Contaminants such as turbine engine oil and hydraulic fluid can be present in air or other gasses in the cabin/cockpit of an aircraft, during flight and/or during ground operation. Certain contaminants can be present in aerosol form, particulate form, and/or gaseous form, and the quantity of contaminants can vary significantly, even over orders of magnitude, leading to sensor fouling and/or delayed sensor response. When a plurality of contaminants are present, they may differ in quantity, such that certain contaminants (present in higher or lower concentrations than other contaminants) are detected while others are not detected. Detecting and identifying the composition or type of contamination is often needed to protect health and/or equipment, detect faults, and help identify the source or cause of the contamination. Inability to detect and identify the contamination may cause the need for a flight diversion, flight cancellation, or emergency landing to ensure the safety of passengers and crew, which, at a minimum, is an inconvenience, and increases costs. US2016327518A1 discloses an exemplary gravimetric type gas sensor to capture and concentrate target gases.

There is a need for improved methods for detection and detection systems. The present invention provides for ameliorating at least some of the disadvantages of the prior art. These and other advantages of the present invention will be apparent from the description as set forth below.

The invention provides a method for determining and classifying by type aircraft air contaminants, the method comprising (a) passing a sample of aircraft air through an aircraft air contaminant analyzer and through at least one aircraft air contaminant collector along a first sample flow path at a first sample flow rate and/or at a first sample flow duration, while passing another sample of aircraft air through the aircraft air contaminant analyzer and through a bypass section along a second sample flow path bypassing the at least one aircraft air contaminant collector at a second sample flow rate and/or at a second sample flow duration, the at least one aircraft air contaminant collector comprising (i) a microporous medium comprising microporous flow-through channels arranged across the first sample flow path, the microporous medium having a chemoselective coating; and, (ii) a thin film resistive heater, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with the microporous medium; the aircraft air contaminant analyzer also including a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for classifying air contaminant type; and wherein the bypass section comprises a bypass channel, the bypass channel including the second sample flow path; (a') the aircraft air contaminant analyzer further comprising a first pump generating flow along the first sample flow path; and a second pump generating flow along the second sample flow path; (b) controlling the first sample flow rate and/or the first sample flow duration through the at least one aircraft air contaminant collector along the first sample flow path while independently controlling the second sample flow rate and/or the second sample flow duration through the bypass section along the second sample flow path, wherein the first sample flow rate and/or the first sample flow duration is/are initially set at a low value for a first measurement of response signal magnitude; (c) capturing air contaminants by the microporous medium; (d) discontinuing passing aircraft air through the at least one aircraft air contaminant collector along the first sample flow path; (e) heating the microporous medium to a temperature sufficient to vaporize the captured air contaminants and desorb the captured air contaminants; (f) receiving the desorbed air contaminants on the gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor; (g) measuring the proportionate resonant frequency response generated by the gravimetric sensor as the air contaminant is added to and removed from the gravimetric sensor, determining the signal magnitude from the proportionate resonant frequency response, determining the air contaminant concentration, classifying the air contaminant type, and outputting the determined air contaminant concentration and classified air contaminant type; (h) executing an air contaminant recognition program stored upon a computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and the first sample flow durations along the first sample flow path; (i) determining a target level for the signal magnitude, and continuously repeating (b)-(h) and measuring response signal magnitudes and adjusting the first sample flow rate and/or the first sample flow duration based upon the previously measured signal magnitude such that the first sample flow rate and/or the first sample flow duration is increased when the signal magnitude is lower than the target level, by an amount proportionate to how much lower the signal magnitude is below the target level, to maintain the signal magnitude at the target level, and the first sample flow rate and/or the first sample flow duration is decreased when signal magnitude is higher than the target level, by an amount proportionate to how much higher the signal magnitude is above the target level, to maintain the signal magnitude at the target value; (j) executing the air contaminant recognition program stored upon the computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and/or the first sample flow durations along the first sample flow path; and, (k) outputting the determined air contaminant concentration and air contaminant type.

Alternatively, the invention provides a method for determining and classifying by type aircraft air contaminants, the method comprising (a) passing a sample of aircraft air through an aircraft air contaminant analyzer and through at least one aircraft air contaminant collector along a first sample flow path at a first sample flow rate and/or at a first sample flow duration, while passing another sample of aircraft air through the aircraft air contaminant analyzer and through a bypass section along a second sample flow path bypassing the at least one aircraft air contaminant collector at a second sample flow rate and/or at a second sample flow duration, the at least one aircraft air contaminant collector comprising (i) a microporous medium comprising microporous flow-through channels arranged across the first sample flow path, the microporous medium having a chemoselective coating; and, (ii) a thin film resistive heater, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with the microporous medium; the aircraft air contaminant analyzer also including a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for classifying air contaminant type; and wherein the bypass section comprises a bypass channel, the bypass channel including the second sample flow path; (a') the aircraft air contaminant analyzer further comprising a first pump generating flow along the first sample flow path; and a second pump generating flow along the second sample flow path; (b) controlling the first sample flow rate and/or the first sample flow duration through the at least one aircraft air contaminant collector along the first sample flow path while independently controlling the second sample flow rate and/or the second sample flow duration through the bypass section along the second sample flow path, wherein the first sample flow rate and/or the first sample flow duration is/are initially set at a low value for a first measurement of response signal magnitude; (c) capturing air contaminants by the microporous medium; (d) discontinuing passing aircraft air through the at least one aircraft air contaminant collector along the first sample flow path; (e) heating the microporous medium to a temperature sufficient to vaporize the captured air contaminants and desorb the captured air contaminants; (f) receiving the desorbed air contaminants on the gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor; (g) measuring the proportionate resonant frequency response generated by the gravimetric sensor as the air contaminant is added to and removed from the gravimetric sensor, determining signal magnitude from the proportionate resonant frequency response, determining the air contaminant concentration, classifying the air contaminant type, and outputting the determined air contaminant concentration and classified air contaminant type; (h) executing an air contaminant recognition program stored upon a computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and the first sample flow durations along the first sample flow path; (i) determining an upper threshold and a lower threshold for the signal magnitude for the contaminant type and continuously repeating (b)-(h) and measuring response signal magnitudes and adjusting the first sample flow rate and/or the first sample flow duration based upon the previously measured signal magnitude such that the first sample flow rate and/or the first sample flow duration is increased when the signal magnitude is lower than the lower threshold, to the next pre-determined higher sensitivity level, to maintain the signal magnitude between the upper threshold and the lower threshold, and the first sample flow rate and/or the first sample flow duration is decreased when signal magnitude is higher than the upper threshold, to the next pre-determined lower sensitivity level, to maintain the signal magnitude to maintain the signal magnitude between the upper threshold and the lower threshold; (k) executing the air contaminant recognition program stored upon the computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and the first sample flow durations along the first sample flow path; and, (1) outputting the determined air contaminant concentration and air contaminant type.

An aircraft air contaminant analyzer according to the invention comprises (a) at least one aircraft air contaminant collector comprising (i) a microporous medium comprising microporous flow-through channels and a chemoselective coating, wherein the microporous medium remains functional and desorbs captured air contaminants while being heated for a controlled time period; (ii) a thin film resistive heater, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with the microporous medium; (b) a bypass section, comprising a bypass channel; (c) a first substrate, having a top surface and a bottom surface; wherein the contaminant collector is associated with the first substrate, the microporous medium and heater being thermally insulated from the first substrate; (d) a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for classifying air contaminant type; (e) a second substrate, having a top surface and a bottom surface; wherein the gravimetric sensor is associated with the top surface of the second substrate, the gravimetric sensor being separated from the contaminant collector by a constant distance, the gravimetric sensor being arranged to receive air contaminants desorbed from the membrane when the membrane is heated; (f) a support comprising a top surface and a bottom surface, the support comprising at least one aircraft air inlet port and a bypass inlet port, the at least one aircraft air inlet port, and the bypass inlet port passing through the top surface and the bottom surface of the support, wherein the bottom surface of the second substrate is associated with the top surface of the support; (g) a first sample flow path, passing through the at least one aircraft air contaminant collector; (h) a second sample flow path, bypassing the at least one aircraft air contaminant collector; (i) a first pump, arranged to generate flow of aircraft air along the first sample flow path through the at least one aircraft air inlet port and through the at least one air contaminant collector before and after the microporous medium is heated; (j) a second pump arranged to generate flow of aircraft air through the bypass inlet port along the second sample flow path through the bypass section and the bypass channel; (k) a resonant frequency measurement device, arranged to measure the proportionate resonant frequency response generated by the gravimetric sensor as the air contaminant is added to and removed from the gravimetric sensor; (1) a computer readable medium bearing an air contaminant recognition program and calibration data; (m) a processor configured to execute the air contaminant recognition program, the contaminant recognition program including a module configured to classify the air contaminant by type and to measure response signal magnitudes, and a module programmed to use the calibration data for comparison with magnitude of the proportionate resonant frequency response generated by the gravimetric sensor to calculate air contaminant concentration and to determine a target value for air contaminant type, and to used measured response magnitudes to adjust first sample flow rates and/or first sample flow durations based upon previously measured response magnitudes.

• Figure 1 is a diagrammatic representation of an illustrative aircraft air contaminant analyzer according to the invention, including first and second pumps, and at least one aircraft air contaminant collector comprising a microporous medium and a thin film resistive heater, wherein the microporous medium and the heater are provided along a first sample flow path; the analyzer also including a gravimetric sensor, and a bypass section comprising a bypass channel, providing a second sample flow path bypassing the aircraft air contaminant collector, wherein the first pump generates sample flow along the first sample flow path, and the second pump generates sample flow along the second sample flow path.
• Figure 2 is a diagrammatic representation showing an illustrative gravimetric sensor (having first and second electrodes), the sensor also including a balance capacitor (having first and second balance capacitor electrodes) as part of an aircraft air contaminant analyzer according to the invention.
• Figure 3A is a diagrammatic top view of the air contaminant collector shown in Figure 1, showing the microporous membrane, also showing a chemoselective coating on the membrane, and the thin film resistive heater, also showing a base, and tethers, wherein the tethers connect the microporous membrane to the base. Figure 3B is a diagrammatic enlarged view of a portion of the air contaminant collector shown in Figure 3A, showing channels in the base providing tethers for connecting the microporous membrane to the base, also showing the thin film resistive heater associated with the top surface of the microporous membrane (surrounding the flow-through channels of the microporous membrane), and on the tethers, also showing electrical traces and the chemoselective coating, wherein only portions of the coating and the heater are shown so that other components can also be shown. Figure 3C shows an enlarged view of the bottom surface of the microporous membrane, also showing the bottom surfaces of the tethers connecting the microporous membrane to the base. Figure 3D is a diagrammatic cross-sectional view of an embodiment of the air contaminant collector with the coating, also showing electrical traces, and an insulator layer, wherein the traces are on top of the heater and insulating layer, and the insulating layer forms the top surface of the microporous membrane.
• Figure 4 shows the signal magnitude versus flow rate (at standard liter per minute (SLM)) along the first sample flow path through the collector for deicing fluid, turbine engine oil, and hydraulic fluid, each at a fixed concentration, wherein the flow along the second sample flow path bypassing the collector is kept constant at 1.0 SLM.
• Figure 5 shows, as a composite graph, a sequence of measurements as the aircraft air contaminant analyzer is challenged with three different concentrations of turbine engine oil, wherein the sampling time and flow rate through the collector are set prior to each challenge, so that the signal magnitudes of all three challenges are approximately equal.
• Figure 6 shows determining the response spectra for deicing fluid, using the air craft air contaminant analyzer according to the invention.
• Figure 7 shows the "thermal subtracted response" resulting from subtracting the response spectra for the absence of a contaminant (deicing fluid) from the response spectra in the presence of deicing fluid.
• Figure 8 shows four features that are calculated from the thermal subtracted responses: a) Maximum frequency shift (MFS); b) Sum before peak (SB); c) Sum after peak (SA); and d) Segment #5 (S5).
• Figures 9 and 10 show using the feature MFS of two aircraft air contaminant collectors with different chemoselective coatings to distinguish between contaminants.

The invention provides a method for determining and classifying by type aircraft air contaminants, the method comprising (a) passing a sample of aircraft air through an aircraft air contaminant analyzer and through at least one aircraft air contaminant collector along a first sample flow path at a first sample flow rate and/or at a first sample flow duration, while passing another sample of aircraft air through the aircraft air contaminant analyzer and through a bypass section along a second sample flow path bypassing the at least one aircraft air contaminant collector at a second sample flow rate and/or at a second sample flow duration, the at least one aircraft air contaminant collector comprising (i) a microporous medium comprising microporous flow-through channels arranged across the first sample flow path, the microporous medium having a chemoselective coating; and, (ii) a thin film resistive heater, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with the microporous medium; the aircraft air contaminant analyzer also including a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for classifying air contaminant type; and wherein the bypass section comprises a bypass channel, the bypass channel including the second sample flow path; (a') the aircraft air contaminant analyzer further comprising a first pump generating flow along the first sample flow path; and a second pump generating flow along the second sample flow path; (b) controlling the first sample flow rate and/or the first sample flow duration through the at least one aircraft air contaminant collector along the first sample flow path while independently controlling the second sample flow rate and/or the second sample flow duration through the bypass section along the second sample flow path, wherein the first sample flow rate and/or the first sample flow duration is/are initially set at a low value for a first measurement of response signal magnitude; (c) capturing air contaminants by the microporous medium; (d) discontinuing passing aircraft air through the at least one aircraft air contaminant collector along the first sample flow path; (e) heating the microporous medium to a temperature sufficient to vaporize the captured air contaminants and desorb the captured air contaminants; (f) receiving the desorbed air contaminants on a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor; (g) measuring the proportionate resonant frequency response generated by the gravimetric sensor as the air contaminant is added to and removed from the gravimetric sensor, determining the signal magnitude from the proportionate resonant frequency response, determining the air contaminant concentration, classifying the air contaminant type, and outputting the determined air contaminant concentration and classified air contaminant type; (h) executing an air contaminant recognition program stored upon a computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and the first sample flow durations along the first sample flow path; (i) determining a target level for the signal magnitude, and continuously repeating (b)-(h) and measuring response signal magnitudes and adjusting the first sample flow rate and/or the first sample flow duration based upon the previously measured signal magnitude such that the first sample flow rate and/or the first sample flow duration is increased when the signal magnitude is lower than the target level, by an amount proportionate to how much lower the signal magnitude is below the target level, to maintain the signal magnitude at the target level, and the first sample flow rate and/or the first sample flow duration is decreased when signal magnitude is higher than the target level, by an amount proportionate to how much higher the signal magnitude is above the target level, to maintain the signal magnitude at the target value; (j) executing the air contaminant recognition program stored upon the computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and/or the first sample flow durations along the first sample flow path; and, (k) outputting the determined air contaminant concentration and air contaminant type.

Alternatively, the invention provides a method for determining and classifying by type aircraft air contaminants, the method comprising (a) passing a sample of aircraft air through an aircraft air contaminant analyzer and through at least one aircraft air contaminant collector along a first sample flow path at a first sample flow rate and/or at a first sample flow duration, while passing another sample of aircraft air through the aircraft air contaminant analyzer and through a bypass section along a second sample flow path bypassing the at least one aircraft air contaminant collector at a second sample flow rate and/or at a second sample flow duration, the at least one aircraft air contaminant collector comprising (i) a microporous medium comprising microporous flow-through channels arranged across the first sample flow path, the microporous medium having a chemoselective coating; and, (ii) a thin film resistive heater, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with the microporous medium; the aircraft air contaminant analyzer also including a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for classifying air contaminant type; and wherein the bypass section comprises a bypass channel, the bypass channel including the second sample flow path; (a') the aircraft air contaminant analyzer further comprising a first pump generating flow along the first sample flow path; and a second pump generating flow along the second sample flow path; (b) controlling the first sample flow rate and/or the first sample flow duration through the at least one aircraft air contaminant collector along the first sample flow path while independently controlling the second sample flow rate and/or the second sample flow duration through the bypass section along the second sample flow path, wherein the first sample flow rate and/or the first sample flow duration is/are initially set at a low value for a first measurement of response signal magnitude; (c) capturing air contaminants by the microporous medium; (d) discontinuing passing aircraft air through the at least one aircraft air contaminant collector along the first sample flow path; (e) heating the microporous medium to a temperature sufficient to vaporize the captured air contaminants and desorb the captured air contaminants; (f) receiving the desorbed air contaminants on a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor; (g) measuring the proportionate resonant frequency response generated by the gravimetric sensor as the air contaminant is added to and removed from the gravimetric sensor, determining signal magnitude from the proportionate resonant frequency response, determining the air contaminant concentration, classifying the air contaminant type, and outputting the determined air contaminant concentration and classified air contaminant type; (h) executing an air contaminant recognition program stored upon a computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and the first sample flow durations along the first sample flow path; (i) determining an upper threshold and a lower threshold for the signal magnitude for the contaminant type and continuously repeating (b)-(h) and measuring response signal magnitudes and adjusting the first sample flow rate and/or the first sample flow duration based upon the previously measured signal magnitude such that the first sample flow rate and/or the first sample flow duration is increased when the signal magnitude is lower than the lower threshold, to the next pre-determined higher sensitivity level, to maintain the signal magnitude between the upper threshold and the lower threshold, and the first sample flow rate and/or the first sample flow duration is decreased when signal magnitude is higher than the upper threshold, to the next pre-determined lower sensitivity level, to maintain the signal magnitude to maintain the signal magnitude between the upper threshold and the lower threshold; (k) executing the air contaminant recognition program stored upon the computer-readable medium, including calculating air contaminant concentration using the measured signal magnitudes and first sample flow rates and the first sample flow durations along the first sample flow path; and, (1) outputting the determined air contaminant concentration and air contaminant type.

In accordance with the method, the air contaminants comprise aerosols and/or particulates, and/or vapor(s).

An aircraft air contaminant analyzer according to the invention comprises (a) at least one aircraft air contaminant collector comprising (i) a microporous medium comprising microporous flow-through channels and a chemoselective coating, wherein the microporous medium remains functional and desorbs captured air contaminants while being heated for a controlled time period; (ii) a thin film resistive heater, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with the microporous medium; (b) a bypass section, comprising a bypass channel; (c) a first substrate, having a top surface and a bottom surface; wherein the contaminant collector is associated with the first substrate, the microporous medium and heater being thermally insulated from the first substrate; (d) a gravimetric sensor arranged to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for classifying air contaminant type; (e) a second substrate, having a top surface and a bottom surface; wherein the gravimetric sensor is associated with the top surface of the second substrate, the gravimetric sensor being separated from the contaminant collector by a constant distance, the gravimetric sensor being arranged to receive air contaminants desorbed from the membrane when the membrane is heated; (f) a support comprising a top surface and a bottom surface, the support comprising at least one aircraft air inlet port and a bypass inlet port, the at least one aircraft air inlet port, and the bypass inlet port passing through the top surface and the bottom surface of the support, wherein the bottom surface of the second substrate is associated with the top surface of the support; (g) a first sample flow path, passing through the at least one aircraft air contaminant collector; (h) a second sample flow path, bypassing the at least one aircraft air contaminant collector; (i) a first pump, arranged to generate flow of aircraft air along the first sample flow path through the at least one aircraft air inlet port and through the at least one air contaminant collector before and after the microporous medium is heated; (j) a second pump arranged to generate flow of aircraft air through the bypass inlet port along the second sample flow path through the bypass section and the bypass channel; (k) a resonant frequency measurement device, arranged to measure the proportionate resonant frequency response generated by the gravimetric sensor as the air contaminant is added to and removed from the gravimetric sensor; (1) a computer readable medium bearing an air contaminant recognition program and calibration data; (m) a processor configured to execute the air contaminant recognition program, the contaminant recognition program including a module configured to classify the air contaminant by type and to measure response signal magnitudes, and a module programmed to use the calibration data for comparison with magnitude of the proportionate resonant frequency response generated by the gravimetric sensor to calculate air contaminant concentration and to determine a target value for air contaminant type, and to used measured response magnitudes to adjust first sample flow rates and/or first sample flow durations based upon previously measured response magnitudes.

In accordance with a method for determining and classifying by type aircraft air contaminants according to the invention, the method including passing a sample of aircraft air through an aircraft air contaminant analyzer along a first sample flow path including a gravimetric sensor at a first sample flow rate and/or at a first sample flow duration through an aircraft air contaminant collector, while passing another sample of aircraft air through the aircraft air contaminant analyzer along a second sample flow path at a second sample flow rate and/or at a second sample flow duration through a bypass section bypassing the aircraft air contaminant collector; while repeatedly measuring the gravimetric sensors' response, the aircraft air contaminant collector microporous medium (e.g., a microporous membrane) is heated to vaporize the collected contaminants passed along a first sample flow path such that the vaporized contaminants are transferred to the gravimetric sensor to be measured. The measurement rate is sufficient to resolve the gravimetric sensor's response, which is in the shape of the sensor's frequency versus time curve as it absorbs and subsequently desorbs contaminants released from the microporous medium.

In accordance with one embodiment, flow rates and/or flow durations are adjusted such that the measured signal magnitude is maintained between upper and lower target thresholds.

Advantageously, the sensitivity of an aerosol composition analyzer can be rapidly tuned to achieve both adequate sensitivity and reduced fouling over a range of contaminant concentrations and vapor pressures, while simultaneously providing rapid response and recovery times. Thus, for example, fouling can be reduced when the contaminant concentration is high and the sampling volume is low, and a rapid response can be provided even when contaminant concentration is low and the sampling volume is high, while maintaining sensitivity under both conditions. In another advantage, when multiple contaminants are present, some having different concentrations and vapor pressures than others, the analyzer can be rapidly tuned to detect the different contaminants.

The aircraft air contaminant analyzer according to the invention is not "single use," e.g., it is resistant to fouling and can be used to repeatedly measure the contaminant concentration(s) and determine the contaminant type(s).

In another advantage, particularly when two or more aircraft air contaminant collectors are utilized, different fluids with similar properties (e.g., vapor pressure and/or density) can be more accurately classified.

An analyzer can be located in the ECS (Environmental Control System) vent or duct since there will be a delay before the contaminant concentration in the large volume cabin increases to the level coming out of the ECS vents. However, a variety of locations are suitable for an analyzer, such as, e.g., in the cockpit, cabin, overhead luggage compartment, storage compartment, galley area, avionics bay, auxiliary power units, etc. Alternatively, an analyzer can be installed in one location and air from another location directed to the analyzer via a variety of air transfer devices including, e.g., piping, tubing, and/or ducts.

Alternatively, or additionally, an analyzer can be located, e.g., at or near a bleed air line, wherein pressurized air from an engine is transferred to the ECS. One benefit of an analyzer at or near the bleed air line is that sampling bleed air from each engine informs and can identify which engine is faulty, allowing the crew to stop supplying contaminated bleed air from a faulty engine to the ECS. In contrast, an analyzer located in the cabin, whether sampling from the cabin or ECS vent or ECS duct will inform there is a contaminant source, but not which engine or APU (auxiliary power unit) is the source of contamination.

The analyzer includes a measurement circuit to measure frequency at a sufficient rate to precisely resolve the gravimetric sensor's response, typically about 10 to about 100 measurements per second per gravimetric sensor. Measurement is synchronized with other analyzer functions, particularly, the function of heating the microporous medium. Measurement is typically over a duration sufficient to resolve the maximum frequency change and the rate of recovery of the gravimetric sensor's response, typically, for example, a duration of about 1 second to about 4 seconds long.

A sufficient volume of sample at a prescribed rate (for example, about 500 to about 2000 standard cubic centimeter per minute (sccm)) for a prescribed period of time (for example, about 10 to about 60 seconds) is flowed through the analyzer to achieve a response magnitude sufficiently over the measurement noise level to resolve the shape of the sensor's frequency versus time curve, typically, a signal-to-noise ratio of about 4:1 or greater.

The kinetics of transfer, adsorption and desorption of the different contaminants results in different response shapes for the different contaminants. Illustratively, if 4 different compounds (e.g., nitromethane triacetone triperoxide, ethylene glcol dinitrate, and 2,3 dimethyl 2,3 dinitrobutane) were superimposed on a single graph for ease of reference, the shape of sensor frequency versus time responses for the compounds would show that the higher vapor pressure (lighter) compounds are released from the membrane more quickly than the lower vapor pressure (heavier) compounds, e.g., nitromethane is released before triacetone triperoxide, ethylene glcol dinitrate, and 2,3 dimethyl 2,3 dinitrobutane.

The flow along the first sample flow path through the microporous medium (generated by the first pump) should be stopped such that it is zero or nearly zero (e.g., about 5 sccm or less) before the microporous medium is heated, e.g., typically, flow should be stopped for at least 0.2 seconds before heating. Flow along the second sample flow path pump (generated by the second pump) can be stopped at the same time, or can be maintained as long as the flow along the second sample flow path does not induce a flow along the first sample flow path (e.g., by the venturi effect).

Preferably, while repeatedly measuring frequency, the microporous medium is heated by applying a voltage step bringing it to a temperature of at least about 400°C in about 0.1 seconds. Typically, the microporous medium is heated to at least about 200°C, more typically, at least about 400°C, in some examples, to about 550°C, for at least about 1 second, preferably, at least about 2 seconds (e.g., up to about 10 second, or more), to vaporize (desorb) the contaminant(s) so that the next measurement can begin from a "fresh start." In order to zero out ("self-zero") gravimetric sensor drift, the sensor's response is referenced to the frequency the sensor had just before heating the microporous medium.

When the microporous medium is not being heated, the analyzer is preferably maintained at a fixed temperature, e.g., a fixed temperature in the range of from about 30°C to about 70°C.

The contaminant(s) can be classified by type using a pattern recognition algorithm to recognize each contaminant by its unique response, the shape of the sensor's frequency versus time curve, which is influenced by the contaminant's material properties such as, but not limited to, one or more of any of the following: vapor pressure, heat capacity, heat of condensation, heat of evaporation, absorption and desorption kinetics, and diffusion rate. A variety of algorithms can be used to classify the contaminant(s) from the contaminant-specific response shape. Suitable algorithms include, for example, neural nets, principal component analysis, support vector machine based classification, linear discriminant analysis and decision tree analysis.

Concentration of the contaminant(s) can be calculated by comparing the magnitude of the response(s) to a pre-determined calibration file, e.g., a curve or lookup table giving values for the contaminant concentration(s) as a function of the magnitude of the response(s).

The signal magnitude is above the sensor noise level for more accurate classification of the contaminant by type. Preferably, the signal magnitude is not so large that excess contaminant is collected as that may decrease sensor life.

The signal magnitude may be a frequency shift (e.g., a maximum frequency shift (MFS)) measured by the gravimetric sensor. For example, the signal magnitude is an MFS, the target level is typically in the range of from about 100 Hz to about 1000 Hz, preferably in the range of from about 300 Hz to about 500 Hz.

The frequency shift may not be a MFS, e.g., the frequency shift could be measured at some fixed time after the collector is energized, or a sum of frequencies at two or more times, or a sum (i.e., area under the curve) over the entire measurement or some portion of it.

The gravimetric sensor (which can comprise a single sensor or a sensor array) generates a precise and proportionate frequency response to mass added or removed from the sensor. Preferably, the response is provided over a wide dynamic range, such that it is not over-dampened by small quantities of transferred contaminant (analyte). The gravimetric sensor is operated as part of an amplified oscillator circuit to maintain it at resonance.

Each of the components of the invention will now be described in more detail below, wherein like components have like reference numbers.

In the illustrative embodiment shown in Figure 1, an aircraft air contaminant analyzer 200 comprises at least one aircraft air contaminant collector 1 providing a first sample flow path 1000, the collector comprising a base 10 comprising a first substrate 1011 and a microporous medium 100 (e.g., a microporous membrane 100A) comprising microporous flow-through channels and a chemoselective coating 150, and a thin film resistive heater 175 (wherein the collector is discussed in more detail below with reference to Figures 3A-3D). The illustrated embodiment of the aircraft air contaminant analyzer also includes a bypass section 2001 providing a second sample flow path 2000, the bypass section comprising a bypass channel port 2002 and a bypass channel 2003.

Using Figures 3A-3D for reference, illustrating an aircraft air contaminant collector 1, the collector comprises a base 10, comprising a first substrate 1011, comprising a first substrate primary layer 101 having a first substrate top layer 101A and a first substrate bottom layer 101B (Fig. 3D), and a microporous medium 100 (e.g., a microporous membrane 100A) on the first substrate, the porous medium having a top surface 111 and a bottom surface 112 (Figs. 3C and 3D), the porous medium comprising microporous flow-through channels 115 (through the top surface and the bottom surface of the porous medium) and a chemoselective coating 150 (shown in Figs. 3A, 3B, and 3D), wherein the porous medium remains functional and desorbs captured air contaminants while being heated for a controlled time period, and a thin film resistive heater 175, capable of heating to a temperature that vaporizes captured air contaminants, wherein the heater is in contact with (in and/or on) the top surface of the porous medium; wherein the layers 101A and 101B, the porous medium 100, the heater 175, wire traces 620 (that can communicate with wirebonds (not shown) communicating with the heater 175) and an optional packaging layer 699 (covering at least a portion of the wire traces, e.g., providing low resistance and allowing the wirebonds to form a reliable electrical contact and more efficiently move heater current from the wirebonds to the heater) are associated with (e.g., mounted to or fabricated on) the first substrate primary layer 101 by, for example, additive processes, and channels 115 and tethers 190 (discussed below), as well as the cavity below the porous medium 100 (shown in Fig. 3D) are fabricated by, for example, subtractive processes.

While Figs. 3A-3D show a first substrate 101' comprising a first substrate primary layer 101 having a first substrate top layer 101A and a first substrate bottom layer 101B, it should be recognized by one of skill in the art that other processes for forming the porous medium 100 may not require layers 101A and/or 101B.

Typically, the chemoselective coating 150 covers all surfaces of the membrane (e.g., top, bottom, the flow-through channels; coating in channels/pores not shown in Fig. 3B) as well as the top of the heater and electrical traces, without covering the packaging layer 699.

Preferably, the porous membrane and heater are thermally insulated from the base 10 and the first substrate 1011, for example, the porous member is thermally insulated from 101, 101A, and 101B (e.g., by tethers 190 connecting the porous member to the substrate, e.g., as shown in Figs. 3A, 3B, and 3D) to reduce conductive heat loss at the edges of the porous member, also allowing rapid and uniform heating with low power. Channels 195 are etched through the first substrate, and define the tethers (e.g., the tethers are portions of the first substrate remaining after channels have been etched therethrough). In contrast with the flow-through channels 115 (typically having a diameter of about 50 micrometers or less), the channels 195 are typically elongated, and define the tethers.

In the embodiment illustrated in Figs. 3B and 3D, the thin film resistive heater 175 is arranged in or on the top surface 111 of the porous membrane (surrounding the flow-through channels 115 of the porous membrane), and on the tethers.

In some embodiments, e.g., as illustrated diagrammatically in Figure 3D, the top surface 111 of the porous member comprises an insulating layer 120 (e.g., SiO₂) underneath the heater (and any other structure carrying current, e.g., electrical traces) to prevent current from shorting through the porous membrane.

In these illustrated embodiment, the aircraft air contaminant collector 1 is associated with (e.g., mounted on) a first support 311, typically a printed circuit board, the first support having a top surface 311A and a bottom surface 311B. As will be discussed in more detail below, the first sample flow path and the second sample flow path pass through separate portions of the first support, and flow through an exit manifold (not shown) downstream of the pumps. Flow along the first flow path is generated by a first pump 1033, and flow along the second flow path is generated by a second pump 2003.

The analyzer 200 includes a gravimetric sensor 3, arranged near each collector along the first flow path to generate a proportionate resonant frequency response when air contaminant mass is added to or removed from the gravimetric sensor, for quantifying the amount of air contaminant and classifying air contaminant by type; and a second substrate 201, having a top surface 201A and a bottom surface 201B; wherein the gravimetric sensor 3 is associated with (e.g., mounted on or fabricated within, e.g., by subtractive and additive processes) the top surface of the second substrate, the gravimetric sensor being separated from the contaminant collector by a constant distance, the gravimetric sensor being arranged to receive air contaminants desorbed from the microporous medium when the microporous medium is heated.

The embodiment of the analyzer shown in Figure 1 also includes a second support 312 comprising a top surface 312A and a bottom surface 312B, the second support comprising at least one aircraft air inlet port 500 (illustrated as a combined aircraft inlet port and bypass inlet port, providing a common inlet for the first sample flow path and the second sample flow path) passing through the top surface and the bottom surface of the support, wherein the bottom surface of the second substrate is associated with (e.g., mounted on) the top surface of the second support. Typically, the second support comprises a printed circuit board. Preferably, as shown in Figure 1, the at least one air inlet port 500 is aligned with bypass channel port 2002, e.g., to allow large particles to pass through the bypass channel port easily.

Separation between the gravimetric sensor and the microporous medium should be kept constant, typically at a distance of about 0.1 mm to about 2 mm, preferably about 0.2 mm to about 0.4 mm. For example, Figure 1 shows spacers 315 between the first support 311 and the second support 312 for maintaining the spacing between the sensor and the microporous medium. Preferably, the length of the spacers is such that the separation between the collector and the gravimetric sensor facing surfaces are about 0.2 mm to about 0.4 mm.

The embodiment shown in Figure 1 also include electronics 600, comprising a power source or a connection to a power source, a power regulator, a measurement circuit 610 comprising a resonant frequency measurement device 610A comprising an oscillator and a field-programmable gate array (FPGA), arranged to measure the proportionate resonant frequency response generated by the resonator array to allow classification of air contaminant type(s); a computer readable medium bearing an air contaminant recognition program; a processor configured to execute the air contaminant recognition program, the contaminant recognition program including a module configured to measure oscillation rate and classify air contaminant type(s), and programmed with a calibration table for comparison with magnitude of the proportionate resonant frequency responses generated by the resonator array(s) to calculate air contaminant concentration(s) and determine air contaminant type(s). If desired, the air contaminant recognition program executed by the processor is stored upon a non-transitory computer-readable medium, and the processor displays (outputs) a value for the determined air contaminant type(s). For example, the value(s) can be displayed through a GUI using a display device (such as a hand-held device) operably arranged with the processor. Alternatively, or additionally, for example, the value(s) can be displayed by an illuminated sensor or communicated audibly.

The electronics can have a variety of arrangements as known in the art. In the illustrated embodiment shown in Figure 1, the electronics provide power when needed to the heater 175, via a cable 601, connector 605, electrical traces 620, fabricated into first support 311 (so traces not visible), wirebonds 625, and traces 630, fabricated onto collector 10 (so traces not visible), and power to the pumps 1003 and 2003 (discussed below) when needed via respective cables 690A, 690B. The electronics with respect to the gravimetric sensor can also include, for example, electrical trace 640, wirebonds 645, electrical traces 650, fabricated into second support 312 (so traces 650, not visible), 660, 670 (as shown in Fig. 5), connector 655, and cable 651.

In those embodiments including additional collectors and gravimetric sensors, each gravimetric sensor would typically have its own oscillator circuit, electrical traces and wirebonds. They may have separate cables and connectors, or signals may be routed into multi-wire cables and connectors. One field programmable gate array (FPGA) is typically capable of counting the resonant frequencies of multiple gravimetric sensors. All collectors can be wired in parallel and heated from the same electronics power circuit, or alternately can be powered by separate circuits and heated independently, for example to different temperatures or durations.

If desired, resonance frequency can be measured using, for example, a phase lock loop or a digital signal processor (DSP) chip to perform frequency sweeps to identify the resonant frequency from the sweep spectra.

Alternatively, if desired, a resonant frequency measurement device comprising a laser and a photodetector can be arranged to measure the proportionate resonant frequency response generated by the gravimetric sensor.

The illustrated embodiment also includes a first pump 1033, arranged to generate aircraft air sample flow along the first sample flow path 1000 through the aircraft air inlet port(s) and through the air contaminant collector(s) before and after the microporous medium/media is/are heated, and a second pump 2033, arranged to generate aircraft sample flow through the aircraft air inlet port(s) and the bypass channel inlet port 2002 and bypass channel 2003 along the second sample flow path 2000 bypassing the air contaminant collector(s).

A variety of pumps are suitable for use in accordance with embodiments of the invention. As shown in Figure 1, the pump 1033 is preferably positioned downstream of the one or more microporous media and the one or more gravimetric sensors (if one or more collectors and sensors are utilized), and the second pump 2033 is preferably located adjacent to the first pump, wherein an optional air-tight cover 434 and/or an optional inlet manifold (not shown), second support 312, first support 311, and spacer 315 isolate the sample to avoid its contamination or dilution, and to ensure that flow generated by the pump 1033 all flows through the microporous medium/media, and the pump is positioned after the gravimetric sensor(s) and the microporous medium/media to ensure that the pump does not contaminate the sample, and the gravimetric sensor(s) is/are positioned upstream of the microporous medium/media with sample flow arranged to avoid flow toward the respective sensor surfaces, thus minimizing the transfer of contaminants and undesirable material (such as dust, aerosols, and/or particulates) onto the surface(s) of the sensor(s).

In those embodiments including two or more aircraft air contaminant collectors and corresponding gravimetric sensors (providing a collector-sensor set), each collector-sensor set is maintained at the same environmental conditions (e.g., temperature, pressure, relative humidity) as the other set(s), as this provides better detection performance by reducing "noise" in the response patterns caused by measuring each set at different times or under different conditions. Preferably, all of the collector-sensor sets are arranged in close proximity.

Each collector-sensor set should have similar sensitivity as the other set(s) such that each provides responses above the noise level to provide good accuracy.

A variety of gravimetric sensors are suitable for use in the invention, including, for example, gravimetric sensors selected from a thin film resonator (TFR), a surface acoustic wave (SAW) resonator, a thickness sheer mode (TSM) resonator (quartz crystal microbalance (QCM) resonator), an acoustic plate mode (APM) resonator, a flexural plate wave (FPW) resonator, a bulk acoustic wave (BAW) resonator, a piezoelectric bimorph resonator array sensor, and a tuning fork sensor.

The sensor can be coated with functionalized SiO₂ nanoparticles (e.g., functionalized with tri-ethyoxysilanes) Suitable tri-ethyoxysilanes for producing functionalized SiO₂ nanoparticles include, for example, 3-[2-(3-Triethoxysilylpropoxy)ethoxy] sulfonlane, 95%; Phenethyltrimethoxysilane, tech-95; 3-Methyoxypropyltrimethoxysilane; N-(Acetylglycl)-3-Aminopropyltrimethoxysilane, 5% in methanol; and Dodecafluorodec-9-Ene-1-Yltrimethoxysilane, 95%. In some examples, the functionalized SiO₂ nanoparticles form self-assembled monolayers that can be deposited on the surface of the sensor.

The gravimetric sensor comprises a piezoelectric bimorph resonator array comprising two active layers, the layers bending under resonance, the resonator array generating a proportionate change in resonant frequency upon the addition or removal of air contaminant mass. One example of such a gravimetric sensor is disclosed in U.S. Patent 6,953,977.

In an example shown in Figure 2, the gravimetric sensor 3 includes a first electrode 3A and a second electrode 3B (collectively forming a resonator) so that motion of the sensor is transduced into an electrical signal via the first electrode on the surface of the sensor, and the signal can be amplified and returned to the second electrode on the sensor surface to drive the sensor at resonance. The gravimetric sensor can further comprise an optional balance capacitor 5 comprising a first balance capacitor electrode 5A and a second balance capacitor electrode (measuring electrode) 5B included adjacent to the resonator to reduce the contributions of parasitic capacitances and resistances from the electrical signal, wherein the balance capacitor has similar or identical materials of construction and dimensions as the gravimetric sensor but is made incapable of motion (e.g., wherein there is no space on the substrate allowing the balance capacitor to move). The balance capacitor can be driven with, for example, a 180° phase shifted signal through a dedicated electrical trace and the first balance capacitor electrode. The signal transduced from the second balance capacitor electrode (measuring electrode) is combined with the signal transduced by the sensor's first electrode as it is routed to the electronics, e.g., a field programmable gate array (FPGA) and firmware that counts the rate of oscillation.

As recognized in the art, a variety of types of electronics are suitable for measuring the proportional frequency responses of the various gravimetric sensors.

A variety of materials are suitable for microporous media (e.g., microporous membranes) for use in accordance with the invention. In addition to microporous membranes, suitable microporous media include fibrous materials, ceramics, printed structures, and micromachined structures. The microporous medium can be supported or unsupported. Typically, in those examples wherein the microporous medium is a microporous membrane, the membrane has a thickness in the range of at least about 20 micrometers to about 500 micrometers, more typically, a thickness in the range of about 50 micrometers to about 200 micrometers, though membranes can have lesser or greater thicknesses for some applications.

The microporous medium, e.g., the microporous membrane, is porous or perforated, providing suitable regular and/or irregular flow through channels and/or pores, e.g., about 5 micrometers to about 50 micrometers, typically, a 10 micrometers to about 30 micrometers, in size and/or diameter, though the pores or perforations can be smaller or larger for some applications. The membrane includes a chemoselective coating associated with (e.g., affixed and/or covalently bonded to) the top and bottom surfaces and the inside of the flow channels and/or pores and/or chemoselective particles in the bulk of the membrane.

A variety of chemoselective coatings are suitable for use in the invention, e.g., porous silica, activated carbon, metal-organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), titania (TiO₂) particles, and zeolites, including hydrophobic zeolites and hydrophilic zeolites. Suitable zeolite coatings include, but are not limited to, Z100 (hydrophobic zeolite); Z110 (hydrophobic zeolite); Z300 (less hydrophobic zeolite); and Z810 (hydrophilic zeolite) (Zeochem LLC, Louisville, KY).

Suitable heaters, preferably, thin film resistive heaters, are known in the art. Illustrative heaters include, for example, platinum (Pt) and tantalum-platinum (TaPt) high temperature compatible thin film resistive heaters, which allow the microporous medium to be ohmically heated to, for example, about 550° C without degradation. Preferably, the heater is fabricated in place onto the substrate, e.g., with a combination of deposition, lithography, and dissolution, processes.

A variety of materials are suitable for use as the first and second supports, the collector die, and the substrates, and suitable materials are known in the art and are readily manufacturable using microelectronics fabrication processes. For example, they can be fabricated from materials such as silicon. Typically, the materials are micromachinable, as they desirably allow micromachining to include, if desired, electrical structures such as traces, electrodes, and interconnects to bring electrical power where needed, and/or include mechanical structures such as suspended plates, tethers and membranes, and fluidic structures such as flow channels.

The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.

This example relates to one aspect of the method and demonstrates that signal magnitude (the change in frequency measured by the gravimetric sensor) changes as flow rate through the collector increases.

An aircraft air contaminant analyzer is set up as generally shown in Figure 1, with a resonator and balance capacitor set up as generally shown in Figure 2.

The gravimetric sensor is a micromachined (MEMS) piezoelectric bimorph SiC-AIN resonator array with a wide mass-loading dynamic range and linear mass-loading response. The resonator has a small tethered plate that provide resonance modes with high mass-loading sensitivities, and includes thin film electrodes deposited onto the surface to electrically drive it to resonance and to transduce the motion back into an electrical signal for readout. The sensor includes metal traces to bring the signals to and from the resonator. The resonant frequencies are in the range of ~1MHz to 30MHz.

The balance capacitor is identical to the gravimetric sensor, but is arranged to be incapable of motion.

The aircraft air contaminant collector includes a microporous silicon membrane having about 25 micrometer diameter flow-through channels, the membrane further having a hydrophobic zeolite powder (Z300; Zeochem LLC, Louisville, KY) coating on the upstream, downstream, and flow-through channel surfaces.

The heater is a tantalum-platinum (TaPt) high temperature compatible thin film resistive heater, fabricated into the substrate and deposited directly on the membrane.

Three contaminants, deicing fluid, turbine engine oil (AEROSHELL 560; Shell), and hydraulic fluid (Exxon HYJET; Exxon), each at a fixed concentration, are passed along the first sample flow path through the collector wherein the flow along the second sample flow path bypassing the collector is kept constant at 1.0 standard liter per minute (SLM). The flow duration is a constant for all measurements.

As shown in Figure 4, higher flow rates along the first sample flow path correspond to more negative signal magnitudes (corresponding to more contaminant being detected), thereby demonstrating how modulating sample flow rate (and thus, sample volume) can be used to change the sensitivity of the aircraft air contaminant analyzer.

This example demonstrates that an embodiment of the aircraft air contaminant analyzer functions over a range of concentrations.

An aircraft air contaminant analyzer is set up as generally described in Example 1.

The aircraft air contaminant analyzer is challenged with clean air, followed by challenges with three different concentrations of turbine engine oil (AEROSHELL 560; Shell), wherein the sampling time and flow rate through the collector are set prior to each challenge, to adjust the sensitivity so that the signal magnitudes of all three challenges are approximately equal.

The bypass flow rate is 1.0 SLM.

Figure 5 shows, as a composite graph, the sequence of measurements of turbine engine oil after the aircraft air contaminant analyzer is challenged with clean air before challenges with the engine oil, wherein measurements 110-137 are at a concentration of 5 mg/m³, 11 seconds sample time, 300 standard cubic centimeter per minute (sccm) flow rate; measurements 175-199 are at a concentration of 0.5 mg/m³, 16.5 seconds sample time, 1200 sccm flow rate; and measurements 242-265 are at a concentration of 50 mg/m³, 11 seconds sample time, 18.5 sccm flow rate.

The data in Figure 5 show that sample time and/or flow rate can be used to sensitize and desensitize an embodiment of the aircraft air contaminant analyzer, allowing it to function over a range of concentrations.

In this Example, an aircraft contaminant analyzer is arranged without a bypass, wherein a pair of gravimetric sensors measures the desorption from a collector comprising a membrane coated with a hydrophobic zeolite coating (Z300; Zeochem LLC, Louisville, KY).

The frequency shift versus time is first determined in the absence of contaminants (for example, using clean laboratory air during calibration or air passing through a sterilizing filter or without first passing air through the collector). For example, the resonance frequency is measured every 0.01 seconds for 4 seconds. Resonance frequency decreases starting at 0.5s when heating power is applied to the collector. Heat transferred to the resonator decreases its resonant frequency. This is also called the "thermal response," and illustrates the response spectra in the absence of contaminants. The response spectra is also determined in the presence of the contaminant (deicing fluid), and both response spectra are shown in Figure 6.

The first response spectra (without a contaminant) is subtracted from the second response spectra (with the contaminant), revealing the frequency shift caused by presence of the contaminant only, illustrating the "thermal subtracted response," as shown in Figure 7.

Various features can be calculated from the "thermal subtracted responses." Four examples of such features are:

1. a) Maximum frequency shift (MFS): the maximum frequency shift seen during the response.
2. b) Sum before peak (SB): the area under the curve before the MFS.
3. c) Sum after peak (SA): the area under the curve after the MFS.
4. d) Segment #5 (S5): the average of the 37^th thru 46^th frequency measurements following the MFS.

These four features are shown in Figure 8.

This example demonstrates how the feature MFS as described in Example 3 can be used by a pattern recognition algorithm to identify the contaminants.

Using an aircraft air contaminant analyzer with gravimetric sensors as described in Example 1, the frequency shift versus time is determined when the analyzer is sequentially challenged with turbine engine oil (AEROSHELL 560; Shell), hydraulic fluid (Exxon HYJET; Exxon), and deicing fluid.

The results are shown in Figure 9, wherein the responses (average MFSs) are similar for oil and hydraulic fluid, and different for deicing fluid.

As shown in Figure 10, the use of the feature MFS shows deicing fluid can be distinguished from hydraulic fluid and turbine engine oil: for hydraulic fluid and turbine engine oil, the ratio of the MFS feature from the gravimetric sensor next to the porous silica coated collector to the MFS feature from the gravimetric sensor next to the Z300 coated collector ranges between 0 and about 2, whereas for deicing fluid the ratio ranges between about 12 and about 23.

The use of the terms "a" and "an" and "the" and "at least one" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term "at least one" followed by a list of one or more items (for example, "at least one of A and B") is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, any combination of the above-described elements in all possible variations thereof is encompassed by the invention insofar as they fall within the scope of the appended claims, and unless otherwise indicated herein or otherwise clearly contradicted by context.