Abstract:
A multiple diffusion channel filter assembly is disclosed. The filter assembly provides filtration of air entering and exiting an electronic enclosure through a breather hole. In one implementation, the filter assembly contains a housing with an adsorbent component and a first diffusion channel and a second diffusion channel. In another implementation, the filter assembly contains a first layer and a second layer which surround an adsorbent component and a first and a second diffusion channel that are in fluid communication with the interior space defined by the first and second layers. In yet another implementation, the invention, includes a filter accessory containing a diffusion channel configured to work in conjunction with a separate filter assembly thereby adding an additional diffusion channel to the existing design.

Description:
FIELD OF THE INVENTION 
   The present invention is, in general, directed to filters and methods of making and using the filters, as well as assemblies having the filters. More particularly, the present invention relates to electronic enclosure filters having at least two diffusion channels. 
   BACKGROUND OF THE INVENTION 
   Filters are useful in a variety of devices. For example, filters are often used in electrical or optical equipment. An air pressure differential between the interior and exterior of a housing containing the equipment can be produced as the electrical or optical equipment heats and cools. Often the housing includes a vent or breather hole to allow airflow that equalizes the pressure. A filter is typically provided over the vent to reduce the flow of contaminants into and/or out of the housing. 
   Computer disk drives, and in particular, hard disk drives, are one example of a device that uses filters in this manner. Disk drives are sensitive to moisture, chemical contamination, and particulate contamination, particularly, as the drive heads become smaller and aerial densities increase. Chemical contaminants, such as hydrocarbons and acid gases, can condense onto a disk and degrade the head/disk interface and/or corrode the heads. Particulate contaminants can lead to stiction and can cause read/write errors and head crashes. 
   Therefore, a need exists for filters that provide beneficial removal of particulate and chemical contaminants. 
   SUMMARY OF THE INVENTION 
   To increase the lifetime of filter material, particularly adsorbent filter material, a long and narrow airflow path is often provided leading into the adsorbent filter media. This flow path may be within the walls of the housing, in a cover disposed against the housing, or in the body of the filter so that air flows along the path, through the filter, and into the interior of the housing. This path is often referred to as a diffusion channel. Example diffusion channels are disclosed and described in Tuma et al. (U.S. Pat. No. 5,997,614), herein incorporated by reference. 
   The presence of a diffusion channel can reduce the amount of chemical contaminants and moisture reaching the adsorbent material of the filter and/or the inside of the disk drive, thereby prolonging the useful life of the adsorbent material. However, current designs only provide for one diffusion channel, in between the filter and the exterior of the housing. The limitations of designs containing only one diffusion channel, in between the filter and the exterior of the housing, include the undesirable reintroduction of contaminants into the interior of the electronic enclosure. When electronic equipment is heating up during operation, the adsorbent contained within filters of current designs typically desorbs. With a diffusion channel in between the filter and the exterior of the housing, and no diffusion channel in between the filter and the interior of the housing, the desorbed contaminants can preferentially diffuse into the electronic enclosure. Furthermore, current designs allow volatile contaminants within the electronic enclosure such as disk lubricants to reach the adsorbent thereby reducing its useful life. 
   Generally, the present invention relates to electronic enclosure filters that contain two or more diffusion channels. Typically, one diffusion channel is disposed in the fluid flow path between the filter and the exterior of the enclosure and another is disposed in the fluid flow path between the filter and the interior of the enclosure. The adsorbent of this multiple diffusion channel filter is disposed in fluid communication with both diffusion channels and is located between the two channels. The dual diffusion channel filter of the invention prevents desorbed contaminants from preferentially diffusing into the electronic enclosure, and further extends the useful life of the adsorbent material. In embodiments with a separate recirculation filter, the filter further prevents volatile contaminants from within the drive enclosure, such as disk lubricants, from reaching the adsorbent material. 
   While not intending to be bound by theory, the rate of diffusion of a fluid is dependent upon the cross-sectional area through which diffusion takes place. Since a diffusion channel provides a smaller cross-sectional area than a standard opening, gases will diffuse through a standard opening at a faster rate than through a diffusion channel. Where a filter has a diffusion channel in fluid communication with the exterior of the drive enclosure and a standard opening in fluid communication with the interior of the drive enclosure, this can lead to preferential diffusion of contaminants that desorb from the adsorbent material back into the electronic enclosure. However, where a diffusion channel is provided upon both sides of the interior of a filter, the rate of diffusion through each channel can be equalized or at least made more equivalent such that contaminants that desorb from the filter material would not preferentially diffuse back into the electronic enclosure. 
   Moreover, the fluid residence time within the interior of a filter is increased where a diffusion channel is provided upon both sides of the filter. This increases the interaction between the fluid and the adsorbent thereby increasing the effectiveness of the adsorbent. 
   In embodiments where there is a separate recirculation filter, the presence of a diffusion channel in between the interior of the electronic enclosure and the adsorbent material of a breather filter prevents water vapor and disk lubricants that may be present inside the electronic enclosure from reaching the adsorbent of the breather filter, thereby increasing the useful life of that adsorbent material. 
   In one embodiment the filter of the present invention includes a housing having a top and a base. The housing further defines an internal volume. Adsorbent filter media is disposed within the internal volume of the housing. The housing has a first diffusion channel configured and arranged to provide fluid communication between the interior chamber of the housing and the exterior of the electronic enclosure. The housing also has a second diffusion channel configured and arranged to provide fluid communication between the interior chamber of the housing and the interior of the electronic enclosure. 
   The housing typically comprises a non-porous material. A porous membrane is typically disposed in the airflow path between the adsorbent filter media and the interior of the electronic enclosure. A porous membrane or other media may also be disposed in the airflow path between the adsorbent filter media and the exterior of the electronic enclosure. The porous membrane can be, for example, a polytetrafluoroethylene membrane. The media could be a woven or nonwoven filter media. 
   A mounting adhesive may also be disposed on the filter and is used to hold the filter in place within an electronic enclosure. The filter media comprises, for example, carbon filter material. Also, a porous support layer can be disposed within the internal volume, and the filter media may be mounted on the porous support layer. 
   The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description that follow more particularly exemplify these embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, wherein like numerals represent like parts throughout the several views, in which: 
       FIG. 1A  is a front perspective view of a filter constructed and arranged in accordance with the invention. 
       FIG. 1B  is a top plan view of the filter of  FIG. 1A , showing dashed lines for the location of a diffusion channel. 
       FIG. 1C  is a bottom plan view of the filter of  FIG. 1A , showing dashed lines for the location of a diffusion channel. 
       FIG. 1D  is a top plan view of a filter constructed in accordance with an implementation of the invention showing an alternative embodiment where dashed lines indicate the location of diffusion channels. 
       FIG. 2  is a side cross-sectional view of the filter of  FIG. 1A . 
       FIG. 3  is an exploded perspective view of a filter constructed and arranged in accordance with an implementation of the invention. 
       FIG. 4  is a front perspective view of a filter constructed in accordance with an implementation of the invention. 
       FIG. 5A  is a side cross-sectional view of the filter of  FIG. 4  taken along line A–A′ constructed and arranged in accordance with the invention. 
       FIG. 5B  is a top plan view of the filter of  FIG. 5A  where dashed lines indicate the location of a diffusion channel. 
       FIG. 5C  is a bottom plan view of the filter of  FIG. 5A  where dashed lines indicate the location of a diffusion channel. 
       FIG. 5D  is a top plan view of an alternative embodiment of a filter constructed in accordance with the invention where dashed lines indicate the location of a diffusion channel. 
       FIG. 5E  is a bottom plan view of the filter of  FIG. 5D  where dashed lines indicate the location of a diffusion channel. 
   

   While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is believed to be applicable to filters and methods of making and using filters to filter a fluid, such as, for example, air or other gases. In particular, the present invention is directed to breather filters having multiple diffusion channels and methods of making and using these breather filters. While the present invention may not be so limited, an appreciation of various aspects of the invention will be gained through a discussion of the example embodiments provided below. 
   Breather filters act to reduce or eliminate contaminants from air that is exchanged with the exterior of an electronic enclosure. Breather filters are typically placed over a hole or port in the electronic enclosure through which air is exchanged. 
   Diffusion channels are regions in the airflow path of a filter that typically have a smaller cross-sectional area than the adjoining regions of the airflow path such that they could be described as constricting the airflow path. The constriction in a diffusion channel typically covers an extended length of the airflow channel. 
   The present invention includes breather filters with multiple diffusion channels, which prevent contaminants from re-entering the interior of the electronic enclosure and help prolong the useful life of the adsorbent filter material. Having at least two diffusion channels prevents contaminants that may desorb from the adsorbent layer from preferentially diffusing back into the interior of the electronic enclosure. Furthermore, in embodiments with a separate recirculation filter, the presence of a diffusion channel in between the interior of the electronic enclosure and the adsorbent material prevents volatile contaminants such as disk lubricants that may be present inside the electronic enclosure from reaching the adsorbent in the breather filter, thereby increasing the useful life of the adsorbent material. 
   One example of a filter  100  of the present invention is shown in  FIG. 1A .  FIG. 1A  shows a cylindrical filter body  101  made up of a top half  112  and a bottom half  120 . The top half  112  and the bottom half  120  of body  101  are attached together. A variety of means can be used for attachment including mechanical compression attachment, adhesive attachment, ultrasonics, etc. An optional release liner  102  is shown adhered to the underside of the bottom half  120 . The release liner  102  is on one side of an adhesive layer (not shown) while the other side of the adhesive layer is adhered to bottom half  120 . The adhesive layer (not shown) facilitates mounting of the filter  100  within an electronic enclosure. A top exterior hole  106  is shown in the top half  112  of the filter and leads to the interior diffusion channel (not shown). 
     FIG. 1B  shows a top plan view of the filter  100  of  FIG. 1A , showing dashed lines for the location of the interior diffusion channel  108  which permits the flow of air from the interior of an enclosure into the interior of filter  100 . The interior diffusion channel  108  is defined by the top half  112  of the filter, in this embodiment. The interior diffusion channel  108  has a top exterior hole  106  on one end and a top interior hole  110  on the other end. The interior diffusion channel  108  provides an airflow path with a cross-sectional area that is small enough to restrict diffusion through the airflow path leading to the interior of the electronic enclosure. The interior diffusion channel may be made with varying cross-sectional areas depending on the amount by which diffusion may be desired to be limited for the particular application. In operation, where the air pressure inside the electronic enclosure is higher than outside the electronic enclosure, air will enter top exterior hole  106 , travel through interior diffusion channel  108  and pass through top interior hole  110  before entering the interior cavity of the filter assembly (not shown). 
     FIG. 1C  shows a bottom plan view of the filter of  FIG. 1A , showing dashed lines for the location of the exterior diffusion channel  116 . The exterior diffusion channel  116  is defined by the bottom half  120  of the filter, in this embodiment. The exterior diffusion channel  116  has a bottom exterior hole  114  on one end and a bottom interior hole  118  on the other end. Bottom exterior hole  114  is disposed to be in fluid communication with a breather hole in the electronic enclosure. While  FIG. 1C  shows an exterior diffusion channel  116  flow path that is similar in shape to the one shown in  FIG. 1B  for the interior diffusion channel  108 , these two diffusion channels may in practice take on different shapes, lengths, and configurations based on the requirements of the particular application. In operation, where the air pressure inside the electronic enclosure is higher than outside the electronic enclosure, air passes through the interior diffusion channel  108  as described above and enters the interior cavity of the filter assembly. From there the air will pass through an adsorbent material and then into the bottom interior hole  118  and travel through the exterior diffusion channel  116  and pass through the top exterior hole  114  before passing through the breather hole of the electronic enclosure and to the exterior of the enclosure. 
   Filters of the present invention with multiple diffusion channels can take on many different configurations. An alternative configuration wherein two diffusion channels are located on the same side of the filter is shown in  FIG. 1D . This alternative configuration filter  121  has a generally cylindrical shape with an end  123  that defines two separate diffusion channels  125  and  127 . Exterior diffusion channel  125  is connected to the outside of the drive enclosure via exterior hole  122 . Exterior hole  122  is configured to be in fluid communication with a breather hole in the electronic enclosure (not shown). There is a first filter hole  124  that connects the exterior diffusion channel  125  with the interior of the filter. Interior diffusion channel  127  is connected to the inside of the drive enclosure via interior hole  126 . There is a second filter hole  128  that connects the interior diffusion channel  127  with the interior of the filter. In situations where filter  121  is positioned with end  123  in contact with the shell of a drive enclosure, interior hole  126  is typically in fluid communication with the interior volume of the drive enclosure via a depression or channel made in the shell of the drive enclosure. However, interior hole  126  may also be in fluid communication with the interior volume of the drive enclosure via other means. Thus filters of the present invention may have multiple diffusion channels on the same side of the filter body or on different sides. 
     FIG. 2  shows a side cross-sectional view of the filter of  FIG. 1A . The housing of filter  100  is formed by a top half  112  and a bottom half  120 . The top half  112  and the bottom half  120  are attached together. A variety of means can be used for the attachment including mechanical compression attachment, adhesive attachment, ultrasonics, etc. 
   Where the air pressure outside the filter is higher than inside the filter, air can flow through the bottom exterior hole  114  and into the exterior diffusion channel  116 . The air can then flow through the exterior diffusion channel  116  and into the interior cavity  138  of the filter assembly. Here the air can interact with the adsorbent material  140 . The air can then flow out of the interior cavity  138  and into the interior diffusion channel  108 . From there the air can flow out the top exterior hole  106  and into the interior of the electronic enclosure. Where the air pressure outside the filter is lower than inside the filter, air will flow in the opposite direction as that described above. 
   Two particulate filter layers  130 ,  132  are shown in  FIG. 2 . An interior side particulate filter layer  130  is shown on top of top half  112 . This particulate filter  130  prevents particulate matter inside the filter assembly from passing into the interior of the electronic enclosure. The particulate filter layer may be constructed of many different materials and is further discussed below. A particulate filter layer may also be positioned in other places in the filter assembly. For example, while the particulate filter layer  130  is shown mounted on the outer surface of the filter housing on top half  112 , the particulate filter layer  130  can also be positioned on the inner surface of the filter housing. Generally, the particulate filter layer  130  is positioned so that all air moving through the filter assembly into the electronic enclosure must pass through it and is positioned in between an adsorbent material  140  and the interior of the electronic enclosure. 
   The adsorbent material  140  can be a chemisorbent and/or physisorbent. The adsorbent material  140  works to remove contaminants from the airflow that moves through the filter assembly. The adsorbent material  140  may have surface features such as one or more projections  142 . In addition to providing a path for fluid flow around the filter media, the projections  142  can also provide an increased surface area for interaction with the fluid. The increase in surface area is typically related to factors, such as, the number of projections, their cross-sectional area, the distance the projections extend from the main body, and the shape of the projections. The projections  142  can also provide obstacles to the direct flow of air over the surface of the shaped adsorbent article and can redirect air flow (e.g., by producing eddy currents) toward that surface, thereby increasing filter efficiency. The adsorbent material  140  may be supported by a layer of scrim  136  on the bottom. The adsorbent material  140  may also, in some embodiments, be supported by a layer of scrim  134  on the top. 
     FIG. 2  shows a cross-sectional view of filter  100 , taken along lines B–B′ of  FIG. 1B . Filter  100  has two diffusion channels,  108 ,  116  with an interior diffusion channel  108  and an exterior diffusion channel  116 . The interior diffusion channel  108  is defined by the top half  112  of the filter, in this embodiment. The interior diffusion channel  108  has a top exterior hole  106  on one end and a top interior hole  110  on the other end. The exterior diffusion channel  116  is defined by the bottom half  120  of the filter, in this embodiment. The exterior diffusion channel  116  has a bottom exterior hole  114  on one end and a bottom interior hole  118  on the other end. Bottom exterior hole  114  is configured to be in fluid communication with a breather hole in the electronic enclosure. 
     FIG. 3  shows an exploded view of an embodiment of the invention. A cylindrical filter body made up of a top half  112  (shown in cross section) and a bottom half  120  (shown in profile). The top half  112  and the bottom half  120  are configured to be attached together. An optional release liner  102  is shown adhered to the underside of the bottom half  120 . The release liner  102  is on one side of an adhesive layer (not shown) while the other side of the adhesive layer is adhered to bottom half  120 . A top exterior hole  106  is shown in the top half  112  of the filter and leads to the interior diffusion channel  108 . 
   The embodiment shown in  FIG. 3  has two adhesive rings,  144 ,  145  for keeping the parts of the filter  100  together. Exterior adhesive ring  144  is configured to be in contact with bottom half  120  and a layer of scrim  136 . Interior adhesive ring  145  is positioned between top half  112  and adsorbent material  140 . The layer of scrim  136  shown is this embodiment is positioned beneath adsorbent material  140  and provides support to the adsorbent. The adhesive rings  144 ,  145  of this embodiment have a ring shape with a center portion that is open. 
   The use of multiple diffusion channels can be applied to many different types of filter construction.  FIG. 4  shows an embodiment of the invention in the context of a label type filter. The label filter  150  has a top layer  154  of a non-permeable material. Top layer  154  defines an interior side hole  152  that leads to an interior diffusion channel (not shown). 
     FIG. 5A  shows a cross-sectional view of the filter of  FIG. 4  taken along lines A–A′ of  FIG. 4 . Top layer  154  is attached to bottom layer  156  at the periphery of the label filter. Top layer  154  and bottom layer  156  define a cavity into which the other components of the filter fit. Top layer  154  defines an interior side hole  152  which leads to an interior diffusion channel  166 . Interior diffusion channel  166  connects to the cavity of the filter via a first filter hole  167 . The interior diffusion channel  166  and the first filter hole  167  are defined by two layers of non-permeable material: a channel layer  170  and a boundary layer  172 . The two layers of non-permeable material  170  and  172  may be attached via a low out-gassing adhesive, although one skilled in the art will appreciate that there are many ways to attach layers of non-permeable material together. 
   Similarly, bottom layer  156  defines an exterior side hole  162  which leads to an exterior diffusion channel  164 . Exterior diffusion channel  164  connects to the cavity of the filter via a second filter hole  165 . The exterior diffusion channel  164  and the second filter hole  165  are defined by two layers of non-permeable material: a channel layer  158  and a boundary layer  160 . The two layers of non-permeable material  158  and  160  may be attached via a low out-gassing adhesive. 
   Inside the cavity defined by the top layer  154  and the bottom layer  156 , and in between the interior diffusion channel  166  and the exterior diffusion channel  164 , is the adsorbent layer  168 . In this embodiment the adsorbent layer  168  is supported by a layer of scrim  174  which underlies the adsorbent layer  168 . 
   In operation, when the air pressure inside the electronic enclosure is greater than outside the electronic enclosure, air will move from the interior of the enclosure through interior side hole  152  and into interior diffusion channel  166 . From there the air will pass through first filter hole  167  before moving into the interior cavity of the filter assembly. The air will pass through the adsorbent layer  168  and a layer of scrim  174  before passing through second filter hole  165 . From there, the air will travel through exterior diffusion channel  164  before exiting through exterior side hole  162 . 
     FIG. 5B  is a top plan view of the filter of  FIG. 5A . Top layer  154  defines an interior side hole  152  that leads to an interior diffusion channel  166 . Interior diffusion channel  166  connects to the cavity of the filter via a first filter hole  167 . 
     FIG. 5C  is a bottom plan view of the filter of  FIG. 5B . Bottom layer  156  defines an exterior side hole  162  that leads to an exterior diffusion channel  164 . Exterior diffusion channel  164  connects to the cavity of the filter via a second filter hole  165 . Exterior side hole  162  is configured to be in fluid communication with a breather hole in the electronic enclosure. 
     FIG. 5D  is a top plan view of an alternative embodiment of a label filter design of the present invention showing an alternative design for the diffusion channels. Top layer  192  defines an interior side hole  194  that leads to an interior diffusion channel  196 . Interior diffusion channel  196  connects to the cavity of the filter via a first filter hole  198 . 
     FIG. 5E  is a bottom plan view of the filter of  FIG. 5D . Bottom layer  200  defines an exterior side hole  202  that leads to an exterior diffusion channel  204 . Exterior diffusion channel  204  connects to the cavity of the filter via a second filter hole  206 . Exterior side hole  202  is configured to be in fluid communication with a breather hole in the electronic device enclosure. 
   The foregoing figures described several embodiments of the present invention. However, one of skill in the art will appreciate that many different embodiments are possible without deviating from the spirit of the invention. Select aspects of the invention will now be discussed in greater detail. 
   Adsorbent Material 
   It will be understood that adsorbent filter material used in accordance with the invention includes materials that adsorb and/or absorb contaminants through physisorption and/or chemisorption. The adsorbent material can include physisorbents and/or chemisorbents, such as desiccants (i.e., materials that adsorb or absorb water or water vapor) and/or materials that adsorb volatile organic compounds and/or acid gas. The adsorbent material may include a single type of material or a combination of materials. Examples of suitable adsorbent materials include, for example, activated carbon, activated alumina, molecular sieves, silica gels, desiccating materials, potassium permanganate, calcium carbonate, potassium carbonate, sodium carbonate, calcium sulfate, or mixtures thereof. The adsorbent material may be in the form of, for example, particles, gels, sheets, webs, tablets, molded articles, or liquids, that are, preferably, held in place within the filter. 
   The adsorbent material may remove a single contaminant or a number of contaminants. Examples of contaminants that may be removed include, for example, water, water vapor, chlorine, hydrogen sulfide, HCl, nitrogen dioxide, acid gases, volatile organic compounds, and hydrocarbon compounds. 
   For typical operation, an adsorbent material that is stable and adsorbs within a temperature range of −40° C. to 100° C. is preferred. Preferably, the adsorbent material is a powder (passes through 100 mesh U.S.S.) or granular material (28 to 200 mesh) prior to forming the adsorbent layer. 
   In some implementations, the adsorbent material is combined with a binder material. The binder is typically dry, powdered, and/or granular and is mixed with the adsorbent. In some embodiments, the binder and adsorbent material are mixed using a temporary liquid binder and then dried. Typically, a binder is used that does not completely coat the adsorbent material. Suitable binders include, for example, microcrystalline cellulose, polyvinyl alcohol, starch, carboxyl methylcellulose, polyvinylpyrrolidone, dicalcium phosphate dihydrate, sodium silicate, latex and polytetrafluoroethylene. 
   The composition of the adsorbent layer can include for example at least about 70%, by weight, and typically not more than about 98%, by weight, adsorbent. In some instances, the adsorbent layer includes 85 to 95%, preferably, approximately 90%, by weight, adsorbent. The adsorbent layer typically includes not less than about 2%, by weight, binder and not more than about 30%, by weight, binder. In some instances, the adsorbent layer includes about 5 to 15%, and, preferably, about 10%, by weight, binder. 
   In some instances, where the adsorbent layer is molded, it may be desirable to include a small amount of lubricant such as PTFE (Teflon® powder) within the composition, in order to facilitate mold release. When PTFE is used, preferably no more than about 10%, and more preferably less than about 3% of the composition, comprises added lubricant. If a lubricant is used, preferably a minimum amount effective to accomplish a desirably reproducible mold release, is used. 
   The adsorbent layer may be supported by one or more support layers, such as a support scrim. Examples of such support layers include woven and non-woven films/fabrics made from, for example, stretched or sintered plastics, such as polyesters, polypropylene, polyethylene, and polyamides (e.g., nylon). In some embodiments, the support layer may be porous and permit substantial cross-flow of fluid across the support layer and into other portions of the filter media. 
   Diffusion Channel Configurations 
   Diffusion channels as used in the present invention may take on many different shapes and may be formed in many different ways. One shape for a diffusion channel is a semi-circular pattern such as that shown in  FIG. 1B . Alternately, a diffusion channel may be substantially straight such as the pattern shown in  FIG. 5D . The channel may also be formed to have a more complex path, such as a winding path or a spiral path. The channel may, in some embodiments, have two or more branches. One skilled in the art will appreciate that there are many different diffusion channel patterns that are possible without deviating from the spirit of the invention. 
   The diffusion channel may be formed with the housing (e.g., molded or compression molded) or may be later formed in the housing by cutting or otherwise removing material from the housing. Alternatively, separate diffusion channel layers, with a diffusion channel defined therein, may be formed as separate pieces and inserted into the interior of the housing or attached, for example, adhesively, to the exterior of the housing. This separate piece may be, for example, a molded article or a polymer film having a channel formed therein. 
   The channel can be formed, for example, by removing a portion of the channel layer. The portion of the channel layer can be, for example, die-cut or otherwise removed using, for example, a stamping apparatus or a rotary press. 
   Such channels have a thickness that typically corresponds to the combined thickness of the channel layer and any adhesives used for attachment. The width of the channel can vary over a wide range. The width of the channel ranges from, for example, 1 mm to 10 mm, although wider or narrower channels may be used. In some embodiments, the width of the channel ranges from 1.5 to 5 mm. The width and thickness of the channel may be chosen to balance the pressure drop of the filter  100  between the channel and the filter media, although this is not necessary to the invention. 
   In some embodiments of the invention, one or more diffusion channels are formed in components that adjoin the filter body and not in the filter body itself. For example, creating a narrow depression in the electronic enclosure itself may form a diffusion channel. In this type of configuration, the breather hole in the electronic enclosure is at one end of the narrow depression and the hole leading into the filter assembly is positioned at the other end of the narrow depression when the filter is attached to the electronic enclosure. 
   Retrofit Applications 
   The present invention may also be used in the context of retrofit applications. Where filters of previous designs may have only one or no diffusion channels, they can be retrofitted with another diffusion channel to improve their performance. The design of the retrofit device will obviously depend upon the particular filter to be upgraded. For example, where one desires to upgrade an old style cylindrical cap type filter so that it is equipped with two diffusion channels, a retrofit cap can be made with a diffusion channel and be configured to simply fit over the top of the old design via a compression type fitting, such as a snap-on fitting. As another example, where the old style filter to be upgraded is a label style filter, a diffusion channel can be fashioned from one or more layers of flexible material and can be adhered to the top of an existing label filter via a layer of a low out-gassing adhesive. One skilled in the art will appreciate that there are numerous ways of crafting retrofit components to convert old style filters into filters with two or more diffusion channels in accordance with the present invention. In the retrofit context, design is only constrained by the shape of the filter to be upgraded. 
   Adhesives 
   The filter of the present invention as constructed in practice may contain a variety of adhesives such as a mounting adhesive, a lamination adhesive, and media lamination adhesive, which may be the same or different. The adhesives are often a single layer of adhesive that is disposed on or applied to, for example, the channel layer, boundary layer, and/or filter media. These adhesives can be disposed on the appropriate layer by, for example, coating, painting, spraying, dipping, or otherwise applying the adhesive to the layer. In some embodiments, adhesive may be pre-applied on a commercially available film. The adhesives can also be double-sided adhesive films that include an adhesive carrier with adhesive disposed on both sides. The adhesive carrier is often a polymer film, such as, for example, a polyethylene, polypropylene, polyester, polycarbonate, polyurethane, or polyvinyl chloride film. 
   In many embodiments, the adhesives include only low out-gassing adhesives. Out-gassing includes the release and/or production of gaseous or other contaminants by the adhesive. Out-gassing by an adhesive or other component of the filter can produce additional contaminants that are often introduced into the fluid and removed by the filter media. Contamination of the fluid by adhesive out-gassing can also be decreased by reducing, and, preferably, minimizing, the exposure of the fluid flowing through the filter to the adhesives. Often, adhesives are chosen which meet ASTM E-595-84 specifications with 1% or less total mass loss and 0.1% or less collected volatile condensable material. This, however, is not necessary to the invention. 
   Typically, the adhesives in the filter have individual thicknesses that range from 10 μm to 150 μm, although thicker or thinner adhesives may be used. Often, the adhesives of the filter have a thickness that ranges from 15 μm to 50 μm. 
   Particulate Filter Layer 
   Filters of the present invention may contain one or more particulate filter layers to prevent particulate contaminants from entering the electronic enclosure from the filter assembly. Such particulate contaminants may originate outside of the electronic enclosure or may be shed from the adsorbent material itself. Filters of the present invention may also include particulate filter layers to prevent particulate contaminants from entering the filter assembly from outside of the electronic enclosure. They may be disposed on the outside of the filter assembly as shown in  FIG. 2  or disposed inside of the filter assembly. 
   The particulate filter layer may comprise a variety of porous or microporous membranes. The size of the pores in the membranes and the thickness of the membranes often determine, at least in part, the size of particles allowed through the membrane and/or filter. 
   Often the porous or microporous membranes are formed from polymers. Examples of suitable porous or microporous membranes include porous or microporous polyethylene, polypropylene, nylon, polycarbonate, polyester, polyvinyl chloride, polytetrafluoroethylene (PTFE), and other polymeric membranes. One particularly suitable membrane is formed using expanded PTFE, which is described as having nodes and fibrils. 
   Filter Housing 
   The filter housing may be, for example, an outer covering, a casing, or a shell. The housing is typically formed from a plastic material, such as, for example, polycarbonate, polyvinyl chloride, nylon, polyethylene, polypropylene, or polyethylene terephthalate (PETG). The housing may be a single piece or, alternatively, the housing may be formed as two or more pieces that are combined together using, for example, an adhesive, mechanical connectors, heat sealing, and/or ultrasonic welding to form, for example, a perimeter seal. 
   The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the instant specification.