Patent Document

FIELD OF THE INVENTION 
   This invention relates to an atomic absorption spectrometer. 
   BACKGROUND OF THE INVENTION 
   Atomic absorption spectrometers analyse a sample by burning the sample in a flame and passing electromagnetic radiation through the flame so that the sample atoms absorb particular wavelengths of the radiation. By detecting the radiation which is absorbed by the ground state atoms in the flame the conception of the elements of interest can be determined. 
   In order to analyse the sample the atomic absorption spectrometer includes a burner assembly which has a spray chamber which is fluid communication with a burner. The spray chamber has a nebuliser bung which receives sample material from an inlet tube. The sample material is generally entrained in a flow of fluid. The nebuliser bung includes a baffle which has an orifice and the tube terminates adjacent the inlet to the orifice. An oxidant line communicates with the nebuliser bung on the inlet side of the orifice and a fuel and air line communicates with the nebuliser bung on the outlet side of the orifice. The bung is in fluid communication with the spray chamber so that when high pressure oxidant passes through the oxidant line, the high pressure oxidant travels through the orifice thereby creating a venturi effect within the sample tube which assists in the drawing of sample through the sample tube into the nebuliser bung and then into the spray chamber. Fuel and air supplied through the fuel line into the nebuliser bung mixes with the oxidant and sample in the spray chamber and the mixture is supplied to the burner where the fuel, air and sample are combusted in a flame for analysis. 
   The spray chamber has an over pressure relief device in the form of an over pressure bung which locates in an aperture in the spray chamber so that in the event of over pressure in the spray chamber the bung is forced out of the opening to relieve that pressure. An over pressure situation can occur if pressure initially drops within the spray chamber so that the flame produced by the burner can move inside the spray chamber and causes flashback. If this occurs the increase in the pressure caused by the flashback will cause the bung to be expelled from the opening thereby reducing the pressure and reducing the damage caused by the flashback. 
   If the bung is not located within the opening, fuel gas can leak from the spray chamber into the environment which, if the burner was to be ignited, can create an explosion. 
   Thus, if the spray chamber assembly components are not inserted correctly, the highly flammable gas produced by the mixture of the fuel and air may be allowed to leak into atmosphere. Such a leak can have two hazardous outcomes;
         the velocity of the gasses exiting burner is reduced to well below the flame velocity causing the flash-back referred to above; and   sufficient fuel gas leaks into the environment to create an explosive atmosphere if, for example, a bung is not located in the opening or incorrectly located in the opening.       

   Similar hazards can also arise from using the wrong burner in the assembly, or allowing the burner to become excessively clogged (from salt formations etc). For these reasons, most gas burner assembly designs rely on interlocks to guard the operator from such hazards. These interlocks are typically realised by means of micro-switches or read switches which sense the presence of components in the assembly. A typical example is a micro-switch which is activated by the presence of the over pressure relief device inserted into the spray chamber body. If the micro-switch is not activated by proper location of the over pressure relief device or bung, the instrument will not ignite the flame or vent fuel into the environment. 
   Burner assemblies are also provided with a liquid trap for draining off aspirated liquid which is supplied to the chamber with the sample. The liquid trap is typically in the form a u-shaped tube into which liquid can drain. The u-shaped tube forms an “s-bend” so that liquid in the s-bend acts as a plug to prevent leakage of flammable gas from the spray chamber through the liquid trap. As liquid flows into the liquid trap, the liquid can flow out of the s-bend but a sufficient amount of liquid remains in the s-bend to form a plug thereby preventing the escape of gasses from the spray chamber through the liquid trap. A magnet may be provided with floats on the surface of the liquid and when the liquid level is at a height sufficient for safe operation (indicating that the liquid plug is in place), the magnet triggers a reed switch to provide a signal indicative of the fact that the integrity of the liquid trap is in tact. 
   SUMMARY OF THE INVENTION 
   The object of a first aspect of the present invention is to improve the safety of atomic absorption spectrometers and in particular the burner assembly of such spectrometers to detect any significant leaks in the spray chamber and to improve the economy and manufacturability of the safely interlocks, and to create an assembly that can be immersed for cleaning without damaging the interlock system. 
   The invention, in a first aspect, may be said to reside in a burner assembly for an atomic absorption spectrometer including;
         a spray chamber for receiving oxidant, fuel and sample material and allowing mixing of the oxidant, fuel and sample material;   a burner locatable on the spray chamber for receiving the mixture so that the mixture can be ignited to produce a flame to facilitate analyses of the sample material by atomic absorption of radiation; and   pressure monitoring means for monitoring the pressure within the spray chamber so that if the pressure is not at a predetermined level thereby indicative of a safety risk, the burner can be shut off by shutting off supply of at least the fuel to the burner assembly.       

   Thus, according to this aspect of the invention the integrity of the burner assembly is monitored by measuring the pressure within the spray chamber. If the pressure changes from a predetermined expected level this will be indicative of the fact of a safety risk, such as leakage caused by failure to locate an over pressure relief device or bung or incorrect location of that device in the spray chamber, or inclusion of an incorrect burner in the burner assembly, or clogging of a burner, or other leakage from the spray chamber. In the event of one of the conditions which may otherwise cause a hazardous situation, the supply of fuel can be shut off to prevent ignition of the burner, or to shut off the burner if already ignited, so the fault can be corrected. 
   Preferably the burner assembly includes;
         a nebuliser bung coupled to the spray chamber, the nebuliser bung having a baffle including an orifice;   a sample tube for supplying sample coupled to the nebuliser bung and terminating adjacent the orifice;   an oxidant line for supplying oxidant, connected to the nebuliser bung on the inlet side of the orifice;   a fuel line connected to the nebuliser bung on an outlet side of the orifice; and   wherein the supply of oxidant through the oxidant line causes flow of the oxidant through the orifice thereby creating a venturi effect to assist in drawing of sample material through the sample tube into the nebuliser and through the orifice, and so that the oxidant, sample and fuel can mix in the spray chamber for supply to the burner.       

   Preferably the spray chamber includes an over pressure relief device. 
   Preferably the spray chamber includes a liquid trap including an s-bend portion for retaining a plug of liquid so that aspirated liquid can drain from the spray chamber through the liquid trap and the plug of liquid can form a seal in the liquid trap to prevent egress of fuel, air and sample mixture from the spray chamber. 
   Preferably the pressure monitoring means is coupled to the oxidant line. 
   Preferably the oxidant line and fuel line communicate with a gas box which includes said pressure monitoring means, said gas box further including;
         an inlet supply line for the input of oxidant, a shut off valve for selectively shutting off supply of oxidant, a pressure regulator for regulating the flow of oxidant, the pressure regulator having an outlet connected to a first flow restrictor, the first flow restrictor being connected to the oxidant line; and   the fuel input supply including a shut off valve, a second flow restrictor and a first flow control valve, an output of the first flow control valve being coupled to the fuel line.       

   Preferably the oxidant supply line is also connected to the fuel line via a second flow control valve so that the fuel line supplies a mixture of fuel and oxidant to the spray chamber. 
   Preferably the fuel supply line includes an igniter branch for supplying fuel to an igniter for creating a flame for lighting the burner. 
   Preferably the igniter branch includes a shut off valve. 
   Preferably a first pressure sensor is coupled across the first flow restrictor for measuring the pressure drop across the first flow restrictor. 
   Preferably a second pressure sensor is coupled across the second flow restrictor for measuring the pressure drop across the second flow restrictor. 
   The first and second pressure sensors provide an indication of the flow rate of oxidant and fuel through the oxidant supply line and the fuel supply line so that the first and second flow control valves can be controlled to provide the desired flow rate of fuel to the fuel line. 
   Preferably the flow restrictors comprise a length of tube through which the oxidant or fuel flows so as to reduce the pressure of the fluid as the fluid flows through the tube by skin friction on the internal surface of the tube. It has been found that using a length of tube to create the pressure drop results in very accurate pressure reduction which is required to ensure that the mixture of the oxidant and fuel supplied to the spray chamber is correct thereby enabling the establishment of a flame of the desired characteristic, or enabling the flame to be modified by accurate measurement of the pressure drop caused by the first and second flow restrictors and adjustment of the flow control valves in response the pressure drop measured by the first and second pressure sensors. 
   In a further aspect the invention also provides an atomic absorption spectrometer having the burner assembly described above. 
   A second aspect of the invention concerns the manner in which a pressure drop is created in the supply of oxidant and/or fuel to the gas burner assembly of an atomic absorption spectrometer. Conventional techniques utilise an orifice in a flow tube in order to create the pressure drop. The control of pressure drop in the supply of oxidant and/or fuel is critical in order to ensure that the burner produces a flame having the correct flame characteristics for a particular analysis. The use of an orifice does not provide sufficient accuracy in the pressure drop of the oxidant or fuel because of tolerances in the formation of the orifice having regard to the small size of the orifice which in required, and therefore conventional machines do exhibit some difficulty in providing a flame of the required characteristics in order to provide good analysis results. 
   A further aspect of the invention may be said to reside in a gas box for an atomic absorption spectrometer, including;
         an oxidant supply line for supplying oxidant to a burner assembly;   a fuel supply line for supplying fuel to the burner assembly; and   a flow restrictor in at least one of the air supply line or fuel supply line, said flow restrictor comprising a length of tubing for reducing the pressure of the oxidant or fuel by skin friction caused by the internal surface of the tube and the flow of oxidant or fuel through the tube.       

   The use of a length of tube of the flow restrictor and the fact that the pressure drop is created by skin friction, results in a very accurate pressure drop which can be precisely controlled and accurately determined simply the length of the tube which creates the flow restrictor. Thus, better control over the flame characteristics can be obtained in an inexpensive manner and also in a manner which is easy to implement. 
   Preferably both the fuel supply line and the oxidant supply include a said flow restrictor. 
   Preferably a pressure sensor is associates with each flow restrictor for measuring the pressure drop across the flow restrictor. 
   Preferably the fuel supply line and the oxidant supply include respective flow control valves and the flow control valves are controlled dependant on the pressure determined by the pressure sensor to control the supply of fuel and oxidant to the gas burner. 
   The present invention may also be said to reside in an atomic absorption spectrometer having a gas box as described above. 
   A third aspect of the invention may be said to reside in a gas box for an atomic absorption spectrometer, the gas box having;
         a fuel supply line;   an oxidant supply;   a first linear solenoid valve in the fuel supply line for controlling flow of oxidant through the oxidant supply line to a gas burner assembly;   a second linear solenoid valve in the fuel supply line for controlling flow of fuel through the fuel supply line to the gas burner assembly.       

   Preferably the gas box includes;
         an input supply for the input of oxidant, a shut off valve for selectively shutting off supply of oxidant, a pressure regulator for regulating the flow of oxidant, the pressure regulator having an outlet connected to a first flow restrictor, the first flow restrictor being connected to the oxidant line; and   the fuel input supply including a shut off valve, a second flow restrictor and a first flow control valve, an output of the first flow control valve being coupled to the fuel line.       

   Preferably the oxidant supply line is also connected to the fuel line via a second flow control valve so that the fuel line supplies a mixture of fuel and oxidant to the nebuliser bung. 
   Preferably the fuel supply line includes an igniter branch for supplying fuel to an igniter for creating a flame for lighting the burner. 
   Preferably the igniter branch includes a shut off valve. 
   Preferably a first pressure sensor is coupled across the first flow restrictor for measuring the pressure drop across the first flow restrictor. 
   Preferably a second pressure sensor is coupled across the second flow restrictor for measuring the pressure drop across the second flow restrictor. 
   The present invention may also be said to reside in an atomic absorption spectrometer having a gas box as described above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described, by way of example, with reference to the accompanying drawings in which; 
       FIG. 1  is a cross sectional view through a gas burner assembly of an atomic absorption spectrometer according to the preferred embodiment of the invention; 
       FIG. 2  is a fluid circuit diagram of a gas box used with the burner assembly of  FIG. 1 ; and 
       FIG. 3  is a block circuit diagram showing the control of the burner assembly and gas box according to the preferred embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1  a gas burner assembly  10  is shown which has a spray chamber  12  which, at one end, is in fluid communication with a nebuliser bung  14 . The nebuliser bung  14  has a baffle  16  which includes an orifice  18 . The baffle  16  divides the nebuliser bung into an inlet chamber  15  and an outlet chamber  19 . The outlet chamber  19  is in free fluid communication with the spray chamber  12 . A sample tube  17  for the supply of sample to the burner assembly  10  passes into the inlet chamber  15  of the bung  14  and terminates adjacent an inlet of the orifice  18 . In other embodiments the tube  17  could terminate just after the outlet side of orifice  18 . The chamber  15  also has a gas connector  21  which connects the chamber  15  to an oxidant line  22 . The chamber  19  has a gas connector  23  which connects the chamber  19  to a fuel line  24 . The fuel lines  22  and  24  connect with an oxidant inlet supply line  28  and a fuel inlet supply line  29  which are shown in FIG.  2 . 
   A gas burner  11  is coupled to the chamber  12  for receiving oxidant, fuel and sample mixture from the spray chamber  12  and for burning the mixture so that the sample can be analysed by directing a beam of electromagnetic radiation through the flame which is detected by a detector. Sample atoms in the flame will absorb radiation and in particular wavelengths and thereby measuring the spectrum of the detected radiation enables the constituents of the sample to be determined. 
   A pressure relief device such as a bung  29  is located in an opening  29   a  in the spray chamber  12  so that in the event of an over pressure situation created in the spray chamber  12 , such as may be caused by flash-back, the bung can be expelled from the opening  29   a  to release the pressure and minimise the damage caused by the backflash. 
   The spray chamber  12  has a liquid trap  90  which is in the form of an s-bend for draining liquid from the spray chamber  12  which is aspirated into the spray chamber  12  with sample material through the sample supply tubes  17 . That liquid can drain through the trap  90  and from the trap  90 . The trap  90  is in the form of an s-bend so that a liquid plug  92  is maintained in the s-bend so as to form a seal to prevent the egress of fuel, oxidant and sample mixture from the chamber  12 . 
   As shown in  FIG. 2 , the fuel supply line  28  has a first inlet branch  31  for the supply of air and a second inlet branch  32  for the supply of nitrus-oxide. The first branch  31  has a first solenoid valve  38  and the branch  32  has a second solenoid valve  39  for selectively shutting off the supply and nitrous-oxide through the branches  31  and  32 . The branches  31  and  32  are coupled to branch  33  which in is coupled to a pressure regulator  35 . The pressure regulator  35  has an outlet branch  36  and an outlet branch  37 . The first outlet branch  36  couples to a flow restrictor  38  which is formed by a significant length of tube (which may be the same as the branch  36 ) and which is coiled to occupy as little space as possible within the gas box and to be neatly retained within the gas box. The flow resistor  38  connects to a branch  39  which couples to the line  22 . The branch  39  and line  22  may form a continuous piece of tube. 
   A pressure transducer  30  for measuring the pressure in the line  22  is connected to the line  22  so as to provide a measure of the pressure in the line  22  and therefore the pressure within the spray chamber  12 . The measurement of the pressure in the spray chamber  12  is indicative of the flow of oxidant through the line  22  and provides an indication of whether the gas chamber  12  is properly interlocked. 
   If the integrity of the gas burner  10  is not intact such as may be caused by the bung  29  not being located in the opening  29   a  or not located correctly in the opening  29 , gas and fuel mixture will escape through the opening  29   a  thereby reducing the pressure within the spray chamber  12 . This reduction in pressure will be measured by the pressure transducer  30  and if the pressure is not about equal to a predetermined pressure the control processing circuitry (shown in FIG.  3  and which will be described in more detail hereinafter), can shut off flow of fuel and air to the spray chamber  12  to prevent operation of the burner  11  or, if the burner  11  is operating, shut off the burner  11 . 
   Typical spray chamber pressures under maximum flow are set out below. 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Spray Chamber Condition 
               p(Pa) 
             
             
                 
                 
             
           
           
             
                 
               Burner or Bungs missing 
                70 
             
             
                 
               Air acet Burner-empty liquid trap 
               200 
             
             
                 
               Air acet Burner-normal operation 
               260 
             
             
                 
               Air acet Burner 20% clogged 
               310 
             
             
                 
               NOX Burner-empty liquid trap 
               510 
             
             
                 
               NOX Burner-normal operation 
               675 
             
             
                 
               NOX Burner 20% clogged 
               875 
             
             
                 
               Pressure drop with 3 mm diameter leak 
                35 
             
             
                 
                 
             
           
        
       
     
   
   With an NOX burner operating normally the above table shows normal operating pressure will be 675 Pa. If the burner is an air acetylene burner the normal operating pressure within the spray chamber is 260 Pa. If the burner or bung is missing the pressure will reduce to 70 Pa and this will be measured by the transducer  30 . If the air acetylene burning is in place the liquid trap is empty the pressure in the spray chamber will drop to 200 Pa. If the burner is, say 20% clogged, the pressure will increase to 310. If the NOX burner is used and the liquid trap  90  is empty the pressure will drop to 510 Pa and if the burner is 20% clogged the pressure will increase to 875 Pa. A 3 mm diameter leak in the system will reduce the pressure 235 Pa. Thus, by measuring these pressures with the transducer  30  an indication can be obtained as to a particular fault condition and the burner shut off or prevented from operating until the fault is rectified. 
   As shown in  FIG. 2 , the branch  37  is also connected to the supply line  22  and a pressure transducer  30  is connected in the branch line  37  for measuring the pressure drop across the flow restrictor  38 . 
   The fuel supply line  29  includes a branch  51  which has a pressure switch  60  which measures whether sufficient fuel pressure is being supplied in the branch  51 . If the pressure measured by the switch  60  is not sufficient to correctly operate the burner  11  the supply of fuel and/or air can be completely shut off as will be described in more detail with reference to FIG.  3 . The branch  51  also includes a third solenoid valve  41  which has an output side connected to a branch  52 . The branch  52  includes a pressure transducer  81 . The output of the solenoid  40  is also connected to a branch  53  which includes a second flow restrictor  54 . The branch  53  also connects to an igniter line  55  which includes a four solenoid valve  41 . The igniter line  55  provides pure fuel to an igniter burner for creating a fuel flame, such as an acetylene flame, for igniting the burner  11  when required. The restrictor  54  connects to a branch  56  which includes a first flow control valve  50 , the branch  52  also joins with the branch  56  so that the pressure transducer  81  is connected across the flow restrictor  54  for measuring the pressure drop across the flow restrictor  54 . The branch  56  connects to fuel line  24 . The oxidant supply line  29  also includes a branch  57  which connects with the branch  37  and also the oxidant line  22 . The branch  57  has a second flow control valve  70  and the branch  57  also joins with the fuel line  24 . Thus, the fuel supplied through the line  24  is a mixture of fuel, such as acetylene and also of oxidant such as air and nitrous-oxide. 
   The pressure transducers  30 ,  80  and  81  may be located on a control circuit board  72  which forms part of the control circuitry of the gas box shown in FIG.  2 . The pressure transducer  30  is different from the pressure transducers  80  and  81  which receive inputs from each side of the respective restrictors  38  and  54  so that the pressure drop across those restrictors can be measured by the transducers  50  and  51 . The pressure transducer  30  simply measures the absolute value of the pressure within the line  22  and therefore within the spray chamber  12 . 
   When fuel and oxidant is supplied to the lines  22  and  24  the supply of oxidant through the line  22  flows through the orifice  18  and into the chamber  19  where it mixes with fuel supplied through the line  24 . The flow of oxidant through the orifice  18  creates a venturi effect at the end of the supply tube  17  for facilitating the drawing of sample material through the tube  17  into the chamber  19  and then into the spray chamber  22  so that the fuel, oxidant and sample can mix in the spray chamber  22  for supply to the burner  11  for combustion by the burner  11 . 
   When the spectrometer is initially turned on, oxidant can be supplied to the oxidant line  22  via the supply line  28  so as the pressurise the spray chamber  12 . The pressure in the spray chamber  12  will be measured by the pressure transducer  30  and if the pressure is within the required predetermined range an indication of the integrity of the spray chamber  12  can be made. Thus, the operating sequence of the spectrometer may continue by supply of fuel to the fuel line  24  and ignition of the burner  11 . However, if the pressure within the spray chamber  12  is outside the predetermined range, fuel will not be supplied through the line  24  and the burner  11  will not be ignited because the low pressure reading will be taken as an indication that the spray chamber  12  has not been properly interlocked and either the bung  29  is missing or not correctly located in the opening  29   a , or a wrong burner  11  has been located on the spray chamber  12  or the spray chamber  12  is otherwise leaking. The burner  11  will therefore not be ignited and fuel will not be supplied through the line  24  until the fault is rectified. 
   If the plug  92  is not in place or, it evaporates away for some reason, the escape of gas through the trap  90  will be detected by the pressure transducer  30  because the pressure within the chamber  12  will drop. Thus, the pressure detected by the transducer  30  will be outside the predetermined range thereby cause a signal to be supplied to the micro-processor  100  which will shut off the solenoid valves  241  to prevent supply of oxidant and fuel to the chamber  12 . 
     FIG. 3  is a block circuit diagram showing the control operation of the spectrometer and in particular of the gas box shown in FIG.  2 . The solenoids  38  to  41  are connected to micro-processor  100  which also receives control signals from the pressure switch  60  the flow control valves  50  and  70  and the pressure transducers  80  and  81 . The micro-processor  100  will initially open solenoid valves  38  and  39  for supply of oxidant through line  28  to oxidant line  22  and then to chamber  12 . If the pressure transducer  30  measures that the pressure within the chamber  12  is within the predetermined range the micro-processor  100  will open solenoid valves  40  and  41  so that fuel can flow through the line  29  to the fuel line  24  so that the oxidant, fuel and sample mixture can mix in the chamber  12  for supply to the burner  11 . The supply of fuel through the branch line  55  will also enable the igniter to be ignited so that the burner  11  can be ignited to produce a flame for analyses. 
   The pressure switch  60  will measure the flow of fuel in the branch line  51  and if the pressure of the fuel is not sufficiently high to create a stable flame then the pressure switch  60  will supply a signal to the micro-processor  100  which will cause the solenoids  38  to  41  to be shut off to stop supply of oxidant and fuel until the fault is rectified. 
   The pressure regulator  35  regulates the pressure supplied in the branch lines  36  and  37  so that the pressure of oxidant supplied to the line  22  and also the line  24  can be a certain pressure to produce a predetermined flow rate to produce a flame at the burner  11  of the desired characteristics. The nature of the flame  11  at the burner can be modified by the pressure of oxidant and fuel supplied through the oxidant line  22  and fuel line  24  and an indication of the supply of oxidant and fuel is made by measuring the pressure drop across the flow restrictors  38  and  54  by the transducers  80  and  81 . The transducers  80  and  81  supply signals to the microprocessor  100  indicative of the pressure drop across the restrictors  38  and  54  and from that information an indication can be made as to the supply of fuel and oxidant to the chamber  12  and therefore the nature of the flame produced at the burner  11 . If it is desired to modify the flame  11  the microprocessor can output signals to the first and second control valves  50  and  70  so as to control those valves to modify the amount of fuel and oxidant supplied to the fuel line  24  to inturn alter the characteristics of the flame. 
   Thus, the supply of the oxidant and fuel can be controlled by the flow control valves  50  and  70  which, inturn control, by measuring the pressure drop across the flow restrictors  38  and  54 . The flow control valves  50  and  70  are linear solenoid valves which, as will be apparent from the above description, can control the flow of fluid into the line  24  from the branch  57  and the branch  56  between a predetermined maximum and a predetermined minimum flow rate. 
   The micro-processor  100  shown in  FIG. 3  is also connected a mains supply sensor M for determining that main supply power is present and also to an infer-red sensor IS which detects that a flame is actually present at the burner  11 . If the infer-red detector IS does not detect the flame a signal is provided to the micro-processor  100  so that supply of fuel and oxidant can be shut of by shutting of the solenoid valves  38  and  41 . 
   The flow restrictors  38  and  54  which are in the form of a length of tube provide a pressure drop by virtue of the length of the tube involved and which is created of skin friction of the fluid passing through the tubes which form the restrictors  38  and  54 . The amount of skin friction and the pressure drop which it creates is dependant on the viscosity of the fluid supplied through the restrictors  38  and  54  an since this is substantially constant, a very accurate and reliable pressure drop can be obtained by the restrictors  38  and  54  simply by making the restrictors  38  and  54  of a desired length. Since it is easy to determine the length of the tubes which will form the restrictors  38  and  54  the very accurate pressure drop can be obtained in a very simple manner because it simply requires the use of a tube to form the restrictors  38  and  54  of a required length. The required length can easily be determined and installed in the supply lines  28  and  29 . 
   Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.

Technology Category: f