# Patent

## Title
Sensor head for a dry powder agent

## Abstract

A sensor head for a dry powder agent according to an exemplary aspect of the present disclosure includes a housing defined along an axis along which light is communicated, the housing defines a multiple of apertures transverse to the axis and in communication with a measurement volume along the axis. Each of the multiple of apertures defines a longitudinal length along the axis which is less than a lateral length defined around the axis. A minor is within the housing to reflect the light through the measurement volume.

## Background
The present disclosure is a Continuation-In-Part of U.S. patent application Ser. No. 12/421,030, filed Apr. 9, 2009 now U.S. Pat. No. 8,004,684 and U.S. patent application Ser. No. 12/420,940, filed Apr. 9, 2009.


BACKGROUND
The present disclosure relates to a measurement system for the measurement of dry powder based agents.
In order to certify a dry powder fire suppression system onboard a vehicle such as an aircraft, the agent is discharged into the protected volume and an analyzer simultaneously records the amount of fire extinguishing agent in various zones of the protected volume. The amount of agent must be above some predetermined level which has been established sufficient to extinguish all possible fires for some period of time simultaneous in all zones.
The analyzer must be calibrated and traceable such that analyzer output proves the dry powder fire suppression system is capable of extinguishing any fire within the protected space. No known systems are capable of both measuring aerosol cloud fire extinguishing agent concentrations and being calibrated so as to measure the agent concentration for an aircraft dry powder fire suppression system certification test.
SUMMARY
A sensor head for a dry powder agent sensor head according to an exemplary aspect of the present disclosure includes a housing defined along an axis along which light is communicated, the housing defines a multiple of apertures transverse to the axis and in communication with a measurement volume along the axis. Each of the multiple of apertures define a longitudinal length along the axis which is less than a lateral length defined around the axis. A minor is within the housing to reflect the light through the measurement volume.
A calibration system for a dry powder agent according to an exemplary aspect of the present disclosure includes a powderizer calibration column. A sensor system includes at least one sensor head at least partially within the powder calibration column, the at least one sensor head having a housing defined along an axis along which light is communicated, the housing defines a multiple of apertures transverse to the axis and in communication with a measurement volume along the axis. Each of the multiple of apertures define a longitudinal length along the axis which is less than a lateral length defined around the axis. A minor within the housing reflects the light through the measurement volume. A light source which communicates light into the measurement volume. A receiver to receive reflected light within the measurement volume from the mirror. A control system in communication with the light source and the receiver.
A measurement system for a dry powder agent according to an exemplary aspect of the present disclosure includes a protected structure test fixture. A sensor system includes at least one sensor head at least partially within the protected structure test fixture, the at least one sensor head having a housing defined along an axis along which light is communicated, the housing defines a multiple of apertures transverse to the axis and in communication with a measurement volume along the axis. Each of the multiple of apertures define a longitudinal length along the axis which is less than a lateral length defined around the axis. A minor within the housing reflects the light through the measurement volume. A light source which communicates light into the measurement volume. A receiver to receive reflected light within the measurement volume from the mirror. A control system in communication with the light source and the receiver.



BRIEF DESCRIPTION OF THE DRAWINGS
Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows:
FIG. 1 is a schematic view of a measurement system with a powderizer calibration column (PCC) for a dry powder agent;
FIG. 2 is a schematic view of a sensor system for a powder based chemical fire suppressant agent installed in a representative protected structure;
FIG. 3A is a schematic view of a powderizer calibration column (PCC) for a dry powder agent;
FIG. 3B is a schematic view of a powder feeder system and a gas distribution system for communication with the powderizer calibration column (PCC);
FIG. 3C is a perspective view of a sensor head within the powderizer calibration column (PCC);
FIG. 4A is an exploded view of a sensor head for a dry powder agent;
FIG. 4B is an expanded perspective view of the sensor head of FIG. 4A;
FIG. 4C is an expanded perspective view of the sensor head of FIG. 4A with a clamp attachment installed thereon;
FIG. 4D is a longitudinal sectional view of the sensor head body;
FIG. 4E is a schematic view of a measurement volume within the sensor head body;
FIG. 4F is a schematic view of a concentration boundary formed by the measurement volume within the sensor head body;
FIG. 4G is a schematic view of the stream lines which are formed generally near each the multiple of apertures within the measurement volume within the sensor head body;
FIG. 4H is a schematic view of the primary particle paths through the multiple of apertures within the measurement volume within the sensor head body;
FIG. 4I is a schematic view of the sensor head in communication with a control system;
FIG. 5 is a schematic view of a sensor head sensing volume;
FIG. 6 is a schematic view of a sensor head sensing volume with possible light paths without multiple reflection;
FIG. 7 is an empirical relationship between mass density concentration and light transmittance for a dry powder agent;
FIG. 8 is a flow chart illustrating calibration of the measurement system to determine an empirical relationship between mass density concentration and transmittance for a desired dry powder agent such as an aerosol cloud fire suppression agent; and
FIG. 9 is a flow chart illustrating operation of the sensor heads located within a protected structure test fixture such as the representative engine nacelle.



DETAILED DESCRIPTION
FIG. 1 schematically illustrates a measurement system 20 for the measurement of dry powder agent based chemical fire suppressant agents. The system 20 generally includes a powder calibration column (PCC) 22, a sensor system 24, and a control system 26. The PCC 22 generally is utilized to calibrate a sensor of the sensor system 24 which may then be installed in a protected structure test fixture 28 such as an engine nacelle (FIG. 2). It should be understood that the engine nacelle is but one representative structure in one non-limiting embodiment, within which a powder based chemical fire suppressant system 30 may be installed and that other protected structures 28 such as a land vehicle engine compartment, cabin or other structure will also benefit herefrom.
Referring to FIG. 3A, the PCC 22 generally includes a powder feeder system 40, a gas distribution system 42, an observation tube 44, a test section 46 within which at least one sensor head 24A, sometimes referred to as a powderizer, is mounted, and a powder capture box 48. The observation tube 44, test section 46 and powder capture box 48 are defined along an axis Z. The observation tube 44 which defines the test section 46 in one non-limiting embodiment is at least twenty diameters in length prior to the at least one sensor head 24A to ensure homogeneous distribution of the powder agent from the powder feeder system 40 and the inert gas from the gas distribution system 42. This facilitates a direct measurement of light transmission through the aerosol cloud by the sensor head 24A.
The powder feeder system 40 in one non-limiting embodiment includes an auger such as that manufactured by Acrison, Inc. of Moonachie, N.J., USA. The powder feeder system 40 defines the rate at which the dry powder agent is communicated into the observation tube 44.
The gas distribution system 42 communicates the inert gas which, in one non-limiting embodiment, includes nitrogen to breakup and fully disperses the dry powder agent. The gas distribution system 42 is located generally above the powder feeder system 40 and transverse thereto (FIG. 3B). The gas distribution system 42 defines the rate at which the inert gas is communicated into the observation tube 44 such that a known flow of dry powder agent and inert gas are received in the sensor head 24A so that a relationship may be determined between the dry powder agent concentration and light transmission. This allows for absolute known rates of dry powder agent feed and inert gas.
The powder capture box 48 provides a relatively large volume to prevent recirculation of the powder agent back into the test section 46. The powder capture box 48 also prevents pressure build up and prevents back flow of the dry powder agent aerosol cloud facilitated by airflow outflow through filters 48F.
The PCC 22 is utilized to calibrate the sensor head 24A light transmission measurement with respect to a concentration of aerosol cloud in mass per volume. The cylindrical cross section of the PCC 22 reduces corner effects and other geometric effects. The dry powder agent is communicated into the observation tube 44 just below the inert gas introduction (FIG. 3B). The high velocity jet of inert gas passes the powder feeder system 40 to cause turbulent mixing of the dry powder agent into the inert gas flow to generate an aerosol cloud as would be typical in an aerosol cloud fire suppression agent. The process which communicates the dry powder agent into the jet of inert gas causes the dry powder agent agglomerates to break up into the principle particles. The particles mixed with the inert gas creates the dry powder agent aerosol cloud.
The aerosol cloud travels downward with gravity in the observation tube 44 to facilitate homogeneous mixing of the aerosol cloud for measurement by the sensor head 24A (FIG. 3C). The downward flow is necessary because different sized particles of the dry powder agent travel at different velocities. Downward flow at steady conditions results in a steady-state concentration over time as the concentration of each particle size stays constant downstream to allow an accurate measurement by the sensor system 24.
Referring to FIG. 4A, one non-limiting embodiment of the sensor head 24A provides for a light transmission measurement through the dry powder agent aerosol cloud. Each sensor head 24A generally includes a housing 50 defined along an axis S though which light is communicated along a light path and a multiple of apertures 52 transverse to the axis S (FIG. 4B). The sensor head 24A generally includes a light source 54, a detector 56, a window 58, a minor window 60, a minor 62, an end block 64, an attachment 66 and gaskets 68A, 68B.
The attachment 66 may include a flanged end to receive a clamp C or other attachment (FIG. 4C). The clamp C is readily attached within the protected structures 28 to position the sensor head 24A (FIG. 2). In one non-limiting embodiment, the gaskets 68A, 68B are electrically conductive gaskets so as to provide an electrical ground between the window 58, the minor window 60, the mirror 62, the housing 50 and the attachment 66 through the clamp C such that the entire sensor head 24A is grounded to the protected structures 28. In this non-limiting embodiment, the housing 50, the end block 64 and the attachment 66 may be formed of a conductive material such as Aluminum coated with alodine. As the sensor head 24A is grounded, static electricity is prevented from accumulation on the window 58 and the mirror window 60 to thereby minimize attraction of dry powder agent. Alternatively, or in addition thereto, the window 58 and the minor window 60 may include an anti-static coating and may be relatively thin. In one non-limiting embodiment, each are less than 1 mm (0.04 inches) in thickness. Such a thickness minimizes distortion and reduces static electricity which may otherwise attract dry powder agent.
The gaskets 68A, 68B cushion the respective window 58 and minor window 60 in response to the end block 64 being threaded to the housing 50 and the attachment 66 being threaded to the housing 50. That is, the end block 64 and the attachment 66 are threaded into the housing 50 to permit disassembly and access to the window 58 and the mirror window 60 for cleaning or other maintenance. The threaded engagement also facilitates the coaxial alignment of the window 58, the minor window 60, and the mirror 62 such that the light source 54 will always provide for a repeatable focus on the same point on the minor 62 to assure alignment in response to the end block 64 and the attachment 66 being threaded into the housing 50. That is, the optical alignment is integral to the design.
A respective shoulder 70A, 70B (FIG. 4D) abuts the respective window 58 and mirror window 60 in response to the end block 64 and the attachment 66 being threaded into the housing 50. The shoulders 70A, 70B further facilitate that length repeatability of assembly such that the distance between the light source 54 and the minor 62 is repeatable. It should be understood that various gaskets, O-rings, seals, neutral density filters for calibration and combinations thereof may alternatively or additionally be provided.
The housing 50 defines a measurement volume 72 which is defined in part by the multiple of apertures 52. The measurement volume 72 is tightly maintained between the light source 54 and the mirror 62 even during the temperature changes, shock and vibration common in the protected structures 28 due to the threaded interface and shoulder 70A, 70B arrangement.
The multiple of apertures 52 are generally rectilinear and transverse to the axis S. In one non-limiting embodiment, each of the multiple of apertures 52 define a longitudinal length along the axis S which is less than a lateral length defined around the axis S, such that for example, the lateral length is less than 10 mm (0.39 inches) and more specifically, may be 3 mm (0.12 inches).
Referring to FIG. 4E, the measurement volume 72 as defined in part by the multiple of apertures 52 is generally trapezoidal in shape. The shape and size of the multiple of apertures 52 readily reduces the optics contamination in high flow and turbulent environments. The shape of the measurement volume 72 forms a concentration boundary which essentially creates a bulge of laminar flow conditions within the sensor head 24A (FIG. 4F). This is represented by the stream lines which are formed generally near each the multiple of apertures 52 (Figure).
Bulk flow from outside the sensor head 24A enters the measurement volume 72 through the multiple of apertures 52 to create the laminar situation such that essentially no turbulence is created inside the measurement volume 72. Flow through the measurement volume 72 form a venturi effect which pulls the dry powder agent from measurement volume 72. Only a relatively small quantity of dry powder agent may be deposited on the optics via concentration gradients. More specifically, an example particle of dry powder agent from the bulk flow which enters the measurement volume 72 will either flow through the multiple of apertures 52 with minimal vector change (A); enter a cavity recirculate and be pulled out of the measurement volume 72 via venturi effects (B); or enter from a turbulent eddy and impact the optics (C) (FIG. 4G). The total amount of dry powder agent that impact the optics is relatively small and is typically less than 2% of the total mass that passed through the sensor head 24A. This does not significantly affect the concentration profile because the outside turbulence which deposits the dry powder agent onto the optics occurs over a short period compared to the concentration measurement time.
The light source 54 may include a red light emitting diode (LED) operating at 650 nm and the detector 56 is a visible spectrum photodiode. The light source 54 and the detector 56 communicate with the sensor head 24A through a fiber optic cable 74 which includes a multiple of fiber optics. In one non-limiting embodiment, 32 fiber optics communicate with the light source 54 and 32 fiber optics communicate with the detector 56. The fiber optics may be bundled to form the fiber optic cable 74 with a SMA905 connection 74C for attachment into the housing 50.
Each sensor head 24A operates on light obscuration principles. Light is communicated from the light source 54 through a multiple of fiber optics of the fiber optic cable 74, passes through the transparent window 58, enters the measurement volume 72 in communication with the aperture 52, reflects off the minor 62 and is communicated through a multiple of the fiber optics of the fiber optic cable 74. The window 58 is generally opposite the minor 62 such that the light passes through the measurement volume 72 twice—from window 58 through measurement volume 72, reflects off minor 62 then through measurement volume 72 and into the detector 56 via window 58 and the fiber optic cable 74. The detector 56 outputs an analog signal proportional to the intensity of light to the control system 26 (FIG. 4I). The intensity of light changes functionally to the concentration of dry powder agent particles in the measurement volume 72. The sensor head 24A is unlike thermocouples, pressure transducers, strain gauges or other instrumentation where signal strength is directly related to the phenomena being measured.
Each sensor head 24A of the sensor system 24 is in communication with the control system 26 which initiates operation of the system 20 and controls the data acquisition from the sensor head 24A. Data in one non-limiting embodiment is acquired at a variable frequency of 1 to 1000 Hz for a maximum of 30,000 total points. The maximum acquisition time is 30,000 divided by the acquisition rate (300 seconds in the case of 100 HZ). It should be understood that alternative data acquisition rates may be utilized by the control system 26.
It should be noted that a computing device can be used to implement various functionality, such as that attributable to the control system 26 (FIG. 4I). In terms of hardware architecture, such a computing device may include a processor, memory, and one or more input and/or output (I/O) device interface(s) that are communicatively coupled via a local interface. The local interface can include, for example but not limited to, one or more buses and/or other wired or wireless connections. The local interface may have additional elements, which are omitted for simplicity, such as controllers, buffers (caches), drivers, repeaters, and detectors to enable communications. Further, the local interface may include address, control, and/or data connections to enable appropriate communications among the aforementioned components.
The processor may be a hardware device for executing software, particularly software stored in memory. The processor can be a custom made or commercially available processor, a central processing unit (CPU), an auxiliary processor among several processors associated with the computing device, a semiconductor based microprocessor (in the form of a microchip or chip set) or generally any device for executing software instructions.
The memory can include any one or combination of volatile memory elements (e.g., random access memory (RAM, such as DRAM, SRAM, SDRAM, VRAM, etc.)) and/or nonvolatile memory elements (e.g., ROM, hard drive, tape, CD-ROM, etc.). Moreover, the memory may incorporate electronic, magnetic, optical, and/or other types of storage media. Note that the memory can also have a distributed architecture, where various components are situated remotely from one another, but can be accessed by the processor.
The software in the memory may include one or more separate programs, each of which includes an ordered listing of executable instructions for implementing logical functions. A system component embodied as software may also be construed as a source program, executable program (object code), script, or any other entity comprising a set of instructions to be performed. When constructed as a source program, the program is translated via a compiler, assembler, interpreter, or the like, which may or may not be included within the memory.
The Input/Output devices that may be coupled to system I/O Interface(s) may include input devices, for example but not limited to, a keyboard, mouse, scanner, microphone, camera, proximity device, etc. Further, the Input/Output devices may also include output devices, for example but not limited to, a printer, display, etc. Finally, the Input/Output devices may further include devices that communicate both as inputs and outputs, for instance but not limited to, a modulator/demodulator (modem; for accessing another device, system, or network), a radio frequency (RF) or other transceiver, a telephonic interface, a bridge, a router, etc.
When the computing device is in operation, the processor can be configured to execute software stored within the memory, to communicate data to and from the memory, and to generally control operations of the computing device pursuant to the software. Software in memory, in whole or in part, is read by the processor, perhaps buffered within the processor, and then executed.
Referring to FIG. 5, the sensor head 24A directly measures light transmission though the dry powder agent aerosol cloud, which can be directly related to a spatial mass density concentration via an empirical or theoretical relationship.
The dry powder agent aerosol cloud enters the measurement volume 72 and passes through the light that travels from the fiber optic cable 74 to the mirror 62 and back to the detector 56. As the dry powder agent aerosol cloud passes through the light, the total light transmitted decreases proportional to the concentration of the aerosol cloud. The minor 62 is concaved and focuses the light back to the fiber optic cable 54.
Referring to FIG. 6, representative paths for the light are schematically illustrated. Reflections off of objects beyond the first fouling layer have been ignored due to the insignificant amount of light return. The intensity of light received by the detector 56 is the sum of Paths 1, 1B, 2, and the stray light from the background. In Path 1 some fraction of the reflected light off the window 58 is received. Path 1B occurs if a fouling layer exists on window 58 and must transmit through window 58 twice.
In Path 2, the light must be transmitted twice through each window 58, 60, each fouling layer, the measurement volume 72 then reflects off the mirror 62. The minor 62 is assumed to reflect all light in these calculations. Both windows 58, 60 have the same properties.
Without a fouling layer, the sensor head 24A output can be expression as:

I=fw1ρwIs+τw4τc12τv2τc22Is+I∞  Equation 1

Prior to a test, the blocked output can be measured setting τv to 0 by blocking the mirror 62 with non-reflective media such that only the light reflected off window 58 and ambient light are measured.

Ibl=fw1ρwIs+I∞  Equation 2

The reference value is found when τv is 1, i.e. no aerosol cloud is in the measurement volume and light is allowed to reflect off the mirror back to the source. Substituting in for the measured blocked value:

Iref=τw4τc12τc22Is+Ibl  Equation 3

By substituting Equation 3 and Equation 2 into Equation 1, the double pass transmittance through the sensing volume during a test can be expressed:




  
    
      
        
          τ
          v
          2
        
        =
        
          
            I
            -
            
              I
              bl
            
          
          
            
              I
              ref
            
            -
            
              I
              bl
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        4
      
    
  




At this point, it is convenient to define a modified sensor head 24A output with the blocked value subtracted off as the blocked value is assumed constant throughout the entire measurement.

I*≡I−Ibl  Equation 5

Equation 4 can be re-expressed:





  
    
      
        
          τ
          v
          2
        
        =
        
          
            I
            *
          
          
            I
            ref
            *
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        6
      
    
  




Mass Density Concentration—Theoretical Relationship
The relationship of light transmission to concentration of particles in a dispersed cloud is given by




  
    
      
        
          
            d
            i
          
          
            d
            x
          
        
        =
        
          
            -
            
              n
              v
              ′′′
            
          
          ⁢
          
            A
            s
          
          ⁢
          i
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        7
      
    
  




Beers' Law, squared here to give the double path transmittance, is a solution to Equation 7 where only the light intensity varies with distance.




  
    
      
        
          
            τ
            v
            2
          
          ≡
          
            
              (
              
                i
                
                  i
                  i
                
              
              )
            
            2
          
        
        =
        
          exp
          ⁡
          
            (
            
              
                -
                2
              
              ⁢
              
                
              
              ⁢
              
                n
                v
                ′′′
              
              ⁢
              
                A
                s
              
              ⁢
              L
            
            )
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        8
      
    
  




Equation 8 can be solved for the number density concentration, when multiplied by the mass of one particle the mass density concentration in the sensing volume is found.




  
    
      
        
          m
          v
          ′′′
        
        =
        
          
            1
            
              2
              ⁢
              L
            
          
          ⁢
          
            
              V
              p
            
            
              A
              s
            
          
          ⁢
          
            ρ
            s
          
          ⁢
          
            ln
            ⁡
            
              (
              
                1
                
                  τ
                  v
                  2
                
              
              )
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        9
      
    
  




The transmission through the sensing volume term is squared because the light travels through the measurement volume 72 twice. Mie Theory identifies the scattering cross sectional area as approaching twice the particle cross sectional area, as the size of the particle increases from three times the wavelength of light. In this example, the average particle diameter is above 3 μm where the wavelength of the light is 0.65 μm. Because the surface area and the mass of the individual particles are of interest, the Sauter Mean diameter is used. This is the diameter of a particle with the same surface area to mass (volume) ratio as the entire aerosol cloud population. Equation 9 reduces to a function of the Sauter Mean diameter:




  
    
      
        
          m
          v
          ′′′
        
        =
        
          
            1
            3
          
          ⁢
          
            
              d
              
                [
                
                  3
                  ,
                  2
                
                ]
              
            
            
              2
              ⁢
              
                
              
              ⁢
              L
            
          
          ⁢
          
            ρ
            s
          
          ⁢
          
            ln
            ⁡
            
              (
              
                1
                
                  τ
                  v
                  2
                
              
              )
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        10
      
    
  




Equation 10 yields a theoretical relationship between the mass density concentration and the transmission if the particle diameter has been characterized. However, this relationship is only valid at high transmissions.
Since the powderizer will measure transmissions less than that valid in Equation 9, the PCC is utilized to derive an empirical relationship between transmission and spatial mass density concentration.
The experiments were conducted in the PCC 22 (FIG. 3A) which provides for constant flows of the dry powder agent and the inert gas. This provides for fully developed flow so that the dry powder agent aerosol cloud is evenly distributed across the observation tube 44 cross section. The fully mixed dry powder agent aerosol cloud then passes by the sensor head 24A where light transmission is measured. By varying the flow rate of inert gas and the feed rate of the dry powder agent, a variety of mass density concentrations could be tested.
The relationship between a transmission and spatial mass density concentration as determined in the PCC is specific to the sensor head 24A design and the dry powder composition and size. If any of these change, a new relationship may be determined.
The measurement error can be found from the scatter in the transmission to spatial mass density concentration data. For the sensor head 24A in FIG. 4C, the error was found to be +/−16 g/m^3. This error would be specific to the sensor head 24A design and the dry powder composition and size.
Referring to FIG. 8, an operational example of the PCC 20 is illustrated in which the measurement system 20 is operated to determine an empirical relationship between mass density concentration and transmittance for a desired dry powder agent such as an aerosol cloud fire suppression agent (FIG. 7). In step 200, the powder feeder system 40 is calibrated to a desired dry powder agent rate. The powder feeder system 40 is then operated at the desired dry powder agent rate (g/sec) (step 202) as the gas distribution system 42 is operated at a desired inert gas rate (m3/sec) (step 204) to provide an aerosol cloud mix. A time period is then allowed for stabilization of the aerosol cloud (step 206). The desired dry powder agent rate may include, in one example, from <10 g/m3 to >300 g/m3 which may be obtained by varying auger feed rates and flow rates. The desired inert gas rate may include, in one example, bulk velocities from  ̃0.5 m/sec to  ̃3.5 m/sec with centerline velocity about 35% greater than bulk velocity.
A sensor head 24A is then inserted into the test section 46 of the PCC 22 (step 208). Data is collected from the sensor head 24A for a predetermined time period (step 210) then the sensor head 24A is removed (step 212). A zero percentage transmission in the PCC 20 from the sensor head 24A is then determined by blocking all light into the sensor head 24A with, for example, a black rubber plug (step 214). Steps 202-214 are then repeated a multiple of times to obtain data points for a curve which represents the empirical relationship between mass density concentration and transmission squared for the desired dry powder agent (Step 218; FIG. 7).
Referring to FIG. 9, once the empirical relationship between mass density concentration and light transmittance for the desired dry powder agent (FIG. 7) is determined, one or more sensor heads 24 may be located within a protected structure test fixture 28 such as the representative engine nacelle (FIG. 2).
In step 300, the sensor heads 24 are installed within the desired protected structure test fixture 28. A reference zero percent transmission output is determined for each sensor head 24A (step 302). That is, the difference between the zero percentage transmission in the PCC 20 and the desired protected structure test fixture 28 is determined. The aerosol cloud fire suppression agent is then activated within the protected structure test fixture 28 and data from each sensor head 24A is recorded by the control system 26 during the test (step 304). The reference zero percent transmission output is subtracted for each sensor head 24A (step 306) to determine post test reference raw output (step 308) and convert that raw output into a light transmittance over time (step 310). The sensor head is relatively small so as to be located in a remote compartment so as to measure the light transmittance of the aerosol cloud with respect to time. The light transmittance over time is then utilized with the empirical relationship between mass density concentration and transmittance for the desired dry powder agent (FIG. 7) to determine concentration over time (step 312).
Through the relatively longitudinally narrow multiple of apertures 52 normal to the optical windows, the turbulent effect of the dry powder agent discharge is significantly reduced into a laminar flow. The narrow multiple of apertures 52 allows for mostly straight aerosol trajectories. Some curved trajectories can deposit aerosol particles on the optical surfaces, however, only relatively few of the possible trajectories that can penetrate through the multiple of apertures 52. Unless an aerosol particle of the dry powder agent travels a trajectory that impacts the optics, venturi effects will carry the particle out of the sensor head 24A as the velocity through the sensing volume will tend to pull out particles of the dry powder agent.
The concentration profile across the light path within the sensing volume can be thought of as normally distributed with a higher than average concentration at the center of each of the multiple of apertures 52 which may be caused by the boundary layer effect inside the sensor head 24A tails to no concentration at the optics. Super-imposing the normal distribution onto a constant-concentration situation yields an equivalent path length. This is the path length of sensing head 24A with a constant concentration across the entire light path length. Note that the equivalent path length may be less than the opening distance due to boundary layer effects. When packets of dry powder agent deposit onto the optics, the dry powder agent will not significantly affect the area under the normal distribution such that the equivalent path length is not affected.







TABLE 2







NOMENCLATURE









SYMBOL
DESCRIPTION
UNITS





As
Scattering cross sectional area of a particle
m2


d
Diameter
m


f
Fraction
—


i
Intensity
lux


I
Sensor head 24A output
Volts


L
Path length of light
m


m
Mass
kg


n
Number
—


V
Volume
m3


x
Dimensional displacement
m


ρ
Density
g/m3


ρw
Window reflection
—


τ
Transmission
—










NOMENCLATURE










SUBSCRIPT
DESCRIPTION







∞
Ambient



[3, 2]
Sauter mean



bl
Blocked



c
Fouling layer



i
Initial



p
Particle



ref
Reference



s
Source, Solid, or Scattering



v
Sensing volume



w
Window













SUPERSCRIPT
DESCRIPTION
UNITS





′′′
Per volume (density concentration)
1/m3


*
Modified sensor head 24A output



(blocked output subtracted off)









It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting.
It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom.
Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present disclosure.
The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.

## Claims

1. A sensor head for a dry powder agent comprising:
a housing defined along an axis along which light is communicated, said housing defines a multiple of apertures transverse to said axis in communication with a measurement volume along said axis, each of said multiple of apertures define a longitudinal length along said axis which is less than a lateral length defined around said axis; and
a mirror within said housing to reflect the light through said measurement volume.

2. The sensor head as recited in claim 1, wherein said measurement volume as defined by said multiple of apertures is generally trapezoidal in shape.

3. The sensor head as recited in claim 1, further comprising an end block and an attachment which is threaded into said housing.

4. The sensor head as recited in claim 3, further comprising a light source which communicates light into said measurement volume through a fiber optic cable which is communicated through said attachment.

5. The sensor head as recited in claim 4, further comprising a lens located opposite said minor, said lens retained between said attachment and a shoulder within said housing.

6. The sensor head as recited in claim 5, further comprising a mirror lens located adjacent to said mirror, said minor lens retained between said end block and a shoulder within said housing.

7. The sensor head as recited in claim 1, wherein each of said multiple of apertures are less than 10 mm along said axis.

8. The sensor head as recited in claim 1, wherein each of said multiple of apertures are 3 mm along said axis.

9. The sensor head as recited in claim 1, wherein adjacent ones of said multiple of apertures are circumferentially aligned with one another.

10. A calibration system for a dry powder agent comprising:
a powderizer calibration column;
a sensor system which includes at least one sensor head at least partially within said powder calibration column, said at least one sensor head comprises:
a housing defined along an axis along which light is communicated, said housing defines a multiple of apertures transverse to said axis, in communication with a measurement volume along said axis, each of said multiple of apertures define a longitudinal length along said axis which is less than a lateral length defined around said axis;
a mirror within said housing to reflect said light through said measurement volume;
a light source which communicates light into said measurement volume;
a receiver to receive reflected light within said measurement volume from said mirror; and
a control system in communication with said light source and said receiver.

11. The system as recited in claim 10, wherein said control system is in communication with said light source and said receiver through a fiber optic cable attached to said housing.

12. The system as recited in claim 10, wherein said control system is operable to determine an empirical relationship between a mass density concentration and a light transmittance for the dry powder agent.

13. The system as recited in claim 10, wherein said control system is operable to calibrate said at least one sensor head.

14. The system as recited in claim 10, wherein said control system is operable to calibrate said at least one sensor head light transmission measurement to a mass density concentration of an aerosol of the dry powder agent.

15. The system as recited in claim 10, wherein said control system is operable to calibrate said at least one sensor head light transmittance measurement to a concentration of an aerosol of the dry powder agent within said powderizer calibration column in mass per volume.

16. The system as recited in claim 10, wherein adjacent ones of said multiple of apertures are circumferentially aligned with one another.

17. A measurement system for a dry powder agent comprising:
a protected structure test fixture;
a sensor system which includes at least one sensor head at least partially within said protected structure test fixture, said at least one sensor head comprises:
a housing defined along an axis along which light is communicated, said housing defines a multiple of apertures transverse to said axis, in communication with a measurement volume along said axis, each of said multiple of apertures define a longitudinal length along said axis which is less than a lateral length defined around said axis;
a mirror within said housing to reflect said light through said measurement volume;
a light source which communicates light into said measurement volume;
a receiver to receive reflected light within said measurement volume from said mirror; and
a control system in communication with said light source and said receiver.

18. The system as recited in claim 17, wherein said control system is operable to measure a concentration of an aerosol of the dry powder agent within said a protected structure test fixture.

19. The system as recited in claim 18, wherein said protected structure test fixture is a gas turbine engine nacelle.

20. The system as recited in claim 17, wherein said protected structure test fixture is a gas turbine engine nacelle.