WO2007121593A1

WO2007121593A1 - Method for measurement and determination of concentration within a mixed medium

Info

Publication number: WO2007121593A1
Application number: PCT/CH2006/000230
Authority: WO
Inventors: Urs Meier; Markus Naegele; Philippe Prêtre; Hubert Brändle
Original assignee: Abb Research Ltd
Priority date: 2006-04-26
Filing date: 2006-04-26
Publication date: 2007-11-01

Abstract

A method for the measurement and determination of the concentration of a certain component within a mixed medium by the absorption of infrared light, comprises the steps of - generating a modulated beam of infrared light; - passing said modulated beam of infrared light through the mixed medium; - receiving said modulated beam after having passed though said mixed medium by means of an infrared detector (17); and - receiving and processing electrical signals of said infrared detector (17), said signals being characteristic of the received modulated infrared beam. The measurement is substantially improved and simplified when - the modulated beam of infrared light is generated by means of a modulated broad band infrared light source; - the modulated beam is sent through a wavelength-selective device (16) after having passed through said mixed medium, and before being received by said infrared detector (17); and - a multi-channel detector array (17) is used as said infrared detector.

Description

DESCRIPTION

METHOD FOR MEASUREMENT AND DETERMINATION OF CONCENTRATION WITHIN A MIXED MEDIUM

FIELD OF THE INVENTION

The present invention relates generally to a method and system for the measurement and determination of the concentration of a certain component within a mixed medium (gas or fluid mixture), based on the optical measurement of the absorption or reflection properties of the medium, and its components, in the infrared wavelength region.

BACKGROUND OF THE INVENTION

Today continuously operated thermal emitters with broadband spectral emission characteristics are used as light sources in infrared photometers. By passing a rotating or oscillating slit (chopper) light pulses are generated, which are needed for the frequency-selective "lock-In" measurement technique with the goal of rotating or oscillating slit (chopper) light pulses are generated, which are needed for the frequency-selective "lock-In" measurement technique with the goal of effective noise suppression (in some devices the sensor signal is based on a photo-acoustic effect, which also requires pulsed light; such devices are not subject of this invention disclosure).

The wavelength selection is achieved by the use of non-dispersive media (optical filters, interference filters, etc.), which can be moved into the optical path (e.g. by being located on the chopper plate). Each component to be measured requires at least one filter to extract the spectral area, where this component has a remarkable (maximum) absorption and thus, suppress all the other radiation. Furthermore, a measurement in a spectral region without any influence of the component(s) to be measured is needed as a reference. Thus, for each possible combination of components a set of dedicated filters is needed.

Due to the poor spectral intensity of thermal light sources and the minimum required light intensity on the detector (which correlates directly with the spectral width of the filter) it is very difficult (or sometimes nearly impossible) to avoid cross interferences between different absorbing gases and to find a reference wavelength without any absorptions of disturbing gases. Nevertheless, this is a must to obtain an optimal measurement result. Today's process photometers are very limited in their areas of application due to these cross interferences. Another drawback is the mechanical sensitivity of these systems, namely the movable parts in the sensor, which have a negative impact in terms of the limitation of the areas of application and the lifetime of the device. However, an advantage of simple and robust prior art process photometers is their rather low price (see ABB Product Brochure, ..Process Photometers - PUV3402 and PIR3502, Applications, Technology and Data" (2002), where the principle of operation and an optical schematic of such a prior art process photometer is disclosed).

Spectrometers, which are equipped with spectral broad band light sources (thermal emitters), have to utilize somewhere in the optical path a dispersive device like an optical grating or a prism and a beam deflection unit in combination with one detector (in this case the wavelength scan will take a certain time period) or lateral interference filters in combination with detector arrays (in this case one gets immediate information about the whole spectrum) to achieve their spectral resolution. Thus, the spectral absorption structure of the interesting component will be obtained. Like in process photometers these spectrometers also use a mechanical modulation of the light beam to apply the ,,Lock-ln" technology for noise suppression.

Due to the rather low spectral energy density of the thermal light sources these devices are limited in terms of the achievable detection limits, especially if a high spectral resolution is required, because the signal-to-noise ratio on the detector is nearly proportional to the incident light intensity. Besides the broad accessible spectral range, which results in many different measurable components and a multi-component capability, a further advantage of these rather simple devices is their maturity and their low price in comparison to other technologies. The required use of movable parts in the device leads to a mechanical sensitivity of these systems and has also a negative impact in terms of the limitation of the areas of application and the lifetime of the device (see for example: US-A-6,072,578, DE- C1-198 13 950, WO-A1 -91/03714, or B. Willing, P. Muralt, N. Setter, and O. Oehler, ,,Gas Spectrometry based on a Pyroelectric Thin Film Array", Ferroelectrics vol. 255, p. 807-813 (1999), or B. Willing, M. Kohli, P. Muralt, O. Oehler, ,,Thin film pyroelectric array as a detector for an infrared gas spectrometer", Infrared Physics & Technology vol. 39, p. 443-449 (1998)).

The Hadamard transformation was originally developed to increase the optical throughput in dispersive spectrometers. A mask with a special combination of slits is located in front of the detector. By the use of a series of masks, which are coded according to the Hadamard method, a set of spectral data will be obtained, which can be post-processed in a Hadamard de-convolution process back to an absorption spectrum. The advantage of this method is the increased light intensity on the detector and thus the increase in signal-to-noise ratio by the use of more than one slits, which ends up in an improved signal-to-noise ratio of the calculated absorption spectrum (see for example: G. Nitzsche, R. Riesenberg, ..Noise, Fluctuation and Hadamard-Transform-Spectrometry", Fluctuations and Noise in Photonics and Quantum Optics, Proc. SPIE, vol. 5111 , p. 273-282 (2003)).

The Hadamard transformation can also be applied in electronic circuits, namely in analogue signal amplifiers, to reduce the noise by coupling multiple channels of a detector together according to the Hadamard method before that signal will be amplified. This will lead also to an improved signal-to-noise ratio, now for the electronic signal instead of the optical signal, which was discussed in the previous section.

In literature, one can find numerous problematic examples of mixtures of different known components, which have to be analyzed to determine the concentration of one component. In the optical spectroscopy, the Beer law leads to the following expression for the transmitted light intensity l(λ), which depends on the wavelength λ, the transmitted light intensity without any absorptions I₀[X), the absorption path length /, the concentration Q of the species / of the mixture, and the extinction coefficient ε{λ) of the species /.

The goal of chemometrics is the determination of the concentration C,- of each component by the means of appropriate measurements. For conventional chemometrics a reference spectrum lo(λ) is required. This could be realized by a stored data set, or a reference channel in the set-up. The measurement over a discrete number of spectral positions A_n leads to a classical linear analytical problem, which can be described as:

In general, ε_nι is not a quadratic matrix, which denotes, that there is no distinct inverse matrix. The method of ..Singular Value Decomposition (SVD)" allows the calculation of a pseudo-inverse matrix, which is also the best solution of the linear system of equations in the sense of the ,,Least Square" method:

This method requires two times the measurement of the whole spectrum: One as reference spectrum and one with the absorbing mixture in the optical path (see for example: N. Benoudjit, E. Cools, M. Meurens, M. Verleysen, ,,Chemometric calibration of infrared spectrometers: selection and validation of variables by nonlinear models", Chemometrics and Intelligent Laboratory Systems, vol. 70, p. 47- 53 (2004)).

Some advantages and drawbacks of prior art process photometers and spectrometers have already been mentioned above. Furthermore, there are some additional problematic aspects concerning the infrared spectral range, which should be dealt with to get: - Insensitivity against cross talk between absorbing species to be able to measure components, which have overlapping absorption areas with others (Fig. 1 shows examples of such overlapping absorption structures in gas mixtures, which consists of water vapor and carbon dioxide (upper part of the Figure) and carbon monoxide, methane and nitrous oxide (lower part of the Figure)).

- Low detection limits to open a large area of applications.

Multi-component capability (simultaneous measurement of different components independent from the spectral position of their absorptions and cross talk effects) - Long lifetime and long calibration intervals to keep the cost of ownership as low as possible. DESCRIPTION OF THE INVENTION

It is therefore an objective of the invention, to find a method and apparatus for measuring and processing the spectral information of a mixture containing a plurality of components in a way, that cross talk has no disturbing influence on the result, or even better, the cross talk information could be used to determine further components in the mixture. Furthermore, this method should provide an optimum signal-to-noise ratio (optically and electronically) and a flexible wavelength selection and analysis to be able to adapt it to different applications. Moreover, the number of moveable parts should be minimized to ensure the longest possible mean time between failures.

The objective is achieved by a method according to claim 1 and an infrared process photometer according to claim 7. It is essential for the invention that - the modulated beam of infrared light is generated by means of a modulated broad band light source;

- the modulated beam is sent through a wavelength-selective device before being received by said infrared detector; and that

- a multi-channel detector array is used as said infrared detector. The modulated beam may thereby be sent through the wavelength-selective device either after or before having passed through said mixed medium.

According to one embodiment of the invention the electrical signals of the infrared detector are processed in accordance with the modulation of the infrared beam using a "Lock in" technique.

Another embodiment of the invention is characterized in that the processed electrical signals from the infrared detector are analyzed using chemometrics to determine the concentration of at least one of a plurality of components within said mixed medium. The implementation of chemometrics allows the determination of the different concentrations, even if their spectral absorption positions are partially overlapping, and improves the signal-to-noise ratio by the use of the whole spectral information about the occurring absorption.

According to another embodiment the signal-to-noise ratio is improved by using a Hadamard transformation for the received electrical signals before said signals are electronically processed, especially amplified. The use of the Hadamard transformation principle results in a remarkably improved signal-to-noise ratio and thus, a reduced noise equivalent power (NEP) of the system, which has for its part a positive impact on the achievable detection limits.

The quality of the measurement can be further improved by doing a reference measurement by means of a reference channel.

Especially, the modulated beam of infrared light is generated by electronically modulating, especially pulsing, a thin-film thermal emitter.

The infrared process photometer according to the invention is characterized in that

- said modulated infrared light source is a broad band light source; - a wavelength-selective device is arranged between said broad band light source and said infrared detector; and

- said infrared detector is a multi-channel detector array.

The means for beam shaping may thereby be arranged between said broad band light source and said infrared detector.

An embodiment of the photometer according to the invention is characterized in that said modulated infrared light source is an electronically pulsed thin-film thermal emitter. According to another embodiment said wavelength selective device is one of an array of optical filters, a lateral linear interference filter or wedge filter, or an optical grating.

In another embodiment said beam shaping means comprise a lens or a mirror.

A further embodiment is characterized in that said signal processing unit uses Hadamard transformation means at its entrance stage to improve the signal-to- noise ratio.

According to another embodiment a data analysis unit is connected to the output of the signal processing unit, and said data analysis unit uses chemometrics to determine the concentration of at least one of a plurality of components within said mixed medium.

A further embodiment of the photometer according to the invention is characterized in that a reference channel is provided for measuring absorption of a reference medium. Especially, said reference channel comprises a reference volume and two sets of mirrors for selectively passing the beam of infrared light either through said volume of said mixed medium or said reference volume.

Furthermore, said reference channel may comprise a reference volume, beam deflecting means for passing said beam of infrared light through said reference volume, and separate infrared beam forming, detecting and processing means arranged behind said reference volume, which are similar to the beam forming, detecting and processing means arranged behind said volume of said mixed medium.

Finally, another embodiment is characterized in that said volume of said mixed medium and/or said reference volume are enclosed in an absorption cell. BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments, which are illustrated in the attached drawings, in which:

Fig. 1 shows examples of overlapping absorption structures in gas mixtures, which consists of water vapor and carbon dioxide (upper part of the Figure) and carbon monoxide, methane and nitrous oxide (lower part of the Figure);

Fig. 2 is a simplified schematic representation of a first embodiment of an infrared process photometer according to the invention;

Fig. 3 is a simplified schematic representation of a second embodiment of an infrared process photometer according to the invention;

Fig. 4 is a simplified schematic representation of a third embodiment of an infrared process photometer according to the invention, which is based on the embodiment according to Fig. 2 and comprises an additional reference channel; and

Fig. 5 is a simplified schematic representation of a fourth embodiment of an infrared process photometer according to the invention, which is similar to the embodiment according to Fig. 4, but comprises a different kind of reference channel.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The basic idea of the invention is a robust system without movable parts and a method for optical measurements followed by the determination of different components and their concentrations in a mixture, even in the presence of strong cross interferences of the different absorption spectra. The mixture could be fluid or gaseous and is typically used in the process industry.

The system consists of a spectral broad band and modulated infrared light source (preferably a novel electronically pulsed thin-film thermal emitter without moveable parts) in combination with a wavelength-selective element (preferably an array of optical filters, a lateral linear interference filter (wedge filter), or a optical grating) and a detector array with an appropriate signal processing, and an absorption path in-between the light source and the detector array. By the use of the Hadamard transformation in the electronics ahead of the first amplifier the number of parts in the circuit can be substantially reduced and the signal-to-noise ratio will be increased.

Furthermore, the utilization of chemometrics allows the determination of the concentrations of the different components, even if their spectral absorption positions are partially overlapping (see Fig. 1). This means, that all data points, where absorption occurs can be used for the calculation of one single concentration. Thus, the inherently maximum possible signal-to-noise ratio/accuracy of the measurement can be achieved.

The realization of a robust multi-channel process photometer for multi-component analysis can be done in different ways. Nevertheless, the fundamental arrangement of the key elements of the system is always the same and is depicted in Fig. 2: Within the infrared photometer 10 of Fig. 2, infrared light is emitted by a modulated infrared light source 11. Preferably, this infrared light source 11 is a novel electronically pulsed thin-film thermal emitter (such a thin-film thermal emitter is for example available from HawkEye Technologies, LLC, under the product name IR-40 or IR-43). The pulse mode is controlled for "lock-in" purposes by a signal processing unit 18 over a control line 22. The emitted infrared radiation passes an optional mechanical light modulator 12 (e.g. a chopper or oscillating shutter). The passing beam is then collimated by means of a first beam shaping element (lens or equivalent mirror optics) 13. The collimated beam runs through an absorbing medium, which may be contained in an absorption cell 14. For loading and unloading of the absorption cell 14, an inlet 23 and an outlet are provided. A second beam shaping element (lens or equivalent mirror optics) 15 at the exit of the absorption cell 14 makes sure, that the following multi-channel detector array 17 is optimally illuminated. A wavelength-selective device 16 (preferably an array of optical filters, a lateral linear interference filter (wedge filter), or a optical grating), which is provided between the broad band light source 11 and the multi-channel detector array 17 for wavelength selection purposes.

Finally, the electrical signals at the output of the multi-channel detector array are processed in a signal processing unit 18 (preferably by the use of the Hadamard transformation method) and then analyzed in a data analysis unit 19, the analysis being based on chemometrics, whereby the concentrations of the different components in the absorbing medium in the absorption cell 14 are calculated. Whereas signal processing as well as signal and data analysis could also be performed within one unit. The results of the calculation process can be displayed on a display unit 21 , or stored in a storage, or used to control a process.

Fig. 3 depicts an optimized version of an infrared (process) photometer 20 according to the invention, without any moveable parts. The infrared photometer 20 is equipped with a modulated (i.e. electronically pulsed) infrared light source 26 and a wavelength-selective multi-channel detector array 28. The Hadamard transformation principle is used for the electronic signals of the detector array 28 within the signal processing unit 29, and chemometrics is implemented for the data analysis within the data analysis unit 30. Again, the results of the measuring process can be displayed on a display unit 31. The whole measuring and analyzing process is controlled by a central control unit 25, which also controls the modulation of the infrared light source 26. The absorbing medium to be analyzed is contained in an absorption cell 36, which has an inlet 38 and an outlet 37. The absorption cell 36 is arranged at the bottom of a housing 39, containing all parts 25, ..,31 of the measuring system. The sides of the absorption cell 36 are closed by a first window 34, through which the infrared light from the infrared light source 26 is reflected by a first (curved) mirror 32, and a second window 35, through which the light from the absorption cell 36 is reflected to the multi-channel detector array 28 by a second (curved) mirror 33. Wavelength selection is effected by means of a filter array 27 at the entrance of the multi-channel detector array 28.

Optionally, a reference channel could be realized according to Fig. 4 and 5 on the basis of the configuration of Fig. 2 by a) the use of a beam splitter 40 after the lens 13, and a reference channel comprising a deflecting mirror 41, a reference absorption cell 14', a beam shaping element 15', a wavelength-selective device 16', a multi-channel detector array 17', a signal processing unit 18' and a data analysis unit 19' (infrared photometer 10' in Fig. 4); or b) the use of switchable deflecting mirrors 43, 45 after the beam shaping element (lens) 13 and after the absorption cell 14, two additional deflecting mirrors 44, 46 and a reference absorption cell 14', to bypass the light beam around the absorbing medium in the absorption cell 14 (infrared photometer 10" in Fig. 5).

The employment of a system as described above leads to the following advantages:

- The utilization of novel electronically pulsed thin-film thermal emitters facilitates the choice of an optimum pulse shape, repetition rate, and duty cycle for the "Lock-In" measurement technique. - For the wavelength selection the deployment of non-dispersive multi- filter arrays, or lateral linear interference filters /wedge filters (preferable located directly ahead of the detector array), or optical gratings open up the possibility of an optimized coverage of the needed spectral range and also represent a very simple and stable wavelength calibration arrangement.

- The use of the Hadamard transformation principle results in a remarkably improved signal-to-noise ratio and thus, a reduced noise equivalent power (NEP) of the system, which has for its part a positive impact on the achievable detection limits. This statement especially applies for measurements close by the noise level of the detectors and the corresponding electronics for the signal processing. - The implementation of chemometries allows the determination of the different concentrations, even if their spectral absorption positions are partially overlapping, and improves the signal-to-noise ratio by the use of the whole spectral information about the occurring absorption (regression of a large number of data points).,

LIST OF REFERENCE NUMERALS

10,10',10",20 infrared photometer

11 ,26 modulated infrared light source (thin film thermal emitter) 12 mechanical light modulator

13,15,15" beam shaping element (e.g. lens)

14,14',36 absorption cell

16,16' wavelength-selective device

17,17',28 multi-channel detector array 18,18',29 signal processing unit (Hadamard transformation)

19,19',30 data analysis unit (chemometrical)

21,31 display unit

22 control line

23,38 inlet (absorption cell) 24,37 outlet (absorption cell)

25 control unit

27 filter array ,33 mirror ,35 window housing beam splitter deflecting mirror reference channel ,45 switchable deflecting mirror ,46 deflecting mirror

Claims

1. Method for the measurement and determination of the concentration of a certain component within a mixed medium by the absorption of infrared light, comprising the steps of generating a modulated beam of infrared light; passing said modulated beam of infrared light through the mixed medium;

- receiving said modulated beam after having passed though said mixed medium by means of an infrared detector (17, 28); and receiving and processing electrical signals of said infrared detector (17, 28), said signals being characteristic of the received modulated infrared beam; characterized in that the modulated beam of infrared light is generated by means of a modulated broad band light source; the modulated beam is sent through a wavelength-selective device (16, 27) before being received by said infrared detector (17, 28); and

- a multi-channel detector array (17, 28) is used as said infrared detector.

2. Method according to claim 1, characterized in that the electrical signals of the infrared detector are processed in accordance with the modulation of the infrared beam using a "Lock in" technique.

3. Method according to claim 1 or 2, characterized in that the processed electrical signals from the infrared detector are analyzed using chemometrics to determine the concentration of at least one of a plurality of components within said mixed medium.

4. Method according to one of the claims 1 to 3, characterized in that the signal-to-noise ratio is improved by using a Hadamard transformation for the received electrical signals before said signals are electronically processed, especially amplified.

5. Method according to one of the claims 1 to 4, characterized in that a reference measurement is done by means of a reference channel (42).

6. Method according to one of the claims 1 to 5, characterized in that said modulated beam of infrared light is generated by electronically modulating, especially pulsing, a thin-film thermal emitter.

7. Infrared process photometer (10, 10', 10", 20) for carrying out the method according to one of the claims 1 to 6, comprising

- a modulated infrared light source (11 , 12, 26);

- means (13, 15, 32, 33) for shaping, especially collimating and/or focusing, a beam of infrared light emitted from said modulated infrared light source (11 , 12, 26); - a volume (14, 36) of the mixed medium to be measured, said volume (14,

36) being arranged, such that a beam of infrared light passes through said volume (14, 36);

- an infrared detector (17, 28) for receiving said light beam after having passed said volume (14, 36); - a signal processing unit (18, 29) for processing the electrical signals of said infrared detector (17, 28); characterized in that

- said modulated infrared light source (11 , 12, 26) is a broad band light source; - a wavelength-selective device (16, 27) is arranged between said modulated infrared light source (11 , 12, 26) and said infrared detector (17, 28); and

- said infrared detector is a multi-channel detector array (17, 28).

8. Infrared process photometer according to claim 7, characterized in that said modulated infrared light source (11 , 12, 26) is an electronically modulated infrared emitter, especially a pulsed thin-film thermal emitter.

9. Infrared process photometer according to claim 7, characterized in that said wavelength selective device (16, 27) is one of an array of optical filters, a lateral linear interference filter or wedge filter, or an optical grating.

10. Infrared process photometer according to claim 7, characterized in that said beam shaping means (13, 15) comprise a lens.

11. Infrared process photometer according to claim 7, characterized in that said beam shaping means comprise a mirror (32, 33).

12. Infrared process photometer according to one of the claims 7 to 11 , characterized in that said signal processing unit (18, 29) uses Hadamard transformation means at its entrance stage to improve the signal-to-noise ratio.

13. Infrared process photometer according to one of the claims 7 to 12, characterized in that a data analysis unit (19, 30) is connected to the output of the signal processing unit (18, 29), and that said data analysis unit (19, 30) uses chemometrics to determine the concentration of at least one of a plurality of components within said mixed medium.

14. Infrared process photometer according to one of the claims 7 to 13, characterized in that a reference channel (42) is provided for measuring absorption of a reference medium.

15. Infrared process photometer according to claim 14, characterized in that said reference channel (42) comprises a reference volume (14') and two sets of mirrors (43, 44; 45, 46) for selectively passing the beam of infrared light either through said volume (14) of said mixed medium or said reference volume (14').

16. Infrared process photometer according to claim 14, characterized in that said reference channel (42) comprises a reference volume (14'), beam deflecting means (40, 41) for passing said beam of infrared light through said reference volume (14'), and separate infrared beam forming, detecting and processing means (15', 16', 17', 18', 19') arranged behind said reference volume, which are similar to the beam forming, detecting and processing means (15, 16, 17, 18, 19) arranged behind said volume (14) of said mixed medium.

17. Infrared process photometer according to one of the claims 7 to 16, characterized in that said volume of said mixed medium and/or said reference volume are enclosed in an absorption cell (14, 14', 36).