Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1995-08-28
1998-07-07
Biegel, Ronald L.
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
7386127, G01F 100
Patent
active
057772371
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a fluid flow meter of the kind comprising a first acoustic transducer upstream of a second acoustic transducer, the time of flight of acoustic waves between the transducers being used to measure the flow velocity of a fluid medium flowing between them.
An ultrasonic fluid movement device which uses this method is described in EP-A-0347096. This may be used to measure the flow velocity of gas through a passage of known dimensions. By multiplying the measured velocity by a velocity dependent co-efficient, the volume flow rate may be calculated. The device described may be used as part of a domestic gas meter.
A disadvantage with the prior art, eg. EP-A-0347096, is that the volume flow rate must be calculated by multiplying the measured velocity by a velocity dependent coefficient. The relationship between measured velocity and volume flow rate is non-linear, making this technique complex and imprecise in practice. It is therefore advantageous to be able to provide a more precise method of calculating the volume flow rate.
As described in "Theory of Transit Time Ultrasonic Flowmeters" (J. Hemp, Cranfield Institute of Technology, 27 Jul., 1981) under certain flow and acoustic conditions, the phase shift of a plane wave travelling down a tube due to the flow in the tube will be proportional to the volume flow rate alone. This is due to the integrating properties of a plane soundwave.
Compensation for off-axis modes is described in W093/00570. In this method a ringaround transmission technique is used in which every fourth wave packet is inverted with respect to the preceding packets in order to annul the propagation effects of high order modes in the duct.
U.S. Pat. No. 4,365,518 discloses a sonic flow meter in which the flow path through the meter is divided into a number of elongate tubes which are sufficiently large with respect to the ultrasonic wavelength so that the ultrasonic pressure wave is propagated through each tube without substantial distortion. The prior art is unable to achieve a linear response over a significant range.
In GB-A-2209216, by providing passages with a diameter below a certain cut-off point, only plane waves will be transmitted along the flow path. The volume flow rate can then be directly calculated without the use of a velocity-dependent co-efficient. Highest accuracy is achieved when the signal measured follows the direct transmission path, without being modified by constructive or destructive echoes which would induce phase errors. Therefore mode suppression also improves accuracy by ensuring that the signals follow the direct path. However, the flow passages are arranged in a close-packed arrangement in the flow tube, so that different fluid flow paths are set up in passages at varying radial distance from the central axis of the flow tube. This can lead to errors in the measured flow rate.
According to a first aspect of the present invention, a fluid flow meter comprises a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having a plurality of parallel fluid flow passages which extend axially in the direction of flow, wherein the cross-sectional dimensions of the passages are chosen such that substantially plane acoustic waves only are transmitted through the passages by the fluid, and wherein under working conditions the fluid flows through each passage such that the ratio of the volume flow rate through the passage to the total volume flow rate remains substantially constant with respect to the total volume flow rate.
In this arrangement each passage samples a representative fraction of the total flow rate.
In contrast to the prior art, the present invention
REFERENCES:
patent: 3817098 (1974-06-01), Brown
patent: 4365518 (1982-12-01), Zacharias, Jr.
patent: 4475406 (1984-10-01), Ansaldi et al.
patent: 4523478 (1985-06-01), Zacharias, Jr.
Collier James Digby Yarlet
Davies Christopher
Fryer Christopher James Newton
Waha Alain Henri
Biegel Ronald L.
G. Kromschroder Aktiengesellschaft
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