Patent Publication Number: US-2018045206-A1

Title: Portable industrial air filtration device that eliminates fan-speed sensor error

Description:
BACKGROUND 
     Air filtration devices are well known and are used to remove impurities, such as particulates, from the surrounding air. Typical air filtration devices include a fan assembly and a filter assembly including one or more filters. When one of these air filtration devices is operating, the fan assembly pulls or pushes air surrounding the air filtration device through the filter assembly. As the air flows through the filter assembly, the filter(s) captures various impurities and removes them from the air. The filtered air is then expelled from the air filtration device. 
     One known air filtration device includes a controller that uses a proportional-integral-derivative (PID) control module to ensure the fan operates at a desired fan speed. The PID control module controls how much electrical current is supplied to the fan motor. The amount of electrical current supplied to the fan motor controls the fan speed. 
     The controller provides the PID control module the following two inputs that enable it to perform this function: (1) the desired fan speed (e.g., as input by the user); and (2) a measured fan speed. The controller determines the measured fan speed by: (1) determining ΔT, which approximates the time it takes the fan blade of the fan to make one complete revolution (based on the output of a fan-speed sensor); and (2) inverting ΔT (i.e., calculating 1/ΔT), which provides the measured fan speed in units of revolutions per unit of time of ΔT (e.g., minutes, seconds, etc.). 
     The PID control module then determines whether the measured fan speed matches the desired fan speed. 
     If the measured fan speed does not match the desired fan speed, the PID control module determines how to vary the electrical current supplied to the fan motor to correct the error. For instance, if the measured fan speed is less than the desired fan speed, the PID control module determines to increase the electrical current supplied to the fan motor to cause the fan to spin faster to reach the desired fan speed. But if the measured fan speed is greater than the desired fan speed, the PID control module determines to decrease the electrical current supplied to the fan motor to cause the fan to spin slower to reach the desired fan speed. 
     As noted above, the controller determines ΔT based on the output of the fan-speed sensor. Ideally, the fan-speed sensor would trip only once per fan blade revolution and, upon each fan-speed sensor trip, the controller would read a free-running timer. In this ideal scenario, since the free-running timer resets to zero following each fan-speed sensor trip, the free-running-timer reading would equal ΔT (i.e., the time elapsed between that fan-speed sensor trip and the immediately previous fan-speed sensor trip, which is the time it took the fan blade to complete one revolution). This way of determining ΔT based on an assumed ideal scenario can be problematic. 
     One problem with determining ΔT based on an assumed ideal scenario is that the fan-speed sensor may trip more than once per revolution of the fan blade (i.e., the ideal scenario of one fan-speed sensor trip per revolution doesn&#39;t exist). For example, if tape on the fan blade trips an optical fan-speed sensor, both the leading and trailing edges of the tape may trip the fan-speed sensor when rotating past it. In this instance, the time elapsed between two consecutive trips of the fan-speed sensor (the leading edge tripping the fan-speed sensor immediately followed by the trailing edge tripping the fan-speed sensor) would be much less than the time it takes the fan blade to complete a single revolution at the desired fan speed. And inverting this time elapsed (i.e., ΔT) would result in a measured fan speed that is much higher than the actual fan speed. This would cause the PID control module to determine to control the electrical current supplied to the fan in an undesired way by unnecessarily decreasing the fan speed. This renders the above-described way of determining the measured fan speed inaccurate, leading to non-ideal fan operation. 
     In one example in which the desired fan speed is 1,000 RPMs and the actual fan speed is 1,000 RPMs, in an ideal scenario, the free-running timer reads 0.001 minutes when the fan-speed sensor trips after the fan blade completes a revolution. Since ΔT is 0.001 minutes, 0.001 minutes elapsed between the previous two consecutive trips of the fan-speed sensor. The controller determines a measured fan speed of 1,000 revolutions per minute (RPMs) by inverting this 0.001 minute ΔT and inputs this measured fan speed to the PID control module. Since the measured fan speed equals the desired fan speed, the PID control module does not vary the electrical current supplied to the fan motor. 
     Modifying the above example for a non-ideal scenario, the free-running timer reads 0.0001 minutes when the fan-speed sensor trips after the fan blade completes a fraction of a revolution. Since ΔT is 0.0001 minutes, 0.0001 minutes elapsed between the previous two consecutive trips of the fan-speed sensor. The controller determines a measured fan speed of 10,000 RPMs by inverting this 0.0001 minute ΔT and inputs this measured fan speed to the PID control module. Since the measured fan speed is 10× larger than the desired fan speed, the PID control module determines to decrease the electrical current supplied to the fan motor to decrease the fan speed. This is problematic because, in reality, the actual fan speed matches the desired fan speed, and the inaccurate measured fan speed (based on the inaccurate ΔT) input to the PID control module causes an unnecessary and undesired decrease in the fan speed. 
     Another problem with determining ΔT based on an assumed ideal scenario is that the fan-speed sensor may not trip during a revolution of the fan blade (i.e., the ideal scenario of one fan-speed sensor trip per revolution doesn&#39;t exist). For example, debris may block the fan-speed sensor and cause it to fail to sense the tape on the fan blade rotating past it. In this instance, the time elapsed between two consecutive trips of the fan-speed sensor would be much greater than the time it took the fan blade to complete a single revolution. And inverting this time elapsed (i.e., ΔT) would result in a measured fan speed that is much lower than the actual fan speed. This would cause the PID control module to determine to control the electrical current supplied to the fan in an undesired way by unnecessarily increasing the fan speed. This renders the above-described way of determining the measured fan speed inaccurate, leading to non-ideal fan operation. 
     In one example in which the desired fan speed is 1,000 RPMs and the actual fan speed is 1,000 RPMs, in an ideal scenario, the free-running timer reads 0.001 minutes when the fan-speed sensor trips after the fan blade completes a revolution. Since ΔT is 0.001 minutes, 0.001 minutes elapsed between the previous two consecutive trips of the fan-speed sensor. The controller determines a measured fan speed of 1,000 revolutions per minute (RPMs) by inverting this 0.001 minute ΔT and inputs this measured fan speed to the PID control module. Since the measured fan speed equals the desired fan speed, the PID control module does not vary the electrical current supplied to the fan motor. 
     Modifying the above example for a non-ideal scenario, the free-running timer reads 0.002 minutes when the fan-speed sensor trips after the fan blade completes two consecutive revolutions. Since ΔT is 0.002 minutes, 0.002 minutes elapsed between the previous two consecutive trips of the fan-speed sensor. The controller determines a measured fan speed of 500 RPMs by inverting this 0.002 minute ΔT and inputs this measured fan speed to the PID control module. Since the measured fan speed is half the desired fan speed, the PID control module determines to increase the electrical current supplied to the fan motor to increase the fan speed. This is problematic because, in reality, the actual fan speed matches the desired fan speed, and the inaccurate measured fan speed (based on the inaccurate ΔT) input to the PID control module causes an unnecessary and undesired increase in the fan speed. 
     Another problem with determining ΔT based on an assumed ideal scenario is that determining ΔT in this manner doesn&#39;t account for run-up to the desired fan speed just after a user powers the air filtration device on. When the user powers the air filtration device on, the fan is not moving. Once the user selects a desired fan speed, the controller controls the fan motor to begin rotating the fan blade and ramp it up to the desired fan speed. Initially, the fan blade rotates (relatively) slowly, so it takes a (relatively) long time for the fan blade to make full revolutions. Inputting this small measured fan speed to the PID control module would result in the same problems described above: an unnecessary increase in electrical current supplied to the fan motor. 
     Accordingly, there is a need for new and improved air filtration devices that solve these problems. 
     SUMMARY 
     The present disclosure describes an air filtration device that operates a fan-speed sensor error elimination process. The air filtration device uses a PID control module to ensure its fan operates at a desired fan speed. The fan-speed sensor error elimination process ensures that the air filtration device&#39;s controller does not send a measured fan speed determined using data that represent the time it takes the fan blade to complete a fraction of a revolution to the PID control module. This ensures the PID control module accurately controls electrical current supplied to the fan motor. 
     Additional features and advantages are described in, and will be apparent from, the following Detailed Description and the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top perspective view of one embodiment of the portable industrial air filtration device of the present disclosure. 
         FIG. 1B  is a side view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 1C  is another side view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 1D  is another side view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 1E  is another side view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 1F  is a top view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 1G  is a bottom view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 1H  is a side cross-sectional view of the portable industrial air filtration device of  FIG. 1A  taken substantially along line  1 H- 1 H of  FIG. 1F , and illustrates the path of air flow through the portable industrial air filtration device. 
         FIG. 1I  is an exploded top perspective view of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 2A  is a top perspective view of the lower housing component of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 2B  is a bottom perspective view of the lower housing component of  FIG. 2A . 
         FIG. 2C  is a top view of the lower housing component of  FIG. 2A . 
         FIG. 2D  is a bottom view of the lower housing component of  FIG. 2A . 
         FIG. 2E  is a side cross-sectional view of the lower housing component of  FIG. 2A  taken substantially along line  2 E- 2 E of  FIGS. 2C and 2D . 
         FIG. 2F  is a partial side cross-sectional view of the lower housing component of  FIG. 2A  taken substantially along line  2 F- 2 F of  FIGS. 2C and 2D . 
         FIG. 3A  is a top perspective view of the fan assembly mounting bracket of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 3B  is a top view of the fan assembly mounting bracket of  FIG. 3A . 
         FIG. 3C  is a bottom view of the fan assembly mounting bracket of  FIG. 3A . 
         FIG. 3D  is a side view of the fan assembly mounting bracket of  FIG. 3A . 
         FIG. 4  is a bottom perspective view of the fan assembly mounted to the fan assembly mounting bracket secured to the lower housing component of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 5A  is a side perspective view of the exhaust screen of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 5B  is a front view of the exhaust screen of  FIG. 5A . 
         FIG. 6A  is a top perspective view of the filter assembly mounting chamber cover of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 6B  is a bottom perspective view of the filter assembly mounting chamber cover of  FIG. 6A . 
         FIG. 7A  is a top perspective view of the air director of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 7B  is a top view of the air director of  FIG. 7A . 
         FIG. 7C  is a bottom view of the air director of  FIG. 7A . 
         FIG. 7D  is a side view of the air director of  FIG. 7A . 
         FIG. 8A  is a top perspective view of the HEPA filter of the portable industrial air filtration device of  FIG. 1A  without the protective mesh. 
         FIG. 8B  is a top perspective view of the HEPA filter of  FIG. 8A  with the protective mesh. 
         FIG. 8C  is a side cross-sectional view of the HEPA filter of  FIG. 8B  taken substantially along line  8 C- 8 C of  FIG. 8B . 
         FIG. 9A  is a top perspective view of the HEPA filter securing bracket of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 9B  is a side view of the HEPA filter securing bracket of  FIG. 9A . 
         FIG. 10A  is a top perspective view of the HEPA filter securing plate of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 10B  is a side view of the HEPA filter securing plate of  FIG. 10A . 
         FIG. 11  is a partial side cross-sectional view of the portable industrial air filtration device of  FIG. 1A  taken substantially along line  11 - 11  of  FIG. 1F . 
         FIG. 12A  is a top perspective view of the pre-filter of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 12B  is a side cross-sectional view of the pre-filter of  FIG. 12A  taken substantially along line  12 B- 12 B of  FIG. 12A . 
         FIG. 12C  is a top perspective view of another example pre-filter. 
         FIG. 12D  is a top perspective view of the pre-filter limit switch actuator of the pre-filter of  FIG. 12A . 
         FIG. 12E  is a top perspective view of another example pre-filter. 
         FIG. 12F  is a side cross-sectional view of the pre-filter of  FIG. 12E  taken substantially along line  12 F- 12 F of  FIG. 12E . 
         FIG. 12G  is a top perspective view of another example pre-filter. 
         FIG. 12H  is a side cross-sectional view of the pre-filter of  FIG. 12E  taken substantially along line  12 H- 12 H of  FIG. 12G . 
         FIG. 12I  is a top perspective view of the pre-filter limit switch actuator of the pre-filter of  FIG. 12G . 
         FIG. 12J  is a top view of the pre-filter limit switch actuator of  FIG. 12I . 
         FIG. 12K  is a side view of the pre-filter limit switch actuator of  FIG. 12I . 
         FIG. 13A  is a top perspective view of the locking cover of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 13B  is a bottom perspective view of the locking cover of  FIG. 13B . 
         FIG. 13C  is a side cross-sectional view of the locking cover of  FIG. 13A  taken substantially along line  13 C- 13 C of  FIG. 13A . 
         FIG. 13D  is the side cross-sectional view of  FIG. 13C  including the pre-filter and the HEPA filter. 
         FIG. 14  is a block diagram showing certain electronic components of the portable industrial air filtration device of  FIG. 1A . 
         FIG. 15  illustrates a flowchart of one example embodiment of an automatic fan speed setting selection process. 
         FIG. 16  illustrates a flowchart of one example embodiment of a dynamic fan speed control process. 
         FIG. 17  illustrates a flowchart of one example embodiment of a pre-filter presence detection process. 
         FIG. 18  illustrates a flowchart of one example embodiment of a HEPA filter presence detection process. 
         FIGS. 19A and 19B  illustrate a flowchart of one example embodiment of a filter occlusion level monitoring process. 
         FIGS. 20A to 20L  are schematic views of the fan blade and the fan-speed sensor at multiple consecutive points in time following a user powering the air filtration device on and setting a desired fan speed. 
         FIG. 21  illustrates a flowchart of one example embodiment of a fan-speed sensor error elimination process 
         FIG. 22  illustrates a flowchart of another example embodiment of a fan-speed sensor error elimination process 
     
    
    
     DETAILED DESCRIPTION 
     1. Components and Structure 
     Referring now to the drawings,  FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H , and  1 I illustrate one example embodiment of the portable industrial air filtration device of the present disclosure, which is generally indicated by numeral  2010  and is sometimes referred to as the air filtration device.  FIGS. 2A to 13D  illustrate the various components of the air filtration device  2010  generally shown in  FIG. 1I , which is an exploded view of the air filtration device  2010 . The Figures include a simplified illustration of the fan assembly  2300  for clarity. 
     As best shown in  FIG. 1I , the air filtration device  2010  includes the following components, each of which is described in detail below: (a) a two-piece housing including a lower housing component  2100  and a locking cover  2200  that is removably attachable to the lower housing component  2100 , (b) a fan assembly mounting bracket  3000  attached to the lower housing component  2100  within a fan assembly mounting chamber defined by an underside of the lower housing component  2100 , (c) a fan assembly  2300  attached to the fan assembly mounting bracket  3000 , (d) a fan assembly mounting chamber cover  2500  attached to the underside of the lower housing component  2100  to substantially cover the fan assembly mounting chamber and enclose the fan assembly mounting bracket  3000  and the fan assembly  2300  within the fan assembly mounting chamber, (e) an exhaust screen  2400  positioned within an exhaust port formed by the lower housing component  2100  and the fan assembly mounting chamber cover  2500 , (f) an air director  3100  attached to the lower housing component  2100  upstream of the fan assembly  2300 , (g) a dual filter assembly installed within the housing between the locking cover  2200  and the lower housing component  2100  and including a removable and replaceable self-supporting outer pre-filter  2900  surrounding a separately removable and replaceable inner high efficiency particulate air (HEPA) filter  2600 , (h) a HEPA filter securing bracket  2700  attached to the lower housing component  2100 , and (i) a HEPA filter securing plate  2800  attached to the HEPA filter securing bracket  2800  that secures the HEPA filter  2600  to the lower housing component  2100 . 
       FIG. 1H  generally illustrates the path air takes when passing through this example embodiment of the air filtration device  2010 . In operation, air surrounding the air filtration device is drawn through the dual filter assembly into the interior cylindrical channel defined or formed by the HEPA filter. More specifically, the air is first drawn through the pre-filter, which initially filters the air by capturing and removing relatively large or coarse impurities from the air as the air is drawn through the pre-filter toward the HEPA filter. The air is then drawn through the HEPA filter, which further filters the air by capturing and removing relatively small or fine impurities from the air as the air is drawn through the HEPA filter toward the interior cylindrical channel. The filtered air exits the HEPA filter into the interior cylindrical channel, and is then drawn through the air director, which directs the filtered air into the fan assembly. The fan draws the filtered air into the fan assembly and expels the air from the fan assembly and through the exhaust channel, exiting the air filtration device. 
     As best illustrated in  FIGS. 2A, 2B, 2C, 2D, 2E, and 2F , the lower housing component  2100  includes: (a) a base  2110 ; (b) a plurality of stabilizers  2120 ,  2130 , and  2140  extending vertically from and circumferentially spaced apart around the base  2110  (with respect to the orientation shown in  FIGS. 2E and 2F ); and (c) an exhaust port upper portion  2150  extending transversely from the base  2110 . 
     The base  2110  includes: (a) a generally cylindrical exterior side surface  2112  to which the stabilizers  2120 ,  2130 , and  2140  are attached; (b) a generally annular exterior upper surface including a plurality of surfaces to which various other components of the air filtration device are mounted (described below); (c) a generally cylindrical interior side surface  2116   a  ; and (d) a generally annular interior upper surface  2116   b . The interior side surface  2116   a  and the interior top surface  2116   b  generally define a fan assembly mounting chamber on the underside of the base  2110 . 
     Turning to the exterior of the base  2110 , as best shown in  FIGS. 2A and 2C , the exterior upper surface of the base  2110  includes a generally annular air director mounting surface  2115  to which the air director  3100  is attached (described below). In this example embodiment, the air director mounting surface  2115  includes four sections: (a) first and second opposing sections  2115   b  and  2115   d , and (b) third and fourth opposing sections  2115   a  and  2115   c  . In this example embodiment, the first and second sections  2115   b  and  2115   d  are recessed with respect to the third and fourth sections  2115   a  and  2115   c  (with respect to the orientation shown  FIG. 2C ). 
     In this example embodiment, the base  2110  defines fastener receiving openings  2114   a ,  2114   b ,  2114   c  , and  2114   d  at least partially therethrough. The fastener receiving opening  2114   a  is partially defined through the third section  2115   a  of the air director mounting surface  2115 , the fastener receiving opening  2114   b  is partially defined through the first section  2115   c  of the air director mounting surface  2115 , the fastener receiving opening  2114   c  is partially defined through the fourth section  2115   c  of the air director mounting surface  2115 , and the fastener receiving opening  2114   d  is partially defined through the second section  2115   d  of the air director mounting surface  2115 . The fastener receiving openings  2114  are substantially equally circumferentially spaced around a vertical axis through the center of the base  2110 . 
     As best shown in  FIGS. 2A, 2C, 2E, and 2F , the exterior upper surface of the base  2110  includes a surface  2111   b  having a “V-shaped” cross-section that defines a pre-filter securing channel around a vertical axis through the center of the base  2110 . The base  2110  defines a pre-filter limit switch actuator receiving opening  2175  at least partially therethrough. The pre-filter limit switch actuator is partially defined through the surface  2111   b , and is sized to receive a pre-filter limit switch actuator of the pre-filter  2900  (described below). Generally, the base  2110  supports a pre-filter limit switch (not shown) that is actuatable by the pre-filter limit switch actuator of the pre-filter  2900 , and the pre-filter limit switch actuator receiving opening  2175  enables the pre-filter limit switch actuator to actuate the pre-filter limit switch when the pre-filter  2900  is installed. 
     As also best shown in  FIGS. 2A, 2C, 2E, and 2F , the exterior upper surface of the base  2110  includes a plurality of annular surfaces  2111   d  and  2111   f  that are connected by an upwardly-protruding sealing rib  2111   e  (with respect to the orientation shown in  FIGS. 2E and 2F ). Together, the surfaces  2111   d  and  2111   f  and the sealing rib  2111   e  form a HEPA filter mounting channel around a vertical axis through the center of the base  2110 . 
     As also best shown in  FIGS. 2A, 2C, 2E, and 2F , the exterior upper surface of the base  2110  includes an annular surface  2111   c  bridging the pre-filter securing channel and the HEPA filter mounting channel and an annular surface  2111   g  partially bridging the HEPA filter mounting channel and the air director mounting surface  2115 . The base  2110  defines a pressure sensor port  2170   b  at least partially therethrough to which one or more pressure sensors may be attached to measure the pressure between the pre-filter and the HEPA filter, as described below. The base  2110  also defines a pressure sensor port  2170   a  at least partially therethrough to which one or more pressure sensors may be attached to measure the pressure downstream of the HEPA filter and upstream of the fan assembly, as described below. The pressure sensor port  2170   b  is partially defined through the surface  2111   c  , and the pressure sensor port  2170   a  is partially defined through the surface  2111   g  . 
     Turning to the interior of the base  2110 , as best shown in  FIGS. 2B and 2D , the interior side surface  2116   a  of the base  2110  includes three fan assembly mounting bracket mounting surfaces  2117   a ,  2117   b , and  2117   c  extending inwardly therefrom (with respect to the orientation shown in  FIG. 2D ) to which the fan assembly mounting bracket  3000  is attached (described below). As best shown in  FIG. 2D , the interior side surface  2116   a  includes a plurality of fan assembly mounting chamber cover mounting surfaces  2118  spaced apart around the interior side surface  2116   a  and extending inwardly therefrom (with respect to the orientation shown in  FIG. 2D ) to which the fan assembly mounting chamber cover  2500  is attached (described below). The interior side surface  2116   a  also defines a pressure sensor port  2119  at least partially therethrough to which one or more pressure sensors may be attached to measure the pressure downstream of the fan assembly, as described below 
     The stabilizers  2120 ,  2130 , and  2140  facilitate attachment of the locking cover  2200  to the lower housing component  2100 , provide structural support for the air filtration device  2010 , and provide protection for the dual filter assembly. Additionally, as best shown in  FIGS. 1B, 1C, 1D, 1E, and 2E , the stabilizers raise the air filtration device off of the ground to enable air to circulate under the air filtration device. While the air filtration device includes three stabilizers in this example embodiment, the air filtration device may include any suitable quantity of stabilizers. 
     To facilitate attachment of the locking cover  2200  to the lower housing component  2110 , in this example embodiment, each of the stabilizers  2120 ,  2130 , and  2140  includes a locking cover mounting tab  2121 ,  2131 , and  2141 , respectively, and a latch mounting surface  2129 ,  2139 , and  2149 , respectively. The locking cover mounting tabs  2121 ,  2131 , and  2141  are received by the locking cover  2200  (described below) and, thereafter, prevent the locking cover  2200  from rotating with respect to the lower housing component  2100 . As shown in  FIGS. 1A, 1B, 1C, 1D, 1E, and 1F , a latch is mounted to each of the latch mounting surfaces  2129 ,  2139 , and  2149 . The latches are attached to corresponding integrated latch strikes on the locking cover  2200  (described below) to secure the locking cover  2200  to the lower housing component  2110 . 
     In this example embodiment, side  2143  of the stabilizer  2140  includes a recessed control panel mounting surface  2144  to which an integrated control panel  2160  is attached. The control panel  2160 , which is shown in  FIGS. 1A and 1B , enables the user to select a desired operating mode of the air filtration device and provides information regarding the status of the air filtration device and the filters. In this example embodiment, the control panel  2160  includes or is otherwise associated with: (i) an operating mode selector  2161 ; (ii) a plurality of operating mode indicators  2161   a ,  2161   b ,  2161   c  ,  2161   d , and  2161   e  that each indicate or identify one of the operating modes of the air filtration device (described below); (iii) a pre-filter fault indicator  2162 ; (iv) a plurality of pre-filter status indicators  2163 ; (v) a HEPA filter fault indicator  2164 ; (vi) a plurality of HEPA filter status indicators  2165 ; (vii) an air filtration device status indicator  2166 ; (viii) an hour meter display  2167 ; and (ix) a dust sensor receiving port  2168  into which a dust sensor (not shown) is fit. Each of these components is described in detail below with respect to  FIGS. 14, 15, 16, 17, 18, and 30 . 
     Additionally, in this example embodiment, side  2122  of the stabilizer  2120  includes a recessed power panel mounting surface  2123  to which a power panel  2170  is attached. The power panel  2170 , which is shown in  FIG. 1E , includes: (a) a plurality of electrical outlets  2172 , (b) a power switch  2176  having “ON” and “OFF” positions, and (c) a strain relief bushing  2174  for a power cord that ends in a plug (not shown). In this example embodiment, to power the air filtration device  2010 , the user plugs the plug of the power cord into an A/C power source (such as a wall electrical outlet), and switches the power switch  2176  to the “ON” position. To cut power to the air filtration device  2010 , the user either unplugs the plug of the power cord from the A/C power source or switches the power switch  2176  to the “OFF” position. In this example embodiment, once the air filtration device  2010  is connected to the A/C power source via the plug of the power cord, the electrical outlets  2172  are powered and the user may plug other electronic devices into the electrical outlets  2172  to power those electronic devices. 
     In other embodiments, the air filtration device includes fewer electrical outlets, more electrical outlets, or no electrical outlets. In other embodiments, the air filtration device is operable using any suitable power source other than and/or in addition to an A/C power source, such as one or more replaceable or rechargeable batteries. 
     As best shown in  FIGS. 2C and 2D , the exhaust port upper portion  2150  extends transversely from the base such that the exhaust port upper portion  2150  is substantially parallel to a plane extending between the stabilizers  2120  and  2130 . The exhaust port upper portion  2150  includes a convex exterior surface  2151  and a concave interior surface  2152 . The interior surface  2152  of the exhaust port upper portion  2150  includes two exhaust screen mounting surfaces  2154  and  2155  to which the exhaust screen  2400  is attached (described below). The base  2110  defines fastener receiving openings  2154   a  and  2155   a  at least partially therethrough. The fastener receiving opening  2154   a  is partially defined through the exhaust screen mounting surface  2154  and the fastener receiving opening  2155   a  is partially defined through the exhaust screen mounting surface  2155 . The base also defines an exhaust screen mounting channel  2153  partially through the interior surface  2152  of the exhaust port upper portion  2150 . 
     In this example embodiment, the lower housing component is dual-walled and rotationally molded out of plastic, though the lower housing component may be made of any suitable material(s) or manufactured in any suitable manner(s). 
     As best illustrated in  FIGS. 3A, 3B, 3C, and 3D  the fan assembly mounting bracket  3000  includes: (a) a generally rectangular fan assembly mounting bracket body  3010  defining: (i) a fastener receiving opening  3012  therethrough proximate each corner of the fan assembly mounting bracket body  3010 , (ii) a fan assembly receiving opening  3040  therethrough proximate the center of the fan assembly mounting bracket body  3010 , (iii) a plurality of fastener receiving openings  3042  therethrough spaced around the fan assembly receiving opening  3040 , and (iv) a fan motor capacitor fastener receiving opening  3032  therethrough; (b) generally rectangular flanges  3070  and  3080  extending substantially perpendicularly in a first direction from opposing edges of the fan assembly mounting bracket body  3010 ; (c) a fan motor capacitor mounting bracket  3030  extending substantially perpendicularly in the first direction from the fan assembly mounting bracket body  3010 ; and (d) a fan-speed sensor mounting bracket  3020  extending substantially perpendicularly in a second direction, which is opposite the first direction, from the fan assembly mounting bracket body  3010 . In this example embodiment, the fan assembly mounting bracket  3000  is made of sheet metal, though the fan assembly mounting bracket may be made of any suitable material. 
     As best illustrated in  FIGS. 5A and 5B , the exhaust screen  2400  includes a plurality of exhaust screen mounting tabs  2454  and  2455  and a flange  2453  spanning the exhaust screen mounting tabs. The exhaust screen mounting tab  2454  includes a base mounting surface  2454   a  and an opposing fan assembly mounting chamber cover mounting surface  2454   b  and defines a fastener receiving opening  2456  therethrough. Similarly, the exhaust screen mounting tab  2455  includes a base mounting surface  2455   a  and an opposing fan assembly mounting chamber cover mounting surface  2455   b  and defines a fastener receiving opening  2457  therethrough. 
     In this example embodiment, the exhaust screen  2400  is an injection molded plastic component, though the exhaust screen may be made of any suitable material or materials or manufactured in any suitable manner or manners. 
     As best illustrated in  FIGS. 6A and 6B , the fan assembly mounting chamber cover  2500  includes: (a) a circular portion  2510  having a slightly concave interior surface  2512  and a slightly convex exterior surface  2514 , and (b) an exhaust channel lower portion  2520  extending transversely from the circular portion  2510  and having a concave interior surface  2522  and a convex exterior surface  2524 . The circular portion defines a plurality of fastener receiving openings  2154  therethrough. The exhaust channel lower portion  2520  includes two exhaust screen mounting surfaces  2554  and  2555  to which the exhaust screen  2400  is attached (described below). The exhaust channel lower portion  2520  defines fastener receiving openings  2524   a  and  2524   b  therethrough. The fastener receiving opening  2524   a  is partially defined through the exhaust screen mounting surface  2554  and the fastener receiving opening  2524   b  is partially defined through the exhaust screen mounting surface  2555 . 
     In this example embodiment, the fan assembly mounting chamber cover  2500  is a thin walled plastic component, though the fan assembly mounting chamber cover may be made of any suitable material. 
     As best illustrated in  FIGS. 7A, 7B, 7C, and 7D , the air director  3100  includes: (a) an annular portion  3110 , (b) a bridging portion  3120  extending downwardly and inwardly from the inner edge of the annular portion  3110  (with respect to the orientation shown in  FIG. 7D ), and (c) a ring-shaped portion  3130  extending downwardly from the inner edge of the bridging portion  3120  (with respect to the orientation shown in  FIG. 7D ). 
     The annular portion  3110  defines fastener receiving openings  3110   a  ,  3110   b ,  3110   c  , and  3110   d  therethrough. In this example embodiment, the fastener receiving openings  3110   a ,  3110   b ,  3110   c  , and  3110   d  are substantially equally circumferentially spaced around a vertical axis through the center of the annular portion  3110 . As best shown in  FIGS. 7A and 7B , the air director  3100  includes rectangular HEPA filter mounting bracket mounting surfaces  3112   a  and  3114   a  proximate the fastener receiving openings  3110   b  and  3110   d , respectively. The HEPA filter mounting bracket mounting surfaces  3112   a  and  3114   a  are recessed relative to the annular portion  3110  (with respect to the orientation shown in  FIG. 7D ). As best shown in  FIG. 7C , the air director  3100  includes rectangular air director mounting surfaces  3112   b  and  3114   b , which are opposite the HEPA filter mounting bracket mounting surfaces  3112   a  and  3114   a , respectively. 
     As best illustrated in  FIGS. 8A, 8B, and 8C , the HEPA filter  2600  includes pleated HEPA filter media  2610  sandwiched between upper and lower ring-shaped end caps  2620  and  2630 , respectively. The HEPA filter media  2610  and the upper and lower end caps  2620  and  2630  form or define a cylindrical interior channel. As shown in  FIGS. 8B and 8C , the HEPA filter  2600  also includes a protective mesh  2640  covering the outer and inner surfaces of the HEPA filter media  2610  around its entire outer and inner circumferences to protect the HEPA filter media  2610 . The protective mesh is not shown in  FIG. 8A  for clarity. 
     The upper and lower end caps  2620  and  2630  each have an exterior diameter De and an interior diameter Di. As best shown in  FIG. 8C , the upper end cap  2620  includes a first surface  2620   a  having a semi-circular cross-section that defines a first channel around the circumference of the upper end cap  2620  at diameter Da. The upper end cap  2620  also includes a second surface  2620   b  having a semi-circular cross-section defining a second channel around the circumference of the upper end cap  2620  at diameter Db. The upper end cap  2620  further includes a generally flat mounting surface  2620   c  around the circumference of the upper end cap  2620  at diameter Dc. The mounting surface  2620   c  is located between and above (with respect to the orientation shown in  FIG. 8C ) the first and second channels. Similarly, the lower end cap  2630  includes a first surface  2630   a  having a semi-circular cross-section that defines a first channel around the circumference of the lower end cap  2630  at diameter Da. The lower end cap  2630  also includes a second surface  2630   b  having a semi-circular cross-section that defines a second channel around the circumference of the lower end cap  2630  at diameter Db. The lower end cap  2630  further includes a generally flat mounting surface  2630   c  around the circumference of the lower end cap  2630  at diameter Dc. The mounting surface  2630   c  is located between and below (with respect to the orientation shown in  FIG. 8C ) the first and second channels. 
     In this example embodiment, both the upper and lower end caps of the HEPA filter include a specific geometry that enables airtight sealing when the HEPA filter is installed. As will be explained in detail below, this specific end cap geometry and, more specifically, the manner in which the end cap geometry enables an airtight seal to be formed, enables the air filtration device to accurately measure various pressures and perform certain functions using those measured pressures. In this example embodiment, the end caps of the HEPA filter are made of molded urethane, though the end caps may be made of any suitable material. While the end caps are substantially identical in this example embodiment, in other embodiments the upper and lower end caps may have different geometries. Further, in this example embodiment, the outer protective mesh is made of plastic and the inner protective mesh is made of a thin gage metal, though the protective mesh may be made of any suitable material. 
     As best illustrated in  FIGS. 9A and 9B , the HEPA filter securing bracket  2700  includes: (a) a rectangular brace  2710 , (b) a first leg  2720  connected to and extending down and away from a first edge of the brace  2710  (with respect to the orientation shown in  FIG. 9B ), (c) a second leg  2730  connected to and extending down and away from a second edge of the brace  2710  that is opposite the first edge (with respect to the orientation shown in  FIG. 9B ), (d) a first HEPA filter securing bracket mounting tab  2740  that is substantially parallel to the brace  2710  and extends away from the edge of the first leg  2720  opposite the edge connected to the first edge of the brace  2710 , and (e) a second HEPA filter securing bracket mounting tab  2750  that is substantially parallel to the brace  2710  and extends away from the edge of the second leg  2723  opposite the edge connected to the second edge of the brace  2710 . 
     The brace  2710  includes an annular, downwardly embossed HEPA filter securing plate nesting surface  2712  (with respect to the orientation shown in  FIG. 9B ) that defines a nut receiving opening  2712   a  therethrough. The nut receiving opening  2712   a  includes an integrated nut  2715  that defines a HEPA filter securing plate fastener receiving opening  2715   a  therethrough. The first and second HEPA filter securing bracket mounting tabs  2740  and  2750  each define HEPA filter securing bracket fastener receiving openings  2740   a  and  2750   a , respectively, therethrough. 
     As best illustrated in  FIGS. 10A and 10B , the HEPA filter securing plate  2800  includes: (a) a first annular portion  2810 , (b) a flange  2815  extending upwardly from an outer edge of the first annular portion  2810  around the circumference of the outer edge of the first annular portion  2810  (with respect to the orientation shown in  FIG. 10B ), (c) a first annular bridging portion  2820  extending downwardly and inwardly from the inner edge of the first annular portion  2810  (with respect to the orientation shown in  FIG. 10B ), (d) a second annular portion  2830  connected to and extending inwardly from the first annular bridging portion  2820  (with respect to the orientation shown in  FIG. 10B ), (e) a second annular bridging portion  2840  extending downwardly and inwardly from the inner edge of the second annular portion  2830  (with respect to the orientation shown in  FIG. 10B ), and (f) a third annular portion  2850  extending inwardly from the second annular bridging portion  2840  (with respect to the orientation shown in  FIG. 10B ). The third annular portion  2850  defines a fastener receiving opening  2850   a  therethrough. 
       FIGS. 12A and 12B  illustrate the pre-filter  2900  including a pre-filter body and a pre-filter limit switch actuator  2990  (such as a plastic piece). In various embodiments, the pre-filter body of the pre-filter  2900  is formed from two different materials: pre-filter media and a rigidized backing. The use of the rigidized backing in combination with the pre-filter media provides structural support to the pre-filter body of the pre-filter, rendering it rigid enough to support itself and stand on its own without deforming, while maintaining enough flexibility to be packed flat for shipping and storage, which enables packaging materials and storage space to be minimized. In one embodiment, the pre-filter body of the pre-filter  2900  is formed by placing the rigidized backing  2920 , which has upper and lower opposing edges and two opposing side edges, onto a sheet of the pre-filter media  2915 , which has upper and lower opposing edges and two opposing side edges. The upper edge of the pre-filter media  2915  is folded over the upper edge of the rigidized backing  2920  and heat sealed to hold it in place. The heat seals are generally indicated by numeral  2950 . Similarly, the lower edge of the pre-filter media  2915  is folded over the lower edge of the rigidized backing  2920  and heat sealed to hold it in place. 
     This above process is performed twice, resulting in two sheets of rigidized pre-filter media  2910  and  2930 . The pre-filter body of the pre-filter  2900  is formed by sewing (e.g., attaching via stitching) the corresponding side edges of the two sheets of rigidized pre-filter media  2910  and  2930  to one another to form an annular or ring-shaped structure (as shown in  FIG. 12A ) or an oval or fish-eye structure (as shown in  FIG. 12C ) such that the two sewed side seams  2970   a  and  2970   b  run lengthwise down the full height of the pre-filter body of the pre-filter  2900 , the rigidized backing  2920  and  2940  forms the interior surface of the pre-filter body of the pre-filter  2900 , and the pre-filter media  2915  and  2935  forms the exterior surface of the pre-filter body of the pre-filter  2900 . The formed pre-filter body of the pre-filter  2900  includes an upper edge formed by upper edges  2912  and  2932  of the sheets of rigidized pre-filter media  2910  and  2930 , and a lower edge formed by lower edges  2914  and  2934  of the sheets of rigidized pre-filter media  2910  and  2930 . 
     In this example embodiment, the pre-filter limit switch actuator  2990  includes a generally rectangular head  2991  and an actuator  2992  extending therefrom. The head  2991  defines a plurality of attachment openings  2993  therethrough. In this embodiment, the pre-filter limit switch actuator  2990  is attached to the pre-filter body the pre-filter  2900  via the attachment openings  2993  (such as by sewing, adhesive, fastener, or any other suitable manner of attachment) such that the head  2991  contacts the exterior surface of the pre-filter body of the pre-filter  2900  and the pre-filter limit switch actuator  2992  extends below the lower edge of the pre-filter body of the pre-filter  2900  formed by the lower edges  2914  and  2934  of the sheets of rigidized pre-filter material  2910  and  2930 . The pre-filter sensor limit switch actuator  2990  is sized to actuate the pre-filter limit switch, as described above, which enables the air filtration device to determine whether an acceptable pre-filter is installed. The pre-filter limit switch actuator may take any suitable shape, be made of any suitable material, and attached at any suitable location on the pre-filter body. 
     In this example embodiment, the pre-filter media is a polyspun material, though any suitable filter media may be employed. Additionally, in this example embodiment, the rigidized backing includes nylon mesh, though any suitable material may be employed, such as a material including vertical, horizontal, or diagonal boning. In this example embodiment, the combination of the polyspun material and the nylon mesh renders the pre-filter flexible enough to fold flat for shipping but rigid enough to support itself and to enable the pre-filter to be slid over and onto the HEPA filter. In other embodiments, a single sheet of rigidized pre-filter media is created and formed into an annular or oval-shaped structure by sewing the two sides of that sheet of rigidized pre-filter media together. That is, in such embodiments, the formation of the pre-filter body causes the pre-filter body to include a single seam. The sides of the rigidized pre-filter media may be joined in any suitable manner other than or in addition to sewing, such as by a heat seal or adhesive. 
       FIGS. 12E and 12F  illustrate another embodiment of the pre-filter  9900   a  including a pre-filter body and a pre-filter limit switch actuator. In this illustrated embodiment, the pre-filter body of the pre-filter  9900   a  is formed from two different materials: pre-filter media  9915  and a rigidized backing  9920 . The use of the rigidized backing in combination with the pre-filter media provides structural support to the pre-filter body, rendering it rigid enough to support itself and stand on its own without deforming, while maintaining enough flexibility to be packed flat for shipping and storage, which enables packaging materials and storage space to be minimized. In this embodiment, a sheet of rigidized pre-filter media  9910  is formed by placing the rigidized backing  9920 , which has upper and lower opposing edges and two opposing side edges, onto a sheet of the pre-filter media  9915 , which has upper and lower opposing edges and two opposing side edges. The upper edge of the pre-filter media  9915  is folded over the upper edge of the rigidized backing  9920  and sewed in place (e.g., via stitching). Similarly, the lower edge of the pre-filter media  9915  is folded over the lower edge of the rigidized backing  9920  and sewed in place, thereby forming the sheet of rigidized pre-filter media  9910 . The sewing is generally indicated by numeral  9950 . The folded-over portions of any of the pre-filters described herein may be secured in any suitable manner other than or in addition to stitching such as, but not limited to, by heat-sealing (as described above), with a plurality of rivets, with a plurality of staples, with a plurality of other fasteners, and the like. 
     The pre-filter body of the pre-filter  9900   a  is formed by sewing the side edges of the sheet of rigidized pre-filter media  9910  to one another to form an annular or ring-shaped structure (as shown in  FIG. 12E ) or alternatively an oval or fish-eye structure (such as that shown in  FIG. 12C ) such that the sewed side seam  9970  runs lengthwise down the full height of the pre-filter body of the pre-filter  9900   a , the rigidized backing  9920  forms the interior surface of the pre-filter body of the pre-filter  9900   a , and the pre-filter media  9915  forms the exterior surface of the pre-filter body of the pre-filter  9900   a  . The formed pre-filter body of the pre-filter  9900   a  includes an upper edge  9912  and a lower edge  9914 . 
     Put differently, in this example embodiment, an upper portion of the rigidized backing is disposed between a first portion of the pre-filter media and a second portion of the pre-filter media, and the first portion of the pre-filter media, the upper portion of the rigidized backing, and the second portion of the pre-filter media are attached via stitching. Additionally, a lower portion of the rigidized backing is disposed between a third portion of the pre-filter media and a fourth portion of the pre-filter media, and the third portion of the pre-filter media, the lower portion of the rigidized backing, and the fourth portion of the pre-filter media are attached via stitching. Further, the first portion of the pre-filter media is connected to the second portion of the pre-filter media and the third portion of the pre-filter media is connected to the fourth portion of the pre-filter media. Additionally, the second portion of the pre-filter media is connected to the third portion of the pre-filter media. Further, the first portion of the pre-filter media terminates in a first free end and the fourth portion of the pre-filter media terminates in a second free end. 
     In this example embodiment, as shown in  FIGS. 12E and 12D , the pre-filter  9900   a  also includes a pre-filter limit switch actuator  9990   a  similar to the pre-filter limit switch actuator  2990  shown in  FIG. 12D . In this example embodiment, the pre-filter limit switch actuator  9990   a  is attached to the pre-filter body of the pre-filter  9990   a  via two rivets such that the head  9991   a  contacts the interior surface of the pre-filter body of the pre-filter  9990   a , though the pre-filter limit switch actuator  9990   a  may be attached to the pre-filter body in any other suitable manner. The pre-filter sensor limit switch actuator  9990   a  is configured to actuate the pre-filter limit switch, as described above, which enables the air filtration device to determine whether an acceptable pre-filter is installed. The pre-filter limit switch actuator may take any suitable shape; be made of any suitable material (such as plastic); be attached at any suitable location on the pre-filter body, such as any suitable location around the circumference of the pre-filter body; and be attached either before or after sewing the side edges of the sheet of rigidized pre-filter media to one another. 
     In this example embodiment, the pre-filter media is a polyspun material, though any suitable filter media may be employed. Additionally, in this example embodiment, the rigidized backing includes nylon mesh, though any suitable material may be employed, such as a material including vertical, horizontal, or diagonal boning. In this example embodiment, the combination of the polyspun material and the nylon mesh renders the pre-filter flexible enough to fold flat for shipping but rigid enough to support itself and to enable the pre-filter to be slid over and onto the HEPA filter. 
       FIGS. 12G and 12H  illustrate another embodiment of the pre-filter  9900   b . The pre-filter  9900   b  includes a pre-filter body that is generally formed in a manner similar to that described above with respect to  FIGS. 12E and 12F . 
     In this example embodiment, the pre-filter  9900   b  also includes a pre-filter limit switch actuator  9990   b . The pre-filter limit switch actuator  9990   b  is “T-shaped” and includes a generally rectangular head  9991   b  and an actuator  9992   b  extending transversely therefrom (such as substantially perpendicularly therefrom). In this embodiment, the head  9991   b  of the pre-filter limit switch actuator  9990   b  is disposed within the lower folded-over portion (with respect to the orientation shown in  FIGS. 12G and 12H ) proximate the lower edge  9914  of the pre-filter body of the pre-filter  9900   b , and the actuator  9992   b  extends from its point of attachment to the head  9991   b  within the lower folded-over portion through the lower edge  9914  and below the lower edge  9914 . The combination of the sewing  9950  and the extension of the actuator  9992   b  from within the lower folded-over portion through the lower edge  9914  ensures the pre-filter limit switch actuator  9900   b  remains substantially in place. The pre-filter sensor limit switch actuator  9990   b  is configured to actuate the pre-filter limit switch, as described above, which enables the air filtration device to determine whether an acceptable pre-filter is installed. 
     In this embodiment, the pre-filter limit switch actuator  9990   b  is inserted within the lower folded-over portion before the lower folded-over portion is sewn in place. In one embodiment, the head fills or substantially fills the entire space within the lower folded-over portion, which minimizes movement of the head within the lower folded-over portion The pre-filter limit switch actuator may take any suitable shape; be made of any suitable material (such as plastic); and be attached at any suitable location on the pre-filter body, such as any suitable location around the circumference of the pre-filter body. For instance, in other embodiments, the head of the pre-filter limit switch actuator may be disc-shaped, square-shaped, sphere-shaped, cylindrically-shaped, and the like. 
     Put differently, in this example embodiment, an upper portion of the rigidized backing is disposed between a first portion of the pre-filter media and a second portion of the pre-filter media, and the first portion of the pre-filter media, the upper portion of the rigidized backing, and the second portion of the pre-filter media are attached via stitching. Additionally, a lower portion of the rigidized backing is disposed between a third portion of the pre-filter media and a fourth portion of the pre-filter media, and the third portion of the pre-filter media, the lower portion of the rigidized backing, and the fourth portion of the pre-filter media are attached via stitching. Further, the first portion of the pre-filter media is connected to the second portion of the pre-filter media and the third portion of the pre-filter media is connected to the fourth portion of the pre-filter media. Additionally, the second portion of the pre-filter media is connected to the third portion of the pre-filter media. Further, the first portion of the pre-filter media terminates in a first free end and the fourth portion of the pre-filter media terminates in a second free end. In this embodiment, the head of the limit switch actuator is disposed between the third portion of the filter media and the fourth portion of the filter media and the actuator extends through the filter media proximate the lower edge of the body. 
     In this example embodiment, the pre-filter media is a polyspun material, though any suitable filter media may be employed. Additionally, in this example embodiment, the rigidized backing includes nylon mesh, though any suitable material may be employed, such as a material including vertical, horizontal, or diagonal boning. In this example embodiment, the combination of the polyspun material and the nylon mesh renders the pre-filter flexible enough to fold flat for shipping but rigid enough to support itself and to enable the pre-filter to be slid over and onto the HEPA filter. 
     As best illustrated in  FIGS. 13A, 13B, 13C, and 13D , the locking cover  2200  includes a generally circular base  2210  including a handle  2212  and a plurality of mounts  2220 ,  2230 , and  2240  circumferentially spaced apart around the base  2210 . Each of the mounts  2220 ,  2230 , and  2240  includes a generally cylindrical surface  2221 ,  2231 , and  2241 , respectively, defining a locking cover mounting tab receiving cavity configured to receive one of the locking cover mounting tabs of the stabilizers (described above). Additionally, each of the mounts  2220 ,  2230 , and  2240  includes an integrated latch strike to facilitate the use of the latches mounted to the stabilizers. 
     As best shown in  FIGS. 13C and 13D , the underside of the locking cover  2200  includes a surface  2211   b  having a “inverted V-shaped” cross-section that defines a pre-filter securing channel around a vertical axis through the center of the locking cover  2200 . The pre-filter  2900  is mounted to the locking cover by being press-fit into the pre-filter mounting channel. The underside of the locking cover  2200  also includes a plurality of generally flat annular surfaces  2211   d  and  2211   f  that are connected by a downwardly-protruding sealing rib  2211   e  (with respect to the orientation shown in  FIGS. 13C and 13D ). 
     In this example embodiment, the locking cover is a rotationally molded plastic component. It should be appreciated, however, that the locking cover may be made of any suitable material or materials or manufactured in any suitable manner or manners. 
     2. Assembly 
     In this example embodiment, each fastener receiving opening of the lower housing component  2100  either: (a) is a threaded fastener receiving opening configured to receive a threaded fastener, or (b) includes an integrated threaded insert (formed into the component or inserted after the component is formed) configured to receive a threaded fastener. It should be appreciated, however, that any suitable fastening mechanisms may be employed to attach the components of the air filtration device to one another. 
     As best illustrated in  FIG. 4 , the fan assembly  2300  is attached to the fan assembly mounting bracket  3000  by: (a) inserting a portion of the bottom of the fan assembly  2300  through the fan assembly receiving opening  3040  of the fan assembly mounting bracket  3000 , and (b) inserting fasteners through the fastener receiving openings  3040  of the fan assembly mounting bracket  3000  and threading those fasteners into fastener receiving openings of the fan assembly  2300 . 
     As also best illustrated in  FIG. 4 , the motor capacitor  2320  is attached to the fan assembly mounting bracket  3000  by: (a) attaching one end of the motor capacitor  2320  to the fan motor capacitor mounting bracket  3020 , such as via any suitable fastener(s); (b) wrapping a fan motor capacitor body securer  2325  around a portion of the body of the fan motor capacitor  2320 ; and (c) attaching the fan motor capacitor body securer  2325  to the fan assembly mounting bracket  3000  via the fastener receiving opening  3032  using any suitable fastener(s). Although not shown, in this example embodiment, a fan-speed sensor (described below) is attached to the fan-speed sensor mounting bracket  3020  using any suitable fastener(s). 
     As also best illustrated in  FIG. 4 , the fan assembly mounting bracket  3000  is attached to the lower housing component  2100  within the fan assembly mounting chamber by inserting fasteners through the fastener receiving openings  3012  and threading those fasteners into corresponding fastener receiving openings of the fan assembly mounting bracket mounting surfaces  2117   a ,  2117   b , and  2117   c  of the interior side surface  2116   a  of the base  2110 . 
     The exhaust screen  2400  and the fan assembly mounting chamber cover  2500  are attached to the base  2110  by: (a) positioning the exhaust screen  2400  such that the flange  2453  is partially disposed within the exhaust screen mounting channel  2153 , the base mounting surface  2454   a  abuts the exhaust screen mounting surface  2154  of the base  2110 , and the base mounting surface  2455   a  abuts the exhaust screen mounting surface  2155  of the base  2110 ; (b) positioning the fan assembly mounting chamber cover  2500  such that the exhaust screen mounting surface  2554  abuts the fan assembly mounting chamber cover mounting surface  2454   b  of the exhaust screen  2400  and the exhaust screen mounting surface  2555  abuts the fan assembly mounting chamber cover mounting surface  2455   b  of the exhaust screen  2400 ; (c) inserting a fastener through the fastener receiving opening  2524   a  of the fan assembly mounting chamber cover  2500  and the fastener receiving opening  2456  of the exhaust screen  2400  and threading that fastener into the fastener receiving opening  2154   a  of the base  2110 ; (d) inserting a fastener through the fastener receiving opening  2524   b  of the fan assembly mounting chamber cover  2500  and the fastener receiving opening  2457  of the exhaust screen  2400  and threading that fastener into the fastener receiving opening  2155   a  of the base  2110 ; and (e) inserting fasteners through the fastener receiving openings  2514  of the fan assembly mounting chamber cover  2500  and threading those fasteners into the corresponding fastener receiving openings  2118  of the base  2110 . 
     Once the fan assembly mounting chamber cover is attached to the base, the fan assembly mounting chamber cover substantially covers the fan assembly mounting chamber and encloses the fan assembly and the fan assembly mounting bracket within the fan assembly mounting chamber. Additionally, once the fan assembly mounting chamber cover is mounted to the base, the exhaust port upper portion of the base and the exhaust port lower portion of the fan assembly mounting chamber cover form an exhaust port that defines an exhaust channel. 
     In this example embodiment, the exhaust port is substantially parallel to a plane extending between the stabilizers  2120  and  2130 . This angle of the exhaust port improves fan efficiency by eliminating turbulence and back pressure within the fan assembly mounting chamber. Further, the fact that the exhaust port is substantially parallel to a plane extending between the stabilizers  2120  and  2130  ensures that the air filtration device will expel the filtered air substantially parallel to the ground regardless of whether the air filtration device is operating in an upright orientation or on its side (i.e., resting on the stabilizers  2120  and  2130 ). 
     The air director  3100  is attached to the base  2110  by: (a) positioning the air director  3100  such that the air director mounting surfaces  3112   b  and  3114   b  abut the first and second opposing sections  2115   b  and  2115   d , respectively, of the air director mounting surface  2115  of the exterior upper surface of the base  2110 ; (b) inserting a fastener through the fastener receiving opening  3110   a  of the air director  3100  and threading that fastener into the fastener receiving opening  2114   a  of the base  2110 ; and (c) inserting a fastener through the fastener receiving opening  3110   c  of the air director  3100  and threading that fastener into the fastener receiving opening  2114   c  of the base  2110 . The use of the air director to direct air drawn through the filters into the fan assembly improves fan efficiency. 
     The HEPA filter securing bracket  2700  is mounted to the base  2110  by: (a) positioning the first and second HEPA filter securing bracket mounting tabs  2740  and  2750  atop the HEPA filter mounting bracket mounting surfaces  3112   a  and  3114   a , respectively, of the air director  3100 ; (b) inserting a fastener through the fastener receiving opening  2740   a  of the HEPA filter mounting bracket and through the fastener receiving opening  3110   b  of the air director  3100  and threading that fastener into the fastener receiving opening  2114   b  of the base  2110 ; and (c) inserting a fastener through the fastener receiving opening  2750   a  of the HEPA filter mounting bracket and the fastener receiving opening  3110   d  of the air director  3100  and threading that fastener into the fastener receiving opening  2114   d  of the base  2110 . 
     To install the HEPA filter  2600 , the HEPA filter  2600  is positioned around the HEPA filter securing bracket  2700  and onto the base  2110  such that the lower end cap  2630  of the HEPA filter  2600  rests within the HEPA filter mounting channel. More specifically, as illustrated in  FIG. 11 , the HEPA filter  2600  is positioned such that the mounting surface  2630   c  of the lower end cap  2630  rests atop securing rib  2111   e  of the exterior upper surface of the base  2110 . 
     The HEPA filter securing plate  2800  is then attached to the HEPA filter securing bracket  2700  by: (a) nesting the second annular bridging portion  2840  and the third annular portion  2850  of the HEPA filter securing plate  2800  within the HEPA filter securing plate nesting surface  2712  of the brace  2710  of the HEPA filter securing bracket  2700 ; and (b) inserting a fastener through the fastener receiving opening  2850   a  of the HEPA filter securing plate  2800  and threading that fastener into the fastener receiving opening  2715   a  of the nut  2715  of the HEPA filter securing bracket  2700 . 
     As best shown in  FIGS. 1H and 1I , after the HEPA filter securing plate  2800  is attached to the HEPA filter securing bracket  2700 , the HEPA filter  2600  is sandwiched between the HEPA filter securing plate  2800  and the base  2110 , thus ensuring that the HEPA filter  2600  will not disengage from the base  2110  until the HEPA filter securing plate  2800  is removed. Further, mounting the HEPA filter securing plate  2800  to the HEPA filter securing bracket  2700  causes the material of the lower end cap  2630  proximate the mounting surface  2630   c  to compress around the securing rib  2111   e  , which creates an airtight seal between the lower end cap  2630  of the HEPA filter  2600  and the base  2110 . 
     The pre-filter  2900  is installed by aligning the pre-filter limit switch actuator  2990  with the pre-filter limit switch actuator receiving opening  2175  of the base  2110  and press-fitting the pre-filter  2900  downward into the pre-filter securing channel of the base  2110  until the pre-filter limit switch actuator  2990  actuates the pre-filter limit switch. 
     The locking cover  2220  is attached to the lower housing component  2100  by: (a) positioning the locking cover  2200  atop the stabilizers such that the locking cover mounting tab receiving opening defined by the surface  2221  of the mount  2220  receives the locking cover mounting tab  2121  of the stabilizer  2120 , the locking cover mounting tab receiving opening defined by the surface  2231  of the mount  2230  receives the locking cover mounting tab  2131  of the stabilizer  2130 , and the locking cover mounting tab receiving opening defined by the surface  2241  of the mount  2240  receives the locking cover mounting tab  2141  of the stabilizer  2140 ; and (b) securing the latches attached to the stabilizers to their respective latch strikes of the locking cover  2200 . Once the locking cover is attached to the lower housing component, the user may carry or otherwise transport the air filtration device via the handle  2212 . 
     As best shown in  FIG. 13D , attaching the locking cover  2200  to the lower housing component  2100  causes: (a) the upper edges  2912  and  2932  of the pre-filter  2900  to be press-fit into the pre-filter securing channel of the locking cover  2200 , which secures the pre-filter  2900  in place; and (b) the material of the upper end cap  2620  of the HEPA filter proximate the mounting surface  2620   c  to compress around the securing rib  2211   e  of the locking cover  2200 , which creates an airtight seal between the upper end cap  2620  of the HEPA filter  2600  and the locking cover  2200 . 
     In another embodiment, the locking cover is attached to one of the stabilizers of the lower housing component via a hinge. Thus, in this embodiment, the locking cover is not completely detachable from the lower housing component. Rather, to remove the filters in this embodiment, the latches are unlocked and the locking cover is rotated via the hinge off of the lower housing component to provide access to the filters. In other embodiments, the locking cover attaches to the stabilizers in any suitable manner, such as through the use of threaded fasteners. 
     In this example embodiment, to replace the pre-filter the user detaches the locking cover from the lower housing component, removes the old pre-filter, and installs a new pre-filter as described above, and attaches the locking cover to the lower housing component. To replace the HEPA filter, the user detaches the locking cover from the lower housing component, detaches the HEPA filter securing plate from the HEPA filter securing bracket, removes the old HEPA filter, installs a new HEPA filter as described above, attaches the HEPA filter securing plate to the HEPA filter securing bracket, and attaches the locking cover to the lower housing component. It should be appreciated that, in this example embodiment, the pre-filter and the HEPA filter are separately replaceable. 
     The geometry of the base, the locking cover, and the HEPA filter end caps that enable airtight sealing when the HEPA filter is installed eliminates need to include an additional gasket to ensure proper sealing. It should also be appreciated that the geometry of the pre-filter securing channels provides improved sealing when the pre-filter is installed. It should further be appreciated that the fact that: (a) the pre-filter securing channel of the lower housing component is lower relative to the HEPA filter mounting channel of the lower housing component, and (b) the pre-filter securing channel of the locking cover is higher than the HEPA filter mounting channel of the locking cover improves the accuracy of the measurements taken by the pressure sensors. 
     3. Electronics 
       FIG. 14  is a block diagram showing certain electronic components of this example embodiment of the air filtration device of the present disclosure. In this example embodiment, the air filtration device  2010  includes: (a) a controller  3650 ; (b) the fan  2310  of the fan assembly  2300 ; (c) at least one sound producing device  3850 ; (d) a control panel  2160  including or otherwise associated with: (i) an operating mode selector  2161 , (ii) a pre-filter fault indicator  2162 , (iii) a plurality of pre-filter status indicators  2163 , (iv) a HEPA filter fault indicator  2164 , (v) a plurality of HEPA filter status indicators  2165 , (vi) an air filtration device status indicator  2166 , and (vii) an hour meter display  2167 ; and (e) a plurality of sensors including: (i) a dust sensor  3910 , (ii) a pre-filter differential pressure sensor  3920 , (iii) a HEPA filter differential pressure sensor  3930 , (iv) a fan differential pressure sensor  3940 , (v) a fan-speed sensor  3950 , and (vi) a pre-filter presence sensor  3960 . The air filtration device  2010  and each of the above-listed electronic components are powered by a power source, such as an A/C power source  20 . 
     In this example embodiment, the controller  3650 : (1) communicates with each of the other electronic components, (2) receives communications from each of the other electronic components, and (3) controls each of the other electronic components. The controller may be any suitable processing device or set of processing devices, such as a microprocessor, a microcontroller-based platform, a suitable integrated circuit, one or more application-specific integrated circuits (ASICs), or any other suitable circuit boards. 
     In certain embodiments, the controller of the air filtration device is configured to communicate with, configured to access, and configured to exchange signals with the at least one memory device or data storage device. In various embodiments, the at least one memory device includes random access memory (RAM), which can include non-volatile RAM (NVRAM), magnetic RAM (MRAM), ferroelectric RAM (FeRAM), and other suitable forms of RAM. In other embodiments, the at least one memory device includes read only memory (ROM). In certain embodiments, the at least one memory device includes flash memory and/or electrically erasable programmable read only memory (EEPROM). The at least one memory device may include any other suitable magnetic, optical, and/or semiconductor memory. 
     As generally described below, in various embodiments, the at least one memory device of the air filtration device stores program code and instructions executable by the controller of the air filtration device to control various processes performed by the air filtration device. The at least one memory device also stores other operating data, such as image data, event data, and/or input data. In various embodiments, part or all of the program code and/or the operating data described above is stored in at least one detachable or removable memory device including, but not limited to, a cartridge, a disk, a CD-ROM, a DVD, a USB memory device, or any other suitable non-transitory computer readable medium. In certain such embodiments, a user uses such a removable memory device to implement at least part of the present disclosure. In other embodiments, part or all of the program code and/or the operating data is downloaded to the at least one memory device of the air filtration device through any suitable data network (such as an internet, an intranet, or a cellular communications network). 
     In this example embodiment, the fan assembly  2300  is a RadiCal R2E250-RB02-15 centrifugal fan, though any other suitable fan assembly may be employed, such as the RadiCal R2E250-RB02-11. 
     In this example embodiment, the sound producing device  3850  is a Mallory Sonalert Products Inc. PB-1224PE-05Q sound producing device, though any suitable sound producing device may be employed. In this example embodiment, as described in detail below, the air filtration device uses the sound producing device  3850  to output the following audible tones: (a) a major air filtration device malfunction tone when the air filtration device determines that a major air filtration device malfunction occurs (described below), (b) a filter change alarm tone when the air filtration device determines that the pre-filter occlusion level exceeds the pre-filter shutdown threshold and needs replacement and/or when the HEPA filter occlusion level exceeds the HEPA filter shutdown threshold and needs replacement (as described below), and (c) a filter fault indicator tone when the air filtration device determines that an acceptable pre-filter is not installed and/or an acceptable HEPA filter is not installed (as described below). 
     In this example embodiment, the major air filtration device malfunction tone, the filter change alarm tone, and the filter fault tone are different. More specifically: (a) the major air filtration device malfunction tone includes a continuous tone; (b) the filter change alarm tone includes a one tone combination (beep-pause, beep-pause); and (c) the filter fault tone includes a two tone combination (beep-beep-pause, beep-beep-pause). In this example embodiment, setting the air filtration device to the standby operating mode or powering the air filtration device off causes the controller to silence the sound producing device  3850 . 
     3.1 Control Panel 
     The operating mode selector  2161  enables the user to select the operating mode in which the user desires the air filtration device to operate. More specifically, in this example embodiment, the operating mode selector  2161  enables the user to select one of the following operating modes: one of the manual fan speed setting operating modes, the automatic fan speed setting selection operating mode, or the standby operating mode, each of which are described below. In this example embodiment, the operating mode selector  2161  includes a control knob that the user may rotate to indicate the desired operating mode. 
     In another embodiment, the operating mode selector includes a touch screen display that enables the user to select the desired operating mode by touching an appropriate area of the touch screen. The air filtration device sets the operating mode to the desired operating mode after receiving such input. In another embodiment, the operating mode selector includes a display and one or more associated buttons. In this embodiment, the user selects an operating mode by using the one or more buttons to select the desired operating mode. The air filtration device sets the operating mode to the desired operating mode after receiving such input. 
     In another embodiment, the air filtration device enables the user to use a computing device, such as (but not limited to) a cellular phone, a tablet computing device, a laptop computing device, and/or a desktop computing device, to select the desired operating mode. That is, in this embodiment: (a) the computing device receives an input of the user&#39;s desired operating mode; (b) the computing device communicates the user&#39;s desired operating mode to the air filtration device, such as (but not limited to) through a wireless network connection, a cellular network connection, a wired network connection, an infrared connection, or a Bluetooth connection; and (c) the air filtration device receives the communication from the computing device and sets the operating mode to the desired operating mode. It should be appreciated that, in this embodiment, the air filtration device enables the user to remotely change the operating mode of the air filtration device, such as from across the room or across the jobsite, which saves the time it would otherwise take the user to travel to the air filtration device to change the operating mode (such as via the control knob). 
     In another embodiment, the air filtration device enables the user to use a remote control to select the desired operating mode. That is, in this embodiment: (a) the remote control receives an input of the user&#39;s desired operating mode; (b) the remote control communicates the user&#39;s desired operating mode to the air filtration device, such as through any of the above-listed connections; and (c) the air filtration device receives the communication from the remote control and sets the operating mode to the desired operating mode. In one such embodiment, the remote control also displays one or more of the pre-filter fault indicator, the HEPA filter fault indicator, the air filtration device status indicator, the pre-filter status indicators, and the HEPA filter status indicators. 
     The air filtration device employs the pre-filter fault indicator  2162  to indicate that there is a problem with the pre-filter. In this example embodiment, the pre-filter fault indicator  2162  includes a red light-emitting diode (LED). As described in detail below, the air filtration device lights the red LED of the pre-filter fault indicator when any of: (a) an acceptable pre-filter is not installed; and (b) the pre-filter occlusion level exceeds the pre-filter shutdown threshold (i.e., when the pre-filter needs replacement). Any suitable pre-filter fault indicator(s) may be employed in addition to or instead of a red LED, such as (but not limited to): a different-colored LED, a light other than an LED, a display screen, a remote control display, a computing device, and/or a non-display indicator such as an audible tone. 
     The air filtration device employs the pre-filter status indicators  2163  to indicate the occlusion level of the pre-filter. In this example embodiment, the pre-filter status indicators  2163  include a green LED, a yellow LED, and a red LED. As described in detail below, the air filtration device: (a) lights the green LED of the pre-filter status indicators when the Clean pre-filter occlusion level range includes the determined pre-filter occlusion level; (b) lights the yellow LED of the pre-filter status indicators when the Slightly Occluded pre-filter occlusion level range includes the determined pre-filter occlusion level; (c) lights the red LED of the pre-filter status indicators when the Highly Occluded pre-filter occlusion level range includes the determined pre-filter occlusion level; and (d) lights the red LED of the pre-filter status indicators in a flashing or blinking manner when the pre-filter occlusion level range exceeds the pre-filter shutdown threshold (i.e., when the pre-filter needs replacement). Any suitable pre-filter status indicators may be employed in addition to or instead of green, yellow, and red LEDs, such as (but not limited to): a single LED that can display a plurality of different colors, different-colored LED, lights other than LEDs, one or more display screens, a remote control display, a computing device, and/or a non-display indicator such as an audible tone. 
     The air filtration device employs the HEPA filter fault indicator  2164  to indicate that there is a problem with the HEPA filter. In this example embodiment, the HEPA filter fault indicator  2164  includes a red LED. As described in detail below, the air filtration device lights the red LED of the HEPA filter fault indicator when any of: (a) an acceptable HEPA filter is not installed, and (b) the HEPA filter occlusion level exceeds the HEPA filter shutdown threshold (i.e., when the HEPA filter needs replacement). Any suitable HEPA filter fault indicator(s) may be employed in addition to or instead of a red LED, such as (but not limited to): a different-colored LED, a light other than an LED, a display screen, a remote control display, a computing device, and/or a non-display indicator such as an audible tone. 
     The air filtration device employs the HEPA filter status indicators  2165  to indicate the occlusion level of the HEPA filter. In this example embodiment, the HEPA filter status indicators  2165  include a green LED, a yellow LED, and a red LED. As described in detail below, the air filtration device: (a) lights the green LED of the HEPA filter status indicators when the Clean HEPA filter occlusion level range includes the determined HEPA filter occlusion level; (b) lights the yellow LED of the HEPA filter status indicators when the Slightly Occluded HEPA filter occlusion level range includes the determined HEPA filter occlusion level; (c) lights the red LED of the HEPA filter status indicators when the Highly Occluded HEPA filter occlusion level range includes the determined HEPA filter occlusion level; and (d) lights the red LED of the HEPA filter status indicators in a flashing or blinking manner when the HEPA filter occlusion level range exceeds the HEPA filter shutdown threshold (i.e., when the HEPA filter needs replacement). Any suitable HEPA filter status indicators may be employed in addition to or instead of green, yellow, and red LEDs, such as (but not limited to): a single LED that can display a plurality of different colors, different-colored LED, lights other than LEDs, one or more display screens, a remote control display, a computing device, and/or a non-display indicator such as an audible tone. 
     The air filtration device employs the air filtration device status indicator  2166  to indicate that the air filtration device is operating normally or to indicate that there is a problem with the air filtration device. In this example embodiment, the air filtration device status indicator  2166  includes an LED that can display a green or red light. As described in detail below, the air filtration device: (a) lights the LED of the air filtration device status indicator green when the air filtration device is operating in any of the manual fan speed setting operating modes, the automatic fan speed setting selection operating mode, or the standby operating mode; and (b) lights the LED of the air filtration device status indicator red when any of: (i) an acceptable pre-filter is not installed; (ii) an acceptable HEPA filter is not installed; (iii) the air filtration device is in shutdown mode and the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode is selected; (iv) the air filtration device is in shutdown mode, the manual medium fan speed setting operating mode or the manual minimum fan speed setting operating mode is selected, and the designated shutdown time period has expired; and (v) a major air filtration device malfunction occurs. In this example embodiment, whenever the air filtration device lights the LED of the air filtration device status indicator red, the power switch must be cycled “OFF” and back “ON” to clear the fault. In certain embodiments, when the air filtration device is in shutdown mode and the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode is selected such that the air filtration device lights the LED of the air filtration device status indicator red, the air filtration device clears the fault when the standby operating mode, the manual medium fan speed setting operating mode, or the manual minimum fan speed setting operating mode is selected. 
     Any suitable air filtration device status indicators may be employed in addition to or instead of an LED, such as (but not limited to): different-colored LED, lights other than LEDs, a plurality of LEDs, one or more display screens, a remote control display, and/or a computing device. 
     The air filtration device tracks or counts the number of hours the fan is operating at any fan speed and displays that number of hours on the hour meter display  2167 . In this example embodiment, the hour meter display  2167  includes a six digit LED display. Additionally, in this example embodiment, the air filtration device does not enable a user to reset the hour count; the air filtration device retains the hour count when the power is disconnected (e.g., when the air filtration device is unplugged); and the air filtration device can roll over the hour counter once the hour meter display reaches a maximum displayed number of hours (such as 99999.9 hours for a six-digit hour meter display including one decimal place). The hour meter display may be any suitable indicator other than or in addition to a six-digit LED display. 
     In certain embodiments, the air filtration device communicates with a computing device of the user, such as (but not limited to) a cellular phone, a tablet computing device, a laptop computing device, and/or a desktop computing device, and causes the computing device to display certain information, such as one or more of: the pre-filter fault indicator, the HEPA filter fault indicator, the air filtration device status indicator, the pre-filter status indicators, the HEPA filter status indicators, and the selected operating mode. For instance, in one example, the user executes an application on the user&#39;s smartphone that syncs and communicates with the air filtration device. The user may then use the application to monitor the status of the air filtration device (such as by viewing one or more of the pre-filter fault indicator, the HEPA filter fault indicator, the air filtration device status indicator, the pre-filter status indicators, the HEPA filter status indicators, and the selected operating mode) remotely, such as from across the room or across the jobsite. Additionally, as described above, in certain embodiments the computing device of the user enables the user to input instructions to control certain aspects of the air filtration device and communicates such instructions to the air filtration device. 
     3.2 Sensors 
     The dust sensor  3910  determines the level of dust or impurities in the air surrounding the air filtration device. In this example embodiment, the dust sensor includes an optical dust sensor, such as a Sharp GP2Y1010AU0F optical dust sensor, though any suitable sensor may be employed to detect the level of dust in the air. 
     The pre-filter differential pressure sensor  3920  measures the differential pressure across the pre-filter. More specifically, the pre-filter differential pressure sensor includes two ports: (1) a first open port; and (2) a second port connected to the pressure sensor port  2170   b  located between the pre-filter and the HEPA filter (i.e., located downstream of the pre-filter and upstream of the HEPA filter). The pre-filter differential pressure sensor determines the differential pressure across the pre-filter by measuring the pressures at the first and second ports and determining the difference between those pressure measurements. 
     The HEPA filter differential pressure sensor  3930  measures the differential pressure across the HEPA filter. More specifically, the HEPA filter differential pressure sensor includes two ports: (1) a first port connected to the pressure sensor port  2170   b  located between the pre-filter and the HEPA filter (i.e., located downstream of the pre-filter and upstream of the HEPA filter); and (2) a second port connected to the pressure sensor port  2170   a  located between the HEPA filter and the fan assembly (i.e., located downstream of the HEPA filter and upstream of the fan assembly). The HEPA filter differential pressure sensor determines the differential pressure across the HEPA filter by measuring the pressures at the first and second ports and determining the difference between those pressure measurements. 
     The fan differential pressure sensor  3940  measures the differential pressure across the fan. More specifically, the fan differential pressure sensor includes two ports: (1) a first port connected to the pressure sensor port  2170   a  located between the HEPA filter and the fan assembly (i.e., located downstream of the HEPA filter and upstream of the fan assembly); and (2) a second port connected to the pressure sensor port  2119  located downstream of the fan assembly. The fan differential pressure sensor determines the differential pressure across the fan by measuring the pressures at the first and second ports and determining the difference between those pressure measurements. 
     In this embodiment, the differential pressure sensors are Freescale +/−1.45 PSI MPXV7002DP differential pressure sensors, though any suitable differential pressure sensors may be employed. 
     In other embodiments, rather than employing three differential pressure sensors, the air filtration device includes absolute pressure sensors and determines the appropriate differential pressures using measured absolute pressures. For instance, in one example embodiment, the air filtration device includes: (a) a first absolute pressure sensor including an open port, (b) a second absolute pressure sensor including a port connected to the pressure sensor port located between the pre-filter and the HEPA filter, (c) a third absolute pressure sensor including a port connected to the pressure sensor port located between the HEPA filter and the fan assembly, and (d) a fourth absolute pressure sensor including a port connected to the pressure sensor port located downstream of the fan assembly. In this example embodiment, the air filtration device: (a) determines the differential pressure across the pre-filter by determining the difference between the pressure measurements of the first and second absolute pressure sensors, (b) determines the differential pressure across the HEPA filter by determining the difference between the pressure measurements of the second and third absolute pressure sensors, and (c) determines the differential pressure across the fan by determining the difference between the pressure measurements of the third and fourth absolute pressure sensors. 
     The fan-speed sensor  3950  measures the speed of the fan  2310 , such as the number of revolutions per minute at which the fan  2310  is spinning. In this example embodiment, the fan-speed sensor includes an optical fan-speed sensor, such as an Optek OPB716Z sensor, though any suitable fan-speed sensor may be employed. In another embodiment, the fan assembly includes an integrated fan-speed sensor and communicates the fan speed to the controller. In this embodiment, the air filtration device does not include a separate fan-speed sensor in addition to the integrated fan-speed sensor of the fan assembly. 
     The pre-filter presence sensor  3960  determines whether an acceptable pre-filter is installed in the air filtration device, as described below with respect to the pre-filter presence detection process  6000 . In this example embodiment, the lower housing component supports or otherwise includes a pre-filter presence sensor in the form of a pre-filter limit switch that is actuatable by the pre-filter limit switch actuator of the pre-filter. In another embodiment, the pre-filter presence sensor is a Hall Effect sensor that detects a metallic element included in the pre-filter, as described below. In another embodiment, the pre-filter presence sensor is a radio frequency identification (RFID) reader configured to read or recognize an RFID tag included in the pre-filter, as described below. Any other suitable pre-filter presence sensor may be employed. 
     4. Operations 
     The below-described operations and processes may be performed regardless of the shapes of the filters. For instance, the below-described operations and processes may be performed in an air filtration device employing two substantially flat filters or semicircular filters positioned one in front of the other. 
     4.1 Power-up Process 
     In this example embodiment, as noted above, the air filtration device includes a power switch  2176  that powers the air filtration device on and off when the air filtration device is connected to a power source (such as an A/C power source). When the air filtration device is connected to a power source and the air filtration device is powered on (i.e., the power switch is switched to “ON”), the air filtration device: (a) displays “CAL” on the hour meter display; (b) lights the LED of the air filtration device status indicator green; (c) lights the green LED of the pre-filter status indicators in a flashing manner; (d) lights the green LED of the HEPA filter status indicators in a flashing manner; and (e) after waiting (if necessary) for the fan speed to fall below 100 revolutions per minute, calibrates the pre-filter differential pressure sensor, the HEPA filter differential pressure sensor, and the fan differential pressure sensor by taking and averaging several pressure measurements. 
     After calibrating the differential pressure sensors: (a) if the standby operating mode is selected, the air filtration device enters full standby mode (described below); and (b) if the automatic fan speed setting selection operating mode or any of the manual fan speed setting operating modes is selected, the air filtration device enters that selected (non-standby) operating mode. 
     This is one example of the power-up process. In other embodiments, the power-up process may include different or additional steps and/or may not include certain of the above-described steps. 
     4.2 Fan Speed Settings 
     In this example embodiment, the air filtration device is operable at any of a plurality of different fan speed settings including at least a minimum fan speed setting and a maximum fan speed setting. Each fan speed setting corresponds to a different desired air flow rate through the air filtration device. For instance, in this example embodiment, the air filtration device is operable at any of three fan speed settings including: (a) a minimum fan speed setting that corresponds to a first desired air flow rate through the air filtration device, (b) a medium fan speed setting that corresponds to a second desired air flow rate through the air filtration device, and (c) a maximum fan speed setting that corresponds to a third desired rate of air flow through the air filtration device. In this example embodiment, the third desired air flow rate through the air filtration device is  600  cubic feet per minute, which is greater than the second desired air flow rate through the air filtration device, which is  400  cubic feet per minute, which is greater than the first desired air flow rate through the air filtration device, which is  200  cubic feet per minute. 
     It should be appreciated that, in other embodiments, the air filtration device may be operable at any suitable number of different fan speed settings. It should also be appreciated that the particular air flow rates associated with the different fan speed settings may be any suitable air flow rates. 
     It should also be appreciated that “current fan speed setting” as used herein refers to the fan speed setting at which the air filtration device is operating at a particular point in time. For instance: (a) at a particular point in time, if one of the manual fan speed setting operating modes (described below) is selected, the current fan speed setting (i.e., the fan speed setting at that particular point in time) is the fan speed setting associated with that selected manual fan speed setting operating mode; and (b) at a particular point in time, if the automatic fan speed setting selection operating mode (described below) is selected, the current fan speed setting (i.e., the fan speed setting at that particular point in time) is the fan speed setting selected by the air filtration device via the automatic fan speed setting selection process (described below). 
     4.3 Operating Modes 
     In this example embodiment, the air filtration device includes a plurality of different user-selectable operating modes including a plurality of different manual fan speed setting operating modes, an automatic fan speed setting selection operating mode, and a standby operating mode. As described above, the operating modes are selectable using the operating mode selector. 
     4.3.1 Manual Fan Speed Setting Operating Modes 
     In this example embodiment, the air filtration device includes a different user-selectable manual fan speed setting operating mode corresponding to each fan speed setting at which the air filtration device may operate. This enables the user to manually select and set the fan speed setting at which the user desires the air filtration device to operate. 
     In this example embodiment, the air filtration device includes: (a) a user-selectable manual minimum fan speed setting operating mode that, when selected by the user, sets the fan speed setting to the minimum fan speed setting (which corresponds to the first desired air flow rate through the air filtration device) and causes the air filtration device to operate at the minimum fan speed setting; (b) a user-selectable manual medium fan speed setting operating mode that, when selected by the user, sets the fan speed setting to the medium fan speed setting (which corresponds to the second desired air flow rate through the air filtration device) and causes the air filtration device to operate at the medium fan speed setting; and (c) a user-selectable manual maximum fan speed setting operating mode that, when selected by the user, sets the fan speed setting to the maximum fan speed setting (which corresponds to the third desired air flow rate through the air filtration device) and causes the air filtration device to operate at the maximum fan speed setting. 
     In this example embodiment, when the air filtration device is operating in either the manual maximum fan speed setting operating mode or the manual medium fan speed setting operating mode such that the fan speed setting is either the maximum fan speed setting or the medium fan speed setting, the air filtration device employs dynamic fan speed control to adjust the fan speed to achieve the desired air flow rate through the air filtration device. Dynamic fan speed control is described in detail below. 
     On the other hand, in this example embodiment, when the air filtration device is operating in the manual minimum fan speed setting operating mode such that the fan speed setting is the minimum fan speed setting, the air filtration device operates the fan at a substantially constant, designated fan speed. In other words, when the air filtration device is operating in the manual minimum fan speed setting operating mode such that the fan speed setting is the minimum fan speed setting, the air filtration device does not employ dynamic fan speed control in this example embodiment. It should be appreciated, however, that in other embodiments the air filtration device employs dynamic fan speed control when the fan speed setting is the minimum fan speed setting. 
     In other embodiments, the air filtration device does not include a manual fan speed setting operating mode associated with each fan speed setting at which the air filtration device may operate. For instance, in one example embodiment in which the air filtration device includes five fan speed settings at which the air filtration device may operate, the air filtration device includes manual fan speed setting operating modes associated with a first, third, and fifth fan speed setting and does not include a manual fan speed setting operating mode associated with a second and fourth fan speed setting. In another embodiment, the air filtration device does not include any manual fan speed setting operating modes. In another embodiment, the air filtration device includes a single manual fan speed setting operating mode. 
     4.3.2 Automatic Fan Speed Setting Selection Operating Mode 
     In this example embodiment, the air filtration device includes a user-selectable automatic fan speed setting selection operating mode. Generally, when the automatic fan speed setting selection operating mode is selected by the user, the air filtration device uses the dust sensor to measure the amount of dust in the air surrounding the air filtration device and, if necessary, automatically increases or decreases the fan speed setting to account for the amount of dust in the air. Thus, when operating in the automatic fan speed setting selection operating mode, the air filtration device dynamically and automatically adjusts the fan speed setting in real-time to account for varying levels of dust in the air surrounding the air filtration device, which eliminates the need for the user to guess the amount of dust in the air and manually select what the user believes to be the most effective and efficient fan speed setting in which to operate the air filtration device to remove that dust. 
     More specifically, in this example embodiment, each of the fan speed settings is associated with a different range of dust levels. The range of dust levels associated with a particular fan speed setting includes the dust levels that the air filtration device may most effectively and efficiently manage or clean when operating at that particular fan speed setting. For instance, in this example embodiment: (a) the minimum fan speed setting is associated with a first range of dust levels beginning at zero and ending at a maximum dust level associated with the minimum fan speed setting; (b) the medium fan speed setting is associated with a second range of dust levels beginning at a minimum dust level associated with the medium fan speed setting, which is greater than the maximum dust level associated with the minimum fan speed setting, and ending at a maximum dust level associated with the medium fan speed setting; and (c) the maximum fan speed setting is associated with a third range of dust levels beginning at a minimum dust level associated with the maximum fan speed setting, which is greater than the maximum dust level associated with the medium fan speed setting, and ending at a maximum measurable dust level, which is the highest dust level measurable by the dust sensor. 
     For instance, Table 1 below includes example ranges of dust levels associated with the minimum, medium, and maximum fan speed settings. In this example, the dust levels range from zero to ten. Each fan speed setting may be associated with any suitable range of dust levels, and that each range of dust levels may include any suitable dust levels. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example Ranges of Dust Levels Associated 
               
               
                 With Example Fan Speed Settings 
               
            
           
           
               
               
               
            
               
                   
                 Fan Speed Setting 
                 Range of Dust Levels 
               
               
                   
                   
               
               
                   
                 Minimum 
                 0 to 3 
               
               
                   
                 Medium 
                 4 to 6 
               
               
                   
                 Maximum 
                 7 to 10 
               
               
                   
                   
               
            
           
         
       
     
     Thus, in this example: (a) when the measured dust level is 0, 1, 2, or 3, the air filtration device most effectively and efficiently manages or cleans the dust when operating at the minimum fan speed setting; (b) when the measured dust level is 4, 5, or 6, the air filtration device most effectively and efficiently manages or cleans the dust when operating at the medium fan speed setting; and (c) when the measured dust level is 7, 8, 9, or 10, the air filtration device most effectively and efficiently manages or cleans the dust when operating at the maximum fan speed setting. 
     At each of a plurality of predetermined dust level sensing time intervals, such as every fifteen seconds (or any other suitable length of time), the air filtration device measures the dust level using the dust level sensor and determines whether the range of dust levels associated with the current fan speed setting includes the measured dust level. If the range of dust levels associated with the current fan speed setting includes the measured dust level, the air filtration device maintains the current fan speed setting. If the measured dust level exceeds the range of dust levels associated with the current fan speed setting, the air filtration device increases the fan speed setting. If the measured dust level falls below the range of dust levels associated with the current fan speed setting for a designated number of consecutive dust level sensing time intervals, the air filtration device decreases the fan speed setting. 
       FIG. 15  illustrates a flowchart of one example embodiment of an automatic fan speed setting selection process or method  4000  of the present disclosure. In various embodiments, the automatic fan speed setting selection process  4000  is represented by a set of instructions stored in one or more memories and executed by the controller. Although the automatic fan speed setting selection process  4000  is described with reference to the flowchart shown in  FIG. 15 , many other processes of performing the acts associated with this illustrated automatic fan speed setting selection process may be employed. For example, the order of certain of the illustrated blocks and/or diamonds may be changed, certain of the illustrated blocks and/or diamonds may be optional, and/or certain of the illustrated blocks and/or diamonds may not be employed. 
     The automatic fan speed setting selection process  4000  starts when the air filtration device receives a selection of the automatic fan speed setting selection operating mode. The air filtration device sets the fan speed setting to the minimum fan speed setting such that the current fan speed setting is the minimum fan speed setting, as indicated by block  4100 . As explained above, each fan speed setting is associated with a different range of dust levels including a minimum dust level and a maximum dust level. The air filtration device sets the variable n equal to zero, as indicated by block  4110 . The variable n represents a number of dust level sensing time intervals in which the measured dust level during that particular dust level sensing time interval is less than the minimum dust level in the range of dust levels associated with the current fan speed setting during that particular dust level sensing time interval. The air filtration device measures the dust level using the dust sensor, as indicated by block  4120 . 
     The air filtration device determines if the measured dust level is greater than the maximum dust level in the range of dust levels associated with the current fan speed setting, as indicated by diamond  4130 . If the air filtration device determines that the measured dust level is greater than the maximum dust level in the range of dust levels associated with the current fan speed setting, the air filtration device increases the fan speed setting, such as by one level (e.g., from the minimum fan speed setting to the medium fan speed setting or from the medium fan speed setting to the maximum fan speed setting), as indicated by block  4140 . The air filtration device determines whether a dust level sensing time interval has elapsed, as indicated by diamond  4150 . If the air filtration device determines that the dust level sensing time interval has elapsed, the process  4000  returns to the block  4120 . If, on the other hand, the air filtration device determines that the dust level sensing time interval has not elapsed, the air filtration device maintains the current fan speed setting, as indicated by block  4160 , and the process  4000  returns to the diamond  4150 . 
     Returning to the diamond  4130 , if the air filtration device determines that the measured dust level is not greater than the maximum dust level in the range of dust levels associated with the current fan speed setting, the air filtration device determines if the measured dust level is less than the minimum dust level in the range of dust levels associated with the current fan speed setting, as indicated by diamond  4170 . If the air filtration device determines that the measured dust level is not less than the minimum dust level in the range of dust levels associated with the current fan speed setting, the air filtration device sets the variable n equal to zero, and the process  4000  proceeds to the block  4160 , described above. 
     If, on the other hand, the air filtration device determines that the measured dust level is less than the minimum dust level in the range of dust levels associated with the current fan speed setting, the air filtration device sets the variable n equal to n+1, as indicated by block  4190 . The air filtration device determines if the variable n is at least equal to a designated number, as indicated by diamond  4200 . If the air filtration device determines that the variable n is not at least equal to the designated number, the process  4000  proceeds to the block  4160 . If, on the other hand, the air filtration device determines that the variable n is at least equal to the designated number, the air filtration device decreases the fan speed setting, such as by one level (e.g., from the maximum fan speed setting to the medium fan speed setting or from the medium fan speed setting to the minimum fan speed setting), as indicated by block  4220 . The air filtration device sets the variable n equal to zero, and the process  4000  proceeds to the diamond  4150 . 
     In this example embodiment, the designated number is four such that the air filtration device decreases the fan speed setting when the air filtration device determines that the measured dust level is less than the minimum dust level in the range of dust levels associated with the current fan speed setting for four consecutive dust level sensing time intervals. It should be appreciated, however, that the designated number may be any suitable number in other embodiments. It should also be appreciated that, in certain embodiments, the designated number is equal to one. Thus, in these embodiments, the air filtration device decreases the fan speed setting when the air filtration device determines that the measured dust level is less than the minimum dust level in the range of dust levels associated with the current fan speed setting. 
     In the example embodiment described above with respect to  FIG. 15 , the air filtration device increases or decreases the fan speed setting one level at a time. In other embodiments, however, the air filtration device may increase or decrease the fan speed level a plurality of levels at a time. For instance, in one example embodiment, if the measured dust level is not within the range of dust levels associated with the current fan speed setting, the air filtration device switches the fan speed setting to the fan speed setting associated with the range of dust levels that includes the measured dust level. For instance, if the current fan speed setting is the minimum fan speed setting and the measured dust level is included in the range of dust levels associated with the maximum fan speed setting, the air filtration device changes the fan speed setting to the maximum fan speed setting (bypassing the medium fan speed setting). Alternatively, if the current fan speed setting is the maximum fan speed setting and the measured dust level is included in the range of dust levels associated with the minimum fan speed setting for a designated number of consecutive dust level sensing time intervals, the air filtration device changes the fan speed setting to the minimum fan speed setting (bypassing the medium fan speed setting). 
     In other embodiments, when operating in the automatic fan speed setting selection operating mode, the air filtration device powers the fan off when the measured dust level is a designated dust level or within a designated range of dust levels. For instance, Table 2 below includes example ranges of dust levels associated with the off, minimum, medium, and maximum fan speed settings. In this example, the dust levels range from zero to ten. Each fan speed setting may be associated with any suitable range of dust levels, and that each range of dust levels may include any suitable dust levels. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Example Ranges of Dust Levels Associated 
               
               
                 With Example Fan Speed Settings 
               
            
           
           
               
               
               
            
               
                   
                 Fan Speed Setting 
                 Range of Dust Levels 
               
               
                   
                   
               
               
                   
                 Off 
                 0 
               
               
                   
                 Minimum 
                 1 to 3 
               
               
                   
                 Medium 
                 4 to 6 
               
               
                   
                 Maximum 
                 7 to 10 
               
               
                   
                   
               
            
           
         
       
     
     Thus, in this example: (a) when the measured dust level is 0, the air filtration device powers the fan off because filtration is not required; (b) when the measured dust level is 1, 2, or 3, the air filtration device most effectively and efficiently manages or cleans the dust when operating at the minimum fan speed setting; (c) when the measured dust level is 4, 5, or 6, the air filtration device most effectively and efficiently manages or cleans the dust when operating at the medium fan speed setting; and (d) when the measured dust level is 7, 8, 9, or 10, the air filtration device most effectively and efficiently manages or cleans the dust when operating at the maximum fan speed setting. Thus, in this example embodiment, when operating in the automatic fan speed setting selection operating mode, the air filtration device only operates fan when the measured dust level is greater than zero (though the threshold minimum dust level that causes operation of the fan may be any suitable dust level). 
     4.3.3 Standby Operating Mode 
     In this example embodiment, the air filtration device includes a user-selectable standby operating mode in which the air filtration device is powered on but in which the fan does not operate. If the air filtration device receives a selection of the standby operating mode upon power-up of the air filtration device, the air filtration device lights the LED of the air filtration device status indicator green. If the standby operating mode is selected after the air filtration device has determined the occlusion levels of the filters (described below) and has indicated such occlusion levels by lighting the appropriate pre-filter and HEPA filter status indicators, the air filtration device maintains those filter occlusion level indicators for a designated period, such as  10  seconds (or any other suitable period of time). Once the designated period expires, the air filtration device enters full standby operating mode. Once in full standby operating mode, when the automatic fan speed setting selection operating mode or any of the manual fan speed setting operating modes is selected, the air filtration device performs the filter occlusion level monitoring process (described below). 
     4.4 Dynamic Fan Speed Control 
     As noted above, in certain instances, the air filtration device employs dynamic fan speed control to adjust the fan speed to achieve a desired air flow rate through the air filtration device. Generally, when employing dynamic fan speed control, the air filtration device uses the differential pressure across the fan and the desired air flow rate through the air filtration device to determine a desired fan speed that achieves the desired flow rate through the air filtration device. This enables the air filtration device to maintain that desired air flow rate through the air filtration device by varying the fan speed as the pre-filter and the HEPA filter occlude during operation of the air filtration device, which prevents the air flow rate through the air filtration device from falling below the desired air flow rate and impairing the air filtration device&#39;s performance. 
     In this example embodiment, the air filtration device employs dynamic fan speed control when the current fan speed setting is one of at least one designated fan speed setting. Here, the maximum fan speed setting and the medium fan speed setting are designated fan speed settings and, therefore, the air filtration device employs dynamic fan speed control when the air filtration device is operating at either of these fan speed settings. The minimum fan speed setting is not a designated fan speed setting in this example embodiment and, therefore, the air filtration device does not employ dynamic fan speed control when the air filtration device is operating at the minimum fan speed setting. It should be appreciated that, in other embodiments: (a) all of the fan speed settings are designated fan speed settings; (b) a plurality, but less than all, of the fan speed settings are designated fan speed settings; (c) one of the fan speed settings is a designated fan speed setting; (d) none of the fan speed settings are designated fan speed settings; and (e) any particular fan speed setting(s) may be a designated fan speed setting(s). 
     It should be appreciated that, in this example embodiment, the air filtration device employs dynamic fan speed control when the current fan speed setting is one of the at least one designated fan speed setting regardless of whether the air filtration device is operating in the automatic fan speed setting selection operating mode or in one of the manual fan speed setting operating modes. 
       FIG. 16  illustrates a flowchart of one example embodiment of a dynamic fan speed control process or method  5000  of the present disclosure. In various embodiments, the dynamic fan speed control process  5000  is represented by a set of instructions stored in one or more memories and executed by the controller. Although the dynamic fan speed control process  5000  is described with reference to the flowchart shown in  FIG. 16 , many other processes of performing the acts associated with this illustrated dynamic fan speed control process may be employed. For example, the order of certain of the illustrated blocks and/or diamonds may be changed, certain of the illustrated blocks and/or diamonds may be optional, and/or certain of the illustrated blocks and/or diamonds may not be employed. 
     The dynamic fan speed control process  5000  starts when the air filtration device begins operating in either the automatic fan speed setting selection operating mode or one of the manual fan speed setting operating modes. The air filtration device determines the current fan speed setting, as indicated by block  5100 . As noted above, each fan speed setting is associated with or corresponds to a desired air flow rate through the air filtration device. The air filtration device determines if the current fan speed setting is the minimum fan speed setting, as indicated by diamond  5110 . If the air filtration device determines that the current fan speed setting is the minimum fan speed setting, the air filtration device sets the fan speed to a designated fan speed, as indicated by block  5120 . 
     The air filtration device determines if a fan speed determination time interval has elapsed, as indicated by diamond  5180 . In this example embodiment, the fan speed determination time interval is  1  second, though any suitable time period may be employed. If the air filtration device determines that the fan speed determination time interval has elapsed, the process  5000  returns to the block  5100 . If, on the other hand, the air filtration device determines that the fan speed determination time interval has not elapsed, the air filtration device maintains the current fan speed, as indicated by block  5190 , and the process  5000  returns to the diamond  5180 . 
     Returning to the diamond  5110 , if the air filtration device determines that the current fan speed setting is not the minimum fan speed setting, the air filtration device determines the differential pressure (such as a pressure drop) across the fan using the fan differential pressure sensor, as indicated by block  5130 . The air filtration device determines a desired fan speed based at least in part on the differential pressure across the fan and the desired air flow rate through the air filtration device, as indicated by block  5140 . The air filtration device determines if the desired fan speed is greater than a maximum allowable speed of the fan, as indicated by diamond  5150 . 
     If the air filtration device determines that the desired fan speed is greater than the maximum allowable fan speed, the air filtration device sets the fan speed to the maximum allowable fan speed, as indicated by block  5160 , and the process  5000  proceeds to the diamond  5180 . If, on the other hand, the air filtration device determines that the desired fan speed is not greater than the maximum allowable fan speed, the air filtration device sets the fan speed to the desired fan speed, as indicated by block  5170 . The process  5000  proceeds to the diamond  5180 . 
     It should be appreciated that, in this example embodiment, the air filtration device determines the desired fan speed based at least in part on the differential pressure across the fan and the desired air flow rate through the air filtration device and does not (directly) use the pre-filter and HEPA filter occlusion levels (described below) to do so. In other words, in this example embodiment, the air filtration device determines the desired fan speed is independent of and without determining the pre-filer and HEPA filter occlusion levels. 
     In other embodiments, the air filtration device determines the desired fan speed based, at least in part, on the determined pre-filter and HEPA filter occlusion levels. That is, in these embodiments the determination of the desired fan speed directly depends on the determined pre-filter and HEPA filter occlusion levels. 
     In another embodiment, the air filtration device determines that a major air filtration device malfunction occurs when the desired fan speed exceeds the maximum fan speed. 
     4.5 Filter Presence Detection 
     4.5.1 Pre-Filter Presence Detection 
     In this example embodiment, the air filtration device determines whether an acceptable pre-filter is installed in the air filtration device using the pre-filter presence sensor, and prevents use of the fan when an acceptable pre-filter is not installed.  FIG. 17  illustrates a flowchart of one example embodiment of a pre-filter presence detection process or method  6000  of the present disclosure. In various embodiments, the pre-filter presence detection process  6000  is represented by a set of instructions stored in one or more memories and executed by the controller. Although the pre-filter presence detection process  6000  is described with reference to the flowchart shown in  FIG. 17 , many other processes of performing the acts associated with this illustrated pre-filter presence detection process may be employed. For example, the order of certain of the illustrated blocks and/or diamonds may be changed, certain of the illustrated blocks and/or diamonds may be optional, and/or certain of the illustrated blocks and/or diamonds may not be employed. 
     The pre-filter presence detection process  6000  starts when the air filtration device receives a selection of one of the manual fan speed setting selection operating modes or the automatic fan speed setting selection operating mode. As described above, in this example embodiment, the lower housing component supports or otherwise includes a pre-filter limit switch that is actuatable by the pre-filter limit switch actuator of the pre-filter. The air filtration device determines whether the pre-filter limit switch is actuated, as indicated by diamond  6100 . If the air filtration device determines that the pre-filter limit switch is actuated, the air filtration device determines that an acceptable pre-filter is installed, as indicated by block  6110 , and the process  6000  proceeds to diamond  6140 , described below. If, on the other hand, the air filtration device determines that the pre-filter limit switch is not actuated, the air filtration device indicates that an acceptable pre-filter is not installed, as indicated by block  6120 , and the air filtration device prevents use of the fan, as indicated by block  6130 . As indicated by the diamond  6140 , once a pre-filter presence detection time interval elapses, the process  6000  returns to the diamond  6100 . In this example embodiment, the pre-filter presence detection time interval is  1  second, though any suitable period of time may be employed. 
     In this example embodiment, the air filtration device indicates that an acceptable pre-filter is not installed by: (a) lighting the red LED of the pre-filter fault indicator, (b) lighting the LED of the air filtration device status indicator red, and (c) outputting the filter fault indicator tone. Any other indications or combinations of indications may be employed instead of or in addition to the above-described indications. 
     In another embodiment, the air filtration device employs the pre-filter differential pressure sensor to determine whether an acceptable pre-filter is installed. In this embodiment, the air filtration device determines the differential pressure across the pre-filter using the pre-filter differential pressure sensor. The air filtration device determines if the differential pressure across the pre-filter is greater than or equal to a minimum allowable differential pressure across the pre-filter. If the air filtration device determines that the differential pressure across the pre-filter is greater than or equal to the minimum allowable differential pressure across the pre-filter, the air filtration device determines that an acceptable pre-filter is installed. If, on the other hand, the air filtration device determines that the differential pressure across the pre-filter is less than (i.e., not greater than or equal to) the minimum allowable differential pressure across the pre-filter, the air filtration device indicates that an acceptable pre-filter is not installed, and the air filtration device prevents use of the fan. 
     In another embodiment, the upper and lower edges of the pre-filter each include an integrated metallic element (such as a 0.003 inch thick×1 inch high element) that substantially spans the pre-filter&#39;s circumference. In this embodiment, the pre-filter presence sensor is a Hall Effect sensor that detects the metallic element. In this embodiment, if the Hall Effect sensor does not detect any metallic element, the air filtration device determines that an acceptable pre-filter is not installed and prevents use of the fan, and if the Hall Effect sensor detects a metallic element, the air filtration device determines that an acceptable pre-filter is installed. 
     In another embodiment, the pre-filter includes at least one RFID tag. In this embodiment, the pre-filter presence sensor is an RFID reader configured to read or recognize the RFID tag included in the pre-filter. In this embodiment, if the RFID reader does not read or recognize an RFID tag or reads or recognizes an improper RFID tag, the air filtration device determines that an acceptable pre-filter is not installed, and if the RFID reader reads or recognizes a proper RFID tag, the air filtration device determines that an acceptable pre-filter is installed. Any other suitable pre-filter presence detection process may be employed. 
     4.5.2 HEPA Filter Presence Detection 
     In this example embodiment, the air filtration device determines whether an acceptable HEPA filter is installed in the air filtration device using the differential pressure across the HEPA filter, and prevents use of the fan when an acceptable HEPA filter is not installed.  FIG. 18  illustrates a flowchart of one example embodiment of a HEPA filter presence detection process or method  7000  of the present disclosure. In various embodiments, the HEPA filter presence detection process  7000  is represented by a set of instructions stored in one or more memories and executed by the controller. Although the HEPA filter presence detection process  7000  is described with reference to the flowchart shown in  FIG. 18 , many other processes of performing the acts associated with this illustrated HEPA filter presence detection process may be employed. For example, the order of certain of the illustrated blocks and/or diamonds may be changed, certain of the illustrated blocks and/or diamonds may be optional, and/or certain of the illustrated blocks and/or diamonds may not be employed. 
     The HEPA filter presence detection process  7000  starts when the air filtration device receives a selection of one of the manual fan speed setting selection operating modes or the automatic fan speed setting selection operating mode. The air filtration device determines the differential pressure (such as a pressure drop) across the HEPA filter using the HEPA filter differential pressure sensor, as indicated by block  7100 . The air filtration device determines if the differential pressure across the HEPA filter is greater than or equal to a minimum allowable differential pressure across the HEPA filter, as indicated by diamond  7110 . If the air filtration device determines that the differential pressure across the HEPA filter is greater than or equal to the minimum allowable differential pressure across the HEPA filter, the air filtration device determines that an acceptable HEPA filter is installed, as indicated by block  7120 , and the process  7000  proceeds to diamond  7150 , described below. 
     If, on the other hand, the air filtration device determines that the differential pressure across the HEPA filter is less than (i.e., not greater than or equal to) the minimum allowable differential pressure across the HEPA filter, the air filtration device indicates that an acceptable HEPA filter is not installed, as indicated by block  7130 , and the air filtration device prevents use of the fan, as indicated by block  7140 . As indicated by the diamond  7150 , once a HEPA filter presence detection time interval elapses, the process  7000  returns to the block  7100 . 
     In this example embodiment, the HEPA filter presence detection time interval is 1 hour, though any suitable period of time may be employed. Additionally, in this example embodiment, the minimum allowable differential pressure across the HEPA filter is equal to the differential pressure across 0.10 inches of water at a fan speed of 3,000 revolutions per minute, though any suitable minimum allowable differential pressure across the HEPA filter may be employed. 
     In this example embodiment, the air filtration device indicates that an acceptable HEPA filter is not installed by: (a) lighting the red LED of the HEPA filter fault indicator, (b) lighting the LED of the air filtration device status indicator red, and (c) outputting the filter fault indicator tone. Any other indications or combinations of indications may be employed instead of or in addition to the above-described indications. 
     In another embodiment, the HEPA filter includes one or more integrated hollow pressure tubes positioned vertically among the pleats of the HEPA filter media. An end of each of these pressure tubes is flush with the bottom of the lower HEPA filter end cap. In this embodiment, the air filtration device includes one or more pressure sensors configured to detect the presence of the pressure tubes. Thus, in this embodiment, if a HEPA filter without such pressure tubes is installed, the air filtration device will determine that an improper HEPA filter is installed, and will not operate. 
     In another embodiment, the HEPA filter includes at least one RFID tag. In this embodiment, the air filtration device includes a HEPA filter presence sensor in the form of an RFID reader configured to read or recognize the RFID tag included in the HEPA filter. In this embodiment, if the RFID reader does not read or recognize an RFID tag or reads or recognizes an improper RFID tag, the air filtration device determines that an acceptable HEPA filter is not installed, and if the RFID reader reads or recognizes a proper RFID tag, the air filtration device determines that an acceptable HEPA filter is installed. Any other suitable HEPA filter presence detection process may be employed. 
     As described below, in certain embodiments, the HEPA filter presence detection process is part of the filter occlusion level monitoring process. 
     4.6 Filter Occlusion Level Monitoring 
     In this example embodiment, the air filtration device monitors the occlusion levels of the pre-filter and the HEPA filter (i.e., the cleanliness levels of the pre-filter and the HEPA filter) and provides feedback regarding the filter occlusion levels to the user to enable the user to quickly and easily determine how clean (or dirty, blocked, or clogged) the pre-filter and the HEPA filter are. When the pre-filter occlusion level exceeds a pre-filter shutdown threshold, the HEPA filter occlusion level exceeds a HEPA filter shutdown threshold, or both, the air filtration device enters a shutdown mode in which the air filtration device eventually prevents any use of the fan until the appropriate filter(s) is(are) replaced. This ensures that the air filtration device does not operate for an extended period of time with a pre-filter and/or a HEPA filter so occluded as to inhibit effective and efficient operation of the air filtration device. 
       FIG. 30  illustrates a flowchart of one example embodiment of a filter occlusion level monitoring process or method  8000  of the present disclosure. In various embodiments, the filter occlusion level monitoring process  8000  is represented by a set of instructions stored in one or more memories and executed by t. Although the filter occlusion level monitoring process  8000  is described with reference to the flowchart shown in  FIG. 30 , many other processes of performing the acts associated with this illustrated filter occlusion level monitoring process may be employed. For example, the order of certain of the illustrated blocks and/or diamonds may be changed, certain of the illustrated blocks and/or diamonds may be optional, and/or certain of the illustrated blocks and/or diamonds may not be employed. 
     The filter occlusion level monitoring process  8000  starts after (such as a designated period of time after (such as 10 seconds or any other suitable time period)) the air filtration device receives a selection of the automatic fan speed setting selection operating mode or any of the manual fan speed setting operating modes either upon power-up of the air filtration device or when the air filtration device is in the full standby mode (described above). The air filtration device increases the fan speed to a differential pressure determination fan speed, such as 3,000 revolutions per minute or any other suitable fan speed, as indicated by block  8105 . The air filtration device determines the differential pressure (such as a pressure drop) across the pre-filter using the pre-filter differential pressure sensor, as indicated by block  8100 , and the differential pressure (such as a pressure drop) across the HEPA filter using the HEPA filter differential pressure sensor, as indicated by block  8110 . 
     The air filtration device determines the pre-filter occlusion level based, at least in part, on the determined differential pressure across the pre-filter and the determined differential pressure across the HEPA filter, as indicated by block  8120 . The air filtration device also determines the HEPA filter occlusion level based, at least in part, on the on the determined differential pressure across the pre-filter and the determined differential pressure across the HEPA filter, as indicated by block  8160 . In this example embodiment, while determining the filter occlusion levels (which includes determining the differential pressures across the pre-filter and the HEPA filter), the air filtration device: (a) lights the yellow LED of the pre-filter status indicators in a blinking or flashing manner; (b) lights the yellow LED of the HEPA filter status indicators in a blinking or flashing manner; and (c) displays “tESt” in the hour meter display. This enables the user to quickly and easily determine when the air filtration device is measuring the filter occlusion levels. Any other indications or combinations of indications may be employed instead of or in addition to the above-described indications. 
     The air filtration device determines if the determined pre-filter occlusion level exceeds a pre-filter shutdown threshold, as indicated by diamond  8130 . The pre-filter shutdown threshold is a maximum allowable pre-filter occlusion level. Once the pre-filter occlusion level reaches the pre-filter shutdown threshold, the air filtration device may no longer efficiently and effectively clean the air (until the pre-filter is replaced). If the air filtration device determines that the determined pre-filter occlusion level exceeds the pre-filter shutdown threshold, the process  8000  proceeds to diamond  8200 , described below. 
     If, on the other hand, the air filtration device determines that the determined pre-filter occlusion level does not exceed the pre-filter shutdown threshold, the air filtration device determines which of a plurality of different pre-filter occlusion level ranges includes the determined pre-filter occlusion level, as indicated by block  8140 . In this example embodiment, each pre-filter occlusion level range is associated with a general indicator of the cleanliness of the pre-filter. For instance, in this example embodiment, the pre-filter occlusion level ranges include: (a) a first or Clean pre-filter occlusion level range, (b) a second or Slightly Occluded pre-filter occlusion level range, and (c) a third or Highly Occluded pre-filter occlusion level range. In this example embodiment, each occlusion level included in the Slightly Occluded pre-filter occlusion level range is greater than each occlusion level included in the Clean pre-filter occlusion level range, and each occlusion level included in the Highly Occluded pre-filter occlusion level range is greater than each occlusion level included in the Slightly Occluded pre-filter occlusion level range. The maximum occlusion level in the Highly Occluded pre-filter occlusion level range is the pre-filter shutdown threshold. For instance, Table 3 below includes example ranges of occlusion levels associated with the Clean, Slightly Occluded, and Highly Occluded pre-filter occlusion level ranges. In this example, the occlusion levels range from zero to ten. Each cleanliness indicator may be associated with any suitable range of pre-filter occlusion levels, and that each range of pre-filter occlusion levels may include any suitable pre-filter occlusion levels. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Example Occlusion Levels Associated With 
               
            
           
           
               
               
               
            
               
                   
                 Cleanliness Indicator 
                 Range of Pre-Filter Occlusion Levels 
               
               
                   
                   
               
               
                   
                 Clean 
                 0 to 2 
               
               
                   
                 Slightly Occluded 
                 3 to 5 
               
               
                   
                 Highly Occluded 
                 6 to pre-filter shutdown threshold 
               
               
                   
                   
               
            
           
         
       
     
     Example Pre-Filter Occlusion Level Ranges 
     Returning to the process  8000 , the air filtration device indicates the pre-filter occlusion level range that includes the determined pre-filter occlusion level, as indicated by block  8150 . In this example embodiment, the air filtration device does so by: (a) if the Clean pre-filter occlusion level range includes the determined pre-filter occlusion level, lighting the green LED of the pre-filter status indicators; (b) if the Slightly Occluded pre-filter occlusion level range includes the determined pre-filter occlusion level, lighting the yellow LED of the pre-filter status indicators; and (c) if the Highly Occluded pre-filter occlusion level range includes the determined pre-filter occlusion level, lighting the red LED of the pre-filter status indicators. This enables a user to quickly and easily determine how clean (or dirty) the pre-filter is. The process  8000  proceeds to the diamond  8200 . 
     Turning to diamond  8170 , the air filtration device determines if the determined HEPA filter occlusion level exceeds a HEPA filter shutdown threshold. The HEPA filter shutdown threshold is a maximum allowable HEPA filter occlusion level. Once the HEPA filter occlusion level reaches the HEPA filter shutdown threshold, the air filtration device may no longer efficiently and effectively clean the air (until the HEPA filter is replaced). If the air filtration device determines that the determined HEPA filter occlusion level exceeds the HEPA filter shutdown threshold, the process  8000  proceeds to the diamond  8200 , described below 
     If, on the other hand, the air filtration device determines that the determined HEPA filter occlusion level does not exceed the HEPA filter shutdown threshold, the air filtration device determines which of a plurality of different HEPA filter occlusion level ranges includes the determined HEPA filter occlusion level, as indicated by block  8180 . In this example embodiment, each HEPA filter occlusion level range is associated with a general indicator of the cleanliness of the HEPA filter. For instance, in this example embodiment, the HEPA filter occlusion level ranges include: (a) a first or Clean HEPA filter occlusion level range, (b) a second or Slightly Occluded HEPA filter occlusion level range, and (c) a third or Highly Occluded HEPA filter occlusion level range. In this example embodiment, each occlusion level included in the Slightly Occluded HEPA filter occlusion level range is greater than each occlusion level included in the Clean HEPA filter occlusion level range, and each occlusion level included in the Highly Occluded HEPA filter occlusion level range is greater than each occlusion level included in the Slightly Occluded HEPA filter occlusion level range. The maximum occlusion level in the Highly Occluded HEPA filter occlusion level range is the HEPA filter shutdown threshold. For instance, Table 4 below includes example ranges of occlusion levels associated with the Clean, Slightly Occluded, and Highly Occluded HEPA filter occlusion level ranges. In this example, the occlusion levels range from zero to ten. Each cleanliness indicator may be associated with any suitable range of HEPA filter occlusion levels, and that each range of HEPA filter occlusion levels may include any suitable HEPA filter occlusion levels. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Example Occlusion Levels Associated With 
               
            
           
           
               
               
               
            
               
                   
                 Cleanliness Indicator 
                 Range of HEPA Filter Occlusion Levels 
               
               
                   
                   
               
               
                   
                 Clean 
                 0 to 2 
               
               
                   
                 Slightly Occluded 
                 3 to 5 
               
               
                   
                 Highly Occluded 
                 6 to HEPA filter shutdown threshold 
               
               
                   
                   
               
            
           
         
       
     
     Example HEPA filter Occlusion Level Ranges 
     Returning to the process  8000 , the air filtration device indicates the HEPA filter occlusion level range that includes the determined HEPA filter occlusion level, as indicated by block  8190 . In this example embodiment, the air filtration device does so by: (a) if the Clean HEPA filter occlusion level range includes the determined HEPA filter occlusion level, lighting the green LED of the HEPA filter status indicators; (b) if the Slightly Occluded HEPA filter occlusion level range includes the determined HEPA filter occlusion level, lighting the yellow LED of the HEPA filter status indicators; and (c) if the Highly Occluded HEPA filter occlusion level range includes the determined HEPA filter occlusion level, lighting the red LED of the HEPA filter status indicators. This enables a user to quickly and easily determine how clean (or dirty) the HEPA filter is. The process  8000  proceeds to the diamond  8200 . 
     Turning to the diamond  8200 , the air filtration device determines if: (a) the determined pre-filter occlusion level exceeds the pre-filter shutdown threshold, and/or (b) the determined HEPA filter occlusion level exceeds the HEPA filter shutdown threshold. If neither: (a) the determined pre-filter occlusion level exceeds the pre-filter shutdown threshold, nor (b) the determined HEPA filter occlusion level exceeds the HEPA filter shutdown threshold, as indicated by diamond  8210 , once a filter occlusion level determination time interval elapses, the process  8000  returns to the block  8100 . In this example embodiment, the filter occlusion level determination time interval is  60  minutes, though any suitable period of time may be employed. 
     If, on the other hand, at least one of: (a) the determined pre-filter occlusion level exceeds the pre-filter shutdown threshold, and (b) the determined HEPA filter occlusion level exceeds the HEPA filter shutdown threshold, the air filtration device indicates that the pre-filter, the HEPA filter, or both need replacement, as indicated by block  8220 . More specifically: (a) if the determined pre-filter occlusion level exceeds the pre-filter shutdown threshold, the air filtration device indicates that the pre-filter needs replacement; (b) if the determined HEPA filter occlusion level exceeds the HEPA filter shutdown threshold, the air filtration device indicates that the HEPA filter needs replacement; and (c) if the determined pre-filter occlusion level exceeds the pre-filter shutdown threshold and the determined HEPA filter occlusion level exceeds the HEPA filter shutdown threshold, the air filtration device indicates that both the pre-filter and the HEPA filter need replacement. The air filtration device enters the shutdown mode, as indicated by block  8230 , and initiates a designated shutdown time period, as indicated by block  8240 . In this example embodiment, the designated shutdown time period is  4  hours, though the designated shutdown time period may be any suitable time period. 
     The air filtration device determines if it is operating in the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode, as indicated by diamond  8250 . If the air filtration device is not operating in either the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode, the process  8000  proceeds to block  8270 , described below. If, on the other hand, the air filtration device is operating in the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode, the air filtration device powers down the fan, as indicated by block  8260 . 
     The air filtration device prevents use of the automatic fan speed setting selection operating mode and prevents use of the manual maximum fan speed setting operating mode, as indicated by the block  8270 . The air filtration device enables operation of the air filtration device in either the manual medium fan speed setting operating mode or the manual minimum fan speed setting operating mode, as indicated by block  8280 . The air filtration device determines if the designated shutdown time period has expired, as indicated by diamond  8290 . If the air filtration device determines that the designated shutdown time period has not expired, the process  8000  returns to the block  8280 . If, on the other hand, the air filtration device determines that the designated shutdown time period has expired, the air filtration device powers down the fan, as indicated by block  8300 , and prevents use of the fan, as indicated by block  8310 . In other words, once the designated shutdown time period expires, the air filtration device prevents use of the automatic fan speed setting selection operating mode and any of the manual fan speed setting operating modes. 
     In this example embodiment, the air filtration device indicates that the pre-filter, the HEPA filter, or both need replacement in a variety of different manners. More specifically, in this example embodiment, if the pre-filter occlusion level exceeds the pre-filter shutdown threshold and the air filtration device is operating in the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode, the air filtration device indicates that the pre-filter needs replacement by: (a) lighting the red LED of the pre-filter status indicators in a flashing or blinking manner, (b) lighting the red LED of the pre-filter fault indicator, (c) lighting the LED of the air filtration device status indicator red, and (d) outputting the filter change alarm tone. In this example embodiment, if the pre-filter occlusion level exceeds the pre-filter shutdown threshold and the air filtration device is operating in the manual medium fan speed setting operating mode or the manual minimum fan speed setting mode, the air filtration device indicates that the pre-filter needs replacement by: (a) lighting the red LED of the pre-filter status indicators in a flashing or blinking manner, (b) lighting the red LED of the pre-filter fault indicator, and (c) lighting the LED of the air filtration device status indicator green or keeping the LED of the air filtration device status indicator lit green. When the designated shutdown time period expires, the air filtration device: (a) lights the LED of the air filtration device status indicator red, and (b) outputs the filter change alarm tone while maintaining flashing the red pre-filter status indicator and lighting the red LED of the pre-filter fault indicator. 
     In this example embodiment, if the HEPA filter occlusion level exceeds the HEPA filter shutdown threshold and the air filtration device is operating in the automatic fan speed setting selection operating mode or the manual maximum fan speed setting operating mode, the air filtration device indicates that the HEPA filter needs replacement by: (a) lighting the red LED of the HEPA filter status indicators in a flashing or blinking manner, (b) lighting the red LED of the HEPA filter fault indicator, (c) lighting the LED of the air filtration device status indicator red, and (d) outputting the filter change alarm tone. In this example embodiment, if the HEPA filter occlusion level exceeds the HEPA filter shutdown threshold and the air filtration device is operating in the manual medium fan speed setting operating mode or the manual minimum fan speed setting mode, the air filtration device indicates that the HEPA filter needs replacement by: (a) lighting the red LED of the HEPA filter status indicators in a flashing or blinking manner, (b) lighting the red LED of the HEPA filter fault indicator, and (c) lighting the LED of the air filtration device status indicator green or keeping the LED of the air filtration device status indicator lit green. When the designated shutdown time period expires, the air filtration device: (a) lights the LED of the air filtration device status indicator red, and (b) outputs the filter change alarm tone while maintaining flashing the red HEPA filter status indicator and lighting the red LED of the HEPA filter fault indicator. 
     In this example embodiment, if the air filtration device receives an input to switch to the standby mode while the air filtration device is determining the pre-filter and HEPA filter occlusion levels, the air filtration device stops such determinations and shuts the fan down. The air filtration device restarts the filter occlusion level monitoring process once the air filtration device receives an input to switch from the standby mode into the automatic fan speed setting selection operating mode or any of the manual fan speed setting operating modes. 
     Further, in this example embodiment, if the air filtration device receives an input to switch from one of: (a) the automatic fan speed setting selection operating mode, and (b) one of the manual fan speed setting operating modes to another one of: (a) the automatic fan speed setting selection operating mode, and (b) one of the manual fan speed setting operating modes while the air filtration device is determining the pre-filter and HEPA filter occlusion levels, the air filtration device ignores this input until the determinations are complete. For instance, if the air filtration device receives an input to switch the air filtration device from the manual medium fan speed setting operating mode to the manual maximum fan speed setting operating mode while the air filtration device is determining the pre-filter and HEPA filter occlusion levels, the air filtration device does not switch from the manual medium fan speed setting operating mode to the manual maximum fan speed setting operating mode until such determinations are complete. 
     In another embodiment, the air filtration device prevents use of the fan once at least one of: (a) the determined pre-filter occlusion level exceeds the pre-filter shut down threshold, and (b) the determined HEPA filter occlusion level exceeds the HEPA filter shut down threshold. That is, in this embodiment, the air filtration device does not enable operation at any of the fan speed settings once the air filtration device determines that at least one of the filters needs replacement. 
     As noted above, in certain embodiments, the HEPA filter presence detection process is part of the filter occlusion level monitoring process. For instance, in one example embodiment, after determining the differential pressure across the HEPA filter using the HEPA filter differential pressure sensor (such as indicated by block  8110  of  FIG. 19A ), the air filtration device determines if the differential pressure across the HEPA filter is greater than a minimum allowable differential pressure across the HEPA filter (such as indicated by diamond  7110  of  FIG. 18 ). If the air filtration device determines that the differential pressure across the HEPA filter is greater than the minimum allowable differential pressure across the HEPA filter, the air filtration device determines that an acceptable HEPA filter is installed (such as indicated by block  7120  of  FIG. 18 ) and proceeds to determine the pre-filter and HEPA filter occlusion levels (such as indicated by blocks  8120  and  8160  of  FIG. 19A ) and the rest of the filter occlusion level monitoring process. If, on the other hand, the air filtration device determines that the differential pressure across the HEPA filter is not greater than the minimum allowable differential pressure across the HEPA filter, the air filtration device indicates than an acceptable HEPA filter is not installed (such as indicated by block  7130  of  FIG. 18 ), prevents use of the fan (such as indicated by block  7140  of  FIG. 18 ), and terminates the filter occlusion level monitoring process and the HEPA filter presence detection process. 
       4 . 7  Eliminating Fan-Speed Sensor Error 
     In various embodiments, the air filtration device—and particularly the controller  3650 —ensures the fan  2310  operates at a desired fan speed by using a proportional-integral-derivative (PID) control module. The PID control module determines how much electrical current is supplied to the fan motor. The amount of electrical current supplied to the fan motor controls the fan speed. 
     The controller  3650  provides two inputs to the PID control module: (1) the desired fan speed, determined as described above; and (2) a measured fan speed. The controller  3650  determines the measured fan speed by: (1) determining ΔT, which approximates the time it takes the fan blade of the fan to make one complete revolution (based on the output of a fan-speed sensor  3950 ), as described below; and (2) inverting ΔT (i.e., calculating 1/ΔT), which provides the measured fan speed in units of revolutions per unit of time of ΔT (e.g., minutes, seconds, etc.). 
     The PID control module assumes that the measured fan speed is equal or generally equal to the actual fan speed at the time the controller determines ΔT. The PID control module then determines whether the measured fan speed matches the desired fan speed. If not, the PID control module determines how to vary the electrical current supplied to the fan motor to correct the error, and the controller  3650  does so. For instance, if the measured fan speed is less than the desired fan speed, the PID control module determines to increase the electrical current supplied to the fan motor to cause the fan to spin faster and attain the desired fan speed. But if the measured fan speed is greater than the desired fan speed, the PID control module determines to decrease the electrical current supplied to the fan motor to cause the fan to spin slower and attain the desired fan speed. 
     As noted above, the controller  3650  determines ΔT based on the output of the fan-speed sensor  3950 .  FIGS. 20A-20L  are schematic views of: (1) the fan blade  2312  of the fan  2310 ; and (2) the fan-speed sensor  3950 . The fan blade  2312  includes a marker  2314  extending radially outward from the center of the fan blade  2312  to its perimeter. These drawings are not to scale, and the sizes of certain of these elements are exaggerated for clarity. The marker  2314  includes a leading edge  2314   a  and a trailing edge  2314   b . The fan-speed sensor  3950  is designed, positioned, or otherwise configured such that rotation of the marker  2314  past the fan-speed sensor  3950  trips the fan-speed sensor  3950 . The marker may be any suitable fan-speed sensor tripping element, such as (but not limited to): paint, tape, ink, a texture molded into the fan blade, a slot cut into the fan blade, or any other material that changes the reflectivity of light sufficiently to trip the optical sensor. The fan-speed sensor tripping element need only extend radially inward from the perimeter of the fan blade far enough to extend beyond the field of view of the optical sensor. That is, the fan-speed sensor tripping element need not extend from the perimeter of the fan blade to its center. In another embodiment, the fan-speed sensor is a Hall-effect sensor and the fan blade includes a magnet configured to trip the Hall-effect sensor when rotating past it. 
     The controller  3650  operates a fan-speed sensor error elimination process to ensure that the controller does not send measured fan speeds determined based on ΔT&#39;s that represent the time it takes the fan blade to complete fractions of a revolution to the PID control module. In certain embodiments, the fan-speed sensor error elimination process to ensure that the controller does not send measured fan speeds determined based on ΔT&#39;s that represent the time it takes the fan blade to complete multiple revolutions to the PID control module. This ensures the PID control module accurately controls electrical current supplied to the fan motor. Additionally, in certain embodiments, the fan-speed sensor error elimination process ensures the controller doesn&#39;t send measured fan speeds based on ΔT&#39;s to the PID control module until the measured fan speed is within a designated range of the desired fan speed. This prevents unnecessarily employing the PID control module. 
       FIG. 21  is a flowchart of one example embodiment of the fan-speed sensor error elimination process  9100  of the present disclosure. In various embodiments, a set of instructions stored in one or more memories and executed by the controller represent the fan-speed sensor error elimination process  9100 . Although the fan-speed sensor error elimination process  9100  is described with reference to the flowchart shown in  FIG. 21 , many other processes of performing the acts associated with this illustrated fan-speed sensor error elimination process  9100  may be employed. For example, the order of certain of the illustrated blocks or diamonds may be changed, certain of the illustrated blocks or diamonds may be optional, or certain of the illustrated blocks or diamonds may not be employed. 
     The fan-speed sensor error elimination process  9100  starts when the air filtration device begins operation at a desired fan speed. The controller starts a free-running timer, as block  9110  indicates. The controller monitors for a trip of the fan-speed sensor following the start of the free-running timer, as diamond  9115  indicates. Once the fan-speed sensor is tripped, the controller reads the free-running timer, as block  9120  indicates. (In certain embodiments, following the first trip of the fan-speed sensor the controller resets the timer but does not proceed to block  9120  until another trip of the fan-speed sensor occurs.) The free-running-timer reading is ΔT. 
     The controller then determines whether ΔT is less than ΔT MIN , as diamond  9125  indicates. ΔT MIN  is a set value that is less than the time it takes the fan blade to complete a single revolution at the maximum fan speed setting. If at diamond  9125  the controller determines that ΔT is less than ΔT MIN , the controller does not input a measured fan speed determined based on ΔT to the PID control module, as block  9130  indicates. The process  9100  then returns to diamond  9115 . In this scenario in which ΔT is less than ΔT MIN , the fan-speed sensor has tripped before the fan blade has completed a full revolution following the previous fan-speed sensor trip. The controller  3650  is thus configured to filter out these small ΔT&#39;s and not use them to calculate measured fan speeds to send to the PID control module. 
     If, on the other hand, the controller determines at diamond  9125  that ΔT is greater than or equal to ΔT MIN , the controller determines whether ΔT is greater than ΔT MAX , as diamond  9135  indicates. ΔT MAX  is a set value that is greater than the time it takes the fan blade to complete a single revolution at the minimum fan speed setting. If at diamond  9135  the controller determines that ΔT is greater than ΔT MAX , the controller resets the free-running timer, as block  9140  indicates, but does not does not input a measured fan speed determined based on ΔT to the PID control module, as block  9130  indicates. The process  9100  then returns to diamond  9115 . In this instance, either: (1) the controller is still running up the fan to the desired fan speed; or (2) the fan-speed sensor did not trip following a full revolution of the fan blade. In either case, the controller prevents unnecessary invocation of the PID control module. The controller  3650  is thus configured to filter out these large ΔT&#39;s and not use them to calculate measured fan speeds to send to the PID control module. 
     If, on the other hand, the controller determines at diamond  9135  that ΔT is less than or equal ΔT MAX , the controller determines the measured fan speed based on ΔT, as block  9145  indicates, and inputs the measured fan speed to the PID control module, as block  9150  indicates. The PID control module compares the measured fan speed to the desired fan speed to determine whether to vary the amount of electrical current sent to the fan motor. The controller then resets the free-running timer, as block  9155  indicates, and the process  9100  then returns to diamond  9115 . The process  9100  ends when the fan motor is powered off. 
       FIGS. 20A-20L  show an example scenario.  FIG. 20A  shows the fan blade  2312  at time T 0  when a user powers the air filtration device on and sets a desired fan speed. At this point, the fan motor begins rotating the fan blade and the controller starts the free-running timer. The controller monitors for a trip of the fan-speed sensor following the start of the free-running timer. 
       FIG. 20B  shows the fan blade  2312  at free-running-timer reading T 1  when the leading edge  2314   a  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . In this embodiment, following the first trip of the fan-speed sensor the controller resets the timer but takes no further action until another trip of the fan-speed sensor occurs. 
       FIG. 20C  shows the fan blade  2312  at free-running-timer reading T 2  when the trailing edge  2314   b  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&lt;ΔT MIN . This means that this sensor trip occurred before the fan blade has completed a full revolution since the previous fan-speed sensor trip. Accordingly, the controller does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
       FIG. 20D  shows the fan blade  2312  at free-running-timer reading T 3  when the leading edge  2314   a  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&gt;ΔT MIN  and that ΔT&gt;ΔT MAX . This means that, at this point, the fan speed is outside of the designated range of the desired fan speed. Accordingly, the controller resets the free-running timer but does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
       FIG. 20E  shows the fan blade  2312  at free-running-timer reading T 4  when the trailing edge  2314   b  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&lt;ΔT MIN . This means that this sensor trip occurred before the fan blade has completed a full revolution since the previous fan-speed sensor trip. Accordingly, the controller does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
       FIG. 20F  shows the fan blade  2312  at free-running-timer reading T 5  when the leading edge  2314   a  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&gt;ΔT MIN  and that ΔT&gt;ΔT MAX . This means that, at this point, the fan speed is outside of the designated range of the desired fan speed. Accordingly, the controller resets the free-running timer but does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
       FIG. 20G  shows the fan blade  2312  at free-running-timer reading T 6  when the trailing edge  2314   b  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&lt;ΔT MIN . This means that this sensor trip occurred before the fan blade has completed a full revolution since the previous fan-speed sensor trip. Accordingly, the controller does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
       FIG. 20H  shows the fan blade  2312  at free-running-timer reading T 7  when the leading edge  2314   a  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&gt;ΔT MIN  and that ΔT&lt;ΔT MAX . This means that, at this point, the fan speed is within the designated range of the desired fan speed. Accordingly, the controller determines the measured fan speed based on ΔT, inputs the measured fan speed to the PID control module, and resets the free-running timer. 
       FIG. 20I  shows the fan blade  2312  at free-running-timer reading T 8  when the trailing edge  2314   b  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&lt;ΔT MIN . This means that this sensor trip occurred before the fan blade has completed a full revolution since the previous fan-speed sensor trip. Accordingly, the controller does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
       FIGS. 20J and 20K  show the fan blade  2312  at free-running-timer readings T 9  and T 10  when the leading and trailing edges  2314   a  and  2314   b  of the marker  2314  on the fan blade  2312  respectively rotate past—but do not trip—the fan-speed sensor  3950 . Debris  2316  blocks the fan-speed sensor  3950  in this scenario. 
       FIG. 20L  shows the fan blade  2312  at free-running-timer reading T 11  (after the debris  2316  has been cleared) when the leading edge  2314   a  of the marker  2314  on the fan blade  2312  trips the fan-speed sensor  3950 . The controller reads the free-running timer to determine ΔT. The controller determines that ΔT&gt;ΔT MIN  and that ΔT&gt;ΔT MAX . This means that, at this point, the fan speed is outside of the designated range of the desired fan speed. Accordingly, the controller resets the free-running timer but does not input a measured fan speed determined based on the free-running-timer reading ΔT to the PID control module. 
     This fan-speed sensor error elimination routine solves the three above-described problems that occur when assuming an ideal scenario. 
     First, ignoring ΔT when ΔT&lt;ΔT MIN  ensures the controller will not send impossibly large measured fan speeds (calculated using impossibly small ΔT&#39;s) to the PID control module. The fan-speed sensor error elimination process thus filters out ΔT&#39;s that are too small to represent the time elapsed during one full revolution of the fan blade. By not sending measured fan speeds calculated using these ΔT&#39;s to the PID control module, the controller prevents the inaccurate, non-ideal fan operation that would otherwise follow. 
     Second, ignoring ΔT when ΔT&gt;ΔT MAX  ensures the controller will not send an unreasonably small measured fan speed (calculated using an unreasonably large ΔT) to the PID control module. The fan-speed sensor error elimination process thus filters out ΔT&#39;s that are too large to represent the time elapsed during one full revolution of the fan blade. By not sending these ΔT&#39;s to the PID control module, the controller prevents the inaccurate, non-ideal fan operation that would otherwise follow. 
     Third, ignoring ΔT when ΔT&gt;ΔT MAX  ensures the controller will not send a measured fan speed (based on ΔT) to the PID control module while the fan is running up to the desired fan speed after a user powers the air filtration device on. The controller therefore prevents unnecessary invocation of the PID control module. 
       FIG. 22  is a flowchart of another example embodiment of the fan-speed sensor error elimination process  9200  of the present disclosure. In various embodiments, a set of instructions stored in one or more memories and executed by the controller represent the fan-speed sensor error elimination process  9200 . Although the fan-speed sensor error elimination process  9200  is described with reference to the flowchart shown in  FIG. 22 , many other processes of performing the acts associated with this illustrated fan-speed sensor error elimination process  9200  may be employed. For example, the order of certain of the illustrated blocks or diamonds may be changed, certain of the illustrated blocks or diamonds may be optional, or certain of the illustrated blocks or diamonds may not be employed. 
     The fan-speed sensor error elimination process  9200  starts when the air filtration device begins operation at a desired fan speed. The controller starts a free-running timer, as block  9210  indicates. At this point, the controller operates two subroutines in parallel: (1) it monitors for a trip of the fan-speed sensor following the start of the free-running timer, as diamond  9215  indicates; and (2) it monitors for the free-running timer reaching a counter-increment threshold, as diamond  9220  indicates. The counter-increment threshold in this example embodiment represents about 1.9 seconds, which is the maximum capacity (or over-run) of the 8-bit free-running timer. In other embodiments, the counter-increment threshold may also be set at a desired threshold beneath the maximum capacity of the free-running timer. For instance, if the free-running timer is a 16-bit or a 32-bit timer, the counter-increment threshold could be set at about 1.9 seconds so the timer signals the controller when it reaches about 1.9 seconds (which is less than the maximum capacity of a 16-bit or a 32-bit timer). 
     Once the fan-speed sensor is tripped, the controller reads the free-running timer, as block  9225  indicates. (In certain embodiments, following the first trip of the fan-speed sensor the controller resets the timer but does not proceed to block  9225  until another trip of the fan-speed sensor occurs.) The free-running-timer reading is ΔT. 
     The controller resets a counter (described below), as block  9230  indicates, and determines whether ΔT is less than ΔT MIN , as diamond  9235  indicates. ΔT MIN  is a set value that is less than the time it takes the fan blade to complete a single revolution at the maximum fan speed setting. If at diamond  9235  the controller determines that ΔT is less than ΔT MIN , the controller does not input a measured fan speed determined based on ΔT to the PID control module, as block  9240  indicates. The process  9200  then returns to diamond  9215 . In this scenario in which ΔT is less than ΔT MIN , the fan-speed sensor has tripped before the fan blade has completed a full revolution following the previous fan-speed sensor trip. The controller  3650  is thus configured to filter out these small ΔT&#39;s and not use them to calculate measured fan speeds to send to the PID control module. 
     If, on the other hand, the controller determines at diamond  9235  that ΔT is greater than or equal to ΔT MIN , the controller determines the measured fan speed based on ΔT, as block  9245  indicates, and inputs the measured fan speed to the PID control module, as block  9250  indicates. The PID control module compares the measured fan speed to the desired fan speed to determine whether to vary the amount of electrical current sent to the fan motor. The controller then resets the free-running timer, as block  9255  indicates, and the process  9100  then returns to diamond  9215 . 
     Returning to diamond  9220 , if the controller determines at diamond  9220  that the free-running timer reached the counter-increment threshold, the controller increments the counter, as block  9260  indicates. The counter starts at zero when the process  9200  begins (though it may start at any suitable number). The controller then determines at diamond  9265  whether that incrementing of the counter caused the counter to reach a first quantity (such as three or any suitable quantity), as diamond  9265  indicates. 
     If the controller determines at diamond  9265  that the incrementing of the counter caused the counter to reach the first quantity (i.e., if the controller determines that a max current condition occurs), the controller inputs a measured fan speed of 0 RPMs (or any other suitable low speed) to the PID control module, as block  9270  indicates. In this scenario, the controller has determined that the fan is either stuck or rotating extremely slowly, and inputting this small fan speed to the PID control module will cause the PID control module to dramatically increase (e.g., maximize or substantially maximize) the electrical current to the fan motor to attempt to free the fan blade. The controller then resets the free-running timer, as block  9275  indicates (or the free-running timer resets itself following overload). 
     If, on the other hand, the controller determines at diamond  9265  that the incrementing of the counter did not cause the counter to reach the first quantity, the controller determines whether that incrementing of the counter caused the counter to reach a second quantity larger than the first quantity (such as six or any suitable quantity), as diamond  9280  indicates. 
     If the controller determines at diamond  9280  that the incrementing of the counter caused the counter to reach the second quantity (i.e., if the controller determines that a shut-down condition occurs), the controller shuts down the fan, as block  9285  indicates. In this scenario, the controller determines that there is a problem with the fan that requires maintenance. If, on the other hand, the controller determines at diamond  9280  that the incrementing of the counter did not cause the overload counter to reach the second quantity, the controller resets the free-running timer, as block  9275  indicates (or the free-running timer resets itself following overload). 
     Ignoring ΔT when ΔT&lt;ΔT MIN  ensures the controller will not send impossibly large measured fan speeds (calculated using impossibly small ΔT&#39;s) to the PID control module. The fan-speed sensor error elimination process thus filters out ΔT&#39;s that are too small to represent the time elapsed during one full revolution of the fan blade. By not sending measured fan speeds calculated using these ΔT&#39;s to the PID control module, the controller prevents the inaccurate, non-ideal fan operation that would otherwise follow. 
     In certain embodiments, ΔT MIN  is equal to the time it takes the fan blade to rotate 30 degrees at the highest available fan speed. 
     In certain embodiments, ΔT MAX  is equal to the time it takes the fan blade to rotate 360 degrees at the lowest available fan speed. 
     In the above-described embodiments, the free-running timer resets in certain scenarios such that each free-running timer reading represents ΔT. In other embodiments, the free-running timer does not overload and runs in perpetuity (until the fan motor is shut down or the air filtration device is powered off). In these embodiments, the controller is configured to store certain free-running timer readings and determine ΔT by calculating the difference between consecutive stored free-running-timer readings. In these embodiments, the controller does not store free-running timer readings that would cause ΔT to be less than ΔT MIN  following a fan speed sensor trip. For instance, if the fan speed sensor trips at T 1  and again at T 2 , the controller would determine ΔT=T 2 −T 1 . If ΔT&lt;ΔT MIN , the controller would not store T 2  and would again monitor for a trip of the fan speed sensor. If ΔT&gt;ΔT MIN , the controller would store T 2 . 
     In another embodiment, the process described with respect to  FIG. 21  also includes the overload subroutine described with respect to  FIG. 22  (i.e., diamond  9220 , block  9260 , diamond  9265 , block  9270 , block  9275 , diamond  9280 , and block  9285 ). 
     4.8 Air Filtration Device Malfunctions 
     In this example embodiment, the air filtration device monitors for a plurality of different major air filtration device malfunctions, such as (but not limited to): (a) a locked fan motor; (b) disconnected differential pressure sensor tubes; (c) disconnected electronic components (e.g., the fan, the operating mode selector, and the like); and (d) an electronics failure (e.g., an hour meter display failure or a pre-filter status indicator failure). In this example embodiment, if the air filtration device determines that one of the major air filtration device malfunctions occurs, the air filtration device: (a) powers down the fan, (b) lights the LED of the air filtration device status indicator red, and (c) outputs the audible major air filtration device malfunction tone. 
     In this example embodiment, the air filtration device also monitors for dust sensor failure. If the air filtration device determines that the dust sensor fails, the air filtration device: (a) enables operation of the air filtration device in any of the manual fan speed setting operating modes; and (b) if the automatic fan speed setting selection operating mode is selected, indicates that a major air filtration device malfunctions occurs, as described above. 
     It should be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present disclosure, and it should be understood that this application is to be limited only by the scope of the appended claims.