Abstract:
There is described a filter arrangement for a particle detector for detecting particles in an environment. The particle detector can include one or more sensors for analyzing fluid in a detection region to produce sensor output. The filter arrangement includes structure defining flow paths, for conveying fluid from the environment towards the detection region, including a first flow path, which includes a filter, and a second flow path bypassing the filter. There is also provided a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path and a controller configured to receive the sensor output corresponding to at least two relative flow rates and apply logic thereto to generate an output indicative of a condition of the filter.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to particle detection. The following description focuses on smoke detectors, and in particular optical smoke detectors, however the skilled person will appreciate that the invention has broader application. 
         [0002]    For the avoidance of doubt, ‘particle detection’ and like terms are used herein to refer to the detection of solid and/or liquid particles. 
       BACKGROUND OF THE INVENTION 
       [0003]    Particle detectors are often used to warn of the presence of smoke emanating from a potential or incipient fire. 
         [0004]    Smoke detectors operate in a variety of environments and include, for example, office environments, factories and manufacturing plant, power stations and clean rooms. Each of which has different levels of background particulate material. In some environments the concentration of background particulate matter will change from time to time. 
         [0005]    A problem can be encountered with such smoke detection apparatus if they are continuously exposed to the relatively high levels of background pollution in the air that can exist in some environments. A large-scale example in recent years has been the high levels of smoke pollution often present in regions of Asia, which is largely attributable to the burning of brown coal. 
         [0006]    Background pollution can cause contamination of components within the detector leading to premature failure, for example due to clogging of air paths or changes in the optical properties of the critical components within the detection chamber. 
         [0007]    Scattering light detectors include a light source arranged to project a beam across the detection chamber. A photoelectric sensor is arranged so that its field of view is traversed by a portion of the beam. The photoelectric sensor receives light scattered from the beam due to the presence of particles in the detection chamber. Over time dust and debris can accumulate on surfaces within the detection chamber and reflect light towards the photoelectric sensor thereby providing a false indication of particles in the detection chamber. Dust and debris may also settle on the light source and/or the photoelectric sensor thereby obscuring the transmission and receipt of light and reducing the sensitivity of the detector. 
         [0008]    One approach to addressing these problems involves the use of an ‘air barrier’. An air barrier is created by directing one or more streams of clean air into the detection chamber to flow over the critical components, such as the light source, the photoelectric sensor and walls within the field of view of the photoelectric sensor, to prevent dust and debris accumulating thereon. 
         [0009]    Aspirated smoke detectors employ a fan, known as an aspirator, to draw air to be studied through the detection chamber. The air to be studied enters the chamber via an inlet(s). A desirable implementation of the air barrier concept employs a filter(s) to create the clean air. The filter is arranged in parallel to the inlet, whereby the clean air is drawn through the filter and into the detection chamber by the aspirator. A common stream of air, e.g. from a network of pipes, may be divided into two portions—one portion being filtered to create the clean air, and the other portion entering the chamber to be studied. 
         [0010]    Another approach to addressing the problems associated with the accumulation of dust and debris in the detection chamber is to obtain a measurement associated with light reflected from the accumulated dust and debris, known as ‘background light’, and to adjust the detection criteria applied to the signal received from the photoelectric sensor in response to the background light. One approach to obtaining a measure of background light involves the use of a second photoelectric sensor within the detection chamber. The second photoelectric sensor is arranged so that its field of view does not include the beam. Signals from the second photoelectric sensor are thereby indicative of light reflected within the detection chamber rather than light scattered directly from the beam. 
         [0011]    The abstract of Japanese patent application 59192940 entitled “Smoke Meter with Purging Device” describes filling a measuring device with clean air and measuring opaqueness in the clean atmosphere to perform calibration. The described device includes a dedicated blower to supply clean air to the detection chamber. A valve controlled by depressible switches is used to close the intake tube to halt the flow of discharge gases to the detection chamber prior to the purging operation. 
         [0012]    New Zealand patent 250497 is concerned with preventing fire suppression measures being activated in response to false alarms. It describes an operating syntax applicable to aspirated smoke detectors. When an alarm condition is detected the chamber is purged with dean air and a background ‘smoke’ signal is measured. If the background reading does not fall below a predetermined threshold a detector fault is indicated. If the background ‘smoke’ falls below the predetermined threshold, the system waits for the detected smoke level to rise above a further threshold before triggering the fire suppression systems. 
         [0013]    Other attempts to overcome the problems associated with operating particle detectors in polluted environments have included dust filters placed in the airstream. Dust filters have been used to filter out particles not associated with the smoke to be detected. Smoke particles may occur in a variety of sizes depending on the fuel used and combustion conditions, and the filter type is chosen according to the type of dust particles expected and the type of smoke to be detected. 
         [0014]    As conventional dust filters clog they begin to remove more particles from the air and eventually begin filtering out smoke particles (or other small particles of interest). This may be due to effective pore size of the filter being reduced as more particles clog the filter. Certain types of filters, particularly foam fitters, can start removing smoke particles before the pressure drop across, or flow rate through, the filter changes appreciably. The result is that the filter may be removing an unknown proportion of smoke long before clogging of the filter is detectable using pressure and/or flow measuring devices. 
         [0015]    In some situations attempts have been made to condition the air sample prior to its introduction into the smoke detector e.g. by diluting the sample flow with clean air. The object of such dilution is to deliver to the detection chamber a sample flow with an unchanged particle distribution, but with a lower particle concentration than the original sample flow. Whilst such dilution arrangements go some way to addressing the problems associated with operating in a polluted environment, the lower particle concentration reduces the sensitivity and accuracy of the detector. 
         [0016]    Dilution presents problems for air sampling smoke detectors that use a pipe network to draw air from a space being monitored, in that the introduction of the diluent air flow into the flow entering the detector reduces the amount of sample air drawn from the region being monitored. This causes an increase in the time taken for the sample air to travel from the region being monitored to the smoke detector, referred to as “transparent time”, and consequently increases detection time. 
         [0017]    The applicant has proposed, in their international patent application WO 2007/095675, an arrangement wherein a first portion of a sample flow is filtered through a HEPA (high efficiency particulate air) filter. The HEPA filter removes substantially all particles from the first portion of the sample flow to form clean air. The clean air is used to dilute a second, unfiltered, portion of the sample flow. The diluted sample flow is in turn carried to a detection region. This arrangement effectively addresses the transit time problem associated with other dilution arrangements and has a desirable ‘fail safe’ operation in that if the filter is allowed to clog to an extent that it becomes more restrictive to flow, the detection region will see an increase, rather than a decrease, in the concentration of particles. The extent of dilution may also vary due to environmental factors such as temperature and humidity. The changing dilution ratio reduces the accuracy of the associated smoke detector. 
         [0018]    Despite these various advances in the art, the known filtration arrangements and dilution arrangements result in a reduced concentration of particles arriving at the detection region and can reduce the sensitivity and accuracy of the particle detector. It is of course desirable that a smoke detector should be sensitive and accurate. It is also desirable that if a filter is used, that it&#39;s condition, and in particular whether it has clogged to an extent that it is removing particles of interest (e.g. smoke particles), is known. 
         [0019]    Objects of the invention include providing improved particle detection, an improved particle detector and components therefor, or at least providing alternatives for those concerned with particle detection. 
         [0020]    Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. 
       SUMMARY OF THE INVENTION 
       [0021]    One aspect of the invention provides a filter arrangement for a particle detector for detecting particles in an environment, the particle detector including one or more sensors for analysing fluid in a detection region to produce sensor output, the filter arrangement including structure defining flow paths, for conveying fluid from the environment towards the detection region, including a first flow path, which includes a filter, and a second flow path bypassing the filter; a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; and a controller configured to receive the sensor output corresponding to at least two relative flow rates and apply logic thereto to generate an output indicative of a condition of the filter. 
         [0022]    The sensor output is preferably indicative of particle concentration. 
         [0023]    In preferred forms of the invention, the mechanism is configured or controlled (e.g. controlled by the controller) to vary the relative flow rates, and the controller is configured to produce the output indicative of the condition of the filter, periodically (e.g. at fixed intervals, randomly from time to time, or according to a predetermined schedule) and/or in response to a sensed variation in particle concentration. 
         [0024]    The controller may be configured to generate a fault signal if the condition of the filter exceeds a predetermined threshold. 
         [0025]    The mechanism is preferably configured or controlled to vary the relative flow rates based on a predicted level of pollution in the environment to control the concentration of pollution reaching the detection region. 
         [0026]    Another aspect of the invention provides a filter arrangement for a particle detector for detecting particles in an environment, the particle detector including one or more sensors for analysing fluid in a detection region to produce sensor output, the filter arrangement including, structure defining flow paths, for conveying fluid from the environment towards the detection region, including a first flow path, which includes a filter, and a second flow path bypassing the fitter, and a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; the mechanism being configured or controlled to vary the relative flow rates based on a predicted level of pollution in the environment to control the concentration of pollution reaching the detection region. 
         [0027]    In preferred forms of the invention the varying the relative flow rates based on the predicted level of pollution includes varying the relative flow rates based on a repeating schedule. Most preferably the schedule includes a night mode, in which a relatively lower proportion of fluid passes through the first flow path for relatively higher detector sensitivity, and a day mode in which a relatively higher proportion of fluid passes through the first flow path for reduced contamination of the detection region. 
         [0028]    Optionally the mechanism is configured or controlled to periodically temporarily increase the relative flow rate through the second flow path to periodically increase sensitivity of the detector whereby a time averaged exposure of the detection region to fluid from the second flow path, and contaminants carried thereby, is reduced for reduced contamination of the detection region. 
         [0029]    Another aspect of the invention provides a filter arrangement for a particle detector for detecting particles in an environment, the particle detector including one or more sensors for analysing fluid in a detection region to produce sensor output, the filter arrangement including: structure defining flow paths, for conveying fluid from the environment towards a detection region, including a first flow path, which includes a filter, and a second flow path bypassing the filter; a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; and a controller configured to control the mechanism to periodically temporarily increase the relative flow rate through the second flow path to periodically increase sensitivity of the detector whereby a time averaged exposure of the detection region to fluid from the second flow path, and contaminants carried thereby, is reduced for reduced contamination of the detection region. 
         [0030]    In preferred forms of the invention the relative flow rate through the second flow path is temporarily increased one or more times per minute. 
         [0031]    Preferably the mechanism is configured or controlled such that, during each periodic temporary increase in relative flow rate through the second flow path, substantially all of the fluid is conveyed via the second flow path. Most preferably the mechanism is configured or controlled such that, between each periodic temporary increase in relative flow rate through the second flow path, substantially all of the fluid is conveyed via the first flow path. 
         [0032]    The structure may include a flow splitting arrangement to receive a common stream of fluid from the environment and direct respective portions of the stream into the first flow path and the second flow path. Preferably the structure includes a flow combining arrangement to receive fluid from the first flow path and the second flow path and convey a combined stream of fluid toward the detection region. 
         [0033]    The mechanism may include a valve, e.g. a solenoid valve. In preferred forms of the invention the mechanism includes an electro-mechanical device. 
         [0034]    Preferably the structure and mechanism together impose a flow restriction substantially greater than a flow restriction imposed by the filter whereby the relative flow rates through the first flow path and the second flow path are substantially independent of filter condition. Preferably the filter is a HEPA filter. 
         [0035]    In preferred forms of the invention the second flow path is configured such that fluid carried thereby is substantially unfiltered of particles of interest. 
         [0036]    Another aspect of the invention provides a particle detector, for detecting particles in an environment, including: structure defining a detection region; one or more sensors for analysing fluid in the detection region to produce sensor output; and the above filter arrangement arranged to convey fluid, from the environment to the detection region, and to cooperate with the sensors. 
         [0037]    Preferably the particle detector includes a controller having a purge mode in which the controller controls the mechanism to deliver fluid to the detection region substantially only from the first flow path to purge the detection region of fluid from the second flow path, and a detection mode in which the controller controls the mechanism to deliver at least some fluid from the second flow path to the detection region; and being configured to apply logic to sensor output from the detection mode to generate a further output; and if necessary adjust the logic, based on the sensor output from the purge mode, to compensate for contamination of the detection region. 
         [0038]    Another aspect of the invention provides a particle detector, for detecting particles in an environment, including structure defining flow paths, for conveying fluid from the environment towards a detection region, including a first flow path, which includes a filter, and a second flow path bypassing the filter, a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; and one or more sensors for analysing fluid in the detection region to produce sensor output; and a controller having a purge mode in which the controller controls the mechanism to deliver fluid to the detection region substantially only from the first flow path to purge the detection region of fluid from the second flow path, and a detection mode in which the controller controls the mechanism to deliver at least some fluid from the second flow path to the detection region; and being configured to apply logic to sensor output from the detection mode to generate a further output; and if necessary adjust the logic, based on the sensor output from the purge mode, to compensate for contamination of the detection region. 
         [0039]    The controller is preferably configured to periodically (e.g. at fixed intervals, randomly from time to time, or according to a predetermined schedule) transition between the purge and detection modes. Most preferably the controller is configured such that the transition from the detection mode to purge mode is conditional on the sensor output. 
         [0040]    The controller may store a plurality of measurements based on the sensor output over an interval of time when the chamber is purged. In preferred forms of the invention the controller is configured to apply further logic, to the sensor output when the chamber is purged, and if necessary generate fault signal. 
         [0041]    The logic may include subtracting a measure of background light from the sensor output. Adjusting the logic may include calculating and substituting a new measure of background light. 
         [0042]    The sensors include one or more photoelectric devices. 
         [0043]    Another aspect of the invention provides a particle detection system for detecting particles in an environment including plumbing defining at least one inlet, for receiving fluid from the environment, and at least one outlet; an aspirator between the inlets and the outlet to move fluid through the plumbing; and the above particle detector arranged to receive fluid from downstream of the aspirator and deliver fluid upstream of the aspirator whereby fluid is moved through the particle detector by the aspirator. 
         [0044]    Another aspect of the invention provides a filter arrangement for receiving sample fluid and supplying fluid to be moved through a detection region of a particle detector, the filter arrangement including structure defining a first flow path and a second flow path, a controllable mechanism for controlling relative flow rates of received sample fluid through the first flow path and the second flow path, and a controller; at least the first flow path including a filter for filtering particles from fluid travelling along the first flow path; the first, filtered, flow path and the second flow path being arranged in parallel, such that fluid travelling along the second flow path bypasses the filter, the first, filtered, flow path and the second, bypass, flow path being arranged to communicate downstream of the filter with the detection region; the controllable mechanism having at least two modes, the at least two modes corresponding to differing relative flow rates through the first, filtered, flow path and the second, bypass, flow path; and the controller being configured or programmed to receive at least one parameter characterising the fluid for each of the at least two modes, and being configured or programmed to apply logic to generate a filter condition signal indicative of filter condition based on the received parameters. 
         [0045]    The at least one parameter is preferably a signal indicative of particle concentration, which is most preferably received from components of the particle detector associated with the detection region, e.g. a photoelectric device. 
         [0046]    According to preferred forms of the invention, the first, filtered, flow path and the second, bypass, flow path converge downstream of the filter and are subsequently communicated with the detection region. 
         [0047]    The controllable mechanism is preferably a valve. The controllable mechanism may be an electro-mechanical device. According to preferred forms of the invention the controllable mechanism is a solenoid valve. Preferably the controllable mechanism is operatively associated with the controller whereby the controller switches between the at least two modes. This allows the controller to be configured or programmed to switch between the at least two modes to produce said filter condition signal. For example, the controller may be configured or programmed to periodically, say once every week, produce said filter condition signal. 
         [0048]    The controller may be configured or programmed to switch between the modes based on actual or predicted pollution levels to control the concentration of pollution reaching the detection region. For example, one mode, wherein substantially all of the received sample fluid is filtered, might be used during day time operation in a factory environment (when high pollution levels are expected), and a second mode, wherein substantially all of the received sample fluid travels along the bypass line, used at night. In this way the particle detector can operate at full sensitivity at night, yet be protected from day time pollution. The controller may have three or more modes corresponding to differing levels of filtration. 
         [0049]    Optionally the controller may be configured or programmed to change between modes to produce the filter condition signal in response to a signal indicative of variation in particle concentration, e.g. if a rapid increase or decrease (e.g. a 50% decrease) in particle concentration is detected, the controller may control the controllable mechanism to obtain said filter condition signal and thereby determine whether the rapid increase or decrease is associated with a change in the received sample fluid or a change in the filter condition. The controller may also be configured or programmed to change between modes in response to a signal indicative of particle concentration for increased detector sensitivity or increased pollution protection. 
         [0050]    Preferably, one of the at least two modes, referred to as a filter-check mode, corresponds to substantially all of the received sample fluid travelling along the second, bypass, flow path. 
         [0051]    According to preferred embodiments of the invention, in at least one of the at least two modes, the structure and/or the controllable mechanism impose a flow restriction substantially greater than a flow restriction imposed by the filter whereby the relative quantity of fluid flowing through the first, filtered, flow path and other flow path(s) is substantially independent of filter condition. 
         [0052]    The controller of preferred forms of the invention is configured or programmed to apply logic to generate a fault signal if the filter condition signal exceeds a predetermined threshold. 
         [0053]    The structure may include a flow splitting arrangement configured to receive, e.g. through a common opening, a common stream of sample fluid, e.g. from a common pipe associated with a piping network, and direct respective portions of the stream into the first, filtered, flow path and the second, bypass, flow path. 
         [0054]    This aspect of the invention also provides a particle detector including the filter arrangement and a detection region, the filter arrangement being communicated with the detection region for supplying fluid thereto. 
         [0055]    This aspect of the invention also provides a method of determining filter condition of a filter upstream of a particle detector, the method including: creating a first fluid flow through at least one of the filter and a bypass arranged to bypass the filter upstream of the particle detector, measuring at least one first parameter associated with the first fluid flow; varying the relative flow rates through the filter and the bypass to generate a second fluid flow; measuring at least one second parameter associated with the second fluid flow; and applying logic to the measured parameters to determine filter condition. 
         [0056]    The measured parameters are preferably particle concentration, and are most preferably measured at the particle detector. 
         [0057]    The varying preferably includes actuating at least one electro-mechanical valve. 
         [0058]    According to preferred forms of the invention, the first and second fluid flows are conveniently created by selectively preventing and allowing the flow of fluid through the bypass. 
         [0059]    In another aspect the invention provides a filter arrangement for receiving sample fluid and supplying fluid to be moved through a detection region of a particle detector, the filter arrangement including, structure defining a first flow path and a second flow path, a controllable mechanism for controlling relative flow rates of received sample fluid through the first flow path and the second flow path, and a controller; at least the first flow path including a filter for filtering particles from fluid travelling along the first flow path; the first, filtered, flow path and the second flow path being arranged in parallel, whereby fluid travelling along the second flow path bypasses the filter, the first, filtered, flow path and the second, bypass, flow path being arranged to communicate downstream of the filter with the detection region; the controllable mechanism having at least two modes, the at least two modes corresponding to differing relative flow rates through the first, filtered, flow path and the second, bypass, flow path; and the controller being configured or programmed to switch between the modes based on actual or predicted pollution levels to control the concentration of pollution reaching the detection region. 
         [0060]    This aspect of the invention also provides a particle detector including the filter arrangement and a detection region, the filter arrangement communicating with the detection region for supplying fluid thereto. 
         [0061]    This aspect of the invention also provides a method of controlling the concentration of pollution in fluid supplied to a detection region of a particle detector, the method including the steps of: creating a fluid flow through at least one of a filter and a bypass arranged to bypass the filter, directing the fluid flow to the detection region; and varying the relative flow rates through the filter and the bypass based on actual or predicted pollution levels to control the concentration of pollution reaching the detection region. 
         [0062]    In broad terms a further aspect of the invention is directed towards a method of monitoring an aspirated smoke/particle detector (in a polluted environment) including: providing a first flow path between a sampled air output and a detection zone of the smoke/particle detector, the first flow path having a filter providing a second flow path between the sampled air output and the detection zone which bypasses the first, filtered, flow path obtaining a first signal associated with particle concentration in the first filtered flow path diverting sampled air through the second flow path obtaining a second signal associated with particle concentration in the second flow path obtaining a measurement representative of the condition of the filter based on the first and second signals. 
         [0063]    The invention further provides a method of operating an aspirated particle detector including: monitoring an aspirated smoke/particle detector(in a polluted environment) including: providing a first flow path between a sampled air output and a detection zone of the smoke/particle detector, the first flow path having a filter providing a second flow path between the sampled air output and the detection zone which bypasses the first filtered flow path switching between the first and second flow paths in response to a signal indicative of the generation of non-indicative particles (e.g. dust particles). 
         [0064]    The signal indicative of the generation of non-indicative particles may be, for example, a timing signal or a detection signal. 
         [0065]    In another aspect the invention provides a particle detector including, a detection region; one or more sensors for detecting, and providing a signal indicative of, particles in the detection region; structure defining a study fluid flow path for conveying study fluid to the detection region; a controllable mechanism for controlling a flow rate along the study fluid flow path; and a controller configured to control the controllable mechanism to selectively move study fluid through the detection region whereby the detection region is exposed to moving study fluid, and contaminants carried thereby, for reduced contamination of the detection region. 
         [0066]    Preferably the structure further defines a sample fluid flow path, for conveying a stream of sample fluid drawn from a sample space, and the controllable mechanism is configured to direct at least a portion of the stream of sample fluid to flow along the study fluid flow path, said portion of the stream of sample fluid forming the study fluid. 
         [0067]    According to preferred embodiments of the invention, the structure further defines a first, filtered, flow path, the study fluid flow path forming a second, bypass, flow path; each of the first, filtered, flow path and the second, bypass, flow path communicating with the detection region. 
         [0068]    The particle detector preferably includes an aspirator downstream of the detection region to create, and move, said stream of fluid. 
         [0069]    The second, bypass, flow path is preferably substantially unfiltered whereby the study fluid is substantially unfiltered. 
         [0070]    The first, filtered, flow path is preferably configured to filter substantially all contaminates from the fluid travelling therealong. By way of example, the first, filtered, flow path may be spanned by a HEPA filter. 
         [0071]    Preferably the controller is configured to move study fluid through the detection region about 20% of the time. Most preferably study fluid is moved through the detection region one or more times per minute. By way of example, the detection region may be alternately exposed to filtered fluid from the first, filtered, flow path for 4 seconds and substantially unfiltered fluid from the second, bypass, flow path for 1 second. 
         [0072]    The controller may be configured to receive the indicative signal and apply logic to generate a further signal. Preferably, the controller is configured to receive the indicative signal during the selective moving of study fluid through the detection region and if necessary, compensate for said contamination of the detection region. 
         [0073]    This aspect of the invention also provides a method of operating an aspirated particle detector having a detection region, the method including the steps of: intermittently introducing a new sample flow to the detection chamber, selectively moving a study fluid through the detection region; and studying the study fluid in the detection region to generate a signal indicative of particle concentration; whereby the detection region is selectively exposed to moving study fluid, and contaminants carried thereby, for reduced contamination of the detection region. 
         [0074]    Said selectively moving preferably includes periodically directing at least a portion, and more preferably substantially all, of a stream of fluid drawn from a sample space for reduced transit time. Said periodically directing preferably includes directing said stream portion from a first, filtered, flow path to a second, bypass, flow path; wherein each of the first, filtered, flow path and the second, bypass, flow path communicate with the detection region; the fluid travelling along the second, bypass, flow path forming the study fluid. The method preferably includes activating an aspirator downstream of the detection region to create, and move, said stream of fluid. 
         [0075]    The second, bypass, flow path is preferably substantially unfiltered such that the study fluid is substantially unfiltered. 
         [0076]    The first, filtered, flow path is preferably configured to filter substantially all contaminants from the fluid travelling therealong. By way of example, the first, filtered, flow path may be spanned by a NEPA filter. 
         [0077]    Preferably the method includes moving a study fluid through the detection region about 20% of the time. Most preferably study fluid is moved through the detection region one or more times per minute. By way of example, the detection region may be alternately exposed to filtered fluid from the first, filtered, flow path for 4 seconds and substantially unfiltered fluid from the second, bypass, flow path for 1 second. 
         [0078]    The method may include applying logic to the indicative signal to generate a further signal; and may also include receiving the indicative signal during the selective moving of study fluid through the detection region and if necessary compensating for said contamination of the detection region. 
         [0079]    In another aspect the invention provides a particle detector including a detection region; one or more sensors for detecting, and providing a signal indicative of, particles in the detection region; a controller; structure defining a first flow path and a second flow path for conveying received sample fluid to the detection region; and a controllable mechanism for controlling relative flow rates of received sample fluid through the first flow path and the second flow path; at least the first flow path including a filter for filtering particles from fluid travelling along the first flow path; the first, filtered, flow path and the second flow path being arranged such that fluid travelling along the second flow path bypasses the filter; the controllable mechanism having a purge mode, wherein the detection region receives substantially only filtered fluid from the first, filtered, flow path to purge the detection region of unfiltered fluid, and a detection mode wherein the detection region receives at least some fluid from the second, bypass, flow path; the controller being configured or programmed to, when in the detection mode, receive the indicative signal and applying logic to generate a further signal; the controller being configured or programmed to receive the indicative signal when the detection region is purged of unfiltered fluid and if necessary adjust the logic in response thereto, whereby to compensate for contamination of the detection region. 
         [0080]    The controller is preferably operatively associated with the controllable mechanism to control the transition between the purge and detection modes. The controller may be configured such that the transition from the detection mode to purge mode is conditional on the indicative signal. For example the detector may be configured to not enter the purge mode if the indicative signal is at or above a threshold. The start purge threshold preferably corresponds to a particle concentration less than, and most preferably about 50%, of an alarm threshold. 
         [0081]    The controller preferably stores a plurality of measurements based on the indicative signal over an interval of time when the chamber is purged. The controller may be configured to generate a fault signal if the indicative signal when the chamber is purged is too low, too high, too variable, and/or too different from the indicative signal during the previous purging and adjusting operation. The logic may include subtracting a measure of background light from the indicative signal. The adjustment of the logic may include averaging the stored indicative signals to calculate a new measure of background light. 
         [0082]    This aspect of the invention also provides a method of operating a particle detector having a detection region, the method including moving a study fluid through the detection region; studying the study fluid in the detection region to generate a signal indicative of particle concentration; and applying logic to the indicative signal to generate the further signal; filtering sample fluid drawn from a sample space to form filtered fluid; moving the filtered fluid through the detection region to purge the detection region of the study fluid; studying the filtered fluid in the detection region to generate a second signal indicative of particle concentration; and if necessary adjusting the logic in response to the second indicative signal whereby to compensate for contamination of the detection region. 
         [0083]    Moving the filtered fluid through the detection region to purge the detection region of the study fluid may be conditional on the indicative signal. 
         [0084]    As used herein, except where the context requires otherwise, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised”, are not intended to exclude further additives, components, integers or steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0085]      FIG. 1  is a schematic illustration of a particle detector in accordance with a preferred embodiment of the invention; 
           [0086]      FIG. 2A  is a schematic illustration of a filter arrangement according to a preferred embodiment of the invention in one operating mode; 
           [0087]      FIG. 2B  is a schematic illustration of filter arrangement of  FIG. 2A  in another operating mode; 
           [0088]      FIG. 3A  is a schematic illustration of a filter arrangement according to a further embodiment of the invention in one operating mode; 
           [0089]      FIG. 3B  is a schematic illustration of the filter arrangement of  FIG. 3A  in another operating mode; 
           [0090]      FIG. 4A  is a schematic illustration of a filter arrangement according to a further embodiment of the invention in one operating mode; 
           [0091]      FIG. 4B  is a schematic illustration of the filter arrangement of  FIG. 4A  in another operating mode; 
           [0092]      FIG. 5  is a schematic illustration of a filter arrangement according to a further embodiment of the invention in one operating mode; 
           [0093]      FIG. 6A  is a schematic illustration of a filter arrangement according to a further embodiment of the invention in one operating mode; 
           [0094]      FIG. 6B  is a schematic illustration of the filter arrangement of  FIG. 3A  in another operating mode; and 
           [0095]      FIG. 7  is a schematic illustration of a fitter arrangement according to a further embodiment of the invention in one operating mode. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0096]      FIG. 1  illustrates a particle detector  12  in accordance with a preferred embodiment of the invention. The particle detector  12  includes a filter arrangement  10 , a detection chamber  30  and an aspirator  14 . 
         [0097]    The aspirator  14  draws air from a piping network  18 . This air is referred to as ‘sample air’. The sample air is exhausted from the aspirator  14  via an exhaust pipe  46  to outlet  16 . A sampling pipe  48  branches from the exhaust pipe  46  and extends to the filter arrangement  10  to communicate the filter arrangement  10  with the aspirator exhaust. A pipe  40  communicates the filter arrangement  10  with the detection chamber  30 . The detection chamber  30  is in turn communicated with the aspirator inlet by pipe  50 . Sample air from the aspirator exhaust is thereby driven through the filter arrangement  10 , through the detection chamber  30 , and returned to the aspirator inlet. This arrangement is referred to as a sub-sampling loop. 
         [0098]    The fitter arrangement  10  includes structure  42  defining a first flow path  22  and a second flow path  24 . The flow paths  22  and  24  are arranged in parallel and extend between a manifold space  20  and a plenum  28 . Sample air from the aspirator exhaust is received within the manifold space  20  where it is divided between the first and second flow paths  22  and  24 . The two flow paths converge and the flows are combined within the plenum  28 . The combined flow is in turn conveyed to the detection chamber  30 . 
         [0099]    A filter  26  is positioned along the first flow path  22 . 
         [0100]    The manifold space  20  includes a valve forming a controllable mechanism for varying the relative flow rates through the first and second flow paths. The valve could be a simple flap or butterfly valve such as flap  120 B illustrated in  FIGS. 2A and 2B . By varying the operation of the valve, the relative portions of filtered and unfiltered air arriving at the plenum  28 , and in turn the composition of the combined air arriving at the detection chamber  30 , can be controlled. 
         [0101]    The filter arrangement  10  further includes a controller  32 . The controller  32  receives from the detection chamber  30  sensor output in the form signal  44  indicative of the concentration of particles within the detection chamber. In some embodiments the controller can be arranged to process the received signals to generate an output. In this embodiment the controller  32  operatively connected with the valve within manifold space  20  to control the relative flow rates through the first flow path  22  and the second flow path  24 . 
         [0102]    The “controller” as described herein can be any arrangement for receiving an input signal, and processing the signal to generate a further useful signal. For example the controller could include, but is not limited to, a microprocessor, field programmable gate array (FPGA), ASIC, microcontroller, or any functionally equivalent analogue or digital realisation. 
         [0103]    The controller  32 , based on the signal indicative of particle concentration  44 , generates an alarm signal (not shown) when certain alarm criteria are met. As will be described, the controller  32  also applies logic to generate a signal  34  indicative of the condition of the filter  26 . 
         [0104]    Smoke detectors according to preferred forms of the invention may be installed along a fire alarm loop (not shown), wherein the signal  34  indicative of the condition of the filter is transmitted via the loop to a fire alarm control panel (FACP; not shown). The FACP may display the condition of filter and signal an error, e.g. by providing an audible signal or flashing light on a display, when the filter condition exceeds a predetermined threshold. 
         [0105]    The filter  26  is configured to produce a known reduction in particle concentration. According to a preferred form of the invention, under the influence of controller  32 , the valve within the manifold space  20  is positioned so that substantially all of the received sample air is directed through the first, filtered, flow path  22  when in a normal, detection, mode. Thereby substantially all of the air arriving at the detection chamber  30  is filtered. The detection chamber  30  is thus protected from pollution. 
         [0106]    The alarm criteria applied to the signal  44  by controller  32  are adjusted based on the known reduction in particle concentration associated with the filter  26 . According to the described preferred form of the invention, the adjustment is automatically performed by the controller  32 , and the amount of adjustment is regularly updated based on the filter condition as determined by the following procedure. 
         [0107]    Once a week, at a fixed time of day, the controller  32  sends a signal to the manifold space  20  to change the position of the valve therein to vary the relative flow rates through the first path  22  and the second path  24  such that substantially all of the received sample air is directed through the bypass flow path  24 . Thereby substantially all of the fluid received by the detection chamber  30  is unfiltered. This condition is referred to as a ‘filter check mode’. 
         [0108]    By comparing the signal  44  in the normal, detection, mode and the filter check mode, the controller  32  is able to make an inference of the filter condition, and in particular the extent to which the filter  26  is filtering particles of interest from fluid passing there through. 
         [0109]    In the described normal, detection, mode, the detection chamber  30  is protected from pollution, whilst the described adjustment maintains an improved degree of accuracy over other filtering arrangements. Nonetheless, by filtering the incoming air the particle concentration reaching the detection chamber  30  is reduced which results in some reduction in sensitivity compared to unfiltered arrangements. 
         [0110]    According to preferred forms of the invention, the controller  32  is operative to control the valve within the manifold space  20  to vary the relative flow rates through the first flow path  22  and the second flow path  24 , and thus control the extent to which fluid received by the detection chamber  30  is filtered, in response to actual or predicted pollution levels. When the relative flow rates are so adjusted, the controller  32  makes a corresponding adjustment to the alarm criteria. The invention thereby allows for maximum protection from pollution when the air is heavily polluted, and for maximum sensitivity when the air is not so polluted. 
         [0111]    By way of example, the particle detector  12  might operate in the normal, detection, mode during the day time operation of a factory when pollution levels are expected to be high; and revert to a “night time” mode when the factory is inactive and the pollution levels should be lower. In the night time mode substantially all, or a selected fraction of, the received sample air is directed through the second, unfiltered, flow path of  24  and a corresponding adjustment to the alarm criteria is made by the controller  32 . The detection chamber  30  is thereby protected from the day time pollution, whilst maximum sensitivity is maintained during the night. The controller  32  might compare the signals  44  on each transition between the normal, detection, mode and the night time mode. Thus the filter condition may be checked, and the alarm criteria appropriately updated, on a twice daily basis. 
         [0112]    According to certain embodiments of the invention, a portion, or substantially all, of the received sample air may pass through the second, unfiltered, flow path  24  during normal operation for maximum sensitivity. According to these embodiments, the amount of filtration may be increased during a pollution event (e.g. the release of theatrical smoke within a theatre or a diesel powered train arriving at a station). The logic applied to the sensor output may be varied as a function of the relative flow rates, e.g. an alarm threshold or alarm delay might be lowered when the degree of filtration is increased. The variation in the amount of filtration might be controlled by a timetable (eg. a train timetable) or on other inputs. For example, in a simple implementation, an operator may provide an input to switch to a highly filtered mode before operating a piece of machinery known to produce a large volume of particle pollution. Certain embodiments may have more than two, e.g. four, discrete modes of operation corresponding to differing relative flow rates and thus differing amounts of filtration. 
         [0113]    The degree of filtration may be adjusted in response to the detected particle concentration. By way of example, the filter  26  may be a foam filter selected to filter dust whilst allowing smoke particles to pass therethrough. According to this embodiment, the controller  32  responds to a signal  44  indicative of an increased concentration of particles by varying the relative flow rate through the flow paths  22  and  24  so that a greater portion of the received sample fluid is filtered. By monitoring the variation in the signal  44 , the controller  32  can make an inference as to whether the detected increase in particle concentration is associated with dust or smoke. 
         [0114]      FIGS. 2A and 2B  schematically illustrate the structure  142  of a filter arrangement in accordance with an embodiment of the invention. Sample air is received via an inlet  138  into a manifold space  120 A. The structure defines two parallel flow paths,  122  and  124 . The flow path  122  is spanned by a foam filter  126 . The flow path  124  is spanned by a simple pivotally mounted flap forming a controllable mechanism for varying the flow rate through the flow path  124 , and in turn the relative flow rates through the flow paths  122  and  124 .  FIG. 2A  shows the flap  120 B in the closed position.  FIG. 2B  shows the flap  120 B in the open position. Flow paths  122  and  124  converge and the flows are thus combined in the plenum  128 , the combined fluid exits the structure  142  via the outlet  140 . 
         [0115]      FIG. 2A  illustrates an arrangement similar to the aforedescribed normal, detection, mode. The flow path  124  is closed by the flap  120 B so that substantially all of the received flow is directed through the filter  126 . The filter  126  has a relatively high impedance such that when the flap  120 B is opened, as in  FIG. 2B , substantially all of the flow is directed through the flow path  124 . 
         [0116]    Foam filters, such as filter  126 , effectively filter out dust particles. Within their normal operating range, such filters remove only a small proportion of the smoke particles. The differential treatment of smoke and dust particles desirably allows for the detection chamber to be protected from dust with only a relatively small reduction in sensitivity. A problem with such foam filters is that as they clog, they begin filtering out smoke particles, and a significant portion of smoke particles may be filtered before there is any appreciable change in pressure drop across the filter. It is thereby difficult to determine when the filter has blocked to an extent that it is filtering out smoke particles. As the skilled person will appreciate, preferred embodiments of the invention address this problem by opening the flap  120 B so that the detection chamber is exposed to unfiltered air. 
         [0117]    The structure of  FIGS. 1 ,  2 A and  2 B lend themselves to two further desirable operating modes. 
         [0118]    According to a first of these two modes, the flap  120 B, and thus the bypass  124 , is opened and closed periodically. In this mode the detector receives periodic bursts of unfiltered air, separated by periods of clean air. This has the advantage of extending the detector life by reducing incoming contaminates in proportion to the duty cycle of the clean air. For example, if the unfiltered air were permitted to enter the detector for one second followed by exposure to the air from the filter  126  for four seconds, then the contamination rate of the downstream detector would be one fifth of the rate that would occur if the detector were fully exposed to the unfiltered air all of the time. This method offers the similar life-extending benefits as the dilution arrangement proposed by the applicant in their international patent application WO 2007/095675. However, preferred forms of this time-dependent method have the advantage that they do not rely on maintaining or measuring ratios of flow. According to preferred forms of the invention, the controller  32  simply applies normal processing techniques to the signal  44  when the chamber  30  is occupied by unfiltered air while ignoring signal  44  when the chamber  30  is occupied by filtered-air. 
         [0119]    A second of these desirable operating modes is particularly well suited to the use of a NEPA filter which effectively filters out substantially all particles from the fluid flowing therethrough. When the flap  120 B is closed such that all of the received sample fluid is directed through the filter  126 , clean air (ie air devoid of particles, or at least devoid of particles of interest) is delivered to the detection chamber  30 . Once the chamber is purged of any unfiltered air and filled with clean air, the signal  44  provides an indication of background light with the detection chamber. This detection chamber can be used to adjust the alarm criteria. By way of example a background light reading might be subtracted from the signal  44  (when the chamber  30  is flooded with unfiltered air) to generate a further signal, and an alarm sounded when the further signal exceeds a predetermined threshold. 
         [0120]      FIGS. 3A and 3B  schematically illustrate a further embodiment of the invention. This embodiment includes three parallel flow paths  222 ,  224  and  236 . The flow path  222  is spanned by a HEPA filter. A second flow path  224  is unfiltered. A third flow path  236  is spanned by a flap  220 B. 
         [0121]    In a normal, detection, mode the flap  220 B is dosed to close the flow path  236  as illustrated in  FIG. 3A . Thereby substantially all of the sample air received via inlet  238  into manifold space  220 A is divided between the flow paths  222  and  224 . From the flow paths  222  and  224 , the air is received into plenum space  228  and exits the structure  242  via the outlet  240 . Accordingly to this arrangement, the relative flow rates through the paths  222  and  224  is determined by the relative impedance of those two flow paths. 
         [0122]    Preferred forms of invention include one or more structural elements, such as an apertured baffle plate (not shown) arranged to control the relative impedance of the flow paths  222  and  224 . The baffle plate preferably has an impedance significantly greater than that of the filter  226 . The relative impedance of the two flow paths  222  and  224  is thereby dominated by the geometry of the baffle plate so that the relative flow rates are more or less independent of the filter condition (at least until the filter becomes extremely clogged). By way of example, the baffle plate may include a single small hole in communication with the unfiltered flow path  224 , and several like holes in communication with the filtered flow path  222 . 
         [0123]    Moreover, this arrangement provides a ‘failsafe’ operation in that a filter clogged to an extent that its impedance changes appreciably (e.g. when recommended service intervals have been ignored) will generate a higher particle concentration signal. As the filter  226  clogs, the impedance of the flow path  222  increases and relatively more air flows through the flow path  224 . Thus, as the filter  226  clogs, the air passing through the structure  242  in the normal, detection, mode becomes less filtered, ie. has a higher particle concentration. 
         [0124]    As in the embodiment of  FIGS. 2A and 2B , the flow path  236  desirably has an impedance significantly lower than that of the flow paths  222  and  224  so that when the flap  220 B is opened, substantially all of the received fluid passes through the unfiltered flow path  236 . Alternatively a further flap  320 C, as illustrated in  FIGS. 4A and 4B  may be provided. 
         [0125]    Periodically, or whenever the detected smoke reading changes by some value, the position of the flap  220 B may be changed from the dosed position of  FIG. 3A  to the open position of  FIG. 3B . In doing so, the smoke detector is now exposed to undiluted smoke and the detector will record the actual sampled smoke value. By forming the ratio from smoke levels taken in the two configurations shown in  FIGS. 3A and 3B , it is possible to determine the actual dilution ratio of when in the normal, detection, mode (ie the ratio of the fluid travelling through the flow paths  222  and  224 ) and the threshold levels or sensitivity gain of subsequent analysing equipment may be adjusted. 
         [0126]    A similar ‘failsafe’ operation could be achieved using the structure of  FIGS. 2A and 2B  by only partly closing the flap  120 B in the normal, detection, mode. 
         [0127]    The embodiment of  FIGS. 4A and 4B  might be modified by the inclusion of a third valve  320 D to selectively close flow path  224  and filter  226  being a HEPA filter as illustrated in  FIG. 5 . By closing flow paths  224  and  236  all of the air is filtered such that a signal from a detection chamber (downstream of the illustrated filter arrangement) provides an indication of background light. 
         [0128]      FIGS. 6A and 6B  illustrate an alternative arrangement wherein the controllable mechanism, in form of flap  320 C′, is arranged to selectively close the filtered flow path  322 , to provide an indication of the condition of the filter  226 ′ the dilution ratio.  FIG. 6A  shows this configuration in normal use, where the filtered air combines with the bypass air causing a dilution of the amount of particulate outlet flow compared with the inlet flow. 
         [0129]    In  FIG. 6B  the flow through the filter is substantially blocked allowing only undiluted air to pass through the outlet. Thus by blocking flow path  324  when required, the actual inlet particulate concentration may be measured and then compared with the concentration measured when the filtered flow path  322  is not blocked. In this way the dilution factor of the overall filter may be determined and then the threshold levels or sensitivity gain of subsequent analysing equipment may be adjusted. 
         [0130]    The skilled person will appreciate that a number of valving arrangements may be used. The applicant contemplates that valve life may be extended by placing the valve down stream of the filter to shield the valve from the dust and debris such as the arrangement illustrated in  FIG. 7  wherein the valve  320 E is positioned downstream of the filter  226 E. 
         [0131]    The invention has been described with reference to embodiments having substantially unfiltered bypass flow paths. It is anticipated that these unfiltered bypass flow paths might be spanned by a coarse filter to filter out very large particulate matter, e.g. insects, and for present purposes such an arrangement would be considered substantially unfiltered. 
         [0132]    Moreover, the skilled person will appreciate that there are commercially advantageous embodiments which do not include an unfiltered bypass. By way of example, with reference to  FIG. 1 , both flow paths  22  and  24  might be spanned by identical foam filters (which, when new, filter out dust but not smoke). In this case, in one possible configuration, one of the filters might be only exposed to fluid, and the particulate matter borne thereby, during the filter check mode. The filter check mode being typically brief and infrequent compared to the routine operation in the normal, detection, mode. Therefore, it can be assumed that this filter is operating as in an “as new” condition whilst receiving flow to check the condition of the other filter. 
         [0133]    It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.