GB2538563A - Gas sensing apparatus - Google Patents

Abstract

To detect and locate leakage sites along utility gas pipework safely, an optical gas sensing apparatus is deployed in a cable ducting enclosure proximal to the pipework. A sensing system comprises a tunable diode laser 320, a control unit therefore 310, a receiver module 360 with at least one photodetector 362, a housing forming a sensor cell 334 having an opening to allow gas in, a first optical fibre 340 for directing light to the housing, a second optical fibre 350 for directing light to the photodetector. First and second lenses are on the sensor housing. The sensor housing is arranged to have a longitudinal axis and a cross section diameter less than the enclosure diameter such that it can be moved through the enclosure to locate gas leaks. The system may be a TDLS system to detect infrared absorption of the gas in the housing.

Description

GAS SENSING APPARATUS

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to the sensing of gas concentrations in enclosed volumes using absorption of incident infrared wavelength electromagnetic waves. In particular, the invention relates to the sensing of gas concentrations in environments where the presence of gas above certain concentrations may indicate leakage in underground gas utility pipeline, pipework within man-made structures, etc. [0002] Gases have characteristic absorption properties with absorption lines at various wavelengths in the electromagnetic spectrum. At infrared wavelengths, for example, methane has convenient absorption lines at wavelengths around 1650nm, which lies in the near-infrared (IR): conveniently, some of these lines do not coincide with significant absorption line characteristics of the major constituents of air (i.e. nitrogen, oxygen, water vapour and/or carbon dioxide).

[0003] To detect the presence of a target gas such as methane, it is known to use tunable diode laser spectroscopy (TDLS). The TDLS technique has two main stages: a) a single frequency diode laser is tuned (by varying the diode current) to generate (spatially and temporally) coherent electromagnetic waves at a range of wavelengths that spans one or more of the characteristic absorption lines of a target species; and b) the absorption of the incident electromagnetic waves as their wavelength varies across the target absorption line is detected (for instance by measuring the ratio of power transmitted through a target gas volume to the power generated) and, the gas concentration is thereby deduced. In the case of methane, it is convenient for the laser to be tunable in a range of infrared 35 wavelengths that span a typical absorption line: this absorption line can be up to two orders of magnitude smaller in bandwidth than the tunable range of the diode.

[0004] In major conurbations there are thousands of 5 kilometers of underground natural gas piping. This pipework has typically been assembled, installed (i.e. laid) and repaired over many decades resulting in a heterogeneous network of pipes of different characteristics and ages. Legacy clay and cast iron gas pipe networks suffer particularly from expansion and contraction with environmental temperature changes, as well as other ageing effects, meaning that natural gas (the majority of which is methane gas) that is delivered via such pipes regularly leaks from these pipes and into nearby service cable ducting. This methane creates potentially explosive mixtures with air.

IS Furthermore escaped natural gas is a component of atmospheric greenhouse gases (GHG).

[0005] In many instances, service cable ducting is disposed in pipe "beds" closer to the surface of the built environment than the gas pipework. Such ducting may carry, for example, cables for telecommunications, cable TV, broadband, traffic systems and controls. Water pipe work is generally laid at greater depth than the gas pipes. To facilitate separate access, each utility pipe/cable is laid at a significant relative horizontal offset substantially parallel to other utility pipe/cables: thus, for instance, an offset of 25cm or more is typically provided between gas pipe beds and cable ducting where possible.

[0006] As a consequence of the proximity of cable ducting to gas pipework, it is not unusual for the first indication of a gas leak to be detected when an engineer lifts a manhole to access an underlying service duct in order to undertake cable maintenance. Natural gas is routinely seeded with a harmless chemical compound (i.e. a thiol compound, such as methyl and/or ethyl mercaptan) that has a garlic-like odour; members of the public can then smell the odour and thus detect the presence of escaped gas.

[0007] Safety protocols then require that the local Gas Distribution Network (GDN) operator should be notified immediately so that the GDN may investigate.

[0008] GDNs then need to locate the source of escaped gas more accurately so that pipework can be replaced or repaired. To do this they resort to drilling holes through the road/footpath at spaced-apart locations (separated by a distance of at least 1m from one another, say) along the expected path of the gas pipework. Holes are drilled at locations between the two nearest access points -where the gas smell is strongest: typically, these access points will be manholes around 30m apart (although the spacings need not be regular and the separation distances may vary greatly depending upon the local setting, so that distances in the range 10m to 150m are not unusual). At each drill hole, point detection is performed using a gas analyser (e.g. an electronic or chemical gas analyser) until the highest gas reading is recorded. The assumption is that the sample sites closest to the highest recorded concentrations correspond to the approximate location of a gas leak: the characteristics of the underground strata may, of course, mean that this assumption is incorrect with escaped gas percolating some distance from a leak site before pooling in higher concentrations. Depending upon the complexity of the pipework and the ease of access to suitable drill locations, this process can take several days. Furthermore the process often causes significant transport disruption, is frequently inconclusive in locating the leak and results in high manpower and associated costs to rectify.

[0009] There is therefore a requirement for readily-deployed, intrinsically safe sensing systems for detecting and more accurately locating natural gas leaks in air-filled underground and interior volumes, such as underground service cable ducts.

[0010] In such applications there is a need for the sensing systems to be physically robust, safe and easily deployed.

[0011] It is an aim of certain embodiments of the present disclosure to solve, mitigate or obviate, at least partly, at least one of the problems and/or disadvantages associated with the related art. Certain embodiments aim to provide at least one of the advantages described below.

SUMMARY OF THE INVENTION

[0012] In one aspect of the present disclosure, there is provided an apparatus for exposing a sample of gas in an enclosed volume of linear extent to incoming coherent light from an optical source, the apparatus comprising: a housing forming a sensing cell, the housing having at least one opening through which gas from the enclosed volume passes; a first optical arrangement configured to receive incoming light via a first optical waveguide and to transmit the incoming light into the sensing cell; and a second optical arrangement configured to collect light that has traversed the sensing cell from the first lens arrangement and to direct the traversing light to a photodetector via a second optical waveguide; wherein the housing encloses the first and second lens arrangements, and wherein the housing is arranged to extend along a longitudinal axis and to have a diameter in cross-section to the longitudinal axis that is less than the diameter of the enclosed volume.

[0013] The term "enclosed volume" refers to the void within a pipe chase or duct not occupied by pipework and/or cables. Nothing in the present disclosure should be understood to limit the deployment to enclosed volumes of substantially circular cross-section, indeed very often the presence of existing TV and/or telecommunications cabling can mean that the enclosed volume presents an irregular cross-section -the housing of the apparatus is therefore conveniently constrained to have a diameter in cross-section to the longitudinal axis that is less than the smallest cross-sectional dimension at any point along the linear extent of the enclosed volume. By ensuring that the apparatus is provided in a housing having a cross-section diameter that is less than the diameter of the enclosed volume at its narrowest point, a sensor assembly including a small form factor measurement sensor can be deployed and the laser output from a master control unit supplied to the measurement sensor via a suitable fibre optic cable, said cable providing the first and second waveguides. It is then possible to detect gas concentrations at otherwise difficult-to-access urderground or interior sites.

[0014] As the control unit is conveniently associated with a remotely located optical source, it is intrinsically safer and simpler to maintain, than alternative sensing technologies, such as pellistors, electrochemical sensors and locally electrically powered gas analyser systems.

[0015] The apparatus may include integral optical cabling to form a sensor assembly with the small form factor measurement sensor at a distal portion. Alternatively, the components of the apparatus may be configured to interface with optical cabling in a modular manner to form a sensor assembly, where the apparatus (i.e. the measurement sensor) is detachable from the optical cabling. The cabling itself may be provided with conventional optical joints allowing the cabling to be extended by appending a further length of similar cabling before being attached to the control unit, thereby addressing occasional requirements for additional length. One or more fibre optic rotary joint (FORJ) may conveniently be provided at the optical joint between the cabled sensor optical connector (on a deployment reel, say) and the linking optical cable attached between the cabled sensor deployment reel and a control unit (CU) that includes the optical source. Using a FORJ allows the cabled sensor deployment reel to be rotated to issue or wind up the cabled sensor while still retaining optical transmission and without twisting the linking cable to the CU as the deployment reel rotates. If a FORJ is not present, the required length of cabled sensor has to be wound off the deployment reel before connecting the linking cable to the CU and inserting the cabled sensor into the duct. It would nevertheless be possible to arrange the cable in an untwisted circular loop to facilitate torsion-free deployment.

[0016] In another aspect of the present disclosure, there is provided a system for sensing concentrations of a target gas species at a plurality of spaced apart locations within an enclosed volume of linear extent, the system comprising: an 20 optical source including a tunable laser diode which is configured to generate a laser beam in accordance with a driving signal; a control unit for generating a control signal for modulating the driving signal applied to the tunable laser diode; a photoreceiver module having at least one photodetector; a housing forming at least one sensing cell, the housing having at least one opening through which gas molecules pass; a first optical waveguide for directing the laser beam from the optical source to the sensing cell; and a second optical waveguide for directing light of the laser beam that has traversed the sensing cell to the photoreceiver module, the traversing light being an attenuated portion of the laser beam generated by the optical source; wherein the attenuation of the input laser beam in the gas sensor cell has a predetermined correlation to the concentration of the target gas species in the sensor cell, wherein the photoreceiver module operates to determine the concentration of the target gas species from the characteristics of the input beam detected at the photodetector in the photoreceiver module, and wherein the housing and the first and second optical waveguides are disposed in a sensor assembly having linear extent and substantially circular cross-section with a diameter less than the diameter of the enclosed volume.

[0017] Conveniently, the tunable laser diode may generate a laser beam having a characteristic wavelength in the infrared.

[0018] Certain embodiments of the present disclosure provide update readings on rapid timescales (with typical update time intervals of around 2 seconds). This allows readings to be taken at many points along the service ducting or other enclosed IS space as the sensor assembly is passed along the ducting without requiring significant pauses before each measurement.

[0019] In certain embodiments, the displacement of the measurement sensor arrangement along the service ducting is 20 measured by means of spaced-apart markings on the casing of optical cabling, where the optical cabling includes the first and second optical waveguides (for example markings at one metre intervals). These can either be counted or read off (where each marking is distinct from the other markings). 25 Certain embodiments use a mechanical counting apparatus that measures the length of cable deployed. In one example, the mechanical counting apparatus engages with the cable as it feeds out from a sheave, for instance, and counts the number of times a gear wheel rotates, the gear wheel being fixed to, and rotating about the same axis as, a roller that engages with the casing of the cable.

[0020] In yet another aspect of the present disclosure, there is provided a method for sensing concentrations of a target gas 35 species at a plurality of spaced apart locations within an enclosed volume of linear extent, the method comprising: generating, at an optical source including a tunable laser diode, a laser beam in accordance with a driving signal; generating a control signal for modulating the driving signal applied to the tunable laser diode; directing the laser beam from the optical source to a sensor cell via a first optical waveguide, the sensor cell being formed in a housing having at least one opening through which gas passes from the enclosed volume; directing light of the laser beam that has traversed the sensing cell to a photodetector via a second optical waveguide; at the photodetector, receiving the traversing light, the traversing light being an attenuated portion of the laser beam generated by the optical source; and determining the concentration of the target gas species from the characteristics of the traversing light detected at the photodetector, wherein the housing, the first optical waveguide and the second optical waveguide are disposed in a sensor assembly having linear extent and substantially circular cross-section with a diameter less than the diameter of the enclosed volume.

[0021] Another aspect of the present disclosure provides a computer program comprising instructions arranged, when executed, to implement a method in accordance with any one of the above-described aspects. A further aspect provides machine-25 readable storage storing such a program.

[0022] Various further aspects and embodiments of the present disclosure are provided in the accompanying independent and dependent claims.

[0023] It will be appreciated that features and aspects of the present disclosure described above in relation to the first and other aspects of the invention are equally applicable to, and may be combined with, embodiments of the invention according 35 to the different aspects of the invention as appropriate, and not just in the specific combinations described above. Furthermore features of the dependent claims may be combined with features of the independent claims in combinations other than those explicitly set out in the claims.

DESCRIPTION OF THE DRAWINGS

[0024] The invention, together with objects and advantages thereof, may best be understood by reference to the following description of exemplary embodiments together with the accompanying drawings in which: [0025] Figure 1 illustrates a gas leak sensor in accordance with the related art; IS [0026] Figure 2 illustrates the deployment of gas leak sensors in accordance with the related art; [0027] Figure 3 illustrates a gas leak sensor system in accordance with an aspect of the present disclosure; [0028] Figure 4 illustrates a typical deployment of gas leak sensors in accordance with embodiments of the present disclosure; [0029] Figure 5 illustrates a gas sensor system having a plurality of sensor assemblies in accordance with a further aspect of the present disclosure; [0030] Figures 6A, 6B and 6C show views of embodiments of measurement sensor apparatus in accordance with further aspects of the present disclosure; and [0031] Figure 7 shows the form factor of the sensor assembly

in accordance with the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0032] The detailed description set forth below in connection with the appended drawings is intended as a description of 35 certain exemplary embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practised. It is to be understood that the same or equivalent functions nay be accomplished by different embodiments that are intended to be encompassed within the scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. Furthermore, terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that module, circuit, device components, structures and method steps that 10 comprises a list of elements or steps does not include only those elements but may include other elements or steps not expressly listed or inherent to such module, circuit, device components or steps. An element or step proceeded by "comprises does not, without more constraints, preclude the existence of 15 additional identical elements or steps that comprises the element or step.

[0033] Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0034] Figure 1 illustrates a gas leak sensor 100 in accordance with the related art.

[0035] This sensor includes a gas analyser that operates by detecting an electrical change or a chemical change in the presence of a target gas (i.e. methane). In the UK, it is known to use a pellistor device (which detects a change in electrical resistance due to the presence of a certain concentration of methane gas) -an example being the GMI Gascoseeker. Pellistor detectors typically require an associated pump to draw in air/gas at a target location into a sensing chamber via a system of tubes and valves. To be effective such sensors are deployed in test holes 120 drilled in the ground along the known layout of the gas pipework 130. In principle, the test holes closest to the leak 140 will provide the highest methane gas reading (but as noted previously characteristics of the ground strata may mean that this is not always the case): the very act of drilling holes in the ground may disturb the local flow of the escaped gas and the conventional technique may incorrectly identify the source of the leak.

[0036] The deployment of gas leak sensors in accordance with the related art is further illustrated in Figure 2. Here the drilling of test holes at 1 metre spacings may be seen to give rise to many inconveniences and potential hazards.

[0037] The present disclosure takes a different approach to the detection of gas leaks in cable ducting: it uses a tuneable diode laser spectroscopy (TDLS) technique instead of the conventional techniques, such as the use of pellistor-based gas 20 analysers.

[0038] In known laser-based gas detection techniques, the laser output of the tunable laser source is directed to a selected gas sensing cell along an optical path (such as an 25 optical fibre) and the laser beam that is transmitted through the cell is detected at a receiver.

[0039] It would be possible to deploy conventional laser-based gas sensing equipment in place of the sensors currently used (e.g. pellistor-type sensors). Such a deployment however shares the same inconveniences as the known deployments. Furthermore conventional laser-based gas sensing equipment is inappropriate for deployment in service ducting as the gas sensing cells are large (60mm by 60mm by 350mm, say) and heavy (typically around 1 kg) meaning that they are essentially static once deployed.

[0040] According to certain embodiments of the present disclosure, TDLS is used in a measurement sensor having characteristics that make it better suited to rapid, field 5 deployment. As in conventional TDLS systems, modulated light of specific wavelength from a laser in a control unit (CU) is transmitted via optical cabling to the measurement sensor. The modulated light propagates down a first optical waveguide in the optical cabling (e.g. an internal single mode optical fibre) to 10 a lens arrangement in the sensor at the distal end of the cabling. The transmitted light is then collected by another lens arrangement within the sensor and returned to the CU along a second optical waveguide (e.g. a second single mode optical fibre in the optical cabling). Methane present in the free space between the sensor lens arrangements will partially absorb the laser light and impose a unique concentration dependent signature onto the return signal received by the CU. Signal analysis in the CU allows the gas concentration in the sensor to be updated within seconds and displayed on the screen.

[0041] To achieve the effective passage of the incoming laser light through the sensor lens arrangements and back along the "return" optical fibre in the confines of a small form factor housing for deployment in service ducting, it is necessary to compact the optical path while not compromising the robustness of the sensor.

[0042] Figure 3 illustrates a gas leak sensor system 300 in accordance with an aspect of the present disclosure.

[0043] In Figure 3, a laser source module 320, controlled by a master control unit (MCU) 310, launches a laser beam into a measurement sensor 330 along a first optic fibre 340. As illustrated in Figure 3, the laser source module 320 may include 35 a laser source and an optical splitter, thereby allowing light from the laser source to be split between more than one optical path.

[0044] The measurement sensor 330 includes a first lens arrangement 332, a sensing cell 334, and a second lens arrangement 336. The first lens arrangement 332 is configured to direct incoming light from the first optic fibre 340 into the sensing cell 334. The second lens arrangement 336 is arranged to gather any light that has traversed the sensing cell 334 and to direct that transmitted light into a second optic fibre 350. The sensing cell 334 is provided with at least one hole through which a gas, such as the surrounding air, may freely circulate.

[0045] The level of absorption of light from the incident beam is related to the concentration of the target gas present in the sensing cell 334. The portion of each laser beam transmitted through the sensing cell 334, and not absorbed, is directed along the second optical fibre 350 to a photodetector 362 in a photoreceiver module 360.

[0046] In certain embodiments, the laser beam generated by the control unit is optically split before one path of the split beam is launched into the measurement cell 330. A further one of the split laser beams may be directed along a reference 25 optical fibre 352 to a further photodetector 364 to provide a reference channel. Values from this reference channel may be used to compensate for wavelength dependent attenuation. The reference channel is not however to be confused with a calibration channel where a gas reference cell is used to obtain sample values associated with the presence of a known concentration of the target gas in a cell of known dimensions. While calibration is not essential in many deployments, there may be cases when it is convenient to perform occasional checks to confirm that factory set calibration values are applied correctly.

[0047] The photoreceiver module 360 of Figure 3 includes photodetectors 362, 364 (where the transmitted beams are sampled) and a signal processor unit 366 (where the level of absorption in the sampled beam is determined). The signal processor unit 366 processes the determined values for the levels of absorption from each associated photodetector, calculates a corresponding target gas concentration from said determined values relative to the reference channel values and generates data packets including the calculated gas concentration levels to the master control unit 310. The master control unit 310 then collates the respective data packets to provide reports (and/or alarms) concerning the concentration of the target gas at each sensing point. In various embodiments, the control unit 310 is electrically connected to a display unit (not shown), upon which the data gathered by the measurement sensor is presented in textual and/or graphical form.

[0048] Various embodiments of the present disclosure provide a system comprising a control unit (CU), a length of fibre optic cabling and at least one measurement sensor, the optical cabling and the at least one measurement sensor being referred to collectively as the "sensor assembly".

[0049] Figure 4 illustrates a typical deployment of a gas leak sensor assembly 400 in accordance with embodiments of the present disclosure. The control unit may, for example, be located in the GDN service vehicle 430 (e.g. a van) at some distance from any potential, gas release zone (>5m away, say) 30 and powered via the service vehicle power supply (typically at 110V). Typically the control unit is provided with a display for presenting the results of the measurements in a graphical form and storage means for storing the measured values for later evaluation and/or further processing. Optionally the control unit may include a communication unit for exchanging information with other devices allowing the results of the measurements to be transmitted to remote locations and/or handheld units operated by the personnel deploying the sensor assembly.

[0050] The optical cabling 420 may include a fibre optic cable supplied on a cable reel with a conventional rugged fibre optic connector at one end and sensor connector, for forming a robust optical connection to the optical measurement sensor 410, at the other end of the cable: the connector is "rugged" in such deployments as it is apt to be exposed to physical force (i.e. stress, strain, friction, etc.) and temperature differences, as well as the ingress of water, mud, etc. In view of the typical spacing of inspection manholes in cable ducting deployments (i.e. approximately every 30m in built-up areas), it is IS sufficient that the fibre optic cable has a length of approximately 40m for most deployments. Clearly, different lengths may be needed in different scenarios; where it is known that typical manhole cover spacings are closer -say every 10m a shorter cable would be appropriate; whereas, for other deployments where the spacings are far greater -say every 50m -a longer cables up to 150m in length may be used. Lengths of cable become increasingly difficult to deploy as they get longer (and thus heavier) and it may be necessary to mount cable sheave on a larger service vehicle for the longer cable lengths.

[0051] As illustrated in Figure 4, the fibre optic cable 420 is connected to the control unit (CU) in the service vehicle 430 via the rugged fibre connector. While not shown, the fibre optic cable may be wound off a cable reel as required and the sensor 410 at the other end of the cable (i.e. the distal end) is inserted into the duct 440 to be investigated. Often the duct 440 is accessed by removing a manhole cover or the like. Pushing and/or pulling the cable through the voids in the duct 440 will move the sensor 410 along the duct and allow the methane concentration profile along the duct to be recorded and examined in real time on the CU display. The pushing or pulling of the cabled sensor assembly is conveniently performed using cable deployment equipment (such as conventional conduit rod equipment) attached to the housing of the measurement sensor; attachment of an external conduit rod is typically implemented by means of an attachment means on the housing arranged to engage detachably with a corresponding engagement means on the conduit rod, with the attachment means being conveniently formed at one end of the housing. Alternatively, the sensor assembly may incorporate an internal structure similar to a conduit rod, allowing the sensor assembly to be man-handled into successive locations along the duct without requiring additional conduit rod equipment. The displacement of the measurement sensor along the duct is measured (by a conventional technique, such as counting printed marks on the casing of the fibre optic cable). The gas concentration measurements are recorded alongside the displacement measurements and together these measurements indicate where the highest concentration of methane exists along the duct (i.e. the presumed gas entry point) so that the gas escape teams can pinpoint where to start surface excavation in order to repair the gas network.

[0052] As previously noted, the deployment of a sensor system in accordance with the present disclosure facilitates rapidly 25 deployable, real time monitoring of gas concentration in narrow underground cable ducts and the like. Optical sensing is intrinsically safe, as optical data transmission gives rise to no voltage, spark or ignition risk in the service duct.

[0053] The detection technique of the present disclosure is self-referencing, so system has inherent calibration stability. There is no need to recalibrate the sensing cell against known concentrations of the target gas, for example.

[0054] Conveniently, the TDLS technique used gives a wide gas measurement range (in a range from 0.05%, i.e. 50Cppm, to 100% methane) with no gas cross-sensitivity.

[0055] The sensor system can be reliably deployed in small 2" (-5cm) diameter cable ducts.

[0056] Various embodiments of the measurement sensor are arranged to have a plurality of holes in the housing to allow the ingress and egress of fluids (especially gases). The holes may be configured to be sufficiently small to exclude any but the smallest particulates (e.g. <100nm); they may alternatively be larger to assist in drainage and cleaning, or provided with protective mesh to exclude the ingress of larger particles (e.g. >lmm). In certain embodiments, the form faccor and the arrangement of holes means that the measurement sensor is easily cleaned and ready to use again following submersion in water, mud, etc. [0057] The reader will readily appreciate that the sensor system of the present disclosure measures actual gas levels in the service duct in real-time. This contrasts with the related art where it is necessary to measure at holes drilled from the surface and the very drilling of such holes may disturb the gas concentration profile in the duct.

[0058] To the GDN provider, this is significant as, typically, the present system requires no drilling during deployment, protecting the subterranean service infrastructure and saving considerable repair time and reinstatement costs. Furthermore, the avoidance of drilling drastically reduces transport disruption as no major excavation work is required until the system of the present disclosure has been deployed and provided confirmation and an accurate location of a leak from a gas supply pipe.

[0059] As the present system allows faster leak location, safety is compromised for a shorter time and the escape of a greenhouse gas to the atmosphere is reduced significantly. Indeed methane is considered to have a potential for contributing to global warming which exceeds the potential of the same mass of carbon dioxide by a factor greater than 30 (over a 100 year time horizon), according to recent (2013) IPCC data.

[0060] When considered as a system that needs to be deployed safely by GDN personnel, it is remarkable that the only electrical elements in the system are the electronics typically contained in the Control Unit (CU); indeed, in certain embodiments the system may be supplied with adequate power from IS a battery pack, fuel cell, or other stable, portable power supply. The CU is of course conveniently sited in a GDN service vehicle >5m away from any potential gas release zone and powered via the service vehicle's low voltage (e.g. 110V) power supply.

[0061] In addition, the optical cabling only transmits very low power optical signals (<20uW of optical power passes through the sensor) back and forth between the sensor and the CU. There are no electrical signals transmitted through the sensor assembly and hence no voltage or spark risk from the optical cabling or the measurement sensor in the potential gas release zone.

[0062] An attractive feature of the system is that the optical sensor assembly only needs to pass through a service 30 duct (i.e. TV/Telecom cable ducting). The system never needs to enter the gas supply pipe. Consequently deployment of the sensor system has no effect upon the integrity of the gas distribution infrastructure.

[0063] In various embodiments, the MCU is provided with at least one optical splitter whereby the laser beam launched by the optical source is split into separate beams. Each of these beams may be applied to a respective one of a plurality of sensor assemblies (Figure 5: 500,500'). In this way more than one measurement sensor may be deployed simultaneously.

[0064] Figure 5 illustrates a gas sensor system having a plurality of sensor assemblies 500, 500' in accordance with a further aspect of the present disclosure. Thus sensor assembly 500 comprises optic fibre 540, 550 and at least one measurement 10 sensor 530, while sensor assembly 500' comprises optic fibre 540', 550' and at least one measurement sensor 530'. In each case, the sensor assemblies 500, 500' cooperate with a control assembly 520 (including an optical source, a control unit and a photoreceiver module). Various different arrangements are possible: the respective sensor assemblies may be provided with respective different optical fibres in corresponding different cables, but they may also be provided by arranging multiple optical fibres in a single cable or in a number of cables fewer than the number of fibres.

[0065] Certain embodiments are "self-referencing", by which is meant they employ mechanisms to compensate for changes in optical losses in the plurality of optical waveguiding paths.

[0066] Certain embodiments require no recalibration after an initial factory setting using gas reference cells at different known target gas concentrations. In essence the source is paired to the MCU. Other embodiments are configured to facilitate occasional re-calibration to fulfil regulatory or other requirements. This may result in a reduction in ongoing maintenance costs.

[0067] As previously noted, a system where the sensing points are addressed only using low-power infrared laser light is intrinsically safe.

[0068] Conveniently, the sensing points do not have a point of saturation; when configured to sense the presence of methane, for example, these sensing points are typically capable of 5 operating at concentrations from <0.05% (i.e. 300ppm) to 100%. While this range is typical fcr many embodiments, detection at lower concentrations than 500ppm is also possible using the same techniques; in certain embodiments, detection at lower concentrations may be achieved by extending the path length 10 through the sensing cell from a typical length of 60mm.

[0069] Certain embodiments implement two sensing techniques: direct absorption measurement and wavelength modulation spectroscopy (WMS). Used in tandem, absorption measurement and WMS effectively detect a target gas at a large dynamic range of gas concentrations (for instance from as little as 100ppm to 100% methane).

[0070] A typical initial installation begins with the physical insertion of the cable of the sensor assembly into the control unit at an optical connector. The distal end of the (or each) sensor assembly is then either pulled or pushed (i.e. rodded) to one of a plurality of sensing points in underground cable ducting. The sensing capabilities of the system are completed with insertion and optical connection of the photoreceiver module(s) to the optical cabling (so that light transmitted across the respective sensor cell is received at a respective photoreceiver module). Once connected in this manner the system is powered.

[0071] Figures 6A, 6B and 6C show views of respective embodiments of the measurement sensor apparatus 600, 600', 600" at the distal end of a sensor assembly in accordance with aspects of the present disclosure.

[0072] In certain embodiments, as illustrated in Figures 6A to 60, the sensor assembly is provided with a measurement sensor 600 at the distal end. The measurement sensor includes a housing 640 forming a sensing cell 634, a first optical arrangement 632 that delivers incoming light to the sensing cell 634 and a second optical arrangement 636 that directs outgoing light from the sensing cell 634: the housing 640 in these Figures is further illustrated with an attachment means 630 that facilitates positioning of the measurement sensor underground.

[0073] The respective optical arrangements 632, 636 and sensing cells 634 in the embodiments illustrated in Figures 6A, 6B and 6C include lens arrangements, optical fibres and/or optical waveguides configured to be compatible with the tight bend radii which are necessary for the turnaround of the optical path at the distal end of the sensor assembly.

[0074] The first and second optical arrangements 632, 636 in Figure 6A are arranged at opposite ends of the sensing cell 634.

The optical path from the second optical arrangemenT. 636 back to the photoreceiver module in the control assembly thus necessarily requires a waveguide arrangement that introduces a 180 degree loop back within the confined form factor of the measurement sensor. Where the wave guide is implemented using single mode optical fibre, the optical fibre must be carefully selected for its characteristics at low radius of curvature: in particular, the fibre should exhibit low loss rates at low radius of curvature. To illustrate, consider a standard single mode optical fibre operating at a typical wavelength of 1650nm and bent with a diameter of 15mm, the typical optical loss for 1 fibre turn is 6dB (which corresponds to a 75% signal loss); by a contrast, optical fibre of a bend-insensitive material (again at 1650nm and 15mm bend diameter) may achieve a typical optical loss for 1 fibre turn of just 0.25dB (amounting to a 5% signal loss).

[0075] In certain alternative embodiments, as illustrated in Figures 6B and 6C, the first and second optical arrangements 632, 636 are arranged at the same ends of the sensing cell 634.

In these alternative embodiments, sensing cell optical arrangements include a reflective surface (i.e. a mirror 610, Figure 6B or a retroreflector 620, Figure 60) at one end of the sensing cell in place of an optical fibre having sufficiently small radius of curvature.

[0076] To implement the respective illustrated measurement sensors, it is also necessary for the optical arrangements 632, 636 to provide suitable collimation of the incoming light transmitted into the sensing cell and the traversing light 15 exiting the sensing cell.

[0077] Furthermore surfaces of the retroreflector 620 in the Figure 60 arrangement are conveniently coated with an anti-reflective coating to prevent unwanted secondary reflection and 20 thus minimise optical losses.

[0078] The environment into which the measurement sensor will be deployed places additional constraints upon the construction and configuration of the optical train (i.e. the sequence of 25 optical arrangements through which the light from the optical source propagates before exiting the measurement sensor). The sensor assembly is likely to experience a degree of vibration and some temperature variation as it passes from the surface through cable ducting. The optical train must therefore be configured to tolerate and/or compensate for likely conditions. To preserve an effective optical train, the ootomechanical mounts upon which the respective optical arrangements are disposed are conveniently constructed of material that has a suitable expansion coefficient in the expected temperature range 35 and significant damping to ensure resilience against vibration and sharp impact.

[0079] In further embodiments, the measurement sensor includes a plurality of sensing cells, each supplied with a respective incoming light beam (i.e. a beam optically split from 5 the light beam launched by the optical source) and each outputting a corresponding outgoing light beam. The optical arrangement in such multi-cell measurement sensors is thus more complex, each sensing cell is served by a respective pair of lens arrangements and associated optical fibres. The optical W fibres delivering incoming light beams to sensing cells and directing outgoing light beams from those cells are disposed in the cable that physically connects the measurement sensor to the control unit.

[0080] Certain embodiments provide additional measurement sensor nodes at locations along the optical cabling offset at a distance from the distal end of the cabling. Each additional node may be provided with one or more sensing cell. These offset sensor nodes differ from the measurement sensor at the distal end of the cable in that they include an optical through-path to allow incoming light beams destined for the measurement sensor at the distal end to pass unhindered (and likewise for the outgoing light beams from the distal measurement sensor to pass unhindered in the return direction). The optical through-path may be formed by providing a longitudinal notch or tube through which optical fibre linking control unit and distal measurement sensor may pass.

[0081] Figure 7 shows an external view of a sensor assembly 700 in accordance with the present disclosure. The sensor assembly 700 includes a length of optical cabling 740 and a measurement sensor apparatus. The housing 730 of the measurement sensor apparatus is shown to have a small cross-section diameter 710 to suit the expected dimensions of voids through which the assembly will need to pass. The housing 730 is illustrated with spaced apart openings 750 that allow air to enter and leave the sensing cell (not shown), thereby facilitating sampling for gas concentration.

[0082] Certain embodiments of the present disclosure use sensor assemblies that have been tested to ensure their inherent safety in explosive environments, by using only ATEX and/or IECEx compliant materials for example, and verified optical powers several orders of magnitude below ignition risk levels.

ATEX is a set of legal requirements set by the European Commission for controlling explosive atmospheres and the suitability of equipment and protective systems used in them; whereas IECEx refers to compliance with the International Electrotechnical Commission Scheme for Certification to Standards Relating to Equipment for use in Explosive Atmospheres and is an internationally recognized certification.

[0083] The measurement sensor (in particular, the physical components housing each sensing cell) may conveniently be 20 provided with one or more features to withstand potentially hazardous and inhospitable environments. These features may include fine mesh and sturdy housings which prevent water damage, dust ingress and physical damage to the sensing element while allowing air circulation to facilitate gas detection and easy cleaning of the measurement sensor for repeated deployment.

[0084] The measurement sensor may further comprise an attachment means 760 arranged to engage detachably with a cable deployment equipment (not shown). Examples of the attachment means include a clip, hook or loop to engage with a loop, clip or hook in a rodding equipment, pulley systems etc.: magnetic attachment systems are also known. Conveniently the attachment means is formed at one end of the housing: the distal end for pulley systems, but the proximal end or the distal end for rodding systems.

[0085] The preceding discussion refers to sensor assemblies having a single measurement sensor with a single sensing cell. The reader will readily appreciate that sensor assemblies may be 5 provided with further measurement sensors without departing from the teaching of the present disclosure. Such additional measurement sensors may be integral within a measurement sensor housing or provided at nodes offset from the distal end of the sensor assembly by a known displacement along the linear extent of the cable. Further measurement sensors may be used to provide redundancy in the measurements at a given displacement from the control unit and/or to allow for measurement in more dynamic scenarios while allowing the sensor assembly to be moved through cable ducting relatively swiftly.

[0086] While the preceding discussion concerns embodiments in which the target gas is methane, the reader will readily appreciate that, without departing from the scope of the invention, the method and apparatus of the present disclosure are equally capable of detecting the presence of a target gas other than methane that has one or more detectable absorption lines at wavelengths accessible to TDLS techniques, for example carbon dioxide (CO2).

[0087] It will be appreciated that embodiments of the sensor system can be realized in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage, for example a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory, for example RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium, for example a CD, DVD, magnetic disk or magnetic tape or the like. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs comprising instructions that, when executed, implement embodiments of the present invention.

[0088] Accordingly, embodiments provide a program comprising code for implementing apparatus or a method as claimed in any one of the claims of this specification and a machine-readable storage storing such a program. Still further, such programs may be conveyed electronically via any medium, for example a communication signal carried over a wired or wireless connection 10 and embodiments suitably encompass the same.

[0089] Features, integers or characteristics described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other 15 aspect, embodiment or example described herein unless incompatible therewith.

[0090] It will be also be appreciated that, throughout the description and claims of this specification, language in the general form of "X for Y" (where Y is some action, activity or step and X is some means for carrying out that action, activity or step) encompasses means X adapted or arranged specifically, but not exclusively, to do Y. [0091] The description of the preferred embodiments of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment disclosed, but covers modifications within the scope of the present invention as defined by the appended claims.

Claims (18)

1. CLAIMS1. Apparatus for exposing a sample of gas in an enclosed 5 volume of linear extent to incoming coherent light from an optical source, the apparatus comprising: a housing forming a sensing cell, the housing having at least one opening through which gas from the enclosed volume passes; a first optical arrangement configured to receive incoming light via a first optical waveguide and to transmit the incoming light into the sensing cell; and a second optical arrangement configured to collect light that has traversed the sensing cell from the first lens 15 arrangement and to direct the traversing light to a photodetector via a second optical waveguide; wherein the housing encloses the first and second lens arrangements, and wherein the housing is arranged to extend along a 20 longitudinal axis and to have a diameter in cross-section to the longitudinal axis that is less than the diameter of the enclosed volume.
2. 2. An apparatus as claimed in claim 1, wherein the cross-25 section is substantially circular.
3. 3. An apparatus as claimed in claim 1 or claim 2, further comprising an attachment means arranged to detachably engage with a cable deployment equipment, the attachment means being 30 formed at one end of the housing.
4. 4. An apparatus as claimed in claim 1, wherein the optical source includes a tunable laser diode.
5. 5. An apparatus as claimed in any one of the preceding claims, wherein the first optical arrangement is a lens arrangement.
6. 6. An apparatus as claimed in any one of the preceding claims, wherein the second optical arrangement includes a lens arrangement.
7. 7. An apparatus as claimed in any one of the preceding claims, further comprising a reflective surface arranged in the sensing cell, the reflective surface directing light from the first optical arrangement to the second optical arrangement, thereby ensuring that the light traverses the sensing cell in a forward and a reflected path.
8. 8. An apparatus as claimed in any one of the preceding claims, wherein the photodetector operates to sample the characteristics of the traversing light detected at the photodetector.
9. 9. An apparatus as claimed in any one of the preceding claims, wherein the housing forms at least one further sensing cell.
10. 10. A system for sensing concentrations of a targec gas species at a plurality of spaced apart locations within an enclosed volume of linear extent, the system comprising: an optical source including a tunable laser diode which is configured to generate a laser beam in accordance with a driving signal; a control unit for generating a control signal for modulating the driving signal applied to the tunable laser 30 diode; a photoreceiver module having at least one photodetector; a housing forming a sensing cell, the housing having at least one opening through which gas molecules pass; a first optical waveguide for directing the laser beam from 35 the optical source to the sensing cell; and a second optical waveguide for directing light of the laser beam that has traversed the sensing cell to the photoreceiver module, the traversing light being an attenuated portion of the laser beam generated by the optical source; wherein the attenuation of the input laser beam in the gas sensor cell has a predetermined correlation to the concentration of the target gas species in the sensor cell, wherein the photoreceiver module operates to determine the concentration of the target gas species from the characteristics 10 of the input beam detected at the photodetector in the photoreceiver module, and wherein the housing and the first and second optical waveguides are disposed in a sensor assembly having linear extent and substantially circular cross-section with a diameter 15 less than the diameter of the enclosed volume.
11. 11. The system of claim 10, wherein the tunable laser diode generates a laser beam having a characteristic wavelength in the infrared.
12. 12. The system of claim 10 or claim 11, wherein the housing includes at least one further sensing cell.
13. 13. The system of any one of claim 10, 11 or 12, the system 25 further comprising: at least one offset sensing node, the or each node having a node housing forming a node sensing cell, the node housing having at least one opening through which gas molecules pass; at least one third optical waveguide for directing the 30 laser beam from the optical source to the or each node sensing cell; and at least one fourth optical waveguide for directing light of the laser beam that has traversed the or each node sensing cell to the photoreceiver module, wherein the node housing, the third optical waveguide and the fourth optical waveguide are disposed in the sensor assembly.
14. 14. The system as claimed in any one of claims 10 to 13, wherein the sensor assembly is provided with markings at predetermined spaced apart intervals along the direction of linear extent, thereby facilitating the measurement of displacement of the measurement sensor along the enclosed volume.
15. 15. The system as claimed in any one of claims 10 to 14, wherein the first optical waveguide interfaces with the optical source at an optical joint, the optical joint being a fibre 15 optic rotary joint, FORJ.
16. 16. A method for sensing concentrations of a -Large?. gas species at a plurality of spaced apart locations within an enclosed volume of linear extent, the method comprising: generating, at an optical source including a tunable laser diode, a laser beam in accordance with a driving signal; generating a control signal for modulating the driving signal applied to the tunable laser diode; directing the laser beam from the optical source to a 25 sensor cell via a first optical waveguide, the sensor cell being formed in a housing having at least one opening through which gas passes from the enclosed volume; directing light of the laser beam that has traversed the sensing cell to a photodetector via a second optical waveguide; at the photodetector, receiving the traversing light, the traversing light being an attenuated portion of the laser beam generated by the optical source; and determining the concentration of the target gas species from the characteristics of the traversing light detected at the photodetector, wherein the housing, the first optical waveguide and the 5 second optical waveguide are disposed in a sensor assembly having linear extent and substantially circular cross-section with a diameter less than the diameter of the enclosed volume.
17. 17. The method of claim 16, further comprising: directing the laser beam from the optical source to a further sensor cell via a third optical waveguide, the third optical waveguide being disposed in the sensor assembly; directing light of the laser beam that has traversed the further sensing cell to a further photodetector via a fourth 15 optical waveguide, the fourth optical waveguide being disposed in the sensor assembly; at the further photodetector, receiving light of the laser beam that has traversed the further sensing cell, the traversing light being an attenuated portion of the laser beam generated by 20 the optical source; and determining the concentration of the target gas species from the characteristics of the traversing light detected at the photodetector and at the or each further photodetector.
18. 18. The method of claim 16 or claim 17, further comprising: measuring displacement of the measurement sensor along the enclosed volume, markings having been provided at predetermined spaced apart intervals along the direction of linear extent of the sensor assembly.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWSCLAIMS1. Apparatus for exposing a sample of gas in an enclosed volume of linear extent to incoming coherent light from an optical source, the volume being an air-filled void within a pipe chase or service cable duct distinct from gas distribution network pipework, the apparatus comprising: a housing forming a sensing cell, the housing having at least one opening through which gas from the enclosed volume 10 passes; a first optical arrangement configured to receive incoming light via a first optical waveguide and to transmit the incoming light into the sensing cell; and a second optical arrangement configured to collect light 15 that has traversed the sensing cell from the first optical cr) arrangement and to direct the traversing light to a photodetector via a second optical waveguide;COwherein the housing encloses the first and second optical arrangements, c)20 wherein the housing is arranged to extend along a longitudinal axis and to have a diameter in cross-section to the longitudinal axis that is less than the diameter of the enclosed volume, and wherein the apparatus is configured for mobility along the 25 linear extent of the enclosed volume.2. An apparatus as claimed in claim 1, wherein the cross-section is substantially circular.3. An apparatus as claimed in claim 1 or claim 2, further comprising an attachment means arranged to detachably engage with a cable deployment equipment, the attachment means being formed at one end of the housing.4. An apparatus as claimed in claim 1, wherein the optical source includes a tunable laser diode.S. An apparatus as claimed in any one of the preceding claims, wherein the first optical arrangement is a lens arrangement.6. An apparatus as claimed in any one of the preceding claims, wherein the second optical arrangement includes a lens arrangement.7. An apparatus as claimed in any one of the preceding claims, further comprising a reflective surface arranged in the sensing cell, the reflective surface directing light from the first optical arrangement to the second optical arrangement, thereby ensuring that the light traverses the sensing cell in a forward and a reflected path. (g)8. An apparatus as claimed in any one of the preceding claims, CY) CD wherein the photodetector operates to sample the characteristics A 20 of the traversing light detected at the photodetector.0 9. An apparatus as claimed in any one of the preceding claims, wherein the housing forms at least one further sensing cell.10. A system for sensing concentrations of a targei gas species at a plurality of spaced apart locations within an enclosed volume of linear extent, the volume being an air-filled void within a pipe chase or service cable duct distinct from gas distribution network pipework, the system comprising: an optical source including a tunable laser diode which is configured to generate a laser beam in accordance with a driving signal; a control unit for generating a control signal for modulating the driving signal applied to the tunable laser 35 diode; a photoreceiver module having at least one photodetector; a housing forming a sensing cell, the housing having at least one opening through which gas molecules pass; a first optical waveguide for directing the laser beam from the optical source to the sensing cell; and a second optical waveguide for directing light of the laser beam that has traversed the sensing cell to the photoreceiver module, the traversing light being an attenuated portion of the laser beam generated by the optical source; wherein the attenuation of the input laser beam in the 10 sensing cell has a predetermined correlation to the concentration of the target gas species in the sensing cell, wherein the photoreceiver module operates to determine the concentration of the target gas species from the characteristics of the input beam detected at the photodetector in the 15 photoreceiver module, cr) wherein the housing and the first and second optical waveguides are disposed in a sensor assembly having linearCOextent and substantially circular cross-section with a diameter less than the diameter of the enclosed volume, and wherein the sensor assembly is configured for mobility along the linear extent of the enclosed volume.11. The system of claim 10, wherein the tunable laser diode generates a laser beam having a characteristic wavelength in the 25 infrared.12. The system of claim 10 or claim 11, wherein the housing includes at least one further sensing cell.13. The system of any one of claim 10, 11 or 12, the system further comprising: at least one offset sensing node, the or each node having a node housing forming a node sensing cell, the node housing having at least one opening through which gas molecules pass; at least one third optical waveguide for directing the laser beam from the optical source to the or each node sensing cell; and at least one fourth optical waveguide for directing light 5 of the laser beam that has traversed the or each node sensing cell to the photoreceiver module, wherein the node housing, the third optical waveguide and the fourth optical waveguide are disposed in the sensor assembly.14. The system as claimed in any one of claims 10 to 13, wherein the sensor assembly is provided with markings at predetermined spaced apart intervals along the direction of linear extent, thereby facilitating the measurement of displacement of the sensor assembly along the enclosed volume.15. The system as claimed in any one of claims 10 to 14,COwherein the first optical waveguide interfaces with the optical source at an optical joint, the optical joint being a fibre O 20 optic rotary joint, FORJ.16. A method for sensing concentrations of a target gas species at a plurality of spaced apart locations within an enclosed volume of linear extent, the volume being an air-filled void within a pipe chase or service cable duct distinct from gas distribution network pipework, the method comprising: generating, at an optical source including a tunable laser diode, a laser beam in accordance with a driving signal; generating a control signal for modulating the driving 30 signal applied to the tunable laser diode; directing the laser beam from the optical source to a sensor cell via a first optical waveguide, the sensor cell being formed in a housing having at least one opening through which gas passes from the enclosed volume; directing light of the laser beam that has traversed the sensing cell to a photodetector via a second optical waveguide; at the photodetector, receiving the traversing light, the traversing light being an attenuated portion of the laser beam generated by the optical source; and determining the concentration of the target gas species from the characteristics of the traversing light detected at the photodetector, wherein the housing, the first optical waveguide and the second optical waveguide are disposed in a sensor assembly having linear extent and substantially circular cross-section with a diameter less than the diameter of the enclosed volume, and wherein the sensor assembly is configured for mobility 15 along the linear extent of the enclosed volume.17. The method of claim 16, further comprising:COdirecting the laser beam from the optical source to a further sensor cell via a third optical waveguide, the third O 20 optical waveguide being disposed in the sensor assembly; directing light of the laser beam that has traversed the further sensing cell to a further photodetector via a fourth optical waveguide, the fourth optical waveguide being disposed in the sensor assembly; at the further photodetector, receiving light of the laser beam that has traversed the further sensing cell, the traversing light being an attenuated portion of the laser beam generated by the optical source; and determining the concentration of the target gas species 30 from the characteristics of the traversing light detected at the photodetector and at the or each further photodetector.18. The method of claim 16 or claim 17, further comprising: measuring displacement of the sensor assembly along the 35 enclosed volume, markings having been provided at predetermined spaced apart intervals along the direction of linear extent of the sensor assembly. (t) CY) ONO