Method and system for the relative referencing of a target gas in an optical measuring system for laser spectroscopy

Abstract

A method for operating an optical measuring system including a wavelength-tunable temperature-stabilized laser light source for measuring the concentration of a target gas in a measured gas, wherein an instantaneous base current I.sub.DC_ZG,act corresponding to a wavelength .sub.ZG of a target gas absorption line is set so that a wavelength distance .sub.DC defined during calibration between a target gas absorption line for a target gas and a reference gas absorption line for a reference gas is maintained. During operation, a temperature difference in the laser light source, defined in advance during calibration, between the operating points selected at the time of calibration of the reference gas, with a base current I.sub.DC_RG,cal, and the target gas, with a base current I.sub.DC_ZG,cal, is maintained by determining the required instantaneous base current I.sub.DC_ZG,act for the target gas, as a function of an instantaneous base current I.sub.DC_RG,act for the reference gas.

Claims

1. A method for operating an optical measuring system for measuring concentration of a target gas in a measured gas, comprising a wavelength-tunable temperature-stabilized laser light source, provided that an instantaneous base current I.sub.DC_ZG,act corresponding to a wavelength .sub.ZG of a target gas absorption line being set so that, after a calibration, a wavelength distance .sub.DC between the wavelength .sub.ZG of the target gas absorption line for a target gas and a wavelength .sub.RG of a reference gas absorption line for a reference gas component is maintained for an instantaneous base current I.sub.DC_ZG,act of the target gas and of an instantaneous base current I.sub.DC_RG,act of the reference gas, wherein carrying out the calibration of the measuring system with a reference gas and a target gas, and during calibration: establishing base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal assigned to the gas absorption lines for the reference gas and for the target gas; determining associated electrical powers P.sub.DC_RG,cal and P.sub.DC_ZG,cal of the laser light source from the base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal and associated internal resistances R.sub.I_RG,cal and R.sub.I_ZG,cal; finding a power difference P.sub.DC,cal by means of the electrical powers P.sub.DC_RG,cal and P.sub.DC_ZG,cal and storing this or equivalent variables in the measuring system, and during operation of the measuring system, maintaining a temperature difference in the laser light source, defined in advance during the calibration, between operating points selected at a time of calibration of the reference gas, with the base current I.sub.DC_RG,cal ascertained during the calibration and the target gas, with the base current I.sub.DC_ZG,cal ascertained during the calibration, by way of determining a necessary instantaneous base current I.sub.DC_ZG,act for the target gas, as a function of the instantaneous base current I.sub.DC_RG,act for the reference gas, wherein ascertaining an instantaneous electrical power P.sub.DC_RG,act of the laser source assigned to the gas absorption line for the reference gas from the instantaneous base current I.sub.DC_RG,act and an instantaneous internal resistance R.sub.I_RG,act; determining an instantaneous electrical power P.sub.DC_ZG,act of the laser source assigned to the gas absorption line for the target gas as the sum from the instantaneous electrical power P.sub.DC_RG,act and the power difference P.sub.DC,cal; and calculating the assigned instantaneous base current I.sub.DC_ZG,act from the instantaneous electrical power P.sub.DC_ZG,act of the laser source, taking the instantaneous internal resistance R.sub.I_RG,act into consideration.

2. The method according to claim 1, wherein during operation of the measuring system, taking into consideration changes in a base laser temperature T.sub.L,cal of the laser light source by adapting the power difference P.sub.DC,cal.

3. The method according to claim 1, wherein determining an internal resistance RI of the laser light source of the reference gas and the target gas from a respective slope of a voltage/current characteristic of the laser light source associated with the instantaneous base current I.sub.DC_RG,act or I.sub.DC_ZG,act.

4. An optical measuring system for measuring concentration of a target gas in a measured gas, comprising a wavelength-tunable temperature-stabilized laser light source, provided that an instantaneous base current I.sub.DC_ZG,act corresponding to a wavelength .sub.ZG of a target gas absorption line is set so that, after a calibration, a wavelength distance .sub.DC between the wavelength .sub.ZG of the target gas absorption line for a target gas and a wavelength .sub.RG of a reference gas absorption line for a reference gas component is maintained for an instantaneous base current I.sub.DC_ZG,act of the target gas and of an instantaneous base current I.sub.DC_RG,act of the reference gas, wherein the measuring system is designed to be calibrated with a reference gas and a target gas, and during calibration: base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal assigned to the gas absorption lines for the reference gas and for the target gas are established; associated electrical powers P.sub.DC_RG,cal and P.sub.DC_ZG,cal of the laser light source from the base currents I.sub.DC_RG,cal and I.sub.DC_ZG,cal and associated internal resistances R.sub.I_RG,cal and R.sub.I_ZG,cal are determined; a power difference P.sub.DC,cal by means of the electrical powers P.sub.DC_RG,cal and P.sub.DC_ZG,cal is found and this or equivalent variables are stored in the measuring system, and wherein the measuring system is designed in such that during operation a temperature difference in the laser light source, defined in advance during the calibration, between operating points selected at a time of calibration of the reference gas, with a base current I.sub.DC_RG,cal ascertained during the calibration and the target gas, with a base current I.sub.DC_ZG,cal ascertained during the calibration, by way of determining a necessary instantaneous base current I.sub.DC_ZG,act for the target gas, as a function of an instantaneous base current I.sub.DC_RG,act for the reference gas is maintained, an during operation: an instantaneous electrical power P.sub.DC_RG,act of the laser source assigned to the gas absorption line for the reference gas from the instantaneous base current I.sub.DC_RG,act and an instantaneous internal resistance R.sub.I_RG,act is ascertained; an instantaneous electrical power P.sub.DC_ZG,act of the laser source assigned to the gas absorption line for the target gas as a sum from the instantaneous electrical power P.sub.DC_RG,act and the power difference P.sub.DC,cal is determined; the assigned instantaneous base current I.sub.DC_ZG,act from the instantaneous electrical power P.sub.DC_ZG,act of the laser source is calculated, taking the instantaneous internal resistance R.sub.I_RG,act into consideration.

5. The optical measuring system according to claim 4, additionally comprising: a modulation device for providing a base current I.sub.DC and a modulation current I.sub.AC for the laser light source; a measured gas; a light detector, and an evaluation unit connected to the light detector and the modulation device; a voltage detector configured to detect a voltage U.sub.L present at the laser light source; a temperature detector configured to detect a laser base temperature T.sub.L; a resistance detector configured to ascertain an internal resistance R.sub.I of the laser light source, and a processor configured to control base currents Ix for a reference gas and the target gas, wherein the laser light source is designed to em it a laser beam of a wavelength .sub.DC having a wavelength modulation amplitude .sub.AC, and the modulation device is designed to periodically vary the wavelength of the laser light of the laser light source by way of the absorption line for the reference gas and the absorption line for the target gas at an operating point and, at the same time, to modulate the same with a frequency and a settable amplitude, the modulation device is directly connected to the laser light source; the light detector is designed to detect the laser beam originating from the laser light source after this has passed through the measured gas, and to generate a reception signal, which is dependent on an intensity of the laser light after it has passed through the measured gas and is supplied to the evaluation unit; wherein the voltage detector for detecting the voltage U.sub.L present at the laser light source, the temperature detector for detecting the laser base temperature T.sub.L, the resistance detector for ascertaining the internal resistance R.sub.I of the laser light source, and the processor for controlling the base currents I.sub.DC for the reference gas and the target gas are configured to cooperate so as to detect the voltage U.sub.L present at the laser light source and determine the internal resistance R.sub.I and, as a function thereof, control the base currents I.sub.DC for the reference gas and the target gas using the modulation device so as to keep the wavelength distance between the reference gas and the target gas constant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described again in more detail hereafter based on the accompanying drawings. In the drawings, in schematic illustrations:

(2) FIG. 1 shows an optical measuring system suitable for carrying out the method according to the invention;

(3) FIG. 2 shows the equivalent circuit for the laser light source;

(4) FIG. 3 shows a recorded voltage/current characteristic curve for determining the internal resistance of the laser light source;

(5) FIG. 4 shows the wavelength .sub.I as a function of the current in a schematic illustration of the target gas position and the reference gas position with a fixed current distance between the reference gas peak and the target gas peak, at a certain temperature, and the shift thereof due to temperature drift;

(6) FIG. 5 shows a measuring curve for methane, serving as the reference gas and a further target gas, and ethane, serving as the target gas, at the target temperature (solid line) and after the drift (dotted line) caused by a change in the laser temperature;

(7) FIG. 6 shows the distance error with respect to the target gas peak compared to the drift of the reference gas peak, measured using a fixed current difference and a fixed relative temperature difference based on an example; and

(8) FIG. 7 shows a flow chart for fixing the wavelength distance to a reference wavelength.

DETAILED DESCRIPTION OF THE INVENTION

(9) FIG. 1 schematically shows the basic design of an optical measuring system 1 for measuring the concentration of a target gas component ZG in a measured gas 2, based on wavelength modulation spectroscopy (WMS). The measuring system 1 comprises a wavelength-tunable temperature-stabilized laser light source 3, a modulation device 4, a light detector 5, and an electronic evaluation unit 6. The laser light source 3 emits a laser beam 7 of the wavelength .sub.DC having a wavelength modulation amplitude .sub.AC. The modulation device 4 periodically varies the wavelength of the laser light of the laser light source 3 by way of a reference absorption line and a target gas absorption line at an operating point and, at the same time, modulates the same in a triangular manner with a frequency (f) and a settable amplitude. This additionally comprises at least one DC and/or AC voltage source or a DC and AC current source 4a, and associated modulation means 4b for operating the laser light source 3. This can be used to variably set the respective base current I.sub.DC and the modulation current I.sub.AC. The modulation device 4 is connected directly to the laser light source 3. The light detector 5 detects the laser beam 7 originating from the laser light source 3 after this has passed through the measured gas 2, and generates a reception signal, which is dependent on the intensity of the laser light after it has passed through the measured gas 2, and is supplied to the evaluation unit 6. The evaluation unit 6 comprises means for the phase-sensitive demodulation of a measuring signal generated by the light detector 5 at the frequency (f) and/or one of the harmonics thereof. The evaluation unit 6 comprises two lock-in amplifiers 6a, 6b and a processing unit 6c. The processing unit 6c evaluates the demodulated reception signal of the light detector 5. Furthermore, an electrical connecting line 9 leads to the lock-in amplifier 6b by way of which the voltage present at the laser light source 3 is detected and the internal resistance is determined. As a function thereof, the control unit controls the base currents I.sub.DC and modulation currents for the reference and target gases using the modulation device 4 so as to keep the wavelength distance between the reference gas and target gas constant. For this purpose, this comprises electrical control lines 8 and 10 to the modulation device 4. In the evaluation, primarily the above-described formulas F 11 or F 15 together with F 10 are used, among other things. Via the line 12, the Peltier temperature is transmitted to the control unit 6. In this exemplary embodiment, the reference gas RG is permanently present in the optical path during calibration. The target gas component ZG is then introduced into the measured gas chamber.

(10) FIG. 2 shows the equivalent circuit for the laser light source 3. The laser light source 3 can thus be arithmetically replaced with a light emitter 3a, and an internal resistance R.sub.I, 3b connected in series thereto. The laser light source 3 is operated in a current-modulated manner with a base current I.sub.DC and a modulation current I.sub.AC. Voltage U.sub.L is present at the laser light source 3 and drops partially across the internal resistance 3b as a partial voltage U.sub.RI and across the light emitter 3a as a partial voltage U.sub.E.

(11) FIG. 3 illustrates a current/voltage characteristic curve 13 recorded during the calibration of the optical measuring system 1 for determining the internal resistance R.sub.I RG of the laser light source 3 at the location of the reference gas. The internal resistance R.sub.I RG is determined from the relationship of the current/voltage characteristics of the laser light source 3 at the operating point 15 I.sub.DC_RG,cal for RG. For this purpose, the current/voltage characteristic curve 13 (solid line) is provided with a linear approximation line 14 (dotted line) at the operating point 15 for determining the internal resistance R.sub.I RG at the location of the reference gas. The slope of the approximation line 14 corresponds to the internal resistance R.sub.I, 3b at the operating point 15 with R.sub.I RG. An analogous procedure is applied at the location of the target gas for ascertaining R.sub.I ZG.

(12) In a schematic illustration, FIG. 4 shows, by way of example, the wavelength position of a target gas and that of a reference gas at a certain target temperature, wherein the two positions have a defined wavelength distance .sub.DC with respect to one another, which via the DC tunability curve leads to a current distance I. The absorption lines are shown with solid lines for the target temperature and with dotted lines for a drift induced by the reduced target temperature. The wavelength distance .sub.DC is typically set by a constant current distance I. Due to the non-linear DC tuning behavior of the laser, a shift (drift) of the operating point, for example as a result of the influence of the outside temperature of the sensor on the temperature stabilization or aging of the laser/electronics, results in a distance error I.sub.F for the position of the gas peak of the target gas GZ, resulting in measurement errors. The figure schematically shows the positions of the reference gas and the target gas at the time of calibration. Drift causes the position of the reference gas line to be shifted. Due to the non-linear relationship between the wavelength and the laser current, a difference, which is to say an incorrect wavelength having a wavelength distance error .sub.F, that is, a distance error between the calculated and the actual target gas position, results for a fixed current distance I. In the most favorable case, the error is small (typically <20 A), and the sensor remains within the specification thereof in terms of the accuracy of the concentration. If the error grows larger, the error increases in a superlinear fashion. In the extreme case, the peak will no longer even be in the tuning range (for example, the target gas is no longer even measured in FIG. 4). The greater the distance between the reference gas and the target gas, the lower the allowed drift must be with this method; otherwise the distance error is >20 A. The actual target gas position (associated base current) is at pos. 1, and the target gas position calculated via formulas F1 to F2 (associated base current) is at pos. 2. These two target gas positions deviate from one another by the distance error I.sub.F. It should be noted that the DC tunability curve itself is shown unimpaired by drift. In reality, the DC tunability curve itself can also change with long-term drift.

(13) FIG. 5 shows, by way of example, a measuring curve, which is to say a spectrum for methane CH.sub.4, having a lower peak, as the reference gas, and ethane C.sub.2H.sub.6, having a higher peak, as the target gas, at a certain target temperature (solid line G 1) and a temperature deviating from the target temperature (dotted line G 2). The deviation of line G 1 from line G 2 is caused by a change in the laser temperature and is intended to illustrate the influence of drift on the distance of the gas peaks. By way of example, the distance error I.sub.F is shown which results when, proceeding from a reference gas peak (here CH.sub.4) having a fixed current distance, a target gas (ethane C.sub.2H.sub.6) is measured at a slightly different laser temperature. A shift in the reference gas peak by 0.96 mA results from the temperature reduction. It is apparent that the actual distance between the reference peak and the target gas peak has decreased from 1.669 mA to 1.568 mA. As was already mentioned above, this is caused by the non-linear DC tunability of the laser light source. A distance error I.sub.F of approximately 100 A thus results. If the proposed method is applied using a fixed relative temperature change (formula (F 15)), the distance error I.sub.F is only 10 A (FIG. 6). As a result, the target gas signal can be evaluated correctly.

(14) FIG. 6 illustrates the dependence of the distance error I.sub.F on the drift values based on an example. This shows the distance error I.sub.F for the use of a fixed current difference according to formula (F 1) and a fixed relative temperature difference according to formula (F 15) for different drift values. The values based on formula F 1 are shown as dots P 1, and the values based on formula F 15 are shown as crosses P 2. It is apparent that, when using the proposed method, the distance error I.sub.F of the base current I.sub.DC never exceeds 20 A, which is sufficiently small to ensure that the measured concentration remains within the specifications.

(15) FIG. 7 shows a flow chart for fixing the wavelength distance to a reference wavelength. In a first method step S1, which is to say during the calibration of the measuring system 1, firstly the DC base currents of the peak positions of the reference gas and the target gas are established and, secondly, the electrical DC power dropping across the internal resistance of the laser is ascertained for the two peak positions by determining the internal resistance of the laser at the respective current position. Thirdly, the power difference P.sub.DC,cal of the DC powers is ascertained and stored in the sensor.

(16) In the subsequent second method step S2, which is to say during operation of the measuring system 1, initially the electrical DC power dropping across the internal resistance of the laser is ascertained for the instantaneous peak position of the reference gas by measuring the internal resistance using lock-in technology, then the instantaneous electrical DC power at the peak position for the target gas is calculated by adding the instantaneous DC power for the reference gas peak and the DC power difference between the reference gas and the target gas ascertained during calibration. Thereafter, the DC current for the peak position thereof is ascertained, wherein according to formula F 11 or F 15 the internal resistance of the target gas from the preceding measurement is used for this purpose. If F 15 is used, additionally the ratio of the instantaneous laser temperature to the calibration laser temperature is used. The laser temperature is stored at the time of calibration.

(17) In the subsequent third method step S3, the actual measuring scan for the target gas is carried out based on the ascertained peak position for the target gas, and the gas concentration is ascertained therefrom.

(18) The method steps S2 and S3 are preferably carried out multiple times in a loop, wherein the base current I.sub.DC_ZG for the target gas position can be adapted between the runs, if the instantaneous base current I.sub.DC_ZG, act deviates from the ideal base current I.sub.DC_ZG, cal, which was ascertained during the calibration of the optical measuring system.