M22A202 - 1 - TEMPERATURE SENSOR FIELD OF THE INVENTION The present invention relates to temperature sensors. BACKGROUND Various different types of temperature sensor are currently in commercial and scientific use. For example, one widely used type of temperature sensor is a thermocouple temperature sensor which makes use of the temperature- dependent voltage characteristics of two dissimilar electrical conductors forming an electrical junction to sense temperature. SUMMARY OF INVENTION In an aspect, there is provided a temperature sensing system comprising a choke formed from a material with one or more temperature-dependent electrical properties, and a controller configured to: pass an electrical signal through the choke, measure one or more electrical parameters of the electrical signal, wherein the one or more electrical parameters vary with temperature as a result of the one or more temperature-dependent electrical properties of the material from which the choke is formed, and determine a temperature of the choke based on the measured one or more electrical parameters. The choke may be a radio frequency choke. The material may be ferrite. The one or more temperature-dependent electrical properties may comprise resistivity and/or permeability. M22A202 - 2 - The one or more electrical parameters may comprise one or more of: an amplitude response of the choke, a standing wave ratio of the electrical signal, and one or more scattering parameters of the electrical signal. The temperature sensing system may further comprise an object, wherein the choke is located proximate to or in contact with the object such that the temperature determined by the controller corresponds to a temperature of the object. The temperature sensing system may further comprise one or more heaters configured to heat the object, wherein the controller is configured to control the heaters based on the determined temperature. The object may be a pipe of a vacuum pumping and/or abatement system. The temperature sensing system may comprise a plurality of chokes, each choke being formed from a material with one or more temperature-dependent electrical properties, and the controller may be configured to pass an electrical signal through the plurality of chokes, measure one or more electrical parameters of the electrical signal, wherein the one or more electrical parameters vary with temperature as a result of the one or more temperature- dependent electrical properties of the materials from which the plurality of chokes are formed, and determine a temperature corresponding to at least one of the plurality of chokes based on the measured one or more electrical parameters. The plurality of chokes may comprise a first choke and a second choke, wherein the behaviour of the first choke in response to an electrical signal passing through it is temperature dependent in a first frequency range of the electrical signal, and the behaviour of the second choke in response to an electrical signal passing through it is temperature dependent in a second frequency range of the electrical signal, the second frequency range being different to the first frequency range. M22A202 - 3 - In another aspect there is provided a vacuum pumping and/or abatement system comprising the temperature sensing system of the above aspect. In yet another aspect, there is provided a method performed by a temperature sensing system, the method comprising: passing an electrical signal through a choke, the choke being formed from a material with one or more temperature- dependent electrical properties, measuring one or more electrical parameters of the electrical signal, wherein the one or more electrical parameters vary with temperature as a result of the one or more temperature-dependent electrical properties of the material from which the choke is formed, and determining a temperature of the choke based on the measured one or more electrical parameters. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic illustration (not to scale) showing a system for sensing temperature; Figure 2 is a schematic illustration (not to scale) showing another system for sensing temperature; and Figure 3 is a flowchart showing steps performed by each of the systems of Figure 1 and Figures 2. DETAILED DESCRIPTION Figure 1 is a schematic illustration (not to scale) showing a system 100 for sensing temperature according to a first embodiment. The system 100 comprises a controller 110, an object 120, a plurality of heating elements (or heaters) 130, first electrical lines 140, a choke 150, a resistor 160, second electrical lines 170, electromagnetic shielding 180, and thermal insulation 190. The controller 110 comprises a temperature sensing module 110a and a power source module 110b. The temperature sensing module 110a is configured to transmit an electrical signal through the choke 150 via the first electrical lines 140. In this embodiment, the electrical signal also passes through the resistor M22A202 - 4 - 160. The temperature sensing module 110a is also configured to monitor characteristics of the electrical signal in order to determine a temperature. The power source module 110b is configured to provide electrical power to the heating elements 130 via the second electrical lines 170 to power the heating elements for them to produce heat. In this embodiment, the power source module 110b provides AC electrical power to the heating elements 130. The controller 110 may be configured to control the operation of the power source module 110b based on temperatures sensed by the temperature sensing module 110a. The object 120 is an object whose temperature is to be measured by the system 100. In this embodiment, the object 120 is a section of pipe. The section of pipe is configured to contain and convey fluid therethrough. For example, the pipe may contain gaseous or liquid chemicals at various different temperatures for industrial use. It will be appreciated that, in general, the object 120 may be any type of object for which a temperature measurement is desired. The plurality of heating elements 130 are configured to heat the object 120. The plurality of heating elements are located proximate to or in contact with the object 120 in order to provide heat to the object 120. In this embodiment, the heating elements 130 are electrically connected in parallel with each other. The first electrical lines 140 and second electrical lines 170 each comprise electrical wiring for conducting electricity. For example, the first and second electrical lines 140, 170 may comprise copper electrical wiring. The choke 150 is an electrical component which blocks or limits some frequencies of AC electricity from passing through, whilst allowing other AC frequencies and/or DC to pass through without limitation. The behaviour of the choke 150 in response to an electrical signal passing through it is temperature dependent in a particular frequency range of the electrical signal. The behaviour of the choke 150 is not temperature dependent or substantially not temperature dependent at some or all frequencies outside of the particular frequency range. In more detail, the choke 150 is formed from a material whose resistivity and/or M22A202 - 5 - permeability, at a given frequency of electricity passing through the material, is dependent on the temperature of the material. In this embodiment, the material is ferrite (e.g. Mn-Zn ferrite or Ni-ZN ferrite) and the choke 150 is a radio frequency (RF) choke. In this embodiment, the choke 150 is in the form of a ferrite bead. As such, when an electrical signal at a given frequency is transmitted through the choke 150, the response of the choke 150 to the given frequency of electricity varies depending on the temperature of the choke 150. The choke 150 is located proximate to or in contact with a portion of the object 120 so that the temperature of the choke 150 is the substantially the same as or similar to the temperature of the portion of the object 120. The resistor 160 is an impedance matching resistor connected in series with the choke 150. The resistance of the resistor 160 is chosen to attempt to match the source and load impedances of the electrical circuit formed by the temperature sensing module 110a, the first electrical lines 140, the choke 150 and the resistor 160. The electromagnetic shielding 180 extends around part of the first electrical lines 140 in order to shield said part of the first electrical lines 140 from external electro-magnetic interference. For example, the electromagnetic shielding 180 may be a sheath of conductive material which forms a Faraday cage around said part of the first electrical lines 140.It will be appreciated that the electromagnetic shielding 180 is optional and may be omitted. The thermal insulation 190 extends around at least part of the object 120, the heating elements 130, the choke 150, the resistor 160, at least part of the first electrical lines 140 and at least part of the second electrical lines 170. The thermal insulation 190 is configured to thermally isolate the part of the object 120 being heated by the heaters 130 and the choke 150 from external thermal influences. It will be appreciated that the thermal insulation 190 is optional and may be omitted. The system 100 is configured to monitor the temperature of the choke 150 in order to estimate the temperature for the portion of the object 120 that the M22A202 - 6 - choke 150 is located proximate to or in contact with. More specifically, the temperature sensing module 110a monitors one or more electrical characteristics of the electrical signal which is passed through the choke 150 via the first electrical lines. The one or more electrical characteristics may represent a measure of an impedance mismatch between the source and load impedances of the electrical circuit formed by the temperature sensing module 110a, the first electrical lines 140, the choke 150 and the resistor 160. The one or more electrical characteristics may comprise one or more of: an amplitude response of the choke 150, a standing wave ratio (SWR) of the electrical signal, and one or more scattering parameters (e.g. S11, S12) of the electrical signal. The temperature sensing module 110a is configured to determine a temperature for the choke 150 based on the one or more electrical characteristics. In one example embodiment, the temperature sensing module 110a is configured to determine a temperature for the choke 150 based on a SWR of the electrical signal passed through the choke 150 using part or all of the information in Table 1 below. Table 1 below shows example temperature values (Temp_C) which map to SWR values (SWR) for a particular material (Mn-Zn ferrite or Ni-Zn ferrite) of the choke 150 at a particular frequency in hertz (Measure_at_hz) of the electrical signal passed through the choke 150. It will be appreciated that, in general, a skilled person will be able to obtain/use more data mapping SWR values to temperature values for a particular material at a particular frequency, in order to configure the temperature sensing module 110a. M22A202 - 7 - Table 1 Figure 2 is a schematic illustration (not to scale) showing a system 200 for sensing temperature according to a second embodiment. The system 200 of the second embodiment is the similar to the system 100 of the first embodiment except that the system 200 of the second embodiment comprises more than one choke 250a, 250b. The system 200 comprises a controller 210, an object 220, a plurality of heating elements (or heaters) 230, first electrical lines 240, a first choke 250a, a second choke 250b, a resistor 260, second electrical lines 270, electromagnetic shielding 280, and thermal insulation 290. The controller 210 comprises a temperature sensing module 210a and a power source module 210b. The temperature sensing module 210a is configured to transmit an electrical signal through the first and second chokes 250a, 250b via the first electrical lines 240. In this embodiment, the electrical signal also passes through the resistor 260. The temperature sensing module 210a is also configured to monitor characteristics of the electrical signal in order to M22A202 - 8 - determine a temperature. The power source module 210b is configured to provide electrical power to the heating elements 230 via the second electrical lines 270 to power the heating elements for them to produce heat. In this embodiment, the power source module 210b provides AC electrical power to the heating elements 230. The controller 210 may be configured to control the operation of the power source module 210b based on temperatures sensed by the temperature sensing module 110a. The object 220 is an object whose temperature is to be measured by the system 200. In this embodiment, the object 220 is a section of pipe. The section of pipe is configured to contain and convey fluid therethrough. For example, the pipe may contain gaseous or liquid chemicals at various different temperatures for industrial use. It will be appreciated that, in general, the object 220 may be any type of object for which a temperature measurement is desired. The plurality of heating elements 230 are configured to heat the object 220. The plurality of heating elements are located proximate to or in contact with the object 220 in order to provide heat to the object 220. In this embodiment, the heating elements 230 are electrically connected in parallel with each other. The first electrical lines 240 and second electrical lines 270 each comprise electrical wiring for conducting electricity. For example, the first and second electrical lines 240, 270 may comprise copper electrical wiring. Each of the first and second chokes 250a, 250b is an electrical component which blocks or limits some frequencies of AC electricity from passing through, whilst allowing other AC frequencies and/or DC to pass through without limitation. The behaviour of the first choke 250a in response to an electrical signal passing through it is temperature dependent in a first frequency range of the electrical signal. The behaviour of the first choke 250a in response to an electrical signal passing through it is not temperature dependent or substantially not temperature dependent at some or all frequencies outside of the first frequency M22A202 - 9 - range. The behaviour of the second choke 250b in response to an electrical signal passing through it is temperature dependent in a second frequency range of the electrical signal. The behaviour of the second choke 250b in response to an electrical signal passing through it is not temperature dependent or substantially not temperature dependent at some or all frequencies outside of the second frequency range. The first frequency range is different to the second frequency range. The behaviour of the first choke 250a in response to an electrical signal passing through it is not temperature dependent or substantially not temperature dependent in the second frequency range. The behaviour of the second choke 250b in response to an electrical signal passing through it is not temperature dependent or substantially not temperature dependent in the first frequency range. In more detail, each of the first and second chokes 250a, 250b is formed from a material whose resistivity and/or permeability, at a respective given frequency of electricity passing through the material, is dependent on the temperature of the material. In this embodiment, the material is ferrite (e.g. Mn- Zn ferrite or Ni-ZN ferrite) and each of the first and second chokes 250a, 250b is a radio frequency (RF) choke. In this embodiment, each of the first and second chokes 250a, 250b is in the form of a ferrite bead. When an electrical signal at a first frequency in the first frequency range is transmitted through the first choke 250a and the second choke 250b, the response of the first choke 250a to the first frequency of electricity varies depending on the temperature of the first choke 250a, but the response of the second choke 250b to the first frequency of electricity does not vary or does not substantially vary with temperature. When an electrical signal at a second frequency in the second frequency range is transmitted through the first choke 250a and the second choke 250b, the response of the second choke 250b to the second frequency of electricity varies depending on the temperature of the second choke 250b, but the response of the first choke 250a to the first frequency of electricity does not vary or does not substantially vary with temperature. Each of the first and second chokes 250a, 250b is located proximate to or in contact with a respective portion of the object 220 so that the M22A202 - 10 - temperature of each of the first and second chokes 250 is the substantially the same as or similar to the temperature of the respective portion of the object 220 that it is proximate to or in contact with. The resistor 260 is an impedance matching resistor connected in series with the first and second chokes 250a, 250b. The resistance of the resistor 260 is chosen to attempt to match the source and load impedances of the electrical circuit formed by the temperature sensing module 210a, the first electrical lines 240, the first and second chokes 250a, 250b, and the resistor 260. The electromagnetic shielding 280 extends around part of the first electrical lines 240 in order to shield said part of the first electrical lines 240 from external electro-magnetic interference. For example, the electromagnetic shielding 280 may be a sheath of conductive material which forms a Faraday cage around said part of the first electrical lines 240. It will be appreciated that the electromagnetic shielding 280 is optional and may be omitted. The thermal insulation 290 extends around at least part of the object 220, the heating elements 230, the first and second chokes 250a, 250b, the resistor 260, at least part of the first electrical lines 240 and at least part of the second electrical lines 270. The thermal insulation 290 is configured to thermally isolate the part of the object 220 being heated by the heaters 230 and the first and second chokes 250a, 250b from external thermal influences. It will be appreciated that the thermal insulation 290 is optional and may be omitted. The system 200 is configured to monitor the temperature of each of the first and second chokes 250a, 250b in order to estimate the temperature for the respective portions of the object 220 that the first and second chokes 250 are located proximate to or in contact with. More specifically, the temperature sensing module 210a monitors one or more electrical characteristics of the electrical signal which is passed through the first and second chokes 250a, 250b via the first electrical lines. The one or more electrical characteristics may represent a measure of an impedance mismatch between the source and load impedances of the electrical circuit formed by the temperature sensing module M22A202 - 11 - 210a, the first electrical lines 240, the first and second chokes 250a, 250b and the resistor 260. The one or more electrical characteristics may comprise one or more of: an amplitude response of each of the first and second chokes 250a, 250b, a standing wave ratio of the electrical signal, and one or more scattering parameters (e.g. S11, S22) of the electrical signal. The temperature sensing module 210a is configured to determine a temperature for each of the first and second chokes 250a, 250b based on the one or more electrical characteristics. For example, in a first scenario, an electrical signal at a frequency in the first frequency range is passed through the first and second chokes 250a, 250b. Since, as described above, the response of the first choke 250a is temperature dependent in the first frequency range but the response of the second choke 250b is not (or substantially not) temperature dependent in the first frequency range, the temperature sensing module 210a is configured to determine the temperature of the first choke 250a in the first scenario. In a second scenario, an electrical signal at a frequency in the second frequency range is passed through the first and second chokes 250a, 250b. Since, as described above, the response of the second choke 250b is temperature dependent in the second frequency range but the response of the first choke 250a is not (or substantially not) temperature dependent in the second frequency range, the temperature sensing module 210a is configured to determine the temperature of the second choke 250b in the second scenario. The system 100 may be configured to operate in the first scenario and then the second scenario or vice versa, to separately detect the temperature of the first and second chokes 250a, 250b. Similarly to the embodiment described with reference to Figure 1, the temperature sensing module 210a may be configured to determine a temperature for the first or second choke 250a, 250b based on a SWR of the electrical signal passed through the first and second chokes 250a, 250b using part or all of the information in Table 1 below. Again, it will be appreciated that, in general, a skilled person will be able to obtain/use more data mapping SWR values to temperature values for a particular material at a particular frequency, in order to configure the temperature sensing module 110a for use with multiple chokes with temperature dependent responses in different frequency ranges. M22A202 - 12 - It will be appreciated that, in other embodiments, even more chokes may be added which have temperature dependent responses in yet further different frequency ranges in order to detect temperatures at even more locations, in accordance with the principles described above in relation to the first and second chokes 250a, 250b. Figure 3 is a flowchart showing a method 300 performed by each of the systems of Figure 1 and Figures 2. At step 310, the method starts. At step 320, a controller of the system passes an electrical signal through a choke. The choke is formed from a material with one or more temperature-dependent electrical properties. At step 330, the controller measures one or more electrical parameters of the electrical signal, wherein the one or more electrical parameters vary with temperature as a result of the one or more temperature- dependent electrical properties of the material from which the choke is formed. At step 340, the controller determines a temperature of the choke based on the measured one or more electrical parameters. Thus, a temperature sensing system is provided. Advantageously, the above-described temperature sensing system does not use thermocouple based temperature sensors. Thermocouple based temperature sensors tend to use relatively large amounts of cabling which tends to be unwieldy and occupies a relatively large amount of space. By allowing avoidance of thermocouple based temperature sensors, the above-described system tends use a lower amount of cabling and is easier to modularise. This in turn tends to simplify maintenance of the system and reduce installation mistakes. It will be appreciated that various modifications/deviations may be made to the above-described embodiments without departing from the scope of the invention. For example, although the specific example given in Table 1 above maps SWR values to temperature values, a skilled person would also be able to use/obtain similar data for other temperature dependent electrical parameters M22A202 - 13 - such as amplitude response and scattering parameters to determine the temperature.

M22A202 - 14 - REFERENCE NUMERAL LIST 100: system 110: controller 110a: temperature sensing module 110b: power source module 120: object 130: heating element 140: first electrical lines 150: choke 160: resistor 170: second electrical lines 180: electromagnetic shielding 190: thermal insulation 200: system 210: controller 210a: temperature sensing module 210b: power source module 220: object 230: heating element 240: first electrical lines 250a, 250b: first and second chokes 260: resistor 270: second electrical lines 280: electromagnetic shielding 290: thermal insulation