Abstract:
Automatic devices that determine when pollutant deposits have accumulated in ductwork may be employed to notify maintenance personnel or automated cleaning equipment of the need for ducts to cleaned or replaced. Various detection devices may be employed to detect a property of accumulated grease and generate an indication of an accumulation. The detection device may present a surface to the fume stream inside a duct. The surface may be cooled to a temperature that represents a worst case temperature so that the accumulation due to condensation on the detector surface is at least as high as the coolest surface in the ductwork which is being monitored. Alternatively, the detection device may be located external to the duct. The detection device may interrogate the surface of the duct through contact or noncontact measurements to determine the thickness of an accumulated grease layer on the interior of the duct.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 60/943,626, entitled “Duct Grease Deposit Detection Devices, Systems, and Methods,” filed Jun. 13, 2007, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to exhaust ventilation systems and, in particular, to exhaust ventilation systems in which material can accumulate inside the exhaust systems causing potential problems, such as fire hazards. 
       BACKGROUND 
       [0003]    Exhaust systems are often used to remove pollutants from a conditioned space. Many of these systems handle aerosols that are imperfectly removed from exhausted air streams permitting the deposit, and accumulation, of materials in exhaust ducting and hoods. For example, kitchen range hoods remove cooking fumes from kitchens. Such fumes often contain grease aerosols that are imperfectly filtered using grease filters. After a long period of operation, some grease inevitably coats the inside of exhaust ductwork. This can pose a fire hazard and have other undesirable consequences. 
         [0004]    There are many devices that have been designed and manufactured for removal of fumes from a kitchen. Canopy and backshelf hoods are common types. These are typically situated above a cooking appliance or appliances and connected through an exhaust duct to a roof-mounted fan that draws air through the hood and discharges to the outside ambient air. Removable cartridge grease filters are usually mounted in the hood just preceding the ductwork. These are normally removed periodically from the hood and washed to remove accumulated grease. Such filters are imperfect in that they are effective for removing the largest particulates, but they tend to leave a substantial amount of grease in the exhausted stream. Grease passing the filters accumulates in the ductwork from the hood and can accumulate on the fan and discharge of the exhaust system as well. 
         [0005]    Once grease builds up in a duct, it is possible to clean the duct. Various systems for doing this are known. Visual inspection is one means of determining whether a duct is in need of cleaning. Another method of detecting buildup is described in U.S. Pat. No. 3,890,827 for “Method and apparatus for monitoring grease buildup within an exhaust system” which describes removable patches that can be installed in a duct and removed for close inspection to determine how much grease has accumulated on the surface. Multiple patches are mounted as a set and one patch is removed at a time to determine the grease accumulation. 
         [0006]    Fire detection and elimination is a well-known solution for exhaust hoods and ducts. Conventional fire detection and suppression systems may be in installed in kitchen exhaust hoods and ductwork. Fire can be suppressed using water or chemical extinguishers. For example, U.S. Pat. No. 4,524,835 for “Fire suppression system” describes a chemical fire suppression system 
         [0007]    There is a need in the art for convenient and reliable mechanisms for detecting the buildup of grease and other contaminants in ductwork. The known methods relying on visual inspection are tedious and unreliable and also difficult to enforce. 
       SUMMARY 
       [0008]    Automatic devices that determine when pollutant deposits have accumulated in ductwork are employed to notify maintenance personnel or automated cleaning equipment of the need for ducts to cleaned or replaced. Various embodiments of detection devices may be employed which detects a property of accumulated grease and generates an indication of an accumulation from it. In most such devices, preferably, a calibration is performed for the type of material that tends to deposit. In preferred embodiments, the detection device presents a surface to the fume stream inside a duct. In the preferred embodiment, the surface is cooled to a temperature that represents a worst case temperature so that the accumulation (due to condensation) on the detector surface is at least as high as the coolest surface in the ductwork which is being monitored. Also, preferably, the detection device is positioned such that, as nearly as possible, it is in a worst-case position for exposure to grease in the fume stream. So, for example, it may be located in a high velocity position or in a region of a reversing or stagnating boundary layer, depending on the properties of the aerosol stream and the configuration of the ductwork. 
         [0009]    In a preferred type of detector, a micro-scale device is used to detect accumulation of grease. Micro-scales are used to measure minute quantities of material by detecting the change in a resonant frequency of an object on which material has been deposited. An example of a micro-scale is one that employs a piezoelectric transducer which is driven over a range of frequencies. By suitably calibrating the device, the change in mass, relative to a baseline, can be determined and compared with a threshold where cleaning is required. 
         [0010]    According to an embodiment, the invention is a method for detecting fouling in a duct, comprising: placing a member with a surface in an exhaust stream, and generating a signal indicating a fouled condition of the surface due to a change in a property of the surface indicative of fouling. In another embodiment, the property is at least one of optical opacity, reflectivity, optical scattering, thermal conductivity, and mass. In another embodiment, the placing includes (i.e., comprises) installing a disposable detector, the method further comprising replacing the detector after the generating. In another embodiment, the method includes cooling the surface. In another embodiment, the method includes cooling the surface to a predetermined temperature. In another embodiment, the property includes mass and the generating includes measuring a resonant frequency of the member. In another embodiment, the placing includes orienting the surface so that it faces an oncoming flow of fumes. In another embodiment, the generating includes comparing a measured property trend with a predetermined trend to identify a correlation. 
         [0011]    According to another embodiment, the invention is a device which may be used to implement any of the foregoing methods. In an embodiment, the device includes a piezoelectric microscale to measure the mass of material accumulated on the surface. 
         [0012]    According to another embodiment, the invention is a system to implement any of the foregoing methods. The system may include a controller to take a sample measurement when an exhaust system is not operating. 
         [0013]    According to another embodiment, the invention is a method for detecting a level of accumulated contamination in a duct including (i.e., comprising) providing a detector in fluid communication with an exhaust stream flowing through the duct. The method may further include determining the level of accumulated contamination in the duct using the detector. The method may further include outputting a signal based on the determining. In another embodiment, the method may further include activating an alarm based on the outputting. In another embodiment, the method may further include displaying to a user the level of accumulated contamination based on the outputting. In another embodiment, the detector may include a sensing element having a surface, and a controller which interrogates the sensing element. In another embodiment, the providing may include orienting the surface of the sensing element in the exhaust stream such that the surface is in a worst-case position for exposure to contaminants in the exhaust stream. In another embodiment, the determining may include using the controller, interrogating the sensing element to obtain a measurement indicative of the level of accumulated contamination in the duct. In another embodiment, the method may further include cooling the detector to a target temperature. In another embodiment, the method may further include determining the target temperature according to a real-time model of a wall of the duct, a temperature of the exhaust stream, and/or ambient temperature. 
         [0014]    According to another embodiment, a method for detecting fouling in a duct may include placing a detector arrangement external to a duct so as to be physically isolated from an exhaust stream flowing through the duct, interrogating the duct using the detector arrangement to generate a detection result, and correlating the detection result with an amount of accumulated fouling on an interior surface of the duct. 
         [0015]    In another embodiment, the detector arrangement may include an acoustic source and an acoustic sensor, said placing may include positioning the acoustic source and the acoustic sensor at a first side on an exterior of the duct, said interrogating may include transmitting an acoustic signal from the source to the first side of the duct and measuring reflected acoustic signals with the acoustic sensor, and said correlating may include calculating an acoustic impedance and relating the acoustic impedance to a thickness of the accumulated fouling. 
         [0016]    In another embodiment, the detector arrangement may include a radioactive source and a radioactive sensor, said placing may include positioning the radioactive source and the radioactive sensor at a first side on an exterior of the duct, said interrogating may include transmitting radioactive energy from the radioactive source and measuring radiation with the radioactive sensor, and said correlating may include relating the measured radiation to a thickness of the accumulated fouling. 
         [0017]    In another embodiment, the detector arrangement may include a radioactive source and a radioactive sensor, said placing may include positioning the radioactive source at a first side on an exterior of the duct and positioning the radioactive sensor at a second side on the exterior of the duct opposite the radioactive source, said interrogating may include transmitting radioactive energy from the radioactive source and measuring radiation with the radioactive sensor, and said correlating may include relating the measured radiation to a thickness of the accumulated fouling. 
         [0018]    Objects, advantages and features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. Throughout the figures, like reference numerals denote like elements. 
           [0020]      FIG. 1  shows a microscale mounted in a duct with a sensor/driver to detect the accumulation of grease on a detector surface by oscillating the detector surface and determining a change in resonant frequency thereof. 
           [0021]      FIG. 2  shows an array of detectors mounted at various angles and positions to mimic multiple duct surfaces on which pollutants may accumulate. 
           [0022]      FIG. 3A  shows a detector with an active cooling device. 
           [0023]      FIG. 3B  shows a detector with a passive cooling device. 
           [0024]      FIG. 4A  shows a detector which uses a change in thermal properties of a detector surface to identify an accumulation of deposits on the surface. 
           [0025]      FIG. 4B  shows a network model that may be used to model the response of the detector of  FIG. 4A . 
           [0026]      FIG. 4C  shows a plot of temperature samples for illustrating the operation of the detector of  FIG. 4A . 
           [0027]      FIG. 5A  shows another type of detector which uses a change in thermal properties of a detector surface to identify an accumulation of deposits on the surface. 
           [0028]      FIG. 5B  shows a network model that may be used to model the response of the detector of  FIG. 5A . 
           [0029]      FIG. 6  shows an optical detector which relies on scattering within a deposit film to detect the accumulation of a specified amount of material. 
           [0030]      FIGS. 7A and 7B  show a detector that detects scattering of light caused by accumulation of grease deposits on a detector. 
           [0031]      FIGS. 8A and 8B  show a passively cooled mechanical balance that can indicate the accumulation of grease on a detection surface by tilting. 
           [0032]      FIGS. 9A and 9B  shows other types of optical devices that indicates the accumulation of material by detecting a change in opacity. 
           [0033]      FIG. 10   a  shows a lever with a strain gauge that can indicate the accumulation of grease on a detection surface by deflection of the free end. 
           [0034]      FIG. 10   b  shows a cantilevered beam with a strain gauge that can indicate accumulation of grease on a detection surface by deflection of the free end. 
           [0035]      FIG. 11   a  shows a schematic of a generalized detector arrangement having a sensing element within the duct for determining the accumulation of fouling material in the duct. 
           [0036]      FIG. 11   b  shows a schematic of a generalized detector arrangement having a source and sensing element collocated external to the duct for determining an accumulation of fouling material in the duct. 
           [0037]      FIG. 11   c  shows a schematic of a generalized detector arrangement having a source on an opposite side of the duct from the sensing element for determining an accumulation of fouling material in the duct. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0038]    Referring now to  FIG. 1 , a duct  112  has an opening  116  through which is inserted a fouling detector  125 . The fouling detector has a plate  108  with a detection surface  109  protected by a removable protector sheet  102 . An oscillator actuator  104 , such as a piezoelectric crystal, causes the plate  108  to vibrate relative to a mounting support  106  attached to the duct  112 . A gas stream  120 , which contains suspended contaminant particles such as grease droplets, passes around the detection surface  109  causing the suspended particles to impinge on the surface. Over time, a coating grows on the detection surface  109 . The coating increases the mass of the plate  108  such that the change in mass can be detected by a change in the resonance frequency of the plate. A sensor/drive  110  provides the driving signal to oscillate the plate  108  and to detect the resonant frequency. 
         [0039]    Suitable detection devices are known in the art and are frequently used for deposition detection where high sensitivity to low deposition rates are required. One name for such devices is microscales. Examples of the applicable technologies are shown in the following patents each of which is incorporated by reference in its entirety herein: U.S. Pat. No. 6,880,402 for “Deposition monitoring system,” U.S. Pat. No. 6,124,927 for “Method to protect chamber wall from etching by endpoint plasma clean,” U.S. Pat. No. 5,985,032 for “Semiconductor manufacturing apparatus,” U.S. Pat. No. 5,897,378 for “Method of monitoring deposit in chamber, method of plasma processing, method of dry-cleaning chamber, and semiconductor manufacturing apparatus,” U.S. Pat. No. 5,843,232 for “Measuring deposit thickness in composite materials production,” U.S. Pat. No. 5,661,233 for “Acoustic-wave sensor apparatus for analyzing a petroleum-based composition and sensing solidification of constituents therein,” U.S. Pat. No. 5,536,359 for “Semiconductor device manufacturing apparatus and method with optical monitoring of state of processing chamber,” U.S. Pat. No. 5,112,642 for “Measuring and controlling deposition on a piezoelectric monitor crystal,” U.S. Pat. No. 5,666,394 for “Thickness measurement gauge,” U.S. Pat. No. 6,701,787 for “Acoustic sensor for pipeline deposition characterization and monitoring of pipeline deposits,” U.S. Pat. No. 5,618,992 for “Device and method for monitoring deposits in a pipe or vessel,” U.S. Pat. No. 3,023,312 for “Radioactive pipe thickness measurement,” and U.S. Pat. No. 4,429,225 for “Infrared thickness measuring device.” 
         [0040]    The mass measurements required for detecting deposition films for the present purposes need not be as precise as required in some industries, such as those discussed in the above patents. In addition, substantial masses of material can provide suitable indications of deposit formation such that oscillating systems other than piezoelectric can be made using, for example, speaker coils and spring or other devices. 
         [0041]    The protector sheet  102  may be, for example, a plastic sheet with an adhesive backing. By providing the protector sheet  102 , the fouling detector can be protected from being permanently coated with material accumulated from the gas stream. The fouling detector  125  may be removed from the duct and the protection sheet  102  replaced at a time after an indication has been generated by the sensor/driver  110 . Preferably, the sensor/driver  110  is configured to run a test on a schedule, such as once per day or once every few days. Thus, the sensor/driver  110  can be provided with an alarm or it may be connected to a computer network to signal one or more remote terminals. 
         [0042]      FIG. 2  shows a support  206  holding multiple fouling detectors  225 A,  225 B, and  225 C. Each fouling detector has a surface  202 A,  202 B, and  202 C, a detector portion  204 A,  204 B, and  204 C, which may be an oscillation actuator as in the embodiment of  FIG. 1  which measures the mass accumulated on plates  208 A,  208 B, and  208 C.  FIG. 2  illustrates that various mounting configurations for fouling detectors, as exemplified by fouling detectors  225 A,  225 B, and  225 C, are possible. Also,  FIG. 2  illustrates that multiple fouling detectors may be combined when it is difficult to predict the configuration corresponding to the worst-case propensity for fouling. For example, fouling detector  225 A is partially “shaded” from the gas flow by fouling detector  225 B. This may induce eddies and stagnation regions which may cause worst-case deposition rates of fumes. The properties of turbulent flow are difficult to predict so that it may not be possible to determine in a real configuration which orientation would produce the worst-case result. Therefore, multiple detectors, each with a different orientation or configuration (for example “shaded”) may be employed in a single device. Note that if fouling detector  225 A were used alone, a shading member could be used instead. 
         [0043]    Other configuration parameters that may be varied include the distance the detector is located downstream of a shading member, the size of the shading member relative to the detector, and the orientation of the shading member (e.g., oblique). Other orientations are also possible such as angled non-rectilinearly and/or non-orthogonally. 
         [0044]    Referring to  FIG. 3A , preferably a detector of any given configuration has a deposition surface that models the worst-case characteristics of the duct other than just the orientation of the surface relative to the flow and the type of flow impinging thereon. For example, grease aerosols often deposit when the temperature of the particles reaches a condensation point. Ducting surfaces which are subject to fouling may be cooler than the flue stream and therefore may cause precipitation of material that is in a vapor phase while in the flue stream. To ensure that a detector collects material at least as effectively as the worst-case duct portion, a mechanism for cooling the deposition surface of the detector may be employed.  FIG. 3A  shows a fouling detector  325  with an active cooling mechanism  332 , for example, a thermoelectric cooler. A sensor driver  110  and detector portion  304  serve to measure the mass of accumulated material on a detection surface  302 . A thermocouple or thermistor or other suitable temperature sensor  330  may be provided as well as a temperature sensor T  336  for a space surrounding the ductwork. 
         [0045]    A controller  340  may, according to known feedback control, regulate a temperature of the detection surface  302  so that its temperature corresponds closely to the worst-case ductwork surface portion, or slightly worse. For example, the temperature may be maintained at the temperature of the lowest air temperature to which the ductwork is exposed. Such temperature, mostly because of film resistance on either side of the duct surface and due to the resistance of insulation, if present, will be lower than any interior duct surface, at least during steady operation. Thus, it may be more representative to use an intermediate temperature between the duct interior (indicated by a temperature sensor  334  for the exhaust flow) and the ambient. 
         [0046]    Preferably, the target temperature may be varied in time according to a model of the duct wall, the temperature of the exhaust flow, and/or the ambient temperature such that a real-time worst-case surface temperature is achieved. Such a real-time model may be implemented readily using a programmable processor and based on the indicated temperature inputs as well as the properties of a suitable duct wall model. For example, a one-dimensional thermal model of the duct wall may be derived using known equations for conductive, convective, and radiative heat transfer. For a given exhaust flow rate, measured exhaust flow temperature and ambient temperature may thus be used with the thermal model to derive the temperature of the surface of the duct. Changes in the measured temperatures can then be correlated to changes in the duct surface temperature. This calculated duct surface temperature may then be used as a target temperature for the cooling of the detection surface. The active cooling mechanism may be applied to any of the foregoing or yet-to-be-discussed fouling detector embodiments, or others. 
         [0047]      FIG. 3B  shows a passively cooled fouling detector device  375 . A support  354  supports a fouling detector  350  in a duct interior  378 . A channel  352  conveys ambient air  380  through it into the duct interior  352 , which may be at a negative pressure relative to the ambient. The flow of ambient air  380  through the channel  352 , which is in contact with the fouling detector  350 , cools the fouling detector  350  relative to the duct interior  378  temperature. An adjustable damper blade  358  blocks the flow  356  through the channel  352  to permit it to be regulated. A sensor/driver  310  controls the fouling detector and also may detect a temperature indicated by a temperature sensor  362  to permit an operator to adjust the damper blade  358  based on the fouling detector  350  temperature. An air pump  366  may be used, with a channel extension  364 , to force air into the channel  352  if the duct interior  378  is under low negative or positive pressure. The passive cooling mechanism may be applied to any of the foregoing or yet-to-be-discussed fouling detector embodiments, or others. 
         [0048]      FIG. 4  shows a fouling detector that employs a thermal effect to determine the quantity of material deposited on a detection surface  412  of a plate  414 . A detector  400  monitors one or more temperatures by receiving corresponding signals from temperature sensors, for example sensors  416 ,  415  which indicate the temperature of the air/gas on a duct side of the plate  414  and the temperature on a heated side of the plate  414 . A heater  410  (under control of the detector  400 ) heats the plate  412  as the temperature of the plate is monitored. Insulation  434  may be provided to reduce cooling of the plate  414  by ambient air  422 . As the temperature of the plate  414  rises, it tracks a time vs. temperature profile which corresponds to the insulation generated by a layer of deposit  419  on the detection surface  412 . The detector  400  may be configured to perform a test when the exhaust system is powered off, for example, to run the test according to a clock indication that off-operating hours are current or by detecting the status of the exhaust system. Preferably, the test is done when the temperature of the duct-side ambient gas (air)  420  is constant and there is no flow, so that the insulation provided by the layer of deposit  419  can be determined. 
         [0049]      FIG. 4B  shows a simple one-dimensional network model for an infinite planar heat source whose power output is Q, which transfers heat to a node whose thermal capacitance is CW, and to an infinite sink at the duct air  420  temperature TD through a thermal resistance equal to that of the deposit RC and the film resistance RF on the duct air side  420 . Referring also to  FIG. 4C , the RC, the quantity that is unknown, can be obtained by solving for the value of RC by fitting a plot (e.g.  430  corresponding to a high value of RC or  432  corresponding to a low value of RC) of the measured temperatures to the unsteady model (t indicating time). Equivalently, a steady state temperature (e.g., T 1 , T 2 ) derived from an interpolation and used in the steady state model. Note that the model may take into account of the change in film coefficient with temperature due to thermal convection, so RF may be a function of temperature and time. For RF, the thickness of the deposited layer may be obtained from calibration data obtained using samples of deposited material. 
         [0050]      FIG. 5A  shows a thermal fouling detector that corresponds to a simpler model than the one of  FIG. 4A . It uses a heated wire  510  whose surface serves as the detection surface. The network model shown in  FIG. 5B  is one-dimensional as in the previous embodiment (and there is a planar equivalent, which is an alternative embodiment). Here, the heat source may be a conducting film over an electrical and thermal insulator. A material with known variation of electrical resistance with temperature may be used, for example platinum. By measuring the voltage and current using a detector  500 , the power dissipation rate and temperature may be obtained and measured over time from a starting time. As in the previous example, by fitting the temperature measurements to a suitable model of the system, the unknown value of RF may be derived and, from that, the thickness of the deposited layer. 
         [0051]      FIG. 6  shows an optical fouling detector  640  which has a plate  618  with an illumination source  606  and a light sensor  604 . A driver/detector  600  powers the illumination source, for example a light emitting diode with a lens, such that the illumination source directs light in a direction normal to a detection surface  616  when no material is deposited on the surface. When material accumulates on the surface as indicated at  612 , light from the illumination source  606  is scattered in the material layer  612  and received by the light sensor  604  as indicated by scattered beam  610 . The greater the thickness of the material layer  612 , the greater the scattering and the more light is received by the light sensor  604 . The driver/detector  600  may be configured to generate an indication of a specified degree of fouling when a threshold quantity of scattered light is detected thereby. The illumination source  606  and light sensor  604  may be complete devices that generate electrical signals through lines  622  or they may be terminals of fiber optic channels also represented by  622 . In the latter case, they may be located very close together. In addition, illumination source  606  and light sensor  604  constitute one pair or there may be more than one of either or both. 
         [0052]      FIGS. 7A and 7B  show another type of optical fouling detector  822  in which a light source  802  directs light such that it does not fall on a detector  806  when the surface of a lens or window  804  is clean, as indicated by arrows (representing beams)  808 . When the surface of the lens or window  804  becomes coated with deposited material, the light from the light source  802  scatters as indicated by arrows  810 . Some of the scattered light falls on the detector  806 . A driver/detector (not shown) functions as in the embodiment of  FIG. 6 , generating an indication of a predefined degree of fouling after the quantity of light falling on the detector  806  reaches a threshold. A support  814  can hold both the light source  802  and the detector  806  in position within the duct. 
         [0053]    Note that the detector  822  may be constructed of low cost materials and design such that it can be replaced each time the duct is cleaned. Thus, the device  822  generates a single indication and then is replaced. The driver/detector associated with it may be a permanent component. A disposable detector may be preferable to avoid the consequences of improper cleaning or change in performance characteristics of the fouling detector over time. All of the discussed embodiments may include single-use disposable components as discussed with regard to  FIGS. 7A and 7B . 
         [0054]    Note that in both of the embodiments of  FIGS. 6 and 7A ,  7 B, rather than triggering an indication of fouling based solely on total amount of light falling on the detector due to scattering, a light intensity curve can be obtained and memorized over time and compared with a representative profile for a detection surface that has become fouled. This may be preferable where the deposited material is not highly transmissive in its dried form, for example, if grease particles contained soot. In such a case, a representative profile may be one where the light intensity on the detector reaches a peak at a certain point in time and then decays due to further blocking by the deposited material. The fouling detection indication may be generated by detecting the peak or, in addition, after a drop in the light intensity that follows it of a certain amount. 
         [0055]      FIGS. 8A and 8B  show a balance device in which a balance  750  has a detection surface  724  exposed to fumes  728  in a duct  726  and a portion  702  outside or shielded from the fumes  728  in the duct  726 . The balance has a rectangular channel shape (but could be other shapes as well) such that a wall  732  projects into a recess defining a flow path  728  between the wall  732  and the balance  750 . Air from outside the duct flows through the flow path  728  to cool the detection surface  724  when fumes flow through the duct  726 . The balance  750  pivots on a knife  714  which is located by a notch  716  defined by an opening  708  such that when the detection surface  724  is clean, the detection surface  724  is horizontal due to a balanced state. Since the pivot point coinciding with the notch  716  is above the center of gravity, the balance  750  will come to equilibrium at different angles depending on how much mass accumulates on the detection surface  724 . The wall  732  prevents the balance  750  from pivoting too far due to dynamic pressure from the fumes  728  during operation of the exhaust system such that the detection surface  724  always remains substantially level as indicated by the outline  704  in  FIG. 8B . When a certain amount of material is deposited on fouling surface  724 , the balance  750  is tipped until contact between it and a contact  710  is made, completing a circuit and triggering an indication of a fouled condition. As in previous embodiments, the test may be performed only when the exhaust system is not operating according to a clock or a detector of the exhaust system state. The detector  700  may be configured such that a constant closed circuit for a minimum period of time must be maintained in order to generate an indication of a fouled condition. The wall  732  and/or knife  714  may include one or more electrical insulators depending on how the electrical circuit is defined by the structure. As in previous embodiments, the balance may be a disposable component which is replaced after a fouled state is indicated. 
         [0056]      FIG. 9A  shows another optical type of fouling detector in which opacity caused by a deposited film is detected and a degree of occultation used as a basis for indicating a fouled condition. A light source  918  shines a light through a window  914  toward a detector  901  located in a well  902 . Air  905  is drawn through the well  902  to keep material from fouling the detector  901 . Light is projected as indicated by arrows  912  toward the detector  901  generating a signal indicative of the amount of light which is received by a detector control  908 , which generates a fouled condition indication when the amount of light received falls below a threshold level. The window  914  may be cooled by a flow of air as indicated by arrow  917  by providing appropriate openings in the housing  919 . The detector  901  and the light source  918  are located on opposite sides of a duct  900  so that fumes are deposited on the window  914 .  FIG. 9B  shows an alternative embodiment of a fouling detector  952  which does not require that portions of the detector be located on opposite sides of the duct. A light source  960  directs light toward a mirror  958  which is reflected back to a detector  962 . The mirror  958  is supported by a low profile arm  958  so that exhaust can flow around it easily causing material in the exhaust to be deposited on the mirror  958 . The fouling detector  952  can be placed in a single access opening of a duct wall  966 . An active or passive cooling mechanism  956  may be provided. The fouling detector  952  can be configured such that the mirror is located at nearly any desirable angle or positioning the light source  960  and the detector. The angle of the mirror can be non-critical if replaced by a diffuse reflector or retroreflector material (typically a bed of spherical particles that return light to the source irrespective of the orientation of the bed). 
         [0057]      FIG. 10   a  shows another embodiment for a fouling detector  1000  employing a lever  1004  with a sensor  1008  that can indicate the accumulation of grease on a detection surface  1018  by deflection of the free end  1020 . The lever  1004  is rotatably fixed at pivot point  1012 . A spring  1010  is provided about pivot point  1012  and adjusted to hold the lever in a parallel orientation when no grease has accumulated on the lever. Support  1014  holds the lever  1004  via pivot point  1012  and spring  1010  at a fixed position with respect to duct wall  1002 . As grease in flow  1006  within the duct accumulates on the detection surface  1018 , the increased mass of the lever  1004  causes the lever to rotate about pivot  1012  in a counter-clockwise manner. A sensor  1008  may be provided in contact with the lever  1004  at a position outside of duct wall  1002 . For example, the sensor  1008  may be a strain gauge. In another example, sensor  1008  may be a force sensor. In yet another example, sensor  1008  may be a displacement sensor, such as a capacitive sensor. The sensor  1008  generates a signal indicative of movement of the lever due to the additional mass of the accumulated grease on the detection surface  1018 . The controller  1016  may then use the signal to determine a fouling condition of the duct, such as the amount of grease accumulated on the detection surface. 
         [0058]      FIG. 10   b  shows another embodiment for a fouling detector  1050  employing a cantilevered beam  1052  with a strain gauge  1054  that can indicate the accumulation of grease on a detection surface  1060  by deflection of the free end  1062 . Support  1056  rigidly fixes the cantilever  1052  adjacent to the duct wall  1002 . As grease in flow  1006  within the duct accumulates on the detection surface  1060 , the increased mass of the cantilever  1052  causes the cantilever to bend. A strain gauge  1054  is provided on a top (or bottom) surface of the cantilever to determine the amount of bending. Calibration of the strain gauge measurement would be necessary to compensate for the natural bending of the cantilever due to its own weight. The strain gauge  1054  thus generates a signal indicative of the degree of bending of the cantilever due to the additional mass of the accumulated grease on the detection surface  1060 . A controller  1058  may then use the signal to determine a fouling condition of the duct, such as the amount of grease accumulated on the detection surface. 
         [0059]      FIG. 11   a  shows a generalized schematic  1100  of a fouling detector arrangement  1108 . An air conveyance  1102 , such as an exhaust duct, is used to carry an exhaust stream  1106  from a source of contamination  1104 , such as a cooking appliance. Exhaust stream  1106  may carry aerosols, such as grease aerosols, which may be deposited on interior surfaces of the air conveyance  1102 . A fouling detector arrangement  1108  may be provided to detect the deposition of aerosols or other pollutants. In particular, fouling detector arrangement  1108  may include a sensing element  1110  and a controller  1112 . 
         [0060]    Sensing element  1110  may be disposed within the exhaust stream  1106  in air conveyance  1102  to allow aerosols or pollutants to interact therewith. For example, sensing element  1110  may have a detection surface exposed to the exhaust stream  1106  which accumulates aerosols and/or pollutants resulting in a change in a property of the detection surface. 
         [0061]    Controller  1112  may be functionally connected to the sensing element  1110 . The controller  1112  may interrogate the sensing element  1110  to obtain a measurement indicative of the level of accumulated aerosol and/or pollutants within the duct. For example, controller  1112  may interrogate sensing element  1110  to determine a change in mass of the detection surface. Controller  1112  may also be configured to provide a subsequent output  1114  based on the interrogation. For example, controller  1112  may activate an alarm system if the amount of accumulated contamination exceeds a predetermined threshold. Controller  1112  may also display a level of accumulated contamination to a user. Such display may take the form of a number or a color-coded display indicating a relative safety level (e.g., green may indicate safe to operate, yellow may indicate clean air conveyance soon, and red may indicate unsafe to operate). Controller  1112  may also provide an output  1114  to other systems, such as an automatic air conveyance cleaning system to provide for cleaning of the air conveyance  1102  when accumulated contamination levels reach a predetermined threshold. 
         [0062]      FIG. 11   b  shows a generalized schematic  1130  of a fouling detector arrangement  1132 . An air conveyance  1102 , such as an exhaust duct, is used to carry an exhaust stream  1106  from a source of contamination  1104 , such as a cooking appliance. In contrast to the schematic of  FIG. 11   a , the fouling detector arrangement  1132  of  FIG. 11   b  may be provided external to the air conveyance  1102  to detect deposition of aerosol or other pollutants on the air conveyance walls. In particular, fouling detector arrangement  1132  may include a source  1134 , a sensing element  1138 , and a controller  1142 . Thus, the fouling detector arrangement  1132  is isolated from the contaminants in the exhaust stream  1106 . The source  1134  interrogates the surface of the air conveyance  1102  by generating a signal  1136  and the sensing element  1138  measures the result  1140  of the interrogation to determine the amount of contaminant accumulated on the surface of the air conveyance  1102 . For example, source  1134  may be a source of acoustic or electromagnetic radiation. The radiation is modified in some form and measured by the sensing element  1138 . Note that both the source  1134  and sensing element  1138  may be located on the same side of the air conveyance  1102  and preferably oriented such that radiation emanating from the source  1134  and modified by the air conveyance  1102  can be received by sensing element  1138 . Controller  1142  may be functionally connected to the sensing element  1138  and may use the measurement of the sensing element to determine a level of accumulated contaminants within the air conveyance  1106 . Similar to controller  1112  in  FIG. 11   a , controller  1142  may also be configured to provide a subsequent output  1144  based on the determination of the level of accumulated contaminants. 
         [0063]    In a particular embodiment, the source  1134  may be an acoustic transmitter and the sensing element  1138  may be an acoustic sensor. The acoustic transmitter may generate an acoustic signal. The acoustic signal interacts with the air conveyance and is reflected. A first reflection occurs at the external surface of the air conveyance. A second reflection occurs at the internal surface of the air conveyance. A third reflection occurs at the surface of the contamination layer accumulated on the internal surface of the air conveyance. The reflected signals are received by the acoustic sensor. The controller may then use the received reflected signals to calculate acoustic impedance, as discussed, for example, in U.S. Pat. No. 6,701,787, which is incorporated by reference herein in its entirety. The acoustic impedance may then be correlated to the thickness of the deposited layer. 
         [0064]    In yet another embodiment, the source  1134  may be a radioactive source and the sensing element  1138  may be a slow neutron detector. For example, neutrons from a radioactive source may be allowed to interact with a wall of the duct having an accumulated contamination on an interior surface thereof. Fast moving neutrons penetrate the pipe wall without significant interaction and may be elastically scattered by hydrogen or carbon atoms in the contamination. The scattering slows the neutrons, causing some neutrons to be reflected and/or diffuse back towards the radioactive source. A detector, such as a BF 3  slow neutron detector, may be placed in proximity to the radioactive source in a position to measure the reflected and/or diffused slow neutrons. The detected slows neutrons thus provide an indication of the thickness of the accumulated contamination. 
         [0065]    In yet another example, the source  1134  may be an electromagnetic radiation source, such as an infrared (IR) transmitter, and the sensing element  1138  may be an electromagnetic radiation sensor. The IR transmitter may generate an IR signal. The IR signal interacts with the air conveyance and is reflected and/or absorbed by the materials it encounters. A first reflection occurs at the external surface of the air conveyance. A second reflection occurs at the internal surface of the air conveyance. A third reflection occurs at the surface of the contamination layer accumulated on the internal surface of the air conveyance. The reflected signals are received by the electromagnetic radiation sensor. The controller may then use the received reflected signals to calculate the thickness of the deposited layer. 
         [0066]      FIG. 11   c  shows a generalized schematic  1160  of a fouling detector arrangement  1162 . An air conveyance  1102 , such as an exhaust duct, is used to carry an exhaust stream  1106  from a source of contamination  1104 , such as a cooking appliance. Fouling detector arrangement  1162  may include a source  1166 , a sensing element  1164 , and a controller  1168  external to the air conveyance  1102 . Thus, the fouling detector arrangement  1162  is isolated from the contaminants in the exhaust stream  1106 . In contrast to the schematic of  FIG. 11   b , the fouling detector arrangement  1162  of  FIG. 11   c  may be provided with source  1166  located at an opposite side of the air conveyance  1102  with respect to the sensing element  1164 . 
         [0067]    The source  1166  interrogates the surfaces of the air conveyance  1102  by generating a signal  1170  and the sensing element  1164  measures the signal  1170 , as modified by the air conveyance  1102 , to determine the amount of contaminant accumulated on the surface of the air conveyance  1102 . For example, source  1166  may be a source of acoustic or electromagnetic radiation. The radiation is modified in some form and measured by the sensing element  1164 . Note that both the source  1166  and sensing element  1164  are located opposite each other and preferably oriented such that radiation emanating from the source  1166  and modified by the air conveyance  1102  can be received by sensing element  1164 . Controller  1168  may be functionally connected to the sensing element  1164  and may use the measurement of the sensing element to determine a level of accumulated contaminants within the air conveyance  1102 . Similar to controller  1112  in  FIG. 11   a , controller  1168  may also be configured to provide a subsequent output  1172  based on the determination of the level of accumulated contaminants. 
         [0068]    In a particular embodiment, the source  1166  may be an acoustic transmitter and the sensing element  1164  may be an acoustic sensor. The acoustic transmitter may generate an acoustic signal. The acoustic signal interacts with the air conveyance and is reflected. The transmitted signal through the air conveyance  1102  is received by the acoustic sensor. The controller may then use the received transmitted signal to calculate the thickness of the deposited layer. 
         [0069]    In yet another example, the source  1166  may be an electromagnetic radiation source, such as an infrared (IR) transmitter, and the sensing element  1164  may be an electromagnetic radiation sensor. The IR transmitter may generate an IR signal. The IR signal interacts with the air conveyance and is selectively absorbed by the materials encountered in traversing the air conveyance. The attenuated transmitted signal is received by the electromagnetic radiation sensor. The controller may then use the received attenuated signals to calculate the thickness of the deposited layer. 
         [0070]    Any of the foregoing embodiments may employ an active or passive cooling mechanism as described with reference to certain embodiments. Any of the above embodiments may take samples during periods of non-operation of the exhaust system based on indications of a clock, an exhaust system state detection (fan power signal, for example), and/or manually. Any of the above embodiments may sample the detected property at intervals and store the values to obtain a trend and use the trend pattern to identify the fouled condition, rather than an instantaneous state. The trend may be derived by studying the properties of the indicator signal compared to the fouling status of the detection surface and providing an appropriate reference to the control. Fouling by different kinds of uses of the exhaust system, which may not be known in advance, may produce different types of results, each associated with a corresponding response by the fouling detector so preferably these variations are taken into account to improve upon the accuracy of the fouled condition indication. 
         [0071]    While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.