BACKGROUND 
     Bulk cryogenic storage of carbon dioxide (CO 2 ) gas is a recent historical development in the beverage industry. Vacuum jacketed storage containers delivering 300 pounds to 750 pounds or more of liquified CO 2  gas are widely used. These containers safely deliver gaseous CO 2  at pressures above 90 pounds per square inch by converting the liquid CO 2  to gas using a natural conversion process through a simple temperature increase effected by ambient temperatures at the location of use. 
     The gas delivered from such tanks is widely used in conjunction with beverage dispensing machines of the type commonly found in restaurants, convenience stores, theaters, amusement parks and the like. The carbon dioxide (CO 2 ) is mixed with water to produce carbonated water under pressure. The carbonated water then is mixed with a syrup at the dispensing machine to produce the finished carbonated beverage. 
     CO 2  in its gaseous state is a tasteless, colorless, odorless gas which naturally displaces oxygen. If this gas is accumulated in sufficient density in a closed space, such as a storage room, it is hazardous, if not lethal. In facilities which initially produce CO 2  gas for ultimate delivery and consumption, multiple safety procedures are employed. Among these are detectors which sense when the CO 2  gas level in a particular area exceeds a safe level, to produce a warning alarm. 
     Bulk storage tanks, however, frequently are located in a confined area adjacent a beverage dispensing machine, frequently, in a small room one wall of which backs onto the location of the machine, or in some other area which is frequented by employees of the establishment using the beverage dispensing machine. CO 2  sensors or safety devices typically are not employed where bulk storage tanks are used to supply CO 2  to a beverage dispensing machine. In such situations, both employees of the establishment and customers may be exposed to unsafe levels of CO 2  gas without their knowledge. 
     If the syrup box or container used to deliver the flavored syrup to the beverage dispensing machine is empty while the CO 2  dispensing line is connected to it, the drop in pressure will allow CO 2  gas to pass outwardly into the surrounding area. Also, if a leak should occur in the gas line for delivering the gaseous CO 2  to the carbonator or beverage box of a beverage dispensing machine, or if for any reason there is a failure to turn off the delivery of CO 2  gas, a drop in pressure, sometimes sudden, takes place at the bulk storage tank. 
     A sudden drop in pressure of CO 2  delivered from the tank causes the liquid CO 2  in the bulk container to turn into “dry ice”. When this occurs, further delivery of gaseous CO 2  from the tank is precluded. This necessitates some type of a service call, since when this occurs, further operation of the beverage dispensing machine ceases. Service calls of this type are unscheduled and are expensive, driving up the operating costs of the entire system. 
     It is desirable to provide a safety system for bulk cryogenic storage tanks which overcomes the disadvantages of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The sole FIGURE of the drawing is a block diagram of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now should be made to the drawing, which is a block diagram of an embodiment of the safety system of the invention. As shown in the drawing, the system is used in conjunction with a bulk cryogenic storage tank  10  of the type used to store and deliver liquid CO 2 , converted to a gaseous state, for use in a variety of applications. One such application is shown in  FIG. 1  as a beverage dispensing unit  32 . 
     The gas delivery line from the tank  10  is connected through a conventional high pressure regulator  12 , which regulates the output gas flow from the tank  10  to a pressure of approximately 90 to 110 pounds per square inch. The pressure regulator and the amount of pressure of gas delivered from the tank is conventional, in a range typically used by beverage dispensing units, such as the unit  32 . After passing through the regulator  12 , the gas line is connected to the input of a safety tank pressure monitor system or unit  14 , which monitors the pressure of gas in the line and particularly includes controls for sensing low pressure, such as caused by a leakage in the gas line or an open CO 2  connection downstream from the unit  14 . 
     The monitor unit  14  includes adjustable electronic circuitry, or other suitable means, for continuously monitoring the pressure in the line as it flows through the system  14 . The operation of the system  14 , in conjunction with other portions of the system, is described in greater detail subsequently. 
     The gas line, after passing through the safety tank pressure monitor system  14 , is connected through a normally closed control valve  16 , from which it then is connected to a conventional carbonator  30 , also supplied with water, as shown in the drawing. The output of the carbonator  30  is supplied to the beverage dispensing machine  32 , along with syrup for selected beverages from a beverage box cluster or a single beverage box  34 , indicated in the drawing. The manner in which syrup is delivered from the beverage boxes  34 , and in which carbonated water is delivered from the carbonator  30  to the machine  32  is standard, and therefore is not discussed in any detail here. 
     As noted above, the gas from the storage tank  10  is supplied to a normally closed valve  16 . In order for the valve  16  to be opened to deliver carbon dioxide gas to the carbonator  30 , a relay  18  must be operated. The relay  18  is electrically operated; and whenever electrical power to the relay  18  is interrupted, the normally closed valve  16  closes to prevent flow of carbon dioxide gas through the system to the carbonator  30 . This is the “fail safe” mode of operation of the system. 
     Whenever the pressure sensed by the pressure sensor in the safety pressure monitor system  14  is above a pre-established level (typically, in the normal pressure range of 90 PSI or more), a signal is supplied to close a normally open switch  22 . This is indicated by the dotted line  36  in the drawing. This signal and the particular type of switch, and the manner in which the switch is closed, may be of any suitable type. The switch  22  is indicated in the drawing diagrammatically as a single-pole-single-throw mechanical switch of the type which may be operated by a relay. The switch  22 , however, may be a micro switch, or a transistor, electronic switch, or any other suitable type of switch. The particular type of switch is not important to the invention; so it has been depicted functionally as a shown in the drawing. 
     When the switch  22  is closed by way of the link shown as the dotted line  36  in the drawing, power is applied from a suitable source of alternating current power  20 , through a rectifier  24 , to operate the relay  18 . When the relay  18  is operated, the valve  16  is opened, and allows gas to pass through the valve to the carbonator  30  causing the system to operate in its normal mode of operation. 
     So long as there are no leaks or an unintentionally left open demand for CO 2  gas from the beverage dispensing unit  32 , the system operates as if the safety tank pressure monitor system  14  was not present. In the event, however, that a sudden and/or prolonged drop in pressure as a result of a leak or other abnormal flow of gas out of the tank  10  takes place, the low pressure condition is sensed by the monitor  14 ; and the switch  22  is opened. When the switch  22  is opened, no further power is delivered to the relay  18 ; and therefore, the normally closed valve  16  again closes. This terminates the delivery of CO 2  gas to the carbonator  30 , so long as this low pressure condition exists. 
     With the valve  16  closed, however, the system stabilizes; and pressure is allowed to build up naturally from the tank  10 . The stabilization of the system at a preselected upper pressure automatically occurs as a result of the nature of the liquid CO 2  in the tank. When the desired operating-pressure is sensed by the unit  14 , the switch  22  is closed; and the valve  16  is opened to again permit flow of CO 2  gas to the carbonator  30 . If the condition which caused the low pressure sensing from the monitor  14  again takes place, however, as a result of a leak or other uncorrected continuous dispensation of the CO 2  gas, the low pressure condition once again will be established. The monitor  14  again senses the low pressure and causes the valve  16  to be closed. 
     Even though the system may cycle back and forth between a closed valve  16  and an open valve  16 , freezing up or icing up of the system is prevented. Obviously, cycling back and forth between the open and closed operation of the valve  16  does not stop leakage, if the condition was caused by leakage. Consequently, repair of whatever caused the leakage still needs to be undertaken. The safety monitor system, however, does provide for operation of the beverage dispenser  32  until such repairs can be made. The operation of the dispenser  32  obviously will be interrupted whenever the valve  16  is closed; so that the persons responsible for the system&#39;s operation are provided with a ready indication of some type of malfunction. The malfunction, however, will not result in a frozen condition of the CO 2  in the tank  10 ; and by the nature of the operation of the monitor  14 , it is possible to schedule a repair and inspection of the system at a more convenient time, rather than under some type of “emergency” situation. 
     The foregoing description of an embodiment of the invention is to be considered as illustrative and not limiting. Various changes and modifications will occur to those skilled in the art for performing substantially the same function, in substantially the same way, to achieve substantially the same result, without departing from the true scope of the invention as defined in the appended claims.
A bulk tank source of pressurized gas supplies gas to a consumption device, such as a beverage dispenser. A control valve is located between the bulk tank and the consumption device. A pressure monitoring system is connected to monitor the gas pressure between the bulk tank and the control valve. The pressure monitor system closes the valve when pressure drops below a first predetermined threshold, and opens the valve when the monitored pressure exceeds a second predetermined threshold greater than the first predetermined threshold.