WO2013009241A1

WO2013009241A1 - New detection method and detector

Info

Publication number: WO2013009241A1
Application number: PCT/SE2012/050768
Authority: WO
Inventors: Vasile-Mihai MECEA
Original assignee: G&M Norden Ab
Priority date: 2011-07-13
Filing date: 2012-07-03
Publication date: 2013-01-17

Abstract

The present invention relates to a real-time method and detector for detection of airborne hard particles, based on the Quartz Crystal Microbalance (QCM) technique. With the invention the particles are detected by employing their capacity to erode an operable part of a QCM arrangement.

Description

NEW DETECTION METHOD AND DETECTOR

Field of invention

The present invention is related to an airborne device that can reveal the presence of a volcanic ash cloud or sandstorm. Based on the Quartz Crystal Microbalance (QCM) technique it detects the erosion produced by the kinetic energy of the hard particles of the volcanic ash or sand. Thus, it provides a real-time, direct information of the adverse effect of the volcanic ash or sand on the airplane body, windows and engines.

Background of the invention

It is known the adverse effect of the volkanic ash and sand on the body, windows and jet engines of the airplanes.

Quartz Crystal Microbalance (QCM) is a very sensitive mass measuring device. The very high mass sensitivity of 10^"9 g to 10^"12 g is explained by the very high intensity of the harmonic inertial field, developed at the surface of the quartz resonator during its vibration. This ideea was developed in the article„Is quartz crystal microbalance really a mass sensor?", published in„Sensors and Actuators A, 128 (2006) 270-277".

As a real-time mass sensing device, QCM was largely used as a particle detector.

US Pat. Nr. 4041768 describes a particle detector based o the QCM technique, where the deposition of the particles, contained in the ambient air, is produced using electrostatic precipitation. US Pat. Nr. 5892141 describes a detector for particles contained in the exhaust of the vehicle engines. Here, particles, mainly soot, are deposited on the surface of a catalyst coated quartz crystal resonator. The deposition is performed using electrostatic precipitation. The analysis of the particles is performed by heating the crystal in an oxidizing atmosphere to oxidize the deposited particles. US Pat. Nr. 6510727 B2 describes a particle detector, based on the QCM technique, where the deposition of the particle is performed by impaction and a movable aperture is used to distribute the particles evenly on the sensitive area of the quartz resonator. US Pat. Nr. 4294105 describes a particle detector, based on the QCM technique, where the particles are entrapped by a mat-like deposition on the quartz surface.

US Pat. No. 3715911 describes a particle detector, based on the QCM technique, where the particles are captured by the sensing quartz crystal coated with a sticky material. US Pat. Nr. 7168292 B2 describes a miniaturized system for particle exposure measurement. It is based on the QCM technique and thermophoresis is used for the particle deposition on the surface of the quartz resonator.

The conventional quartz crystal microbalance collect the particulate matter on the surface of the quartz resonator using electrostatic precipitation, thermophoresis, or jet-to plate impaction, either simple, or in combination with a sticky layer. However, because of the low adhesion of the particle matter to the quartz crystal surface, only a small total accumulated mass capacity is possible. Therefore, the quartz crystal requires frequent crystal cleaning and surface restoration. Such requirements for cleaning and resurfacing are impractical for the use in the field. It is therefore apparent that there is need for improvements for determination levels of air borne particles with the QCM technologies and adaptations to the specific needs of direct, real time measurements of potentially harmful levels of particles appearing in airstreams.

Summary of the invention The present invention generally employs the QCM technique in a reversed mode. Instead of the deposition of the particulate matter on the quartz resonator surface, the erosion effect of the hard particles, for example contained in volcanic ash and sand, is employed. Hard particles are represented by silicon dioxide and aluminium oxide particles. These particles represent about 70% of the total particles contained in the volcanic ashes. In a first general aspect, the invention relates to a real-time method of detecting airborne solid, hard particles. As a first the step, the method provides at least one Quartz Crystal

Microbalance (QCM) arrangement including at least one oscillating quartz crystal with attached electrodes. The method further includes subjecting the QCM arrangement to an airstream including solid particles, while kinetically energized particles are admitted to eroding at least one electrode. The eroded electrode mass is assessed and thereby directly or indirectly it can be determined if the electrode mass loss relates to a particle density harmful to a part of an airborne vehicle. The method so described also provides for a determination of the particle density in an airstream containing solid hard particle with a general capacity of eroding solid materials. Preferably, the described QCM arrangement comprises a first operating oscillating quartz crystal with attached electrodes adapted to be impacted by the particle carrying airstream and a second reference oscillating quartz crystal with attached electrodes adapted to be shielded from exposure of the particle carrying airstream. In this respect "QCM arrangements" described useful herein are based on oscillating quartz crystals, however the person skilled in this field would realize that the term "quartz crystal" be held equivalent with crystals of other materials such as langasite, langatite, gallium phosphate, lithium niobate and the similar. Likewise, methods and equipments to bring QCM arrangements into a suitable oscillation described with the present invention are also well known by the skilled person.

Also in general terms, the present invention includes measuring the frequency of the operating crystal and the reference crystal and such measured value serve as a general basis for calculating and optionally displaying a real erosion mass loss value or the decrease in electrode thickness; and calculating and optionally displaying any of the rate of electrode mass decrease or the rate of electrode thickness decrease. Such calculations of mass changes are differently embodied with the present invention. According to a first embodiment, the inventive method comprises separately calculating the electrode mass loss of the operating crystal and the apparent mass change of the reference crystal, as a result of the temperature change, and calculating the erosion mass value.

According to another embodiment, the inventive method comprises measuring the difference of the frequencies and computing a reconstructed frequency value for calculating a real erosion mass value.

It is preferable with the present invention to calculate the so discussed mass changes with an equation derived from the Energy Transfer Model, as explained in more detail in the exemplifying part of this specification.

According to a yet another embodiment, the inventive method comprises measuring the difference of the frequencies, converting the frequency difference to a voltage, comparing obtained voltage values to predetermined operating electrode mass loss values and calculating the decrease in electrode thickness and the rate of the electrode thickness .

The method can further comprise a step of increasing the velocity of the particle carrying airstream to obtain sufficiently kinetically energized particles.

Also preferred with the method is an operating quartz crystal with an electrode directly exposed to the particle carrying airstream and that is made from a conductive material capable of being eroded when subjected to kinetically energized particles, having a size range about 1 to 200 microns and comprising silicon dioxide, but not limited to this. In general terms, the electrode material is adapted to the kinetic energy of the particles to become determined and it is conceivable within the inventive context to construe different eroding electrodes adapted to the nature of the particles to determine. It is also conceivable within all contexts of the present invention to obtain the electrode erosion with an erosion layer on the electrode which, not necessarily needs to be conductive, but is designed to generate a detectable mass loss when subjected to the hard energized particles. As an example, such an erosion layer may be coated on an electrode surface intended to face the airstream to be assessed.

In another general aspect, the present invention relates to a detector capable of detecting the presence of airborne solid, hard particles. The detector is generally suitable for performing any of the earlier described methods.

The detector includes a sensor generally comprising a first operating oscillating quartz crystal with attached electrodes adapted to be impacted by the particle carrying airstream and a second reference oscillating quartz crystal with attached electrodes adapted to be shielded from exposure of the particle carrying airstream. The electrode of the first operating crystal erodes by the impact of the particle and the detector is capable of determining the loss of mass resulting from the electrode erosion. Preferably, the first and second crystal have matched frequency to temperature characteristics.

Preferably, the sensor has an aperture directing a particle carrying airstream to an electrode of the first operating crystal.

Also preferably, the sensor has an air pumping mechanism for accelerating ambient air to a particle carrying airstream with so the particles become sufficiently kinetically energized The erodible electrode of the first oscillating crystal preferably is made from a conductive material capable of being eroded when subjected to kinetically energized particles having a size range about 1 to 200 microns and comprising silicon dioxide, or other hard particles. Alternatively, as earlier described the erodible electrode is made of a conductive material having an erosion layer adapted to be impacted by the particle containing airstream to be assessed.

Accordingly, the skilled person can envisage a number of designs and materials for electrodes. By way of a non-limiting example the erodible electrode comprises gold and the airstream comprises particles from volcanic ash or sand having a velocity exceeding 200 km/h

Among many advantages as detectors for airborne particles, the presently invented detector is useful on an aircraft or an airborne vehicle for detecting the level of particularly harmful particles arriving from volcanic ash or sand which may have an immediately damaging impact on such vehicles. Skilled artisans may readily cutline other advantageous applications of the invention as it conveniently in real-time give an accurate readout of solid airborne particles.

Brief description of the drawings

Fig. l . is a schematic view of the airborne detector for volcanic ash and sand.

Fig.2. is a schematic view of the first alternative (A) for the measurement of the volcanic ash and sand. Fig.3. is a schematic view of the second alternative (B) for the measurement of the volcanic ash and sand.

Fig.4. is a schematic view of the third alternative (C) for the measurement of the volcanic ash and sand.

Fig.5. is schematic view of the test system used to reveal the functionality of the airborne detector for volcanic ash and sand.

Fig.6. is a graph illustrating the experimental eroded mass produced by the impact of the volcanic ash collected from Eyjafjallajokull in Island during April-May 2010. Fig.7. is a graph illustrating the response of the quartz sensor during an experiment performed in the wind tunnel.

Description of the preferred embodiment

In the article "Characterization of Eyjafjallajokull volcanic ash particles and a protocol for rapid risk assessment" published in "PNAS, May 3, 2011, Vol. 108, Nr. 18, pag. 7307-7312" is revealed that the volcanic ash consists of 58% silica, which is a hard material. In addition the volcanic may contain about 15 % aluminium oxide, which also is a hard material. The impact of such particulate matter with the metallic electrode of the quartz resonator will erode the metallic electrode, reducing its mass and causing an increase of the frequency of the quartz resonator. The metallic electrode used with the examples of the present invention has a good adhesion to the quartz surface and can be several tens of microns in thickness, which is significantly larger than conventionally used electrodes. With the present studies a gold-based electrode is used, but many other materials are conceivable for the present applications. This provides a long life of the quartz sensor and a large total accumulated response

Where The mass of the electrode eroded by the impact of the hard particles can be calculated from the frequency change of the exposed quartz resonator using various approximative equations. As the mass removed by erosion is quite large, it is preferably calculated using the equation derived from the Energy Transfer Model, presented in the article "Loaded vibrating quartz sensors" published in "Sensors and Actuators A, 40 (1994) 1-27", hereby incorporated as a reference, in order to obtain a preferred accuracy that is not limited by the thickness of the quartz electrode.

The preferred equation, referred as the equation derived from the Energy Transfer Model is: wherein f_q is the frequency of the quartz resonator before erosion, f_c is the frequency of the quartz resonator after erosion, rrif is the mass of the electrode eroded by the hard particles and m_q is the mass of the quartz resonator.

The adverse effect of the temperature variation on the frequency is greatly reduced by performing differential measurements. This arrangement consists of a measuring crystal exposed to the erosion of the volcanic ash or sand and a reference crystal that is not exposed to erosion, but is subjected to the same temperature changes.

Three different measuring systems are proposed. The flow charts for these systems are illustrated in Fig.2 to Fig.4. Appropriate computer software programs calculate the measuring crystal electrode mass loss, produced by erosion, the diminishing of the measuring crystal electrode thickness and the rates of the electrode mass and thickness change. In this way information about the particle density in the volcanic ash cloud or sand is obtained.

As shown in Fig. l, the detector consists of a sensing head 1 and a housing 4. Inside the sensing head 1 are located the measuring crystal 2 and the reference crystal 3. They are close in frequency and have matched frequency to temperature characteristics. An aperture in front of the measuring crystal electrode allows for the impact of the volcanic ash particles or sand during the flight. The housing 4 contains the electronic circuitry 5.

Fig.2. is a schematic view of an alternative A for the measurement of the volcanic ash and sand. The measuring crystal and the reference crystal are maintained in vibration using appropriate oscillators. The frequency of the measuring crystal F_M and the frequency of the reference crystal F_R are measured independently with appropriate frequency counters. The frequency of the measuring crystal will change as a result of the electrode erosion and temperature change. The frequency of the reference crystal will change only as a result of the temperature change, with the same amount the temperature has influenced the frequency of the measuring crystal. Thus, a total mass response M_t can be calculated using the Energy Transfer Model. The total mass response comprises the real mass response M_r, due to erosion, and the apparent mass response M_a, due to the temperature change. The reference crystal will measure only the apparent mass M_a, due to the temperature change. Out of the real mass M_r is calculated the decrease of the electrode thickness by erosion. Finally, the rate of both real mass decrease and the thickness decrease are calculated in order to indicate the particle density of the volcanic ash cloud or sand.

Fig.3. is a schematic view of an alternative B for the measurement of the volcanic ash and sand. Here, the frequencies F_M and F_R from the two crystals are mixed together and the difference AF is obtained. The frequency difference is measured by a single frequency counter. It is not dependent on the temperature when the two crystals have similar frequency to temperature characteristics. However, the frequency difference will change as a result of the electrode erosion. In order to calculate the mass loss caused by erosion it is necessary to reconstruct the actual value of the measuring frequency. This can be accomplished by adding the current value of the frequency difference AF to the known initial value of the reference frequency F_R. From the current value of the reconstructed measuring frequency FMR it is possible to calculate the real eroded mass M_r, using the equation based on the Energy Transfer Model. The same model is used to calculate the decrease in electrode thickness and, further, the rate of the mass and electrode thickness decrease. Fig.4. is a schematic view of an alternative C of the volcanic ash and sand detector. Here the difference in frequency between the two crystals is converted into a voltage, using a frequency-to- voltage converter. The resulting voltage is related to the mass eroded from the measuring crystal electrode. This mass is further converted into a decrease of the electrode thickness. Further, the rate of erosion and the rate of the electrode thickness decrease is obtained using a derivative system. This alternative is appropriate for an analog output signal.

Fig.5. is a schematic illustration of the test system used to reveal the functionality of the airborne detector for volcanic ash and sand. The ambient air is sucked through a funnel. In front of the funnel is placed a wind velocity measuring device. An air duct allow the introduction of short pulses of air containing volcanic ash, sand or other types of hard particles, like aluminium oxide. A vacuum pump is used to suck air from the ambient. The funnel is directing the air jet, with a certain velocity, towards the sensing crystal. When a short pulse of air, containing volcanic ash or other particles, is injected into the constant airflow, an increase in the frequency of the sensing crystal is recorded and the mass of the eroded electrode material is calculated using the equation derived from the Energy Transfer Model. The result of a typical test is illustrated in Fig.6. Fig.7. is an illustration of an experiment in the wind tunnel where the silver electrode of the quartz sensor was eroded by aluminium oxide particles with a size of 9 micron at an air speed of 590 km/h. The concentration of these particles was 1.12 mg/m³, lower than the

concentration range of the volcanic ash of 2 mg/m³ to 4 mg/m³ admitted, with restrictions, for the flight of the airplanes.

Claims

1. A real-time method of detecting airborne solid, hard particles comprising:

(a) providing at least one quartz crystal microbalance (QCM) arrangement comprising at least one oscillating quartz crystal with attached electrodes;

(b) subjecting the QCM arrangement to an airstream comprising solid particles while admitting kinetically energized particles eroding at least one electrode;

(c) assessing an eroded electrode mass as a representation of an eroding amount of

particles present in the airstream.

2. A method according to claim 1 comprising determining from the eroded electrode mass the density of particles present in the airstream.

3. A method according to claim 1 or 2, comprising determining if the eroded electrode mass or determined density of particles is harmful to part of an airborne vehicle.

4. A method according to any previous claim, wherein the QCM arrangement comprises a first operating, measuring, oscillating quartz crystal with attached electrodes adapted to be impacted by the particle carrying airstream and a second reference oscillating quartz crystal with attached electrodes adapted to be shielded from exposure of the particle carrying airstream.

5. A method according to claim 4, comprising measuring the frequency of the operating crystal and the reference crystal.

6. A method according to claim 5, comprising calculating the eroded mass and the

change in the electrode thickness using the equation derived from the Energy Transfer Model, and optionally displaying a real erosion mass loss value or the decrease in electrode thickness; and calculating and optionally displaying any of the rate of electrode mass decrease or the rate of electrode thickness decrease.

7. A method according to any of claims 6, comprising separately calculating the

electrode mass loss of the operating crystal and the mass change of the reference crystal and calculating the erosion mass value using the equation derived from the Energy Transfer Model (Fig. 2).

8. A method according to claim 7, comprising the steps of: independently measuring the frequency of the operating crystal and the reference crystal; calculating the total mass response from the frequency of the operating crystal by the using the equation form the Energy Transfer Model, and calculating the apparent mass response from the frequency of the reference crystal; calculating the real eroded mass from the total mass response and the apparent mass response and the corresponding decrease in electrode thickness from erosion; and calculating at least one of the rate of the decrease of the electrode thickness or the rate of the decrease of the electrode mass. (Fig. 2)

9. A method according to claim 5 or 6, comprising measuring the difference of the

frequencies between the frequency of the operating crystal and the frequency of the reference crystal. (Fig. 3).

10. A method according to claim 9, comprising the steps of: measuring the frequency difference with a single frequency counter; reconstructing a present value of the frequency by adding the present frequency difference to the known initial value of frequency of the reference crystal; and calculating the real eroded mass from the reconstructed frequency value by using the equation derived from the Energy Transfer Model. (Fig. 3)

11. A method according to claim 10, comprising calculating at least one of the decrease in electrode thickness and the rate of electrode thickness decrease.

12. A method according to claim 9, comprising, converting the frequency difference to a voltage, comparing obtained voltage values to predetermined operating electrode mass loss values and calculating the decrease in electrode thickness and the rate of the electrode thickness. (Fig.4)

13. A method according to any previous claim further comprising increasing the velocity of the particle carrying airstream.

14. A method according to any of claim 3 to 13 wherein the operating quartz crystal has an electrode directly exposed to the particle carrying airstream and is made from a conductive material, capable of being eroded when subjected to kinetically energized particles having a size range from 1 to 200 microns and comprising silicon dioxide, or other hard particles.

15. A method according to any of claims 3 to 13, wherein the operating quartz crystal has an electrode directly exposed to the particle carrying airstream coated with a layer which is capable of being eroded when subjected to kinetically energized particles having a size range from 1 to 200 microns and comprising silicon dioxide, but not limited to this.

16. An airborne volcanic ash and sand detector, based on the QCM technique, where the detection is accomplished by the erosion of the measuring crystal electrode, as a result of the impact of the volcanic ash or sand particles with a high kinetic energy.

17. A detector capable of detecting the presence of airborne solid, hard particles according to claim 16, suitable for performing any of the methods according to claims 3 to 15 including a sensor comprising:

(a) a first operating oscillating quartz crystal with attached electrodes adapted to be impacted by the particle carrying airstream and

(b) a second reference oscillating quartz crystal with attached electrodes adapted to be shielded from exposure of the particle carrying airstream, wherein an electrode of the first operating crystal erodes by the impact of the airborne particles and the detector is capable of determining the loss of mass resulting from the electrode erosion.

18. A detector according to claim 17, wherein the sensor has an aperture directing a

particle carrying airstream to an electrode of the first operating crystal.

19. A detector according to claim 17 or 18, comprising air pumping mechanism for

accelerating ambient air to a particle carrying airstream whereby the particles obtain a sufficient kinetic energy to erode the electrode of the operating crystal.

20. A detector according to any of claims 17 to 19, performing thermo-compensation, preferably by admitting the first and second crystal to have matched frequency to temperature characteristics.

21. A detector according to any of claims 17 to 20, wherein the erodible electrode is made from a conductive material capable of being eroded or comprising a surface layer capable of being eroded when subjected to kinetically energized particles, having a size range of 1 to 200 microns and comprising silicon dioxide, or other hard particles.

22. A detector according to any of claims 17 to 20, wherein the erodible electrode

comprises gold and the airstream comprises particles from volcanic ash or sand.

23. The use of a detector according to any of claims 16 to 22 on an aircraft or an airborne vehicle for detecting the level of airborne solid particles.