,  403   d  differs from the metal  704 ,  706  used to form the electrodes of the electrolyte sensors  403   f ,  403   g ,  403   h , the pH sensor  403   e , the oxygen sensor  403   a , and the reference electrode  407 . Therefore, in the preferred embodiment, the electrodes of the hematocrit sensor  403   c ,  403   d  are formed by depositing a third metallic layer  1001 . Since no polymeric membrane is to be placed over the metallic layer  1001  of the hematocrit electrodes  403   c ,  403   d , the hematocrit electrodes  403   c ,  403   d  preferably only have one metallic layer. In the preferred embodiment of the present invention, the metal used to form the electrodes for the hematocrit sensor  403   c ,  403   d  is a cermet platinum conductor, such as part number ESL 5545, available from Electro-Science Laboratories, Inc. The diameter of the metallic layer  1001  of each hematocrit sensor electrode is 0.054 inches. The hematocrit sensor electrodes  403   c ,  403   d  are preferably spaced approximately 0.15 inches apart. 
     After forming the metallic layer  1001  of the hematocrit sensor electrodes  403   c ,  403   d , the cathode conductor  707  (see FIG. 8) is deposited. In accordance with the preferred embodiment of the present invention, the cathode conductor  707  is formed from a gold paste, such as part number ESL 8880H, available from Electro-Science Laboratories, Inc. It will be understood by those skilled in the art that the cathode conductor  707  may be fabricated from any metal commonly used to form a cathode of a conventional amperometric cell, However, it should be noted that the level of contaminants in the paste will effect the sensor characteristics. Furthermore, in an alternative embodiment of the present invention the particular geometry of the cathode conductor  707  may vary from that shown in FIG.  8 . At the same time that the cathode conductor  707  is deposited, a pair of laser targets  417 ,  418  are preferably deposited. The laser targets  417 ,  418  provide a reference which is used to form a cathode  717 , as will be discussed in greater detail below. Once deposited, the cathode conductor  707  is dried and fired at a temperature of 800°-950° C. for approximately 1 to 20 minutes. 
     Once the cathode conductor  707  has been dried and fired, a resistor  412  is preferably deposited on the back side of the substrate  405 , as shown in FIG.  5 . The resistor  412  is coupled in series with the heater  601  in order restrict the current to an appropriate level through the heater during electrical conduction. Next, a first layer of an encapsulant is deposited on the front side of the substrate  405 . FIG. 11 is a cross-sectional view of a sensor  403  showing the first layer of encapsulant  901 . FIG. 12 is a cross-sectional view of one of the hematocrit sensors  403   c  showing the first layer of encapsulant  901 . It should be noted that FIGS. 10 and 11 are not to scale and that the first layer of encapsulant  901  is preferably very thin (i.e., preferably only a few microns). The encapsulant  901  is deposited essentially over the entire front side of the substrate  405  in order to prepare the surface of the substrate to receive a polymer, as will be discussed in more detail below. In accordance with the preferred embodiment of the present invention, the encapsulant  901  is deposited through a screen using a conventional thick film technique. The screen preferably has a density of 250 wires per inch (with a wire diameter of approximately 0.0016), and an emulsion thickness of 0.0007 inches. The screen masks the encapsulant  901  from forming over the thermistor  409  and metallic layers  704 ,  706  of each of the sensors. However, in the preferred embodiment, the distal end  703 ,  703 ′ of the anode  701 ,  701 ′ and the entire cathode conductor  707  are encapsulated, as shown for example in FIG. 8. A high quality encapsulant is preferably used which will not undergo chemical alteration in the presence of a caustic solution (such as blood or other aqueous solvents). For example, in the preferred embodiment, the encapsulant is part number ESL 4904, available from Electro-Science Laboratories, Inc. However, the thermistor  409  is preferably not encapsulated with the higher quality encapsulant, since such high quality encapsulants typically require firing at high temperatures (850° C., for example in the case of encapsulant used in the preferred embodiment). Such high temperatures will cause the thermistor  409  to deform. Therefore, only after firing the high quality encapsulant can the thermistor be encapsulated. Accordingly, in the preferred embodiment of the present invention, the thermistor  409  is encapsulated with an encapsulant which may be fired at a low temperature. 
     In the preferred embodiment of the present invention, a second layer of encapsulant  905  is deposited only over the cathode conductor  707  in order to ensure that the cathode conductor is securely isolated. In one embodiment of the present invention, the second layer of encapsulant  905  is applied in two screening procedures in order to provide a total desired thickness for both the first and second layers of encapsulant of approximately 27-47 μM. While alternative embodiments of the present invention may employ an encapsulant layer which differs in thickness, a thickness in the range of approximately 27-47 μM provides satisfactory isolation of the cathode conductor  707 . Furthermore, i single layer of encapsulant provides sufficient treatment of the surface of the substrate  405  to allow a polymer to be deposited and bonded to the substrate  405 , as further explained below. 
     After the encapsulant  901 ,  905  are deposited over the cathode conductor  707 , a hole is preferably laser drilled through the encapsulant  901 ,  905  to expose a portion of the cathode conductor  707 , and thus form the cathode  717 . The cathode may be laser drilled either before or after firing the encapsulant. The laser targets  417 ,  418  are used to visually align the laser apparatus in order to drill the hole at the correct location. That is, the lower horizontal edge of the target  417  identifies a line in the horizontal direction. Likewise, the leftmost edge of the laser target  418  identifies a line in the vertical dimension. The cathode is then formed at the intersection of these two lines. Alternatively, the cathode  717  is formed by masking a portion of the cathode conductor  707  in order to prevent the encapsulant  901  from forming over that portion of the cathode conductor  707 . In yet another embodiment of the present invention, the cathode  717  may be exposed by a chemical etch. It will be clear to those skilled in the art that numerous other methods may be used to expose a portion of the cathode conductor  707  in order to form a cathode  717 . 
     After applying the first and second encapsulant layers to the front of the substrate  405 , a thermistor encapsulant  413  is deposited over the thermistor  409 . The thermistor encapsulant  413  can be fired at a relatively lower temperature (such as approximately 595° C.) and thus firing of the thermistor encapsulant  913  does not disturb the geometry of the thermistor  409 . In one embodiment of the present invention, the thermistor encapsulant  413  is applied in two screenings in order to achieve a desired thickness and to ensure that no pores are formed in the encapsulant  413 . It will be understood by those skilled in the art that the encapsulant over the thermistor  409  should remain relatively thin in order to avoid adding any delay in the sensing of the temperature of the sensor assembly  400 . In addition, a resistor encapsulant  415  is deposited over the resistor  412  on the back side of the substrate  405 . The resistor encapsulant  415  is preferably the same material as the thermistor encapsulant  413 . 
     After the resistor encapsulant  413  has been deposited on the back side of the substrate  405 , a first polymer layer  1101  is deposited on the front side of the substrate  405 . The first polymer layer (together with the first encapsulation layer  901 ) forms the lower wall  902  of a plurality of sensor cavities  903  (see FIGS.  10  and  11 ). The polymer of the preferred embodiment of the present invention is screen printable, absorbs minimal moisture, chemically isolates the membrane chemistries of adjacent cavities, and produces a strong solution bond with the polymeric membrane also forms a strong bond with the dialectic layers when exposed at the inside surface of the cavity by an appropriate solvent (such as tetrahydrofuran, xylene, dibutyl ester, and carbitol acetate or any cyclohexanone solvent) in the membrane formation, as will be discussed in further detail below. 
     The polymer used to form the layer  1101  is preferably a composition of 28.1% acrylic resin, 36.4% carbitol acetate, 34.3% calcined kaolin, 0.2% fumed silica, and 1.0% silane, noted in percentage by weight. The acrylic resin is preferably a low molecular weight polyethylmethacrylate, such as part number 2041, available from DuPont Elvacite. The calcined kaolin is preferably a silaninized kaolin, such as part number HF900, available from Engelhard. The silane is preferably an epoxy silane, such as trimethoxysilane. Silane bonds to the hydroxyl groups on the glass encapsulant over the substrate, and yet is left with a free functional group to crosslink with the resin&#39;s functional group. In accordance with one embodiment of the present invention, the first polymer layer  1101  is deposited in three screening processes in order to attain the desired thickness (i.e., preferably approximately 0.0020 inches). The first polymer layer is dried after each screening process. A second polymer layer  1103  is deposited to form an upper wall  904  of the sensor cavities  903 . The first and second polymer layer  1101 ,  1103  differ only in the diameter across the cavity at the lower cavity wall  902  and at the upper cavity wall  904  and the number of screening processes that are required to achieve the desired depth. In the case of the second polymer layer,  10  screening procedures are performed. The second polymer layer is dried after each screening procedure. In addition, after the last two procedures, the polymer is both screened and cured. In the preferred embodiment of the present invention, the last screening procedure may be omitted if the second polymer layer has achieved the desired thickness (i.e., preferably 0.0075-0.0105 inches after curing). 
     The diameter of the cavities are preferably carefully controlled to aid in controlling the deposition of the membranes which are placed over the electrodes of the sensors (i.e., the shape and thickness of the membranes). That is, the sensor cavities enable a droplet of polymeric membrane solution to be captured and formed into a centrosymmetric form over the electrode with sufficient surface contact with the walls of the cavity to assure that the membrane remains physically attached. 
     Preferably, the sensor cavities  903  for the pH sensor  403   e , the electrolyte sensors  403   f ,  403   g ,  403   h , and the hematocrit sensor  403   c ,  403   d , each have a total depth of approximately y=0.0075 inches, a diameter at the upper wall  904  of approximately x 1 =0.070 inches, and at the lower wall of approximately x 2 =0.06 inches (see FIG.  1 l). The diameter x 3  of the carbon dioxide sensor cavity  903  is slightly larger than the diameter x 1  of the electrolyte sensors  403   e - 403   f  and the hematocrit sensor electrodes  403   b ,  403   c . In the preferred embodiment, the diameter X 3  is equal to 0.078 inches (see FIG.  12 ). It should be understood that a membrane of the same thickness may be produced by increasing the diameter of the sensor cavity  903  and increasing the volumetric quantity of the membrane solution that is applied to the sensor in proportion to the increase in the volume of the cavity. Likewise, the same thickness can be maintained by decreasing the diameter of the sensor cavity  903  and proportionally decreasing the volumetric quantity of the membrane solution. It will be clear to those skilled in the art that in an alternative embodiment of the present invention, the sensor cavities may have a shape other than the generally cylindrical shape disclosed above. For example, in accordance with one embodiment of the present invention, the electrodes are formed in an oval shape to reduce the required volume of a sample. However, in the preferred embodiment, the sensor cavities are either cylindrical or generally conical. 
     Once the sensor cavities  903  have been formed and the polymer layers dried, each silver potentiometric electrode is chemically chlorodized to create a layer of silver chloride. The cavity  903  of each ion sensitive sensor is filled with an electrolyte which is appropriate to the particular type of sensor  403 . In the preferred embodiment of the present invention, the electrolyte used in the sodium, potassium and calcium electrolyte sensors are ions of inorganic salts that disassociate in solution, such as NaCl, KCl, or CaCl 2 . In accordance with one embodiment of the present invention, the electrolyte solution is evaporated to a solid form. Alternatively, the electrolyte remains a liquid or a gygroscopic water insoluble gel that acts as a support to immobilize the electrolyte. In accordance with one embodiment of the present invention, such a gel may crosslinked after transfer to the cavity  903 . Furthermore, in accordance with one embodiment, the gel undergoes polymerization by a catalyst contained within the solution. In one such embodiment, the gel is polymerized by activating a catalyst with heat or radiation. 
     The gelled polymer is preferably one of the following, or a mixture of these: (1) starch, (2) polyvinyl, (3) alcohol, (4) polyacrylamide, (5) poly (hydroxy ethyl methacrylate), or (6) polyethylene glycol or polyethylene oxide ether, or another long chained hygroscopic polymer. Hygroscopic polysaccarides or natural gelatin are preferably added to the electrolyte solution. 
     The electrolyte used in the pH sensor preferably has an acidic pH in the range of about 3-7. In accordance with one embodiment, the electrolyte is an aqueous solution of potassium hydro phosphate (KH 2 PO 4 ), preferably has 13.6 grams of potassium hydro phosphate in one liter of deionized water. The electrolyte suppresses the reaction of carbon dioxide and water to minimize the extent to which the carbon dioxide influences the pH of the electrolyte. This favors the pH response for pH measurement and minimizes the response of CO 2 . The electrolyte for pCO 2  sensor is initially at an alkaline in the range of approximately 7-14. However, in the preferred embodiment of the present invention, the electrolyte is approximately 8 due to the presence of bicarbonate ions. In accordance with the present invention, the electrolyte for the pCO 2  sensor is preferably 0.02 moles of sodium bicarbonate in a liter of deionized water. Solutions in either liquid or gel phase may be used. A sensor which includes such an electrolyte is also described in U.S. Pat. No. 5,336,388, assigned to PPG Industries, Inc, which is incorporated in its entirety by this reference. 
     The electrolyte of the oxygen sensor  403   a  provides a low impedance contact across the anode and cathode and not to create a standard chemical potential as is the case in the aforementioned potentiometric sensors. Suitable electrolytes are NaCl and KCl. The electrolyte may be either a fluid or a gel. The preferred use of the electrolyte is in a buffered solution such as one having 0.1 mole potassium hypophosphite (KH 2 PO 3 ). 
     All of the aforementioned electrolytes are preferably encapsulated by a selectively permeable, hydrophilic membrane that serves to trap the electrolyte against the electrode. Such membranes include a polymer, a plasticizer, an ionophore, a charge screening compound, and a solvent. The membranes are selective permeable barriers that restrict the free passage of all but the desired ion. The membrane preferably comprises an inert iypophilic polymer dispersed in an organic plasticizer. 
     Water molecules will rapidly diffuse across these membranes. In accordance with one embodiment of the present invention, the inert polymer is polyvinlychoride (PVC). However, in an alternative embodiment, other ion permeable polymers may be used, such as (1) copolymeric vinyl ethers, (2) porous polytetraflourethelene (PTFE), (3) silicones, (4) cellulose acetate, (5) poly (methlymethacrylate), (6) polystyrene acrylate, (7) methacrylate copolymers, (8) polyimides, (9) polyamides, (10) polyurethanes, (11) polybisphenol-A carbonate (polysiloxane/poly(bisphenol-A carbonate) blocked copolymer, (12) poly(vinylidenechloride); and (13) lower alkyl acrylate and methacrylate copolymers and polymers. It will be clear to those skilled in the art that this list is not exhaustive, and that other such ion permeable polymers may be used. 
     Furthermore, suitable plasticizers include (1) dioctyl adipate, (2) bis(2-ethylhexyl)adipate, (3) di-2-ethlylhexyladipate, (4) dioctyl phthalate, (5)  2- nitrophenyl octyl ether (NPOE), (6) diotcyl sebacate, (7) nitrobenzene, (8) tri(2-ethylhexyl) phosphate, (9) dibutyl sebacate, (10) diphenyl ether, (11) dinonyl phthatlate, (12) dipenyl phthalate, (13) di-2-nitrophenyl ether, (14) glycerol triacetate, (15) tributyl phosphate, (16) dioctyl phenyl phosphate, and similar long chained ethers and hydrocarbons, and combinations thereof. In the preferred embodiment, a combination of bis(2-ethylhexyl)adipate, 2-nitrophenyloctylether or 0-nitrophenyloctylether (NPOE), and nitrobenzene are used as the plasticizer for the pH and CO 2  sensor. Dioctyl Phthalate is preferably used as the plasticizer in the calcium, potassium and sodium sensors. 
     The membrane polymer and plasticizers are preferably soluble in organic solvents, such as cylohexanone, tetrahydofuran, xylene, dibutyl ester, and carbital acetate. In accordance with one embodiment of the present invention, such solvents are removed from the membrane after application over the electrode by vacuum drying at ambient temperatures or low temperatures less than 100° C. The solvent softens the organic layer on the substrate that supports the membrane and encapsulates the internal electrolyte over the electrode while allowing penetration of the membrane by the ion via the complexing agent or ionophore. In accordance with one embodiment of the present invention, after encapsulation, the internal electrolyte is hydrated for a predetermined period prior to use to allow water vapor to permeate the membrane and form an internal electrolyte solution producing a chemically and physically uniform distribution of charge on the electrode. 
     It will be understood by those skilled in the art that any ionophore or ion exchanger that mediates the interaction of the ion with environment and which facilitates the translocation of the ion would be suitable for use in the membrane of the present invention. For example, in the present invention the ionophore or ion exchanges may be another of the following: (1) tridodecylamine (TDDA), (2) tri-n-dodecylamine, (3) valinomycin (K + ); (4) methyl monesin (Na + ), or (5) tridodecylmethyl-ammonium chloride (Cl − ). A lipophilic organic anion serves as a balancing specie, such as tetraphenyl borate is preferably present to provide electroneutrality. The membranes of the present invention provide accurate detection and fast response over long periods of use. 
     The oxygen sensor membrane restricts access of electroactive materials other than oxygen to the electrode surface while allowing free diffusion of oxygen to the electrode surface. 
     All membrane solutions arc dispenses in the sensor cavities using automated fluid dispensing systems. These systems have three main parts: (1) a horizontal x-y-z motorized and programmable table (such as those available from Asymtek of Carlsbad, Calif.); (2) a precision fluid metering pump (such as those available from Fluid Metering, Inc. of Oyster Bay, N.Y.); and (3) a personal computer control unit. All three parts are linked by a digital communication protocol. Software for set-up and dispensing a sequence of liquid microvolumes communicates the x, y, and z positions to the table, and timing of the dispensing pump controller. At each cavity, the metering pump transfers a preset volume of electrolyte or membrane solution through fine diameter tubing from a supply reservoir to a needle or nozzle mounted on the motorized axes of the table and then to the cavity. The fluid may be successfully dispensed with a number of different pumps; pinch tube, rotary positive displacement or diaphragm valves. The drop size is generally no larger than one diameter of the sensor cavity. 
     After dispensing the aqueous or organic solution, the membrane is formed by drying or curing liquid. Drying removes the solvent components by evaporation. The drying process may be performed by heating or applying a vacuum pressure. Some organic solutions may be cured either thermally or by exposure to ultra-violet radiation. 
     The combination of the geometry, membrane composition, and aqueous or organic internal electrolyte have been found to yield membranes of minimal thickness, with controlled diffusion paths so that potentiometric sensor to a varying concentration of gas. Elimination of in-plane electrical connections to the electrode by use of a subminiature through hole assures better control of the electro-chemical process. In addition, the use of subminiature through holes improves the flatness of the bonding surface of the polymer coating laminated on the substrate for better bonding and sealing of the flow cell. 
     FIG. 13 is a top plan view of the sensor assembly  400  installed within a plastic encasement  1200 . FIG. 14 is a cross-sectional view of the sensor assembly  400  installed in the plastic encasement  1200 . After each of the sensors have been completed, the pads  411  are plated with solder. The solder provides an electrical and mechanical interface between the pads  411  and contacts  1209  of a conventional electrical surface mount connector  1205 . The contacts  1209  of the surface mount connector  1205  are soldered to the pads  411  in a conventional manner. In addition, the connector  1205  is preferably secured to the substrate  405  by an adhesive, such as an epoxy glue. Electrically conductive pins  1207  of the conventional connector  1205  permit the sensor assembly  400  to be easily installed and in, and removed from, a blood analyzer (not shown). Use of a conventional surface mount connector  1205  result in a reliable interface to the blood analyzer instrumentation, provides a simple design, low cost construction, an simple test interface, and allows critical connections to be spaced apart to ensure high electrical resistance between each critical connection. Furthermore, the conventional surface mount connector  1205  allows the present invention to be mass produced at low cost, and makes the present invention analogous to familiar semiconductor dual-in-line packages. 
     The front side of the sensor assembly  400  is enclosed in the plastic encasement  1200  which forms a flow cell  1201  and a reference cell  1203 . A lap joint  1211  is preferably formed between the sensor assembly  400  and the encasement  1200 . In accordance with the preferred embodiment of the present invention, an adhesive, such as epoxy glue, is used to secure the sensor assembly  400  in the encasement  1200 . The encasement  1200  is formed with inlet and output ports  1202 ,  1204 , respectively. The inlet and outlet ports  1202 ,  1204  allow a sample to be injected into, and discharged from, the flow cell  1201 . The adhesive seals the reference cell  1203  and the flow cell  1201  along the lap joint, such that fluid can only enter and exit through the inlet and outlet ports  1202 ,  1204 . 
     The encasement is preferably formed of a material having low oxygen permeability, low moisture permeability which is transmissive to ultraviolet radiation, and which is resistant to color change upon exposure to ultraviolet radiation, such as a composition of acrylic, styrene, and butadene. Because even the preferred composition absorbs oxygen, the encasement  1200  is preferably formed with a third cell  1213 . The third cell  1213  reduces the amount of encasing material which is adjacent to the flow cell  1201 . However, it will be clear to those skilled in the art that such a third cell  1213  is not necessary for the proper operation of the present invention. In addition, in one embodiment of the present invention the amount of encasing material is reduced to a minimum to reduce the absorption of oxygen from a sample which is present in the flow cell  1201 . 
     The flow cell  1201  is formed to ensure that a sample which enters the flow cell comes into contact with each of the sensors  403 . Furthermore, the flow cell  1201  is very shallow, thus the volume of the flow cell  1201  is very small (i.e., 0.05 milliliters in the preferred embodiment). A very thin reference channel  1206  (preferably 0.005-0.010 inches in diameter) between the reference cell  1203  to the flow cell  1201  provides electrical contact between the reference medium which resides within the reference cell  1203 . The reference medium may be any well known reference electrolyte in solution or gel form. However, in the preferred embodiment, the reference medium is preferably a natural polysaccharide, such as agarose, gelatin, or polyacrylamide. The greater viscosity of the reference medium used in the preferred embodiment retards evaporation of the reference medium, as well as preventing the reference medium from intermingling with the fluids in the flow cell  1201 . The reference medium is preferably introduced into the reference cell  1203  after the sensor assembly  400  is installed in the encasement  1200 . In accordance with the present invention, a vacuum is created in the flow cell  1201  and the reference cell  1203  by applying a low pressure source to either the inlet or outlet port  1204 ,  1206 . The reference medium is then applied to the other port  1206 ,  1204 . Preferably, the reference medium is heated to approximately 37°-50° C. by the heater  601  or by application of heat through an external heat source to reduce the viscosity of the reference medium, and thus allow the reference medium to completely fill the reference cell  1203 . Once the gel has filled the reference cell  1203 , any excess reference medium is gently flushed from the flow channel prior to allowing the reference medium to cool. In an alternative embodiment of the present invention, the viscosity of the reference medium may be increased in response to a chemical reaction between the medium and a catalyst which is placed into the reference channel either before or after the reference medium. 
     It should be noted that when the height of the fluid column over the sensor array has been minimized to conserve sample volume (0.10 inches, for example), measurement is preferably made within 10-15 seconds after the sample has entered the flow cell  1201 . 
     It will be seen from the above description of the present invention, that the sensors are not separable into parts, but rather form a signal modular unit, designed for a predefined life, installed once, and then discarded. Discarding the unit is economically feasible due to the low cost at which such sensor assemblies can be fabricated. The present invention makes it possible to provide a low cost system which is built around standardized electronic assemblies by providing a low cost, mass producible sensor assembly that has highly accurate and reproducible results. 
     It should be clear to those skilled in the art that the use of subminiature through holes to route electrical signals from the electrodes of the sensors to the opposite side of the substrate allow a chemically selective membrane overlaying the planar electrode to function with the desired sensor reaction mechanism while providing a means for packing a number of sensors into a relatively small area on the surface of the substrate. The use of the subminiature through holes also allows for excellent physical isolation of the sample from the conductors that carry the electrical signals between the sensor electrodes and the instrumentation used to process those signals. This physical isolation results in very high electrical isolation between signals generated by each of the sensors 
     FIGS. 15 a - 15   c  illustrate three alternative embodiments of the present invention in which the relative positions of the sensors differ from those shown in FIG.  4 . 
     New Sensor Cartridge 
     FIG. 16 a  is an assembly views of a disassembled sensor cartridge  1600  in accordance with another embodiment of the present invention. The sensor cartridge  1600  shown in FIG. 16 a  has four of the four basic component parts as in the previous embodiment; (1) a housing  1602 ; (2) a housing cover  1604 ; (3) a pump tube assembly  1606 ; (4), a sensor assembly  400 , the same as in the prior embodiment; and (5) a novel direct input fluid aspiration port assembly  1608 . This new cartridge has a direct input aspiration port  1608  wherein the fluid sample is introduced directly into the cartridge rather than routed through the analyzer as in the prior embodiment. The sensor  400  is rotated or turned around one hundred eighty degrees in the cartridge showing from its position in the prior embodiment so that the pump tube assembly  1606  is connected to the sensor outlet  1204  rather than the inlet as in the prior embodiment. 
     The housing  1602  shown in FIGS. 16 a  and  16   b  is similar in many respects to the prior embodiment and has a floor  1601 , four walls  1603 ,  1605 ,  1607 ,  1609 , an opening  1610 , and in addition, a construction on one end for mounting the articulated intake aspiration stylus. Male electrical contact pins  1207  (FIG. 1 a ) of an electrical connector  1205  of the sensor assembly  400  protrude through the opening  1610 . The walls of the opening  1610  generally conform to the shape and size of the body of the connector  1205 FIG. 1 b . Thus, the sensor assembly  400  is constrained from movement in the plane of the floor  1601  of the housing  1602 . Preferably, the connector body  1616  of the sensor assembly fits loosely within the opening  1610 . 
     The pump tube assembly  1606  is substantially as in the prior embodiment and preferably comprises a right angle end fluid coupling  1626 , a straight end fluid coupling  1624 , and a pump tube  1636 . In accordance with one embodiment of the present invention, the end fluid couplings  1624 ,  1626  are formed (such as by a conventional molding process) from an elastomer. The pump tube  1636  is preferably very resilient in order to allow the pump tube  1636  to exit and enter the housing at openings  1638  and properly interface with a roller to form a peristaltic roller pump, as is described below in greater detail. A fluid path is formed through the pump tube assembly  1606  such that fluid enters at one end of the pump tube assembly and exits from the other end. Walls  1622  may be provided to retain the pump tube assembly  1606  in position within the housing  1602 . 
     The direct input aspiration port assembly  1608  includes a rotatable fluid coupling which in one embodiment comprises a tube  1640  mounted in and extends through a body  1642  and mounts in a flexible or elastomeric tube  1644  at the end of sensor assembly  400 . The tube  1644  flexes to allow rotation of the tube  1640  up to about 90 degrees. The input aspiration port assembly  1608  is preferably formed as a right angle coupling with the major portion of tube  1640  at right angle to the end mounted in tube  1644  and the pivot or rotating axis. That is, the coupling provides a means by which tube  1640  rotates or pivots through about 90 degrees from a recessed positioned as shown FIG. 16 c  to a position extending outward from the surface of the housing as shown in FIG. 16 d . The housing  1602  is provided with a wall  1615  parallel to wall  1603  and aligned openings  1616  which journal the pivoting body  1642 . The parallel walls also form a recess  1617  into which the aspiration tube or stylus  1640  is normally recessed. The housing is also formed with an extension  1618  which forms a recess for an actuating lever or tab  1619  for manually rotating the aspiration tube  1640 . A slot  1621  is formed in the extension  1618  to allow a tab  1625  on the back of lever  1619  to extend and retract and to activate some signal such as a switch or block a light beam to prevent operation. The tab can block a signal such as a light beam to or from a source or sensor  1627 . A removable protective elastomeric cap  1623  covers the inlet end of tube  1644 . In accordance with one embodiment of the present invention, port  1204  of the sensor assembly is directly coupled to the pump tube assembly  1606 . The inlet port  1202  of the sensor assembly  400  is coupled to the input aspiration port assembly  1608 . The cover  1604  is preferably translucent or clear and has a transparent window to enable viewing of the sensors. Furthermore, as will be described in greater detail below, a plastic encasement  1200  (see FIG. 14) is also preferably either translucent or clear. Since the cover and the plastic encasement are either translucent or clear, the user can view the movement of analytes gas bubbles, and reagents through the sensor assembly within the cartridge. In accordance with one embodiment of the present invention, illustrated in FIG. 16 b , the cover  1604  has an opening  1670  which allows the user of a blood analyzer into which the cartridge is to be installed to view the sensor assembly directly. Accordingly, the user may directly observe an analyte gas bubbles and reagents flowing through the sensor assembly. 
     Two reinforced holes  1650 ,  1652  are provided through the cover  1604 . The holes  1650 , 1652  align with two hollow generally cylindrical bosses  1654  which extend up from the floor  1601  of the housing  1602  to accept retaining devices, such as screws, which secure the cover  1604  to the housing  1602 . In an alternative embodiment of the present invention, studs extend from the cover in alignment with the bosses  1654 . Each stud fits tightly within the opening in one of the bosses  1654  in order to secure the cover  1604  to the floor  1601  of housing  1602 . 
     In accordance with one embodiment of the present invention, the cartridge of the present invention is assembled by coupling the input aspiration port assembly or inlet  1608  to a first port  1202  of the sensor assembly  400 . The fluid coupling  1624  is coupled to the other port  1204  of the sensor assembly  400 . The combination of input aspiration port assembly  1608 , sensor assembly  400 , and pump tube assembly  1606  are then lowered into the housing  1602  and the protrusion  1628  is inserted into the opening  1634 . The pump tube  1636  is inserted into openings  1638  in the wall  1609  of the housing  1602 . A latch member  300  is also provided and mounted in the housing as in the FIG. 3 embodiment. The cover  1604  is then placed over, and secured to, the housing  1602 . 
     Once the cartridge  1600  is assembled, it may be installed in a blood analyzer, such as the blood analyzer  1700  illustrated in FIG.  17 . The blood analyzer of the present invention has a fluid connector (not shown, but like connectors  202  and  204  of FIG. 2 a  and  2   b ) for connection to port  1626  on the cartridge. The direct input aspiration port assembly  1608  provides a fluid flow path via inlet  1202  into cartridge sensor housing  400 . The fluid flow path continues through the cartridge sensor housing via outlet  1404 , through the pump tube assembly  1606 , through the right angle end fluid coupling  1626  and via the male fluid connector which mates with the fluid coupling  1626  to complete a fluid flow path into the analyzer. 
     Fluids are pumped along the fluid flow path by a peristaltic roller pump which includes a roller  1702  that massages the pump tube  1636 . That is, the pump tube  1636  is preferably resilient enough to be stretched over the roller  1636 . The roller  1702  applies areas of alternating greater and lesser pressure to the pump tube  1636 , causing those portions of the pump tube  1636  that lie over an area of greater pressure to be internally constricted and those areas of the pump tube  1636  that lie over an area of lesser pressure to be relaxed to essentially the full unstressed diameter of the channel through the interior of the pump tube  1636 . As the roller  1702  rotates, the areas of alternating greater and lesser pressure traverse the pump tube to generate a peristaltic action in the pump tube  1636 . 
     It can be seen from the above description of the disposable cartridge that the present invention provides a cartridge that: (1) is very easy to install, and thus may be installed with virtually no training; (2) establishes both electrical and fluid connections in one installation process with little or no risk of misaligning the electrical or fluid connections of the cartridge with the corresponding connections of the blood analyzer; (3) includes an integral inexpensive and reliable pump tube assembly; (4) allows the user of the blood analyzer to see the movement of an analyte, gas bubbles, or reagent during analysis; (5) is inexpensive and thus may be disposed of without concern for excessive cost; (6) facilitates rapid, reliable replacement of the sensors of the blood analyzer; (7) reduces contact between blood elements and the analyzer; (8) is compact in size; (9) can be used for sensors with different analyte panels; and (10) allows one type of analyzer to accept many different types of sensors. 
     It should be understood that the cartridge of the present invention may be provided in numerous alternative configurations. For example, a plurality of sensor assemblies may be coupled in series to provide redundancy or to increase the number or type of sensors that are provided within the cartridge. Furthermore, straight fluid couplings may replace the right angle fluid couplings, and flexible tubing may be used to alter the direction of the flow path. Furthermore, the pump tubing may be directly coupled to the sensor assembly without the need for a fluid coupling between the pump tubing and the sensor assembly. Furthermore, a wide variety of latching mechanisms may be used to securely latch the cartridge to a blood analyzer. 
     SUMMARY 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, while the present invention is described generally as being fabricated using a thick film technique, any other well known layered circuit technique may be used, such as thin film, plating pressurized laminating, and photolithographic etching. Furthermore, substrates for a number of sensor assemblies may be fabricated concurrently on a single section of ceramic material which has preferably been scored to allow for easy separation into individual substrates after deposition of all of the components of the sensor assembly, and prior to installation in an encasement. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims