Abstract:
A fluid system pressure indicator is adapted for use in fluid systems having a filter element. The fluid system force indicator includes a housing partitioned to be exposed to the fluid system to provide transmission of a mechanical force between these partitioned environments to indicate the fluid pressure. This exterior shape change is such that it can be identified through tactile means not requiring visual identification.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation in part of U.S. application Ser. No. 10/930,970 filed on Aug. 31, 2004, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present disclosure relates to fluid systems having a filtration device in which restriction of the filtration device can be determined by measurement of pressure. More specifically, the disclosure relates to devices for indicating whether a fluid filter is effective and/or requires replacement.  
       BACKGROUND  
       [0003]     Fluid systems requiring filtration apparatus are an integral part of the automotive and heavy equipment industries. Engine systems, hydraulic systems and various other collateral systems require fluids such as air, oil, fuel and coolants to be at least partially contained and directed to their functional end points.  
         [0004]     For instance, in engine systems utilizing diesel as fuel, extremely high pressure pumps are utilized. These pumps have very close tolerances and may be easily damaged or disabled if particulate laden fuel is passed through them. In addition, the fuel injectors of these engines are configured to deliver a spray of fuel in a specifically designed pattern. Interference with the passages, orifices or other structures of the injectors may result in a decrease in engine efficiency and/or damage to the engine itself. As such, many diesel fuel systems require at least one filter to be present between the fuel storage compartment and the high pressure pump.  
         [0005]     Depending on such things as preventative maintenance scheduling, fuel quality, operating conditions, and the like, fuel filters become restricted or clogged at various rates. Filter occlusion may adversely impact engine efficiency, and in some cases, may damage or destroy components of the engine. In other cases, restriction of the filter can result in filter failure which may allow highly contaminated fluid to reach portions of the pump or injector system, resulting in extremely high repair costs for those devices.  
         [0006]     Typically, the status of a filter, be it a gas or liquid filter, is determined through use of a pressure gauge, which is incorporated between the filter and a pump. As the filter becomes occluded with particles, the pump must maintain a higher pressure differential across the filter to maintain the same level of fluid flow required for proper engine function. As this pressure differential increases, the conventional filter monitor moves an indicator contained within a housing. The position of the indicator can be viewed through a sight window and the percent of filter occlusion can typically be determined by marks located on the gauge housing relative to the indicator within the gauge housing.  
         [0007]     A wide variety of filter monitors or indicators exist conventionally. Some of the conventional devices utilize colors as indicators. These monitors fall into two general categories of gauges that are observed while the engine is running, and 2 gauges which maintain statically a reading of the highest differential pressure encountered during engine operation. These conventional devices have several drawbacks. They often must be cleaned of material build-up covering the sight window or be otherwise manipulated so as to allow visualization through the sight window in order to accurately determine the level of filter occlusion.  
         [0008]     The direct visional observation requirement means that the device has to be located such that it can be viewed during pre-startup and/or post-running maintenance. As is well known in the relevant arts, the normal operation of equipment and associated fluid systems results in a buildup of material on equipment components that is often composed of oils and other fluids mixed with dust, dirt and particulates. Accordingly, the sight window of conventional pressure monitors often becomes sufficiently covered with the dust, grime, grease or other material so that the indicator is no longer visible. The inability to readily observe the indicator markings may lead to the filter check step of normal maintenance being eliminated, thus resulting in severe damage to the equipment during operation.  
         [0009]     In addition, the principal composite materials of these conventional devices are limited to transparent plastic or glass material. In particular, the plastic materials may be damaged by heat and/or abrasion to the point that visibility through the material is significantly degraded or no longer achievable.  
         [0010]     Several conventional filter monitoring devices utilize electronic means for the detection of pressure differentials. These devices require that the detector be energized and typically employ pressure transducers. In some instances, these electronic devices are not as dependable as mechanical indicators since a failure of the pressure transducer may occur without warning, thereby allowing an engine to be run with a heavily occluded filter, which can result in engine and/or injection system damage.  
         [0011]     Another problem associated with detecting pressure differentials is the susceptibility to false indications caused by transient pressure pulses. Pressure spikes are commonly generated from the throttle changes and cold fuel conditions.  
       SUMMARY  
       [0012]     Briefly stated, one preferred form is generally directed toward a fluid system pressure indicator for use in fluid systems having a pump and a filter element. The fluid system pressure indicator includes a housing providing a space in which fluid pressure of the fluid system can be monitored and a pressure communicator to provide a transmission of pressure from the monitored space to an electrical indicator element. The transmission of pressure causes the filter change indicator to initiate an electrical signal when a greater than normal pressure differential across the filter is encountered. The presence or absence of the electrical signal can be monitored through such things as gauges, lights, and/or sound generating means remote from the filter change indicator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a cut-away view of a filter change indicator;  
         [0014]      FIG. 2  is a cut-away view of the filter change indicator in a position with the projectable indicator extended;  
         [0015]      FIG. 3  is a view of the filter change indicator of  FIG. 2  in a non-extended position;  
         [0016]      FIG. 4  is a cut-away view of a filter change indicator having a one-piece pressure communicator and diaphragm;  
         [0017]      FIG. 5  is an exploded view of the filter change indicator showing various elements of the filter change indicator;  
         [0018]      FIG. 6  is a top plan view, partially in phantom, of a filter change indicator;  
         [0019]      FIG. 7  is a bottom plan view of a filter housing from the end with an associated filter change indicator;  
         [0020]      FIGS. 8A and 8B  are a top plan view, partially in phantom, and a cut away view respectively of a filter change indicator;  
         [0021]      FIGS. 9A and 9B  are a top plan view, partially in phantom, and a cut away view respectively of a filter change indicator;  
         [0022]      FIGS. 10A and 10B  are a top plan view, partially in phantom, and a cut away view respectively of a filter change indicator;  
         [0023]      FIG. 11  is a cut-away view of a filter change indicator having a crash safety feature;  
         [0024]      FIGS. 12A and 12B  are a top view of a force communicator and a side view of a force communicator;  
         [0025]      FIGS. 13A and 13B  are a side plan view and a top plan view respectively of a filter change;  
         [0026]      FIG. 13C  is a cut away view of a filter change indicator as shown in  FIGS. 13A and 13B ;  
         [0027]      FIGS. 14A and 14B  are cut away views of filter change indicators each having a different embodiment of a normally open style electrical contact assembly; and  
         [0028]      FIGS. 15A and 15B  are cut away views of filter change indicators each having a different embodiment of a normally closed style electrical contact assembly. 
     
    
     DETAILED DESCRIPTION  
       [0029]     With reference to the drawings wherein like numerals represent like parts throughout the several figures, a filter change indicator is generally designated by the numeral  10 . The filter change indicator  10  is preferably incorporated into a filter system to provide a tactile indication of the filter condition to aid in the determination as to whether the filter requires replacement. The filter change indicator  10  has an efficient and low cost construction and is, for example, constructed from combinations of low cost materials such as plastic, metal, ceramic or other materials. For example, the principal material may be molded ABS plastic.  
         [0030]     The filter change indicator  10  includes a graduated multi step housing  12 , which partially forms a containment vessel or fluid portion  18 . The exterior of the housing is preferably configured to be integrated into a fluid line, plenum or housing through such features as a threaded plug portion  13 . The housing  12  has, for example, a fluid passage  14  extending from the interior of the fluid system, into the interior portion of the housing  12 . Within the housing  12  is a pressure communicator  16 , which at least partially interacts with the fluid in the fluid system. The pressure communicator  16  has a portion designed to maintain fluid contact and a portion which is to be isolated from the fluid. The interior of the housing  12  is divided into a fluid portion  18 , which is in fluid communication with the fluid system of interest, and a non-fluid portion  22 . This partitioning is achieved with a fluid diaphragm  20 , which can be associated with the pressure communicator  16 . The fluid diaphragm  20  is made of a membrane material such as metal, rubber, rubber-coated woven material, plastic, silicones, fluorosilicones, and/or other polymeric material, having varying degrees of flexibility. The fluid diaphragm  20  acts as a sealing element between the fluid portion  18  and the non-fluid portion  22  of the housing  12 , and may also act as a flexible element to allow for the pressure communicator  16  to be axially displaceable.  
         [0031]     In one embodiment, the fluid diaphragm  20 B is an integral part of the pressure communicator  16 A. For example, as shown in  FIG. 4 , the fluid diaphragm  20 B is formed of the same material as the pressure communicator  16 A and is preferably formed out of plastic or metal. In a one-piece configuration, the diaphragm  20 B has a substantially planar form which extends radially away from an axial center line  40  of the pressure communicator  16 A. In another embodiment, as illustratively shown in  FIGS. 9 and 10 , a fluid diaphragm  20 A has a cross-section with alternating ridges and troughs. These alternating ridges and troughs, in some cases, function to define the flexure of the fluid diaphragm  20 A during operation. The fluid diaphragm  20 A and  20 B in the one-piece configuration may exhibit differing degrees of flexibility due to characteristics and/or proportions of its composite material. For example, the thickness of the material can be varied, or the material can be suitably treated to enhance or suppress the rigidity of the material to provide the selected pre-established flexure characteristics. The selected characteristics allow the diaphragm to be tuned at a desired damping rate to dampen out certain pressure spikes for pre-defined transient time intervals.  
         [0032]     The non-fluid system portion  22  of the housing  12  contains a projectable indicator  24 . This projectable indicator  24  is selectively moveable such that a portion or portions of the indicator  24  can project through an opening or openings  26  in a cap  28  of the housing  12 . The projectable portion or portions extend past the exterior surface of the housing  12  as shown in, for example,  FIG. 2 . The projectable portion, when projected, changes the overall configuration of the filter change indicator  10 . For instance, the projectable indicator  24  in one position is entirely housed within the housing  12 . Thus, no significant portion of the projectable indicator  24  extends outside of the housing  12 . The indicator  24  moves into a second projected position either in a single step or through incremental steps. In the projected position, a portion of the indicator  24  protrudes exteriorly from the housing  12 . This protrusion changes the shape of the exterior of the housing  12 . In one form, the indicator moves in a lateral direction relative to the axial center line  40  of the filter change indicator.  
         [0033]     In one embodiment, the indicator  24  has a portion, which is contained within the housing  12 , and a portion that is extended from the housing through, for example, openings  26 . When actuated by a higher than normal pressure differential, the contained portion and extended portion of the indicator  24  are simultaneously moved into and out of the housing cap openings  26 . For example, one portion may be colored green and the other portion may be colored red. This color-coding provides a visual indication as to the filter status in addition to the exterior tactile-shape change.  
         [0034]     The indicator  24  has an engagement surface  30 , which mechanically engages an actuator portion  32  of the pressure communicator  16 . The pressure communicator  16  in operation moves due to pressure generated in the fluid system. The engagement between the actuator  32  and the indicator  24  is altered by movement of the pressure communicator  16 . The altered engagement allows the indicator  24  to be moved in, for example, a lateral direction which is assisted by a first tensioning spring  34 , which exerts a constant biasing pressure on the indicator  24  in a substantially unidirectional fashion.  
         [0035]     In one embodiment, the pressure communicator  16  may have a first end  36  and a second end  38  with an axial centerline  40  extending between these ends. The pressure communicator can be configured to include differing cross-sectional portions intermediate the first end  36  and the second end  38 , as illustratively shown in  FIG. 8 . The pressure communicator  16  is displaceable along the axial center line  40 . A web portion  42  extends radially relative to the axial center line  40  and supports and interacts with the fluid diaphragm  20  in order to maintain the sealing integrity of the diaphragm  20 . The web portion  42  may also act to modulate the flexibility of the fluid diaphragm  20  such that portions of the diaphragm  20  are not deformed with similar flexibility characteristics. The web portion  42  of the pressure communicator  16  may extend out toward an interior wall of the housing  12  equidistantly such that a gap  56  exists between an edge  58  of the web portion  42  and an interior wall or surface of the housing  12 . In one embodiment, as illustratively shown in  FIG. 10 , a first portion  43  of the pressure communicator  16 C is associated with a second portion  147  of the pressure communicator. The second portion  147  of the pressure communicator is formed integral with the fluid diaphragm  20 C.  
         [0036]     The pressure communicator  16 , as illustratively shown in  FIGS. 2 and 3 , may also have an engagement surface  44 , such as an arcuate slot, which interacts with an inner lip portion  52  of the fluid diaphragm  20 . The inner lip portion  52  ensures that fluid cannot pass between the fluid diaphragm  20  and the engagement surface  44  under normal circumstances. The fluid diaphragm  20  may also have a housing sealing lip  46 . With the fluid diaphragm  20  in place, the interior of the housing is divided by the fluid diaphragm  20  into a fluid portion  18  and a non-fluid portion  22 . A portion of the pressure communicator  16  extends through the diaphragm  20  such that the pressure communicator  16  is present in both fluid  18  and non-fluid  22  portions of the housing  12  as shown in at least  FIG. 1 . The portion of the pressure communicator  16  extending into the non-fluid portion  22  of the housing may have cylindrical protrusion  50 , wherein a portion of the cylindrical protrusion includes the actuator  32 . A sealing washer  54  is provided over the protrusion  50  such that it can be screwed or pressed into place to create or aid in creating a fluid and/or pressure tight seal between the fluid diaphragm  20  and pressure communicator  16 . For example, the sealing washer contacts the inner lip portion  42  such that the inner lip portion is held in firm contact with the pressure communicator  16 . The sealing washer  54 A can extend away from the axial centerline  40  toward the interior surface of the housing  12 . The sealing washer  54 A, in some cases, may extend radially a distance which is substantially less than, co-extensive with, or greater than the distance the web portion  42  extends radially.  
         [0037]     The pressure communicator  16 , in one embodiment, interacts with a second spring  48  or pressure communicator spring which biases the pressure communicator  16  in a unidirectional fashion such that engagement portion  32  is maintained in a stable position relative to the engagement surface  30  of the pressure indicator  24  during normal pressure ranges in the fluid system. The pressure communicator spring  48  is mounted in association with a tubular portion  100  of the housing. The tubular portion may define part of the fluid pathway between the fluid portion  18  and the fluid system.  
         [0038]     The fluid diaphragm  20  is associated with the pressure communicator  16  and may be disk-shaped with a diameter greater than that of a pressure communicator web  42 . The fluid diaphragm  20  extends beyond the interior surface plane of the housing  12 . The portion extending beyond the interior surface plane of the housing includes a housing sealing lip  46  which fits into an arcuate slot  47  formed in the housing wall or it may interact with a similar structure in the housing  12 . The housing sealing lip  46  is fluidly secured through use of the cap  28 , which may be pressed or screwed down against the housing sealing lip  46  of the fluid diaphragm  20 . The fluid diaphragm  20  has a flexible portion  60 , which spans the gap  56  between the end  58  of the web and the interior wall of the housing. This can, among other things, allow for displacement of the pressure communicator  16  while still maintaining the fluidly separated environment within the housing.  
         [0039]     In one embodiment, the housing has a shelf  70  upon which a portion of the fluid diaphragm  20 , the housing sealing lip  46 , and/or an  0 -ring  74  rests. The shelf  70  may have vertical sealing ridges  72 , which interact with the resting element, for instance, when the cap  28  is associated with the housing  12 .  FIG. 10  illustratively shows two variants of the sealing ridges  71 .  
         [0040]     When connected to a fluid system, a portion  102  of the pressure communicator  16  extends into the fluid pathway  14 . This can allow for such things as a modulation of the fluid flow rate between the fluid portion  18  and the fluid system. In some cases, it is necessary to ensure that the fluid portion  18  of the housing  12  is fully flooded with the fluid from the fluid system. The exterior of the housing  12  may, as shown in  FIGS. 6, 13A , and  13 B, have a knurled portion  29  to facilitate removal to vent air from the fluid system. In this regard, the filter change indicator functions as an effective air vent as well as a change indicator.  
         [0041]     During operation, for instance, when the fluid system is pressurized, the fluid system pump is moving fluid through the filter. A differential pressure is created across the filtration membrane or other filtering structure. This differential pressure typically is of a nature that a lower pressure exists on the filtrate side of the filter. The fluid diaphragm  20  and associated pressure communicator  16  of the filter change indicator  10  are subjected to a differential pressure proportionally relative to this pressure differential across the filter. As the filter becomes occluded, and the differential of pressure across the filter changes due to a restriction of fluid flow through the filter, the pressure communicator  16  is forced against the second spring  48 . As the filter occlusion increases over time due to an increased accumulation of material filtered out of the fluid, the actuator portion  32  disengages from the engagement surface  30  of the indicator  24 , thus allowing the indicator  24  to be forced by the first spring  34  into a second or subsequent position. For instance, the engagement surface  30  of the indicator may have a step-like configuration as shown in, at least,  FIG. 1 . This step-like configuration is designed so as to allow the actuator portion  32  to retreat away from the indicator engagement surface  30  incrementally.  
         [0042]     In operation, the height of each step of the engagement surface  30  and the lateral displacement distance of the indicator  24  can be correlated with different filter occlusion levels. The step-like configuration allows for an identification of differing degrees of filter occlusion as increased pressure is exerted upon the pressure communicator and fluid diaphragm. In operation, for example, the pressure communicator  16  is pulled downward against the biasing pressure of the second spring  48 . The indicator  24  is then allowed to move incrementally laterally along the subsequent step-like engagement surface  30  as the pressure communicator  16  descends.  
         [0043]     In addition, during operation in, for instance, low temperature environments a transient increase in the pressure differential may occur. For instance, in extremely cold weather, fluid may thicken, gel, or may contain solidified waxes or other non-fluid components. These temperature-related changes create transient higher pressure differentials across the filter along with a higher pressure differential between the fluid portion  18  and non-fluid portion  22  of the filter change indicator housing  12 . Often these higher than expected differentials last for only a short time duration and dissipate when fluid warmed by, for example, engine heat, and/or a fluid heater reaches the filter element. Once warmed up, the thickening, wax formations, and/or gelling dissipate and the pressure differential drops to within a normal range.  
         [0044]     In one embodiment, these transient higher differential pressures are moderated or otherwise compensated for with a delay in the filter change indicator actuation. The delay can be anywhere from about 0.25 seconds to about 5.0 seconds but preferably between about 1 to 2 seconds. This delay may be accomplished by, for example, the flexible portion  60  of the fluid diaphragm  20  which spans the gap  56  between the end  58  of web portion  42  of the pressure communicator  16  and an interior structure or surface of the containment vessel  12 . This flexible portion  60  allows for a certain amount of buffering of the higher short term pressure differentials. For example, in a system that has a fluid heater in addition to a filter and pump, the fluid heater may warm the fluid such that the viscosity and/or other properties are kept within normal operational ranges. However, often a small volume of the fluid may not have been warmed due to the distance from the heater. This non-warmed fluid may then be caused to pass through the filter by the pump. Compensation or buffering of the higher differential pressure can occur through the pre-established flexure resistance of the flexible portion  60  of the fluid diaphragm  20  which, due to its elasticity, creates a lag in the transmission of pressure to the pressure communicator  16 . Thus, the pressure communicator  16  does not move within the short time duration that it takes warmer fuel to reach the filter element. This lag time can prevent misleading actuation of the filter change indicator.  
         [0045]     In one embodiment, the lag in response can be effectuated through use of dimensioning of certain elements to produce a combination of surface areas and/or mass that will move with a delayed fashion in response to transient high pressure differentials. For instance, the mass of components and/or spring tension profiles may be designed to resist sudden movement. In addition, the fluid passages leading from the fluid passage may be dimensioned such that the internal structures of the filter change indicator are not subjected to sudden pressure changes due to fluid transfer limitations.  
         [0046]     In one embodiment, a fluid change indicator, which has changed shape through protrusion of a portion of the indicator  24 , is reset by pressing, for example, with a finger on the exterior of the extended portion of the indicator  24  in a direction toward the housing  12 . This pressing moves the indicator  24  in a direction counter to the first spring  34  and back into the housing  12 . Since the pressure communicator is under continuous biasing pressure by the second spring  48 , the actuator portion  32  re-engages with the engagement portion  30  of the indicator. For example, in operation a mechanic can push the extended portion of the indicator  24  back into the housing  12 , resetting the indicator portion, and then operate the fluid system to monitor and determine whether or not the indicator is again actuated.  
         [0047]     Mechanically, in one embodiment, the pressure generated by the second spring  48  is designed to be greater than the pressure exerted on the pressure communicator  16  during normal fluid system operation. As an example, the second spring  48  has a biasing pressure, which keeps the actuator portion  32  engaged with the indicator engagement portion  30  at all pressure differentials below a predetermined filter occlusion level. The differential pressure across the filter can be determined for different filter occlusion states and can be correlated to differential pressures developed in the housing  12 . A filter with, for instance, a 75% occlusion can be correlated with a certain pressure differential across the fluid portion  18  and the non-fluid portion  22  of the filter change indictor  10 . This correlated pressure is then used for selection of an appropriate biasing device such as the second spring  48 . Also factored into this equation, among other things, can be the force of the engagement surface  30  against the actuator  32  generated by the first spring  34 . Thus, a frictional force may be present between the actuator portion  32  and the indicator engagement surface  30  such that second spring  48  does not require the total biasing force of the correlated pressure due to the occlusion level.  
         [0048]     In one embodiment a crash safety feature is present. The crash safety feature operates to prohibit or retard fuel flow through the filter change indicator  10  in the event the filter change indicator is damaged by, among other things, an impact event. For example, a filter change indicator present on a vehicle may be damaged in the event of the vehicle crashing. Such damage may result from, for example, a crushing and/or a shearing force being applied to the filter change indicator  10 . This force may cause structural failure of one or many portions of the filter change indicator. The structural failure of the one or many portions can result in a fluid containment breach. The fluid escaping from the filter change indicator if flammable or corrosive may pose a serious safety hazard.  
         [0049]     The crash safety feature may include a pressure communicator  16   x  having a tapering configuration along portions of its axial length. This tapering configuration is configured to cooperatively associate with a tapered bore  300  during, for example, application of a crushing force. The tapered bore  300  forms a portion of the fluid passage  14 . In the event of a crushing force being applied to the filter change indicator, the tapered portion of the pressure communicator  16   x  is driven into cooperative association with the tapered bore  300 . This cooperative association may seal off, or otherwise inhibit fluid flow through the fluid passage  14 . The pressure communicator  16   x  may also have a connecting surface such as a threaded portion, dimensioned portion, and/or a cavity  302 . For example, the cavity  302  may be configured to receive a screw  304  having a head  306 .  
         [0050]     In one embodiment the pressure communicator  16   x  may be formed of a material that will operate to bend out of axis in response to, for example, a shearing force being applied to the filter change indicator  10 . In the event a shearing force is applied to the filter change indicator a portion of the pressure communicator  16   x  may bend and a portion of the pressure communicator may be pulled through the tapered bore  300 . As the portion of the force communication is pulled through the tapered bore  300 , the screw head  306  is forced into association with the tapered bore  300 . It should be understood that the second end  39  of the pressure communicator  16   x  may have various configurations, and may be associated with a variety of shaped elements which serve to associate with the tapered bore  300 .  
         [0051]     In another embodiment, an electrical contact unit is employed for the purpose of indication. It should be understood that any of the above components may be used either alone or in combination with the below-described electrical contact unit. The electrical contact unit can be of a normally open or normally closed circuit type. A normally open circuit type electrical contact is configured to operate with an electrical indicator circuit that is designed to be incomplete. A normally open circuit system requires that the circuit be closed in order to allow a signal to be generated. In contrast, a normally closed circuit type electrical contact is configured to operate with an electrical indicator circuit that is designed to be complete. A normally closed circuit system requires that the circuit be opened in order to allow a signal to be generated. With reference to  FIGS. 14A and 14B , a filter change indicator may include a normally open type electrical contact having, for example, a pre-stressed contact unit  400  ( FIG. 14   a ), or a prong contact unit  401  ( FIG. 14B ). With reference to  FIGS. 15A and 15B , a filter change indicator may include a normally closed type electrical contact having, for example, a pre-stressed contact unit  402  ( FIG. 15A ), or a prong contact unit  403  ( FIG. 15B ).  
         [0052]     The electrical contact units  400 ,  401 ,  402 ,  403  may be used with any of the components previously described. In this regard, the filter change indicators of  FIGS. 14A, 14B ,  15   a  and  15 B may each include a housing  12   a  partially forming a containment vessel or fluid portion  18   a . A fluid passage  14   a  extends into the fluid portion  18   a  from the exterior of the housing  12   a . A pressure communicator  16   a  is positioned within the housing  12   a  as previously described. A fluid diaphragm  20   a  separates the fluid portion  18   a  of the housing from a non-fluid portion  22   a . A spring  48   a  is provided in connection with the pressure communicator  16   a  to bias the pressure communicator in an unidirectional fashion as previously described. The housing  12   a  may include an externally threaded portion  405  as shown.  
         [0053]     As shown in  FIG. 14A , the pre-stressed normally open contact unit  400  utilizes an electrical actuator in the form of a pre-stressed metal plate  406 . The pre-stressed metal plate  406  is flexible between an open position “b” and a closed position “a”. The pre-stressed metal plate  406  is designed to remain in a natural closed position unless acted on by an external force. For example, an external force applied to the center of the pre-stressed metal plate  406  causes the plate  406  move into the open position “b”. The pre-stressed metal plate  406  is supported by plate rests  404 .  
         [0054]     In the embodiment as shown in  FIG. 14A , electrical contacts  408  protrude through a connector  409  attached to the cap  28   a  such that they may be selectively contacted by the pre-stressed metal plate  406  as shown. The electrical contacts  408  may be terminals that can be connected to a remainder of an electrical indicator circuit. A portion of the pre-stressed metal plate  406  is in contact with the pressure communicator  16   a . The pre-stressed metal plate  406  may be fixed to the pressure communicator  16   a  by a fastener, such as a screw or rivet. The pressure communicator  16   a  exerts a force against the pre-stressed metal plate  406  by transmission of the force from spring  48 , thus keeping the pre-stressed metal plate  406  in a normally open position. In operation, as the pressure communicator  16   a  descends, due to filter occlusion, as previously discussed, the pre-stressed metal plate  406  is allowed to move to its natural closed position “a”, and thus make contact with electrical contacts  408  and closing the circuit.  
         [0055]     As shown in  FIG. 15A , the pre-stressed normally closed contact unit  402  utilizes a pre-stressed metal plate  406   a . The pre-stressed metal plate  406   a  may be fixed to the pressure communicator  16   a  by a fastener  410 . The fastener  410  may be, for example, a screw or rivet. Electrical contacts  408  protrude through a connector  409  attached to the cap  28   a  such that they may be selectively contacted by the pre-stressed metal plate  406   a . The pressure communicator  16   a  is biased by the spring  48   a  such that the pre-stressed metal plate  406   a  is held against the contacts  408 , thus closing the circuit. In operation, as the pressure communicator descends, as previously discussed, the pre-stressed metal plate  406  is moved away from contacts  408 , thus opening the circuit. The pre-stressed metal plate  406   a  may also, in one embodiment, be pulled against plate rests (not shown) which are similar to those shown in  FIG. 14A . When present, the plate rests act as a fulcrum, wherein the force of the descending pressure communicator  16   y  cause the pre-stressed metal plate  406  to flex between a closed and an open position.  
         [0056]     With reference to  FIG. 14B , in this embodiment an electrical signal is initiated by completion of an electrical circuit across two C-shaped probes  414 . The contact across the probes  414  is through a conductor attached to the pressure communicator  16   a . For example, the conductor may be a conductive disk  412  mounted on the pressure communicator  16   a  by a fastener  416 . The disk  412  extends into the opening of the C-shaped probes  414  as shown. The probes  414  may have a spring section  424  which, among other things, serves to cushion contact between the conductive disk  412  and the probes  414 . The probes  414  may be attached to electrodes  422  that are embedded in, or otherwise affixed to, a coupling  420 . The coupling  420  may be connected to an electrical component (not shown) that provides electrical connection between electrodes  422  and a remainder of the electrical indicator circuit. The coupling  420  may be mounted to the cap  28   a  of the filter change indicator. In operation, the disk  412  is maintained in its normal position spaced from the probes  414  by the pressure communicator  16   a . As the pressure communicator  16   a  descends, the conductive disk  412  is brought into electrical contact with the probes  414 , thereby closing the circuit.  
         [0057]     With reference to  FIG. 15B , in another embodiment an electrical signal is initiated by opening an electrical circuit across two probes  426 . The contact across the probes  426  is through a conductor. For example, the conductor may be a conductive disk  412   a  that is held against the probes  426  by a conductor disk spring  413 . The conductive disk  412   a  may have a central hole  415  through which a fastener  410   a  passes. The fastener  410   a  is fixed to the pressure communicator  16   a , for example by threading, and has a head  411  that has a larger diameter than the central hole  415 . The probes  426  may contact an upper surface of the conductive disk  412 . The probes  426  may be attached to electrodes  428  that are embedded in, or otherwise affixed to, a coupling  420   a . The coupling  420   a  is mounted to the cap  28   a  of the filter change indicator. In operation, as the pressure communicator  16   a  descends, the fastener  410   a  passes through central hole  415  until the fastener head  411  contacts the conductive disk  412   a . As the pressure communicator  16   a  continues to descend, the conductor disk  412   a  descends with the pressure communicator  16   a  and loses contact with the probes  426 , thereby opening the circuit.  
         [0058]     While preferred embodiments have been shown and described, various modifications and substitutes may be made thereto. Accordingly, it is to be understood that the present embodiments have been described by way of illustration and not limitation.