MEASUREMENT DEVICE FOR ABSORPTION SPECTROMETRY GAS MEASUREMENT, USE OF SPINEL, POLYCRYSTALLINE ALUMINUM OXIDE, OR ALUMINUM OXYNITRIDE, AND METHOD FOR ABSORPTION SPECTROMETRY GAS MEASUREMENT - Patent application

Abstract

The present invention relates to a measurement device for absorption spectroscopy gas measurements, to the use of spinel, polycrystalline aluminum oxide, or aluminum oxynitride, and to a method for absorption spectroscopy gas measurements.

Description

[Technical Field]

[0001] The present invention relates to a measurement device for absorption spectroscopy gas measurements, the use of spinel, polycrystalline aluminum oxide, or aluminum oxynitride, and a method for absorption spectroscopy gas measurements. [Background technology]

[0002] Absorption spectroscopy gas measurements are typically performed by laser absorption spectroscopy, specifically diode laser absorption spectroscopy using a tunable laser, also known as TDLAS (Tunable Diode Laser Absorption Spectroscopy). This spectroscopy is particularly suitable for gas measurements, particularly for determining the chemical and / or physical parameters of the gas. Laser absorption spectroscopy is particularly advantageous for determining the concentration of a gas in a gas mixture. In addition to concentration, the temperature, pressure, velocity, and mass flow of the gas can also be determined, for example.

[0003] A measurement device for absorption spectroscopy gas measurement using diode laser absorption spectroscopy (TDLAS) using a tunable laser specifically includes an emission device capable of generating a laser beam, a reflection element capable of reflecting the laser beam, and at least one detection element capable of detecting the laser beam. The emission device specifically may be a laser diode capable of generating a tunable laser beam. The laser beam generated by the emission device can be guided to the reflection element along a first optical axis, and then the laser beam reflected by the reflection element can be guided from the reflection element to the detection element along a second optical axis.

[0004] For absorption spectroscopy gas measurements using such a TDLAS, the gas mixture being measured is illuminated with a laser beam as the laser beam travels along a first optical axis and a second optical axis. During this illumination of the gas, the emission wavelength of the tunable diode laser is tuned to a characteristic absorption band of the gas being measured in the gas mixture, which results in a reduction in the radiation intensity of the laser beam due to absorption. This reduction in radiation intensity results in a reduction in the signal intensity measured by the detection element, which can then be used for gas measurements, specifically to determine the gas concentration in the gas mixture. The gas concentration can be determined, for example, using the Beer-Lambert law.

[0005] The diode laser used can be selected depending on the gas being measured and the desired tuning range: for example, DFB lasers (Distributed Feedback Lasers) can cover a wavelength range between 700 nm and 3 μm, VCSEL lasers (Vertical Cavity Surface Emitting Lasers) can cover a wavelength range up to about 2.1 μm, ICL lasers (Interband Cascade Lasers) can cover a wavelength range between 2.8 μm and 5.8 μm, and QCL lasers (Quantum Cascade Lasers) can cover a wavelength range above 3.5 μm.

[0006] TDLAS can measure, for example, oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NO X ), ammonia (NH3), hydrogen sulfide (H2S), sulfur oxides (SO X It can be used to measure gases that have at least one characteristic absorption band or line in these wavelength ranges, such as benzene, benzene, toluene, toluene, toluene-sulfonyl ether ...

[0007] A particular difficulty in the use of TDLAS is the use of a suitable reflective element, since the reflective element must have a number of very specific properties. For example, the reflective element must be transparent and reflective in particular in the wavelength range of the laser beam generated by the emitting device and directed towards the reflective element. Furthermore, the reflective element must be resistant to aggressive gases that it may come into contact with during gas measurements and must also be able to withstand high temperatures. Furthermore, the reflective element must be easy to clean in the event of contamination.

[0008] Reflective elements made from sapphire are commonly used in state-of-the-art technology. However, this reflecting element has many drawbacks. For example, sapphire is birefringent, which causes the reflected laser beam to split into several beams. Another drawback is the technically complex processing of sapphire. Finally, sapphire is also disadvantageous from an economic point of view due to its high price.

[0009] Reflective elements made from YAG (yttrium aluminum garnet) are also commonly used in state-of-the-art technology, however they are disadvantaged by their high raw material costs.

[0010] Because of these drawbacks, quartz is sometimes used as a reflective element, but it can only be used in the wavelength range below about 3 μm. Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the present invention to provide a measuring device for absorption spectroscopy gas measurements, particularly in the form of a TDLAS, with an alternative reflective element. In particular, the reflective element should be usable at wavelengths above 3 μm, particularly in the mid-infrared wavelength range. According to a further object, the reflective element should not be birefringent. According to a further object, the reflective element should be chemically and thermally resistant, particularly resistant to aggressive gases and high temperatures. According to a further object, the reflective element should be easy to clean. Finally, according to a further object, the reflective element should be economically advantageous to provide, i.e., easy to process and available at the lowest possible cost. [Means for solving the problem]

[0012] To achieve these objectives, a measuring device for absorption spectroscopy gas measurements is provided, comprising: an emission device capable of generating a laser beam; a reflective element capable of reflecting a laser beam; at least one detection element by which the laser beam can be detected; a first optical axis along which a laser beam generated by the emitting device can be guided to the reflective element; and a second optical axis, along which the laser beam reflected by the reflecting element can be guided to the detecting element; and Equipped with A measuring device is provided, wherein the reflective element is selected from the following group: spinel, polycrystalline aluminum oxide, aluminum oxynitride.

[0013] The present invention is based on the surprising discovery that spinel, polycrystalline aluminum oxide, or aluminum oxynitride can be used as a reflective element for measurement devices for absorption spectroscopy gas measurements, particularly in the form of TDLAS.

[0014] Spinel, or more precisely, magnesia spinel, is a mineral of the spinel family with the chemical composition MgAl2O4 that crystallizes in the cubic system. Surprisingly, the present invention has shown that spinel can be used particularly advantageously as a reflective element in measuring devices for absorption spectroscopy gas measurements, particularly in TDLAS. In this regard, spinel is transparent to laser beams in a broad wavelength range, allowing the laser beam to be reflected by the spinel. In this regard, spinel is transparent in the wavelength range of approximately 0.3 to 5.6 μm, and therefore is also transparent in the mid-infrared wavelength range, particularly above 3 μm.

[0015] Polycrystalline aluminum oxide, also known as "PCA" (polycrystalline alumina), is a high-density ceramic made from aluminum oxide. Typically, polycrystalline alumina is produced by pressing and subsequent sintering of powdered alumina. Preferably, the polycrystalline aluminum oxide used in the present invention is produced from very fine aluminum oxide powder, particularly preferably with a particle size of less than 1 μm. Particularly preferably, the polycrystalline alumina is present as transparent polycrystalline alumina, i.e., as so-called TPCA (transparent polycrystalline alumina). Surprisingly, it has been found by the present inventors that polycrystalline aluminum oxide can be advantageously used as a reflective element in measuring devices for absorption spectroscopy gas measurement, particularly in TDLAS. In this regard, polycrystalline aluminum oxide is transparent to laser beams in a wide wavelength range, and the laser beam can be reflected by polycrystalline aluminum oxide. In this regard, polycrystalline aluminum oxide is transparent in the wavelength range from about 0.3 μm to about 5.6 μm, and therefore is also transparent in the mid-infrared wavelength range, particularly above 3 μm.

[0016] Aluminum oxynitride is a ceramic material composed of the elements aluminum, oxygen, and nitrogen, also known as "ALON." Surprisingly, the present invention has shown that aluminum oxynitride can be advantageously used as a reflective element in measurement devices for absorption spectroscopy gas measurements, particularly in TDLAS. In this regard, aluminum oxynitride is transparent to laser beams in a wide wavelength range, allowing the laser beam to be reflected by aluminum oxynitride. In this regard, aluminum oxynitride is transparent in the wavelength range from about 0.3 μm to about 5.2 μm, and therefore is also transparent in the mid-infrared wavelength range, particularly above 3 μm.

[0017] However, spinel, polycrystalline aluminum oxide, and aluminum oxynitride, in particular, are not birefringent, so that a laser beam is not split into several beams when reflected by each of these materials, allowing the laser beam to be reflected particularly advantageously by these materials. Furthermore, spinel, polycrystalline aluminum oxide, and aluminum oxynitride are particularly resistant to aggressive gases and high temperatures. Finally, spinel, polycrystalline aluminum oxide, and aluminum oxynitride are easy to clean and process, in particular easier than sapphire, and therefore can be provided economically and at low cost, in particular lower than sapphire. However, an advantage of using these materials as reflective elements in measurement devices for absorption spectroscopy gas measurements, in particular in TDLAS, is that they can be very well coated, in particular with anti-reflection coatings (AR coatings), as will be explained in more detail below.

[0018] Although spinel, polycrystalline aluminum oxide, and aluminum oxynitride are equally advantageous as reflective elements in measurement devices for absorption spectroscopy gas measurements, in particular in TDLAS, as described above, spinel is particularly preferred as such reflective elements according to the present invention, since it has the best properties in terms of transmission, reflection, resistance, cleanability, coatability, and cost.

[0019] According to a preferred embodiment, the reflective element has a coating. In a particularly preferred embodiment, the reflective element has an anti-reflection coating (AR coating). Alternatively, or additionally, the reflective element may have an anti-scratch coating or a heat-shielding coating.

[0020] According to a preferred embodiment, the reflective element has a coating selected from the following group: Al2O3 / SiO2, TiO2 / Al2O3, TiO2 / SiO2, Ta2O5, or MgF2. According to the invention, it has been found that each of the above substances or mixtures of substances can be used to provide an AR coating that also provides scratch protection and temperature protection for the reflective element. According to the invention, the reflective element particularly preferably has a coating of Ta2O5 (tantalum pentoxide). According to the invention, it has surprisingly been found that by coating the reflective element with Ta2O5, the reflection in the wavelength range of approximately 3,425 to 3,650 nm can be reduced to less than 0.05%.

[0021] To coat the reflective elements with a coating, in particular with a coating of one of the above-mentioned substances or mixtures of substances, techniques known from the state of the art for applying coatings can be used, such as, for example, chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0022] According to a particularly preferred embodiment, the reflective element is a retroreflector. As is known, a retroreflector (also known as a triple prism) is a device in which incident electromagnetic waves, in particular light or laser beams, are largely reflected in their direction of origin, regardless of the direction of incidence relative to the orientation of the device. The design of the reflective element as a retroreflector also has the particular advantage that laser beams reflected by the reflective element can be particularly easily and efficiently reflected towards the detection element.

[0023] According to a preferred embodiment, the reflecting element has an entrance surface, through which the laser beam generated by the emitting device can be introduced into the reflecting element, the entrance surface being designed as a flat surface aligned at an angle unequal to 90° relative to the first optical axis. Such an inclination of the entrance surface to the first optical axis also has the particular advantage that any radiation components of the laser beam reflected at the entrance surface are not reflected in the direction of the incident laser beam, thereby avoiding interference with the incident laser beam. According to a preferred embodiment, the normal is aligned to the entrance surface at an angle relative to the first optical axis ranging from more than 0° to 5°, more preferably at an angle in the range of 1 to 5°, and particularly preferably at an angle of 3°.

[0024] Preferably, the radiation device of the measuring device according to the invention is a laser diode, particularly preferably a tunable laser diode. According to a preferred embodiment, the laser diode is used in the form of a laser diode, in particular a tunable laser diode selected from the following group: interband cascade lasers, quantum cascade lasers.

[0025] Interband cascade lasers, also known as ICLs (Interband Cascade Lasers), are laser diodes for emitting wavelengths in the range from about 2.8 μm to about 5.8 μm.

[0026] Quantum cascade lasers, also known as QKLs or QCLs (Quantum Cascade Lasers), are laser diodes for emitting wavelengths in the range from about 3.5 μm up to at least about 14 μm.

[0027] Although interband cascade lasers and quantum cascade lasers are equally advantageous as lasers in the measuring device according to the invention, as explained above, interband cascade lasers are particularly preferably used as lasers according to the invention.

[0028] Preferably, the emitting device, in particular the tunable laser diode, is a laser diode tunable from 2.0 μm to 5.0 μm.

[0029] According to the invention, it has been found that laser diodes tunable to this wavelength range interact particularly advantageously with reflective elements in the form of spinel, polycrystalline aluminum oxide, or aluminum oxynitride, since these materials are optically transparent to laser beams in this wavelength range and reflect the laser beam particularly advantageously in this wavelength range.

[0030] In principle, the detector element can take the form of any detector element known from the prior art for detecting a laser beam. According to a preferred embodiment, at least one detector element comprises at least one photodetector. As is known, a photodetector is an electronic component that converts a signal into an electrical signal using the photoelectric effect or that exhibits an electrical resistance that depends on the incident radiation. According to a particularly preferred embodiment, at least one detector element comprises at least one photodetector in the form of a photodiode.

[0031] The laser beam detected by the detecting element can be converted by the detecting element into an electrical signal, which is then evaluated according to known methods for gas measurement. To measure the concentration of the gas to be measured, the laser beam detected by the detecting element can be evaluated, for example, using the Beer-Lambert law, to determine the gas concentration. Preferably, the evaluation is performed using evaluation electronics. The evaluation electronics can be a component of the measuring device according to the invention or can exist in the form of a separate evaluation electronics. Such evaluation electronics can exist according to known evaluation electronics for TDLAS.

[0032] According to a preferred embodiment, the laser beam generated by the radiation device can be guided through the gas mixture along a section of the first optical axis. Along this section of the first optical axis, the laser beam can interact with the gas to be measured in the gas mixture, or the radiation intensity of the laser beam can be reduced due to absorption by the gas in this section. According to a further development of this concept of the invention, the laser beam generated by the radiation device can also be guided through the gas mixture along a section of the second optical axis, or the laser beam reflected by the reflecting element can be guided through the gas mixture along a section of the second optical axis. Due to absorption by the determined gas, the radiation intensity of the laser beam is further reduced during the second pass through the gas mixture along a section of the second optical axis. This greater reduction in radiation intensity can be more clearly detected by at least one detection device, and therefore the gas measurement can be measured and evaluated more reliably.

[0033] According to a preferred embodiment, the measuring device according to the invention comprises a probe, in which a section of the first optical axis extends. According to a further development of the inventive concept, a section of the second optical axis also extends in the probe. In particular, the probe can be designed in such a way that it can be introduced into a gas mixture. By introducing the probe into the gas mixture, the laser beam generated by the emission device can be guided through the gas mixture along the section of the first optical axis and the second optical axis.

[0034] According to a preferred embodiment, the measuring device according to the invention further comprises a device for applying a purge gas, with which the reflective element or the spinel, polycrystalline aluminum oxide or aluminum oxynitride can be surrounded with a purge gas, which can protect the reflective element from aggressive gases in the gas mixture.

[0035] Furthermore, the measuring device according to the present invention may comprise any other components known from the prior art for TDLAS. In particular, the measuring device according to the present invention may, for example, have a propagation optics, i.e., one or more lenses and / or mirrors, by which the laser beam generated by the emission device may be optically shaped. Furthermore, the measuring device may, for example, have a receiving optics, i.e., one or more lenses and / or mirrors, by which the laser beam reflected by the reflection element may be shaped before it hits the at least one detection element.

[0036] Preferably, one or more mirrors are used as the transmitting and receiving optics, particularly preferably no lenses are used, particularly preferably an interband cascade laser or quantum cascade laser is used as the laser, since lenses always have a certain amount of reflection to which such lasers are very sensitive, which can be reduced or substantially completely suppressed by using mirrors as the receiving optics.

[0037] It is also an object of the present invention to use, in measuring devices for absorption spectroscopy gas measurements, in particular in diode laser absorption spectroscopy using a tunable laser (TDLAS), reflective elements from which a laser beam can be reflected, selected from the following group: spinels, polycrystalline aluminum oxides, aluminum oxynitrides. This use is particularly preferred, as previously described, provided that spinels are used.

[0038] In practical use, the measuring device according to the invention can be operated as follows.

[0039] The emitting device, specifically a tunable laser diode, generates a laser beam, specifically in the wavelength range from 2.0 μm to 5.0 μm. The generated laser beam, after beam shaping by an optical transmission section if necessary, can be guided along a first optical axis to a reflecting element. During this path, the laser beam is preferably guided along a section of the first optical axis through a gas mixture, the gas mixture containing the gas to be determined, specifically the gas whose concentration in the gas mixture is to be determined. The laser beam is reflected by the reflecting element. The reflecting element, preferably in the form of spinel, polycrystalline aluminum oxide, or aluminum oxynitride, is preferably a retroreflector, preferably has an anti-reflection coating, and the incident surface of the reflecting element is preferably oriented at an oblique angle relative to the first optical axis. The laser beam reflected by the reflecting element is reflected along a second optical axis to at least one detecting element, where the laser beam is optionally shaped by a receiving optics and impinges on at least one detecting element, particularly preferably a photodiode of at least one detecting element. Along the second optical axis, the laser beam is preferably guided again through the gas mixture along the area, so that the laser beam again interacts with the gas to be measured in the gas mixture and thereby further loses radiation intensity. The laser beam impinging on at least one detection element, in particular a photodiode, is then detected by the at least one detection element, in particular converted into an electrical signal by the at least one detection element, which is then preferably evaluated by evaluation electronics, and the gas to be measured, in particular the concentration of the gas to be measured in the gas mixture, is determined.

[0040] According to the invention, it has been found that the measuring device according to the invention can be advantageously used to determine the gas concentration, in particular the concentration of at least one of the following gases in a gas mixture, in particular at least one of the following gases: C2H2, C2H4, C2H6, CHO, CH3Cl, CH4, CO, CO2, CS, CS2, H2, H2O, H2S, HBr, HCl, HCN, HF, HI, HOCl, N2, N2O, NH3, NO, NO2, O3, OCS, PH3, SO2, SO3.

[0041] Preferably, the measuring device according to the invention is used in particular to determine the gas concentration of at least one of the following gases in a gas mixture, in particular at least one of the following gases: C2H4, C2H6, CHO, CO, NO, NO2, SO2, SO3.

[0042] The measuring device according to the invention is particularly preferably used for determining the gas concentration of formaldehyde (CH2O), in particular in gas mixtures.

[0043] Another object of the invention is the use of the measuring device according to the invention for determining the concentration of gaseous formaldehyde in a gas mixture.

[0044] Another object of the present invention is to provide a method for absorption spectroscopy gas measurement, comprising: providing a measuring device according to the invention; generating a laser beam by an emitting device; detecting the laser beam with a detection element; performing a gas measurement based on the detection; A method for absorption spectroscopy gas measurement comprising:

[0045] Generating the laser beam, detecting the laser beam, and performing gas measurements based on the detection can be performed as described herein.

[0046] Further features of the invention can be seen from the claims, the figures and the associated description of the figures.

[0047] All of the features of the present invention can be combined with one another as desired, either individually or in combination.

[0048] One exemplary embodiment of the invention will now be described in more detail with reference to the following description of the figures. [Brief explanation of the drawings]

[0049] [Figure 1] 1 is a schematic diagram of an exemplary embodiment of a measuring device according to the present invention; [Figure 2] 1 is a perspective view of an exemplary embodiment of a spinel used as a reflective element in accordance with the present invention. FIG. [Figure 3] 10 is a diagram of another exemplary embodiment of a measuring device according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0050] 1 shows an embodiment of a measuring device according to the invention for absorption spectroscopy gas measurements. The measuring device, whose entirety is marked with the reference numeral 1, is designed to perform tunable diode laser absorption spectroscopy (TDLAS). The measuring device 1 comprises an emission device 2 in the form of a laser diode, by which a laser beam can be generated. The measuring device 1 also comprises a reflecting element 3 in the form of a spinel, by which the laser beam generated by the emission device 2 can be reflected. Finally, the measuring device 1 comprises a detecting element 4 in the form of a photodiode, by which the laser beam can be detected. The laser beam generated by the emission device 2 can be guided to the reflecting element 3 along a first optical axis 5. The laser beam generated by the reflecting element 3 can be guided to the detecting element 4 along a second optical axis 6.

[0051] The spinel or reflective element 3 is designed as a retroreflector or triple prism and has an anti-reflective coating of Ta2O5 applied by means of PVD.

[0052] The spinel or reflective element 3 has an entrance surface 7 through which the laser beam generated by the radiation device 2 can be directed onto the spinel 3, whereby the entrance surface 7 is designed as a flat surface whose normal is aligned at 3° to the first optical axis 5.

[0053] The laser diode 2 is an interband cascade laser that can be tuned over wavelengths from 3,627 nm to 3,633 nm (for the gas being measured, here formaldehyde).

[0054] The detection element 4 consists of a photodiode by which the laser beam detected by the detection element 4 can be converted into an electrical signal. The detection element 4 is connected via an electronic data line 8 to an electronic evaluation unit 9, which is able to evaluate the electrical signal generated by the detection element 4. In the illustrated embodiment, the evaluation electronics is in the form of an electronic data processing device.

[0055] The measuring device 1 is designed to determine the concentration of gaseous formaldehyde (CH2O) in a gas mixture.

[0056] In order to be able to determine the concentration of formaldehyde in a gas mixture by means of the measuring device 1, the measuring device 1 expediently comprises a probe, which is realized in a practical embodiment of the measuring device 1 as shown in Figure 3. The laser beam generated by the emitting device 2 can be guided through the gas mixture along a section of a first optical axis 5 and along a section of a second optical axis 6, each of which extends in the probe.

[0057] In practical use, the measuring device 1 is used as follows to determine the concentration of formaldehyde in a gas mixture.

[0058] The measuring device 1 is initially positioned such that a section of the first optical axis 5 and a section of the second optical axis 6 extend through the gas mixture in which the concentration of formaldehyde is to be determined. A laser beam is generated by the emitting device 2. The laser beam is initially directed along the first optical axis 5 toward the reflective element or spinel 3, introduced into the reflective element via the entrance surface 7, and reflected by the reflective element 3. The path of the laser beam in the reflective element 3 is indicated by a dashed line. The reflected laser beam is then guided along the second optical axis 6 toward the detecting element 4, where it is detected and converted into an electrical signal, which is transmitted via an electronic data line 8 to the evaluation electronics 9. The laser beam generated by the emitting device 2 is periodically modulated within a predetermined wavelength range, whereby this wavelength range includes at least one absorption band of formaldehyde. This reduces the radiation intensity of the laser beam as it passes through the gas mixture. This reduction in the radiation intensity of the laser beam is detected by the detection element 4 and the concentration of formaldehyde in the gas mixture is determined by the evaluation electronics 9 on the basis of this detection.

[0059] By using spinel as the reflective element 3, this determination of the concentration of formaldehyde in the gas mixture can be performed particularly reliably, since spinel is known not to fragment the laser beam and to be resistant to aggressive gas mixtures at high temperatures. Furthermore, spinel is known to be transparent for the required wavelengths. Since spinel is also designed as a retroreflector, the laser beam entering the spinel is reflected in the direction or parallel to the direction in which the laser beam entered the spinel along the first optical axis 5.

[0060] Furthermore, the inclination of the incidence face 7 relative to the first optical axis 5 can prevent interference of the laser beam introduced into the spinel with radiation components of the laser beam reflected at the incidence face 7. In this regard, any radiation components of the laser beam reflected at the incidence face 7 are not reflected in the direction of the incident laser beam, but are reflected obliquely relative to the incidence face 7, as indicated by arrow 10.

[0061] The spinel of the reflective element 3, which is shown only diagrammatically in Figure 1, is shown in more detail in the respective diagram in Figure 2. Figure 2 clearly shows that the reflective element 3 is designed as a retroreflector or triple prism. The beam path in the reflective element 3 is indicated by arrows in Figure 2, whereby the laser beam is first introduced into the reflective element 3 along a first optical axis 5, where it is reflected three times and then exits the reflective element 3 along a second optical axis 6.

[0062] FIG. 3 shows a measuring device according to FIGS. 1 and 2 in an example of a practical embodiment.

[0063] In the example embodiment shown in FIG. 3, identical or similarly acting elements are marked with the same reference numerals as in FIGS.

[0064] 3, the measuring device 1 also has propagation optics 11 in the form of a parabolic mirror, through which the laser beam generated by the emission device 2 can be shaped. Furthermore, the measuring device 1 has receiving optics 12 in the form of a parabolic mirror, through which the laser beam reflected by the reflecting element 3 can be shaped before reaching the detection element 4.

[0065] The measuring device 1 as shown in Fig. 3 has a steel housing 13 in which the emitting device 2, the propagation optics 11, the reflecting element 3, the receiving optics 12, the detecting element 4 and the evaluation electronics 9 are arranged. Furthermore, the measuring device 1 has a flange 14, via which the measuring device 1 can be attached to a device (not shown). In particular, this device contains a gas mixture with the gas to be measured by the measuring device 1.

[0066] The measuring device 1 according to Fig. 3 also comprises a probe 15 which can be inserted into the gas mixture to be analyzed. The probe 15 is suitably designed so that it can be inserted into the gas mixture when the measuring device 1 is attached to the device via the flange 14. The probe 15 has an elongated opening 16 in the form of a process window.

[0067] A section of the first optical axis 5 and a section of the second optical axis 6 extend through the probe 15, so that the laser beam traveling along these sections can be guided through the gas mixture by the gas mixture entering the probe 15 through the opening 16 in the probe 15.

[0068] The emitting device 2, the propagation optics 11, the receiving optics 12, the detection element 4 and the evaluation electronics 9 are arranged in a specially protected housing 17 of the measuring device. The housing 17 is shielded from the probe 15 by glass windows 18, 19 that are transparent to the laser beam generated by the emitting device 2.

[0069] The measuring device 1 according to FIG. 3 also has a device (not shown) for releasing a purge gas, with which the reflective element 3 can be flushed with purge gas.

[0070] In practical use, the measuring device 1 shown in Fig. 3 is used as follows: a laser beam generated by the emitting device 2 is shaped and refracted by the propagation optics 11 and propagates along a first optical axis 5 through two glass windows 18, 19 to the reflecting element 3. The laser beam is reflected by the reflecting element 3 and propagates along a second optical axis 6 through two glass windows 18, 19 to the detecting element 4. Before arriving at the detecting element 4, the laser beam is shaped and refracted by the receiving optics 12. The laser beam is detected by the detecting element 4, and an electrical signal generated in the process is transmitted to the evaluation electronics 9. The evaluation electronics performs a gas measurement based on the detection of the detecting element 4.

[0071] In a section of the first optical axis 5 and in a section of the second optical axis 6 passing through the probe 15, the laser beam is guided through the gas mixture to be analyzed, whereby, as explained above, the signal intensity of the laser beam is reduced by interaction with formaldehyde. Based on this reduction in the signal intensity of the laser beam, the concentration of formaldehyde in the gas mixture is determined as explained above. [Explanation of symbols]

[0072] 1. Measuring Device 2. Radiating devices, laser diodes 3 Reflective element, spinel 4. Detection Elements 5 First optical axis 6 Second optical axis 7 Entrance plane 8 Electronic Data Line 9 Electronic evaluation unit, evaluation electronics 11 Propagation optics 12 Receiving optical system 13 Steel casing 14 flange 15 probes 16 Aperture 17. Cabinet 18, 19 Glass windows

Claims

1. A measuring device (1) for absorption spectroscopy gas measurements, comprising: 1.1 a radiation device (2) capable of generating a laser beam; 1.2 a reflective element (3) capable of reflecting the laser beam; 1.3 at least one detection element (4) by which the laser beam can be detected; 1.4 a first optical axis (5), along which the laser beam generated by the radiation device (2) can be guided to the reflecting element (3); and 1.5 a second optical axis (6), along which the laser beam reflected by said reflecting element (3) can be guided to said detecting element (4); Equipped with 1.6 Measuring device (1), wherein said reflective element (3) is selected from the following group: spinel, polycrystalline aluminum oxide, aluminum oxynitride.

2. 2. The measuring device (1) according to claim 1, wherein the reflective element (3) is a retroreflector.

3. 3. The measuring device (1) according to claim 1 or 2, wherein the reflective element (3) has a coating.

4. 2. The measuring device (1) according to claim 1, wherein the reflecting element (3) has an entrance surface (7) through which the laser beam generated by the radiation device (2) can be introduced into the reflecting element (3), the entrance surface (7) being designed as a flat surface aligned at an angle unequal to 90° with respect to the first optical axis (5).

5. 2. The measuring device (1) according to claim 1, wherein the emitting device (2) is a laser diode.

6. 2. The measuring device (1) according to claim 1, wherein the emitting device (2) is a tunable laser diode.

7. 2. The measuring device (1) according to claim 1, wherein the emitting device (2) is a laser diode tunable from 2.0 μm to 5.0 μm.

8. 2. The measuring device (1) according to claim 1, wherein the at least one detection element (4) comprises at least one photodetector.

9. 2. The measuring device (1) according to claim 1, wherein the laser beam generated by the radiation device (2) can be guided through the gas mixture along a section of the first optical axis (5).

10. 10. The measuring device (1) according to claim 9, wherein the laser beam generated by the radiation device (2) can also be guided through the gas mixture along a section of the second optical axis (6).

11. 10. The measuring device (1) according to claim 9, further comprising a probe (15), wherein the section of the first optical axis (5) extends in the probe (15).

12. 12. The measuring device (1) according to claim 11, wherein the section of the second optical axis (6) also extends in the probe (15).

13. 2. The measuring device (1) according to claim 1, further comprising a device for delivering a purge gas, with which the reflective element (3) can be flushed with a purge gas.

14. 1. Use of spinel, polycrystalline aluminum oxide, or aluminum oxynitride as a reflective element (3), by means of which a laser beam can be reflected in a measuring device (1) for absorption spectroscopy gas measurements.

15. A. Providing a measuring device (1) according to claim 1; B. generating a laser beam by said emitting device (2); C. A step of detecting said laser beam by said detection element (4); D. performing a gas measurement based on said detection; 1. A method for absorption spectroscopy gas measurement, comprising:

Images