WO2018096091A1

WO2018096091A1 - Measuring device and method for operating the measuring device

Info

Publication number: WO2018096091A1
Application number: PCT/EP2017/080325
Authority: WO
Inventors: Herbert REINGRUBER; Klaus-Christoph Harms
Original assignee: Avl List Gmbh
Priority date: 2016-11-25
Filing date: 2017-11-24
Publication date: 2018-05-31
Also published as: AT518830B1; AT518830A1; DE112017005966A5

Abstract

The invention relates to a measuring device (1) for measuring a measurement property (C) of a measurement component (2) in a measurement gas (3) and to a method for operating the measuring device (1). The measurement gas (3) is photoacoustically excited in a measurement chamber (4), the sound thus produced is captured by means of a sound sensor (5), and the captured signal is evaluated in an evaluation unit (6) in order to determine the measurement property (C). The evaluation unit (6) corrects the determined measurement property (C) by means of a correction function. In order to determine at least one parameter of the correction function, a determination pressure (P_E) is set in the measurement chamber (4) at least once and the absolute pressure (P) in the measurement chamber (4) is determined. At said absolute pressure (P), at least one determination measurement is performed with determination excitation, and the at least one parameter is determined on the basis of the absolute pressure (P) and the determination measurement result (C_E) of the determination measurement.

Description

Measuring device and method for operating the measuring device

The invention relates to a method for operating a measuring device which is suitable for measuring a measuring property of a measuring component in a measuring gas, wherein the measuring gas is photoacoustically excited in a measuring chamber, the sound generated thereby is recorded with a sound pickup and the received signal in an evaluation unit for Determining the measurement property is evaluated, the evaluation unit corrects the determined fair property by means of a correction function.

The invention further relates to a measuring device for measuring a measuring characteristic of a measuring component in a measuring gas, wherein the measuring device has a measuring chamber, at least one excitation device for photoacoustic excitation of the measuring gas in the measuring chamber, a sound pickup and an evaluation unit, with the evaluation of the unit recorded signal for determining a measurement property evaluable and the measurement property is correctable by means of a correction function, and wherein a pressure measuring unit for determining the absolute pressure in the measuring chamber is provided on the measuring chamber.

For research and development on internal combustion engines, measurements are carried out for the analysis of the components of the exhaust gas and, among other things, photoacoustic measuring instruments are used. These can have a high and selective measuring sensitivity for certain, present in low concentration in the measuring gas solid, liquid or gaseous components and work according to the following known principle:

The components to be measured of the measuring gas (which are referred to herein as measuring components) flow with this into a measuring chamber which is preferably embodied in such a geometric shape that at least a clearly pronounced acoustic resonance can be excited in it. In particular, the gaseous, liquid or solid constituents present in the measurement gas are referred to as the measurement component. As a measuring property of this measuring component, for example, the concentration or the number of particles can be measured in a time-resolved manner by the photoacoustic measuring device. A matched in their spectrum on the absorption capacity of the measuring component electromagnetic radiation, in particular "light" from the near ultraviolet (UV) to the middle infrared (IR) with wavelengths from 200 to 10,000 nm can be used with such a pulsation frequency in the measuring chamber irradiates that the measuring component and thus the surrounding measuring gas periodically heats up by the absorption of the periodically irradiated light, whereby corresponding sound pressure pulsations are generated. excited sound in the measuring chamber is detected with a sound pickup or microphone and evaluated by the evaluation unit.

In the context of the subject invention, the term "measuring gas" refers to the gas which is passed through the measuring chamber during the measurement and / or during other operating functions, such as a determination of parameters of the correction function, or is currently located in the measuring chamber If appropriate, the term also encompasses a calibration gas or zero gas which has known properties and is located in or flows through the measuring chamber during calibration, adjustment or another reference measurement.

Known measuring systems of the type mentioned above are often designed very specifically for applications with only one gas type and measuring component. In general, in order to keep the system parameters and in particular the measurement sensitivity as constant as possible, the measuring chamber and the measuring gas are thermostated and the gas in the measuring chamber is maintained at a constant pressure (measuring chamber pressure).

However, this can not always be fully realized. Due to technical limitations, for example, the pressure difference between the pressure in the measuring chamber and the ambient pressure may be subject to restrictions; for example, in prior art devices, the measuring chamber pressure may be limited to the ambient pressure + 60 / -100 mbar. In the event of exceeding or falling below this limitation, for example due to strong pressure pulsations in the exhaust gas, it may be necessary to close the inlet valve and thus prevent damage to the components. When using such known devices, for example for mobile measurement of the soot particles contained in the exhaust gas of an incinerator, but the problem may occur that the ambient pressure changes more, and thus generally the pressure in the measuring chamber must change. For example, when using the device aboard a vehicle and on mountain roads, the barometric pressure may be very low, for example 783 mbar at 2000 m above sea level or 533 mbar at 5000 m above sea level, compared to 1013 mbar at sea level. With a change in the ambient pressure, however, the measuring chamber pressure can change accordingly. Nevertheless, the reported soot readings should be independent of pressure and therefore comparable.

In principle, it is known to measure the absolute pressure in the measuring chamber and to take it into account in the measurement result of the photoacoustic measuring device. For this purpose, on the one hand, the concentration of the measured component measured at a certain absolute pressure is converted to standard conditions using the (ideal) gas equation, for example at 0 ° C. and 1013 mbar. On the other hand, correction algorithms or correction functions, eg polynomials, become first or higher order, with which a non-linear dependence of the measurement sensitivity of the absolute pressure can be considered.

In known devices, however, the coefficients of such algorithms must always be specified a priori. They are determined, for example, during manufacture and calibration of the device for altitudes up to 2000 m above sea level and stored as fixed values for approximate consideration of the absolute pressure in the measuring system. Or the user must manually enter them before taking the measurements, which is generally not the case, as in the field, in the current measurements, no metrological basis for such a parameter change can be obtained.

Also, in known devices is not taken into account that the microphone sensitivity is subject to changes, these changes are caused on the one hand by aging, on the other hand by pressure differences. Thus, apart from the above-described decrease in the mass concentration (according to the "ideal gas equation"), the microphone sensitivity also decreases with decreasing pressure.

It is therefore an object of the invention to provide a method which can avoid the disadvantages of the prior art and allows a check and possibly also a correction of the coefficients for the consideration of the absolute pressure in the measurement result of the photoacoustic measuring system.

These and other objects are achieved according to the invention by a method of the type mentioned above in that at least once a determination pressure in the measuring chamber is set to determine at least one parameter of the correction function, the absolute pressure in the measuring chamber is determined at this determination pressure, at this absolute pressure at least one determination measurement is performed during a determination initiation, and the at least one parameter is determined on the basis of the absolute pressure and the determination measurement result of the determination measurement. Any excess or negative pressure (relative to the ambient pressure) can be used as the determination pressure, wherein the determination of parameters can also be repeated in several steps at several different pressures, if several measurements are required to determine the parameters. As a determination excitation any excitation can take place, which either corresponds to or differs from the photoacoustic excitation which takes place during the "regular" measurement.

In a variant of the invention, the determination pressure is defined as a relative pressure to the ambient pressure. This permits a very simple setting of the determination pressure, whereby relatively large tolerance ranges are permissible, since a measurement of the absolute pressure in the measuring chamber for determining the parameters is provided anyway. As detection excitation, an excitation strength reproducible and independent of the measurement component acoustic detection oscillation is excited in an advantageous manner, it being preferred if this detection excitation is realized in the simplest possible way and with minimal operator intervention.

Preferably, the determination excitation is generated acoustically by means of a sound generator, wherein the sound generator used for this purpose does not affect the functionality of the measuring chamber in the measuring operation.

In a variant of the invention, the detection excitation is generated photoacoustically.

In a further variant of the invention, the detection excitation is generated by means of an absorber arranged in a beam path of an electromagnetic excitation radiation used to generate the photoacoustic effect. The known application of an absorber can also be carried out in the field by the user in a simple manner.

In a further embodiment, the determination pressure in the measuring chamber is generated by a pump device, preferably by the measuring gas pump present in the measuring device. Thereby, a desired positive or negative pressure (determination pressure) in the

Measuring chamber with the pump device present in the device and with appropriate throttling or shut-off of the gas connection are relatively easily generated, which allows a cost-effective application of the method. For example, with an existing suction pump, a negative pressure can be generated in the measuring chamber, as is known per se in a "leak test" in which the measuring chamber is evacuated using the existing suction pump and the subsequent pressure increase to determine the leak rate as a result of any existing By reversing the effective direction of the suction pump, for example, with valves, of course, an overpressure in the measuring chamber can be generated, if necessary.

Advantageously, at least one parameter of the correction function is a coefficient of a polynomial function of first, second or higher order, for example, the term

Ko + Ki ^* ((P - Po) / Po) + K ₂ ^* ((P - Po) / Pof

may contain. Here, P corresponds to the absolute pressure in the measuring chamber, P _{0 to} the reference pressure under standard conditions, preferably 1013 mbar, and K ₀ , Ki and K ₂ correspond to coefficients of the polynomial function. Second order and higher order polynomial terms allow the consideration of particular effects, such as a change in microphone sensitivity.

Furthermore, the objects of the invention are achieved by the measuring device mentioned at the outset in that it has a determination unit with which a determination pressure in the measuring chamber can be set to determine at least one parameter of the correction function, and the absolute pressure can be determined at this determination pressure At least one determination measurement in a determination excitation can be carried out in accordance with this absolute pressure, wherein at least one parameter of the correction function can be determined from at least the measured absolute pressure and the determination measurement result of the determination measurement with the determination unit. Such a measuring device allows the advantageous implementation of the above-mentioned method.

In a variant of the invention, a sound generator for acoustic detection excitation is arranged on the measuring chamber. This makes it possible to acoustically obtain the detection excitation with sufficient accuracy. An acoustic excitation with a sound generator can, in a conventional manner, also be used to determine the resonance frequency of the system. As a sound generator, for example, a microphone can be used, the sounder may be similar to the sound pickup, since it can be operated electrically as a speaker because of the reciprocity of the transducer. The transfer function of this arrangement is thus determined twice by an approximately equal characteristic, on the one hand by the sensitivity in the receiving mode and, on the other hand, by the sensitivity in the transmitting mode.

In order to determine the at least one parameter of the correction function, an absorber is advantageously arranged in a beam path of the electromagnetic excitation radiation used to generate the photoacoustic effect. As an absorber for the defined photacoustic excitation of sound waves in the measuring chamber, an absorber window has proved to be suitable, which can be introduced in a simple way into the beam path of the light source. Thus, inter alia, a so-called "span check" for determining or monitoring a possible, possibly age-related drift of the microphone sensitivity can be performed.

In a further variant of the invention, the measuring device has a pump device, preferably a sample gas pump, by means of which the determination pressure in the measuring chamber can be set.

Advantageously, the determination unit is set up to re-determine the at least one parameter of the correction function before each test run. The measuring device thus has a determination unit with which the at least one parameter of the correction function can be determined again before each test run. In this case, the functionality of determining the at least one parameter of the correction function is favorably integrated into the measuring device and requires no operator intervention. As a result, incorrect measurements due to operator errors can be minimized.

The subject invention is explained in more detail below with reference to Figure 1, which shows an example, schematically and not limiting an advantageous embodiment of the invention. It shows 1 shows a schematic circuit diagram of a measuring device according to the invention.

The measuring device 1 shown schematically in FIG. 1 comprises a measuring chamber 4, through which a measuring gas 3 is passed. The measuring gas 3 contains a measuring component 2, for example exhaust gas particles, wherein a measuring characteristic C of this measuring component 2 is to be determined by the measuring device. The measuring characteristic C can in particular be a value which allows conclusions to be drawn about the concentration and / or the number of particles of the measuring component 2 in the measuring gas 3. As a measuring property, however, a mass and / or size distribution of particles or other properties of the measuring component 2 could be determined, provided that these properties are accessible to a photoacoustic measurement. The sample gas 3 can, for example, be branched off from the exhaust gas contained in an exhaust line 18 at an extraction point 17 and fed to the measuring cell 4, it being possible for a conditioning unit 19, in particular for diluting or cooling the exhaust gas, to be provided in a manner known per se. The conditioned exhaust gas is fed via an inlet valve 20 into the measuring chamber. On the other hand, with a throttle valve 15 of adjustable cross-section of the measuring chamber pressure P _{M can be} adjusted.

The flow of the measuring gas 3 through the measuring chamber 4 is maintained during the measurement of a downstream of the measuring chamber 4 pumping device in the form of a sample gas pump 9. For purposes of determining the at least one parameter of the correction function according to the invention, the conveying direction of the sample gas pump 9 can be reversed by means of a switching valve 16. This reversal of the direction of conveyance is only relevant to the determination function described below and is not used during the measurement.

In order to determine the time course of the measuring characteristic C of the measuring component 2, a pulsating-ie intensity-periodically modulated-electromagnetic radiation, preferably laser radiation in the visible or non-visible spectrum, is introduced into the measuring chamber 4 by an exciting device 11 , wherein the Pulsationsfre- frequency to the measuring chamber 4, the measuring gas 3 and the measuring component 2 is tuned. This can be at least a clearly pronounced acoustic resonance excited and thereby the achievable accuracy can be improved. The wavelength of the electromagnetic radiation can be tuned specifically to the properties of the measuring component 2 and is generally chosen such that the radiation is optimally absorbed by the measuring component, but not by the measuring gas 3 or by other components carried by the measuring gas 3 become. The pulsation frequency is also matched to the dimensions of the measuring chamber 4 and the properties of the measuring gas 3 and the measuring component 2 in order to achieve the best possible measuring amplification by exploiting resonance behavior. The photoacoustically excited sound is picked up by a sound pickup 5 or microphone and evaluated by an evaluation unit 6 for determining the measurement property C.

The absolute pressure P in the measuring chamber can be determined via a pressure measuring unit 13. As is generally customary, this can be used to determine the absolute pressure or a differential pressure, in particular relative to the ambient pressure Pu. In connection with the subject invention, the "absolute pressure" P is the absolute pressure determined from the measurement result of the pressure measuring unit 13. On the other hand, the relative pressure in the measuring chamber relative to the ambient pressure Pu is referred to as the measuring chamber pressure P _M.

Furthermore, for the purpose of determining at least one parameter of the correction function, the measuring device 1 comprises an absorber 8 which can be inserted into the beam path of the stimulating radiation 7 of the excitation device 11 and a sound generator 10 with which the measuring chamber can be inserted in a conventional manner (ie without photoacoustic excitation and in the absence of the measuring component 2), it can be acoustically excited.

Optionally, by appropriate valves (which are not shown for clarity in Fig. 1) instead of the exhaust gas, a purge gas and / or a zero or calibration gas via the throttle valve 15 and inlet valve 20 are passed into the measuring chamber, if in the measuring device. 1 corresponding devices are provided.

The individual electronic components of the measuring device 1 are connected to a control unit 14 which, in addition to the evaluation unit 6, also comprises a detection unit 12. The control unit 14 is in communication with the sound pickup 5, the sample gas pump 9, the sounder 10, the pickup device 1 1, the pressure measuring unit 13, the throttle valve 15, the switching valve 16 and the inlet valve. Thus, the control unit 14 can centrally acquire measurement data of these units and output control commands to these units.

The photoacoustic measurement is carried out in a manner known per se, wherein the sample gas 3 is sucked by the sample gas pump 9 through the measuring chamber 4, while the excitation device 1 1 pulsating into the measuring chamber for exciting the photoacoustic effect at a predetermined by the control unit 14 Pulsationsfrequenz 4 brings. The exciting radiation 7 is absorbed by the measuring components 2 and these, or the surrounding areas of the measuring gas 3, are thereby periodically heated, which causes corresponding sound pressure pulsations. The sound generated thereby is detected by the sound pickup 5 and evaluated by the evaluation unit 6, wherein the evaluation unit 6 determines a time-resolved value for the measurement characteristic C from the signal recorded by the sound pickup 5. In this case, known methods for digital and / or analog signal processing can be used. During the determination of the coefficients K _n according to the invention, the measuring chamber pressure P _M is essentially determined by the control of the sample gas pump 9 and of the throttle valve 15. The type of wiring, with the throttle valve 15 before and the sample gas pump 9 after the measuring chamber 4, is shown in Fig. 1 only by way of example. In fact, usually more complex systems are used, such as to avoid throttling of the exhaust gas in front of the measuring chamber 4. Such complex circuits are well known to those skilled in the art, so that only the simplified circuit shown in FIG. 1 will be described for reasons of clarity. Optionally, further temperature and / or pressure sensors for measuring the media flowing in the measuring device 1 can be provided at all points of the flow paths, if this is necessary.

In general, it is not always possible to set the pressure in the measuring chamber to a predetermined constant absolute pressure P, since too great a deviation of the measuring chamber pressure P _M from the ambient pressure Pu would falsify the measurement result and could even lead to damage of the components. Therefore, the measuring chamber pressure P _M is generally measured as a relative pressure to the ambient pressure Pu and kept as constant as possible over a narrow interval. This has the consequence that the absolute pressure P in the measuring chamber can change not only with the potentially violently pulsating pressure in the exhaust gas, but also with the ambient pressure Pu during a measuring run.

The measuring device 1 is usually calibrated to standard conditions (for example to a pressure P ₀ of 1013 mbar and a temperature T ₀ of 0 ° C). Deviations from these standard conditions are therefore monitored by the evaluation unit 6 on the basis of the pressure measuring unit 13 and, if appropriate, a temperature measuring unit (not shown in FIG. 1) and the value for the measuring characteristic C is corrected by means of correction functions which, among other things, can also take into account the ideal gas equation ,

The correction functions can be in polynomial form, for example, where the deviation ((P-Po) / Po) from the normal pressure, the deviation ((TT ₀ ) / T ₀ ) from the normal temperature or combinations thereof can be used as a variable of the polynomial. The coefficients of the polynomial are currently usually determined at the factory, but in practice currently only first degree polynomials are used. A determination of coefficients for higher-order polynomial terms from the factory has so far proved too inaccurate.

The solution according to the invention makes it possible to adapt the correction function very precisely to the actual conditions immediately before a measurement, so that, for example, even higher-order coefficients can be used meaningfully and advantageously. For this purpose, the control unit 14 comprises a determination unit 12, which performs the following steps before the measurement: • In the measuring chamber 4, a desired positive or negative pressure (determination pressure) is produced.

Under this operating condition, an excitation of the measuring gas which is reproducible in the excitation intensity and independent of the measuring component takes place to acoustic oscillations (detection excitation).

• The absolute pressure P in the measuring chamber is measured and the acoustic vibrations are detected and evaluated by the evaluation unit 6 as in the actual measurement (determination measurement).

• If necessary, the above steps can be repeated with a different determination pressure if a determination measurement is not sufficient to determine the coefficients or parameters.

• The desired coefficients or parameters of the correction function are determined from the dependence of the measurement result on the upcoming absolute pressure.

Thereafter, in the actual measurement, the correction function can be used with the thus determined coefficients or parameters.

The reproducible and independent of the measurement component excitation of the sample gas can be done either photoacoustically using an introduced into the excitation radiation 7 absorber 8, or via the sounder 10th

According to the invention, the coefficients of the correction polynomial can thus be determined in a simple manner, for example before each measurement run, without requiring a factory-side intervention. As a result, additional factors in the correction function can be taken into account in a more complex manner, such as an aging-related change in the microphone sensitivity or a dependence of the microphone sensitivity on the pressure.

The determination of coefficients immediately before the measurement run also makes it possible to reliably correct the measurement result with the help of polynomials of second and higher order.

In addition to polynomials, other types of correction functions can be used whose parameters can be determined or adjusted in the same way immediately before the measurement run.

Functionality and significance of the method according to the invention and the associated device are particularly clear in connection with the concrete application example described below. Priority is given to the question of which variables and operating conditions have an influence on the calibrated measuring sensitivity of the measuring device 1, with which methods and devices they are determined and checked, and how the known possibilities by the inventive method and the associated device can be useful supplemented.

"Calibration" is the comparison of the displayed measured value of a sensor with that of a calibrated reference (according to DIN 1319-1) Equation of the displayed measured value with that of the calibrated reference - more precisely: minimizing the measured value deviation - becomes (according to DIN 1319 -1) is referred to as "adjustment." Since calibration alone is generally meaningless for a meter, "calibration" will always be used in the sense of "calibration and adjustment".

The following values can be calibrated at the factory and, if necessary, by a trained service technician:

> Analog outputs: zero value, end value

> Sensors:

- Temperature of the measuring chamber: 2-point calibration with a "lower" and an "upper" comparison value. If necessary, this calibration should be performed before the calibration of the other sensors.

Absolute pressure: current ambient air pressure; essential for the inventive method

Pressure ("relative pressure") in or immediately after the measuring chamber: 2-point calibration with "zero value" and an "upper" comparison value; essential for the inventive method

Sample gas flow, differential pressure sensor: 2-point calibration with "zero value" and an "upper" comparison value

Note: Error in the calibration of the relative pressure, which must be below 50 mbar and therefore will typically have an error of <5 mbar (0.5% of the ambient pressure), and errors in the calibration of the flow have no effect on the correctness of the measured value.

> Measured value: Relationship between sensor signal in mV and exhaust gas load with soot in

mg / m ³

When calibrating the measured value, the sensor signal (in mV) is related to the soot concentration of a stable soot source. For example, the "Compression Aerosol Standard" (CAST) device from Matter Engineering can be used for this purpose. It produces soot, whose properties resemble those of diesel soot. The carbon black concentration emitted by CAST is determined gravimetrically, the result being based on standard conditions (pressure 1013 mbar, temperature 0 ° C.). This method corresponds to the standard measurement method for soot calibration according to VDI guideline 2465, sheet 1. A simpler calibration method would be desirable, but because of the complex physicochemical nature of carbon black, a well-accepted alternative (such as the calibration gas bottles known in gas analyzers) has not been developed to date.

The result of calibration and adjustment is a calibration factor used by the device to convert the microphone's internal measurement (mV) to concentration (g / m ³ ). The calibration factor is composed of a fixed quantity and a variable factor F _ka i. The fixed size is a rough conversion for a "medium" device sensitivity. For the first factory calibration, the default value F _ka i = 1 is replaced by a device-specific value.

First, the phase position φ _{Μ of} the microphone signal relative to the phase position of the exciting laser signal at a sufficiently high soot concentration in the measuring cell is to be determined and set as "reference phase." The phase position φ _Μ is for measuring signals over 100 mV, or for soot concentrations of more than 1 mg / m ³ , constant within ± 3 ° and must be observed by the device during the phase-sensitive evaluation of the measuring signals.

Thereafter, the display value of the device (in mg / m ³ ) with the result z. B. compared to a coulometric Rußbestimmung. The new calf replacement factor F _ka i is determined from the old calibration factor and the ratio of the current display and the reference measured value based on standard conditions (1013 mbar, 0 ° C) according to the following formula:

F _ka i _(new ) = F _ka i _(a | _t) ^* Reference / Display

The measured value calibration is usually stable for years, as comparative measurements have shown. However, should it be suspected that the sensitivity has changed and therefore a new calibration is required, the factor F _ka i, and possibly also the phase angle φ _Μ , can be _reset . It is important to ensure that the windows are freshly cleaned and therefore the lowest possible zero signal is present.

To check the factory calibration, or its validity, without having to perform the complete calibration procedure, a special method can be used. It is based on the reasonable and generally accepted assumption that the calibrated relationship between sensor signal (in mV) and mass concentration of soot (in mg / m ³ ) does not change when the characteristics of those elements that generate the sensor signal remain unchanged. In practice this means that the following properties remain stable:

- Microphone sensitivity

- Intensity of the laser beam

- Linearity of the microphone - Linearity of the laser intensity with the current

Note on the linearity of microphone sensitivity and laser control: As shown on the one hand different experiments and on the other hand can be derived theoretically from the photoacoustic principle, the sensor signal (in mV) and the soot mass concentration (in mg / m ³ ) are linearly related. A deviation from this linearity can only occur in the case of non-linear behavior of the components of the measuring chain, in particular of the microphone.

To check these properties, the following functions, which are described in more detail below, are already available and can be advantageously supplemented by the method according to the invention:

> Calibration check: To check the correct calibration of the measured value, the sensitivity of the measuring chain (the intensity of the laser beam together with the sensitivity of the microphone) is checked by means of an absorber window.

> Resonance Frequency Scan: The sensitivity of the microphone (and the speaker also integrated in the system) can be checked by checking the signal value displayed after the resonance frequency scan is completed.

> Linearity check of the microphone

> Linearity check of the laser

To understand the calibration and verification functions you need to know the overall system. Its core is the photoacoustic measuring chamber 4, as shown schematically in a simplest design for achieving a good measuring sensitivity in Fig. 1.

In the center of the measuring chamber 4 is an acoustic resonance tube, which is designed as a so-called "open whistle", with the following characteristic features: The diameter is small compared to the length L and at both ends widens the

Diameter (notch filter). This forms a standing acoustic wave with pressure nodes at the ends of the resonance tube and a pressure maximum in the middle. The amplitude of the acoustic wave is considerably weaker in the area of the "notch filter" with a relatively larger diameter than in the resonance tube.

Based on these principles:

^■ The resonant frequency of the cell (and thus the associated lambda wavelength) is determined by the length L _{R of} the resonator. Approximately, lambda = L _R / 2.

^■ The maximum of the sound pressure is in the middle of the cell, which is why the microphone is also there. ^■ The sound velocity becomes zero at the end of the "Notch Filter", ie the optical windows. This is associated with a maximum of the sound pressure. Together with the first point it follows that the total length of the cell space L is approximately an integer multiple of lambda / 2.

■ Gas inlet and gas outlet are placed in the pressure nodes of the standing wave, so as not to disturb them by the unavoidable flow noise at the inlet edges.

The measuring chamber shown schematically in FIG. 1 has various advantageous features. For example, the "AVL Micro Soot Sensor" device, which has been on the market for many years, reveals well-designed roundings at the inlets and outlets and the cross-sectional changes, and there is another narrow tube section and a further chamber ("notch filter") A larger cross-sectional area is provided which allows flow through the measuring cell by two inlets at the windows and a common outlet, thus ensuring that the optical windows are not flown by the particle flow and therefore less easily pollute.

The device can be set in different operating states:

o REST

PAUSE

o STANDBY (including ZERO ADJUST)

o MEASUREMENT (with the possible PEAK MEASUREMENT)

o ZERO POINT CHECK

And depending on the operating state, additional test and maintenance functions can be carried out:

o BACKWASHING (the sampling line)

o LEAK TEST (checking the gas path for leaks)

o RESONANCE FREQUENCY SCAN (Check Resonance Frequency and Signal Intensity)

o WINDOW CLEANING (after checking window and cell contamination) o "SPAN CHECK" CALIBRATION CHECK (checking the validity of the calibration)

o LI N EARITY TEST for microphone and laser When in the REST state, the microprocessor system is ready for operation, but no measurement or control functions are executed. The solenoid valves are de-energized and connect the components of the measuring system with clean filtered air.

Even in PAUSE mode, the solenoid valves are de-energized and connect the measuring system with clean filtered air. The temperature controls of the measuring cell are switched on and after a three-minute stabilization time, the various temperatures, the acoustic resonance frequency and the contamination are checked. In the event of a limit violation of the newly determined zero value, a warning is issued that the optical windows must be cleaned.

In the STANDBY operating state and after reaching the setpoint temperatures, the pump is started, but the solenoid valves remain de-energized and connect the measuring system with clean filtered air. This particle-free "zero gas" - in special cases also filtered exhaust gas can be used - flows in the sequence through the measuring cell and allows after 10 seconds stabilization time to determine the zero value by averaging the sensor signal (here: "zero signal") for at least 20 seconds ( maximum 260 seconds). In the event of a limit violation of the newly determined zero value, a warning is issued that the optical windows must be cleaned. In addition, the resonance frequency is determined and checked. Thereafter, the system is ready for measurement, and as long as the MEASUREMENT operating state is not called, "zero gas" is still passed through the measuring cell and the zero value is continuously determined over the last 20 seconds (or the currently set zero balance time).

A ZERO ADJUST should be performed before each measurement cycle. Measuring cycles typically take no longer than 30 minutes and it is recommended to switch back to the STANDBY operating state after a measuring period of 30 to 60 minutes in order to determine the zero value again. The new zero value compensates for any newly added soiling of the optical windows.

The inlet valve is only switched over in the MEASURE mode, so that sample gas (undiluted or diluted exhaust gas) is pumped from the sampling line to the measuring cell and to the bypass. The measured values are determined continuously, with the previously determined zero value being subtracted in a phase-correct manner ("vectorial"). With the device-specific calibration factor, the measured values are converted into soot concentrations.

Sometimes, during the MEASURE mode, a PEAK MEASUREMENT is required, for example, to determine the peak occurring during acceleration or load application. This is done by a start and a stop command which activates an interval for the peak detection during the current measurement so that after the stop command the peak value measured during the interval can be retrieved.

In the ZERO POINT CHECK mode, a solenoid valve is activated to allow "zero gas" to flow into the measuring chamber, and the readings are still continuously read. In this case, the last determined zero value is subtracted phase-correct ("vectorial") and the primary sensor signals are converted into soot concentrations using the device-specific calibration factor. If now - for example, as a result of added pollution of the windows - an inadmissibly high reading occurs, for example, with an amount greater than 0.001 mg / m ³ , or the original zero value is not reached, the newly obtained zero gas reading for baseline correction of the previously obtained soot measured values are used. In such a case, it is recommended to switch to the STANDBY operating state before a further measurement and to re-calculate the zero value.

If the zero point check is called from a high steady state soot load measurement state, it may take a few seconds to reach the stable zero point: The fall time to 10% of the last measured value lasts approx. 1 s, but significantly longer at 1% of the last measured value.

The need for a zero point check is related to possible window contamination. Therefore, the system issues a zero calibration warning that a zero point check should be performed if a total soot concentration of eg 100000 s ^* mg / m ³ summed up since the last time the meter was operated in STANDBY mode (eg 10 mg / m ³ during 10000 s) was conveyed through the measuring cell. After such a conveyed amount of soot, the signal caused by the window contamination should still be less than 0.01 mg / m ³ . This is a rough estimate of typical operating conditions; the actual window contamination depends on various operating conditions, for example pressure fluctuations, stationary measurements or measurements with varying soot concentrations.

If the value is above 0.05 mg / m ³ , for example, massive contamination has occurred during the test run and / or the device has not been operated in the STANDBY operating state for a very long time. When such high zero values occur, it is necessary to clean the windows.

With the BACKWASH service function, all valves are closed and then the solenoid valve is opened for 30 seconds to allow the compressed air to enter. This purge air then flows through the sampling line and sampling probe back into the dilution unit. The sampling probe and sampling line are cleaned of deposited coarse particles. The leak test function ("leak test", checking the gas path for leaks) can be called to automatically check the leak rate of the system and it can also be used to implement the method according to the invention.

The internal leak test is used to check the internal device components (from the inlet valve to the pump) and should be carried out after working inside the device (window cleaning, measuring cell cleaning, filter replacement).

The external leak test is also used to check the external components (total distance from the manually closed sampling probe or pipe to the pump) and should be performed after each reinstallation of the entire system or after work on the sampling system.

With the start of the leak test, the gas path is evacuated to a negative pressure of approx. -1 10 mbar (relative to the ambient pressure). After a short stabilization time, the increase in pressure possibly caused by a leak is checked. This increase must be so small that, even at the maximum permitted negative pressure of -1 10 mbar, the leakage air flowing through the leak is less than 1% of the flow in the measuring operation of the system. An error message is output when the limit value of 0.5 ml / s is exceeded. This provides a safe distance to the tolerated leak rate of 1% of the flow 4 l / min = 0.66 ml / s.

With the RESONANCE FREQUENCY SCAN function, the system shuts off the laser as at the end of the PAUSE mode and checks the resonance frequency of the measuring cell (duration approx. 20 seconds). A second microphone built into the measuring cell, which is operated as a loudspeaker, generates sound with a constant amplitude but a variable frequency between 3750 Hz and 4500 Hz. The maximum amplitude received by the sensor microphone defines the resonance frequency, its value (in mV) means device-specific microphone sensitivity. By repeatedly calling this function (which is meaningful only after reaching a stable state of the temperature control) and note the microphone signal for a long time can be checked whether a drift of the microphone sensitivity has taken place.

Moreover, with the method according to the invention, an improved determination and consideration of the microphone and loudspeaker sensitivity dependent on the relative pressure in the measuring chamber can be realized.

At the beginning of the operating state PAUSE and the operating state STANDBY, the device independently carries out an evaluation of the zero value. In the event of a limit violation, a warning is issued that the optical windows must be cleaned. The SPAN CHECK function checks the overall sensitivity of the sensor (the intensity of the laser beam and the sensitivity of the microphone) using an absorber window.

Moreover, with the method according to the invention, an improved determination and consideration of the microphone sensitivity dependent on the relative pressure in the measuring chamber can be realized.

The function corresponds to the "span check" with calibration gases in gas analyzers and means a check of the correct calibration of the measuring instrument. It should be done between once a week and once a month, depending on the application.

A robust implementation of the original idea suitable for use on test benches, occasionally introducing an optical absorber into the acoustic resonance tube and thus stimulating the resonance photoacoustically, is not possible since the absorber would have to have microscopically small dimensions so as not to disturb the acoustic properties of the resonance cell , In addition, the absorption would have to be less than 1% in order to obtain a signal within the measuring range of the measuring device 1 according to the invention, e.g. as "AVL Micro Soot Sensor Plus".

On the other hand, it has proven useful to insert an object with high absorption outside the resonance tube in a photoacoustically less sensitive area of the measuring cell near the entrance or exit window of the laser beam. The absorber is fastened directly to the measuring cell window, which enables simple, robust and reliable insertion. However, due to manufacturing tolerances of the absorber and measuring cell, it is not intended to exchange the absorber windows of different devices with one another. This means that an absorber window is only used together with a specific measuring cell. The unit's absorbance or default value is determined during factory calibration and stored in the firmware, and only that unit is used to calibrate a particular device.

The calibration check can be performed after the warm-up phase. For this purpose, the laser is activated and the value obtained with the absorber window and its deviation from the reference value are displayed. The repeatability of this calibration check is approximately 5%, and as long as the displayed value within this limit matches the reference value, no further steps are necessary. In the case of a larger deviation, the calibration check should be repeated in order to rule out statistical outliers. With a bias of between 10% and 50%, the user can apply the newly determined calibration factor to the instrument, and any deviation greater than 10% should consider re-measurement calibration. The function LINEARITY TEST serves on the one hand to check the linearity between the response of the microphone and the activation power of the built-in loudspeaker to ensure the correct functioning of the sensor module in the measuring cell, and on the other hand to check the linearity between laser power and current above the threshold for the laser diode laser activity ,

The linearity check of the microphone causes the loudspeaker in the measuring cell to be operated sequentially at a power of 10 to 100% of the power used during the resonance frequency scan. The response of the microphone is recorded and a linear regression between the speaker power and the microphone signal is calculated. The displayed regression coefficient should be above 0.95. Smaller regression coefficients indicate errors in the speaker or microphone. If the integrity of the speaker and microphone is not guaranteed, the measuring cell must be replaced.

So that the linearity check of the laser can be carried out with a reproducible signal, the absorber window is installed. The laser is operated with currents of 10 to 120% of the difference between the threshold of the laser activity current and the nominal current of 1 W in standard operation. The response of the microphone is recorded and a linear regression between current and microphone signal is calculated. The displayed regression coefficient should be above 0.95. Smaller regression coefficients indicate errors from lasers or laser drivers. In this case, laser and / or laser drivers must be replaced.

Reference number: Measuring device 1

Measuring component 2 measuring gas 3

Measuring chamber 4 Sound sensor 5 Evaluation unit 6 Stimulating radiation 7 Absorber 8

Measuring gas pump 9 Sounder 10 Starting device 1 1 Determining unit 12 Pressure measuring unit 13 Control unit 14 Throttling valve 15 Changeover valve 16 Sampling point 17 Exhaust pipe 18 Conditioning unit 19 Intake valve 20

Measurement Characteristic C Coefficient K _n Ambient pressure Pu Measuring chamber pressure P _M Detection pressure P _E

Claims

claims

1 . Method for operating a measuring device (1), which is suitable for measuring a measuring characteristic (C) of a measuring component (2) in a measuring gas (3), wherein the measuring gas (3) is photoacoustically excited in a measuring chamber (4) which produced thereby Sound is recorded with a sound pickup (5) and the recorded signal is evaluated in an evaluation unit (6) for determining the measurement property (C), wherein the evaluation unit (6) corrects the determined measurement property (C) by means of a correction function, characterized in that for determining at least one parameter of the correction function at least once a determination pressure (P _E ) in the measuring chamber (4) is set, the absolute pressure (P) in the measuring chamber (4) at this determination pressure (P _E ) is determined at this absolute pressure (P ) is carried out at least one determination measurement in a determination initiation, and the at least one parameter based on the absolute pressure (P) and the Ausmittlungsmessergeb (C _E ) of the determination measurement is determined.

2. The method according to claim 1, characterized in that the determination pressure (P _E ) is defined as a relative pressure to the ambient pressure (Pu).

3. The method according to claim 1 or 2, characterized in that is excited as a detection excitation reproducible in the excitation strength and independent of the measuring component acoustic detection oscillation.

4. The method according to any one of claims 1 to 3, characterized in that the detection excitation is generated acoustically by means of a sound generator (10).

5. The method according to any one of claims 1 to 3, characterized in that the detection excitation is generated photoacoustically.

6. The method according to claim 5, characterized in that the detection excitation by means of a in a beam path of an electromagnetic excitation radiation used for generating the photoacoustic effect (7) arranged absorber (8) is generated.

7. The method according to any one of claims 1 to 6, characterized in that the detection pressure (P _E ) in the measuring chamber (4) by a pumping device, preferably of the measuring device (1) existing sample gas pump (9) is generated.

8. The method according to any one of claims 1 to 7, characterized in that at least one parameter of the correction function is a coefficient (K _n ) of a polynomial function of the first, second or higher order.

9. The method according to claim 8, characterized in that the polynomial function the term

o + Ki ^* ((P - Po) / Po) + K ₂ ^* ((P - Po) / Pof

where P is the absolute pressure in the measuring chamber, P _{0 is} the reference pressure at standard conditions, preferably 1013 mbar, and K ₀ , Ki and K _{2 are} coefficients of the polynomial function.

10. Measuring device (1) for measuring a measuring characteristic (C) of a measuring component (2) in a measuring gas (3), wherein the measuring device (1) has a measuring chamber (4), at least one starting device (1 1) for photoacoustic excitation of the measuring gas ( 3) in the measuring chamber (4), a sound pickup (5) and an evaluation unit (6), wherein with the evaluation unit (6) from the sound pickup (5) recorded signal to determine a fair property (C) evaluable and the fair property (C ) is correctable by means of a correction function, and wherein at the measuring chamber (4) a pressure measuring unit (13) for determining the absolute pressure (P) in the measuring chamber (4) is provided, characterized in that the measuring device has a detection unit (12), with the determination of at least one parameter of the correction function, a determination pressure (P _E ) in the measuring chamber (4) adjustable, the absolute pressure (P) at this determination pressure (P _E ) determined and at this absolute pressure (P) at At least one parameter of the correction function can be determined from at least the measured absolute pressure (P) and the determination result (C _E ) of the determination measurement with the determination unit (12).

1 1. Measuring device (1) according to claim 10, characterized in that a sound generator (10) for acoustic detection excitation is arranged on the measuring chamber (4).

12. Measuring device (1) according to claim 10 or 1 1, characterized in that for determining the at least one parameter of the correction function, an absorber (8) is arranged in a beam path of the electromagnetic stimulation radiation used for generating the photoacoustic effect (7).

13. Measuring device (1) according to one of claims 10 to 12, characterized in that the measuring device (1) has a pumping device, preferably a sample gas pump (9), by means of which the determination pressure in the measuring chamber (4) is adjustable.

14. Measuring device (1) according to any one of claims 10 to 13, characterized in that the determining unit (12) is adapted to determine the at least one parameter of the correction function before each test run again.