Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1994-02-02
1996-02-27
Chilcot, Richard
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
7386127, G01F 166
Patent
active
054939160
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to ultrasonic flow meters and in particular, to a method by which errors in measurement caused by high order acoustic modes are minimised.
BACKGROUND TO THE INVENTION
It is known to measure the velocity of fluid flow in a duct by transmitting acoustic signals through the duct and measuring the time of propagation of the signals along the duct. For a known distance, the velocity can be calculated. Generally, measurements are taken in both directions along the duct to provide for increased accuracy of the result.
However, when a pulsed acoustic signal is propagated within a duct, the acoustic wave packet breaks into a number of transition modes each having a frequency corresponding to the transmitting frequency but having a difference phase whereby at different points along the duct, each mode has its own set of null points at which the amplitude of the mode wave is zero. Also, the different modes have different acoustic velocities.
In order to obtain flow measurements, the acoustic wave must be received by a transducer or like apparatus arranged at some point within the duct. The receive transducer however does not receive a single pulse of acoustic energy as was transmitted, but however, it receives the various modes each having a different energy level and being received at different times. Depending on the energy level of the received signal, it will be apparent that errors arise in determining a point in time in which a particular wave packet is received and hence a flow measurement can be determined. Because of this, the errors are carried over to the resulting flow velocity calculation.
It is an object of the present invention to substantially overcome or ameliorate, the abovementioned problems through provision of a method by which flow velocity errors can be substantially reduced.
DISCLOSURE OF THE INVENTION
In accordance with a first embodiment of the present invention there is disclosed a method for measuring time of flight of a plurality of acoustic wave packets between two locations in a fluid having at least one acoustically reflective surface in the path of said packets, said method comprising the steps of: between the locations and measuring time of flight thereof; and respect to the immediately preceding packet or having an inverted polarity with respect to the immediately preceding packet to substantially reduce the effects of high order acoustic modes on the immediately preceding wave packet(s) having the first phase and/or immediately following wave packet(s) having the first phase, and measuring a time of flight thereof.
Generally, the extremities of the path are defined by a duct for carrying the fluid, and preferably is a conduit.
Advantageously, steps (a) and (b) are repeated a plurality of times (eg 1 to .sup.10.sup.6, typically 32 to 64) and an average time of flight may be calculated which can then be utilised for determining parameters related to the fluid medium such as fluid flow rate, for example.
Generally, the high order modes of the inverted wave packet act to annul the high order modes of succeeding wave packets.
Typically, the acoustic wave packets are transmitted between two or more transducers which are typically ultrasonic transducers, and the transmission of successive wave packets is initiated immediately on reception of an acoustic wave packet by one of the transducers.
In accordance with a second embodiment of the present invention there is disclosed a method for determining fluid flow velocity in a duct in which first and second acoustic transducers are spaced by a predetermined distance within said duct, the method comprising the steps of: first transducer to said second transducer; recording a time of travel of said acoustic wave packet between said transducers; successive transmissions are initiated upon the detection of the immediately preceding acoustic wave packet; having a (second) phase or polarity inverted with respect to said first phase; second transducer and detecting at said first transducer; and
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Chilcot Richard
Commonwealth Scientific and Industrial Research Organisation--AG
Dougherty Elizabeth L.
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