Arrangement for measuring gas concentrations

Abstract

An arrangement for measuring gas concentrations in a gas absorption method, wherein the arrangement includes a plurality of light sources, a measuring cell, at least one measuring receiver and an evaluation apparatus. The measuring cell has a narrow, longitudinally-extended beam path with an entrance-side opening diameter B and an absorption length L with L>B, wherein the measuring cell has a gas inlet and a gas outlet wherein a plurality of light sources of different wavelength spectra is grouped into a first light source group wherein an optical homogeniser is interposed between the first light source group and the measuring cell, wherein, in particular, the homogeniser is coupled to the light source group directly or via a common optical assembly.

Claims

1. An arrangement for measuring gas concentrations in a gas absorption method in which light from light sources of various wavelengths in the visible region, the UV region and/or IR region, is conducted through a measuring cell with a gas mixture to be analysed, and gas concentrations of gases of the gas mixture to be measured are determined via a measurement of an attenuation of the light conducted into the measuring cell at various wavelengths due to absorption in the various gases of the gas mixture, the arrangement comprising: a plurality of light sources having different wavelength spectra that are adjusted to absorption bands, absorption gaps and/or transition regions between absorption bands and absorption gaps of the gases to be measured, wherein the plurality of light sources is grouped into a first light source group with a first plurality of the light sources of different wavelength spectra and a second light source group with one or a second plurality of the light sources of different wavelength spectra, a measuring cell having openings at each of its two ends, wherein the light of the first light source group and of the second light source group is conducted on two independent beam paths through the measuring cell and the light of both beam paths exiting the measuring cell respectively conducted through wavelength-selective beam dividers to corresponding first and second measuring receivers, wherein each of the first and second measuring receivers measures, at an exit of the measuring cell, a light intensity in one or a plurality of the wavelengths emitted, and an evaluation apparatus configured to determine the gas concentrations from the measured light intensities, wherein at least one beam path through the measuring cell is a narrow, longitudinally-extended beam path with an entrance-side opening diameter B and an absorption length L with L>B, wherein the measuring cell has a gas inlet and a gas outlet, wherein an optical homogeniser is interposed between the first light source group and the measuring cell.

2. An arrangement according to claim 1, wherein the light sources of the first light source group are LED light sources with a characteristic radiation angle, and the LED light sources of the first light source group are arranged in front of the homogeniser, such that a radiation cone of the LED light sources of the first light source group enters the homogeniser substantially complete.

3. An arrangement according to claim 2, wherein the LED light sources of the first light source group are arranged in front of the homogeniser, such that a radiation cone of the LED light sources of the first light source group, after passing through a common optical assembly, enters the homogeniser substantially complete.

4. An arrangement according to claim 1, wherein the homogeniser is a shaped, transparent solid light conductor on the basis of total reflection on the surface or of refractive index gradients in the substrate or as a hollow reflector arrangement with a transparent medium in the interior, and reflective lateral boundary surfaces, wherein the homogeniser is shaped linear or curved with a circular, oval or polygonal cross-section.

5. An arrangement according to claim 4, wherein the homogeniser alters, in the cross-section in the direction towards the measuring cell.

6. An arrangement according to claim 1, wherein defects are arranged in or on the homogeniser.

7. An arrangement according to claim 6 wherein defects are: imperfections in the substrate, dispersion bodies in a mirror cavity, rough patches on boundary surfaces, or rough patches on mirror surfaces.

8. An arrangement according to claim 1, wherein at its entrance opening, the measuring cell and/or a member of the arrangement adjacent to the measuring cell has a combined light inlet and light outlet window, and, facing the light inlet window and light outlet window, a light-reflecting wall.

9. An arrangement according to claim 1, wherein, at the entrance and/or at the exit of the measuring cell, is arranged one or a plurality of beam dividers, with which light of different light sources of the first light source group and/or a second light source group is conducted to two or more different measuring receivers.

10. An arrangement according to claim 1, wherein the two beam paths partly or wholly overlap in a measuring volume of the measuring cell.

11. An arrangement according to claim 1, wherein: the first light source group is adjacent an opening at a first end of the measuring cell; and the second light source or the second light source group is adjacent an opening at a second end of the measuring cell.

12. An arrangement according to claim 1, wherein the one light source or at least one of the light sources of the second light source group is a MQW LED with a temperature-stable emission spectrum, and the beam path belonging to the second light source group comprises a wavelength-selective beam divider and two measuring receivers at the exit of the measuring cell, wherein the wavelength-selective beam divider is configured to split the emission spectrum of the MQW LED into two or more portions and to conduct the portions separated from each other to the two measuring receivers.

13. An arrangement according to claim 12, wherein the emission spectrum of the at least one MQW LED and a wavelength characteristic of the wavelength-selective beam divider are adjusted to an absorption spectrum of a gas to be measured, such that a first portion of the emission spectrum of the MQW LED undergoes a greater absorption in the gas than a second portion.

14. An arrangement according to claim 1, further comprising a pressure measuring device and/or temperature measuring device connected to the measuring cell, the pressure measuring device and/or temperature measuring device configured in order to measure a pressure and/or a temperature of the gas mixture in the measuring cell, wherein the evaluation apparatus is configured to take into account the influence of at least one of: a measured level of pressure, pressure fluctuations, the temperature, and temperature fluctuations, when determining the gas concentrations to extrapolate them to a normal pressure and/or a normal temperature.

15. An arrangement according to claim 1, wherein light is coupled into the measuring cell and/or light is decoupled out of the measuring cell, by using additional light conductors.

16. An arrangement according to claim 1 wherein the measuring cell can be taken out.

17. An arrangement according to claim 1, wherein the homogeniser, directly or via a common optical assembly, is coupled to the light source group.

18. An arrangement according to claim 1, wherein at its entrance opening, the measuring cell and/or a member of the arrangement adjacent to the measuring cell has a light inlet window and a light outlet window respectively, with or without reflecting walls between the light inlet window and the light outlet window, wherein the light inlet window or light inlet windows and/or light outlet window or light outlet windows, is inclined compared to a longitudinal extension of the measuring cell.

19. An arrangement according to claim 1, wherein the two beam paths are in opposite directions to each other.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Without being restricted to the general idea of the invention, the invention is described below by means of exemplary embodiments with reference to the drawings, and the drawings are expressly referred to with respect to all the details according to the invention, which are not explained in greater detail in the text. In the figures:

(2) FIGS. 1a), b) show emission spectra and absorption spectra of measuring gases and LEDs

(3) FIG. 2 shows a schematic representation of an arrangement according to the invention,

(4) FIG. 3 shows examples of homogenisers according to the invention,

(5) FIG. 4 shows a schematic representation of a part of an arrangement according to the invention,

(6) FIG. 5 shows a schematic representation of a homogenisation without dispersion centres,

(7) FIG. 6 shows a schematic representation of a homogenisation with dispersion centres,

(8) FIGS. 7a)-d) show schematic representations of light conduction principles of homogenisers according to the invention and

(9) FIG. 8 shows the spectral overlapping principle and dividing principle when using MQW LED is to measure gas absorption.

DETAILED DESCRIPTION

(10) In each of the drawings the same members and/or parts, or members and/or parts of the same type, are provided with the same reference numbers so that a re-introduction is omitted.

(11) In FIGS. 1a) and 1b) each of the absorption spectra of the gases nitrogen monoxide (NO), sulphur dioxide (SO.sub.2) and nitrogen dioxide (NO.sub.2) are shown together with the emission spectra of six different LEDs, which are arbitrarily normalised to a common maximum emission. A gas mixture such as this occurs, for example, in the exhaust gas of vehicles for example, the ratios of NO and NO.sub.2 depending on temperature. The wavelengths begin at 200 nm, that is, in the UV region and go as far as 600 nm in FIG. 1a). The section in FIG. 1b) is completely located in the UV region between 200 nm and 270 nm.

(12) The absorption spectrum of NO.sub.2 is particularly broadband and has a maximum at approx. 400 nm. Both the LED3 at 330 nm and the LED4 at approx. 405 nm undergo a significant absorption in NO.sub.2. The longer-wave LEDS at 580 nm can serve as a reference since it undergoes very much less absorption in NO.sub.2. The narrower absorption spectrum of SO.sub.2 with a width of approx. 60 nm is located around the maximum at 285 nm in the UV region. The LED1 and its emission spectrum are centred thereon. A further LED2 and its emission spectrum are centred around a maximum at approx. 246 nm and lie in a local minimum of the absorption spectra of all three gases shown. Hence this LED2 wavelength is suited to being a reference wavelength and has the further advantage compared to LEDS for example, in that not only is the absorption low, but also the dispersive effects relative to the wavelengths of, for example, LED1 and LED3, also LED 4, are less.

(13) As can be seen clearly from FIG. 1b), the NO absorption spectrum consists of a plurality of narrow bands with widths of few nanometres at 205 nm, 215 nm and 226 nm. An MQW LED labelled LED6 generates light in a region with a width of approx. 8 nm around 248 nm. The spectrum is divided into two regions labelled LED6a and LED6b, which are generated with steep edge, by splitting in a wavelength-selective interferometric beam divider. In this case the shorter-wave portion LED6a has a large overlap with an absorption peak of NO at 226 nm, whereas the longer-wave portion LED6b has scarcely any overlap with the absorption peak and can therefore be used as a reference from the same light source. The principle is illustrated more clearly in FIG. 8 below.

(14) FIG. 2 shows a schematic presentation of an arrangement according to the invention, in the centre of which stands a measuring cell 3 with a measuring gas or respectively gas mixture. The measuring cell 3 is a slim, elongated, for example cylindrical measuring cell 3, which has the advantage that, with little volume and therefore a possible high exchange rate of the measuring gas, a considerable absorption length is achievable. For this, the measuring cell 3 has a gas inlet 6 and a gas outlet 7.

(15) A light source group 10 is shown on the right-hand side, in which a plurality of light sources of various colours, for example LEDs, are grouped in a small space according to the invention. The light they emit reaches the measuring cell 3 through a homogeniser 11, which will be explained in still greater detail in the following, and via beam divider 12, 4. The beam divider 12 can fulfil the function of conducting a part of the light emitted into a reference measuring receiver 13 as a reference. Moreover, the beam divider 12 can, but does not have to, be wavelength-selective. The remaining portion of the light from the light source group 10 passes through the entire length of the measuring cell 3, undergoes a wavelength-dependent absorption in the gas mixture in so doing and reaches the measuring receiver 14 at the other end, which is designed to measure the intensity of the light falling onto it. The attenuation is then calculated or determined by means of comparison with a target value or by means of comparison with the intensity in reference measuring receiver 13 and the gas concentrations of the gases to be investigated determined therefrom.

(16) Connected to the measuring cell 3 is a pressure measuring device and/or temperature measuring device 20, which measures pressure and/or temperature of the gas in the measuring cell 3 and transmits it to the evaluation apparatus not shown, which, from this, can make corrections to the determination of the gas concentrations.

(17) The invention according to the arrangement according to FIG. 2 has a second measuring path which is oriented in the opposite direction to the first, afore-described measuring path. The second measuring path starts with a light source group 1, which, at its simplest, comprises or has a single MQW LED. A plurality of MQW LEDs can also be comprised or mixtures of various LEDs with or without MQW LED. The exemplary embodiment shown comprises at least one MQW LED, for example LED6 of FIG. 1. A beam divider 2 directs the light of light source group 1, that is, the MQW LED, into the measuring cell 3 and separates the beam paths of the first measuring path and the second measuring path from each other. After passing through the measuring cell, a beam divider 4, which can be a wavelength-selective beam divider, again separates the two measuring paths or respectively beam paths from each other and directs the light from light source group 1 into a measuring arrangement with a measuring receiver 5. This measuring arrangement can comprise a further beam divider which performs a division with a steep edge in accordance with the description above and FIG. 8 here below. In this case two optical measuring receivers, for example photodiodes, are used.

(18) A common beam divider can also be used in place of two beam dividers 4, 12, which is accordingly designed to decouple both beam paths in a wavelength-selective manner, for which it should be partly transparent and partly reflecting for the wavelengths of the first light source group 10. In such a case the measuring receivers 5 and 13 are arranged on opposite sides of the main beam path.

(19) In FIG. 3 various examples of homogenisers 21 to 25 according to the invention are shown schematically. The homogeniser 21, which is shown in the cross-section on the left-hand side and from the side on the right-hand side, is a conventional solid light conductor with a six-sided or respectively hexagonal cross-section. The homogeniser 22 differs from this in that it tapers from the entrance towards the exit, which contributes to greater homogenisation. Hexagonal cross-sections such as this have the property that more light losses occur on the abutting edges between two smooth surfaces whereas they are more distributed over the entire area in round cross-sections.

(20) In contrast to the homogeniser 2, the homogeniser 23 has a round cross-section. Although it tapers, the homogeniser 24 also has a round cross-section. Finally, although it curves through 360, the homogeniser 24 has a round cross-section and a constant diameter, which results in considerable homogenisation. All these are solid homogenisers made out of glass or plexiglass for example.

(21) In FIG. 4 the combination of the light source group 10 and the homogeniser 11 is shown schematically. The light source group 10 has a plurality of single LEDs of various wavelengths or respectively colours, which are indicated as short dashes and placed next to each other in a small space. This light source group 10 will usually have a diameter of less than 1 to 2 mm. Like most LEDs, these LEDs have a radiation angle 31, which leads to a complete radiation cone 30 of the light source group 10, which exits the entire area of the light source group 10 and broadens towards the entrance area of the homogeniser 11. The distance between light source group 10 and the entrance area of homogeniser 11 has been selected such that, when entering homogeniser 11, the cone 30 has a diameter which is smaller than the opening aperture D.sub.1 of the homogeniser 11. Inside the homogeniser, which tapers to a smaller diameter D.sub.2 towards the exit, light is conducted with no or little loss of intensity.

(22) A relatively larger light source group 10, which is arranged much nearer to the homogeniser 11, also spans the same radiation cone 30. For the purpose of miniaturisation the choice of making the homogeniser 11 roughly as large as large as the light source group 10 and placing it right in front of the light source group 10 accordingly is more favourable.

(23) Each of FIGS. 5 and 6 show how the phase space, in this case the occupancy of the spatial distribution, is compensated by the respective homogeniser 11. The spatial distribution of the light emitted is at first the spatial arrangement of the LEDs of the light source group 10, shown on the left in each of FIGS. 5 and 6. After passing through the homogeniser, which is provided without defects in FIG. 5 and with defects 32 in FIG. 6, all the LEDS, which substantially cover the exit area of the homogeniser 11 uniformly, are reflected in many ways in the first case. However, the individual LEDs can still be substantially identified as such. If, as in FIG. 6, defects 32 are present, the additional dispersion of the light in the homogeniser 11 generates complete blurring, such that the individual light sources are no longer identifiable. However, this is at the expense of a loss of intensity due to the light dispersed out of homogeniser 11. In place of defects 32 the exterior surface can also be roughened, such that defects arise in the total reflection, which have a similar effect to embedded defects 32.

(24) FIGS. 7a) to d) again show cross-sections of various embodiments of homogenisers which can be used according to the invention. According to FIG. 7a) the homogeniser 40 has a round cross-section and is solid. In this case, light conduction takes place by means of total reflection on the outer surface. The homogeniser 42 from FIG. 7 b) differs from this only in its shape which is hexagonal in this case. Here losses of light are concentrated on the edges between the individual lateral surfaces.

(25) FIG. 7c) shows an example of the homogeniser 43 designed as a reflector arrangement of which the outer contours do not differ from those in FIG. 7b). However, it is hollow inside and has reflected interior surfaces 45 around its interior space 44. Finally, FIG. 7d) again concerns a solid homogeniser 46, which, unlike the exemplary embodiment of FIG. 7a), has a higher refractive index gradient in the centre than at the edge, so that light is conducted by means of refraction due to the refractive index gradients. Moreover, on the boundary surface to the surrounding air total reflection takes place.

(26) In FIG. 8 the principle of path division with MQW LED is clearly illustrated schematically once more. A wavelength is again shown on the horizontal axis in arbitrary units, an absorption amplitude, transmission amplitude or an emission amplitude respectively given in arbitrary units on the vertical axis. The absorption spectrum of a measuring gas with a comparatively steep edge is labelled with reference number 50. The emission spectrum 51 of a MQW LED, for example LED6 in FIG. 1, is selected such that it only partially overlaps with the falling edge of the settling spectrum 50. Accordingly, a wave length characteristic of an interferometric beam divider with steep edge is drawn as a dashed line 52, the edge of which substantially divides the emission spectrum 51 into two parts which are transmitted to different measuring receivers, for example photodiodes. These two portions are shaded differently and labelled with reference numbers 53 and 54 for a signal portion and a reference portion. The signal portion 53 has a large overlap with the falling edge of absorption spectrum 50, whereas the reference portion 54 has hardly any overlap therewith. These portions 53 and 54 are labelled LED6a and LED6b in FIG. 1a) and FIG. 1b) and shown separated from each other, even though they originate from a single light source. A bandpass filter can also be used instead of the short-pass filter shown.

(27) All the features mentioned, also the features to be inferred from the drawings alone, as well as individual features which are disclosed in combination with other features, are regarded as essential to the invention individually and in combination. Embodiments according to the invention can also be complied with by individual features or a combination of a plurality of features. Within the context of the invention, features which are described with in particular or preferably are to be understood to be optional features.

REFERENCE LIST

(28) 1 Light source group with MQW LED 2 Spectral beam divider 3 Measuring cell 4 Spectral beam divider 5 Measuring receiver 6 Gas inlet 7 Gas outlet 10, 10 Light source group 11 Homogeniser 12 Beam divider 13 Reference measuring receiver 14 Measuring receiver 20 Pressure and/or temperature measuring device 21-25 Homogeniser 30 Radiation cone 31 Radiation angle 32 Defects 34, 35 Intensity distribution at the exit of the homogeniser 41 Homogeneous round homogeniser 42 Homogeneous hexagonal homogeniser 43 Inwardly reflected homogeniser 44 Cavity 45 Inward reflection 46 Homogeniser with refractive index gradient 50 Absorption spectrum of a gas 51 Emission spectrum of an MWQ LED 52 Wave length characteristic of an interferometric beam divider 53 Signal portion 54 Reference portion