Sensor Apparatus

Abstract

A sensor apparatus for use in obtaining information relating to the presence of metal particles within a sample of a substance to be tested includes a substrate comprising an aperture for receiving a sample of a substance to be tested. The sample may be an extracted or dynamic sample. The substrate has an electrically conductive coil printed thereon, which surrounds the aperture. The coil is arranged to generate a magnetic field for application to a sample received in the aperture in use, and may also sense the result of interaction between the generated magnetic field and the sample. The sensed result of the interaction is usable to determine information relating to the presence of metal particles in the sample.

Claims

1. A sensor apparatus for use in obtaining information relating to the presence of metal particles within a sample of a substance to be tested, the sensor apparatus comprising: a substrate comprising an aperture for receiving a sample of a substance to be tested, the substrate having one or more electrically conductive coils printed thereon; magnetic field generating means for generating a magnetic field for application to a sample received in the aperture in use; and sensing means for sensing the result of interaction between the generated magnetic field and a sample received in the aperture in use, the sensed result of the interaction being usable to determine information relating to the presence of metal particle(s) in the sample; wherein the substrate includes an electrically conductive coil printed thereon that circumferentially surrounds said aperture for receiving the sample, the electrically conductive coil forming part of said sensing means.

2. The sensor apparatus of claim 1 wherein the substrate comprises multiple substrate layers, and wherein at least the coil surrounding the aperture and forming part of the sensing means comprises coil portions printed on more than one layer of the substrate, each coil portion being printed at a different respective level of the substrate.

3. The sensor apparatus as claimed in claim 1, wherein the printed conductive coil circumferentially surrounding said aperture that forms part of the sensing means also forms part of said magnetic field generating means.

4. The sensor apparatus as claimed in claim 1, wherein said sensing means is arranged to sense a change in impedance of the conductive coil resulting from said interaction between the sample fluid and the generated magnetic field.

5. The sensor apparatus of claim 1 further comprising processing means for using the sensed result of said interaction in obtaining information relating to the presence of metal particles within the sample, wherein the processing means comprises means for correlating the result sensed by the sensing means with one or more of: (i) the presence, (ii) the quantity and (iii) one or more properties of a metal particle or particles within the sample.

6. A system comprising the sensor apparatus of claim 1 and processing means remote from the sensor apparatus for using the sensed result of said interaction in obtaining information relating to the presence of metal particles within the sample.

7. The sensor apparatus of claim 1 wherein the result of interaction between the sample and the generated magnetic field sensed by the sensor apparatus may be used to count individual metal particles in a dynamic sample provided by a volume of fluid flowing through the aperture between first and second times.

8. The sensor apparatus of claim 1 further comprising a flow sensor to provide data indicative of the flow rate of a fluid through the aperture and processing means for using the sensed result of said interaction in obtaining information relating to the presence of metal particles within the sample, wherein the processing means is arranged to determine a count of individual metal particles in the sample fluid as a function of volume.

9. The sensor apparatus of claim 8 wherein the volume of fluid flowing through the aperture represents only a portion of a fluid flow through the system in the region of the sensor apparatus.

10. The sensor apparatus as claimed in claim 1, further comprising a reference component for calibrating or balancing said sensing means, wherein said reference component comprises a reference coil printed on the substrate.

11. A sensor apparatus as claimed in claim 1, further comprising electrostatic shielding disposed between the conductive coil of said sensing means and the aperture, wherein said electrostatic shielding comprises an incomplete ring printed on said substrate.

12. A sensor apparatus for use in determining a count of individual metal particles with respect to volume for a dynamic sample provided by a volume of fluid flowing through a sensing region of the apparatus between first and second times, the apparatus comprising: magnetic field generating means for generating a magnetic field for application to a sample in the sensing region; sensing means comprising one or more electrically conductive coils for sensing the result of interaction between the generated magnetic field and a dynamic sample provided by a volume of fluid flowing through the sensing region between first and second times; and a flow sensor for determining data indicative of a rate of fluid flow through the sensing region; the sensed result of the interaction and the data indicative of the rate of fluid flow through the sensing region determined by the flow sensor being usable in determining a count of individual metal particles with respect to volume for the dynamic sample.

13. The apparatus of claim 12 wherein the or each electrically conductive coil of the sensing means is a printed coil.

14. The apparatus of claim 13 wherein the sensor apparatus comprises a substrate comprising an aperture for receiving a sample of fluid to be tested, the substrate having one or more electrically conductive coils printed thereon, and the magnetic field generating means is arranged for generating a magnetic field in use for application to a sample received in the aperture; and the sensing means is arranged for sensing the result of interaction between the generated magnetic field and a fluid sample received in the aperture in use, the substrate including an electrically conductive coil printed thereon that circumferentially surrounds the aperture for receiving the sample, the electrically conductive coil forming part of said sensing means.

15. The sensor apparatus of claim 12 wherein the sensor means forms part of a sensor unit, the sensor unit having a first portion intended to be immersed in a fluid to be tested in use, the first portion of the sensor apparatus being configured such that when the first portion is immersed in fluid, fluid may pass through the sensing region of the sensor apparatus to provide a dynamic sample and to contact the flow sensor.

16. The sensor apparatus of claim 15 wherein the first portion of the sensor unit comprises a passage therethrough through which fluid may flow in use to pass through the sensing region of the sensor apparatus to provide the dynamic sample and to contact the flow sensor.

17. The apparatus of claim 16 wherein the flow sensor is disposed in the passage for contacting fluid in use.

18. The sensor apparatus of claim 12 wherein the sensor unit has a second portion that is intended to be disposed out of contact with the fluid to be tested in use.

19. The sensor apparatus of claim 12 wherein the sensor apparatus comprises means for generating data indicative of the sensed result of the interaction between the sample and the generated magnetic field and means for generating data indicative of the flow rate sensed by the flow sensor, wherein the sensor apparatus is arranged to transmit the generated data to remote processing means for determining the count of individual metal particles with respect to volume, or wherein the sensor apparatus comprises processing means for determining the count of individual metal particles with respect to volume using the generated data.

20. The sensor apparatus of claim 12 mounted to a part of a system comprising a fluid to be tested, wherein the sensor apparatus is configured to sample only a portion of the fluid flow through the part of the system to which the apparatus is mounted.

21. A sensor apparatus for use in obtaining information relating to the presence of metal particles within a sample of a substance to be tested, the sensor apparatus comprising: a substrate comprising an aperture for receiving a sample of a substance to be tested, the substrate having an electrically conductive printed coil, circumferentially surrounding the aperture; a magnetic field generator configured to generate a magnetic field for application to a sample received in the aperture in use; and a sensor configured to sense the result of interaction between the generated magnetic field and a sample received in the aperture in use, the sensed result of the interaction being usable to determine information relating to the presence of metal particle(s) in the sample; wherein the sensor includes the coil.

22. The sensor apparatus of claim 21 wherein the substrate comprises multiple substrate layers, and wherein the coil surrounding the aperture and forming part of the sensor comprises printed coil portions on more than one layer of the substrate, each coil portion being at a different respective level of the substrate.

23. The sensor apparatus as claimed in claim 21, wherein the printed conductive coil circumferentially surrounding said aperture that forms part of the sensor also is part of said magnetic field generator.

24. The sensor apparatus as claimed in claim 21, wherein said sensor t is configured to sense a change in impedance of the conductive coil resulting from the interaction between the sample fluid and the generated magnetic field.

25. The sensor apparatus of claim 21 further comprising a processor connected to the sensor for using the sensed result of said interaction in obtaining information relating to the presence of metal particles within the sample, wherein the processor is configured to correlate the result sensed by the sensor with one or more of: (i) the presence, (ii) the quantity and (iii) one or more properties, of a metal particle or particles within the sample.

26. A system comprising the sensor apparatus of claim 21 and a processor connected remotely to the sensor for using the sensed result of said interaction in obtaining information relating to the presence of metal particles within the sample.

27. The sensor apparatus of claim 21 wherein the result of interaction between the sample and the generated magnetic field sensed by the sensor may be used to count individual metal particles in a dynamic sample provided by a volume of fluid flowing through the aperture between first and second times.

28. The sensor apparatus of claim 21 further comprising a flow sensor to provide data indicative of the flow rate of a fluid through the aperture and a processor connected to the sensor for using the sensed result of said interaction in obtaining information relating to the presence of metal particles within the sample, wherein the processor is configured to determine a count of individual metal particles in the sample fluid as a function of volume.

29. The sensor apparatus of claim 28 wherein the volume of fluid flowing through the aperture represents only a portion of a fluid flow through the system in the region of the sensor apparatus.

30. The sensor apparatus as claimed in claim 21, further comprising a reference component for calibrating or balancing said sensor unit, wherein said reference component comprises a reference coil printed on the substrate.

31. A sensor apparatus as claimed in claim 31, further comprising electrostatic shielding disposed between the conductive coil of said sensor and the aperture, wherein said electrostatic shielding comprises an incomplete ring printed on said substrate.

32. A sensor apparatus for use in determining a count of individual metal particles with respect to volume for a dynamic sample provided by a volume of fluid flowing through a sensing region of the apparatus between first and second times, the apparatus comprising: a magnetic field generator for generating a magnetic field for application to a sample in the sensing region; a sensor comprising one or more electrically conductive coils for sensing the result of interaction between the generated magnetic field and a dynamic sample provided by a volume of fluid flowing through the sensing region between first and second times; and a flow sensor for determining data indicative of a rate of fluid flow through the sensing region; the sensed result of the interaction and the data indicative of the rate of fluid flow through the sensing region determined by the flow sensor being usable in determining a count of individual metal particles with respect to volume for the dynamic sample.

33. The apparatus of claim 32 wherein the electrically conductive coil of the sensor is a printed coil.

34. The apparatus of claim 33 wherein the sensor apparatus comprises a substrate comprising an aperture for receiving a sample of fluid to be tested, the substrate having one or more electrically conductive printed coils, and the magnetic field generator is configured for generating a magnetic field in use for application to a sample received in the aperture; and the sensor is configured for sensing the result of interaction between the generated magnetic field and a fluid sample received in the aperture in use, the substrate including an electrically conductive printed coil that circumferentially surrounds the aperture for receiving the sample, the electrically conductive coil forming part of said sensor.

35. The sensor apparatus of claim 32 wherein the sensor has a first portion intended to be immersed in a fluid to be tested in use, the first portion of the sensor being configured such that when the first portion is immersed in fluid, fluid may pass through a sensing region of the sensor apparatus to provide a dynamic sample and to contact the flow sensor.

36. The sensor apparatus of claim 35 wherein the first portion of the sensor comprises a passage therethrough through which fluid may flow in use to pass through the sensing region of the sensor apparatus to provide the dynamic sample and to contact the flow sensor.

37. The apparatus of claim 36 wherein the flow sensor is disposed in the passage for contacting fluid in use.

38. The sensor apparatus of claim 12 wherein the sensor has a second portion configured to be disposed out of contact with the fluid to be tested in use.

39. The sensor apparatus of claim 32 wherein the sensor apparatus comprises means for generating data indicative of the sensed result of the interaction between the sample and the generated magnetic field and means for generating data indicative of the flow rate sensed by the flow sensor, wherein the sensor apparatus is arranged to transmit the generated data to remote processing means for determining the count of individual metal particles with respect to volume, or wherein the sensor apparatus comprises processing means for determining the count of individual metal particles with respect to volume using the generated data.

40. The sensor apparatus of claim 32 mounted to a part of a system comprising a fluid to be tested, wherein the sensor apparatus is configured to sample only a portion of the fluid flow through the part of the system to which the apparatus is mounted.

Description

DESCRIPTION OF DRAWINGS

[0270] Various embodiments of the present invention will now be described by way of example only, and by reference to the accompanying drawings in which:

[0271] FIG. 1 schematically shows the coil arrangement in a conventional sensor apparatus;

[0272] FIG. 2 schematically shows the coil arrangement in an embodiment of the present invention;

[0273] FIG. 3 shows the coil arrangement similar to that of FIG. 2 from an alternate angle;

[0274] FIG. 4 shows a system including a coil arrangement like that shown in FIGS. 2 and 3;

[0275] FIG. 5 shows one example of a product including a sensor apparatus according to the present invention;

[0276] FIG. 6 shows another example of a product including a sensor apparatus according to the present invention;

[0277] FIG. 7A is a perspective view of a sensor unit in accordance with another embodiment of the present invention;

[0278] FIG. 7B is a cross sectional view of the sensor shown in FIG. 7A;

[0279] FIG. 8 shows the sensor unit of FIGS. 7A and B mounted within a section of pipe;

[0280] FIG. 9 shows an alternative mounting of the sensor unit of FIGS. 7A and B; and

[0281] FIG. 10 shows a pipe section including the sensor unit of FIGS. 7A and B and a stator mixer.

DETAILED DESCRIPTION

[0282] FIG. 1 illustrates a sensor apparatus like that described in U.S. Pat. No. 6,051,970. A solenoidal coil 1 is driven with current to generate a magnetic field which interacts with metal particles suspended in a fluid 3 contained within a sample test tube 4. The coil 1 is surrounded by ferrite shields 5 which act to confine the lateral extent of the generated field. For clarity, further details of the housing or shielding have been omitted. A ferrite shield is formed from ferrite, which is a ceramic material composed of iron oxides.

[0283] It can be seen that the coil 1 has a relatively long axial extent, and that each turn of the coil 1 varies continuously in each of the x, y and z directions. To manufacture such a coil, wire must be mechanically wound onto a bobbin 2. The mechanical winding process is fundamentally time consuming, and the operation and parts associated with the machining are relatively expensive. The winding process may also suffer from a lack of reproducibility from coil to coil, particularly where operating conditions change. This may in particular make it difficult to manufacture a suitable reference coil, and where a wound reference coil is used it may be necessary to include additional electrical components to ensure that the coil 1 can be appropriately balanced. Of course, any deviations or mechanical instabilities of the coil could lead to a reduction in sensitivity of the apparatus if not accounted for.

[0284] FIG. 2 illustrates the coil arrangement for a sensor according to embodiments of the present invention, with a sample provided in a test tube located in the aperture thereof. Other features may be provided on the substrate which are not shown in FIG. 2. FIG. 2 shows the portion of the substrate in the vicinity of the sensing coil and the aperture which it surrounds. In the present invention, instead of mechanically winding a solenoidal coil of wire, as shown in FIG. 2 the coil 21 is printed directly onto a flat substrate 22. The coil 21 provides both the sensing means and magnetic field generating means. The coil 21 is printed in a circular spiral pattern surrounding a circular aperture 23 formed in the substrate 22. The substrate 22 may be a standard printed circuit board formed of glass-reinforced plastic. For instance, the substrate 22 may comprise glass fibres embedded in a plastic matrix, e.g. an epoxy resin matrix, a polyimide matrix or a PTFE matrix. However, it will be understood that other materials may suitably be used as a substrate, for instance, plastics or ceramics. The substrate 22 should ideally have a thermal coefficient of expansion suitable to provide stability under intended operating conditions. The substrate 22 should be rigid enough to support the coil and to avoid mechanical or thermal deformations in use. The printed coil 21 may be etched out of a layer of e.g. copper, or may be printed directly onto the substrate 22 using conductive ink. Other suitable printing techniques may also be used to pattern the coil 21 onto the substrate 22.

[0285] The aperture 23 is sized to receive a sample test tube 24 containing a suspension of metal particles 25. The test tube is shown in FIG. 2. It will be appreciated however that a sample may also be introduced by any other suitable means. In particular, a sample may be introduced as a continuous flow through a pipe in which case the aperture 23 may be sized so that it may be mounted about the pipe, or otherwise to sample a desired portion of the flow through the pipe. The example described will be in relation to ferrous metal particles, which are most commonly found in a sample obtained from a system for the purposes of assessing wear.

[0286] As with FIG. 1, an electrical current is supplied to the coil 21 to generate a magnetic field. The generated field is strongest (and primarily directed along the z axis) at the centre of the coil 21, i.e. within the aperture 23. The field strength decreases with distance away axially from the coil 21. As the coil 21 is essentially planar, and relatively short compared to the solenoidal coil shown in FIG. 1, the sensing region of the FIG. 2 sensor apparatus within which sensing coil 21 is adequately sensitive to sense the result of interactions between ferrous particles and the generated field, is relatively small so that the suspension of metal particles 25 extends throughout and beyond the sensing region. A sensed volume is defined. This is the volume of the sample within the sensing region, and whose interaction with the magnetic field may be detected. The magnitude of the generated magnetic field, and hence the sensitivity of the sensor apparatus, will depend on the number of turns of the coil. In the present invention the number of turns can be increased by printing the coil 21 in a spiral pattern.

[0287] The number of turns can be further increased by using a multi-layered substrate with coil portions being printed onto more than one, or each layer, with the portions of the coil on adjacent layers being connected through a via. Coil portions may also be printed onto different sides of one layer. The ‘coil’ therefore comprises multiple coil portions on different levels, i.e. layers or sides, of the substrate. Naturally, in this case the coil turns on different levels should be such that the current flows in the same sense on each layer, such that the magnetic fields generated by each turn reinforce each other. The layers of the substrate are joined to one another e.g. laminated to provide a unitary substrate. Each coil portion may have any number of turns. The coil portion on the surface shown in FIG. 2 is a spiral coil. Preferably, each of the coil portions is a spiral coil.

[0288] In these ways, the number of coil turns can be increased if desired to a comparable number of an extended solenoid coil whilst still being confined to the substrate. When a multi-layer substrate it used, the number of layers is not particularly limited, but standard 8-layer printed circuit boards may suitably be used. Printed circuit boards are available with 16, 24, etc. layers. It will be understood that a standard n-layer printed circuit board includes n conductive layers, but also includes a number of non-conductive, e.g. spacer or core layers (consistent with how the term “layers” is normally used in relation to printed circuit boards). Any known multi-layer board architecture may suitably be used to form the substrate. It is envisaged that multiple standard printed circuit boards may also be laminated together to provide a thicker printed circuit board and hence substrate. Of course, it is not essential that the substrate is multi-layered, and a single layer substrate may be used. By way of example, an 8 turn spiral may be printed onto each layer of an 8-layer board to form a 64 turn coil. This may provide a good balance between cost and sensitivity. In one exemplary embodiment the printed tracks defining the turns of each coil portion are about 0.2 mm wide, and the spacing between the turns is also about 0.2 mm. For an 8 turn spiral coil this gives a total coil annular width of about 3 mm.

[0289] Where a sample is introduced through the aperture 23 in sample test tube 24 as shown in FIG. 2, it has been found that aperture diameters in the range between about 1 and 2 cm, for instance between 1.25 cm and 1.5 cm may be suitable. However, the diameter of the aperture may vary widely, particularly if the sample is introduced through a flow pipe.

[0290] The sensor apparatus of the present invention may also include shielding to reduce interference from external magnetic fields or radiation, or from metal particles external to the sample, but for reasons of clarity this is not shown in FIG. 2. The shielding may take the form of a conductive shield, grid or box surrounding the coil and sample. Because the coil is relatively small, e.g. compared to the solenoidal coil shown in FIG. 1, the shielding can be placed nearer to the coil without causing currents or fields in the shield to interfere with the measurement. Typically, the shielding may be disposed about 2 or 3 coil diameters away from the sample. The grid of box will typically be made from a non-ferrous metal, e.g. aluminium. This serves to shield the sensor from alternating fields. The shielding has no substantial effect on static fields, but these also do not effect the measurement. The measurement is also not significantly affected by alternating fields at frequencies away from the frequency at which the coils are driven as these can easily be tuned out at the signal processing stage.

[0291] It is noted that unlike U.S. Pat. No. 6,051,970 the sensor illustrated in FIG. 2 does not include any form of ferrite shielding. It is in fact preferred in the present invention to not use ferrite shielding because it has been found, unexpectedly, that this may actually reduce the performance of the sensor apparatus. The advantage of ferrite shielding is that since it is not very conductive it can generally be placed closer to the sample than typical non-ferrite shielding. However, ferrite shielding is not itself very effective at shielding external fields. Furthermore, because the ferrite shielding has non-linear magnetic behaviour, static external fields, or off-frequency alternating fields, which would normally have no effect on the measurement, may interact with the ferrite material to produce spurious signals at or near to the measurement frequency. Additionally, because the ferrite shield has a high magnetic permeability, any movement of the ferrite shield e.g. due to mechanical disturbances or temperature fluctuations may in turn generate an alternating field. Thus, the ferrite shield may itself provide new mechanisms for introducing noise or interference to the measurement. Removing the ferrite shielding also allows a further reduction in bulk, and cost, of the sensor apparatus. It is also noted that standard sizes and shapes of ferrite shielding are not readily compatible with the substrate-based coil arrangements described herein and incorporation of such shielding would require modification of the standard shielding adding to processing complexity.

[0292] An electrostatic shield may also be disposed between the coil and the sample to reduce electric field coupling between the coil 21 and the sample, and hence to reduce sensitivity to variations in the dielectric constant of the fluid being tested. The electrostatic shield is connected to ground and arranged to avoid or reduce any current flow that would otherwise act to cancel the applied magnetic field. For example, the shield may be arranged so as to not form a complete loop. Alternatively, the shield may form a complete loop provided it is made of a high resistance material. The shield may be provided as a separate sheet of material to the coil/substrate or may be formed from an extra copper track on each PCB layer. For instance, the shield may take the form on an incomplete ring on each PCB layer within the spiral coil that is connected to ground. This may be the most practical arrangement and can be readily integrated with the manufacturing process for the coil. Such a shield may not be required when the test fluid shows little variation in dielectric properties, e.g. oils, but is more useful where there could be a mixture of different fluids e.g. oil containing droplets of water (water has a very high dielectric constant of around 80, whereas the dielectric constant for a typical oil is only around 2.4). The electrostatic shield may be most useful when the sensor apparatus is used to count individual particles in a continuous flow of sample fluid, where the sample fluid may be more likely to contain a (potentially varying) mixture of fluids.

[0293] FIG. 3 shows a coil arrangement similar to that described in relation to FIG. 2 from a top-down perspective. In this embodiment, the substrate 22 is printed with two spaced-apart coils: a sensing coil 21 as described above, and a reference coil 28 that is used to calibrate the measurements. Optionally, the reference coil 28 circumferentially surrounds a second aperture 25. The reference coil 28 may be arranged to monitor a reference sample, or may simply be used as an electrical dummy. Preferred measurement techniques are similar to those set out in U.S. Pat. No. 6,051,970 and described in relation to FIG. 4 below, where the coils 21 and 28 together form one half of a bridge circuit, and are both driven by a single A.C. current supply. The bridge may be driven by a digitally synthesised sinusoid. The other half of the bridge circuit is formed by two resistors, and the bridge may further comprise a digital potentiometer for adjusting the balance. In this arrangement, the output of the bridge circuit represents the difference in impedance between the sensing coil 21 and the reference coil 28. The use of printing facilitates the manufacture of a reference coil 28 that is identical to the sensing coil 21, which in turn facilitates balancing of the bridge such that the output is zero when no sample is present. Any change in conditions affecting both coils equally will not lead to an output. It will be appreciated that multiple reference components or coils may be used and that the invention is not limited to the particular type of bridge circuit described herein. For instance, the substrate may include one sensing coil 21 and three reference coils. It will also be appreciated that the reference coil 28 need not be used, for instance an electrical component having the same electrical characteristics as the sensing coil 21 could be used instead. Furthermore, the sensor apparatus need not have any reference components.

[0294] FIG. 4 illustrates some of the components of an exemplary system employing the sensor apparatuses described herein. A common source 41 provides an alternating signal to the sensing coil 21 and reference coil 28, which together form a half-bridge circuit as described above. This circuit senses the result of an interaction between the sample fluid and the sensing coil 21. The signal detected by this circuit may be passed through a number of signal processing stages. For example, as shown in FIG. 4, the detected signal is first passed through a differential amplifier 44. The amplified signal and a phase reference signal 45 are then passed to a phase sensitive detector 46 to help tune out any off-frequency noise.

[0295] The processed signal output from the phase sensitive detector 46 is indicative of the signal detected by the sensing circuit comprising the sensing coil 21 and the reference coil 28. The processed signal may then optionally be provided to any of a processing unit 47, a display 48, an internal storage unit (not shown), or to an external computer 49. For instance, the processed signal may first be passed through an internal processing unit 47 before being provided to a display 48 within the housing. Alternatively, or additionally, the processed signal may be passed to an external computer 49 for processing and/or display in which case either or both of the internal processor 47 and the display 48 may be omitted. In this case the unit may also comprise transmitting means for providing the signal to the external computer 49. The signal may be provided through a wired or wireless connection. The processed signal may also be provided to an internal storage means, for instance, so that it may be accessed or downloaded after a measurement is complete. For example, data indicative of the signal detected by the sensing coil 21, or data indicative of the processed signal output from the phase sensitive detector 46, may be passed to a removable memory for subsequent transfer to an external computer. Alternatively, the data may be stored on an internal memory that is accessible via a USB port, or other suitable connection (including wireless). Any or all of these components (other than the external computer 49) may be contained within the same housing 50 as the sensor apparatus, as shown in FIG. 4, or may be provided externally. Any external components may be situated remote from the housing, and may be connected wirelessly or through a network, or may be situated local to the housing and e.g. connected by a cable.

[0296] It will be appreciated that the arrangement of components illustrated in FIG. 4 is not intended to be limiting and any suitable circuitry may be used for measuring a change in the sensing coil 21 responsive to a sample being introduced or for providing an output to the user. It will also be appreciated that although the details of some components are not shown (for brevity), the circuitry may also include any suitable filtering, amplifying or demodulating circuitry, for example, any of the circuitry described in U.S. Pat. No. 6,051,970 (e.g. in FIG. 1 thereof).

[0297] When a sample containing metal particles is introduced through the aperture 23 into the sampling volume of the sensor, i.e. into the magnetic field generated by the sensing coil 21, the metal particles will magnetically couple to the coil 21. This interaction will result in a measureable change in the coil impedance. There are a number of factors that may cause the coil impedance to change and these are known in the art.

[0298] A first contribution results from the alternating magnetic field inducing eddy currents in the conductive metal particles, which in turn produces a secondary magnetic field which may be detected by the sensing coil 21. Since all metals are conductive, these effects will always occur although for some materials they may be relatively weak and contribute little to the measurement. The contribution resulting from this effect will generally increase with particle size.

[0299] The imaginary, or reactive, part of the coil impedance will also be influenced by changes in inductance of the coil due to the magnetic properties of the sample. For instance, ferromagnetic e.g. ferrous materials will act to reinforce the generated field and increase the magnetic induction experienced by the sensing coil 21. This will cause an increase in the inductance of the sensing coil 21, and hence its impedance, relative to the reference coil 28.

[0300] For non-ferromagnetic magnetic materials the magnetic coupling may be weaker and a combination of different effects may contribute to the measured impedance.

[0301] For the bridge circuit described in relation to FIG. 4 above, a change in inductance of the sensing coil 21 relative to the reference coil 28 produces an alternating signal at the junction of the coils, with the amplitude and phase of this signal being measured relative to the reference signal 45 at the phase sensitive detector 46. The contribution resulting from the increase in magnetic induction due to the presence of ferromagnetic e.g. ferrous particles will generally be at the same frequency and in phase with the reference signal (with the coils connected as shown in FIG. 4). Changes in impedance resulting from eddy current induction will generally result in a signal at the same frequency as but out of phase with the reference signal 45, for instance, for small particles and/or low measurement frequencies this signal may be 90 degrees out of phase with the reference signal 45, and for progressively larger particles, or higher measurement frequencies, the phase will change until it is eventually 180 degrees out of phase with the reference signal 45, i.e. acting to oppose the ferromagnetic effect and reduce the inductance of the sensing coil 21.

[0302] In general the total signal generated by the sensor apparatus for subsequent storage, processing or transmission results from the sum of both of the effects described above. The amplitude and phase of this signal will depend on the sensed interaction between the magnetic field and the metal particles, i.e. on the size, the conductivity and the magnetic permeability of the particles within the sample. The generated signal may thus be analysed by decomposing the signal into two sinusoidal signals each at the measurement frequency, one signal being in phase with the reference signal 45 and the other being 90 degrees out of phase. The in-phase component includes the ferromagnetic contribution, and for sufficiently small particles the in-phase component essentially only includes the ferromagnetic contribution. In this case, the change in inductance, and hence the amplitude of the signal, increases linearly with the amount of ferromagnetic e.g. ferrous material. The out-of-phase component includes the contribution resulting from the induced eddy currents. The relative amplitudes of the two components will vary depending on the nature of the sample, i.e. on which effects are contributing to the sensed change in impedance.

[0303] Two main applications that make use of these techniques will now be described.

[0304] The first application involves measuring the total content of ferrous particles per unit volume of sample of fluid, typically provided in ppm (parts per million) or mg/ml. For this application, the sample fluid is preferably introduced in a sample test tube. As discussed above, the introduction of ferrous particles will cause an increase in coil impedance. It can generally be assumed that the magnetic coupling between the ferromagnetic moments and the applied field will dominate the measurement so that other, relatively weak, contributions can be ignored. Provided that the sample extends throughout the entire sensing region (and preferably extends axially beyond the sensing region) the sensed interaction will result from the same amount of sample i.e. a sensed volume. It is thus possible to calibrate the measurement using one or more standard reference samples containing a known quantity of iron powder, e.g. embedded within epoxy resin. A calibration curve can thus be determined. The calibration data may be stored by the sensor apparatus or where the obtained information is transmitted to an external processor may be stored at the external processor. It has been found that for typical ferrous particles the change in inductance essentially scales linearly with the content of ferrous particles within the sensing region. To achieve best accuracy it is desirable for the sample to be uniformly distributed within the sensing region. For a relatively short sensing coil 21 it is relatively easy to provide a longer sample such that the ends of the sample contribute very little to the measurement. Shaking the sample prior to the measurement may also help achieve a more uniform distribution.

[0305] A long solenoidal coil has an apparent advantage over a short coil for this first application, where a sample containing many distributed particles is to be measured, because the fractional change in the signal can be larger, i.e. the sample can be made to extend the length of the coil and thus more coil turns can be provided adjacent to the sample. However, it has been found that in reality the major factor that limits sensitivity is the mechanical stability of the coil which depends on the materials and construction. As discussed above, a planar printed coil is better from a constructional viewpoint than a wound solenoid. The materials used to form the substrate may be chosen to give a fairly similar performance to typical materials used to form the bobbin. The sensitivity of the planar coils described herein may therefore be at least comparable to that of a conventional solenoidal coil.

[0306] While this example has been given in relation to determining a total content of ferrous particles per unit volume, as these are most commonly found in samples, the system will determine a total content of any ferromagnetic particles sensed. Thus a total content of ferromagnetic particles, including or consisting of ferrous particles per unit volume will be determined. Suitable calibration should be used based upon the metal material(s) expected to be present in the sample.

[0307] A second application is the counting and optionally categorisation of individual particles passing through the aperture 23, e.g. in a continuous fluid flow. Further details of this application are given below

[0308] The frequency of the alternating current supplied by the power supply 41 to drive the sensing coil 21, and thus to generate the magnetic field may affect the sensitivity of the measurement. A suitable frequency may be chosen based on the type or size of particles to be detected and/or the type of measurement to be performed. For instance, where the sensor apparatus is used to measure the total content, or to quantify the size, of ferrous or ferromagnetic metal wear particles in a sample, a typical driving frequency might be less than 1 MHz, for example about 100 kHz. This is because the linear relationship between the change in inductance of the sensing coil 21 and the amount of ferromagnetic e.g. ferrous material in the sample being measured may start to break down at high frequencies, e.g. above 5 MHz. On the other hand, it may be preferable to use higher frequencies, e.g. in the range 1 MHz to 100 MHz, where the sensor apparatus is used to simply count individual metal particles in a flow of sample fluid, as the sensitivity (i.e. ability to detect smaller particles) generally improves at higher frequencies. Furthermore, because the coil impedance depends on the product of frequency and inductance, and so at higher frequencies fewer coil turns are needed to achieve a convenient value of impedance for measuring. Of course, the present invention is not limited to these frequencies, and other suitable frequencies may be used.

[0309] It will be understood that other techniques may be used to detect metal particles not employing a bridge circuit. Although most of the discussion herein is in the context of sensing a change in impedance of the coil, other suitable measuring techniques will also be apparent to the skilled person that may also be used. For instance, it is also contemplated that the sensing means may measure a change in coil inductance more directly, for instance, by monitoring the resonant frequency of an LC circuit comprising the conductive coil and a known capacitor. Other ways of measuring the interactions between metal particles and an applied magnetic field will also be apparent to the skilled person.

[0310] FIG. 5 shows an example of a sensor unit incorporating a sensor apparatus of the type described herein. A housing 50 is provided with a display 48 and an orifice 51 for receiving a sample tube. The orifice 51 is coaxial with the aperture in the substrate and hence the sensing coil. The FIG. 5 product is particularly suited to the first application described above where the aim is to determine the total content of ferrous material per unit volume in a sample. The unit shown in FIG. 5 is portable and self-contained, so that all of the components, including the processor, are provided within the housing 50 alongside the sensor apparatus.

[0311] A second application of the type of sensor apparatus described above is the counting and categorisation of individual particles passing through the aperture 23, e.g. in a continuous fluid flow. For instance, the sensor apparatus may be mounted onto a flow pipe to monitor machine wear debris in situ without having to extract any fluid. As above, whenever metal particles pass into the sensing region of the coil, their presence will result in a change in impedance of the sensing coil 21 and the coil impedance can thus be monitored to assess the rate of metal wear debris generation. Furthermore, by making certain assumptions it is possible to correlate a change in impedance with size and/or type of metal particle. For instance, the same principles used in the first application can be employed to determine the size of ferrous particles passing through the aperture (assuming that the measured signal corresponds to a single metal particle). Additionally, by analysing the shape of the measured impedance change, e.g. by analysing the in-phase and out-of-phase contributions to the measured signal, it is possible to determine information about the magnetic properties of the particles. Particularly, it is possible to determine whether the particle is ferromagnetic or non-ferromagnetic. In order to accurately correlate the change in impedance with the size and/or type of metal particle flowing through the aperture 23 it may be necessary to assume that the measured signal is associated with a single particle. The validity of this assumption may be improved by controlling the flow rate. In the present invention, because the coil 21 is essentially planar and the sampling volume is relatively short, this approximation is more likely to be valid. With conventional coil arrangements like that shown in FIG. 1 the signal may become blurred by the presence of other metal particles in the sample fluid. For example, engine fluids will typically contain tens of thousands of relatively fine particles resulting from normal wear. These are typically too small to be detected individually, but together constitute background noise. These particles are generally distributed fairly uniformly within the fluid and so the size of the background noise increases with the sampling volume. The FIG. 2 coil arrangement may therefore provide a lower signal to noise ratio than an extended solenoidal coil like that shown in FIG. 1. Additionally, because the sampling volume is shorter than would be the case for a solenoidal coil, the signal will be sharper and the sensor may provide a relatively high resolution and/or may be compatible with higher flow rates. In one exemplary embodiment the background particles may have a typical size of less than about 10 microns, and the sensitivity of the sensor apparatus may be such that particles having a typical size of greater than around 40 or 50 microns are detected. However, the sensor apparatus may have a sensitivity for detecting particles of any suitable size for a particular application.

[0312] It will be appreciated that the larger the sampling volume, the greater the probability that two or more particles will be sensed at the same time and incorrectly classified as a single larger particle. The ability to provide a smaller sampling volume that is associated with the use of a printed coil may help to improve the accuracy with which individual particles may be counted, and, where appropriate, sized. Improved reliability in counting may be obtained even at higher particle concentrations. This makes the apparatus of the present invention particularly useful for detecting smaller particles as they are typically present at relatively high concentrations.

[0313] To give an indication of the distribution of particle sizes within a typical flow of lubricating fluid or engine oil, then it is noted that even new oil may contain over a hundred particles per millilitre greater than about 14 microns in size, and more than a thousand particles per millilitre greater than about 4 microns in size. Used oil may contain an order of magnitude greater number of particles per millilitre. During normal engine wear, metal particles in around the 1-15 microns range are typically generated. The onset of abnormal wear will tend to produce larger particles, e.g. in the 25-100 microns range.

[0314] It will be appreciated that the distribution will be skewed towards small particles, i.e. that smaller particles will be vastly more common than larger ones. This is partly because smaller particles are generated constantly by normal wear, partly because any larger particles will eventually break up into smaller particles and partly because larger particles (>about 10 microns) may be filtered out by the oil filtration system, which may be insensitive to smaller particles. Furthermore, larger, heavier particles tend to settle out of the oil very quickly e.g. a 100 micron steel particle may settle in a matter of seconds. Whilst large particles tend to settle, break up or be filtered out, smaller particles will continue to circulate in suspension within the fluid. This removal or larger particles from the flow is problematic for monitoring machine wear debris by analysing the metal particle content of the lubricating oil or grease. For instance, for offline (lab) tests it is difficult to know whether the extracted sample is really indicative of the distribution of particles being generated at the moving parts in the engine. Naturally, this is also problematic for online sensors that are only capable of detecting relatively large particles, as again, unless the sensor is located in close proximity to the source of the fault, any particles that are large enough to be detected, and would indicate abnormal wear, may not reach the sensor.

[0315] As described above, the presence of many thousands of small particles within the fluid, which may be too small to be individually detected but whose aggregate signal and fluctuations thereof is a source of noise that limits the ability of the sensor to count large particles, limits the size of particles that can be detected by a sensor apparatus. Naturally, the size of this noise increases with the size of the sensing region of the sensor apparatus. Thus, it will be appreciated that the use of a relatively small or planar sensor apparatus, substantially of the type described above, is particularly advantageous for this second application, because the relatively small sensing region which may be provided (e.g. compared to a conventional wound coil arrangement) allows the detection of much smaller particles with a higher signal to noise ratio. Thus, a relatively small sensor apparatus with a relatively small sensing volume is capable of detecting particles that remain in suspension and give early indications of developing problems.

[0316] An example of a sensor unit that may incorporate a sensor apparatus as described herein for use in the second application is shown in FIG. 6. Here, the sensor unit is arranged to be mounted between sections of or around a flow pipe so that sample fluid may continuously flow through the aperture of the sensor apparatus. The housing 60 may contain a flow conduit disposed coaxially within the aperture and extending between opposite sides of the housing parallel to the substrate. Connectors 61 may be provided at the ends of the flow conduit and/or on the sides of the housing to enable the housing 60 to be connected between sections of an existing, external flow pipe 62. The sensor apparatus shown in FIG. 6 connects or bridges between a first upstream and second downstream portion of a pipe, and thus forms part of a continuous flow path. The sensor unit being mounted onto a flow pipe is particularly suited to the second application described above where the aim is to count and/or categorise individual metal particles flowing through the sensor. The unit shown in FIG. 6 does not provide a display to the user, but instead provides an output to a remote display. Preferably the output is provided wirelessly to the remote display. The unit can thus be left in situ and continuously monitored by a remote user. The output may comprise a count of particles, and may further comprise indicating a change in count e.g. over a period of time.

[0317] FIGS. 7A and B show a perspective and cross sectional view respectively of another example of a sensor unit intended for use in the second application described above. A portion of the sensor unit shown in FIGS. 7A and B is arranged to be immersed within a dynamic flow of fluid for counting individual metal particles dispersed within the fluid and passing through the sensor. The sensor apparatus of the sensor unit of FIGS. 7A and B may thus be mounted within a pipe for in situ i.e. online measurements.

[0318] The sensor unit 70 comprises a first portion 69 arranged to be immersed within a flow of fluid and including a passage 71 through which a portion of the total sample fluid flow within a pipe passes. At least a portion of a sensor apparatus having a coil arrangement of the type described above e.g. in FIG. 2 or 3, having one or more coil units printed onto a flexible substrate around an aperture, is located within the first portion so that fluid passing through the passage 71 then passes through the aperture (i.e. into the sensing region) of the sensor apparatus so that a determination of the presence of individual metal particles within the fluid can be made. This may be achieved in various manners. For instance, the passage may be defined by a tube, with the aperture surrounding the tube. In this way, the tube, and fluid passing therealong, will flow through sensing region of the sensor apparatus. Alternatively, the portion of the substrate defining the walls of the aperture of the sensor apparatus may at least in part define the flow passage 71.

[0319] A reference coil may also be provided and positioned within the sensor apparatus 70 such that it is exposed to substantially the same temperature and pressure conditions within the pipe but arranged so that metal particles within the fluid flow do not pass through the reference coil. For instance, a baffle may be provided as part of the sensor body to prevent the metal particles passing through the reference coil. Alternatively, the reference coil may be arranged so that it does not encounter i.e. is sealed off from or not immersed within the fluid flow. The reference coil may generally be provided on the same substrate as the sensing coil, and spaced apart from the sensing coil by approximately a coil diameter, substantially as shown in FIG. 3.

[0320] The sensor unit 70 also comprises a second portion 74 that is arranged to be outside of the fluid flow i.e. outside of the pipe, and to house the electronics or processing means of the sensor unit 70. The second portion 74 further includes any connections necessary to transmit the sensed information to a user or to an external processing unit. For instance, in the embodiment illustrated in FIGS. 7A and B the second portion 74 includes a wired external connection.

[0321] In order to facilitate its mounting within a pipe, the sensor unit shown in FIGS. 7A and B also contains a collar or mounting ring 73 that may, for example, be threaded to allow the sensor 70 to be screwed into a suitable opening in the pipe. The mounting ring 73 and/or the shoulder between the second portion 74 and the mounting ring 73 provide a fluid-tight seal around the mounting.

[0322] FIG. 8 illustrates the sensor unit 70 mounted in situ within a pipe 81. In the arrangement illustrated in FIG. 8 a collar 82 for receiving the sensor unit has been welded onto the pipe 81. The collar 82 on the pipe 81 engages the mounting ring 73 of the sensor unit. For instance, the collar 82 and the mounting ring 73 may be correspondingly threaded, or otherwise connected. The shoulder between the second portion 74 and the mounting ring 73 abuts the end of the collar 82. However, it will be appreciated that any other suitable arrangements for mounting the sensor unit 70 onto a pipe may be used. For example, FIG. 9 shows an arrangement where the sensor unit 70 is provided integrally with a section of pipe 90. The pipe section 90 terminates in mounting flanges 91, 92 to enable it to be mounted to adjacent pipe sections. The pipe section 90 can thus be mounted as a unit within an existing flow pipe. Providing the sensor unit 70 as part of a pipe section 90 may advantageously allow further components to be positioned adjacent to the sensor apparatus. For example, in FIG. 10, which illustrates an alternative embodiment in which the sensor unit 70 is provided mounted to a section of pipe, a stator mixer 100 is provided upstream of the sensor apparatus. The use of an upstream mixer 100 may help to ensure that the fraction of the fluid passing into and sensed by the sensor unit 70 more accurately reflects the total composition of the fluid, i.e. the dispersion of metal particles within the fluid and that the measurement is not skewed by the (radial) position of the sensor apparatus in the pipe due to e.g. variations in flow profile across the cross section of the pipe.

[0323] It will be appreciated that the sensor unit 70 shown in FIGS. 7-10 is arranged to sample only a portion or fraction of the fluid flow through the pipe or pipe section, the sampled portion being defined by the diameter of the passage 71. This is by contrast to arrangements where a larger diameter sensor is mounted around the circumference of the pipe and used to monitor the entire flow. It will also be appreciated that the sensor unit shown in FIG. 6 may also be arranged to sample only a fraction of a larger flow, e.g. where the flow pipe 62 is part of bypass arrangement. As explained above, a flow of lubricating oil or engine grease will generally contain a distribution of particle sizes including a vast number of relatively small particles. By arranging for the sensor apparatus to see only a fraction of the fluid flow, the size i.e. diameter of the sensor apparatus can be reduced (at least relative to the pipe diameter), thus reducing the sensing volume and background noise further, again allowing the detection of much smaller particles. It has been found that the small bore sensors which may be provided in accordance with the invention may be sensitive enough to provide a count that is indicative of wear rate. In contrast, larger bore sensors that are mounted around the flow pipe and thus sample the whole cross-sectional flow may only be capable of detecting particularly large particles that may only be present in the case of imminent catastrophic failure. Furthermore, such large particles may drop out of suspension relatively quickly or be filtered out, before reaching the sensor. Thus, it will be appreciated that sampling only a fraction of the total flow may advantageously allow better detection of developing faults, i.e. particle sizes indicative of the onset of abnormal wear.

[0324] By way of example only, relating to a sensor for use in the context of monitoring metal particles within a marine engine, a flow passage through the sensor unit, which may correspond to the diameter of the aperture in the substrate in accordance with the invention, may be within the range about 3-5 mm, and ideally towards the lower end of this range to provide a relatively high sensitivity. However, in some circumstances it may be advantageous to use a larger diameter, e.g. towards the upper end of this range, or higher, so that a larger proportion of the flow can be sampled. This is by contrast to larger bore sensors in this context, which may have a diameter of e.g. 25 or 38 mm, that are mounted around the flow pipe and thus sample the whole cross-sectional flow. The typical detection limit for a large bore (i.e. >25 mm) sensor may be around 250 microns for ferrous particles and between about 500-750 microns for non-ferrous particles. These dimensions are merely exemplary, and the sensor of the present invention may be arranged to have a flow passage/aperture with diameter selected over a wide range of values, depending upon the particular application in which it is to be used. Generally, there is a balance between the size of the passage (or aperture), the flow speed and the measurement interval to ensure that a count of particles per volume can be obtained with a sufficiently high confidence level. The uncertainty in the count will generally vary as the square root of the total number of particles counted in a given period.

[0325] In this context, where the particles being detected are entrained within a continuous fluid flow, and may remain in suspension for multiple passes through the sensor apparatus, it is desirable to be able to count not just the absolute number of particles flowing through the sensor over time (e.g. the counts per minute or hour), but also to determine the count as a function of volume. For instance, outputting the count as a function of volume allows more accurate comparisons to be made between measurements obtained using an online sensor unit of the type described above with offline lab results, or with other particle counting methods. Particularly, obtaining the count as a function of volume allows for consistent measurements to be obtained under different operating conditions e.g. different pumping speeds. For example, if the output was provided only as a count as a function of time, a sharp increase in the number of metal particles being counted could be caused by an increase in flow rate, rather than an increase in wear.

[0326] In order to achieve this, a flow sensor 72 is provided integrated within the sensor unit 70 (e.g. in accordance with the embodiments of any one of FIGS. 7 to 10) for determining the volume of fluid that is measured by the sensing coil within a test period. The volume may for instance be determined using the cross-sectional area of the aperture of the sensor apparatus and a measured flow velocity through this aperture. In the FIGS. 7A and B embodiment the flow sensor 72 is located within the passage 71, i.e. within the same flow passage as the sensing region of the coil, such that the flow sensor 72 measures the flow velocity of the same fraction of fluid that is to be measured by the sensing coil. Typically, the flow sensor 72 may be mounted just upstream or downstream of the coil sensor. It will be appreciated that the flow sensor 72 may be secured via a suitable adaptor to the substrate upon which the coil sensor is printed, or could be provided as part of the substrate. Alternatively, the flow sensor 72 may be secured within the flow passage, but not directly to the substrate. Other suitable ways of mounting the flow sensor 72 will be apparent to the skilled person. The flow sensor 72 should be located in close proximity to the sensing coil to ensure that the measured flow velocity can be taken to be indicative of the fraction of the flow that is sensed by the coil sensor. This is important because the sampling fraction may be affected by variations in the flow profile across the width of the pipe. It will be appreciated however that the flow sensor 72 may be positioned sufficiently far from the sensing coil to minimise any interactions between any metallic components of the flow sensor and the magnetic field of the coil sensor. That is, the flow sensor 72 may be substantially outside of the sensing region of the coil sensor. By way of example only, for a printed coil sensor of the type described above having an aperture of around 3-5 mm diameter, the flow sensor may be a few millimetres away from the coil sensor; however, the most appropriate distance will of course depend on the size of the sensor apparatus. Generally, there may be some compromise between keeping the coil sensors small, and hence sensitive to smaller particles, and the risk of there being significant interactions with the flow sensor.

[0327] There are various known ways of measuring flow that are suitable for use with these embodiments. For example, a preferred embodiment would be the use of planar thermal mass flow anemometer-type sensor that senses the cooling effect of the flow. A suitable planar thermal mass flow sensor may be that available from Innovative Sensor Technology designated as the FS5 Thermal Mass Flow Sensor. This type of planar sensor is shown in FIGS. 7A and B, with the plane of the flow sensor 72 being oriented in the radial direction when mounted in the pipe. However, the orientation of the flow sensor 72 is not particularly important. It will also be appreciated that any other suitable flow sensors may also be used. For instance, a linear or hotwire type anemometer sensor may also be used. As another example, a capillary thermal mass flow sensor including a central heater with upstream and downstream temperature sensors for comparing the heat transfer with and against the direction of flow may be used. Other (non-thermal) sensors may also be used to measure the flow. For instance, a pressure sensor may be used to measure a pressure drop between two points. For example, an orifice type sensor measuring the pressure drop across the coil may be used. As another example, a venturi-type may be used to measure the pressure difference between the coil, which provides the throat of the venturi sensor, and the main flow. Other types of sensors, including simple rotating impeller type sensors may also suitably be used.

[0328] Whilst the embodiment described above in relation to FIGS. 7-10 is advantageously implemented using a sensor apparatus of the type described above in relation to and shown in FIGS. 2 and 3 especially, it will be understood that the technique of counting particles within a dynamic flow of fluid and outputting the count as a function of volume is not limited to this type of sensor. For instance, a conventional wound bobbin-type coil sensor like that shown in FIG. 1 could also be used in conjunction with a flow sensor in a similar manner. However, it will be appreciated that the advantages at least in sensitivity obtainable using a sensor including one or more coil portions printed around an aperture are particularly useful in the context of the second main application described herein.

[0329] It will be understood that any of the sensor units described herein may internally process the output signal generated by the sensing means, or an output signal indicative of the output of the sensing means, or may transmit such signals to a remote processor. It will also be understood that any of the sensor units described herein may comprise a display, or may provide data to a remote system for display. For any of the sensor units described herein data may be transmitted to a remote system wirelessly, via a cable or via a removable storage means. The sensor units FIGS. 5-10 are not intended to be limiting in these respects and merely illustrate a number of possible arrangements. It is also noted that the features described above in relation to FIG. 4, including any modifications to what is illustrated in FIG. 4, are compatible with both the sensor unit of FIG. 5 and the sensor units of FIG. 6-10.

[0330] Embodiments of the present invention have been described with respect to determining a total quantity (mass) of ferromagnetic e.g. ferrous particles, e.g. per given/unit volume of an extracted sample. Typically the ferromagnetic particles will be ferrous. The total quantity may then be one of ferrous particles. However, where non ferrous ferromagnetic particles are present they may contribute to the sensed interaction, and hence to the determined total quantity of ferromagnetic particles. Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.