1. Field of the Invention
The present invention is directed to an ultrasonic flowmeter comprising an elongate duct through which fluid flows when the flowmeter is in use, ultrasonic transducers arranged respectively to transmit and receive ultrasonic pulses propagated through such fluid when the flowmeter is in use, and electronic circuitry connected to the transducers to provide a measure of the time delay between emission of an electronic pulse from one of the transducers and reception of the pulse by the other of the transducers, and constructed to enable an output to be provided which is dependent upon that delay and which is indicative of the rate of flow of fluid through the duct, in which the transducers are located respectively at opposite ends of the duct and are arranged to transmit and receive ultrasonic pulses propagated through the fluid substantially parallel to the duct from one end thereof to the other.
2. Description of the Invention
An example of such a flowmeter is illustrated and described in GB 2259571 A. Thus in that specification there is disclosed a fluid flow measurement system which includes two transducers which are located in a duct to provide a measuring path through a measuring tube. The transducers are switchable to allow each to be used for both transmission and reception and the speed of fluid passing through the measuring tube will be dependent on the difference in time periods for a signal from the transducers passing with or against the fluid flow, and including correction means for substantially eliminating transducer delay errors to ensure accuracy. The system may comprise a calibration mechanism employing an auxiliary transducer with its own ultrasonic paths. It allows accurate measurement of speed of sound values to be determined for the particular fluid flowing and the particular system parameters including transducer delay.
A disadvantage of such a construction is the spread of the transmitted ultrasonic pulse in such a manner that there is a relatively high dispersion of each pulse before it is received by the second transducer, thus impairing the accuracy of the flowmeter.
The present invention seeks to provide a remedy.
Accordingly the present invention is directed to an ultrasonic flowmeter having the construction set out in the opening paragraph of the present specification, characterised in that the transducers are provided with respective vibratory surfaces which transmit and/or receive ultrasound, which vibratory surfaces have a diameter which exceeds the cross-sectional diameter of the duct.
Both transducers may be isolated from fluid which flows through the flowmeter when the latter is in use.
This provides the advantage that the transducers are not subject to any corrosive effects from such fluid.
Such isolation may be effected by respective caps to which the transducers are acoustically coupled.
Fluid which flows through the flowmeter when it is in use may be in contact with that side of each cap which is further from the associated transducer, the transducers being in respective cavities filled with air or other gas or other low density material.
The thickness of the caps may be such as to optimise coupling between the transducers and such fluid.
The caps may be mounted in or on damping mountings to reduce signal degradation owing to ringing of the caps.
Both transducers may be constructed to both transmit and receive ultrasonic pulses, and the electronic circuitry may be such as to enable the flowmeter to operate by transmitting and receiving ultrasonic pulses which pass within the fluid from one transducer to the other in the direction of flow of fluid, and also in the opposite direction, and to enable the difference in transmission time between pulses transmitted in the direction of flow and those transmitted opposite to the direction of flow to be used to calculate the rate of flow of fluid through the duct.
This provides the advantage that substantially all the ultrasound transmitted and received by the transducers is propagated substantially parallel to the duct. This in turn reduces the amount of scattered ultrasound, and preserves the integrity of the ultrasound waves as they pass from one transducer to the other.
There may be a circular symmetry in the construction of the flowmeter at the transducers.
There may be a reflection symmetry in the construction of the flowmeter about a central transverse plane thereof.
The flowmeter may be provided with an input aperture and an output aperture through which fluid flows when the flowmeter is in use, the diameters of the input and output apertures being substantially equal, but being larger than the cross-sectional diameter of the duct.
This results in an increase in the speed of fluid flow in the duct relative to the speed it has when it passes into and out of the flowmeter, which in turn increases the accuracy of the measurement of fluid flow through the input and the output apertures.
The input and output apertures may be located laterally of the duct, between the ends of the duct, and the flowmeter may be provided with fluid-flow passageways from the inlet aperture to one end of the duct, and from the other end of the duct to the outlet aperture. This again reduces the amount of turbulence of the fluid within the flowmeter.
The flowmeter may be constructed so that the diameter of the duct is substantially five wavelengths of the ultrasound transmitted by the transducers or at least one of the transducers, being the wavelengths of the ultrasound as transmitted within the fluid the flow of which is to be measured. This enables the duct to act as an ultrasonic waveguide, facilitating directional propagation of the ultrasound. However, to effect this a suitable balance is required between on the one hand the absorption of ultrasonic waves that strike the material which defines the duct, so that reflection or scattering of such soundwaves does not unduly corrupt the ultrasonic pulse or wave packet travelling along the duct, and on the other hand sufficient reflection of ultrasonic waves from the material which defines the duct to ensure that the strength of the wave packet travelling along the duct is not unduly attenuated.
Thus the material which defines the duct may be such that it absorbs ultrasound, as transmitted by the transducers or at least one of the transducers, having an angle of incidence to the material which is less than a predetermined angle, but reflects such ultrasound having an angle of incidence to the material which is greater than a predetermined angle.
This again preserves the integrity of the wave packets of ultrasound as they pass through the fluid which flows through the flowmeter when the latter is in use.
The ratio of the length of the duct to its cross-section diameter is preferably in the range from 20 to 30, preferably about 23.
Preferably the duct is cylindrical and of uniform cross-section, although it may be defined by material the internal walls of which have a relief pattern to achieve the desired absorption/reflection properties of the walls of the duct.
Preferably the diameter of the duct is about 5.0 mm, and the spacing between the transducers is about 113 mm.
To this end the speed of sound through the material which defines the duct is preferably greater than the speed of sound through the fluid which flows through the flowmeter when the latter is in use, and the density of that material is preferably greater than the density of the fluid which flows through the flowmeter when the latter is in use. It is desirable that this remains so throughout the range of operating temperatures and fluid compositions.
The said material may comprise glass filled polytetrafluoroethylene (PTFE). This is able to withstand attack from fluids which may flow through the flowmeter, especially for example engine fuel such as petroleum or petroleum derivatives including diesel fuel. The glass content can be varied to tailor the acoustic impedance to a desired value.
The fluid which flows through the flowmeter when the latter is in use may comprise engine fuel.
The present invention extends to a method of measuring the flow of fluid using an ultrasonic flowmeter as set out in one or more of the preceding paragraphs relating to the present invention.