Methods and apparatus for mechanical resonance monitoring a mass concentration of particulate matter

A particulate mass monitor includes a controller that monitors a change in a resonant oscillation frequency of a taut metallic membrane, as caused by deposition of the particulate matter on the metallic membrane. The metallic membrane, such as a foil or metallized plastic film, is substantially mechanically stable under tension. Application of a tension to the periphery of the metallic membrane generates a substantially constant tension within the membrane, thereby allowing the particulate mass monitor to detect a particulate mass concentration of the air sample with a relatively high degree of accuracy. Additionally, the particulate mass monitor includes a membrane transporter that automatically advances the metallic membrane within the particulate mass monitor. The membrane transporter minimizes the necessity for manual replacement of the metallic membrane over time and allowing long term, unattended operation of the particulate mass monitor.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to a system and method for measuring and monitoring the mass concentration of airborne particles.

BACKGROUND

Ambient air quality affects the health of people breathing the ambient air. The lower the air quality, the greater the risk for health-related problems induced by the ambient air. Conventional particulate matter monitors measure the mass concentration of particulate matter within ambient air, gases, or other fluids to detect the quality of the ambient air or gaseous fluid. A conventional particulate matter monitoring device can provide a warning to a user when the device detects a relatively low air quality (e.g., a relatively large particulate mass concentration within the air) or a decrease in the ambient air quality based upon an increase in particulate mass concentration measured over a specific time period.

Certain sensing techniques, such as in mechanical resonance sensing, detect the particulate mass concentration of an air sample. For example, a conventional particulate matter monitor includes a collector that collects particulate matter within an air sample and that detects the mass of the particulate matter based upon the mass-spring principle. If the collector is part of a mechanically resonating system, the natural resonance frequency of the system decreases as the collected mass of the particulate matter increases. The general equation governing this behavior is:
Δm=m0[(f0/ff)2−1]
where Δm is the mass increment of collected particulate matter, m0is the total initial mass of the resonating system, and f0and ffare the initial (before particle collection) and final (after particle collection) resonance frequencies of the oscillating system (e.g., of the collector).

One type of particulate matter monitor utilizing mechanical resonance sensing includes a quartz crystal microbalance. Conventional quartz crystal microbalances have a thin piezoelectric quartz crystal sandwiched between two metal electrodes. The conventional quartz crystal microbalances collect particulate matter on the surface of the quartz crystal from an air sample using either electrostatic precipitation or jet-to-plate impaction. As the piezoelectric quartz crystal receives the particulate matter, the quartz crystal microbalance provides an alternating electric field to the piezoelectric quartz crystal, causing the quartz crystal to generate a shear-induced acoustic wave. Changes in the mass of the quartz crystal, as caused by deposition of particulate matter, affect the frequency of the wave. In such a configuration, the quartz crystal microbalance allows for detection of and quantification of relatively small masses of particulate matter within an air sample.

Another type of particulate matter monitor utilizing mechanical resonance sensing includes a tapered-element oscillating microbalance (TEOM). In a conventional TEOM, a filter cartridge attached to a tapered element of the TEOM receives an air or gas sample pumped at a known flow rate. The TEOM produces an alternating voltage signal that oscillates the tapered element at the resonant frequency of the tapered element/filter cartridge combination. As the filter cartridge, attached to the tapered element, removes particulate matter from the air or gas sample, the mass change of the tapered element causes a frequency shift in the resonant frequency of the tapered element. The frequency shift of the signal relates to the mass accumulated by the tapered element/filter cartridge combination of the TEOM and relates to the amount of particulate matter within the air sample.

Another type of particulate matter monitor utilizing mechanical resonance sensing includes a resonant taut filter membrane sensing system. The conventional taut filter membrane sensing system includes an annular piezoelectric driver to impart an oscillation to a taut filter and a miniature microphone to detect the resonance frequency of the filter. The taut filter membrane sensing system detects the mass of particulate matter collected on the taut filter by oscillating the filter, thereby causing the filter to resonate at its natural mechanical resonant oscillation frequency (e.g., characteristic fundamental frequency or fundamental mode). As particulates within an air sample accumulate on the filter, the resonant frequency of the filter's oscillation decreases due to the additional particulate mass. The taut filter membrane sensing system calculates the mass concentration of airborne particulates within the air sample based upon the decrease in the natural resonant frequency of the taut filter.

Additionally, the conventional taut filter membrane sensing system is configured to oscillate the taut filter at higher, non-harmonic modes related to the fundamental mode or characteristic fundamental frequency of the filter. As particulates within an air sample accumulate on the filter, the oscillation of the filter at the higher, non-harmonic modes decreases due to the additional particulate mass. By using the higher, non-harmonic modes to oscillate the taut filter, the resonant taut filter membrane sensing system allows for relatively sensitive measurements of particulate matter concentrations within an air sample, compared to measurements taken using fundamental mode oscillations.

SUMMARY

Conventional mechanisms and techniques that provide monitoring of airborne particulate mass concentration levels suffer from a variety of deficiencies.

As indicated above, one type of particulate matter monitor utilizing mechanical resonance sensing includes a quartz crystal microbalance. The conventional quartz crystal microbalances collect particulate matter on the surface of the quartz crystal from an air sample using either electrostatic precipitation or jet-to-plate impaction. However, because of the configuration of quartz crystal microbalances, the use of quartz crystal microbalances is typically constrained to laboratory use. For example, the relatively small total accumulated mass capacity of the quartz crystal requires frequent crystal cleaning or surface restoration. Such requirements for cleaning or resurfacing can be impractical to a user while in the field (e.g., away from the laboratory environment).

Also, another type of particulate matter monitor utilizing mechanical resonance sensing includes a tapered-element oscillating microbalance (TEOM). While the TEOM allows for ambient air monitoring of particulate matter, measurements made by the TEOM are susceptible to positional and vibrational effects. For stationary, fixed-point applications (e.g., within a laboratory) positioning or vibration of the TEOM minimally affects the measurements made by the TEOM. However, in relatively severe environments (e.g., blasting sites, coal mines, etc.) vibration of the TEOM can have a relatively substantial effect on the TEOM measurements. Additionally, over time, the filter cartridge attached to the tapered element receives a relatively large quantity of particulate matter. As such, due to particle volatilization and/or reception of moisture present within the air samples, the accuracy of the TEOM measurements can become compromised. Also, conventional TEOMs require routine manual replacement of the particle collection medium (i.e., the filter cartridge) of the tapered element thereby minimizing long term unattended operation.

Another type of particulate matter monitor utilizing mechanical resonance sensing includes a resonant taut filter membrane sensing system. The resonant taut filter membrane sensing system detects the mass of particulate matter collected on a taut filter by oscillating the taut filter to resonate at its natural mechanical resonant oscillation frequency. As particulates within an air sample accumulate on the filter, the resonant frequency of the filter's oscillation decreases due to the additional particulate mass. The filter, however, is typically formed from a moisture absorbent fiber mesh. During operation, as the filter receives particulate matter within the air sample, the filter also absorbs any moisture present within the air sample as well. The moisture changes the resonant frequency of the taut filter during operation, thereby affecting the accuracy of the resonant taut filter membrane sensing system in detecting the particulate mass concentration within an air sample.

Additionally, the material properties of the filter in the resonant taut filter membrane sensing system can affect the accuracy of the system's detection of the particulate mass concentration of an air sample. For the resonant taut filter membrane sensing system, the precision or accuracy of a particulate mass measurement depends upon the constancy of a radial tension of the filter membrane. However, as indicated, the filter is typically formed from a fiber mesh. Application of a radial tension to the fiber mesh, therefore, can cause weakening or tearing of the fiber mesh (e.g., mechanical instability within the filter) within certain areas of the membrane. The weakening of the filter in certain areas reduces the constancy of the radial tension within the filter (e.g., the uneven distribution of tension within the filter membrane). As such, a non-constant radial tension generated within the filter decreases the precision or accuracy of a particulate mass measurement made by the resonant taut filter membrane sensing system.

Also as described above, the conventional taut filter membrane sensing system is configured to oscillate the taut filter at higher, non-harmonic modes to allow for relatively sensitive measurement of particulate matter concentrations within an air sample. However, conventional filter membranes are formed from a fibrous material. During operation, therefore, the resonant taut filter membrane sensing system can generate a non-constant (i.e., inconsistent) radial tension within the filter. When the conventional taut filter membrane sensing system attempts to oscillate the taut filter at higher, non-harmonic modes, the non-constant radial tension within the filter limits the ability for the filter to vibrate at the desired non-harmonic modes. Such limited, upper mode vibration minimizes the accuracy of the membrane sensing system in detecting the particulate mass concentration within an air sample.

By contrast, embodiments of the present invention significantly overcome the described deficiencies and provide mechanisms and techniques for monitoring a mass concentration of particulate matter within a gas sample. A particulate mass monitor includes a controller that monitors a change in a resonant oscillation frequency of a taut metallic membrane, as caused by deposition of the particulate matter on the metallic membrane. The metallic membrane, such as a foil or metallized plastic film, is substantially mechanically stable under tension. Application of a tension to the periphery of the metallic membrane generates a substantially constant tension within the membrane, thereby allowing the particulate mass monitor to detect a particulate mass concentration of the air sample with a relatively high degree of accuracy. Additionally, the particulate mass monitor includes a membrane transporter that automatically advances the metallic membrane within the particulate mass monitor. The membrane transporter minimizes the necessity for manual replacement of the metallic membrane over time and allowing long term, unattended operation of the particulate mass monitor.

In one arrangement, a particulate mass monitor includes a metallic membrane, a particle collector directed at the metallic membrane, and a controller in electrical communication with the metallic membrane. The particle collector (e.g., electrostatic precipitator or impactor) is configured to direct particulate matter within an air sample to the metallic membrane. The controller is configured to generate and detect an oscillation frequency in the metallic membrane where the oscillation frequency of the metallic membrane is based upon the particulate matter collected by the metallic membrane. The metallic membrane is formed from a metallic material, such as a stainless steel material. As such, the metallic membrane is substantially mechanically stable under tension. When placed under tension, the metallic membrane maintains a substantially constant tension over time. Therefore, the use of the metallic membrane within the particulate mass monitor allows detection a particulate mass concentration of the air sample with a relatively high degree of accuracy, compared to a conventional taut filter membrane sensing system.

In one arrangement, the particulate mass monitor includes a membrane transporter coupled to the metallic membrane. The membrane transporter is configured to advance the metallic membrane relative to the particle collector such that a first metallic membrane portion orients in proximity to the particle collector during a first particulate mass monitor test and a second metallic membrane portion orients in proximity to the particle collector during a second particulate mass monitor test. The membrane transporter automatically advances the metallic membrane within the particulate mass monitor. The membrane transporter, therefore, minimizes the necessity for manual replacement of the metallic membrane over time and allows long term, unattended operation of the particulate mass monitor.

DETAILED DESCRIPTION

Embodiments of the present invention provide mechanisms and techniques for monitoring a mass concentration of particulate matter within a gas sample. A particulate mass monitor includes a controller that monitors a change in a resonant oscillation frequency of a taut metallic membrane, as caused by deposition of the particulate matter on the metallic membrane. The metallic membrane, such as a foil or metallized plastic film, is substantially mechanically stable under tension. Application of a tension to the periphery of the metallic membrane generates a substantially constant tension within the membrane, thereby allowing the particulate mass monitor to detect a particulate mass concentration of the air sample with a relatively high degree of accuracy. Additionally, in one of the embodiments of the present invention, the particulate mass monitor includes a membrane transporter that automatically advances the metallic membrane within the particulate mass monitor. The membrane transporter minimizes the necessity for manual replacement of the metallic membrane over time and allowing long term, unattended operation of the particulate mass monitor.

FIG. 1shows an arrangement of a particulate mass monitor20. The particulate mass monitor20is configured to monitor a fluid sample42, such as a gas or air sample, from a fluid source for particulate matter (PM), such as within PM10(i.e., particulate matter having a size smaller than 10 micrometers), or PM2.5(i.e., particulate matter having a size smaller than 2.5 micrometers) particle size ranges. The particulate mass concentration of the air sample42relates to the particulate mass concentration of the fluid source. For example, as the particulate mass monitor20continuously receives the air sample42from an ambient air source, detection of a particulate mass concentration within the air sample42generally relates to the particulate mass concentration of the ambient air source. The particulate mass monitor20utilizes mechanical resonance mass sensing to monitor the particulate mass concentration of the air sample42, as will be described below.

The particulate mass monitor20includes a fluid inlet32and a fluid outlet34. The particulate mass monitor20also includes a taut metallic membrane36, a particle collector38, and a controller40in electrical communication with the metallic membrane36.

The fluid inlet32is configured to direct an air sample or gas sample42toward the particle collector38of the particulate mass monitor20. For example, in one arrangement, the fluid inlet32is configured with a geometry (e.g., as a nozzle or funnel) that directs a portion or sample42of ambient air, flowing relative to the particulate mass monitor20, toward the particle collector38. The fluid outlet34is configured to vent the air sample42from the particulate mass monitor20to the atmosphere.

The ambient air (e.g., the air sample42) flows at a substantially constant flow rate through the particulate mass monitor20. For example, the fluid inlet32or the fluid outlet34includes pump that draws an air sample42from the ambient air surrounding the particulate mass monitor20toward the particle collector38. The pump maintains a substantially constant flow rate of fluid (e.g., a constant flow rate of the air sample42) through the particulate mass monitor20.

The metallic membrane36is formed from a metallic material, such as a stainless steel material or a metallized plastic material, such as Mylar plastic having a metal coating, for example. The metallic membrane36is configured as a substantially thin foil or film having a thickness of between approximately 0.0001 and 0.001 inches. The metallic membrane36orients relative to the particle collector38to receive particulate matter44within the air sample42carried by the fluid inlet32. The metallic membrane36is configured to receive particulate matter44within the air sample42.

InFIG. 1, the metallic membrane36is configured as a tensioned or taut metallic membrane36within the particulate mass monitor20. Such tensioning results from application of a load or tension47to a periphery of the metallic membrane36. As indicated above, for a mechanical resonance system, the precision or accuracy of a particulate mass measurement depends upon the constancy of tension of the sensing membrane (e.g., the metallic membrane36). Because the metallic membrane36is formed from a metallic material, such as a stainless steel material or a metallized plastic material, the metallic membrane36is substantially mechanically stable under tension. Application of the load47to a periphery of the metallic membrane36, therefore, generates a substantially constant tension within the metallic membrane36(e.g., a substantially even distribution of tension within the metallic membrane36). Therefore, the use of the taut metallic membrane36within the particulate mass monitor20allows the particulate mass monitor20to detect a particulate mass concentration of the air sample with a relatively high degree of accuracy, such as compared to a conventional taut filter membrane sensing system.

InFIG. 1, the particle collector38orients in proximity to the metallic membrane36and is configured to direct particulate matter44within the air sample42toward the metallic membrane36. The particle collector38aids in adhering the particulate matter44within the air sample42to the metallic membrane36.

In one arrangement, the particle collector38is configured as an electrostatic precipitation device in combination with the metallic membrane36. For example, in such an arrangement, the electrostatic precipitation device38is a sharp metallic point either at a positive or negative potential relative to the metal membrane36and creates a corona discharge that electrically charges the particulate matter44within the air sample42(i.e., the particulate matter44between the sharp point electrostatic precipitation device38and the metallic membrane36). Based upon the potential difference between the electrostatic precipitation device38and the metallic membrane36, the metallic membrane36electrostatically attracts the charged particulate matter44such that the charged particulate matter44travels from the electrostatic precipitation device38toward the metallic membrane36and adheres to the metallic membrane36.

In one arrangement, the particulate mass monitor20having the electrostatic precipitation device38includes a pump. The pump creates a pressure differential between the particulate mass monitor20and the atmosphere to draw an air sample42from the surrounding ambient air into the particulate mass monitor20. The fluid inlet32then directs the air sample42toward the electrostatic precipitation device38and past the corona discharge generated by the electrostatic precipitation device38. Use of the pump and the fluid inlet32maximizes the particle collection efficiency of the particulate mass monitor20by forcing the air sample42through an ion stream (i.e., corona discharge) generated by the electrostatic precipitation device38.

In one arrangement, the particulate mass monitor20having the electrostatic precipitation device38does not utilize a pump. In order to direct an air sample42into the particulate mass monitor20and past the electrostatic precipitation device38, the electrostatic precipitation device38, when ionizing the air molecules, is configured to create an ion wind effect. The ion wind effect causes the air sample42to flow relative to (i.e., into) the particulate mass monitor20, thereby providing the particulate mass monitor20with a substantially continuously flowing air sample42during operation.

For example, assume the particulate mass monitor20is configured such that the fluid inlet32is open to the ambient air (i.e., without any pre-collection stage). As the electrostatic precipitation device38produces a corona discharge, the corona discharge creates an air flow directed from the electrostatic precipitation device38towards the metallic membrane36, as caused by an ion wind effect. The ion wind results from collisions between the ions generated in the gas (e.g., air) traveling from the electrostatic precipitation device38and the air molecules in the vicinity of the electrostatic precipitation device38. The ion wind induces an air or gas flow rate within the particulate mass monitor20of approximately 10 liters/minute or more at a corona current of approximately of 5 μA (the flow rate being proportional to the square root of the corona current). As such, the ion wind generated by the electrostatic precipitation device38during operation draws the air sample42into the particulate mass monitor20in a substantially continuous manner.

With the electrostatic precipitation device38configured to create an ion wind effect relative to the particulate mass monitor20, the electrostatic precipitation device38allows the particulate mass monitor20to operate without the use of a pump or mechanically dynamic components. As such, the electrostatic precipitation device38allows substantially silent operation particulate mass monitor20, such as applicable to indoor air quality measurements where pump noise is typically unacceptable.

In another arrangement, the particle collector38is configured as a jet impactor. For example, in such an arrangement, the particle collector38is configured either as a single nozzle or a multiplicity of nozzles to increase a velocity of the air sample40flowing through the particulate mass monitor20. The particle collector38nozzle (or nozzles) directs both the air sample40and the particulate matter44within the air sample42toward the metallic membrane36at a relatively high velocity, thereby causing the particles44to impact onto the metallic membrane36. An adhesive coating on the surface of the metallic membrane36can be used to ensure the retention of the particles collected on the metallic membrane36.

Another variation using jet impaction to collect the particulate matter on the metallic membrane36includes a series of consecutive impaction stages wherein the nozzle (or nozzles) at each consecutive impaction stage have decreasing openings (i.e. decreasing opening diameters) resulting in correspondingly increasing air velocities. This configuration, called a cascade impactor separates particulate matter by size. The combination of this impaction configuration with mass sensing of the taut metal membrane resonance provides a measurement of the particle size distribution of the sampled particulate matter.

The controller40is configured to detect a mass concentration of particulate matter44within the air sample42based upon a mechanical resonance or oscillation of the metallic membrane36. For example, the controller40electrically couples with the metallic membrane36and is configured to generate an oscillation frequency in the metallic membrane36. The controller40oscillates or induces a vibration in the metallic membrane36where the resonant oscillation of the metallic membrane36is based upon the overall mass of the metallic membrane36(i.e., the mass of the metallic membrane36and any particulate matter44collected by the metallic membrane36over time). Furthermore the controller40is configured to detect or monitor the resonant oscillation frequency of the metallic membrane36over time. Changes in the resonant oscillation frequency of the metallic membrane36, as detected by the controller40and caused by deposition of the particulate matter44on the metallic membrane36, relate to a particulate mass concentration of the air sample42.

The controller40, in one arrangement, includes an output device54configured to provide, to a user, a particulate mass concentration value56associated with the particulate mass concentration of the air sample42. The particulate mass concentration value56indicates, to the user, a level or indicator as to ambient air quality. In one arrangement, the output device54is configured as a display, such as a liquid crystal display or a light emitting diode display, to provide the particulate mass concentration value56to a user. In another arrangement, the output device54is configured as a digital data output port to provide the particulate mass concentration value56to a user.

In one arrangement, the controller40includes an oscillation element46coupled to the metallic membrane46and electrically coupled to the controller40. The oscillation element46is configured to generate the oscillation frequency in the metallic membrane36. The oscillation element46, in one arrangement, induces an oscillation frequency, such as a resonance frequency, in the metallic membrane36using a piezoelectric element. In another arrangement, the oscillation element46induces an oscillation frequency, such as a resonance frequency, in the metallic membrane36using an oscillatory electric or magnetic field.

In one arrangement, the controller40includes a detector element48oriented in proximity to the metallic membrane36and electrically coupled to the controller40. The detector element48is configured to detect the resonant oscillation frequency of the metallic membrane36, as induced by the controller40. The detector element48, in various arrangements, is configured as an optical, acoustic, or electric detector, to detect the oscillation frequency of the metallic membrane36during operation.

FIG. 2is a flow chart100of a procedure performed by the particulate mass monitor20.FIG. 3, taken withFIGS. 1 and 3, illustrates a graph representing resonant oscillation frequencies induced in the metallic membrane36by the controller40during operation. The controller40monitors a change in the resonant oscillation frequency of the metallic membrane36, as caused by deposition of the particulate matter44on the metallic membrane36. Based upon the change in the resonant oscillation frequency of the metallic membrane36, the controller40detects a mass concentration of particulate matter44within the air sample42, the mass concentration indicative of ambient air quality.

In step102, at a first time period T1, the controller40generates and detects a first resonant oscillation frequency70in the metallic membrane46. The first resonant oscillation frequency70of the metallic membrane36provides the controller40with a baseline resonant oscillation measurement related to the metallic membrane36. As the metallic membrane36collects particulate matter within the air sample42over time, the controller40uses the baseline resonant oscillation measurement as a reference to measure changes in the resonant oscillation frequency of the metallic membrane36caused by collection of the particulate matter44by the metallic membrane36.

In one arrangement, the controller40is configured to induce the first resonant oscillation frequency70in the metallic membrane36. The resonant frequency is defined as the natural oscillation of a mechanical system (e.g., the metallic membrane36) proportional to the square root of the ratio of the system stiffness constant (e.g., a metallic membrane stiffness constant) and the system mass (e.g., a metallic membrane mass). In such an arrangement, the controller40, the oscillation element46, and the detector element48form a mechanical resonance feedback loop system to iteratively detect and generate the resonant frequency in the metallic membrane36.

For example, referring toFIG. 3, the controller40transmits an input signal72, such as a sine wave having a particular frequency, to the oscillation element46, thereby causing the oscillation element46to oscillate the metallic membrane36at or near the resonant frequency of the metallic membrane36. In a substantially simultaneous manner, the controller40retrieves an output signal74from the detector element48where the output signal74indicates the vibration or oscillation of the metallic membrane36in response to the input signal70. The controller40then iteratively adjusts the frequency of the input signal72, based upon the output signal74from the detector element48(i.e., using the mechanical resonance feedback loop system). When the controller40detects, based upon the output frequency74measured by the detector element48, that a given input signal frequency produces a maximal vibration or oscillation of the metallic membrane36, such as the first resonant oscillation frequency70, the controller40maintains the oscillation (i.e., the first oscillation frequency) in the membrane36. Such maximal vibration or oscillation of the metallic membrane36represents the resonant frequency of the metallic membrane36.

Returning toFIG. 2, in step104, at a second time period T2, when the particle collector38directs the particulate matter44within the air sample42to the metallic membrane32, the controller40generates and detects a second resonant oscillation frequency76in the metallic membrane36. For example, as the particle collector38deposits particulate matter44onto the metallic membrane36during operation, the mass of the metallic membrane36increases (i.e., as a combination of the mass of the metallic membrane36and the mass of the particles44). As the mass of the metallic membrane36increases, the resonant frequency of the metallic membrane36decreases. To generate the resonant frequency of the metallic membrane36having the additional particulate matter, the controller40decreases (i.e., by sweeping) the frequency of the input signal72.

For example, as the resonant frequency of the metallic membrane36changes after receiving the particulate matter within the air sample42, the controller40iteratively decreases the input frequency72to the membrane36, via the oscillation element46and detects the output signal74, via the detector element48. When the controller40detects that the decreased input signal frequency72produces a shifted maximal vibration or oscillation of the metallic membrane36, such as the second resonant oscillation frequency76, the controller40maintains the oscillation (i.e., the second resonant oscillation frequency76) in the membrane36.

In step106, the controller40calculates a particulate mass concentration of the air sample42based upon a relation between the first resonant oscillation frequency70and the second resonant oscillation frequency76. For example, assume the controller40stores, such as in a memory location, the first resonant oscillation frequency70and the second resonant oscillation frequency76. Using the following relationship:
Δm=m0[(f0/ff)2−1]
where m0is the total initial mass of the resonating system, f0is the first resonant oscillation or resonance frequency70of the metallic membrane36(e.g., before particle collection), and ffis the second resonant oscillation frequency76of the metallic membrane36, the controller40calculates Δm where Δm is the mass increment of collected particulate matter that represents the particulate mass concentration of the air sample42. After calculating the particulate mass concentration, the controller40provides the result as a particulate mass concentration value56using the output device54, thereby providing, to a user, an indicator as to ambient air quality.

As indicated above, the metallic membrane36is configured as a tensioned or taut metallic membrane36within the particulate mass monitor20. For a mechanical resonance system, the precision or accuracy of a particulate mass measurement, and therefore, accurate detection of a particulate mass concentration of the air sample42, depends upon the constancy of tension of the metallic membrane36. Because the metallic membrane36is formed from a substantially uniform metallic material (i.e., having substantially uniform material properties), such as a stainless steel material or a metallized plastic material, the metallic membrane36is substantially mechanically stable under tension. Application of the load47to a periphery of the metallic membrane36, therefore, generates a substantially constant tension within the metallic membrane36(i.e., a substantially even distribution of tension within the metallic membrane36). Therefore, the use of the taut metallic membrane36within the particulate mass monitor20allows the particulate mass monitor20to detect a particulate mass concentration of the air sample with a relatively high degree of accuracy (e.g., such as compared to a conventional taut filter membrane sensing system).

FIG. 4illustrates, in one arrangement, steps performed by the controller40as the controller40detects a mass concentration of particulate matter44within the air sample42. For example,FIG. 4illustrates the controller40detecting the first and second resonant oscillation frequencies of the metallic membrane36when providing particulate matter44to the metallic membrane36in an interrupted manner.

In step110, at the first time period, the controller40generates and detects the first resonant oscillation frequency in the metallic membrane36prior to the particle collector38directing the particulate matter44to the metallic membrane36. The controller40, as such, measures the resonant oscillation of the metallic membrane36before the metallic membrane collects particulate matter44from the air sample42. For example, as indicated above, when the particle collector38is configured as an electrostatic precipitation device38, the electrostatic precipitation device38creates an ion wind effect when ionizing the gas molecules within an air sample42. The ion wind results from collisions between the ions (i.e., ionized gas molecules) traveling from the electrostatic precipitation device38and the air molecules in the vicinity of the electrostatic precipitation device38. As the ionic wind effect causes the air sample42to flow within the particulate mass monitor20, the air sample42impacts the metallic membrane36. Such impaction between the air sample and the metallic membrane36affects the resonant oscillation frequency of the metallic membrane36.

In the case where the metallic membrane36oscillates at the resonant frequency, contact between the air sample42and the metallic membrane36causes the metallic membrane36to oscillate at a shifted resonant frequency, thereby affecting the accuracy of a particulate mass measurement made by the particulate mass monitor20. By generating and detecting the first resonant oscillation frequency in the metallic membrane36prior to the particle collector38directing the particulate matter44to the metallic membrane36, the controller40allows the metallic membrane36to oscillate at the resonant frequency, without influence of the ionic wind effect on the oscillation of the membrane36. As such, the controller40provides a relatively high level of accuracy to the particulate mass measurement made by the particulate mass monitor20.

In step111, at the second time period, the controller40generates and detects the second oscillation frequency in the metallic membrane36after the particle collector38directs the particulate matter38to the metallic membrane36. Similar to step110, to minimize the effect of the ion wind on the resonant frequency oscillation of the metallic membrane36, the controller40measures the second resonant oscillation frequency in the metallic membrane36once the particle collector38has completed generation of the electrostatic discharge to transmit the particulate matter44to the metallic membrane36. Therefore, as the controller40detects the second resonant oscillation frequency in the metallic membrane36, the controller performs such detection in the absence of an ionic wind. By generating and detecting the second resonant oscillation frequency in the metallic membrane36after the particle collector38completes the step of directing the particulate matter44to the metallic membrane36, the controller40allows the metallic membrane36to oscillate at the resonant frequency, without influence of the ionic wind effect on the resonant oscillation of the membrane36. As such, the controller40provides a relatively high level of accuracy to the particulate mass measurement made by the particulate mass monitor20.

FIG. 5illustrates, in one arrangement, steps performed by the controller40as the controller40detects a mass concentration of particulate matter44within the air sample42. For example,FIG. 5illustrates the controller40detecting the first and second resonant oscillation frequencies of the metallic membrane36when providing particulate matter44to the metallic membrane36in a substantially uninterrupted manner.

In step112, at the first time period, the controller40generates and detects the first resonant oscillation frequency in the metallic membrane36while the particle collector38directs the particulate matter44to the metallic membrane36. The controller40, as such, measures the first resonant oscillation frequency of the metallic membrane36while the metallic membrane36collects particulate matter44from the air sample42.

In step113, at the second time period, the controller40generates and detects the second resonant oscillation frequency in the metallic membrane36while the particle collector continues to direct the particulate matter to the metallic membrane. The controller40, as such, measures the second resonant oscillation frequency of the metallic membrane36while the metallic membrane36continuously collects particulate matter44from the air sample42. By detecting the first and second resonant oscillation frequencies of the metallic membrane36as the metallic membrane36collects particulate matter, the controller40allows substantially continuous operation of the particulate mass monitor20.

Returning toFIG. 1, as indicated above, the particle collector38, in one arrangement, is configured as an electrostatic precipitation device. Also as indicated above, the oscillation element46induces an oscillation or resonance frequency in the metallic membrane36using a piezoelectric element or using an oscillatory electric or magnetic field. Additionally the detector element48is configured as an acoustic or electric detector, to detect the resonant oscillation frequency of the metallic membrane36during operation.FIGS. 6 and 7illustrate example arrangements of the particulate mass monitor20having combinations of the described configurations of the particle collector38, the oscillation element46, and the detector element48.

FIG. 6illustrates an arrangement of the particulate mass monitor20where the particle collector38is configured as a point-to-plane electrostatic precipitation device80, the oscillation element46is configured as a mechanical oscillator such as a piezoelectric driver82, and the detector element48is configured as a microphone84. The controller40includes a constant current, high-voltage power supply86coupled to the point-to-plane electrostatic precipitation device80, a sine wave synthesizer88coupled to the piezoelectric driver82, and a resonance sensing circuit90coupled to the microphone84. A user utilizes the particulate mass monitor20configured with the point-to-plane electrostatic precipitation device80for ambient air monitoring or for indoor air quality measurements, for example.

The piezoelectric driver82, such as a ring-shaped piezo crystal, mounts in direct mechanical contact with the taut metallic membrane36. For example, in one arrangement, the piezoelectric driver82forms part of a membrane frame92having a membrane tensioning mechanism94. The piezoelectric driver82is configured to receive an input frequency (i.e., variable frequency) from the sine wave synthesizer88. The piezoelectric driver82vibrates or oscillates based upon the input frequency from the sine wave synthesizer88and transmits the oscillation to the metallic membrane36.

The microphone84is configured as an acoustic detector to detect the oscillation of the metallic membrane36based upon the sine wave input frequency provided by the sine wave synthesizer88. The microphone84, in one arrangement, orients in proximity to the metallic membrane36in a location downstream to a direction of the flow of the air sample42. During operation, as the piezoelectric driver82oscillates the metallic membrane36, the microphone84receives an acoustic signal generated by the oscillating membrane36and transmits a corresponding output signal to the resonance sensing circuit90. The resonance sensing circuit90(i.e., a processor of the controller40) utilizes the output signal to adjust the input frequency provided from the sine wave synthesizer88to the piezoelectric driver82. When the resonance sensing circuit90detects, based upon the output signal provided by the microphone84, that a given input frequency produces a maximal vibration or oscillation of the metallic membrane36, the resonance sensing circuit90directs the sine wave synthesizer88to maintain the input frequency to the membrane36to maintain maximal vibration or oscillation.

The point-to-plane electrostatic precipitation device80is configured to generate and deliver a corona discharge relative to the metallic membrane36(i.e., a conically shaped corona discharge). The geometry of the point-to-plane electrostatic precipitation device80is configured to minimize or limit the presence of sharp edges in order to prevent generation of corona discharges at unwanted locations of the electrostatic precipitation device80. For example, the point-to-plane electrostatic precipitation device80includes a point electrode96and a delivery element98. The point electrode96defines either a positive or negative electrical potential relative to the metal membrane36and is configured with a corona discharge point97facing the metallic membrane36. The point-to-plane electrostatic precipitation device80also includes a delivery element98that delivers current at a high positive or negative voltage from the power supply86to the point electrode96. The delivery element98is configured as having a sphere shape (e.g., an anti-corona sphere) to minimize or limit the presence of sharp edges of the electrostatic precipitation device80. The delivery element98couples to the constant current, high voltage power supply86of the controller40. The power supply86maintains a substantially constant corona current for the electrostatic precipitation device80, thereby minimizing the effect of changes in operating parameters, such as air density, humidity, surface condition of the corona point, on the generation of the corona discharge.

During operation, the delivery element98delivers the substantially constant current to the point electrode96. The point electrode96receives the current, and the corona discharge point97of the point electrode96ionizes the air or gas molecules within the air sample42in the vicinity of the point electrode96which ions, in turn impart an electric charge to the particulate matter44within the air sample42. With the point electrode96forming either a positive or negative potential relative to the metal membrane36, the metal membrane36electrostatically attracts the oppositely charged particulate matter44.

In one arrangement, in the case where the particulate mass monitor20includes the point-to-plane electrostatic precipitation device80, the fluid inlet32includes an electrically conductive inner surface99having an electrical potential equal to the electrical potential of the point electrode96. Such a configuration minimizes collection of electrically charged particulate matter44by the inner surface99of the fluid inlet and maximizes the amount of particulate mater44directed toward the metallic membrane36during operation.

FIG. 7illustrates an arrangement of the particulate mass monitor20where the particle collector38is configured as a wire-to-plane electrostatic precipitation device120, the oscillation element46is configured as an electrical oscillator such as a capacitive driver122, and the detector element48is configured as an electrical detector, such as a capacitive charge detector124. In one arrangement, the controller40operates the capacitive driver122and the capacitive charge detector124. A user utilizes the particulate mass monitor20configured with the wire-to-plane electrostatic precipitation device120for continuous source emission monitoring. For example, the user orients the particulate mass monitor20relative to a flow direction of the air sample42such that the air sample42flows though the particulate mass monitor20without the use of a pump associated with the particulate mass monitor20(e.g., such as in stack gas monitoring).

Because the oscillating element within the particulate mass monitor20is configured as a taut metallic membrane36(e.g., where the tensioning mechanism94applies a tension to the metallic membrane36about a periphery of the membrane36), the metallic membrane36functions as one of two electrodes of a parallel plate capacitor. The particulate mass monitor20, therefore, includes a conductive plate126defining a substantially planar surface128. The conductive plate126orients in proximity, and substantially parallel, to a substantially planar surface130defined by the metallic membrane36.

The capacitive driver122electrically couples with the metallic membrane36and the conductive plate126. The capacitive driver122, in one arrangement, is configured as an electronic circuit, such as a variable frequency sine wave oscillator, configured to apply an alternating electrical potential between the metallic membrane36and the conductive plate126. When applying the alternating potential between the metallic membrane36and the conductive plate12, the capacitive driver122creates a corresponding, periodic deflection in the metallic membrane36. During operation, the capacitive driver122imposes a drive frequency to the metallic membrane36where the drive frequency is one-half the oscillation frequency of the metallic membrane36. For example, if the capacitive driver122provides a drive frequency of 5,000 Hz to the metallic membrane36, the metallic membrane36oscillates at a frequency of 10,000 Hz because for every cycle of the drive signal, the metallic membrane36deflects twice from a rest position.

The capacitive charge detector124electrically couples with either the conductive plate126, or the metallic membrane36, or both the conductive plate126and the metallic membrane36. The capacitive charge detector124, in one arrangement is configured as an electronic circuit that measures the varying capacitance that results from the oscillation or time-varying deflection of the metallic membrane36. Based upon the varying capacitance between the conductive plate126and the metallic membrane36, the capacitive charge detector124detects the resonant oscillation frequency in the metallic membrane36.

The wire-to-plane electrostatic precipitation device120is configured to generate and deliver a corona discharge relative to the metallic membrane36(i.e., a triangular or “tent-shaped” corona discharge). The geometry of the wire-to-plane electrostatic precipitation device120is configured to minimize the presence of sharp edges of the precipitation device120in order to prevent corona discharges at unwanted locations of the electrostatic precipitation device80. The wire-to-plane electrostatic precipitation device120includes a wire electrode134and current delivery elements136. The wire electrode134defines either a positive or negative electrical potential relative to the metallic membrane36(e.g., a rectangularly-shaped metallic membrane36). The delivery elements136are configured as having a sphere shape (e.g., an anti-corona sphere) to minimize or limit the presence of sharp edges of the electrostatic precipitation device80. The delivery element98couples to a constant current, high voltage power supply, such as associated with the controller40. The power supply86maintains a substantially constant corona current for the electrostatic precipitation device120, thereby minimizing the effect of changes in operating parameters, such as air density, humidity, surface condition of the corona point, on the generation of the corona discharge.

During operation, the delivery elements136provide the substantially constant corona current to the wire electrode134. The wire electrode134produces, in turn, a corona discharge to charge or ionize the particulate matter44within the air sample42in the vicinity of the wire electrode134. With the wire electrode134forming either a positive or negative potential relative to the metal membrane36, the metal membrane36electrostatically attracts the oppositely charged particulate matter44.

FIGS. 1,6, and7illustrate configurations of the particulate mass monitor20having a single metallic membrane36that forms part of a single sensing stage. However, during operation, variations in temperature of the air sample42over time can affect the resonant oscillation frequency of the metallic membrane36. In turn, such an effect on the resonant oscillation frequency of the metallic membrane36reduces the accuracy of the measurement of the particulate mass concentration of an air sample42.

FIG. 8illustrates an arrangement of the particulate mass monitor20where the particulate mass monitor20includes two sensing stages140,142. The particulate mass monitor20has a first sensing stage140having a first metallic membrane36-1and a second sensing stage142having a second metallic membrane36-2. In such an arrangement, the particulate mass monitor20substantially isolates the second metallic membrane36-2from the particulate matter delivered to the first metallic membrane36-1by the particle collector38. For example, in one arrangement, the particulate mass monitor20includes the first sensing stage140and the second sensing stage142arranged in series and separated by a barrier146. The barrier146provides physical and acoustic isolation of the first sensing stage140from the second sensing stage142. In one arrangement, the barrier146is configured as a particle filter to minimize or eliminate the presence of particulate matter within the air sample144.

During operation, the controller40compares the resonant oscillation frequencies of the first metallic membrane36-1and the second metallic membrane36-2both before and after the particle collector38directs the particulate matter44to the first metallic membrane36-1. Such comparison minimizes or eliminates the effect that a variance in the temperature of the air sample42has on the air sample mass concentration measurements made by the particulate mass monitor20over time. As such the use of the two sensing stages140,142increases the accuracy of the air sample mass concentration measurements made by the particulate mass monitor20. The following outlines the theory behind the use of two sensing stages.

If the metallic membranes36-1,36-2have similar masses and tensions, their respective resonance frequencies will also be similar (although the resonance frequencies of the metallic membranes36-1,36-2are unlikely to be identical). As indicated above, the basic mass sensing equation for a resonating metallic membrane, is given as:
Δm=m0[(f0/ff)2−1]  (1)
where Δm is the mass increment, f0is the initial or tare resonant frequency, ffis the resonant frequency after particle collection, and m0is the initial or tare mass of the metallic membrane.

For a two-stage system, an initial ratio of resonant frequencies R0is given as:
R0=f0m/f0r(2)
where f0mis the initial or tare frequency of the first or mass sensing stage140and f0ris the initial frequency of the second or reference stage142. After sampling, or during the run, a new ratio Rfis defined by:
Rf=ffm/ffr(3)
where ffmand ffrare the resonant frequencies of the metallic membranes36-1,36-2of the mass sensing stage140and the reference stage142, respectively, after particle collection has occurred on the first metallic membranes36-1in the first stage140. Equation (1), therefore, becomes:
Δm=m0[(R0/Rf)2−1]  (4)
where R0is the reference ratio of the resonant frequencies, (e.g., as stored in memory associated with the controller40during a run and updated at the start of every run). The value Rfdecreases as particle collection by the first metallic membranes36-1occurs. Equation (4), therefore, is based upon the relative resonant oscillation frequencies of the first36-1and second36-2metallic membranes. Any drifting or change in the resonant frequencies of the first metallic membrane36-1and the second metallic membrane36-2as caused by changes in temperature, air density, or gravity, for example, are minimized or eliminated because the changes affects equally both R0and Rf.

FIG. 9is a flow chart160of a procedure performed by the particulate mass monitor20illustrated inFIG. 8. During the procedure, the controller40compares the resonant oscillation frequencies of the first metallic membrane36-1and the second metallic membrane36-2both before and after the particle collector38directs the particulate matter44to the first metallic membrane36-1. Such comparison minimizes or eliminates the effect that a variance in the temperature of the air sample42has on the air sample mass concentration measurements made by the particulate mass monitor20over time.

In step162, the controller40at a first time period, generates and detects a first resonant oscillation frequency in the first metallic membrane36-1and a first resonant oscillation frequency in the second metallic membrane36-2. For example, the controller40generates a resonant oscillation frequency in the first metallic membrane36-1and in the second metallic membrane36-2. In such an arrangement, the controller40utilizes a first oscillation element46-1and first detector element48-1to iteratively generate and detect a resonant oscillation frequency within the first metallic membranes36-1until the oscillation frequency of the first metallic membranes36-1reaches the resonant frequencies of the metallic membrane36-1. Additionally, the controller40utilizes a second oscillation element46-2and second detector element48-2to iteratively generate and detect an oscillation frequency within the second metallic membranes36-2until the oscillation frequency of the second metallic membranes36-2reaches the resonant frequencies of the second metallic membrane36-2.

In step164, the controller40calculates a baseline ratio based upon a ratio between the first resonant oscillation frequency in the first metallic membrane36-1and the first resonant oscillation frequency in the second metallic membrane36-2. For example, as indicated above, an initial ratio of the first and second resonant oscillation frequencies R0is given as:
R0=f0m/f0r
where f0mis the initial or tare resonant frequency of the first metallic membrane36-1and f0ris the initial resonant frequency of the second metallic membrane36-2.

In step166, the controller40at a second time period, after the particle collector38directs the particulate matter44within the air sample42to the first metallic membrane36-1and the particulate mass monitor20directs the air sample144to the second metallic membrane36-2, generates and detects a second resonant oscillation frequency in the first metallic membrane36-1and a second resonant oscillation frequency in the second metallic membrane36-2. For example, during operation, the particle collector38, such as an electrostatic discharge device, produces a corona discharge to electrically charge particulate matter44within the air sample42(i.e., the air sample oriented in the vicinity of the electrostatic discharge device). The charged particulate matter44travels toward, and adheres to, the metallic membrane36. The corona discharge also creates an airflow144directed from the electrostatic precipitation device38towards the metallic membrane36, as caused by an ion wind effect. With the electrically charged particulate matter44collected by the first metallic membrane36-1, the electrostatic discharge device provides the air sample to the second metallic membrane36-2where the air sample144is substantially free of particulate matter.

The controller40utilizes the first oscillation element46-1and first detector element48-1to iteratively generate and detect a second oscillation frequency within the first metallic membrane36-1until the oscillation frequency of the first metallic membrane36-1reaches the resonant frequencies of the metallic membrane36-1. Additionally, the controller40utilizes a second oscillation element46-2and second detector element48-2to iteratively generate and detect an oscillation frequency within the second metallic membranes36-2until the oscillation frequency of the second metallic membranes36-2reaches the resonant frequencies of the second metallic membrane36-2.

In step168, the controller40calculates a sampling ratio based upon a ratio between the second resonant oscillation frequency in the first metallic membrane36-1and the second resonant oscillation frequency in the second metallic membrane36-2. For example, the controller40(e.g., a computer processor associated with the controller40) calculates the sampling ratio using the equation:
Rf=ffm/ffr
where ffmand ffrare the second resonant frequencies of the metallic membranes36-1,36-2of the mass sensing stage140and the reference stage142, respectively, after particle collection has occurred on the first metallic membranes36-1in the first stage140.

In step170, the controller40detects a particulate mass concentration of the air sample42based upon a relation between the baseline ratio and the sampling ratio. For example, the controller40detects or calculates the particulate mass concentration of the air sample42using the equation:
Δm=m0[(R0/Rf)2−1]
where Δm is based upon a ratio between R0and Rf, the relative resonant oscillation frequencies of the first36-1and second36-2metallic membranes. Any drifting or change in the resonant frequencies of the first metallic membrane36-1and the second metallic membrane36-2as caused by changes in temperature, air density, or gravity, for example, are minimized or eliminated because the changes affect equally both R0and Rf.

In one arrangement, the particulate mass monitor20includes a membrane transporter45coupled to the metallic membrane36. The membrane transporter45is configured to advance the metallic membrane36relative to the particle collector38such that a clean or unused metallic membrane36orients relative to the particle collector38prior to the particulate mass monitor20measuring the particulate mass concentration of the air sample42. In one arrangement, the membrane transporter45advances the metallic membrane36in a substantially automated manner, thereby minimizing the necessity for manual replacement of the metallic membrane over time and allowing for long term, unattended operation of the particulate mass monitor20.

FIG. 10, for example, illustrates an arrangement of the membrane transporter45and metallic membrane36where the particulate mass monitor20includes two sensing stages140,142as inFIG. 8. As shown, the metallic membrane36is configured as a metallic membrane tape62. The membrane transporter45advances the metallic membrane tape62from a source (i.e., source roll)64to a destination (i.e., destination roll)66. During operation, the membrane transporter45advances the metallic membrane36within the particulate mass monitor20such that a first metallic membrane portion36-1orients in proximity to the particle collector38at the mass sensing stage140and membrane portion36-2at the reference sensing stage142.

During operation, the controller40compares the resonant oscillation frequencies of the first metallic membrane portion36-1and the second metallic membrane portion36-2both before and after the particle collector38directs the particulate matter44to the first membrane portion36-1. Such comparison minimizes or eliminates the effect that a variance in temperature of the air sample42has on the air sample mass concentration measurements made by the particulate monitor20over time. The same theory described above for the configuration ofFIG. 8applies to the configuration ofFIG. 10.

As shown inFIG. 10, the membrane transporter45adjusts a position of the metallic membrane tape62relative to the particle collector38such that the first metallic membrane portion36-1is configured to receive particulate matter44from the particle collector38during a first test.

After a given period of time, or after exposure to a relatively large amount of particulate matter44, the first metallic membrane portion36-1becomes saturated with particulate matter44. Such saturation can affect the particulate mass measurements made by the particulate mass monitor20, as a limited amount of particular matter44can be collected on the first metallic membrane portion36-1. The membrane transporter45, therefore, is configured to advance the metallic membrane36within the particulate mass monitor20such that a second metallic membrane portion36-2orients in proximity to (e.g., becomes aligned with) the particle collector38during a second particulate mass monitor test. The membrane transporter45advances a “clean” or unused portion of the metallic membrane tape62relative to the particle collector38, thereby allowing the metallic membrane tape to receive a maximal amount of particulate matter44from the particle collector38. Correspondingly, a new clean portion of the metallic membrane tape moves to the reference sensing stage to provide the new reference resonant oscillation frequency against which the resonant oscillation frequency of the mass sensing stage is compared to compensate for any temperature effects. As such, the membrane transporter45acts to maintain the measurement accuracy of the particulate mass monitor20over time.

In one arrangement, the particulate mass monitor20includes a tensioning mechanism50configured to generate the tension47within the metallic membrane36relative to a substantially planar surface defined by the metallic membrane. For example, as shown inFIG. 10, the membrane transporter45includes the tensioning mechanism50. During operation, the membrane transporter45applies the tensioning mechanism50to the first metallic membrane portion36-1and the second metallic membrane portion36-2to generate a tension within the portions36-2,36-2. In one arrangement, the membrane transporter45utilizes a sensor (not shown) coupled to the controller40to adjust the tension of the portions36-1,36-2(e.g., to obtain a specific resonant frequency of the metallic membrane portions36-1,36-2).

FIG. 11illustrates a more detailed architecture of a particulate mass monitor20configured as a computerized device180. The computerized device180includes the controller40formed of a memory182and a processor184. A computer program product186includes an application or logic instructions that are loaded into the computer device180to configure the device180to perform as a particulate mass monitor20.

The particulate mass monitor20, in this example, includes an interconnection mechanism188such as a data bus and/or other circuitry that interconnects the controller memory182and the processor184, and one or more communications interfaces190. The communication interface188connects with the output device54via connections192.

The memory182may be any type of volatile or non-volatile memory or storage system such as computer memory (e.g., random access memory (RAM), read-only memory (ROM), or other electronic memory), disk memory (e.g., hard disk, floppy disk, optical disk and so forth). The memory182is encoded with logic instructions (e.g., software code) and/or data that form a mass concentration sensing application194configured according to embodiments of the invention. In other words, the mass concentration sensing application194represents software code, instructions and/or data that represent or convey the processing logic steps and operations as explained herein and that reside within memory or storage or within any computer readable medium accessible to the particulate mass monitor20.

The processor184represents any type of circuitry or processing device such as a central processing unit, microprocessor, application-specific integrated circuit, programmable gate array, or other circuitry that can access the mass concentration sensing application194encoded within the memory182over the interconnection mechanism188in order to execute, run, interpret, operate or otherwise perform the mass concentration sensing application194logic instructions. Doing so forms the mass concentration sensing process196. In other words, the mass concentration sensing process196represents one or more portions of the logic instructions of the content portion reception application while being executed or otherwise performed on, by, or in the processor184within the particulate mass monitor20. The particulate mass monitor20inFIG. 1collectively represents either one or both of the mass concentration sensing application194and the mass concentration sensing process196.

For example, in one arrangement described above, the controller40is configured to induce, as the first oscillation frequency70, a fundamental resonant oscillation frequency in the metallic membrane36. The resonant frequency is defined as the natural oscillation of a mechanical system (e.g., the metallic membrane36) proportional to the square root of the ratio of the system stiffness constant (e.g., a metallic membrane stiffness constant) and the system mass (e.g., a metallic membrane mass). Such description is by way of example only. In one arrangement the controller40is configured to generate a modal oscillation frequency in the metallic membrane36where the modal oscillation frequency is based upon a fundamental resonant oscillation frequency of the metallic membrane36.

FIG. 12illustrates oscillations nodes associated with the resonant oscillation of the metallic membrane36.FIGS. 12B–12Lillustrate oscillation nodes associated with oscillation of the metallic membrane36at modes higher than the fundamental resonant oscillation frequency. The higher modes of resonant oscillation are not harmonically related to the fundamental resonant frequency of the metallic membrane36. InFIG. 12Awhere the metallic membrane oscillates at the fundamental resonant frequency, the nodal line (i.e., static line)200associated with the oscillation is the circular periphery of the metallic membrane36. Oscillation at higher modes, as illustrated inFIGS. 12B through 12L, involves further diametrical and/or circular (i.e., concentric) nodes in addition to the peripheral node. By oscillating the metallic membrane36at higher resonance modes, the controller40increases the sensitivity and accuracy of the particulate mass measurement of the air sample42(i.e., the higher resonance modes produce a larger change in frequency in the metallic membrane36for a given mass increment).

The metallic membrane36has substantially uniform material properties throughout and, when placed under tension, substantially maintains the tensile load47over time. Because of the material and mechanical properties of the metallic membrane36, when the controller40provides a modal resonant oscillation frequency input signal to the metallic membrane36, the metallic membrane36responds by providing a modal resonance oscillation output that substantially corresponds to the input signal.

As indicated above, in one arrangement, the particulate mass monitor20includes a membrane transporter45coupled to the metallic membrane36. As illustrated inFIG. 10, the metallic membrane36is configured as a metallic membrane tape62. During operation, the membrane transporter45advances the metallic tape62relative to the particle collector38such that, at a certain time, a clean or unused portion of the metallic membrane tape62orients relative to the particle collector38. Such description is by way of example only. In one arrangement, the metallic membrane36is configured as a pre-tensioned cartridge. In such a configuration, the membrane transporter45advances multiple cartridges within the particulate mass monitor20relative to the particle collector38. The membrane transporter45, therefore, provides clean or unused cartridges relative to the particle collector38over certain time intervals.

As described above with reference toFIG. 10, the metallic membrane36is configured as a metallic membrane tape62where the membrane transporter45advances the metallic membrane tape62from a source (i.e., source roll)64to a destination (i.e., destination roll)66. Such description is by way of example only. In another arrangement, the metallic membrane36is configured as magazine of pre-tensioned metallic membrane cartridges. During operation, the membrane transporter45advances individual magazine cartridges within the particulate mass monitor20, relative to the particle collector38, during testing.