Light source for a gas analyzer

Abstract

Light source for a gas analyzer (1), in particular an extractive gas analyzer, wherein the gas analyzer (1) has at least one measuring section (2) and at least one detector (4) for measuring an intensity of the light source (3), characterized in that the light source (3) comprises a primary (3a) and a secondary (3b) light source, wherein the secondary light source (3b) is configured to homogenize the emission characteristics of the primary light source (3a).

Description

[0001] The invention relates to the technical field of gas analysis, in particular to gas analyzers designed as extractive absorption photometers. This exploits the fact that the interaction of electromagnetic radiation in the optical spectral range with the molecules of a gas depends very specifically on the molecular structure of the gas molecules. When gas molecules are irradiated with electromagnetic radiation, i.e. with light in the visible or non-visible range, the molecules are excited by absorbing the energy of the radiation, which is why characteristic absorption bands form in the intensity profile of this radiation transmitted to a detector. In the infrared spectral range, all heteroatomic gas components such as carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen monoxide (NO) have a characteristic absorption spectrum. SO2 and NO also have one in the ultraviolet spectral range.In this way, the concentration of a gas component, such as CO, in a gas to be analyzed can be determined.

[0002] In a gas analyzer, an optical filter selects the wavelength range of light from a light source relevant for measuring a specific gas component. This selected light is radiated through a measuring section, e.g., a measuring cuvette, through which the gas to be measured flows. The selected light interacts with the gas component to be measured, e.g., the CO molecules. This means that part of the light radiated through the measuring section is absorbed by the CO molecules. Due to the absorption, the intensity originally emitted by the light source is reduced in the selected wavelength range, thus providing a measure of the concentration of CO molecules in the gas to be analyzed.To measure this intensity reduced by the CO molecules, the light hits a detector, which is followed by electronic signal processing, which calculates the concentration of CO molecules in the gas to be analyzed from the measured intensity.

[0003] In the described gas analyzer design, even the slightest changes in the light source and the detector's sensitivity would result in unacceptably high zero-point errors. In other words, the device would not be reliably calibrated for operation, or the zero point of the calibration would shift due to these changes, such as a change in the light source's radiation characteristics.

[0004] The radiation characteristics of the light source include in particular the radiation angle and the intensity per solid angle, as well as the radiation spectrum of the light source.

[0005] To avoid zero point errors, common measurement setups of gas analyzers therefore use a comparison standard.

[0006] When using a comparison standard, an additional filter can alternately provide a first and a second frequency, with which measurements are then alternately taken, wherein the filter selects the first and second frequency from the spectrum of the light source, for example by rotating a filter wheel arranged in the beam path of the gas analyzer, such as an interference filter wheel.

[0007] In one application, this corresponds to what is known as interference filter correlation (IFC). In this method, a measurement beam and a reference beam are generated in the relevant frequency range by alternately scanning two different interference filters into the beam path of the gas analyzer. Alternatively, gas filter correlation can be used, for example, for a NO measurement. In this method, a reference beam is alternately scanned by scanning a gas filter filled with the gas to be analyzed into the beam path. For a joint measurement of, for example, NO and other gases, both measurement methods are then combined.

[0008] Alternatively or additionally, the beam path of the gas analyzer can be modified so that a specific frequency or frequency range of the light source selected by the filter is guided along a first path through a measuring section and along a second path through a reference measuring section having a predetermined, fixed gas concentration. In this case, it is referred to as a dual-beam photometer.

[0009] However, the described methods for correcting zero-point drift are based on the fundamental assumption that the light source itself is always the same, i.e., ideally, it emits with the same radiation characteristic relevant to the respective frequency range, e.g., intensity per solid angle. If this is not the case, a change in the gas analyzer's zero point may occur despite the described correction methods, thus leading to a measurement inaccuracy in the sense of zero-point drift.

[0010] This is particularly the case when a gas discharge lamp is used as a light source, as this is typically subject to an aging process that affects the radiation characteristics of the lamp. This can be due, for example, to a reduction or change in the gas in the discharge lamp, which influences the spatial extent of the lamp's plasma and thus its spatial radiation characteristics, i.e., the angular range in which the lamp emits radiation to the outside. If the radiation angle of the light source changes, more or less light is coupled into the beam path of the gas analyzer, which also corresponds to a change in intensity and thus overlays the change in intensity resulting from the concentration of the gas component to be measured. Furthermore, the change in intensity can vary across the frequency range of the gas discharge lamp, i.e.not be homogeneously distributed, resulting in a measurement error even with the same solid angle radiation. According to these measurement errors, the intensity decreases or increases during a measurement, especially during a long-term measurement. Even with compensation of the overall intensity, this change may lead to a change in the relative intensities of the measurement and reference beams imaged on the detectors, since their relationship to one another changes. This change cannot be compensated for by the measured value calculation and thus leads to a drift of the calculated measured value. Thus, the long-term stability of the gas analyzer in particular is not guaranteed, even if the device is equipped with a reference cuvette, for example. Causes of drift in gas analyzers include the aforementioned aging effect, temperature-related mechanical changes and contamination.A further problem arises from the heat generated during operation of the light source, which can lead to excessive temperatures in the gas analyzer. This affects the optical components in the beam path as well as the gas being analyzed in the measuring section or the measuring cell. To minimize the thermal influence, a filter wheel unit, for example, which filters out the spectral range relevant to a gas component from the light source, is thermostatted.

[0011] It is therefore an object of the invention to reduce the long-term drift of a gas analyzer in addition to the existing correction methods mentioned in the prior art.

[0012] The object is achieved by the light source according to claim 1 and the gas analyzer according to claim 6. Further embodiments are the subject of the dependent claims.

[0013] The light source according to the invention for a gas analyzer, in particular an extractive one, has at least one measuring section and at least one detector for measuring an intensity of the light source and is characterized in that the light source comprises a primary and a secondary light source, wherein the secondary light source is configured to homogenize the radiation characteristic of the primary light source.

[0014] In the present case, a gas analyzer is understood to be an absorption photometer that operates in particular extractively and can be designed in particular as a two-beam photometer.

[0015] A measuring section is understood in particular to be a measuring cuvette or sample cuvette which contains a gas to be measured as a gas sample for a measurement. The measuring cuvette is an at least partially optically transparent or optically semi-transparent chamber or cell which can hold a specific amount of the measuring gas to be analyzed. The measuring cuvette is typically made of materials that are non-reactive towards the gases to be analyzed in order to avoid distorting the results. Suitable materials are glass or special plastics. The measuring cuvette is at least partially optically transparent or semi-transparent so that the light from the light source can pass through it to measure the gas concentration of the gas component to be measured in the gas sample. The measuring cuvette has a fixed volume capacity which allows it to hold a specific amount of the gas sample for the measurement.The volume and especially the optical length can vary depending on the application. The sample cuvette has inlet and outlet ports for introducing the sample into the chamber and removing it after measurement. These ports can be connected to tubing, valves, or other devices to control the gas flow through the sample cuvette. To ensure accurate measurements, the sample cuvette is essentially gas-tight, preventing sample escape, which is particularly relevant for toxic measurement gases. In some gas analyzers, sample cuvettes may have temperature control to optimize sample conditions or to account for temperature-dependent measurements.

[0016] In this context, a light source is understood to be an active source that generates electromagnetic radiation, in particular from electrical power. Furthermore, a light source is understood to be a passive source that receives and / or reflects and / or scatters and / or partially absorbs and emits, radiates, or re-emits electromagnetic radiation. Furthermore, it is understood to be a light source that acts as both an active and a passive source.

[0017] The term light refers to electromagnetic radiation that covers a broad spectrum of wavelengths, from infrared to visible light to ultraviolet light and beyond.

[0018] In this case, a primary light source is preferably understood to be an active source that generates electromagnetic radiation from electrical current and emits it as light. A secondary light source is preferably understood to be a passive light source. The primary light source preferably generates light from electrical current and transmits this light at least partially to the secondary light source.

[0019] A gas discharge lamp is a primary light source that generates light from electrical current and then radiates or emits it. Gas discharge lamps use electrical discharges in gases to generate light. Examples include neon lamps, mercury vapor lamps, and sodium vapor lamps.

[0020] The radiation characteristic of a light source is the pattern or distribution of the emitted light at a specific solid angle around the light source. The radiation characteristic specifies how the light is emitted by the light source and how the intensity of the light is distributed in different directions in space, for example how the intensity of the light source is distributed along the different solid angles of a sphere. There are different types of radiation characteristics depending on how the light is emitted by the light source. An omnidirectional light source emits light evenly in all directions, so that the light intensity is approximately the same in every direction. An example of an omnidirectional light source is an incandescent light bulb. With directional radiation, the light intensity is higher in one or more specific directions, i.e. solid angles, than in others.The light is emitted in a focused beam within a specific angular range. Examples of this are spotlights or floodlights.

[0021] Aging processes in a light source affect its radiation characteristics. This can change the intensity of the light source emitted at different solid angles, as well as the solid angles into which the light is emitted.

[0022] In this context, a detector is understood to be a measuring element that can detect the intensity of electromagnetic radiation. This can also include semiconductor-based detectors, particularly those designed as detector arrays.

[0023] In this case, homogenized means that a non-uniform radiant power, ie a radiant flux distributed non-uniformly over solid angles or, photometrically expressed, a non-uniform luminous flux, is collected from all spatial directions and converted into a uniform radiant flux, or, photometrically expressed, into a uniform illuminance, ie into a uniform or homogeneous, area-related luminous flux.

[0024] The effect of homogenizing the light source's radiation characteristics is, among other things, that directed radiation is converted into diffuse radiation. In other words, the radiation from the primary light source, directed into specific solid angles, is converted into diffuse radiation by the secondary light source, for example, through reflection, absorption, re-emission, scattering, diffraction, or refraction, and thus homogenized.

[0025] By homogenizing the radiation characteristics of the primary light source, the secondary light source couples light with a consistent irradiance, particularly across the relevant frequency range, into the beam path of the gas analyzer. This prevents or reduces zero-point drifts in the gas analyzer, which result from fluctuations in the radiation characteristics of the primary light source, triggered, for example, by aging effects of the primary light source, as is typically the case with gas discharge lamps. This particularly improves the long-term stability of the gas analyzer. Furthermore, if zero-point drift does occur, troubleshooting can be further narrowed down, making it possible to find the cause of any observed zero-point drift more quickly and thus complete it with less effort.Finally, the homogenization of the radiation allows more light to be coupled into the beam path of the gas analyzer, thereby improving the measurement accuracy of the gas analyzer.

[0026] Dividing the light source into a primary and a secondary light source also improves heat dissipation from the primary light source. For example, the primary light source generates the light by absorbing electrical power, some of which is converted into heat. This amount of heat can advantageously also be transferred to the secondary light source, whereby the secondary light source does not absorb any electrical power and therefore does not produce any heat. This means that the secondary light source also acts as a heat sink, particularly when the secondary light source has a large surface area, which can be further increased by adding cooling fins. This is particularly advantageous when the secondary light source at least partially surrounds the primary light source.Alternatively, the primary light source can be positioned further away from the secondary light source and connected to it, for example, via an optical fiber, which can very efficiently reduce thermal influences on the measurement.

[0027] In a preferred embodiment, the secondary light source encompasses the primary light source at least in sections. By integrating the secondary light source into the primary light source, the structure can be realized in a particularly space-saving, i.e., compact manner.

[0028] In a particularly preferred embodiment, the secondary light source comprises an integrating sphere or is designed as an integrating sphere. The use of an integrating sphere has the advantage that it generates diffuse radiation from directed radiation. This makes it possible to collect the radiation from light sources with spatially strongly divergent radiation through the integrating sphere and emit it as diffuse radiation through the integrating sphere without foregoing a significant portion of the overall radiant output. Furthermore, the use of an integrating sphere has the advantage that the spherical shape results in a large surface area, and this large surface area can absorb and dissipate heat emitted by the primary light source. Due to the spherical design of the secondary light source and the material of the sphere, the sphere itself already acts like a heat sink.Particularly for applications in the UV range, the interior of the sphere can be coated to absorb the thermal radiation emitted by the primary light source in the infrared range, rather than reflecting it, thus transporting it to the outside. Good heat dissipation is particularly important when using a gas discharge lamp, as a resulting thermal gradient influences the gas being measured in the measuring section and the accuracy of the beam guidance.

[0029] In a further preferred embodiment, the primary light source comprises a gas discharge lamp and / or at least one light-emitting diode (LED), or is designed as such. The use of an LED as an alternative to a gas discharge lamp has the advantage that aging processes change the radiation characteristics less significantly. Furthermore, power consumption can be reduced compared to that of a gas discharge lamp. Moreover, due to its compact and smaller design, an LED can be more easily integrated into a secondary light source, in particular into an integrating sphere. Reduced power consumption also has the advantage that the amount of heat emitted by the light source during operation is reduced. Since a thermal gradient influences the accuracy of the beam guidance and the gas to be measured in the measuring section, this influence is reduced, thus improving the accuracy and long-term stability of the analyzer.

[0030] In a further preferred embodiment, the primary light source comprises a plurality of primary light sources, wherein the plurality of primary light sources emit light simultaneously and / or sequentially. Advantageously, several LEDs with different wavelength ranges can be combined with one another or combined with a gas discharge lamp. Or several LEDs of the same type can be combined with one another, thereby creating a very homogeneous radiation pattern. Furthermore, specific primary light sources such as LEDs can be operated sequentially for specific gas components, or a broader frequency spectrum can be generated by simultaneously operating light sources of different designs.

[0031] The invention further relates to a gas analyzer comprising the light source according to the invention, which comprises a primary and a secondary light source.

[0032] In a particularly preferred embodiment of the gas analyzer, it further comprises a reference measuring section.

[0033] In a particularly preferred embodiment of the gas analyzer, it further comprises a reference detector.

[0034] Further preferred embodiments of the light source according to the invention and of the gas analyzer according to the invention for homogenizing a light source for use in a gas analyzer will become apparent from the following description of the exemplary embodiments in conjunction with the figures and their description. Identical components are essentially identified by identical reference numerals unless otherwise stated or apparent from the context. Fig. 1 shows a schematic representation of an embodiment of the light source according to the invention in a gas analyzer, wherein the light source comprises a gas discharge lamp as a primary light source and an integrating sphere as a secondary light source and the gas analyzer is designed as a two-beam photometer. Fig. 2 shows a schematic representation of an embodiment of the light source with an integrating sphere as a secondary light source, an LED and a gas discharge lamp as the primary light source. Fig. 3 shows a schematic representation of an embodiment of the light source with an integrating sphere as a secondary light source and with three LEDs as respective primary light sources.

[0035] In Fig. 1 is a schematic representation of an embodiment of the light source 3 according to the invention with a gas discharge lamp 7 as the primary light source 3a and an integrating sphere 8 as the secondary light source 3b in a gas analyzer 1, which is designed as a two-beam photometer. The gas discharge lamp 7 radiates light 10 in the direction of the integrating sphere 8, which absorbs the incident light 10 and reflects it multiple times and finally radiates it as a homogeneous light source 3, i.e. with constant illuminance, in the direction of the filter wheel 11. The filter wheel holder 11 is thermostatted so that the influence of temperature fluctuations is kept to a minimum. The filter wheel holder 11 has a first 12 and a second 13 filter wheel. The first filter wheel 12 comprises optical interference filters 14. The second filter wheel comprises a gas filter 15, which can be pivoted into the beam path, in particular for the measurement of NO.

[0036] The beam path 16 has a lens 17, a beam splitter cube 18, and a mirror 19, which are arranged to form a first path 16a and a second path 16b. In the first path 16a, a measuring cuvette 2 is arranged such that the light 10 from the light source 3 radiates through the measuring cuvette 2 along its longitudinal axis. In the second path 16b, a reference measuring cuvette 5 is arranged such that the light 10 from the light source 3 radiates through the reference measuring cuvette 5 along its longitudinal axis. In this way, a reference measurement is possible in addition to the measurement of the gas component in the measuring cuvette 2. In the first path 16a and second path 16b, lenses 17 are located adjacent to the respective cuvettes 2, 5, which image the beam onto a respective detector 4, 6 for measuring the intensity of the light 10. In addition, an adjustment unit 20 with adjustment filters 21 is located in the first path. The process gas flows through the measuring cuvette 2, with the arrows 22 indicating the inflow or outflow.Sketch the outflow of the process gas from the measuring cuvette 2.

[0037] In Fig. Figure 2 is a schematic representation of an embodiment of the light source 3 with an integrating sphere 8 as a secondary light source 3b, an LED 9, and a gas discharge lamp 7 as the primary light source 3a. The LED 9 is arranged adjacent to the gas discharge lamp 7 such that the radiation characteristics of both primary light sources 3a are homogenized overall when they are operated jointly and / or the light of that primary light source 3a is homogenized when the light sources are operated sequentially. Fig. 2, the secondary light source 3b, i.e. the integrating sphere 8, comprises the primary light sources 3a, i.e. the LED 9 and the gas discharge lamp 7, at least partially. The integrating sphere 8 is in particular arranged spatially around the primary light sources 3a. The arrangement is preferably designed such that a respective essentially heat-generating section of the respective primary light sources 3a is arranged outside the integrating sphere 8, or at least a heat-dissipating section of the respective light source is located outside the integrating sphere. In a further alternative embodiment, the active, primary light source 3a, for example the LED 9, can be coupled to the integrating sphere 9 via an optical waveguide.

[0038] In Fig.Figure 3 is a schematic representation of an embodiment of the light source 3 with an integrating sphere 8 as a secondary light source 3b and with three LEDs 9 as respective primary light sources 3a.

[0039] The arrangement of the LEDs 9 can be such that the radiation characteristics of the LEDs 9 are homogenized overall when the LEDs are operated together, or such that the radiation characteristics of the respective LEDs 9 are homogenized when the LEDs are operated sequentially. Sequential operation offers the advantage that less heat is generated when the primary light sources 3a are operated than when the light sources 3a are operated together. The LEDs can have different or identical spectral properties. Alternatively, the LEDs can be coupled to the integrating sphere 8 via an optical waveguide. The LEDs can have different designs and emit light at different solid angles, i.e., have different radiation characteristics. List of reference symbols 1 gas analyzer 2 measuring cuvettes 3 Light source 3a primary light source 3b secondary light source 4 Detector 5 Reference measuring cuvette 6 Reference detector 7 Gas discharge lamp 8 integrating sphere 9 LEDs 10 light 11 filter wheel holder 12 first filter wheel 13 second filter wheel 14 interference filters 15 gas filters 16 Beam path 16a first path 16b second path 17 lens 18 beam splitter cubes 19 mirrors 20 Adjustment unit 21 adjustment filters 22 Process gas

Claims

[1] Light source for a, in particular extractive, gas analyzer (1), wherein the gas analyzer (1) has at least one measuring section (2) and at least one detector (4) for measuring an intensity of the light source (3), characterized by that the light source (3) comprises a primary (3a) and a secondary (3b) light source, wherein the secondary light source (3b) is arranged to homogenize the radiation characteristic of the primary light source (3a). [2] Light source according to claim 1, characterized by that the secondary light source (3b) encompasses the primary light source (3a) at least in sections. [3] Light source according to one of the preceding claims, characterized by that the secondary light source (3b) has an integrating sphere (8) or is designed as an integrating sphere (8). [4] Light source according to one of the preceding claims, characterized bythat the primary light source (3a) comprises a gas discharge lamp (7) and / or at least one light-emitting diode (LED) (9). [5] Light source according to one of the preceding claims, characterized by that the primary light source (3a) has a plurality of primary light sources (3a), wherein the plurality of primary light sources (3a) emit light simultaneously and / or sequentially. [6] Gas analyzer (1) comprising a light source (3) according to one of claims 1 to 5. [7] Gas analyzer according to claim 6, further comprising a reference measuring section (5). [8] Gas analyzer according to claim 6 or 7, further comprising a reference detector (6).

Images