Method of determining the concentration of a gas component and a spectrometer for this purpose

Abstract

The invention relates to a method of determining the concentration of a gas component comprising the steps: generating and guiding a light beam having a wavelength variable in a wavelength range through a measurement volume in which the gas component having an absorption in the wavelength range is present; tuning the wavelength range; detecting the intensity of the light beam after passage through the measurement volume; storage of measurement points during the tuning that respectively consist of a point in time and an associated intensity value, to obtain a direct absorption line; generating an artificial measurement curve from the stored measurement points by shifting the measurement points on the time axis; wherein the shift takes place so that an artificial modulation results in the wavelength time extent; and evaluating the artificial measurement curve in accordance with the method of the wavelength modulation spectroscopy and determining a first concentration value therefrom.

Claims

1. A method of determining the concentration of a gas component, the method comprising the steps of: generating a light beam with a light source, the light beam having a wavelength variable in a wavelength range; guiding the light beam through a measurement volume in which the gas component to be determined is present, wherein the gas component has an absorption in the wavelength range; tuning the wavelength range with a light source controller; detecting an intensity of the light beam with a light detector after passage of the light beam through the measurement volume; storing measurement points in memory, the measurement points respectively consisting of a point in time and the associated intensity value during the tuning, whereby a direct absorption line is obtained; generating an artificial measurement curve from the stored measurement points with an evaluation unit, the evaluation unit shifting the measurement points on the axis of time to generate the artificial measurement curve, wherein the shift takes place such that an artificial modulation of the wavelength results following the shift in the wavelength time extent; and evaluating the artificial measurement curve, with the evaluation unit, in accordance with the methods of wavelength modulation spectroscopy and determining a first concentration value therefrom.

2. The method in accordance with claim 1, in which an evaluation of the direct absorption line is carried out in accordance with the method of the direct absorption spectroscopy and a second concentration value is determined following the storage of the measurement points.

3. The method in accordance with claim 2, in which the two generated concentration values are used for the purpose of achieving an increased safety with respect to a functional safety by a plausibilization of the values with respect to one another.

4. The method in accordance with claim 2, in which a common concentration value is generated from the two concentration values.

5. The method in accordance with claim 1, in which the tuning of the wavelength range takes place without an additional high frequency modulation of the wavelength during the tuning in such a way that a monotonous wavelength time extent results without a high frequency modulation.

6. The method in accordance with claim 1, in which the tuning of the wavelength range in the positive direction is varied with a not necessarily constant velocity 0 and subsequently a change in the negative direction is varied with a not necessarily constant velocity 0, in this respect both directions or only one direction can be drawn on for the evaluation.

7. The method in accordance with claim 1, in which the artificial modulation is one of sinusoidal, rectangular or triangular.

8. The method in accordance with claim 1, in which the amplitude and/or the frequency of the artificial modulation is adapted for the evaluation in accordance with the wavelength modulation spectroscopy.

9. The method in accordance with claim 1, in which the phase of the artificial modulation is adapted in such a way that, on the evaluation in accordance with the wavelength modulation spectroscopy, no further phase shift or signal rotation has to be added in order to achieve an ideal result.

10. A spectrometer, comprising: a light source for the generation of a light beam with a wavelength variable in a wavelength range, a measurement volume in which the gas component to be determined is present and through which the light beam propagates; a controller for the light source for tuning the wavelength range without an additional high frequency modulation of the wavelength during the tuning in such a way that a monotonous wavelength time extent results without high frequency modulation; a light detector for detecting the intensity of the light beam after passage through the measurement volume; memory for the storage of measurement points that respectively consist of a point in time and an associated intensity value during the tuning, whereby a direct absorption line is obtained; an evaluation unit for generating an artificial measurement curve from the stored measurement points by shifting the measurement points on the axis of time, wherein the shift takes place such that following the shift an additional high frequency modulation results in the wavelength time extent, the evaluation unit further evaluating the artificial measurement curve in accordance with the methods of the wavelength modulation spectroscopy and determining a first concentration value therefrom.

11. The spectrometer in accordance with claim 10, wherein the evaluation unit is configured in such a way that the measurement points that respectively consist of a point in time and an associated intensity value can be evaluated in accordance with the method of direct absorption spectroscopy and in this way a second concentration value can be determined.

12. The spectrometer in accordance with claim 10, wherein the light source is a laser whose emission wavelength can be changed by a control current or a control voltage.

13. The spectrometer in accordance with claim 12, wherein the tuning takes place by a linear change of the control current or of the control voltage.

14. The spectrometer in accordance with claim 12, wherein the tuning takes place by a nonlinear change of the control current or of the control voltage.

15. The spectrometer in accordance with claim 10, wherein the tuning of the wavelength range in the positive direction is varied with a not necessarily constant velocity 0 and subsequently a change in the negative direction is varied with a not necessarily constant velocity 0, in this respect both directions or only one direction can be drawn on for the evaluation.

Description

(1) In the following the invention will be described by means of an embodiment with reference to the drawing in detail. In the drawing there is shown:

(2) FIG. 1 a schematic illustration of the spectrometer in accordance with the invention;

(3) FIG. 2 a qualitative, schematic illustration of the laser control;

(4) FIG. 3 a qualitative, schematic illustration of an absorption signal of a measurement gas component;

(5) FIG. 4 an illustration like FIG. 2 after shifting the points on the scale in time;

(6) FIG. 5 an illustration of an extract of FIG. 4 for clarifying the shifting of the points on the scale in time; and

(7) FIG. 6 an illustration of the artificial measurement curve after shifting the points at the time scale.

(8) A spectrometer 10 in accordance with the invention schematically illustrated in FIG. 1 has a light source 12 that is preferably configured as a tunable diode laser (TDL), that can be controlled with control means 14. The tunable range corresponds to a wavelength range [1, 2]. For tuning, a control current IA is provided at the diode laser 12 by means of the control means 14 in such a way that a corresponding wavelength is generated in dependence on the current intensity. In FIG. 2 the current intensity IA of the control current is applied in dependence on the time t. When the current intensity IA changes this also changes the wavelength shown as should be indicated by the two ordinate axes in FIG. 2 and the comparison of FIG. 2 and FIG. 3.

(9) Furthermore, the spectrometer 10 has a measurement volume 16 that can be formed from a measurement cell 18 having a measurement gas inlet 20 and a measurement gas outlet 22. Other arrangements e.g. open systems (open path) or a tube line (cross duct) that conducts the measurement gas connected thereto are plausible. A measurement gas 30 having a gas component whose concentration shall be measured is present in the measurement cell 18.

(10) The light of the laser 12 is coupled into the measurement cell 18. The optical path within the measurement cell forms an optical measurement part 28. The optical path can be extended via one or more reflectors within or outside of the measurement cell, for example in the shape of a white cell or Herriott cell in order to thus obtain a longer optical measurement path 28.

(11) The measurement gas 30 that has the gas component to be measured is present in the measurement cell 18. The gas component has an absorption A in the tunable wavelength range in such a way that the measurement path 28 absorbs light of the laser 12 propagating along the measurement path 28 at the absorption wavelength A. This is illustrated in FIG. 3 that shows the light intensity I transmitted through the measurement cell 18 in dependence on the wavelength .

(12) Furthermore, a light detector 32 is provided that adapts the light that has propagated along the measurement path. The detector 32 can be a photodiode, an avalanche photodiode or a photo multiplier (PM). The light detector 32 generates a received signal in dependence on the intensity of the incoming light.

(13) The one electrical received signal then includes all pieces of information. It is optionally amplified and/or filtered and supplied to the evaluation unit 36. From a received signal finally the concentrations of the gas components are determined in the evaluation unit 36.

(14) The significance of the individual components and their particular designs and functions will become evident in the subsequent description when the functional principle of the spectrometer 10 in accordance with the invention is described. In this connection it is assumed that the functional principle of the DAS and the WMS as they were also initially explained, are known in principle.

(15) In accordance with the invention the tunable laser 12 is controlled by means of the control means 36. The control generally takes place via the control current IA having regard to diode lasers. The laser emits a certain wavelength in accordance with the control current IA. The laser 12 controlled in this way covers the wavelength range [1, 2] (FIGS. 2 and 3).

(16) The control of the laser 12 takes place with a current ramp, such as it is shown in FIG. 2. In this example the current ramp is linear, this means that the control current IA changes linearly with respect to time. For the measurement, the current ramp is repeatedly adjusted with a repetition rate and in this way the measurement is repeated at the repetition frequency.

(17) During the tuning of the wavelength range measurement points P are recorded that together form the intensity extent I and of which only three are illustrated by way of example in FIG. 3. Each measurement point P consists of a point in time tp and an associated intensity value Ip.

(18) The intensity extent I shows the direct absorption line A at the absorption wavelength A. The direct absorption line A is evaluated in the evaluation unit 36 in accordance with the methods of the direct absorption spectroscopy and provides a second concentration value.

(19) In a next step the stored measurement points P of a tuning are now taken and from these a new artificial intensity extent is generated, this means an artificial measurement curve is generated. In this respect one proceeds as follows. And indeed the measurement points P are shifted on the axis in time (abscissa) and are quasi newly strung together or expressed differently are resorted. In this respect the shift takes place in such a way that in a wavelength time diagram resulting after the shift (new stringing together) as is illustrated in FIG. 4 an additional high frequency modulation f having a small amplitude would show and not a linear extent as is shown in FIG. 2. The small amplitude and high frequency can be recognized in FIG. 4 only in an enlarged section.

(20) The shift (resorting or respectively the newly stringing together) should be explained with reference to FIG. 5. There only a small section is illustrated from the wavelength time diagram of the FIG. 4. The points P.sub.DAS and/or P.sub.DAS drawn in FIG. 4 are bijectively associated via the points in time tp with respect to the stored measurement points P. Each point P.sub.DAS, P.sub.DAS has a coordinate in time tp and a wavelength coordinate. Having regard to a linear tuning the points P.sub.DAS and/or P.sub.DAS lie on a straight line G as is shown in FIG. 5.

(21) However, the shift now takes place (new stringing together) and indeed in such a way that the sinusoidal extent S now results on the straight line G modulated thereon. For this purpose it requires a shift of the point P.sub.DAS on the axis in time (horizontal) to a new point in time tw so that the point P.sub.DAS is displaced to P.sub.WMS. This takes place in an analog manner for the other points. In order to obtain a clean sinusoidal extent some of the points P.sub.DAS have to be shifted a plurality of times to different new points in time. Thus, for example the time P.sub.DAS has to be shifted from its point in time tp, on the one hand, to tw and, on the other hand to tw. The point P.sub.DAS has thus quasi been doubled and following the new stringing together appears twice. Depending on the frequency and the amplitude of the sinusoidal extent S to be achieved and the inclination of the straight line G it can also occur that measurement points P.sub.DAS are shifted more than twice.

(22) By way of the thus shifted and newly strung together measurement points P that each also have an intensity value, an artificial measurement curve Mk is now constructed in the evaluation unit, such as it is illustrated in FIG. 6. The measurement curve Mk of the FIG. 6 is composed of a plurality of individual measurement points P.sub.WMS that are generated by a shift on the axis in time corresponding to the previously mentioned explanations and which, due to the high frequency of the modulation, are not individually recognizable, but only in such an illustration that illustrates the total tunable range (in this way one can see the absorption) and only appear as a black mass.

(23) The artificial absorption curve Mk is now evaluated in the evaluation unit 36 in accordance with the methods of the wavelength modulation spectroscopy and therefrom a first concentration value is determined.