Fluid flow measurement correcting system and methods of constructing and utilizing same

[0002] This application is a continuation-in-part of U.S. patent application Ser. No. 08/685,502 filed Jul. 24, 1996, now U.S. Pat. No. 5,691,914, issued Nov. 25, 1997.

[0003] 1. Field of Invention

[0004] The present invention relates to a system for measuring parameters relating to fluid flow in a channel, and in particular to a system which provides an accurate measurement of average fluid flow rate which takes into account the effect of drawdown.

[0005] Flow of material through a partially filled channel or pipe is expressed by the equation

[0006] Q=A*V,

[0007] where Q represents the rate of flowable material through the channel; V represents the average velocity of material in the channel; and A represents the cross-sectional area of material in the channel, or wetted cross-sectional area. For a channel or pipe having a substantially circular cross-section, the wetted cross-sectional area may be expressed as

[0008] A=R²* cos⁻¹((R−L)/R)−(R−L) * (2*R*L−L²)⁵,

[0009] where R represents the actual channel or pipe radius and L represents the depth of the flowable material in the channel (FIG. 3). It is noted that similar equations exist for calculating the wetted cross-sectional area of channels having different cross-sectional shapes. As a result, an accurate measurement of the depth of flowable material in the channel is essential in determining flow through a partially filled channel.

[0010] Pressure sensitive devices exist which, when placed in a fluid channel, determine fluid depth by measuring the pressure exerted on the device due to the fluid overhead. Once the fluid pressure is measured, fluid depth L may be calculated as

[0011] L=27.681*P,

[0012] where P represents the measured pressure acting on the probe, in psi.

[0013] Pressure sensitive devices which measure fluid depth in a channel are often placed in a stilling well—a calm, isolated area which is adjacent a stream of flowing fluid and which has a fluid level which is substantially the same as the fluid level in the stream. In some instances, however, the pressure sensitive devices are placed directly into the stream of flowing fluid, in which case their fluid depth measurements are affected by a phenomenon known as drawdown.

[0014] Drawdown is caused by the presence of objects in the stream of fluid flow. Specifically, drawdown is an effect that occurs in nonhydrostatic conditions in which the pressure exerted on the probe by the flowing fluid is actually less than the ambient pressure in the stream due to the streamlines of flow being disturbed by the probe. If the presence of the probe causes any disturbance in the streamlines of flow (FIG. 4), drawdown will occur regardless of the streamlined nature of the probe. With the pressure sensitive probe measuring fluid pressure that is less than the actual ambient pressure in the stream of fluid, fluid depth in the channel is under-represented, thereby leading to an inaccurate flow rate computation. The present invention is directed at substantially eliminating the inaccuracies associated with fluid depth measurements by taking into consideration the effects of drawdown.

[0015] 2. Description of the Relevant Art

[0016] There are known probes which measure fluid flow. For example, U.S. Pat. No. 5,506,791 discloses a multi-functional device having a pressure sensitive probe for measuring fluid depth in a channel.

[0017] The above-identified reference, however, fails to disclose or otherwise suggest a system for measuring fluid velocity in a channel which takes into account the effects of drawdown.

[0018] The present invention overcomes the above-discussed shortcomings of prior fluid flow measuring devices and satisfies a significant need for accurately registering fluid level by considering the effects of drawdown acting on pressure sensitive devices.

[0019] According to the invention, there is provided a fluid flow rate measurement system, comprising a submerged probe member having a means for determining fluid depth in a channel and having a height to width ratio of less than 0.52, wherein said probe height and width are positioned substantially perpendicular to the direction of the main fluid flow in said channel; a means for measuring average fluid velocity in the channel; a means for correcting the fluid depth determination to account for drawdown conditions acting on the probe member; and a means for calculating average fluid flow rate based upon the fluid depth correction and the average fluid velocity measurement.

[0020] In use, the probe member is first analyzed in order to determine and record the extent of drawdown acting thereon over a range of fluid velocities. Next, the probe member is placed within the fluid channel and connected to the fluid depth determining means. Thereafter, the probe member measures fluid pressure and transmits the measured pressure data to the determining means for computing fluid depth. The fluid velocity measuring means transmits average fluid velocity measurements to the fluid depth correcting means so that it computes a fluid depth value which accounts for drawdown. The flow rate calculating means then computes average fluid flow rate based upon the average fluid velocity measurement and the corrected fluid depth value.

[0021] It is an object of the present invention to provide a system which accurately measures fluid flow rate in a channel.

[0022] Another object of the present invention in to provide such a system which considers the effect of drawdown in measuring average fluid flow rate.

[0023] It is another object of the present invention to provide a system which provides accurate fluid flow rate measurements in real time.

[0024] A further object of the present invention is to provide a system as described above wherein very low profile probes are used such that the height to width ratio of the probe is less than 0.52.

[0025] Yet another object of the present invention is to provide a system described above, wherein the offset fluid depth equation is a fourth order equation.

[0026] Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, disclose preferred embodiments of the present invention.

[0027] FIG. 1 is a perspective view of a probe member of the present system.

[0028] FIG. 2 is a perspective view of another probe member of the present system.

[0029] FIG. 3 is a cross-sectional view of a closed channel in which fluid flow is to be measured.

[0030] FIG. 4 is a side elevational view of a closed channel having therein a preferred embodiment of the present invention.

[0031] FIG. 5 is a flowchart depicting the correcting means of a preferred first embodiment of the present invention.

[0032] FIG. 6 is a graph showing the relationship between measured average fluid velocity and fluid level error due to drawdown, for a probe member of the present system.

[0033] FIG. 7 is a block diagram showing components of some of the preferred embodiments of the present invention.

[0034] FIGS. 8 through 12 show various views of a very low profile pressure transducer area velocity probe having a height to width ratio of less than 0.52.

[0035] FIGS. 13 through 16 show various views of a very low profile bubbler area velocity probe having a height to width ratio of less than 0.52.

[0036] FIG. 17 illustrates a schematic of a pipe setup for testing the very low profile probes depicted in FIGS. 8-16.

[0037] FIGS. 18 and 19 show further details of the custom fitted pipe bracket used in the testing procedure.

[0038] FIG. 20 shows a graph depicting the relationship between velocity and level correction for the bubbler probe depicted in FIGS. 13-16.

[0039] FIG. 21 shows a graph depicting the relationship between velocity and level correction for the pressure probe illustrated in FIGS. 8-12.

[0040] Referring to FIGS. 1-7, there is shown a fluid flow rate measuring system according to the preferred first embodiments of the present invention, comprising a submerged probe member; a means for measuring average fluid velocity in a channel; a means for determining fluid depth in the channel; a means for correcting the determined fluid depth based upon the average fluid velocity measurement in order to account for drawdown; and a means for calculating fluid flow rate based upon the average fluid velocity measurement and the corrected fluid depth.

[0041] Probe member 1 preferably comprises a pressure sensitive device, such as a bubbler-type probe (FIG. 2) in which the probe housing is isolated from the fluid flow and tubing is connected between the housing and the channel in which fluid flows for measuring fluid pressure therefrom; or a pressure transducer probe (FIG. 1) which is directly placed into the fluid stream. Alternatively, probe member 1 may suitably comprise other submerged measuring devices which measure fluid levels in a channel.

[0042] In a preferred first embodiment of the present invention, probe member 1 is sized and shaped in order to minimize any disturbance in fluid flowing in the channel. The pressure probe of FIG. 1 preferably has a length of 5.21″, a width of 1.5″, and a height of 0.9″. The bubbler probe of FIG. 2 preferably has a length of 3.66″, a width of 1.5″, and a height of 0.79″. As shown in FIGS. 1, 2 and 4, probe member 1 is preferably elongated having a curved end 1A which faces upstream when placed within the channel. Probe member 1 measures fluid pressure acting thereon and transmits the measurement to the fluid depth determining means. Wire end 1B extends from probe member 1 in order to provide electrical communication to the determining means.

[0043] The present invention preferably, but not necessarily, includes computing means for performing arithmetic operations relating to calculating fluid depth, average fluid velocity, average flow rate, and drawdown effects on fluid depth. As shown in FIG. 7, the computing means preferably, but not necessarily, comprises microprocessor 2 having, among other things, an arithmetic logic unit for performing arithmetic and boolean logic operations; memory 3 for storing both data and program instructions, comprising both random access memory (RAM) and read only memory (ROM); and interface and control circuitry for suitably connecting microprocessor 2 to fluid level measuring devices, fluid velocity measuring devices, signal processing circuitry, etc. Alternatively, the computing means of the present invention comprises other arithmetic processing devices.

[0044] The fluid depth determining means receives electrical signals from probe member 1 corresponding to the measured fluid pressure in the channel, and calculates fluid depth L using the equation identified above. In a preferred embodiment of the present invention, the fluid depth determining means comprises software stored in memory 3 which cooperates with microprocessor 2 in translating measured fluid pressure to fluid depth, as shown in FIG. 7. The fluid depth determining means preferably calculates fluid depth in real time, but alternatively the fluid depth determining means stores the raw fluid pressure measurements in memory 3 for processing at a later time.

[0045] According to a preferred embodiment of the present invention, the fluid velocity measuring means measures the average velocity of the fluid flowing in the channel. As shown in FIG. 7, the velocity measuring means preferably, but not necessarily, comprises software stored in memory 3 which suitably cooperates with microprocessor 2; and sensing unit 4 which is suitably connected to microprocessor 2.

[0046] Sensing unit 4 of the fluid velocity measuring means preferably, but not necessarily, comprises one or more piezoelectric transducers which transmit and receive high frequency signals, such as ultrasonic signals; and signal processing circuitry, operably associated with microprocessor 2, for controlling the transducers and measuring the Doppler frequency shift between the transmitted and received signals, including amplifiers, mixing circuits, filters, and a spectrum analyzer. Microprocessor 2 and memory 3 preferably cooperate with the signal processing circuitry in recording and calculating the average fluid velocity in the channel. Alternatively, sensing unit 4 comprises circuitry which utilizes principles other than Doppler principles in measuring average fluid velocity. In a preferred embodiment of the present invention, the fluid velocity measuring means measures average fluid velocity substantially in real time.

[0047] The flow rate calculating means preferably computes average flow rate in the channel according to the equation:

[0048] Average Flow Rate=Average Velocity×A

[0049] where A is the wetted cross-sectional area which is dependent upon fluid depth and channel geometry information as described above. In a preferred embodiment of the present invention, the flow rate calculating includes software stored in memory 3 which utilizes microprocessor 2, memory 3 and associated control and interface circuitry in calculating average flow rate. In a preferred embodiment of the present invention, the flow rate calculating means computes average flow rate in real time.

[0050] According to the preferred embodiments of the present invention, the flow rate measuring system includes means for correcting measured fluid depth to account for the drawdown effect. The correcting means preferably, but not necessarily, comprises a correcting factor which adjusts or offsets the raw fluid depth measurement to account for drawdown.

[0051] Since the effect of drawdown on an object varies depending upon fluid velocity in the stream, the correcting means in a preferred embodiment adjusts the raw fluid depth measurement as a function of the average fluid velocity measurement. Once corrected, the adjusted fluid depth value, together with the measured average fluid velocity value, is used to calculate flow rate in the channel, using the above equation for average flow rate.

[0052] In a preferred embodiment, the correcting means adds a correction factor to offset the raw fluid depth measurement. The correcting means preferably, but not necessarily, comprises an equation that represents the drawdown effect for the particular probe member 1 that is used to measure fluid depth. The correcting means preferably comprises system software which cooperates with microprocessor 2, memory 3 and necessary control and/orinterface circuitry so that the drawdown correction equation is accessed from memory 3, the fluid depth offset value is calculated based upon the measured average fluid velocity value, and the offset fluid depth value is thereafter added to the raw fluid depth value. The correcting means preferably, but not necessarily, adjusts the raw fluid depth value substantially in real time.

[0053] In addition, the correcting means preferably, but not necessarily, includes system software and necessary control circuitry to retrieve the raw fluid depth value from either memory 3 or the fluid depth determining means and to check to see if the fluid velocity measuring means is connected into the system. If the fluid velocity measuring means in not connected so that an average fluid velocity measurement is unavailable, then the correcting means does not calculate or apply an offset fluid depth value in order for the flow rate calculating means to calculate average flow rate. A flowchart representing the operation of the correcting means is shown in FIG. 5.

[0054] The fluid depth offset equation representing the drawdown effect is preferably, but not necessarily, derived from an analysis of the particular probe member 1 used in the system. The particular probe member 1 is analyzed at a plurality of fluid velocities so that an accurate drawdown profile for the probe member is obtained. The test results are then curve fitted, such as by using a least squares approximation, in order to derive a drawdown equation from the test data points. In one preferred embodiment of the present invention, the offset fluid depth equation is a second order equation. In other preferred embodiments as shown in FIGS. 8 through 21, the offset fluid depth equation is a fourth order equation. A graph showing the fluid depth offset curve as a function of fluid velocity for a probe member 1 is shown in FIG. 6.

[0055] In an alternative embodiment, the fluid depth offset values are stored directly in memory 3 rather than an equation being derived therefrom and stored in memory 3 instead. In this way, the measured average velocity value is used as the address pointer for the appropriate fluid depth offset value when the correcting means adjusts the measured fluid depth value.

[0056] Installation and operation of the system is as follows. First, probe member 1 is placed into position in the channel. Next, probe member 1 is connected to microprocessor 2 and/or memory 3 via interface circuitry in order to communicate with the fluid depth determining software. Sensing unit 4 of the fluid velocity measuring means is inserted into the channel and is connected to microprocessor 2 and/or memory 3 via interface circuitry so that it can communicate with the fluid level correcting means software and the flow rate calculating means software.

[0057] Once the system is installed, probe member 1 measures fluid pressure acting thereon and transmits the measured data to microprocessor 2 and/or memory 3; and the fluid depth determining software computes fluid depth, based upon this data. Concurrently, sensing unit 4 measures average fluid velocity and transmits the measured data to microprocessor 2 and/or memory 3. Thereafter, the flow rate correcting software computes the fluid level offset value based upon the average fluid velocity measurement and adjusts the fluid depth measurement accordingly. Next, the flow rate calculating software computes average flow rate in the channel based upon the average fluid velocity measurement and the adjusted fluid depth value.

[0058] Prior to being used in the system, probe member 1 is initially analyzed by measuring for drawdown at a plurality of different velocities and by deriving a formula representing the relationship between fluid level error due to drawdown and fluid velocity.

[0059] The embodiments of the present invention described in connection with FIGS. 8-21 emphasize the implementation of very low profile probe members 10 and 20 which minimize disturbance of laminar flow. It is has been discovered that the dimensions of the probe member, and/or the ratio of the main dimensions of the probe member are critical to minimizing disturbance of laminar flow. In accordance with these preferred embodiments, it is has been discovered that probe members having a height to width ratio of less than 0.52, and preferably closer to 0.333, are fluid dynamically superior.

[0060] FIGS. 8-12 show various views of a preferred submerged pressure transducer area velocity probe 10. In this very low profile probe member 10 embodiment, the length dimension L is 4.10 inches, the width dimension W is 1.35 inches, and the height dimension H is 0.45 inches. It should be borne in mind that the probe when placed in its operable position in the channel is positioned so that the length dimension is oriented parallel to the direction of main fluid flow. This is best seen in FIG. 4.

[0061] The embodiment illustrated in FIGS. 13-16 shows a very low profile velocity bubbler probe 20. This very low profile bubbler probe member 20 has a length dimension L′ of 3.250 inches, a width dimension W′ of 1.350 inches, and a height dimension H′ of 0.450 inches.

[0062] FIGS. 17-19 relate to the calibration and testing of the bubbler area velocity probe 20 and the submerged pressure transducer area velocity probe 10 illustrated in FIGS. 8-16.

[0063] Data was collected at various open channel depths in a 10-inch PVC pipe 30 set at three different slopes. Indicated velocity, flow and depth readings were compared to actual measurements. Indicated flow meter measurements were averaged over approximately twenty minutes for each test condition.

[0064] FIG. 17 illustrates a schematic of the pipe setup for the test. Approximately seventy feet of straight 10-inch pipe 30 was installed upstream of the measurement location 35 to provide near uniform flow to the sensor 31. The sensor 31 was placed in a PVC pipe 30 having an inside diameter of 10.02 inches. Access holes 32 (see FIG. 18) were cut in the top of the pipe 30 for ease in installing the meter, making depth measurements and photography.

[0065] Due to the pristine water used in the laboratory conditions, air was injected in the rising elbow 33 seventy feet upstream of the sensor 31. The purpose of the air was to simulate reflectors normally found in sewer and stormwater application. The air pressure was maintained between 5 psi and 20 psi using a regulator in the compressor hose. However, no air bubbles were actually visible.

[0066] Slope changes were made by supporting the pipe 30 along its length using adjustable pipe stands and shims. The entire pipe line was able to pivot in the far upstream dresser coupling for each slope change. Steel I-beams were placed under the pipe section containing the sensor 31 and the pipe 30 immediately upstream. The I-beam uniformly supported the pipe 30 to eliminate the possibility of sagging.

[0067] No downstream control valve was installed. Flow settings were made using a 16-inch gate valve. This valve was located upstream of the test piping at the outlet of the constant level tank. The laboratory's constant level tank was used in ensure that test flows remain constant over an extended period of time by maintaining a constant supply pressure.

[0068] The laboratory flow depth measurement was made using a precision point gauge 34 (FIG. 10) readable to the nearest 0.001 feet. It was mounted to a custom fitted 10-inch pipe bracket as shown in FIGS. 18 and 19. Reference point gauge measurements were made for each setting. Reference measurements were made on the invert of the pipe. The point gauge depth measurements were made for specific locations upstream of the sensor 31. These depths were averaged to determine one level to be compared to the flow meter indicated level. Level measurements were performed in a manner consistent with the Doppler technology. Other point gauges were permanently bolted to the crown of the pipe 30, and were used to set flow rates and to observe water surface profiles.

[0069] Flow measurements were made with a 30,000 lb. capacity weigh tank. This primary flow measuring device is traceable to the National Institute of Standards and Technology (NIST). A net weight of water (in pounds) was recorded over a measured time period (in seconds). Using the specific gravity of water (lbs/ft³), the flow rate was accurately calculated using the following equation:

[0070] Q=netweight÷(time * γ)

[0071] where Q is the flow rate in cfs.

[0072] The actual mean velocity was calculated for each run using the flow and level measurements in the continuity equation V=Q ÷A.

[0073] The flow depth in inches, the velocity in feet per second, and the flow rate in cubic feet per second were recorded from the flow meter output during the flow rate measurement. The indicated flow rate data and the actual laboratory measurement were immediately put into a computer spreadsheet so that deviations from actual could be monitored.

[0074] Three different flow depths were set for each of the three test slopes. These depths were set by adjusting the upstream control valve. The depths were set at approximately two-, four- and six-inches.

[0075] Tables 1 and 2 present the results from the flow meter test. In Table 1, the bubbler area velocity probe 20 data is presented. In Table 2, the submerged pressure transducer area velocity probe 10 data is presented. Actual deviations between the actual and indicated depth, velocity, and flow are provided.

[0076] FIG. 20 is a graph illustrating the relationship between level correction and velocity for the bubbler probe 20. The Series 1 curve in FIG. 20 is a plot of the original data. The Poly. (Series 1) curve is a best fit smooth curve representing the original data, and such best fit curve is represented by the equation:

[0077] y=−0.0016×⁴+0.0379׳−0.2418ײ+0.9792×−1.6628.

[0078] Table 3 represents data for the FIG. 20 graph.

[0079] FIG. 21 is a graph depicting the relationship between level correction and velocity for the submerged pressure probe 10. The Series 1 curve in FIG. 21 represents a plot of the original data. The Poly. (Series 1) curve is the best fit smooth curve for such data, and is represented by the equation

[0080] y=0.0012×⁴−0.0199׳+0.0977ײ+0.0564×−1.0839.

[0081] Table 4 represents data for the FIG. 21 graph.

[0082] Although there have been described what are currently considered to be the preferred embodiments of the present invention, it will be understood that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof.

[0083] The described embodiments are, therefore, to be considered in all aspects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description.