Gas detection system and method

Abstract

A gas detection system 100 and method for detecting and sensing a target gas within a gaseous sample comprising first 104 and second 106 waveguides, an input port 114 connected to the first waveguide

Description

28 05 25 FIELD OF THE INVENTION This invention relates to the field of gas detection and analysis. More particularly, it relates to a system and method for detecting and sensing gases such as, but not limited to, methane. BACKGROUND TO THE INVENTION Methane gas is colourless and odourless which makes it difficult to detect. It is also highly flammable. The gas is widely used as a fuel in industries, homes and vehicles. It is also released during coal mining and flaring in both underground and surface mines. The accumulation of methane in underground coal mines can have grave consequences. Gas leaks in the oil and gas industries can cause explosions and may have environmentally damaging consequences. Detection of methane gas is therefore important for managing safety and environmental concerns in industry and mining. Hallil et al. (2009) have described a resonator-based sensor for gas detection. The sensor includes coplanar waveguides for microwave propagation and a dielectric resonator coated with SnO2. Adsorption of gas onto the SnO2 layer changes its permittivity during use, which in turn modifies the resonance frequency of the resonator. Cismaru et al. (2016) have described a device for detecting methane gas using an electromagnetic band gap resonator coated with multi-walled carbon nanotubes having gold nano-islands. The device includes a coplanar waveguide and relies on the adsorption of gas molecules onto the nanotubes. Shifts in resonance frequency and phase occur in the presence of methane. Siddiqui et al. (2015) have described dielectric sensors based on electromagnetic energy tunnelling. Metallic wires are embedded in narrow waveguide bends and channels. Zarifi etal. (2017) have described a non-contact microwave sensor for monitoring the interaction of zeolite 13x with CO2 and methane in gaseous streams. The zeolite is combined with a planar microwave microstrip resonator sensor, and sensing is performed by detecting permittivity changes during adsorption of the target gas onto the zeolite. Shifts in resonance frequency can be correlated with the concentration of the target gas. 28 05 25 Ramzan etal. (2019) have described a wire-based energy tunnelling waveguide which supports an impedance-matched resonance mechanism over the frequencies at which embedded wires become a half-wavelength long. A parallel-plate implementation of a 180° waveguide bend is described. Material samples for testing must be placed into narrow resonant channels before the device is operated. US10281423B1 teaches a fuel quality sensor for detecting contaminants in fuel flowing in a conduit. The contaminants are detected using electromagnetic energy tunnelling. The sensor includes two stacked parallepiped waveguides with a wire extending through an intermediate wall between the upper and lower waveguide cavities. A fluid conduit also extends through the upper waveguide cavity adjacent the wire. The sensor operates by measuring changes in the dielectric constant of fuel flowing in the conduit, which in turn cause a change in tunnelling frequency and / or Tan 5. Energy tunnelling sensors fall into the class of microwave sensors of the resonant type, which estimate electrical properties by detecting changes in their resonance frequencies and quality factors. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY OF THE INVENTION In accordance with an aspect of the invention there is provided a gas detection system for detecting and sensing a target gas within a gaseous sample, the system comprising: first and second waveguides; an input port connected to the first waveguide and configured to transmit electromagnetic radiation; an output port connected to the second waveguide and configured to receive an output signal of the electromagnetic radiation; a gas sampling component connected or connectable to the first and second waveguides; at least one elongate tunnelling element arranged within the gas sampling component, the tunnelling element being configured to provide energy coupling between the first and second waveguides; a processing component arranged to analyze the output signal received by the output port, thereby to detect a presence of the target gas in the gaseous sample. 2 28 05 25 The processing component may be arranged to analyze the output signal received by the output port to detect a presence and concentration of the target gas in the gaseous sample. The gas detection system may further comprise an output component arranged to provide results of the analysis to a user. The tunnelling element and the gas sampling component may be configured to promote resonant tunnelling of an electromagnetic field associated with the electromagnetic radiation, from the first to the second waveguide. The tunnelling element may comprise a metallic wire. The tunnelling element may be at least partially coated with a coating material. The coating material may comprise at least one substance selected from the group consisting of metallic oxides, semiconductors, and nanomaterials. The coating material may comprise Pt / AI2Os as a catalyst on SnO2. The processing component may be arranged to analyze the output signal to calculate a property selected from the group consisting of permittivity, tunnelling frequency, dielectric constant, and loss tangent (Tan 5). The processing component may be arranged to generate and analyze data correlated to variations in either or both of (i) tunnelling frequency as the permittivity of either or both the tunnelling element and its coating changes; and (ii) dielectric loss in the gaseous sample. The system may be configured to detect and measure a dielectric constant and dielectric loss associated with the target gas or a gas other than the target gas. The gas sampling component may be configured to be detachable from at least one of the waveguides, optionally in a tool-free manner. The gas sampling component may define an internal chamber. The internal chamber may define a channel of reduced diameter. The channel may be defined in part by an aperture, hole or orifice defined through a dividing plate which partitions two separate regions of the internal chamber from each other. The tunnelling element may be arranged to extend at least partially through the channel. The tunnelling element is typically electrically isolated from the dividing plate and other components of the system. The waveguides may be arranged generally parallel to each other and the channel may define a bend of about 180e between the waveguides. 28 05 25 A gas flow aperture may be defined through at least one side of the gas sampling component. At least two gas flow apertures may be defined through different respective sides of the gas sampling component. At least one of the gas flow apertures may be covered with a grating, net or gauze. The grating may be arranged in a plane aligned generally perpendicularly to a primary longitudinal axis of the tunnelling element. The gas detection system may comprise at least one substrate integrated waveguide (SIW). The system may comprise first and second planar SIWs. The SIWs may have circuits with a common ground. Embodiments of the system which comprise at least one SIW (referred to as SIW based systems) may further comprise microstrip ports configured to be connected to the SIWs. The input port may comprise an electromagnetic radiator. The electromagnetic radiator may be configured to radiate electromagnetic radiation in the microwave frequency region of the electromagnetic spectrum. The electromagnetic radiator may be configured to radiate electromagnetic radiation in the THz frequency region of the electromagnetic spectrum. The tunnelling element may have a radius and a length defining a resonance frequency of electromagnetic energy tunnelling from the first waveguide cavity into the second waveguide cavity. The system may include a plurality of tunnelling elements, each being adapted to sense a different target gas. The system may include at least one calibration component. The calibration component may comprise a calibration die. The calibration component may be configured to be movable in relation to at least one of the tunnelling elements. The calibration component may comprise at least one material having a known permittivity, or a known value of loss tangent (Tan 5), or both. The calibration component may have a plurality of different regions, segments, sectors or zones. Each such zone may, respectively, have a different known value of permittivity, known value of loss tangent (Tan 5), or both. These different zones may be applied for sensing and calibrating the system in respect of a plurality of different target gases. Each separate zone of the calibration component may be configured to be sensitive to a different gas. 28 05 25 The different zones of the calibration component may be coated with different materials in order to provide the different sensitivities, with each coating being sensitive to a different gas. In such embodiments the tunnelling wire is not required to have a coating. In use, if the test sample contains a specific gas, then when the zone of the calibration die that is sensitive to that gas is brought into proximity with a tunnelling wire, e.g., by rotating the die in the case of a cylindrical die, a shift in frequency will be observed. This shift may then serve as an indicator of the presence of that specific gas in the sample. Accordingly, it is not necessary to change the system or tunnelling wire in order to test for different gases. A plurality of calibration components may be provided. A first calibration component may comprise a material having a known value of permittivity and a second calibration component may comprise a material having a known value of loss tangent (Tan 5). At least one of the calibration components may have a plurality of zones, each zone having a different known value of permittivity. At least one other of the calibration components may have a plurality of zones, each zone having a different known value of Tan 5. The target gas may, without limitation thereto, comprise an alkane gas such as methane. According to a further aspect of the invention there is provided a sampling apparatus adapted for detection and sensing of a target gas in a gaseous test sample, the apparatus comprising a plurality of cooperating gas detection systems as described above. At least one of the gas detection systems may provide a test sensor module configured to sense test data relating to the target gas in the test sample. At least one other of the plurality of gas detection systems may provide a reference sensor module configured to sense reference data relating the target gas in the test sample. The reference sensor module may include at least one movable calibration component as described above. The output component may be arranged to provide the test data and reference data to the user. The sampling apparatus may be configured to interrogate a reference look-up table or calibration database. The calibration database may contain data which cross-reference permittivities of different zones of the calibration component against corresponding, predetermined concentration levels of the target gas. The calibration database may be stored on a non-transient state computer-readable data storage medium configured to be accessible by the processing component. The processing component may be configured to determine concentrations of the target gas based on data obtained from the calibration database. In use, for standardisation purposes and to populate the calibration database, the target gas may be provided in a known concentration in a standard gas sample to which the sampling apparatus is exposed. 28 05 25 According to a further aspect of the invention there is provided a method of detecting and sensing a target gas within a gaseous sample, the method comprising: transmitting electromagnetic radiation from an input port of a first waveguide to an output port of a second waveguide, the output port being configured to receive an output signal of the electromagnetic radiation; channelling the electromagnetic radiation via at least one elongate tunnelling element arranged within a gas sampling component connecting the first and second waveguides, the tunnelling element being dimensioned and configured to provide energy coupling between the first and second waveguides; admitting or receiving the gaseous sample into the gas sampling component; and analyzing the output signal received by the output port to detect a presence of the target gas in the gaseous sample. The method may comprise analyzing the output signal received by the output port to detect a presence and concentration of the target gas in the gaseous sample. The method may include analyzing the output signal to compare values of at least one property selected from the group consisting of tunnelling frequency, permittivity, dielectric constant, and loss tangent (Tan 5). The method may include comparing shifts in measured resonance frequencies associated with variations in at least one of these properties. The method may include engaging the gas sampling component with at least one of the waveguides, optionally in a tool-free manner. The method may include detaching the gas sampling component from at least one of the waveguides, optionally in a tool-free manner. The method may include exchanging or swapping out at least one tunnelling element in the gas sampling component, or a coating on the tunnelling element, by replacing it with a different tunnelling element or coating. The exchange may be done prior to engaging the gas sampling component with the waveguide. The method may include admitting or receiving the gaseous sample into the gas sampling component via at least one gas flow aperture defined through a side thereof. The gas flow aperture may be covered with a grating, net or gauze. The grating, net or gauze may be arranged 6 28 05 25 in a plane aligned substantially perpendicularly to a primary longitudinal axis of the tunnelling element. According to a further aspect of the invention there is provided a calibration apparatus for a detection system adapted to detect and sense a target substance within a fluid test sample, the detection system comprising first and second waveguides and a sampling component configured to receive the fluid test sample, with at least one elongate tunnelling element arranged within an internal chamber of the sampling component, the detection system being configured to measure and compare resonance frequencies of electromagnetic radiation tunnelled from the first waveguide to the second waveguide via the internal chamber; wherein the calibration apparatus comprises at least one movable calibration component arranged proximate the tunnelling element. At least a portion of the calibration component may comprise a material having a known value of permittivity. At least a portion of the calibration component may comprise a material having a known value of loss tangent (Tan 5). A plurality of calibration components may be provided. A first calibration component may have a plurality of zones, each zone having a different known value of permittivity. A second calibration component may have a plurality of zones, each zone having a different known value of loss tangent (Tan 5). Further details of the calibration component and other features of the calibration apparatus may be as described above. The target substance may optionally comprise a gas. The target substance may comprise methane. However, the calibration apparatus may also be useful for calibrating similar detection systems configured to detect target substances in fluids other than gases, such as liquids. According to a further aspect of the invention there is provided a method of calibrating a detection system adapted to detect and sense a target substance within a fluid test sample, the detection system comprising first and second waveguides and a sampling component configured to receive the fluid test sample, with at least one elongate tunnelling element arranged within an internal chamber of the sampling component, the detection system being configured to measure and compare resonance frequencies of electromagnetic radiation tunnelled from the first waveguide to the second waveguide via the internal chamber; the method comprising the steps of: positioning at least one movable calibration component in proximity to the tunnelling element; exposing the tunnelling element to a standard fluid sample to serve as a reference, the standard fluid sample containing a known concentration of the target substance; 28 05 25 moving the calibration component relatively to the tunnelling element to measure and compare a range of resonance frequencies associated with the target substance, thereby to generate reference data; and preparing a calibration database containing the reference data, for use in analysing the fluid test sample to establish a presence of the target substance. The calibration method may comprise preparing a calibration database containing the reference data, for use in analysing the fluid test sample to establish a presence and concentration of the target substance. The calibration method may comprise moving the calibration component from a first position to a second position relatively to the tunnelling element; comparing measured resonance frequencies in each of the first and second positions of the calibration component; and determining a permittivity value associated with the tunnelling element and the target substance based on the known permittivity of the calibration component material and the comparison of the measured resonance frequencies. The calibration method may include a step of rotating the calibration component relatively to the tunnelling element. It may include a step of linearly displacing the calibration component relatively to the tunnelling element. The calibration method may include positioning a first movable calibration component in proximity to the tunnelling element and moving said first calibration component, the first calibration component comprising a material having a known value of permittivity. The calibration method may include positioning a second movable calibration component in proximity to the tunnelling element and moving said second calibration component, the second calibration component comprising a material having a known value of Tan 5. Further details of the calibration component or components may be as described above. The target substance may be as described above. Embodiments and modes of performing the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: 28 05 25 Figure 1 is a schematic three-dimensional view of a portion of a gas detection system according to the invention, showing a waveguide stack or sensor box comprising two waveguides having input and output ports, and an elongate tunnelling element mounted in a detachable gas sampling component fitted to the waveguide stack; Figure 2 shows a schematic three-dimensional view and corresponding plan view of the elongate tunnelling element illustrated in Figure 1, the tunnelling element comprising a wire coated with a gas-sensitive material; Figure 3 is a schematic cross-sectional side view of the gas detection system shown in Figure 1, illustrating a squeezing effect of the tunnelling element on an electromagnetic field propagated through the system, as well as the strength and configuration of the field in various zones and components of the system; Figure 4 is a similar schematic cross-sectional side view of the disclosed system, illustrating respective distances of the input and output ports from a front conducting wall of the waveguide stack, and a distance of the tunnelling element from a back conducting wall of the system; Figure 5 is a schematic cross-sectional back view of the disclosed system, illustrating the dimensions of the two waveguides which make up the waveguide stack, as well as the length and position of the tunnelling element in the gas sampling component of the system; Figure 6 is schematic chart which illustrates how the disclosed system can be scaled to operate across a broad range of frequencies, that is, how it can be adapted for use in the GHz (e.g., microwave) region of the spectrum as well as the THz region and beyond; Figure 7 is a schematic exploded view of the disclosed system which illustrates various sections or parts of the system, including meshes or gratings over gas flow apertures in the top and bottom sides of the gas sampling component, and further illustrating how the gas detection component can be detached from the waveguide stack; Figure 8 is a schematic top view of the disclosed system, illustrating how the waveguide stack and the gas sampling component can have complementary cooperating 9 28 05 25 fastening formations such as notches, allowing them to be engaged with or detached from each other; Figure 9 is a schematic display output of a hypothetical plot, illustrating how tunnelling frequency measured by the disclosed system shifts left and right in the frequency scale with changes in concentration of methane gas in a gaseous sample; Figure 10 is a schematic display output of a hypothetical plot, illustrating how amplitude measured in dB decreases with an increase in dielectric loss of a gas sample under test; Figure 11 is a schematic cross-sectional rear view of an embodiment of the disclosed system in which multiple tunnelling wires are arranged in the gas sampling component, with each wire being configured and tuned to sense a different type of gas in a gaseous sample; Figure 12 is a hypothetical plot which illustrates, schematically, how the frequency response may be expected to vary for each of a series of different tunnelling wires as exemplified in Figure 11; Figure 13 is a schematic three-dimensional view of an alternative embodiment of the disclosed system, in which the waveguides are provided by substrate-integrated waveguides (SIWs); Figure 14 is a schematic top view of the embodiment shown in Figure 13, illustrating various components and modules of the embodiment integrated on a circuit board; Figure 15 is a schematic three-dimensional vew of a further embodiment of the disclosed system which includes a cylindrical calibration die mounted for rotation in proximity to a tunnelling wire; Figure 16 shows diagrammatic detail of the calibration die and tunnelling wire of Figure 15, illustrating how the height of the cylindrical calibration die typically matches or exceeds the vertical length of the tunnelling wire; and further how the die may be divided into a series of axially distributed sectors; 28 05 25 Figure 17 shows diagrammatic detail of alternative variants of the disclosed calibration die, comprising a calibration ribbon and a calibration bar, which may be moved linearly instead of rotationally relatively to the tunnelling wire; Figure 18 shows schematic cross-sectional top views of two exemplary calibration dice arranged in proximity to their respective tunnelling wires, illustrating how the each die may have different cylindrical sectors with different respective values of permittivity £1 and / or loss tangent (Tan 5), and illustrating further how the effect of the permittivity or loss tangent of the cylindrical sector positioned closest to the tunnelling wire will dominate over the effects of the other two illustrated sectors of the applicable die; Figure 19 is a schematic top view of a sampling apparatus adapted for calibration-free detection and sensing of a target gas, the apparatus comprising a pair of gas detection systems of the SIW type as shown in Figures 13 and 14, with both systems being printed on a same, single circuit board, and with one system serving as a test sensor module while the other serves as a reference sensor module; and Figure 20 is a schematic flowchart i 11 u strati ng\ a method of detecting and sensing a target gas within a gaseous sample. Similar reference numerals and characters denote corresponding features consistently throughout the attached drawings. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS Embodiments of the disclosed system and method are explained in greater detail in the following description. Figure 1 illustrates an exemplary embodiment of a gas detection system 100. The system may also be referred to as a sensor module. The system 100 exploits the microwave tunnelling phenomenon in order to detect gas samples under test. The operation of the system 100 accordingly relies on energy tunnelling (ET). Resonant transmission involving microwave tunneling through bent channels can be applied to inhibit transmission losses. The channels squeeze and intensify electromagnetic fields associated with radiation passing through the channels. Metallic wires embedded in narrow waveguide bends and channels demonstrate resonance behaviour at specific frequencies. The electromagnetic energy at these resonance 11 28 05 25 frequencies tunnels through the waveguide channels with low propagation losses. Under the tunnelling behavior, high-intensity electromagnetic fields are produced in the vicinity of the metallic wires. The system 100 is able to detect, sense and measure the concentration of gases (e.g. methane) in a gaseous sample. The sample may, for example, be air that is contaminated with methane. The system 100 includes at least one elongate tunnelling element 102, as well as first and second rectangular parallepiped waveguides 104,106 stacked one atop the other with a common ground 108. The waveguides 104, 106 are typically short-circuited. Together they form a waveguide stack, which may also be referred to as a sensor box 118. The system 100 is configured so that the first waveguide 104 is operatively positioned above the second waveguide 106. The first waveguide may be considered to be an operatively upper or upstream waveguide, and the second waveguide may be considered to be an operatively lower or downstream waveguide. The upper and lower waveguides 104,106 may be arranged in abutment with each other. In the embodiment shown, the upper and lower waveguides 104, 106 have the same dimensions so that they resonate at the same frequency as each other. The waveguides 104,106 are arranged generally parallel to each other so that a reflexive bend is defined between them. The bend typically typically has an angle of approximately 180e. In use, this configuration allows electromagnetic radiation propagating through the first waveguide to follow a path countercurrent to that of electromagnetic radiation propagating through the second waveguide. Each waveguide may comprise a rectangular parallelepiped enclosure defined by a top plate, side plates, a front plate, a back plate and a bottom plate. The upper and lower waveguides may together make up a single rectangular parallelepiped housing. The bottom plate of the upper waveguide may overlie the top plate of the lower waveguide, or a single shared plate may separate the upper and lower waveguides. This shared plate may be referred to as a common or intermediary plate and may serve as the common ground between the two waveguides. The waveguide stack (sensor box) 118 shown in the drawings therefore comprises a single housing bisected by the common or intermediary plate which defines the two waveguides and provides the common ground 108. The waveguides may share other plates with each other, such as their side plates and front plate. In other embodiments (not shown) the waveguide stack may be formed by two separate waveguides held in abutment with each other. In such embodiments the bottom plate of the upper waveguide may be fastened, clamped, welded, bonded, or otherwise fixed to the lower waveguide. 28 05 25 The system 100 further includes a gas sampling component 110. Typically, the gas sampling component 110 comprises a housing with a generally box-like construction, and can also be referred to as a wire-holding box. This component is connected (or releasably connectable) to the first and second waveguides 104,106 of the waveguide stack. The gas sampling component 110 may define an internal chamber which typically communicates with interior cavities of the two waveguides. Thus, the two waveguides can be connected to each other via the internal chamber of the gas sampling component 110. (Figures 3 and 4 show a broken vertical line separating the two stacked waveguides from the gas sampling component (wire-holding box). This line does not represent any physical feature but is intended to illustrate a notional plane along which the cavities of the two waveguides connect to and communicate with the cavity (internal chamber) of the wireholding box. Advantageously there is no hurdle, wall, plate, grid, mesh or other physical impediment separating the cavities of the three components from one another there, that is, the cavities of the waveguides are typically each open on one side and connect with the cavity of the wire-holding box so that electromagnetic waves can enter and exit unhindered from the wireholding box.) Referring back to Figure 1, the tunnelling element 102 is mounted in the internal chamber of the gas sampling component 110. It promotes energy coupling between the two waveguides at a pretuned frequency. The tunnelling element 102 is typically made of a conductive metal. It may comprise a thin metallic wire or rod, for example. The wire may be arranged generally perpendicularly to the top and bottom sides of the waveguides. The wire may accordingly be oriented in an operatively upright configuration within the internal chamber of the gas sampling component. The wire 102 may have a radius and a length defining a resonance frequency of electromagnetic energy tunnelling from the first waveguide cavity into the second waveguide cavity. The wire 102 is typically electrically isolated from other components of the system. During operation of the disclosed system 100, microwave tunnelling occurs in the wire-loaded internal chamber of the gas detection system 100 and is accompanied by intense electric fields and a highly-selective frequency response. The system can be tuned by changing the length and / or radius of the tunnelling wire (or multiple wires) accommodated in the internal chamber of the gas sampling component 110, depending on the wavelength of the electromagnetic energy supplied to the input port 114. For example, the length of a wire can be made equal to a halfwavelength of a desired frequency. Figure 2 shows how the thin metallic wire 102 may advantageously have a coating 203 comprising a material that is sensitive to contact with the target gas. The coating may have a 13 28 05 25 diameter dc. The wire may have a diameter dw. The coating 203 may be provided as a layer which at least partially covers the outer surface of the tunnelling element. The coating material may have properties which vary in response to contact with the target gas. The coating material may be preselected with reference to these properties. For example, the coating material may be preselected with reference to an amount by which its permittivity changes in response to contact with methane or another target gas. The coating may be characterised by having a permittivity which varies when the coating is contacted with varying concentrations of the target gas. The coating material may comprise at least one material selected from the group consisting of metallic oxides, metals, semiconductors, and nanomaterials. For example, the coating material may comprise Pt / AhOs as a catalyst on SnO2. During use, when the sensitive coating material is exposed to methane gas molecules, the resultant permittivity of the sensitive layer may change. This phenomenon can be exploited to detect methane or other types of gases depending on the material of the coating. Referring back to Figure 1, two waveguide ports 114,116 are provided for electromagnetic wave launching and receiving, respectively. Ports are used to define the planes of excitation for waveguide structures. The port 114 is an input port connected to the first or upper waveguide 104. It is configured to transmit electromagnetic radiation and is connected to a source of electromagnetic radiation such as an electromagnetic radiator. In certain embodiments, the electromagnetic radiator may be positioned in an interior cavity of the first waveguide. The input port 114 is used to excite the system 100 by introducing a microwave, THz or higher frequency signal into the first waveguide 104. In the embodiment shown, the input port 114 extends downwardly from the top of the first waveguide 104. The length of the input port is advantageously \ / 2. The port 116 is an output port connected to the second or lower waveguide 106. This is a receiving port and is configured to receive an output signal of the electromagnetic radiation. The output port 116 may accordingly comprise an electromagnetic receiver element for receiving the microwave (or other frequency) signal that tunnels along the tunnelling wire 102 from the upper waveguide into the lower waveguide after being modified by the capacitance of a gaseous sample present in the internal chamber of the gas sampling component 110. The output port may be positioned in an interior cavity of the second waveguide 106 and may extend upwardly from the bottom of the waveguide. Advantageously, the output port 116 also has a length A / 2. 28 05 25 The waveguide ports 114, 116 can be traditional coaxial waveguide launching ports with a backshort. Coaxial cable connectors (not shown) may be mounted on the plate walls of the waveguides, for connecting the ports in the waveguide cavities to exterior cabling and components. The internal chamber of the gas sampling component 110 may define a narrow channel of reduced diameter to contain an electromagnetic field associated with the electromagnetic radiation. In use of the system, propagated radiation may tunnel through the channel. The channel may define the 180e angle between the waveguides. The gas sampling component 110 and the tunnelling element may be configured to promote resonant tunnelling of the electromagnetic field between the first and second waveguides, that is, to tunnel the electromagnetic radiation from the first to the second waveguide. The tunnelling wire 102 is typically backshorted. The gas sampling component 110 may accordingly incorporate a backshort. This may be provided by a back conducting wall of the system. It may accordingly be positioned on an opposite side of the tunnelling wire 102 relative to the waveguides. The backshort may have a reflective face. It may be arranged perpendicularly to the waveguide channels so as to reflect the high-frequency signal towards the tunnelling wire. In use, the backshort may reflect the signal in the waveguides into a standing wave pattern. In selected embodiments, the backshort may comprise a tunable backshort. Gas flow apertures may be defined through sides of the gas sampling component. They may be positioned on opposite sides of the internal chamber. Optionally, they may be positioned diametrically opposite each other, as shown in Figures 1 and 7. The gas flow apertures may respectively be positioned proximate opposite ends of the elongate tunnelling element, such that a gaseous sample flowing through the internal chamber may flow in a direction generally parallel to a longitudinal axis of the tunnelling element. The gas flow apertures may each be covered with a grating, gauze, net or mesh 112 configured to allow gas to flow into the internal chamber and optionally through it. The meshes 112 may be aligned in planes which are generally perpendicular to a primary longitudinal axis of the tunnelling wire 102. Each mesh 112 may be made from conductive wires arranged perpendicularly to one another, similar to a metallic screen. Each mesh 112 must be dimensioned and configured to inhibit electromagnetic signal leakage from the internal chamber. To avoid wave leakage, the hole size or physical pitch (distance between adjacent wires) of the of the mesh 112 must be smaller than 15 28 05 25 Ao / 4 where Ao is the wavelength of the electromagnetic energy (e.g., microwave signal) applied to the input port 114. In some embodiments, the hole size may be less than one-tenth the wavelength (Ao / 10). The meshes 112 are typically arranged in the same orientations as the gas flow apertures, that is, substantially perpendicularly to the longitudinal axis of the tunnelling element 102 and parallel to the common ground plate of the waveguides. This configuration permits air or gas entering and exiting the internal chamber chamber to follow flowlines that are parallel to the longitudinal axis of the tunnelling wire. It will be appreciated, however, that the gratings could instead be arranged so that the direction of gas flow will be transverse to the longitudinal alignment of the tunnelling wire. The disclosed system 100 may be configured to hold a gaseous sample (e.g., ambient air) within the internal chamber of the gas sampling component. The gas sampling component may be configured to receive the gaseous sample into the internal chamber passively under the influence of ambient air currents, or instead as a forced gas stream. For example, the system could be configured to force the gaseous sample through the internal chamber using positive or negative pressure, e.g., by using a fan to blow air through the chamber or by using a vacuum pump to draw a partial vacuum in the chamber. In the former configuration the chamber can left open to ambient air and separated from the surrounding environment by the gratings only. Air to be tested can then flow passively into and out of the gas sampling component through the gratings. For embodiments in which a flowing gas stream is to be sampled, a gas conduit (not shown) could be connected to the top and bottom sides of the gas sampling component, separated from the chamber by the top and bottom gratings. In certain embodiments the gas conduit could be made of rubber, plastic, or other electrically nonconductive material. The system 100 may be configured to measure or calculate a property selected from the group consisting of permittivity, tunnelling frequency, dielectric constant, and loss tangent (Tan 5). The tunnelling element or coating material may have measurable properties in these categories, which may be variable depending upon differing levels of exposure of the tunnelling wire or coating material to atoms, molecules or ions of the target gas. The coating material may already have some permittivity, say 'a', and after reaction with the target gas it changes to 'b'. Changes in the permittivity of the coating material only may therefore be measured to assess the concentration of the target gas. As best seen in Figures 14 &19, the gas detection system also includes a processing component 1408, 1908 arranged to receive and analyze the output signal received by the output port and thereby detect the presence of the target gas, and optionally its concentration, in the gaseous 16 28 05 25 sample. The processing component can be configured to identify the target gas and calculate its concentration in the gaseous sample, based on variations in the output signal. The processing component may comprise a frequency detection and processing circuit connected to an electromagnetic radiation receiver of the output port. The processing component may comprise a digital signal processor (DSP) 1408, optionally connected to the receiver. The processing component may be arranged to calculate data correlated to variations in either or both of (i) tunnelling frequency as the permittivity of either or both the tunnelling element and its coating changes; and (ii) dielectric loss in the gaseous sample. The processing component may comprise a vector network analyzer. A wide range of software is available for signal processing and information extraction. The processing component of the disclosed system may comprise a data analyzer or reader such as a spectrum analyzer, vector network analyzer (VNA), or other suitable measurement device (not shown). The data analyzer may be configured to analyze frequency shifts between the transmitted and received electromagnetic energy. It may be configured to measure and analyze vector properties, e.g., phase and magnitude (amplitude) of the output signal compared to those of the input signal. During operation of the system, measurements of received electromagnetic energy can provide data for processing by the data analyzer to compute both the relative dielectric permittivity (Dk) and Tan 5 of the coated wire and / or the gaseous test sample in proximity to the coated wire. Dk represents the relative permittivity (or dielectric constant) of the gaseous sample when in proximity to the coated tunnelling wire. This is the permittivity of the sample expressed as a ratio with the electric permittivity of a vacuum. Permittivity is a property that affects the Coulomb force between two point charges in a material (the gas sample when in proximity to the coated tunnelling wire in this case), and relative permittivity is the factor by which the electric field between the charges is decreased relative to vacuum. It is a measure of the material’s ability to store electric energy in an electrical field. Dielectric loss can be described as the loss of energy that goes into heating a dielectric material in a varying electric field, and it is a function of frequency. Tan 5 refers to the loss of tangent, which is a way of measuring dielectric loss. It is a measure of the loss-rate of energy of a mode of oscillation in a dissipative system. It can also be referred to as loss tangent or dissipation factor. The disclosed system measures the shift in tunnelling frequency and corresponding reduction in amplitude of the coupled energy due to the dielectric permittivity and Tan 5 of the gaseous sample under test when in proximity to the coated tunnelling wire. Pure air and air containing gas 17 28 05 25 contaminants in varying concentrations have their own unique dielectric permittivity and Tan 5. Therefore, the type and amount of contamination can be determined by measuring Dk and Tan 5 of the air or other gaseous sample under test and comparing it to known values. The known values can be obtained by using the system to test known gases (with and without contaminants) and recording the results, or from a reference lookup table or database stored in memory and accessible by the analyzer. The amplitude of energy coupled between waveguides will depend upon the Tan 5 of the gaseous sample under test. The concentration of a target gas in a gaseous sample under test can be estimated by the value of the change in frequency (Af) and the change in Tan 5 (A Tan 5). Large Af and ATan 5 indicate a larger amount of contamination in a sample. The disclosed gas detection system may be configured to detect and measure a dielectric constant and dielectric loss of gas atoms, molecules, compounds or ions associated with the target gas or a substance other than the target gas. The system may further comprise at least one microstrip port configured to be connected to at least one of the waveguides. The system may also incorporate at least one micro-electro-mechanical system (MEMS) (not shown). Figure 3 shows a schematic cross-sectional side view of the embodiment 100 shown in Figure 1. The arrows represent an electric field 302, 304, 306 and the proximity of the arrows to one another illustrates variations in the field strength. The tunnelling element 102 establishes a narrow channel for energy coupling between the first and second waveguides. Microwave radiation propagating in the first waveguide 104 converges into a narrow channel around the wire 102, generating a strong electric field 304 around the wire 102. Thus, at the tunnelling frequency, the electric field 302 in the first waveguide 104 gets squeezed and tunnelled as it passes along or past the wire 102 into the second waveguide 106. This contributes to the intense electric field 304 established around the wire 102. In that region the concentration of electric field is extremely high by comparison with other regions in the waveguides. The tunnelling energy (or frequency) depends on the length of the tunnelling wire 102 and the permittivity of the coating material 203 in accordance with the following Equation I: f = ^-1 21^ (I) Where: 28 05 25 ft is the tunnelling frequency c is the speed of light 2 / is the total length of the tunnelling wire 102 £r is the static relative permittivity of the medium (coating material) around the wire By applying Equation I to preset the length of the wire and to select the coating material based on its static relative permittivity, the tunnelling frequency can be tuned to desired frequency ranges without changing the dimensions of the gas detection system. However, it will also be appreciated that the dimensions of the system, including the dimensions of the waveguides 104, 106 and the gas sampling component or box 110 can be be varied according to design parameters required for a desired frequency range. Figure 4 indicates representative dimensions and distances of the ports and tunnelling wire from the conducting walls of the system 100. The dimensions 402, 404 represent respective distances of the input and output ports 114,116 from a front conducting wall of the waveguide stack (sensor box) 118. The dimension 406 represents a distance of the tunnelling wire 102 from a back conducting wall of the system 100, also referred to herein as a backshort. The dimensions 402, 404, 406 are selected to transmit a field of maximum intensity in one direction. Usually, these distances (from the sides to the ports and tunneling wire) are a quarter-wavelength from the shorted ends of the waveguide from all sides to maximize field transfer. Figure 5 indicates representative dimensions 502, 504, 506 of the waveguides. The dimension 502 is the width of each waveguide and the dimension 504 is the height of each waveguide. The dimension 502 should be significantly greater than the distance 504. The dimensions of the waveguides can be selected such that only the dominant transverse electric mode (TEw mode) propagates through them. The length of the tunnelling wire is represented by dimension 508, which is less than a height 506 of the stacked waveguides. Distances 510 and 512 are the upper and lower gaps between the wire and the top and bottom sides of the waveguide stack. The distances 510 and 512 are equal. The sum of 508, 510 and 512 is equal to the height 506. Dimensions 514 and 516 are the same length, meaning that the wire is centrally arranged. The operating frequency depends on the length of the wire. Similar device arrangements can be used to design the system for terahertz (THz) or higher frequency ranges. The system size reduces with an increase in the operating frequency. 28 05 25 To inhibit radiation losses, the wall plates of the waveguides are typically made of a conducting material. They may be metallic. For example, they may be formed of aluminium or other electrically conductive metal. Figure 6 is a schematic chart 600 which illustrates the relationship between the size of the system and the operating frequency. The chart illustrates, schematically, how a required operating frequency of a given gas detection system can be increased by reducing the height of the waveguides and the length of the tunnelling wire. The vertical axis 602 of the main chart represents the size, in millimetres, of one waveguide (dimension 504 in Figure 5) of an embodiment provided by way of example only, while the horizontal axis 604 represents the operating frequency in GHz. The curve of the plot 606 reduces generally exponentially as the operating frequency increases. As the overall size of the system decreases, the length 508 of the thin metallic wire 102 also decreases. When designing systems for THz applications, the sample size must be smaller, so THz devices can be more sensitive. The inset diagram in the chart shows a cross-section of one of the two waveguides making up a typical waveguide stack. To visualize the decrease in the size, only the upper waveguide is shown. It has a height 504 which is equal to half of 506 shown in Figure 5. The vertical bar in the inset diagram represents an upper portion of the tunnelling wire 102 protruding into the waveguide, with this portion having a height or length equal to half of dimension 508 shown in Figure 5. As best seen in Figures 5, 7 and 11, the internal chamber of the gas sampling component 110 may be partitioned by a dividing plate 702, preferably arranged in the same plane as the wall separating the two waveguides This configuration effectively extends the lengths of the two waveguides to the backshort behind the tunnelling wire when the gas sampling component 110 is connected to the waveguides 104,106. Figure 7 illustrates, for example, how the dividing plate 702 divides upper and lower halves of the internal chamber of the gas sampling component 110 from each other. The dividing plate 702 defines an orifice, hole or aperture 704 through which the elongate element 102 extends. This configuration therefore differs from that shown in Figure 1, which has no dividing plate. In Figure 1, the area around the wire 102 is open and there are chances that some E-Fields will pass through the gap. The arrangement with the dividing plate 702 provides better isolation between 20 28 05 25 the two waveguides and ensures that most or all energy is coupled through only the tunnelling wire 102. The tunnelling wire 102 is typically electrically isolated from the dividing plate 702. Thus, the orifice 704 is sized so that the tunnelling wire 102 does not make contact with the dividing plate 702. The orifice 704 can be left open and unobstructed to permit a free flow of the gaseous sample through the internal chamber, from the entry gas flow aperture to the exit gas flow aperture of the gas sampling component. The metallic wire 102 may extend approximately equal distances into the respective halves of the internal chamber on either side of the dividing plate 702 in the gas sampling component 110. Figure 7 also illustrates how, in certain embodiments, the system 100 can be configured so that the waveguide stack 118 (and associated electronic components) can be releasably engaged with, and detached from, the gas sampling component or wire-holding box 110. This facilitates cleaning of the system components and protection of the electronic components. It also gives users access to the box 110 so that the tunnelling wires (or optionally only their respective coatings) can be swapped out. The system can be configured so that detachment of the wireholding box 110 from the waveguide stack (sensor box) 118 can be performed in a tool-free manner. For example, these components may be equipped with complementary cooperating fastening formations such as notch locks, tongue-and-groove formations, manually releasable snap-locking clips, or other suitable temporary retainers or fasteners. Figure 8 shows a top view of an embodiment 800 of this type, configured to permit releasable engagement and detachment of the wire-holding box 110 from the sensor box 118. As can be seen, a set of flexible, resilient notch locks 802 are provided on either side of the gas sampling component 110. These allow the gas sampling component to be releasably locked to the waveguide stack during use. Detachment of the two components can be done by sliding or unclipping the respective tongue formations of the notch locks out of their cooperating notches or grooves. Figure 9 illustrates a normalized frequency response for a hypothetical set of measurements taken during operation of the disclosed system 100. The display output 900 represents the amplitude responses of the transmitted signal when methane is present or absent from a test sample. At tunnelling frequency, the thin metallic wire 102 provides full-wave coupling. As discussed earlier, the tunnelling frequency mainly depends on the length of the tunnelling wire and the permittivity of the coating material surrounding the wire. If the permittivity of the material around the metallic wire changes, the tunnelling frequency shifts to lower or higher values over the frequency scale, depending on the type, number and concentration of gases mixed in with the 21 28 05 25 air. The plot 902 is the response at £m (f), which is the permittivity of air as a function of frequency. Thus, 902 represents the normalized amplitude response of the system when the wire-loaded gas sampling component 110 is filled with uncontaminated air only. The plot 904 is the response at £m+ (f) = £m (f)+5(f), and the plot 906 is the response at £m-(f)= £m (f)-5(f). These plots represent the frequency responses when methane gas molecules are present in the sampled air. The 5(f) is a change in tunnelling frequency due to the methane gas molecules in the air. The horizontal axis 908 represents the normalized frequency scale and the vertical axis 910 represents the amplitude in dB. The permittivity of the coated material changes as a result of its reaction with the gas sample under test. The system will sense the resultant change in the permittivity of the coated material. A relatively small change in the relative permittivity (£r) of the material (wire coating and gaseous sample) results in a comparatively large frequency shift in the response curve of the tunnelling frequency of the electromagnetic energy received at the output port, relatively to the frequency of the electromagnetic energy transmitted into the input port 114. Figure 10 illustrates the effect of dielectric loss on amplitude response for a hypothetical set of measurements. The amplitude of energy coupled between waveguides depends upon the Tan 5 of the gaseous sample under test. Tan 5 can therefore also be used detect the presence of gas molecules in the air. The display output 1000 shown in Figure 10 represents the amplitude responses of the transmitted signal when methane is either present or absent from a test sample. The amplitude 1002 represents the response in uncontaminated air. The amplitude 1004 is the value at Tan bi (f) and the amplitude 1006 is the value at Tan b2 (f), where Tan bi (f) is less than Tan b2 (f). The peak amplitude therefore decreases as the value of Tan 5 increases. The horizontal axis 1008 in Figure 10 represents the normalized frequency scale while the vertical axis 1010 represents the amplitude in dB. In summary, there is typically a large shift in tunnelling frequency with a small change in the relative permittivity of the gas in contact with the coating on the wire. There is also a marked reduction of amplitude with an increase in Tan 5. Together, these large variations enable the disclosed gas detection system to be highly sensitive. The disclosed system may include a plurality of tunnelling wires, each being adapted to sense a different target gas. Each tunnelling element may have a length and resonance frequency different to the others. Each may have a coating material different than the others. Figure 11 shows part of an embodiment 1100 of this type. It includes a plurality of tunnelling elements 1102, 1104,1106,1108 held within a gas sampling component 1110. This variant can 22 28 05 25 be referrred to as a multi-resonance device. The tunnelling elements comprise thin metallic wires or rods mounted within the internal chamber of the gas sampling component 110. Each tunnelling wire has a different length and / or radius and a unique resonance frequency. Each can also have a different coating material. Each of these different materials can be preselected for its ability to detect and measure the concentration of a different type of gas under test. The combination of multiple wires can therefore be used to detect a variety of different types of gases simultaneously. As mentioned previously, the gas sampling component 110 can be configured so that it is detachable from the waveguide stack (sensor box) 118. This configuration allows the tunnelling wires or their coatings to be exchanged or swapped out, which in turn enables the system to be used for detecting multiple different sets of target gases. The metallic wires 1102, 1104, 1106, 1108 (or in some cases just their coatings) may accordingly be configured to be removable from the gas sampling component so that they can be replaced with others. Figure 12 illustrates a normalized frequency response of an exemplary embodiment of the multiresonance device 1100, based on a hypothetical set of measurements. The display output 1200 shows a plot of N different curves 1202, which can be used to detect different types of gas species depending on the coating of the sensitive material. If N is the number of wires with different lengths, then N curves are received. On account of its size, the first variant 100 of the gas detection system described above (the device shown in Figure 1) is preferred for sampling in larger locations with more room for installation. The rectangular waveguide topology offers excellent immunity against radiation losses and presents low insertion losses. However, rectangular waveguides in their classical form are not compatible with the miniaturization required by modern applications. To address this shortcoming, alternative embodiments of the disclosed system are provided wherein at least one of the waveguides comprises a substrate integrated waveguide (SIW). Such embodiments may comprise first and second planar SIWs. The SIWs may have circuits with a common ground. A SIW is composed of a thin dielectric substrate covered on both faces by a metallic layer. The substrate embeds two parallel rows of metallic via holes delimiting the wave propagation area. An SIW can have reduced height compared to the standard 2:1 width:height ratio of classical waveguides. SIWs present a platform for integrating all the components of a microwave circuit inside a single substrate with a rectangular cross-section. Using a single substrate guarantees a limited volume and a simplicity of manufacture, while the rectangular cross-section of the line provides the advantages of the waveguide topology in terms of losses. Figure 13 illustrates an exemplary embodiment 1300 of such a system. This variant is smaller than the system 100 and is more suitable for operation in locations with limited space available 23 28 05 25 for installation, or where integration with planar microcircutry is a required design parameter. For example, the variant 1300 is better configured than the first variant 100 for integration of the wave launching component into planar electrical circuits. The variant 1300 is compatible with standard CMOS integrated circuits. The variant 1300 is an SIW-based embodiment. Thus, the most notable difference from the classical waveguide-based variant 100 is that the 180e shifted rectangular waveguides are replaced by two SIWs stacked one atop the other, with a common ground 1302 arranged between them. Waveguide ports 1304 and 1306 are connected to a microwave source and receiver, respectively (not shown). Reference numeral 1308 indicates metallic vias delimiting the wave propagation area. The SIWs are coupled through a thin metallic wire. The substrate in the SIW-based device can be provided by air. Figure 14 is system-level conceptual diagram showing selected components of an SIW-based embodiment 1400 in plan view. Reference numeral 1402 indicates a front portion of an SIW stack comprising a pair of commonly grounded SIWs. Reference numeral 1404 indicates a radiofrequency (RF) electronic circuitry module configured for generating the source electromagnetic waves. Reference numeral 1406 indicates a power module configured to provide power to the RF module 1404. Reference numeral 1408 indicates a processing component comprising a DSP module and reference numeral 1410 indicates a display module. All components may be integrated along the same general plane. The SIW embodiment 1300 can be reconfigured to operate in higher frequency ranges such as the THz (or higher) frequency range. Calibrating the sensor module of the gas detection system is essential for accuracy. The disclosed system may accordingly include a calibration component, which may comprise a calibration die. The calibration die may comprise at least one material having a known permittivity and / or or a known value of loss tangent (Tan 5). The calibration die may have a plurality of different regions, segments, sectors or zones. Each such zone may, respectively, have a different known value of permittivity or a different known value of loss tangent (Tan 5) (or both). These different zones may be applied for sensing and calibrating the system in respect of a plurality of different target gases. The calibration die may have various shapes. Without limiting the generality of possible shapes, the die may be shaped as a cylinder, ribbon, bar, beam, disk, or sheet. In certain embodiments the calibration component comprises a cylindrical die mounted for rotation relatively to the tunnelling element. Supplementary to or instead of the cylindrical die, a calibration die comprising a ribbon may be used. In such embodiments, the die may be configured to be displaced linearly along a vector or arc relatively to the tunnelling wire. It may be configured to be displaced operatively vertically, i.e., up and down relatively to the tunnelling wire or wires. 28 05 25 The calibration die is typically positioned in close proximity to the tunnelling wire, inside the wireholding box, so that it is exposed to the gaseous test sample along with the tunnelling wire. The calibration die is typically mounted so that it extends through a wall of the waveguide stack, projecting to the exterior of the stack, thereby permitting it to be rotated or otherwise moved so that its different zones can be brought into proximity with the wire. Whenever a dielectric medium is brought into proximity with a tunnelling wire, the tunnelling energy shifts left or right in the frequency spectrum depending on the material of the dielectric medium. This is the principle upon which the calibration method is based. The effect of the rotation or linear displacement of the calibration die is to shift the measured resonance frequency left or right in the scale, depending on the permittivity values and loss tangents of the tunnelling element and of the gas in the internal chamber of the gas sampling component. Figure 15 illustrates a further SIW-based embodiment 1500 which is configured to permit calibration. The embodiment 1500 includes an SIW stack 1502. A calibration component is provided which comprises a cylindrical, pre-calculated calibration die 1504 rotatably mounted in the SIW stack 1502. The calibration die 1504 may be used to calibrate the system 1500. The die 1504 can be used to accurately detect the concentration of the methane or other target gas. The die permits the system 1500 to work for different types of sensing. Different gases react with different materials in different unique ways. If the calibration die 1504 incorporates a collection of different materials (optionally in the form of coatings) which can react with different gases, the system can detect multiple gases depending on their unique reactions with the different component materials. For embodiments of the disclosed system which incorporate calibration dies, it is not necessary that the tunnelling wire or wires should be coated, i.e., the wire or wires can be bare metallic wires. This is because the coatings with different permittivities (or sensitivity to different gases) are provided as part of the different zones of the calibration die and therefore do not need to be provided on the wire or wires as well. Figure 16 shows detail of the cylindrical calibration die 1504 and its positioning and distance from a metallic tunnelling wire 102. The calibration die 1504 is positioned at a distance 1606 from the wire 102. The distance 1606 is kept as small as practically possible and cannot be more than A / 4. The die 1504 is has a series of sectors or zones 1606,1608,1610,1612,1614,1616,1618,1620 arranged around its axis, each sector having a different permittivity. The values of the permittivities are preselected and predefined. The length of the cylindrical calibration die should 28 05 25 at least match the length of the tunnelling element with which it is associated, and typically exceeds it. The calibration die 1504 may be used for the detection of different gases. If materials with various permittivities are selected for the zones 1606, 1608, 1610,1612,1614,1616, 1618,1620, each material being suitable for reacting with a different gas, then a single sensor module can be used to detect multiple different gases. The change in the material dielectric constant close to the wire is sensed. By rotating the die in either direction, different types of gases can be detected using the same sensor module. Figure 17 illustrates how a flexible calibration ribbon 1700A orcalibration bar 1700B can be used instead of the cylindrical calibration die 1504. The ribbon 1700A comprises a series of segments 1703 of different permittivities. The length of each segment 1703 equals the length of the metallic wire 102. For calibration, the ribbon 1700A or bar 1700B can be moved up ordown in the direction 1705. The calibration ribbon or bar can be used to detect the unknown permittivity of a gas sample under test. Figure 18 illustrates the working of a calibration die 1504a and a calibration die 1504b. The die 1504a has three axially distributed sectors or zones 1706, 1708, 1710, each having a different permittivity. The effect of the permittivity of sector 1706 will dominate the impacts of the permittivities of the two other sectors 1708,1710 because the sector 1706 is positioned closer to the tunnelling wire 102. When the die 1504a is rotated in a clockwise or anticlockwise direction, the tunnelling frequency will shift left or right in the frequency scale depending on the permittivity value of the wire 102. This phenomenon can be used for calibration and the concentration of the methane or other gas under test can be determined. Similarly, the die 1504b has three axially distributed sectors or zones 1712, 1714, 1716. In this case each sector has a different value of Tan 5. This die can be used to increase the accuracy of the disclosed system. When two parameters, permittivity and Tan 5, of a sample can be detected, the readings can be cross-verified and hence more accuracy can be achieved in measuring the concentration of the gas. For example, if two sample materials or concentrations have very similar permittivities, they may nevertheless differ more substantially in their values of Tan 5, allowing better distinctions to be drawn. The calibration principles described above may be extended to calibrate detection systems designed to sample fluids other than gases. Thus, the present invention also provides a calibration apparatus for generalised detection systems (not restricted to gas detection) which have similar components to those described for the disclosed gas detection system. Such systems may, for 26 28 05 25 example, incorporate waveguides, one or more tunnelling wires, etc., but could be intended to sample liquids instead of gases. Using the principles and components discussed in this specification, a calibration apparatus could, for example, be constructed to calibrate a fuel quality sensor such as that taught by US10281423B1. Such a calibration apparatus could comprise at least one movable calibration component arranged proximate a tunnelling element of the applicable detection system. Other features of the calibration component and its use for such applications may be as described elsewhere herein. The present invention also provides a sampling apparatus adapted for calibration-free detection and sensing of a target gas in a gaseous test sample. This apparatus can comprise a plurality of cooperating gas detection systems or sensor modules as described herein. At least one of the gas detection systems may be configured as a test sensor module capable of sensing test data relating to the target gas in the test sample. At least one other of the gas detection systems may be configured as a reference sensor module capable of sensing reference data According to a further aspect of the invention there is provided a sampling apparatus adapted for calibration-free detection and sensing of a target gas in a gaseous test sample, the apparatus comprising a plurality of cooperating gas detection systems as described above. At least one of the gas detection systems may provide a test sensor module configured to sense test data relating to the target gas in the test sample. At least one other of the plurality of gas detection systems may provide a reference sensor module configured to sense reference data relating to the target gas in the test sample. The reference sensor module may include one or more movable calibration components as described above. The output component may be arranged to provide the test data and reference data to the user. The processing component may comprise a digital signal processor (DSP). The DSP may be arranged to receive and compare the test data and the reference data. The processing component may be configured to permit comparison of the test and reference data, e.g., by way of a display of the data in visual form, for example by presentation of plots or curves which may overlap each other and be shifted relatively to each other to facilitate comparison and matching as the calibration component is rotated or otherwise moved. In use, comparison of the test and reference data may be implemented to evaluate one or more conditions relating to the target gas in the test sample. The processing component may, for example, be arranged to establish a presence or absence of the target gas in the test sample or its concentration therein, or both, based on the comparison of data. Typically in use, the gas sampling component of the test sensor module and of the reference sensor module are exposed to the same gas sample. However, it will be appreciated that the 27 28 05 25 apparatus may be configured to receive different gas samples into the gas sampling component of the test sensor module and of the reference sensor module, respectively. For example, the sampling apparatus may be configured to receive the gaseous test sample into the gas sampling component of the test sensor module for detection and sensing of the target gas therein, and to receive a standard gas sample into the gas sampling component of the reference sensor module, for accumulation of data corresponding to the known concentration of the target in the standard gas sample. The sampling apparatus may share components between the two gas detection systems, that is, between the test and reference sensor modules. Thus, the processing components of the test and reference sensor modules may be integrated as a single, common or shared processor. Similarly, the output components of the two modules may be integrated as a single, shared output component. At least a portion of each of the test and reference sensor modules may be integrated into a same, single circuit board. The circuit board may be printed. Figure 19 illustrates one embodiment 1900 of the type of sampling apparatus discussed above, referred to as a dual sensor. Dual sensors may be suitable for providing calibration-free detection and sensing of target gases. The dual sensor 1900 includes two separate gas detection systems of the SIW type implemented on a same, single circuit board 1902. A first SIW-based system 1904 serves as test sensor module and is used to measure the sample under the test, and a second SIW-based system 1906 is used as a reference sensor module. Each sensor module incorporates a tunnelling wire 102a, 102b. The reference sensor module 1906 is equipped with a rotatable cylindrical calibration die 1504 mounted in proximity to its tunnelling wire 102b. With the help of the calibration die 1504, which may be configured as a knob, the reference sensor can be tuned and hence the permittivity of the sample under test can be checked from the value of the permittivity from the calibration die. In the presence of the target gas, both sensors will resonate at the same frequency once tuned, which can provide calibration free measurements. As discussed further below, by comparing the output of two sensor modules it is possible to accurately measure quantities of gaseous atoms, molecules, compounds or ions in an air sample, that is, their concentration. A standard gas sample can be used for reference; however, during normal operation both sensor modules 1904, 1906 are typically exposed to the gaseous test sample. With the arrangement shown in Figure 19, the value of permittivity and Tan 5 can be measured and calculated, representing a signature of a particular quantity of gas atoms, molecules, compounds or ions in the air. The tunnelling frequencies of the two sensor modules are compared in a DSP module forming part of circuitry 1908 shared by the two modules. The calibration die 28 28 05 25 1504 can be rotated in a clockwise or anticlockwise direction to match the tunnelling frequencies exactly. When both the frequencies match (i.e., when the measured frequency curves overlap), the permittivity and the value of Tan 5 may be determined through the position of the die facing the tunnelling wire. Figure 20 illustrates the main steps needed to perform a method 2000 of detecting and sensing a target gas within a gaseous sample. In a first step 2001, electromagnetic radiation is transmitted from an input port of a first waveguide to an output port of a second waveguide. The output port is configured to receive an output signal of the electromagnetic radiation. In a second step 2002, the electromagnetic radiation is channelled via (through and / or past) at least one elongate tunnelling element arranged within a gas sampling component connecting the first and second waveguides. The tunnelling element is dimensioned and configured to provide energy coupling between the first and second waveguides. In a third step 2003, the gaseous sample is admitted or received into the gas sampling component. In a fourth step 2004, the output signal received by the output port is analyzed to detect a presence, and optionally concentration, of the target gas in the gaseous sample. It will be appreciated that the steps of this method may be performed in a different order than the order listed above, e.g., the gaseous sample may be received into the gas sampling component before, after or together with the steps of transmitting and channelling the electromagnetic radiation. The steps of the method 2000 can be performed using embodiments 100, 1300, 1400, 1500, 1900 of the disclosed gas detection system. In certain modes of carrying out the method 2000, electromagnetic energy having a first wavelength Ao can be transmitted into the interior cavity of a first waveguide through its input port using a conventional RF / microwave generator 1404. Electromagnetic energy at a second wavelength At then tunnels into and radiates inside the interior cavity of the second waveguide through the tunnelling wire and is received through the output port. A tunable microwave signal or wideband time domain pulse can be applied to the input port via an input connector. Accordingly, in certain modes of carrying out the method 2000, the input port may be excited with a wideband time domain pulse. The response may be detected at the output port using a processing component, which may comprise a vector network analyzer. A Fourier transform of the received signal can be taken to find the frequency response H(f) of the received output signal. The spectral location of the H(f) peak can then be located. Three dB points can be detected and a Q-factor can be determined using a standard table or curve. The three dB and the 28 05 25 Q-factor can be used to extract Tan 5. The peak frequency can be detected and mapped using a standard table or curve. The peak frequency can be used to extract relative permittivity (£r). If a target gas is not present in the gas sampling component, the output signal will show no change from the tunnelling frequency of uncontaminated air. If a target gas is present in the gaseous sample, a change in the relative permittivity of the wire coating and in the sample’s dielectric constant will result in a change in the tunnelling frequency and / or Tan 5 at the receiver. By observing the frequency shift and amplitude of the received electromagnetic energy compared to the transmitted electromagnetic energy, the dielectric permittivity and Tan 5 of the gas sample in contact with the wire or its coating can be determined. By comparing the dielectric permittivity and Tan 5 of the gaseous sample to known values for air contaminated with different gases in different quantities, the amount and type of gas contamination can be determined. Details of the configuration of the gas detection system which can be used to implement the mettod 2000 may be as previously described. The tunnelling element and other components of the system may be configured such that the energy coupling is provided at a pre-tuned frequency. The method 2000 may include analyzing the output signal to compare values of at least one property selected from the group consisting of tunnelling frequency, permittivity, dielectric constant, and loss tangent (Tan 5). The method may include comparing shifts in measured resonance frequencies associated with variations in at least one of these properties. The method 2000 may include releasably engaging the gas sampling component with at least one of the waveguides, optionally in a tool-free manner. The method 2000 may include detaching the gas sampling component from at least one of the waveguides, optionally in a tool-free manner. The method may include exchanging or swapping out at least one tunnelling element in the gas sampling component by replacing it with a different tunnelling element. The exchange may be done before the sampling chamber is engaged with the waveguide or waveguides of the waveguide stack. The method may include admitting or receiving the gaseous sample into the gas sampling component via at least one aperture defined through a side thereof. The aperture may be covered with a grating, net or gauze. The grating, net or gauze may be arranged in a plane aligned substantially perpendicularly to a primary longitudinal axis of the tunnelling element. The method may include a step of causing the gaseous sample to flow through an internal chamber of the gas sampling component. 28 05 25 The invention also povides a method of calibrating a generalised detection system adapted to detect and sense a target substance within any suitable test sample comprising a fluid (not necessarily only in gas form). Thus, this calibration method can be applied outside the field of gas sampling. The disclosed calibration method can be used to calibrate any suitable detection system having the following features: • first and second waveguides; • a sampling component configured to receive the fluid test sample; • at least one elongate tunnelling element arranged within an internal chamber of the sampling component; • the detection system being configured to measure and compare resonance frequencies of electromagnetic radiation tunnelled from the first waveguide to the second waveguide via the internal chamber. The calibration method comprises the steps of: positioning a movable calibration component in proximity to the tunnelling element; exposing the tunnelling element to a standard fluid sample to serve as a reference, the standard fluid sample containing a known concentration of the target substance; moving the calibration component relatively to the tunnelling element to measure and compare a range of resonance frequencies associated with the target substance, thereby to generate reference data; and preparing a calibration database containing the reference data, for use in analysing the fluid test sample for the presence, and optionally concentration, of the target substance in other fluid samples, e.g., the test sample referred to above. Further details of the calibration component and other features of this calibration apparatus may be as described above. For example, the calibration component may comprise a calibration die shaped as a cylinder, ribbon, bar, beam, disk, or sheet. The calibration die may have a plurality of different or regions, segments, sectors or zones. In use, the different zones may be applied to detect, sense and measure a plurality of target substances present in the sampling component. Each zone of the calibration die may be configured to be predominantly sensitive to the presence of a different, respective gas or other substance, for example. The system may comprise a plurality of calibration components. The target substance may optionally comprise a gas such as methane. As mentioned, however, the calibration apparatus may also be used for calibrating detection systems which are configured to detect target substances in fluids other than gases, such as liquids. The calibration apparatus 28 05 25 may accordingly be useful for calibrating systems configured to detect substances in non-gaseous physical states. The calibration method may comprise moving the calibration component from a first position to a second position relatively to the tunnelling element; comparing measured resonance frequencies in each of the first and second positions of the calibration component; and determining a permittivity value associated with the tunnelling element and the target substance based on the known permittivity of the calibration component material and the comparison of the measured resonance frequencies. The calibration method may include a step of rotating the calibration component relatively to the tunnelling element. It may include a step of linearly displacing the calibration component relatively to the tunnelling element. In the context of gas sensing, the disclosed calibration method can be carried out for various purposes: 1. For example, one goal could be to tune a gas detection system (“sensor”) for the detection of different gases. This calibration method could be applied when using the SIW-based embodiment 1500 shown in Figure 15, for example. 2. A second goal could be to calibrate a dual sensor for the evaluation of the concentration of a single gas. Example: Referring to the dual sensor 1900 shown in Figure 19, assume that the tunnelling wire 102a in the top SIW sensor 1904 (main or test sensor module) is coated with a single material sensitive to a single gas (e.g., methane), and the calibration die 1504 in the bottom SIW sensor 1906 (reference sensor module) has zones with different permittivities (£1, £2, £3, etc.). Both the top and bottom SIW sensors can be exposed to the same gaseous test sample. At a given low concentration of the target gas in the sample, the test sensor 1604 may, for example, resonate at a frequency of 2.4 GHz. If the calibration die 1504 is then rotated, the resonant frequency of the reference sensor 1906 will shift left and right in the frequency scale, for example from 2.2 GHz to 2.6 GHz. If the outputs of both sensors are superimposed, then at the point where the output curves overlap while the calibration die 1504 is being rotated clockwise or counterclockwise, the permittivity can checked by looking at or sensing the permittivity value of the part of the calibration die that is in front of the tunnelling wire in the reference sensor 1906 at that point. 28 05 25 For all the values of £1, £2, £3, etc., a concentration chart or database can be kept, where the concentration of the target gas can be checked. The circuitry of the dual sensor 1900 may incorporate a digital signal processor (DSP) which can look up the concentration based on permittivity. For example, £2 may correspond to 20% of the target gas in the atmosphere, while £3 may correspond to 40% of the target gas in the atmosphere. From the rotation of the calibration die 1504 and by checking where the frequency curves overlap, it is thus possible to determine the concentration of the gas. Mechanisms of Operation Resonant transmission occurs with microwave tunneling through extremely squeezed or bent channels. The operation of wire-based electron tunnelling waveguide structures is based on impedance-matched resonance mechanism over the frequencies at which the embedded wire or wires become a half-wavelength long. The wire tunnels substantially all of the energy that enters the waveguide as transverse electromagnetic waves. Electric field distributions in and around the tunnelling wire shed light on the mechanism of tunnelling and the effect of dielectric changes in the high-field regions. Electric fields may be enhanced along the resonant channel, in particular at the wire edges, because of higher charge accumulation. For a thinner-diameter wire, more charges are accumulated on its smaller surface area to preserve the charge conservation. Consequently, much-augmented field distributions are observed compared with a thicker wire. A narrower tunnelling path supports more intense fields. A larger resonance shift is typically noted for thinner wires. Zarifi et al. (2017) have proposed that changes in permittivity may be due to a cumulative effect of increased temperature (heat of adsorption) and occupied pores of the adsorbents. For present purposes the methane or other target gas could be considered to be an adsorbent occupying pores of the wire coating. The gas detection system described herein is distinguishable from existing technologies in several important ways. For example, previously described detection systems based on microwave resonators have relied upon ring and patch resonators which are significantly less sensitive than the disclosed gas detection system. Furthermore, the presently disclosed system differs from the device of US10281423B1 insofar as its structural and configurational aspects are concerned, and also because the present system is intended to be used for gas detection rather than the testing of liquid fuel, requiring critical design changes. US10281423B1 does not teach that the metallic wire can be coated, nor that the 33 28 05 25 material from which it is made should be selected based on changes in permittivity (and resonance frequency) in the presence of different target gases. The presently disclosed system incorporates a gas sampling component (the wire-holding box) which can be detached from the sensor module comprising the waveguides, and some embodiments allow for this to be done in a tool-free manner. The detachability of the gas sampling component provides an important benefit because it allows the tunnelling wire or wires, and the nano-material coating, to be changed out more easily. This allows the system to be used for detecting, sensing and measuring a variety of target gases, not only methane. The ability to swop out one or more tunnelling wires coated with different types of materials (adaped to detect different gases), therefore makes the present system versatile and adaptable. The fuel quality sensor of US10281423B1 does not have a detachable gas sampling component. Moreover, a fuel conduit passes through an upper waveguide adjacent the thin wire and between the input port and the thin wire. By contrast, in the presently disclosed gas detection system, the internal chamber of the gas sampling component contains the tunnelling element (or elements). The sample position during operation is therefore entirely different. In the present system it is at the area of maximum field intensity where squeezing takes place due to the tunnelling effect. Air or gas can flow through the gas sampling component in close proximity to the coated tunnelling wire (or wires), and there is no physical barrier preventing adsorption of the gas onto the wire or its coating. The configuration of the gas sampling component and the positioning and orientation of the tunnelling element or elements therefore promote sensitivity of the disclosed system. The geometry, configuration and dimensions of the grating or net holes which allow gas into the internal chamber of the gas sampling component are entirely different from the configuration and hole-dimensions of the gauzes in the fuel conduit of the fuel quality sensor disclosed in US10281423B1. Moreover, the positioning of the gratings and the planes in which they lie are distinctly different. In the device of US10281423B1 the fuel conduit is arranged so that fuel flows in a direction transverse to the alignment of the thin wire, and to one side of it. By contrast, in the presently disclosed system the gratings or nets are arranged so that air can flow substantially parallel to the longitudinal axis of the tunnelling element and along its exterior surface. There is no discussion in US10281423B1 of any possibility to use SIWs in the propagative structure. The SIW-based embodiments of the presently disclosed system provide important benefits because they define a flatter (more planar) profile than conventional waveguide stacks, 28 05 25 and are therefore suited to integration into standard PCBs and CMOS integrated circuit processes. The wave launching mechanism of US10281423B1 is also different, and the ports have a different configuration than those of the presently disclosed system. Embodiments of the presently disclosed system can measure the dielectric constant and dielectric loss of methane gas molecules in gaseous samples. Depending on the type of wire and coating material used, the system can also be used for sensing other gases. The system is also highly tunable and can be tuned to a desired frequency by varying the length of the tunnelling wire. As discussed, the presently disclosed invention also extends to a calibration technique which provides further accuracy to the detection system. The innovation disclosed herein is highly scalable and suitable for high-resolution sensing. The system is suitable for GHz, THz, and higher frequency dielectric detection applications. The disclosed gas detection system and method may be suitable for detecting the presence of small amounts of alkanes. The wire coating may be preselected based on a high degree of sensitivity to the presence of alkanes selected from the group consisting of methane, ethane, propane, and butane gases, amongst others. The ability of conventional microwave sensors to resolve smaller-than-wavelength samples is limited because the size of such resonators is comparable to the operating wavelength. The spatial resolution of microwave sensors can be increased by using tunnelling (ET) structures, such as that used for the presently disclosed system. In these structures, waves are guided through much narrower (than wavelength) channels that are impedance-matched, leading to resonant field enhancement. The confinement of the electric field into the narrow resonant channels supports highly sensitive detection and sensing of test samples that are positioned in the channels. The disclosed system can also provide cost effective, contactless operational capability. It can serve either as a portable or a fixed-installation instrument for detecting, sensing and monitoring for the presence of multiple gases, including methane. It may be configured to provide remote wireless sensing in industrial and off-the shelf applications. Some of the disclosed embodiments allow for calibration without the need to duplicate components such as waveguides. Overall, the disclosed gas detection system may be expected to provide a desirable level of sensitivity to methane and other gases in sampled atmospheres, resistance to fouling, long service life, a simple manufacturing process and a good detection range. 28 05 25 The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention. Finally, throughout the specification and claims, unless the context requires otherwise: • “comprise” or variations such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers; • the term “wire” will be understood broadly to include reference to a wire, rod, pin, probe or similar elongate element, whether flexible or inflexible; and • the term “via”, when used in relation to the tunnelling of radiation via a tunnelling element, is intended to be interpreted broadly and will be understood to include channelling the radiation along or past the tunnelling element. REFERENCES Cismaru et al. (2016): Cismaru, A., Aldrigo,M., Radoi, A., and Dragoman,M. (2016). Carbon nanotube based electromagnetic band gap resonator for CH4 gas detection. J. Appl. Phys. 119, 1801-1804. doi: 10.1063 / 1.4944708 Hallil efal. (2009): Hallil, H., Menini, P., and Aubert, H. (2009). Novel Microwave Gas Sensor using Dielectric Resonator With SnO2 Sensitive Layer. Procedia Chemistry, Volume 1, Issue 1, 2009. Pages 935-938. ISSN 1876-6196. https: / / doi.Org / 10.1016 / j.proche.2009.07.233 Ramzan et al. (2019): R. Ramzan, O. Siddiqui, M. Omar and O. M. Ramahi, Energy Tunnelling: A Way to Achieve Highly Sensitive Material Detection With Sub-Wavelength Resolution in IEEE Microwave Magazine, vol. 20, no. 11, pp. 32-48, Nov. 2019, doi: 10.1109 / MMM.2019.2935390. Siddiqui et al. (2015): O. Siddiqui, M. Kashanianfard and O. Ramahi, Dielectric sensors based on electromagnetic energy tunnelling. Sensors, vol. 15, no. 4, pp. 7844-7856, 2015. Zarifi et al. (2017): Zarifi, M. H., Shariaty, P., Hashisho, Z., and Daneshmand, M. (2017). A noncontact microwave sensor for monitoring the interaction of zeolite 13X with CO2and CH4 in gaseous streams. Sens. Actuat. B Chem. 238, 1240-1247. doi:0.1016 / j.snb.2016.09.047 28 05 25

Claims

28 05 251. A gas detection system for detecting and sensing a target gas within a gaseous sample, the system comprising:first and second waveguides;an input port connected to the first waveguide and configured to transmit electromagnetic radiation;an output port connected to the second waveguide and configured to receive an output signal of the electromagnetic radiation;a gas sampling component connected or connectable to the first and second waveguides;at least one elongate tunnelling element arranged within the gas sampling component, the tunnelling element being configured to provide energy coupling between the first and second waveguides;a processing component arranged to analyze the output signal received by the output port to detect a presence of the target gas in the gaseous sample; andan output component arranged to provide results of the analysis to a user.

2. The gas detection system according to claim 1, wherein the processing component is arranged to analyze the output signal received by the output port to detect a presence and concentration of the target gas in the gaseous sample.

3. The gas detection system according to claim 1 or claim 2, wherein the tunnelling element and the gas sampling component are configured to promote resonant tunnelling of an electromagnetic field associated with the electromagnetic radiation, from the first to the second waveguide.

4. The gas detection system according to any one of claims 1 to 3, wherein the tunnelling element comprises a metallic wire.

5. The gas detection system according to claim 4, wherein the tunnelling element is at least partially coated with a coating material selected with reference to a degree by which its permittivity changes in response to contact with the target gas.

6. The gas detection system according to claim 5, wherein the coating material comprises Pt / AI2O3 as a catalyst on SnO2.28 05 257. The gas detection system according to any one of claims 1 to 6, wherein the processing component comprises a digital signal processor (DSP) connected to a receiver of the output port, the DSP being arranged to generate and analyze data correlated to variations in either or both of (i) tunnelling frequency; and (ii) dielectric loss in the gaseous sample.

8. The gas detection system according to any one of claims 1 to 7, wherein the processing component is configured to identify the target gas and calculate its concentration in the gaseous sample, based on variations in the output signal.

9. The gas detection system according to any one of claims 1 to 8, wherein the gas sampling component is configured to be detachable from at least one of the waveguides in a tool-free manner.

10. The gas detection system according to any one of claims 1 to 9, wherein the gas sampling component comprises a chamber defining a channel of reduced diameter and the tunnelling element is arranged to extend at least partially through the channel.

11. The gas detection system according to any one of claims 1 to 10, wherein a gas flow aperture is defined through at least one side of the gas sampling component.

12. The gas detection system according to any one of claims 1 to 11, wherein at least one of the waveguides comprises a substrate integrated waveguide (SIW).

13. The gas detection system according to any one of claims 1 to 12, which incorporates at least one micro-electro-mechanical system (MEMS).

14. The gas detection system according to any one of claims 1 to 13, which includes a plurality of tunnelling elements, each being adapted to sense a different target gas.

15. The gas detection system according to any one of claims 1 to 14, which includes at least one calibration component configured to be movable in relation to the at least one tunnelling element, the calibration component comprising a plurality of different zones, each zone respectively having a different known value of permittivity or Tan 5, or both.

16. The gas detection system according to claim 15, which includes a first calibration component comprising a material having a known value of permittivity and a second calibration component comprising a material having a known value of Tan 5.28 05 2517. The gas detection system according to any one of claims 1 to 16, wherein the target gas is methane.

18. A sampling apparatus adapted for detection and sensing of a target gas in a gaseous test sample, the apparatus comprisingat least one gas detection system according to any one of claims 1 to 14, providing a test sensor module configured to sense test data relating to the target gas in the test sample; andat least one gas detection system according to any one of claims 15 to 17, providing a reference sensor module configured to sense reference data relating to the target gas in the test sample, the reference sensor module including the calibration component;and wherein the output component is arranged to provide the test data and reference data to the user.

19. The sampling apparatus according to claim 18, wherein the processing component comprises a digital signal processor arranged to receive and compare the test data and the reference data, and the processing component is furthermore configured to interrogate a calibration database containing data which cross-reference permittivities of different zones of the calibration component with corresponding, predetermined concentration levels of the target gas.

20. A method of detecting and sensing a target gas within a gaseous sample, the method comprising:transmitting electromagnetic radiation from an input port of a first waveguide to an output port of a second waveguide, the output port being configured to receive an output signal of the electromagnetic radiation;channelling the electromagnetic radiation via at least one elongate tunnelling element arranged within a gas sampling component connecting the first and second waveguides, the tunnelling element being configured to provide energy coupling between the first and second waveguides;receiving the gaseous sample into the gas sampling component; andanalyzing the output signal received by the output port to detect a presence of the target gas in the gaseous sample.28 05 2521. The method according to claim 20, which includes analyzing the output signal to compare values of at least one property selected from the group consisting of tunnelling frequency, permittivity, dielectric constant, and Tan 5.

22. The method according to either one of claims 20 and 21, which includes detaching the gas sampling component from at least one of the waveguides in a tool-free manner, and exchanging at least one tunnelling element in the gas sampling component by replacing it with a different tunnelling element.

23. A calibration apparatus for calibrating a detection system adapted to detect and sense a target substance within a fluid test sample, the detection system comprising first and second waveguides and a sampling component configured to receive the fluid test sample, with at least one elongate tunnelling element arranged within an internal chamber of the sampling component, the detection system being configured to measure and compare resonance frequencies of electromagnetic radiation tunnelled from the first waveguide to the second waveguide via the internal chamber; the calibration apparatus comprising at least one movable calibration component arranged proximate the tunnelling element.

24. The calibration apparatus according to claim 23, which includes a first calibration component comprising a material having a known value of permittivity and a second calibration component comprising a material having a known value of Tan 5.

25. A method of calibrating a detection system adapted to detect and sense a target substance within a fluid test sample, the detection system comprising first and second waveguides and a sampling component configured to receive the fluid test sample, with at least one elongate tunnelling element arranged within an internal chamber of the sampling component, the detection system being configured to measure and compare resonance frequencies of electromagnetic radiation tunnelled from the first waveguide to the second waveguide via the internal chamber; the method comprising the steps of:positioning at least one movable calibration component in proximity to the tunnelling element;exposing the tunnelling element to a standard fluid sample containing a known concentration of the target substance;moving the calibration component relatively to the tunnelling element to measure and compare a range of resonance frequencies associated with the target substance, thereby to generate reference data; andpreparing a calibration database containing the reference data, for use in analysing the fluid test sample for the presence of the target substance.28 05 25

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