WO2008053193A1

WO2008053193A1 - Ultrasonic flow-rate measurement device and system

Info

Publication number: WO2008053193A1
Application number: PCT/GB2007/004130
Authority: WO
Inventors: Clive Henry Buckberry; Richard John Nighy
Original assignee: Imi Vision Limited
Priority date: 2006-10-31
Filing date: 2007-10-31
Publication date: 2008-05-08
Also published as: EP2130004A1

Abstract

An ultrasonic flow-rate measurement device (10) comprising a measurement conduit (12), a first and second capacitive micro-fabricated ultrasonic transducers (cMUTs) (14, 16), and a flexible printed circuit board (PCB) (18). A transducer housing (20) extends longitudinally along the conduit (12). First and second recesses (22, 24) are provided within the housing (20), for receiving the first and second cMUTs (14, 16) respectively. The flexible PCB (18) is formed with a plurality of folds to fit against the housing (20). A pressure sensor (32), located mid-way between and in-line with the cMUTs (14, 16). A cover member (38) of a complementary shape to the housing (20) is provided for protecting and securing the flexible PCB (18), the cMUTs (14,16) and the pressure sensor (32) in position. In the measurement system (90) a controller (92) is operable to alternatingly actuate the cMUTs (14, 16) to generate and receive ultrasonic signals, and, from the times-of-flight with and against the flow, to calculate the flow rate.

Description

Ultrasonic flow-rate measurement device and system

The invention relates to an ultrasonic flow-rate measurement device and to an ultrasonic flow-rate measurement system incorporating the device.

Flow sensors are well known, the majority of flow sensors requiring some form of physical interaction with the fluid, for example a turbine or orifice plate which can be undesirable in many applications where a non-contact flow measurement is required.

The use of ultrasound as a non-contact means of measuring fluid flow rate is known. Several methods are known, one of which is time of flight in which the time taken for a pulse of ultrasound to travel from the transmitter at a first position to a receiver at a second position is timed. This is then either compared to a reference time or is compared to the time taken for a pulse of ultrasound to travel from the second position to a receiver at the first position. The difference in the two times, caused by the flow of the fluid in a direction from one position towards the other, is then used to calculate the flow rate given the material properties of the flowing medium. This may be done a number of times to calculate an average time as shown in European Patent EP 0 440 701 of J. Delsing.

Ultrasonic transducers are well known and commonly comprise piezo ceramics to produce the ultrasound. Certain micro electro-mechanical systems (MEMS) are also known which are capable of producing and sensing ultrasound. However the problem with these devices is that they are generally expensive and require great care in manufacture, particularly in the alignment of parts. In addition to this the required geometry often requires complex wiring of components. The overall result is a sensor which, although functional, is often too expensive to compete with, for example, turbine flow meters.

The object of the present invention is to provide an ultrasonic flow-rate measurement device that can be simply and economically manufactured and assembled so as to be competitive with more common intrusive methods of flow measurement. According to a first aspect of the present invention there is provided an ultrasonic flow- rate measurement device comprising: a measurement conduit through which fluid to be measured flows; a first ultrasonic transducer provided at a first position on the conduit and a second ultrasonic transducer provided at a second position on the conduit, the first and second positions being separated in the flow direction; an electrical track circuit to which the first and second transducers are electrically coupled, the electrical tracks being adjacent to and conforming to the shape of at least part of the external wall of the conduit; and first and second recesses, formed in the wall of the conduit at the first and second positions, in which the first and second transducers are respectively received such that the transducers are sonically coupled to the fluid flow through the bottoms of the recesses.

In one arrangement the electrical track circuit is preferably provided on a printed circuit board, the printed circuit board being coupled to and conforming to the shape of the at least part of the external wall of the conduit, and the first and second ultrasonic transducers being mechanically coupled to the printed circuit board. The printed circuit board is preferably a flexible printed circuit board. The electrical track circuit can be on either side of the printed circuit board, either the side adjacent the external wall of the conduit or the side facing away from the external wall of the conduit, in the second case the electric track, although spaced from the external wall of the conduit by the board is adjacent the external wall and conforms to its shape.

In an alternative arrangement, the electrical track circuit may be provided directly on the external wall of the conduit, the first and second ultrasonic transducers being mechanically coupled to the external wall of the conduit. In this arrangement the first and second ultrasonic transducers may be moulded into or adhered to the external wall of the conduit.

In another alternative arrangement the electrical track circuit may be provided directly on a structural component of the device, preferably a conduit cover which attaches directly to the external wall of the conduit. The first and second ultrasonic transducers may be moulded into or adhered to the surface of the conduit cover such that when the conduit cover is attached to the conduit the transducers are pre aligned with one another.

The ultrasonic transducers are preferably operable to generate ultrasonic signal pulses. The pulses preferably have steep rising edges. The ultrasonic transducers are preferably micro electro-mechanical systems transducers, and are most preferably capacitive micro-fabricated ultrasonic transducers. The capacitive micro-fabricated ultrasonic transducers are preferably approximately one millimetre square to three millimetres square in size. The ultrasonic signals generated by the capacitive micro- fabricated ultrasonic transducers preferably have a beam width of substantially the same cross-sectional size as the transducers.

The ultrasonic transducers may alternatively comprise piezo-electric transducers.

The ultrasonic transducers are preferably operable to generate ultrasonic signals having a signal frequency in the region of 100KHz to 40 MHz more preferably in the region of 5 Megahertz to 10 Megahertz and a signal bandwidth of 1-2 Megahertz.

Preferably, the first and second positions are co-axially arranged with one another and an ultrasonic reflector is provided on the measurement conduit diagonally opposite and generally mid-way between the first and second positions. The ultrasonic reflector is preferably a metallic reflector, and may be a metallic flat plate or a metallic curved, preferably parabolic, reflector. Preferably the ultrasonic reflector is a flat plate as this ensures that the distance travelled by the ultrasound is substantially the same, irrespective of where on the plate it reflects from. As the ultrasound is transmitted in a beam that may have a width up to several millimetres, using a flat plate ensures a clean signal is received, where as using a curved reflector will result in some noise on either side of the received signal due to the difference in path length caused by reflection from different points on the curved reflector. The use of a flat plate, and the associated cleaner signal received enables more accurate measurement of flow. A further advantage is that using a flat reflector requires alignment in less axis and therefore enables a simpler manufacturing process. The ultrasonic reflector may be provided within a recess formed in the internal wall of the measurement conduit, and is most preferably over-moulded with an ultra sound transparent plastic film to retain it within the recess and separate it from fluid flowing through the measurement conduit. Alternatively the ultrasonic reflector may be integrally moulded into the measurement conduit or may be positioned in a recess formed in the external wall of the measurement conduit. Any space within the recess between the ultrasonic reflector and the bottom of the recess is preferably filled with ultrasound coupling gel.

The measurement conduit may be substantially circular in cross-section. The measurement conduit may alternatively be substantially rectangular in cross-section, most preferably having a width that is substantially the same as the beam width of an ultrasonic signal generated by the transducers.

In one preferred arrangement, a generally frusto-conical inlet tube is provided, extending from the, in-use, up-stream end of the measurement conduit for coupling a fluid flow from a delivery conduit, generally of substantially circular cross-section, into the measurement conduit. The inlet tube is preferably of substantially circular cross- section at its inlet end and rectangular cross-section at its delivery end, coupled to the measurement conduit. A generally frusto-conical outlet tube is also preferably provided, extending from the, in-use, down-stream end of the measurement conduit. The outlet tube is preferably of rectangular cross-section at its inlet end and of substantially circular cross-section at its outlet end.

The cross sectional area of the measurement conduit is preferably less than that the cross-sectional areas of the inlet tube and the outlet tube forming a Venturi, thereby accelerating the flow of fluid through the flow-rate measurement device. This is advantageous when the flow rate of the fluid is very low or when the flow borders between laminar and turbulent flow, since accelerating the flow can ensure the flow is turbulent therefore removing any ambiguity.

In an alternative arrangement the device comprises an inlet tube, measurement conduit and an outlet tube. Preferably the internal cross section of the inlet tube has a substantially circular inlet end and rectangular outlet end, the internal cross section of the measurement section is substantially rectangular and the internal cross section outlet section has a substantially rectangular inlet and a substantially circular outlet. Preferably the cross section of the measurement conduit is substantially equal to the cross sectional areas of the inlet tube and the outlet tube thereby ensuring that the flow does not accelerate through the flow-rate measurement device and therefore there is substantially no pressure drop created by the flow-rate measurement device. Preferably the width (the dimension perpendicular the direction of propagation of the ultrasound) of the rectangular cross section of the measurement conduit is less than the diameter of the circular inlet of the inlet tube and the height of the rectangular cross section of the measurement conduit is greater than the diameter of the circular inlet of the inlet tube. In this manner the path length of the ultrasound as it passes from the first transducer, across the fluid flow to the reflector and back across the fluid flow to the second transducer is greater for the same cross sectional area than were a measurement conduit having a substantially circular cross section used. In addition as the width of the internal cross section of the measurement conduit is reduced, the ultrasound passes through a greater percentage of the fluid than it would were a measurement conduit having a circular internal cross section used. This helps to eliminate inaccuracies in flow measurement due to boundary effects of the fluid flowing through the conduit. Furthermore, using a rectangular cross having a width approximate to the width of the band of ultrasound enables the ultrasound to pass through substantially all of the flow, resulting in the measure signal taking into consideration any boundary effects. This would not be possible in a circular cross section. Preferably if the cross section of the measurement section is wider than the band of ultrasound, the transducers and reflectors are positioned to pass through a section of flow representative of the average flow, e.g. if the main flow is off centre then the transducers can be located to take this into account.

A further benefit of having a substantially rectangular cross section is that attenuation of the signal is greatly reduced in comparison to a circular cross section. With a circular cross section a portion of the signal is reflected at an angle, not directly at the transceiver receiving the signal. A large amount of the signal therefore gets lost which makes it much harder to isolate the signal from any background noise. In addition, as some of the attenuated part of the signal gets converted to background noise, reducing the amount of attenuation by using a rectangular cross section also reduces the background noise.

The ultrasonic flow-rate measurement device preferably further comprises a pressure sensor, most preferably a capacitive membrane pressure sensor. The pressure sensor is preferably carried by and electrically coupled to the flexible circuit board.

The measurement conduit preferably further comprises a transducer housing provided along the at least part of the external wall of the conduit, in which the first and second recesses are provided. The transducer housing is preferably integrally formed with the external wall of the measurement conduit. The first and second recesses are preferably formed at an angle to the longitudinal axis of the measurement conduit such that, when located in or adjacent their respective housings, the ultrasonic transducers are orientated at an angle to the longitudinal axis of the measurement conduit, and are thereby angled towards the ultrasonic reflector and each other. The ultrasonic transducers are most preferably orientated at an angle of less than 85 degrees to the longitudinal axis of the measurement conduit. Preferably the recesses have a thin layer of material separating the recesses from the interior if the measurement conduit, through which the fluid passes, thereby forming a fluid impenetrable boundary between the fluid on one the side and the ultrasonic transducer on the other. Preferably the thin layer of material has a thickness in the range 50 - 150 micrometers, more preferably in the range 80 - 100 micrometers resulting in its absorption of the ultrasound being negligible compared to the size of the signal. The thin layer of material prevents any fluids within the conduit from coming into direct contact with the transducers and also enables the pressure within the conduit to be isolated fro the transducers to eliminate or minimise any effect of changing pressures within the conduit on the frequency or amplitude of the emitted ultrasound.

In one preferred arrangement the thin layer is integrally moulded with the measurement conduit. In this arrangement the measurement conduit.

Alternatively, the measurement conduit and the transducer housing may be made of a plastics material having a through hole in place of the two recesses and a thin film of material may be attached to the inside of the measurement conduit to separate the fluid flowing therethrough from the ultrasonic transducers.. The plastics material is preferably polyethylene, most preferably medium density polyethylene.

The first and second recesses preferably comprise an external part and an internal part, the external part being generally V-shaped and the internal part comprising an enclosed recess extending from the wall of the external part which generally faces the ultrasonic reflector and having an ultrasound transparent base, wherein the respective ultrasonic transducers are located on the said wall of the external part, over the internal part enclosed recess. The dimensions of the internal part enclosed recess are preferably smaller than the dimensions of the ultrasonic transducer, such that the transducer is mounted across the enclosed recess, on the said wall of the external part, but large enough so that substantially all of an ultrasonic signal generated by the ultrasonic transducer is transmitted through the internal part. The internal part enclosed recess is preferably substantially filled with ultrasound coupling gel.

The pressure sensor is preferably provided substantially in-line with and between the first and second ultrasonic transducers. The transducer housing preferably further comprises a third recess, which is generally U-shaped, the pressure sensor being provided at the bottom of the recess such that it is coupled to the fluid flow. An aperture is preferably provided in the bottom of the third recess through which the pressure sensor is coupled to the fluid flow. Preferably, a non-permeable coating is provided on the pressure sensor to isolate fluid acting on it from the circuit board.

The measurement conduit preferably further comprises a cover member provided on the transducer housing, the cover member being of a complementary shape to the external surface of the transducer housing, such that the flexible circuit board, the ultrasonic transducers and the pressure sensor are clamped in position between the transducer housing and the cover member.

The flexible circuit board is preferably provided with a plurality of fold lines along which it is folded such that it is shaped to conform with the external surface of the transducer housing, the parts of the flexible circuit board on which the ultrasonic transducers are provided being located within the V-shaped external recesses and the part carrying the pressure sensor being located within the U-shaped recess. Alternatively, circuit board retaining means may be provided on the transducer housing or the cover member for attaching the flexible circuit board to the transducer housing or the cover member such that when the cover member is located on the transducer housing the flexible circuit board is thereby deformed to conform to the shape of the external surface of the transducer housing.

Alternatively, the flexible circuit board may be spiral wrapped around the external wall of the measurement conduit.

The first and second positions may alternatively be diagonally opposite one another. The flexible circuit board is preferably spiral wrapped around the external wall of the measurement conduit.

Alternatively, the measurement conduit may comprise two dogleg bends connected by a straight measurement section, the first and second positions being located linearly opposite one another, on the verticals of the dogleg bends, at either end of the measurement section. The flexible circuit board preferably being folded along a plurality of fold lines such that it is shaped to fold around the external surface of the measurement conduit in the linear direction.

Clamping means may be provided for fixing the flexible circuit board in position around the measurement conduit.

The ultrasonic flow-rate measurement device may further comprise a fluid temperature sensor thermally coupled to the fluid flow. The fluid temperature sensor preferably comprises a temperature coefficient of resistance temperature sensor, most preferably a Titanium resistor temperature sensor. The fluid temperature sensor is preferably a membrane mounted temperature sensor. The fluid temperature sensor is preferably provided on the flexible circuit board. The ultrasonic flow-rate measurement device may further comprise a background temperature sensor.

According to a second aspect of the invention there is provided an ultrasonic flow-rate measurement system comprising: an ultrasonic flow-rate measurement device according to the first aspect of the invention; and control means operable to altematingly actuate the ultrasonic transducers to generate and receive ultrasonic signals, and to monitor the transmission of the ultrasonic signals through the fluid flow to thereby determine the flow rate of the fluid through the measurement conduit.

The control means is preferably operable to determine the time-of-flight of each ultrasonic signal through the fluid flow, and from the times-of-flight in each direction to determine the flow rate of the fluid.

The control means is preferably operable to actuate the ultrasonic transducers to each generate and receive a plurality of ultrasonic signals, most preferably between 100 and 400 signals each. The control means is preferably further operable to store the time-of- flight of each signal, and to calculate the average time-of-flight in each direction through the measurement conduit.

The control means may alternatively or additionally be operable to cause the ultrasonic transducers to generate a frequency chirped ultrasonic signal and to compare the ultrasonic signals transmitted in each direction through the measurement conduit to determine any phase difference between counter propagating signals, from which the flow rate of the fluid is then determined.

The control means may be further operable to change from measuring phase difference to measuring time-of-flight when a 2π phase-difference between counter propagating signals is reached.

The control means may alternatively or additionally be operable to determine a Doppler effect induced shift in the frequency of an ultrasonic signal propagating through the fluid flow, from which the flow rate of the fluid is determined.

The control means preferably comprises an application specific integrated circuit. Preferably, the control means further comprises memory means for storing calibration data.

The pressure sensor is preferably integral with or mounted on the application specific integrated circuit. The temperature sensor may be integral with or mounted on the application specific integrated circuit. Alternatively, the temperature sensor may be provided separately ensuring that the temperature reading is not affected by the heat produced by the application specific integrated circuit.

The application specific integrated circuit is preferably provided on the flexible circuit board of the ultrasonic flow-rate measurement device.

According to a third aspect of the invention there is provided a flow measurement device according to the first aspect of the invention further comprising a

means for, in use, exciting an ultrasonic transducer to emit an ultrasonic signal ^• into the measurement conduit; means of, in use, receiving said ultrasonic signal in a transducer once it has passed through at least a portion of the fluid flowing in the measurement conduit, and converting it into an electrical signal; means of comparing the electric signal to a reference value to determine a characteristic of the fluid.

In one preferred embodiment the emitting transducer and the receiving transducer are the first and seconds transducers.

In an alternative embodiment the emitting and receiving transducers are one and the same and comprise a transceiver that transmits the ultrasonic signal which is reflected back onto it by the reflector, whereupon it receives it. In this arrangement the transceiver is located opposite the reflector and is mounted in a similar manner to the first and second transducers, with a thin layer of material forming a fluid impenetrable boundary barrier between the fluid passing through the conduit and the transceiver.

Preferably the ultrasound crosses the fluid substantially perpendicular to the direction of fluid flow, thereby minimising the effect of fluid flow on the ultrasound received.

In a first preferred arrangement the device further comprises a temperature sensor and an processor means, the processor means including a clock, configured to measure the time of flight of the ultrasound from the transducer to the receiver, calculate the speed of sound in the fluid and generate an electrical signal corresponding thereto, the electrical signal indicative of the bulk modulus of the fluid at a given temperature.

In this arrangement the transmitter emits substantially a single wavelength or a narrow band of wavelength and is operated at a resonant frequency.

In this arrangement preferably the transmitter and receiver send and receive a plurality of signals over a time period. Preferably the average speed of sound is calculated for the plurality of send/receive cycles. This may be done by averaging the time of flight prior to the calculation of the speed of sound or calculating the speed of sound for the plurality of send/receive cycles and then averaging it.

In a second preferred arrangement the device comprises a broadband transmitter and receiver (or transceiver), arranged to transmit and receive a multiple frequency impulse, and a processor to process the received signal and to convert it into an electrical signal indicative of a characteristic of the fluid. Preferably, the processor performs a Fourier transform on the received signal resulting in a signature specific to the fluid passing through the conduit. The signature is compared to reference values to determine a characteristic of the fluid, for example the concentration of a solute therein. The signature can then be monitored for variation from a desired value

In a third arrangement the transmitter may be activated to emit a frequency sweep and a processor to process the received signal and to convert it into an electrical signal indicative of a characteristic of the fluid. In one preferred arrangement the processor creates a signature, preferably by conducting a Fourier transform on multiple data sampled during the frequency sweep for the fluid. The signature can then be compared to a reference value to determine if the fluid characteristics differ from the required fluid characteristics. Alternatively, or in addition, the processor compares the signature to a plurality of reference signals to determine a characteristic of the fluid, for example the fluid concentration. Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic side view of an ultrasonic flow-rate measurement sensor according to a first embodiment of the invention;

Figure 2 is a diagrammatic plan view of the sensor of Fig. 1 ;

Figure 3 is a diagrammatic cross-sectional view along line A-A of Fig. 2;

Figure 4 is a diagrammatic part-exploded view of the sensor of Fig. 1 ;

Figure 5 is a diagrammatic view of the flexible printed circuit board of the sensor of Fig. 4;

Figure 6 is a diagrammatic plan view of the flexible printed circuit board of Fig. 5;

Figure 7 is a diagrammatic view of the cover member of the sensor of Fig. 5;

Figure 8 is a diagrammatic plan view of the cover member of Fig. 7;

Figure 9 is a diagrammatic representation of an ultrasonic flow-rate measurement sensor according to a second embodiment of the invention;

Figure 9A is a diagrammatic representation of an ultrasonic flow-rate measurement sensor according to a third embodiment of the invention;

Figure 10 is a diagrammatic representation of an ultrasonic flow-rate measurement sensor according to a fourth embodiment of the invention;

Figure 11 is a diagrammatic representation of an ultrasonic flow-rate measurement sensor according to a fifth embodiment of the invention;

Figure 12 is a diagrammatic representation of an ultrasonic flow-rate measurement sensor according to a sixth embodiment of the invention; and Figure 13 is a schematic representation of an ultrasonic flow-rate measurement system according to a seventh embodiment of the invention;

Figure 14 is a cross section of a device according to the third aspect of the invention.

Referring to Figures 1 to 8, a first embodiment of the invention provides an ultrasonic flow-rate measurement device 10 comprising a measurement conduit 12, a first ultrasonic transducer 14, a second ultrasonic transducer 16, and a flexible printed circuit board (PCB) 18.

The first and second ultrasonic transducers 14, 16 comprise capacitive micro-fabricated ultrasonic transducers (cMUTs). Each cMUT 14, 16 has a 2mm square signal output aperture and is operable to generate ultrasonic signal pulses having a central frequency of 1 to 10MHz, a signal bandwidth of 2MHz and a duration of less than 10 microseconds. It will be appreciated that the central frequency of the ultrasonic signal pulses is chosen to produce a measurable time-of-f light through a fluid flow, as will be described in more detail below. The frequency will therefore at least in part depend upon the speed of sound through a particular fluid, the duration of the pulses, the time resolution of the measurement electronics and the speed of the processing electronics.

The measurement conduit 12 in this example comprises a circular cross-section cylinder of medium density polyethylene (MDPE). A transducer housing 20 is integrally moulded with the conduit 12 and extends longitudinally along the conduit 12. First and second recesses 22, 24 are provided within the housing 20, for receiving the first and second cMUTs 14, 16 respectively. Each recess 22, 24 comprises an external, V- shaped part 22a, 24a and an enclosed recess 22b, 24b (shown most clearly in Figure 3).

One wall of each external recess 22a, 24a is angled towards the corresponding wall of the other external recess 22a, 24a, and the enclosed recesses 22b, 24b extend from these walls to the internal wall of the conduit 12. The bottoms of the enclosed recesses 22b, 24b comprise plastic membranes having a thickness of 0.1 -0.5mm. The centre-to- centre spacing between the bottoms of the first and second recesses 22, 24 is 19.4mm. The cMUTs 14, 16 are located generally adjacent the said walls of the external recesses 22a, 24a, across the open ends of the enclosed recesses 22b, 24b, such that the cMUTs 14, 16 are orientated at an angle to the longitudinal axis of the conduit 12, and are thereby angled towards each other and the opposite side of the conduit 12. In this example, the second cMUT 16 is orientated at an angle of 58° and the first cMUT 16 is orientated at an angle of 122° (measured in the same direction) to the longitudinal axis of the conduit 12. An ultrasonic reflector in the form of a 3mm square metal plate 26 is provided mid-way between the cMUTs 14, 16, on the opposite side of the conduit 12, at the point where ultrasonic signals generated by the cMUTs 14, 16 strike the internal wall of the conduit 12. The reflector 26 is mounted within a recess formed in the internal wall of the conduit 12 and is over-moulded with an ultrasound transparent plastic membrane having a thickness of 0.1-0.5 mm.

The enclosed recesses 22b, 24b have a cross-sectional area equivalent to the active area of the cMUT's, and as such are smaller than the overall dimensions of the cMUTs 14, 16, such that the cMUTs 14, 16 are mounted across the respective enclosed recesses 22b, 24b, but are large enough so that substantially all of an ultrasonic signal generated by a cMUT 14, 16 is transmitted through the enclosed recess 22b, 24b to the fluid flow. The enclosed recesses 22b, 24b are filled with polydimethylsiloxane (PDMS) 46 which ultrasonically couples the cMUTs 14, 16 with the fluid flow via a thin portion of the inner wall of the conduit 12. In an alternative option (not shown) there may be a break in the wall of the conduit enclosing the recesses 22b, 24b enabling the PDMS to be in direct contact with the fluid flowing through the conduit 12.

The flexible PCB 18 is formed with a plurality of folds (as seen best in Figs. 4 and 5), so that the flexible PCB 18 is shaped to conform with and be located over the lowermost (as shown in the drawings) face of the housing 20. Apertures 28, 30 of a corresponding size and shape to the cross-section of the enclosed recesses 22b, 24b are formed in the flexible PCB 18, for location over the enclosed recesses 22b, 24b. The first and second cMUTs 14, 16 are mechanically and electrically coupled to the underside (as shown in the drawings) of the flexible PCB 18, over the respective apertures 28, 30.

The flow-rate measurement device 10 also comprises a pressure sensor 32, located mid-way between and in-line with the cMUTs 14, 16. The pressure sensor 32 in this example is a capacitive membrane pressure sensor. The pressure sensor 32 is located at the bottom of a third, U-shaped recess 34, formed in the housing 20. The bottom of the U-shaped recess 34 comprises a plastic membrane having a thickness of 0.1 mm, so that the fluid flow through the measurement conduit 12 is mechanically coupled to the pressure sensor 32. The pressure sensor 32 is provided on the flexible PCB 18. Similarly to the cMUTs 14, 16, an aperture 36 is provided in the PCB 18 and the pressure sensor 32 is coupled to the underside of the PCB 18.

The flow-rate measurement device 10 is also provided with a temperature sensor (not shown) for monitoring the temperature of fluid flowing through the measurement conduit 12.

A cover member 38 of a complementary shape to the lowermost surface of the transducer housing 20 is provided for protecting and securing the flexible PCB 18, the cMUTs 14,16 and the pressure sensor 32 in position. Recesses 40, 42, 44 are provided on the uppermost (as shown in the drawings) face of the cover member 38 for receiving the cMUTs 14, 16 and the pressure sensor 32 respectively.

Figure 9 shows an ultrasonic flow-rate measurement device 50 according to a second embodiment of the invention. The device 50 of this embodiment is substantially the same as the device 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

In this embodiment, the measurement conduit 52 has a rectangular cross-section, having a width only slightly greater than the width of the ultrasonic signals generated by the cMUTs 14, 16. The narrowed measurement conduit 52 provides better coverage of the fluid flow by the ultrasonic pulse width, which produces a more accurate flow-rate measurement because the ultrasonic pulses extend further into the fluid/conduit boundary areas of the fluid flow.

An inlet tube 54, in the form of a Venturi tube, is provided at the in-use up-stream end of the measurement conduit 52. The Venturi tube 54 has a substantially circular cross- section at its inlet end and rectangular cross-section at its delivery end, where it is coupled to the measurement conduit 52. The inlet tube 54 can thereby couple a fluid flow from a circular cross-section delivery conduit (not shown) into the measurement conduit 52. A generally frusto-conical outlet tube 56 is provided at the in-use, downstream end of the measurement conduit 52. The outlet tube 56 has a rectangular cross- section at its inlet end and a substantially circular cross-section at its outlet end.

The cross sectional area of the measurement conduit 52 is less than that the cross- sectional areas of the inlet tube 54 and the outlet tube 56, thereby accelerating the flow of fluid through the flow-rate measurement device. This is advantageous when the flow rate of the fluid is very low or when the flow borders between laminar and turbulent flow, since accelerating the flow can remove this ambiguity by ensuring the flow is turbulent. Accelerating the flow rate results in a greater difference in the time of flight f the signals going in each direction resulting in any errors equating to a smaller percentage error in the flow reading.

Figure 9a shows an ultrasonic flow-rate measurement device 58 according to a third embodiment of the invention. The device 58 of this embodiment is substantially the same as the device 50 of the previous embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

In this embodiment the inlet tube 54 is circular in cross-section at its inlet end and is rectangular in cross-section at its delivery end, where it is coupled to the measurement conduit 52. The cross-sectional shape of the inlet tube 54 changes along its length (towards the measurement conduit 52) such that the inlet conduit 54 has a constant cross-sectional area along its length, as it changes from a circular cross-section to a rectangular cross-section. The outlet tube 56 has a rectangular cross-section at its inlet end and a substantially circular cross-section at its outlet end. Similarly to the inlet conduit 54, the outlet conduit 56 has a constant cross-sectional area along its length and a changing cross-sectional shape. The shape of the inlet conduit 54 and the outlet conduit 56 result in a constant flow-rate of fluid through the flow-rate measurement device 58, i.e. the fluid flow does not accelerate through the flow-rate measurement device 58 and therefore there is no pressure drop created by the flow-rate measurement device 58.

An ultrasonic flow-rate measurement device 60 according to a fourth embodiment of the invention is shown in Figure 10. The device 60 of this embodiment is substantially the same as the device 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

In this embodiment the flexible PCB 62 is spiral wrapped around the external wall of the measurement conduit 62. Clamps (not shown) are provided to hold the flexible PCB 64 in place around the conduit 62. The cMUTs 14, 16 and the pressure sensor 32 are carried on the underside of the flexible PCB 64 and are provided within recesses (not visible in the drawing) formed in the external wall of the measurement conduit 62. As previously, the recesses extend to the internal wall of the conduit 62, the bottoms of the recesses comprising plastic membranes having a thickness of 0.1 -0.5mm and the recesses being filled with ultrasound coupling gel.

An ultrasonic flow-rate measurement device 70 according to a fifth embodiment of the invention is shown in Figure 11. The device 70 of this embodiment is substantially the same as the device 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

In this embodiment the measurement conduit 72 is formed with two dog-leg bends 74, 76 connected by a straight section 78. The cMUTs 14, 16 are provided on the vertical (as shown in the drawing) sections of the conduit 72 at either end of the straight section 78, so that they are linearly opposite one another. The cMUTs 14, 16 and the pressure sensor 32 are carried on the underside of the flexible PCB 80 and are provided within recesses (not visible in the drawing) formed in the external wall of the measurement conduit 72. As previously, the recesses extend to the internal wall of the conduit 72, the bottoms of the recesses comprising plastic membranes having a thickness of 0.1- 0.5mm and the recesses being filled with ultrasound coupling gel. Clamps (not shown) are provided to hold the flexible PCB 80 in place around the conduit 72.

An ultrasonic flow-rate measurement device 100 according to a sixth embodiment of the invention is shown in Figure 12. The device 100 of this embodiment is substantially the same as the ultrasonic flow-rate measurement device 70 of the previous embodiment, with the following modifications. The same reference numbers are retained for corresponding features. In this embodiment the measurement conduit 102 is arcuate in shape. The arcuate shape of the measurement conduit 102 results in a reduced fluid flow pressure loss as compared with the previous embodiment.

First and second ultrasonic transducer housings 104, 106 are provided towards either end of the measurement conduit 102. The housings 104, 106 are integrally formed with the wall of the measurement conduit 102 and extend tangentially from the conduit 102. The housings 104, 106 respectively define recesses 108, 110 in which the cMUTs 14, 16 are provided. The recesses 108, 110 are separated from the fluid flow by ultrasonic transparent plastic membranes 112, 114.

The cMUTs 14, 16 are arranged against the vertical (as depicted in the drawing) back walls of the recesses 108, 110, such that they face one another and are co-linearly aligned with each other. The space within the recesses 108, 110 between the cMUTs 14, 16 and the membranes 112, 114 is filled with PDMS 46.

The location of the cMUTs 14, 16 defines an ultrasonic pulse transmission path between the cMUTs 14, 16 which transects the arc defined by the measurement conduit 102, without contacting the walls of the measurement conduit 102. The pulse transmission path thereby passes through both the exterior and interior regions of the fluid flow radius, resulting in a measurement of the average flow-rate being recorded.

Figure 13 shows an ultrasonic flow rate measurement system 90 according to a seventh embodiment of the invention. The system 90 comprises an ultrasonic flow rate measurement device 10 according to the first embodiment and a controller 92.

The controller 92 is electrically coupled to the flexible PCB 18 and is operable to altematingly actuate the cMUTs 14, 16 to generate and receive ultrasonic signals, as follows: the first cMUT 14 generates an ultrasonic signal pulse which propagates through the fluid flow, to the reflector plate 26 and from the reflector plate 26 through the fluid flow again to the second cMUT 16 where it is received; and vice versa.

The controller 92 in this example is an application specific integrated circuit (ASIC) designed specifically for controlling and calculating the flow rate at a processing speed short enough to be compatible with the high frequency (short wavelength) ultrasound signals required for the short flight path of the ultrasonic flow rate measurement device 10. In this arrangement the pressure sensor 32 and the temperature sensor are integral with the ASIC 92 and the ASIC is mounted on the circuit board.

In this example, the rate of flow of the fluid is determined from the difference in the time- of-flight of the ultrasonic signal pulse from the first cMUT 14 (A) to the second cMUT 16 (B), and from the second cMUT 16 (B) to the first cMUT 14 (A). This method of measuring the rate of flow of a fluid is based on the fact that the ultrasound signal pulses travel faster through the moving fluid in the direction of the flow (AB) and slower when they are propagating through the fluid in the opposite direction to the flow (BA).

The time-of-flight of the ultrasound signal pulses in each direction, AB and BA, is given by the following:

t_AB = 2,L/(C+ V_f|_uid)

t_BA= 2.L/(C-V_f|_uid)

From which the flow rate of the fluid can be calculated as:

Vfluid = L((t_BA-tAB) / (W X UB))

Where:

C = Speed of sound in the fluid

L = Path length

Vfiuid = speed of fluid along pipe axis

The controller 92 is operable to store the time-of-flight in each direction and to calculate the flow rate of the fluid using the above equations, since the other variables are known.

In order to improve the accuracy of the measurement of the flow rate the controller is operable to actuate each cMUT 14, 16 400 times and to store each time-of-flight in each direction (T_AB and T_BA) in separate parts of its memory 94; the times-of-flight in the flow direction (T_AB in this example) are stored in a first memory part 94a, together with a count of the number of measurements made in that direction, and the times-of-flight in the direction opposite the flow direction are stored in a second memory part 94b, together with a count of the number of measurements made in that (opposite) direction. At the end of a measurement sequence of 400 measurement loops (known as "sing- arounds"), the controller 92 adds up all of the TAB times-of-flight and divides it by the count (i.e. 400 in this example) to obtain the average TAB time-of-flight, and similarly with the TBA times-of-flight to obtain the average T_BA time-of-flight. The average times- of-flight are then used to determine the flow rate of the fluid.

As an alternative to a time-of-flight based measurement of the flow rate, the controller is additionally operable to cause the cMUTs 14, 16 to generate an ultrasonic signal pulse having a chirped frequency spectrum. As the ultrasonic signal pulses propagate through the fluid flow the phase of the signal will be changed, and changed differently for each direction of propagation (i.e. with and against the flow). The controller 92 is then operable to compare the ultrasonic signal pulses following their propagation through the fluid in each direction to determine the phase difference between the signals, from which the flow rate of the fluid is determined.

If the flow rate goes beyond a certain speed, the phase difference between the ultrasonic signal pulses will reach 2π. When a 2π phase difference is reached the controller 92 changes its mode of operation from measuring phase difference to measuring time-of-flight.

As an alternative, for use with fluids containing particles or bubbles, the controller is operable to determine a Doppler effect induced shift in the frequency of an ultrasonic signal propagating through the fluid flow, from which the flow rate of the fluid is determined.

Various modifications may be made to the described embodiments without departing from the scope of the invention. For example, the ultrasonic transducers may alternatively comprise piezo-electric transducers. The measurement conduit may be of a different size and/or cross-section to those described. The ultrasonic signal pulses may have a different central frequency and/or bandwidth to those described.

The controller may be operable to carry out a different number of measurement loops (sing-arounds). The controller may alternatively comprise a stand alone device coupled to the flexible PCB. The pressure sensor may be provided separately to the ASIC. The temperature sensor may alternatively be provided separately from the ASIC, to ensure that the temperature reading is not affected by the heat produced by the ASIC.

The measurement conduit may alternatively take a form between the dogleg of Figure 11 and the radial section of Figure 12, with the ninety degree turns of the dogleg being replaced with radiused turns, or the inlet and outlet being provided at an angle.

Referring to Figure 14 a cross section of a measurement device according to the third aspect of the invention is shown comprising a flow sensor as described with reference to Figures 1 to 8 and furthermore having a cMUT transceiver 200 arranged opposite the reflector 26. The transceiver 200 is located in recess 202 separated from the measurement conduit 12 by a thin barrier layer 204 of the housing material in a similar manner as described above.

In a first mode of operation an ASIC 206 has an in built clock and uses the time taken for the emitted signal to be received back to calculate the speed of sound in the fluid passing through the measurement conduit. The ASIC 206 then compares the speed of sound at the measured temperature to a reference value to determine a characteristic of the fluid.

and, as the bulk modulus K has a much larger magnitude and is in the denominator then small changes in the density p can largely be ignored and c becomes a function of K. The ASIC 206 has a memory module 208 associated therewith that stores a look up table relating speed of sound to bulk modulus for a given temperature thereby enabling the ASIC 206 to determine the bulk modulus of the fluid passing therethrough at the measured temperature. As bulk modulus dependant of solute strength the sensor can be used to measure the solute strength of a fluid passing through a pipe. As the sensor is non contact, it can be used online in a process to continually monitor the fluid. In a second mode of operation the cMUT 200 is a high bandwidth cMUT and is activated with a voltage square wave to emit a mixed frequency impulse of ultrasound having wavelengths between 1 an 40 MHz. The ASIC 206 performs a fast Fourier transform (FFT) on the signal produced when the cMUT 200 receives the reflected signal to generate the Fourier spectrum. The Fourier spectrum forms a signature particular to an individual fluid and the signature can them be compared to values in a look up table in the memory module 208 to determine if the fluid is of the required characteristics or if it has changed in characteristic. Thus a sensor may be formed either to determine a characteristic of a fluid, for example a concentration of a solute, or for example to detect if the fluid has changed in character over time by comparing the signal to a reference and looking for a shift or change in the Fourier spectrum.

In a third mode of operation the cMUT 200 is a high bandwidth cMUT and is activated with constant amplitude frequency sweep to emit a linear chirp of ultrasound having wavelengths between 1 an 40 MHz. The ASIC 206 samples the received signal at a number of frequencies during the sweep and performs a fast Fourier transform (FFT) on the sampled to generate a Fourier spectrum for the fluid. The Fourier spectrum forms a signature particular to an individual fluid and the signature can them be compared to values in a look up table in the memory module 208 to determine if the fluid is of the required characteristics or if it has changed in characteristic. Thus a sensor may be formed either to determine a characteristic of a fluid, for example a concentration of a solute, or for example to detect if the fluid has changed in character over time by comparing the signal to a reference and looking for a shift or change in the Fourier spectrum.

One specific application of this sensor is for the monitoring of the concentration of a solute flowing through a pipe or tube. This application is especially applicable to the food and beverage production industry as it enables fluids to be constantly monitored for variation enabling a very high level of control over product quality.

A specific application of this sensor is for the monitoring of the quality of oil in an engine. Over time engine oil deteriorates for a number of reasons including cracking of hydrocarbons therein and the absorption of pollutants, for example the absorption of some exhaust gasses which might blow by the piston rings in the engine. The cumulative effect is that there is a degradation of the performance of the oil over time and the life of the engine is partially dependent on the quality of the oil being used. There present invention therefore enables the quality of the oil in an engine to be continuously monitored over time and a warning to be issued if the quality falls below an acceptable level.

The described embodiments provide various advantages, as follows. Firstly the device is non intrusive and can simply be fitted in line into existing systems. Secondly, and most importantly, a flow rate measurement device is provided which is non intrusive and which can be produced and assembled without the need for complex assembly processes as the alignment of the functional components is effected by the design. Thirdly, a non intrusive flow rate measurement device can be produced consisting primarily of parts which can be simple and efficiently manufactured by cost effective techniques, primarily plastic moulding, and fourthly it enables not only the flow rate to be measured but also the actual quality of the fluid flowing through the device.

Claims

1. An ultrasonic flow-rate measurement device comprising: a measurement conduit through which fluid to be measured flows; a first ultrasonic transducer provided at a first position on the conduit and a second ultrasonic transducer provided at a second position on the conduit, the first and second positions being separated in the flow direction; an electrical track circuit to which the first and second transducers are electrically coupled, the electrical tracks being adjacent to and conforming to the shape of at least part of the external wall of the conduit; and first and second recesses, formed in the wall of the conduit at the first and second positions, in which the first and second transducers are respectively received such that the transducers are ultrasonically coupled to the fluid flow through the bottoms of the recesses.

2. An ultrasonic flow-rate measurement device as claimed in claim 1 , wherein the electrical track circuit is provided directly on the external wall of the conduit, the first and second ultrasonic transducers being mechanically coupled to the external wall of the conduit.

3. An ultrasonic flow-rate measurement device as claimed in claim 2, wherein the first and second ultrasonic transducers are moulded into or adhered to the external wall of the conduit.

4. An ultrasonic flow-rate measurement device as claimed in claim 1 , wherein the electrical track circuit is provided on a printed circuit board, the printed circuit board being coupled to and conforming to the shape of the at least part of the external wall of the conduit, and the first and second ultrasonic transducers being mechanically coupled to the printed circuit board.

5. An ultrasonic flow-rate measurement device as claimed in claim 4, wherein the printed circuit board is a flexible printed circuit board.

6. An ultrasonic flow-rate measurement device as claimed in claim 1 wherein the electrical track circuit is provided directly on a conduit cover which attaches directly to the external wall of the conduit.

7. An ultrasonic flow-rate measurement device as claimed in claim 6 wherein the first and second ultrasonic transducers are moulded into, or adhered to, the surface of the conduit cover such that when the conduit cover is attached to the conduit the transducers are pre aligned with one another

8. An ultrasonic flow-rate measurement device as claimed in any preceding claim, wherein the ultrasonic transducers are operable to generate ultrasonic signal pulses having a steep rising edge.

9. An ultrasonic flow-rate measurement device as claimed in any preceding claim, wherein the ultrasonic transducers are micro electro-mechanical systems transducers.

10.An ultrasonic flow-rate measurement device as claimed in claim 9, wherein the ultrasonic transducers are capacitive micro-fabricated ultrasonic transducers.

11.An ultrasonic flow-rate measurement device as claimed in claim 10, wherein the ultrasonic signals generated by the capacitive micro-fabricated ultrasonic transducers have a beam width of substantially the same cross-sectional size as the transducers.

12.An ultrasonic flow-rate measurement device as claimed in claim 9, wherein the ultrasonic transducers comprise piezo-electric transducers.

13.An ultrasonic flow-rate measurement device as claimed in any preceding claim, wherein the measurement conduit is substantially circular in cross-section.

14. An ultrasonic flow-rate measurement device as claimed in any of claims 1 to 11 , wherein the measurement conduit is substantially rectangular in cross-section, having a width that is substantially the same as the beam width of an ultrasonic signal generated by the transducers.

15.An ultrasonic flow-rate measurement device as claimed in claim 13, wherein a generally frusto-conical inlet tube is provided, extending from the, in-use, up-stream end of the measurement conduit for coupling a fluid flow from a delivery conduit, generally of substantially circular cross-section, into the measurement conduit.

16.An ultrasonic flow-rate measurement device as claimed in claim 14, wherein the inlet tube is a Venturi tube, of substantially circular cross-section at its inlet end and rectangular cross-section at its delivery end, coupled to the measurement conduit.

17. An ultrasonic flow-rate measurement device as claimed in any of claims 13 to 15, wherein a generally frusto-conical outlet tube is provided, extending from the, in-use, down-stream end of the measurement conduit and being of rectangular cross- section at its inlet end and of substantially circular cross-section at its outlet end.

18.An ultrasonic flow-rate measurement device as claimed in claim 16, wherein the cross sectional area of the measurement conduit is less than that the cross-sectional areas of the inlet tube and the outlet tube, thereby accelerating the flow of fluid through the flow-rate measurement device.

19. An ultrasonic flow-rate measurement device as claimed in claim 16, wherein the cross section of the measurement conduit is substantially equal to the cross sectional areas of the inlet tube and the outlet tube thereby ensuring that the flow does not accelerate through the flow-rate measurement device and therefore there is no pressure drop created by the flow-rate measurement device.

20.An ultrasonic flow-rate measurement device as claimed in any preceding claim, wherein the ultrasonic flow-rate measurement device further comprises a pressure sensor.

21.An ultrasonic flow-rate measurement device as claimed in claim 19, wherein the pressure sensor is a capacitive membrane pressure sensor.

22. An ultrasonic flow-rate measurement device as claimed in any preceding claim, wherein the first and second positions are co-axially arranged with one another and an ultrasonic reflector is provided on the measurement conduit diagonally opposite and generally mid-way between the first and second positions.

23.An ultrasonic flow-rate measurement device as claimed in claim 22 wherein the ultrasonic reflector is preferably a metallic reflector, and may be a metallic flat plate.

24. An ultrasonic flow-rate measurement device as claimed in claim 22 wherein the ultrasonic reflector is a metallic curved reflector.

25. An ultrasonic flow-rate measurement device as claimed in claim 23 or claim 24 wherein the ultrasonic reflector is provided within a recess formed in the internal wall of the measurement conduit.

26.An ultrasonic flow-rate measurement device as claimed in claim 25 wherein the reflector is over-moulded with a plastic film to retain it within the recess and separate it from fluid flowing through the measurement conduit.

27.An ultrasonic flow-rate measurement device as claimed in claim 25 wherein the ultrasonic reflector is integrally moulded into the measurement conduit.

28. An ultrasonic flow-rate measurement device as claimed in claim 23 or 24 wherein the reflector is positioned in a recess formed in the external wall of the measurement conduit.

29. An ultrasonic flow-rate measurement device as claimed in claim 28 wherein any space within the recess between the ultrasonic reflector and the bottom of the recess is preferably filled with ultrasound coupling gel.

30. An ultrasonic flow-rate measurement device as claimed in claim 22, wherein the measurement conduit further comprises a transducer housing provided along the at least part of the external wall of the conduit, in which the first and second recesses are provided, the transducer housing being integrally formed with the external wall of the measurement conduit.

31.An ultrasonic flow-rate measurement device as claimed in claim 30, wherein the first and second recesses are formed at an angle to the longitudinal axis of the measurement conduit such that the ultrasonic transducers are orientated at an angle to the longitudinal axis of the measurement conduit, and are thereby angled towards the ultrasonic reflector and each other.

32.An ultrasonic flow-rate measurement device as claimed in claim 31 , wherein the measurement conduit and the transducer housing are made of an ultrasonicaily transparent material.

33. An ultrasonic flow-rate measurement device as claimed in claim 31 , wherein the measurement conduit and the transducer housing are made of an ultrasonicaily non- transparent plastics material.

34.An ultrasonic flow-rate measurement device as claimed in claim 33, wherein the first and second recesses comprise an external part and an internal part, the external part being generally V-shaped and the internal part comprising an enclosed recess extending from the wall of the external part which generally faces the ultrasonic reflector and having an ultrasound transparent base, wherein the respective ultrasonic transducers are located on the said wall of the external part, over the internal part enclosed recess.

35.An ultrasonic flow-rate measurement device as claimed in claim 34, wherein the internal part enclosed recess is substantially filled with ultrasound coupling gel.

36. An ultrasonic flow-rate measurement device as claimed in any of claims 21 to 26, wherein the pressure sensor is provided substantially in-line with and between the first and second ultrasonic transducers.

37. An ultrasonic flow-rate measurement device as claimed in claim 36, wherein the transducer housing further comprises a third recess, which is generally U-shaped, the pressure sensor being provided at the bottom of the recess such that it is coupled to the fluid flow.

38. An ultrasonic flow-rate measurement device as claimed in claim 37, wherein an aperture is provided in the bottom of the third recess through which the pressure sensor is coupled to the fluid flow and a non-permeable coating is provided on the pressure sensor to isolate fluid acting on it from the circuit board.

39. An ultrasonic flow-rate measurement device as claimed in any of claims 30 to 38, wherein the measurement conduit further comprises a cover member provided on the transducer housing, the cover member being of a complementary shape to the external surface of the transducer housing, such that the flexible circuit board, the ultrasonic transducers and the pressure sensor are clamped in position between the transducer housing and the cover member.

40.An ultrasonic flow-rate measurement device as claimed in claim 39, wherein the flexible circuit board is provided with a plurality of fold lines along which it is folded such that it is shaped to conform with the external surface of the transducer housing, the parts of the flexible circuit board on which the ultrasonic transducers are provided being located within the V-shaped external recesses and the part carrying the pressure sensor being located within the U-shaped recess.

41.An ultrasonic flow-rate measurement device as claimed in claim 39, wherein circuit board retaining means are provided on the transducer housing or the cover member for attaching the flexible circuit board to the transducer housing or the cover member such that when the cover member is located on the transducer housing the flexible circuit board is thereby deformed to conform to the shape of the external surface of the transducer housing.

42. An ultrasonic flow-rate measurement device as claimed in claim 21 , wherein the flexible circuit board is spiral wrapped around the external wall of the measurement conduit.

43. An ultrasonic flow-rate measurement device as claimed in any of claims 6 to 20, wherein the first and second positions are diagonally opposite one another.

44. An ultrasonic flow-rate measurement device as claimed in claim 43, wherein the flexible circuit board is spiral wrapped around the external wall of the measurement conduit.

45. An ultrasonic flow-rate measurement device as claimed in claim 6, wherein the measurement conduit comprises two dogleg bends connected by a straight measurement section, the first and second positions being located linearly opposite one another, on the verticals of the dogleg bends, at either end of the measurement section.

46.An ultrasonic flow-rate measurement device as claimed in claim 45, wherein the flexible circuit board is folded along a plurality of fold lines such that it is shaped to fold around the external surface of the measurement conduit in the linear direction.

47.An ultrasonic flow-rate measurement device as claimed in any of claims 44 to 46, wherein clamping means are provided for fixing the flexible circuit board in position around the measurement conduit.

48. An ultrasonic flow-rate measurement device as claimed in any preceding claim, wherein the ultrasonic flow-rate measurement device further comprises a fluid temperature sensor thermally coupled to the fluid flow.

49. An ultrasonic flow-rate measurement device as claimed in claim 48, wherein the fluid temperature sensor comprises a membrane mounted Titanium temperature sensor.

50. An ultrasonic flow-rate measurement system comprising: an ultrasonic flow-rate measurement device as claimed in any preceding claim; and control means operable to alternatingly actuate the ultrasonic transducers to generate and receive ultrasonic signals, and to monitor the transmission of the ultrasonic signals through the fluid flow to thereby determine the flow rate of the fluid through the measurement conduit.

51. An ultrasonic flow-rate measurement system as claimed in claim 50, wherein the control means is operable to determine the time-of-flight of each ultrasonic signal through the fluid flow, and from the times-of-flight in each direction to determine the flow rate of the fluid.

52. An ultrasonic flow-rate measurement system as claimed in claim 33, wherein the control means is operable to actuate the ultrasonic transducers to each generate and receive a plurality of ultrasonic signals, to store the time-of-flight of each signal, and to calculate the average time-of-flight in each direction through the measurement conduit.

53. An ultrasonic flow-rate measurement system as claimed in any of claims 50 to 52, wherein the control means is operable to cause the ultrasonic transducers to generate a frequency chirped ultrasonic signal and to compare the ultrasonic signals transmitted in each direction through the measurement conduit to determine any phase difference between counter propagating signals, from which the flow rate of the fluid is then determined.

54. An ultrasonic flow-rate measurement system as claimed claim 54, wherein the control means is further operable to change from measuring phase difference to measuring time-of-flight when a 2π phase-difference between counter propagating signals is reached.

55. An ultrasonic flow-rate measurement system as claimed in any of claims 20 to 54, wherein the control means is operable to determine a Doppler effect induced shift in the frequency of an ultrasonic signal propagating through the fluid flow, from which the flow rate of the fluid is determined.

56. An ultrasonic flow-rate measurement system as claimed in any of claims 52 to 54, wherein the control means comprises an application specific integrated circuit.

57. An ultrasonic flow-rate measurement system as claimed in claim 56, wherein the control means comprises memory means for storing calibration data.

58. An ultrasonic flow-rate measurement system as claimed in claim 56 or 57, wherein the pressure sensor is integral with or mounted on the application specific integrated circuit.

59. An ultrasonic flow-rate measurement system as claimed in any of claims 58 to 59, wherein the temperature sensor is integral with or mounted on the application specific integrated circuit.

60. An ultrasonic flow-rate measurement system as claimed in any of claims 58 to 59, wherein the application specific integrated circuit is provided on the flexible circuit board of the ultrasonic flow-rate measurement device.

61. An ultrasonic flow-rate measurement system as claimed in any of claims 56 to 58, wherein the temperature sensor is provided separately to the application specific integrated circuit, ensuring that the temperature reading is not affected by the heat produced by the application specific integrated circuit.

62. An ultrasonic flow-rate measurement system according to according to any of claims 50 to 61 further comprising: means for, in use, exciting an ultrasonic transducer to emit an ultrasonic signal into the measurement conduit; means of, in use, receiving said ultrasonic signal in a transducer once it has passed through at least a portion of the fluid flowing in the measurement conduit, and converting it into an electrical signal; and means of comparing the electric signal to a reference value to determine a characteristic of the fluid.

63. An ultrasonic flow-rate measurement system according to claim 62 wherein the emitting transducer and the receiving transducer are the first and seconds transducers.

64. An ultrasonic flow-rate measurement system according to claim 62 wherein the emitting and receiving transducers are one and the same and comprise a transceiver that transmits the ultrasonic signal, the signal reflected back onto it by an ultrasonic reflector whereupon it receives it.

65. An ultrasonic flow-rate measurement system according to claim 64 wherein the ultrasonic signal crosses the fluid substantially perpendicular to the direction of fluid flow.

66. An ultrasonic flow-rate measurement system according to claim 62 wherein the device comprises a temperature sensor and a processor means, the processor means including a clock configured to measure the time of flight of the ultrasound from the transmitter to the receiver, calculate the speed of sound in the fluid and generate an electrical signal corresponding thereto, the electrical signal indicative of the bulk modulus of the fluid at the measured temperature.

67. An ultrasonic flow-rate measurement system according to claim 66 wherein transmitter emits substantially a single wavelength or a narrow band of wavelength and is operated at a resonant frequency.

68. An ultrasonic flow-rate measurement system according to claim 66 or claim 67 wherein the transmitter and receiver send and receive a plurality of signals over a time period and the average speed of sound is calculated for the plurality of send/receive cycles.

69. An ultrasonic flow-rate measurement system according to any one of claims 62 to 65 wherein the transmitter is a broadband transmitter, arranged to transmit and receive a multiple frequency impulse

70. An ultrasonic flow-rate measurement system according to claim 69 having a processor, the processor configured to process the received signal and to convert it into an electrical signal indicative of a characteristic of the fluid.

71. An ultrasonic flow-rate measurement system according to claim 70 wherein the processor performs a Fourier transform on the received signal resulting in a signature specific to the fluid passing through the conduit and the processor compares the signature to reference values to determine a characteristic of the fluid.

72. An ultrasonic flow-rate measurement system according to claim 71 wherein the processor monitors the signature and/or characteristic to detect a variation from a desired value range.

73. An ultrasonic flow-rate measurement system according to any one of claims 62 to 65 wherein the transmitter is activated to emit a frequency sweep and a processor processes the received signal to convert it into an electrical signal indicative of a characteristic of the fluid.

74. An ultrasonic flow-rate measurement system according to claim 73 wherein the processor samples the received signal during the frequency sweep and conducts a Fourier transform on the multiple data sampled during the frequency sweep for the fluid.

75. An ultrasonic flow-rate measurement system according to claim 75 wherein the signature is compared to a reference value to determine a characteristic of the fluid

76. An ultrasonic flow-rate measurement system according to claim 75 wherein the signature is monitored to detect if the fluid characteristics differ from the required fluid characteristics.