Improved noise canceling detection device

Abstract

The present invention relates to a photoacoustic gas detection device including a gas measurement space and a gas reference space. The spaces are separated by at least one flexible membrane that responds to a pressure difference between the spaces. The detection device also includes a light source that emits light within a predetermined wavelength range into the measurement space. The wavelength range is selected based on the absorption spectrum of the gas to be measured. The gas detection device also includes a measurement unit for measuring the displacement of the membrane. The measurement space and the reference space together define a specific center of mass. The location of the center of mass is selected so that the pressure on the membrane is balanced when the membrane is displaced in at least one predetermined first direction.

Description

[Technical Field]

[0001] The present invention relates to an optical gas detection device for detecting gases that absorb light at specific wavelengths. [Background technology]

[0002] In known absorption-based gas sensors, light is transmitted through a gas mixture to a detection device, and specific gases absorb light at characteristic wavelengths, allowing the concentration of the gas to be determined by knowing both the transmission spectrum and the detection spectrum. However, measuring the spectrum of transmitted light is a complex process that is difficult to incorporate into small, low-cost devices such as those used to measure alcohol concentration.

[0003] Alternative solutions have been developed to exploit absorption of specific wavelengths in gases, which can cause, for example, temperature increases or pressure fluctuations in the gas. Bruel and Kjar's photoacoustic gas detector (U.S. Pat. No. 4,818,882) demonstrated the ability to detect very low levels of gas. This concept has been further developed, for example, in International Publications WO 2017 / 055219 and WO 2017 / 089624, which describe solutions using symmetrically arranged membranes to reduce noise sensitivity. Breguet J et al., "Photoacoustic detection of trace gases with an optical microphone" (Sensors and Actuators A: Physical, Elsevier BV, NL, Vol. 48, No. 1, May 1, 1995), describes the use of two membranes, one of which contains an optical fiber sensor. JP 01-277000 A describes a solution using two symmetrically arranged microphones connected to a sealed container. Other examples of photoacoustic sensors are described in EP 1546684B1, EP 3483589, U.S. Pat. No. 9360417, and U.S. Pat. No. 7245380, as well as Kuusela et al., "Photoacoustic Gas Analysis Using Interferometric Cantilever Microphone," Applied Science Spectroscopy Reviews, 42:5, 443-474, 2007, DOI: 10.1080 / 00102200701421755, and de Paula et al., "Optical microphone for photoacoustic spectroscopy," Journal of applied physics 64, 3722 / 1988, DOI: 10.1063 / 1.341416.

[0004] Typically, in known solutions, a gas volume is illuminated by pulsed electromagnetic radiation having a specific wavelength corresponding to the absorption wavelength of the gas to be detected. The gas is illuminated by directing a light beam having an appropriate wavelength and beam shape towards the gas volume. If gas is present, each pulse heats the gas and generates a pressure wave, which exerts a force on one or more membranes, beams, doors or similar elements, allowing the measurement to be performed. The sensitivity depends on the concentration of the gas and the geometry of the system connecting the illumination to the gas volume. The object of the present invention is to provide a solution that improves the sensitivity of the measurement.

[0005] As described in International Publication No. WO 2017 / 055219, arranging two symmetrically arranged membranes side by side can reduce vibration sensitivity to acceleration perpendicular to the membrane surfaces. However, as described in International Publication No. WO 2017 / 089624, when this is incorporated into an enclosed space, the vibration sensitivity becomes highly dependent on the size and shape of these spaces (e.g., the reference space and the measurement space). Therefore, an object of the present invention is to provide a stable photoacoustic device that reduces the effects of vibration and noise from the surrounding environment. Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to provide a small, low-cost photoacoustic gas sensor that is low in sensitivity to external vibrations and capable of high-sensitivity gas detection. This object is achieved by an optical gas detection device having the features set forth in the appended claims. [Means for solving the problem]

[0007] The present invention is therefore based on two principles for reducing the effects of vibrations and movements in photoacoustic gas detection devices, where the reference space and the gas space are balanced so that equal pressures act on both sides of the membrane even if movements cause pressure fluctuations in these gas spaces. Furthermore, when two identical membranes are used, the principles described in WO 2017 / 055219 and WO 2017 / 089624 can be used for such movements.

[0008] This is achieved by balancing the center of mass of the gas space. TIFF2026501450000002.tif8170 and center of mass in space V with total mass M The definition of TIFF2026501450000003.tif8170 is as follows: TIFF2026501450000004.tif13170

[0009] If the mass density is uniform, such as a gas in the volume of the sensor, this can be simplified to: TIFF2026501450000005.tif12170Here, the integral TIFF2026501450000006.tif10170 represents the centroid or geometric center of space V. It is therefore useful to refer to the center of mass of an air space, and if the density of air is uniform, the center of mass is defined by the shape of that space.

[0010] The aforementioned article by Kuusela et al. describes how acceleration affects pressure in a space (Kuusela, Equation 62, item 469). TIFF2026501450000007.tif9170Here, p(x) is the pressure at distance x from the center of mass in the direction of acceleration, p0 is the equilibrium pressure, ρ gasis the mass density of the gas, and a is the acceleration of the space. This shows that if the center of mass of the gas is aligned on the inside and outside of the membrane, the pressure change due to acceleration will be equal on both sides of the membrane. Therefore, the net force acting on the membrane is zero, making it insensitive to vibrations or accelerations. The geometry of the space can be balanced according to acceleration in one or more directions.

[0011] Therefore, preferred embodiments of the present invention provide a gas detection device that can balance the shape and location of the gas space to provide noise cancellation in three dimensions without limiting the size of the measurement or reference space. The present invention also provides a highly efficient photoacoustic measurement space. [Brief explanation of the drawings]

[0012] The invention will now be described with reference to the accompanying drawings, which show, by way of example, the invention. [Figure 1a] 1 is a diagram schematically illustrating an embodiment of a gas detection device according to the present invention. [Figure 1b] FIG. 10 is a diagram schematically illustrating another embodiment of a gas detection device according to the present invention. [Figure 2] FIG. 1 shows a measurement cell according to a preferred embodiment of the present invention. [Figure 3] 1 shows a preferred embodiment of a gas detection device according to the present invention; [Figure 4] FIG. 4 is a bottom view of the embodiment shown in FIG. 3. DETAILED DESCRIPTION OF THE INVENTION

[0013] As shown in FIG. 1a, the present invention relates to a photoacoustic gas detection device 5. The gas detection device 5 includes a measurement cell or measurement space 1 and an interrogation unit 2. The interrogation unit 2 responds to pressure fluctuations in the measurement space 1 due to gas freely passing between the measurement space 1 and the interrogation unit 2. The interrogation space 1 is composed of one or two membranes 2a and 2b, and preferably includes an optical interrogation unit 6 that measures pressure fluctuations in the measurement space 1 based on the distance or movement between the membranes, as described, for example, in WO 2017 / 055219 and WO 2017 / 089624. In the illustrated example, the interrogation unit is an optical sensor 6, and a light source 7, preferably a laser or a sufficiently coherent light source, is located opposite the sensor 6. The interrogation unit can measure fluctuations in the distance between the membranes 2a and 2b caused by interference between the membranes 2a and 2b. Other measurement means capable of measuring pressure fluctuations are also conceivable, for example where at least one of the membranes is an optical microphone, but to reduce sensitivity to vibrations in the x-direction perpendicular to the surface, symmetrical solutions such as those described in WO 2017 / 055219 and WO 2017 / 089624 are preferred.

[0014] Outside the reading space 2, reference spaces 3a, 3b are provided. Preferably, the reference spaces 3a, 3b are connected by a channel 3c. This provides a single space with equal pressure on both sides of the reading space. Preferably, the channel 3c should be sufficient to allow free gas movement within the reference spaces 3a, 3b and to avoid the accumulation of pressure fluctuations due to movement perpendicular to the membranes 2a, 2b.

[0015] The drawing shows a cross section of the present invention in the xy plane, with the z-axis extending out of the plane of the drawing. Preferably, the yz plane between surfaces 2a and 2b and the xy plane perpendicular to surfaces 2a and 2b are symmetrically arranged planes across the entire space 1, 2, 3a, and 3b of the gas detection device unit of the present invention. In this way, vibration or movement in the x direction perpendicular to the surfaces does not affect the relative movement or distance between surfaces 2a and 2b, as described in WO 2017 / 055219. Furthermore, no pressure gradients are generated at the center of the surfaces due to vibration or movement in the z direction parallel to surfaces 2a and 2b.

[0016] Additionally, the y coordinates of the centers of mass CM of the gas in the reference spaces 3a, 3b and in the measurement space 1, including the reading space 2, must be equal, so that any movement or vibration in the y direction does not create a pressure gradient beyond the center of the surfaces 2a, 2b.

[0017] Figure 1b shows an alternative embodiment that corresponds to the embodiment of Figure 1a, except for the structure of the reading unit 2. In Figure 1b, the reading unit 2 consists of one movable membrane 8 and a reflector 9. The light source 7 and the sensor 6 are arranged on opposite sides of the measurement space 1 and the reading unit 2. As shown in Figure 1a, the back spaces 3a, 3b and the measurement space 1 are essentially symmetrical in the x and z directions, but since the membrane 8 is sensitive to movements in the y direction, it is necessary to adjust the y coordinate of the center of mass CM of the measurement space or back space.

[0018] 2 shows a preferred embodiment of a measuring cell that can be preferably used in the present invention, where the measuring cell has a selected radius r cell and height h cell It has a disk shape having

[0019] As shown in the figure, the light source 4 is located within a radius r of the wall that defines the circumference of the disk-shaped space 1. source This space is placed at the opening of the cell and height r cellThe light source is selected according to the absorption wavelength of the gas to be detected and is attached to the opening in the wall. Preferably, the inner wall of the cell is coated with a reflective surface, such as Au, to increase the light intensity in the relevant wavelength range within the cell and increase the efficiency of light absorption. A disk shape is also preferred to maximize the light distribution within the cell. The size r of the opening and the light source 4 source should be minimized to reduce losses.

[0020] Figure 3 shows an embodiment of the invention including openings 11, 13a, 13b from the ambient environment into the reference and measurement spaces 1, 3a, 3b. Gas from the ambient environment may diffuse or migrate into these spaces. Periodic pressure fluctuations occur within the measurement spaces. This causes a pressure P between surfaces 2a, 2b. i and the pressure P in the reference spaces 3a and 3b o The pressure fluctuations can be generated by focusing a light source having an appropriate wavelength spectrum into the measurement volume, pulsing the light source, or preferably by tuning the central wavelength of the light source to be in or out of the absorption spectrum of the gas in the volume.

[0021] Preferably, the openings 11, 13a, and 13b are provided with a sintered filter 14. The sintered filter 14 functions as an acoustic low-pass filter, thereby preventing the external pressure P o and the internal pressure P in the reading space 2 i While equalizing the acoustic noise P noise Furthermore, if the reference space includes two equal and symmetrically arranged parts 3a, 3b, the acoustic resistance R of the sintered filter connected to the different spaces 1, 2, 3a, 3b can be reduced. i , R o corresponds to the above space as follows: TIFF2026501450000008.tif14170This causes the pressure changes due to acoustic noise to be the same both inside and outside the reading space 2, effectively canceling out the acoustic noise.

[0022] FIG. 4 is a bottom view of the gas detection device, showing the relative sizes of filters 11, 13a, 13b.

[0023] In summary, the present invention relates to a photoacoustic gas detection device that includes a gas measurement space and a gas reference space. The spaces are separated by at least one flexible membrane that responds to a pressure difference between the spaces. The detection device also includes a light source that emits light within a predetermined wavelength range into the measurement space. The wavelength range is selected based on the absorption spectrum of the gas to be measured. Absorption increases the temperature and therefore the pressure in the measurement space, resulting in membrane movement. The gas detection device also includes a measurement unit for measuring the membrane movement.

[0024] The measurement space and the reference space together define a particular center of mass, the location of which is selected to balance the pressure on the membrane when the membrane is moved in at least one predetermined first direction, such that the pressure fluctuations on both sides of the membrane are the same when moved in the first direction within the detection device.

[0025] Preferably, the gas measurement space 1 is a gas measuring chamber of a given radius r enclosed in a vessel having a reflective inner surface, preferably coated with Au. cell and height h cell The detector is composed of a disk-shaped space having a diameter of 1 / 4. A light source 4 is attached to the circumference of the disk and emits light into the space. In this way, the light is distributed throughout the space, improving the coverage and sensitivity of the detection device.

[0026] Thus, the location of the center of mass may be selected such that the pressure on the membrane is balanced when the membrane moves in a predetermined first direction and a second direction perpendicular to the first direction.

[0027] The reference space may be composed of two spaces arranged symmetrically with respect to the membrane that communicate with each other via a channel between the spaces on either side of the membrane.

[0028] According to a preferred embodiment, the detection device comprises two membranes, the measurement space comprising a reading space separating the membranes, the two membranes being symmetrically arranged along an axis perpendicular to a first direction perpendicular to a plane defined by the surfaces of the membranes.

[0029] The measurement cell and the reference space have openings to the ambient environment to equalize the pressure and allow gas to enter the spaces. The reference space may contain the same gas as the measurement space, the only difference being that only the gas in the measurement space absorbs the light emitted by the light source.

[0030] The reference space V0 may be composed of two connected spaces, where each of the reference spaces has an opening area Ab and the measurement space has an opening area Am. If the total volume of the measurement spaces is V i When R is the opening size in the reference space and the measurement space, preferably with a sintered filter, i / R0=V0 / V i where R i and R0 are the respective spaces V i and V0, and the effect of the sintered filter is also taken into account.

Claims

1. 1. A photoacoustic gas detection apparatus including a gas measurement space and a gas reference space, the gas measurement space and the gas reference space being separated by at least one flexible membrane that is responsive to a pressure difference between the gas measurement space and the gas reference space, the gas detection apparatus further comprising a light source that emits light in a predetermined wavelength range into the gas measurement space, the wavelength range being selected based on an absorption spectrum of a gas to be measured, the gas detection apparatus further comprising a measurement unit that measures the movement of the membrane; the gas measurement space and the gas reference space together define a particular center of mass, the location of the center of mass being selected such that pressure on the membrane is balanced when the membrane is moved in at least one predetermined first direction; Gas detection equipment.

2. 2. The gas detection device according to claim 1, wherein the gas measurement space is a disk-shaped space enclosed in a container having a reflective inner surface, and the light source is attached to a circumference of the disk-shaped space and radiates light into the disk-shaped space.

3. 2. The gas detection device of claim 1, wherein the location of the center of mass is selected so that pressure on the membrane is balanced when the membrane moves in the predetermined first direction and in a second direction perpendicular to the first direction.

4. 3. The gas detection apparatus of claim 2, wherein the reflective inner surface is coated with Au.

5. 2. The gas detection device according to claim 1, wherein the gas reference space is composed of two spaces arranged symmetrically with respect to the membrane and communicating with each other via a channel.

6. 2. The gas detection device of claim 1, wherein the gas detection device comprises two membranes, the gas measurement space includes a reading space separating the two membranes, and the two membranes are arranged symmetrically along an axis perpendicular to the first direction.

7. 2. The gas detection device of claim 1, wherein the gas measuring cell and the gas reference space have openings that open to the ambient environment for pressure equalization.

8. 8. The gas detection device according to claim 7, wherein the opening is provided with a sintered filter.

9. The gas reference space V ０ is composed of two connected spaces, each of which has an opening area Ab, the gas measurement space has an opening area Am, and the total volume of the gas measurement spaces is V ｉ When the gas reference space and the gas measurement space V ｉ , V ０ The size of the opening in the ｉ , R ０ is selected to provide R ｉ / R ０ =V ０ / V ｉ 9. The gas detection device according to claim 7, wherein the temperature is selected so as to satisfy the following expression.

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