Abstract:
A switching solenoid valve for use in a fluid handling apparatus comprising two input ports for inputting fluid samples, and a third port for outputting fluid from one of the first and second ports. The valve switches one of the two input ports to the third port. The valve is such that the path between the ports which is normally closed when the solenoid is unactuated, has at least one of a significantly lower dead space, significantly less flow perturbations, and significantly lower total included volume between ports, than the path which is normally closed. Such a valve provides operational advantages especially for use in capnographic systems for analyzing exhaled breath.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     Reference is made to U.S. Provisional Patent Application 60/575,174, filed May 27, 2004, entitled “MINIATURE SOLENOID VALVE”, the disclosure of which is hereby incorporated by reference and priority of which is hereby claimed pursuant to 37 CFR 1.78(a) (4) and (5)(i). 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to capnography generally and more particularly to capnographs employing solenoid valves.  
       BACKGROUND OF THE INVENTION  
       [0003]     The following U.S. Patent Documents are believed to represent the current state of the art:  
         [0004]     U.S. Patent Nos. U.S. Pat. Nos. 5,085,402 and 6,024,114.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention seeks to provide capnography apparatus and a solenoid valve particularly advantageous for use therein.  
         [0006]     There is thus provided in accordance with a preferred embodiment of the present invention a capnograph including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the passageway between the patient sample inlet and the gas analysis chamber having significantly less dead space than the passageway between the reference sample inlet and the gas analysis chamber.  
         [0007]     There is also provided in accordance with another preferred embodiment of the present invention a capnograph including a patient sample inlet, a reference sample inlet, a gas analysis chamber, a manifold and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the manifold defining a socket for the solenoid valve and the passageways being defined in the manifold and jointly between the solenoid valve and the manifold at the socket.  
         [0008]     There is further provided in accordance with yet another preferred embodiment of the present invention a capnograph including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the capnograph being characterized in that it has a rise time which does not exceed 50 milliseconds at a flow rate of 50 ml/min.  
         [0009]     More preferably, the rise time does not exceed 30 milliseconds at a flow rate of 50 ml/min. Most preferably, the rise time does not exceed 10 milliseconds at a flow rate of 50 ml/min.  
         [0010]     There is yet further provided in accordance with still another preferred embodiment of the present invention a capnograph including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the capnograph being characterized in that it has a rise time which does not exceed 10 milliseconds at a flow rate of 50 ml/min.  
         [0011]     There is also provided in accordance with another preferred embodiment of the present invention a capnograph including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet and including a magnet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, wherein the passageway between the patient sample inlet and the gas analysis chamber is maintained open at least partially by a force applied by the magnet.  
         [0012]     Preferably, the passageway between the patient sample inlet and the gas analysis chamber has significantly less dead space than the passageway between the reference sample inlet and the gas analysis chamber.  
         [0013]     Preferably, the solenoid valve includes a partially hollow plunger. Additionally or alternatively, the solenoid valve includes a push valve. Alternatively or additionally, the solenoid valve includes a magnet operative to maintain the passageway between the patient sample inlet and the gas analysis chamber open irrespective of the orientation of the solenoid valve, when the solenoid valve is not actuated.  
         [0014]     Preferably, the capnograph is characterized in that it has a rise time which does not exceed 50 milliseconds at a flow rate of 50 ml/min. More preferably, the rise time does not exceed 30 milliseconds at a flow rate of 50 ml/min. Most preferably, the rise time does not exceed 10 milliseconds at a flow rate of 50 ml/min.  
         [0015]     There is further provided in accordance with yet another preferred embodiment of the present invention a gas analyzer including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the passageway between the patient sample inlet and the gas analysis chamber having significantly less dead space than the passageway between the reference sample inlet and the gas analysis chamber.  
         [0016]     There is even further provided in accordance with still another preferred embodiment of the present invention a gas analyzer including a patient sample inlet, a reference sample inlet, a gas analysis chamber, a manifold and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the manifold defining a socket for the solenoid valve and the passageways being defined in the manifold and jointly between the solenoid valve and the manifold at the socket.  
         [0017]     There is still further provided in accordance with another preferred embodiment of the present invention a gas analyzer including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the gas analyzer being characterized in that it has a rise time which does not exceed 50 milliseconds at a flow rate of 50 ml/min.  
         [0018]     More preferably, the rise time does not exceed 30 milliseconds at a flow rate of 50 ml/min. Most preferably, the rise time does not exceed 10 milliseconds at a flow rate of 50 ml/min.  
         [0019]     There is also provided in accordance with yet another preferred embodiment of the present invention a gas analyzer including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, the gas analyzer being characterized in that it has a rise time which does not exceed 10 milliseconds at a flow rate of 50 ml/min.  
         [0020]     There is further provided in accordance with still another preferred embodiment of the present invention a gas analyzer including a patient sample inlet, a reference sample inlet, a gas analysis chamber and a solenoid valve governing the supply of gas to the gas analysis chamber from the patient sample inlet and the reference sample inlet and including a magnet, the solenoid valve being operative for defining a normally-open passageway between the patient sample inlet and the gas analysis chamber and a normally-closed passageway between the reference sample inlet and the gas analysis chamber, wherein the passageway between the patient sample inlet and the gas analysis chamber is maintained open at least partially by a force applied by the magnet.  
         [0021]     Preferably, the passageway between the patient sample inlet and the gas analysis chamber has significantly less dead space than the passageway between the reference sample inlet and the gas analysis chamber.  
         [0022]     Preferably, the solenoid valve includes a partially hollow plunger. Additionally or alternatively, the solenoid valve includes a push valve. Alternatively or additionally, the solenoid valve includes a magnet operative to maintain the passageway between the patient sample inlet and the gas analysis chamber open irrespective of the orientation of the solenoid valve, when the solenoid valve is not actuated.  
         [0023]     Preferably, the gas analyzer is characterized in that it has a rise time which does not exceed 50 milliseconds at a flow rate of 50 ml/min. More preferably, the rise time does not exceed 30 milliseconds at a flow rate of 50 ml/min. Most preferably, the rise time does not exceed 10 milliseconds at a flow rate of 50 ml/min.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0025]      FIG. 1  is a simplified pictorial illustration of a capnograph constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0026]      FIG. 2  is an exploded view illustration of part of the capnograph of  FIG. 1 , including a solenoid valve constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0027]      FIG. 3  is an assembled view illustration of the part of the capnograph shown in exploded view in  FIG. 2 ;  
         [0028]      FIGS. 4A and 4B  illustrate gas flow through part of the capnograph of  FIGS. 1-3  in respective patient sampling and reference sampling modes of operation; and  
         [0029]      FIGS. 5A and 5B  illustrate gas flow through part of a variation of the capnograph of  FIGS. 1-3  in respective patient sampling and reference sampling modes of operation.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]     Reference is now made to  FIG. 1 , which is a simplified pictorial illustration of a capnograph constructed and operative in accordance with a preferred embodiment of the present invention, to  FIG. 2 , which is an exploded view illustration of part of the capnograph of  FIG. 1 , including a solenoid valve constructed and operative in accordance with a preferred embodiment of the present invention, and to  FIG. 3 , which is an assembled view illustration of the part of the capnograph shown in  FIG. 2 .  
         [0031]     As seen most clearly in  FIG. 1 , the capnograph comprises a main housing element  10 . A patient breath input tube  20 , having an input connector  22 , which is connectable to a source of patient breath, is attached to a patient gas input port  24  ( FIGS. 2 &amp; 3 ) formed in main housing element  10 . A spiraled cable  26  typically is operative to transmit data in electronic form between input connector  22  and a microprocessor  28  which governs the operation of the capnograph. A reference gas input tube  30  is attached to a reference gas input port  32  formed in the main housing element  10 .  
         [0032]     Threadably mounted onto main housing element  10  is a solenoid valve assembly  34 , communicating with a patient sample input bore  36  and a reference input bore  38  formed in main housing element  10  and connected, via additional bores (not shown) formed in the main housing element  10 , to the patient gas input port  24  and the reference gas input port  32  respectively.  
         [0033]     Gas entering the capnograph from either of patient breath input tube  20  and reference gas input tube  30  passes through the solenoid valve assembly  34  and thence via a gas supply bore  40  to a gas analysis chamber  42  formed within main housing element  10 . In the gas analysis chamber  42  the gas is analyzed using an infrared lamp assembly  44  emitting infrared light which passes through a window portion  46  formed in a wall  48  of gas analysis chamber  42 . Gas leaves the gas analysis chamber  42  via a bore  50 , formed in main housing element  10 , leading to a gas output port  52  which is connected to a gas output tube  54 .  
         [0034]     It is appreciated that the patient sample input bore  36 , reference input bore  38  and gas supply bore  40 , as well as other bores referred to herein, may extend in various planes of the main housing element  10 , and typically do not all extend in a single plane of the main housing element  10  as depicted for the sake of clarity, in the drawings.  
         [0035]     The solenoid valve assembly  34  governs the supply of gas to gas analysis chamber  42  from the patient sample input bore  36  and the reference input bore  38 .  
         [0036]     Infrared lamp assembly  44  preferably includes an infrared lamp (not shown) which is threadably connected to a threaded bore  56  formed in the main housing element  10 , and receives electrical power from a power source  58 . Typically, main housing element  10 , infrared lamp assembly  44  and power source  58  are mounted onto a base element  60 .  
         [0037]     As seen with particular clarity in  FIG. 2 , the solenoid valve assembly  34  includes a valve subassembly  70  and a solenoid subassembly  80 .  
         [0038]     Main housing element  10  is configured to accommodate the valve subassembly  70  and the solenoid subassembly  80  and includes a generally cylindrical bore  102  which is in fluid flow communication with patient sample input bore  36  and gas supply bore  40 .  
         [0039]     Rearward of cylindrical bore  102 , in the sense of  FIG. 2 , there is formed a generally cylindrical bore  104 , which has a larger cross-section than that of cylindrical bore  102 , and a shoulder  106  is defined between bores  102  and  104 . Cylindrical bore  104  is in fluid flow communication with reference input bore  38 .  
         [0040]     Rearward of bore  104  in the sense of  FIG. 2 , there is formed a generally threaded cylindrical bore  110 , having a cross-section which is larger than that of bore  104 . Forward and rearward ends of bore  110 , designated by reference numerals  112  and  114  respectively, have somewhat larger cross sections than the remainder of bore  110 . Bores  104  and  110  accommodate solenoid subassembly  80 , and a sealing ring  116  is located at end  114 .  
         [0041]     Valve subassembly  70  includes a body portion  120  which is loosely and slidingly accommodated within cylindrical bore  102  of main housing element  10 . Body portion  120  is formed with a bore  122  extending axially therethrough, and includes a first generally cylindrical portion  124  having a first cross-section, and a second generally cylindrical portion  126  having a second cross section which is generally larger than that of cylindrical portion  124 .  
         [0042]     A shoulder  128  is defined between cylindrical portions  124  and  126  and defines a seat for a compression spring  130 , disposed about cylindrical portion  124 .  
         [0043]     A seal  132  is located in a recess  134  formed at a rearward facing surface of cylindrical portion  126 .  
         [0044]     Disposed at a forward end of bore  122  is an additional bore  136  which has a larger cross section than that of bore  122 . A flexible elastomeric sealing element  138  is sealingly seated within bore  136  and extends rearwardly into a forward portion of bore  122 .  
         [0045]     A shaft  140  is fixedly seated within bore  122  and is axially rearwardly spaced from elastomeric sealing element  138 . Shaft  140  extends rearwardly through seal  132  and out of bore  122 . Alternatively, shaft  140  may be integrally formed with body portion  120 .  
         [0046]     Solenoid subassembly  80  includes a forward element  150 , a forward portion of which is seated within cylindrical bore  104  of main housing element  10 . Forward element  150  is formed with a bore  152  extending axially therethrough, and includes a forwardly facing generally cylindrical portion  154 . Cylindrical portion  154  is formed with a transverse bore  156 , which is arranged to be in fluid flow communication with reference input bore  38  formed in main housing element  10 .  
         [0047]     At a forward end thereof, cylindrical portion  154  includes a ring shaped protrusion  158 , which is best seen in  FIGS. 4A and 4B , described hereinbelow. Ring shaped protrusion  158  is adapted to sealingly engage seal  132 .  
         [0048]     Forward element  150  also includes, integrally formed with cylindrical portion  154  and rearwardly thereof, a disc portion  160 , rearwardly of which there is formed a generally cylindrical portion  162 .  
         [0049]     At a forward end thereof, bore  152  slidingly accommodates a rearwardly facing end of shaft  140  of valve subassembly  70 . A shaft  164 , having a forward facing surface  168  and a rearward facing surface  170 , is slidingly disposed within bore  152  rearwardly of shaft  140 . Forward facing surface  168  of shaft  164  engages a rearward facing surface of shaft  140 , and rearward-facing surface  170  of shaft  164  extends rearwardly and axially outwardly of forward element  150 .  
         [0050]     Solenoid subassembly  80  additionally includes a tubular coil support element  180  having a tubular portion  182 . At a forward end thereof, tubular coil support element  180  includes a flange portion  184 . Tubular coil support element  180  is disposed about cylindrical portion  154  of forward element  150  and extends rearwardly thereof. A solenoid  190  is wound about tubular portion  182  of tubular coil support element  180 .  
         [0051]     A plunger  192 , which is preferably partially hollow and which defines a forward facing surface  194 , is slidingly disposed within tubular portion  182  of tubular coil support element  180 . Forward facing surface  194  of plunger  192  engages rearward facing surface  170  of shaft  164 .  
         [0052]     A solenoid housing  200  includes a generally cylindrical tubular portion  202  which terminates at a rearward end thereof in a wall portion  204 . Wall portion  204  is formed with a generally circular aperture  206  which accommodates a rearward facing portion of tubular portion  182 . Solenoid housing  200  defines at a forward end thereof a flange portion  210  which abuts against disk portion  160 .  
         [0053]     A nut  220 , which surrounds solenoid housing  200 , is threadably seated within bore  110  of main housing element  10 , thus retaining valve subassembly  70  and solenoid subassembly  80  therein.  
         [0054]     Reference is now made to  FIGS. 4A and 4B , which illustrate gas flow through part of the capnograph of  FIGS. 1-3  in respective patient sampling and reference sampling modes of operation.  
         [0055]      FIG. 4A  illustrates a patient sampling mode of operation, during which current does not flow through solenoid  190 , and valve subassembly  70  is in a rearward, normally open position. In this normally open position, a fluid flow passageway designated by arrows  250  extending from patient sample input bore  36  of main housing element  10  to gas supply bore  40  is open. In this mode of operation, seal  132  sealingly engages protrusion  158  of forward element  150 , thus minimizing the dead space in the fluid flow passageway. The valve subassembly  70  is maintained in this open position by the force of compression spring  130  and does not require electrical power.  
         [0056]     In the patient sampling mode of operation, as shown in  FIG. 4A , a gas sample which is supplied to the solenoid valve assembly  34  flows freely from patient sample input bore  36  to gas supply bore  40 , with little or no interference. It is a particular feature of the present invention that in the patient sampling mode of operation, there is very little dead-space in the passageway designated by arrows  250 , thus reducing distortion of the waveform reaching the gas analysis chamber  42  and causing the rise-time thereof to be relatively low, preferably less than 50 milliseconds, more preferably less than 30 milliseconds and most preferably not exceeding 10 milliseconds.  
         [0057]     In the patient sampling mode of operation, a passageway defined between reference input bore  38  and gas supply bore  40  is normally closed, and the passageway designated by arrows  250  has significantly less dead space than the passageway defined between reference input bore  38  and gas supply bore  40 .  
         [0058]      FIG. 4B  illustrates a reference sampling mode of operation, during which a current flows through solenoid  190 , thereby pushing plunger  192  axially forward against the force applied by compression spring  130 , in a direction indicated by an arrow  260 . Forward motion of plunger  192  results in respective forward motion of shaft  164 , which causes forward motion of shaft  140  and of body portion  120 , resulting in elastomeric sealing element  138  sealingly engaging patient sample input bore  36 .  
         [0059]     In this closed position, a fluid flow passageway, designated by arrows  270 , extending from reference input bore  38  of main housing element  10  to gas supply bore  40  is open. In this mode of operation, seal  132  does not engage protrusion  158  of forward element  150 . The valve subassembly  70  is maintained in this closed position by the force of the magnetic field created by passing a current through solenoid  190 .  
         [0060]     In the reference sampling mode of operation, as shown in  FIG. 4B , a gas sample which is supplied to the solenoid valve assembly  34  flows generally freely from reference input bore  38  to gas supply bore  40 . Although there is dead space surrounding the fluid passageway indicated by arrows  270 , this dead-space does not affect the accuracy of the analysis of the reference gas, as the waveform of the reference gas is of no importance in the testing.  
         [0061]     Reference is now made to  FIGS. 5A and 5B , which illustrate gas flow through part of a variation of the capnograph of  FIGS. 1-3  in respective patient sampling and reference sampling modes of operation.  
         [0062]     As seen in  FIGS. 5A and 5B , the capnograph comprises a main housing element  510 . Threadably mounted onto main housing element  510  is a solenoid valve assembly  534 , communicating with a patient sample input bore  536  and a reference input bore  538  formed in main housing element  510  and connected, via additional bores (not shown) formed in the main housing element  510 , to a patient gas input port (not shown) and a reference gas input port (not shown) respectively.  
         [0063]     Gas entering the capnograph from either of a patient breath input tube and a reference gas input tube passes through the solenoid valve assembly  534  and thence via a gas supply bore  540  to a gas analysis chamber (not shown) formed within main housing element  510 .  
         [0064]     In a similar manner to that described hereinabove with reference to  FIG. 1 , the gas is analyzed in a gas analysis chamber by infrared light emitted from an infrared lamp assembly. Gas leaves the gas analysis chamber via a bore formed in main housing element  510  and a gas output port which is connected to a gas output tube.  
         [0065]     It is appreciated that the patient sample input bore  536 , reference input bore  538  and gas supply bore  540  as well as other bores referred to herein may extend in various planes of the main housing element  510 , and typically do not all extend in a single plane of the main housing element  510  as depicted for the sake of clarity, in  FIGS. 5A and 5B .  
         [0066]     The solenoid valve assembly  534  includes a valve subassembly  570  and a solenoid subassembly  580 . Main housing element  510  is configured to accommodate the valve subassembly  570  and the solenoid subassembly  580  and includes a generally cylindrical bore  602  which is in fluid flow communication with patient sample input bore  536  and gas supply bore  540 . Rearward of cylindrical bore  602 , in the sense of  FIG. 5A , there is formed a generally cylindrical bore  604 , which has a larger cross-section than that of cylindrical bore  602 , and a shoulder  606  is defined between bores  602  and  604 . Cylindrical bore  604  is in fluid flow communication with reference input bore  538 .  
         [0067]     Rearward of bore  604 , there is formed a generally threaded cylindrical bore  610 , having a cross-section which is larger than that of bore  604 . Forward and rearward ends of bore  610 , designated by reference numerals  612  and  614 , respectively have somewhat larger cross sections than the remainder of bore  610 . A sealing ring  616  is located at end  614 .  
         [0068]     Valve subassembly  570  includes a shaft portion  620  defining a rearward facing end portion  621  and having an elastomeric sealing element  622  mounted on a forward end thereof. Elastomeric sealing element defines a forward facing surface  624  and a rearward facing surface  626 , and is loosely and slidingly accommodated within cylindrical bore  602 .  
         [0069]     Solenoid subassembly  580  includes a forward element  650 , a forward portion of which is seated within cylindrical bore  604  of main housing element  510 . Forward element  650  is formed with a bore  652  extending axially therethrough, and includes a forwardly facing generally cylindrical portion  654 . Cylindrical portion  654  is formed with a transverse bore  656 , which is arranged to be in fluid flow communication with reference input bore  538  formed in main housing element  510 .  
         [0070]     At a forward end thereof, cylindrical portion  654  includes a ring shaped protrusion  658 , which is best seen in the enlarged portions of  FIGS. 5A and 5B . Ring shaped protrusion  658  is adapted to sealingly engage rearward facing surface  626  of elastomeric sealing element  622 .  
         [0071]     Forward element  650  also includes, integrally formed with cylindrical portion  654  and rearwardly thereof, a disc portion  660 , rearwardly of which there is formed a generally cylindrical portion  662 . A recess  664  is formed at a rearward facing surface of cylindrical portion  662 , defines a spring seat for a compression spring  666 , which is disposed about shaft  620 .  
         [0072]     Bore  652  loosely and slidingly accommodates shaft  620  of valve subassembly  570 .  
         [0073]     Solenoid subassembly  580  additionally includes a tubular coil support element  680  having a tubular portion  682  terminating at a wall portion  683 , rearward of which there is formed a cylindrical portion  684 . At a forward end thereof, tubular coil support element  680  includes a flange portion  685 . Tubular coil support element  680  is disposed about cylindrical portion  654  of forward element  650  and extends rearwardly thereof. A solenoid  690  is wound about tubular portion  682  of tubular coil support element  680 .  
         [0074]     A plunger  692 , which defines a forward surface  694 , is slidingly disposed within tubular portion  682  of tubular coil support element  680 . Forward surface  694  of plunger  692  defines a rear spring seat for compression spring  666 . A bore  696 , formed in a forward facing portion of plunger  692 , fixedly accommodates rearward facing end  621  of shaft  620 . Preferably a magnet  698  is seated within cylindrical portion  684  against wall portion  683 , thus maintaining plunger  692  in its rear position when the solenoid  690  is not actuated.  
         [0075]     A solenoid housing  700  includes a generally cylindrical tubular portion  702  which terminates at a rearward end thereof in a wall portion  704 . Wall portion  704  is formed with a generally circular aperture  706  which accommodates a rearward facing portion of tubular portion  682 . Solenoid housing  700  defines at a forward end thereof a flange portion  710  which abuts against disk portion  660 .  
         [0076]     A nut  720 , which surrounds solenoid housing  700 , is threadably seated within bore  610  of main housing element  510 , thus retaining valve subassembly  570  and solenoid subassembly  580  therein.  
         [0077]     An essential difference between the embodiment of  FIGS. 1-4B  and that of  FIGS. 5A and 5B  is that the compression spring is placed in a more rearward position in the solenoid valve subassembly  534 , resulting in a further reduction of dead space in the system.  
         [0078]      FIG. 5A  illustrates a patient sampling mode of operation, during which current does not flow through solenoid  690 , and valve subassembly  570  is in a rearward, normally open position. In this normally open position, a fluid flow passageway designated by arrows  750  extending from sample input bore  536  of main housing element  510  to gas supply bore  540  is open. In this mode of operation, ring shaped protrusion  658  of forward element  650  sealingly engages rearward facing surface  626  of elastomeric sealing element  622 , thus minimizing the dead space in the fluid flow passageway. The valve subassembly  570  is maintained in this open position by the force of compression spring  666  and does not require electrical power. Additionally, magnet  698  maintains plunger  692  in its rear position, thus ensuring that the valve subassembly  570  remains in its open position irrespective of its orientation, when solenoid  690  is not actuated.  
         [0079]     In the patient sampling mode of operation, as shown in  FIG. 5A , a gas sample which is supplied to the solenoid valve assembly  534  flows freely from sample input bore  536  to gas supply bore  540 , with little or no interference. It is a particular feature of the present invention that in the patient sampling mode of operation, there is very little dead-space along or in communication with the passageway designated by arrows  750 , thus ensuring that unnecessary distortion of the waveform reaching the gas analysis chamber is avoided and the rise-time is relatively low, preferably less than 50 milliseconds, more preferably less than 30 milliseconds and most preferably not exceeding 10 milliseconds.  
         [0080]     In the patient sampling mode of operation, a passageway defined between reference input bore  538  and gas supply bore  540  is normally closed, and the passageway designated by arrows  750  has significantly less dead space than the passageway defined between reference input bore  538  and gas supply bore  540 .  
         [0081]      FIG. 5B  illustrates a reference sampling mode of operation, during which a current flows through solenoid  690 . The force of the magnetic field formed by the current flowing through the solenoid  690  initally enables the release of plunger  692  from magnet  698 , and thereafter enables motion of plunger  692  axially forward against the force applied by compression spring  666 , in a direction indicated by an arrow  760 . It is a particular feature of the present invention that the force required to displace the plunger  692  away from magnet  698  is equal to or less than the force required to push the plunger forward against the force applied by compression spring  666 . Forward motion of plunger  692  results in respective forward motion of shaft  620  and elastomeric sealing element  622  and in sealing engagement between forward facing surface  624  of elastomeric sealing element  622  and sample input bore  536 .  
         [0082]     In this closed position, a fluid flow passageway designated by arrows  770  extending from reference input bore  538  of main housing element  510  to gas supply bore  540  is open. In this mode of operation, rearward facing surface  626  of elastomeric sealing element  622  does not engage protrusion  658  of forward element  650 . The valve subassembly  570  is maintained in this closed position by the force of the magnetic field created by passing a current through solenoid  690 .  
         [0083]     In the reference sampling mode of operation, as shown in  FIG. 5B , a gas sample which is supplied to the solenoid valve assembly  534  flows generally freely from reference input bore  538  to gas supply bore  540 . Though there is dead space surrounding the fluid passageway indicated by arrows  770 , this dead-space does not affect the accuracy of the analysis of the reference gas, as the waveform of the reference gas is of no importance in the testing.  
         [0084]     It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.