Method for measuring the concentration of a gas component in a measurement gas

Abstract

A method for measuring the concentration of a gas component in a measurement gas using a gas analyzer comprises varying the wavelength of the light of a wavelength-tunable light source within periodically consecutive scan intervals for wavelength-dependent scanning of a gas component absorption line of interest. The method also comprises modulating the wavelength of the light of the wavelength-tunable light source with a frequency, guiding the modulated light through the measurement gas onto a detector and demodulating a measurement signal generated by the detector in the event of a harmonic of the frequency. The method further comprises producing a measurement result by fitting a desired curve to the profile of the demodulated measurement signal. A function orthogonal to the desired curve is provided, and an orthogonal component of the measurement result is produced by fitting the orthogonal function to the profile of the demodulated measurement signal.

Claims

1. A method for measuring the concentration of a gas component in a measurement gas using a gas analyzer, the method comprising: varying the wavelength of the light of a wavelength-tunable light source within periodically consecutive scan intervals for wavelength-dependent scanning of a gas component absorption line of interest; modulating the wavelength of the light of the wavelength-tunable light source with a frequency; guiding the modulated light through the measurement gas onto a detector; demodulating a measurement signal generated by the detector in the event of a harmonic of the frequency; and producing a measurement result by fitting a desired curve to the profile of the demodulated measurement signal, wherein a function orthogonal to the desired curve is provided, and an orthogonal component of the measurement result is produced by fitting the orthogonal function to the profile of the demodulated measurement signal, wherein, given a known concentration of the gas component to be measured, an interfering parameter is varied and it is determined in the process an error, which comprises an inphase component (N.sub.in), in the form of the difference between the obtained measurement result and a desired measurement result, and the orthogonal component, wherein a relationship between the inphase component of the error and its orthogonal component is further determined, and wherein, in the case of the measurement of an unknown concentration of the gas component, the measurement result obtained in the process is corrected with an inphase component which is determined, with the aid of the relationship determined in the measurement calibration, from the likewise obtained orthogonal component.

2. The method of claim 1, wherein the relationship determined in the measurement calibration is determined in the form of a functional relationship between the inphase component of the error, its orthogonal component and the measured or known interfering parameter.

3. The method of claim 1, wherein the operating temperature of the gas analyzer is varied as interfering parameter.

4. The method of claim 1, wherein the concentration of an interfering gas component in the measurement gas used for the calibration is varied as interfering parameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Reference is made below to the figures of the drawing for the purpose of further explanation of the invention as follows:

(2) FIG. 1 shows a gas analyzer for carrying out the method of the invention in accordance with one embodiment of the present invention;

(3) FIG. 2 shows an example of the interference of a demodulated measurement signal in accordance with one embodiment of the present invention;

(4) FIG. 3 shows an example of a desired curve corresponding to the profile of the demodulated measurement signal ideally to be expected, and a function orthogonal to the desired curve in accordance with one embodiment of the present invention;

(5) FIG. 4 shows an example of a temperature-dependent measurement result obtained by fitting the desired curve and the orthogonal function to the demodulated measurement signal in accordance with one embodiment of the present invention; and

(6) FIG. 5 shows an example of changes in the measurement result within different temperature ranges in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

(7) The gas analyzer shown in the form of a simplified block diagram in FIG. 1 is a laser spectrometer for measuring the concentration of at least one gas component of interest in a measurement gas 1 which is contained in a measuring volume 2, such as a sample cell or a process gas line. The spectrometer includes a light source 3 in the form of a laser diode whose light 4 falls on a measuring detector 5 after traversing the measurement gas 1.

(8) A current source 7 controlled by a modulation device 6 feeds the laser diode 3 with an injection current i, the intensity and wavelength of the generated light 4 depending on the current i and the operating temperature of the laser diode 3. The modulation device 6 comprises a first signal generator 8 which drives the current source 7 periodically with the aid of a prescribed function 9, such as, preferably, a ramp function or a triangular function 9, in order to scan a selected absorption line of the gas component of interest with the aid of the wavelength of the generated light 4, which follows the profile of the current i more or less linearly. A second signal generator 10 generates a sinusoidal signal 11 of higher frequency f.sub.0, which is used to modulate the ramp function or triangular function 9 in an adding element 12.

(9) The measuring detector 5 generates a measurement signal 13 as a function of the detected light intensity, which measurement signal is demodulated in a lock-in amplifier 14 at a harmonic nf.sub.0 (n=1, 2, 3 . . . ), here 2f.sub.0 for example, of the modulation frequency f.sub.0. In a downstream evaluation device 15, the demodulated measurement signal 13 for each scan interval is evaluated to form a measurement result. For this purpose, a desired curve corresponding to the ideal demodulated measurement signal 13 is fitted to the demodulated measurement signal 13 in a first arithmetic logic unit 16, and a function orthogonal to the desired curve is fitted to the demodulated measurement signal 13 in a second arithmetic logic unit 17.

(10) As already explained at the beginning, temperature changes within the gas analyzer can lead to drifting of the measurement results, and the cause of the drift is etalons in the optical beam path that lead to periodic structures in the profile of the demodulated measurement signal 13.

(11) By way of example, FIG. 2 shows an interference-free measurement signal 13a demodulated at the second harmonic 2f.sub.0 of the modulation frequency f.sub.0, a periodic interference 18, and the measurement signal 13b, on which the interference 18 is superposed. It can be seen at once that fitting a desired curve (19 in FIG. 3) corresponding to the ideal demodulated measurement signal 13a to the disturbed measurement signal 13b does not lead to a correct determination of concentration.

(12) By way of example, FIG. 3 shows the desired curve 19 corresponding to the profile of the demodulated measurement signal 13 ideally to be expected and a function 20 orthogonal thereto. In contrast to the desired curve 19, the orthogonal function 20 correlates not with the demodulated measurement signal 13 or 13a, but with the orthogonal components of interference signals whose inphase components for their part appear directly as interference signal components in the demodulated measurement signal 13 or 13a.

(13) Returning to FIG. 1, the arithmetic logic unit 16 supplies a more or less disturbed measurement result M, more precisely an inphase component M.sub.in of the measurement result M comprising a useful component S=S.sub.in and an inphase interference component N.sub.in. The arithmetic logic unit 17 produces an orthogonal component M.sub.ortho of the measurement result M consisting of the orthogonal interference component N.sub.ortho: M=M.sub.in+M.sub.ortho=(S+N.sub.in)+N.sub.ortho, where S=S.sub.in and S.sub.ortho=0.

(14) By way of example, FIG. 4 shows a phasor diagram of the measurement result M with its inphase and orthogonal components. For the purpose of measurement calibration given a known concentration of the gas component to be measured, the operating temperature of the gas analyzer is varied and there is determined in the process an error N which comprises the inphase component N.sub.in, in the form of the difference between the inphase measurement result M.sub.in and the desired measurement result S.sub.in, and of the orthogonal component M.sub.ortho.

(15) In a next step, a relationship between the inphase component N.sub.in of the error N and its orthogonal component N.sub.ortho is determined. FIG. 4 shows a simple case in which the error N based on etalon effects describes over the temperature T an approximate circle about the desired measurement result S.sub.in. The relationship is stored in a memory 18 (FIG. 1) of a further arithmetic logic unit 19, downstream of the arithmetic logic units 16, 17, which, in the course of the measurement of an unknown concentration of the gas component, corrects the inphase measurement result M.sub.in obtained in the process with the aid of an inphase component which is determined by using the stored relationship from the orthogonal component M.sub.ortho likewise obtained. In the simple example shown in FIG. 4, the radius R=|N| of the circle is determined during the measurement calibration, and when an unknown concentration of the gas component is being measured the inphase measurement result M.sub.in obtained in the process is corrected as follows: M.sub.in.sub._.sub.korr=M.sub.in(R.sup.2M.sup.2.sub.ortho).sup.1/2.

(16) FIG. 5 shows an example of changes in the measurement result M within different temperature ranges T1 and T2, for example T1=45 C. to 49 C. and T2=6 C. to 2 C. In reality, in some circumstances a plurality of etalons with various dependencies and sensitivities concerning temperature are active during measurement, and so the errors they cause can rotate in the phasor diagram at different speeds.

(17) Furthermore, the amplitudes (phasor lengths) can change because of changes in intensity (alignment dependence of the temperature) or through changes in reflectivities (for example coatings of optical surfaces). The amplitudes also change whenever other interfering influences are added which do not behave like etalons. The measurement results M(T1), M(T2) then comprises components M.sub.trans.sub._.sub.T1, M.sub.trans.sub._.sub.T2 which move by translation in the phasor diagram as a function of the temperature, and components, M.sub.rot.sub._.sub.T1, M.sub.rot.sub.T2 which move by rotation.

(18) Although the present invention has been described above with reference to presently preferred embodiments, it is not limited thereto but rather can be modified in a wide variety of ways. In particular, the invention can be altered or modified in multifarious ways without departing from the essence of the invention.