MEASURING DEVICE AND METHOD FOR DETECTING DIFFERENT GASES AND GAS CONCENTRATIONS

Description

[0001] The invention relates to a measuring device for detecting different gases and gas concentrations, which comprises a radiation source, a measuring channel having an optical path and a gas interaction path, and radiation detectors arranged along the measuring channel.

[0002] The invention also relates to a method for measuring gases and gas concentrations, which uses the measuring device according to the invention.

[0003] There are two basic types of gas sensors: interacting and non-interacting gas sensors. In the former, a gas must interact physically or chemically with a sensing element. The gas comes into contact with one or more components, e.g., electrodes, electrolytes, or sensor surfaces of the gas sensor via oxidation, reduction, or physical adsorption. These interactions inevitably lead to a change in the gas sensor, i.e., a change in sensor parameters depending on the interacting component of the sensor, e.g., a change in the electrolyte interacting with the gas, which requires regular calibration and ultimately the replacement of the gas sensor. The most commonly used interacting gas sensors are, for example, electrochemical sensors, solid-state sensors, and catalytic sensors.

[0004] Non-interacting, also known as interaction-free, gas sensors are optical gas sensors. In these sensors, only electromagnetic radiation comes into contact with the gas or interacts with it. Part of the radiation is absorbed by the gas molecules, causing them to change their excited state. However, collisions with other gas molecules or the sample chamber cause the excited gas molecules to immediately return to their ground state, so that the state of the gas undergoes neither physical nor chemical changes. The wavelength λ of the electromagnetic radiation ranges from the ultraviolet to the far infrared spectral range (λ = 0.2 µm to 20 µm). Non-dispersive infrared (NDIR) gas sensors are the most widely used interaction-free gas sensors. In addition to their simple design, they are characterized by high measurement resolution, a long service life, and good long-term stability.The method utilizes the excitation of energy states in molecules, i.e., the vibrational excitation of molecular bonds, by infrared radiation. Infrared radiation is absorbed at these molecule-specific rotational and vibrational frequencies. Due to its unique molecular structure, each molecule possesses very specific absorption bands in the infrared spectral range, which allows it to be uniquely identified. The infrared spectral range λ = (2 ... 20) µm is of technical interest because the characteristic absorption bands of many compounds lie within this spectral range.

[0005] The first practically usable NDIR gas sensor was developed in 1938 and is described in patent DE730478. In this setup, the radiation emitted by two radiation sources is periodically interrupted by a motor-driven aperture wheel and guided into two separate tubes. One tube contains the gas or gas mixture to be measured, and the other contains a reference gas. The radiation then enters two measuring chambers, which contain the gas to be detected as a receiver layer. These two measuring chambers are separated from each other by a thin membrane. Gas-tight means that no gas is exchanged between the chambers. The membrane, together with an insulated counterplate, forms an electrical capacitor whose capacitance can be read with a measuring instrument. The absorption of the infrared radiation by the gas is thus detected as a pressure difference using a very sensitive microphone.The main disadvantages of this NDIR gas sensor, known as a photoacoustic gas measuring cell, are its size and its mechanical susceptibility to vibrations and shocks.

[0006] With the development of non-dispersive, very narrowband optical filters, a technology finally became available that enabled significantly smaller and more robust NDIR gas detection devices. These so-called interference filters use the effect of interference to filter electromagnetic radiation according to frequency or wavelength. In a bandpass filter design, a specific wavelength band is transmitted, while shorter and longer wavelengths are reflected or absorbed. The transmission maximum is defined as the center wavelength (CW) of the bandpass filter. The filter's bandwidth is specified as the full width at half maximum (FWHM), i.e., the difference between the two argument values ​​for which the function values ​​have dropped to half the maximum.Finally, the transmission spectrum of the bandpass interference filter is selected to correspond to a characteristic absorption band of the gas being measured. The absorption of infrared radiation by the sample gas is measured using a highly sensitive radiation detector located behind the bandpass interference filter.

[0007] The radiation attenuation caused by the gas due to radiation absorption is ultimately a measure of the gas concentration. The radiation intensity IM at the measurement wavelength changes depending on the gas concentration c according to the Lambert-Beer law: I M = I 0 ⋅ e α ⋅ c ⋅ l where α is the gas-specific absorption coefficient, l is the absorption path length and I 0 is the basic intensity of the radiation, ie in the absence of the measuring gas (c = 0).

[0008] A simple NDIR gas sensor therefore consists of an infrared radiation source, a measuring chamber (cuvette) containing the gas or gas mixture to be measured, and an infrared detector with a bandpass interference filter whose transmission spectrum corresponds to the absorption band of the gas to be measured ( Fig. 1 These components are mounted along an optical axis. Such a setup is described, for example, in the publications DE10221708B4 and DE10013374A1. The infrared radiation source is typically an electrically modulated thermal radiator, which, due to its temperature, emits electromagnetic radiation with a continuous spectrum that includes all wavelengths of the technically interesting spectral range λ = (2 ... 20) µm. A wide range of infrared detectors with sufficient signal-to-noise ratios and low costs are available, e.g., thermopile sensors and pyroelectric detectors.

[0009] Modern NDIR gas sensors, such as those known from documents DE 10 2008 005 572 B4, DE 20 2005 010 475 U1, DE 102 21 708 B4 and DE 296 02 282 U1, are usually operated according to the so-called dual-frequency method ( Fig. 2). In addition to the measurement at a measuring wavelength matched to the measuring gas, measurement is also taken at a second wavelength, the so-called reference wavelength, which lies in a spectral range in which no absorption by other gases present in the gas mixture or in the environment occurs. This requires two infrared detectors with different bandpass interference filters arranged in the beam path. By forming the quotient of the two detector signals, a significant improvement in stability is achieved. This can compensate for signal changes due, for example, to intensity drifts of the radiation source or dirt deposits in the measuring chamber. A disadvantage, however, is the necessary distribution of the radiation flux emitted by the radiation source between the two infrared detectors, which reduces the radiation intensity at the infrared detector and thus the detection limit of the gas sensor. Figure 2shows the state-of-the-art dual-frequency method and the necessary distribution of the radiation emitted by the radiation source S to the detectors D1 and D2.

[0010] However, with the above-mentioned measurement methods and suitable measuring devices, only one gas can be measured. In many gas analysis applications, such as exhaust gas, flue gas, or anesthetic gas measurements, it is necessary to measure several gases simultaneously and determine their concentrations in the gas mixture.

[0011] US 2012 / 0235038 A1 presents a so-called multispectral detector for NDIR gas sensors, which features a multitude of detector elements with bandpass interference filters. This enables a simple gas sensor design according to the dual-frequency method described above for simultaneous measurement of multiple gases. Similar arrangements are also known from the documents DE 34 06 175 A1, DE 41 33 481 A1, US 6,201,245 B1, and DE 101 40 998 C2. Significant disadvantages of such NDIR gas sensors with multispectral detectors are a constant absorption path or cuvette length for all spectral channels and the distribution of the radiation intensity across the individual detector elements. For example, with a 4-channel detector, ideally only 25% of the incident radiation intensity per detector element is available for signal generation. In reality, this is usually less than 10%.Consequently, the detection limit of the gas sensor is greatly reduced for all gases to be measured.

[0012] A constant absorption path length for all spectral channels limits both the measurement range and the detection limit of the gas sensor. This results from the fact that, firstly, each gas has a gas-specific absorption coefficient; secondly, gases are usually present in different concentrations in a gas mixture; and thirdly, gases have different toxicities, which is why different limit values ​​apply, which in turn require different measurement resolutions. For example, the respiratory toxins carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), nitrogen monoxide (NO), and carbon monoxide (CO) contained in the flue gas from an oil-fired furnace are present in the following concentrations: (125,000 ... 140,000) ppm CO 2 , (180 ... 220) ppm SO 2 , (80 ... 150) ppm CO and (50 ... 100) ppm NO.

[0013] The maximum workplace concentrations (MAK values) of these flue gas components can be found in the technical rules for hazardous substances (TRGS 900) and are: 5000 ppm for CO 2 , 0.5 ppm for SO 2 , 25 ppm for NO and 30 ppm for CO.

[0014] Consequently, the radiation attenuation caused by the gas, according to the Beer-Lambert law, is unique for each gas, and therefore a specific absorption path length is advisable for accurate concentration determination. Otherwise, a compromise must always be made regarding measurement range and resolution.

[0015] DE19604167A1 proposes a gas sensor device for detecting gas concentrations in a complex gas mixture. The individual radiation detectors are arranged rotationally symmetrically around a radiation source, whereby the distance to the radiation source and thus the absorption path length can vary. A significant disadvantage of this arrangement remains the distribution of the emitted radiation flux among a large number of radiation detectors, resulting in only a very small fraction of the radiation intensity reaching the detectors and a significant reduction in the detection limit of the gas sensor device. The same applies to the arrangement described in US5222389A, in which the individual radiation detectors are arranged along the measuring chamber to achieve different absorption path lengths.However, there is also a significant disadvantage that only a fraction of the measuring radiation hits the detectors, which also depends on the respective reflection on the measuring chamber wall.

[0016] KR 1020100052691 A also discloses an NDIR gas sensor with only one radiation detector, in which the measurement wavelength is selected using a filter wheel. This filter wheel can be equipped with suitable bandpass interference filters depending on the gases to be measured, allowing a large number of gases to be identified using a simple setup. However, this can only be done sequentially. Simultaneous detection of the concentration of different gases in a gas mixture is not possible. Furthermore, the absorption path length is the same for each gas, making the arrangement difficult to miniaturize. Another arrangement for the sequential detection of gases is disclosed in US Pat. No. 3,797,942.

[0017] To improve the detection limit of optical gas sensors, it is necessary to focus the highest possible radiation intensities onto the radiation detector. Furthermore, a specific absorption path length must be provided for each sample gas to ensure optimal determination of the individual gas concentrations in a complex gas mixture and to minimize the size of the gas sensor. For many applications, simultaneous determination of the components in a gas mixture is also required.

[0018] It is therefore an object of the present invention to provide an optical measuring device with a significantly improved detection limit, which is compact, simple in design, and cost-effective to manufacture. The measuring device should operate without radiation intensity losses and have an individually adjustable absorption path length for different gas-specific absorption coefficients and gas concentrations, enabling the simultaneous determination of the concentration of different gases in a gas mixture.

[0019] The object is achieved in terms of the arrangement by a measuring device according to independent claim 1. In the measuring device according to the invention, the optical path has at least two deflection points, wherein a first narrow-bandpass interference filter is arranged at a first deflection point and a first radiation detector is arranged immediately downstream of the narrow-bandpass interference filter, and wherein the optical path and the gas interaction path in the measuring channel are identical. A deflection point defines a boundary of the measuring channel and limits the interacting measuring channel length, i.e., the absorption path length of a gas to be specifically measured. The gas interaction path is the path along which the gas or gas mixture to be measured interacts with the radiation in the measuring channel.At the deflection point in the optical path, the measuring radiation strikes a first narrow-bandpass interference filter, whereby only radiation of a wavelength specified by the first narrow-bandpass interference filter strikes a radiation detector arranged downstream of the narrow-bandpass interference filter. This means that only a specific narrow wavelength band is transmitted through the filter, while shorter and longer wavelengths are reflected. In the context of this description, narrowband means a waveband of < 10% of the center wavelength of the narrow-bandpass interference filter. The transmission maximum is defined as the center wavelength of the narrow-bandpass interference filter (ZWL). The transmission spectrum of the narrow-bandpass interference filter is selected so that it corresponds to a characteristic absorption band of the gas to be measured.The absorption of infrared radiation by the measuring gas is measured using a very sensitive radiation detector located behind the narrow bandpass interference filter. The radiation reflected by the filter is passed on from the deflection point in the measuring channel until it reaches the next deflection point. This, and the fact that the gas interaction path and the optical path are identical, i.e. congruent with one another, also ensures that no radiation splitter is required in the measuring device to direct the radiation to multiple radiation detectors or to split radiation needed for a specific gas detection and make it available to a separate detector for measurement. A beam splitter splits the radiant power emitted by the radiation source into two parts, independent of the wavelength, and reduces the radiation intensity accordingly (. Fig. 3The elimination of beam splitters has the particular advantage that the radiation intensity of the radiation source does not have to be divided among different detectors as in the prior art; instead, the maximum radiation intensity reaches each radiation detector. This results in significantly improved resolution because the signal at the detector is larger.

[0020] The optical path from the radiation source to the first deflection point has an absorption path length L 1 . The absorption path length L must be designed according to the gas to be detected. This is necessary because each gas has a gas-specific absorption coefficient and gases are usually present in different concentrations in a gas mixture. Furthermore, a gas-specific measurement resolution is required due to, for example, gas-specific toxicity.

[0021] Consequently, an individual absorption path length or measuring channel length is required for each gas so that the measuring device can be adapted to the required measuring resolutions and measuring ranges of the gases to be measured.

[0022] In all embodiments of the measuring device according to the invention, the optical path of the measuring device has N deflection points, wherein from the radiation source to the Nth deflection point the optical path has an absorption path length LN, where N is a natural number greater than 1. This means that the number of deflection points allows the absorption path length of the optical path and thus the length of the gas interaction path to be individually adjusted, so that different gases with different absorption path lengths can be measured simultaneously. The gas to be examined can be introduced into the measuring channel over the entire length of the optical path. Furthermore, according to the invention the narrow bandpass interference filter has an optical axis, wherein the optical path and the optical axis of an Nth narrow bandpass interference filter FN enclose an angle φ N.

[0023] The angle φ allows for a compact design of the measuring device, and the measurement radiation reflected by the narrow bandpass interference filter FN is completely directed to the filter F N+1 downstream in the measuring channel. This ensures that the maximum radiation intensity reaches each detector. The angle φ ranges from greater than 0° to less than 50°, more preferably from 30° to 50°. The angle can be individually adjusted for each deflection point N.

[0024] For optimized radiation detection at the radiation detector, a radiation concentrator can be arranged between the narrow bandpass interference filter and the radiation detector, although this embodiment does not fall within the scope of the claims.

[0025] This radiation concentrator is a type of collimator and is designed in such a way that the radiation transmitted through the filter is directed as completely as possible to the radiation detector.

[0026] It is advantageous if the substrate material of the narrow-bandpass interference filter is made of silicon (Si), germanium (Ge), calcium fluoride (CaF2), barium fluoride (BaF2), or zinc selenide (ZnSe). These materials exhibit a particularly high transmittance, particularly in the infrared spectral range, so that, depending on the narrow-bandpass interference filter used, a high proportion of radiation reaches the detector behind it. However, other substrate materials can also be used and / or implemented.

[0027] To further variably adjust the absorption path length, an optical mirror can be arranged at a second deflection point to extend the optical path, i.e., the absorption path length. Thus, the path length of the measurement radiation can be variably extended to L = L 1 +L 2 +...+LN by arranging mirrors at the deflection points in the measurement channel, depending on the absorption path length L required for the gas to be detected.

[0028] Furthermore, according to the invention, a reference detector can be arranged at a second, further deflection point. This has the advantage that, for example, changes in the intensity of the radiation source or impurities in the measuring channel can be monitored. For example, at a wavelength of approximately 4 µm there is no absorption by gases. This wavelength can be used by a reference detector to detect changes in the measuring device that are not caused by the gas being analyzed. Based on changes in the signals detected by the radiation detectors and a reference detector, it can be determined to what extent changes are caused, for example, by aging or drift of the radiation source or due to deposits, e.g. on the optical mirrors, by the gas to be measured in the measuring channel. The reference measurement, which is carried out synchronously with the gas concentration measurement, ensures the long-term stability of the gas concentration or gas flow measurement.Gas measurement is improved and maintenance requirements for the gas sensor are reduced as calibration is required less frequently.

[0029] The problem with this type of reference measurement is that spectral changes in the radiation flux cannot be compensated or can only be compensated insufficiently, especially if the absorption wavelength of the gas is not close to the reference wavelength.

[0030] In order to fully compensate for spectral changes in the radiation flux caused, for example, by deposits in the optical measurement channel, intensity drifts and emissivity changes of the radiation source, as well as ambient temperature influences, thereby significantly improving the long-term stability of NDIR gas sensors, minimizing maintenance and calibration effort and the associated costs, and achieving a compact design, it is particularly advantageous if the reference measurement is performed at the absorption wavelength of the gas to be measured. In this type of reference measurement according to the invention, the reference detector with the reference narrow-bandpass interference filter is arranged in the measurement channel downstream of the radiation detector for the gas to be measured.The reference narrow bandpass interference filter has a larger bandwidth than the narrow bandpass interference filter for the gas to be measured, whereby the transmission ranges of the two filters overlap and the reference narrow bandpass interference filter always transmits a range outside the absorption band of the gas to be measured to the reference detector.

[0031] The overlap of the narrow-bandpass interference filter in front of a reference detector and the narrow-bandpass interference filter in front of a radiation detector is understood to be a spectral range in which the passbands of the two filters partially, but not necessarily completely, overlap. An overlap therefore occurs when the bandpasses of the filters used partially overlap, forming a common intersection in the spectral range.

[0032] It is particularly advantageous if the narrow-bandpass interference filter in front of a reference detector and / or the narrow-bandpass interference filter in front of a radiation detector detects, with its respective passband, an edge of an absorption band of a gas to be detected. This means that either the narrow-bandpass interference filter in front of a reference detector detects, with its respective passband, an edge of an absorption band of a gas to be detected, or the narrow-bandpass interference filter in front of a radiation detector detects, with its respective passband, an edge of an absorption band of a gas to be detected.It is also conceivable that both narrow bandpass interference filters, namely the narrow bandpass interference filter in front of a reference detector and the narrow bandpass interference filter in front of a radiation detector, detect a flank of an absorption band of a gas to be detected, but it must be ensured that one of the detectors detects a larger spectral range than the other detector, whereby the first detector along the measuring channel must have the smaller passband.

[0033] Due to the conditions listed above, an offset occurs between the signal detection by the radiation detector and the signal detection by the reference radiation detector. This offset is caused by the larger bandwidth of the narrow-bandpass interference filter upstream of the reference detector, or more generally, the narrow-bandpass interference filter upstream of the second detector, which is arranged downstream of the first detector along the measurement channel.

[0034] Interference, such as intensity drift of the radiation source, affects the measurement and reference channels equally, so that the signal ratio remains unchanged. Changes in the gas concentration, however, lead to a change in the signal ratio. A particularly advantageous feature is that previously used compensation methods, such as generating different absorption path lengths or integrating a saturation cell, can be dispensed with (see US 8,003,944 B2 and US 8,143,581 B2). This allows for a significantly more compact and simpler design and significantly better compensation of interference, resulting in gas detection devices with significantly improved long-term stability and consequently lower maintenance costs.

[0035] In another embodiment, the narrow-bandpass interference filter in front of a radiation detector and the narrow-bandpass interference filter in front of a reference detector can be designed such that they fully capture a gas absorption band to be detected. In this case, too, it must be ensured that one of the detectors detects a larger spectral range than the other detector arranged downstream along the measurement channel.

[0036] It is also advantageous if the narrow-bandpass interference filter upstream of a radiation detector has a first center wavelength and the narrow-bandpass interference filter upstream of a reference detector has a second center wavelength, wherein the first and second center wavelengths are identical. It is important that the bandwidths, i.e., the passbands, of the two narrow-bandpass interference filters are designed differently, and that a narrow-bandpass interference filter transmits at least a portion of the spectrum that lies outside the absorption band of the gas to be detected, so that a radiation detector sees a larger spectral radiation component, thus generating an offset as described above.

[0037] However, the first and second center wavelengths do not have to be identical. They can also be different, although it is advantageous if the center wavelengths are in the range of the gas absorption band to be detected. Depending on the gas to be detected and its characteristic spectral absorption behavior, the narrow-bandpass interference filter in front of a radiation detector and the narrow-bandpass interference filter in front of a reference detector must be selected in such a way that the generation of an offset is ensured.

[0038] The first and second center wavelengths can also lie within the absorption band of the gas to be detected. To achieve this, the narrow-bandpass interference filter in front of a reference detector must always transmit a portion of the radiation to the radiation detector that lies outside the characteristic absorption band of the gas. For example, the reference detector detects 30% of the transmission spectrum that lies outside the absorption band of the gas to be detected and 70% of the transmission spectrum that lies within the absorption band of the gas to be detected.

[0039] In one embodiment, the radiation detector and the reference radiation detector can be arranged next to one another at a single measuring location, for example, at a deflection point along the measuring channel. This allows the measuring device to be implemented in a particularly compact design. Furthermore, both detectors are located on the same heat sink, allowing ambient temperature fluctuations to be fully compensated. It is important that a narrow-bandpass interference filter is arranged in front of each detector, with the passband of the narrow-bandpass interference filter in front of the radiation detector and the passband of the narrow-bandpass interference filter in front of the reference detector overlapping, and the narrow-bandpass interference filter in front of the reference detector or the narrow-bandpass interference filter in front of the radiation detector having a larger bandwidth than the other narrow-bandpass interference filter.

[0040] A particularly advantageous feature is that the measuring device is modular in design, and the optical path and gas interaction path can be extended in a modular manner. Modular means that the measuring device can be extended by an optical path length L and a deflection point with a narrow-bandpass interference filter and a detector located at this deflection point, but also optionally by a mirror arranged at the deflection point, until, for example, a desired absorption path length is achieved. These mirrors can have, for example, a planar, concave, or convex surface.

[0041] Furthermore, the optical path can be curved, for example, helical. This allows the design of the measuring device according to the invention to be made very compact by arranging radiation detectors at defined points along the measuring path to adjust different absorption path lengths for the gases to be detected.

[0042] It is advantageous to design the measuring channel with a nearly perfectly reflective layer. Gold-plated and chrome-plated surfaces, as well as reflective surfaces made of aluminum or stainless steel, are particularly suitable for this. However, the measuring channel does not necessarily have to be understood as a physically defined space. The measuring channel, as defined in this description, is a space in which the gas or gas mixture to be measured can interact with the radiation from the radiation source, e.g., in the form of a channel.

[0043] For a compact design, the measuring device with the detectors, the measuring channel, the narrow bandpass interference filters, the concentrators and the mirrors as well as the reference detectors can be integrated on a circuit board. The radiation source and the detectors are arranged in such a way that the most space-saving and compact arrangement can be achieved. The measuring channel is formed on the circuit board, e.g. integrated into the circuit board. In order to guide the measuring radiation from the radiation source into the measuring channel, a mirror is used to deflect the measuring radiation into the measuring channel. The same applies to the beam deflection of the measuring radiation onto the detectors at the deflection points along the optical path within the measuring channel. For this purpose, a mirror or similar is installed behind the narrow bandpass interference filter, which the measuring radiation initially hits.for beam deflection, which directs the transmitted radiation portion to the detector arranged on the circuit board. This allows for a significantly more compact design, as the vertical integration of the measuring device components reduces the horizontal dimensions because, for example, the radiation source and the detectors do not have to be arranged in the same plane as the measuring channel. The portion of the measuring radiation reflected by the narrow-bandpass interference filter is guided further along the optical path through the measuring channel until the measuring radiation encounters another narrow-bandpass interference filter.

[0044] In a particularly advantageous embodiment of the invention, the radiation source is a broadband laser with parallel radiation. This has the advantage that the full intensity of the radiation always hits the detector at the deflection points. Reflection and absorption losses due to scattering of the radiation in the measurement channel are thus minimal.

[0045] Furthermore, in a particularly advantageous embodiment of the measuring device according to the invention, a focusing device can be arranged in front of the radiation source for focusing and parallelizing the radiation emitted by the radiation source, especially when no laser is used. This has the effect that the maximum radiation intensity impinges on the respective deflection points and the functional units arranged at the deflection points, such as the radiation detector, mirror, reference detector, etc.

[0046] The object of the present invention is achieved in terms of the method in that a measuring radiation is introduced into a measuring channel containing the gas or gas mixture to be measured through the radiation source, the measuring radiation strikes a first narrow bandpass interference filter arranged at a first deflection point, wherein only radiation of a wavelength predetermined by the first narrow bandpass interference filter strikes a radiation detector arranged downstream of the narrow bandpass interference filter, the portion of the radiation reflected by the narrow bandpass interference filter is guided along the optical path in the measuring channel to a second radiation detector and / or mirror, wherein the gas concentrations measured with the radiation detectors are then evaluated. Due to the design of the measuring channel orDue to the characteristics of the radiation source, the radiation from the radiation source is directed directly to the narrow-bandpass interference filter and the downstream radiation detector. By arranging various radiation detectors at the deflection points along the optical path, the simultaneous measurement of different gases or gas components of a gas mixture under investigation is possible.

[0047] In one embodiment of the method according to the invention using the measuring device according to the invention, a reference detector with a reference narrow bandpass interference filter arranged upstream of it is arranged in the measuring channel after the radiation detector with the narrow bandpass interference filter for the gas to be measured, wherein the reference narrow bandpass interference filter has a larger bandwidth than the narrow bandpass interference filter of the radiation detector, the transmission ranges of the two narrow bandpass interference filters are designed to overlap and the reference narrow bandpass interference filter allows a surrounding part outside the absorption band of the gas to be measured to be transmitted to the reference detector and a signal ratio is formed between a detector signal of the radiation detector and a detector signal of the reference detector and the signal ratio is evaluated and thus a deviation in the signal ratio can be compensated.It is important that for the narrow bandpass interference filters arranged upstream of the detectors, the first narrow bandpass interference filter along the measuring channel always has a smaller passband than the second narrow bandpass interference filter, i.e. the narrow bandpass interference filter arranged downstream of the first narrow bandpass interference filter in the measuring channel.

[0048] The offset is generated by the narrow-bandpass interference filter in front of a reference detector transmitting a larger spectral range, while the reference detector detects a range outside the gas absorption band. Spectral changes resulting, for example, from deposits in the optical measurement channel or ambient temperature influences, as well as intensity drifts and spectral changes in the radiation source, can thus be compensated.

[0049] It is advantageous if the narrow-bandpass interference filter in front of a reference detector transmits at least 30% of a wavelength range that lies outside the absorption band of the gas to be measured. However, this range can also be smaller or larger than 30%. At a minimum, it must be ensured that the reference radiation detector detects a larger spectral range than the radiation detector, with a spectral radiation component that lies outside the absorption band of the gas to be detected.

[0050] In the following, the invention will be explained in more detail using exemplary embodiments.

[0051] The accompanying drawings show Fig. 1 Schematic representation of the basic structure of a simple NDIR gas sensor according to the prior art; Fig. 2 Schematic representation of the basic structure of a simple NDIR gas sensor with reference measurement according to the prior art; Fig. 3 Radiation intensity with and without the use of a beam splitter; Fig. 4 Schematic representation of the measuring device according to the invention for the simultaneous measurement of different gases and gas concentrations; Fig. 5 Reflected radiation intensity at the deflection points / bandpass interference filters FN arranged sequentially in the measuring channel according to the gases to be detected; Fig. 6 Transmission curves of the exemplary bandpass interference filters for various gases; Fig. 7 Schematic representation of the measuring device according to the invention with a reference measurement; Fig.8Schematic representation of an embodiment of a measuring device with optical concentrators arranged in front of the detectors and optionally also in front of the radiation source, wherein an arrangement of concentrators in front of the detector does not fall within the scope of protection of the claims; Fig. 9Schematic representation of an embodiment of the measuring device according to the invention with optical mirrors Sp at selected deflection points N to increase the absorption path length L; Fig. 10Schematic representation of an embodiment of a measuring device with a curved optical path, which does not fall within the scope of protection of the claims, i.e. a curved measuring channel, a) sectional representation, b) perspective representation; Fig.11Schematic representation of an embodiment of a measuring device which does not fall under the scope of protection of the claims, wherein the measuring device is integrated with the detectors and the optical path on a circuit board, a) perspective top view, b) side view; Fig. 12Spectral profile of the transmission spectra of the radiation narrow-bandpass interference filter, the reference narrow-bandpass interference filter, and the gas to be detected, wherein the center wavelengths of the bandpass interference filters are identical; Fig. 13Signal ratio between the signal of the radiation detector and the signal of the reference detector.

[0052] Figure 4shows schematically a possible embodiment of the measuring device according to the invention. The gas is admitted into the measuring channel 1 by laminar flow via a gas inlet 4. The gas can also be introduced into the measuring channel over the entire length of the optical path. The gas inlet can also be designed as a gas-permeable membrane, where the gas diffuses into the measuring channel 1 of its own accord. The IR radiation source S emits a beam of rays with a continuous spectrum, which is guided, for example, in a waveguide, the measuring channel 1, in which the gas or gas mixture is located, along an optical path 2 and, after an individually adjustable path length L 1, strikes a narrow bandpass interference filter F 1 at the angle φ 1. The location of the filter defines a first deflection point 6 of the measuring radiation and only allows a specific wavelength ora very narrow wavelength band of the radiation and reflects the remainder back into the measuring channel 1 at an angle φ' 1 . The transmission wavelength of the filter F 1 corresponds to the absorption wavelength of a gas G1 to be measured. The . Figures 4 , 5 and 6 show, by way of example, the operation and effect of the measuring device for measuring gas components in a gas mixture to be examined, whereby the gas components are detected simultaneously at the successive detectors D, which are arranged at the deflection points 6 in the measuring channel 1 with corresponding narrow-bandpass interference filters F. In Figure 5 The radiation intensities reflected at the respective deflection points 6 are shown. The transmitted wavelength band is defined by the respective narrow-bandpass interference filter. The radiation portion transmitted through the first narrow-bandpass interference filter F 1 is detected by the detector D 1 ( Fig. 5a ). The reflected beam component, after a path length L 2, hits another narrow-bandpass interference filter F 2 at the angle φ 2 ( Fig. 4 ). At this second deflection point 6, a further radiation component of a gas G2 to be measured is filtered out and measured by the detector D 2 ( Fig. 5b ). This can be continued ( Fig. 5c - e ). From the Fig. 5a-e It becomes clear that the full radiation intensity always hits the detectors D. The required narrow-bandpass interference filters F are arranged in the measuring channel 1 according to the gas fraction to be detected and the required absorption path lengths. For example, a longer path length L is required for the detection of SO 2 than for CO 2 . The gas can leave the measuring channel again via a gas outlet 5. The gas outlet 5 can also be designed as a gas-permeable membrane, where the gas diffuses spontaneously out of the measuring channel 1.

[0053] Fig. 6shows examples of the transmission curves of the narrow bandpass interference filters used for different gases.

[0054] Figure 7 shows a preferred embodiment of the measuring device according to the invention, wherein a reference measurement is performed at any deflection point 6 in the measuring channel 1 with a reference detector 9 at a wavelength at which no absorption by other gases present in the gas mixture or in the environment occurs (e.g., at 3.95 µm). This reference measurement achieves a significant improvement in stability, which, for example, allows signal changes due to intensity drifts of the radiation source S or dirt deposits in the measuring chamber to be compensated.

[0055] In an example not falling within the scope of the claims, an optical concentrator KS or a similar optical component (e.g. mirror, lens) is arranged on the radiation source S, whereby the generated radiation is focused into the measuring channel 1. This is shown in Figure 8 shown schematically. In addition, an optical concentrator K 1 , K 2 , etc. is arranged behind each narrow-bandpass interference filter F 1 , F 2 , etc., which focuses all rays transmitted through the filter onto the detector element D so that a maximum detector signal is generated.

[0056] In a further variant of the measuring device according to the invention, in order to increase the absorption path length L, a mirror Sp can be arranged at one or more deflection points 6 instead of a narrow bandpass interference filter F, which mirror Sp completely reflects the incident radiation so that the radiation is forwarded to the next deflection point 6 ( Fig. 9 ).

[0057] In another exemplary measuring device, which does not fall under the scope of the claims, the optical path 2 or the measuring channel 1 can have a curved path, which can also be designed three-dimensionally, e.g. helically, so that the largest possible absorption path length L can be accommodated in a small volume. This is shown schematically in Figure 10a, b At suitable locations, ie at the deflection points 6 along the curved optical path 2, detectors D are arranged according to the measuring device. The curved design of the optical path 2 allows an even more compact design of the measuring device. For illustration, Fig. 10 a, b only one detector D is shown at the measuring channel 1 along the optical path 2, namely at a first deflection point 6.

[0058] Figure 11shows a further embodiment of an exemplary measuring device which does not fall under the scope of protection of the claims, wherein the measuring device is integrated with the detectors D and the optical path 2, i.e. the measuring channel 1, on a printed circuit board LP. The radiation source S and the detectors D are arranged in such a way that the most space-saving and compact arrangement possible can be realized. The optical path 2, i.e. the measuring channel 1, is formed on the printed circuit board LP. In order to guide the measuring radiation 3 from the radiation source S into the measuring channel 1, a mirror Sp is used to deflect the measuring radiation 3 into the optical path 2. The same applies to the beam deflection of the measuring radiation 3 onto the detectors D at the deflection points 6 along the measuring channel 1. For this purpose, a mirror Sp or similar is arranged behind the narrow bandpass interference filter F, which the measuring radiation initially strikes.for beam deflection, which directs the transmitted radiation portion onto the detector D arranged on the circuit board LP. This allows for a significantly more compact design, since the vertical integration of the measuring device components reduces the horizontal dimensions. The portion of the measuring radiation reflected by the narrow-bandpass interference filter F is guided further along the optical path 2 through the measuring channel 1 until the measuring radiation 3 strikes another narrow-bandpass interference filter F.

[0059] The position of the respective bandpass interference filter of the radiation and reference detector to the absorption band of the gas to be examined is shown in Fig. 12The center wavelengths of the bandpass interference filters F1, F2 can coincide. It is important that the bandwidths of the two bandpass interference filters F1, F2 are different, and that a bandpass interference filter detects at least a portion of the spectrum 11, 12 that lies outside the absorption band 10 of the gas to be detected, so that a bandpass interference filter sees a larger spectral radiation component, thus generating an offset as described above.

[0060] Fig. 13Shows the signal ratio of the radiation or gas detector and the reference radiation detector, and the resulting signal ratio as a function of the gas concentration. Interference, such as intensity drift of the radiation source, affects the measurement and reference channels equally, so the signal ratio remains unchanged. Changes in the gas concentration, however, lead to a change in the signal ratio. Measuring device and method for detecting different gases and gas concentrations List of reference symbols

[0061] FNarrow bandpass interference filter, in short: Filter F 1 , F 2 , ..., FN Filter 1, Filter 2, ..., Filter N GGas G1, G2, ..., GNGas 1, Gas 2, ..., Gas N DDetector D 1 , D 2 , ..., DN Detector 1, Detector 2, ..., Detector N SRadiant source LAbsorption path length L 1 , ..., LN Absorption path length 1, ... N KConcentrator K 1 , ..., KN Concentrator 1, ..., Concentrator N KConcentrator in front of the radiation source SpMirror Sp 1 , ..., Sp N Mirror 1, ..., Mirror N LPPrinted circuit board φAngle of incidence φ 1 , ..., φ N Angle of incidence at the Nth narrow bandpass filter 1Measurement channel 2Optical path 3Direction of propagation of the measurement radiation 4Gas inlet 5Gas outlet 6Deflection point 8Optical axis 9Reference detector 10Gas absorption band 11One flank of a gas absorption band 12Another flank of a gas absorption band

Claims

1. Measuring device for detecting different gases and gas concentrations, comprising a radiation source (S), a measuring channel (1) having an optical path (2) and a gas interaction path, and radiation detectors (D) arranged along the measuring channel (1), characterized in that the optical path (2) has at least one deflection point (6), wherein a first narrow bandpass interference filter (F) is arranged at a deflection point (6) and a first radiation detector (D) is arranged immediately downstream of the first narrow bandpass interference filter (F), and wherein the optical path (2) and the gas interaction path in the measuring channel (1) are identical, wherein the optical path (2) has N deflection points (6) and an absorption path length L = L1 ...+ LN from the radiation source (S) up to the Nth deflection point, wherein N is a natural number greater than 1, wherein an individual absorption path length L1, ... LN is provided for each sample gas and different gases and gas concentrations can be detected simultaneously, wherein the narrow bandpass interference filter (F) has an optical axis (8), wherein the optical path (2) and the optical axis (8) of an N-th narrow bandpass interference filter (F) enclose an angle φN, wherein the angle φN has a size in the range greater than 0° to less than 50°, wherein a further narrow bandpass interference filter (F) is arranged at a further deflection point and a further radiation detector (D) is arranged immediately downstream of the further narrow bandpass interference filter (F).

2. Measuring device according to claim 1, characterized in that the optical path (2) has an absorption path length L1 from the radiation source (S) to the first deflection point (6).

3. Measuring device according to one of the preceding claims, characterized in that the optical path (2) is curved, in particular helical.

4. Measuring device according to claim 1, characterized in that the radiation source (S) is a broadband laser with parallel radiation.

5. Measuring device according to one of the preceding claims, characterized in that a focusing device for focusing or parallelizing the radiation emitted by the radiation source (S) is arranged in front of the radiation source (S).

6. Measuring device according to one of the preceding claims, characterized in that the substrate material of the narrow bandpass interference filter (F) consists of silicon, germanium, calcium fluoride, barium fluoride, or zinc selenide.

7. Measuring device according to one of the preceding claims, characterized in that an optical mirror (Sp) for extending the optical path (2) can be arranged at a second further deflection point and the mirror (Sp) has a planar or concave or convex surface.

8. Measuring device according to one of the preceding claims, characterized in that a reference detector (9) can be arranged at a second further deflection point.

9. Measuring device according to one of the preceding claims, characterized in that the measuring device is constructed modularly and the optical path (2) and the gas interaction path are modularly extendable.

10. A method of measuring gases and gas concentrations, which uses the measuring device according to claims 1 to 9, the method comprising the following steps: - Introduction of a measurement radiation into a measuring channel (1) containing the gas or gas mixture to be measured, - Incidence of the measurement radiation on a first narrow bandpass interference filter (F) arranged at a first deflection point (6), wherein only radiation of a wavelength specified by the first narrow bandpass interference filter (F) is incident on a radiation detector (D) arranged immediately downstream of the narrow bandpass interference filter (F), - Relay of the radiation reflected at the first narrow bandpass interference filter (F) along the optical path (2) in the measuring channel (1) to a further narrow bandpass interference filter with a further radiation detector (D) arranged immediately downstream, and - Evaluation of the gas concentrations measured using the radiation detectors (D).

11. Method for measuring gases and gas concentrations according to claim 10, characterized in that a reference detector (D) with a reference narrow-bandpass interference filter (F) arranged in front of the reference detector (D) is arranged in the measuring channel (1) after the radiation detector (D) with the narrow bandpass interference filter (F) for the gas to be measured, wherein the reference narrow bandpass interference filter (F) has a greater bandwidth than the narrow bandpass interference filter (F) of the radiation detector (D), the transmission ranges of the two narrow bandpass interference filters (F) are designed to overlap and the reference narrow bandpass interference filter (F) allows a surrounding part outside the absorption band of the gas to be measured to be transmitted to the reference detector (9, D), and a signal ratio is formed between a detector signal of the radiation detector (D) and a detector signal of the reference detector (9, D), the signal ratio is evaluated and a deviation in the signal ratio is thus compensated.