Abstract:
A gas feed system for supplying a gas such as chlorine to a water system for chlorinating the water. The gas feed system includes multiple containers and provides for automatic switch over from one container to a second container once the first container is empty and such that the first containers can be completely emptied. 
     The invention also includes a gas feed regulator for controlling the supply of gas from a container such as a chlorine cylinder, the regulator having a simplified construction.

Description:
This application is a continuation of application Ser. No. 08/981,242, filed Apr. 3, 1998, titled Low Capacity Chlorine Gas Feed System, now U.S. Pat. No. 6,105,598, which is a 371 of PCT/US96/10315, filed Jun. 14, 1996. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to low capacity gas feed systems of the type for use in feeding chlorine gas to a water supply to chlorinate the water. More specifically the invention relates to gas flow regulators for controlling the flow of gas from gas cylinders and valves for controlling gas flow from one gas supply to another gas supply. 
     BACKGROUND PRIOR ART 
     Low capacity chlorine gas feed systems provide for the supply of gas from chlorine gas containers through a gas pressure regulator device to an injector wherein the chlorine gas is delivered to a water supply conduit. One prior art chlorine feed system is illustrated in the assignee&#39;s Technical Data Sheet 910.250 titled “SONIX 100™ Chlorinator.” Attention is also directed to the Konkling U.S. Pat. No. 3,779,268 illustrating a prior art regulator valve for a chlorine gas system. 
     One limitation of prior art chlorine gas supply systems is the amount of chlorine which can be delivered to the water supply. Use of a single gas cylinder permits the discharge of chlorine gas only at a limited flow rate before frosting of the valve makes the gas regulator valve inoperative. 
     In many areas, chlorine gas suppliers require that chlorine tanks be emptied completely before they can be returned to the supplier for refilling. Prior art gas regulation systems have not provided an effective mechanism for insuring efficient use of all of the chlorine in the tanks. In other areas, chlorine gas suppliers require that chlorine tanks returned for refilling contain a predetermined quantity of chlorine in the tanks. Prior art gas regulation systems do not provide an effective mechanism for controlling the amount of gas left in the gas supply cylinders. 
     Another limitation of prior art chlorine gas systems is that they have not provided an effective and efficient system for switching over from one chlorine supply container to another chlorine supply container once the supply in the first container is exhausted. 
     Another limitation of prior art gas feed systems including an arrangement for switching from one gas supply cylinder to another cylinder is that they do not insure complete use or controlled use of the gas in the first container. 
     Another disadvantage of prior art gas supply systems is that they require mechanically complex regulator valve assemblies and are expensive to manufacture and can be unreliable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a gas feed system for supplying a gas, and can be used to supply gas such as chlorine to a water system for chlorinating the water. The gas feed system includes a pair of gas containers or multiple banks of containers and provides for automatic switch over from one container or a bank of containers to a second container or bank of containers once the first container or bank of containers is empty and such that the first containers can be completely emptied. The gas feed system of the invention also provides for automatic switch over from one bank of containers to a second bank of containers while providing for complete emptying of the first bank of containers. 
     The gas feed system of the invention facilitates the use of two sets or banks of multiple tanks of gas. When used to supply chlorine to a water system, the gas supply system can have one bank of tanks supplying chlorine to an injector while the other bank of tanks can remain in a standby condition and such that the second bank of tanks will automatically supply chlorine to the water supply when the amount of gas in the first bank of tanks falls below a predetermined level. Additionally, the tanks in each bank of tanks will discharge even quantities of gas. Gas discharged from a single tank can be limited by frosting that occurs in the control valves. The provision of multiple tanks in parallel permits the discharge of sufficient amounts of gas, and the provision of an even draw-down device embodied in the invention provides for uniform simultaneous discharge from a pair of gas tanks or cylinders. 
     Another principle feature of the invention is the provision of a gas feed regulator for controlling the supply of gas from a container such as a chlorine cylinder, the regulator having a simplified construction. In one preferred embodiment of the invention, the gas feed regulator includes a retractable center pin extending through the center of a pressure responsive diaphragm, the center pin being movable to provide for manual shutoff of the regulator to interrupt gas flow from the gas supply. The regulator includes a manual control lever connected to the center pin, the lever being rotatable 180° to manually shut off the valve. 
     The gas feed regulator embodying the invention further includes the provision of a manual control/operation indicator switch mounted on the regulator housing and engaging the operating lever, the switch being rotatable to rotate the operating lever and the center pin between a manual “off” and a “standby” operating position. The indicator switch further cooperates with the operating lever to form a detent assembly. The detent assembly holds the center pin in a stand-by position until a differential pressure caused by vacuum on the diaphragm causes the center pin to move to an “on” or operating position wherein gas can flow through the regulator from the gas container. When the container is exhausted of gas, the vacuum on the regulator diaphragm will move the center pin to a position where the detent assembly and indicator switch move to an “empty” position. The indicator switch can be rotated manually to a “off” position where the gas flow through the regulator is manually interrupted. The vacuum regulator of the invention further includes a primary check valve operated by the central pin and the vacuum operated diaphragm and further includes a secondary pressure check valve also operated by the center pin and diaphragm. 
     One of the advantages of the vacuum regulator included in the gas supply system embodying the invention is that the vacuum regulator has an efficient construction, has a minimum number of components and can be economically assembled and manufactured. 
     The gas feed system embodying the invention further includes a remote automatic switchover device connected to two gas containers or two banks of gas containers and providing for switch over from one container or bank of containers to the other container or bank of containers when the first empties. The remote automatic switchover device includes a valve housing and a chamber, two inlets communicating with respective ones of the banks of gas cylinders and an outlet communicating with a gas injector supplying gas to a water source. A double acting spool is housed in the chamber and selectively closes one or the other inlet. A manually operable arm connected to the double acting spool is movable between a position opening one outlet and a detent is provided for maintaining the spool in that position until gas pressure supplied through the one inlet decreases to a pressure wherein pressure supplied from the other inlet on the spool member overcomes the detent and opens the other inlet leaving the spool member in a position where both inlets are open. 
     The gas feed system further includes at least one even drawdown device operably connected to two gas cylinders and connecting the regulators of those two cylinders to the remote switchover device. The even drawdown device provides for even flow of gas from the two gas cylinders connected to the even drawdown device. 
     One of the principal features of the invention is the provision in the vacuum regulator of a diaphragm assembly including a diaphragm made of Teflon sheet, the Teflon sheet being heat formed to include concentric grooves. A concentric groove at the periphery of the diaphragm is housed in a groove provided in the opposed two halves in the regulator body and secured in place by an O-ring seal. A concentric groove in the central portion of the diaphragm is similarly clamped using an O-ring between a central diaphragm backing plate and an opposed backing plate nut. The construction of the heat formed diaphragm and O-ring seals permits the use of fewer mechanical components to secure the diaphragm and the use of lower clamping pressures on the diaphragm while also providing a reliable long lasting diaphragm configuration. The diaphragm arrangement is an improvement over prior art constructions where heat can cause variations in the thickness of the diaphragm membrane and loosening of clamping screws. This permits the membrane to pull away from the supporting structure causing wrinkling of the membrane and permitting air leakage into the vacuum regulator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a gas supply system embodying the invention. 
     FIG. 2 is a perspective view of a vacuum regulator and cylinder mounting bracket included in the gas feed system shown in FIG.  1 . 
     FIG. 3 is an exploded perspective view of a gas flow control valve assembly included in the vacuum regulator shown in FIG.  2 . 
     FIG. 4 is an enlarged cross section view of a vacuum regulator included in the gas feed system shown in FIG.  1  and showing the vacuum regulator in a “standby” position. 
     FIG. 5 is a side view of the vacuum regulator shown in FIG.  4 . 
     FIG. 6 is a view similar to FIG.  4  and illustrating the vacuum regulator in an “on” position. 
     FIG. 7 is a view similar to FIG.  7  and showing the vacuum regulator in the “on” position. 
     FIG. 8 is a view similar to FIGS. 4 and 6 and showing the vacuum regulator in an “empty” position. 
     FIG. 9 is a view similar to FIGS. 5 and 7 and showing the vacuum regulator in an “empty” position. 
     FIG. 10 is a view similar to FIG.  4  and showing the vacuum regulator in an “off” position. 
     FIG. 11 is a view similar to FIG.  5  and showing the vacuum regulator in the “off” position. 
     FIG. 12 is an enlarged cross section view of an even drawdown valve included in the gas supply system shown in FIG.  1 . 
     FIG. 13 is an enlarged cross section view of a remote switchover valve included in the gas supply system shown in FIG.  1 . 
     FIG. 14 is a side view of the remote switchover device shown in FIG.  13 . 
     FIG. 15 is a cross section taken along line  15 — 15  in FIG.  14 . 
     FIG. 16 is an enlarged cross section view of a gas injector included in the gas supply system shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Gas Feed System 
     FIG. 1 illustrates a gas feed system embodying the invention and including a plurality of gas cylinders  12 . In the illustrated arrangement the gas cylinders  12  are conventional chlorine gas containers. The gas feed system  10  further includes a vacuum regulator  14  mounted on each cylinder  12 , each of the vacuum regulators  14  comprising a vacuum operated valve intended to control the supply of chlorine gas from the gas cylinders  12 . The vacuum regulators  14  are connected through plastic tubing or conduits  16  to supply chlorine gas to a chlorine gas injector  18 . The chlorine gas injector  18  is best shown in FIG.  16  and has a conventional construction. The gas injector  18  provides for mixing of gas into water flowing through a water supply conduit  20  and facilitates the injection of chlorine gas into the water supply. At the injector  18 , metered gas entering port  22  is dissolved at chamber  23  in the water stream flowing through passage  24  from the water supply conduit  20 . The resultant solution is discharged through passage  26  to the point of application and the flow of water through the injector  18  generates a vacuum at port  22  and in the tubing or conduit  28 . It is this vacuum in the tubing  28  which draws gas through the conduits  16 ,  30  and  32  into the injector  18  and which operates the vacuum regulators  14  connected to the cylinders  12 . 
     In the illustrated arrangement of the gas feed system, a rotameter  34  is provided between the gas feed cylinders  12  and the injector  18 . The rotameter  34  indicates the volume or rate of the flow of gas through the tubing  32  and  28  to the injector  18 . The rotameter  34  can also include a control valve  36  for controlling the rate of flow through the tubing  32  and  28  to the injector  18 . The construction of the rotameter  34  and the control valve  36  is conventional and will not be described in detail. While in the illustrated arrangement the rotameter  34  is mounted remote from the vacuum regulators  14 , in other arrangements a rotameter  34  could be mounted directly on each vacuum regulator to indicate the flow of gas from the individual gas cylinders  12  to the tubing  16 . 
     The gas supply system  10  shown in FIG. 1 further includes a remote switchover device  38  for providing for supply of chlorine gas from a first bank  40  of cylinders during initial operation of the chlorine gas system while maintaining a second bank  42  of cylinders in a standby condition. The remote switchover device  38  includes a valve which isolates the second bank  42  of cylinders during initial operation of the cylinders and then, when the gas in the first bank  40  of cylinders nears an empty condition, the remote switchover device  38  opens to provide for supply of gas from the second bank  42  of cylinders to the injector  18  while also maintaining the first bank  40  of cylinders in communication with the injector  18  so that all of the gas in the first bank  40  of cylinders can be used. 
     The remote switchover device  38  can then be manually switched over to connect only the second bank  42  of cylinders to the injector  18  and to isolate the first bank  40  of cylinders. The cylinders  12  in the first bank  40  can then be removed from the system for refilling and be replaced with full gas containers. The remote switchover device  38  can then maintain those containers  12  in the standby condition until the second bank  42  of cylinders nears an empty condition. 
     In the gas supply system  10  illustrated in FIG. 1, each bank of cylinders  40  and  42  further includes an even drawdown device  44  connecting the two vacuum regulators  14  in that bank of cylinders to the tubing  30  communicating with the remote switchover device  38  and the injector  18 . The even drawdown device  44  provides for simultaneously even or equal flow of gas from the two cylinders  12  in the bank of cylinders  40  to the remote switchover device  38 . 
     Vacuum Regulator 
     Referring more particularly to the vacuum regulators, they each include a housing  46  clampingly mounted to respective ones of the gas cylinders by a yoke clamp or bracket assembly  48 . The bracket assembly  48  for mounting the regulators  14  to the gas cylinders is conventional and will not be described in detail. Each vacuum regulator  14  also includes a control knob/indicator  50  which is positionable as shown in FIG. 11 in an “off” position preventing flow of gas through the regulator  14 . The control knob  50  can be manually rotated counterclockwise 180° from the “off” position shown in FIG. 11 to a “standby” position shown in FIG.  2  and FIG.  5 . As will be explained below, when the control knob  50  of the vacuum regulator  14  is in the “standby” position, the regulator valve is closed until vacuum in the tubing  16  actuates the regulator valve to cause the control knob  50  to move downwardly to the “on” position shown in FIG.  7  and wherein the regulator valve will then permit discharge of chlorine gas in response to vacuum in the tubing  16 . When the cylinder  12  connected to that regulator  14  is empty of gas, vacuum in the tubing  16  will then actuate the regulator to cause the control knob to move to the “empty” position shown in FIG. 9 to thereby indicate depletion of the gas in the chlorine cylinder  12 . The operator can then manually rotate the control knob to the “off” position of FIG. 11, and the cylinder  12  can then be disconnected from the regulator  14  and then replaced with a full cylinder. 
     Referring now more specifically to the construction of the vacuum regulator  14 , as seen in FIG. 4, the vacuum regulator includes a front housing  52  supporting a front cover  54 . The cover  54  in turn supports the control knob  50  for vertical slidable movement between the “standby”, “on” and “empty” positions and also for rotation of the control knob  50  to the “off” position. 
     The vacuum regulator  14  also includes a rear housing  56  fixed to the rear face  58  of the front housing  52 . A flexible diaphragm  60  has a periphery  62  clamped between the front  52  and rear  56  housing. The diaphragm includes a central opening housing a diaphragm backing plate assembly  64  comprised of a diaphragm backing plate  66  and a diaphragm backing plate nut  68  which clampingly engages the inner portion  70  of the diaphragm  60  therebetween. The diaphragm backing plate assembly  64  is housed in the chamber  72  defined by the rear housing  56 , and the diaphragm backing plate assembly  64  is movable with the diaphragm in the chamber  72  between the positions shown in FIGS. 4,  6 ,  8  and  10 . The backing plate nut  68  is threaded onto a projecting threaded extension  74  of the backing plate  66  such that the backing plate nut  68  clampingly engages the diaphragm  60  and clamps it against the backing plate  66  in fluid tight relation. 
     The diaphragm backing plate  66  includes a circular groove  76  in its front face  78 , the groove  76  housing a projecting circular flange  80  of the front housing  52  such that the diaphragm backing plate assembly  64  is supported for movement in the chamber  72  of the rear housing  56  toward and away from the front housing  52 . 
     The vacuum tubing  16  communicates with the chamber  72  through a port  82 , and a coupling  84  (FIG. 2) connects the tubing to the rear housing  56 . The vacuum in the tubing  16  thus draws a vacuum in the chamber  72  defined by the rear housing  56 . The front face of the diaphragm  60  is subjected to atmospheric pressure in the space  86  between the front housing  52  and the diaphragm  60  and diaphragm backing plate  66 . When vacuum is applied in the chamber  72  defined by the rear housing  56 , atmospheric pressure on the diaphragm  60  and diaphragm backing plate  66  will tend to force the diaphragm backing plate assembly  64  rearwardly into the rear housing  56 . 
     The vacuum regulator  14  also includes a valve assembly  90  fixed to the rear housing  56  and controlling flow of chlorine gas from the gas cylinder through the inlet port  92  and into the vacuum chamber  72  where it can then be drawn through the port  82  to the vacuum line or tubing  16 . 
     The valve assembly  90  includes a secondary valve housing  94  having one end housed in a bore  96  in a sleeve  98  projecting rearwardly from the rear housing  56 . A valve housing retainer nut  100  is provided to secure the secondary valve housing  94  to the sleeve  98  and rear housing  56 . The secondary valve housing  94  includes a central bore  102  housing a regulator nipple  104  which is threaded into the secondary valve housing  94 . The regulator nipple  104  includes a central bore  106  housing a valve seat  108  and a valve body  110  biased against the valve seat  108  by a first compression spring  112 . The secondary valve housing  94  also houses a secondary valve seat  114  and a secondary valve body  116  biased against that valve seat by a second compression spring  118 . The second compression spring  118  is supported by a stop member  120  slidably housed in the bore  102  in the secondary valve housing  94 . A rod  122  connected to the first valve body engages the stop  120  to provide a connection between the stop  120  and the first valve body  110 . A second rod  124  extends from the secondary valve body  116  and projects forwardly into the vacuum chamber  72  provided by the rear housing. The regulator nipple  104  also includes the inlet port  92  which communicates through the clamping bracket to the gas cylinder  12 . 
     The regulator also includes an operating pin or shaft  130  threaded into a central bore  132  of the diaphragm backing plate  66  and located centrally with respect to the diaphragm  60 . The operating pin  130  has an end  134  adapted to move with the diaphragm backing plate assembly  64  and to selectively engage the end of the rod  124  extending from the secondary valve body  116  and to provide for movement of the secondary valve body  116  away from the secondary valve seat  114 . The operating pin  135  is threaded into the diaphragm backing plate  66  such that it moves with the diaphragm backing plate  66  in the direction of its longitudinal axis. The threads  136  connecting between the operating pin  130  and the diaphragm backing plate assembly  64  permits the operating pin  130  to be rotated 180° to an “off” position as shown in FIG. 10 where it is backed out of the diaphragm backing plate  66  such that it cannot engage the rod  124  extending from the secondary valve body  116 . 
     The opposite end of the operating pin  130  includes a cavity or bore  138  housing an operating lever pawl  140  and a compression spring  142 . The operating lever pawl  140  is connected to the operating pin  130  by a cross pin  144  and is supported by the operating pin  130  such that the pawl  140  is resiliently biased by the compression spring  142  into engagement with cam surfaces  142  provided in a recess  144  in the end of a lever  146 . The cross pin  144  connecting the operating lever pawl  140  to the end of the operating pin  130  also pivotally connects the lever  146  to the operating pin  130 . 
     In operation of the vacuum regulator  14 , when the operating lever  50  is in the “standby” position shown in FIGS. 4 and 5, and when there is no vacuum applied through the port to the vacuum chamber  72 , the components of the vacuum regulator  14  will assume the position illustrated in FIG. 4, with both the first valve body  110  and second valve body  116  in engagement with the respective valve seats  108  and  114  thereby precluding flow of gas from the inlet port  92  into the vacuum chamber  72 . 
     When the remote switchover valve  38  actuates to cause vacuum to be drawn in the vacuum tubing  16  and the vacuum chamber  72 , vacuum in the vacuum chamber  72  will cause the diaphragm  60  and the diaphragm backing plate assembly  64  to move to the position shown in FIG.  6 . The operating pin  130  is carried by the diaphragm backing plate assembly  64  and such that the end  134  of the operating pin  130  will engage the rod  124  projecting from the secondary valve body  116 . This movement of the operating pin  130  opens both the secondary valve  116  and the first valve body  110  to provide for flow of gas through the inlet port  92  into the vacuum chamber  72  where it will be drawn by vacuum in the tubing  16  through the port  82 . 
     As the opposite end of the operating pin  130  moves to the left as seen in FIGS. 4 and 6, the end of the operating lever pawl  140  will move with respect to the lever  146  from engagement with the cam surface  150  shown in FIG. 4 to engagement with the cam surface  152  shown in FIG. 6 thereby causing the operating lever  50  to be moved from the “standby” position shown in FIG. 5 to the “on” position shown in FIG.  7 . 
     The chlorine gas cylinder  12  will then continue to supply gas to the injector  18  until the cylinder  12  is completely empty. When the cylinder  12  is empty, the vacuum in the vacuum chamber  72  will increase causing the diaphragm  60  and the diaphragm backing plate assembly  64  to move from the position shown in FIG. 6 to the position shown in FIG.  8 . When the diaphragm backing plate assembly  64  moves to this position, the operating pin  130  and operating lever pawl are moved to the cam position shown FIG.  8  and the operating lever  50  will be caused to move by the operating lever pawl  140  and the cam surface  154  of the operating lever to the “empty” position shown in FIGS. 8 and 9. 
     The operator can then rotate the operating lever 180° from the “empty” position shown in FIG. 9 to the “off” position shown in FIG.  11 . Rotation of the operating lever  50  to the “off” position causes rotation of the operating pin  130  with respect to the diaphragm backing plate  66  and threadably backs the operating pin  130  out of the diaphragm backing plate  66  thereby pulling the end  134  of the operating pin  130  away from the rod  124  connected to the secondary valve body  116 . As shown in FIG. 10, the check valves  110  and  116  can then move to a closed position. 
     One of the principle features of the invention is the construction of the vacuum regulator to provide both a primary and a secondary backup check valve  110  and  116  operated by a single diaphragm  60 . In the event one of the check valves fails to close fully, the other check valve will insure complete sealing of the valve assembly. But, while a second check valve  116  can be provided, the construction of the regulator of the invention facilitates the use of only a single diaphragm  60  to provide for movement of both valve assemblies. 
     The vacuum regulator also includes a pressure relief valve  160  for discharging gas from the regulator in the event that a gas pressure develops in the vacuum chamber  72 . A gas discharge port  162  in the rear housing  56  communicates through a spring biased check valve with a discharge port  166 . The check valve includes a flexible diaphragm  164  biased against the port  162  by a pin  168  and a compression spring  170 . The compression spring  170  is backed by a plug  172  threaded into a bore  174  provided in the rear housing. 
     Remote Switchover Valve 
     The remote switchover valve  38  is illustrated in greater detail in FIGS. 13-15 and includes a T-shaped valve body  180  including a pair of inlets  182  and  184  connected to the tubing  30  extending from the banks of chlorine tanks and an outlet port  186  connected by tubing  32  to the rotameter  34  and injector  18 . The remote switchover device  38  includes a reciprocally movable elongated valve member  190  having opposite ends, the opposite ends of the elongated valve member supporting resilient valve cups  192  and  194 . The elongated valve member is movable from the intermediate position shown in FIG. 13 to a position wherein the resilient valve cup  192  at one end of the elongated member  190  is engageable with a seat surface  196  to selectively prevent gas flow through the inlet  182 . The elongated valve member  190  is also movable from the intermediate position to the right as shown in FIG. 13 to a position wherein the resilient valve cup  194  sealingly engages a second seat surface  198  to selectively prevent gas flow through the inlet  184 . 
     A pair of compression springs  200  and  202  are provided for biasing the elongated valve member  190  toward the centered or intermediate position shown in FIG.  13 . 
     A detent device is also provided for releasably restraining the elongated valve member  190  in a selected position where the valve member  192  seats against the seat  196  or alternatively for releasably restraining the elongated valve member  190  in a second position wherein the valve member  194  seats against the opposite seat  198  at the opposite end of the valve. The detent device includes a rack  204  formed integrally with the central portion of the elongated valve member  190  and a pinion  206  engaging the rack  204 . The pinion  206  is mounted on the end of a manually rotatable shaft  208  (FIG.  15 ), and a control knob  210  is mounted on the opposite end of the rotatable shaft  208 . The control knob  210  can be manually rotated between a first position wherein the elongated valve member  190  is moved to a position where the cup valve  192  engages the valve seat  196 . In that position (FIG. 14) a spring biased detent ball  214  engages a notch  216  provided in a collar  218  mounted on the shaft  208 . The detent ball  214  releasably holds the elongated valve member  190  in that position. The manual control knob  210  can be rotated in the opposite direction wherein a second spring biased detent ball  220  will engage the notch  216  in the collar  218  to hold the elongated valve member  190  in a position wherein the cup valve  194  engages the other valve seat  198 . 
     In operation of the remote switchover device, the control knob  210  can be rotated to a position wherein the detent ball  214  will hold the elongated valve member  190  in a position wherein one of the cup valves engages a valve seat to block the flow of gas through that inlet  182 . The elongated valve member is held in that position by the force of the detent  214  and by the pressure of gas at inlet  184  from the other bank of cylinders. When the gas pressure at inlet  184  from the other bank of cylinders falls below a predetermined level, gas pressure from the other bank of cylinders and the force of the return spring  200  will overcome the force of the detent ball  214  and the elongated valve member  190  will be shifted by the compression springs  200  and  202  to a central position. In this position chlorine gas can then be drawn from the second bank of cylinders while the first bank of cylinders is also connected to the vacuum tubing and the injector  18 . 
     FIG. 12 illustrates in greater detail the even drawdown device  44  which includes a pair of housing portions  230  and  232  defining chambers  234  and  236  separated by a diaphragm  238 . The periphery of the diaphragm  238  is clamped between the halves  230  and  232  of the housing and an O-ring  240  provides a fluid tight seal. The left housing portion  230  shown in FIG. 12 includes a boss or sleeve  242  threadably housing a valve seat holder  244 . A Teflon valve seat  246  is housed in the valve seat holder  244  and a reducing bushing  248  provides for connection of the tubing  16  with bore  249 . The right housing portion  232  includes a boss or sleeve  250  housing a valve seat  252 , and a reducing bushing  254  is provided for connecting the other tubing  16  to the inlet bore  256 . 
     The even drawdown device  44  further includes a valve spool  260  having a diaphragm hub  262  clampingly engaging the central portion of the diaphragm  238  such that the valve spool  260  is movable with the diaphragm. One end of the valve spool  260  includes a valve body  264  selectively engageable with the valve seat  246  and the opposite end of the valve spool  260  includes a second valve body  266  engageable with the second valve seat  252 . The second valve seat  252  includes a plurality of small orifices  268  between the valve body  266  and the valve seat  252  to permit controlled gas flow past the valve seat  252  when the valve member  266  engages the valve seat  252 . The left and right housing portions  230  and  232  are provided with discharge ports  270  and  272 , respectively which communicate with the tube  30  providing flow of gas to the rotameter and the injector  18 . 
     In operation of the even drawdown device, vacuum in the tube  30  communicating with rotameter  34  applies a vacuum in the chambers  234  and  236  on both sides of the diaphragm  238 , causing gas to be drawn initially through the orifices  268  around the valve body  266 . The pressure differential caused by gas flow into the right chamber  236  as seen in FIG. 12 will create a pressure on the diaphragm  238  causing movement of the valve body  264  away from the valve seat  246  to cause flow of gas into the chamber  234  and until the gas pressure in the chambers on  234  and  236  opposite sides of the diaphragm  238  is equal. The gas flow from the tubes  16  communicating with the two gas cylinders  12  will thus be equalized to provide for uniform and even flow from those cylinders  12  to the injector  18 .