Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1997-11-19
2001-03-27
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C137S386000
Reexamination Certificate
active
06208943
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
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.
Flow of material through a partially filled channel or pipe is expressed by the equation
Q=A*V,
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
A=R
2
* cos
−1
((R−L)/R)−(R−L) * (2*R*L−L
2
)
5
,
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.
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
L=27.681*P,
where P represents the measured pressure acting on the probe, in psi.
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.
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.
2. Description of the Relevant Art
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.
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.
SUMMARY OF THE INVENTION
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.
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.
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.
It is an object of the present invention to provide a system which accurately measures fluid flow rate in a channel.
Another object of the present invention in to provide such a system which considers the effect of drawdown in measuring average fluid flow rate.
It is another object of the present invention to provide a system which provides accurate fluid flow rate measurements in real time.
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.
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.
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.
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patent: 5691914 (1997-11-01), Randolph
Kaiser Donald F.
Liebe John
Lyndaker David W.
Randolph Gary L.
Steen Michael D.
American Sigma, Inc.
Burt Pamela S.
Hoff Marc S.
Nguyen Linh
Weiner Irving M.
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