Filter having a change indicator

Disclosed is a change indicator for a filter for use in a gaseous circulation system, such as a forced air heating and cooling system. The change indicator includes an air impervious patch that is secured to the filter. The portion of the filter media adjacent to the change indicator changes appearance in response to the accumulation of contaminants at a much slower rate than the remainder of the air filter. The accumulation of contaminants progresses inwardly from the region adjacent the periphery of the change indicator towards the region adjacent to the center. Once the appearance of the filter media beneath the change indicator fully changes from periphery to the center, that is an indication that the air filter is in need of cleaning or replacement.

FIELD OF THE INVENTION 
This invention relates generally to filters and more specifically to 
filters having change indicators. 
BACKGROUND OF THE INVENTION 
Filters have been devised in the past to remove various particulate 
contaminants from a moving gas stream, and specifically for air streams. 
Such contaminants may, for example, include dust, lint, smoke, pollen or 
dander. For instance, in conventional Heating, Ventilating and Air 
Conditioning (HVAC) Systems, a filter is inserted into the air stream to 
remove contaminants carried along in the air stream. The filtered air may 
then be recirculated throughout the HVAC system. 
Conventional air filters generally include a filter media constructed from 
a porous material, such as open cell foam or a non-woven web of materials 
such as fiberglass. As a stream of air is induced to flow through the 
filter, particulate contaminants are entrapped or separated from the air 
stream and accumulated on the filter media. After a period of use in this 
manner, the accumulated contaminants begin to interfere with the flow of 
the air stream through the filter. The increased pressure drop across the 
filter impedes the airflow through the air filter and potentially reduces 
the effectiveness of the entire HVAC system. This requires that the air 
filter be removed and either cleaned or replaced with a new air filter. 
However, typically the filter is hidden from view during normal use, 
making it difficult to determine the condition of the filter without 
removing it from the HVAC system for visual inspection. Visual inspection 
may be done periodically or on a predetermined schedule. However, visual 
inspection alone may not be entirely reliable since a filter may appear to 
be fully loaded with contaminants, when in fact only a portion of the 
capacity of the filter has been utilized. 
However, devices have been provided in the past that generate a change 
signal to alert a user to the clogged condition of the filter and indicate 
the need for inspection, cleaning or replacement. Such devices include 
simple pressure sensors that measure the pressure on the downstream side 
of the filter and generate a signal if the pressure drops below a 
predetermined point. In another example, U.S. Pat. No. 4,321,070, issued 
to Bede, provides a whistle mounted in the filter. When the filter becomes 
sufficiently clogged, the air stream is induced to flow through the 
whistle portion at an increasing rate until an audible signal is produced. 
U.S. Pat. No. 2,782,747, issued to Alderfer, U.S. Pat. No. 2,753,831, 
issued to Davies, and PCT patent application Ser. No. 88/01050 to 
Horowitz, all also disclose an audible change signal device. 
U.S. Pat. No. 2,804,839 discloses a device external to the filter that 
produces both an audible change signal and visual change signal responsive 
to an elevated differential pressure drop on either side of the filter. 
The visual signal is evident external of the filter and does not require 
removal of the filter itself and visual inspection. However, this device 
is complicated and expensive relative to the price of the air filter 
itself and thus does not lend itself to widespread use. 
U.S. Pat. No. 3,027,865, issued to Kuntz et al. incorporates a diaphragm 
that shifts position responsive to a change in the pressure drop across an 
air filter. The shift in the position of the diaphragm may be used to 
provide a visual signal or to close an electrical contact in order to 
provide an electrical signal externally of the filter. U.S. Pat. No. 
3,916,817 also discloses a device for providing a visual change signal. 
However, none of these existing air filter change indicators are completely 
satisfactory. For instance, those that require elaborate and complicated 
apparatus to provide a change signal are relatively expensive, and may 
require setup at installation and periodic maintenance to ensure reliable 
operation. Those devices that depend on a reduction in airflow to provide 
an audible signal may be susceptible to clogging by the very contaminants 
that the air filter is designed to entrap. It is also generally not 
desirable to require modification to the air circulation system or to the 
air filter, as many of these approaches teach. 
Moreover, all of these devices provide only one signal. That is, they only 
indicate when the air filter has reached a predetermined level of lowered 
performance. It may be desirable in certain situations to have a change 
indicator that provides a continuous and progressive indication of the 
performance of the filter. 
SUMMARY OF THE INVENTION 
The present invention provides a filter for filtering particulate 
contaminants from an air stream. The filter includes a filter media for 
filtering the particulate contaminants from the air stream. The filter 
media has a first major surface and a second major surface for airflow 
through the filter media from the first major surface to the second major 
surface. The filter includes an air impervious indicator patch having a 
first major surface and a second major surface, a periphery and a center. 
Means are provided that are adapted to support the indicator patch at a 
desired location adjacent to a portion of at least one of the first major 
surface or second major surface of the filter media. The indicator patch 
is adapted to impede the air flow through the filter media and divert the 
air stream so that contaminants are progressively accumulated in the 
filter media from the periphery towards the center of the air impervious 
indicator patch as an indication of use of the air filter. 
In one embodiment of the invention, the indicator patch is attached to the 
filter with the second major surface of the indicator patch adjacent to 
the first major surface of the filter. At least a portion of the indicator 
patch is light transmissive to enable visual perception of the portion of 
the filter media adjacent the indicator patch. 
In another embodiment of the invention, the indicator patch includes a 
layer of pressure sensitive adhesive on the second major surface of the 
indicator patch and the indicator patch is adhered to the first major 
surface of the filter media by the layer of pressure sensitive adhesive. 
In another embodiment of the invention, the indicator patch is attached to 
the second major surface of the filter and the indicator patch may be 
opaque.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 1 and 2, there is shown a filter 10 according to the 
present invention. For the purposes of this invention, the filter 10 will 
be generally referred to as an air filter, but it is to be understood that 
the present invention is equally applicable to any filter for use with a 
gaseous stream having particulate contaminants. Filter 10 includes a 
filter media 12 having a first major surface 14 and a second major surface 
16 (not shown in FIG. 1). The filter media may be composed of any suitable 
material that may be utilized to filter particulate contaminants from a 
gas stream. The following is a non-exclusive list of such materials: 
Fiberglass, cotton/polyester blends, carded, air laid, spunbonded or 
needled nonwoven polymeric webs, blown microfibers, expanded foam, 
reticulated foam, woven or punched plastic screens, or combinations of the 
above materials. 
In one preferred embodiment of the present invention, the filter media is 
comprised of a nonwoven web having electret fibers. Nonwoven webs of 
electret fibers are typically formed of loosely associated 
electret-charged fibers. The filters can be electrostatically charged 
prior to, during, or after being formed into a nonwoven web. For example, 
post-formation charging is described in U.S. Pat. No. 4,588,537 which 
charges a lofty nonwoven web which can be formed by a variety of methods 
including carding and melt blowing. The webs are charged while under 
compression and then permitted to return to their original loft. 
Fibers can also be charged while they are being formed as disclosed in U.S. 
Pat. No. 4,215,682 (Kubik et al.), where melt-blown fibers are bombarded 
by ions or electrons immediately after being extruded from melt-blowing 
orifices. The fibers solidify extremely rapidly in the atmosphere and are 
collected as a semi-coherent mass of entangled microfibers as the fiber 
web. The fiber webs are described as preferably open to provide a 
low-pressure drop for liquid passing through a filter formed of the 
fibrous web. 
Fibers can also be charged as described in U.S. Pat. No. 4,798,850. This 
patent describes blending different fibers together, which when properly 
selected will induce an electrostatic charge in the fibers as the fibrous 
web is formed. Other patents relating to charging fibers or fibrous webs 
include U.S. Pat. Nos. 4,904,174; 5,122,048; 5,401,446; 4,592,815; and 
5,496,507. 
A particularly effective method of forming a nonwoven electret fiber filter 
web is described in U.S. Reissue Pat. No. 30,782 (Van Turnout et al.). The 
electret fibers in this patent are formed from a corona charged film that 
is fibrillated to form the charged fibers. The charged fibers can then be 
formed into a nonwoven filter web by common methods such as carding or air 
laying. This charging method provides a particularly high density of 
injected charges in the finished fibers. However, problems are often 
encountered with forming webs from these precharged fibers. The fibers are 
generally quite large and uncrimped. They also have a resistance to 
bending. Due in part to these properties, the fibers resist formation into 
a uniform coherent web, particularly at low basis weights. This problem is 
partially addressed in U.S. Pat. No. 4,363,682, which proposes the use of 
such fibrillated fiber webs in facemask applications. In order to provide 
a more coherent web, as well as one that resists shedding fibers, this 
patent proposes a post-embossing treatment. This post-embossing welds the 
outer surface fibers together allegedly providing a more coherent and 
comfortable web for use as a facemask. However, this treatment will also 
tend to result in a more condensed web, which would increase pressure-loss 
across the filter web and decreases filter life. 
An improvement over the embossing treatment is disclosed in U.S. Pat. No. 
5,230,800. This patent proposes needle-punching the fibrillated electret 
fiber filter webs (e.g., prepared via the Van Turnout et al. method) onto 
a scrim support. The result is a consolidated coherent fibrous filter 
composite material with improved uniformity and filtration performance. 
A nonwoven electret fiber filter media and filter is obtained by forming 
the filter media using a nonwoven web of electret fibers on an open scrim 
support. An unsupported nonwoven fibrous filter web is placed onto the 
open, substantially non-extensible, scrim support material. The scrim 
support material has individual discrete open areas with an average 
cross-sectional area as viewed from the plane of the filter media of at 
least 0.25 mm.sup.2, preferably 1.0 mm.sup.2, and a pressure drop across 
the scrim support, without the filter web, of less than 1.5 mm H.sub.2 O 
measured at 98.4 meters/min face velocity. The unsupported fibrous filter 
web and the scrim support are joined to form the filter media by 
needle-punching the filter web and scrim support material to provide a 
highly uniform nonwoven fibrous filter media with enhanced lifetime and 
filtration performance. At least certain of the fibers forming the 
nonwoven fibrous filter web are provided with an electret charge. 
Referring now again to FIGS. 1 and 2, arrow 18 indicates the flow of air 
through the air filter 10. In one application of the present invention 
(not shown), the filter media is applied or suspended from a pre-existing 
structure. For instance, the filter of the present invention may take the 
form of a generally planar sheet mounted over the intake of a room air 
conditioner. Means may be provided to secure the filter in this position, 
such as mechanical clips or pressure sensitive adhesive tape. Another form 
of a filter to be used in regard to the present invention is a bag filter 
that is mounted about an aperture in a structure in a manner known in the 
art and through which a gaseous stream is ejected. Other filter forms may 
be utilized as part of the present invention as are found convenient or 
advantageous in a particular application. 
More commonly, filter media 12 is a sheet like member supported by a 
surrounding frame 20 to which the periphery of the web is secured. For 
instance, as shown in FIG. 1, the frame may have a generally u-shaped 
peripheral cross-section (not shown) for receipt of the periphery of the 
filter media 12. Any suitable material may be used for the frame 20, such 
as metal or rigid polymeric materials. Most preferably, cardboard or 
pasteboard is formed into the desired shape and the web secured thereto, 
such as by a suitable adhesive. 
In one embodiment of the invention, the filter media includes a supporting 
polymeric mesh or scrim (not shown) that is positioned adjacent one or 
both of the first major surface 14 or second major surface 16 of the web. 
The scrim acts to reinforce the filter media against the force of the 
airflow passing through the filter media. The scrim may be formed from a 
polymeric material or may be constructed of any other suitable material, 
such as an expanded metal. In the preferred embodiment of the invention, 
as illustrated in FIGS. 1 and 2, frame 20 includes an integral lattice 22 
on one or both sides of the web 12 to provide the necessary support 
against the pressure of the air stream against the air filter 10. 
The filter media need not be planar. As illustrated in FIGS. 1 and 2, the 
filter media may preferably be pleated or otherwise formed into a three 
dimensional surface to increase the amount of filter media surface area 
exposed to the air stream for a given cross sectional area of the filter. 
The change indicator for the present invention is shown at 30 and in 
greater detail in FIGS. 3-5 and 6-8. Change indicator 30 includes air 
impervious patch 32 having a first major surface 34 and an opposing second 
major surface 36. Patch 30 represents a relatively small portion of the 
effective surface area of the filter media 12 so that the overall 
effectiveness of air filter 10 is not significantly impaired. For the 
purposes of this invention, the term "air impervious" includes not only 
materials that are solid and preclude the passage of air through them such 
as polymeric films, but also air resistive materials. By "air resistive" 
it is meant materials that are not totally impervious to the flow of air 
through them, but impede the flow of air through themselves to the extent 
needed to provide the shadowing qualities described hereinafter in regard 
to the change indicator of the present invention. Examples of air 
resistive materials include cloths, non-woven fabrics, paper or any other 
materials having similar properties. 
Means are provided to secure the patch 32 on or adjacent to filter media 
12. For the purposes of this invention, the term "adjacent` includes 
placing the indicator patch directly in contact with the filter media of 
the filter, as well as placing the indicator patch spaced from but in 
sufficient proximity to the filter media as to engender the "shadowing" 
effect described herein. 
Preferably, indicator patch 32 is a flexible material, such as a flexible 
polymeric film, to enable the indicator patch to be placed directly on and 
conform closely to the profile of the filter media to which the indicator 
patch is secured, as shown particularly in FIGS. 1 and 2. However, it is 
within the spirit and scope of the present invention to mount the 
indicator patch to the frame 20 or the lattice 22 or any other suitable 
structure with the indicator patch spaced from the filter media 12. 
Mechanical clips (not shown) may be provided that engage both the patch and 
the filter media 12, frame 20 or lattice 22 to secure the patch thereto. 
Any other suitable method may be employed to secure the patch 32 to the 
filter, such as ultrasonic bonding, heat sealing, stapling, sewing, and 
stitching, or the like. Preferably, a layer of adhesive 38 may be utilized 
to secure the patch to the filter 10. Any suitable adhesive may be 
employed, such as a heat-activated adhesive. Most preferably, a pressure 
sensitive adhesive is employed. Conveniently, a portion of a pressure 
sensitive adhesive tape may be used to construct the change indicator, 
with the polymeric film backing of the tape acting as the air impervious 
patch and the pressure sensitive adhesive layer of the tape being used to 
secure the tape portion to the air filter. The following is a 
non-exclusive list of pressure sensitive adhesive tapes that may be used 
to form the change indicator 30 of the present invention: Nos. 355, 810, 
371, 373, 605, 610 and 821 pressure sensitive adhesive tapes available 
from the Minnesota Mining and Manufacturing Company of St. Paul, Minn. 
Change indicator 30 further includes means for progressively indicating use 
and decreasing performance of the air filter. As is shown sequentially in 
FIGS. 3-5 and correspondingly in FIGS. 6-8, contaminants carried in the 
airflow in direction 18 encounter the air filter 10. The majority of the 
filter media 12 that is exposed to the airflow enables the airflow to 
travel through the filter media from first major surface 14 to the second 
major surface 16 and acts to entrap or remove contaminants therefrom in a 
normal fashion. As is expected, the greater the amount of contaminants 
trapped in the filter media, the greater the change in appearance of the 
filter. Generally, the filter media is a lighter color that gradually 
darkens as entrapped contaminants builds up in the filter media, although 
in some applications it may be desirable for the filter media to be a 
darker color and the contaminants act to gradually lighten the appearance 
of the filter. The change in color and appearance is dependent on the 
nature of the contaminants encountered. In most residential applications, 
it is preferable to provide a filter media that is white or light gray, 
with the normally encountered contaminants being dust and lint that 
gradually turn the appearance of the filter media to a dark gray or black. 
It is sufficient that the normal color of the filter media and the color 
of the contaminants be contrasting to enhance visual perception of the 
accumulation of the contaminants. 
When the airflow encounters the relatively small area covered by patch 32, 
the airflow is diverted about the patch to adjacent uncovered portions of 
the filter media. As the diverted airflow travels about the periphery 40 
of the patch, there is a zone 42 of relatively low pressure (compared to 
ambient pressure in the air stream) created by this "shadowing" effect 
adjacent the second major surface 36 of the patch and second major surface 
16 of the filter media. This low-pressure zone 42 tends to divert the 
airflow adjacent the periphery 40 of patch 32 towards the low-pressure 
zone indicated generally at the center 44 of second major surface 34 of 
the patch. Over time, contaminants 46 from the diverted airflow also 
accumulate in the portion of the filter media 12 covered by the patch. 
This process of accumulation progresses from the portion of the filter 
media at the periphery 40 of the patch inwardly towards the portion 
adjacent center 44 of the patch. The remainder of the filter media 
accumulates contaminants at a relatively uniform rate at all portions 
exposed to the air stream. 
In the embodiment illustrated in FIGS. 1-5 (excluding the embodiment of the 
present invention shown in FIG. 4a, discussed hereinafter), at least a 
portion of the patch 32 is light transmissive that extends generally from 
the periphery towards the center of the indicator patch. Preferably, the 
entire indicator patch is light transmissive. For the proposes of this 
invention, the term "light transmissive" includes materials that are 
transparent or translucent to a degree sufficient for visual perception of 
the filter media underlying the patch 32 through first major surface 34 of 
the patch 32. 
Thus, as the major portion of the filter media changes appearance (e.g. 
darkens) during use due to the uniform accumulation of contaminants, the 
portion covered by the change indicator gradually changes appearance 
progressively from the periphery towards the center of the patch. The 
graduated change in appearance permits periodic visual inspection through 
the light transmissive qualities of the patch to determine the state of 
the filter and a convenient gage of the effectiveness of the air filter 
and the point at which the air filter should be cleaned or replaced. 
Referring now to FIG. 4a, an alternative embodiment 10' of the present 
invention is illustrated in which the change indicator 30' is mounted on 
or adjacent to the second major surface 16 of the filter media 12. In 
contrast to the low pressure zone created behind the indicator patch 30 
shown in FIGS. 3-5, the indicator patch 30' of FIG. 4a impedes the 
encountered air stream so as to create a relatively high pressure zone 48 
in front of patch 32. This relative high-pressure zone 48 acts to divert 
the air stream away from the patch and about the periphery 40 of the 
patch, in a reverse shadowing affect. In this manner, the accumulation of 
contaminants 46 in the filter media from the air stream progresses from 
the periphery towards the center 44 of the patch as described hereinabove 
with respect to the embodiment illustrated in FIGS. 3-5. It will be 
appreciated that in regard to this embodiment, the indicator patch need 
not be light transmissive, since the accumulation of contaminants is 
directly perceptible by looking at the first major surface 14 of the 
filter media. Thus, in all other respects, this embodiment 30' of the 
indicator patch is essentially the same as the embodiment 30. 
Referring now to FIGS. 9 and 10, there is shown an embodiment 10" of the 
present invention with a change indicator 30" having an indicator patch 
32" that may be placed on either first major surface 14 or second major 
surface 16 of the filter media. As illustrated in FIGS. 9 and 10, the 
indicator patch is preferably attached to the first major surface 14 of 
the filter media 12. Preferably, indicator patch 32" is constructed of an 
"air resistive" material and is not completely impervious to air or gas 
streams. Most preferably, the indicator patch material may be constructed 
of filter media material that accumulates or loads contaminants at a rate 
that may be different from that of the filter media 12 of the filter 10". 
Any suitable means may be employed to attach the indicator patch 30" to the 
filter 10" or filter media 12. In the illustrated embodiment, it has been 
found convenient to employ a strip of doubled sided pressure sensitive 
adhesive tape 60. The double-sided pressure sensitive adhesive tape 60 
includes a film backing 62 having a first major surface 64 and an opposing 
second major surface 66. A layer of pressure sensitive adhesive 68 is 
coated on both major surfaces 64 and 66 of the film backing strip 62 to 
secure the indicator patch to the filter. The following is a non-exclusive 
list of double sided pressure sensitive adhesive tapes that may be 
employed with this embodiment of the invention: Nos. 665, 136 and 137 
available from the Minnesota Mining and Manufacturing Company of St. Paul, 
Minn. 
It will also be appreciated that an alternative to double sided pressure 
sensitive adhesive tape is to employ a strip of pressure sensitive 
adhesive transfer tape (not shown) that has no film backing, but is a 
layer of pressure sensitive adhesive tape that may be used to adhere the 
indicator patch to the filter. The following is a non-exclusive example of 
a pressure sensitive adhesive tape that may be used in conjunction with 
the present invention: No. 924 available from the Minnesota Mining and 
Manufacturing Company of St. Paul, Minn. 
The presence of the adhesive strip 60 or adhesive transfer tape acts as an 
air impervious layer that blocks the passage of the air stream through the 
indicator patch and creates the shadowing effect on the indicator patch 
itself. 
Indicator patch 30" is particularly well adapted to embodiments of the 
filter in which the filter media includes two or more layers of filter 
media (not shown). For instance, the layer of filter material closest to 
first major surface 14 may have a filtering capacity for larger diameter 
contaminants, whereas the subsequent encountered layers of filter material 
are constructed to filter smaller diameter contaminants. In such a case, 
it may be desirable to provide an indicator patch that is representative 
of and may be calibrated to disclose the condition of one or more of the 
layers of the filter media that are not directly visually perceptible from 
the first major surface. 
It will also be appreciated that although the illustrated shape of the 
indicator patches of the present invention is round, any suitable shape, 
such as square, rectangular, or triangular may be employed as is found 
advantageous in a particular application. In particular, a triangular 
shaped indicator patch enables an expanded scale to be established 
(preferably with indicia) that more efficiently displays the progressive 
accumulation of contaminants adjacent the indicator patch. The continuous 
and progressive indication of the accumulation of contaminants will still 
proceed from the periphery towards the center of whatever shape the 
indicator patch takes. 
In another example, severing a segment of pressure sensitive adhesive tape 
from a roll of tape may conveniently form the patch and the rectangular 
segment produced thereby applied to a filter to form the indicator patch. 
This eliminates the need to form the patch by die cutting or a similar 
process that produces a preferred shape such as circular, but that also 
produces waste or weed material. 
It will also be appreciated that the size and configuration of an indicator 
patch for a particular application may be calibrated to take into the 
account the characteristics of the filter media, the HVAC systems, the 
expected contaminants to be encountered and the air impervious qualities 
of the indicator patch material, along with any other pertinent factors to 
provide an accurate indication of the accumulation of the contaminants in 
the air filter. 
Although not shown, various indicia may be imprinted on the indicator patch 
to allow a user to estimate the amount of use and remaining life of a 
filter. For instance, a row of parallel lines or concentric circles may be 
added to the patch extending from the periphery inwardly towards the 
center of the patch to facilitate quick and accurate reading of the 
progressive accumulation of the contaminants. Another approach would be to 
have words or alphanumeric markings that are in contrasting colors to the 
filter in both a new or completely used states. For instance, if the 
normal color of the filter media is white, indicia in a dark color may be 
added to the patch, such as "use", that is easily visible against the 
background of the filter media. When the filter media is loaded with 
accumulated contaminants, the appearance of the filter media changes to a 
darker color. The "use" indicium is then much harder to distinguish 
against the darker background. Correspondingly, indicia may be added to 
the patch, such as "change", in a light color that is hidden from view 
when viewed against the matching light color of the unused filter media. 
When the accumulation of contaminants is sufficient, the darker "use" 
indicium is hidden and the lighter colored "change" indicium contrasts 
against the darker color of the filter media and is easily discernable. 
It will also be appreciated that although the filter of the present 
invention has generally been described as being an air filter, the 
indicator patch of the present invention may be configured and constructed 
for use in filtering particulates from any gaseous flow, and is not 
limited to air. 
The following are several non-exclusive examples of embodiments of the 
present invention: 
EXAMPLES 
Round patches of transparent tape were applied to the upstream face of 3M 
Filtrete brand furnace filters 16 inches by 20 inches by 1 inch (Catalog 
No. 9800). The tapes used were 3M brand #355 Packaging Tape and 3M brand 
Energy Saving Products Sealing Tape. Round patches of both 2" and 1" 
diameter of the #355 tape and 1" diameter round of the Energy tape were 
applied to the surface of the pleated filters with the adhesive side 
against the filter media and lightly pressed down. The filters were 
measured for pressure drop at 300 feet per minute (fpm) when clean or 
unused. The filters were then used in a residential environment for a 
period of two months with a fan continuously running to accelerate the 
filter loading. The filters were periodically measured for pressure drop 
in a laboratory setting at a velocity of 300 fpm. The progression of the 
discoloration was measured along with the pressure drop. Each of the tape 
spots showed similar progression of the discoloration, regardless of spot 
size or tape type. 
______________________________________ 
Pressure drop 
Color penetration from edge 
Day (Pa) (Centimeters) 
______________________________________ 
1 60.0 0 
12 62.5 .25 to .50 
21 67.5 .50 to .75 
33 70.0 .50 to .75 
42 75.0 .75 to 1.00 
63 82.5 .75 to 1.00 
______________________________________ 
The present invention has now been described with respect to several 
embodiments thereof. It will be apparent to those skilled in the art that 
many changes can be made to the embodiments described herein without 
departing from the spirit and scope of the present invention. Thus the 
scope of the present invention should not be limited to the structures 
described herein, but only by the structures described in the language of 
the claims and equivalents of those structures. It is also to be 
understood that the contents of all patents identified herein are 
incorporated by reference in their entirety. 
For instance, although the filter and change indicator of the present 
invention may be used as shown and described hereinabove without any 
supplemental apparatus and relying solely on visual inspection to monitor 
the state of use of the filter, it will be recognized that it is within 
the spirit and scope of the present invention to provide remote sensing 
apparatus, such as electronic optical sensors that monitor the color of 
the indicator patch. The optical sensor may provide a signal automatically 
when the color of the indicator patch achieves a predetermined state. This 
arrangement, although requiring additional apparatus, may be found to be 
advantageous in certain applications. 
It will also be appreciated that, if desired, more than one indicator may 
be employed as part of the change indicator, with each of the indicator 
patches having differing characteristic sensitivity to loading of 
contaminants on the filter media.