Abstract:
An air filter sensor kit and method of using the same for permitting a person to modify the filtering portion of a heating or cooling system that uses an air filter to enable the determination of when the air filter is dirty and needs changing. The air filter sensor kit includes an air filter sensor with a portion shaped for insertion through the air filter and a portion with an indicator for indicating the condition of the air filter. The kit includes an air filter sensor connecting and support member for connecting the air filter sensor to an air filter grill and for supporting the air filter sensor and a cutting tool for cutting a hole in an air filter grill to allow the passage of a portion of the air filter sensor connecting and support member to permit a portion of the air filter sensor to be inserted through the air filter. The air filter sensor kit also includes a cutting tool guide that is adjustable to conform to various configurations of air filter grills.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Heating and cooling systems for houses and other dwellings and for at least some commercial establishments include a system for transmitting heated or cooled air throughout the dwelling or commercial establishment. These systems for transmitting heated or cooled air also include filtration equipment for removing particulate matter from the heated or cooled air and this filtration equipment, particularly for dwellings, normally includes one or more mechanical filters formed from fibrous materials.  
           [0002]    These filter materials serve the function of mechanically blocking particulate matter that is in the air flowing in circuitous paths through the filter material. The particulate matter becomes attached to individual fibers and, over a period of time, progressively restricts the air flow paths. The difference in pressure between the inflow and outflow sides of the air filter increases as a function of the quantity and size of particulate matter trapped and retained by the filter. This trapped particulate matter simultaneously causes a reduction of the air flow rate through the filter.  
           [0003]    This reduced air flow rate reduces the efficiency of the heating or cooling system and the effective heating or cooling of the dwelling or commercial establishment. The partially clogged filter also causes increased back pressure to be applied to the blower or fan which generates the air flow and this back pressure increases the work that must be performed and the energy consumed by the blower or fan unit. The resulting increased load increases the wear rate of the moving parts in the heating or cooling system and also results in increased operating costs. In view of these undesirable effects, it is important that partially clogged or dirty air filters be promptly replaced.  
           [0004]    In order to determine when an air filter needs to be changed, a person normally must gain access to the filter. The filter must then be removed and visually inspected. If it is evident through this visual inspection that there is a significant build up of particulate matter on the outside surface of the air filter it is usually replaced with a new filter. This procedure has various deficiencies. This procedure means that the air filter must be periodically checked in order to determine when the filter needs to be changed. This is time consuming and can often result in dirty filters not being changed on time due to the failure to remember to check the filter. Also, the mere visual inspection of the air filter does not in many cases result in an accurate determination if the filter needs to be replaced. In this connection, one person may believe the filter is dirty but the next person may believe it is fine. The mere visual inspection of the surface of the filter is not that reliable in determining the condition of the filter because visual surface contamination or the lack of visual surface contamination may not be representative of contamination plugging microscopic flow paths inside the filter material.  
           [0005]    In view of these problems, various devices have been developed over the years to determine when an air filter is dirty and needs replacement and to provide an indication of the need for replacement of an air filter in an heating and air conditioning system. Examples of such devices are set forth in U.S. Pat. Nos. 2,753,831 and 4,321,070 that describe a device with a tube which extends through an air filter and incorporates a whistle. In these devices, air flows continuously through a tube and as the air flow through the tube increases as a result of increasing clogging of the surrounding air filter, the whistle generates a sound when the air flow rate is of a sufficient magnitude. These devices have potential problems since contamination and clogging of the tube may occur and may have a negative effect upon the operation of the whistle. U.S. Pat. No. 2,804,839 discloses a device for providing a visual and audible signal indicative of the clogging of an air filter. The device uses a magnet for retaining a pivotable member in place that provides a visually perceivable indication and actuates further structure capable of energizing an audible alarm. All of these devices are difficult to install and require at least some support from the air filter itself which is undesirable since it can distort and damage the air filter.  
           [0006]    These problems are overcome with the present air filter sensor apparatus kit that permits an air filter sensor to be easily installed to eliminate the need for actual inspection of the filter to determine when it is becoming clogged and needs to be replaced. This means that clogged or dirty air filters are more likely to be replaced on time. Moreover, the air filter sensor is supported by the air filter grill and not by the air filter and hence possible air filter damage is eliminated.  
         SUMMARY OF THE INVENTION  
         [0007]    This invention relates to air flow filters and more particularly to sensors for air flow filters.  
           [0008]    Accordingly, it is an object of the invention to provide an air filter sensor apparatus kit and method that eliminate the need to periodically visually check heating and cooling air filters.  
           [0009]    It is an object of the invention to provide an air filter sensor apparatus kit and method that can be used with a wide variety of types and sizes of heating and cooling air filters.  
           [0010]    It is an object of the invention to provide an air filter sensor apparatus kit and method that provide an air filter status indicator in a visible location.  
           [0011]    It is an object of the invention to provide an air filter sensor apparatus kit and method that provide an air filter status indicator that is located in plain sight of the occupants of a dwelling.  
           [0012]    It is an object of the invention to provide an air filter sensor apparatus kit and method that do not use the air filter to support a sensor.  
           [0013]    It is an object of the invention to provide an air filter sensor apparatus kit and method that use an existing air duct grill to support a sensor.  
           [0014]    It is an object of the invention to provide an air filter sensor apparatus kit and method that can be used with a variety of air duct grills.  
           [0015]    It is an object of the invention to provide an air filter sensor apparatus kit and method that has an air duct grill drilling guide that can be used with different shaped air duct grills.  
           [0016]    It is an object of the invention to provide an air filter sensor apparatus kit and method that has an air duct grill drilling guide that is adapted to conform to the shape of various air duct grills.  
           [0017]    It is an object of the invention to provide an air filter sensor apparatus kit that includes all items that are required for installation.  
           [0018]    It is an object of the invention to provide an air filter sensor apparatus kit that permits a dwelling occupant to easily install the air filter sensor apparatus.  
           [0019]    It is an object of the invention to provide an air filter sensor apparatus kit and method that do not require the removal of the air filter.  
           [0020]    It is an object of the invention to provide an air filter sensor apparatus kit and method that do not require the removal of any structure associated with the air filter.  
           [0021]    It is an object of the invention to provide an air filter sensor apparatus kit and method that permit the easy removal of the air filter sensor apparatus.  
           [0022]    It is an object of the invention to provide an air filter sensor apparatus kit and method that permit the easy removal of the air filter sensor apparatus without removing the air filter.  
           [0023]    It is an object of the invention to provide an air filter sensor apparatus kit and method that permit the easy removal of the air filter sensor apparatus without removing any structure associated with the air filter.  
           [0024]    It is an object of the invention to provide an air filter sensor apparatus kit and method that are specifically designed for easy use by a home owner.  
           [0025]    It is an object of the invention to provide an air filter sensor apparatus kit that is simple in its operation.  
           [0026]    It is an object of the invention to provide an air filter sensor apparatus kit that is reliable in its operation.  
           [0027]    It is an object of the invention to provide an air filter sensor apparatus kit that is easy to manufacture.  
           [0028]    It is an object of the invention to provide an air filter sensor apparatus kit that normally requires no service.  
           [0029]    These and other objects of the invention will become apparent from the following described air filter sensor apparatus kit and method that includes an air filter sensor for sensing differential air pressure caused by the air filter. The air filter sensor or probe is a generally elongated cylindrical member that is sized and shaped with a pointed end portion to be easily inserted through an air filter. The air filter sensor has an indicator for providing an indication of the differential air pressure sensed by the air filter sensor. The air filter sensor apparatus kit also includes air filter sensor connecting and support means for connecting and supporting the indicator end of the air filter sensor in the air filter grill. The air filter sensor apparatus kit also includes a cutting tool and a cutting tool guide and positioner for cutting a hole in an air filter grill to permit the passage of a portion of the air filter sensor connecting and support means and a portion of the air filter sensor through the grill.  
           [0030]    The method includes providing an air filter, an air filter sensor with an indicator, an air filter grill cutting tool and a cutting tool guide and positioner and connecting and support means for connecting and supporting the air filter sensor in an air filter grill. The method also includes cutting a hole in the air filter grill using the air filter grill cutting tool and the cutting tool guide and positioner, inserting a portion of the connecting and support means and the air filter sensor into the hole that has been cut in the air filter grill with a portion of the air filter sensor being inserted through the air filter. The method also includes monitoring the indicator periodically and changing the air filter when the indicator provides an indication that the air filter needs to be changed. During the step of changing the air filter, the air filter sensor is removed from the air filter that is dirty and is being changed and is inserted into the new clean replacement air filter. During the step of changing the dirty air filter, the grill will normally be removed as well as the connected sensor. Then the grill will be replaced and the air filter sensor inserted into the air filter grill. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The invention will be hereinafter more fully described with reference to the accompanying drawings in which:  
         [0032]    [0032]FIG. 1 is a side elevational view of the air filter sensor apparatus kit invention;  
         [0033]    [0033]FIG. 2 is a front elevational view of a cutting tool and cutting tool guide and positioner that forms part of the air filter sensor apparatus kit set forth in FIG. 1;  
         [0034]    [0034]FIG. 3 is an end elevational view of an air filter sensor that forms part of the sensor apparatus kit set forth in FIG. 1;  
         [0035]    [0035]FIG. 4 is an end elevational view of an air filter sensor connecting and support member that forms part of the sensor apparatus kit set forth in FIG. 1;  
         [0036]    [0036]FIG. 5 is a side elevational view of the air filter sensor of the air filter sensor apparatus kit invention installed in an air filter grill and an air filter with certain portions thereof broken away;  
         [0037]    [0037]FIG. 6 is an enlarged view of the end elevational view of an air filter sensor illustrated in FIG. 3 with certain portions thereof broken away for clarity;  
         [0038]    [0038]FIG. 7 is a sectional view of an air filter sensor structure illustrated in FIG. 6 taken on the line  7 - 7  thereof; and  
         [0039]    [0039]FIG. 8 is an enlarged side elevational view of a portion of the structure illustrated in FIG. 7 taken in the direction of the line  8 - 8  thereof. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0040]    Referring first to FIG. 1, the air filter sensor apparatus kit is illustrated and is designated generally by the number  10 . The air filter sensor apparatus kit  10  comprises an air filter sensor  12 , an air filter sensor connecting and support member  14 , an air filter grill cutting tool  16 , and an air filter grill cutting tool guide  18 .  
         [0041]    The air filter sensor  12  has an elongated hollow slender tubular portion  20  with a sharp point  22  on one end and an enlarged circular disk shaped indicator head portion  24  on the other end. The elongated hollow slender tubular portion  20  has a vent hole  26  in its side wall near the pointed end  22  and another atmospheric reference vent hole  28  is located in the side of the enlarged indicator head portion  24 . The air filter sensor connecting and support member  14  comprises a cylindrical support means comprises a resilient flexible member having a circular cross section insert portion  30  having a centrally located circular cross section hole  32  that is sized and shaped to receive and lightly grip the slender tubular portion  20  of the air filter sensor  12 . An enlarged disk shaped stop portion  34  is located at one end of the cylindrical insert portion  30 .  
         [0042]    As illustrated in FIGS. 1 and 2, the air filter grill cutting tool  16  comprises an elongated circular cross section cylindrical hard plastic member  36  that has a slot  38  in its inner end portion  40  that is sized and shaped to receive and hold a flat tool steel cutting head  42  that has an outward extending point  44 .  
         [0043]    Also, as illustrated in FIGS. 1 and 2 the air filter tool cutting tool guide  18  comprises an elongated rectangular shaped guide member  46  that has a circular shaped guide hole  48  through it along its long axis A that is sized and shaped to slidably receive the cutting tool  16  with its elongated member  36  and its cutting head  42 . The tool cutting guide  18  also has two spaced apart elongated rectangular thin positioning members  50  and  52  whose respective end portions  54  and  56  are rigidly secured to the respective side portions  58  and  60  of the guide block  46 . The portions  61  and  63  of the positioning members  50  and  52  that are not attached to the guide member  46  are located parallel to each other and form a gap to receive the respective end portions  62  and  64  of identical generally rectangular guide members  66  and  68 .  
         [0044]    Each positioning member  50  and  52  has two elongated respective slots  70  and  72  and  74  and  76  located in their portions  61  and  63 . These elongated slots  70  and  72  and  74  and  76  receive portions of the respective identical bolts  78  and  80  that pass through and are secured to the end portions  62  and  64  of the respective guide members  66  and  68 . These bolts  78  and  80  have respective enlarged head portions  82  and  84  at one end and respective threaded portions  86  and  88  at the other end that are threaded to receive identical wing nuts  90  and  92  that are used to clamp the guide members  66  and  68  between the positioning members  50  and  52 .  
         [0045]    It will be noted, particularly in FIG. 1, that the guide members  66  and  68  each have pointed end portions  94  and  96  that have respective forward sloping surfaces  98  and  100 . These pointed end portions  94  and  96  and their associated surfaces  98  and  100  are placed against the louvers  102  of an air filter grill  104  that is illustrated in FIG. 5 to position the air filter grill cutting tool guide  18 . Since the spacing or distance D between the louvers  102  may be different with different air filter grills  104  and since the angle S of the outer surface of the louvers  102  may be different, the air filter grill cutting tool guide  18  has means for adjusting to these differences in spacing D and the difference in the outer surface angles S of the louvers  102 .  
         [0046]    This means for adjusting to the differences in spacing D of the louvers  102  and the differences in the outer surface angles S of the louvers comprises the bolts  78  and  80 , the elongated slots  70  and  72 , and  74  and  76 , and the wing nuts  90  and  92  as well as the movable guide members  66  and  68  and their sloping surfaces  98  and  100 . In this connection, as illustrated in FIG. 1, by loosening the wing nuts  90  and  92  the guide members  66  and  68  can be spread apart to change the distance D 1  between the guide members  66  and  68  as illustrated in dashed lines in FIG. 1 where the distance has been changed from D 1  to a greater distance D 2 . As also illustrated in FIG. 1, the angle that the surface surface  98  or  100  makes with the long axis A of the cutting tool guide hole  48  can be changed from B 1  to B 2  as illustrated by the surface  100  in dashed lines. This is possible since the guide members  66  and  68  can be rotated by loosening the wing nuts  90  and  92  as well as spreading them apart. When the guide members are in the desired position for the particular air filter grill  104  and its louvers  102  the wing nuts  90  and  92  would be tightened by hand.  
         [0047]    If it is desired due to space limitations or due to a particular air filter grill  104  configuration, the guide members  66  and  68  can be inverted, so that the sloping surfaces  98  and  100  face upward as viewed in FIG. 1, by removing the wing nuts  90  and  92  and removing the associated bolts  78  and  80 . The guide members  66  and  68  are then removed from between the positioning members  50  and  52  and then inverted. The bolts  78  and  80  are then reinserted through the holes  79  and  81  in the guide members  66  and  68  (see FIG. 2) and through the appropriate slots  70  and  72  in the positioning members  50  and  52  and the wing nuts  90  and  92  are replaced and tightened.  
         [0048]    Additional flexibility can also be incorporated into the guide members  66  and  68  by providing the holes  97  and  99  illustrated in FIG. 1 in the opposite end of the guide members  66  and  68  from the holes  79  and  81  that normally receive the respective bolts  78  and  80 . This will allow the guide members  66  and  68  to be flipped from their positions illustrated in FIG. 1 so that the respective blunt ends  101  and  103  of the guide members  66  an  68  can be pressed against the louvers  102  of the air filter grill  104 . These holes  79  and  81  also allow the guide members  66  an  68 , after the bolts  78  and  80  have been removed, to be rotated 180 degrees from their positions illustrated in FIG. 1 and then moved laterally so that the bolts  78  and  80  can be inserted into the holes  79  and  81  and the respective slots  70  and  72  in the positioning members  50  and  52 . Then the wing nuts  90  and  92  would be replaced and tightened. This results in shorter pointed end portions  94  and  96  extending from the positioning members  50  and  52 . In this configuration, the cutting tool  16  would be inserted into the opposite end of the guide member  46  from that illustrated in FIG. 1. This alternative configuration could allow easier access to the louvers  102  of the air filter grill  104  in certain confined situations.  
         [0049]    As illustrated in FIG. 3, the outer surface  105  of the enlarged head portion  24  of the air filter sensor  12  has a visual indicator arm  106  that indicates when an air filter is satisfactory or when it is dirty or not good, and needs to be replaced. FIG. 4 also illustrates the centrally located circular cross section hole  32  that is present in the air filter sensor connecting and support member  14  for receiving the slender tubular portion  20  of the air filter sensor  12 .  
         [0050]    [0050]FIG. 5 illustrates how the air filter sensor  12  is installed so that it can sense the condition of an air filter that is designated by the number  108 . As illustrated, the air filter  108  is located in a conventional manner adjacent to and in back of or behind an air filter grill  104  and its louvers  102 . Some of these adjacently located louvers  102  have a circular shaped cross section hole  110  cut through them that is cut through the use of the previously discussed air filter grill cutting tool  16  and the associated air filter grill cutting tool guide  18 . The air filter sensor connecting and support member  14  is located in the air filter grill  104  with its cylindrical circular cross section insert portion  30  being pressed into the hole  110  in the louvers  102  until its stop portion  34  rests upon the outside surface  112  of the louvers  102  adjacent to the hole  110 .  
         [0051]    As illustrated in FIG. 5, the slender tubular portion  20  of the air filter sensor  12  is inserted into the hole  32  in the air filter sensor connecting and support member  14  and the pointed end  22  is pushed through the air filter  108  so that the hole  26  in the tubular portion  20  is located on the opposite side of the air filter  108  then the air filter grill  104  and the enlarged head portion  24  rests upon the disk shaped stop portion  34  of the air filter sensor connecting and support member  14 . As illustrated, in this position, the hole  28  in the enlarged head  24  of the air filter sensor  12  is located outside of the air filter grill and hence is exposed to the ambient or atmospheric air pressure. Portions of the tubular portion  20  of the air filter sensor  12  are broken away and illustrate that the tubular portion  20  is hollow and has a chamber  114  extending through most of its length.  
         [0052]    [0052]FIGS. 6, 7 and  8  illustrate in detail the head portion  24  of the air filter sensor  12  and its pressure sensing means  116 . As illustrated, the pressure sensing means  116  has a circular flat hollow housing  118  that forms part of the enlarged indicator portion  24 . The flat rear housing wall  120  has an aperture  122  that is in fluid communication with the aperture or chamber  114  located in the elongated hollow slender tubular portion  20 . The pressure sensing means  116  includes a hollow cylindrical shaped flexible bellows  124  that extends outward from the outer face or surface  126  of the flat rear wall  120  of the housing  118 . A ramp member  128  that is part of the pressure sensing means  116  is secured to the outer surface  130  of the bellows  24 .  
         [0053]    A circular cross section projection  132  projects outward at a right angle from the outer surface  126  of the rear wall  120  of the housing  118 . The inner end portion  134  of the elongated indicator arm  106  that is part of the pressure sensing means  116  is rotatably secured to the outer end portion  138  of the projection  132  by the shoulder screw  140 . As indicated, the elongated indicator arm  106  is sufficiently long so that its outer end portion  142  can contact the sloping surface  144  of the ramp member  128  (see FIG. 8). A circular shaped flat faceplate  146  covers a substantial portion of the front portion of the housing  118 .  
         [0054]    As illustrated in FIG. 6, the indicator arm  106  is biased in a downward direction by a spring  150  that has one end portion  152  connected to a hole  154  in the inner end portion  134  of the indicator arm  106  and the other end portion  156  of the spring  150  is connected to a hole  158  in the faceplate  146 . As indicated in FIG. 6, this spring  150  exerts a downward biasing force on the indicator arm  106  that is represented by the letter FS. The faceplate  146  has an OK or good indicator  160  and a NG or no good indicator  162  on its front surface. The housing  118  also has the atmosphere reference vent  28  in it. The arrows B in FIG. 7 indicate that the bellows  124  moves inward or outward in response to the differential in the pressure in the chamber  166  inside the bellows  124  and the chamber  168  formed by the housing  118  outside of the bellows  124 .  
         [0055]    To use the pressure sensing means  116 , it is armed or set by moving the indicator arm  106  so that the outer end portion  142  is located on the OK position on the faceplate  146  as indicated in FIG. 6. In doing this, the indicator arm  106  is caused to slide up the inclined sloping surface  144  of the ramp member  128  stretching the spring  150  in the process until the indicator arm  106  clicks or moves into position on the upper flat side  170  of the ramp member  128  which holds the indicator arm  106  and prevents it from moving due to the spring  150  force FS. Then, as the blower runs, in time due to the filter  108  getting dirty, the air pressure on the inside of the bellows  124  becomes lower than the air pressure outside the bellows  124 . This causes the bellows  124  to deflect toward the low pressure pressure or to collapse inward toward the surface  126 . When the differential pressure becomes large enough and the bellows  124  deflates or collapses sufficiently, the indicator arm  106  slips past the upper flat side  170  of the ramp member  128  that is connected to the flat outer surface  130  of the bellows and the indicator arm  106  snaps to the NG position on the faceplate  146  due to the force FS of the spring  150 .  
         [0056]    As indicated in FIG. 5, when the pressure sensor  116  is installed it is exposed to both the ambient pressure through the hole  28  and the air pressure beyond or down stream from the filter  108  through the hole  26  near the point of the tubular portion  20 . It will be noted that air flows in the direction of the arrow F and hence from the ambient air outside of the air filter grill  104  through the louvers  102  and then through the air filter  108 . Consequently, when the air filter gets clogged with dirt and the like it restricts the passage of air and this results in the air pressure at the hole  26  being less than the air pressure at the hole  28  in the head portion  24  of the air filter sensor  12 . This difference in pressure is sensed by the pressure sensing means  116  and causes the pressure sensing means  116  to operate in the previously indicated manner.  
         [0057]    The air filter sensor apparatus kit  10  is used and method of the invention is practiced in the following manner. In order to use the invention initially, it is necessary to make a hole in the louvers  102  of the air filter grill  104 . This is conveniently accomplished using the air filter sensor apparatus kit  10  that includes the previously described tool guide  18  and the air filter grill cutting tool  16 . The user of the air filter sensor apparatus kit  10  selects a convenient location on the grill  104  and adjusts the tool guide  18  in the previously indicated manner so that it properly fits the louvers  102 . Then the air filter grill cutting tool  16  is inserted into the guide hole  48  after it has been properly chucked into a standard hand drill (not shown). Then a hole  110  is drilled through the louver or louvers  102  with the cutting tool  16 . Then the air filter sensor connecting and support member  14  is manually inserted into the resulting hole  110  in the louvers  102  of the air filter grill  104  in the previously indicated manner. Normally, the dirty air filter  108  is then removed if this had not been done previously and a new clean filter  108  installed in a conventional manner. Then the air filter grill  104 , without the air filter sensor  12  but with the air filter sensor connecting and support member  14 , is replaced in a conventional manner. Then the air filter sensor  12  is inserted into the air filter sensor connecting and support member  14  in the air filter grill  104  and in doing this the tubular portion  20  of the air filter sensor  12  is inserted into and through the new air filter  108 .  
         [0058]    After this, prior to operating the system (not shown) that uses the air filter  108 , the user manually sets or arms or activates the air filter sensor  12  by setting the indicator arm  106  in the manner previously indicated in connection with FIGS. 6 through 8 by moving it upward along the sloping surface  144  of the ramp member  128  until it clicks into position on the upper flat side  170  of the ramp member  128  which holds it from moving due to the spring  150  force FS. Then, the system that uses the air filter  108  is operated and in time the air filter  108  will become clogged with dust and other particulate matter and this will cause a drop in the air pressure on the down wind side of the air filter  108 . As previously indicated, in connection with FIGS. 6, 8 and  8 , this drop in air pressure causes a drop in air pressure within the chamber  166  inside of the bellows  124 .  
         [0059]    When the pressure drop in the chamber  166  is sufficiently large due to a dirty air filter  108 , the bellows  124  will collapse sufficiently so that its movable outer surface  130  and the connected ramp member  128  pulls away from the outer end portion  142  of the indicator arm  106  so that the outer end portion  142  of the indicator arm  106  is no longer held in place by the upper flat side  170  of the ramp member  128 . When this occurs, the spring  150  pulls the indicator arm  106  downward so that its outer end portion  142  points to the NG or no good indicator  162  on the faceplate  146 .  
         [0060]    An important feature of the invention is that once the indicator arm  106  is in the position where its outer end portion  142  points to NG or the no good indicator  162  on the faceplate  146 , it is maintained in that position by the force FS of the spring  150  until it is manually reset or armed. Consequently, the NG indication is maintained for visual observation even if the differential air pressure acting on the air filter sensor  12  changes or even becomes zero when the system is shut down or if the fan that causes air to pass through the filter  108  is turned off. Therefor, the user is always able to observe that the filter is not in good condition even if there is no power to the system that uses the air filter  108 .  
         [0061]    In order to properly use the sensor  12  of the invention, periodically the user will visually monitor the location of the outer end portion  142  of the indicator arm  106 , and when the outer end portion  142  points to the NG indicator  162  on the faceplate  146 , the user will know that the air filter  108  should be changed. When the outer end portion  142  of the indicator arm  106  is on NG, the user who notices that the outer end portion  142  of the indicator arm  106  has moved to this position first removes the air filter sensor  12  and then the air filter grill  104  with the connected the air filter sensor connector and support member  14  that is located in the hole in the air filter grill  104  in a conventional manner to expose the old air filter  108 . The old air filter  108  is then manually removed and replaced with a new air filter  108 . The air filter grill  104  along with the connected the air filter sensor connecting and support member  14  is then reinstalled. The user then manually inserts the air filter sensor  12  into the air filter sensor connector and support member  14  which results in the tubular portion  20  of the air filter sensor  12  with its sharp point  22  piercing the new air filter  108 .  
         [0062]    The user then manually arms or resets the indicator arm  106  in the manner previously indicated in connection with FIGS. 6 through 8 by moving it upward along the sloping surface  144  of the ramp member  128  until it clicks into position on the upper flat side  170  of the ramp member  128  which holds it from moving due to the spring  150  force FS. Then the system that uses the air filter  108  is restarted and operated in a conventional manner. This procedure associated with replacing the air filter  108  is then repeated each time that the user sees that the outer end portion  142  of the indicator arm  106  is on NG which indicates that the air filter sensor  12  has determined that the air filter  108  is dirty and needs to be replaced.  
         [0063]    In view of the foregoing, the method of the invention for determining the need to change an air filter  108  and for changing the air filter  108  located behind an air filter grill  104  includes the following steps: providing a cutting tool  16 , an air filter sensor  12  and an air filter sensor connecting and support member  14 ; using the cutting tool  16  to cut a hole  110  in the air filter grill  104  for a portion of the air filter sensor connecting and support member  14 ; inserting a portion of the air filter sensor connecting and support member  14  in the hole  110  in the air filter grill  104 ; locating an air filter  108  in place behind the air filter grill  104 ; inserting the air filter sensor  12  through the hole  32  in the air filter sensor connecting and support member  14  in the air filter grill  104  and through the air filter  108 ; arming or activating the air filter sensor  12 ; exposing the air filter  108  to particulate matter while operating the system using the air filter  108 ; monitoring the air filter sensor  12  for an indication from the air filter sensor  12  when the air filter  108  is dirty and needs to be changed; and changing the air filter  108  when the air filter sensor  12  provides an indication when air filter  108  is dirty.  
         [0064]    The method of the invention for determining the need to change an air filter and for changing the air filter  108  located behind an air filter grill  104  can also include providing a cutting tool guide  46  in addition to the cutting tool  16  and using the cutting tool guide  46  to orient the cutting tool  16  with respect to the air filter grill  104  during the step of using the cutting tool  16  to cut a hole  110  in the air filter grill  104  for a portion of the air filter sensor connecting and support member  14 .  
         [0065]    The method of the invention for determining the need to change an air filter and for changing the air filter  108  located behind an air filter grill  104  also includes manually resetting the air filter sensor  12  after the air filter sensor  12  has given an indication that the air filter  108  is dirty and the dirty air filter  108  has been replaced with a clean air filter  108 .  
         [0066]    The method of the invention for determining the need to change an air filter and for changing the air filter  108  located behind an air filter grill  104  also includes removing the elongated hollow slender tubular portion  20  of the air filter sensor  12  from the hole  32  in the air filter connecting and support member  14  for servicing or replacement of the air filter sensor  12  whenever the air filter sensor  12  becomes defective.  
         [0067]    Although the invention has been described in considerable detail with reference to a certain preferred embodiment, it will be understood that variations or modifications may be made within the spirit and scope of the invention as defined in the appended claims.