Method of optimising the output of a sensor for indicating the relative location of a mettalic object

Abstract

Method of optimizing output of sensor for indicating location of metallic object. Sensor having primary electromagnetic coil to generate time varying magnetic field; secondary electromagnetic coil to detect time varying magnetic field as affected, by object to output, on basis of detected time varying magnetic field, signal indicative of location of object. Method includes steps of: supplying primary coil with alternating-current to result generated time varying magnetic field; locating object in first-position and recording signal output by secondary electromagnetic coil for range of frequencies of supplied alternating-current; locating object in second-position and recording signal output by secondary electromagnetic coil for range of frequencies of supplied alternating-current; calculating, for each of frequencies, a value for span to offset ratio of measured signals on basis of respective signals measured for object in first and second positions; determining frequency of supplied alternating-current which provides maximum span to offset ratio on basis of calculations.

Claims

1. A method of optimising the output of a sensor for indicating the relative location of a metallic object, the sensor being of the type having a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object, the method including the steps of: supplying the primary coil with an alternating current to result in the generated time varying magnetic field; locating the object in a first position and recording the signal output by the secondary electromagnetic coil for a range of respective frequencies of the supplied alternating current; locating the object in a second position and recording the signal output by the secondary electromagnetic coil for the range of respective frequencies of the supplied alternating current; calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions, the calculating step including, for each respective frequency calculating the difference between the amplitudes of the signals measured for the object in the first and second positions, and dividing the difference by the amplitude of the signal measured for the object in the second position; and determining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations.

2. A method according to claim 1, wherein when the object is in the first position, the output from the secondary coil is a maximum; and/or when the object is in the second position, the output from the secondary coil is a minimum.

3. A method according to claim 1, wherein at least one of the primary and secondary electromagnetic coils of the sensor is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object.

4. A method according to claim 1 wherein the primary and secondary coils of the sensor are arranged coaxially.

5. A method according to claim 1, wherein the primary coil of the sensor is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object.

6. A method according to claim 1, wherein the secondary coil of the sensor is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object.

7. A method according to claim 1, wherein the primary and secondary coils of the sensor are each wound about the same core body formed of a material having the same conductivity and/or magnetic permeability as the object.

8. A method according to claim 1, wherein the primary coils of the sensor are mutually arranged in electrical series; and/or wherein the secondary coils of the sensor are separately mutually arranged in electrical series.

9. A method according to claim 1 wherein the primary and/or secondary coils of the sensor are formed of an alloy comprising 86% copper, 12% Manganese and 2% Nickel.

10. A method according to claim 1 wherein a core body of the sensor is formed of a material having the same conductivity and/or magnetic permeability as the object.

11. A method according to claim 1 wherein a core body of the sensor is formed of the same material as the object.

12. A method according to claim 1, wherein the metallic object is attached to a nuclear reactor control rod.

13. A method according to claim 1, wherein the sensor is positioned within a metallic tube and the metallic object is arranged to move relative to the tube between a position of minimum overlap and a position of maximum overlap of the tube and the object.

14. A method according to claim 13, wherein the first position is a position where there is minimum overlap between the tube and the object.

15. A method according to claim 13, wherein the second position is a position where there is maximum overlap between the tube and the object.

16. A method according to claim 1, wherein the sensor includes a plurality of primary electromagnetic coils.

17. A method assembly according to claim 16, wherein the sensor includes a plurality of secondary electromagnetic coils.

18. A method assembly according to claim 17, wherein the plurality of primary and secondary coils are arranged in a mutually alternating sequence of primary and secondary coils.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1A shows (half) a cross section of a sensor in situ in a probe tube for measuring the relative location of a control rod leadscrew, where the leadscrew is “uncovered” (i.e. is located distally to the sensor);

(3) FIG. 1B shows (half) a cross section of a sensor in situ in a probe tube for measuring the relative location of a control rod leadscrew, where the leadscrew is “uncovered” (i.e. is located proximally to the sensor);

(4) FIG. 2 shows a cutaway view of a nuclear reactor probe tube and indicates the likely field lines generated;

(5) FIG. 3 shows a plot indicating the effect, at an arbitrary 400 Hz, on the signal span to offset ratio when the material of the core body is matched to that of the object to be detected;

(6) FIG. 4 shows a plot indicating the AC supply current frequency dependence of the signal span to offset ratio of the output signal for various material of core body;

(7) FIG. 5 shows a plot indicating the effect, at an optimum 7.5 kHz, on the signal span to offset ratio when the material of the core body is matched to that of the object to be detected;

(8) FIG. 6 shows a plot of the output signal span at a range of frequencies with the leadscrew in a first predetermined position;

(9) FIG. 7 shows a plot of the output signal span at a range of frequencies with the leadscrew in a second predetermined position;

(10) FIG. 8 shows a plot of the signal span to offset ratio for the signals across the range of frequencies indicated in FIGS. 6 and 7

(11) FIG. 9 shows a plot of the output signal at a range of positions of the leadscrew at a maximum sensor temperature of 20° C. and 200° C.;

(12) FIG. 10 schematically shows a cross section of a further sensor;

(13) FIG. 11 schematically shows a cross section of the sensor of FIG. 10;

(14) FIG. 12 shows a plot of the output signal from the sensor of FIG. 10 at a range of positions of the leadscrew at a maximum sensor temperature of 20° C. and 200° C.; and

(15) FIG. 13 schematically shows a processor for calibrating an output from the sensor of FIG. 10.

DETAILED DESCRIPTION

(16) FIG. 1 shows schematic cross sections of a sensor 10. The schematic only shows half of the full arrangement; the full arrangement being mirrored about the dashed line A-A to shown in FIG. 1.

(17) The sensor 10 comprises a plurality of primary coils 12, coaxially arranged with a plurality of secondary coils 14. One or more primary coils 12 may be provided. One or more secondary coils 14 may be provided.

(18) Where a plurality of primary coils 12 are provided, the respective primary coils may be connected in electrical series. Where a plurality of secondary coils 12 are provided, the respective secondary coils may be connected in electrical series.

(19) The primary and secondary coils are arranged in a mutually alternating (physical) series or sequence, such that the sequence of coils along the long axis of the series alternates between individual primary and secondary coils. The primary and secondary coils are not in electrical connection. In other words, between each pair of adjacent primary coils 12 a secondary coil 14 may be provided; and/or between each pair of adjacent secondary coils 14 a primary coil 12 may be provided.

(20) In the embodiment shown, each coil 12, 14 is wound around a single core body 16. However, the coils may be each be wound around a respective core body 16. Or plural sets of two or more of the coils may be wound around respective core bodies.

(21) The coils 12, 14 may be wound around a supporting body, which is itself mounted on to the core body 16. However, the coils 12, 14 may be wound directly on to the core body 16. In either case the coils 12, 14 may be referred to as bobbins.

(22) In one particular use, the sensor 10 is mounted inside a probe tube 18 which extends or projects into a region containing the primary water surrounding a nuclear reactor. In this example, for safety reasons the probe tube must be metallic.

(23) Within the aforementioned region the nuclear reactor control rods (not shown) are movable, to be inserted into or withdrawn from the nuclear reactor itself. Typically, each control rod is attached to a leadscrew 20, such that movement of the nuclear rod causes movement of its respective leadscrew. It is the accurate detection of the movement, or more accurately the relocation, of the leadscrew that the present disclosure aims to provide.

(24) As the control rod is moved, the leadscrew 20 moves along the probe tube 18. At one extreme, the leadscrew may not cover any part of the probe tube, as shown in FIG. 1A. This may occur for example when the control rod is fully inserted into the nuclear reactor core. At another extreme, the leadscrew may fully cover the probe tube, as shown in FIG. 1B, for example when the control rod is fully withdrawn from the nuclear reactor core.

(25) Therefore, to assist in the understanding of the present example, FIG. 1A shows the leadscrew in the “uncovered” position, whereby the leadscrew 20 is withdrawn from the probe tube 18; whereas FIG. 1B shows the leadscrew in the “covered” position, whereby the leadscrew 20 is arranged proximate to the sensor, e.g. to cover the probe tube 18. In FIG. 1A the nuclear reactor control rod (not shown) to which the leadscrew 20 is attached may be at a maximum insertion in the nuclear reactor core for example. Whereas, in FIG. 1B the nuclear reactor control rod (not shown) to which the leadscrew 20 is attached may be at a maximum extent of withdrawal from the nuclear reactor core, for example.

(26) In order to control the reaction within the nuclear reactor core in a reliable and safe manner it is important to know the relative location of the leadscrew 20, and therefore of the control rod, with a high degree of accuracy.

(27) During operation of a sensor, the primary coils 12 of the sensor 10 are supplied with an alternating (AC) current so as to result in a time varying magnetic field being produced by the primary coils 12. The time varying magnetic field interacts with the local environment, including the probe tube 18, the core body 16 and the leadscrew 20.

(28) The time varying magnetic field, as affected by the local environment, induces in the secondary coils 14 a corresponding AC current, and the secondary coils therefore output a corresponding signal indicative of the time varying magnetic field which induced the AC current in the secondary coils.

(29) Changes in the local environment, such as relocation, or repositioning, of the leadscrew 20 will alter the time varying magnetic field, and therefore will consequently alter the current induced in the secondary coils 14. Thus the corresponding output signal will be changed.

(30) This change in the signal output of the secondary coils is detectable, and can be used to establish the relative location of the leadscrew 20, and thus of the control rods.

(31) As discussed above, similar prior art sensor arrangements (e.g. U.S. Pat. No. 5,563,922) suffer from disadvantages that mean the accuracy of the determination of the relative location of the leadscrew can be improved significantly. The present inventor has realised that an important factor when considering how to improve the accuracy of said determination is the (signal) span to offset ratio.

(32) The signal span is the measurable signal span from the minimum signal to the maximum signal, and the offset is the minimum achievable signal.

(33) It is often difficult, if not impossible, to achieve a zero offset in a measured signal. Noise and residual signal inducing effects (e.g. residual magnetic fields in the context of the present discussion) mean that a non-zero signal offset is almost inevitable in any measurement system.

(34) Systems such as that shown in U.S. Pat. No. 5,563,922 often suffer from relatively small signal spans and undesirably large signal offsets, meaning that the overall accuracy of the measurement system can suffer.

(35) The present inventor currently considers that the (static) local environment around the sensor 10 is responsible for disadvantages discussed above. For example, as shown in FIG. 2, the time varying magnetic field 22 generated by the primary coils 12 results in a secondary electromagnetic field 24 in the metallic probe tube 18 (due to the produced eddy current 26) which can adversely affect the signal output by the secondary coils 14 by reducing the signal span to offset ratio for example. Other aspects of the local environment can also affect the signal span and signal offset. For example, the core body 16 about which the respective coils are wound.

(36) The present inventor has realised that one way to significantly improve the (signal) span to offset ratio to achieve excellent accuracy in determining the relative location of the leadscrew 20, is to ensure that the core body 16 is formed of a material having the same permeability and/or conductivity as the material from which the leadscrew 20 is formed.

(37) Indeed, in particularly preferred embodiments, the core body 16 is formed of the same material as the leadscrew 20. In such embodiments, ideally, the core body would be formed of the same production batch of material as the leadscrew 20, although this is not strictly necessary for the sensor arrangement to be worked.

(38) FIG. 3 shows a plot demonstrating the advantageous effect on the SoR (signal span to offset ratio) of the output signal of the secondary coils 12 when the material (or the permeability and/or conductivity characteristics) of the core body 16 is matched to the material from which the leadscrew is formed. Line 28 indicates the output signal V.sub.S against leadscrew position P for the sensor of the present embodiment. Line 30 is provided for comparison purposes and indicates an output signal against leadscrew position for a sensor where the material of the core 16 is different to the material of the leadscrew 20 (including having a different conductivity and a different magnetic permeability).

(39) To produce FIG. 3, an arbitrary frequency of 400 Hz for the AC current supply to the primary coils 12 was chosen.

(40) To calculate the SoR at the arbitrary frequency of 400 Hz, the output signal from the secondary coils 14 was measured for the uncovered leadscrew arrangement (i.e. where the leadscrew is distal to the sensor as in FIG. 1A) and separately for the covered leadscrew (i.e. the leadscrew at least partially ensheathing the sensor 10 and probe tube 16 as shown in FIG. 1B). Typically this provides values representative of the maximum output signal and the minimum output signal respectively.

(41) The difference between the measured values was then calculated to obtain the signal span. The result was then divided by the measured signal corresponding to the covered leadscrew (i.e. at least partially ensheathing the sensor) which typically corresponds with the offset of the measured signal.

(42) The result of the division operation gives the span to offset ratio (SoR) for the output signal at the chosen 400 Hz.

(43) For a typical prior art arrangement without core matching (without matching the material characteristics of the core body 16 to that of the leadscrew 20), the SoR at 400 HZ was determined to be around 0.8 only.

(44) However, for a sensor arrangement according to the present embodiment, which adopts the principle of matching the permeability and/or conductivity characteristics of the core body material to that of the leadscrew material (for example, by matching the material of the core body 16 to that of the leadscrew 20), the SoR at 400 HZ was determined to be 2.26.

(45) Thus, the described sensor arrangement can provide a very significant improvement in the SoR of the output signal from the secondary coils 14.

(46) This is particularly advantageous where the output signal of the secondary coils may be fed to a measurement system via a data acquisition card having a maximum input voltage. For example, such data acquisition cards may have a maximum input voltage of 5V. Therefore, improving the SoR within the available 5V range means that the resolution of the acquired signal is improved, and thus the subsequent processing can produce a more accurate result for the determination of the relative location of the leadscrew 20.

(47) To demonstrate that matching the conductivity and/or magnetic permeability of the core body material to that of the leadscrew 20 is particularly advantageous in achieving an optimum SoR for the output signal of the secondary coils 14, the present inventor has conducted extensive finite element analysis, a resulting plot of the SoR for various metals against the frequency of the AC current supply to the primary coils 12 is shown in FIG. 4.

(48) In FIG. 4, line 30 is the plot of HAS4104; line 32 is the plot for grey cast iron; line 34 is the plot for ingot iron; line 36 is the plot for powdered iron; line 38 is the plot for supermendur (a cobalt-iron alloy); line 40 is the plot for pure iron; line 42 is the plot for Sinimax (a nickel-iron alloy); line 44 is the plot for Mumetal® (a nickel-iron alloy); line 46 is the plot for Inconel 625 (a nickel-chromium alloy); and line 48 is the plot for stainless steel.

(49) The finite element analysis has shown that the particular characteristics of the material of the core body 16 which contribute to the significant improvement of SoR are the conductivity of the core body material and the magnetic permeability of the core body material. In particular, the finite element analysis has shown that the improvement in the SoR of the output of the sensor 10 to be most significant when the magnetic permeability and/or the conductivity values of the core body material is/are matched closely to the magnetic permeability and/or conductivity values of the material from which the object to be detected is formed—here, the object to be detected typically being a leadscrew 20 formed of a particular metal.

(50) To demonstrate this effect, FIG. 4 shows the primary coils 12 AC current frequency dependency of the SoR of the output signal of the secondary coils 14 for various different materials of core body 16 for a leadscrew formed of a material referred to as HAS 4104, or DGS MS HAS 4104, which is a stainless having a high magnetic permeability. This material was chosen for the leadscrew material in this study because it is the typical material from which the leadscrews in nuclear reactors are formed.

(51) As can be seen from FIG. 4, the highest SoR is achieved for a core material of HAS4104, i.e. a material matching the material of the leadscrew which is also formed of HAS4104.

(52) So, where leadscrews are typically formed of HAS4104, embodiments for use in nuclear reactors employing such leadscrews may also have a core body 16 formed of HAS 4104.

(53) The SoR is also dependent on frequency. Not only will the electrical circuitry typically demonstrate a resonance peak, but the materials in the local environment will demonstrate different responses depending on the frequency of the time varying magnetic field generated by the primary coils.

(54) For example, a peak at around 7.5 KHz is observed in FIG. 4 when the core body 16 is formed of HAS4104 material. The SoR at this frequency is calculated to be around 11.

(55) This result for the SoR is calculated as follows, taking the suitable voltage values from FIG. 5 (in FIG. 5, line 50 is the line plotted for a core body of HAS4104, and line 52 is for a core body of stainless steel 316): Core body of HAS4104: (2.35V−0.19V)/0.19V=˜11

(56) As shown in FIG. 5, if an alternative material is used for the core body 16, which does not have conductivity and/or permeability characteristics which match with the HAS4104 of the leadscrew, the SoR is shown to be only around 2.8. This result for the SoR is calculated as follows, taking the suitable voltage values from FIG. 5: Core body of stainless steel 316: (0.27V−0.07)/0.07=˜2.8

(57) Therefore, the described sensor arrangement surprisingly offers an improvement in the SoR of almost four times. Interestingly, this is achieved with an alternative material which is not a wildly different material to HAS4104, but which is another stainless steel: stainless steel 316.

(58) The present inventor has therefore demonstrated that a careful selection of the material for the core body 16 can have a surprisingly large advantageous effect on the SoR of the output signal of the secondary coils 14.

(59) As can be seen from FIG. 4, the optimum SoR is provided at a particular frequency, and so the present disclosure also proposes a method for determining the frequency at which the optimum SoR exists for a particular system.

(60) The object to be detected, for example the leadscrew 20, is arranged distally from the sensor 10; for example at its furthest distance from the sensor 10. In the case of the leadscrew, the control rod may be fully inserted into the nuclear reactor, for example.

(61) With the leadscrew 20 in this position, the primary coils are provided with AC current at a range of (two or more) discrete frequencies f, and the output signal V.sub.S from the secondary coils measured and recorded for each respective frequency.

(62) The result of such an exercise is shown in FIG. 6 for example.

(63) The object to be detected, for example the leadscrew 20, is also arranged at proximally to the sensor 10; for example at its nearest position to the sensor 10. In the case of the leadscrew 20, the control rod may be at its maximum withdrawal from the nuclear reactor for example.

(64) With the leadscrew in this position, the primary coils 12 are provided with AC current at the same range of the same (two or more) discrete frequencies f, and the output signal V.sub.S from the secondary coils 14 measured and recorded for each respective frequency.

(65) The result of this exercise is shown in FIG. 7 for example, where the effect of the object (the leadscrew) on the signal output by the secondary coils 14 can clearly be seen by comparison of FIG. 7 with FIG. 6.

(66) Then the SoR at each frequency is determined in accordance with the calculation discussed above in relation to FIG. 5, to determine the frequency f at which the SoR is a maximum. In other words, for each frequency, the minimum measured output signal is subtracted from the maximum measured signal to generate a difference value, and the difference value is divided by the minimum value to generate the SoR value.

(67) For the range of frequencies f measured, FIG. 8 shows a plot of the SoR. As can be seen, for the particular arrangement used in the demonstration, the SoR reaches a maximum value of around SoR=11.4 at around 6.75 kHz.

(68) Therefore, for the particular sensor and the local environment in which the sensor was located in this demonstration, the AC current should ideally be supplied to the primary coils 12 at around 6.75 KHz in order to maximize the SoR of the output signal of the secondary coils.

(69) Accordingly, the present embodiment provides a position sensor which provides an output signal indicative of the relative position of an object to be detected with a higher resolution than equivalent sensor arrangements in the prior art.

(70) This is achieved by winding the primary coil(s) and secondary coil(s) around one or more core bodies formed of a material having similar characteristics to the material of the object to be detected. In particular, it is preferred that the material of the one or more core bodies has a conductivity and/or magnetic permeability which matches the material of the object to be detected. In most preferred embodiments, the material of the one or more bodies is the same as the material of the object to be detected.

(71) In this way, a sensor arrangement according to the present embodiment provides a higher SoR and span output signal when detecting metallic objects through another metallic body. This provides major advantages in high accuracy and resolution measurement systems. The ability to provide the downstream instrumentation detection electronics with good resolution sensor signals enables errors to be reduced significantly, thereby allowing the overall system to be more accurate and to offer better resolution.

(72) In particular, a sensor according to the present embodiment, especially when used in conjunction with the SoR optimisation technique disclosed herein, offers a greatly improved means to measure linear displacement of a metallic device through another metallic device.

(73) In the sense that a sensor 10 according to the present embodiment generates a signal for interaction with the local environment and measures the effect on the signal in order to output a signal indicative of a change in the local environment, the sensor 10 may be considered to be a transducer, and may be referred to as such.

(74) As mentioned above, a sensor according to the present embodiment is particularly suited to use in a nuclear reactor, where the temperature of the local environment may fluctuate to a large extent. A large fluctuation in temperature will likely change the resistive properties of the primary and/or secondary coils, and therefore will likely change the SoR of the output signal of the secondary coils.

(75) Referring to FIG. 9, the output V.sub.S from the secondary coil 14 at varying positions of the leadscrew 20 is illustrated for a sensor 10 where the primary coil 12 and the secondary coil 14 are made from copper. The line 60 indicates the variation of the output V.sub.S from the secondary coil for varying positions P of the leadscrew at 200° C. and line 62 indicates the variation of the output from the secondary coil for varying positions of the leadscrew at 20° C. The temperatures given are measured at a position on the sensor having a maximum temperature, in this case this is in a region at the bottom of the probe tube 18. It can be seen from FIG. 9, that a change in maximum temperature from 200° C. to 20° C. significantly affects the output from the secondary coil. This means that the output from the secondary coil is undesirably dependent upon the temperature of the system. Furthermore, at lower temperatures the change in output for a given position change of the leadscrew decreases, which in turn impacts the sensitivity of the sensor.

(76) Referring to FIG. 10 a sensing arrangement that attempts to address this temperature dependence problem is illustrated. The arrangement of FIG. 10 is similar to the arrangement previously described, and similar features are given a similar reference numeral but with a pre-fix “1” to distinguish between embodiments. Only the differences between the embodiments will be described in detail.

(77) As illustrated in FIG. 10, similar to the previously described embodiment, the arrangement includes a sensor 110 positioned in a probe tube 118, and the probe tube 118 is moveable relative to a leadscrew 120.

(78) Referring now to FIG. 11, the sensor 110 is shown in more detail. Similar to the previously described sensor, the sensor 110 includes a series of primary coils 112 and secondary coils 114. In the presently described embodiment, the primary coils and the secondary coils are made from a copper-manganese-nickel alloy. In particular, the primary and secondary coils are made from Manganin®. Manganin® is a Copper-manganese-nickel alloy, and is generally provided in the ratio of 86:12:2 by weight.

(79) FIG. 12 illustrates the output V.sub.S from the secondary coil 114 at varying positions P of the leadscrew. Line 64 illustrates the output from the secondary coil at 200° C. and line 66 illustrates the output from the secondary coil at 20° C. As can be seen from FIG. 12, the use of manganin wire significantly reduces the temperature dependence of the output from the secondary coil both in terms of magnitude for a given position of the leadscrew as well as in terms of the change in magnitude for a step change in position of the leadscrew.

(80) However, it can be seen from FIG. 12 that there is still some variation in the output V.sub.S from the secondary coil 114. It is believed that this variation is due to probe tube and leadscrew thermal effects, primarily probe tube thermal effects.

(81) Referring again to FIG. 11, the sensor 110 is further optimised to include a tertiary coil 115. The tertiary coil is arranged substantially coaxially with the primary coil 112 and is positioned radially outside of the primary coil. However, in alternative embodiments the tertiary coil could surround the secondary coil or surround both the primary and secondary coils, or be positioned at any other suitable position on the core 116. The tertiary coil is made from copper or an alloy thereof.

(82) Referring now to FIG. 13, the sensor is connected to a processor 168. The voltage V.sub.p from the primary coil, the Voltage V.sub.S from the secondary coil and the Voltage V.sub.T from the tertiary coil are transmitted to the processor. The processor receives the voltage from the primary coil, secondary coil and tertiary coil and performs a compensation procedure. The algorithm for the compensation procedure can be established using techniques known in the art and will vary depending on the specific environment in which the sensor is used. Once the processor has performed the compensation procedure, the processor outputs a calibrated secondary coil output V.sub.SC. The compensation procedure can remove the variation in output from the secondary coil illustrated in FIG. 12, so that the calibrated secondary coil output V.sub.SC is independent of the temperature of the sensor.

(83) In alternative embodiments, the primary and secondary coils may be made from an alloy such as constantan (a copper-nickel alloy). However, the inventor has found Manganin® to provide an optimum SoR.

(84) In the present embodiment, the tertiary coil is provided with an AC current, but in alternative embodiments the tertiary coil may be provided with a DC current. In further alternative embodiments, the tertiary coil may be replaced with another type of temperature indicator.

(85) It will be appreciated by one skilled in the art that, where technical features have been described in association with one or more embodiments, this does not preclude the combination or replacement with features from other embodiments where this is appropriate. Furthermore, equivalent modifications and variations will be apparent to those skilled in the art from this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting.