Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1995-10-27
1998-08-11
Biegel, Ronald L.
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
G01F 166
Patent
active
057929653
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates generally to ultrasonic flow measurements of fluids and, in particular, to the controlling and/or suppression of the effects, on the measurements, of non-fundamental acoustic modes.
BACKGROUND ART
It is known that the velocity of the flow of a gas as well as the velocity of sound in the gas can be determined by the taking of two measurements of the transit time of a pulse of ultrasound, one upstream and one downstream in the flowing stream. This is the principle of operation of a transit time ultrasonic gas meter. In order to perform this timing with small uncertainty, it is necessary to select some precise feature of the pulse which can serve as a timing marker. The crossing of the signal through zero is capable of very accurate location in time and makes a good timing marker. There are, however, a number of zero-crossings from which one such crossing must be reliably selected as the timing marker.
FIG. 1E shows a typical ultrasonic measurement arrangement 1 in which two transducers 2,3 face each other through a duct 4 having a cylindrical shape and circular cross-section with gas flowing in the direction indicated by the arrow 6.
FIGS. 1A-1D show a signal typically received from a pulse of ultrasound launched into the circular duct 4. The received signal is initially shown in FIG. 1A and extends for some time shown in FIGS. 1B, 1C and 1D where markers indicate times, 2 times, 3 times and 4 times respectively past the initial arrival of the start of the signal.
A particular negative going zero-crossing has been chosen as the event on which to do the timing. International Patent Application No. PCT/AU92/00314 (WO 93/00569) discloses an electronic fluid flow meter which incorporates circuitry developed which select this particular crossing in a two-stage process. Using the signal amplitude, a time is chosen before the arrival of the required crossing at which a negative-going zero-crossing detector is enabled. There is a reasonable latitude allowable in the timing for this preliminary event but it is apparent that it must occur before the chosen zero-crossing but after the preceding one. Otherwise, it would be used instead for the precise timing marker.
Such an arrangement has been found to operate satisfactorily. As it is based on the amplitude of the signal and how it varies with time, anything which changes the amplitude of the signal has the potential to interfere with the choice of the initial time and thus in the selection of the correct zero-crossing. One cause of amplitude changes in electronic systems are gain changes. These can be due to aging, temperature, or other environmental effects. It is normal to compensate for such changes by some form of automatic gain control (AGC) which almost eliminates these amplitude changes. There are, however, other causes of amplitude change that affect the individual peak heights in the signal even when, as a result of the AGC, the maximum peak height is constant. Furthermore, when amplitude changes cause an incorrect choice of zero-crossing the timing error which results will be at least one whole period of the signal. This represents a serious error because it is systematic and averaging will not produce an unbiased mean of smaller uncertainty. A significant contributor to these amplitude changes is the existence and propagation of, non-fundamental acoustic modes.
SUMMARY OF THE INVENTION
It is an object of the present invention to substantially overcome, or ameliorate, the abovementioned problems through provision of a means by which the effects of non-fundamental acoustic modes on the received signal, typically the amplitude thereof, can be further reduced.
Throughout the specification reference to "non-fundamental acoustic modes" is to be taken as including a reference to higher order acoustic modes, and vice versa.
In accordance with one aspect of the present invention there is disclosed a fluid flow measurement system comprising a duct for carrying a fluid, the duct having two transducers spaced
REFERENCES:
patent: 4003252 (1977-01-01), Dewath
patent: 4164865 (1979-08-01), Hall et al.
patent: 4445389 (1984-05-01), Potzick et al.
patent: 5383369 (1995-01-01), Khuri-Yakub
International Search report mailed 6 Jul. 1994.
AGL Consultancy Pty. Ltd.
Biegel Ronald L.
Commonwealth Scientific and Industrial Research Organisation
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