Abstract:
Flow sensors for measuring the flow of an ion-containing fluid may be implemented using mechanical or electrical techniques. Mechanical flow sensors are have moving parts and therefore may be unreliable after some time and are expensive to manufacture. Hall-effect type flow sensors typically require a reversible magnetic field to compensate for electrochemical effects. A flow meter including such a sensor uses an electromagnet. A flow sensor ( 100 ) is described using a capacitive sensor ( 10 ) and processor ( 12 ) to determine the flow rate from a change in capacitance and a magnetic field. Such a flow sensor may be implemented using CMOS technology. The flow sensor may operate in a magnetic field generated by a permanent magnet and measure the flow reliably.

Description:
[0001]    The invention relates to a flow sensor for measuring the rate of flow of a fluid. 
         [0002]    Fluid flow meters used for example for measuring water flow rate are widely available and work on different physical principles of operation. One class of flow meter is referred to as magnetic flow meters and relies on the principle of the Lorenz Force by applying a magnetic field to a channel carrying a fluid containing ions. A magnetic field which is orthogonal to the direction of flow will displace or separate positive ions (cations) and negative ions (anions) in the fluid. This results in a potential difference across the channel which is proportional to the fluid flow. This potential difference may be detected by sensor electrodes at either side of the channel as a DC measurement. The material forming the channel must be an insulator so that the potential difference can be detected. Electrochemical and other effects at the electrodes may make the potential difference drift and consequently the component of the potential difference due to the fluid flow is difficult to determine. This can be mitigated by using an electromagnet and constantly reversing the magnetic field. However, this can only be done at relatively low frequencies since at higher frequencies the effect of the lorenz force on the ions will be difficult to measure. 
         [0003]    Various aspects of the invention are defined in the accompanying claims. In a first aspect there is described a flow sensor for detecting the rate of flow of an ion-containing fluid in a channel, the flow sensor comprising a capacitive sensor and a processor coupled to the capacitive sensor, and wherein the capacitive sensor is operable to detect changes in capacitance value due to the deflection of the ions in the fluid by a magnetic field, and the processor is operable to determine a flow speed of the fluid from the detected change in capacitance value and a predetermined value of magnetic field strength. 
         [0004]    The capacitive sensor allows the flow sensor to be positioned on a single plane either within a channel or parallel to the direction of flow. This is because detecting the change in capacitance value does not require the electrodes to be positioned on different planes. The use of capacitive sensing reduces sensitivity to electro migration effects and consequently a permanent magnet rather than an electromagnet may be used to create a magnetic field resulting in a lower cost sensor. 
         [0005]    In embodiments of the flow sensor the capacitive sensor comprises a plurality of nano-electrodes. A plurality of nano-electrodes may be used to detect very small changes in capacitance, which may be a few attoFarads, in a fluid flowing across the sensor in the presence of a magnetic field. 
         [0006]    In embodiments of the flow sensor the capacitive sensor further comprises a plurality of charge pump circuits and a plurality of integration capacitors and wherein each of the plurality of charge pump circuits is coupled to a respective one of the plurality of nano-electrodes and a respective one of the plurality of integration capacitors. 
         [0007]    In embodiments the flow sensor includes a temperature sensor coupled to the processor. This allows the flow sensor to compensate dynamically for any effect of temperature on the measured capacitance value for a given flow rate. 
         [0008]    In embodiments the flow sensor includes a magnetic field sensor coupled to the processor, This allows the flow sensor to compensate dynamically for any variation in magnetic field strength on the measured capacitance value for a given flow rate. 
         [0009]    In embodiments of the flow sensor the processor is further operable to calculate the volume of fluid flowing in a channel of a predetermined cross-sectional area past the capacitive sensor. 
         [0010]    If the flow sensor is placed in a fluid channel where the cross sectional area is known. The processor can determine the volume of fluid flowing past the sensor by integrating the determined fluid flow speed multiplied by the cross sectional area over time. 
         [0011]    In embodiments the flow sensor may include a touch sensor coupled to the processor and the flow sensor is operable to change from a standby mode of operation to a normal mode of operation in response to the touch sensor being touched by a user. This may reduce the power consumption requirements when flow sensing is not required. 
         [0012]    In embodiments, the flow sensor may include a near field communication (NFC) receiver coupled to the processor. The NFC receiver may be used to receive data regarding a predetermined volume or flow rate, or maybe used to update default values of flow rate or magnetic field strength transmitted from an NFC transmitter. The NFC receiver may also be used to actuate a valve for example on a tap, as well as initiate the calibration or first measurement of the flow sensor. 
         [0013]    Embodiments of the flow sensor of may include a radio frequency transponder coupled to the processor. The RF transponder or transceiver allows the flow-sensor to communicate wirelessly to remote control and/or monitoring devices. The remote control and/or monitoring devices maybe on a dedicated wireless network or may be connected via the internet. Embodiments of the flow sensor including the RF transponder may communicate according to the Zigbee standard or other protocol used in building control systems. 
         [0014]    Embodiments of the flow sensor may be implemented as an integrated circuit allowing a robust flow sensor to be produced at low cost. 
         [0015]    Embodiments of the flow sensor may form part of a flow measurement apparatus including a magnet arranged such that in operation the magnetic field displaces one of the positive ions and the negative ions in the fluid towards the capacitive sensor. 
         [0016]    For maximum sensitivity a magnet for providing a magnetic field can be arranged so that the magnetic field is orthogonal to the direction of flow of the fluid to be measured. However, provided the magnetic field deflects either anions or cations in a general direction towards the flow sensor, a change in capacitance may be detectable. 
         [0017]    The magnet may be an electromagnet or permanent magnet for example Nd 2 Fe 14 B or other ferromagnetic material. 
         [0018]    Embodiments of the flow sensor may include a pH sensor coupled to the processor. 
         [0019]    Embodiments of the flow sensor together with a magnet may be incorporated into a water tap which may also be referred to as a spigot or faucet. 
         [0020]    A dispensed water volume may be determined and displayed by the flow sensor so that the user of the tap can dispense a known volume of water into a container. This may reduce energy waste for example when filling an electric kettle usually results in too much water as the fill level is hard to see while tapping. In cooking recipes one needs to quantify a certain volume of water which is usually done via a graduated beaker. A tap including a flow sensor with a display showing the amount of water dispensed removes the requirement to have a graduated measuring beaker. 
         [0021]    Embodiments of the flow sensor including a near field communication receiver may be included in a tap. The container may include a tag containing the volume of that container which can be communicated to the flow sensor. The tap may generate an alert when the required amount of water has been dispensed. 
         [0022]    Embodiments of a tap with a flow sensor including a touch sensor, or a NFC receiver may be coupled to an actuator for controlling a valve. The tap is operable to open and/or close the valve in response to the touch sensor being touched or a container with a NFC transmitter being placed in proximity to the tap. 
         [0023]    In a second aspect there is described a method for measuring the flow rate of an ion-containing fluid, the method comprising: detecting a capacitance value due to the deflection of the ions in the fluid by a magnetic field, and determining a flow speed of the fluid from the detected capacitance value. 
     
    
     
         [0024]    Embodiments of the invention are now described in detail, by way of example only, illustrated by the accompanying drawings in which: 
           [0025]      FIG. 1  shows a flow sensor according to an embodiment. 
           [0026]      FIG. 2  illustrates an equivalent circuit showing the principle of operation of the flow sensor. 
           [0027]      FIG. 3  shows an example flow sensor circuit according to an embodiment. 
           [0028]      FIG. 4  illustrates the principle of operation of the flow sensor circuit of  FIG. 3 . 
           [0029]      FIG. 5  shows a method of operation of a flow sensor according to an embodiment. 
           [0030]      FIG. 6  shows a flow sensor including a temperature sensor according to an embodiment. 
           [0031]      FIG. 7  shows a flow sensor including an NFC receiver and a display according to an embodiment. 
           [0032]      FIG. 8  shows a flow sensor including a wireless transceiver according to a further embodiment. 
           [0033]      FIG. 9  illustrates a flow sensor including a Hall Effect sensor according to an embodiment. 
           [0034]      FIG. 10  shows a tap or spigot or faucet including a flow sensor according to an embodiment. 
           [0035]      FIG. 11  illustrates a tap or spigot or faucet, including a flow sensor coupled to the valve according to a further embodiment. 
       
    
    
     DESCRIPTION 
       [0036]      FIG. 1   a  shows flow sensor  100  which includes a capacitive sensor  10  which may be coupled to a processor  12 . The capacitive sensor  10  may include multiple small capacitor electrodes which may be nano-electrodes. In operation capacitive sensor  10  may be positioned to sense the flow of a fluid containing ions in a channel as illustrated in  FIG. 1   b . This fluid may for example be water but can be any other fluid which contains ions. In operation, for example to detect water flow speed Φ, a magnetic field B may be applied orthogonally to the direction of water flowing in the channel at the point where the capacitive sensor is positioned. The magnetic field will physically separate positive ions, for example H 3 0 + , and negative ions, for example OH − , in the flowing water due to the Lorenz force. This separation may be detected by the capacitive sensor  10 . The measured capacitance is proportional to the flow speed of the water in the tube, the measured capacitance change is a function of the drift velocity of the water in the tube. The processor  12  may convert the detected capacitance value into a value representative of the flow speed of the water. 
         [0037]    To reduce the possible effect of lime scale on the sensor where the fluid to be measured is water, the flow sensor may be placed on the portion of the channel surface where cations are being displaced away from the sensor. 
         [0038]    In embodiments the processor may be an analogue to digital converter combined with a logic circuit to generate a digital value representative of the flow rate. In embodiments the processor may be a microprocessor or microcontroller. Embodiments of the capacitive sensor may include a number of nano-electrodes formed on a single semiconductor substrate. The processor  12  and the capacitive sensor  10  may be implemented as a single integrated circuit. 
         [0039]      FIG. 2  illustrates the principle of operation of an example capacitive sensor. The measured electrode capacitance C is determined by the admittance path from the capacitive sensor to a ground potential at sample frequencies of around 50 MHz. The admittance measured by the sensor can be modelled as a fluid impedance Z E  consisting of a fluid resistance R E  arranged in parallel with a fluid capacitance C E . The fluid resistance R E  and fluid capacitance C E  are in series with an electrode capacitance C n . The electrode capacitance C n  may be a nano-electrode. At frequencies greater than 300 MHz the equivalent circuit may be modelled as fluid capacitance C E  in series with an electrode capacitance C n . 
         [0040]    C n  is the Debye capacitance at the surface of an electrode, R E  is the water resistance and C E  is the water capacitance. Are all determined by the geometrical size of the electrode, as can be seen by the expressions given below: 
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         [0041]    Here d is the diameter of the electrode (capacitor), c 0  is the Debye capacitance (due to the presence of ions), σ E  is dc conductivity of the water, ε o  is the permittivity of vacuum (8.854×10 −12  C/V−m) and ε E  is the relative permittivity of the water solution. The impedances of equation 1, equation 2 and equation 3 can be considered as the fundamental impedances relevant for the flow sensor. These three impedances may be influenced by the presence of the external magnetic field in combination with the flow of water. The displacement of ions in the water, due to the Lorenz force acting upon them will alter c 0 , σ E  and ε E . Hence the displacement will become dependent on the magnetic field B and the flow rate Φ). Therefore the flow rate can be determined by measuring a change in the capacitance of the electrode. 
         [0042]    If the orientation of the magnetic field is such that negative ions are deflected towards the electrode, the capacitance will increase with increasing flow rate. If the orientation of the magnetic field is such that positive ions are deflected towards the electrode, the capacitance will decrease with increasing flow rate. For nano-electrodes which may be less than 100 nm in diameter for an electrode with circular cross section or less than 100 nm in width, the detected change an individual electrode can be very small, for example in the atto-Farad range (10 −18  Farads). The capacitive sensor  10  may be formed from arrays of hundreds of individual nano-electrodes allowing statistical algorithms and frequency signal modulation to be used to provide a robust signal despite various possible low frequency parasitic influences. 
         [0043]    Where the flow sensor is located to detect the flow in a channel of known cross-sectional area, the fluid volume may be determined from the measured flow rate integrated over flow time duration and cross-sectional area. 
         [0044]      FIG. 3  ( a ) illustrates a flow sensor  200 . The capacitive sensor has a plurality of nano-electrode sensors  300 . The output of each of the nano-electrode sensors  300  may be connected to an analog to digital convertor  13  which may include multiplexing circuitry to select a subset of the plurality of sensors  300 . The output of the analog to digital converter  13  may be connected to a processor  12 . Alternatively the analog to digital converter may be included in the processor.  FIG. 3  ( b ) shows an example circuit for a nano-electrode sensor  300 . A nano-electrode is shown as a capacitor having a Debye capacitance C n . A charge pump circuit may be formed by a series arrangement of a first charge pump transistor CPT 1 , and a second charge pump transistor CPT 2 . The nano-electrode may be connected to a source of first charge pump transistor CPT 1  and the drain of second charge pump transistor CPT 2 . The source of second charge pump transistor CPT 2  may be connected to a supply rail VD. A source follower circuit may be formed by a series arrangement of a first source follower transistor SFT 1  and a second source follower transistor SFT 2 . The drain of first charge pump transistor CPT 1  may be connected to the source of the second source follower transistor SFT 2 . The drain of the second source follower transistor SFT 2  may be connected to the source of the first source follow transistor SFT 1 . The drain of the first source follower transistor SFT 1  may be connected to a first electrode of integration capacitor Cint. A second electrode of capacitor Cint may be connected to a ground rail. The first electrode of capacitor Cint may be connected to a first voltage rail Vreset via a switch S 1  which may be implemented using a transistor. The first electrode of capacitor Cint may be connected to a second voltage rail Vcall via a switch S 2  which may be implemented using a transistor. The first electrode of capacitor Cint may be connected to the input of an analog to digital convertor. The arrangement of integration capacitance Cint, first source follower transistor SFT 1  and second source follower transistor SFT 2  may form an analog charge detector. In embodiments C int  may have a capacitance of 480 fF. In embodiments C int  may have a range of 100 fF to 5 pF. Transistors CPT 1 , CPT 2 , SFT 1  and SFT 2  may be NMOS transistors. 
         [0045]    The first electrode of integration capacitor C int  may be connected to a voltage buffer B 1 . The output of the voltage buffer may be connected to an analog to digital converter. 
         [0046]    In operation the capacitive sensor is placed adjacent to a channel containing a fluid such as water which contains ions. The nano-electrode may not be in direct contact with the fluid but may be separated by an isolation layer between 1 nm to 1000 nm in thickness. The equivalent impedance may be due to C n  and a fluidic impedance Z E  as described in  FIG. 2 . 
         [0047]    At the start of a measurement the integration capacitor C int  may be charged to voltage V reset  which may be a voltage of 1.2 volts by closing the reset switch  51 . Subsequently the integration capacitor C int  may be discharged by the charge pump by sequentially opening and closing transistor CPT 1  and opening and closing transistor CPT 2 . For each cycle a quantity of charge is transferred, which may be three orders of magnitude lower than the total charge residing on the initialized integration capacitor C int . The exact quantity of charge transferred may depend on the capacitance of the nano-electrode C n , the connected fluidic impedance Z E  and on chip impedance. A change in C n  and/or Z E  can be induced by the deflection of ions towards or away from the nano-electrode C n , thereby changing the amount of charge transferred for each charge pump cycle. The fluid potential V L  may be set by a reference electrode in contact with the fluid. Alternatively one or more of the nano-electrodes of the inactive nano-electrode sensors  300  may act as the reference electrode. By setting the fluid potential V L  using the inactive nano-electrodes the capacitive sensor  10  can be placed on one side of the channel providing a simple and robust solution for fluid flow measurement. 
         [0048]    The total amount of charge Q INT  that is pumped away from the integration capacitor C int  in a number of pump cycles N i  may be determined by measuring the final voltage V INT  across the integration capacitor by the Analog to Digital Converter  13  (ADC). 
         [0049]    The number of pump cycles N i  may be chosen such that the integration capacitor C int  is discharged as far as possible. This may be for example until the remaining voltage V INT  of the integration node is just high enough to keep the voltage buffer alive which may be a voltage of 0.6 Volts. As an example with a discharge voltage VD=0.1 V and a charge pump modulation amplitude of 0.2 V, the maximum voltage at the source of the lower cascode source follower transistor SFT 2  is VT=0.3V. The gate of source follower transistor SFT 1  may be biased to a voltage Vfollow. The gate of source follower transistor SFT 2  may be biased to a voltage Vtransfer. The two cascode source follower transistors may require a potential difference of 0.2V between the drain of SFT 1  having a voltage of V cascode  and the source of SFT 2  having a voltage of V T  in total. Added to the 0.3V at the maximum of the modulation voltage this is 0.5V. The supply voltage may be 1.2V. So this leaves a theoretical 0.7V available for the voltage swing over the integration capacitor. Subtracting a safety margin of 0.1V to accommodate for the required voltage needed by the buffer, the maximum applicable voltage swing over the integration capacitor may be 0.6 V. For a value of the integration capacitor C int  of 480 fF, the maximum charge Q INT  that can be pumped away from the integration capacitor is approximately 0.6 Volt×480 fF=0.288 pC. 
         [0050]    The nano-electrode Cn may be coated with a layer such as Teflon, conductive or insulation polymers and/or an inorganic layer such as SiO2, Si3N4, TiOx, TaOx. For applications where water flow is to be detected coating the nano-electrode may reduce the effect of scaling on the sensor. The nano-electrode may be gold or copper. The skilled person will appreciate that the capacitive sensor of  FIG. 2  may be implemented using a CMOS process. 
         [0051]      FIG. 4  shows the basic operation principle of the charge pump of the nano-electrode sensor circuit  300 . Firstly the discharge switch formed by second transistor CPT 2  which may be an NMOS transistor is closed that is CPT 2  is switched on. This may discharge the nano-electrode capacitor Cn to the supply rail V D  which may be set to a ground potential. At a time t=0 the gate voltage φ T  of transistor CPT 1  which may be an NMOS transistor is at a voltage such that the transistor CPT 1  switches on, thereby enabling the nano electrode capacitor Cn to be charged to a voltage V T . At this time t=0 transistor CPT 2  is switched off. At a time t=t1 the gate voltage φ T  of transistor CPT 1  is at a voltage such that transistor CPT 1  is switched off. This may electrically isolate the drain of CPT 1  from the nano electrode capacitor Cn. At t=t2 the gate voltage φ D  of transistor CPT 2  transistor is pulled high to switch on CPT 2  in order to discharge the nano-electrode capacitor Cn to the discharge voltage V D  again. At time t=t3 both CPT 1  and CPT 2  are switched off. At time t=t4 CPT 1  is switched on and the charge transfer cycle repeats. 
         [0052]    The charge Q cycle  transferred though the charge pump after one cycle is: 
         [0000]        Q   cycle =( V   T   −V   D ) C   Equation 4
 
         [0053]    Where C is the effective capacitance due to the nano electrode capacitance Cn and the fluid impedance Z E  nano electrode capacitance and V T -V D  is the charge pump modulation amplitude. After N i  discharge/transfer cycles the total amount of charge transferred from the integration capacitor is: 
         [0000]        Qn   cycle   =N   i ( V   T   −V   D ) C   Equation 5
 
         [0054]    When a change occurs in the effective capacitance C by an amount ΔC, a corresponding change ΔQINT in the transferred charge from the integration capacitor will follow. Hence a change in capacitance caused by a change in flow speed of a fluid containing ions in the presence of a magnetic field can be detected, since a change in flow speed will alter the amount of deflection or displacement of the ions and therefore the resulting effective capacitance. 
         [0055]    The source follower transistors SFT 1  and SFT 2  between the integration capacitor C int  and the charge pump maintain a substantially constant voltage at the input of the charge pump, despite the continuously declining voltage V INT  due to the discharging of the integration capacitor. Both cascode source follower transistors may work in deep sub-threshold. By biasing in deep sub threshold, the drain currents may be about 6 nanoamps. In embodiments the threshold voltage may be 0.3 volts. The drain current of SFT 1  and SFT 2  in deep sub threshold operation may be a few nanoamps whereas if the gate source voltage difference of SFT 1  and SFT 2  is greater than the threshold voltage, the drain current may be several microamps. The gate source voltage of SFT 1  and SFT 2  may be less than 70 millivolts in deep sub-threshold operation. 
         [0056]    In a deep sub-threshold bias regime the transistors CPT 1  and CPT 2  may need only a drain-source voltage of about 0.1V to operate. So the two source follower transistors cascode may only need about 0.2 V in total. 
         [0057]    The voltage buffer between the analog to digital converter and the integration capacitor C int  may prevent the influence of the state of charge of the integration capacitor, as represented by V INT . However the voltage buffer may induce a potential drop V X  of several tens of Volts between the analog to digital converter and the integration capacitor voltage V INT . As V X  may change over time, a read out by the analog to digital converter may be followed by a calibration measurement. The analog to digital convertor may be a 10 bit convertor. A measurement performed by the A/D converter consists of a measurement of V INT  which may be followed by a calibration measurement. The calibration measurement may be performed by closing the calibration switch S 2  thereby connecting a calibration voltage Vcal to the input of the A/D converter. This procedure may eliminate possible drifts and 1/f noise in the A/D converter and may be used to subtract the potential drop and 1/f noise of the source follower from the measurement. The sequence of both measurements first yields: 
         [0000]        V   ADC1   =V   int   −V   x   Equation 6
 
         [0000]    where V X  represents the unknown voltage drop across the single source follower. The second calibration measurement yields information on V X : 
         [0000]        V   ADC2   =V   cal   −V   x   Equation 7
 
         [0058]    From equations 6 and 7 the value of the integration capacitance voltage Vint can be determined and hence the charge Q INT  that has been pumped away from integration capacitor C int  can be calculated as: 
         [0000]        Q   INT =( V   reset   −V   adc1   +V   adc2   −V   cal )* C   int   Equation 8
 
         [0059]    A processor coupled to the digital output of the analog to digital convertor can therefore calculate the charge Q INT  using the above measurement steps and given a known value for V reset , V cal  and C int . This may be used to determine the effective capacitance C due to the nano-electrode capacitance C n  and the fluid capacitance C e  by combining equations 5 and 8 
         [0000]        C =( V   reset   −V   adc1   +V   adc2   −V   cal )* C   int /( N *( V   T   −V   D ))  Equation 9
 
         [0060]    Embodiments of the capacitive sensor may have many nano electrodes and associated charge pump circuits and integration capacitors. Embodiments may have more than one analog to digital convertor. A capacitive sensor may have an array of 256×256 nano electrodes arranged in a rows 256 coupled to 8 analog to digital convertors. A single analog to digital convertor may read 32 columns in approximately 40 microseconds. A read out of a value for Qint for 256 nanoelectrodes may take 140 microseconds. Reading the entire array may take 40 mS consequently each nano electrode may be samples at a rate of 25 samples per second. 
         [0061]    An example of flow sensor calibration is now described assuming water is the fluid. This could be done for example just before the water starts to flow for example when a person touches the opening valves. The following illustrates the relationship between the volumetric flow rate and the detected change in measured capacitance.
       1. An initial capacitance measurement may be made for B=0 (no magnetic field) and Φ=0 (no water flow). For a single nanoelectrode this may give a measured capacitance of 100 aF. With an electrode radius of 65 nm, this amounts to a debye layer capacitance density of approximately 8×10 −3  Farad/m2.   2. In J.-L. Fraikin at al, “Probing the Debye Layer: Capacitance and Potential of Zero Charge Measured using a Debye-Layer Transistor”, published in Physical review letters, 17 Apr. 2009, a change in the Debye layer capacitance is obtained of ΔC DL =4×10 −5  F/m 2 /mV, where the voltage refers to a dc voltage difference between the nano-electrode and the fluid (parameter V L ).   3. Next the Hall voltage has to be calculated when water is flowing: suppose a maximum flow rate Φ=100 ml/sec, a maximum magnetic field of B=1 Tesla and a square tube with a cross-section A=w×h=1 cm×1 cm=1×10 −4  m 2 . Then the maximum flow velocity v d,max =Φ/A=1 m/s. The maximum Hall voltage V H,max =v d,max ×B×h=1×1×10 −2 =10 MV.   4. From 2 and 3 follows the maximum induced Debye layer change: ΔC DL,max =4×10 −4  F/m 2 . This corresponds to 5% (4×10 −4 /8×10 −3 ×100%) of the initial signal under 1).   5. From 4) and 1) the maximum signal is about 20% of 100 aF, which is 5 aF. With a noise of ˜1 aF for a single nano-electrode and 10 −2  aF for a 256×256 array of nano-electrodes, the change in value may be detected.       
 
         [0067]    Hence it will be appreciated by the skilled person that the flow rate may be determined from the capacitance change relative to the value when no fluid is flowing in a magnetic field of known value. This is illustrated in  FIG. 5  where an initial calibration phase starts at step  50 . A measurement of capacitance is taken by the flow sensor placed in the channel with fluid present but not flowing in step  52 . The magnetic field strength B is a known reference value Bref which may be predetermined. The calibration phase ends at step  52 . The calibration phase may be repeated each time the flow sensor is powered up or it may be done once when the flow sensor is initially installed. Once the capacitance value with no flow is known, the flow may be determined firstly by measuring the capacitance value C in step  56  and then calculating the difference between the measured capacitance C and the reference capacitance Cref at zero flow in step  58 . The difference between C and Cref can be used to derive the Hall voltage in step  60 . Once the Hall voltage has been calculated the velocity and therefore the flow rate is determined in step  62 . This method may then return to step  56  and the cycle repeats. 
         [0068]      FIG. 6  shows flow sensor  400  including a capacitive sensor  10  which may be coupled to a processor  12 . The capacitive sensor  10  may include multiple small capacitor electrodes which may be nano-electrodes and formed using CMOS technology. In operation capacitive sensor  10  may be positioned to sense the flow of a fluid containing ions in a channel. This fluid may for example be water but can be any other fluid which contains ions. In operation, for example to detect water flow speed, a magnetic field may be applied orthogonally to the direction of water flowing in the channel at the point where the capacitive sensor is positioned. Positive ions, for example H 3 0 + , and negative ions, for example OH − , in the fluid may be separated due to the magnetic field. This separation may be detected by the capacitive sensor  10 . Since the measured capacitance may be proportional to the flow speed of the water in the tube, the processor  12  may convert the detected capacitance value into a value representative of the flow speed of the water. Temperature sensor  14  may be connected to processor  12 . Processor  12  may determine the flow speed by firstly determining a capacitance value corresponding to the stationary fluid when a magnetic field is applied which may use a permanent magnet  20 . This may for example be a user initiated calibration sequence when the user knows that the sensor is in position and the fluid is stationary in a channel. This capacitance value may correspond to the situation of maximum displacement or separation of the ions in the fluid due to the Lorenz force resulting from the magnetic field. This initial calibration may only need to be done once but may also be repeated. Once the fluid is flowing, the separation of the ions by the magnetic field will on average decrease as the rate of flow increases and consequently the capacitance will change. The difference between the capacitance measured and the reference capacitance value when there is no flow may give an indication of flow speed. The processor may also use the temperature of the fluid measured by the temperature sensor to compensate the measured capacitance value for temperature. 
         [0069]      FIG. 7  shows flow sensor  500  including a capacitive sensor  10  which may be coupled to a processor  12 . The capacitive sensor  10  may include multiple small capacitor electrodes which may be nano-electrodes and formed using CMOS technology. Near Field Communication (NEC) Receiver  16  may be connected to processor  12 . Display  18  may be connected to processor  12 . In embodiments the display  18  may use LED, LCD or other known display technologies. Processor  12  may determine the flow speed by firstly determining a capacitance value corresponding to the stationary fluid when a magnetic field is applied which may use a permanent magnet  20 . Alternatively or in addition, an electromagnet may be used to provide a magnetic field. If an electromagnet is used, the magnetic field may be periodically reversed or the field strength changed. This may reduce sensitivity to parasitic DC drifts. 
         [0070]    In operation of flow sensor  500  capacitive sensor  10  may be positioned to sense the flow of a fluid containing ions in a channel which may have a predetermined cross-sectional area. This fluid may for example be water but can be any other fluid which contains ions. In operation, for example to detect water flow speed, a magnetic field may be applied orthogonally to the direction of water flowing in the channel at the point where the capacitive sensor is positioned. The magnetic field will physically separate positive ions, for example H 3 0 + , and negative ions, for example OH − , in the flowing water due to the Lorenz force. This separation may be detected by the capacitive sensor  10 . Since the measured capacitance may be proportional to the flow speed of the water in the tube, the processor  12  may convert the detected capacitance value into a value representative of the flow speed of the water. In operation the NFC receiver may receive a signal from an NFC transmitter to provide power to the flow sensor  500 . The NFC receiver may receive a signal from an NFC transmitter indicating a predetermined volume of fluid. In embodiments, the NFC receiver  36  may include a secure element. The processor  12  may calculate the volume of fluid which has passed the flow sensor after a period of time following a start condition which may involve for example applying a reset signal to the processor from the flow rate, cross sectional area and the flow time duration. The processor  12  may display the volume on the display  18 . The processor  12  may indicate when a predetermined volume has been reached on the display  18 . 
         [0071]      FIG. 8  shows flow sensor  600  including a capacitive sensor  10  which may be coupled to a processor  12 . The capacitive sensor  10  may include multiple small capacitor electrodes which may be nano-electrodes and formed using CMOS technology. Touch sensor  32  may be connected to a power management unit  28 . A battery  30  may be connected to the power management unit  28 . A power management unit  28  may be connected to a capacitive sensor  10 , a temperature sensor  24 , a processor  12  and a transponder  22  which may be configured to transmit/or receive signals according to the Zigbee standard. The transponder  22  may be connected to antenna  26 . The transponder  22  may be connected to processor  12 . Temperature sensor  24  may be connected to processor  12 . The processor  12  may be connected to the display  18 . 
         [0072]    In operation of flow sensor  600  capacitive sensor  10  may be positioned to sense the flow of a fluid containing ions in a channel which may have a predetermined cross-sectional area. This fluid may for example be water but can be any other fluid which contains ions. In operation, for example to detect water flow speed, a magnetic field may be applied orthogonally to the direction of water flowing in the channel at the point where the capacitive sensor is positioned. The magnetic field will physically separate positive ions, for example H 3 0 + , and negative ions, for example OH − , in the flowing water due to the Lorenz force. This separation may be detected by the capacitive sensor  10 . Since the measured capacitance may be proportional to the flow speed of the water in the tube, the processor  12  may convert the detected capacitance value into a value representative of the flow speed of the water. The processor may compensate the detected value for temperature using the temperature value detected by the temperature sensor  24  which may be a PTat temperature sensor. The power management unit  28  may supply power to the capacitive sensor  20 , the temperature sensor  24 , the processor  12 , the transponder  22  and the display  18  in response to the touch sensor being touched. The capacitive sensor  10  and processor  12  may be periodically powered in a low power or sleep mode. The power management unit  28  may change to a normal power mode in response to a rapid change in detected capacitance corresponding to a large increase in flow rate. This may be caused for example by a tap or faucet being turned on. The flow sensor  500  may display one or more of the fluid flow rate, fluid temperature and volume of fluid flowing past the flow sensor during a certain time period. 
         [0073]    The transponder  22  may transmit data on the flow speed, volume of fluid and temperature to a receiver. The transponder  22  may receive control information such as a signal to power down or reset the flow sensor  500 . The transponder  22  may receive data indicating a predetermined flow rate and/or volume of fluid to be measured. This may be used for example to remotely control and/or monitor water flow rate or water volume dispensed through a tap or shower. The transponder  22  may transmit data to the internet via a wireless router. 
         [0074]    One or more of the power management unit  28 , the capacitive sensor  10 , the temperature sensor, the processor  12  and the transponder  22  may be integrated on a CMOS integrated circuit. The processor  12  maybe a microprocessor or microcontroller configured to calculate the flow rate from the detected capacitance. Alternatively the processor  12  may be implemented as logic hardware. 
         [0075]      FIG. 9  shows flow sensor  700  including a capacitive sensor  10  which may be coupled to a processor  12 . The capacitive sensor  10  may include multiple small capacitor electrodes which may be nano-electrodes and formed using CMOS technology. Touch sensor  32  may be connected to a power management unit  28 . A battery  30  may be connected to the power management unit  28 . Power management unit  28  may be connected to a capacitive sensor  10 , a temperature sensor  24 , a processor  12  and a transponder  22 . The transponder  22  may be configured to transmit/or receive signals according to the Zigbee standard. The transponder  22  may be connected to antenna  26 . The transponder  22  may be connected to processor  12 . Temperature sensor  24  may be connected to processor  12 . Magnetic field sensor  34  which may be a Hall-Effect sensor may be connected to processor  12 . NFC Transceiver  36  may be connected to processor  12 . In operation the NFC transceiver  36  may receive a signal from an NFC transmitter to provide power to the flow sensor  400 . The NFC transceiver  36  may receive a signal from an NFC transmitter indicating a predetermined volume of fluid. In embodiments, the NFC transceiver  36  may include a secure element. 
         [0076]    The operation of the flow sensor  700  is similar to that of flow sensor  600 . In addition the magnetic field may be sensed by magnetic field sensor  34  which may be a Hall Effect sensor. This output of the Hall Effect sensor may be used by the processor  12  to determine the magnetic field strength so providing the predetermined magnetic field strength value for calculating the flow rate. In addition the processor may compensate for the effect of any magnetic field strength variation when determining the fluid flow rate. In operation the NFC transponder  38  may receive a signal from an NFC transmitter to provide power to the flow sensor  700 . The NFC transceiver  36  may receive a signal from an NFC transmitter indicating a predetermined volume of fluid. In embodiments, the NFC transceiver  36  may include a secure element. This may be used for example to authenticate whether or not a valve controlling the fluid in a channel may be opened or adjusted. 
         [0077]    In embodiments, one or more of the power management unit  28 , the capacitive sensor  10 , the temperature sensor  24 , the magnetic sensor  34 , the RF transceiver  22 , the processor  12  and the NFC transceiver  36  may be integrated on a CMOS integrated circuit. 
         [0078]      FIG. 10  shows a tap or spigot or faucet in longitudinal cross-section  800  and in transverse cross-section  800 ′. Tap  700  has a tube  46  for carrying fluid and a valve  40  located within the tube  46  which controls the flow of fluid through the channel formed by the tube and out of the tap through an outlet  48 . The direction of fluid flow is shown in  44  which is orthogonal to the direction of the magnetic field direction B. Capacitive sensor  10  is located in the channel and is coupled to processor  12 . Processor  12  may be coupled to battery  30 . Battery  30  may be connected to display  18  and capacitive sensor  10 . The flow of water or other ion containing fluid may be controlled by valve  40 . Permanent magnet  20  is positioned such that the magnetic field B is approximately orthogonal to the direction of flow of the fluid. In operation, when the valve  40  is opened allowing water to flow, the capacitive sensor  10  detects changes capacitance due to the displacement of the ions in the water by the magnetic field provided by the permanent magnet  20 . This change in capacitance is processed by the processor  12  which converts the capacitance value change into a measure of the flow of fluid. This information is displayed on display  18 . The cross-sectional area of the channel in the tap through which the fluid flows is known and therefore the processor  12  may also send data on the total volume of water or other ion containing fluid that has flowed following the opening of the valve  40  to the display  18 . Any significant change of the measured capacitance value may indicate that either the flow has started i.e. the valve  40  has been opened. Alternatively any change of the measured capacitance value may indicate that the fluid flow has stopped i.e. the valve has been shut. The magnet  20  may be positioned such that the magnetic field is orthogonal to the sensing plane of the capacitive sensor  10 . This results in the maximum displacement of ions either directly towards or directly away from the capacitive sensor  10 . This positioning therefore gives maximum sensitivity of detection of the flow rate. However other relative positioning of the magnet and sensor is possible at the expense of reduced sensitivity. The capacitive sensor  10  may be connected by a wire to processor  12  alternatively it may be a wireless connection for example via a NFC link. In the latter case the capacitive sensor  12  may include a NFC receiver to receive power via the NFC link. 
         [0079]      FIG. 11  shows a tap or spigot or faucet longitudinal cross-section  900  and in transverse cross-section  900 ′. Tap  900  includes a flow sensor  700  of the embodiment of  FIG. 9 . Tap  900  has a tube  46  for carrying fluid and a valve  40  located within the tube  46  which controls the flow of fluid through the channel formed by the tube and out of the tap through an outlet  48 . Flow sensor  700  may be connected to an actuator  42 . Actuator  42  may be connected to valve  40 . Permanent magnet  20  may be positioned on the body of the tap  900  such that the magnetic field may be orthogonal to the direction of flow of the fluid  44 . Alternatively the tap may be at least partially formed from a hard magnetic material. 
         [0080]    The magnetic field will physically separate positive ions, for example H 3 0 + , and negative ions, for example OH − , in the flowing water due to the Lorenz force. This separation may be detected as change in capacitance by the flow sensor  700 . Since the measured capacitance may be proportional to the flow speed of the water in the tube, the flow sensor  700  may convert the detected capacitance value into a value representative of the flow speed of the water. The flow sensor  700  may compensate the detected value for temperature using the temperature value detected by the temperature sensor  24  and the measured magnetic field detected by the magnetic field sensor  34 . The flow sensor  700  may signal the actuator  42  to open the valve  40  in response to the touch sensor being touched. 
         [0081]    The transponder  22  in flow sensor  700  may transmit data on the flow speed, volume of fluid and temperature to a receiver. The transponder  22  may receive control information such as a signal to power down or reset the flow sensor  700 . The transponder  22  may receive data indicating a predetermined flow rate and/or volume of fluid to be measured. This may be used to remotely control and/or monitor fluid flow rate or fluid volume dispensed through the tap  800 . The transponder  22  may transmit data to the Internet via a wireless router. 
         [0082]    In operation the NFC transponder  38  may receive a signal from an NFC transmitter to provide power to the flow sensor  700 . The NFC transponder  38  may receive a signal from an NFC transmitter indicating a predetermined volume of fluid. The NFC transmitter may be incorporated in a fluid containing vessel, for example a cup. In operation, for example to detect water flow speed, a magnetic field may be applied orthogonally to the direction of water flowing in the channel at the point where the capacitive sensor is positioned. The magnetic field will physically separate positive ions, for example H 3 0 + , and negative ions, for example OH − , in the flowing water due to the Lorenz force. This separation may be detected by the capacitive sensor  10 . Since the measured capacitance may be proportional to the flow speed of the water in the tube, the processor  12  may convert the detected capacitance value into a value representative of the flow speed of the water. The processor may compensate the detected value for temperature using the temperature value detected by the temperature sensor  24  which may be a PTat temperature sensor. The power management unit  28  may supply power to the capacitive sensor  20 , the temperature sensor  24 , the processor  12 , the transponder  22  and the display  18  in response to the touch sensor being touched. In embodiments the capacitive sensor  10  and processor  12  may be periodically powered in a low power or sleep mode. The power management unit  28  may change to a normal power mode in response to a rapid change in detected capacitance corresponding to a large increase in flow rate. This may be caused for example by a tap or faucet being turned on. The flow sensor  700  may display one or more of the fluid flow rate, fluid temperature and volume of fluid flowing past the flow sensor during a certain time period. 
         [0083]    The transponder  22  may transmit data on the flow speed, volume of fluid and temperature to a receiver. The transponder  22  may receive control information such as a signal to power down or reset the flow sensor  700 . The transponder  22  may receive data indicating a predetermined flow rate and/or volume of fluid to be measured. This may be used for example to remotely control and/or monitor water flow rate or water volume dispensed through a tap or shower. For example a cup containing a NFC transmitter or an RFID tag may include data representing the volume of water that the cup can contain. When the cup is moved near to the tap  900 , the volume of water required to fill the cup may be transmitted from the NFC transmitter and received by the NFC transponder in the flow sensor  700 . The flow sensor  700  may then signal to the valve actuator  42  to open the valve  40 . The flow sensor  700  may sense the volume of water flowing through the tap following the opening of the valve  40 . Once the predetermined volume has been reached, the flow sensor  700  may signal to the valve actuator  42  to close the valve  40 . The transponder  22  may transmit data to a remote location via a wireless router connected to a computer network. In embodiments, the NFC transponder  38  may include a secure element. This may be used for example to authenticate whether or not a valve controlling the fluid in a channel may be opened or adjusted. 
         [0084]    Embodiments of the flow sensors may be included in a tap or water meter. Embodiments of the flow sensor may also be included in control systems for irrigation. Embodiments of the flow sensor may also be used to detect leaks. The flow sensor may also include other types of sensors. For example a pH sensor may be coupled to the processor and the flow sensor may then be used for example to detect changes in the chemical composition of water. The flow sensor may be formed on an integrated circuit together with other analogue and digital circuits. 
         [0085]    Embodiments of the flow sensor may be included in a tap or water meter. Embodiments of the flow sensor may be included in a water e-meter. For instance the e-meter may be installed at the main water pipe to a house, or at several pipes inside a house such as the kitchen, bathroom, etc. The e-meters may communicate with a hub via RF, and deliver data of how much water is consumed to the hub. The hub may keep an overview of water consumption in the house. 
         [0086]    The flow sensor may also be included in other machines that use water in the process, such as automatic coffee machines, washing machines and dishwashers. 
         [0087]    Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
         [0088]    Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. 
         [0089]    The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
         [0090]    For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.