Abstract:
A gas cylinder valve is opened by application of a pilot gas pressure. The gas cylinder valve connects with a yoke for engaging a valve assembly of a gas cylinder The yoke comprising a pilot gas passage that connects a pilot gas chamber in the cylinder valve assembly to a source of pressurized pilot gas and the cylinder gas port of the gas cylinder valve to an external cylinder gas passage. The valve and yoke enable the gas cylinder to be tamper-resistance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/936,920 filed 4 Jan. 2011 and claims priority from International Application Number PCT/GB2009/000915 filed Apr. 8, 2009 which claims priority from British Application Number 0806530.2 filed Apr. 10, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to tamper-resistant valves for use on gas cylinders and to connection devices for use therewith. 
     Gas cylinders are vessels, typically but not necessarily of cylindrical shape, which are used for the storage of gas under a pressure, of at least 40 bar, and sometimes up to 300 bar, and typically in the range of 80 to 200 bar. The gas cylinder has a mouth which engages a valve closing the cylinder. The engagement usually of complementary screw-threads is required to prevent the elevated pressure within the cylinder from ejecting the valve. The valve has an internal configuration which enables the cylinder to be filled with gas and which enables gas to be supplied from the cylinder to a user. The valve is opened and closed manually. 
     Nitrous oxide is used in admixture with oxygen, typically as a 50/50 mixture, for short anaesthetic applications. It is extremely good, giving almost immediate pain relief, and flushes out of the body very quickly. In some countries the use of a pre-mixed gas is allowed, but in others, notably the USA, the use of pre-mixed gas is not permitted. In those circumstances, oxygen and nitrous oxide can be supplied separately and mixed on demand in a mixing device. 
     It is common for the nitrous oxide to be supplied in a pin index cylinder. In practice, however, it has been found that nitrous oxide supplied in this form is sometimes abused by staff, who could crack the valve open, and breathe the gas for “recreational” purposes. Also the gas can be stolen, and abused by people outside. The level of this abuse has been serious enough that the use of nitrous oxide has been discontinued in many establishments. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a gas cylinder valve able to be opened by application of a pilot gas pressure. 
     The valve of the invention enables the initiation of a flow of gas from the gas cylinder to be made essentially dependent upon the detection of a pilot gas pressure. The pilot gas pressure can be generated from another source of gas, especially a gas with which the gas from the cylinder is to be mixed. That allows tamper-resistance of a gas cylinder, for example a cylinder of nitrous oxide, to be improved. 
     In one embodiment of the invention, the valve comprises a pilot gas chamber having an inlet connectible to a source of pressurised pilot gas, and a displaceable member able to act in concert with a valve member to open and close a gas passage placing the interior of the cylinder in communication with a cylinder gas port in the gas cylinder valve, wherein the displaceable member is normally biased into a valve closing position, but is able to be displaced by the pilot gas pressure against the bias from the valve closing position to a position in which the gas cylinder valve is open. The displaceable member is advantageously a piston or a diaphragm, especially a piston. 
     In some embodiments, the piston and the valve member may be discrete and separate members. Advantageously, the valve member has opposed ends that are both in communication with the atmosphere. Advantageously, the valve member has a passage therethrough extending from one opposed end to the other. 
     In certain other embodiments it may be expedient for the piston to be integral with or connected to the valve member. 
     In some embodiments, the valve member is a spool. 
     In practice, the displaceable member will preferably be biased. Advantageously, the bias is provided by a spring, preferably a compression spring, and the spring together with the area of the displaceable member and the friction in the seals determines the pilot gas pressure required to move the displaceable member to a position where the valve is in an open position. Advantageously, the pilot gas pressure acts against one face of the displaceable member and the compression spring is seated against the other face of the displaceable member. 
     In a preferred embodiment a second valve is provided. Advantageously, the second valve comprises a mechanical poppet. 
     Advantageously, the gas port for exit of gas from the cylinder is positioned at the side of the valve. 
     Advantageously, the cylinder valve has at its head a tamper-proof cover preventing unauthorised manual access to the displaceable member. 
     The invention also provides a yoke for engaging with a valve of a gas cylinder, the yoke being able to connect a pilot gas chamber in the cylinder valve to a source of pressurised pilot gas and the cylinder gas port of the gas cylinder valve to an external cylinder gas passage. The yoke may be connectible to a valve of a gas cylinder by any suitable means. Suitable means may include, for example, interengageable structures provided on the yoke and the valve. In one embodiment, the yoke has recessed regions arranged for engaging with cooperating structures on the valve, for example, the valve assembly may have a pair of external locating studs engageable with the yoke. 
     The yoke of the invention is preferably adapted for use with the valve of the invention. Advantageously, the yoke comprises a pilot gas passage for connecting the pilot gas chamber to a source of pressurised pilot gas and a cylinder gas transport passage for connecting the cylinder gas port to the external cylinder gas passage. 
     Furthermore, the invention provides an assembly of: 
     a gas cylinder fitted with a gas cylinder valve able to be opened by application of a pilot gas pressure and including a pilot gas chamber; and 
     a yoke able to connect the pilot gas chamber to a source of pressurised pilot gas and the cylinder gas port of the gas cylinder valve to an external cylinder gas passage. 
     Advantageously, the yoke comprises a handle and a connector portion for connection to the cylinder valve, the handle being movable relative to the connector portion from a first position in which the gas cylinder is not locked to the yoke to a second position in which the gas cylinder is locked to the yoke. In a preferred embodiment of the assembly according to the invention, the yoke engages with external locating studs of the gas cylinder valve. In that case, it is advantageous for the yoke to have a pair of arms, both of which have cam surfaces which when the yoke is in position engage the external locating studs. More preferably, the arms are connected to a handle such that operation of the handle causes the cam surfaces to move over the location studs from a first position in which the yoke engages the gas cylinder valve but is not locked thereto to a second position in which the yoke is locked to the gas cylinder valve. 
     Advantageously, the cam surfaces have, in use, a snail cam action. 
     In certain preferred embodiments of the yoke according to the invention, the yoke has a pilot gas passage containing a pilot gas valve. Advantageously, the pilot gas valve has a valve member, for example, a poppet, normally biased into a valve closing position, but able to be displaced from the valve closing position by the action of locking the yoke to the gas cylinder valve. In some embodiments of the assembly according to the invention, the cylinder gas port has a nozzle through which extends from a face thereof to the pilot gas chamber a pilot gas conduit, the nozzle being adapted to displace the valve member, for example the poppet, of the pilot gas valve so as to open the pilot gas passage thereby permitting flow of pilot gas from the pilot gas passage to the pilot gas chamber via the pilot gas conduit. Advantageously, the pilot gas passage downstream of the pilot gas valve communicates with a locking chamber in the yoke, gas pressure in the locking chamber urging a retractable stop into a position in which it is received in a complementary aperture in one of the said arms so as to lock the yoke in said second position to the gas cylinder valve. For example, one or both of the said arms of the yoke may have, as a said aperture, a detent which when the cam surfaces are in said second position engages a spring loaded ball or a biased elongate element carried in the yoke constituting a said retractable stop. 
     In one especially preferred embodiment described below, a stop feature in the middle of the snail cam prevents the possibility of the user pressing the valve and making a connection without the cylinder being locked in place, thus preventing release of pressure that could make the cylinder jump out of the yoke. 
     The snail cam can give a very large mechanical advantage, so small force on the handle gives very strong force to push the valve connections together. 
     Within the motion of the snail cam, connections can be made on attachment, and un-made on detachment, so that the cylinder can be attached and removed safely. In certain preferred embodiments, some of which are described below and in which nitrous oxide is typically the gas to be delivered from the cylinder and oxygen is typically the pilot gas, when the cylinder is pushed into the yoke, none of the connections are made, and the protruding portion in the middle means they cannot be unless the user starts to turn the handle. As the handle starts to turn to lock the cylinder in place:
         1. The cylinder is locked. All the connections are un-made and the poppet is not actuated.   2. The N 2 O connection is made (pilot connection not made, poppet not actuated).   3. The pilot gas connection is made (poppet not actuated).   4. Poppet actuated.   5. On detachment, as the handle is turned:   6. Poppet closed (so gas to pilot is off)   7. O 2  connection is un-made, so pilot gas is vented (this will turn off and vent the N 2 O)   8. N 2 O connection is un-made   9. Cylinder is free to be removed.
 
This sequence gives a great advantage in the safety of making the connection.
       

     Other sequences of operation within the movement of the cam are possible. 
     Preferably, the gas cylinder valve has a pair of guide flanges to facilitate location of the yoke. 
     In one advantageous embodiment of the invention, the cylinder is a nitrous oxide cylinder. 
     The valve of the invention, especially when used with a yoke according to the invention, may be used to improve the tamper-resistance of a gas cylinder, especially a nitrous oxide cylinder for use in an N 2 O/O 2  mixing system. In particular, the system is advantageously arranged in such a way that the supply of N 2 O out of the cylinder is initiated only if the cylinder valve is attached to the yoke and the presence of the second gas (oxygen in the case of an N 2 O/O 2  mixing system) is detected in the valve in the form of the existence of the pilot gas pressure. 
     Thus, in a preferred embodiment, the invention provides a nitrous oxide supply system comprising a gas cylinder containing nitrous oxide fitted with a gas operated valve in which a pressure of a further gas is required to be present at the valve before the delivery of N 2 O is initiated. 
     The term “pilot” is used herein, with reference to a gas, to refer to a supply of gas that is subsidiary to, but indicative of the presence of, a main supply of the gas. The term “pilot gas pressure” is analogously used herein to refer to a gas pressure generated by a pilot gas supply that is indicative of the presence of a pressurised source of the gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain illustrative embodiments of the invention are explained in detail below with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an assembly according to the invention, having a gas cylinder connected to a valve assembly, and further having a yoke; 
         FIG. 2  shows the cylinder and valve of  FIG. 1  with the yoke, before clamping into position; 
         FIG. 3  is a side view of the yoke, shown partly in section; 
         FIG. 4  is a side view of the yoke, partly in section, with a handle in a clamping position; 
         FIG. 5 a    is a side view of the valve assembly and yoke before connection; 
         FIG. 5 b    is a side view of the valve assembly and yoke during an initial stage of connection; 
         FIG. 5 c    is a side view of the valve assembly and yoke during a final stage of connection; 
         FIG. 5 d    is a side view of the valve assembly and yoke after connection; 
         FIG. 6  is a side view of a further assembly according to the invention, shown partly in section, when there is no pilot gas pressure; 
         FIG. 7  is a side view of the assembly of  FIG. 6 , partly in section, when there is a pilot gas pressure present; 
         FIG. 8  is a vertical section through an arm of a yoke with a first detent arrangement; 
         FIG. 9 a    is a vertical section through an arm of a yoke with a pressure actuated locking arrangement, when there is no pilot gas pressure in the yoke; 
         FIG. 9 b    is a vertical section through the yoke arm of  FIG. 9 a   , when there is a pilot gas pressure in the yoke; 
         FIG. 10  is a side view of a further assembly according to the invention, in which a pilot gas valve is present in a pilot gas passage of the yoke, before clamping of the yoke to the valve; 
         FIG. 11  shows the assembly of  FIG. 10 , with the yoke clamped to the valve; 
         FIG. 12  is a section through another form of valve in accordance with the invention; 
         FIG. 13  is a section through a further form of valve according to the invention; 
         FIG. 14  is a section through yet another form of valve according to the invention; 
         FIG. 15  is a section through further illustrative embodiment of valve according to the invention; and 
         FIG. 16  is a sectional view of an especially preferred form of valve according to the invention including a mechanical poppet arrangement for opening and closing the supply of cylinder gas; 
         FIG. 17  is a further sectional view of the valve of  FIG. 16 ; and 
         FIG. 18  is an expanded view of a portion of  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a cylinder  1  is fitted with a cylinder valve  2 . The cylinder may advantageously contain nitrous oxide, although it is to be understood that the cylinder  1  may contain any gas that is to be used in admixture with a gas from another cylinder. For example, in the case of nitrous oxide, the gas is mixed with a gas from another source and supplied for its intended use, which in the case of nitrous oxide is advantageously use, in admixture with oxygen, for anaesthetic purposes. 
     The valve  2  has an upper flange  3  with a flange lower guide face  3   a , and a lower flange  4  with a flange upper guide face  4   a . Between the upper flange  3  and lower flange  4  the valve is of generally rectangular configuration in horizontal cross-section. On side face  5  of the valve assembly there is provided a protruding external locating stud  6 . A similarly configured locating stud  7  is arranged on the opposite face of the valve assembly but is not visible in  FIG. 1 . 
     Additionally shown in  FIG. 1  is a yoke  8 , the purpose of which is to connect a pilot gas chamber in the valve to a source of pressurised pilot gas and the cylinder gas port of the gas cylinder valve to an external cylinder gas passage 
     The yoke  8  has a connector portion  9  comprising opposed clamp arms  10 ,  11  which are so spaced apart that the yoke is engageable with the valve assembly  2  with the clamp arms  10 , 11  embracing the valve assembly. As will be described in more detail below with reference to  FIG. 3  the clamp arm  10  includes a through-hole in which there is rotatably received a cam arm  12  including a snail cam engageable with the locating stud  7  when the yoke  8  is fully engaged with the cylinder valve  2 . A further cam arm  13  is similarly rotatably mounted in a through-hole in the clamp arm  11 . The through-holes in the clamp arms  10 ,  11  are coaxial. The cam arm  13  is configured to have a mirror image configuration relative to the cam arm  12 . Each of the cam arms  12 ,  13  is provided with an extension  14 ,  15  protruding outwardly from the respective clamp arm. A cam handle has a grip portion  16  and arms  17 ,  18  extending laterally from the grip portion  16  at opposed ends thereof. The arm  17  is attached to the extension  14  and the arm  18  is attached to the extension  15 . The cam handle is pivotable from the position shown in  FIG. 1  to a lower position in which the grip portion  16  is in the vicinity of the cylinder wall (as shown in, for example,  FIG. 6 ). That pivoting is made possible by means of rotation of the cam arms in the clamp arms  10 ,  11 . 
     In  FIG. 2 , the yoke  8  is shown in engagement with the cylinder valve  2 , the studs  6 ,  7  received in the cam arms  12 ,  13 . The cam handle is directed upwards, and in that position the yoke  8  is not clamped to the valve  2 . 
       FIG. 3  is a vertical section through the yoke  8 , showing the inner side of cam arm  12 . The construction of only the cam arrangement in the cam arm  12  is described in detail below, but it is to be understood that the cam arrangement in the cam arm  13  is similarly constructed in mirror-image relationship. With reference to  FIG. 3 , the inner end surface of the cam arm has a cam arrangement, which operates in snail cam manner. The cam arrangement has a channel having an entrant portion  19  and an inner portion  20 . The entrant portion  19  is a straight channel extending horizontally along the inner surface of the clamp arm. An enlarged mouth  21  is provided at the inlet to the entrant portion  19 , serving to guide the connector portion  9  onto the studs  6 ,  7 . The inner portion  20  consists of a curved region of channel and a straight region  22 , the curved region communicating directly with entrant portion  19  via the straight region  22  when the cam arm  12  is in the position shown in  FIG. 3 . The other end  23  of the curved region is closed by an end wall. The curved channel region has an upper wall defining an arcuate cam surface  24  and a lower wall which forms a protrusion  25  which is positioned to obstruct the path of an object in the event of relative movement horizontally of the object within the entrant portion  19 . That configuration has the advantage that, unless or until the cylinder is locked into the yoke, the connection between the valve and connector cannot be made. The inner portion  20  is wholly located within the surface of the cam arm  12 . The inner portion  20  and in particular the cam surface  24  are so configured that, on rotation of the cam arm  12  around a central axis of the cam arm by means of rotation of the handle in an anticlockwise direction in the view shown in  FIG. 3 , the connector portion  9  is moved relative to the cylinder valve  2 , with the connector portion being guided by means of the cooperation of the pins  6 ,  7  of the cylinder valve with the cam surface  24  of cam arm  12  and corresponding cam surface of cam arm  13 . 
       FIG. 4  shows the handle in a lowered position in which the connector portion  9  is clamped to the cylinder valve  2 . On rotation of the cam arm  12 , the connector portion  9  has been so moved, by means of cooperation of the cam surface  24  of the yoke  8  and the pin  7  of the valve (and corresponding cam surface of cam arm  13  and the pin  6 ), that a gas connection arrangement in the connector portion  9  is brought into communicatable relationship with the valve  2 . In this position, the pin  7  is located at the closed end  23  of the curved region of inner portion  20 . The connector portion  9  may be locked in position by any suitable arrangement, for example by means of a recess provided in the cam arms  12 ,  13  to receive a biased detent element, as described further below with reference to  FIG. 8  or  FIGS. 9 a  and 9 b   . It will be appreciated that the pin  7  is shown in  FIGS. 3 and 4  for ease of explanation, whilst the remainder of the valve is omitted for the sake of clarity. 
     The affixing of the connector portion  9  to the valve  2  is shown in  FIGS. 5 a  to 5 d   , each of which is partly in section. In  FIG. 5 a   , the yoke  8  is brought towards the valve assembly, with the clamp arms  12 ,  13  aligned with and between the upper and lower flanges  3 ,  4  of the valve, and embracing the valve between them. The handle (not shown) is in the up position. There are shown schematically in  FIGS. 5 a  to 5 d    a cylinder gas supply passage  25  and pilot gas inlet passage  26  in the valve  2  and a cylinder gas transport passage  27  and a pilot gas passage  28  in the connector portion  9 . At the outlet port of the cylinder gas supply passage  25  there is provided a sealing ring  29 . A further sealing ring  30  is provided in the connector portion  9 . The cylinder gas transport passage  27  terminates at a nozzle  31  which is dimensioned and configured to be sealably joined to the end of the cylinder gas supply passage  25 .  FIG. 5 b    shows the connector portion  9  located around the valve assembly, but the handle (not shown) is still in the up position so that clamping has not yet been effected. In  FIG. 5 c   , the handle (not shown) is part way through lowering to the downward position, drawing the connector portion  9  against the valve assembly  2  and inserting the nozzle  31  through the sealing ring  29  into cylinder gas supply passage  25 . Simultaneously, a wider, nozzle arrangement comprising both the cylinder gas port terminating the cylinder gas passage  25  and the pilot gas inlet passage  26  advances through, and forms a seal with, sealing ring  30 . In  FIG. 5 d   , the handle (not shown) is in the downward position, with the connector portion  9  firmly engaged with the valve assembly such that the cylinder gas transport passage  27  is in communication with the cylinder gas supply passage  25 , and the pilot gas passage  28  is in communication with the pilot gas inlet passage  26 . 
     A first illustrative embodiment of valve assembly  2  is shown in  FIG. 6 , attached to a yoke  8  of the kind shown in  FIGS. 5 a  to 5 d   . The valve assembly  2  has a body  32 , a lower part  33  of which is engageable in the mouth  34  of a gas cylinder  1 . As mentioned above, the valve assembly includes an upper flange  3  and a lower flange  4 , defining between them the region in which the valve assembly is engageable with the connector portion  9 . Extending vertically through the body  32  is a bore, which is closed at its upper end by a sealed cap  35  and at its lower end by sealed cover  36 . The bore consists of a number of portions of varying diameter. At the top, adjacent to the cap  35 , is the region of largest diameter, forming piston chamber  37 . Extending downwardly from the piston chamber  37  is a central region housing a valve arrangement generally of the sliding-spool type. Beneath the central region is a lower bore region, which is of greater diameter than the central region and houses a return spring arrangement. The piston chamber  37  houses a piston  38  which is vertically displaceable within the chamber. The piston is connected with a piston stem  39 , which is connected to a spring arrangement housed in the lower bore region. The piston stem  39  includes a longitudinal passage  40  having a vent  41  into piston chamber  37  to allow there to be a certain pressure of cylinder gas in the piston chamber. The chamber  37  is vented to the external atmosphere in such a manner as to allow the gas in the system to vent part way through the lifting of the handle. That reduces the risk of the cylinder jumping on disconnection but also allows for there to be some pressure in chamber  37  to aid the piston moving to a closed position. The spring arrangement has a compression spring  42 , the lower end of which is supported on the cover  36 . At the upper end of the spring is a spring head  43 , a lower extension of which is received within the upper coils of the spring. When the spring is in its maximum permitted state of extension, the spring head  43  abuts against a shoulder  44  at the narrower entrance to the central region of the bore. 
     A second, narrower, bore  45  is provided in the body  32  extending from the lower face of the body vertically upwardly to a position generally level with the central bore region, and thence transversely to an inlet into the bore wall in the central bore region. 
     In the central region of the bore there is a spool valve arrangement which serves to permit gas to pass from the cylinder into the connector portion  9  when the presence of another gas supply, for example from another gas cylinder or a wall socket, is detected. The piston stem  39  extends vertically through the entirety of the central region of the above. In that region, the piston stem  39  has a narrowed region  46 . Three sealing rings  47   a ,  47   b ,  47   c  are provided, vertically spaced from one another, in the central bore region, surrounding the piston stem. The piston stem is a snug fit within the sealing rings except in the narrowed region  46  in which the diameter of the piston stem is smaller than the internal diameter of the sealing rings  47   a ,  47   b ,  47   c . The sealing rings are maintained in vertically spaced relationship by means of seal spacers  48 . In the embodiment of  FIGS. 5 a  to 5 d   , the piston may, instead of the narrowed region  46 , be provided with elongate slots, which offer the advantages of reduced friction and reduced damage to the sealing rings, particularly at high pressure. 
     A horizontal passage  49  extends across the body  32  from the central bore region for carrying the cylinder gas via cylinder gas passage  25  (which is only partly visible in  FIG. 6 ) to the cylinder gas outlet port. The connector portion  9  is generally of the construction described with reference to  FIGS. 5 a  to 5 d   . When the connector portion is locked in place, the passage  49  is in communication via the passage  25  to the cylinder gas transport passage  27  of the connector portion  9   
     The pilot gas inlet passage  26  communicates, via an inlet  50 , with the upper part of the piston chamber  37 . 
     When a second pressurised gas source, for example, an oxygen source, is connected to the pilot gas passage  28 , causing a flow of pilot gas along the passage  28 , pilot gas enters the piston chamber  37  above the piston head  38  and, when sufficient pressure builds up, the piston head is forced downwardly against the biasing spring  42 , as shown in  FIG. 7 . As a result of the downward movement of the piston  38  and the attached stem  39 , the region of narrowing  46  is displaced downwardly (see  FIG. 7 ) with the result that a gas flow channel is created between the outlet of vertical conduit  45  and the horizontal channel  49 , allowing cylinder gas to flow uninterruptedly into the cylinder gas transport passage  27 . 
       FIG. 8  shows schematically a first detent arrangement for indicating correct location of the yoke  8  on the valve assembly  2 . Recesses  51 ,  52  are provided in the circumferential surface of the cam arm  12 . A detent ball  53 , mounted on a compression spring  54 , is able to engage selectively in either the recess  51  or the recess  52 , according to the rotational position of the cam arm (and associated handle). The compression spring is mounted within a bore in the connector block assembly, and bears on the inner end wall of the bore. When the handle is so positioned that the detent ball is opposite one of the recesses  51 ,  52 , the spring loaded detent ball automatically engages in that recess, indicating to the user that the handle is correctly positioned in the clamping position or the release position, respectively. The arrangement is such that withdrawal of the detent ball from the recess is permitted when the user moves the handle away from the clamping or release position, respectively. 
       FIGS. 9 a  and 9 b    show a detent arrangement in which the detent ball of  FIG. 8  is replaced by an elongate element  53 ′ that can engage in the recesses  51 ,  52  (see  FIG. 9 b   ). The elongate element is automatically displaced outwardly to engage in the detent  52  when there is a pilot gas pressure in the yoke, with the spring  54  serving to retract the piston when there is no pilot gas pressure. The connection portion  9  can be disengaged only when there is no pilot gas supply. 
     In  FIG. 10 , there is shown an embodiment in which the pilot gas passage in the yoke  9  includes a pilot gas valve in the form of a poppet valve. The poppet valve closes the pilot gas passage  28  when the connector portion  9  is not attached to the valve assembly  2 . A poppet  55  is mounted at the front end of a poppet closure spring  56 , which bears against a bearing surface (not shown) within the conduit  28 . The poppet is normally biased by spring  56  into a valve closing position, in which poppet  55  is snugly located within a seal  57 , but is able to be displaced from the valve closing position by the action of locking the yoke  8  to the gas cylinder valve  2 . The poppet valve is opened by means of the protruding nozzle arrangement of the valve assembly  2  which, on advancing the connector portion  9  towards the valve assembly, forces the poppet back through the valve seat against the return force of the spring  56 .  FIG. 11  shows the poppet in the open position just after application of a pilot gas pressure, and before commencement of the lowering of the piston head  38 . In a variant of the embodiment of  FIGS. 10 and 11 , the poppet may be displaced by an appropriately located pin instead of by the nozzle arrangement. 
     In use of a yoke having a poppet valve as described above, as the handle starts to turn to lock the cylinder in place:
         1. The cylinder is locked. All the connections are un-made and the poppet is not actuated.   2. The N 2 O connection is made (pilot connection not made, poppet not actuated).   3. The pilot gas connection is made (poppet not actuated).   4. Poppet actuated.   5. On detachment, as the handle is turned:   6. Poppet closed (so gas to pilot is off)   7. O 2  connection is un-made, so pilot gas is vented (this will turn off and vent the N 2 O)   8. N 2 O connection is un-made   9. Cylinder is free to be removed.       

     Such an arrangement, connected and unconnected according to the above sequence, gives great advantages in the safety of making the connection. 
       FIGS. 12 to 15  show a number of further illustrative forms of valve assembly. 
     The valve assembly of  FIG. 12  is similar in some respects to that described above with reference to  FIG. 6 . In the arrangement of  FIG. 12 , however, the return spring for the piston acts directly on the piston and a valve arrangement is actuated by the piston in order to open and close the flow of cylinder gas. Additionally, a circular projection is provided on the upper surface of the piston. That can reduce any tendency of the piston to stick to the upper wall of the piston chamber  37 , potentially making the piston more responsive to the arrival of the pilot gas into the chamber. 
       FIG. 13  shows a valve assembly in which the piston operates in the opposite direction, the oxygen being admitted on top of the piston and urging the piston downwards against a spring below, thereby opening the cylinder gas passage.  FIG. 14  shows a variant on the arrangement of  FIG. 13 , in which the cylinder gas passage is provided by lifting the piston (by means of admission of pilot gas) such that the lower extremity of the stem clears the valve seat.  FIG. 15  shows an arrangement in which oxygen enters the piston chamber above the piston, the descent of which, by virtue of a poppet valve arrangement  58 , opens a passage for the nitrous oxide. The poppet valve  58  may include a return spring (not shown in  FIG. 15 ). 
     In an advantageous embodiment shown in  FIG. 16 , a second poppet valve  60  may be added between the connector portion  9  and the valve assembly  2 . The poppet valve  60  operates in a generally similar manner to the poppet valve  55  provided in the connector block assembly in connection with the oxygen supply conduit. In the case of the poppet valve  60 , however, the poppet is provided in the valve assembly in connection with the supply conduit for cylinder gas from the cylinder  1 , and is operable to open and close the feed of the cylinder gas into the spool valve, thus providing a second valve between the cylinder and the yoke. The poppet valve is opened by means of longitudinal displacement of poppet  61  by means of a cooperating actuating structure, for example a pin, provided on the connector portion  9 , thereby allowing cylinder gas to flow out of the cylinder through valve seat  62  when the connector portion  9  is connected to the valve assembly. For there to be a flow of cylinder gas through the valve assembly  2  into the connector portion  9 , both the poppet valve  60  and the main valve arrangement must be open. The poppet valve  60  adds additional security. In this arrangement, the passage for carrying cylinder gas from the cylinder to the spool valve is arranged on the opposite side of the valve from the outlet port to provide for location of the poppet valve upstream of the spool valve. This is operated by a corresponding pin on the yoke. 
       FIG. 17  shows a further section through the valve of  FIG. 16 , viewed from the opposite direction. The construction of the valve is in many respects similar to that of the valve of  FIG. 6 . As already mentioned, however, the cylinder gas passage  45  is in the embodiment of  FIGS. 17 and 18  located on the opposite side of the valve from the outlet port. Pin  63  is arranged to actuate a poppet valve on the connector portion  9 . As shown in more detail in the expanded view of  FIG. 18 , gas from the passage  45  can pass via an outlet (not shown) around the piston stem  39  by means of elongate slots  64  provided in the piston stem, into passage  49 . 
     In the valves and assemblies of the invention having a balanced valve arrangement, the pilot gas pressure at which the valve is arranged to open is expediently a little lower than the minimum design pressure for the pilot gas so that, at the minimum design pressure of the pilot gas, opening of the valve will reliably ensue to overcome the effects of seal friction varying over time and spring manufacturing tolerances. The minimum design pressure might be the lowest input pressure to the system, for example, in the case of oxygen, the lowest pressure that would be expected from a cylinder regulator or wall outlet. However, if the system, for example, a gas mixer, uses a lower pressure internally, that may be the minimum design pressure. In the embodiment described, typical pressures of the pilot gas to obtain valve opening were in the range of 1.5 to 2.6 bar, for example, 2.2 to 2.6 bar. Other opening pressures may be appropriate in many circumstances. 
     Although the foregoing invention has been described in detail by way of illustration and example for purposes of understanding, it will be obvious that changes and modifications may be practised within the scope of the appended claims.