In-situ infrared and ultraviolet photometers

Description

Technical Field

[0001] The invention relates to gas monitoring / analysis techniques, and more particularly to an improved in-situ photometer with combined Infra-Red (IR) and Ultra Violet (UV) measurement, and to a method for analysing the composition of a sample gas.Background

[0002] A gas stream may be required to be analysed to determine the species and concentrations of gases therein. Gas analysis is typically required for exhaust gases in applications such as power generation, incineration, shipping, steel production, etc. Such industries are under increasing pressure from environmental legislation to reduce pollution levels, which is driving a reduction in the monitoring ranges for gaseous emissions and a requirement for greater accuracy / sensitivity of measurement devices.

[0003] It is known to provide a prior art Infra-Red (IR) photometer that is located in-situ in an exhaust stack (i.e. chimney) to monitor gaseous pollutant emissions. Such a system may be termed an in-situ Continuous Emission Monitoring System (CEMS). A known CEMS can utilise up to eight wavelengths of light to monitor up to six different gases within the exhaust stream, whereby the wavelength used for monitoring is dependent on the gases to be detected. For specific gases the CEMS may also use the known Gas Filter Correlation (GFC) technique.

[0004] An example of the known prior art in-situ IR photometer is shown in Figure 1 (Prior Art), generally designated 8. A sampling probe 10 is shown which is inserted in an exhaust stack (not shown) up to a flange 12 such that a body 14 of the photometer is on an outside of the exhaust stack. A sampling cell 16 is provided in the probe 10 which comprises a sintered assembly. A lens 18 is also shown in the probe 10. An IR source 20 is provided in the body 14 which transmits IR light to exhaust gas in the gas cell 16. The IR light passes through one of eight filters provided on a filter wheel 22, which is selected under the control of a motor 24. After the IR light has passed through the gas sample and back it impinges on an IR detector 26. This known photometer 8 is a multi-component device that uses eight wavelengths, typically using two wavelengths for each gas (reference and measure) and therefore working on the principle of "dual wavelength single beam" detection.

[0005] A problem with the known in-situ CEMS is that the limit of performance to measure gaseous emissions over a smaller monitoring range has been reached. This may be a particular problem with a marine Exhaust Gas Cleaning System (EGCS), where the measurement of exhaust stack gases is difficult to achieve after the exhaust gas has been cleaned (e.g. cleaning using sea water Flue Gas Desulfurization (FGD) systems) to remove Sulphur Dioxide (SO 2 ). For example, the known in-situ CEMS typically provides a range measurement for SO 2 in the range 0 - 150ppm, and a range measurement for NO 2 in the range 0 - 300ppm, but these ranges are not low enough (i.e. do not provide sufficient accuracy / sensitivity) to meet the latest legislation.

[0006] Further, drift in the measurement of gases using photometers is a known problem. To remove drift, two different techniques are known - "single wavelength dual beam" and "dual wavelength single beam". The "single wavelength dual beam" technique uses a common light source and detector, whereby the beam is split into two paths. One path is passed through a sample cell containing the gas sample and the other path is passed through a parallel cell containing air. By comparing the signals from each path, the concentration of a particular gas can be calculated. The "dual wavelength single beam" technique uses a common source and detector, whereby a reference wavelength and a measured wavelength are alternatively sent through the sample cell as a single beam. By comparing the signals from each of the wavelengths, the concentration of a particular gas can be calculated. In general it is considered that the "dual wavelength single beam" is more effective at reducing or avoiding drift that the "single wavelength dual beam".

[0007] It is known from US20140361171 (SICK AG) to provide a two-channelled measurement apparatus for the determination of gas concentrations. The apparatus comprises two spectral channels, wherein the channels are separated by a single chopper wheel. The chopper wheel brings the light of the two light sources onto the same measuring path, and associates the light to the associated receiver.

[0008] It is known from JP5349996 (CENTRAL RES INST ELECT) to provide an apparatus for continuously measuring a concentration of a gas in real time. The apparatus calculates the concentration of sulfur oxide in a target gas on the basis of intensities of both of a signal light and a reference light measured by the photodetector.

[0009] It is known from WO03019160 (SPX Corporation) to provide an optical path structure for open path emissions sensing using an open path device for measuring particulate matter and / or gaseous components in an air sample. Particulate matter and / or gaseous component concentrations are derived from light intensities measured by a detection unit using the Lambert-Beer law.

[0010] It is known from EP0193718 (HARTMANN & BRAUN AG) to provide a rotating modulation device to achieve alignment of the phases of a measurement beam and a comparison beam. The rotating modulation device has a disc with openings and a structural part with corresponding openings.

[0011] It is known from JPH05332933 (ANIMA CORP) to provide a CO 2 concentration measuring device for the expiration gas of a person using an infrared light source.

[0012] It is known from US3162761 (ONERA (OFF NAT AEROSPATIALE) to provide an apparatus for analysing a mixture of substances by selective absorption of infrared radiation.

[0013] It is known from US5386295 (EASTMAN KODAK Company) to provide a postacquired spectrophotometer device, which provides modulation, wavelength selection and switching of one more pairs of sample and reference light beams.

[0014] It is known from US2013039811 (SICK AG) to provide an apparatus for the determination of a concentration of a component to be measured in a gas. The gas to be analysed is supplied to a measurement cuvette and to a reference cuvette via an absorption apparatus, which includes a substance which completely absorbs the component to be measured.

[0015] It is known from US3999062 (IBM) to provide a spectrophotometer for dual mode fluorescence analysis. The spectrophotometer includes a wide band radiant energy source and a detector for providing an output signal proportional to the intensity of the radiant energy received.

[0016] It is broadly an object of the present invention to address one or more of the above mentioned disadvantages of the previously known apparatus and methods.Summary

[0017] What is required is a photometer and method for analysing the composition of a sample gas, which may reduce or minimise at least some of the above-mentioned problems.

[0018] According to the invention there is provided a photometer and a method according to the claims.

[0019] According to a first aspect, there is provided a photometer for analysing the composition of a sample gas. The photometer comprises an infra-red (IR) source configured to direct a first plurality of pulses of IR radiation through the sample gas to an IR detector. The photometer further comprises an ultraviolet (UV) source configured to generate a second plurality of pulses of UV radiation for conveyance to a UV detector. The UV source comprising two UV diodes configured to be operated sequentially at different wavelengths. A path selection arrangement configured to selectively convey different ones of the second plurality of pulses through the sample gas and to the UV detector. The photometer further comprises processing circuitry coupled to the IR source, the UV source, the IR detector, the UV detector and the path selection arrangement. The processing circuitry is configured to (i) select the wavelength to be used for a given UV pulse of the second plurality of pulses, (ii) receive a plurality of detection signals from each of the IR detector and the UV detector and (iii) based on the detection signals, determine a concentration of at least one component of the sample gas.

[0020] Such an apparatus provides the advantage that, at least in embodiments, using UV monitoring to measure SO 2 significantly improves the performance of the in-situ IR photometer in terms of range measurement. For example, the in-situ CEMS of the invention typically provides an improved range measurement for SO 2 in the range 0 - 30ppm, and a range measurement for NO 2 in the range 0 - 30ppm. The advantages provided by the UV diode include being able to use the in-situ IR photometer in the marine industry to measure and analyse exhaust stack gases after being cleaned by the known marine Exhaust Gas Cleaning Systems using sea water Flue Gas Desulfurization. Such an improved in-situ infra-red photometer may also improve the capability of the apparatus in land based FGD applications for SO 2 monitoring, or land based Selective Catalytic Reduction (SCR) applications for NO 2 monitoring. It will be appreciated that for item (iii) above the determination of the concentration of at least one component of the sample gas is based on the level (i.e. amplitude, intensity, or magnitude) of the detection signals.

[0021] According to the invention at least two of the second plurality of pulses are of different wavelength. Such arrangements provide a reference and a measure wavelength, and have the advantage of an improved stability of the measured signal as per the "dual wavelength single beam" detection technique.

[0022] Preferably the path selection arrangement includes a rotating member, the rotating member having at least one annular inner slot and at least one annular outer slot, the inner slot and outer slot being at radially different positions. Preferably the path selection arrangement includes a conveyance post having a first port and a second port, whereby conveyance of UV radiation from the UV source to the first port and the second port is enabled when the inner slot and the outer slot overlap with the first port and the second port, respectively. Preferably the path selection arrangement further includes a first light path coupling the first port to a transmitter part of a light transmitter / receiver module, for directing the UV pulse to the sample gas. Preferably the path selection arrangement further includes a second light path coupling a receiver part of the light transmitter / receiver module to the UV detector. Preferably the path selection arrangement further includes a third light path coupling the second port direct to the UV detector.

[0023] In one embodiment, the first light path, the second light path and / or the third light path comprise light guides. Preferably at least one of the light guides is an optical fibre.

[0024] Preferably the processing circuitry is configured to control the timing of the pulses of the second plurality of pulses and select the wavelength to be used for a given UV pulse of the second plurality, whereby successive pulses through the first port and / or successive pulses through the second port are of different wavelengths.

[0025] In one embodiment, the processing circuitry is configured to control the timing of the pulses of the second plurality of pulses and / or select the wavelength to be used for a given UV pulse of the second plurality, whereby the plurality of detection signals received by the UV detector include: (i) a reference reading, corresponding to a path of UV radiation direct from the UV source to the UV detector, (ii) a dark reading, corresponding to the sample gas not being illuminated by UV radiation, and / or (iii) a measurement reading, corresponding to a path of UV radiation from the UV source to the UV detector via the sample gas.

[0026] Preferably the measurement reading corresponds to the inner slot overlapping with the first port, the reference reading corresponds to the outer slot overlapping with the second port, and / or the dark reading corresponds to no overlapping of the slots and ports.

[0027] In one embodiment, the path selection arrangement includes at least one pair of inner slots and / or at least one pair of outer slots. Preferably, for a given pair, the inner slots and / or outer slots are spaced apart around the rotating member. Preferably, for a given pair, the inner slots and / or outer slots are diametrically opposed.

[0028] In one embodiment, the rotating member includes thereon a plurality of timing indicia and wherein the processing circuitry is configured to (i) detect, during rotation of the rotating member, the passing of the indicia past an indicium detector; and (ii) determine the angular position of the inner slots and / or outer slots based on the detected passing of indicia. Preferably the timing indicia include a primary indicium and a plurality of secondary indicia. Preferably the timing indicia comprise a plurality of through holes or cut-outs angularly spaced apart around the rotating member. Preferably the timing indicia comprise a plurality of through holes or cut-outs equally angularly spaced apart around the rotating member. Preferably the timing indicia comprise cut-outs disposed at the periphery of the rotating member. Preferably the primary indicium is larger than the secondary indicia. Preferably the indicium detector comprises an optical transmitter / receiver.

[0029] Preferably the rotating member comprises a filter wheel having a plurality of filter elements spaced apart around the filter wheel; wherein the filter elements are disposed at a different radial position to the inner slots and / or outer slots.

[0030] According to another aspect there is provided a method of analysing the composition of a sample gas. The method comprises providing a photometer according to any of claims of the appended claims. The method further comprises operating the processing circuitry to (i) receive a plurality of detection signals from each of the IR detector and the UV detector and (ii) based on the detection signals, determine a concentration of at least one component of the sample gas.

[0031] Preferably the method further includes operating the processing circuitry to select the wavelength to be used for a given UV pulse of the second plurality of pulses.

[0032] According to an alternative characterisation there is provided a photometer for analysing the composition of a sample gas, the photometer comprising: an infra-red (IR) source configured to direct a first plurality of pulses of IR radiation through the sample gas in a sample cell to an IR detector; an ultraviolet (UV) source configured to direct a second plurality of pulses of UV radiation through the sample gas to a UV detector; and processing circuitry coupled to the IR source, the UV source, the IR detector and the UV detector, the processing circuitry being configured to (i) receive a plurality of detection signals from each of the IR detector and the UV detector and (ii) based on the detection signals, determine a concentration of at least one component of the sample gas.

[0033] According to an alternative characterisation there is provided a photometer for analysing the composition of a sample gas, the photometer comprising: an infra-red (IR) source configured to direct a first plurality of pulses of IR radiation through the sample gas to an IR detector; an ultraviolet (UV) source configured to direct a second plurality of pulses of UV radiation through the sample gas to a UV detector,; and processing circuitry coupled to the IR source, the UV source, the IR detector and the UV detector, the processing circuitry being configured to (i) receive a plurality of detection signals from each of the IR detector and the UV detector and (ii) based on the detection signals, determine a concentration of at least one component of the sample gas.

[0034] According to an alternative characterisation there is provided a photometer for analysing the composition of a sample gas, the photometer comprising: an infra-red (IR) source configured to direct a first plurality of pulses of IR radiation through the sample gas to an IR detector; an ultraviolet (UV) source configured to direct a second plurality of pulses of UV radiation through the sample gas to a UV detector; and processing circuitry coupled to the IR source, the UV source, the IR detector and the UV detector, the processing circuitry being configured to (i) receive a plurality of detection signals from each of the IR detector and the UV detector, (ii) determine a first concentration of a first component of the sample gas using detection signals from the IR detector, (iii) determine a second concentration of a second component of the sample gas using detection signals from the UV detector, and (iv) based on the detection signals, determine a corrected concentration of the second component, the corrected concentration being the second concentration corrected for cross-sensitivity based on the first concentration.Brief Description of the Drawings

[0035] Other features of the invention will be apparent from the following description of preferred embodiments shown by way of example only with reference to the accompanying drawings, in which; Figure 1 (PRIOR ART) shows a known form of gas photometer; Figure 2 is a perspective view of a photometer according to an embodiment of the invention; Figure 3 is an enlarged view of a path selection arrangement using in the photometer of Fig. 2; Figure 4 is an axial view of the path selection arrangement of Fig. 3; and Figure 5 shows a method according to an embodiment of the invention. Detailed Description

[0036] Figures 2, 3 and 4 show an improved in-situ Infra-Red (IR) photometer with combined IR and Ultra Violet (UV) measurement according to an embodiment of the invention, generally designated 30. In Figures 2 to 4 like features to the prior art arrangement of Figure 1 are shown with like reference numerals. In Figures 2 and 3 a UV source 32 (e.g. a UV light emitting diode) is provided in the body 14 which transmits UV light to exhaust gas in the gas cell 16 through one of two light slots 45, 47, which are at different radial positions in the filter wheel 22. The two light slots 45, 47 are through-holes in the filter wheel 22, and are part-circular (i.e. arcuate) and elongate. After the UV light has passed through the gas sample and back it impinges on a UV detector 36. Also shown in Figure 2 is a UV signal 38 produced from the UV detector, and IR signal 40 produced from the IR detector 26. Also shown are the eight filters 21 of the filter wheel 22 for selecting one of the eight wavelengths as described above using the IR source 20.

[0037] As shown in Figures 3 and 4 the UV source 32 alternatively illuminates the two pairs of slots 45, 47 as the filter wheel 22 rotates, shown at 23. The UV light passes through the two pairs of slots 45, 47 and is then input to the two optic fibres 44, 46 which are connected to a pillar 42, which is mounted on a support 43. After passing through the spinning filter wheel 22 the UV light illuminates the two optic fibres 44, 46 in turn. As shown in Figure 4 the pillar 42 provides the optic fibre entry point for the optic fibres 44, 46 one above the other so that one UV light beam is provided via the optic fibre 44 to the detector 36, and the other UV light beam is provided via the optic fibre 46 to a transmitter / receiver module 48. Figure 4 shows the arrangement of the filter wheel 22 relative to the pillar 42 for the optic fibres 44, 46. Also shown in Figure 4 are the two light slots 45 on the filter wheel 22, which are diametrically opposed to each other and offset from the two light slots 47 on the filter wheel 22 which are also diametrically opposed to each other. The light slot 45 is to provide the UV beams to the UV detector 36 via the optic fibre 44, and the light slots 47 is for the UV beam to the transmitter / receiver module 48. The transmitter / receiver module 48 is a lens pillar module and operates to transmit UV light from the optic fibre 46 through the gas sample in the gas cell 16, and to receive reflected UV light from the gas sample in the gas cell 16 (as shown by the double headed arrow 49) and to pass it to the detector 36 via an optic fibre 50.

[0038] As the motor 24 rotates the filter wheel 22 the two light slots 45, 47 for the UV light provide the possibility of using two UV light beams, which are alternatively switched. The UV source 32 is for SO 2 monitoring and operates at a wavelength of 250 nm to 325 nm (and may be centred around 280 nm) in a dual path configuration, which meets the requirement of using a single source and detector as required for the "dual wavelength single beam" technique mentioned above.

[0039] The UV source 32 contains two UV diodes that sequentially switch and operate at different wavelengths (one to monitor SO 2 operating between 250 nm to 325 nm the other to monitor NO 2 operating between 350 nm to 450 nm). Such an arrangement for UV monitoring combined with the IR capabilities permits two gases (i.e. SO 2 and NO 2 ) to be monitored, and also provides the advantage that lower NO 2 concentrations can be monitored.

[0040] The data from each of the UV and IR systems is also used to correct for cross sensitivity (i.e. water vapour effect on the NO 2 ). This is achieved by monitoring the NO 2 in the UV signal and correcting for any cross sensitivity using the IR range for H 2 O. In this manner the UV and IR capabilities are combined to produce an improved monitoring compared to ether the UV or IR capability when used alone.

[0041] The signal processing of the UV signal 38 and IR signal 40 is carried out on an analyser PCB (not shown), which utilises the existing sixteen timing slots 51 on the circumference of the filter wheel 22 to determine the position of the UV light slots 45, 47 with respect to the UV source(s). Using the arrangements shown in Figures 2 to 4 the UV beam from the UV source 32 is alternatively sent through the sample cell 16 and directly to the detector 36. With such an arrangement any drift caused by either the UV detector 36 or the UV source 32 can be removed. This is achieved by the two off-set slots 45, 47 in the filter wheel 22 whereby one slot 45 is aligned with the fibre 46 to the gas cell 16, and the other slot 47 is aligned with the fibre 44 to the UV detector 36. It will be appreciated that UV pulses are alternatively sent to the UV detector 36 via the gas cell 16 (i.e. fibre 46 out, fibre 50 return), and directly to the UV detector 36 (i.e. via the fibre 44). The timing and switching of the UV beams are achieved by using the filter wheel 22 with the slots 45, 47 to provide a reference reading, a dark reading, and a measure reading. All signals are combine electronically using a microprocessor device (not shown) to provide measurement readings as required on a display (not shown) which may be in the form of a graph of as numerical values.

[0042] It will be appreciated that whereas the use of optic fibre 44, 46, 50 is described above, in an alternative arrangement other types of waveguide or light guides may be used to achieve the dual UV paths. It will also be appreciated that the filter wheel 22 continuously rotates as shown at 23 when performing both UV and IR monitoring.

[0043] Figure 5 shows steps of a method according to an embodiment of the invention, generally designated 60. It will be appreciated that the steps may be performed in a different order, and may not necessarily be performed in the order shown in Figure 5.

[0044] Firstly, a photometer is provided (block 62) in-situ where the gas is to be analysed, for example as in the measurement scenarios mentioned hereinabove. Processing circuitry (e.g. the above-mentioned analyser PCB (not shown)) controls the timing and wavelength of pulses, and the method proceeds with the processing circuitry selecting (block 64) the wavelength of a given UV pulse (i.e. the measure and reference UV pulses), using the aforementioned timing mechanisms. In one arrangement the UV wavelengths are dynamically selected. Next, the IR pulses from the IR source and the UV pulses from the UV source are directed (block 66) at the sample gas in gas cell. Using the IR detector and UV detector, detection signals are received (block 68). Finally, the processing circuitry determines (block 70) a concentration of gas component of the sample gas based on the detection signals.

[0045] The arrangements described herein provide a photometer with a sufficiently acceptable performance in terms of drift and noise, and also permit the use of UV and IR measurement in a combined unit with only one sampling cell 16. It will be appreciated that whereas the UV diode 32 operates at a narrow fixed wavelength, the combination with IR measurement permits the apparatus to be used with the conventional "dual beam" principle in an "in-situ" gas analyser and over a broader wavelength range. In summary, embodiments of the invention provide: 1. Combined UV and IR optical system with combined UV and IR timing system; 2. Combined UV and IR measurement in the same sample cell; 3. Combined signal processing of the UV and IR signals; and 4. Combined cross sensitivity corrections.

[0046] Furthermore, the improved in-situ IR photometer provides an additional range capability (i.e. two additional UV channels), which increase the prior art system from 6 to 8 channels. In addition the improved in-situ IR photometer releases IR wavelengths, which reduces compromises on gas measurement for certain applications. A major limitation of the prior art apparatus of Figure 1 is that there are only 8 wavelengths available so when monitoring 6 gas species it is necessary to make compromises on the wavelengths allocated to each gas depending on the gases to be monitored. When monitoring NO and CO (using GFC) and SO 2 (using a Measure Filter & Wide Band Filter method), which takes up two wavelength slots each, then for cross sensitivity correction it is necessary to monitor CO 2 & H 2 O. Using the prior art apparatus of Figure 1, as there are only two slots left it is necessary to use a fixed reference number for CO 2 & H 2 O which is not ideal and can compromise the accuracy of the measurement. When using the prior art apparatus of Figure 1 a measure filter is often used for one gas as a reference filter for a second gas, which adds significantly to calibration time and accuracy of the measurement. Using embodiments of the invention shown in Figures 2 to 4, the inclusion of a UV SO 2 channel frees up two filter slots for IR ranges. Such an arrangement provides additional operational flexibility for the apparatus of the invention, and also improves the cross sensitivity correction for gas monitoring.

Claims

1. A photometer (30) for analysing the composition of a sample gas in a gas sample cell (16), the photometer comprising: an infra-red (IR) source (20) and an IR detector (26), the IR source (20) configured to direct a first plurality of pulses of IR radiation through the sample gas to the IR detector (26); an ultraviolet (UV) source (32) and a UV detector (36), the UV source (32) configured to generate a second plurality of pulses of UV radiation for conveyance to the UV detector (36); the ultraviolet (UV) source (32) comprising two UV diodes (32) configured to sequentially switch and operate at different wavelengths; a path selection arrangement (22) configured to provide said conveyance and to alternatively convey different ones of the second plurality of pulses through the sample gas and to the UV detector (36); the first and second plurality of pulses being provided by the path selection arrangement (22); and processing circuitry coupled to the IR source (20), the UV source (32), the IR detector (26), the UV detector (36) and the path selection arrangement (22), the processing circuitry being configured to (i) select the wavelength to be used for a given UV pulse of the second plurality of pulses, (ii) receive a plurality of detection signals from each of the IR detector (26) and the UV detector (36) and (iii) based on the detection signals, determine a concentration of at least one component of the sample gas, wherein at least two of the second plurality of pulses are of different wavelength.

2. The photometer according to claim 1, wherein at least two of the first plurality of pulses are of different wavelength.

3. The photometer according to claim 1 or 2, wherein the path selection arrangement (22) includes a rotating member, the rotating member having at least one annular inner slot (45) and at least one annular outer slot (47), the inner slot (45) and outer slot (47) being at radially different positions.

4. The photometer according to claim 3, wherein the path selection arrangement (22) includes a conveyance post (42) having a first port and a second port, whereby conveyance of UV radiation from the UV source (32) to the first port and the second port is enabled when the inner slot (45) and the outer slot (47) overlap with the first port and the second port, respectively.

5. The photometer according to claim 4, wherein the path selection arrangement (22) further includes a first light path (46) coupling the first port to a transmitter part of a light transmitter / receiver module (48), for directing the UV pulse to the sample gas.

6. The photometer according to claim 5, wherein the path selection arrangement (22) further includes a second light path (50) coupling a receiver part of the light transmitter / receiver module (48) to the UV detector (36).

7. The photometer according to claim 5 or 6, wherein the path selection arrangement (22) further includes a third light path (44) coupling the second port direct to the UV detector (36).

8. The photometer according to claim 5, 6, or 7, wherein the first light path (46), the second light path (50) and / or the third light path (44) comprise light guides.

9. The photometer according to claim 1 and 4, or any claim dependent thereon, wherein the processing circuitry is configured to control the timing of the pulses of the second plurality of pulses and select the wavelength to be used for a given UV pulse of the second plurality, whereby successive pulses through the first port and / or successive pulses through the second port are of different wavelengths.

10. The photometer according to any of the preceding claims, wherein the processing circuitry is configured to control the timing of the pulses of the second plurality of pulses and / or select the wavelength to be used for a given UV pulse of the second plurality, whereby the plurality of detection signals received by the UV detector (36) include: (i) a reference reading, corresponding to a path of UV radiation direct from the UV source (32) to the UV detector (36), (ii) a dark reading, corresponding to the sample gas not being illuminated by UV radiation, and / or (iii) a measurement reading, corresponding to a path of UV radiation from the UV source (32) to the UV detector (36) via the sample gas.

11. The photometer according to claim 3, or any claim dependent thereon, wherein the path selection arrangement (22) includes at least one pair of inner slots (45) and / or at least one pair of outer slots (47).

12. The photometer according to claim 11, wherein, for a given pair, the inner slots (45) and / or outer slots (47) are spaced apart around the rotating member; or wherein, for a given pair, the inner slots (45) and / or outer slots (47) are diametrically opposed.

13. The photometer according to claim 3, or any claim dependent thereon, wherein the rotating member comprises a filter wheel having a plurality of filter elements (21) spaced apart around the filter wheel; wherein the filter elements (21) are disposed at a different radial position to the inner slots (45) and / or outer slots (47).

14. A method of analysing the composition of a sample gas in a gas sample cell, (16) the method comprising: providing a photometer (3) according to any of the preceding claims; and operating the processing circuitry to (i) receive a plurality of detection signals from each of the IR detector (26) and the UV detector (36) and (ii) based on the detection signals, determine a concentration of at least one component of the sample gas, wherein at least two of the second plurality of pulses are of different wavelength.

15. A method according to claim 14, and further including operating the processing circuitry to select the wavelength to be used for a given UV pulse of the second plurality of pulses.