Field of the invention

[0001] The present invention relates to ultrasonic flowmeters where the transducers switch between being used as transmitters and receivers for each one or few measurements.

Background of the invention

[0002] Ultrasonic flowmeters often involve various ways to decouple the transducers from the driver circuit between each one or few measurements, in order to be able to utilize all transducers as both transmitters and receivers. At the connection of a transducer, when being switched from receiver to transmitter role, it is preferable to wait for some time, a settling time, before performing the next measurement in order for the system, both the electrical and acoustic system, to reach a steady state of sufficiently low noise to obtain an acceptable measurement accuracy. However, it is desired to reduce the overall measurement time to save power and/or allow for more frequent measurement.

Summary of the invention

[0003] The inventors have identified the above-mentioned problems and challenges related to settling time in ultrasonic flowmeters and identified further causes to the challenges, in particular in relation to flowmeters that are powered by single supplies. The inventors have subsequently made the below-described invention by which may be achieved in various embodiments one or more of reducing the settling time, carry out measurements faster, reduce power consumption and/or increase measurement accuracy.

[0004] The invention relates to a utility meter comprising communication means, and a power supply. The utility meter further comprising an ultrasonic flowmeter comprising one or more ultrasonic transducers, a driver circuit and a receiver circuit. A switching arrangement is configured to selectively couple the driver circuit and/or the receiver circuit to at least one of the one or more ultrasonic transducers. The ultrasonic flowmeter further comprises a transducer biasing circuit arranged to establish a transducer bias voltage over the at least one of the one or more ultrasonic transducers before the switching arrangement couples the driver circuit to the at least one of the one or more ultrasonic transducers.

[0005] When connecting an ultrasonic transducer in a flowmeter to a driver circuit to perform a flow measurement, there may typically be different bias or DC point on each terminal of the transducer. One terminal may for example be connected to ground, while the other is connected to the idle output voltage of the driver circuit, for example half the supply voltage. The different bias or DC points make the ultrasound transducer, which has a capacitive property, to start charging. Charging a capacitive property requires a settling time until equilibrium and steady state is achieved. A simple connection without controlling the current makes the charging to happen fast, but also unstable and causes voltage overshoot and current to oscillate for a while, which is experienced as electrical and acoustical noise obstructing measurements until faded out. Charging through an impedance at the connection time offers a controlled charging but it is undesirable for the driver circuit to deliver the measurement signal through a high, current-impeding impedance and undesirable for the receiver circuit with non-matching transmission and receiving impedances. By means of a transducer biasing circuit, on the other hand, the present invention offers a controlled charging of the transducer before it is connected to the driver circuit. Controlling the charging also enables controlling when the transducer is charged, how long it takes, and how much noise it generates.

[0006] Establishing a transducer bias voltage over an ultrasonic transducer before it is coupled to a powered driver circuit may be advantageous, as the settling time or startup time may be reduced or even avoided, thereby making such a system ready to carry out ultrasonic measurements faster than a system without transducer bias voltage.

[0007] As ultrasonic flowmeter configurations involve various ways to decouple the transducers from the driver circuit between each one or few measurements, the reduction of settling time provided by embodiments of the present invention may thereby allow each measurement to be carried out faster than with conventional ultrasonic flowmeters.

[0008] The overall power consumption of an ultrasonic flowmeter of the invention may be reduced, as the measurement circuits do not have to be powered for as long a time for each measurement sequence. The reduced power consumption may be exploited for smaller batteries, for longer battery lifetime, or for other purposes, for example for more frequent measurements.

[0009] The measurement accuracy of an ultrasonic flowmeter of the invention may be improved over conventional ultrasonic flowmeters because the shorter settling time makes it feasible to wait for a less noisy state of the system before performing a measurement, without spending too much time and/or spending too much idling power.

[0010] There is a trade-off between the accuracy of a measurement and the time and power spent on that measurement. Embodiments of the present invention make it possible to improve one side of this balance without worsening the other or improve both sides.

[0011 ] The settling time or startup time include at least the time it takes after coupling a transducer to a driver circuit output before the transducer voltage and the idle voltage of the driver circuit output get equalized, the transducer and surrounding system stop oscillating from the settling process, and the system reaches a stable state ready for measurement. During the settling time, because of the voltage changes, the transducer may generate undesired artifacts such as acoustic and/or electric oscillations, which are referred to as noise. The smoother the voltage change, the less noise is produced and the faster the system reaches a stable state. The settling time or startup time may also include the preceding time it takes the driver circuit and other circuits to reach a stable state after connecting it to the power supply. As it is up to the designer of the system to decide at which level the system is sufficiently stable as defined as when it is capable of making measurements with acceptable accuracy, the settling time is in principle not objective, but is rather determined by the designer. It may either be set as a predetermined time, or as a threshold of noise level or other runtime assessable value.

[0012] Bias voltage refers to a voltage that offsets a signal or circuit node to a different reference level. Many electronical components, e.g. operational amplifiers, are typically more linear towards the middle of their dynamic range than near the extremes, and it is therefore preferred to process signals around the middle of the range, i.e. with the middle value as reference potential.

[0013] Transducer bias voltage refers, according to the present invention, to a bias voltage applied to the ultrasonic transducers to achieve that the transducers are charged with a DC potential when disconnected from the driver circuit. Thereby, when subsequently being connected to the powered driver circuit, there already exists a potential drop over the transducer, which therefore does not need to be charged from the driver circuit output. Thereby is avoided the current spikes and voltage changes that make ultrasonic transducers emit ultrasound and thereby introduce noise in the system each time it is connected or disconnected in conventional ultrasonic flowmeters.

[0014] A transducer biasing circuit is an arrangement for applying the transducer bias voltage to the transducers. Various embodiments are described below.

[0015] A utility meter may for example be a consumption meter, such as for example a water meter or any other fluid metering device for measuring flow and/or volume of a fluid, such as water, such as cold water, hot water, central heating water, district heating water, waste water, gas, etc. The communication means may be configured to communicate a measured consumption to for example a concentrator, gateway or server over local or remote networks, and may in some embodiments also be used for configuration, error messages, etc. The power supply may preferably be a battery of any kind, but may alternatively be a power supply connected to mains or other external power. The power supply may alternatively or in addition comprise or receive power from an alternative energy source such as a solar panel, near-field or far-field wireless power transfer, etc. The power supply may be a dual supply or a single supply. The utility meter may comprise electronics to implement the communication means, driver circuit, receiver circuit, switching arrangement, power supply, transducer biasing circuit, etc. The electronics may preferably comprise one or more processors and memory to store instructions for measurement control, calculations and communication, etc.

[0016] The ultrasonic flowmeter part of the utility meter makes ultrasound-based measurements which can be used to determine a flow rate, volume or other related values of a fluid flow in order to determine consumption of the utility. The measured value relating to the fluid may represent a current flow rate, an average flow rate, an accumulated volume, a fluid temperature, etc. The person skilled in the field of ultrasonic flowmeters knows several typical configurations and measurement schemes. In the context of the present invention, the reliance on ultrasound measurements using ultrasonic transducers, typically ultrasonic piezo transducers, has made it relevant to optimize how the ultrasonic transducers are utilized in order to reduce power consumption, make more accurate measurements, achieve other advantages, or provide an alternative to other solutions.

[0017] The driver circuit refers to the electronics that are arranged to establish a driver signal to be transmitted by an ultrasonic transducer. The driver circuit may in various embodiments generate the signal itself, e.g. from a memory, or may receive the desired signal at an input, e.g. from a measurement controller. The driver circuit preferably comprises an operational amplifier or other means for establishing, at a low- impedance output, an ultrasonic transducer driver signal with a suitable power. The output of the driver circuit is preferably biased to a steady state DC level which make the driver circuit operate in the middle of its dynamic range, where linearity is typically best. The biasing of the driver circuit output makes conventional transducer configurations generate noise when being connected and disconnected from the driver circuit. This may be solved or reduced by embodiments of the present invention.

[0018] The receiver circuit refers to the electronics that are arranged to measure ultrasound resulting from the transmission from the driver circuit, through the ultrasonic transducers and fluid. The receiver circuit preferably has a low-impedance, high sensitivity, input for receiving an ultrasound signal from an ultrasonic transducer, and the receiver circuit preferably comprises an operational amplifier or other means to achieve this while generating a measurement output for further processing. In various embodiments the receiver circuit may be configured similar to the driver circuit, or the driver circuit and receiver circuit may be configured as a single circuit sharing for example an operational amplifier. In various embodiments, a measurement input and/or reference input of the receiver circuit is preferably biased to a steady state DC level to make the receiver circuit operate in the middle of its dynamic range, where linearity is typically best. The biasing of the receiver circuit input makes conventional transducer configurations generate noise when being connected and disconnected from the receiver circuit. This may be solved or reduced by embodiments of the present invention.

[0019] The switching arrangement is provided to allow switching between the two transducers, which are physically fixed in relation to each other, and when in use, also physically fixed in relation to a fluid pipe on which the flowmeter is mounted. The switching arrangement, of which the person skilled in the art of ultrasonic flowmeters are aware of several known configurations, selects which of the transducers are connected to the driver circuit and which of the transducers are connected to the receiver circuit, and when those connections are established.

[0020] In some configurations, for example, the switching arrangement connects the driver circuit to one transducer and the receiver circuit to the other transducer for one measurement, and then reverses this to the opposite transducers for the next measurement, thereby causing each transducer alternate between the roles of sender and receiver. An example of another configuration is to have the switching arrangement connected one transducer for transmission, then disconnect that transducer and instead connect the other transducer for reception while the ultrasonic signal is underway through the fluid. Other configurations are also known.

[0021] However, switching arrangements in ultrasonic flowmeters generally and frequently connects and disconnects ultrasonic transducers from driver circuits and receiver circuits, causing establishment of current spikes and voltage changes, which makes the transducers generate noise in the system in conventional flowmeters. This may be solved or reduced by embodiments of the present invention.

[0022] In an embodiment, said driver circuit is powered by a single supply.

[0023] When the driver circuit is power by a single supply, e.g. a battery, it may be particularly advantageous to establish a transducer bias voltage over an ultrasonic transducer before it is coupled to the single supply-powered driver circuit. Single supply driven circuits are typically biased on the input side, thereby producing significantly different DC points on each side of the transducer upon connecting it to the driver circuit output, and therefore experience the biasing and transducer charging problem to a larger degree than circuits with dual supply.

[0024] Single supply powering of the driver circuit refers to the driver circuit being powered by the potential between the system’s reference potential, e.g. a ground plane GND, and a single DC voltage different from the reference potential. This is contrary to dual supply powering, also referred to as +/- supply, where the system’s reference potential is between the two supply potentials, typically in the middle between the supply potentials. In other words, a single supply may typically be said to have a single DC rail and a GND rail, whereas a dual supply is said to have a positive DC rail and a negative DC rail.

[0025] Processing of signals in the most linear range around the middle of the dynamic range of components such as operational amplifiers, i.e. around their reference potential, may be automatically or easily achieved by dual supply powering. However, with singly supply powering, where the system’s reference potential is at one of the extremes of the supply, it has to be decided whether to perform the processing in a less linear range or instead apply a bias voltage to offset the processing to reference the middle potential to utilize the better linearity.

[0026] As accuracy is pursued in ultrasonic flowmeters, and as the power supplies are typically single supplies for simplicity as the power usually comes from batteries, it is typically relevant to utilize biasing techniques to offset the processing in the driver circuit to about the middle of the operating range, i.e. about half the potential difference, as these circuits are often based on operational amplifiers. Therefore, single supply powered ultrasonic flowmeters offset the output of the driver circuit by a bias voltage to improve linearity. When the DC rail voltage is referred to as for example Vcc, the desired steady state DC level at the output of the driver circuit to achieve optimum linearity may thereby be Vcc/2, which may be achieved by applying a corresponding bias voltage to the output, making the driver signal transmitted to the transducer alternate around the level of Vcc/2 instead of around 0 V.

[0027] In an embodiment, said transducer biasing circuit comprises a high impedance connection between said power supply, such as said single supply, and said at least one ultrasonic transducer, preferably wherein said high impedance connection has an impedance in the range of 100 Q to 1 MQ, preferably in the range of 1 kQ to 100 kQ, such as for example 5 kQ, 10 kQ, 20 kQ or 30 kQ.

[0028] The high impedance connection is preferably configured to be able to establish and maintain a transducer bias voltage over the transducer, while not allowing too high an idle current to flow. The mentioned examples of impedances may achieve this for typical ultrasonic transducers. The high impedance connection may typically be a simple resistor as the purpose is to establish a steady state DC level. The skilled person may for example determine a suitable impedance value, e.g. a suitable resistor, on the basis of the impedance and capacitance of the ultrasonic transducer at the relevant voltage level, and in consideration of a suitable charging time, as elaborated below.

[0029] In an embodiment, said transducer biasing circuit has higher impedance than said driver circuit, such as least 10 times higher, for example at least 100 or 1000 times higher, e.g. at least 10,000 or 50,000 times higher.

[0030] As the transducer bias voltage is only used for applying and maintaining a charge to an ultrasonic transducer, and not used for supply to power-requiring applications, the impedance may be high, such as exemplified above.

[0031] In an embodiment, said transducer biasing circuit is configured to establish said transducer bias voltage without exciting any of said ultrasonic transducers. [0032] It may further be advantageous to only allow a low current to be provided from the transducer biasing circuit, as it may be preferable to charge the transducer slowly to not cause ultrasound to be emitted and add to the noise. On the other hand, it is not desirable to wait too long to accomplish the charging, because it may, as described above, also be an overall goal to minimize the time in which the system is powered to reduce power consumption. Therefore, the charging time, and thereby the components involved in the charging should be selected from a suitable balancing between not spending too much time and on the other hand not charging so quickly that noise is emitted by the transducer. A guideline for the charging time may also be to consider the time spent on preparing the driver circuit for transmission, for example by establishing a driver circuit bias voltage. This time frame may be suitable for also establishing the transducer bias voltage, so that the time where the system is unavailable for measurements is utilized efficiently to stabilize both the driver circuit and the relevant transducer(s) with bias voltages. The skilled person is able to determine a suitable balance based on the disclosure herein and the parameters and goals of his or her particular project. As an example, with a 1MHz transducer, a suitable charging time to design for, with regard to some embodiments, may be 10ps-50ps, for example 30ps.

[0033] In an embodiment, frequency content introduced by said establishment of said transducer bias voltage by said transducer biasing circuit is significantly lower than a resonance frequency of said ultrasonic transducers, such as less than half the resonance frequency, preferably less than a fifth of the resonance frequency, e.g. less than a tenth, a twentieth, a thirtieth or a fiftieth of the resonance frequency, e.g. less than a hundredth or thousandth of the resonance frequency.

[0034] Frequency content introduced by establishment of a bias voltage is related to the timing of the voltage change, as a changing voltage resembles an AC signal with a frequency content determined by the rate of change. The changing of a voltage level may also be referred to as a ramp-up or ramp-down of a voltage. The faster the voltage change, the higher the frequencies included in the frequency content. As ultrasonic transducers are easier excited at relatively higher frequencies approaching to their resonance frequency, e.g. 100kHz, 1MHz, lOMhz, and thereby generates ultrasound, it may be advantageous to avoid generating high frequency content, which in turn means that the voltage changes are in a preferred embodiment performed relatively slowly, e.g. without generating frequency content above 50kHz, preferably not above 20kHz, such as not above 10kHz, depending on the specific transducers of the application. As described above, the selected charging time is a trade-off between the stability achieved and the time spent, in other words between noise produced and energy spent. Therefore, the charging time should preferably neither be too slow, nor too fast. Reference is made to examples and further comments above.

[0035] In an embodiment, said transducer biasing circuit comprises a voltage divider between a ground plane and a DC voltage rail of power supply, such as said single supply, with said at least one of said one or more ultrasonic transducers connected between said ground plane and the output of said voltage divider.

[0036] A voltage divider between the supply voltage and ground is a simple and efficient way to establish a DC voltage usable as bias voltage, in particular in embodiments where the reference level is defined from the supply voltage, e.g. as half the supply voltage. Although a voltage divider applies a constant power draw, its advantages in terms of simplicity and stability may still be preferrable, in particular where the voltage divider can be arranged with a high impedance so that the power draw becomes negligible. Further, in circuits implementing a bleed resistor to discharge the ultrasound transducers in case a charge is produced thereon by pyroelectric effects or otherwise, the bleed resistor may be utilized as the lower part of the voltage divider, as the impedance magnitude typically used for bleeding may also be suitable for the transducer biasing circuit. Thereby only the upper part of the voltage divider has to be implemented to obtain a transducer biasing circuit.

[0037] In an embodiment, said transducer biasing circuit is configured to establish said transducer bias voltage at the level of an operational average voltage of the output of said driver circuit. [0038] Advantageously, the transducer bias voltage may be selected to be close to the average voltage, idle voltage or steady state voltage of the driver circuit output. Thereby, when the switching arrangement establishes the connection between transducer and driver circuit output, there is high probability that they are at approximately the same potential.

[0039] In an embodiment, said transducer biasing circuit is configured to establish said transducer bias voltage at the level of half the output voltage of said power supply, such as said single supply.

[0040] A transducer bias voltage of about half of the supply voltage, for example referred to as Vcc/2, may be an advantageous bias level in several embodiments, as this level is often the same or very close to the steady state output level of the driver circuit.

[0041] In an embodiment, the driver circuit comprises an output arranged to be coupled to said at least one of said one or more ultrasonic transducers by said switching arrangement and wherein said output of the driver circuit is a low-impedance output.

[0042] It may in many embodiments be advantageous to configure the driver circuit with low output impedance in order to transfer as much power as possible to the ultrasonic transducer when emitting ultrasound for measurements.

[0043] In an embodiment, said driver circuit comprises a single-supply operational amplifier, preferably wherein said output of said driver circuit is an output of said single-supply operational amplifier.

[0044] Op-amp outputs are typically low-impedance outputs, and thereby suitable for providing driver signals to ultrasonic transducers in ultrasonic flowmeters. Preferably there is not applied further impedances between the op-amp output and the ultrasonic transducer. In some embodiments, a small resistive, capacitive and/or inductive impedance may be applied. However, the driver circuit output impedance should be kept sufficiently low to drive the transducer with as much power as available. [0045] In an embodiment, said driver circuit is provided with a driver circuit bias voltage, and wherein said transducer bias voltage is substantially equal to said driver circuit bias voltage.

[0046] As described above, it is typically relevant to utilize biasing techniques to offset the processing in battery-driven driver circuits to about the middle of the operating range. Therefore, single supply powered ultrasonic flowmeters may offset the output of the driver circuit by a bias voltage to improve linearity. It may be advantageous that the biasing level of the driver circuit output and the transducer are similar, e.g. as close to each other as possible, e.g. substantially equal, e.g. within 20%, 10%, 5%, 2% or 1% of each other, in order to minimize sudden current spikes and voltage changes when the switching arrangement couples them together. It may be advantageous that the transducer biasing circuit and the circuit to establish the driver circuit bias voltage are dimensioned with corresponding time constants or other to achieve corresponding charging times, so that the time where the system is unavailable for measurements is utilized efficiently to stabilize both the driver circuit and the relevant transducer(s) with bias voltages.

[0047] In an embodiment, said power supply, such as said single supply, provides a ground plane and a DC voltage rail different from said ground plane, preferably wherein said DC voltage rail is in the range of 0.8 V to 12 V, preferably in the range of 1.5 V to 6 V, such as for example approximately 1.8V, 3.6 V or 5 V.

[0048] In an embodiment, said power supply, such as said single supply, comprises a battery.

[0049] In an embodiment, a power supply switch is configured to selectively disconnect said driver circuit from said power supply, preferably wherein said power supply switch is configured to also disconnect said transducer biasing circuit from said power supply together with disconnecting said driver circuit.

[0050] The driver circuit may in various embodiments consume power even when idling, for example in preferred embodiment where the driver circuit is based on an operational amplifier. It is preferred to provide a power supply switch with the power supply to be able to disconnect the driver circuit when the flowmeter is not performing measurements in order to save power. In such an embodiment, it may be advantageous to further disconnect the transducer biasing circuit at the same time, to disable a possible current draw for example when implemented as a voltage divider. The power supply switch may be implemented to disconnect the power supply completely, or only disconnect selected sub-circuits, such as the driver circuit, receiver circuit and transducer biasing circuit. The power supply switch may in various embodiments comprise a load switch, an analog switch, GPIO of for example a microcontroller, or any other switching arrangement that can be electronically or programmatically operated and is suitable to handle a current draw of the subcircuits to be disconnected.

[0051] In an embodiment, a controller connected to said switching arrangement is configured to control each of said two ultrasonic transducers to act as transmitter or receiver.

[0052] In an embodiment, said controller is configured to control said power supply switch to connect and disconnect the power supply from one or more of said driver circuit, said transducer biasing circuit and said receiver circuit.

[0053] In an embodiment, said transducer biasing circuit is arranged to further establish said transducer bias voltage over said at least one of said one or more ultrasonic transducers before said switching arrangement couples said receiver circuit to said at least one of said one or more ultrasonic transducers.

[0054] It may be advantageous to ensure a similar transducer bias voltage over the transducers before they are connected to an input of the receiver circuit to avoid sudden voltage changes and current spikes and thereby noise generation. As the receiver circuit in preferred embodiments are build around a single supply operational amplifier like the driver circuit, the same advantages related to biasing apply to the receiver circuit, e.g. to improve linearity when measuring the ultrasound. For example, it may be advantageous to apply a bias voltage to the receiver circuit input to offset the reference level to half the supply voltage, e.g. Vcc/2. Thereby, according to the present invention, it may be advantageous to provide a similar transducer bias voltage before connecting a transducer to the receiver circuit input.

[0055] In an embodiment, said flowmeter comprises a driver biasing circuit arranged to establish a driver bias voltage as reference voltage before said switching arrangement couples said driver circuit to said at least one of said one or more ultrasonic transducers

[0056] In an embodiment, said utility meter comprises a display, optionally as part of said communication means.

[0057] The display may be configured to show the measured consumption and/or status information.

The drawings

[0058] Various embodiments of the invention will in the following be described with reference to the drawings wherein: figs. 1-4 are simplified circuit diagrams illustrating different prior art circuits, fig. 5 is a plot of transducer current and voltage around the time of coupling the transducer to the driver circuit in the prior art circuit of fig. 4, fig. 6 is a block diagram of an embodiment of the present invention, fig. 7 is a simplified circuit diagram illustrating an embodiment of the present invention, fig. 8 is a plot of transducer current and voltage around the time of coupling the transducer to the driver circuit in the embodiment of fig. 7, fig. 9 is a partial circuit diagram of a driver circuit and single transducer with a biasing circuit according to an embodiment of the invention, figs. 10-11 illustrate a utility meter comprising an embodiment of a flowmeter according to the invention, and figs. 12-14 are simplified circuit diagrams illustrating the application of embodiments of the invention to various flowmeter concepts.

Detailed description

[0059] Many ultrasonic flowmeters for measuring the flow rate of a fluid are based on a principle method where a signal is generated by a driver circuit and an ultrasonic transducer, transmitted through the fluid and measured by another ultrasonic transducer and a receiver circuit, repeated in the opposite direction through the fluid, and finally calculating a flow rate based on a difference between the measured signals.

[0060] In order to easily perform the opposite measurement without having duplicate driver and receiver circuits or more than two transducers and without means to reverse the fluid flow or relocate the transducers, different solutions have been employed where switches are used to rearrange the electrical connections to the transducers so that they can alternate between the sending and receiving roles simply by controlling the switches electronically.

[0061] Figs. 1-3 illustrate by simplified circuit diagrams a few examples of how switching solutions have been employed for the above-described purpose in ultrasonic flowmeters. In the example of Fig. 1, a driver circuit 3 and a receiver circuit 4 can by means of a switching arrangement 5 independently be coupled to any of two ultrasonic transducers 2a, 2b which are located in acoustic connection with the fluid flow 9 to be measured. At the instance shown in the example, transducer 2b is connected to the driver circuit 3, which receives a measurement signal Ms, generates a driver output signal Dout for the transducer 2b to generate an ultrasonic wave in the fluid flow 9. When the ultrasonic wave reaches the transducer 2a it generates an electrical signal and as transducer 2a is coupled to the receiver circuit 4 by means of the switching arrangement, the electrical signal form a receiver input signal Rin, and the receiver circuit 4 generates a measurement output Mout. A moment later, the switching arrangement 5 decouples transducer 2a from the receiver circuit 4 and instead connects it to driver circuit 3, and vice versa for transducer 2b, and the measurement procedure is repeated, thereby causing an ultrasonic wave to be measured in the fluid flow in the opposite direction as indicated by the dashed arrow. [0062] The example of Fig. 2 has a single common transceiver circuit operating as both driver circuit 3 and receiver circuit 4. This is possible in flowmeters with sufficient ultrasonic wave propagation delay, i.e. distance between the transducers, to allow the common transceiver circuit to change role from driving to measuring during the ultrasound propagation. Although the driver circuit 3 and receiver circuit 4 are implemented around a common operational amplifier, the operation principle of the switching arrangement 5 is the same as described above in relation to Fig. 1. In order to ensure that only one of the transducers is connected when sending, and only the other transducer is connected when receiving, the switching arrangement needs an additional switch position that is either floating or tied to ground to completely decouple one transducer at a time. The example of Fig. 3 is very similar to the example of Fig. 2 described above but allows to avoid the additional switch position due to a common connection from both transducers to the driver output Dout and receiver input Rin. The switching principle to produce the two measurements of opposite direction is however the same as described above for Figs. 1-2.

[0063] Fig. 4 is a highly simplified diagram of the switching principle used in the prior art, such as the examples of Figs. 1-3, in order to illustrate some of the problems and challenges identified by the present inventors. The diagram of Fig. 4 illustrates a driver circuit 3, here reduced to an amplifier symbol, an ultrasonic transducer 2 connectable between the output of the driver circuit 3 and a ground potential, and a controllable switching arrangement 5 to couple and decouple the transducer 2 from the output of the driver circuit 3. A power supply 6 provides a supply voltage Vcc for powering the driver circuit 3.

[0064] A driver circuit 3 is typically and preferably implemented so that its idle output potential is around the middle between the low and high supply potentials, i.e. around Vcc/2 in this example, thereby allowing the output to swing in both directions with maximum dynamic range. Further, many electronical components, e.g. operational amplifiers, are typically more linear towards the middle of their dynamic range than near the extremes, and it is therefore preferred to process signals around the middle of the range, i.e. with the middle value as reference potential. [0065] The transducer 2, on the other hand, typically experiences a floating or ground-tied position before being coupled to the output of the driver circuit 3, for example when being switched away from a receiver circuit connection or from a floating switch position.

[0066] At the instance the switching arrangement 5 establishes connection between the output of the driver circuit 3 and the transducer 2, there will thus be established a voltage difference V(transducer). As ultrasonic transducers 2 have capacitive properties, the voltage difference V(transducer) will also generate a charging current I(transducer) until the capacitive element of the transducer reaches equilibrium. The transducer is also generating ultrasound due to the charging current. After a while, herein referred to as settling time, the V(transducer) and I(transducer) will idle out around Vcc/2 V and 0 A, respectively, and stop generating ultrasound until a driver signal is received.

[0067] Fig. 5 is a plot of transducer current I(transducer) and voltage V(transducer) around the time of coupling the transducer 2 to the driver circuit 3 in the prior art circuit described above with reference to Fig. 4. A simulation was conducted where the driver circuit 3 at the time toN= lOps, was provided with a supply voltage Vcc = 3.6V and an input signal bias of Vcc/2 = 1.8V to cause the idle output of the driver circuit 3 to settle also at around Vcc/2 = 1.8V. At a switching time tsw = 25ps, the switching arrangement 5 coupled the floating terminal of the transducer 2 to the driver circuit output.

[0068] As seen from the plots in Fig. 5, the I(transducer) was 0mA and the V(transducer) was 0.0V during the period of floating transducer terminal, until the switching time tsw. Then the capacitive property of the transducer 2 was charged very quickly upon the connection being made, but the quick charging resulted in a peak current I(transducer) of 58.5mA, and an unstable period starting with current swings having peak-to-peak amplitude of about 700pA and taking at least 20ps to settle. The transducer voltage V(transducer) had an overshoot of about 49m V, i.e. to about 1.849V, upon connection, an unstable period starting with voltage swings with peak- to-peak amplitude of about 5mV, and spending at least 20ps to settle. As the high peak amplitude, the voltage overshoot, and the swinging for at least 20ps will be superimposed on any measurement signals during this period, it is unreasonable to start using the measuring circuit until it has stabilized. Moreover, the ultrasound generated by the transducer because of the high current peak and current oscillations during this time causes acoustic and physical oscillations that also have to fade out before undisturbed measurements can be made. The charging could be controlled by adding a resistor between the driver circuit output and the transducer to reduce the peak current, but this is undesirable because of the added impedance to signals, and because of the prolonged settling time.

[0069] Prior art flowmeters with a switching arrangement to avoid duplicate components as exemplified above, typically perform periodic measurement cycles involving that a first ultrasonic signal is transmitted through the water or other fluid in one direction, and then a second ultrasonic signal is transmitted in the opposite direction. The flow rate is determined based on the difference in transit time of the first and second ultrasonic signal. Multiple variants of this general scheme is in use in different ultrasonic flowmeters. Variants may for example include different number of repetitions at different intervals, averaging at different stages, etc.

[0070] In a generalized form, a measurement cycle in a flowmeter may typically include the following steps, here only considering the actions of the driver circuit:

1. ton: Connect driver circuit to power supply

For power saving purposes, several components including the driver circuit are typically disconnected or in a sleep mode when not in use.

2. Wait for steady state

There is a settling time for the driver circuit and other components that have been disconnected or in sleep more to become stabilized and ready for measuring.

3. tsw.i : Connect first transducer to driver circuit

The transducer may come from idling and is connected to the driver circuit by means of the switching arrangement.

4. Wait for steady state for first transducer The unstable period starting from tsw as described above and shown in the plots of Fig. 5. For prior art systems, usual waiting times for achieving low noise in both the electronic and acoustic systems at this point may lie between 10 - 500 ps, such as 50 - 300 ps, for example 150 ps.

5. Transmit first ultrasonic signal from first transducer

When everything is stabilized, the driver circuit produces a driver output signal causing the transducer to generate a well-defined ultrasound signal.

6. Wait while measuring

During this time, the second transducer and the receiving circuit are active. The ultrasound propagation delay between the two transducers may for example be in the order of 30ps - lOOps, depending on the distance between them. Another for example 20ps may be delayed in order to allow the oscillations caused by the measurement wave to fade out.

7. Disconnect first transducer from driver circuit

Using the switching arrangement.

8. tsw, 2: Connect second transducer to driver circuit

The transducer comes from the role as receiving transducer and is connected to the driver circuit by means of the switching arrangement.

9. Wait for steady state for second transducer

The unstable period starting from tSW as described above and shown in the plots of Fig. 5. For prior art systems, usual waiting times for achieving low noise in both the electronic and acoustic systems at this point may lie between 10 - 500 ps, such as 50 - 300 ps, for example 150 ps.

10. Transmit second ultrasonic signal from second transducer

When everything is stabilized, the driver circuit produces a driver output signal causing the transducer to generate a well-defined ultrasound signal.

11. Wait while measuring

During this time, the first transducer and the receiving circuit are active.

12. Disconnect second transducer from driver circuit

Using the switching arrangement.

13. [optionally repeat from step 3 if more measurements are desired for averaging]

14. Disconnect driver circuit from power supply for power saving 15. Wait for next measurement cycle

16. [repeat from step 1]

[0071] Fig. 6 is a block diagram of an embodiment of the invention. It comprises a driver circuit 3 configured to receive a measurement signal Ms and generate a driver output signal Dout. It also comprises receiver circuit 4 configured to receive a receiver input signal Rin and produce a measurement output Mout. Two ultrasonic transducers 2a, 2b, are provided in acoustic connection with a fluid flow 9 to be measured. A switching arrangement 5 is arranged to couple the driver output signal Dout to one of the transducers 2a, 2b, for establishment of ultrasound in the fluid flow, and to couple one of the transducers 2b, 2a, to the receiver input signal Rin to establish an electrical measurement signal from received ultrasound. A power supply 6 provides a supply voltage Vcc to the driver circuit 3, the receiver circuit 4, and possibly other electronic components of the flow meter. The driver circuit 3, receiver circuit 4, switching arrangement 5, transducers 2a, 2b, and power supply 6 may be implemented and arranged according to the prior art, for example as shown and described with reference to Figs. 1-3, including embodiments with a driver circuit 3 and receiver circuit 4 sharing a common amplifier, and may be configured to perform measurement cycles in accordance with the generalized measurement procedure of steps 1-16 described above.

[0072] Unlike the prior art flowmeters, however, the embodiment of Fig. 6 further comprises at least one transducer biasing circuit 7 configured to mitigate the instability starting at the switching time tsw by pre-charging the capacitive element of the transducer at a biasing voltage potential Vb similar to the idle output of the driver circuit 3, so that a steady state can be achieved significantly faster in steps 4 and 9 of the measuring procedure above. Examples of how this can be achieved are discussed below. By this feature, embodiments of the invention enable reducing the waiting times during measuring, making the measurements faster. Faster measuring means that the active components such as the driver circuit and receiver circuit amplifiers are only active for shorter periods and thereby consume less power. Alternatively, embodiments of the invention may utilize the same waiting times as usual, and thereby achieve even better stabilization during the same time and thereby enable more accurate measurement from a less noisy system. A balancing of reducing some of the waiting time but still achieving a less noisy system may also be an advantageous option enabled by the present invention. Note that ‘noisy system’ may refer both to electrical noise until a steady state is achieved, and acoustic noise from ultrasound generated by the transducer due to electrical noise or other stimuli.

[0073] Fig. 7 is a simplified circuit diagram illustrating how a transducer biasing circuit 7 can be added to an ultrasonic flowmeter in an embodiment. This embodiment is based on the same flowmeter circuit as described with reference to Fig. 4 above, and thus comprises a driver circuit 3, a transducer 2, a switching arrangement 5 configured to couple and decouple the driver circuit 3 and transducer 2, and a power supply 6 delivering a supply voltage Vcc to the driver circuit 3. The driver circuit 3 is implemented in such a way that its idle output is around the middle between its supply voltages, i.e. around Vcc/2, as described above with reference to Fig. 4, and for the same reasons.

[0074] The embodiment of fig. 7 further comprises a transducer biasing circuit 7 comprising a simple voltage divider consisting of a resistor 71 between the upper supply voltage Vcc and the upper transducer terminal, and a resistor 72 between the upper transducer terminal and the lower supply voltage reference, in this embodiment ground GND. Both the resistors 71, 72 have the same resistance R, thereby dividing the supply voltage Vcc evenly, and ensuring a biasing voltage Vb at the same level as Vcc/2 at the upper transducer terminal. Instead of the upper transducer terminal being floating as in Fig. 4 or tied to ground, it is by the present invention tied to, or biased with, the same voltage potential as the idle output of the driver circuit.

[0075] In an embodiment which, for any reason, is having a different idle output voltage of the driver circuit than Vcc/2, the transducer biasing circuit 7 may be modified to provide the same biasing voltage Vb as the idle output voltage of the driver circuit, for example by selecting an appropriate ratio between the resistors 71, 72. The biasing voltage Vb may in an embodiment be made adjustable by replacing one or both of resistors 71, 72 with adjustable resistors. [0076] With the embodiment of Fig. 7, as soon as the power supply is switched on or connected, there will be established a voltage difference V(transducer) equal to the biasing voltage Vb. The voltage difference V(transducer) will also generate a charging current I(transducer) until the capacitive element of the transducer reaches equilibrium, and the transducer may generate ultrasound due to the charging current. After a while, the V(transducer) and I(transducer) will idle out around Vb = Vcc/2 V and 0 A, respectively, and stop generating ultrasound until a driver signal is received. When the switching arrangement 5 later establishes connection between the output of the driver circuit 3, having an idle output level of Vcc/2, and the transducer 2, also having V(transducer) of Vcc/2, the V(transducer) does not change, does not generate a charging current I(transducer), and thereby does not generate ultrasound. Accordingly, the instability will occur for a period after time toN when the power supply is turned on or connected, and substantially not at time tsw when the transducer is coupled to the driver circuit.

[0077] Further, the transducer biasing circuit 7 makes it possible to control the charging time, peak current and oscillations by controlling the current delivered by the transducer biasing circuit 7. In the voltage divider example of Fig. 7, the resistance of resistors 71, 72, define the possible charging current I(transducer), and thereby also the settling time.

[0078] Fig. 8 is a plot of transducer current I(transducer) and voltage V(transducer) around the time of coupling the transducer 2 to the driver circuit 3 in the embodiment of the invention described above with reference to Fig. 7. A simulation was conducted with the transducer biasing circuit 7 resistor values R = 30kQ each. At the time toN+BiAS = 10ps the driver circuit 3 was provided with a supply voltage Vcc = 3.6V and an input signal bias of Vcc/2 = 1.8V to cause the idle output of the driver circuit 3 to settle also at around Vcc/2 = 1.8V, and at the same time the transducer biasing circuit 7, i.e. in this embodiment the voltage divider consisting of the resistors 71, 72, was provided with the supply voltage Vcc in order to establish a biasing voltage Vb = Vcc/2 = 1.8V. 15ps later, at a switching time tsw = 25ps, the switching arrangement 5 coupled the Vb-biased terminal of the transducer 2 to the driver circuit output.

[0079] As seen from the plots in Fig. 8, the transducer biasing circuit 7 caused the V(transducer) to start a controlled rise from 0.0V at the time toN+BiAs= 10ps towards the level of Vb = 1 ,8V which was reached around the time tsw = 25ps. During this precharging of the capacitive element of the ultrasonic transducer 2, the peak amplitude was experienced right at the time ION+BIAS at around 120pA, but fell quickly and without oscillation. At the time tsw = 25ps when the switching arrangement coupled the transducer to the driver circuit, an insignificant current peak of around 20pA and an insignificant voltage overshoot of around 0.25mV was detected. The I(transducer) and V(transducer) stabilized at 0A and 1.8V during less than Ips after time tsw = 25ps.

[0080] Note that the resolutions of the main vertical axes (0-60mA and 0-2 V) are the same in the plots of the prior art simulation Fig. 5 and the embodiment of the invention simulation Fig. 8, but the magnified inserts of current have different scales, being -400pAto 400pA in Fig. 5, but only OpA to 125pA in Fig. 8. On the major 60mA scale the peak charging current in Fig. 5 of 58.5mA is very significant, and even the following current oscillations and the 49mV overshoot can be seen on the nonmagnified plot in Fig 5. On the other hand, in Fig. 8, the tiny peak charging current of 120pA at lOps can barely be detected on the 60mA-scale, and the controlled voltage progress is clearly recognized. Comparing the Fig. 4+5 prior art simulation with the Fig. 7+8 simulation of an embodiment of the invention makes it clear that the latter achieves a steady state far earlier, and at the same time generates less noise both electrically and ultrasonic acoustically. With the present invention it may thereby be reasonable to start measuring after a much shorter waiting time in steps 4 and 9 of the measurement procedure described above, and/or expect more accurate results due to reduced noise.

[0081] Fig. 9 is a circuit diagram of the transmiter part of an ultrasonic flowmeter in an embodiment of the invention. A switching arrangement 5 (SW1) is provided to controllably couple an ultrasonic transducer 2 (T) to the output of a driver circuit 3 based around an operational amplifier (Ul) for transmitting ultrasonic signals in a fluid flow for flow measurement possibly by conventional principles.

[0082] A power supply 6 is provided to power the electronics, including the operational amplifier Ul although power supply connections are not shown in the drawing. Thereby the power supply 6 indirectly controls or affects the upper and lower output levels of the driver circuit 3. In this embodiment, the power supply 6 comprises a voltage source (VI), for example a battery, and a controllable power supply switch 61 (SW2) allowing to shut down some or all of the electronics for power saving when not performing measurements. The power supply switch may in various embodiments comprise a load switch, an analog switch, GPIO of for example a microcontroller, or any other switching arrangement that can be electronically or programmatically operated and is suitable to handle a current draw of the subcircuits to be disconnected. Alternative embodiments may be configured so that the power supply and/or some or all of the electronics can be set to a sleep mode or otherwise reduce power consumption. The power supply 6 may for example provide DC at levels of 3.6V, 5V, 1.8 V or other voltages depending on the circuit design and component specifications.

[0083] A driver biasing circuit 8 is provided to establish a desired reference voltage level for the driver circuit, in this case for the positive input terminal of the operational amplifier Ul. In this example, the driver biasing circuit 8 comprises a simple voltage divider (R5, R4) and a noise filtering capacitor (C2). The driver biasing circuit 8 may for example and preferably provide a reference voltage in the middle of the power supply range, e.g. Vcc/2, such as for example 1.8V, 2.5V or 0.9V, which may in this example be achievable by selecting R4 = R5.

[0084] A transducer biasing circuit 7 is further provided in order to pre-charge the capacitive element of the transducer 2 (T) as described above with reference to figs. 6- 8. In this example, a simple voltage divider (R3, R2) divides the supply voltage to establish a biasing voltage Vb to charge the transducer 2 (T). The transducer biasing circuit 7 may for example and preferably provide a reference voltage equal to the reference voltage of the driver circuit, preferably in the middle of the power supply range, e.g. Vcc/2, such as for example 1.8V, 2.5V or 0.9V, which may in this example be achievable by selecting R3 = R2.

[0085] As understood from the plots of Fig. 8 and the description of Figs. 6-8, the transducer biasing circuit 7 achieves a controlled charging of the transducer 2, so that when the switching arrangement 5 (SW1) couples the transducer to the driver circuit 3, there will ideally be no voltage difference, and thereby no current surge at the connection time. The controlled charging, in this example through resistor R3, may preferably be designed to take longer than a simple short circuit to Vcc/2. An advantage of this is that instability, overshoot and oscillation that was evident in for example the plot of Fig. 5 above, and in turn ultrasound oscillations resulting therefrom, is reduced to nothing or at least an insignificant amount, resulting in the smooth charging curve evident from the plot of Fig. 8 above. While the controlled charging takes longer than a high-current charging, the total time from charging starts to all noise and oscillations have reliably faded out, is often shorter and more predictable than for high-current charging.

[0086] A further advantage is that the controlled charging by means of the transducer biasing circuit 7 of the present invention can be started at any time, preferably prior to coupling the transducer to the driver output by the switching arrangement.

[0087] Preferably the driver biasing circuit 8 and the transducer biasing circuit 7 may be coordinated to aim for similar settling time or charging time. This may be achieved by selecting the resistors R3, R2 according to a time constant or capacitance properties of the transducer 2, while selecting the resistors R5, R4 according to similar considerations about the driver circuit, primarily the operational amplifier Ul. In an embodiment R5 = R4 = R3 = R2. When the components are selected for similar settling or charging time, the preparation of the driver circuit, i.e. the settling time after turning on the driver circuit, may be similar to the preparation time of the transducer, i.e. the charging time after powering connecting the transducer biasing circuit. When both are started at the same time, both preparations may, as enabled by the present invention, be conducted in parallel, thereby reducing the total waiting time to only one settling period, instead of first waiting for the driver circuit, then waiting for the transducer.

[0088] In terms of additional components, the present invention is very simple and inexpensive to implement, takes up very little space, does not introduce disturbances, and consumes an insignificant amount of power depending on the resistor values, as it merely requires an inexpensive voltage divider designed for very low current. Further it is mentioned that some prior art flowmeter implementations already include a resistor in parallel with the transducer, a so-called bleed-resistor, in order to mitigate current generated by the transducer in case of temperature changes. The bleed-resistors have relatively high resistance in order to not draw too much current from the measurement signals, and may thereby correspond to resistor 72 of the transducer biasing circuit 7 of the embodiments of the present invention. Modifying a flowmeter circuit with bleed-resistors to obtain an embodiment of the present invention, thereby only requires one resistor 71 per transducer and a configuration enabling connecting and disconnecting resistor 71 from the power supply for example in synchrony with turning the driver circuit on and off. Even if preferring not to implement a voltage divider, other voltage biasing solutions exists which are suitable for transducer biasing circuit 7 for establishing Vb, which also are relatively inexpensive and with small footprints, and relatively easy to add to existing flowmeter circuit designs.

[0089] Figs. 10-11 illustrate an example of a utility meter 10 for measuring a fluid flow 9 through a pipe by means of a ultrasonic flowmeter as described herein. The flowmeter comprises ultrasonic transducers 2a, 2b, located in acoustic connection with the fluid through a housing wall, a membrane or in direct contact. In the example shown, the acoustic distance between the transducers have been extended by rotating the transducers so that they ‘shoot’ towards the opposite side of the fluid pipe, where acoustic mirrors, e.g. metal sheets, cause the ultrasonic wave to zigzag through the fluid flow before reaching the other transducer. The invention is not limited or significantly influenced by any specific configuration of the ultrasound path or fluid path. The utility meter 10 comprises electronics 12 configured to perform the flow meter tasks using driver circuits, receiver circuits and the ultrasonic transducers 2a, 2b, for example as described. The electronics 12 may preferably comprise a processor and memory to perform the controlling and calculation parts of the flow metering. The utility meter 10 further comprises one or more batteries 6 acting as power supplies or a part of a power supply, as the power supply may preferably further comprise DC/DC converters, filters, etc. to produce a stable supply voltage at a desired voltage level from the battery voltage. The utility meter 10 may further comprise one or more displays 11 to provide measurement values and messages to a user. The utility meter may also comprise wireless communication means as part of the electronics 12 to communicate measurement values and messages to a user, and/or receive configuration and control messages. In an embodiment a utility meter needs not both a display 11 and a wireless communication means.

[0090] Figs. 12-14 illustrate how a transducer biasing circuit 7 may be added to the prior art circuit examples described above with reference to Figs. 1-3. The transducer biasing circuits 7 should be added to each transducer, for example as voltage dividers or other voltage biasing means, e.g. reference voltage providers capable of providing a suitable small charging current of for example 50-200pA.

[0091] In terms of the measuring process steps described above, the ability by embodiments of the present invention to charge the transducers already when starting up the driver circuit, and maintain the charge when switching between driver and receiver roles, means that most of the waiting time in step 4 advantageously can be carried out at the same time as step 2, leaving none or only a short waiting period in step 4 to be safe. This may for example be achieved by coordinating the control of the switches and powering of the circuit elements in the embodiment of Fig. 9. Preferably step 1 may include powering up all of the driver circuit, driver biasing circuit and transducer biasing circuit (and remaining necessary circuits, for example receiver circuit) by means of power supply switch 61 (SW2). Thereby the waiting for steady state in step 2 in parallel achieves the pre-charging of the transducer and settlement of the driver output. When the transducer is connected to driver circuit by switch 5 (SW1) in step 3, there will be none or only insignificant current surge, allowing none or a very short waiting time in step 4, before the first measurement signal can be transmitted in step 5.

[0092] Various embodiments of the invention as described above may thereby enable a modified, even more efficient measurement cycle for a flowmeter, again here only considering the actions of the driver circuit to be able to compare with the steps 1-16 in the method described above:

101. toN+BiAs: Connect driver circuit 3, driver biasing circuit 8 and transducer biasing circuit 7 to power supply.

Thereby the transducer can pre-charge while other components are waking up.

102. Wait for steady state

There is a settling time for the driver circuit and other components that have been disconnected or in sleep more to become stabilized and ready for measuring During the same time, the transducer charges to the same level as the driver idle output, for example as shown in the plot of Fig. 8.

103. tsw.i : Connect first transducer to driver circuit

No significant transition is experienced because the voltage levels are equalized before the connection, for example as shown in the plot of Fig. 8.

104. Wait for steady state for first transducer

In practice no unstable period is experienced, for example as shown in the plot of Fig. 8, so this waiting period may be reduced to a very short delay for example between 0 - 20 ps.

105. Transmit first ultrasonic signal from first transducer

The driver circuit produces a driver output signal causing the transducer to generate a well-defined ultrasound signal.

106. Wait while measuring

During this time, the second transducer and the receiving circuit are active. The ultrasound propagation delay between the two transducers may for example be in the order of 3 Ops - lOOps, depending on the distance between them. Another for example 20ps may be delayed in order to allow the oscillations caused by the measurement wave to fade out.

107. Disconnect first transducer from driver circuit

The transducers are preferably still be connected to the transducer biasing circuits when being switched between driver and receiver and even non-connected states, thereby maintaining the bias voltage, for example Vcc/2.

108. tsw,2: Connect second transducer to driver circuit

The transducer comes from the role as receiving transducer and is connected to the driver circuit by means of the switching arrangement. The second transducer is already charged to Vb as it is continuously connected to the transducer biasing circuit.

109. Wait for steady state for second transducer

As in step 104, in practice no unstable period is experienced, for example as shown in the plot of Fig. 8, so this waiting period may be reduced to a very short delay for example between 0 - 20 ps.

110. Transmit second ultrasonic signal from second transducer

The driver circuit produces a driver output signal causing the transducer to generate a well-defined ultrasound signal.

111. Wait while measuring

During this time, the first transducer and the receiving circuit are active.

112. Disconnect second transducer from driver circuit

Using the switching arrangement.

113. [optionally repeat from step 103 if more measurements are desired for averaging]

114. Disconnect driver circuit, driver biasing circuit and transducer biasing circuit from power supply for power saving.

115. Wait for next measurement cycle

116. [repeat from step 101] [0093] As seen, the waiting periods in steps 104 and 109 may be significantly reduced or even completely dispensed with compared to the method of steps 1-16 described previously. It is noted, however, that also when following the previously described method of steps 1-16, shorter waiting periods may be achieved by implementing transducer biasing circuits according to the present invention.

[0094] List of reference signs:

1 ultrasonic flowmeter

2, 2a, 2b ultrasonic transducer

3 driver circuit

4 receiver circuit

5 switching arrangement

6 power supply, single supply, battery

7 transducer biasing circuit

8 driver biasing circuit

9 fluid flow

10 utility meter

11 display

12 electronics, processor, communication means

61 power supply switch

71, 72 resistors of voltage divider

Vcc supply voltage

GND ground potential

Ms measuring signal

Mout measuring output

Dout output of the driver circuit

Rin input of the receiver circuit

Vb transducer bias voltage

I(transducer) transducer current

V(transducer) transducer voltage