CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing based upon International PCT Application No. PCT/EP2018/097061, filed 28 Dec. 2018, which claims the benefit of priority to Austria application No. A 51084/2017, filed 28 Dec. 2017.

BACKGROUND

The present invention relates to a measuring device for determining a measured variable of a measuring gas using a photoacoustic method, wherein the measuring device has a flow channel for the measuring gas having at least one feed line, a photoacoustic measuring cell and a discharge line.

Furthermore, the invention relates to a method for suppression of sound interferences in a measuring device for determining a measured variable of a measuring gas by means of a photoacoustic method, wherein the measuring gas is conducted via a flow channel which runs through at least one feed line, a photoacoustic measuring cell and a discharge line.

Photoacoustic spectroscopy (PAS) is used very successfully for measurement methods in many areas of gas and particle measurement technology. The measuring gas to be examined (aerosol) is excited to density or pressure oscillations in an acoustically resonant measuring cell by means of an intensity-modulated laser. This sound signal is received by a microphone and converted into an electrical signal. Corresponding systems are used, for example, for determining the mass concentration [mg/m] of soot particles in the exhaust gas of internal-combustion engines.

The acoustic frequency spectrum recorded by the microphone also includes unwanted interfering frequencies from various sources, e.g. flow noise of the inflowing measuring gas, engine noise or various ambient noises and vibrations (structure-borne sound). The sound can take the following paths in particular:Via the feed/discharge lines to the measuring cell as airborne sound or as structure-borne sound, which is transmitted from the walls of the linesVia the surroundings of the measuring cell also as airborne sound or as structure-borne sound, which is transmitted, for example, via the mounting of the measuring cell

For interference suppression, the structure-borne sound at the measuring cell mounting can be suppressed, for example, by the use of attenuators such as rubber buffers, the apparatus housing can be provided with double walls to shield airborne sound coming from the surroundings, and potential internal sources of interference of the apparatus (pumps, fans, etc.) can be stored accordingly (for example, via rubber buffers) or they can be installed in an external housing. Furthermore, the frequency response of the microphone and subsequent amplifier can be selected according to the useful frequency. The interference influences can also be specifically influenced by a suitable material selection.

Despite numerous measures already implemented, there is still a need for additional improvements for interference suppression. In particular, interferences that enter the PAS measuring cell via the airborne sound path via the feed or discharge line represent a major problem. The higher the extraction flow is chosen (which contributes to the improvement of dynamics), the higher the flow noise.

The present invention has the object to effectively suppress interference noises even at high flow velocities and to increase the measuring accuracy and reliability of the measuring device.

SUMMARY OF THE INVENTION

These and further objects are solved by a measuring device of the type mentioned above according to the invention in that the measuring device has at least one acoustic filter member tuned to the useful frequency of the measuring cell, which comprises at least one cavity resonator arranged on the flow channel. Preferably, the interior of the cavity resonator is in connection with the interior of the flow channel. The acoustic filter member effectively eliminates interference frequencies in the range of the useful frequency. The preferred cavity resonators are cavity resonators open on one side towards the flow channel. The desired properties of the acoustic filter can be specifically influenced to one another via the shape, size and, if appropriate, the relative arrangement of individual cavity resonators.

In the context of the present disclosure, “useful frequency” is the frequency that is evaluated for determining the measurement property. In general, this is a resonant frequency of the measuring cell and in general, the useful frequency coincides with the excitation frequency of the laser of the measuring cell. If necessary, a measuring cell can also be operated with different useful frequencies. In measuring devices for determining the mass concentration of soot particles in the exhaust gas of internal-combustion engines, for example, a useful frequency in the range of about 1000 to 12000 Hz has proven to be advantageous, although other frequency ranges can also be used reasonably depending on the respective application.

Advantageously, the axis of the cavity resonator is aligned normally to the axis of the flow channel. This enables a simple construction of the cavity resonator, the acoustic properties of which can be easily determined.

Advantageously, at least one cavity resonator is formed as a double-sided cavity resonator having a first and a second neck portion, wherein the first neck portion is preferably arranged opposite to the second neck portion. This allows the value peaks of the negative sound pressure amplification to be widened in the sound pressure course.

In another advantageous embodiment, the acoustic filter member has a plurality of cavity resonators which enter consecutively into the flow channel. This allows several negative value peaks to be realized at different frequencies in the sound pressure course. The exact position of the value peaks in the frequency course depends essentially on the length of the resonator necks. The consequences that a certain shape of a cavity resonator or a certain arrangement of cavity resonators have on the sound pressure course can be determined by appropriate routine tests. The person skilled in the art is thus able to create suitable acoustic filter members for a given measuring device, provided that he is aware of the teachings of the present disclosure.

Advantageously, at least one acoustic filter member is designed for negative sound pressure amplification in a first frequency range which comprises the fundamental frequency of the useful frequency. In this way, the interference noise is specifically reduced in the frequency field in which the respective measuring cell operates.

In another advantageous embodiment, at least one acoustic filter member is designed for negative sound pressure amplification in at least one second frequency range which comprises a multiple of the fundamental frequency of the useful frequency. By combining a plurality of cavity resonators, one or more frequency ranges can thus be effectively attenuated and suppressed.

In a preferred embodiment, at least one cavity resonator is formed to be cylindrical. A cylindrical cavity resonator is easy to manufacture and can also be easily cleaned. Furthermore, high accuracy can be achieved for the design of the acoustic filter members with relatively simple calculation and simulation models.

Advantageously, at least two cavity resonators are formed cylindrically and have a different length and/or diameter. By the use of several cavity resonators having slightly varying lengths, for example, it is possible to achieve wider but not as deep indentations or negative value peaks in the sound pressure course.

Another advantageous embodiment provides that the length of at least one cavity resonator is adjustable. This allows the properties of the acoustic filter member to be adapted, for example, to changed ambient conditions (e.g. a different pressure, temperature or measuring gas). The length adjustment can be realized in a simple way, for example, by means of appropriate screw connections.

In another advantageous embodiment, the measuring cell is arranged in an airtight housing. Airtight here means that the housing is essentially pressure-tight achieved and can be brought to negative pressure and/or vacuum. For example, a vacuum pump can be provided on the measuring device with which the housing can be evacuated. This attenuates the airborne sound affecting the measuring cell and reduces the interference noise transmitted in this way. According to the technical definition, negative pressure is defined as a pressure level below normal pressure; a pressure level below 300 hPa is referred to as rough vacuum.

In another aspect, the method for solving the problem mentioned at the beginning provides, according to the invention, that at least one acoustic filter member is provided in the flow channel, which is adjusted to a negative sound pressure amplification in a first frequency range, which comprises the fundamental frequency of the useful frequency.

Advantageously, at least one acoustic filter member is provided in the flow channel, which is adjusted to a negative sound pressure amplification in at least one second frequency range, which comprises a multiple of the basic frequency of the useful frequency.

In another advantageous embodiment, the adjusting of the at least one acoustic filter comprises a constructive defining and/or adjusting of at least one or a plurality of the following features of a cavity resonator: Shape, position, length, diameter, length of a cylindrical cavity resonator, diameter of a cylindrical cavity resonator. This allows acoustic filter members to be created in a targeted manner by the use of cavity resonators having known properties, such as Helmholz resonators or similar. This also allows a constructive and computationally simple implementation of the acoustic filter members used in the method.

A further preferred embodiment of the method according to the invention provides that a negative pressure and/or vacuum is generated in a housing which can be brought to negative pressure and/or vacuum and in which the measuring cell is arranged.

