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    • 专利摘要:
    The invention relates to a measuring device (1) for measuring a measuring characteristic (C) of a measuring component (2) in a measuring gas (3) and to a method for operating the measuring device (1). The measuring gas (3) is photoacoustically excited in a measuring chamber (4), the sound generated thereby is recorded with a sound pickup (5) and the recorded signal is evaluated in an evaluation unit (6) for determining the measuring characteristic (C). The evaluation unit (6) corrects the determined measurement property (C) by means of a correction function. To determine at least one parameter of the correction function, a determination pressure (PE) in the measuring chamber (4) is set at least once, thereby determining the absolute pressure (P) in the measuring chamber (4). In this absolute pressure (P), at least one determination measurement is carried out in a determination excitation and the at least one parameter is determined on the basis of the absolute pressure (P) and the determination result (CE) of the determination measurement.
    公开号:AT518830A1
    申请号:T51073/2016
    申请日:2016-11-25
    公开日:2018-01-15
    发明作者:Dr Reingruber Herbert；Dr Harms Klaus-Christoph
    申请人:Avl List Gmbh；
    IPC主号:
    专利说明:

    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, wherein with the evaluation unit the signal picked up by the sound pickup can be evaluated to determine a measurement property and the measurement property can be corrected 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 of the measuring gas to be measured (which are referred to herein as measuring components) flow with this into a measuring chamber, which is preferably designed 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. An electromagnetic radiation matched in its spectrum to the absorption capacity of the measuring component, wherein in particular "light" from the near ultraviolet (UV) to the middle infrared (IR) with wavelengths from 200 to 10000 nm can be used, is introduced into the measuring chamber with such a pulsation frequency irradiated that the measuring component and thus the surrounding measuring gas heated periodically by the absorption of the periodically irradiated light, whereby corresponding sound pressure pulsations are generated. The sound optically excited in this way in the measuring chamber as a function of the concentration of the measuring component 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 includes a calibration or zero gas which has known properties and is located in or flows through the measuring chamber during a calibration, adjustment or other 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 output 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 measuring component measured at a certain absolute pressure is converted to standard conditions by means of the (ideal) gas equation, e.g. at 0 ° C and 1013 mbar. On the other hand, correction algorithms or correction functions, e.g. Polynomials of the first or higher order, used 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 enter them before making the measurements, which does not happen in general, since 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 by a method of the type mentioned in the present invention 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 carried out at a detection initiation, and the at least one parameter is determined based on the absolute pressure and the determination result of the determination measurement. Any excess or negative pressure (based on the ambient pressure) can be used as the determination pressure, whereby 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. A determination measurement is taken to mean the measurement made in connection with the determination initiation.
    In a variant of the invention, the determination pressure is defined as a relative pressure to the ambient pressure. This allows 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 stimulation 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. As a result, a desired overpressure or negative pressure (determination pressure) in the measuring chamber with the pump device present in the device and with corresponding throttling or blocking of the gas connection can be generated relatively easily, which allows a cost-effective use of the method. For example, with an existing mammalian pump, a suppression in the measuring chamber can be produced, as is done in a known manner in a "leak test" in which evacuated the measuring chamber using the existing mammal pump and evaluated the subsequent pressure increase to determine the leak rate due to any existing leaks becomes. By reversing the effective direction of the mammal 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
    may contain. Here, P corresponds to the absolute pressure in the measuring chamber, P0 to the reference pressure under standard conditions, preferably 1013 mbar, and K0, and K2 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, the absolute pressure can be determined at this determination pressure and at this absolute pressure at least a determination measurement in a determination excitation is feasible, wherein at least one parameter of the correction function can be determined from at least the measured absolute pressure and the determination result of the determination measurement with the determination unit. Such a meter allows the advantageous implementation of the above 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 therefore 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 transmission 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 proven useful, which can be introduced in a simple manner in 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 adjusted.
    Advantageously, the at least one parameter of the correction function can be determined again before each test run with the determination unit, wherein the functionality of determining the at least one parameter of the correction function can be integrated into the measurement 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
    Fig. 1 is a schematic 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 measuring gas 3 can be branched off from the exhaust gas contained in an exhaust pipe 18, for example, at an extraction point 17 and the measuring cell 4 are supplied, wherein optionally in a conventional manner, a conditioning unit 19, in particular for dilution or cooling of the exhaust gas can be provided. The conditioned exhaust gas is fed via an inlet valve 20 into the measuring chamber. On the other hand, with a throttle valve 15 having an adjustable cross section of the measuring chamber pressure PM 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 profile of the measuring characteristic C of the measuring component 2, a starting device 11 generates a pulsating - i. a periodically modulated in intensity - electromagnetic radiation, preferably a laser radiation in the visible or non-visible spectrum, introduced into the measuring chamber 4, wherein the pulsation 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 so that the radiation is absorbed optimally by the measuring component, but not by the measuring gas 3 or by other components carried by the measuring gas 3. 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 utilizing 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 is referred to as "absolute pressure" P determined from the measurement result of the pressure measuring unit 13 absolute pressure. In contrast, the measuring chamber pressure PM refers to the relative pressure in the measuring chamber relative to the ambient pressure Pu.
    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 starting 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) can be excited acoustically.
    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 11, the pressure measuring unit 13, the throttle valve 15, the switching valve 16 and the inlet valve. Thus, the control unit 14 can collect measurement data of these units centrally 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 Anregevorrich-device 11 a stimulating radiation 7 to achieve the photoacoustic effect under a predetermined by the control unit 14 Pulsationsfrequenz pulsating in the Introduces measuring chamber 4. The on Regestrahl ung 7 is absorbed by the measuring components 2 and these, or the surrounding areas of the measuring gas 3, thereby periodically heated, causing 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 Kn according to the invention, the measuring chamber pressure PM 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 PM from the ambient pressure Py would falsify the measurement result and could even lead to damage of the components. Therefore, the measuring chamber pressure PM is generally measured as a relative pressure to the ambient pressure Py and kept as constant as possible in a narrow interval. This has the consequence that the absolute pressure P in the measuring chamber can change not only with the possibly violently pulsating pressure in the exhaust gas, but also with the ambient pressure Py during a measuring run.
    The measuring device 1 is usually calibrated to standard conditions (for example to a pressure P0 of 1013 mbar and a temperature T0 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 ((T-To) / T0) 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 overpressure or negative pressure (determination pressure) is established. 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 may be repeated with a different detection pressure if a determination measurement for determining the coefficients or parameters is insufficient. • 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 are meaningfully supplemented by the method according to the invention and the associated device can.
    "Calibration" means (according to DIN 1319-1) the comparison of the displayed measured value of a sensor with that of a calibrated reference. Equating the displayed measured value with that of the calibrated reference-more precisely, minimizing the measured value deviation-is referred to as "adjustment" (according to DIN 1319-1). Since calibration alone is generally meaningless for a measuring instrument in the following, "calibration" is always 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:> Analogue outputs: zero value, final 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 method according to the invention - pressure ("relative pressure") in or immediately after the measuring chamber: 2-point calibration with "zero value" and an "upper" comparison value; essential for the method according to the invention - flow of the sample gas, differential pressure sensor: 2-point calibration with "zero value" and an "upper" comparison value
    Note: Error in relative pressure calibration, which must be less than 50 mbar and therefore will typically have an error of <5 mbar (0.5% of 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 / m3
    When calibrating the measured value, the sensor signal (in mV) is related to the soot concentration of a stable soot source. For example, the device "Combustion Aerosol Standard" (CAST) from Matter Engineering can be used. 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 complies with the standard reference calibration method for soot calibration to VDI Guideline 2465, Sheet 1. A simpler calibration method would be desirable, but due to the complex physicochemical nature of soot, a generally accepted alternative (such as the calibration gas cylinders 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 size (g / m3). The calibration factor is composed of a fixed quantity and a variable factor Fkai. The fixed size is a rough conversion for a "medium" device sensitivity. During the first factory calibration, the default value Fkai = 1 is replaced by a device-specific value.
    First, the phase angle φΜ of the microphone signal relative to the phase position of the exciting laser signal at a sufficiently high soot concentration in the measuring cell to be determined and set as a "reference phase". The phase angle φΜ is constant within ± 3 ° for measuring signals over 100 mV, or for soot concentrations of more than 1 mg / m3, 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 / m3) with the result z. B. compared to a coulometric Rußbestimmung. The new calf replacement factor Fkai 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:
    Fkai (new) = Fkai (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 Fkai, and possibly also the phase position φΜ, 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 the sensor signal (in mV) and the mass concentration of soot (in mg / m3) does not change if 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 the microphone sensitivity and the 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 / m3) 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 described in more detail below are already available and can be advantageously supplemented by the method according to the invention:> Calibration check: For "checking the correct calibration of the measured value" the sensitivity of the measuring chain (the intensity of the laser beam together with sensitivity of the microphone) using an absorber window. > Resonance Frequency Scan: The sensitivity of the microphone (and the speaker also integrated in the system) can be determined by checking the signal level displayed after completing the Resonance Frequency scan. > Linearity check of the microphone> Linearity check of the laser
    To understand the calibration and verification functions you need to know the whole 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 wavelength lambda) is determined by the length LR of the resonator. Approximately Lambda = LR / 2.
    The maximum of the sound pressure is in the middle of the cell, which is why the microphone is also there.
    The speed of sound is at the end of the "notch filter", so the optical windows,
    Zero. 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.
    The 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 device "AVL Micro Soot Sensor", which has been on the market for many years, has fluidically designed rounded edges at the inlets and outlets and at the cross-sectional changes. And there is another narrow pipe section and a further chamber ("notch filter") provided with a larger cross-sectional area, which allow the flow of the measuring cell through two inlets at the windows and a common outlet. This ensures that the optical windows are not affected 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 MEASURE (with the possible PEAK MEASUREMENT) o ZERO POINT CHECK
    And, depending on the operating status, additional test and maintenance functions can be performed: o RINSING (sampling line) o LEAK TEST (checking the gas path for leaks) o RESONANCE SCAN (checking resonance frequency and signal intensity) o WINDOW CLEANING (after checking the window and cell contamination) o CALIBRATION CHECK "SPAN CHECK" o LINEARITY 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 operating state, a PEAK MEASUREMENT is needed, for example, to determine the peak occurring during acceleration or load-up. A start and a stop command are used to activate a peak detection interval during the current measurement so that after the stop command the peak value measured during the interval can be called up.
    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 determined. 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, for example, as a result of an added contamination of the windows, an inadmissibly high measured value occurs, for example with an amount of greater than 0.001 mg / m3, or the original zero value is not reached, then the newly obtained zero gas measured value for the baseline correction of the previously obtained Soot readings 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 outputs a zero calibration warning that a zero point check should be performed if, since the last operation of the meter in the STANDBY mode, a total shot concentration of e.g. 100,000 s * mg / m3 (which corresponds, for example, to 10 mg / m3 for 10,000 s) 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 e.g. 0.05 mg / m3, so massive contamination has occurred during the test run, and / or the device has not been operated in STANDBY mode 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 components (from the inlet valve to the pump) and should be carried out after working inside the unit (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 line 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. -110 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 allowable negative pressure of -110 mbar, the leakage air flowing through the leak is less than 1% of the flow rate during 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 attachment of the absorber is done directly on the measuring cell window, which allows a 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 be re-measured.
    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 ,
    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.
    In linearity testing of the microphone, the loudspeaker in the measuring cell is sequentially operated at a power of 10 to 100% of the power used during the resonant 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.
    Designation: Measuring instrument 1 Measuring component 2 Measuring gas 3 Measuring chamber 4 Sound pickup 5 Evaluation unit 6 Stimulating radiation 7 Absorber 8 Measuring gas pump 9 Sounder 10 Starting device 11 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 Kn Ambient pressure Pu Measuring chamber pressure PM Determination pressure PE
    权利要求:
    Claims (14)
    [1]
    claims
    1. A 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), the Sound generated with a sound pickup (5) is recorded 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 (PE) in the measurement chamber (4) is set, the absolute pressure (P) in the measurement chamber (4) is determined at this determination pressure (PE), 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 Ausmittmessgebennni sses (CE) of the determination measurement is determined.
    [2]
    2. The method according to claim 1, characterized in that the determination pressure (PE) is defined as a relative pressure to the ambient pressure (Pu).
    [3]
    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]
    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]
    5. The method according to any one of claims 1 to 3, characterized in that the detection excitation is generated photoacoustically.
    [6]
    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]
    7. The method according to any one of claims 1 to 6, characterized in that the detection pressure (PE) in the measuring chamber (4) by a pumping device, preferably of the measuring device (1) existing sample gas pump (9) is generated.
    [8]
    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 ") of a polynomial function of the first, second or higher order.
    [9]
    9. The method according to claim 8, characterized in that the polynomial function the term

    where P is the absolute pressure in the measuring chamber, P0 is the reference pressure at standard conditions, preferably 1013 mbar, and K0, and K2 are coefficients of the polynomial function.
    [10]
    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) comprises a measuring chamber (4), at least one exciting device (11) for photoacoustic excitation of the measuring gas (3 ) in the measuring chamber (4), a sound pickup (5) and an evaluation unit (6), the evaluation unit (6) being able to evaluate the signal picked up by the sound pickup (5) to determine a measuring characteristic (C) and the measuring characteristic (C) can be corrected by means of a correction function, and wherein a pressure measuring unit (13) for determining the absolute pressure (P) in the measuring chamber (4) is provided on the measuring chamber (4), characterized in that the measuring device has a detection unit (12) with which for determining at least one parameter of the correction function, a determination pressure (PE) in the measuring chamber (4) adjustable, the absolute pressure (P) at this determination pressure (PE) determined and at this absolute pressure (P) at least one e determination measurement in a detection excitation is feasible, wherein at least one parameter of the correction function from at least the measured absolute pressure (P) and the determination result (CE) of the determination measurement with the determination unit (12) can be determined.
    [11]
    11. Measuring device (1) according to claim 10, characterized in that at the measuring chamber (4) a sounder (10) for acoustic detection excitation is arranged.
    [12]
    12. Measuring device (1) according to claim 10 or 11, 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]
    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]
    14. Measuring device (1) according to one of claims 10 to 13, characterized in that the at least one parameter of the correction function can be determined again before each test run by the determination unit (12).
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    同族专利:
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    DE112017005966A5|2019-08-01|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    DE19516974A1|1995-04-07|1996-10-10|Landis & Gyr Tech Innovat|Photo-acoustic gas sensor for monitoring air-conditioned room|
    US20020194897A1|2001-06-22|2002-12-26|William Patrick Arnott|Photoacoustic instrument for measuring particles in a gas|
    WO2008072167A1|2006-12-12|2008-06-19|Koninklijke Philips Electronics N.V.|Sample concentration detector with temperature compensation|
    US20120279279A1|2011-05-04|2012-11-08|Honeywell International Inc.|Photoacoustic Detector with Long Term Drift Compensation|
    US7710566B2|2005-05-27|2010-05-04|Board Of Regents Of The Nevada System Of Higher Education On Behalf Of The Desert Research Institute|Method and apparatus for photoacoustic measurements|FR3089250B1|2018-11-30|2020-12-18|Faurecia Systemes Dechappement|Exhaust line comprising a photoacoustic sensor for measuring at least one pollutant gas concentration and motor vehicle comprising such an exhaust line|
    法律状态:
    优先权:
    申请号 | 申请日 | 专利标题
    ATA51073/2016A|AT518830B1|2016-11-25|2016-11-25|Measuring device and method for operating the measuring device|ATA51073/2016A| AT518830B1|2016-11-25|2016-11-25|Measuring device and method for operating the measuring device|
    DE112017005966.6T| DE112017005966A5|2016-11-25|2017-11-24|Measuring device and method for operating the measuring device|
    PCT/EP2017/080325| WO2018096091A1|2016-11-25|2017-11-24|Measuring device and method for operating the measuring device|
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