Electrical detection, electrical admittance sensing and tomography apparatus and methods

Abstract

Apparatus for use in determining an electrical admittivity of a subject of interest has a first electrode and a second electrode configured to apply electric excitation to the subject of interest and one or more electric potential sensors provided about the subject of interest, each sensor configured to simultaneously measure an electric potential at a surface of the subject of interest and / or one or more magnetic field sensors configured to simultaneously measure a magnetic field close to a surface of the subject of interest. A controller o simultaneously monitors current or voltage at the electrodes and the sensors to determine an electrical admittivity of the subject.

Description

TECHNICAL FIELD This disclosure relates to electromagnetic detection or imaging systems and has application to electrical admittance sensing and tomography although not being limited to that application. BACKGROUND Electrical impedance tomography is a non-invasive type of detection or imaging in which the electrical conductivity, permittivity, and impedance of a region of interest of a subject or object is inferred from surface electrode measurements. The detected conductivity or impedance may be used to form an image such as a tomographic image of a part, or parts, of the subject or object of interest. In this document the terms subject and object and medium are used interchangeably to refer to an article or volume which is being investigated whether for imaging or other detection purposes. An example of a known system is disclosed in PCT / NZ2018 / 050029 (now US patent US11,181,318). In this publication the current and voltage distribution is determined in real-time within a length of timber (for example, a log) undergoing Joule heating. It achieves this by using the electric field imposed for Joule heating to cause electric current to flow through thetimber. Knowledge of these current and voltage distributions can be used either to implicitly infer the presence and / or dimensions of different regions within the log, such as dimensions of a heartwood volume or sapwood volume of the log. In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art. SUMMARY It is an object of this disclosure to provide an electromagnetic detection or imaging system which goes at least some way towards improving one or more existing systems, or to at least provide the industry or public with a useful choice. One or more aspects of the invention is set forth in the appended claims. Any of the claimed features, or disclosed embodiments or aspects may be combined with one or more of the other features or embodiments or aspects as described herein. The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner. It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner. As used herein the term "and / or" means "and" or "or", or both. As used herein "(s)" following a noun means the plural and / or singular forms of the noun. The invention in one aspect comprises several steps. The relation of one or more of such steps with respect to each of the others, the apparatus embodying features of construction, and combinations of elements and arrangement of parts that are adapted to affect such steps, are all exemplified in the following detailed disclosure. The term 'connected to' as used in this specification in relation to data or signal transfer includes all direct or indirect types of communication, including wired and wireless, via a cellular network, via a data bus, or any other computer structure. It is envisaged that there may be intervening elements between the connected integers. Variants such as 'in communication with', 'joined to', and 'attached to' are to be interpreted in a similar manner. Related terms such as 'connecting' and 'in connection with' are to be interpreted in the same manner. The term 'connected to' as used in this specification in relation to electric power envisages that there may be intervening elements between the connected integers. Variants such as 'in communication with', 'joined to', and 'attached to' are to be interpreted in a similar manner. Related terms such as 'connecting' and 'in connection with' are to be interpreted in the same manner. The terms 'component', 'module', 'system', 'interface', and / or the like as used in this specification in relation to a processor are generally intended to refer to a computer-related entity, either hardware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and / or a computer. By way of illustration, both an application running on a controller and the controller can be a component. One or more components may reside within a process and / or thread of execution and a component may be localized on one computer and / or distributed between two or more computers. To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. The disclosure consists in the foregoing and also envisages constructions of which the following gives examples only. Features disclosed herein may be combined into new embodiments of compatible components addressing the same or related inventive concepts. The invention allows determination, in real-time, of the current and voltage distribution within a heterogeneous, and potentially anisotropic, medium. It achieves this by using the electric field imposed to cause electric current to flow through the medium. The medium may be solid (e.g., a log) or liquid (e.g., an aqueous food mixture, such as milk). In the latter case, the medium may be constrained by a vessel, such as a pipe or tank. The electric current may be conducted by ions, as in the previous examples, or by electrons or holes (e.g., in semiconductors, metal films or semimetals). The resulting electric current distribution, and the voltage drops it causes, can be measured in multiple ways, according to the invention. Knowledge of these current and voltage distributions can be used either to implicitly infer the presence and / or dimensions of expected heterogeneities (e.g., knots in a log), or unexpected heterogeneities (e.g., metal or plastic foreign objects in an aqueous food medium) or to build a 2 or 3 dimensional map of electrical conductivity, or admittivity, from which, in conjunction with known properties of the medium / media, images may be constructed (e.g., of a foreign body in a food medium). The current distribution at certain points (e.g., at the electrodes which apply the field across the medium) can be directly measured, using the "Smart Electrodes" disclosed in PCT / NZ2018 / 050029 or a variation thereon. Electrode segments may be turned on or off, altering the excitation applied to the medium. The voltage of the medium at any segment injecting current may also be measured, as disclosed here (e.g., co-axial electrode segment, with current on outside and voltage in centre, or vice-versa - see Figures 1A and 2). Multiple excitation patterns can be superposed by applying different frequency and / or phase signals to any combination of electrode segments. Multiple frequencies may be applied to any given segment - for instance, by driving a segment from a Class-D switching amplifier, using unipolar pulse-width modulation (PWM), the modulation frequency, fm, and certain harmonics of the carrier frequency, fc, may be applied simultaneously (e.g., fm and 2fc + / - fm). The current distribution within the medium, at other points than the excitation electrodes, can be inferred by one or more magnetic field sensors (such as search coils), which may include sensors sensitive to field components in a single axis, or 2 or 3 orthogonal axes, external to the medium. These sensors are inherently of the noncontact type. The voltage distribution across the surface of the medium, at other points than the excitation electrodes, can be measured by one or more point contacts, or by non-contact displacement current sensors. In the latter case, the sensors may be embedded in a conductive plane held at ground potential, where the excitation voltage may be unipolar or bipolar with respect to ground. Alternatively, the sensors may be embedded in a plane (e.g., made from thin metal film or conductive plastic) whose voltage distribution mimics that of the medium for the homogeneous case. Alternatively, the sensors may be embedded in discrete sections of a plane which follow the voltage of the medium for the homogeneous case, but in a discretized fashion. Note that "plane" in this sense may be a flat plane, or a cylinder surrounding a pipe, or any other shape related to the shape of the object or medium or its container. The electric potential of the plane affects the exact displacement current value, but in all cases a reading related to the surface potential is obtained. Notably, all sensors (and there can be a large number of them, comprising some or all of directly applied currents and voltages, directly measured surface potentials, using point contacts, as well as non-contact sensed magnetic flux density and surface potential), may be measured / sampled simultaneously. BRIEF DESCRIPTION OF THE FIGURES Preferred embodiments of the disclosure will be described by way of example only and with reference to the accompanying drawings, each of which includes a brief accompanying description. Figure 1 is an overall schematic; Figures 1A and 2 are views of electrode assemblies Figure 3 shows views of an embodiment Figures 4-5 show return conductor arrangements Figure 6 shows another embodiment Figures 7 and 10 shows plot of field strength Figures 8 and 9 show circuit diagrams for sensors Figure 11 shows plots of field strength Figures 12-14 show an example of foreign object detection Figure 15 shows examples of sensing arrangements and circuits Figure 16 shows an example of a heat exchanger Figure 18 shows a sensor matrix and a PCB assembly Figures 19 and 20 shows examples of sensors for conduit applications Figure 21 shows a cross-section relating to a timber sensing application Figure 22 shows 2-dimensional view of electric equipotential lines surrounding a metallic sphere in a typical aqueous food mixture application DETAILED DESCRIPTION Various embodiments are described with reference to the Figures. Throughout the Figures and specification, the same reference numerals may be used to designate the same or similar components, and redundant descriptions thereof may beomitted. Electrical impedance tomography (EIT) is a well-developed technique. In its most common form, a number of electrodes, commonly in a ring pattern around a pipe or circular tank, are alternately excited in pairs, typically by an AC current, while the voltage between all other pairs is measured. In this way, a 2, or 3 (if there are at least 2 planes of electrodes) dimensional map of the electrical impedance or admittance of the medium is gradually built up. This is slower than for the present invention in which all the sensors are sampled simultaneously for a given excitation. One other aspect of existing EIT systems is the constraint of external fields, or rather the lack thereof, such that any excitation stimulus creates magnetic and electric fields which are not confined to the immediate locality. In some typical embodiments of the present invention, e.g., in a cylindrical pipe sensor, the electric field is applied axially between two electrode rings, causing longitudinal current flow (e.g., Figure 3). In the middle part of the pipe, assuming a homogeneous medium, the current density is constant across the cross section, and the electric field gradient is constant along the length. The resulting external fields can be greatly reduced, or eliminated, using the electromagnetic shielding system disclosed herein. This ensures that all external magnetic and electric fields from the excitation can be controlled or contained. The electromagnetic screen disclosed also makes it possible for the magnetic flux sensors to only be sensitive to deviations in electrical properties from a homogeneous medium case, rather than their absolute values. Replacing the grounded electrostatic screen disclosed with discrete rings of conductor, or a continuous partially conductive material, energized to the same electric field gradient as the pipe contents in the homogeneous medium case, also makes it possible to measure only deviations in electrical properties from a homogeneous medium case, rather than their absolute values. Thisgreatly improves the sensitivity and dynamic range of the system and is not possible with conventional EIT. The present system provides electrical voltage excitation to the whole medium continuously (at least while measurements of any or all of current, voltage, magnetic flux / flux density, displacement current / current density, are being taken), whereas conventional EIT only applies localized current excitation, and measures resultant voltages, in a time-sequenced fashion. Hence, for similar measurement signal levels, greater power is applied in the present method. This means that Joule heating effects will be more pronounced; however, in applications where the medium has a significant thermal mass (e.g., unseasoned timber), or is in motion (e.g., aqueous food mixture) this is unlikely to be a problem. Furthermore, in the case of deliberate Joule heating, the heating source also provides the excitation for sensing. Since the principal mechanism of detecting heterogeneity is change / s in electrical current density, J, for a given electric field, E, electrical admittance sensing and tomography is used herein as a term to clarify the present invention and assist with distinguishing it from EIT methods and systems. Referring now to the drawings, Figure 1 shows an example of a detection system 100 which may be used to detect electrical admittivity of a subject or object of interest 102 which may in embodiments be a solid or a fluid and can in embodiments be heterogeneous, and potentially anisotropic. For example, subject of interest may comprise a solid article, for example a wood length or log, or may comprise a fluid in a tank or pipe, for example an aqueous food mixture, such as milk. If subject 102 comprises a solid article, electrodes 104 may be electrically connected to the article, preferably at opposing locations, for example 104a and 104b on the article 102 in a manner similar to that shown in Figure 1. In an embodiment, if the subject 102 is provided in a container or vessel, then electrodes 104 may be configured to extend into or through a wall of the container or vessel such that the electrodes 104 make an electrical connection with the subject 102. In some embodiments the electrodes 104 may not be located at opposing positions or locations. In embodiments the electrodes 104 are connected to the subject 102 at locations which cause a current to flow in the subject between the electrode locations once a potential is applied between the electrodes. Electrodes 104 are electrically connected to a power supply 108 by conductors such as cables 105. In an embodiment one or more return conductors 109 are provided as will be described further below. The power supply is configured to electrically excite the electrodes so that the electrodes 104 apply electrical excitation to the subject 102. In an embodiment the power supply 108 comprises an AC voltage source, a DC voltage source, or an AC / DC voltage source. In an embodiment the power supply 108 comprises an AC Z-source, a DC Z-source, or an AC / DC Z-source. In an embodiment, where the electric power supply 108 provides AC electric power, the frequency matches a locally available mains supply. In an embodiment the frequency is not harmonically related to the mains supply. In an embodiment the frequency is in the approximate range of 50-75 Hz. In an embodiment the frequency is in the approximate range of 375 Hz to 425 Hz. In an embodiment the frequency is approximately 405 Hz. In some embodiments the frequency is in the range of 1 - 100kHz, for example approximately 10kHz. This is suited to cases where properties of a heterogeneous fluid, flowing in the velocity range of 0.1 to lOm / s are being sensed. Additionally, multiple frequencies may be applied, either concurrently or sequentially. In the former case, a modulated frequency and / or a signal related to the modulation frequency and / or a carrier frequency may be simultaneously applied, for example using a Class D amplifier as the electric power supply. In general, the excitation frequency / ies, or range of frequencies, may be selected to reveal as much information as possible regarding the subject or object, taking into account the geometry and material properties of the overall system. Higher frequencies may reveal additional information due to capacitive admittance increasing with frequency and inductive admittance falling with frequency, while resistive admittance (or conductance) is frequency independent. Conductors 105 are shown in broken lines because alternative electrode arrangement can be used depending on the subject or the nature of the required detection or imaging. For example, in an embodiment, electrodes 106 (connected to the power supply 108 by conductors 107) may be used in place of, or additionally to, the electrodes 104. The power supply is configured to electrically excite the electrodes so that the electrodes 106a,b apply electrical excitation to the subject 102. The excitation that is applied by the electrodes is configured to cause a current 116 in the subject 102. Current 116 is shown for purposes of illustration as flowing in one direction, but it will be understood that the current 116 may alternate in use. The electrode arrangement, positioning and / or the excitation applied to the electrodes is configured in some embodiments to produce currents 116 that predominantlyflow longitudinally between longitudinally separated electrodes 104a and 104b, or 106a and 106b, such as the axial or longitudinal flow illustrated in Figure 1. As illustrated in Figure 1, a variation in the medium comprising subject 102 will typically alter the admittance of a part of the subject, causing a change in behaviour of the current 116. This may also cause a change in surface potential of a part or parts of the electrically excited subject 102. In Figure 1, the variation in 102 is shown as a nonhomogeneity 118, which may for example comprise a knot if the subject 102 is a log or may comprise a solid object if the subject is otherwise a liquid. In an embodiment one or more electric potential sensors 112 is provided adjacent to the subject 102 and between the electrodes to detect a surface potential of the subject 102. In an embodiment the sensor 112 may comprise a capacitive plate as described further below. In an embodiment sensor 112 can comprise a direct surface contact to the subject or object of interest. In an embodiment one or more magnetic fields sensors 110 is provided between the electrodes to detect one or more magnetic fields resulting from current 116. The sensors 110 and 112 are in communication with a controller 114 which may for example comprise one or more processors configured to do one or more of: control or instruct the power supply 108 to energise one or more of the electrodes at a given magnitude, sequence and / or frequency; receive data from the electrodes regarding current and / or voltage magnitudes in response to the excitation; receive data from the sensors 110 and 112; provide an output to apparatus for processing the data, or process the data and provide an output. In an embodiment an output may be provided to a display device 120. Turning to Figure 6, a diagrammatic illustration of an embodiment part of system 100 is shown in which the subject of interest is provided within an electrically insulated housing or body 122. The electrodes 106 are configured to extend through the body 122 so that they may be electrically connected to subject of interest 102 located within body 122. An example of an application of use for the embodiment illustrated in Figure 6 is for body 122 to be disposed in a conduit or pipe, or for body 122 to comprise a section of a conduit or pipe, so that a fluid present in the conduit may comprise the subject of interest 102. As shown in Figure 6, a plurality of magnetic field sensors 110 and / or electric potential sensors 112 can be provided. In an embodiment the sensors 110 and / or 112 may be provided circumferentially about the subject of interest 102. Although the sensors 110 and / or 112 are shown as being evenly spaced circumferentially in Figure 6, in some embodiments they may be provided at irregular spacings, or only provided at one or more specific circumferential locations. In an embodiment the sensors 110 and / or 112 may be provided longitudinally along a longitudinal axis that extends through the subject of interest 102 and between the first group of electrodes 106a and the second group of electrodes 106b. Although the sensors 110 and / or 112 are shown as being evenly spaced longitudinally in Figure 6, in some embodiments they may be provided at irregular spacings, or only provided at one or more specific longitudinal locations. In an embodiment the sensors 110 and / or 112 are provided on a sensing structure that may for example be cylindrical in form and which carries sensing matrix 128 external to the subject of interest 102. The structure may comprise an outer surface of housing 122 or may comprise a discrete structure. Figures 3a-3c show an embodiment in which the electrodes 106a,b comprise electrode assemblies 130a and 130b which incorporate the construction shown in Figure 2. In an embodiment the electrode assemblies 130a, 130b may include active or passive components configured to measure current and / or voltage at one or more individual electrodes that comprise the respective assembly. The sensors 110 and / or 112 are provided between the assemblies 130a, 130b. The apparatus comprises a structure having part 128 which substantially encompasses a sensing region configured to receive the subject of interest, for example a substance that may flow through the apparatus. The structure supports the first and second electrodes or electrode assemblies 130a and 130b in this example which are provided on the structure at opposed ends of the sensing region. The structure part 128 which comprises the sensing region has the sensor(s) 110 and 112 are provided thereon between the first and second electrodes. Further detail is seen in Figures 3a and 3b in which data acquisition boards are removed to reveal more detail of the sensing matrix or array comprising sense coils 110 and potential sense plates 112. In an embodiment sense coils 110 are located outside plates 112 i.e. coils 110 extend further from the sensing region than the electric potential sensing plates 112. Figure 4 shows an embodiment in which the subject of interest comprises a substance or object to which electrodes 104a,b are applied. For example, electrode assembly 132a is applied to a side or end of the subject of interest so that the individual electrodes 104a that comprise the assembly make electrical contact with the portion of the subject 102 to which they are proximate. Similarly, electrode assembly 132b is applied to another (for example opposing) side or end of the subject of interest so that the individual electrodes 104b that comprise the assembly make electrical contact with the portion of the subject 102 to which they are proximate. As disclosed in PCT / NZ2018 / 050029, electrode assemblies such as 132a,b may be applied to either end of a length of woodand energised to provide a current through the wood length. Referring again to Figure 4, the system 100 may in some embodiments, without being limited to the arrangement shown in Figure 4, include an electromagnetic screen 136 and / or electrostatic screen 138 which will be described further below with reference to Figure 9. In an embodiment the electrode assemblies 132a,b may comprise an array or mosaic of electrodes 104a,b which may or may not be arranged in a regular pattern. An example is illustrated in Figure 1A in which there are n electrodes 104a,b that are arranged in a pattern. A current and / or voltage distribution across the electrodes can be used to determine admittance or regions of the subject 102. Turning to Figure 21, the arrangement of Figure 4 is shown in cross section through the middle of subject 102 (which in this example comprises a log). Upon energisation of electrodes 104, current will flow in the log, alternating in a predominant direction in and out of the page. If careful attention is not paid to the layout of the current path, considerable electric (E) and magnetic (H) fields will exist outside the log, potentially leading to undesirable coupling with, and interference to, other equipment and processes in the vicinity. International standards define the allowable levels of emitted fields, such that non-complying products and processes can be identified. Also, external fields from other apparatus for example can impinge on the subject or sensors and affect detection parameters. In one or more embodiments, external fields can be largely or essentially eliminated by ensuring that the subject 102, and the conductors feeding the electrodes 104, 106, are surrounded by electrostatic 138 and / or electromagnetic 136 screens, or shields. In an embodiment the electromagnetic (EM) screen 136 around the subject 102 is a conducting cylinder at a known potential 140, for example approximately ground potential. In order to have evenly distributed current through the cylinder, to avoid current crowding, the cylinder can in an embodiment be made up from a number of individual conductors with small series balancing impedances to balance the current in each conductor. In an embodiment a separate electrostatic (ES) screen 138 may also be employed, again coaxially, inside the EM screen. The purpose of this screen is to return displacement currents, caused by alternating voltage on the subject 102 surface, directly to supply ground 140 rather than allowing them to flow through the EM screen. As can be seen in Figure 21, in an embodiment the surface potential sensors 112 can be provided adjacent to the electrostatic screen. Figure 8 shows an example of a surface potential sensor 112, which comprises conductive plate 156 which is provided in the electrostatic shield 138 but insulated therefrom by a non-conductive border 158. The sensor 112 also comprises a transimpedance amplifier 160. If alternating current (AC) excitation is used between the electrodes, the absolute potential, or voltage, v, relative to ground, of any part of the subject's surface 103 can be measured by means of capacitive displacement current. Each member of a cylindrical array of electrically conductive plates 156, such as those shown in Figure 8, is connected to transimpedance amplifier 160, such that the output voltage of each amplifier is proportional to the rate of change of potential between the adjacent surface 103 and ground reference 140. The governing equation relating current through, to voltage across, a capacitoris: i = C dv / dt where i is the displacement current, C is the capacitance and dv / dt is the rate of change of applied voltage. Assuming the excitation is sinusoidal with frequency f and the amplitude of the subject voltage (with respect to ground) at the nth sensor location is Vn, the amplitude of the nth displacement current, Ieo, is given by: IEn = 2nfCnVn The capacitance, Cn, between the nth conductive plate 156 and the surface of the log is defined as: dn where eo is the permittivity of free space, Er is the relative permeability of the gap between the subject surface and the conductive plate 156, An is the cross-sectional area of the nth conductive plate and dn is the distance between the surface of the subjectand the nth conductive plate. Normally, as the two "plates" of each capacitor are not the same size, there would be fringing flux which would vary the effective value of area An. However, in the example shown in Figure 8, the cylindrical surface between the array of electrically conductive plates 156 is grounded, with the plates themselves being held at virtual ground by their transimpedance amplifiers 160, as shown in Figure 8. Thus, there is no preferential electric flux path other than the direct one from the surface to the nearest portion of the cylindrical surface, leading to a complete lack of fringing flux. Thus, the output voltage amplitude of the nth transimpedance amplifier is given by — lEnRn —          — where Rn is the feedback resistance, or transimpedance, of the amplifier. In an embodiment the conductive plates 156 and their associated transimpedance amplifiers 160 make up a complete cylindrical array of n sensors, as shown in cross section in Figure 21, and that each sensor is calibrated to give the same voltage output when a second, smaller diameter cylinder, at a fixed voltage other than ground is inserted into the centre of the array, i.e. all sensors are identical in terms of their capacitance Cn and their amplifier's transimpedance Rn. Now, when a subject 102, which is not perfectly cylindrical, is inserted into the cylindrical array and held at a fixed voltage other than ground, each sensor 112 in the array will output a different voltage which will be inversely proportional to the distance, d, between it and the nearest part 103 of the subject's surface, i.e. each effective capacitance, Cn, will be different, because dn will be different. In this way a three-dimensional image of the subject's surface can be formed, which can be used to determine properties relating to the volume of the subject. (In conjunction with real-time mass measurement, this can also yield an estimate of density and / or approximate moisture content figures for example). Additionally, this image can in an embodiment yield a unique "signature" for each subject that can be used for tracking it if required, for example, tracking an object through a manufacturing process or a treatment system, or positioning an object in a required orientation. Once the image has been formed an automatic calibration can be carried out. The output of the nth sensor, Vzn, is normalized to the highest sensor output, Vzh, by multiplication (for example in software by controller 114) by a factor Vzh / Vzn, such that, when the surface 103 adjacent to each sensor is at the same potential, the output of each sensor is the same. Now the output of each sensor will be proportional to the subject'ssurface potential, regardless of dn. When the subject is energized with a potential difference across it, rather than a fixed potential throughout, the sensor array will yield a real-time picture of the voltage distribution over the surface of the subject. Thus, the voltage drop across the surface of any part of the subject can be determined. Additionally, by considering the shape of the equipotentials that can be calculated from the sensor array outputs, the direction of current flow can also be determined. This allows the detection of non-uniformities which cause the current to diverge from a straight longitudinal path. Current, i, flowing through the subject produces a magnetic field, H, with lines of magnetic flux, cp, circling the subject, through free space with relative permeability po, around loops of length, I. i = <p Hdl If we assume that the subject, or at least the wires connected to the electrodes at either end, is / are very long, the magnetic flux density, B, is inversely proportional to the distance, R, from the centre line of the current flow. ®          uoi b=-a = ^ = £r (In practice the return current loop to / from the excitation source will also cause magnetic flux lines which will interfere with the subject or object's flux, unless they are cancelled in some way). Assuming that AC excitation is used, the flux density, B, around the subject can be measured by means of an array of magnetic field sensors 110 which may comprise coils, as shown in Figure 4. In an embodiment, such as that shown in Figure 9, the magnetic sensors comprise coils 162, which can be selectively oriented if required to be sensitive to magnetic fields in certain directions, dependent for example on the subject 102 or that property or feature being detected. The coils may be air cored so as not to distort the magnetic field. Those skilled in the art will understand that other forms of magnetic sensor can be used. When one or more coils 162 is used, the coil embodies Faraday's law of EM induction and produces a voltage, v, at its terminals, proportional to the rate of change of flux, cp, linking the N turns of its windings. Assuming the excitation is sinusoidal, with frequency f, and the amplitude of the flux density component normal to the nth sensor location is Bn, the amplitude of the nth search coil output, with effective area Ann is given by: VHn = 2nfNAHnBn The output of each search coil is fed to a voltage amplifier 164 with gain Gvn, such that the output voltage amplitude of the nth voltage amplifier 164 is given by Vv = Gv VH As mentioned above, in the absence of shielding, stray fields from other conductors in the excitation system and from other loads running from the same excitation supply, or located nearby can affect the field being received by any one or more coils 162 in use. A more sensitive and accurate approach, which has the benefit of cancelling all external fields, while also ensuring that the E field does not affect the search coil readings is now described and shown with respect to Figures 4 and 21. As disclosed above, the E field is measured by one or more virtually grounded capacitor plates 156, into which the relevant displacement current is directed. The plates 156 may be provided in a cylindrical array, which in conjunction with the ground plane in between each plate element 156 forms a complete cylindrical electrostatic (ES) screen 138 (or Faraday cage) around the subject 102, such that the external E field is approximately zero. If the "current compensating" conductive cylinder (CC) 136 is added around this ES screen 138, and a uniformly distributed current equal, but opposite in direction, to the current flowing in the log is made to flow in it, the external H field far from the subject 102 will also be approximately zero. This is because the subject and the CC effectively form a coaxial conductor system, with the magnetic field from one cancelling that from the other when the distance from the axis of conduction is large relative to the radii of the two conductors. Hence the CC 136 acts as an electromagnetic (EM) screen, minimizing external H field caused by the apparatus as can also be seen in Figure 7. As long as both this EM screen and the ES screen 138 have no appreciable potential between them (typically they are both at approximately ground potential) there is negligible E field between them. If the subject 102 and EM screen 136 currents are perfectly coaxial there will be no external H field at all. However, the H field close to the EM screen 136 will not be zero if the two currents are not perfectly symmetrical. If the current in the EM screen is perfectly distributed then the H field will be disturbed by any asymmetry in the current flow in the subject 102. For instance, assuming that the subject current 116 flows predominantly down one side of the subject, the H field will be more positive outside the EM screen on this side and less positive outside the EM screen on the other side. These deviations from zero of the external H field can be picked up using a cylindrically arranged array of coils 110 close to the outside of the EM screen, as shown in Figures 4 and 24. As there is no appreciable E field, capacitive displacement current pick-up by the coils 162 will not occur. Figure 5a shows an embodiment in which a unipolar high voltage (HV) alternating current (AC) supply is applied to a subject 102. Note that HV is relative to the low voltage (LV) source applied to the transformer shown and the actual LV and HV amplitudes are determined by the application. Current flows from the dot end of the HV winding 170 through the centre conductor of a triaxial cable 172 (red line and arrow), through the left hand (red) electrode 104a or 106a and into the subject 102. The current continues through the right hand (yellow) electrode 104b or 106b and into the magnetic screen (blue) 136, through which it returns coaxially to the earthed end of the HV winding 170 (blue arrow), thus cancelling the magnetic field around the subject 102 and cable. The electromagnetic (EM) screen 136 around the log is a conducting cylinder at approximately ground potential. Note that, in order to have evenly distributed current through the cylinder, to avoid current crowding, the cylinder can be made up from a number of individual conductors with small series balancing impedances 150 to balance the current in each conductor, as shown in Figure 5a. The Figure 5a topology may be used in the embodiment illustrated in Figure 4, but with the exception of the balancing impedances. In the Figure 4 embodiment the magnetic field sense coils 110, and / or other current sensors, are configured to sense the imbalance in currents in the return conductors that form the EM shield, so the balancing impedances are not added. Despite the return current variation, the coaxial nature of the return conductors still constitutes an effective EM screen, as is discussed further below with respect to Figure 21. A separate electrostatic (ES) screen 138 may also be employed, again coaxially, inside the EM screen 136. The purpose of screen 138 is to return displacement currents, caused by alternating voltage on the log surface, directly to supply ground rather than allowing them to flow through the EM screen 136. Figure 5b shows another embodiment, in which the current flows through the high voltage cable 172 and electrode (red arrow) 104a or 106a and returns through the return cable and electrode (yellow arrow) 174 to ground. The primary winding of a current transformer (CT) 176, with unity turns ratio, is placed in series with the earth end of the HV winding 170. The secondary winding of the CT 176, which may be earthed as shown, drives an exact replica of the current through subject 102 around the EM screen (blue arrows) 136 to oppose the subject current and cancel the external magnetic field. The CT 176 is configured to drive the burden impedance of the EM screen 136, including any balancing impedances 150. An ES screen 138 may again be employed, as shown, in which case it is possible to allow the EM screen 136 to float or be connected to any convenient potential, rather than being grounded. Figure 5c shows an embodiment, in which a centre-ground bipolar HV supply 180 is connected to the subject 102. Again the CT 176 supplies an exact replica of the current in subject 102, to cancel the external magnetic field, and allows the EM screen 136 to be at any desired potential, including ground, while the ES screen 138 prevents any displacement currents flowing through the CT windings. The arrangement of Figure 3 is similar to that of Figure 5c, except that electrodes 106a, b are of the ring type, as Figure 2. Figure 5d shows an alternative embodiment, in which the passive CT arrangement of Figure 5c is replaced by an active system. A CT 176, or other current sensor, provides a signal corresponding to the current (red and yellow arrows) in subject 102 to an amplifier 178 which drives a compensating current through the EM screen (blue arrows) 136, to eliminate external magnetic field. Again, the electromagnetic screen 136 may be grounded, or held at any other desired potential. This embodiment removes the need for the CT 176 to be capable of driving the impedance of the EM screen 136. It will be understood that other arrangements for arranging an EM screen 136 are possible, depending on whether the excitation is unipolar or bipolar and whether the compensation is passive or active. Calibration of the H field system can be carried out, if required, by using a conductive rod, geometrically centred between the electrodes and passing a current through it. By passing a current equal in magnitude, but opposite in sign to that through the EM screen, the null of each element of the search coil array can be set. By deliberately passing currents of known unequal magnitudes and opposite sign, the gain can be set (if necessary in both polarities). It should be noted that in one or more embodiments the arrays of potential sensors 112 and coils 110 can be fixed or movable rotationally and / or longitudinally, to increase the number of apparent sensor locations, for increased resolution. In the limit a single ring of sensors and coils, or even a single sensor and coil can be used with means for rotational and longitudinal movement, to give any sized array of apparent sensing locations. In an embodiment the electrostatic shield 138 may all be at some reference ground potential, or it can be composed of discrete rings, each held at a different potential, for instance the same potential as the undisturbed sensed medium at the same position in the pipe (thus following a discretised version of the excitation voltage gradient along the sensor), or it can be made from some partially conductive material, such as "conductive" 3D-printing plastic, following the continuous function of the excitation voltage gradient. The latter two options tend to cause the displacement current into each capacitive sensor to follow more closely the actual disturbance in surface potential, relative to the undisturbed case, caused by heterogeneity in the medium. By this means the "commonmode" component of displacement current is also either reduced, oreliminated. In an embodiment the magnetic flux sensors may advantageously be arranged to measure in one, two, or all three of the possible orthogonal directions - in the case of a cylindrical pipe these are: circumferential; longitudinal; and radial. In an embodiment the magnetic flux sensors may advantageously be situated outside the electromagnetic shield (if fitted). For the circumferential component of flux, this leads to removal of the "common-mode" component, caused by the total current flowing between the two rings of electrodes. Figure 10 illustrates theoretical and experimental current and voltage distributions which validate the expected conductive behaviours discussed above. Figure 10 shows simulated a current distribution 200 and a voltage distribution 202 by externally measured magnetic and electric fields for a block of plastic 204 and graphite 206 in a brine solution. The measured flux density distribution 208 and measured voltage distribution 210 are also shown in Figure 10. The plastic is less electrically conductive than the brine, so it causes current diversion, reduced magnetic flux density and higher voltage drop. The graphite is more electrically conductive than the brine, so it causes current crowding, increased magnetic flux density and lower voltage drop. Testing has been conducted using an experimental apparatus 100 which is similar to the Figure 3 embodiment. This embodiment, and others disclosed herein, may have electrodes 104 or 106 which comprise a contact portion 111 that makes electrical contact with the subject 102, and a measurement portion 113 which includes a sensor to measure voltage and / or current, such as the coaxial electrode segments shown in the ring electrode of Figure 2. Therefore, any or all of each of the electrode segments 104 and / or 106 may be provided such that they have two parts - a driven part, through which excitation current flows and can be measured, and a measurement part, at which a parameter,for example the actual voltage of the medium, can be measured. In the case of the coaxial segments of Figure 2, using the outer annulus as the contact part 111 may be advantageous to maximize contact area with the medium in the tube at excitation frequencies where skin depth becomes small relative to electrode segment dimensions. The central part of the apparatus 100 in the embodiment shown includes a substantial cylinder having sensors 110 and 112 as disclosed above. Other shapes may be used. Figure 14a and 14b shows two objects for use in testing apparatus. The object 220 is a plastic object and 230 is a metal object. Lines 222 and 232 show the expected current flow with respect to each object. 240 represents a thin insulating rod or line used to move the object through the liquid food mixture. Figure 12 is a plan view showing the location of each object 220, 230 in use as the object is raised upwardly out of the page through a liquid food mixture. Figure 12 identifies three sensors 110 (channel 1, 2 and 3), in this case two circumferential and one axial magnetic field sensors, and the output of those sensors is recorded on the plot in Figure 13 of sensor output voltage against axial distance between the plane of the sensors and the geometric centre of the object. Further testing has been carried out with the Figure 3 embodiment. Figure 14c shows the demodulated outputs of just nine of the sensors in the sensor matrix 128 in response to a 6 mm radius lead sphere falling through the 100mm internal diameter pipe around which the sensor matrix is installed. The pipe is filled with a brine with an electrical conductivity of about 0.5 Sm4, while a 10 kHz sinusoidal electric field of 40 Vm4 is applied between the two electrode rings 130 which contact the food mixture with an axial spacing of 0.5 m. The top traces show the displacement currents measured by the top three surface potential sensors 112 in one sensing column. The middle traces show the disturbance to the circumferential magnetic flux density in the top three circumferential magnetic flux density sensors 110 in the same sensing column. The bottom traces show the axial magnetic flux density linking the top three axial magnetic flux density sensors 110 in an adjacent sensing column. Some applications of the apparatus and methods disclosed above are described below, by way of example. 1. Knot detection and / or saowood / heartwood discrimination in loos. It is known that the sapwood of unseasoned timber is anisotropically electrically conductive, and that the conductivity in the longitudinal direction (in the order of 0.01 to 0.1 Snr1 at room temperature) is substantially higher than in the radial and tangential directions. Thus, when an AC voltage is applied across the ends of a log, a current flows along the log, but deviates around the knots which are roughly at right angles to the current path and hence present radial / tangential resistivity. With a suitable number of magnetic field sensors (e.g., search coils) around the log, a picture describing the current flow and revealing the location and relative size of the knots can be produced. Additionally, by splitting the electrodes applied to the ends of the log into multiple segments, the distribution of current can be used to distinguish the log's sapwood (more conductive) from the heartwood (less conductive), giving accurate dimensions of both timber zones and the overall shape of the log end. Note that this mensuration can be carried out at the same time as Joule heating (high power), when elevating the log's temperature (e.g., for phytosanitary, pre-peel or pre-saw treatment) or at low power solely to obtain data. In the the former case, knowledge of the knot dimensions and locations can be used to help control the heating process, as well as to grade the log. Since there are knots all along a log's length, conventional EIT is not appropriate. Figure 4 gives a diagrammatic representation of this application. The segmented electrodes (another version is shown in Figure 1A) simultaneously differentiate between the various timber zones within the log, as per the existing patent. 2. Online foreign object detection and / or assessment of flow homogeneity and / or temperature in a food medium in a circular pipe. A food mixture such as milk is an aqueous solution with electrical conductivity in the order of 0.01 to 10 Sm'1 at room temperature. A metallic object passing through the pipe will have a conductivity of up to 8 orders of magnitude higher than this, while a plastic or glass object may be up to 16 orders of magnitude less conductive. Thus, electric current preferentially flows through metallic objects and diverts around plastic / glass ones. The sensing pipe section is typically made from a food-grade engineering plastic material, such as PTFE or PEEK, or alternatively glass, although it may be covered by an external metallic shield. This ensures that the sensing pipe does not itself affect the current and voltage distribution in the food medium. However, the balance of the system pipe-work is generally 304 or 316 stainless-steel. Assuming the pipe is excited at a fixed voltage applied between two electrode rings, spaced some distance apart along the length of the pipe, that each ring consists of multiple electrode segments, and that the medium is homogeneous other than for the foreign object, the following is true: i. A metallic object entering the pipe between the electrodes will increase the total current through the medium. This will drop back to the original current once the object has passed the second electrode. ii. A metallic object entering or leaving the pipe closer to one electrode segment than the others will cause that segment to have the highest current as it passes. Hi. Any section of pipe between the electrode rings will have a lower electricfield strength (i.e., voltage drop per metre) if there is a metallic object present. iv. A plastic object entering the pipe between the electrodes will reduce the total current. This will rise back to the original current once the object has passed the second electrode. v. A plastic object entering or leaving the pipe closer to one electrode segment than the others will cause that segment to have the lowest current as it passes. vi. Any section of pipe between the electrode rings will have a higher electric field strength (i.e., voltage drop per metre) if there is a plastic object present. vii. When the electromagnetic shield is incorporated, apart from fixed end effects, the only disturbance to the measured magnetic flux and / or the surface potential will be due to the object. viii. The ratio of the total current to the electrode voltage provides a real-time average electrical conductance value for the medium between the electrode rings,from which average conductivity is readily calculable. The effects seen in i and iv above may be very small if the object's dimensions are small in respect to the pipe diameter, but the effect will be there, even if the object is at the geometric centre of the pipe. The same will be true for Hi and vi above; however, if there are multiple rings of electric potential sensors between the excitation electrodes, the moving disturbance, which will be related to the flow velocity, may be relatively easy to detect and correlate with the effects of i and iv. In this application, the minimum rate of sensor sampling is important and is related to the flow velocity of the mixture. However, the same is true of conventional EIT in which the multiple excitation and response patterns also need to be carried out; hence, EIT is severely limited in this application. Also, if two such sensors are installed in a pipeline in a plant, for instance one upstream and one downstream of a heat exchanger, the disturbance will appear in the downstream sensor a certain time later than in the upstream one, dependent on flow rate. Additionally, with some imaging software, the present system can be used to infer a 3dimensional electrical conductivity map of the medium at any instant. For an isothermal medium, such variations will be due to heterogeneity of the mixture, whereas, for a homogeneous medium they will be due to heterogeneity of temperature distribution. Figure 3 shows one embodiment of a sensor for this system. The electrodes used may be of the two part type shown in Figure 2; in this case, the outer current injecting annulus 111 is electrically connected to the inner surface of the electrode segment board, while the centre voltage measuring electrode 113 is electrically connected to the outer surface of the electrode board. In Figure 3, 8 rings of sensors are arranged in 16 columns. Since each of the visible search coils has a surface potential sensor underneath it, there are 128 search coils and 128 surface potential sensors in this embodiment. Adding 16 electrode currents and 16 electrode voltages for each of the two electrode rings, there are 320 measurements taken each sample. More complex embodiments exist, where there are more sensors, which may be smaller and more closely spaced, to improve spatial resolution. The use of displacement current measurement for surface potential determination, as used in the embodiment of Figure 3, is now explained in more detail, with reference to accompanying figure 18A. Figure 18A (i), top, shows a schematic representation of the sensing array / matrix shown In Figure 3, surrounding a pipe. Effectively, this array can be regarded either as 16 columns, each with 8 equally displaced vertical elements, or as 8 rings, each with 16 equi-angularly displaced circumferential elements. (This particular embodiment is thus hexadecagonal in nature and is shown affixed to a pipe with a hexadecagonal outer wall and a circular inner one - in other embodiments both walls could be hexadecagonal, or circular, or another shape appropriate to the number of columns). In this case we will consider the 8-ring case, as each of the 16 sensors in any given ring has the same reference ground. This local reference ground is different for each of the 8 rings and its absolute value is chosen to match the expected absolute voltage of the medium inside the pipe, in the undisturbed case, on the circular plane at the geometric centre of the ring. In the case where a bipolar, centre ground excitation voltage is applied, with the top electrode being positive, the top ring will thus have the most positive local ground potential (relative to "system ground" which is the centre point of the bipolar supply), while the bottom ring will have the most negative local ground. Figure 15B shows one embodiment of a scheme for providing the necessary local grounds for the 8 sensor rings of Figure 18A(i), as used in the embodiment of Figure 3. Bipolar 10 kHz voltage excitation is applied across the upper and lower electrode rings (106a, b) via an AC coupled power amplifier and an isolation transformer, resulting in an electric field of approximately 40 Vm4 along the pipe between the electrode rings. An autotransformer with a suitable total number of turns and taps provides an appropriate local ground potential to each of the 8 rings. The electronics of each ring are powered by a bipolar supply from the outputs of one of 8 isolated DC-DC converters designed or chosen to minimize displacement currents, through their isolation barrier, at the excitation frequency / ies. Assuming the pipe diameter is constant, the electrodes are far enough away from the sensing array for the E field gradient to be constant and that the sensor ring spacing is constant, the potential difference between each adjacent local ground will be the same. Considering just the top ring in the 8-ring array, each of the 16 elements incorporates an electrically conductive sensor pad embedded in a printed circuit board (PCB) as shown schematically, but not to scale, in Figure 18A(ii), bottom, connected to the inverting input of a transimpedance amplifier similar to the simplified circuits of Figures 8 and 15 (iv). The non-inverting terminal is referred to the local ground, rather than the system ground - hence, when the pipe medium is uniform there is no net displacement current into the sensor pad, as it is at the same potential as the adjacent pipe contents, so the outputs of all the transimpedance amplifiers are at zero volts relative to local ground. The sensor pad is accessible to displacement current from the surface of the medium through the electrically insulating pipe wall (typically PEEK, PTFE, Polycarbonate, PMMA or some other plastic, or glass, or ceramic) and a "hole" in the local ground-connected conductor with which it is otherwise "guarded" (typically copper, including copper layers on the surface of and within the PCB, which is likely to be of a multilayer construction, and a shielding can), to protect it from electric field interference from other, unintentional sources. The wanted electric flux lines thus pass through the pipe wall and the insulating layers of the PCB, which are usually made from FR4 fibreglass, to couple with the sensor pad, so the effective capacitance is affected by the geometry and the relative permittivities of the various insulating materials. Generally, some demodulation scheme synchronous with the excitation frequency / ies is used, such as quadrature demodulation, with bandwidth limited appropriately to minimize non-coherent noise coupling. Simplified embodiments also exist, where the number and orientation of the electric potential and / or magnetic flux sensors are reduced. There are six limiting cases, all shown for a cylindrical pipe geometry, which are shown in Figure 15 (a-g), with related amplifier circuits shown as (i - v). These have the potential advantage of being of low cost, relative to the embodiment already discussed, for simple foreign object detection. a) One or more (one may be sufficient) circumferential search coil encompassing the entire perimeter. This is similar to a Rogowski coil, and integrates all the circumferential flux which links its turns. Here, the total voltage induced can be amplified with a single-ended amplifier (i), as in the multiple small search coil system, or the electrical centre of the search coil can be grounded with the start and finish connected to the inverting and non-inverting inputs of a differential voltage amplifier (ii). There are other possibilities too, such as weighted summing amplifier connecting various turns of the coil to the amplifier. As the foreign body passes through the coil, the total flux linking the coil's turns will vary, changing the induced voltage (iii). b) One or more axial search coil extending along the entire sensing region. This / ese search coil / s integrate all the axial flux which links its / their turn / s. In the central sensing region there will be no axial flux unless a heterogeneity is present. Again various amplifier configurations are possible. Here, the total voltage induced can be amplified with a single-ended amplifier, as in the multiple small searchcoil system, or the electrical centre of the search coil can be grounded with the start and finish connected to the inverting and non-inverting inputs of a differential voltage amplifier. In the latter case, a change in amplifier signal output sign can be expected as a foreign body passes through the plane of the coil. There are other possibilities too, such as weighted summing amplifier connecting various turns of the coil to the amplifier. This / ese axial search coils can be made using a double-sided, or multilayer, printed circuit board (PCB) with the turns made from conductive traces linked through the board by vias. c) A special case of b) involves a single coil wound around the outside of the pipe, which can again be connected to various amplifier configuration (i - iii). In a further special case, the single coil can itself be made with multiple turns, like a Rogowski coil, i.e. a hybrid of a and c, such that it is sensitive to both axial and circumferential flux components and can again work with multiple amplifier configurations. d) One or more radial search coil extending along the entire sensing region. This / ese search coil / s integrate all the radial flux which links its / their turn / s. In the central sensing region there will be no radial flux unless a heterogeneity is present. e) One or more circumferential ring plate surrounding the entire perimeter of the sensing area. This ring captures all the displacement current from the inner surface of the pipe. This current is typically amplified by a transimpedance amplifier which holds the plate at a fixed potential relative to theadjacent medium, when homogeneous. This ring may be made with a small slit, or with an overlap, to avoid inadvertently creating a single turn shorted search coil. If correctly biased, there will be no net displacement current unless a heterogeneity is present. A single such ring plate can be connected to a transimpedance amplifier, such as that of (iv). Pairs of ring plates can be connected to the circuit of (v), such that the difference between two displacement currents is amplified. For instance, if the centre pair of ring plates of (e) were connected as in (v), under homogeneous conditions, with the centre point of the medium in the pipe at 0 V (same potential as transimpedance amplifier non-inverting input), the two displacement currents would cancel, leading to a null output. If a foreign object passes through, the two displacement currents will be altered, such that the net current through the transimpedance amplifier's feedback resistor will cause the output voltage to first swing in one direction as the object nears the upper of the two rings, pass through zero as it passes the pipe's mid-point between the two rings, and then swing in the opposite direction as the object nears and passes the lower of the two rings. f) One or more axial plate extending along the entire sensing region. Again, if correctly biased, there will be no net displacement current unless a heterogeneity is present. Any such displacement current may be converted to a voltage signal by the transimpedance amplifier, such as that of (iv). g) Combining case d) and case e), if correctly biased, there will be no net displacement current into the single circumferential ring plate unless a heterogeneity is present. Any such displacement current may be converted to a voltage signal by the transimpedance amplifier, such as that of (iv). 3. Online imaging of food composition or temperature variation across a rectangular extruded sample. Minced meat may be extruded through a die of any shape, while plant-based meat analogues are often extruded while undergoing cooking or other heat-treatment. Perhaps the simplest implementation of the present invention is to embed the segmented electrodes in the large faces of the rectangular extruder. In this manner, the electrical current distribution can be directly read from each face. This is analogous to the case of imaging one or both ends of a log with a smart electrode / s, as in PCT / NZ2018 / 050029, US 11,181,318 B2. Assuming the extruder nozzle is relatively thin in comparison to its width, the current distribution in the segments provides a pixelated image closely correlated with the average electrical conductivity between opposite electrode pairs on the two faces. Assuming the mixture temperature is relatively well controlled at the entry to the extruder, the resulting conductivity map will show the presence of high and low areas of ionic (salt) concentration and / or foreign objects, as well as the cooling process as the mixture proceeds through the die. Alternatively, assuming the mixture is relatively homogeneous, the conductivity map will show hot and cold spots. 4. Online monitoring of fouling build-up and / or foreign object detection in food processing. This modality of operation depends on the plant conditions and layout, particularly heat exchanger type. In the following example a tube-in-tube heat exchanger is assumed. For the purposes of explanation it is further assumed that countercurrent flow heating is used, such that the heating medium (e.g., hot water or process steam) flows in the outer jacket, while the food medium (e.g., milk) flows in the opposite direction through the inner pipe, which is most likely made from 304 or 316 stainless steel. As in example application 2, the two electrode rings in the sensing pipe have an electric field applied between them, causing electrical current to flow through the medium. The ratio between the average total current and the applied voltage (note that the voltage may be independently measured by the sensing element of the two-part electrode segment, discussed elsewhere in this document) yields the average conductance of the current path, from which the mixture's electrical conductivity can be deduced, according to the sensing pipe geometry. The apparatus consists of two sensing pipes, one on the cold, the other on the hot side of the heat exchanger, so the likely increase of conductivity of the medium can be measured, also inferring the temperature rise in realtime. For this application, the sensing pipes, especially the one on the hot side, can advantageously be mounted close to part of the process pipe in which fouling films are most likely to start building up first (e.g., close to a heat exchanger). The process pipework (which is typically constructed from stainless-steel, although this is sometimes lined with PTFE) is connected to electrical ground, either directly or through electrical traps, resonant at the excitation frequency / ies. Such parallel resonant traps exhibit extremely high impedance to current at the resonant frequency. Figure 16 shows a typical multi-section tube-in-tube or double tube heat exchanger 300, where the medium flows through the centre pipe and the heating fluid (e.g., hot water or process steam) flows through the outer jacket in the opposite direction. In a typical application the medium enters horizontally at bottom left and leaves horizontally at top right, while the heating fluid enters vertically at top right and leaves vertically at bottom left. The inside of the hot medium pipe (top right) is likely to be the first location to experience fouling build-up when the process is run after Clean-In-place (CIP). Figure 19 gives a schematic cross-sectional diagram of the suggested arrangement. Note that each of the pipe sensors (left and right) can also simultaneously detect foreign objects and medium / temperature homogeneity, just as in example 2 above. Although each of the pipe sensors can be similar to the arrangement of Figure 3, the implementation shown in Figure 19 can be achieved using four modified sight-glasses or similar bespoke mechanical arrangement, as shown in Figure 19A and 19B. Electrodes Za, b are formed by the metal flanges of the sight-glass, while the fluid flows through insulating pipe P. The positive terminal of excitation voltage source, V, can be applied to electrode Zb, for instance via pressure contact with metallization X on one side of double sided printed circuit board (PCB) B, with the negative applied via metallization Y on the opposite side of the PCB, via clamping rods Ra to Rn which are electrically insulated from Zb to electrode Za. Figure 19B-D also shows how, assuming all voltage sources (Vi - V4) have the same phase and frequency, the currents I2 and I3 which flow in the heat exchanger metalwork, can be made to cancel by controlling the ratio between V2 and V3. Thus, assuming the heat exchanger is not fouled initially, as fouling starts to build up at the hot end, I2 will start to reduce, meaning that V2 needs to be increased to maintain current cancellation. Thus the change in the voltage ratio is an indicator of fouling initiation and build-up. Figures 19 and 19A and 19B also show an arrangement wherein the excitation current due to each of the voltage sources flows through the medium inside the pipe and returns through a number of conductors surrounding the pipe. Hence, for example, current Ii, due to voltage source Vi, returns as components Iia to Im, where there are n return conductors, Ra to Rn (NB. Only four are shown in Figure 19B, but any reasonable number are feasible). This is similar to the arrangement of Figure 5a, where all of these conductors together comprise EM screen 136. If, for instance, a foreign object Q is present between electrodes Za and Zb, or a fouling film initiates only on part of the surface of electrode Za and / or Zb, the path of current Ii will be diverted accordingly. Depending on the excitation frequency, the physical geometry of the arrangement and the foreign object / s and / or fouling, the return current distribution in Ra to Rn will be modulated according to the relative admittances / impedances of the multiple possible current loops. Figure 19C shows a simplified electrical circuit diagram of the arrangement of Figure 19B. Zcontact represents a contact impedance between the electrodes and the medium, while Rn, Ln and Cn (where set n comprises elements a, b, c & d in this case) represents the impedance / admittance elements associated with current flow through the four possible return loops. For a given location of a foreign object or fouling film with different electrical conductivity and / or permittivity to the medium, the path of current Ii will deviate relative to the uniform (no foreign object or fouling) case. This will cause the lengths of the current loops through the four return paths to become unbalanced. At low excitation frequencies, the admittances of Cn are very low, while those of Ln are very high, such that the elements of return current set Im are largely determined by Rn. However, as the excitation frequency rises the admittances of Ln reduce significantly until their magnitude approaches those of Rn, thus "choking" the flow of Im. Any path with lower Ln will hence have greater current flow. Since the magnitude of each member of Ln is proportional to the area of the loop enclosed by Im, the shortest loop will have the highest admittance and hence highest current flow. Hence, if spherical foreign object Q in Figure 19BA is more conductive than the medium, Rb may be slightly smaller than Ra, for instance, but Lb may be substantially smaller than La, causing Im to have a significantly higher magnitude than ha. (Although the admittances of Cn will increase with frequency, and hence will also influence the total current, Ii, the overall system geometry can be such that changes between Ca and Cb will have less influence on return current redistribution). Hence, measuring the current in each return conductor, at one or more frequency, may be used to determine the presence of foreign objects and / or fouling, either in conjunction with other sensors as previously described or alone. Despite this return current variation, the coaxial nature of the return conductors still constitutes an effective EM screen, 136, minimizing magnetic field at significant distances from the sensor. Figure 19D shows the electrical equivalent circuit of the four voltage source excitation 5 arrangement. Note that initially (clean start), fouling resistances Rfc and Rm will be very close to zero, with Rfh expected to start to increase first with the initiation of a fouling film at the hot end of the heat exchanger. Figure 19D also shows how suitable selection and control of each voltage source (assuming they are all the same frequency and phase) can eliminate excitation currents flowing 10 through the plant pipework or electrical ground. This can be expressed mathematically as: V2 - Vi + V4 - V3 = 0. V4 - Vi = V3 - V2                                                    (1) VH = Vi - V2 = V4 - V3. To ensure that I2 - I3 = 0, V2 / (R2 + Rm) = V3 / (R3 + Rfc)                                         (2) A controller can satisfy (1) & (2) simultaneously. One simple stand-alone implementation of each of the hot and / or cold side sensors can be realized according to Figure 20. Here, the pair of voltage sources Vi and / or V2, or V3 and V4, are replaced by a single source, V, which gives rise to two currents, in this case Ii and I2. Note that the connections from source V to the common positive centre electrode El and to the two negative electrodes E2a, b can be made as in Figure 21A. Any difference in the total conductance, or admittance, between the media in the two conduction paths will result in a difference between Ii and I2. For instance, a foreign object flowing from right to left will modulate first k and then Ii, causing a change in sign of the difference signal I2 - Ii. Fouling build-up at the right electrode will cause I2 to be depressed relative to Ii. One advantage of this scheme is that each sensor can be independent and can be mounted in electrically grounded pipework without disturbing the plant grounding scheme. Electrode El is the only metalwork at a potential other than plant ground. Nevertheless, depending on the frequency / ies of the excitation source, V, some current components may flow through the plant grounding system. These can be minimized by the addition of a high frequency inductor, or choke, C, which can be a toroidal permeable core (such as ferrite, amorphous or nanocrystalline metal) located around one or both end / s of the sensor pipework. An ES screen 138 can also be added to the sensor to contain any electric fields. Note that, in this case, the ES screen 138 is outside the EM screen 136. 5. Online monitoring of average electrical properties (e.q. conductivity, permittivity) in media flowing in a pipe. This relates to applications 1 and 4 above, where the arrangement of electrodes (and additional sensors, if any) is adapted to simply measure electrical properties of a flowing fluid medium. Applications could include food, water, blood / plasma, petrochemicals and so on, possibly both before and again after an intermediate process (e.g., heating, evaporation, emulsification, etc.). Calibration There are a number of ways in which automatic calibration of the surface potential can be carried out on the various embodiments. i. Where voltage point contacts are used to measure surface potential, excitation can be applied to just one electrode / electrode ring, with zero volts applied for zero calibration and full-scale volts applied for gain calibration. ii. Where displacement current is used to measure surface potential: a. If a fixed screen potential is used, excitation can be applied to just one electrode / electrode ring, with zero volts applied for zero calibration and full-scale volts applied for gain calibration. b. If a variable screen potential (either discretized, or continuously variable along the sensing region) is used, excitation can be applied to just one electrode / electrode ring, with zero volts applied for zero calibration and appropriate full-scale volts for each successive set of sensors applied for gain calibration. Here, some sensors will necessarily be overloaded, but as the displacement currents are small, this will merely result in amplifier saturation and will not result in any damage. Additionally, in the case of applications such as 1 above, an ideal cylinder can be substituted for the log and excited from just one electrode to calibrate the displacement current sensors to the symmetrical case. When a real log is substituted for the ideal cylinder, and also excited from just one electrode, the change in measured displacement current can be used to infer the degree to which the real log deviates from a cylinder and can thus be used to ascertain it's true shape and volume. This is illustrated in cross section in Figure 21. There are a number of ways in which automatic calibration of the magnetic field sensors can be carried out on the various embodiments. i. For instance, in typical applications 1 and 2, a straight rod can be run down the axis between the two electrodes and first no current and then a known full-scale current passed through it. The equal and opposite field cancellation current in the magnetic shield, if fitted, can be turned on to fine tune the effective "zero" reading, by removing the signal due to the not-completely cancelled circumferential flux portion, especially when the search coils are very close to, but outside, the magnetic shield. See Figure 7 in which the lower plot includes the effect of the magnetic shield. Application of Artificial Intelligence (AI) and Machine Learning (ML) Figure 22 shows a 2-dimensional view of electric equipotential lines surrounding a metallic sphere in a typical aqueous food mixture excited by a voltage of -10 V along the left side surface and +10 V along the right side surface. The top and bottom sides of the figure are the points at which surface potential can be measured (either by direct contacts, or by displacement current sensors). The metallic object at the left end causes the equipotential lines, spaced 0.5 V apart, to spread out, while the plastic one at the right end causes them to converge, resulting in a distortion of the measured potentials, relative to the case without the object, in which the equipotential lines are parallel and equally spaced. In the case of a moving object in a flow, the changing surface potential distribution will be related to the velocity of the object. The computer / data processor will be connected to plant supervisory control and data acquisition systems (SCADA) and will thus have access to flow rate data for the process medium. Foreign objects would be expected to flow at a rate equal to, or somewhat slower than the main flow, dependent on factors such as size, shape and density. By detecting the moving surface potential distribution information concerning the object's presence, nature and velocity can be inferred. In the case of multiple sensor rings and / or two sensor units (as in application 4 above), AI and ML techniques can be used to train the computer / data processor to recognizethe passage of such objects from the time-sequenced patterns of measured data, which greatly increases immunity to noise. Similar cases exist for AI assisted detection using just the segment current and voltage readings from the electrode rings, where a perceived object's transit time between the electrodes can be measured - if this is inconsistent with medium flow velocity, a false positive is much less likely to be flagged. Figure 11 shows the conductance of an aqueous medium measured by dividing the sensed electrode current by the applied excitation voltage for the test system shown in Figure 3. In this particular test, a brass object was dropped down the centre of the pipe filled with a saline solution of 0.5 Snr1 conductivity. The increase in conductivity is picked up by each of the individual electrode segments (in this case two diametrically opposite electrode segments in a ring of 24, labelled "Electrode 1" and "Electrode 2", have their individual current measurements multiplied by 24 and divided by the excitation voltage) and by the total measured current, divided by the excitation voltage (labelled "Average"). In this case, the increase in conductance is around 1.5 %, so the change is quite evident. Smaller changes, e.g., due to smaller objects, could potentially be lost in noise, but correlation between rise and fall in each segment's current and the time between events due to flow velocity will help to "dig" the signal from the noise. The same is true of signals from search coil measurements, e.g., from the arrangement shown in Figure 12, which results in the measurements shown in Figure 13 as a foreign object moves axially relative to the search coils. For an object moving at a constant velocity, the x-axis becomes time and the three resultant waveforms shown, potentially augmented by many others, may be fed into an algorithm that will allow the passage of very much smaller objects to be reliably sensed. Summary of some salient features of the present invention. To recapitulate, the invention comprises the combination of some or all of the below: 1. One or more voltage excitation signal generators or power supplies. These may beneficially be based on Class D amplifiers, for efficiency and flexibility, and may incorporate one or more filter stage / s, and / or resonant trap / s (e.g., for electrical isolation of segment from medium when not energized), and / or isolation transformer / s. 2. At least one "smart electrode" (usually 2, but one or a plurality are also appropriate in certain applications), which allows measurement of simultaneous current and / or voltage distribution at the contact point / s between excitation and the medium. 3. Means for connecting / disconnecting excitation from one or more "smart electrode" and / or any or all of the electrode segments making up said "smart electrode". 4. Means for measuring the voltage and / or current of each electrode segment, with the addition of each segment possibly consisting of two electrically isolated conductive parts, through one of which the excitation current passes and at the other of which the contact voltage with the medium is measured. One viable arrangement is a coaxial design, as shown in Figure 2. 5. A 3-dimensional array of magnetic field sensors, which may be oriented in any or all of the 3 orthogonal axes. E.g., for a cylindrical pipe sensor, sensitive to circumferential flux, sensitive to longitudinal flux, sensitive to radial flux. 6. An electromagnetic shield, which actively or passively cancels magnetic fields outside the sensor. 7. Return conductors, which effectively form a passive electromagnetic shield, in which the current is measured in any or all of said conductors. 8. A 3-dimensional array of electric field sensors, including, but not limited to, sensors parallel or tangential to the surface of the medium, or the vessel (e.g., pipe, tank) containing the medium. E.g., for a cylindrical pipe sensor, conductive pad / s tangential to the outer wall of the pipe, which intercept / s displacement current from the medium in contact with the adjacent inner surface of the pipe. 9. An electrostatic shield in which the sensors of 7 above may be embedded, with the addition that this shield may all be at some reference ground potential, or it can be composed of discrete rings, each held at a different potential, for instance the same potential as the undisturbed sensed medium at the same position in the pipe (thus following a discretised version of the excitation voltage gradient along the sensor), or it can be made from some partially conductive material, such as "conductive" 3D-printing plastic, following the continuous function of the excitation voltage gradient. 10. One or more array / s of amplifiers and ADCs to amplify and digitize the signals from the electrode segment sensors and the magnetic and / or the magnetic and / or electric field sensor array / s. 11. One or more controller / s (e.g., microcontroller or FPGA based) to control the excitation signal generator / s (1 above), the electrode segment switching, if any (3 above), the operation of the ADCs, any DSP required, and data communication with a network and / or host computer. The following is a description of how these features are exploited in typical application 4 above (assuming phase and frequency of all four voltage sources are locked). The application calls for two complete pipe sensor units, as per typical application 2 above. Each of these is capable of detecting foreign objects and determining homogeneity of medium and / or temperature. Each of these may comprise its own external magnetic field cancellation system, and electric field shielding. The excitation frequency and phase of each sensor is locked, using a synchronization signal / s between the units, and / or between each unit and a host computer / controller. Additionally, the host computer / controller receives excitation amplitude signals from both units and can control the amplitude of either or both excitation signals (e.g., amplitude of Vi and / or V2 in Figures 21 & 22). In addition, each unit either incorporates, or can communicate with an, additional excitation generator / s, which provide fouling detection excitation signals (V2 & V3 in Figures 21 & 22). Either one or both of the units, or the host computer / controller, controls the relative magnitudes of V2 & V3. As shown in Figure 22, there is no need to change the grounding of existing input and output pipework, assuming a retrofit installation. However, it is likely that the heat exchanger metalwork will need to be grounded for electrical safety, so this can be done with a trap resonant at the excitation frequency (as shown in Figure 22), to reduce / eliminate excitation current flowing to ground. If multiple excitation frequencies are used (either simultaneously or otherwise), multiple traps, for each frequency, can be placed in series. There will be a requirement to limit the grounding impedance at low frequencies (e.g., 50 / 60 Hz), so this will have an effect on inductor design and construction. Where, in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth. Although the present disclosure has been described in terms of certain embodiments, other embodiments apparent to those of ordinary skill in the art also are within the scope of this disclosure. Thus, various changes and modifications may be made without departing from the spirit and scope of the disclosure. For instance, various components may be repositioned as desired. Moreover, not all of the features, aspects and advantages are necessarily required to practice the present disclosure. Accordingly, the scope of the present disclosure is intended to be defined only by the claims that follow. Aspects of the invention are set forth in the clauses below. 1. Apparatus for use in determining an electrical admittivity of a subject of interest, comprising: a first electrode and a second electrode configured to apply electric excitation to the subject of interest; one or more electric potential sensors provided about the subject of interest, each sensor configured to simultaneously measure an electric potential at a surface of the subject of interest and / or one or more magnetic field sensors configured to simultaneously measure a magnetic field close to a surface of the subject of interest; a controller configured to simultaneously monitor current or voltage at the electrodes and the sensors to determine an electrical admittivity of the subject of interest. 2. The apparatus of clause 1 comprising a structure defining a sensing region configured to receive the subject of interest, wherein the first and second electrodes are provided on the structure at opposed ends of the sensing region. 3. The apparatus of clause 2 wherein the sensor(s) are provided on the structure between the first and second electrodes. 4. The apparatus of any one of the preceding clauses wherein the magnetic field sensor(s) extend further from the sensing region than the electric potential sensor(s). 5. The apparatus of any one of the preceding clauses wherein determining admittivity comprises detecting an inhomogeneity or a foreign object. 6. The apparatus of any one of the preceding clauses further comprising one or more return conductors provided about the subject. 7. The apparatus of clause 6 wherein current in the return conductors is configured to flow in the opposite direction to current flow between the electrodes. 8. The apparatus of clause 6 or clause 7 wherein a plurality of return conductors are provided. 9. The apparatus of clause 8 wherein the return conductors extend about the subject of interest and are configured to conduct unbalanced or unbalanced return currents which sum to the total current passing through the subject of interest, ignoring any displacement currents. 10. The apparatus of clause 8 or 9 wherein the controller is configured to monitor the current in any or all of the return conductor(s). 11. The apparatus of any of clauses 8 to 10 wherein the magnetic field sensor(s) are provided externally of the return conductor(s). 12. The apparatus of any of the preceding clauses wherein the field sensors sense a field component in a single axis, or in 2 or more axes. 13. The apparatus of any one of clauses 6-10 wherein the one or more conductors comprise a ground plane. 14. The apparatus of any preceding clause wherein each electric potential sensor senses an electric potential by sensing a displacement current. 15. The apparatus of any preceding clause wherein the subject of interest comprises an object, for example a log. 16. The apparatus of any preceding clause wherein the subject of interest is a region in a flow path, for example a region or volume of a pipe carrying a flowable substance. 17. The apparatus of clause 8 wherein the flow path includes solidobjects. 18. The apparatus of any preceding clause wherein the controller is further configured to provide a 2D or 3D map of the electrical conductivity or admittivity of the subject. 19. The apparatus of any preceding clause wherein the first electrode and second electrode are spaced apart along a longitudinal axis. 20. The apparatus of any one of the preceding clauses wherein the first and second electrodes each comprise a substantially continuous conductor. 21. The apparatus of any preceding clause wherein the first electrode and second electrode each comprise a plurality of individual electrode elements, and the electrode elements are optionally energised individually or simultaneously to apply the electric excitation. 22. The apparatus of any one of the preceding clauses Wherein the excitation is applied at different frequencies and / or phases to one or more combinations of electrode elements. 23. The apparatus of clause 20 or 21 wherein the electrode elements are arranged in the form of a circle. 24. The apparatus of clause 20 or 21 wherein the electrode elements each comprise a radial segment. 25. The apparatus of any one of clauses 2-23 wherein the structure comprises a substantial cylinder. 26. The apparatus of any preceding clause wherein the apparatus comprises an electrical admittance tomography system. 27. The apparatus of any preceding clause wherein the electric potential sensors and / orthe magnetic field sensors are provided externally of the subject of interest and optionally arranged in a cylindrical shape externally of the subject ofinterest. 28. The apparatus of any preceding clause wherein the electrodes create longitudinal currents in the subject of interest. 29. The apparatus of any preceding clause wherein the return current is arranged to flow in the opposite direction to the excitation current. 30. A method for determining an electrical conductivity or admittivity of a subject of interest, and / or the presence of a foreign object and / or a fouling film, comprising: energising a first electrode and a second electrode to apply electric excitation to the subject of interest; simultaneously monitoring current or voltage at the electrodes and / or current in one or more return conductors; simultaneously monitoring one or more electric potential sensors provided about the subject of interest and / or one or more magnetic field sensors, to detect an electric potential or magnetic field at a surface of the subject of interest; determining an electrical conductivity oradmittivity of the subject of interest from the monitored current or voltage and / or the surface electric potentials and / or external magnetic fields. 31. The method of clause 30 further comprising monitoring a plurality of magnetic field sensors to sense a field component in a single axis, or in 2 or more axes. 32. The method of clause 30 or 31 further comprising sensing each electric potential using a displacement current. 33. The method of any of clauses 30-32 wherein the subject of interest comprises an object, for example a log. 34. The method of any of clauses 30-33 wherein the subject of interest is region in a flow path, for example a region or volume of a pipe carrying a flowable substance. 35. The method of any of clauses 30-34 wherein the flow path includes solid objects. 36. The method of any of clauses 30-35 further comprising using the electrical conductivity oradmittivity of the subject to detect or identify a nonhomogeneity in the subject. 37. The method of any of clauses 30-36 further comprising providing a 2D or 3D map of the electrical conductivity or admittivity of the subject. 38. The method of any of clauses 30-37 wherein the first electrode and second electrode are spaced apart along a longitudinal axis. 39. A method for detecting a shape of an object comprising: energising a first electrode and a second electrode to apply electric excitation to the object; simultaneously monitoring current or voltage at the electrodes and / or current in one or more return conductors; simultaneously monitoring one or more non-contact electric potential sensors provided exteriorly of the object to detect an electric potential(s) at a surface of the object; processing the detected electric potentials to determine a shape of the object. 40. The method of clause 38 further comprising determining a volume of the object from the determined shape. 41. The method of clause 38 further comprising determining a position of the object from the determined shape. 42. Apparatus for use in determining an electrical admittivity of a subject of interest, comprising: a first electrode and a second electrode spaced axially apart and configured to apply electric excitation to the subject of interest; a plurality of return conductors extending about the subject of interest and configured to conduct unbalanced or unbalanced return currents which sum to the total current passing through the subject of interest, ignoring any displacement currents; a controller configured to simultaneously monitor current or voltage at the electrodes or the current in the return conductors to determine an electrical admittivity of the subject of interest. 43. The apparatus of clause 42 further comprising one or more field sensors to detect an external magnetic field or an electric potential at a surface of the subject. 44. The apparatus of clause 42 or 43 wherein the subject comprises a flowable substance in a flow path. 45. A conduit flow sensing apparatus comprising a conduit segment having a first end and a second end, the ends being configured for connection of the sensing apparatus into a fluid flow path, a first electrode at or near the first end a second electrode at or near the second end, each electrode configured to make electrical contact with a substance which may flow in the conduit segment, and a controller configured to energise the electrodes at a selected frequency and simultaneously monitor current or voltage at the electrodes to determine an electrical admittivity within the conduit segment to thereby detect the presence of one or more of an inhomogeneity, a foreign object and a fouling film. 46. The apparatus of clause 45 further comprising one or more return conductors provided about the subject. 47. The apparatus of clause 46 wherein current in the return conductors is configured to flow in the opposite direction to current flow between the electrodes. 48. The apparatus of clause 46 or clause 47 wherein a plurality of return conductors are provided. 49. The apparatus of clause 48 wherein the return conductors extend about the subject of interest and are configured to conduct unbalanced or unbalanced return currents which sum to the total current passing through the subject of interest, ignoring any displacement currents. 50. The apparatus of clause 48 or 99 wherein the controller is configured to monitor the current in any or all of the return conductor(s). 51. Sensing apparatus comprising a first flow sensor according to any of clauses 45-50 and a second flow sensor according to any of clauses 45-50 fluidly interconnected. 52. The sensing apparatus of clause 51 further comprising a controller configured to control a first power supply to excite the electrodes of the first flow sensor and a second power supply to excite the electrodes of the second flow sensor whereby the excitation voltages or currents of the power supplies sum to zero. 53. A heat exchanger, for instance a double tube heat exchanger, having an apparatus according to clause 51 or 52 fluidly connected at an inlet to the heat exchanger and another apparatus according to clause 51 or 52 fluidly connected to an outlet of the heat exchanger. 54. Apparatus for sensing an electrical admittivity, comprising a structure defining a sensing region in which admittivity is to be sensed; a first electrode and a second electrode spaced axially apart on the structure and configured to apply electric excitation to the sensing region; one or more magnetic field sensors and / or one or more electric field sensors located on the structure externally of the sensing region; and a controller configured to simultaneously monitor current or voltage at the electrodes and the sensor(s). 55. Any novel feature or combination of features disclosed herein.

Claims

1. Apparatus for use in determining an electrical admittivity of a subject of interest, comprising: a first electrode and a second electrode configured to apply electric excitation to thesubject of interest;one or more electric potential sensors provided about the subject of interest, each sensor configured to simultaneously measure an electric potential at a surface of the subject of interest and / or one or more magnetic field sensors configured to simultaneously measure a magnetic field close to a surface of the subject of interest;a controller configured to simultaneously monitor current or voltage at the electrodes and the sensors to determine an electrical admittivity of the subject of interest.

2. The apparatus of claim 1 comprising a structure defining a sensing region configured to receive the subject of interest, wherein the first and second electrodes are provided on the structure at opposed ends of the sensing region.

3. The apparatus of claim 2 wherein the sensor(s) are provided on the structure between the first and second electrodes.

4. The apparatus of any one of the preceding claims wherein the magnetic field sensor(s) extend further from the sensing region than the electric potential sensor(s).

5. The apparatus of any one of the preceding claims wherein determining admittivity comprises detecting an inhomogeneity or a foreign object.

6. The apparatus of any one of the preceding claims further comprising one or more return conductors provided about the subject.

7. The apparatus of claim 6 wherein current in the return conductors is configured to flow in the opposite direction to current flow between the electrodes.

8. The apparatus of claim 6 or claim 7 wherein a plurality of return conductors areprovided.

9. The apparatus of claim 8 wherein the return conductors extend about the subject of interest and are configured to conduct unbalanced or unbalanced return currents which sum to the total current passing through the subject of interest, ignoring any displacement currents.

10. A method for determining an electrical conductivity or admittivity of a subject of interest, and / or the presence of a foreign object and / or a fouling film, comprising:energising a first electrode and a second electrode to apply electric excitation to the subject of interest;simultaneously monitoring current or voltage at the electrodes and / or current in one or more return conductors;simultaneously monitoring one or more electric potential sensors provided about the subject of interest and / or one or more magnetic field sensors, to detect an electric potential or magnetic field at a surface of the subject of interest;determining an electrical conductivity oradmittivity of the subject of interest from the monitored current or voltage and / or the surface electric potentials and / or external magnetic fields.

11. Apparatus for use in determining an electrical admittivity of a subject of interest, comprising:a first electrode and a second electrode spaced axially apart and configured to apply electric excitation to the subject of interest;a plurality of return conductors extending about the subject of interest and configured to conduct unbalanced or unbalanced return currents which sum to the total current passing through the subject of interest, ignoring any displacement currents;a controller configured to simultaneously monitor current or voltage at the electrodes or the current in the return conductors to determine an electrical admittivity of the subject of interest.

12. The apparatus of claim 11 further comprising one or more field sensors to detect an external magnetic field or an electric potential at a surface of the subject.

13. The apparatus of claim 11 or 12 wherein the subject comprises a flowable substance in a flow path.

14. A conduit flow sensing apparatus comprising a conduit segment having a first end and a second end, the ends being configured for connection of the sensing apparatus into a fluid flow path, a first electrode at or near the first end a second electrode at or near the second end, each electrode configured to make electrical contact with a substance which may flow in the conduit segment, and a controller configured to energise the electrodes at a selected frequency and simultaneously monitor current or voltage at the electrodes to determine an electrical admittivity within the conduit segment to thereby detect the presence of one or more of an inhomogeneity, a foreign object and a fouling film.

15. The apparatus of claim 14 wherein current in the return conductors is configured to flow in the opposite direction to current flow between the electrodes.

16. The apparatus of claim 15 wherein the return conductors extend about the subject of interest and are configured to conduct unbalanced or unbalanced return currents which sum to the total current passing through the subject of interest, ignoring any displacement currents.

17. Sensing apparatus comprising a first flow sensor according to any of claims 14-16 and a second flow sensor according to any of claims 14-16 fluidly interconnected.

18. The sensing apparatus of claim 17 further comprising a controller configured to control a first power supply to excite the electrodes of the first flow sensor and a second power supply to excite the electrodes of the second flow sensor whereby the excitation voltages or currents of the power supplies sum to zero.

19. A heat exchanger, for instance a double tube heat exchanger, having an apparatusaccording to claim 17 or 182 fluidly connected at an inlet to the heat exchanger and another apparatus according to claim 17 or 18 fluidly connected to an outlet of the heat exchanger.

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