IN-SITU INFRA-RED ULTRA-VIOLET PHOTOMETER

Abstract

The invention relates to a photometer (30) for analysing the composition of a sample gas. The photometer comprises an infra-red (IR) source (20) configured to direct a first plurality of pulses (40) of IR radiation through the sample gas to an IR detector (26), at least two of the first plurality of pulses being of different wavelength. The photometer further comprises an ultraviolet (UV) source (32) configured to generate a second plurality of pulses (38) of UV radiation for conveyance to a UV detector (36), at least two of the second plurality of pulses being of different wavelength. A path selection arrangement (22, 42-50) is configured to selectively convey different ones of the second plurality of pulses (38) to one of the sample gas and the UV detector (36). The photometer further comprises 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, 42-50). The processing circuitry is configured to (i) select the wavelength to be used for a given UV pulse of the second plurality of pulses (38), (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. A method for analysing the composition of a sample gas is also disclosed.

Claims

1. A photometer for analysing the composition of a sample gas in a gas sample cell, the photometer comprising: an infra-red (IR) source and an IR detector, the IR source configured to direct a first plurality of pulses of IR radiation through the sample gas to the IR detector; an ultraviolet (UV) source and a UV detector, the UV source configured to generate a second plurality of pulses of UV radiation for conveyance to the UV detector, wherein the UV source comprises two UV diodes operable at different wavelengths; a path selection arrangement configured to alternatively convey different ones of the second plurality of pulses through the sample gas and to the UV detector, wherein the first and second plurality of pulses being provided by the path selection arrangement; and processing circuitry coupled to the IR source, the UV source, the IR detector, the UV detector and the path selection arrangement, the processing circuitry being configured to (i) select a 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, 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, wherein 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 at least one annular inner slot and the at least one annular outer slot being at radially different positions.

4. The photometer according to claim 3, wherein the path selection arrangement includes a conveyance pillar 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 at least one annular inner slot and the at least one annular outer slot overlap with the first port and the second port, respectively.

5. The photometer according to claim 4, wherein 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 given UV pulse to the sample gas.

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

7. The photometer according to claim 6, wherein the path selection arrangement further includes a third light path coupling the second port directly to the UV detector.

8. The photometer according to claim 7, wherein the first light path, the second light path and/or the third light path comprise light guides.

9. The photometer according to claim 5, wherein the processing circuitry is further configured to control a timing of the pulses of the second plurality of pulses, whereby successive pulses through one or both of the first and second ports are of different wavelengths.

10. The photometer according to claim 1, wherein the processing circuitry is further configured to control a timing of the pulses of the second plurality of pulses, 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.

11. The photometer according to claim 4, wherein the path selection arrangement includes at least one pair of inner slots and/or at least one pair of outer slots.

12. The photometer according to claim 11, wherein, for a given pair, the at least one pair of inner slots and/or the at least one pair of outer slots are either spaced apart around the rotating member or are diametrically opposed to each other on the filter wheel.

13. The photometer according to claim 4, wherein 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 annular inner slots and/or annular outer slots.

14. A method of analysing the composition of a sample gas in a gas sample cell, the method comprising the steps of: providing a 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 generate a second plurality of pulses of UV radiation for conveyance to a UV detector, the UV source comprising two UV diodes operable 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 first and second plurality of pulses being provided by the path selection arrangement; and processing circuitry coupled to the IR source, the UV source, the IR detector, the UV detector and the path selection arrangement; operating the processing circuitry to receive a plurality of detection signals from each of the IR detector and the UV detector; and determining a concentration of at least one component of the sample gas based on the detection signals, wherein at least two of the second plurality of pulses are of different wavelength.

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

16. A photometer for analysing the composition of a sample gas in a gas sample cell, the photometer comprising: an infra-red (IR) source and an IR detector, the IR source configured to direct a first plurality of pulses of IR radiation through the sample gas to the IR detector; an ultraviolet (UV) source and a UV detector, the UV source configured to generate a second plurality of pulses of UV radiation for conveyance to the UV detector; a path selection arrangement configured to alternatively convey different ones of the second plurality of pulses through the sample gas and to the UV detector, wherein the first and second plurality of pulses being provided by the path selection arrangement; and processing circuitry coupled to the IR source, the UV source, the IR detector, the UV detector and the path selection arrangement, the processing circuitry being configured to (i) select a 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, wherein 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 at least one annular inner slot and the at least one annular outer slot being at radially different positions.

17. The photometer according to claim 16, wherein at least two of the first plurality of pulses are of different wavelength.

18. The photometer according to claim 16, wherein at least two of the second plurality of pulses are of different wavelength.

19. The photometer according to claim 16, wherein the path selection arrangement includes a conveyance pillar 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 at least one annular inner slot and the at least one annular outer slot overlap with the first port and the second port, respectively.

20. The photometer according to claim 19, wherein 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 given UV pulse to the sample gas.

21. The photometer according to claim 20, wherein the path selection arrangement further includes a second light path coupling a receiver part of the light transmitter/receiver module to the UV detector.

22. The photometer according to claim 20, wherein the path selection arrangement further includes a third light path coupling the second port directly to the UV detector.

23. The photometer according to claim 20, wherein the first light path, the second light path and/or the third light path comprise light guides.

24. The photometer according to claim 19, wherein the processing circuitry is further configured to control a timing of the pulses of the second plurality of pulses, whereby successive pulses through one or both of the first and second ports are of different wavelengths.

25. The photometer according to claim 16, wherein the processing circuitry is further configured to control a timing of the pulses of the second plurality of pulses, 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.

26. The photometer according to claim 16, wherein the path selection arrangement includes at least one pair of inner slots and/or at least one pair of outer slots.

27. The photometer according to claim 26, wherein, for a given pair, the at least one pair of inner slots and/or the at least one pair of outer slots are either spaced apart around the rotating member or are diametrically opposed to each other on the filter wheel.

28. The photometer according to claim 16, wherein 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 annular inner slots and/or the annular outer slots.

29. A method of analysing the composition of a sample gas in a gas sample cell, the method comprising: providing a 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 generate a second plurality of pulses of UV radiation for conveyance to a UV detector, 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 first and second plurality of pulses being provided by the path selection arrangement; and processing circuitry coupled to the IR source, the UV source, the IR detector, the UV detector and the path selection arrangement; operating the processing circuitry to receive a plurality of detection signals from each of the IR detector and the UV detector; and determining a concentration of at least one component of the sample gas, wherein 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.

30. A method according to claim 29, further comprising the step of operating the processing circuitry to select a wavelength to be used for a given UV pulse of the second plurality of pulses.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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;

[0028] FIG. 1 (PRIOR ART) shows a known form of gas photometer;

[0029] FIG. 2 is a perspective view of a photometer according to an embodiment of the invention;

[0030] FIG. 3 is an enlarged view of a path selection arrangement using in the photometer of FIG. 2;

[0031] FIG. 4 is an axial view of the path selection arrangement of FIG. 3; and

[0032] FIG. 5 shows a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0033] FIGS. 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 FIGS. 2 to 4 like features to the prior art arrangement of FIG. 1 are shown with like reference numerals. In FIGS. 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 FIG. 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.

[0034] As shown in FIGS. 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 FIG. 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. FIG. 4 shows the arrangement of the filter wheel 22 relative to the pillar 42 for the optic fibres 44, 46. Also shown in FIG. 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.

[0035] 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.sub.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.

[0036] The UV source 32 contains two UV diodes that sequentially switch and operate at different wavelengths (one to monitor SO.sub.2 operating between 250 nm to 325 nm the other to monitor NO.sub.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.sub.2 and NO.sub.2) to be monitored, and also provides the advantage that lower NO.sub.2 concentrations can be monitored.

[0037] 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.sub.2). This is achieved by monitoring the NO.sub.2 in the UV signal and correcting for any cross sensitivity using the IR range for H.sub.2O. 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.

[0038] 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 FIGS. 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.

[0039] 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.

[0040] FIG. 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 FIG. 5.

[0041] 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.

[0042] 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: [0043] 1. Combined UV and IR optical system with combined UV and IR timing system; [0044] 2. Combined UV and IR measurement in the same sample cell; [0045] 3. Combined signal processing of the UV and IR signals; and [0046] 4. Combined cross sensitivity corrections.

[0047] 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 FIG. 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.sub.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.sub.2 & H.sub.2O. Using the prior art apparatus of FIG. 1, as there are only two slots left it is necessary to use a fixed reference number for CO.sub.2 & H.sub.2O which is not ideal and can compromise the accuracy of the measurement. When using the prior art apparatus of FIG. 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 FIGS. 2 to 4, the inclusion of a UV SO.sub.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.