Abstrict A peak flow meter for measuring a peak flow or air exhaled by a
patient. The preferred peak flow meter includes a substantially
hollow housing having a top portion, a bottom portion, an air inlet
and at two air outlets. A flow restriction is disposed within the
housing and in fluid communication with the air inlet. A vane assembly
is disposed within the housing and includes a vane, a post to which
the vane is fixedly attached, and a hub attached to the bottom portion
of the housing. A torsion spring is engaged at one end to the hub
and at the other to the post A visual indictor is movably disposed
within the slot for indicating a peak flow rate of air based upon
a movement of the vane and a scale is disposed proximate to the
slot such that the location of the visual indicator may be related
to a corresponding flow rate.
Claims What is claimed is:
1. A peak flow meter comprising: a substantially hollow housing
consisting of a top portion having a slot and a scale disposed proximate
to said slot, a bottom portion, an air inlet and at least one air
outlet; a flow restriction disposed within said housing and in fluid
communication with said air inlet, said flow restriction being dimensioned
to create a back pressure within said housing; a vane assembly disposed
within said housing, said vane assembly comprising a vane, a post
to which said vane is fixedly attached, and an adjustable hub attached
to said bottom portion of said housing, wherein said adjustable
hub is dimensioned to allow said post to be rotated to a predetermined
position; a torsion spring comprising a first end engaged with said
adjustable hub and a second end engaged with said post; and an indicator
for indicating a peak flow rate of air based upon a movement of
said vane, wherein said indicator is a visual indictor movably disposed
within said slot, said visual indicator being dimensioned to be
moved by said vane when said vane is rotated by said stream of air
and to maintain a peak flow position within said slot upon cessation
of said stream of air; wherein a user blows a stream of air into
said air inlet, a first portion of said stream of air passes through
said flow restriction and is vented through said at least one air
outlet, a second portion of said stream of air contacts said vane
and causes said vane to rotate against said torsion spring, and
said indicator indicates the peak flow rate of said stream of air
based upon said movement of said vane.
2. The peak flow meter as claimed in claim 1 wherein said visual
indicator is a unitary plastic indicator having a flexible tab for
maintaining said peak flow position within said slot upon cessation
of said stream of air.
3. The peak flow meter as claimed in claim 1 wherein said slot
forms an arc about an axis defined by a centerline of said post
of said vane assembly, said arc subtending an angle of more than
one hundred and eighty degrees.
4. The peak flow meter as claimed in claim 1 wherein said at least
one air outlet comprises two air outlets disposed within a flow
portion of said housing, said air outlets being dimensioned to allow
air to freely exit said housing, and wherein said housing further
comprises a back vent.
5. The peak flow meter as claimed in claim 1 further comprising
an air filter assembly removably attached to said air inlet.
6. The peak flow meter as claimed in claim 5 wherein said air filter
assembly comprises a substantially hexagonal filter portion.
Description FIELD OF THE INVENTION
The present invention relates to the field of spirometry and, in
particular, to a portable peak flow meter and kit for measuring
the peak flow of air exhaled by a user.
BACKGROUND OF THE INVENTION
Asthma is a disease that effects about 4% of the population of
the United States or about 15 million people. The incidence of asthma
has grown at an alarming rate, up nearly 75% in the past decade.
It is a very complex disease, having multiple etiologies involving
inflammatory cells, mediators, and the cells and tissues that line
airways. Symptoms are many and varied including wheezing, shortness
of breath, chest tightness and cough. Several classes of medications
have been developed to combat asthma such as beta agonists, anti-inflammatory
and anti leukotrienes. Aggressive management plans have been designed
that include medication, monitoring and lifestyle adjustments to
maintain patients at optimal levels of health.
The diagnosis of asthma includes a thorough history and physical
exam as well as objective tests of pulmonary function using simple
spirometry. Evidence of reversible airway obstruction is a hallmark
of the disease. Occasionally patients do not present with airway
obstruction at the time of exam. In these cases, asthma is confirmed
by a positive response to the inhalation of methacholine, a known
asthma provocateur.
Asthma is typically monitored using a simple device to measure
Peak Expiratory Flow Rate (PEFR). Peak flow meters are widely used
for this purpose. Peak flow meters measure the muscular effort to
exhale forcibly from fully inflated lungs as well as the elastic
recoil of the lungs and airways function. Thus, peak expiratory
flow rate is a simple expression of a complex process of lung emptying.
A wide variety of peak flow meters are commercially available and
many more have been patented, but not commercialized. One popular
type of peak flow meter is the linear, or "in-line", peak
flow meter. These flow meters typically include a cylindrical housing
with an air inlet having an integral air restrictor at one end,
an air outlet at the other end, and slot into which an indicator
is movably disposed. A piston is disposed within the housing and
in contact with a compression spring. When the user blows into the
housing, the flow restrictor restricts the flow of air and the piston
moves against the force of the air, causing it to move the indicator
within the slot as the spring is compressed. When airflow is ceased,
the spring forces the piston back to its original position, but
the indicator remains at its "peak flow" position within
the slot. The position of the indicator is then compared to graduations
along the side of the slot to determine the "peak flow"
of air from the user.
Although these flow meters are relatively popular, they have a
number of drawbacks. First, in order to obtain an acceptable level
accuracy, a relatively long spring and housing must be utilized,
making the meter cumbersome to transport. Second, the arrangement
of the spring, and variations in effective lengths due to manufacturing
tolerances, makes these meters difficult to zero. Accordingly, these
meters require the use of springs with extremely tight manufacturing
tolerances. Third, the location of the air outlet at the end of
the meter makes them prone to blockage or influence by the fingers
of the user, resulting in corruption of test results. Finally, the
location of the air restrictor within the air outlet makes them
prone to blockage or influence as well, again resulting in corruption
of test results.
A second type of peak flow meter is the "electronic type"
of peak flow meter. These meters are typically battery powered and
electronically measure the pressure drop across a flow restriction.
This peak flow of air from the user is then displayed on a liquid
crystal display, or other visual display, and may be stored for
later download into a personal computer or other electronic device.
Electronic peak flow meters are relatively accurate and are easy
to use. However, they are also generally too large to be truly portable
and are much more expensive than non-electronic versions. Further,
these units will not operate without properly charged batteries,
creating the risk that user will not detect the onset of an asthmatic
attack when the batteries are low or completely depleted.
A third type of peak flow meter is the "rotary" peak
flow meter. An example of a "rotary" peak flow meter is
currently marketed by the assignee of the present invention, Spirometrics,
Inc. of Gray, Me., under the trademark "SPIRO-FLOW.TM.".
Rotary peak flow meters typically include a relatively thin rectangular
housing having an air inlet at one end, an air outlet having an
integral air flow restriction extending from the other, and a semicircular
slot disposed along the side of the housing into which an indicator
is movably disposed. A vane assembly is rotatably attached to a
torsion spring and is dimensioned such that it interferes with the
indicator when it is rotated. The torsion spring is fixedly attached
to the housing and is oriented to counteract the force of the air
on the flag when the user blows into the housing. In operation,
the flag is caused to rotate by the force of the air, causing the
indicator to move within the slot. When airflow is ceased, the torsion
spring causes the flag to rotate back to its original position,
but the indicator remains at its "peak flow" position
within the slot. The position of the indicator is then compared
to graduations along the side of the slot to determine the "peak
flow" of air from the user.
Rotary peak flow meters do not require the use of long compression
springs to operate and, therefore, can be made small enough to be
truly portable. However, conventional rotary peak flow meters are
not without drawbacks. First, the accuracy of the measurement is
dependent, in part, upon the relative positions of the ends of the
torsion spring. Given the tolerances inherent in the process of
manufacturing the torsion springs, the variation in this positioning
can result in significant inaccuracies. These inaccuracies can be
counteracted by customizing each scale for each particular spring.
However, this process requires that each unit be tested to generate
the values for this custom scale, causing a corresponding increase
in cost. Second, like the linear, or "in-line", peak flow
meters, rotary peak flow meters include a single air outlet. As
was the case with these meters, this outlet is prone to blockage
by the finger of the user, resulting in corruption of test results.
Finally, the location of the air flow restriction within the air
inlet makes them prone to the same corruption as their linear counterparts.
Therefore, there is a need for a peak flow meter that is small
enough to be truly portable, that is sufficiently accurate and repeatable
to provide meaningful results, that does not require a customized
scale to provide the necessary level of accuracy, that is not prone
to corruption of test results through blockage by a user's finger
of the air outlet or flow restrictor, that may be manufactured and
sold at a relatively low cost, and that does not require the use
of batteries.
SUMMARY OF THE INVENTION
The present invention is a peak flow meter for measuring a peak
flow or air exhaled by a patient. In its preferred form, the peak
flow meter includes a substantially hollow housing having a top
portion, a bottom portion, an air inlet and two air outlets. A flow
restriction is disposed within the housing and in fluid communication
with the air inlet.
A vane assembly is also disposed within the housing and preferably
includes a vane, a post to which the vane is fixedly attached, and
an adjustable hub that is rotatably attached to the bottom portion
of the housing. A torsion spring is fixedly engaged at one end with
the hub and at the other end with the post of the vane assembly.
The adjustable hub is dimensioned to allow the post to be rotated
to a predetermined position relative to the ends of the torsion
spring in order to compensate for differences in the relative positions
of the ends of the spring caused by manufacturing tolerances.
A slot is preferably disposed through the top portion of the housing
and forms an arc about an axis defined by a centerline of the post
of the vane assembly. This preferred arc subtends an angle of more
than one hundred and eighty degrees in order to provide enhanced
resolution. A visual indictor is movably disposed within the slot
for indicating a peak flow rate of air based upon a movement of
the vane. The preferred visual indicator is a unitary plastic indicator
that is dimensioned to be moved by the vane when the vane is rotated
by the stream of air and to maintain a peak flow position within
the slot upon cessation of the stream of air. Finally, a scale is
disposed proximate to the slot such that the location of the visual
indicator may be related to a corresponding flow rate.
In operation, a user blows a stream of air into the air inlet.
A first portion of the stream of air passes through the flow restriction
and is vented through the air outlets. A second portion of the stream
of air contacts the vane and causes the vane to rotate against the
torsion spring. The vane then contacts the indicator and moves it
along the slot. Upon cessation of the stream of air, the indicator
remains in its furthermost position. The location of the indicator
relative to the scale indicates a numerical value of peak flow rate
of the stream of air.
Therefore, it is an aspect of the invention to provide a peak flow
meter that is small enough to be truly portable.
It is a further aspect of the invention to provide a peak flow
meter that is sufficiently accurate and repeatable to provide meaningful
results.
It is a further aspect of the invention to provide a peak flow
meter that does not require a customized scale to provide the necessary
level of accuracy.
It is a further aspect of the invention to provide a peak flow
meter that is not prone to corruption of test results through blockage
of the air outlet or flow restrictor by a user's finger.
It is a further aspect of the invention to provide a peak flow
meter that may be manufactured and sold at a relatively low cost.
It is a still further aspect of the invention to provide a peak
flow meter and that does not require the use of batteries.
These aspects of the invention are not meant to be exclusive and
other features, aspects, and advantages of the present invention
will be readily apparent to those of ordinary skill in the art when
read in conjunction with the following description, appended claims
and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of the one embodiment of the peak flow
meter of the present invention.
FIG. 2 is an isometric view of the embodiment of FIG. 1 in which
the top portion of the housing has been removed to reveal the vane
assembly.
FIG. 3 is an isometric view of one embodiment of the indictor of
the peak flow meter of the present invention.
FIG. 4 is a cut away side view of the preferred peak flow meter
showing of the relationship between the bottom portion of the housing,
the spring, and the vane assembly.
FIG. 5 is a side view of one embodiment of the peak flow meter
utilizing an air filter assembly.
FIG. 6 is an end view of the preferred air filter assembly utilizing
a hidden line to show a second end of the assembly.
FIG. 7 is a side view of the preferred indicator prior to bending
into its operating position.
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIGS. 1 and 2 one embodiment of the preferred
peak flow meter 10 of the present invention is shown. In the embodiment
of FIGS. 1 and 2 the peak flow meter 10 includes a substantially
hollow housing 12 having a top portion 14 a bottom portion 16
an air inlet 18 and two air outlets 22 24. A vane assembly 30 is
disposed within the housing 12 and includes a vane 34 a post 32
to which the vane 34 is attached, a hub 42 that is attached to the
bottom portion 16 of the housing 14. A torsion spring (shown in
FIG. 4) that is disposed between the hub 42 and the post 32 and
a mechanical indicator 38 is provided for indicating a peak flow
of air into the meter 10.
The housing 12 preferably includes a disc-like measurement portion
13 and an elliptical flow portion 15 that includes the air inlet
18 and the two air outlets 22 24. In the preferred embodiment,
finger pads 19 are provided on the top portion 14 and bottom portion
16 of the housing 12 in order to reduce the risk that the air outlets
22 24 will be covered by a user's finger. Similarly, the preferred
housing 12 includes a raised ridge 21 that indicates the maximum
insertion point of the air inlet 18 into a user's mouth, which also
acts to prevent inadvertent covering of the air outlets 22 24.
However, some embodiments of the housing 12 may take different shapes,
and may or may not include the finger pads 19 and ridge 21 of the
preferred embodiment. For example, some embodiments may include
a rectangular or triangular measurement portion 13 while still
others may utilize round, square, triangular, or hexagonal flow
portions 15.
The housing 12 is preferably formed of a thermoplastic, such as
a polystyrene or ABS. These materials are preferred for a number
of reasons. First, these materials are capable of withstanding dishwasher
cleaning, which typically involves exposure to temperatures of about
one hundred and sixty degrees Fahrenheit for periods of about an
hour, without deformation or swelling of the material. This ability
to thoroughly clean the meter 10 in the dishwasher is unique to
the present invention and is important in preventing the growth
of bacteria therein. Second, each of these materials can be molded
to achieve a very smooth finish, allowing mechanical embodiments
of the indicator 38 to slide smoothly within the slot 26. Finally,
these materials may be purchased at low cost, are aesthetically
pleasing, are easily moldable and ultrasonically weldable, and provide
sufficient mechanical strength to withstand use by children and
adolescents. Notwithstanding the fact that it is preferred that
the housing be made of a thermoplastic material, such as polystyrene
or ABS, it is also understood that the housing 12 may be made of
other art recognized materials common to the field of peak flow
meters 10.
The peak flow meter 10 includes at least one flow restriction 40
to create a back pressure within the housing 12. Such a flow restriction
40 is dimensioned to provide a necessary level of back pressure
while preventing undue limitation of the high end of the peak flow
scale. In the embodiment of FIGS. 1 and 2 a pair of flow restrictions
40 are disposed within, and formed integral to, the flow portion
15 of the housing 12 at a location between the air inlet 18 and
measurement portion 13 of the housing 12. This internal disposition
of the restrictions 40 eliminates the risk of blockage by the user
and the attendant corruption of the test data.
Because of the inclusion of this internal flow restriction 40
the air outlets 22 24 of the preferred housing are dimensioned
to prevent any substantial restriction to the flow of air exiting
the meter 10. Accordingly, the preferred air outlets 22 24 include
generously sized openings 23 that are protected by grill members
25. These grill members 25 are disposed such that it is difficult
to completely cover either of the air outlets 22 24 with a finger.
Although the preferred embodiment of the meter 10 utilizes two
internally disposed flow restrictions 40 and two air outlets 22
24 it is recognized that other embodiments may utilize a single
flow restriction 40 or multiple flow restrictions 40 to achieve
similar results. Likewise, in some embodiments, the flow restriction
40 is not formed integral to the housing 12 but rather is a separate
piece that may be disposed within the flow portion 13 of the housing
12 at the air inlet 18 or at a separate air outlet (not shown).
The preferred housing 12 also includes a back vent 29 disposed
adjacent a flow barrier 37 and opposite one of the air outlets 24
for venting air from the backside of the vane 34. The inclusion
of such a back vent 29 is preferred in order to relieve the pressure
on the back side of the vane 34 and corresponding torque acting
to reduce the travel or swing of the vane 34. This back side pressure
is most significant as the vane 34 approaches its maximum travel.
This is due to the fact that the slot 26 is generally effective
as a backside vent for small swing angles but, at maximum vane 34
travel, there is little slot 26 remaining to vent air from the back
side of the vane 34. The resulting back side pressure tends to lower
the performance curve such that readings obtained at high flows
with a back vent 29 are actually higher than readings obtained without
the vent 29.
It is recognized that back side pressure is not entirely detrimental
and may be used to advantage. This is due to the fact that a fully
linear curve forces a compromise between resolution at the low end
and the ability to read a high end flow. With a linear scale 31
actuation pressure must be reduced to keep the vane 34 from overtravel,
commonly referred to a "pegging", at high flows. However
this reduction of actuation pressure leads to poor low flow performance.
Lowering the top end of the curve permits good resolution at the
low end while suppressing the travel at the higher flows. Testing
has shown that this suppression of travel does not have a significant
negative impact on the accuracy on high flows. Accordingly, in order
to achieve optimal instrument performance, the preferred back vent
29 is designed to vent air at a rate that prevents the vane 34 from
pegging while allowing for accurate measurements to be made at the
lowest flows.
In the preferred peak flow meter 10 a slot 26 is disposed through
the top portion 14 of the housing 12 and forms an arc about an axis
28 defined by a centerline of the post 32 of the vane assembly 30.
As shown in FIG. 1 the preferred slot 26 forms an are that subtends
an angle of more than one hundred and eighty degrees. This allows
for a greater distance between graduations 33 on the scale 31 that
is disposed proximate to the slot 26 providing enhanced measurement
resolution. In fact, test results have indicated that the inclusion
of this extended slot 26 allows the peak flow meter 10 of the present
invention to meet American Thoracic Society (ATS) standards for
accuracy and repeatability of peak flow meters. However, in other
embodiments, a slot 26 that forms an arc that subtends and angle
less than, or equal to, one hundred and eighty degrees may be utilized,
provided there is sufficient space for the scale 31 to allow readable
and repeatable measurements within a desired flow range, even if
such measurements do not meet ATS standards.
In the embodiment of FIG. 1 a mechanical visual indictor 38 is
movably disposed within the slot 26 for indicating a peak flow rate
of air. Such a mechanical indicator 38 must be designed to allow
the indictor 38 move substantially freely within the slot 26 while
maintaining its peak flow position in the slot 26 upon cessation
of the stream of air. In the preferred embodiment, the location
of the slot 26 in the top portion 14 of the housing 12 tends to
eliminate gravitational forces upon the indictor 38 such as those
encountered when the slot is located upon the side of the meter
10 during operation. This location of the slot 26 and the attendant
reduction in gravitational forces that would tend to cause the indicator
38 to move within the slot 26 allows the retention force generated
by the indicator 38 to be significantly reduced.
In embodiments utilizing a mechanical indicator 38 it is preferred
that the indictor 38 be of very low mass so as to contribute little
to the overall inertia of the system. By utilizing an indicator
having low mass, and consequently low inertia, the indicator 38
has less of a tendency to continue moving along the slot 26 upon
cessation of the airflow within the meter 10. In the preferred embodiment,
the indicator 38 is designed to contribute between eleven and twenty-six
gmm.sup.2 to the overall inertia of the vane assembly 30. However,
it is recognized that other embodiments of the meter 10 may have
indicators 38 having higher or lower overall inertial contributions
and still achieve acceptable results.
As described more fully with reference to FIG. 3 the visual indicator
38 of the embodiment of FIG. 1 is a unitary plastic indicator 38
having a flexible tab 48 for maintaining the indicator 38 at a peak
flow position within the slot 26 upon cessation of said stream of
air. The indicator of FIG. 3 also includes a pointer 50 that is
oriented within the slot 26 such that it points toward the graduations
33 on the scale 31. In the preferred embodiment, a standard unitary
plastic indicator 38 having a living hinge design, similar to those
currently utilized in other peak flow meters, is utilized. As shown
in FIG. 7 such an indictor 38 includes a pair of ends 39 41 that
are folded together about a central notch 45. Once folded in this
manner, the indicator 38 is readily movable along the slot, but
the tendency of the plastic to flex outward causes the indicator
38 to remain at its peak flow position.
In other embodiments, a spring-loaded mechanical indicator 38
such as those commonly utilized in other peak flow meters, may be
utilized. Further, although mechanical visual indictors 38 have
heretofore been described, it is also recognized that the indictor
38 may take the form of an electronic indictor that senses the position
of the vane 34 of the vane assembly 30 and indicates the peak flow
rate via a liquid crystal, or other art recognized display. In these
embodiments, the slot 26 is unnecessary and, hence, is eliminated
from the top portion 14 of the housing 12.
As shown in FIGS. 2 and 4 the preferred vane assembly 30 is disposed
within the housing 12 and includes a vane 34 a post 32 to which
the vane 34 is fixedly attached, and an adjustable hub 42 that is
engaged with the bottom portion 16 of the housing 12 via a mating
detail 17. The adjustable hub 42 is dimensioned to accept and secure
one end 62 of a torsion spring 60 (shown in FIG. 4) and to press
fit within an opening and corresponding mating detail 17 in the
bottom portion 16 of the housing 12. In the preferred embodiment,
the mating detail 17 includes a plurality of ribs 46 about its inner
circumference in order to enhance the press fit with the hub 42.
However, in other embodiments, the outer circumference of the adjustable
hub 42 includes ribs, while in still other embodiments the mating
detail 17 and hub 42 are substantially smooth. The bottom of the
adjustable hub 42 will typically include a slot 43 to accept a screwdriver,
Allen wrench, or the like, which allows the location of the end
62 of the torsion spring 60 and consequently the position of the
vane assembly 30 to be adjusted once the hub 42 has been press
fit within the mating detail 17.
The post 32 is rotatably captured at one end within the adjustable
hub 42 and includes a catch 35 dimensioned to accept and secure
the second end of the torsion spring 60. In the preferred embodiment,
the post 32 and vane 34 are formed integral to one another from
a thermoplastic material. However, in other embodiments, the post
32 and vane 34 are formed separately and are attached via glue,
mechanical fastening, ultrasonic welding or the like.
The preferred vane 34 is dimensioned to approximate the inside
dimension of the housing and includes a pair of stabilizing ribs
70 72. In order to reduce the rotational inertia of the assembly
30 it is preferred that the vane 34 be extremely thin. This is
preferred so as to prevent inertial overshoot of the vane 34 and
corresponding high peak flow readings. In the preferred embodiment,
the vane 34 is 0.010 inches thick and contributes approximately
forty gmm.sup.2 to the overall inertia of the vane assembly 30.
However, in other embodiment, the thickness and mass of the vane
34 may be altered to achieve similar results.
In the preferred embodiment, the vane 34 contacts the indictor
38 via a tab 74 that extends upward from the rib 70 at a position
along the vane 34 that corresponds to the position of the slot 26
in the top portion of the housing. However, in other embodiments,
the tab 74 is eliminated, and an extension (not shown) is provided
from the indicator such that the bottom of the extension is below
the top of the vane 34.
The preferred torsion spring 60 is a manufactured of stainless
steel having a diameter of 0.012 inches and is formed into eight
active coils having a separation of between 0.006 inches and 0.009
inches between coils and an outer diameter of 0.169 inches. However,
it is recognized that a wide variety of torsion springs 60 may be
utilized. The selection of such a spring 60 is based upon a number
of variables, including the size of the vane 34 the size of the
air flow restrictions 40 and the printed scale 31 on the unit.
Therefore, the above specifications should be understood as being
illustrative of one embodiment, but not limiting the overall scope
of the claims with respect to the spring.
Once the preferred vane assembly 30 has been assembled, the post
32 may be rotated to a predetermined position, preferably corresponding
to a zero graduation 33 on the scale 31. In this manner, the position
of the vane 34 may be adjusted in order to compensate for differences
in the relative positions of the ends 62 64 of the spring 60 caused
by manufacturing tolerances.
Although the preferred peak flow meter includes an adjustable hub
42 it is recognized that other embodiments of the invention may
include a fixed hub. In some embodiments, for example, the position
of the vane 34 may be adjusted via a press fit between the vane
34 and post 32. In other embodiments, the post 32 may be adapted
to allow adjustment of the vane 34. In other embodiments, the vane
assembly 30 is not adjustable and the variations in the initial
positions of the vane 34 are compensated for via adjustments in
the positioning of the scale 33. In still other embodiments, torsion
springs 60 manufactured to more exacting tolerances are utilized
in order to eliminate the need to adjust the position of the vane
assembly 30.
In operation, a the peak flow meter is held in the hand and brought
to the mouth of the user, who blows a stream of air into the air
inlet 18 of the housing 12. A first portion of the stream of air
passes through the flow restrictions 40 and a second portion of
the stream of air is vented through the air outlets 22 24. The
first portion of the stream of air contacts the vane 34 and causes
the vane 34 to rotate against the torsion spring 60. The vane 34
then contacts the indicator 38 and moves it along in the slot 26.
Upon cessation of the stream of air, the indicator 38 remains in
its furthermost position within the slot 26. The location of the
indicator 38 relative to the graduations 33 on the scale 31 indicates
a numerical value corresponding to the peak flow rate of the stream
of air.
Referring now to FIGS. 5 and 6 an alternative embodiment is shown
in which the peak flow meter 10 is provided with an air filter 80.
The preferred air filter 80 is a disposable plastic filter 80 which
includes two substantially cylindrical end portions 84 88 and a
hexagonal central filter portion 82 into which the hexagonal filter
90 is disposed. The filter 90 may be manufactured of any conventional
filter media, but is preferably manufactured of a composition of
polypropylene and modified acrylic that results in a filter having
electrostatic properties. In the preferred embodiment, the end portions
84 88 and filter portion 82 are thermoformed. However, in other
embodiments, they may be injection molded or manufactured using
other art recognized techniques.
A hexagonal central filter portion 82 is preferred for a number
of reasons. First, the hexagonal shape is unique and is aesthetically
pleasing. Second, this shape reduces filter waste over that produced
when manufacturing circular filters. Third, the hexagonal shape
provides enhanced flow characteristics over conventional square
filters. Finally, this shape reduces shipping costs by allowing
the filters to be nested within one another during shipping.
Although the present invention has been described in considerable
detail with reference to certain preferred versions thereof, other
versions would be readily apparent to those of ordinary skill in
the art. Therefore, the spirit and scope of the appended claims
should not be limited to the description of the preferred versions
contained herein. |