Abstract:
A system and method of measuring volumetric flow through a meter. One step of the method involves providing a flow meter connected to a duct having fluid flowing therethrough. A memory having at least two Roshko/Strouhal curves stored therein is also provided. A viscosity of the fluid is measured. One of the Roshko/Strouhal is selected curves based on the method viscosity. A volumetric flow through the meter is determined utilizing the selected Roshko/Strouhal curve.

Description:
PRIORITY CLAIM 
     The present application is based upon and claims the benefit of U.S. provisional application Ser. No. 61/787,250, filed Mar. 15, 2013, incorporated herein by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to turbine flow meters for use in fuel dispensing environments. More particularly, the invention relates to a turbine flow meter adapted to have enhanced accuracy across a range of fluids with varying viscosities. 
     Examples of turbine flow meters are shown and described in U.S. Pat. Nos. 8,381,597, 8,096,446, 6,854,342, 6,692,535, 5,831,176 and 5,689,071 (each of which is incorporated herein by reference in its respective entirety for all purposes). A turbine flow meter can be used to measure the flow rate of a liquid, which can be used to derive the volume of the liquid that has passed through the meter. In some such meters, two turbine rotors are positioned in a meter housing along the liquid flow path. As liquid passes across the rotors, they rotate. The liquid passes through the first turbine rotor and is directed into the second turbine rotor such that the second turbine rotor rotates in a direction opposite from the first turbine rotor. 
     During calibration of the turbine flow meter, a known volumetric flow rate of liquid is typically passed through the meter. The rotational frequency of the turbine rotors is measured at various flow rates and frequencies to arrive at a combined “Strouhal” number. The combined Strouhal number is the frequency of the rotor(s) A and B (FA and FB) divided by the volumetric flow rate (Vf) as follows: 
     
       
         
           
             Strouhal 
             = 
             
               
                 FA 
                 + 
                 FB 
               
               Vf 
             
           
         
       
     
     The combined “Roshko” number for each of the Strouhal numbers is determined by dividing the sum of frequencies (FA+FB) by the viscosity (v) of the liquid, as follows: 
     
       
         
           
             Roshko 
             = 
             
               
                 FA 
                 + 
                 FB 
               
               v 
             
           
         
       
     
     The Strouhal and corresponding Roshko numbers are plotted on a Roshko-Strouhal (R-S) curve and/or are stored in an array of finite points with the Strouhal numbers being plotted in one axis or an array, typically in the y-axis, and the corresponding Roshko numbers being stored in another axis or corresponding array, typically the x-axis. During operation, the R-S curve is used to determine a Strouhal number from a calculated Roshko number as discussed below. 
     First, the rotational frequencies of the turbine rotors are measured. As is known in the art, pick-off coils or other sensing devices, such as Hall-effect sensors, are employed in the turbine meter to detect the rotation of the turbine rotors. The sensors detect the movement of each blade on the turbine rotor and can therefore determine the frequency of rotation. Once the rotation frequencies of the turbine rotors are measured, the Roshko number can be determined according the formula for the Roshko number shown above. After the Roshko number is calculated, the corresponding Strouhal number is determined. The Strouhal number and the frequency of the turbine rotor are then used to determine the volumetric flow rate according to the rearranged Strouhal formula below: 
     
       
         
           
             Vf 
             = 
             
               
                 FA 
                 + 
                 FB 
               
               Sr 
             
           
         
       
     
     The volumetric flow rate calculation is repeated continuously so that the volumetric flow rate of liquid flowing through the turbine meter is known at any given time. The volume of the liquid can be derived from the volumetric flow rate using time as is well known. 
     If the turbine flow meter is used in an application in which the liquid flow is of varying viscosities, such as in a fuel dispenser, it can be more difficult to accurately measure flow rate and volume. In known systems, a single Roshko/Strouhal curve is used by assuming a “standard” viscosity. Although the use of standard viscosity may be effective at higher flow rates, it may introduce significant error into the volumetric measurements at low flow rates (e.g., less than 2 gpm). This is because differences in viscosity result in smaller Roshko/Strouhal differences at higher flow rates rather than at lower flow rates. 
     Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the present invention provides a method of measuring volumetric flow through a meter. One step of the method involves providing a flow meter connected to a duct (i.e., flow passage) having fluid flowing therethrough. A memory having at least two Roshko/Strouhal curves stored therein is also provided. A viscosity of the fluid is measured. One of the Roshko/Strouhal is selected curves based on the measured viscosity. A volumetric flow through the meter is determined utilizing the selected Roshko/Strouhal curve. 
     Another aspect of the present invention provides a turbine flow meter including a controller in which multiple calibration curves corresponding to multiple viscosities are stored. During use, a actual viscosity is determined during valid flow, and is used to select a calibration curve. 
     Other objects, features and aspects of the present invention are provided by various combinations and subcombinations of the disclosed elements, as well as methods of practicing same, which are discussed in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
         FIG. 1  is a diagram of a fuel dispenser using a pair of turbine flow meters constructed in accordance with an embodiment of the present invention; 
         FIG. 2  is a perspective diagrammatic view of a turbine flow meter constructed in accordance with an embodiment of the present invention; 
         FIG. 3  is an illustration of a flow pattern of a turbine flow meter constructed in accordance with the embodiment of  FIG. 2 ; and 
         FIG. 4  is a graphical representation of Roshko-Strouhal (R-S) curves that may be utilized with the flow meter of  FIG. 2 . 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
       FIG. 1  illustrates a pair of turbine flow meters  10 A and  10 B in a fuel dispenser  40 . The purpose of a fuel dispenser  40  is to dispense and measure fuel delivered to a vehicle (not shown). Accurate meters are required to measure fuel dispensing to comply with Weights &amp; Measures regulatory requirements. 
     Fuel dispenser  40  may be a blending type fuel dispenser wherein a low-octane fuel  41  stored in a low-octane underground storage tank  42  and a high-octane fuel  43  stored in a high-octane underground storage tank  44  are blended together by fuel dispenser  40  to deliver either a low-octane fuel  41 , high-octane fuel  43 , or a mixture of both to a vehicle. Low-octane fuel  41  is supplied to fuel dispenser  40  through a low-octane fuel supply conduit  46 . Likewise, high-octane fuel  43  is delivered to fuel dispenser  40  through a high-octane fuel supply conduit  48 . Both low-octane fuel  41  and high-octane fuel  43  pass through fuel dispenser  40  in their own independent flow paths. Each fuel  41 ,  43  encounters a respective valve  50 ,  52  that controls whether the fuel is allowed to enter into fuel dispenser  40 , and if so at what flow rate. Valves  50 ,  52  may be proportionally controlled and may be under the control of a controller  60  in fuel dispenser  40  via control lines  62 ,  64 . 
     Controller  60  determines when a fueling operation is allowed to begin. Typically, a customer is required to push a start button  78  to indicate which octane of fuel  41 ,  43  is desired, and controller  60  thereafter controls valves  50 ,  52  to allow low-octane fuel  41  or high-octane fuel  43  (or both) to be dispensed, depending on the type of octane of fuel selected by the customer. After fuel  41 ,  43  passes through both valves  50 ,  52  (if a blended octane fuel was selected by the customer), fuels  41 ,  43  flow through turbine meters  10 A,  10 B. (If only a low-octane fuel  41  or high-octane fuel  43  was selected by the customer to be dispensed, controller  60  would only open one of the valves  50 ,  52 .) As fuels  41 ,  43  flow through turbine meters  10 A,  10 B, sensors  29 ,  30  ( FIG. 2 ) on each of turbine meters  10 A,  10 B produce a respective signal  66 ,  68  that is input into controller  60 . Using the technique described herein, controller  60  determines the amount of flow of fuel flowing through turbine meters  10 A,  10 B for the purpose of determining the amount to charge a customer for delivery of such fuel. Controller  60  uses the data from the signals  66 ,  68  to generate a totals display  70 . Totals display  70  comprises an amount to be charged to the customer display  72 , the quantity of gallons (or liters) dispensed display  74 , and the price per unit of fuel display  76 . 
     Also, U.S. Pat. No. 4,876,653 entitled “Programmable multiple blender” describes a blender operation like that illustrated in  FIG. 1  and is incorporated herein by reference in its entirety for all purposes. As either low octane fuel  41 , high-octane fuel  43 , or both pass through their respective turbine meters  10 A,  10 B, the fuels come together in the blend manifold  54  to be delivered through a hose  56  and nozzle  58  into the vehicle. 
     In other embodiments, meter  10  may be used in a vent stack of a underground storage tank at a service station. It may be desirable to measure the amount of air flowing through a vent stack using meter  10  to determine how often and how much air is separated by a membrane and released to atmosphere for any number of diagnostic or information purposes. The membrane may either permeate hydrocarbons or permeate oxygen or air as disclosed in U.S. Pat. Nos. 5,464,466 and 5,985,002, each of which is incorporated herein by reference in their entirety for all purposes. In other embodiments, meter  10  may measure the amount of vapor being returned to the underground storage tank in a Stage II vapor recovery system. 
       FIG. 2  illustrates meter  10  constructed in accordance with an embodiment of the present invention. Meter  10  includes a housing  12  that forms an inlet port  14  and an outlet port  16  for ingress and egress of fluid (liquid or gas), respectively. A stationary shaft  18  is located inside of housing  12  to support a pair of turbine rotors  20  and  21  that rotate in a plane perpendicular to the axis of shaft  18 . Generally, a bearing set will be interposed between each of the rotors and the shaft  18  to facilitate the respective rotor&#39;s rotation. As shown, rotor  20  is located slightly upstream of rotor  21 . Accordingly, rotor  20  may be referred to as the “first turbine rotor” (rotor A) with rotor  21  being referred to as the “second turbine rotor” (rotor B). 
     Rotor  20  includes one or more vanes  22  (also known as blades) that cause rotation when impinged by the flowing fluid. Similarly, rotor  21  includes one or more vanes  23 . Referring now also to  FIG. 3 , vanes  22  and  23  are preferably spaced evenly around the periphery of the respective rotor hub. In addition, vanes  22  of rotor  20  are preferably canted oppositely from vanes  23  of rotor  21 . This orientation of vanes  22  and  23  causes the two rotors to rotate in opposite directions (shown by arrows  32  and  36 ) at a rotational speed proportional to the fluid flow rate. For example, a controller can determine the ratio of the rotation speed of one turbine rotor to the other in order to determine the fluid flow rate. 
     In the illustrated embodiment, fluid entering housing  12  through inlet port  14  will encounter rotor  20 . Because vanes  22  are canted, the straight fluid flow is converted into a generally swirling pattern with an angular trajectory based on angle  27  of vanes  22 . This angular trajectory is generally oblique to the longitudinal axis of meter  10  (shown as “A-A”). After passing through rotor  20 , the fluid impinges vanes  23  of rotor  21 . The angular trajectory of the flow due to rotor  20  increases the fluid&#39;s angle of incidence with vanes  23 . As the angle of incidence increases, the driving force used to impart rotational movement on turbine rotor  21  also increases. 
     The rotational speeds of rotor  20  and rotor  21  are determined by counting the vanes  22  and  23  passing by respective sensors  29  and  30  mounted to housing  12 . In some embodiments, the ratio of the respective rotational speeds may be used to determine the flow rate of the liquid or gas passing through the meter  10 . Instead of sensors  29  and  30 , any other suitable technique for detecting the rotation of the rotors  20  and  21  may be used. 
     In some embodiments, sensors  29  and  30  are pickoff coils that generate a magnetic signal which penetrates through housing  12  to reach the turbine rotors  20  and  21 . As the rotors  20  and  21  rotate, vanes  22  and  23  superimpose a pulse signal on the magnetic signal generated by pickoff coils  29  and  30 . The pulse signal may then be delivered to controller  60 . Controller  60  uses the pulse information to determine the frequency (Fr) of the rotor(s). At lower flow rates, rotor  20  may not be spinning. In such instances, the frequency of rotor  21  may be used by controller  60 . Controller  60  also determines the viscosity (v) of the fluid flowing through meter  10  using any acceptable technique, such as a viscosity sensor. 
     As shown in  FIG. 4 , multiple R-S curves are stored in memory  61  associated with the meter and, preferably, with controller  60 . The R-S curves  100 ,  102 , and  104  are shown in a graphical representation of a line in two-dimensions, where for a given Roshko number in the x-axis  106 , a corresponding Strouhal number in the y-axis  108  can be ascertained. 
     Each R-S curve shown in  FIG. 4  corresponds to a different fluid viscosity. For instance, curve  102  represents a R-S curve with a first viscosity, curve  102  represents a R-S curve with a second viscosity, and curve  104  represents a R-S curve with a third viscosity. The number of curves stored in memory is limited only by practical limitations, such as memory space and processing speed. It is desirable to have a large number of R-S curves stored into memory. Therefore, certain embodiments will contain more than the three R-S curves displayed in  FIG. 4 . 
     The range of viscosities represented by the R-S curves will typically include the values of viscosities to be expected during fueling applications. These differing R-S curves are produced during the calibration process. It is understood that the R-S curves ray be produced using different techniques during calibration. For instance, in some embodiments process fluids with differing viscosities may be used to ascertain the R-S curves. In other embodiments the same process fluid may be used throughout the calibration process but at varying temperatures and utilizing some temperature compensation. At lower flow rates, the curves may be determined using only FB (which may be the only one available as FA is zero at low flow rates). The combined Roshko number is preferably used for higher flow rates. 
     During operation, the viscosity (v) of the fluid flowing through meter  10  is measured during a valid flow rate (as soon as FA&gt;0). Based on measured viscosity (v) of the fluid flowing through meter  10 , controller  60  determines the R-S curve that represents that particular viscosity. In the event a R-S curve representing the measured viscosity is not stored in memory, controller  60  will preferably select the R-S curve representing the viscosity of a fluid that is closest to the measured viscosity. One skilled in the art would appreciate that by increasing the number of R-S curves stored in memory, the overall accuracy of the meter will tend to improve. 
     The selected R-S curve can then be used to determine the volumetric flow rate (Vr) of the liquid flowing through the meter and in turn calculate the volume (V) of the liquid. To determine volumetric flow rate (Vr), controller  60  utilizes the R-S curve representative of the viscosity closest to the measured viscosity of the flowing fluid. The Roshko number can then be determined according the rotor frequency and fluid viscosity, as shown in the formula for the Roshko number shown above. Viscosity (v) in the denominator will vary for each curve stored in memory. After the Roshko number is calculated, the corresponding Strouhal number is determined using the selected R-S curve or equation. The Strouhal number and the frequency of the turbine rotor are then used to determine the volumetric flow rate according to the equation discussed above. 
     The volumetric flow rate calculation is repeated continuously in periodic time increments so that the volumetric flow rate of liquid flowing through the turbine meter is known at any given time. The volume of the liquid can be derived from the volumetric flow rate using time as is well known. In the example of a fuel dispenser such as the dispenser shown in  FIG. 1 , the volume (V) of liquid can then be communicated to a control system or other electronic circuitry to display the volume of fuel dispensed on the dispenser. 
     While preferred embodiments of the invention have been shown and described, modifications and variations may be made thereto by those of ordinary skill in the art without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limitative of the invention as further described in the appended claims.