Abstract:
An apparatus for measuring the differential pressure of fluid in filter. The apparatus comprises a housing defining a pressure chamber. A differential pressure gauge divides the pressure chamber into first and second fluid chambers. The differential pressure gauge is arranged to measure a differential pressure between fluid in the first chamber and fluid in the second chamber. The differential pressure gauge has a variable output.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to filters, and more particularly, to a differential pressure gauge for filters.  
       BACKGROUND  
       [0002]     Filters are commonly used in many different applications to ensure that a liquid meets a certain standard of purity or cleanliness. In one example, fluids such as gasoline and hydraulic fluid is filtered to ensure that there are no particles in the fluid that might damage an engine or a pump. In another example, gas such as an exhaust is filtered to minimize the pollution generated by an engine. The applications in which such filters are used are endless and include automobiles, tractors, farm equipment, construction equipment, and machinery.  
         [0003]     In a typical application, fluid flows through a filter, which removes foreign matter or particles from the fluid. These filtered particles accumulate in the filter element. As these particles accumulate, the filter element becomes plugged or clogged and loses its effectiveness. As result, the amount of foreign matter in the fluid that escapes through the filter will increase to a dangerous level and may damage the equipment that uses the fluid or may allow an unacceptable level of contaminates to escape into the atmosphere.  
         [0004]     In the past, one structure that has been used to monitor the effectiveness of the filter is a pressure differential switch. Such a switch monitors the fluid pressure on both sides of the filter element As the filter element becomes plugged, the pressure differential across the filter increases. Accordingly, the pressure differential switch is calibrated to close when the pressure differential rises above a predetermined level. The switch can then cause some event to occur such as activating a warning alarm or a warning light One example of such a pressure differential switch is disclosed in U.S. Pat. No. 4,480,160, which is entitled Differential Pressure Switch and issued on Oct. 30, 1984, the disclosure of which is hereby incorporated by reference.  
         [0005]     One difficulty with these preexisting pressure differential switches is that they typically have only a single output They are either opened or closed, and their state or output changes only when the pressure differential across the filter element crosses single predetermined threshold value. As a result, the preexisting pressure switches can provide only limited information For example, they cannot provide both a warning signal that indicates a filter element is at the lower limits of acceptable performance and also a warning signal that indicates when the filter has failed. Thus the switch must be set to either provide an operator with a warning signal that the filter is reaching is failure point or a warning that the filter has actually failed. In the first scenario, the operator does not have any warning that the filter has failed. In the second scenario, the operator does not have warning the filter is reaching its limit of acceptable performance and thus does not have any warning to perform preventative maintenance until after the filter actually fails and exposes equipment to damage.  
         [0006]     Another difficulty is that the output or warning signal of typical pressure differential switches for filters is mechanical. The switch merely provides a visual indicator for an operator when it is tripped. The filter does not provide electronic accumulation of information, which can be used for a variety of useful purposes. For example, such information would enable a computer to control operation of the equipment utilizing the filter and prevent damage if a filter fails. In another example, such information could be used to monitor filter maintenance for warranty purposes.  
       SUMMARY  
       [0007]     The current disclosure provides techniques that can be applied to a pressure differential sensor that has a variable output. In other words, the sensor output includes one or more signals that convey information regarding a pressure differential measurement across a filter element. The output can be communicated to a variety of devices such as visual indicators or a computer.  
         [0008]     One aspect of the present invention is directed to an apparatus for measuring the differential pressure of fluid in filter. The apparatus comprises a housing defining a pressure chamber. A differential pressure gauge divides the pressure chamber into first and second chambers. The differential pressure gauge is arranged to measure a differential pressure between the first chamber and the second chamber. The differential pressure gauge has a variable output.  
         [0009]     Another aspect of the present invention is also directed to an apparatus for measuring the differential pressure of fluid in filter. The apparatus comprises a housing defining a pressure chamber. A gauge member divides the chamber into first and second chambers. A sensor has an electrical output, the sensor is arranged to detect the gauge member and output an electric signal in response to detection of the gauge member.  
         [0010]     Yet another aspect of the present invention is directed to a method of measuring differential pressure in a filter head The method comprises inputting fluid into first and second chambers; creating the differential fluid pressure between the fluid in the first chamber and fluid in the second chamber, and outputing a variable output in response to creation of the differential fluid pressure, the variable output indicative of at least two predetermined differential pressures. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates an operating environment for a filter assembly embodying the present invention.  
         [0012]      FIG. 2  is a partially exploded perspective view of a filter assembly embodying the present invention.  
         [0013]      FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2 .  
         [0014]      FIG. 4  is a partial cross-sectional view taken along line  4 - 4  of  FIG. 2 .  
         [0015]      FIG. 5  illustrates the output of one type of sensor that can be used with the present invention.  
         [0016]      FIG. 6  illustrates one magnet and sensor arrangement that can be used with the present invention  
         [0017]      FIG. 7  illustrates one circuit having warning lights that can be used with the present invention  
         [0018]      FIG. 8  illustrates an alternative embodiment of the filter assembly shown in  FIG. 2 .  
         [0019]      FIG. 9  is a cross-sectional view of an alternative embodiment of the filter head and pressure differential gauge shown in assembly shown in  FIG. 2 .  
         [0020]      FIG. 10  shows the relationship between a magnet and sensor included within the filter assembly embodiment shown in  FIG. 9 .  
         [0021]      FIG. 11  shows the output signal of the sensor shown in  FIG. 10 .  
         [0022]      FIG. 12  shows an alternative embodiment of the filter head and pressure differential gauge shown in  FIG. 9 .  
         [0023]      FIG. 13  shows a top elevational view of the differential pressure gauge housing illustrated in  FIG. 12 .  
         [0024]      FIG. 14  is a cross-sectional view of the pressure differential gauge housing shown in  FIG. 13 , taken along line  14 - 14 .  
         [0025]      FIG. 15  is a cross-sectional view of the differential pressure gauge housing shown in  FIG. 13 , taken along line  15 - 15 .  
         [0026]      FIG. 16  is an alternative embodiment of the filter head and pressure differential gauge shown in  FIG. 9 .  
         [0027]      FIG. 17  illustrates a relationship between a magnet and a sensor illustrated in  FIG. 16 .  
         [0028]      FIG. 18  illustrates one circuit that can be connected to the sensors illustrated in  FIGS. 6, 10 ,  17 .  
         [0029]      FIG. 19  illustrates one circuit that can process signals output from the sensors illustrated in  FIGS. 6, 10 , and  17 .  
         [0030]      FIGS. 20   a  and  20   b  illustrate look up tables stored on a microcontroller shown in  FIG. 19 .  
         [0031]      FIGS. 21   a  and  21   f  is a flow chart illustrating operations of a look up table routine stored in an executable by the microcontroller illustrated in  FIG. 19 . 
     
    
     DETAILED DESCRIPTION  
       [0032]     Preferred embodiments of the present invention will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. This description does not limit the scope of the invention, which is limited only by the scope of the attached claims.  
         [0033]     Referring to  FIG. 1 , an application for the filter assemblies  100  described herein are to remove foreign mater from hydraulic fluid that is used in vehicles such as tractors  102 . An example of such a tractor has a hydrostatic transmission and a hydraulic system, both of which require a filter. Such a hydraulic system can be used for a variety of purposes including raising and lowering farm implements such as plows. A hydraulic system can also be used to raise and lower earth moving equipment such as blades, buckets, back hoes, scrapers. Additionally, the filter assembly  100  can be used with tractors that have a variety of engine sizes and pump sizes.  
         [0034]     There are many other applications for the filters and differential pressure gauges that are described herein. For example, such filters and differential pressure gauges can be used for other farm equipment, construction equipment, skidders, loaders, other off-road vehicles, heavy-duty highway trucks, automobiles, and other vehicles, industrial machines requiring hydraulic filtering, and all other equipment or mechanical devices that require the filtering of fluids. Additionally, the filters described herein can be used to remove foreign mater from a variety of different fluids. Examples of liquid fluids include other hydraulic fluids, engine lube oil, diesel fuel, gasoline, engine coolant, automatic transmission fluid, and any other types of fluid. The filter can also be used with gaseous fluids such as air and exhaust.  
         [0035]      FIGS. 2-4  illustrate a filter assembly, generally shown as  106 , that has a differential pressure gauge assembly  108  having a perpendicular sensor arrangement as described below. More particularly, the filter assembly  106  includes a filter  110  and a filter head  112 . The filter head  112  is formed from a non-ferrous material and forms a substantially flat mounting surface  114  and two bolt holes  116 . Aluminum is one example of such a material. In alternative embodiments, the filter head  112  can be formed with other material including ferrous materials.  
         [0036]     The filter head  112  also defined an input passage  118  and an output passage  120 . A pressure-relief valve  122  is positioned between the input passage  118  and the output passage  120 . The pressure-relief valve  122  opens when the pressure of fluid in the input passage  118  reaches a predetermined level and provides fluid communication directly between the input and the output passages  118  and  120 .  
         [0037]     A valve housing  124  defines a passageway and is positioned within the mouth  126  of the output passage  120 . A portion of the valve housing  124  extends downward from the output passage  120 . A one-way, anti-drain valve  127  extends downward  128  defined in the value housing and prevents fluid from flowing backwards into the filter  110 . If the filter  110  is detached from the filter head  112 , the one-way valve  127  prevents fluid from passing onto the ground.  
         [0038]     The filter head  112  also defines an upper fluid chamber  130  that functions as a pressure chamber and is generally cylindrical in shape, although other shapes and configurations for the upper fluid chamber  130  are possible. A piston  132  is positioned in the upper fluid chamber  130  and divides the chamber into a high-pressure portion  134  and a low-pressure portion  136 .  
         [0039]     The upper fluid or pressure chamber  130  is generally cylindrical and has first and second ends  138  and  140 . The first end  138  is at the high-pressure portion  134 , and the second end  140  is at the low-pressure portion  136 . The lengths of the high-pressure portion  136  and low-pressure portions  136  will change as the piston  132  moves along the length of the pressure chamber  130 . The second end  140  is open, and is sealed with a threaded plug  142  and o-ring  144 .  
         [0040]     The diameter of the pressure chamber  130  slightly increases proximal the second end  140 . The portion of pressure chamber  130  with the increased diameter forms a fluid receiving area  146 . A first passage  148  provides fluid communication between the input passage  118  and the high-pressure portion  134  of the pressure chamber  130 . A second passage  150  provides fluid communication between the output passage  120  and the low-pressure portion  136  of the pressure chamber  130 . In one embodiment, the outlet of the first passage  148  is as close as possible to the first end  138  of the pressure chamber  130 . Similarly, the outlet of the second passage  150  is as close to the plug  142  as possible and opened into the fluid receiving area  146  of the pressure chamber  130 . This configuration maximizes the piston&#39;s  132  range of motion.  
         [0041]     The filter head  112  described herein is only one possible embodiment that can incorporate a differential pressure gauge. The filter head  112  can include any other structure that provides fluid flow through a filter element.  
         [0042]     The filter  110  includes a filter housing  152  that has a closed end  154  and an open end  156 . The open end  156  is attached to the filter head  112 . An o-ring  158  creates a seal between the filter housing  152  and the filter head  112 . A filter element  160  is positioned within the filter housing  152 . In one embodiment, the filter housing  152  can include a drain plug (not shown) for draining fluid from the filter housing  152  before it is detached from the filter head  112 .  
         [0043]     In one embodiment the filter element  160  is tubular in shape and has a lower edge  162  proximal to the closed end of the filter housing  152  and upper end  164  that is proximal to the filter head  112 . Other filter and filter elements can be used in conjunction with a pressure differential gauge. For example, the pressure differential gauge described herein can be used with filter elements that have a shapes and geometry other than tubular shapes.  
         [0044]     The upper end  164  of the tubular filter element  160  circumscribes the valve housing  124 . An o-ring  166  creates a seal between the upper end  164  of the filter element  160  and the valve housing  124 . Additionally, a gasket  168  creates a seal between the lower edge  162  of the filter element  160  and the wall  170  of the filter housing  152 . In this configuration, the filter element  160  divides the filter  110  into outer and inner chambers  172 , through the filter element  160 , into the inner chamber  174 , through the one-way valve  127 , the through the output passage  120 .  
         [0045]     The tubular filter element  160  can have a variety of sizes and dimensions. For example, one possible range of diameters for the filter element  160  is from about 2 inches to about 40 inches. A possible range of heights for the filter element  160  is from about 2 inches to about 40 inches. A possible range of fluid flow capacity for the filter element  160  is from about 0 gallons per minute to about 250 gallons per minute. However, the precise dimensions and fluid flow capacity for a filter element  160  will vary greatly from application to application. In a typical example such as the tractor discussed above, for example, the filter element  160  is about 4.75 inches in diameter, is about 11 inches tall, and has a fluid flow rate from about 0to about 45 gallons per minute.  
         [0046]     The piston  132  is formed from a magnet  176  positioned within a sleeve  178  and is sized to slide along the length of the pressure chamber  130 , but still maintain a seal between the high-pressure portion  134  and the low-pressure portion  136 . In one embodiment, the magnet  176  is cylindrical and has a centerline that is co-linear with the centerline of the upper fluid or pressure chamber  130 . In this embodiment, the magnetic field generated by the magnet  176  is symmetrical around the centerlines of both the magnet  176  and the pressure chamber  130 . However, other embodiments do not have a cylindrical magnet or a magnet having a centerline that is co-linear with the pressure chamber  130 .  
         [0047]     One embodiment of the piston sleeve  178  is generally cylindrical in shape and conforms to the shape of the pressure chamber  130 . A protrusion  180  extends from the end  182  of the sleeve  178  and into the high-pressure portion  134  of the pressure chamber  130 . The diameter of the protrusion  180  is smaller than the diameter of the sleeve  178 . This configuration prevents the sleeve  178  from extending all the way to the first end of the pressure chamber  130  and sealing the outlet portion of the first passage  148 . In other words, the protrusion  180  holds open the high-pressure portion  134  of the pressure chamber  130  so that fluid can flow in from the first passage  148 .  
         [0048]     One embodiment of the sleeve  178  is formed with a resin material such as nylon. One type of nylon material that can be used is “HYTREL” brand polyester elastomers, which is commercially available from E. I. DuPont de Nemours and Company of Wilmington, Del. Other embodiments can include any other material that will stand up to fluids and allow the piston  132  to slide against the aluminum that forms the surface of the pressure chamber  130 .  
         [0049]     A spring  184  extends between the piston  132  and the plug  142 . The spring  184  biases the piston  132  toward the first end  138  of the pressure chamber  130 . The end of the spring  184  that engages the piston  132  extends into the sleeve  178  and rests against the magnet  176 . The spring constant and the extension of the spring  184  while in a relaxed state will vary depending on the desired sensitivity and range of the differential pressure gauge.  
         [0050]     In one embodiment, the piston  132  has a range of motion between about ½ inch to about 2 inches within the pressure chamber  130 . In another embodiment, the range of motion for the piston  132  is between about ¾ inch and about 1¼ inches. Yet another embodiment of the piston  132  has a range of motion of about 1 inch.  
         [0051]     In this configuration fluid, from the input passage  118  fills the high-pressure portion  134  of the pressure chamber  130  and the outer chamber  172  of the filter  110 . Similarly, fluid in the outer chamber  172  of the filter  110  flows through the filter element  160  and into the inner chamber  174  of the filter  110 . From the inner chamber  174 , the fluid flows into the low-pressure portion  136  of the pressure chamber  130  and through the output passage  120 .  
         [0052]     As the filter element  160  becomes clogged, the fluid does not flow through it as easily and fluid pressure within the outer chamber  172  of the filter  110  increases. This increased pressure also causes the fluid pressure within the high-pressure portion  134  of the pressure chamber  130  to increase relative to the fluid pressure in the low-pressure chamber  136  of the pressure chamber  130 . The increased fluid pressure drives the piston  132  against the spring  184  and causes it to move toward the second end  140  of the pressure chamber  130 .  
         [0053]     Referring to  FIGS. 2 and 4 , a sensor chamber  186  is also formed in the filter head  112  and is perpendicular to the pressure chamber  130 . The sensor chamber  186  has a first end  188  proximal to, but not in fluid communication with, the pressure chamber  130 . The sensor chamber  186  also has a second end  190  that is open.  
         [0054]     There is a thin, intermediate wall  192  formed from non-ferrous aluminum between the sensor chamber  180  and the pressure chamber  130 . One embodiment of the intermediate wall  192  has a thickness between about 1/16 inch and about 3/16 inch. Other embodiments have a thickness of about ⅛ of an inch.  
         [0055]     A sensor assembly  194 , together with the piston  132 , spring  184 , and pressure chamber  130  form a differential pressure gauge. The differential pressure gauge can detect a broad range of differential pressures. One embodiment is sensitive to a pressure range of about 2 differential pounds per square inch (pisd) to about 100 psid. Another embodiment is sensitive to a pressure range of about 7 psid to about 80 psid.  
         [0056]     The sensor assembly  194  includes a sensor plug  196  and is positioned snugly within the sensor chamber  186  so that it has minimal movement. The sensor plug  196  has first and second ends  198  and  200 . The sensor plug  196  is secured within the sensor chamber  130  with a nut  202  that is threaded to the filter head  112 .  
         [0057]     A hall-effect sensor  204  is mounted to the end of the plug  196  and is positioned at the first end  188  of the sensor chamber  186 . The plug  196  and surface of the sensor chamber  186  have a slot  206  and key  208  arrangement that properly orients that the hall-effect sensor  204 . In another embodiment, the hall-effect sensor  204  is potted within the sensor chamber  186 . That is, the hall-effect sensor is positioned against the intermediate wall  192  and the sensor chamber is then filled with a liquid that hardens and seals the hall-effect sensor  204  in place. The potting can be formed from a variety of materials. Examples include epoxies and urethanes that are not conductive to electricity. In this configuration, the sensor  204  is isolated from the pressure chamber  130  and hence isolated from the fluid.  
         [0058]     The hall-effect sensor  204  has a front surface  210  that is active or sensitive to magnetic fields and a rear surface  212 . The rear surface  212  opposes the plug  196  and the active, front surface  210  faces away from the plug  196 . In this configuration, the active surface  210  is positioned facing, and in close proximity to the intermediate wall  192  that is between the pressure chamber  130  and the sensor chamber  186 . In one embodiment, the active surface  210  of the sensor  204  lies against the intermediate wall  192 . The sensor  204  also has a lower surface  214 , which faces downward toward the filter  110 .  
         [0059]     One hall-effect sensor  204  that can be used is model no. UGN3235K, which is commercially available from Allegro MicroSysterns, Inc. of Worcester, Mass. Referring to  FIG. 7 , this hall-effect sensor  204  has four pins  205   a - 205   d , one for a voltage supply  205   a , one ground  205   d , and two outputs  205   b  and  205   c . Referring back to  FIGS. 2 and 4 , a four-wire cable  216  is connected to the four pins, extends through the plug  196 , through a hole  218  defined in the nut  202 , and terminates in an electrical connector  220 . The connector  220  can be used to electrically connect the hall-effect sensor  204  to a variety of different analog or digital circuits such as warning lights or a variety of different programmable circuits such as a computer, a microprocessor, a microcontroller, or a programmable logic array.  
         [0060]     The first output of the hall-effect sensor  204  is in a normally low state and switches to a high state in response to detecting a threshold level of positive flux from the south pole of the piston magnet  176 . The second output also is in a normally low state and switches to a high state in response to detecting a threshold level of positive flux from the south pole of the magnet  176 .  
         [0061]     A graph of the output for the UGN3235K hall-effect sensor is illustrated in  FIG. 5 , when used in the embodiment described above. The graph illustrates the sensor output verses the displacement of the magnet in the piston. The displacement of the magnet  176  is 0.0 when the piston  132  is positioned against the first end  138  of the pressure chamber  130 . The first and second traces  222  and  224  illustrate the signal transmitted through the first and second outputs, respectively, when the voltage supply to the hall-effect sensor is 5 Volts. The third and fourth traces  226  and  228  illustrate the signal transmitted through the first and second outputs, respectively, when the voltage supply to the hall-effect sensor is 8 Volts.  
         [0062]     Referring to  FIGS. 5 and 6 , the movement of the piston  132 , and hence the magnet  176 , is perpendicular to the hall-effect sensor  204 . The flux lines  230  from the magnet field generated by the magnet  176  flow from the south pole  232  of the magnet  176  to the north pole  234 . As the south pole  232  of the magnet  176  approaches the hall-effect sensor  204 , the magnetic flux  230  from the south pole  232 , which is a positive flux, will flow through it in one direction. When the strength of the field to which the sensor  204  is exposed reaches a threshold level the positive magnetic flux  230  causes the first output of the hall-effect sensor  204  to change from a low state to a high state.  
         [0063]     As the midpoint of the magnet  176  approached the hall-effect sensor  204 , the magnetic flux  230  begin to run parallel to, or near parallel to the active face  210  hall-effect sensor  204 . As a result, the magnetic flux  230  does not pass through the hall-effect sensor  204 , and the positive flux  230  to which the hall-effect sensor  204  is exposed falls below the threshold level. The first output then returns to a low state. As the north pole  234  of the magnet  176  approaches the hall-effect sensor  204 , the magnetic flux  230  from the north pole  234 , which is a negative flux, will flow through it in an opposite direction. When the strength of the field to which the sensor  204  is exposed reaches a threshold level, the negative magnetic flux causes the second output of the hall-effect sensor  204  to change from a low state to a high state.  
         [0064]     Magnets of various strengths can be used. In one embodiment, the strength of the magnet has a range between about 200 gauss and about 800 gauss. In other embodiment, the magnet has a strength between about 400 gauss and about 800 gauss. One type of magnet that can be used is an ALNICO8 magnet such as model number S8A632, which is commercially available from Arnold Magnet of Marango, Ill. One of many dimensions that can be used for the magnet  176  has a diameter of about ⅜ inch and a length of about ⅜ inch.  
         [0065]     In one embodiment, as shown in  FIG. 7 , the outputs of the hall-effect sensor  204  drives a circuit that illuminates various LED&#39;s. The circuit includes the dual output hall-effect sensor  204 , a green LED  236 , an amber LED  238 , and a red LED  240 . A power supply  242  provides 5 Volts D.C. As one skilled in the art will recognize, the power is supplied through a voltage regulator (not shown). An alternative embodiment can provide other power levels, such as 8 volts as discussed above. The power supply  242  also provides power to the hall-effect sensor  204 , through the Vcc and ground terminals  205   a  and  205   d.    
         [0066]     The cathodes of the three LED&#39;s  236 ,  238 , and  240  are in direct electrical communication with ground. The anode of the green LED  236  is in electrical communication with positive terminal of the power supply  242 . Thus the green LED  236  indicates that power is being supplied to the circuit, including the hall-effect sensor  204 . The anode of the amber LED  238  is in electrical communication with the first output  205   b  of the hall-effect sensor  204 . Similarly, the anode of the red LED  240  is in electrical communication with the second output  205   c  of the hall-effect sensor  204 . Depending on the voltage and current output by the power supply  242  and the hall-effect sensor outputs  205   b  and  205   c , other embodiments might include resistors connected in series with the LED anodes.  
         [0067]     In operation, the first and second outputs  205   b  and  205   c  of the hall-effect sensor  204  are normally low, which prevents the amber and red LED&#39;s  238  and  240  from illuminating. As pressure within the high-pressure portion  134  of the pressure chamber  130  increases and drives the piston  132  toward the hall-effect sensor  204 . As south pole  232  of the magnet  176  approaches the hall-effect sensor  204 , the flux will cause the hall-effect sensor  204  to jump to a high state. This action increases the voltage potential across the amber LED  238  and causes it to illuminate signaling that the filter element  160  is approaching its operating limits.  
         [0068]     As the filter element  160  continues to clog, the pressure in the high-pressure portion  134  of the pressure chamber  130  continues to increase and drive the north pole  234  of the magnet  176  toward the hall-effect sensor  204 . As the midpoint of the magnet  176  approaches the midpoint of the hall-effect sensor  204 , the magnetic flux  230  runs parallel to the sensor  204 . As a result, the sensor  204  is not subject to either a substantial positive and negative flux and both outputs  205   b  and  205   c  of the sensor  204  return to a low state. Neither the amber nor the red LED&#39;s  238  or  240  are illuminated at this point.  
         [0069]     As the fluid pressure in the high-pressure portion  134  of the pressure chamber  130  continues to increase, the north pole  234  of the magnet  176  approaches the hall-effect sensor  204  and exposes the hall-effect sensor  204  to the flux. This flux causes the second output  205   c  of the hall-effect sensor  204  to jump to a high state and illuminate the red LED  240 , which indicates that the filter element  160  has clogged or failed and is no longer adequately filtering the fluid.  
         [0070]     Travel of the magnet  176  is limited by the second end  140  of the pressure chamber  130  so that the north pole  234  of the magnet  176  will not travel past the hall-effect sensor  204  and the red LED  240  will not stop emitting light as pressure within the high-pressure portion  134  of the pressure chamber  130  continues to build.  
         [0071]     In an alternative embodiment, the first and second outputs  205   b  and  205   c  from the hall-effect sensor  204  are input to a programmable circuit such as a computer that is onboard a tractor, vehicle, or other machinery. In this scenario, the onboard computer detects the state changes from the first and second outputs  205   b  and  205   c  of the hall-effect sensor  204  and are programmed to perform certain tasks in response thereto. For example, the onboard computer might be programmed to control the illumination of warning lights, similar to those described above. The onboard computer might keep a log of how long an engine or other machinery has run with a filter that has failed. In yet another embodiment, the onboard computer might even be programmed to send a signal to an engine control module that causes the engine to shut down when the second output  205   c  of the hall-effect sensor  204  changes states because the filter element  160  failed. The programmable circuit might also communicate with other computers that are onboard a vehicle or control a machine.  
         [0072]      FIG. 8  illustrates another alternative embodiment of a filter assembly, generally shown as  244 . The filter assembly  244  is similar to the filter assembly  106  described above and includes a filter  110  and a filter head  112  that defines an input passage  118 , an output passage  120 , an upper fluid chamber  130  that functions as a pressure chamber, and first and second passages  148  and  150 . The filter head  112  also has a pressure-relief valve  122 , a one-way valve  127 , an intermediate wall  192 , a piston  132 , a spring  184 , and a plug  196  and sensor arrangement  204 . The filter  110  includes a filter housing  152  and a filter element  160  that divides the filter housing  152  into inner and outer chambers  174  and  172 .  
         [0073]     Additionally, a housing  246  that contains electronics is mounted on the filter head  112  and replaces the sensor chamber. The hall-effect sensor  204  is mounted in the housing  246 , and the active face  210  of the sensor  204  is positioned proximal to or against the intermediate wall  192 . The LED&#39;s  236 ,  238 , and  240 , are mounted on the housing  246  so that they are visible to an operator. The filter assembly  244  also includes a four-wire cable  216  so that the hall-effect sensor  204 , in addition to the LED&#39;s, is in electrical communication with remote electronics or a computer.  
         [0074]      FIG. 9  illustrates an alternative embodiment of a filter head  248  that has a differential pressure gauge with a linear sensor arrangement. The filter head  248  is similar to the filter head  112  described above in that it defines an input passage  118 , an output passage  120 , an upper fluid chamber  130  that functions as a pressure chamber, a fluid receiving portion  146  formed in the pressure chamber  130 , and first and second passages  148  and  150 . The filter head  248  also has a pressure-relief valve  122 , a one-way valve  127 , and an intermediate wall  192 . A piston  132  has a sleeve  178 , a magnet  176 , and a protrusion  180 , and a spring  184  that biases the piston  132  toward the first end  138  of the pressure chamber  130 .  
         [0075]     Additionally, a sensor housing  250  has first and second portions  252  and  254  and first and second ends  256  and  258 , respectively. The first end  256  is closed and the second end  258  is open. The first portion  252  has a smaller diameter than the second portion  254  and is threaded within the opening at the first end  256  of the pressure chamber  130 . An o-ring  260  creates a seal between the pressure chamber  130  wall and the sensor housing  250 . The sensor housing  250  is formed from a non-ferrous material such as aluminum.  
         [0076]     The cylindrical sensor housing  250  defines a sensor chamber  262 , and the first end  256  of the housing  250  forms an intermediate wall  264  between the pressure chamber  130  and the sensor chamber  262 . One embodiment of the intermediate wall  264  has a thickness between about 1/16 inch and about 3/16 inch. Other embodiment have a thickness of about ⅛ of an inch.  
         [0077]     A hall-effect sensor  266  has first and second faces  268  and  270 . The first face  268  is active and is responsive to magnetic flux. The hall-effect sensor  266  is positioned in the sensor chamber  262  so that the first face  268  is against the intermediate wall  264 . Additionally, the center of the first, active face  268  is aligned with the centerlines of the magnet  176  and the pressure chamber  130 . The hall-effect sensor  266  is potted  267  within the sensor chamber  262  with an electrically non-conductive material such as an epoxy or a urethane.  
         [0078]     Given this arrangement, the hall-effect sensor  266  is linearly aligned with path of the piston  132  and hence the magnet  176 . Furthermore, the magnet  176  is oriented in the sleeve  178  so that the south pole  232  of the magnet  176  faces the active surface  268  of the hall-effect sensor  266 . The movement of the piston magnet  176  relative to the hall-effect sensor  266  is illustrated in  FIG. 10 . As the pressure within the high-pressure portion  134  of the pressure chamber  130  increases, the piston  132 , and hence the magnet  176 , will move toward the hall-effect sensor  266 . The voltage output from the hall-effect sensor  266  increases as the magnet  132  moves toward the sensor  266  and the magnitude of the flux  230  to which the sensor  266  is exposed increases.  
         [0079]     One hall-effect sensor  266  that can be used is model no. A3515LUA, which is also manufactured by Allegro MicroSystems, Inc. This hall-effect sensor  266  has three pins  272   a - 272   c , one for a voltage supply  272   a , one ground  272   b , and one output  272   c . A cable  274  is in electrical connection to the three pins  272   a - 272   c , extends through the potting  267 , and terminates in an electrical connector  276 . The voltage output from the hall-effect sensor  266  is continuous and proportional to the strength of the magnetic flux to which it is exposed. The connector  276  can be used to electrically connect the hall-effect sensor  266  to a circuit such as warning lights or to a programmable circuit.  
         [0080]     A graph of the output for the A3515LUA hall-effect sensor, when used in the embodiment described above is illustrated in  FIG. 11 . The graph illustrates the sensor output verses the distance of the magnet from the hall-effect sensor  266 . The first trace  278  illustrates the voltage generated by the output when the voltage supply is 8 volts. The second trace  280  illustrates the voltage generated by the output when the voltage supply is 5 volts.  
         [0081]     The hall-effect sensor  266  also can be programmable. An example of such a programmable hall-effect sensor is the HAL800 Programmable Linear Hall-Effect sensor, which is commercially available from Micronas Intermetall GmbH of Freiburg, Germany. Several aspects of the output voltage range of the HAL800 hall-effect sensor can be adjusted. For example, the low and high voltage output levels can be adjusted. Using a programmable hall-effect sensor in this manner simplifies calibration of the differential pressure gauge during the manufacturing process. The pressure chamber  130  can be loaded with a predetermined pressure differential and the hall-effect sensor  266  then can be programmed to output the correct, predetermined high and low output voltage.  
         [0082]     In an alternative embodiment, the hall-effect sensor  266  is mounted on the end of a plug similar to the plug  196  described above. In this embodiment, the plug and sensor  266  would be secured within the sensor chamber  262  with a nut, through which the cable  274  would extend. In yet another embodiment, the hall-effect sensor  266  and electrical connector  276  are insert molded. In other words, the hall-effect sensor  266  and connector  276  are molded into a single and unitary plastic housing that takes the place of sensor housing  250 . This insert-molded unit then can be threaded into the second end  140  of the pressure chamber  130 . The insert molding also can include electrical circuitry. Alternatively, the inset-molded unit can be plugged or snapped into the second end  140  of the pressure chamber  130 .  
         [0083]     Many other embodiments of the filter head  248  are possible as well. In one such embodiment, for example, second end  140  of the pressure chamber  130  is sealed with a threaded plug similar to plug  142  or other sealing structure. A sensor cavity (not shown) is then defined in the outer surface of the filter head  248  at a location  141  adjacent to and opposing the first end  138  of the pressure chamber  130 . In this embodiment, the sensor cavity is formed in the outer wall  139  of the filter head  248  but does not extend all the way through the filter head  248  and does not open to the pressure chamber  130 . In another possible embodiment, a bracket (not shown) is fastened to the filter head  248  at the location  141 . The sensor housing  250  is then fastened into the sensor chamber or to the bracket The sensor housing  250  can be fastened to the sensor cavity or the bracket by threads or similar mechanism such as a snap-lock fitting. This embodiment may permit existing filter head designs to be manufactured with hall-effect sensors using minimal changes to tooling designs and may also permit existing filter heads to be retrofitted with hall-effect sensors  266 .  
         [0084]      FIG. 12  illustrates an alternative embodiment of a filter head  282  and differential pressure gauge housing, generally shown as  284 . The filter head  282  is similar to the filter head  248  described above and defines an input passage  118 , an output passage  120 , an upper fluid chamber  130 , a fluid receiving portion  146  formed in the upper fluid chamber  130 , and first and second passages  148  and  150 . The filter head  282  also has a pressure-relief valve  122 .  
         [0085]     Referring to  FIGS. 13-15 , the differential pressure gauge housing  284  has first and second portions  286  and  288 . The first portion  286  has an outer diameter sized to slide into the upper fluid chamber  130  of the filter head  282 . The outer diameter of the second portion  288  is greater than the outer diameter of the first portion  286 . A radial shoulder  290  extends between the first and second portions  286  and  288  of the differential pressure gauge housing  284 . In one embodiment, the outer diameter of the first portion  286  of the differential pressure gauge housing  284  ranges from about 0.4 inch to about 0.9 inch. Another embodiment has a diameter of about 0.6 inch In another embodiment the length of the first portion  286  is between about 0.9 inch and about 3.5 inches. Another embodiment has a length of about 2.3 inches.  
         [0086]     A pressure chamber  292  is defined in the first portion  286  of the housing  284 , and a sensor chamber  294  is defined in the second portion  288  of the housing  284 . The pressure and sensor chambers  292  and  294  are generally cylindrical in shape and are axially aligned. The pressure chamber  292  has inner and outer ends  296  and  298 . The sensor chamber  294  also has inner and outer ends  300  and  302 .  
         [0087]     An intermediate wall  304  is formed between the inner ends  296  and  300  of the pressure and sensor chambers  292  and  294 , respectively. The surface  306  of the intermediate wall  304  that forms the inner end  296  of the pressure chamber  292  is generally concave in shape. In one embodiment, the thickness of the intermediate wall  304  is between about 1/16 inch and about 3/16 inch. In another embodiment &amp; the thickness is about ⅛ inch.  
         [0088]     Threads  308  are formed in a portion  308  of the outer surface of the first portion  286  and extend from the radial shoulder  290 . The threads mate with threads formed on the inner surface of the upper fluid chamber  130  of the filter head  282 . The length of the threaded portion  308  is shorter than the length of the fluid receiving portion  146  of the upper fluid chamber  130  in the housing  282 . Additionally, a first linear groove  310  is formed in the outer surface of the housing  284  and extends along the length of the threaded portion  308 . The first groove  310  is deeper than the minor diameter of the threads. A second groove  312  is similarly formed on the opposite side of the housing  284  from the first groove  310 .  
         [0089]     In one embodiment, a first hole  313  extends between the first groove  310  and the concave surface  306  of the intermediate wall  304 . A second hole  314  similarly extends between the second groove  312  and the concave surface  306  of the intermediate wall  304 . The first and second holes  312  and  314  are as close to the inner end  296  of the pressure chamber  292  as possible.  
         [0090]     Returning to  FIG. 12 , when the differential pressure gauge housing  284  is attached to the filter head  282 , the second passage  150  is in fluid communication with the first and second grooves  310  and  312 . In this configuration, fluid flows from the second passage  150 , into the fluid receiving portion  146  of the upper fluid chamber  130 , into the first and second grooves  310  and  312 , through the first and second holes  313  and  314 , and into a low-pressure portion  316  of the pressure chamber  292 . In one embodiment, the outlet port of the second passage  150  is as close as possible to the first and second holes  313  and  314 . In yet another embodiment, the outlet port of the second passage  150  is in direct fluid communication with the holes  313  and  314 .  
         [0091]     A piston  318  is formed from a sleeve  320  and a magnet  176  positioned within the sleeve  320 . The piston  318  is positioned in the pressure chamber  292  and divides the pressure chamber  292  into a high-pressure portion  322  and the low-pressure portion  316 . A spring  184  extends between the piston  318  and the intermediate wall  304  and biases the piston  318  toward the outer end  298  of the pressure chamber  292 . In one embodiment, the magnet  176  is generally cylindrical and has a centerline that is co-linear with the centerline of the pressure chamber  292 .  
         [0092]     A plug  324  is threaded into the outer end  298  of the pressure chamber  292  of the housing  284 . The plug  324  defines a fluid passage  326 . Additionally, the sleeve  320  has a closed end  327  that defines a concave surface. The concave surface opposes the fluid passage  326  in the plug  324  and provides a space to receive fluid.  
         [0093]     The length of the first portion  286  of the pressure differential gauge housing  284  is sized so that when it is fully inserted in the upper fluid chamber  130  of the filter head  282 , there is a gap  328  between the plug  324  and the first end  138  of the upper fluid chamber  130 . The first passage  148  is in fluid communication with the gap  328 . In this configuration, fluid can flow from the first passage  148 , through the fluid passage  326  in the plug  324 , and into the high-pressure portion  322  of the pressure chamber  292 .  
         [0094]     The housing  284  is threaded to the surface of the upper fluid chamber  130  proximal to the second end  140  of the upper fluid chamber  130 . A first o-ring  330  is adjacent the radial shoulder  290  and is positioned between the outer surface of the first portion  286  of the housing  284  and the inner surface of the upper fluid chamber  130 . An second o-ring  332  rests in a groove  334  that is formed around the circumference of the first portion  286  of the housing  284  and is positioned so that it is between the first and second passages  148  and  150  when the housing  284  is inserted into and engaging the upper fluid chamber  130  of the filter head  282 .  
         [0095]     The sensor chamber  294  is similar to the sensor chamber  262  discussed above. A continuous output hall-effect sensor  266  is positioned within the sensor chamber  294  with the first, active face  268  opposing the intermediate wall  304 . Additionally, the center of the first face  268  is aligned with the centerline of the magnet  176  and the pressure chamber  292 . The hall-effect sensor  266  is potted  267  with an electrically non-conductive material such as an epoxy or urethane. This configuration isolates the hall-effect sensor  266  from the pressure chamber  292  and the fluid. A cable  274  extends from the sensor  266 , through the potting  267 , and terminates in an electrical connector  276 .  
         [0096]      FIG. 16  illustrates yet another alternative embodiment of a filter head  336 , which is similar to the filter head  282 .  
         [0097]     The filter head  336  defines an input passage  118 , an output passage  120 , and first and second passages  148  and  150 . The filter head  336  defines an upper fluid chamber  338  similar to the upper fluid chambers  130  described above. However, a first end  340  of the upper fluid chamber  338  is open and a second end  342  of the upper fluid chamber  130  is closed. A fluid receiving portion  344  is proximal the first end  340  of the upper fluid chamber  338 .  
         [0098]     A differential pressure gauge housing  284  includes pressure and sensor chambers  292  and  294 , first and second portions  286  and  288 , an intermediate wall  304 , a radial shoulder  290 , and a threaded portion  308 . The threaded portion  308  is threaded to mating threads formed in the inner surface of the upper fluid chamber  338  of the filter head  336  and is sealed with o-rings  330  and  332 .  
         [0099]     First and second grooves  310  and  312  are formed in the threaded portion  308  of the differential pressure gauge housing  284 . First and second holes  313  and  314  pass from the first and second grooves  310  and  312 , respectively, to a concave surface  306  formed in an inner end  296  of the pressure chamber  292 . In one embodiment, the first and second holes  313  and  314  are positioned as close as possible to the inner end  296  of the pressure chamber  292 . Additionally, the outlet of the first passage  148  is in fluid communication with the first and second grooves  310  and  312 .  
         [0100]     A plug  324  is threaded into the outer end  298  of the pressure chamber  292 . The plug  324  defines a fluid passage  326 . The length of the first portion  286  of the pressure differential gauge housing  284  is sized so that when it is fully inserted in the upper fluid chamber  338  of the filter head  336 , there is a gap  346  between the plug  324  and the second end  342  of the upper fluid chamber  338 . The second passage  150  is in fluid communication with the gap  346 .  
         [0101]     A piston  348  is formed from a sleeve  350  and a magnet  176  positioned within the sleeve  350 . The piston  348  has first end  352  formed by a flat wall. In one embodiment, the flat wall forming the first end  352  is between about 0.08 inch and about 0.25 inch. In another embodiment, the flat wall about 0.12 inch thick. A spring  184  extends between a second end  356  of the piston  348  and the outer end  298  of the pressure chamber  292 . The spring  184  urges the piston  348  and the magnet  176  toward the intermediate wall  304 .  
         [0102]     The piston  348  divides the pressure chamber  292  into a high-pressure portion  322  and a low-pressure portion  316 . In this configuration, fluid flows from the first passage  148 , into the fluid receiving portion  146  of the upper fluid chamber  130 , into the first and second grooves  310  and  312 , through the first and second holes  313  and  314 , and into the high-pressure portion  322  of the pressure chamber  292 . In one embodiment, the outlet port of the first passage  148  is as close as possible to the first and second holes  313  and  314 . In yet another embodiment, the outlet port is in direct fluid communication with at least one of the holes  313  and  314 . Similarly, fluid from the second passage  180  flows into the gap  346 , through the fluid passage  326  in the plug  324 , and into the low-pressure portion  316  of the pressure chamber  292 .  
         [0103]     The sensor chamber  294  is similar to the sensor chamber  262  discussed above. A continuous output hall-effect sensor  266  is positioned within the sensor chamber  294  with the first, active face  268  opposing the intermediate wall  304 . Additionally, the center of the first, active face  268  is aligned with the centerline of the magnet  176  and the pressure chamber  292 . The hall-effect sensor  266  is potted  267  with an electrically non-conductive material such as an epoxy or urethane. This configuration isolates the hall-effect sensor  266  from the pressure chamber  292  and the fluid. A three wire cable  274  extends from the sensor  266 , through the potting  267 , and terminates in an electrical connector  276 .  
         [0104]     Referring to  FIG. 17 , the motion of the piston magnet  176  is reversed relative to the motion of the magnet  176  illustrated in  FIG. 10 . When there is no pressure differential, the magnet  176  is close to the hall-effect sensor  266 . As the differential pressure increases, the fluid pressure in the high-pressure portion  322  of the pressure chamber  292  drives the piston  348 , and hence the magnet  176  away from the sensor  266 .  
         [0105]     Similar to the hall-effect sensor  204 , the output of the hall-effect sensor  266  can drive a variety of different analog or digital circuits or provide input for a variety of different programmable circuits. In one embodiment, as shown in  FIG. 18 , for example, the output of the hall-effect sensor  266  drives a circuit that that generates at least one discrete output. The exemplary circuit has an input  354 , a first op amp  356 , and a second op amp  358 . The input  354  is in electrical communication with the output of the hall-effect sensor  266 . The input is also in electrical communication with the noninverting input of the first op amp  356  via a 10 kΩ resistor and is in electrical communication with the noninverting input of the second op amp  358  via a 10 kΩ resistor.  
         [0106]     The inverting input of the first op amp  356  is tied to a 5 volt power supply via a 1.5 kΩ resistor and to ground via a 3.5 kΩ resistor. The inverting input of the second op amp  358  is tied to a 5 volt power supply via a 750 Ω resistor and to ground via a 4.25 kΩ resistor. Additionally, the noninverting input and the output of the first op amp  356  are tied together with a 1 MΩ resistor, and the noninverting input and output of the second op amp  358  are tied together with a 1 MQ resistor.  
         [0107]     In this circuit, the first op amp  356  is in a normally low state, but jumps to a high state of 5 volts when the voltage of the signal output by the hall-effect sensor  266  reaches 3.5 volts. The second op amp  358  is also in a normally low state, but jumps to a high state of 5 volts when the voltage of the signal output by the hall-effect sensor  266  reaches 4.25 volts. Each op amp  356  and  358  provides a discrete output that correlates or corresponds to a differential pressure and is indicative of the condition of a filter. In use, when the filter element  160  is reaching the end of its useful life, the output of the first op amp  356  will change from a low state to a high state. When the filter element  160  fails or has reached the end of its useful life, the output of the second op amp  358  will change states from a low state to a high state. In other embodiments, the circuit includes only a single op amp, which would provide information regarding one differential pressure. Alternatively, the circuit could include more than two op amps, which would provide information about more than two differential pressures.  
         [0108]     The output of the first and second op amps  356  and  358  can input into a programmable circuit such as a computer or an engine control module that controls the machine, activates an alarm, and/or records data. Alternatively, the outputs of the first and second op amps  356  and  358  can drive amber and red LED&#39;s  360  and  362 , respectively, which provide visual warning lights.  
         [0109]      FIG. 19  illustrates another circuit that can process the signal output by the continuous output hall-effect sensor  266 . This circuit includes an attenuator  364 , a low-pass filter  366 , an analog-to-digital (A/D) converter  368 , and a microcontroller  370 . The A/D converter  368  can be multiplexed between two inputs  372  and  374 . The microcontroller  370  includes input circuitry  376 , output circuitry  378 , processing circuitry  380 , and memory  382 . The memory  382  is loaded with lookup tables  384  and a lookup table routine  386 .  
         [0110]     During operation of the programmable circuit, the output signal generated by the hall-effect sensor  266  is passed through the attenuator  364  to attenuate or scale the voltage of the signal so that its maximum value is between 0 and 5 volts. The signal is then filtered  366 , which reduces any oscillations or spikes in the signal. Oscillations and spikes in the signal can result from rapid movement in the piston caused by sudden movement in, or impacts to, the filter assembly. Additional filtering can be performed in software executed by the microcontroller  370 . The filtered signal is input into the first input  372  of the A/D converter  368 .  
         [0111]     The voltage of the power supply for hall-effect sensor is input into the second input  374  of the A/D converter  368 . The output of the A/D converter  368  is then input into the microcontroller  370 . In one embodiment, additional inputs into the microcontroller  370  include a binary code or digital word that identifies the sensor type  388  and a binary code or digital word that sets an alarm threshold  390 . A variety of structures can be used to input the sensor type and alarm threshold. Examples of inputs include dip switches and jumpers. In another embodiment, the identity of the sensor and the alarm threshold also can be downloaded into the microcontroller  370  electronically and stored in memory  382 .  
         [0112]     Additionally, there might be a variety of alarm threshold values stored in memory  382  and used by the lookup table routine  386 . For example, one alarm threshold might correspond to a warning that signals the filter element  160  is approaching the end of its useful life and should be changed. Another alarm threshold might correspond to a failure of the filter element  160 . Other embodiments include other inputs or no inputs at all. For example, a microcontroller programmed to interface with only one particular type of sensor does not need an input that identifies the sensor.  
         [0113]     The microcontroller  370  has several outputs including an alarm output  392  and a serial output  394 . The alarm output  392  is configured to send an alarm signal. In one possible, embodiment, the alarm signal will cause a warning light to illuminate. The serial output  394  is configured to communicate a signal to other programmable devices such as a computer or an engine control module for vehicles. Examples of other computers include an onboard computer for vehicles and a computer controller for manufacturing equipment. Additionally, these other computers can perform a variety of functions such as diagnostics, collection and recordation of data, the generation of alarm conditions, or even disabling an engine or a pump.  
         [0114]     Communication through the serial output  394  can be over a dedicated link, a data bus, or radio frequency (Rf) transmission. Furthermore, other embodiments include programmable circuits such as microprocessors or programmable logic arrays in place of the microcontroller.  
         [0115]     In yet other embodiments, the memory  382  of the microcontroller  370  is used to store data as well as look up tables  384  and code for the lookup table routine  386 . For example, various alarm conditions and the value of various sensor outputs might be recorded in memory  382  and downloaded at a later time. In another example, circuitry different than that illustrated in  FIG. 18  is used to condition the sensor signal before it is input in the microcontroller.  
         [0116]     Referring to  FIGS. 20A and 20B , two sets of lookup tables  396   a  and  396   b  are stored in memory. The first set of lookup tables includes a plurality of tables. Each table  384   a  relates a sensor output voltage to the distance between the sensor and the piston magnet for a given sensor type and a given voltage supply. An advantage of having a plurality of tables is that both the sensitivity of a sensor and signal strength of a sensor&#39;s output will vary on the sensor&#39;s make, model, and supply voltage. As a result, a single microcontroller can be used with a variety of different sensors, which simplifies manufacturing and inventory requirements.  
         [0117]     Each table  384   a  within the first set of lookup tables  396   a  include a two column array of data that correlates the sensor output voltages to the displacement of the piston in the pressure chamber. Each column includes data words that range from word(0) to word(N), and are ordered such that word(I+1)&gt;word(I). A first pointer P 0  points to data in the first column of the table in the first set of tables, and a second pointer P 1  points to data in the second column of the first set of tables.  
         [0118]     The second set of tables  396   b  includes a single table  384   b  that relates the differential pressure to the distance between the sensor and the piston magnet. Only a single table  384   b  is required because the relationship between the differential pressure and the piston of the piston magnet does not depend on the type of hall-effect sensor that is used. In an alternative embodiment, however, the second set of tables can include multiple sets of tables if a single microcontroller is used with different differential pressure gauges.  
         [0119]     The second table  384   b  also is a two column array of data that correlates the displacement of the piston in the pressure chamber to the actual pressure differential. Each column includes data word that range from word(0) to word(N), and are ordered such that word(I+1)&gt;word(I). A third pointer P 2  points to data in the first column of the table in the second set of tables, and a fourth pointer P 3  points to data in the second column of the second set of tables.  
         [0120]     Many alternative embodiments of the tables are possible. For example, a microcontroller that has a design dedicated to a single sensor can include a single lookup table that relates sensor output voltage to pressure. Yet other embodiments might calculate the differential pressure from the sensor output and not include any lookup tables. Yet other embodiments might merely record data or set alarm conditions without determining differential pressures.  
         [0121]      FIGS. 21A-21F  illustrate the operation of the lookup table routine. Generally, programmed operations perform a particular task. Operation  398  reads the sensor type  388  from the binary input code, and operation  400  outputs a signal to the A/D converter  368  that selects the second input  374 . The A/D converter  368  then converts the sensor&#39;s supply voltage and inputs the digital word corresponding to that voltage into the microcontroller  370 . Operation  402  selects the table  384   a  from the first set of tables  396   a  that corresponds to that sensor and supply voltage. Operation  404  then sets pointer P 0  to the start of the first column in the first table  384   a  and outputs another signal to the A/D converter  368  that selects the first input  372 , which corresponds to the signal output from the hall-effect sensor. The A/D converter  368  then converts the signal output from the sensor. Operation  406  reads the digital word output by the A/D converter  368  that corresponds to the sensor output  
         [0122]     Hall-effect sensors typically output a signal having a nominal value even when they are not exposed to a magnet-c signal. Accordingly, operation  408  determines whether the value of the sensor output is below the typical nominal value. If the signal is below the nominal value, operations  410  and  412  generate an error signal and output the error signal. The computer or engine control module that interfaces with the microcontroller  370  then receives the signal and takes appropriate steps such as activating a warning signal or disabling an engine.  
         [0123]     If the signal output by the hall-effect sensor is equal to or greater than the nominal output level, operation  412  sets pointer P 1  to the start of the second column, which corresponds to sensor distance. Operations  416 - 420  increment pointers P 0  and P 1  until the value of the digital word at pointer P 0  equals the value of the digital word received from the A/D converter  368 . These operations also ensure that pointers P 0  and P 1  are aligned. Operation  422  sets the value of a variable distance equal to the value pointed to by pointer P 1 .  
         [0124]     Operations  423  and  425  set pointers P 2  and P 3  to start of the distance column and the pressure column, respectively, in the second table  384   b . Operations  426 - 430  then align and increment pointers P 2  and P 3  until the value at pointer P 3  equals is greater than the value of the variable distance. Operation  423  then sets the value of a variable D(I+1) equal to the value at pointer P 3 , and operation  20  sets the value of the variable P(I+1) equal to the value at pointer P 4 .  
         [0125]     After the value of variable D(I+1) is set, operation  436  decrements the pointer P 3  so that it points to the next lower distance in the distance column. Similarly, operation  438  decrements the pointer P 4  so that it points to the next lower pressure in the distance column. After the pointers P 3  and P 4  are decrements, operation  440  then sets the value of variable D(I) to equal the value in the table at pointer P 3 . Similarly, operation  442  sets the value of variable P(I) to equal the value in the table at pointer P 4 . After the value of the variables are set, operation  444  uses a linear interpolation to calculate the pressure. The equation used in the interpolation is: 
 
PRESSURE={[ P ( I+ 1)− P ( I )]/[ D (I+1)− D ( I )]}*{DISTANCE− D ( I )}+ P ( I ). 
 
         [0126]     In one embodiment, operation  446  then communicates the calculated value of PRESSURE over the serial bus to a computer, which records the data for historical and diagnostic purposes. Alternatively, the value of the variable PRESSURE is locally stored in the memory of the microcontroller so that it can be downloaded at a latter time. Additionally, operations  448  and  450  read the alarm threshold  390  and compare it to the value of the variable PRESSURE. If the value of the variable PRESSURE is equal to or greater than the alarm threshold  390 , operations  452  and  454  generate and output an alarm signal  392 . The alarm signal  392  then activates an alarm such as a warning lamp.  
         [0127]     Many different embodiments of the lookup table routine  386  are also possible. For example, one embodiment does not use interpolation to determine the pressure. In this embodiment, every digital word in the second column of the first table has a matching value in the first column of the second table. The lookup table routine  386  merely indexes the third and fourth pointers P 3  and P 4  until the value at P 3  matches the value at the second pointer P 2 . The measured differential pressure is the corresponding differential pressure at the pointer P 4 .  
         [0128]     The various embodiments described herein are exemplary only, and many different embodiment are possible. For example, the differential pressure gauge can use any other structure other than a piston to gauge the pressure differential. Examples of such other structures include membranes and diaphragms. The differential pressure gauge described herein can also be used with filters that are adapted for back flow trough the filter element.  
         [0129]     Additionally, any other sensor or measurement device that measures the displacement of a gauging structure and outputs an electrical signal can be used in place of a hall-effect sensor. For example, other types of sensors that can be used with a pressure differential gauge includes sensors made with gigantic magnetoresistive (GMR) materials. Yet other embodiments might include a completely electronic arrangement for measuring the pressure differential across a filter element and output a variable signal. Electrical signals can include any signal that can be detected and processed by another piece of equipment such as electrical signals, radio frequency signals, and light signals.  
         [0130]     Additionally, other embodiments of the hall-effects sensors can be used. For example, one embodiment uses two single output hall-effect sensors, one responsive to positive flux and the other responsive to negative flux. The two hall-effect then can be spaced along the path of the magnet to eliminate the dead zone in the response of the dual output hall-effect sensor described above. Another example of an alternative hall-effect sensor is a programmable-type of hall-effect sensor that can be programmed or calibrated to output a certain voltage given a particular relative position of a magnet. An advantage of such a device is accuracy, the programming can account for spring variations, magnet variations, and sensor orientation. An example of such a programmable hall-effect sensor is model no. A3150, which is a prototype being developed by Allegro MicroSystems.  
         [0131]     Furthermore, embodiments having a perpendicular sensor arrangement as described above are not limited to only hall-effect sensors having a discrete output Similarly, embodiments having a parallel sensor arrangement as described above are not limited to only hall-effect sensors having a continuous output. Still other embodiments of the hall-effect sensors described above are possible. For example, other embodiment might use a hall-effect sensor that has a normally high output. An advantage of this structure is that any computer that interfaces with the hall-effect sensor can more easily determine that there is a sensor failure when fluid is not flowing by merely checking the sensor output.  
         [0132]     The foregoing description of various embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. It is intended that the scope of the invention not be limited by the specification, but defined by the claims set forth below: