Patent Application
20250164393
The present disclosure relates generally to absorption spectroscopy, and more particularly, to an optical extinction analyzer (OEA) with continuous airflow sampling.
In known methods of measuring optical extinction (also referred to as optical loss) using cavity ringdown spectroscopy, a baseline measurement of optical loss is obtained while an optical cavity of an optical cell is filled with filtered air. A sample measurement of optical loss is then obtained by bringing unfiltered sample air into the optical cavity using a sampling fan. Repeated baseline measurements (while the optical cavity is filled with filtered air) and sample measurements (while the optical cavity is filled with unfiltered sample air) are obtained by cycling the sampling fan on and off.
With reference to
With this known analyzer, baseline measurements are obtained while the sampling fan is off and sample measurements are obtained while the fan is on. Due to the on/off cycling of the sampling fan, baseline measurements are obtained under different airflow conditions, e.g., flow rate (also referred to as velocity) and flow path, than the sample measurements. Furthermore, because the filtered air and unfiltered sample air can have different temperatures, the baseline measurements and sample measurements may be obtained under different temperature conditions. More specifically, when the sample fan is off, only filtered air having a temperature equal to the temperature of the filtered air source is input to the optical cavity through the filtered air inputs. The filtered air moves along the surface of the mirror at a flow rate and into a filtered-airflow path along the longitudinal axis of the optical cavity. When the sample fan is on, unfiltered sample air having a temperature different than the filtered air is brought into the optical cavity by the sample fan at a flow rate different than the flow rate of the filtered air. The sample air moves along a sample-airflow path transverse to the filtered-airflow path of the filtered air, which continues to be input through the filtered air inputs.
Due to the transverse airflow conditions of the filtered air and the sample air, the different flow rates of the filtered air and the sample air, and the different temperatures of the filtered air and the sample air, air having different temperatures are mixed in the region of space in the cavity where the laser beam is propagating. When the laser beam encounters the region of mixed temperature and airflow, differential refractivity of the laser beam results, e.g., the laser beam goes off-axis. The differential refractivity resulting from the different airflow conditions and temperature conditions can contribute inaccuracies in the optical extinction measurements on the order of 1-30 Mm−1.
Thus, there is a need for an optical extinction analyzer that substantially reduces or eliminates inaccuracies in the optical extinction measurements due to differential refractivity of the laser beam.
In one aspect, the disclosure relates to a system for measuring optical extinction. The system includes an optical cell, a duct system, a sampling fan, and a controller. The optical cell has an optical cavity, a sample inlet, and a sample outlet. The duct system has an interior that is in fluid communication with the optical cavity through the sample inlet of the optical cell. The duct system includes an unfiltered inlet, and a filtered inlet with a filter and an inline duct fan that is arranged to draw a sample from a sample source and force the sample through the filter. The sampling fan is associated with the sample outlet. The controller is configured to cycle the inline duct fan on and off while the sampling fan is on to thereby cycle filtered samples and unfiltered samples through the optical cavity.
In another aspect, the disclosure relates to a method of measuring optical extinction. The method includes continuously drawing air from a duct system into and through an optical cavity under same airflow conditions, during a measurement period that includes alternating first times and second times. The method further includes intaking, during the first times of the measurement period, filtered air into the duct system such that only filtered air is drawn from the duct system into and through the optical cavity during the first times. The method further includes, refraining from intaking filtered air into the duct system during the second times of the measurement period, such that only unfiltered air is drawn from the duct system into and through the optical cavity during the second times.
In another aspect, the disclosure relates to a duct system for use with an optical cell having an optical cavity and a sample inlet. The duct system includes a filtered inlet, a filter associated with the filtered inlet, an inline duct fan associated with the filtered inlet and arranged to draw air from a sample source and force the air through the filter, an unfiltered inlet arranged relative to the filtered inlet such that the air forced through the filter can follow an outlet flow path that exits the interior of the duct system through the unfiltered inlet, and a duct outlet configured to couple with the sample inlet of the optical cell and arranged relative to the filtered inlet and the unfiltered inlet to provide a sample flow path for the air into the optical cavity of the optical cell.
It is understood that other aspects of apparatuses and methods will become readily apparent to those skilled in the art from the following detailed description, wherein various aspects of apparatuses and methods are shown and described by way of illustration. As will be realized, these aspects may be implemented in other and different forms and its several details are capable of modification in various other respects. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.
Various aspects of apparatuses and methods will now be presented in the detailed description by way of example, and not by way of limitation, with reference to the accompanying drawings, wherein:
Disclosed herein is a system for measuring optical extinction. The system (also referred to as an optical extinction analyzer) includes an optical cell having an optical cavity, a sampling fan, a duct system having an interior, and a controller. The sampling fan is associated with a sample outlet of the optical cell, while the interior of the duct system is in fluid communication with an optical cavity of the optical cell through a sample inlet of the optical cell. The duct system includes an unfiltered inlet and a filtered inlet. A filter and an inline duct fan are arranged relative to the filtered inlet to draw a sample, e.g., air or gas, from a sample source and to force the sample through the filter into the interior of the duct system. The controller is configured to cycle the inline duct fan on and off while the sampling fan is continuously on to thereby cycle filtered samples (for baseline measurements) and unfiltered samples (for sample measurements) through the optical cavity.
Continuous operation of the sampling fan results in continuous airflow through the optical cavity for both the filtered samples and the unfiltered samples, where the air flows under same airflow conditions. These same airflow conditions include the flow rate of air through the optical cavity and the flow path of air through the optical cavity. The “same flow rate” means the flow rate of the filtered samples through the optical cavity and the flow rate of unfiltered samples through the optical cavity are equal or within 5 percent of each other. The “same flow path” means the path of filtered samples through the optical cavity and the path of unfiltered samples through the optical cavity are identical.
As disclosed further below, the continuous drawing of air from the duct system improves the accuracy of low extinction coefficient values, reduces the problems associated with mirror cleanliness, and eliminates the need for optical cavity opening/closing mechanisms. The system can be deployed to monitor both indoor and outdoor visibility and air quality, and can be used to measure the wavelength dependence of the optical extinction coefficient, which would otherwise suffer issues due to wavelength-dependent refractivity.
With reference to
The light source 102 generates the primary optical beam along path 102a. Continuous or pulsed lasers are suitable. The light source 102 generates light of a wavelength corresponding to an optical region of interest in a sample. For gases containing particles, the wavelength of light generated by the light source 102 corresponds to the optical region where information on light extinction can be obtained. For traces gases, the wavelength of light generated by light source 102 corresponds to optical region with an absorption feature. The light source 102 can be, for example, a 674 nm diode laser, a 450 nm diode laser, a 450 nm light emitting diode.
The optical isolator 106 is positioned in path 104a of the optical beam to reduce optical feedback caused by back reflections into the optical beam along path 104a, and to control the polarization of the optical beam along path 104a. The optical isolator 106 can be, for example, a Faraday isolator.
The optical component 104 is placed in front of the light source 102 to direct the optical beam along path 104a to the optical isolator 106. The optical component 108 is placed in front of the optical isolator 106 to direct the optical beam along path 106a to the optical cell 110. The optical beam from the light source 102 may also be directed to the optical cell 110 by other light directing means such as fiber optic delivery. The optical path from the light source 102 to optical cell 110 may also include optics to focus the light, or otherwise improve the coupling of light into the optical cell.
The optical cell 110 includes an optical cavity 110e formed between an entrance mirror 126 and an output mirror 128. The optical cavity 110e is further formed by a hollow core or hollow cross-sectional structure, e.g., a tube 124, the extends between the entrance mirror 126 and an output mirror 128. The tube 124 is configured receive a sample gas having differing dielectric properties than the cavity itself. To this end, the tube 124 includes a sample inlet 124a through which a sample gas enters the cavity and a sample outlet 124b through which the sample gas exits the cavity. Each of the sample inlet 124a and sample outlet 124b may be in the form of a linear slot with a predetermined width and extending along a predetermined length of tube 124. The sample inlet 124a and the sample outlet 124b may be positioned on opposing sides of tube 124 to provide a linear opening in the tube along a transverse direction.
A cavity entrance mirror chamber 118 is located at the entrance of the optical cell 110 and a cavity output mirror chamber 120 is located at the exit of the optical cell 110. The curved entrance mirror 126 is positioned within the cavity entrance mirror chamber 118 and the curved output mirror 128 is positioned within the cavity output mirror chamber 120. The curved entrance mirror 126 and the curved output mirror 128 are positioned facing each other along a longitudinal axis 110c to form the optical cavity 110e.
The curved entrance mirror 126 and the curved output mirror 128 are configured to reflect light between one another within the optical cavity 110e. The mirrors 126, 128 can be spherical, elliptical, or another curved shape. The curved entrance mirror 126 and the curved output mirror 128 are highly reflective so as to minimize light loss within the optical cavity 110e of the optical cell 110. In one configuration the, curved entrance mirror 126 and curved output mirror 128 are concave dielectric mirrors of 1 meter radius of curvature having a dielectric coating (e.g., Advanced Thin Films, Boulder, Colo., USA) optimized for maximum reflectivity of about 99.9993% at laser wavelength of about 674 nm.
The curved entrance mirror 126 may include a small area of reflective coating removed to form an aperture 126a positioned at the center of the mirror 126. The curved output mirror 128 may also include a small area of reflective coating removed to form an aperture 128a positioned at the center of the mirror 128. The apertures 126a, 128a may also be positioned off center of the mirror surface. The mirror chambers 118 and 120 include inlet ports 118a and 120a for receiving air capable of purging the optical cavity 110e to keep contaminate away from the surfaces of the mirrors 126, 128 and thereby keep the mirrors clean. The inlet port 118a is positioned to direct air in front of the curved entrance mirror 126 and the inlet port 120a is positioned to direct air in front of the curved output mirror 128. The air through each inlet port 118a, 120a flows along the surface of its respective mirror 126, 128 and in a direction along the longitudinal axis 110c of the optical cavity 110e.
With continued reference to
With reference to
The duct system 202 is coupled to the sample inlet 124a of the optical cell 110 so the interior of the duct system is in fluid communication with the optical cavity 110e of the optical cell. The coupling may be made using a two-inch diameter hose together with a hose clamp to provide an air tight, seal between a duct outlet of the duct system 202 and the sample inlet 124a. The duct system 202 is characterized by an open split configuration, e.g., a T-shape, having an unfiltered inlet 206 and a filtered inlet 208. The unfiltered inlet 206 and the filtered inlet 208 are spaced apart, opposite each other and generally axially aligned. The filtered inlet includes a filter 210 and an inline duct fan 212 arranged to draw a sample from the sample source and force the sample through the filter. In some embodiments the filter 210 is a high efficiency particulate air (HEPA) filter. The sampling fan 216 is associated with, e.g., coupled to, the sample outlet 124b of the optical cell 110.
The controller 218 is configured to control the on/off operation of the sampling fan 216 and the inline duct fan 212 during measurement. To this end, and with reference to
With reference to
During baseline measurements, purge air is input into the optical cavity 110e to keep the cavity mirrors of the optical cell clean. The purge air can be provided, for example, by a single head miniature diaphragm pump passed through a 0.1 micron Teflon membrane filter. After contacting the surfaces of the mirrors, the flow paths 222a, 222b of the purge air extend generally in the direction of the longitudinal axis 110c and converge with the flow path 220a of the filtered air. As a result, the cavity flow rate FC of the filtered air within the optical cavity 110e is generally equal to the sample flow rate FS of filtered air drawn from the duct system 202 by the sampling fan 216 plus the flow rates FP1, FP2 of the purge air input by the purge fans. Baseline measurements are obtained under particle-free conditions based on the filtered air flowing through the optical cavity 110e along the flow path 220a at a cavity flow rate FC.
With continued reference to
With reference to
During sample measurements, purge air is input into the optical cavity 110e by purge fans (not shown) to keep the cavity mirrors of the optical cell clean. After contacting the surfaces of the mirrors, the flow paths 222a, 222b of the purge air extend generally in the direction of the longitudinal axis 110c and converge with the flow path 220b of the filtered air. As a result, the cavity flow rate FC of the filtered air within the optical cavity 110e is generally equal to the sample flow rate FS of unfiltered air drawn from the duct system 202 by the sampling fan 216 plus the flow rates FP1, FP2 of the purge air input by the purge fans. Sample measurements are obtained based on the unfiltered air flowing through the optical cavity 110e along the flow path 220b at a cavity flow rate FC.
In comparing the air flow rates of
At block 402, during a measurement period comprising alternating first times and second times, air is continuously drawn from a duct system into an optical cavity under the same airflow conditions, e.g., same flow rate and same flow path. To this end and with reference to
At block 404, during the first times (which may have a duration between 30 seconds and 60 seconds) of the measurement period, filtered air is taken into the duct system 202 such that only filtered air is drawn from the duct system into the optical cavity 110e; and baseline measurements of optical loss are obtained during the first times. To this end and with reference to
The output of filtered air through the unfiltered inlet 206 prevents unfiltered air from entering the duct system. Thus, only filtered air flows into the optical cavity 110e. Within the optical cavity 110e the filtered air flows at a cavity flow rate FC, which can be equal to the sample flow rate FS (100 liters per minute). However, as mentioned above the cavity flow rate FC may be affected by the flow of purge air within the optical cavity 110e. For example, purge air may be input to the optical cavity 110e at a flow rate of 1 liter per minute, in which case the cavity flow rate FC is the sum of the sample flow rate FS (100 liters per minute) plus the flow rates FP1 (1 liter per minute) and FP2 (1 liter per minute) of purge air.
At block 406, during the second times (which may have a duration between 60 seconds and 300 seconds) of the measurement period, filtered air is not taken into the duct system 202 such that only unfiltered sample air is drawn from the duct system into the optical cavity 110e; and sample measurements of optical loss are obtained during the second times. To this end and with reference to
The prominent advantage of the system 200 disclosed herein is the improvement of the accuracy of aerosol optical extinction coefficient measurements due to the reduction of differential refractivity. Since differential refractivity can contribute anywhere between 1 and 20 Mm−1 to the measured extinction coefficients, depending on airflow conditions (flow rate, flow path, temperature), alignment of the laser beams relative to the input cavity mirror, mirror cleanliness, and homogeneity of the reflectivity of the mirror surface, a typical aerosol extinction coefficient measurement of 20 Mm−1 is subject to inaccuracies up to 100%. Measurements where the extinction coefficient is much higher are less affected. For example, a measured value of 500 Mm−1 would have an uncertainty of 20/500=4%, which is typically an acceptable amount of error.
The upper bound for wavelength dependent inaccuracies due to differential refractivity that are typically seen in known optical extinction analyzers are as follows: ˜20 Mm−1 at 405 nm; ˜10 Mm−1 at 520 nm; ˜7 Mm−1 at 635 nm; ˜5 Mm−1 at 1060 nm; and ˜2 Mm−1 at 1650 nm. These values establish the magnitude of the inaccuracy that is eliminated by the system 200 disclosed herein.
With reference to
With reference to
With continued reference to
With reference to
Other advantages of the system 200 disclosing herein include:
1) In known optical extinction analyzers the magnitude of the effect of differential refractivity is highly dependent on mirror cleanliness, which suggests that refraction causes the laser beam to explore a different portion of the mirror surface, which will give rise to either more or less optical loss depending on the exact conditions at the mirror. An advantage of the system and method disclosed herein is a reduction in the dependence of measurement accuracy on keeping the mirrors scrupulously clean. In other words, the system disclosed herein produces accurate values under suboptimal mirror conditions, and less time and effort are needed to clean the cavity mirrors.
2) The system disclosed herein eliminates the need to open and close the optical cavity, as required by known optical extinction analyzers, such as disclosed in U.S. Pat. No. 9,709,491. This reduces the likelihood of contamination of interior components, such as the mirrors. It also eliminates the need for an optical cell with an opening/closing mechanism and thereby reduces the complexity, cost and weight of the analyzer.
3) The system 200 disclosed herein, with its continuously running the sampling fan 216, establishes the same airflow conditions for both the filtered purge air and unfiltered sample air. In known optical extinction analyzers, the purge air flow path empties into an optical cavity that lacks a well-defined outlet path. This makes it more difficult to control and correctly model air flow through the optical cavity.
4) The duct system 202 can attach to the remainder of the system 200 via a flexible hose. Thus, the duct system 202 component of the system 200 can be placed remote from the rest of the system, e.g., such as the light source 102, the optical isolator 106, the optical components 104, 108, the optical cell 110, the collimator 112, the photodetector 114 and the processor 116 shown in
The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Various modifications to exemplary embodiments presented throughout this disclosure will be readily apparent to those skilled in the art. Thus, the claims are not intended to be limited to the various aspects of this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the various components of the exemplary embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”
