Abstract:
A method and apparatus for measuring flow rates is disclosed. At least one of a non-vortex based low-flow measurement and a vortex based flow measurement is taken. The measurement or measurements are communicated to a central processor. The processor analyzes the data provided and makes a determination as to the more accurate measurement, if there is more than one measurement. The flow rate is calculated by the processor and is based on either the singular or the more accurate measurement. Further enhancements include diagnosis of meter performance, redundancy of measurements, and re-calibration of either measurement device based on information from the other device.

Description:
This application is a Continuation of U.S. patent application Ser. No. 09/557,352 entitled Low-Flow Extension For Flow Measurement Device, filed on Apr. 25, 2000 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to measuring devices, and more particularly to the combination of multiple measurement devices to seamlessly measure a range of flow rates. 
     BACKGROUND OF THE INVENTION 
     In general, fluid flows through an area, for example a pipe, in a substantially axial flow pattern if there are no obstructions or other external forces. An object placed in that axial flow pattern creates a disturbance. Vortices then result along both sides of the object as the fluid flows past. Each vortex created sheds from the object as the fluid flow carries it downstream. The generation and shedding of the vortices alternates between the two sides of the object and is continuous with the flow of fluid past the object. 
     It is possible to sense and measure a low-pressure area associated with the vortex in the fluid flow. It is this characteristic that is the foundation for vortex flowmeters. In a vortex flowmeter the design includes an object, otherwise known as a bluff body, placed in the flow of the fluid. Both sides of the bluff body alternately generate vortices and subsequently shed them. A pressure sensor, such as, for example, a pressure transducer, positioned downstream of the bluff body senses each vortex that is shed from the object. Each time a vortex flows past the pressure transducer, it causes the pressure transducer to generate a pulse having an amplitude proportional to the fluid density and the square of the fluid flow rate. The vortex shedding frequency, i.e., the rate at which vortices are shed, is proportional to the fluid flow rate. 
     When there is sufficient Reynolds Number and fluid velocity to consistently generate vortices, for example a Reynolds value of 5,000 or higher, simple calculations utilize the vortex shedding frequency to determine the flow rate of a fluid, so long as the rate is constant or has a relatively slow rate of change. 
     However, if the Reynolds Number of the fluid is generally less than 5,000, the generation of vortices will be either inconsistent, too miniscule for the sensor to measure, or non-existent. A Reynolds Number higher than 5,000 with a low fluid velocity will also create such conditions because fluid velocity that is too low will prevent the sensor from functioning correctly. This is a common problem in vortex metering, which makes it prohibitive to utilize vortex meters for metering situations in which a wide range of flow rates is occurring that includes low-flow rates less than the flow velocities at which consistent, measurable vortices are generated. 
     This barrier creates several inconveniences to users of vortex flowmeters, which results in the elimination of vortex metering as an option for many applications. Some examples include applications having start-up modes, batching, or intermittent flow rates. 
     For the foregoing reasons, as well as others not discussed, there is a need for a measuring device an instance of which is a flowmeter having the reliability and features of a vortex-metering device at normal flow conditions, with the added feature of being able to measure flow rate during low-flow to zero-flow conditions. 
     SUMMARY 
     The present invention is directed to a measuring device for measuring, e.g. fluid flow. The basic structure of one embodiment includes a vortex flow measuring device, and a non-vortex measuring device. The two metering devices are in communication with a common processor to form a flowmeter. 
     In a further embodiment, the flowmeter includes a low-flow measuring device with a usable range of flow measurement which at least partially overlaps with a usable range of flow measurement of the vortex flow measuring device. In still a further embodiment, the vortex flow measuring and non-vortex measuring devices are each maintained in separate housings. In still another embodiment, the vortex flow measuring and non-vortex measuring devices are each maintained in a single housing. In yet further embodiments, the processor is maintained in one of the separate housings, or alternatively, in the single housing. In still further embodiments, the low-flow measuring device utilizes thermal flow, pressure drop, ultrasonic, or magnetic sensing technology. 
     The flowmeter is utilized in measuring flow rate of a fluid flow. To do so, in one embodiment, the flowmeter utilizes at least one low-flow measurement device and at least one vortex measuring device to attempt measurement of a fluid flow rate. To determine a flow rate, a measurement value is obtained from one of the measuring devices. This measurement is communicated said measurement value to a processor, and interpreted by the processor. An indication of flow rate is determined from the measurement value. 
     In still another embodiment, multiple measurement values are obtained and interpreted with the processor to determine which value is a more substantially accurate representation of the flow rate. A flow rate is then identified based on the representation. In yet another embodiment, multiple measurement values are obtained and interpreted with the processor to determine which is more substantially accurate. Subsequently, the more accurate measurement value is utilized in re-calibrating other of the measuring devices. In still a further embodiment, a plurality of measurement values are interpreted and manipulated to create various reports, charts, tracking information, and analyses of a flow stream. In still a further embodiment, multiple sensors provide measurement values to a network in which a network processor performs desired calculations to determine output such as flow rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned features and advantages, and other features and aspects of the present invention, will become better understood with regard to the following detailed description and accompanying drawings, wherein: 
         FIG. 1  is a block diagram illustrating a first example configuration of a flowmeter embodiment; 
         FIG. 2  is a block diagram illustrating a second example configuration of a flowmeter embodiment; 
         FIG. 3  is a block diagram illustrating a third example configuration of a flowmeter embodiment; 
         FIG. 4  is a graph illustrating an example low-flow measurement in conjunction with vortex flow measurement without electronic correction; 
         FIG. 5  is a graph illustrating an example low-flow measurement in conjunction with vortex flow measurement and with electronic correction; and 
         FIG. 6  is a flow diagram of one embodiment of a flowmeter with electronic correction pursuant to the present invention; 
         FIG. 7  is a block diagram illustrating a fourth example configuration of a flowmeter embodiment; 
         FIG. 8  is a block diagram illustrating a fifth example configuration of a flowmeter embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings wherein like parts are designated by like reference numerals throughout,  FIGS. 1-7  illustrate a vortex-shedding-based flow measurement device  12  in combination with a non-vortex-shedding-based flow measurement device  14 . Both devices are in communication with each other, and/or a common processor, to form a single metering unit  19 . To summarize, the vortex-shedding-based flow measurement device  12  measures relatively higher flow rates; while the non-vortex-shedding-based flow measurement device  14  measures relatively low flow rates; the two measurements being electronically combined into a single, wide flow range measurement. 
     As previously discussed, vortex flowmeters rely on the physical principle that as a generally axial flow fluid flows past an object, vortices are created along each of the two sides of the object. The vortex shedding frequency is directly proportional to the rate of the flow. Some other sensing means capable of utilization for detecting vortex shedding include but are not limited to, pressure (including capacitive and piezoelectric), thermal, and ultrasonic. 
     In  FIG. 1 , a block diagram illustrates a first example embodiment of the present invention. Fluid (not shown) flows in the direction of Arrow A through pipe  10 . A vortex-shedding-based flow measuring device  12  is placed in line with pipe  10 . The fluid flows through vortex measuring device  12 . If the Reynolds Number and/or fluid velocity are sufficient to create measurable vortices, vortex measuring device  12  senses the vortices and generates a signal relating to a flow rate. A processor  18  receives the signal. Different methods for communicating the signal include, but are not limited to, wire, fiber-optics, radio frequency, infra-red, internet, intranet, or even telephonic including cellular or digital transmission. 
     If the fluid Reynolds Number and/or velocity are less than that required for generation of vortices that the measuring device is capable of sensing, then vortex flow measuring device  12  will not register any vortices and an appropriate signal will be communicated to processor  18  indicating that no measurement is available. Alternatively, the measuring device  12  sends no signal to processor  18 , which would similarly indicate a condition of no available measurement. 
     Whether or not vortex flow measuring device  12  obtains a vortex measurement, the fluid continues past vortex flow measuring device  12 , through pipe  10 , to non-vortex-shedding-based flow measuring device  14 , acting as the low-flow device, to complete the metering unit  19 . In this example, non-vortex flow measuring device  14  measures low-flow to zero flow conditions. If the rate of the flow is such that it is low enough to register on non-vortex flow measuring device  14 , non-vortex flow measuring device  14  will measure the flow and communicate a flow rate related signal to processor  18 . If the flow rate is such that it is faster than a rate measurable by non-vortex flow measuring device  14 , then non-vortex flow measuring device  14  will not register a valid flow rate. At such time, either no signal will be communicated by non-vortex flow measuring device  14  to processor  18 , or perhaps a signal indicating that no reading is available will be communicated to processor  18 . The fluid will continue to flow downstream. 
     In  FIG. 2 , a block diagram illustrates a second example embodiment. In this embodiment, the fluid flows in the direction of Arrow A through pipe  10 . Combined flowmeter  16  is placed in line with pipe  10 . As fluid flows through pipe  10 , it enters combined flowmeter  16 . Combined Flowmeter  16  is comprised of a vortex shedding type measuring device (not shown), and a non-vortex shedding type measuring device (not shown) as the low-flow device, both contained within one flowmeter body and in combination with the processor  18 , forming the entire metering unit  19 . The fluid flows through the two measuring devices, and out the downstream side of the combined flowmeter  16 . Dependant upon the Reynolds Number and/or velocity of the fluid, one of the two measuring devices within combined flowmeter  16  will provide an accurate reading of the flow rate, and a signal will be sent to processor  18 , or no signal will result. As illustrated, processor  18  is not physically a part of the flowmeter  16 ; rather it is in a separate location upstream or downstream of the flowmeter  16 . Alternatively, processor  18  could be internal to flowmeter  16 . 
     In  FIG. 3 , a block diagram illustrates a third example embodiment. In this embodiment, the fluid again flows in the direction of Arrow A through pipe  10 . However, in this example, the low-flow non-vortex flow measuring device  14  is first in the pipeline and is the low-flow device. The second meter through which the fluid flows is the vortex shedding type flowmeter. Both meters still function together to create a single metering unit  19 . Again, depending on the fluid flow rate, one or the other of the two flow measuring devices  12 ,  14  will obtain a more accurate measurement and deliver a signal to processor  18 . 
     The choice, based on operating specifications, of actual vortex flow measuring device  12  and non-vortex flow measuring device  14  as the low-flow device, or the combined flowmeter  16 , is important to the final effect of the two devices functioning together along with processor  18  as one metering unit  19 . The range of flow velocities that will be measured is also a consideration in making the choice of device  12 ,  14 . Ideally, there should be some overlap  20  ( FIGS. 4 and 5 ) in the range of flow velocities measurable by each of the vortex flow measuring device  12  and the non-vortex flow measuring device  14 . One of the two devices will act as the primary measurement device. As a flow rate increases or decreases through overlap  20 , a transition occurs for the primary measurement device from vortex flow measuring device  12  to non-vortex flow measuring device  14 , or vice versa, depending on the direction of the change in fluid flow rate. The existence of overlap  20  makes it possible to take measurements from either of the flow measuring devices  12 ,  14 , when they are not at their extreme measurement capabilities where there is an increased risk of obtaining less accurate readings. Rather, as the flow rate approaches one flow measuring device&#39;s  12 ,  14  range limit, it simultaneously enters the other flow measuring device&#39;s  12 ,  14  usable range. This way the primary measurement is always taken from the device that is more likely to have a more accurate measurement based on its performance specifications and the Reynolds Number and/or velocity of the fluid. 
     Processor  18  will take the signals it receives from each of the meters, and transform the data into the desired information. Among some of the tasks that could be performed by the processor  18  and any surrounding electronics is adjusting the span of the low-flow measuring device to remove any discontinuities. A chart in  FIG. 5  depicts an adjusted span. As stated previously, the primary flow measurement at any one point in time is the more accurate flow measurement. While the measurements indicated are in the overlap  20  area, there will likely exist a deviation between what is acting as the primary measurement and the other measuring device. Adjusting or calibrating the non-primary measurement device produces agreement between the primary and non-primary measurements. Alternatively, assuming one has prior knowledge that a bias is likely, a adjusting for that bias brings the two measurements into agreement. If two points in the overlap  20  area are available with sufficient space between, then a linear fit with bias and span brings about the desired result. For the span adjustment process to occur, the processor first must interpret readings from both measuring devices and select one or the other as the primary measurement. 
     Modification of signals also adjusts for hysteresis issues passing through the overlap  20  area. In the overlap  20  area automatic or triggered re-calibration of the low-flow measurement by the primary measurement is also possible. Conversely, redundant measurement for a diagnostic of the primary measurement is also possible utilizing the overlap  20  with the low-flow measurement. 
     These various options of redundancy and re-calibration make possible a highly accurate, highly reliable, flow-measuring device without the high cost of more complex metering devices. Further, the use of meters such as a vortex flow measurement device to periodically re-calibrate the low-flow measurement device allows the utilization of a less expensive low-flow technology without sacrificing accuracy of measurements. 
     In  FIG. 6  a flow diagram represents an example of how one embodiment of the present invention processes measurement signals. Measurement devices send vortex signal  30  and a non-vortex low-flow signal  32  to processor  18 .  FIG. 6  illustrates the decision tree that processor  18  implements to determine a final course of action. If processor  18  determines a low-flow condition exists, the processor  18  accesses the low-flow signal, passes the signal through the span adjust  34 , and under the low flow zone branch  36  of the decision tree, transmits the flow rate. If processor  18  determines a flow condition in overlap  20  range, then according to the overlap flow zone branch  38  of the decision tree either the processor  18  re-calibrates the low-flow measuring device  14  based on the vortex measuring device  12 , or validates the vortex measuring device  12  as the primary sensor and transmits the flow rate. If processor  18  determines there to be a flow rate in the normal or primary range, the processor  18  implements the primary flow zone branch  40 , accesses the vortex measuring device  12 , and transmits the flow rate. 
     In general, vortex measuring devices have very simple construction. The accuracy of vortex measuring devices is typically +−1 percent or better. Vortex measuring devices work equally well on liquids and gases. In addition, the measured fluid powers the vortex meter. 
     The low-flow measuring devices utilized in this type of application can vary in their measuring technology. The sensors should be relatively inexpensive and non-complex. Suitable technologies will depend on the actual environment in which the measuring is taking place, but could include thermal flow sensing, pressure drop/drag force, ultrasonic, magnetic, or a less accurate utilization of the vortex shedding sensor signal. 
     This invention anticipates the use of a plurality of vortex and non-vortex flow measuring devices on a single flowstream able to communicate with a central processor. The devices would measure the flow in different ranges. As the flow velocity moves into a known overlap range, the primary flow measuring device downloads a function block from the secondary flow measuring device which instructs the secondary device whether to become the primary device. Meanwhile, the secondary device downloads a calibration function block from the primary device to calibrate its measurements when desired. In fact, a plurality of flow measurement devices could exist along a flow process. Given the various flow rate measurements, and the position of each of the flow measuring devices, the processor could determine the primary measurement device at any one point in time, and calibrate the remaining devices based on the primary measurement. The processor could use the data from the multiple measuring devices to determine the flow rate via an averaging scheme. The processor could generate various charts, graphs, and plots of the flow data at each measurement position for analysis and comparison of flow conditions. 
     To further expand upon this feature of the invention, this vortex-shedding-based flow measurement and a non-vortex-shedding-based low-flow measurement seamless measuring device can be an element in an all digital, serial, two-way communication system or network  22  interconnecting sensors, actuators, arid controllers. As illustrated in  FIG. 7 , this invention anticipates meters  12 ,  14  in a flow stream. The meters  12 ,  14  transmit measurement readings into the network  22 . The network  22  could even be a smaller portion of a larger industrial control system. A plurality of meters and meter combinations could all contribute measurement readings to a network  22 . The processor  18  would then communicate with the network  22  to obtain the necessary data. The physical placement of the network  22  and the processor  18  with respect to the meters  12 ,  14  becomes irrelevant in this embodiment so long as there is a manner by which they all can communicate. Whatever information is gathered is simply contributed to the network  22 , and the processor  18  then communicates with the network  22  to obtain whatever measurements it requires. The information could also include measurements of individual pressure sensors, valve positioners, etc. The processor  18  simply gathers information from various points of the network  22  and generates various calculations, analyses, data manipulations, and reports. The sensors that are positioned throughout a system can measure any number of different conditions. 
     In one embodiment, processor  18  could be a processor within the network. The processor  18  executes a function block which receives measurements from the sensors or meters  12 ,  14 , combines them in accordance with the invention, and produces a flow output accessible by other entities and/or function blocks in the system. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved. It is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.