Abstract:
A dispensing system for dispensing volatile liquids such as hydrocarbon fuel for vehicles into a tank having a filler neck also collects the vapors to reduce atmospheric pollution. A fuel delivery hose includes a hand-held fuel valve and nozzle for insertion in the opening of the tank. A means delivers fuel under pressure to the fuel delivery hose, and another means provides electrical pulses corresponding to the volumetric flow of liquid through the fuel delivery hose when the fuel valve is open. A vapor recovery hose includes a vapor intake connected to the hand-held nozzle for insertion in the opening of the tank without sealing with the tank, and a motor driven vapor pump produces a volumetric flow through the vapor recovery hose corresponding to a signal applied to the motor. A processing means produces the signal applied to the motor in response to the electrical pulses to produce a volumetric flow of vapor slightly greater than the volumetric flow of fuel to the tank.

Description:
RELATED APPLICATIONS 
   This application is a continuation-in-part of application Ser. No. 07/946,741 filed Sep. 16, 1992, now U.S. Pat. No. 5,955,915 which is a continuation-in-part of application Ser. No. 07/824,702 filed Jan. 21, 1992, (now U.S. Pat. No. 5,156,199 issued Oct. 20, 1992) which is a continuation of application Ser. No. 07/625,892 filed Dec. 11, 1990, now abandoned. The disclosures of application Ser. No. 07/946,741 and U.S. Pat. No. 5,156,199 are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   U.S. patent application entitled “Vapor Recovery System for Fuel Dispenser” filed on May 21, 1990, in the name of Kenneth L. Pope, and bearing Ser. No. 07/526,303 now U.S. Pat. No. 5,040,577. 
   The field of the present invention relates generally to fuel dispensers, and more particularly to vapor recovery systems for use when dispensing a volatile fuel such as gasoline. 
   BACKGROUND OF THE INVENTION 
   Vapor recovery fuel dispensers, particularly gasoline dispensers, have been known for quite some time, and have been mandatory in California for a number of years. The primary purpose of using a vapor recovery fuel dispenser is to retrieve or recover the vapors which would otherwise be emitted to the atmosphere during a fueling operation, particularly for motor vehicles. The vapors of concern are generally those which are contained in the vehicle gas tank. As the liquid gasoline is pumped into the tank, the vapor is displaced and forced out through the filler pipe. Other volatile liquids such as hydrocarbon fluids raise similar issues. 
   The traditional vapor recovery apparatus is known as the “balance” system, in which a sheath or boot encircles the liquid fueling spout and connects with tubing back to the fuel reservoir. As the liquid enters the tank, the vapor is forced into the sheath and back toward the fuel reservoir where the vapors can be stored or recondensed. 
   Balance systems have numerous drawbacks, including cumbersomeness, difficulty of use, ineffectiveness when seals are poorly made, and slowed fueling rates. 
   As a dramatic step to improve on the balance systems, Gilbarco, Inc., assignee of the present invention, patented an improved vapor recovery system for fuel dispensers, U.S. Pat. No. 5,040,577 to Kenneth L Pope. The Pope patent discloses a vapor recovery apparatus in which a vapor pump is introduced in the vapor return line, driven by a motor. The liquid flow line includes a pulser, conventionally used for generating pulses indicative of the amount of liquid fuel being pumped. This permits computation of the total sale and the display of the volume of liquid and the cost in a conventional display, such as, for example as shown in U.S. Pat. No. 4,122,524 to McCrory et al. A microprocessor translates the pulses indicative of the liquid flow rate into a desired vapor pump operating rate. The effect was to permit the vapor to be pumped at a rate correlated with the liquid flow rate so that, as liquid is pumped faster, vapor is also pumped faster. 
   While the apparatus described in detail in the Pope patent is significant and quite workable, various improvements and refinements have been discovered to further enhance the usability of it and similar vapor recovery systems. 
   In particular, since the vapor pump is independently driven, in the event of a malfunction so that the vapor pump is operating when the liquid pump is not, there is a possibility of drawing large volumes of air into the liquid storage tank. When the quantity of air reaches a high enough level, the air/vapor mixture in the tank can reach dangerously explosive proportions. Accordingly, safety features are needed to assure that excessive amounts of air are not drawn in. 
   Further, it has been found that if liquid is pumped back through the vapor pump line in large quantities, damage to the vapor pump can result, so that a need is present to deal with that circumstance. 
   In dispensing systems for vaporizable liquid, the liquid flows to the tank being filled through a tube and vapor is sucked by a recovery pump from the tank via an adjacent tube. If the temperature of the liquid and the temperature of the vapor in the tank are the same, the volumetric flow V R  of the vapor recovery pump can be made more or less equal to the volumetric flow of the liquid, V L . However, if the temperatures are different, a heat exchange takes place between the liquid and the vapor in the vehicle tank so that the vapor expands or contracts in accordance with the universal gas law PV=mRT. Therefore, in order to evacuate all of the vapor that is displaced from the tank as the liquid enters it and yet not suck in excess air by sucking too hard, the volumetric flow of the vapor recovery pump must be varied. By way of example, if the temperature of the vapor in the tank being filled is colder than the liquid being pumped into it from an underground reservoir, as may well occur during winter, the vapor in the vehicle tank will be heated and will expand, thereby requiring an increase in the volumetric flow of the vapor pump. The opposite effects may take place during the summer. Compensation of the vapor flow rate to account for these differences is needed. 
   SUMMARY OF THE INVENTION 
   The present invention fulfills these needs in the art by providing a vapor recovery fuel dispenser for dispensing volatile liquids such as hydrocarbon fuel for vehicles into a tank having a filler neck while collecting the vapors to reduce atmospheric pollution. The dispenser includes at least one liquid dispensing means including a hand-held nozzle and liquid valve means disposed at the end of a flexible hose for flowing liquid into the fuel tank of a vehicle under the control of an operator operating the liquid valve, and it includes a vapor collection means. The vapor collection means includes a vapor intake means manipulated with the hand-held nozzle so as to be positioned closely adjacent, but not sealed with, the fuel tank during delivery of fuel to the tank, a normally closed vapor valve operable when liquid is flowing through the liquid valve of the nozzle, and vapor suction means including a vapor pump driven by an electrical motor and coupled to draw vapor through the vapor intake and the vapor valve and deliver the vapor to vapor storage means. A flow meter produces a first electrical signal representative of the rate of flow of liquid being dispensed from the nozzle, and processing means receives the first electrical signal and operates the electric motor at a controlled rate to draw vapors through the vapor intake at a volumetric rate slightly greater than the volumetric rate at which liquid is being flowed from the nozzle. Thus, substantially all fuel vapor displaced from the tank will be delivered to the vapor storage means while minimizing delivery of air to the vapor storage means. The electrical signal may take the form of electrical pulses at a repetition rate corresponding to the volumetric flow of liquid through the fuel delivery hose when the fuel valve is open. 
   In a preferred embodiment a different grade of hydrocarbon fuel is dispensed from each of a plurality of nozzle and liquid valve means, and the processing means includes a point of sale display indicating the volume and cost of the fuel dispensed. Most often, there are three grades of fuel dispensed from three nozzle and liquid valve means. Each grade of hydrocarbon fuel may be dispensed from a different storage tank, and vapor within the storage tanks may be in fluid communication, with the collected vapors returned to the storage tanks. A pump may be provided for each of the delivery hoses for providing a flow of fuel of a different grade to their respective nozzles. 
   The vapor return piping for a dispenser is typically configured so that the hoses operable from one side of the dispenser—usually three in number—have their vapor return hoses manifolded together and connected to a single vapor return pump and motor leading to the underground storage tanks from whence the liquid fuel is pumped. Non-active hoses on the side of a dispenser are closed off through the use of the vapor valves associated with those hoses. These valves may be mechanically actuated or of the electrical solenoid type and may be located either in the dispenser housing or in the nozzle. 
   Each hand-held nozzle and liquid valve means may include a vapor valve operated in synchronization with and in response to manual operation of the liquid valve means whereby only the vapor intake associated with the nozzle from which liquid is being dispensed will function to collect vapor. Preferably, a hand-operated liquid valve is attached to the nozzle, and the vapor collection means includes a vapor valve between the vapor pump and the end of the nozzle, which is opened only when fuel is being delivered. For example, each vapor valve may be attached to the respective hand-held nozzle and be opened in response to the user causing fuel to be delivered through the nozzle. The vapor pump may be operated in such a manner as to produce a volumetric flow in the respective vapor hose slightly greater than that in the respective fuel delivery hose. Or, the vapor pump may be operated in such a manner as to produce a volumetric flow in the respective vapor hose less than that in the respective fuel delivery hose. 
   One embodiment includes a liquid fuel pump for pumping liquid fuel from a fuel reservoir along a fuel delivery line to an outlet, a vapor pump for returning fuel vapors from proximate the outlet along a vapor return line to a vapor repository, and a controller operably interposed between the liquid fuel pump and the vapor pump which monitors when both pumps are operating and disables operation of the vapor pump when the liquid pump is not operating. 
   In a preferred embodiment the signal indicative of operation of the motor is a pulse train and the controller counts pulses in the pulse train during periods when the signal to operate the vapor pump is absent and disables operation of the vapor pump motor when a threshold number of pulses is counted. 
   In a further aspect, the invention provides a vapor recovery fuel dispenser including a vapor pump for returning fuel vapors from proximate a liquid fuel outlet along a vapor return line to a vapor repository, an electric motor driving the pump, and a controller which monitors the electrical current to the motor and disables operation of the vapor pump motor when the monitored current indicates a system error, such as liquid fuel blocking the vapor return line. 
   In an alternate embodiment, the invention provides a vapor recovery fuel dispenser system including a liquid fuel pump for pumping liquid fuel from a fuel reservoir along a fuel delivery line to an outlet, a vapor pump for returning fuel vapors from proximate the liquid fuel outlet along a vapor return line to a vapor repository, an electrically-activatable valve in the vapor return line, and a controller which monitors when the liquid fuel pump is operating and outputs an electrical signal to open the valve when the liquid fuel pump is operating and to close the valve when liquid fuel pump operation is not detected. In a preferred embodiment the signal indicative of operation of the liquid fuel pump is a pulse train and the controller converts pulses in the pulse train to a logic level corresponding to a desired valve open or valve closed condition. 
   The invention may also be summarized as a dispensing system for dispensing volatile liquids such as hydrocarbon fluids for vehicles while collecting the vapors to reduce atmospheric pollution including a plurality of liquid dispensing means. Each dispensing means includes a hand-held nozzle and liquid valve means disposed at the end of a flexible hose for flowing liquid into the fuel tank of a vehicle under the control of an operator. Also included is a vapor collection means including a vapor intake means positioned to be closely adjacent, but not sealed with, the fuel tank including a normally closed vapor valve associated with each hand-held nozzle and operable in response to opening of the respective liquid valve of the respective nozzle for collecting vapors displaced from the fuel tank as the liquid is flowed through the liquid valve into the tank at a variable, controlled rate. A vapor suction means includes a vapor pump driven by an electrical motor and coupled to draw vapors from all of the plurality of vapor intakes associated with the plurality of liquid dispensing means, and delivering the vapor to vapor storage means. 
   A plurality of flow meter means are included, each for producing a first electrical signal representative of the rate of flow of liquid being dispensed from one of the respective nozzles. A digital processing means receives each of the first electrical signals and operates the vapor collection means at a controlled rate to pump vapors through the vapor intake at a volumetric rate having a predetermined relationship to the volumetric rate at which liquid is being flowed from the nozzles whereby substantially all fuel vapor will be delivered to the vapor storage means. 
   A different grade of hydrocarbon fuel may be dispensed from each of the nozzle and liquid valve means, and the digital processing means may include a point of sale display indicating the volume and cost of the fuel being dispensed. Preferably, each grade of hydrocarbon fuel is dispensed from a different storage tank, vapor within the storage tanks are in fluid communication, and the collected vapors are returned to the storage tanks. 
   In one embodiment, each hand-held nozzle and liquid valve means includes a vapor valve operated in synchronization with and in response to manual operation of the liquid valve means whereby only the vapor intake associated with the nozzle from which liquid is being dispensed will function to collect vapor. 
   If desired, the system may be configured so that more than one of the plurality of liquid dispensing means can be operated simultaneously and the vapor pump means is operated at a vapor flow rate to the total liquid volume being simultaneously dispensed from the plurality of liquid dispensing means. If so, preferably the liquid valve means and the vapor valve means are each proportioning valves which dispense liquid and collect vapor at a variable rate determined by the operator, and the liquid valve and vapor valve are interconnectively operated to maintain a predetermined ratio of vapor volume collected to liquid dispensed through each nozzle being operated regardless of the rate of flow of fuel through the respective nozzle. 
   The invention also provides several improved vapor recovery methods. These include a method of recovering fuel vapor in a vapor recovery fuel dispenser comprising pumping liquid fuel with a liquid fuel pump from a fuel reservoir along a fuel delivery line to an outlet, pumping fuel vapors from proximate the outlet along a vapor return line to a vapor repository with a pump that is not mechanically actuated by the liquid pump, monitoring the liquid and vapor pumping to ascertain whether liquid and vapor pumping are taking place substantially simultaneously, and disabling the vapor pump when it is ascertained that vapor pumping is taking place and liquid pumping is not taking place. 
   Another method of recovering fuel vapor in a vapor recovery fuel dispenser includes pumping fuel Vapors from proximate a liquid fuel outlet along a vapor return line to a vapor repository with a vapor pump, driving the vapor pump with a motor by providing a signal to operate the vapor pump, monitoring when the motor is operating, and disabling the vapor pump motor when motor operation is detected while not signaled to operate. 
   A further method of recovering fuel vapor in a vapor recovery fuel dispenser includes pumping fuel vapors from proximate a liquid fuel outlet along a vapor return line to a vapor repository with a vapor pump, driving the vapor pump with an electric motor, monitoring the electrical current to the motor, and disabling operation of the vapor pump motor when the monitored current indicates a system error. 
   The invention provides a method of dispensing a plurality of grades of liquid fuels from a corresponding plurality of liquid storage tanks at a single point of sale through a corresponding number of hand-held nozzles each having a normally closed fuel valve and a normally closed vapor valve into a fuel tank having a filler neck. 
   The method includes on demand from a customer&#39;s simultaneous operation of the fuel and vapor valves of a selected nozzle, pumping fuel from the corresponding storage tank through a meter to the customer&#39;s fuel tank having a filler neck while producing an electrical signal representative of the volume flow rate of the fuel. 
   The method also includes digitally processing the electrical signal and operating an electrically driven vapor pump connected to allow the vapor valve, which is positioned closely adjacent to, but not sealed with, the fuel tank, when open to collect vapors displaced from the fuel tank by a vacuum intake disposed adjacent but not sealed with the filler neck of the customer&#39;s fuel tank at a vapor volume flow rate having a predetermined relationship to the fuel flow rate represented by the electrical signal. 
   The pumped vapors are discharged to a vapor manifold interconnecting all of the storage tanks. Preferably, the method also includes digitally processing the electrical signal to calculate the total volume of the selected fuel being dispensed to the customer&#39;s tank and the total cost, and displaying the volume and cost information to the customer at the point of sale. 
   The thermal expansion or contraction of the vapor may be compensated for by controlling the volumetric rate of the vapor pump to a level higher or lower than otherwise projected. The amount of adjustment is determined in large part by the ratio of-the absolute temperature of the liquid to the absolute temperature of the vapor. Appropriately located conventional transducers may be used in making the temperature measurements. Also, in a practical system, it is preferred to use the ambient or atmospheric temperature as an estimate of the initial vapor temperature T V . In most situations the initial temperature, T V , of the vapor in the tank being filled is approximately the same as the atmospheric or ambient temperature T A . A thermistor or other appropriate type transducer, for example, mounted in the product flow path may be used to measure the product temperature T L . 
   In accordance with another aspect of this invention, compensation is made for any difference between the actual volumetric flow, V A , of the recovery pump and the ideal flow, V R , that can be caused by such things as pump wear and differences between pumps due to variations within tolerance limits. This is accomplished by measuring the actual flow V A , deriving the difference between it and V R , and controlling the recovery pump so that V A =V R . 
   An advantage of the invention as disclosed herein and in our earlier patent applications Ser. No. 625,892 filed Dec. 11, 1990 and its continuation Ser. No. 824,702 filed Jan. 21, 1992 (now U.S. Pat. No. 5,156,199), is that the nozzle used to fill the automobile tank need not be equipped with bellows or other face sealing means. Gasoline customers who use vapor recovery fuel dispensers have found such bellows or seals to be cumbersome and difficult to use. Also, when the seals are used in a balance system vapor recovery installation, if the seal is not perfect, vapor can leak, defeating the environmental advantages sought to be obtained. By doing away with such seals, applicant has been able to do away with the problems attendant thereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from a reading of the detailed description of the preferred embodiments along with a study of the drawings in which: 
       FIG. 1  is a schematic representation of the delivery system for volatile liquid constructed in accordance with the invention; 
       FIG. 2  is a flow chart used in explaining the operation of the delivery system shown in  FIG. 1 ; 
       FIG. 3  is an alternate schematic representation of the delivery system, emphasizing use of the vapor recovery components in a multi-product, multi-hose fuel dispenser; 
       FIG. 4  is a further schematic representation of the delivery system, emphasizing use of the vapor recovery components in a multi-product, multi-hose fuel dispenser; 
       FIG. 5  is a somewhat more detailed schematic view of the delivery system of  FIGS. 3 and 4 , further emphasizing use of the vapor recovery components in a multi-product, multi-hose fuel dispenser; and 
       FIGS. 6A and 6B  are block circuit diagrams showing electronic control components as used in the apparatus depicted in FIG.  4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Temperature Compensation 
   In the embodiment of the invention shown in  FIG. 1 , liquid is pumped from a reservoir  2  by a pump  4  with a volumetric flow V L  that is determined by the position of a trigger  6  of a nozzle  8 . The nozzle  8  is a bellows-free and seal-free nozzle, such as the one described in U.S. Pat. No. 4,199,012, for example, and is inserted into the fill pipe  10  of a tank  12  that is to be filled with liquid  13 . The liquid flows to the nozzle from the pump  4  via a tube  14 , a temperature transducer  16 , a flow meter  18 , and a tube  20 . As vapor  15  is forced from a tank  12 , it is drawn through a tube  22  by a pump  24  that forces it through a flow meter  26  and a tube  28  to the reservoir  2 . 
   As described below, means are provided for initially driving the recovery pump  24  at such speed that its volumetric flow, V V , equals the volumetric flow, V L , of the liquid produced by the pump  4 . Signals from the flow meter  18  are applied via a lead  31  to a microprocessor  30  that is programmed to supply a control signal to a drive pulse source  32  that supplies drive pulses to a motor  34 . The motor  34  is mechanically coupled via a rod  36  to drive the recovery pump  24 . The frequency of the drive pulses supplied by the source  32  is such that the motor  34  drives the recovery pump  24  at such a speed a to cause V V =V L . 
   The volumetric flow of the recovery pump  24  may be modified as follows to accommodate the change in volume of the vapor emanating from the tank  12 . The signal provided by the temperature transducer  16  representing the temperature, T L , of the liquid flowing to the tank  12  is conducted to the microprocessor  30  via a lead  38 . A temperature transducer  40  supplies a signal representing the atmospheric or ambient temperature T A  to the microprocessor  30  via a lead  42 . The microprocessor  30  modifies the control signal supplied in the drive pulse source  32  in a manner described in  FIG. 2  so as to change the nominal volumetric flow V V  of the recovery pump  24  to the ideal value V R . 
   Reference is now made to the flow chart of FIG.  2 . At the start of the program, the microprocessor  30  reads the signal V L  on the lead  31  as indicated by a block  44 . A determination is made as to whether any liquid is flowing by comparing V L  with zero, block  46 . If V L  = 0 , the process returns to the block  44 , via line  48 . 
   When block  46  indicates that V L &gt;0, a block  50  indicates that the microprocessor  30  reads the signals on the leads  38  and  42  respectively representing the temperature, T L , of the liquid and the temperature, T A , of the atmosphere. In block  52 , the signal supplied to the pulse drive source  32  is changed, if required, to a value reflecting the ratio of the liquid temperature to the vapor temperature. 
   Thus far, it has been assumed that the actual volumetric flow V A  of the recovery pump  26  corresponds precisely to the ideal value V R , but, as indicated previously, this may not always be the case because of differences between pumps and wear. If desired, the ideal value of V R  can be attained by the microprocessor reading the signal on the lead  27  representing actual vapor flow, V A , as Indicated by block  54 , comparing it with the Ideal value V R , which it has computed from T L , T A  and if need be by block  56 , and changing the signal supplied to the drive pulse source  32  to a value such that V A =V R , as indicated by a block  58 . The process then returns to the start at the block  44 . 
   Note that in the embodiment of the invention shown in  FIG. 1 , the electrical apparatus is enclosed in a non-hazardous zone  33  above a vapor barrier  35 . The fluid handling mechanical apparatus are enclosed below vapor barrier  35  in a hazardous zone  37 . Such an arrangement is useful for a gasoline dispenser, for example. 
   If correction for deviation of the actual volumetric flow, V A , from the ideal volumetric flow is not desired, the procedure can be returned to its start after the block  52  as indicated by the dashed line  62 . In either case, the process is repeated rapidly enough to follow changes in the volumetric flow of liquid V L  as well as changes in other parameters such as T L  and T A . 
   Multi-product Dispensers 
     FIG. 3  illustrates an embodiment similar to  FIG. 1 , focusing more on the application of the invention to a multi-product dispenser. The dispenser  200  is capable of dispensing different grades of gasoline through different hoses, supplied from different underground pipes  202 , 204 , 206 . Each underground pipe extends to a separate underground tank, as is conventional. In the depiction of  FIG. 3 , pipes  202 , 204  are shown merely in schematic form, but pipe  206  extends into the dispenser pass a meter  218  and is shown in schematic form extending to each of three hose assemblies  240 , 242 , 244 . As will be apparent, the depiction of  FIG. 3  is abbreviated for the sake of simplicity. A practical embodiment would have product supply pipe  202  connected with hose  240 , product supply pipe  204  connected with hose  242 , and product supply pipe  206  connected with hose  244 , each having a meter and without product intermingling. Each of the hoses  240 , 242 , 244  has a liquid supply path and a vapor recovery path terminating in a nozzle  8 , all the same as described above with respect to the embodiment of FIG.  1 . 
   Each hose is affixed to the upper piping housing  246  of the dispenser through a vapor valve/hose casting  248 . The vapor valve can be any suitable valve designed to shut off inactive hoses, when desired. For example, the valves may be product-operated valves, solenoid valves or the like. The vapor return lines extend past the valves  248  to a common manifold  250  which extends back down to the lower part of the dispenser  200  to vapor pump  224 . As can be appreciated, the single vapor pump  224  services all three of the hoses  240 , 242 , 244 . The inactive ones of the hoses are closed off by the closing of their associated vapor valves. If desired, vapor flow pressure sensors  252  may be included upstream of the pump  224  to provide pressure data back to controller  254 . 
   Also supplied to controller  254  are the outputs of pursers  256  linked to the liquid flow meters  218  in conventional fashion. Pulsers  256  are the conventional pulsers used in modern gasoline dispensers to provide an indication of the amount of liquid gasoline being dispensed. The output of the pulser is used to derive the display to a customer, not shown in  FIG. 3  of the amount of gas dispensed and its cost. The ambient temperature may be sensed by temperature sensor  240  and applied to the controller  254 , and the temperatures of the respective gasolines being dispensed may be sensed by sensors  258  and similarly applied to the controller  254 . 
   Controller  254  acts on the volumetric liquid flow rate output by pulser  256  to output a control signal to motor  234 . If desired, it may also act on the atmospheric temperature signal from sensor  240  and the product temperature from sensors  258 , as discussed with reference to FIG.  1 . Motor  234  has its shaft connected to vapor pump  224  across air gap  235 , as in the embodiment of FIG.  1 . Thus, the liquid flow rate as determined by the pulsers is used to drive the vapor pump  224  to retrieve all of the vapors generated approximate the nozzle  208  of the active hose. The vapor being drawn by the pump  224  comes only from the active hose by virtue of the closure of the valves of the two inactive hoses. The vapor may be pumped at a rate further modulated by the temperature sensing as indicated above, or by the pressure as sensed by pressure sensors  252 . 
     FIG. 5  discloses a further view of virtually the same invention as invention disclosed in  FIG. 3 , differing in only the level of detail depicted. Three underground tanks T 1 ,T 2 ,T 3  are provided having outlets driven by pumps P 1 ,P 2 ,P 3 , respectively, past flow meters M 1 ,M 2 ,M 3  to nozzles N 1 ,N 2 ,N 3 , respectively. The output of the three flow meters, entirely analogous to the pulsers and meters shown in  FIG. 3 , is applied to a digital processor  332 . The digital processor  332  drives a visual display  333 , as is conventional. Digital processor  332  also outputs a signal to a speed control  350  which, in turn, controls a motor  348  having its drive shaft connected to a vapor pump  346 . 
   Each nozzle has a liquid dispensing spout  338  and a chamber  337  to receive vapor displaced by the liquid being dispensed into an automobile nozzle. The chamber  337  communicates with a passageway  339  back to a manifold  344 . The spout  338  communicates with a liquid passageway  331  extending back to the respective flow meter M for that nozzle N. Each nozzle includes valves  334 , 335 , both actuated by a nozzle lever  336 . The valve  334  selectively permits passage of liquid out through the spout  338 . The valve  335  selectively permits return of vapor from the chamber  337 . As will be appreciated, the nozzle described in U.S. Pat. No. 4,199,012 or U.S. Pat. No. 4,429,725 may be substituted, in which the vapor valve opens as a response to the movement of liquid through the liquid portion of the nozzle, as distinct being rigidly linked as depicted in FIG.  5 . 
   The respective vapor paths  339  are joined at a manifold  344  from which the vapor is drawn by the vapor pump  346  back to a manifold  348  connected with the three tanks T 1 ,T 2 ,T 3 . A vapor pump operation sensor  352 , such as one or more of those described below with respect to  FIG. 4  may be included. These will shut down the operation of the dispenser by outputting a signal to the processor  332  when an error is detected. 
   In operation, the selection of one of the nozzles N 1 ,N 2 ,N 3  by a customer may permit the beginning of vapor recovery fuel dispensing. For example, if nozzle N 3  is selected, it is inserted into the filler pipe of the automobile gas tank. The plunger  336  is depressed, thereby opening the valves  334 , 335  of nozzle N 3 . Pump P 3  is activated in conventional fashion to pump liquid from tank T 3  through meter M 3  out through the spout  338  of nozzle N 3 . The movement of the liquid through the flow meter M 3  applies a signal to digital processor  332  to output a display of the quantity dispensed and its cost on display  333 . The digital processor  332  also outputs a signal to the speed control  350  to drive the pump motor  348  at a speed appropriate to draw vapor through the vapor pump  346  to retrieve substantially all of the vapor being displaced by the liquid emanating from the nozzle N 3 . Since the vapor valves  335  of nozzles N 1 ,N 2  are closed, all of the vapor being pumped by the pump  346  is drawn from the chamber  337  of nozzle N 3 . Downstream of the vapor pump  346 , the vapor is directed through the manifold  348  and is made available to the three tanks T 1 ,T 2 ,T 3 . However, since it is tank T 3  which is being depleted by the liquid pumping, the vapor in manifold  348  is preferentially deposited in the head space of tank T 3 , although it is also free to pass into tanks T 1 ,T 2 . 
   If desired, the output of the digital processor  332  and/or the speed control  350  may be modified in accordance with the temperature compensation or pressure compensation or other control features described above. 
   A modified embodiment of the invention is shown in schematic form in FIG.  4 . The fuel dispenser  110 , preferably a gasoline dispenser, is connected to a multiplicity of turbine pumps  8  in gasoline storage tanks  112 , 114 , 116  through pipes  118 , 120 , 122 , respectively. The pipes draw gasoline from the tanks and the respective liquid flow rates are measured in meters  124 , 126 , 128 . The fuel from the pipes is mixed in mixing manifold  130 . The mixing manifold has downstream of it a pipe  132  which outlets to a hose  134 , terminating in a controllable dispensing nozzle  138 . The nozzle  138  is provided with a vapor return line which connects with a vapor return hose  136  in the hose  134 , preferably concentrically within it. The vapor return line  136  connects with a vapor line  140  extending to a vapor pump  144 . An electrically operated solenoid valve  142  is provided in line  140  to close off the vapor line when not in use. 
   A conventional handle  164  is mounted in the outside wall of the dispenser  110 , on which the nozzle  138  can rest when not in use. As is conventional, the handle  64  is pivotally mounted, so it can be lifted after the nozzle is removed to activate a switch, and the activation of the switch is signalled along line  162  to a transaction computer  166 . 
   Controller  150  is provided with electrical connections  156  with the meters  124 , 126 , 128 , so that signals indicative of the liquid flow rate can be transmitted from the meters to the controller  150 . Preferably the meters  124 , 126 , 128  include pulsers, such as are commonly used in gasoline dispensers made by Gilbarco, Inc. The pulsers emit a pulse for every {fraction (1/1000)}th of a gallon of gasoline passed by the pump. Thus, as the fuel is being pumped, a pulse train is delivered on the respective lines of the connections  156 , with the pulse train frequencies corresponding to the liquid flow rate. The liquid pumps may, of course, be located in the dispenser  110 , or elsewhere, and may have the metering devices integral with them. As is conventional, the pulser data is accumulated to show the amount of fuel dispensed and its cost. This is not shown in  FIG. 4 , for simplicity. 
   Various other tank, pump and meter arrangements can also be used. In particular, the invention is useful for dispensers in which the output of each meter is passed to a separate hose, without any mixing. In such a case, the signals output on lines  156  will be exclusive; i.e. there will be a signal indicative of liquid flow only on one of the lines at a time. Dispensers of this type are sold by Gilbarco, Inc. under the MPD designation. 
   The vapor of the vapor pump  144  is transmitted along line  148  back to a storage vessel. The returning vapor can be transmitted via a manifold system to the plurality of tanks  112 , 114 , 116  as shown in  FIG. 5  or, as shown more simply in  FIG. 4 , to one tank. The manifold system is preferred. 
   Controller  150  also has a connection  141  to the valve  142  to open or close that valve, as desired. Controller  150  also has connections  158 , 160  to the transaction computer  166  which controls the overall operation of the dispenser  110 , in conventional fashion. Line  158  transmits signals from the transaction computer  166  to the controller  150  indicating that pumping is desired, and line  160  transmits signals from the controller  150  to disable pumping, when the controller  150  has ascertained that pumping should be disabled. This will be discussed in more detail later. 
   The vapor pump  144  is preferably a positive displacement pump, such as the Blackmer Model VRG3/4. It is driven by a motor  146 , preferably a brushless three-phase DC motor. The brushless DC motor  146  includes three hall effect sensors, one for each phase of the three-phase motor. These are used in conventional motor drive electronics in the controller  150  to apply appropriately phased power to the three phase motor  146 . The hall effect signals are a form of feedback and indicate the angular displacement of the motor. Rates of change of angular displacement signalled by the hall effect sensors by a pulse frequency are sent over lines  152  to the controller  150 . That is, the lines  152  provide a tachometer reading of the rate of rotation of the motor  146 . The motor drive electronics portion of the controller  150  outputs three-phase power over lines  154  to the motor to drive the motor as desired. Of course, if desired, the motor can be separately driven with a separately denominated motor drive which takes its instructions from the controller  150 . 
   The controller  150  plays a number of important roles which will be described in more detail in subsequent sections. However, to generalize, the flow rate of the liquid being pumped through the lines  118 , 120 , 122  as controlled by the transaction computer  166 , via a connection not shown, is transmitted to the controller  150  over lines  156 . The controller  150  evaluates the pulse trains  156  and output signals over lines  154  to the motor  146  to drive the vapor pump  144  at a rate correlated with the liquid pumping rate. Thus, generally the faster the liquid is pumped out, the faster the vapor is retrieved. 
   However, the controller  150  also includes circuitry to compare whether liquid is passing the meters  124 , 126 , 128  with whether the motor  146  is being driven. In the event that the motor  146  is running, and therefore pumping vapor back to the tank  116 , when liquid is not passing, the controller can disable the motor  146  to prevent the air from being pumped into the tanks  112 , 114 , 116 . Similarly, the controller  150  can combine the flow rates of multiple meters whose output is mixed, to get an overall liquid flow rate to output a proper vapor pump flow rate to the motor  146 . Further, the controller  150  ascertains when the liquid is passing the meters (or in an alternative embodiment, when the motor  146  is being driven) and passes a signal on line  141  to open the valve  142 . Further, the controller  150  includes circuitry which monitors the current drawn by the motor  146 . When the current is drawn at a rate which is uncharacteristic of normal vapor pumping, it can determine an error condition, such as liquid clogging the vapor return line and disable the vapor pump. 
   Vapor Line Valve 
     FIGS. 6A and 6B  depicts a circuit for opening the solenoid valve  142  ( FIG. 4 ) when vapor pumping is to be implemented. Various other hardware and software embodiments may be employed. In this embodiment, vapor pump rotation is detected by combining the tachometer feedback signals  152  from the hall effect sensors of motor  146  in exclusive OR gates. Thus, rotation becomes noticed by transitions at the output of the exclusive OR gate. One shot then converts the pulse train into a stable logic level signal by functioning as a retriggerable one shot whose period is greater than the typical minimum pulse width produced by the motor feedback signals during operation. This signal, the output of the one shot, is then used to gate the vapor solenoid valve by outputting the signal on line  141 . 
   It should be noted that alternately (or in conjuncton) the presence or detection of liquid fuel flow (i.e., the signals on line  156 ) may be substituted for (or logically combined with) the presence or detection of vapor pump motor rotation. This substitution (or combination) is possible because in a working system, vapor pump motor rotation will be a function of liquid fuel flow. 
   During periods of motor rotation where the vapor pump is actively moving vapors from the nozzle to the vapor return lines, the signal output on line  141  is true, and the vapor solenoid valve  142  may be opened with assured direction of flow. During periods of no motor rotation, that signal becomes false, closing the valve and preventing the escape of vapors via system back pressure. 
   The system eliminates the escape of vapors into the atmosphere during idle dispensing periods and eliminates the need for a check valve in the vapor return line or dispensing nozzle. Also, since the valve is not located in the nozzle, which is subject to accident, breakage and abuse, the cost of replacement of the nozzle is lessened by locating the valve in the dispenser. 
   The circuit shown in  FIGS. 6A and 6B  also includes an Error Status Latch  104 , which latches an error signal out to AND gate  106  to disable the motor drive electronics whenever any of the error conditions are noticed in NOR gate  108 . The latch is reset by a clearing input form the signals  156  when the liquid pump is next restarted. If the error is cleared, operation may resume. If not, the error will be detected and again disable the dispenser. 
   While the invention has been disclosed with respect to a particularly preferred embodiment, those of ordinary skill in the art will appreciate that the functionalities obtained can be obtained through numerous other systems, electrical, mechanical and hardware. The present invention is deemed to be broad enough to encompass apparatus of such sort. Similarly, the invention includes methods of operation of the vapor recovery liquid fuel dispenser as outlined herein. The circuitry has largely been described with reference to analog operation, but those of ordinary skill in the art will be able without undue experimentation to devise digital circuitry to accomplish the same functionalities, and such digital circuits are deemed to be within the scope of this invention.