Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1996-01-10
1998-03-17
Dougherty, Elizabeth L.
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
7386129, G01F 166
Patent
active
057289484
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates generally to the construction of electronic fluid flow meters and, in particular, to a housing for such a meter, a housing for a transducer, a system for insertion as a section in a fluid carrying pipe and a system for determining the velocity of a fluid within a pipe.
BACKGROUND ART
Occasions arise where it is necessary to determine the velocity of flow of a fluid within a pipe. This can be achieved by placing at least two transducers within the pipe and causing at least one of the transducers to transmit ultrasonic signals along or across the pipe for reception by the other transducer. Subject to variations in the time of flight of the ultrasonic signals between the transducers, the velocity of fluid flow can be determined.
Such methods invariably require variations in the cross-section of the pipe in the vicinity of the transducers giving rise to pressure variations at or about each transducer location. In some cases, the pressure drop about the transducer can be irrecoverable.
Furthermore, in order to determine this time of flight 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 101 in which two transducers 102, 103 face each other through a duct 104 having a cylindrical shape and circular cross-section with gas flowing in the direction indicated by the arrow 106.
FIGS. 1A-1D show a signal typically received from a pulse of ultrasound launched into the circular duct 104. 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 (WO93/00549) discloses an electronic fluid flow meter which incorporates circuitry developed which selects 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. However, as the arrangement 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.
OBJECT OF THE INVENTION
It is an object of the present invention to substantially overcome, or ameliorate, one or more of t
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International Search Report mailed 6 Jul. 1994 (Jul. 06, 1994).
Bignell Noel
Brew Alan William
Collings Anthony Francis
Hews-Taylor Kenneth James
Peterson Michael Ross
AGL Consultancy Pty. Ltd.
Commonwealth Scientific and Industrial Research Organisation
Dougherty Elizabeth L.
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