Note: Descriptions are shown in the official language in which they were submitted.
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RESISTIVE ELEMENI AND
CALIBRATED AIR TUBE FOR SPIROMETER
Backqround of the Invention
The present invention relates to resistive
elements and air tubes for use with spirometers, and to
spirometers using such resistive elements and air
tubes. More particularly, the present invention
relates to resistive elements and air tubes which are
disposable and preferably at least partially
biodegradable, to spirometers, preferably differential
pressure spirometers, which employ such elements and
tubes, and to calibration techniques for ensuring a
high level of accuracy when the disposable air tubes
are used with the spirometers.
Spirometers are devices used to measure the volume
and flow rate of gas exhaled and inhaled by a user or
patient, for example, a human being. Two general types
of spirometers measure volume and flow, respectively.
For the flow type, the actual port of the spirometer
used to measure flow is the pneumotach of which Fleisch
is one type. These measurements are important for
physiological studies and for diagnostic analysis of
the pulmonary performance of the spirometer user. For
example, the effects of various medicines used to treat
patients with pulmonary or asthmatic problems can be
analyzed by monitoring the volume and flow rate of gas
exhaled before and after the administration of
2~ medication. Several devices are available on the market
~hich are known as pneumotachs, such as the Fleisch
Pneumotach. These devices depend on a laminar air flow
past a resistance element. Other spirometers employ
more sophisticated electronics so that laminar flow is
not needed.
Measuring the pressure difference or differential
pressure of exhaled gas across an element which creates
or causes the pressure difference is the basis for
differential pressure spirometers. In such
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differential pressure spirometers, it is important that
the air tube (pneumotach) be precisely configured and
positioned, for example, relative to the pressure
sensing and electronics systems of the spirometers so
that measurements can be reliably and reproducibly
made. Such precisely configured pneumotachs, rather
than being disposable, are made out of metals or
durable plastics to be long lasting and effective after
many uses without structural degradation. See, for
example, Waterson et al U.S. Patent 5,137,026, the
disclosure of which is hereby incorporated in its
entirety by reference herein.
Since most spirometers involve passing exhaled gas
directly from the respiratory system of a user into the
instrument for measuring, one important complication of
using such devices is contamination from one patient to
another patient if the same spirometer is employed by
both. Various approaches to overcoming this
contamination problem have been suggested. A
particularly popular approach is to use a disposable
mouthpiece and/or bacterial filter over the inlet to
the spirometer. The patient using the spirometer comes
in contact only with the mouthpiece and/or bacterial
filter and is able, at least in theory, to avoid
contaminating the remainder of the device. Drawbacks
to this approach include the relative expense of such
mouthpieces/filters, and the relative inefficiency of
such systems.
Another approach to overcoming this contamination
problem is to sterilize, in-between patients, the
portion or portions of the spirometer which come in
contact with the user and/or exhaled air. Drawbacks to
this approach include having to spend additional
capital on sterilization equipment and supplies, having
to monitor the operation and efficacy of the
sterilization equipment, and having to purchase
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relatively durable and expensive spirometers to
withstand the sterilization procedures.
~ A third alternative that has been suggested is the
use of disposable spirometer components. See, for
example, Norlien et al U.S. Patent 5,038,773; Acorn et
al U.S. Patent 5,305,762; Karpowicz U.S. Patent Des.
272,184; Boehringer et al U.S. Patent 4,807,641; and
Bieganski et al U.S. Patent 4,905,709. Such previous
disposable spirometer components have generally been
made out of durable plastics or medical grade metals so
that, even though they are disposable, the cost of
producing such components is relatively high. In
addition, such disposable components are relatively
difficult to dispose of, for example, because they are
made of durable and long lasting materials.
The economical manufacture of a relatively
inexpensive spirometer component from a low cost and/
or biodegradable material, however, has heretofore been
prohibitive because of, for example, quality control
concerns. General industry specifications require high
quality spirometer components but the quality of these
components can decrease as the components are made
biodegradable, for example, placement of these
components within the spirometer can also present
problems. The placement of the resistive element within
each air tube can affect the performance of the overall
spirometer, for example. The resistive element is
often placed in a normal or perpendicular configuration
relative to the interior wall of the air tube and,
further, should be placed at exact, predetermined
dlstances from the two opposing ends of the air tube.
Prior art resistive elements often do not exhibit
linear resistance-versus-flow-rate responses. More
particularly, resistive elements configured to exhibit
good resistance at high flow rates often do not perform
adequately at low flow rates and, on the other hand,
resistive elements configured to perform well at low
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flow rates often do not provide ideal resistance at
high flow rates. Thus, any possibility of manufacturing
a relatively inexpensive spirometer, as an alternative
to the existing durable plastic or metal non-
biodegradable components of the prior art, would appear
to be vitiated due to manufacturing and performance
concerns. These manufacturing concerns include the
inconsistencies between various disposable,
biodegradable spirometer components that may be
produced on an assembly line and, further, include
subsequent performance variances between the spirometer
components resulting from these inconsistencies.
Inconsistencies in these components may be
augmented when they are assembled together or placed
into the spirometer. For example, a throughport of an
air tube may not be perfectly formed, and the
subsequent placement of this throughport onto the
spirometer may introduce abnormally low pressure
readings due to air leakage around the pressure port.
Even placement of the resistive element within the air
tube, as another example, may not be exact between
various assemblies and, accordingly, a problem of
accuracy may even be prevalent among existing durable
plastic or metal non-biodegradable components as well.
Accordingly, it would be advantageous to provide a
means of ensuring high performance quality and
consistency between various spirometer components from
an assembly line, regardless of whether the spirometer
components are metal, plastic, or biodegradable.
A typical resistive element for a spirometer
includes a disk-shaped member with a large aperture
through the center thereof. Other resistive elements
of the prior art may include disk-shaped elements
formed of a mesh material. Another prior art device
includes a diamond-shaped window formed in a center
portion of the disk shaped member. The diamond-shaped
window is secured to a portion of the disk shaped
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element, and is adapted for opening and shutting to
various extents or degrees, depending upon the air flow
~ rate.
A prior art resistive element formed of a mesh is
5 typically rendered inoperative or inaccurate by
moisture and sputom from the patient~s breath,
resulting in clogging of the mesh. Prior art resistive
elements comprising a diamond-shaped window have been
somewhat effective for low air flow rates, but have not
provided fully effective resistance-versus-pressure
responses at both high and low flow rates.
It would be advantageous to provide spirometers
and spirometer components which exhibit linear
characteristics and which can be economically,
conveniently and effectively produced and used.
SummarY of the Invention
New resistive elements and air tubes, preferably
calibrated air tubes, for use in spirometers and
spirometers including such resistive elements and air
tubes have been discovered. The present resistive
elements and air tubes are disposable so that after use
by a patient, they are removed from the spirometer and
disposed of. The resistive elements and air tubes are
almost completely biodegradable, can be manufactured
relatively economically, and are capable of yielding
high and consistent performance characteristics.
As used herein, the term "biodegradable" means
that the component or material is decomposable into
more environmentally acceptable components, such as
carbon dioxide, water, methane and the like, by natural
biological processes, such as microbial action, for
example, if exposed to typical landfill conditions, in
no more than five years, preferably no more than three
years, and still more preferably no more than one year.
Having the resistive elements and air tubes
biodegradable provides substantial advantages. First,
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when the resistive elements and air tubes are disposed
of, the burden on the environment of such disposal is
reduced relative to, for example, a non-biodegradable
air tube, such as those made out of conventional
plastics or metals. In addition, because the resistive
elements and air tubes are biodegradable, they can be
made of materials which are inexpensive and plentiful
(readily available). Thus, the present resistive
elements and air tubes are relatively inexpensive, easy
and straightforward to produce. Subsequent calibration
of the air tubes accounts for any discrepancies in
size, shape, and performance of the air tubes.
Since the present resistive elements and air
tubes can be made economically, replacing a used air
tube with a new air tube is done without substantial
economic impact. In addition, the present resistive
elements and air tubes can be replaced in the
spirometer very easily. These advantages promote
operator compliance in that the spirometer operator
(for example, the care provider or the patient
operating the spirometer) is more likely to change the
present resistive elements and air tubes after each
patient or treatment, thus reducing the risks of
contamination and the spread of diseases, for example,
tuberculosis and other respiratory system disorders,
AIDS, other systemic conditions and the like.
Spirometers employing the present air tubes,
preferably calibrated air tubes, provide cost
effective, reliable and reproducible (from air tube to
air tube) measurements of the pulmonary performance of
the user, with reduced risk of contamination. In
short, the present disposable, biodegradable resistive
elements and air tubes, preferably calibrated air
tubes, are inexpensive and easy to produce to
acceptably precise specifications (for reproducible
performance), are effective and reliable in use, and
are conveniently and effectively disposed of in an
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environmentally acceptable or safe manner to reduce the
risks of contamination caused by spirometer use.
In one broad aspect, the present invention is
directed to air tubes for use in spirometers. The
present air tu~es comprise a tubular portion which
defines an open inlet, an open, preferably opposing,
outlet and a hollow space therebetween. The tubular
portion is sized and adapted to be removably coupled to
the housing of a spirometer. The air tube is
disposable, i.e., can be removed or decoupled from the
spirometer housing and disposed of without disposing of
the housing. Substantially all of the tubular portion
is preferably biodegradable. The open inlet is sized
and adapted to be received in the mouth of the user of
the spirometer. Thus, this open inlet and the area of
the tubular portion near the open inlet act as a
mouthpiece for the spirometer so that the user or
patient using the spirometer can exhale into the air
tube directly through the open inlet. No separate
and/or specially configured (relatively expensive)
mouthpiece/filter is needed when using the present air
tubes.
The present air tubes include a resistive element
which is located in the hollow space of the tubular
portion. This resistive element is sized to cause a
pressure difference or differential as air flows in the
hollow space across this element, and is adapted for
providing an alinear flow-versus-pressure response.
This response is subsequently linearized with software.
The resistive element includes a planar portion having
a first face and a second face, and a parameter
connecting the first face to the second face. An
aperture is formed in a center of the planar portion
for connecting the first face to the second face. A
plurality of slots in the planar portion extend
radially from the aperture, thereby forming a plurality
of hinged windows in the planar portion. Each of the
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slots includes a central end and a distal end. The
resistive element further includes a plurality of hinge
slots. Each hinge slot is connected to a distal end of
a slot, and extends generally perpendicularly to an
axis of the slot. A total number of hinge slots
corresponds to a total number slots.
According to one aspect of the present invention,
the slots and hinge slots form arrowhead-shaped, hinged
windows. Each hinged window includes a point, which
points toward the center of the planar member, and a
neck, which controls the flexibility of the window. A
large neck reduces the flexibility of the hinged
window, and a small neck increases the flexibility of
the hinged window. The resistive element has an
approximately linear pressure response over a range of
flow rates from zero liters per second to 15 liters per
second.
According to another aspect of the present
invention, an air tube is formed of a first tube, a
second tube, and a collar tube. The first tube has a
proximal end, a distal end, and a first diameter. The
second tube, similarly, has a proximal end, a distal
end, and a second diameter that is approximately equal
to the first diameter. A resistive element contacts
the proximal end of the first tube and the distal end
of the second tube, and has a third diameter that is
approximately equal to the first diameter. A collar
tube fits over both the proximal end of the first tube
and the distal end of the second tube. The collar tube
has an inner diameter that is approximately equal to
the first diameter, and has an outer diameter that is
larger than the first diameter. A through port is
formed in the second tube. The through port opens
directly into a hollow space defined by the tube
assembly and is spaced from the resistive element. The
through port provides communication between the hollow
space of the tubular assembly and a pressure sensing
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assembly of a spirometer. The pressure sensing
assembly of the spirometer compares a pressure from the
hollow space with an atmospheric pressure.
The tubular portions and resistive elements of the
present air tubes preferably comprise biodegradable
materials, and are more preferably 99~ biodegradable.
Preferred biodegradable materials of construction
include cardboard, paper, biodegradable polymeric
materials and the like and mixtures thereof. In one
particularly useful embodiment, the tubular portion is
made of cardboard or paper or mixtures thereof, more
preferably produced by methods analogous to those
conventionally used to produce tubes around which are
wound bathroom tissue. Such production methods often
include forming a cardboard or paper tube over a
mandrel or a like implement and then cutting the
resulting tube to the desired length. In the event
that the tubular portion is made from a biodegradable
polymeric material such tubes can be formed by
conventional polymer molding techniques.
The resistive element is placed relative to the
tubular portion so that the pressure difference for any
given rate of flow of air across the resistive element
is the same from air tube to air tube. The resistive
element is preferably located transverse to the
longitudinal axis of the tubular portion. The
resistive element can be placed in the tubular portion
by adhering (for example, using biodegradable
adhesives) the resistive element to the interior wall
of the tubular portion or by joining two separate
segments of the tubular portion together with the
resistive element therebetween. Other methods or
techniques for placing the resistive elements in the
tubular portions may be employed. Preferably, the
resistive elements of the present air tubes designed
for use in the same spirometer are structured and
configured essentially the same, so that no
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recalibration of or other adjustment to the spirometer
is needed because one air tube is replaced by another
air tube.
In a preferred embodiment, the present air tubes
further comprise a positioning means or sub-system
adapted to cooperate with the housing of the spirometer
to properly position the air tube relative to the
housing of the spirometer. Any suitable positioning
means may be employed to properly orient the air tube
relative to the housing of the spirometer, for example,
so that the through port of the air tube is properly
aligned with the pressure sensing assembly of the
spirometer.
In one specific embodiment, the positioning means
includes a notch sized and adapted to cooperate with a
projection on the housing of the spirometer. In
another specific embodiment, the positioning means
includes a positioning port in the tubular portion
sized and adapted to cooperate with a positioning
projection in the housing of the spirometer. This is
a particularly useful embodiment since the positioning
port can be easily placed in the tubular portion of the
air tube. Also, since the housing of the spirometer is
often a molded polymeric component, the positioning
projection can be easily formed in the spirometer
housing.
An air tube in accordance with the present
invention can be snugly fitted into a hollow open space
defined by a spirometer housing tube so that the
through port of the tubular portion is properly aligned
with the pressure sensing assembly of the spirometer.
To insure such proper alignment, the projections of the
housing can be placed in the notch of the tubular
portion such that the through port of the tubular
portion is properly aligned with the pressure-sensing
assembly of the spirometer.
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A suction-cup shaped interface of the pressure
sensing assembly fits snugly around the through port.
The snugness of the fit between the air tube and the
housing tube of the spirometer insures that the air
tube can be used in conjunction with the spirometer
without disturbing the through port/pressure sensing
assembly alignment. After use, the air tube can be
relatively easily removed from the spirometer housing
tube and replaced by a new air tube.
The present air tubes can be designed and
structured to be used with a retrofitted existing
spirometer or with a spirometer specifically built for
use with the air tubes. It is particularly useful to
have the tubular portion longer than the housing of the
spirometer so that in use the tubular portion extends
beyond at least one end of t:he component of the housing
of the spirometer to which the tubular portion is
removably coupled. This feature is very attractive in
preventing undue contamination of the spirometer
housing by the user of the spirometer. Thus, the air
which is exhaled by the patient passes through the
tubular portion and does not come into significant or
intimate contact with the housing of the spirometer.
In another broad aspect of the present invention,
new spirometers are provided. The present spirometers
comprise a housing, an air tube as described herein, a
pressure sensing assembly positioned relative to a
through ports of the air tube to sense the pressure at
the through port, and an electronic assembly coupled to
the pressure sensing assembly for generating signals,
preferably electrical signals, indicative of the
differential between the pressure sensed at the through
port and an atmospheric pressure. The electronic
- assembly can be disposed in the housing or can be
located remote from the housing. For example, the
housing can be a hand held component which is
connected, for example, by wire, cable, or an RF path,
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to an electronic processing system which includes a
substantial portion of the electronic assembly of the
present spirometer. Alternately, the electronic
assembly can be completely disposed in the spirometer
housing so that a fully self-contained unit can be
provided.
Although many of the features of the present
invention are described separately, more than one or
all of such features can be used in various
combinations, provided that such features are not
mutually inconsistent, and all of such combinations are
within the scope of the present invention. These and
other aspects and advantages of the present invention
are set forth in the following detailed description and
claims, particularly when considered in conjunction
with the accompanying drawings in which like parts bear
like reference numerals.
Brief Description of the Drawinqs:
Figure 1 is a side view of a spirometer in
accordance with the present invention showing a portion
of the electronics disposed apart from the hand held
unit.
Figure lA is a front side view of the spirometer
shown in Figure 1.
Figure 2 is an exploded view of the air tube of
the present invention;
Figure 3 is a cross-sectional view of the air tube
of the present invention;
Figure 4 is a top planar view of the resistive
element of the present invention;
Figure 5 is a partially cut away, top front view,
in perspective, of the air tube used in the spirometer
shown in Figure 1.
Figure 6 is a somewhat schematic illustration
showing a spirometer in accordance with the present
invention.
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Figure 6A is a cross-sectional view taken
generally along line 6A-6A of Figure 6.
Figure 7 is a cross-sectional view taken generally
along line 7-7 of Figure 1.
Figure 8 is a side view of an alternative
embodiment of a spirometer in accordance with the
present invention.
Figure 9 is a back side view of the spirometer
shown in Figure 8.
Figure 10 is a perspective view illustrating the
bar code reading assembly of the spirometer of the
presently preferred embodiment;
Figure 11 is a circuit diagram illustrating a
specific implementation of the bar code reading
assembly of Figure 10;
Figure 12 is a schematic representation of a
linear array of photodiodes for receiving light from a
- bar code label according to the presently preferred
embodiment; and
Figure 13 is a perspective view of a self focusing
lens array used for focusing light onto the linear
array of photodiodes, according to the presently
preferred embodiment.
Figures 14 and 15 illustrate perspective views of
a spirometer design according to the presently
preferred embodiment.
Detailed DescriPtion of the Drawinqs
Referring to Figures 1 and lA, a spirometer in
accordance with the present invention, shown generally
at 10, includes a disposable, biodegradable air tube
12, a housing 14 and control electronics 16.
Spirometer 10 is what is commonly known as a
differential pressure spirometer and, in general,
operates in a manner similar to the spirometer
disclosed in the above-noted Waterson et al U.S. Patent
5,137,026.
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14
The air tube 12 is described with reference to
Figures 2 and 3. The air tube 12 includes a first
tubular segment 18, a second tubular segment 20, and a
collar tube 21. A resistive element 22 fits between
the first tubular segment 18 and the second tubular
segment 20. The air tube 12 and resistive element 22
are preferably approximately ninety-nine percent
biodegradable. The tubular segments 18, 20, and 21 are
made of biodegradable cardboard or heavy paper, for
example, in a manner similar to how cardboard tubes are
conventionally made, such as for use with bathroom
tissue and the like products. These segments 18, 20,
and 21 are preferably coated with a glossy layer. The
resistive element 22 preferably comprises biodegradable
material having good memory characteristics. As
presently embodied, the resistive element 22 comprises
a Nomex material. The resistive element 22 material
may, alternatively, comprise any nylon or other
material which is somewhat resistant to moisture. As
presently embodied, the resistive element 22 is
approximately .003 inches thick, but other thicknesses
may be used according to design parameters.
The resistive element 22 is first secured to
either the first tubular segment 18 or the second
tubular segment 20, and then the other tubular segment
18 or 20 is then secured to the resistive element 22.
A biodegradable adhesive is preferably used. As
presently embodied, an outer diameter of the first
tubular segment 18 is equal to an outer diameter of the
second tubular segment 20, and the outer diameter of
the resistive element 22 is equal to the outer diameter
of the first tubular segment 18.
An inner diameter of the collar tube 21 is
approximately equal to the outer diameter of the first
tubular segment 18. The collar tube 21 is adapted to
fit over both the first tubular segment 18 and the
second tubular segment 20. Although adhesives are
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preferably used for securing the resistive element 22
between the first tubular segment 18 and the second
tubular segment 20, the close, frictional fit of the
collar tube 21 over the first tubular segment 18 and
the second tubular segment 20 may be sufficient, alone,
to secure the resistive element 22 between the first
tubular segment 18 and the second tubular segment 20.
The distal end 23 of the collar tube 21 is flush
with the distal end 25 of the first tubular segment 18,
when the collar tube 21 is properly secured over both
the first tubular segment 18 and the second tubular
segment 20. Additionally, a notch 27, which preferably
comprises a punched out semicircle in the distal end 23
of the collar tube 21, is preferably lined up with a
port 24 of the second tubular segment. The port 24 of
the second tubular segment 20 preferably comprises a
punched out circle in the second tubular segment 20.
The notch 27 and/or the port 24 may be formed in the
collar tube 21 and/or the second tubular segment 20
either before or after assembly of the three pieces 18,
20, and 21. After assembly of the three elements 18,
20, and 21. The port 24 opens directly into a hollow
space (Figure 3) of the air tube 12.
Figure 3 illustrates the air tube 12 in an
assembled state. Although a three piece configuration
of the air tube 12 is presently preferred, these three
pieces 18, 20, and 21 may be replaced by a single tube,
for example, and/or the resistive element 22 may be
secured to an annular ring (not shown), which is
inserted within the single tube.
Figure 4 illustrates a top planar view of the
resistive element 22, according to the presently
preferred embodiment. The resistive element 22
comprises a center aperture 32 and a plurality of slots
34 extending radially from the center aperture 32.
Each pair of adjacent slots 34 forms a hinged window
36, which as presently embodied comprises an arrowhead
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16
shape. Each arrowhead-shaped hinged window 36
comprises a point located near the center aperture 32
and a neck 38 located distally of the center aperture
32. As presently embodied, the resistive element 22
comprises eight hinged windows 36, but greater or fewer
numbers of hinged windows 36 may be used according to
design parameters. The width of each neck 38 controls
the flexibility of the corresponding hinged window 36.
A larger neck renders the corresponding hinged window
36 less flexible, and a smaller neck 38 renders the
corresponding hinged window 36 more flexible.
A human patient blowing into an end of the air
tube 12 generates an air flow through the resistive
element 22 which, typically, may comprises an air flow
rate of between zero and 16 liters per second. The
resistance provided by the resistive element 22 should,
ideally, be approximately linear among these various
air flow rates. Prior art resistive elements,
comprising a disk with a single aperture therein, for
example, do not have linear pressure versus flow rate
relationships. A prior art disk shaped resistive
element having a good resistance of less than 1.5
centimeters of water per liter per second at
approximately 12 liters per second, for example, will
not have a good resistance at lower flow rates. More
particularly, such a conventional disk shaped resistive
element would have a very low resistance at low flow
rates, which is unacceptable.
The resistive element 22 of the present invention
utilizes unique hinged windows 36 having necks 38,
which can be engineered to tailor the resistance of the
resistive element 22 at various flow rates. The
resistive element 22 of the present invention is
adapted to provide an ideal resistance of less than 1.5
centimeters of water per liter per second at a flow
rate of approximately 12 liters per second but, in
contrast to a conventional disk shaped resistive
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17
element, the resistive element 22 of the present
invention also provides good resistance at low flow
rates. Generally speaking, the resistive element 22
provides a very good, approximately linear flow-rate-
versus-resistance response for flow rates between zero
and 16 liters per second. At high flow rates, the
hinged windows 36 open widely to provide a good
resistance that is not too high. At low flow rates,
the hinged windows 36 open very little, to thereby
provide a good resistance that is not too low.
According to the presently preferred embodiment,
an angle between two of the slots 34 is approximately
degrees, and each of the slots 34 has a width of
approximately .02 inches. A preferred width of each of
the perpendicular hinged portions 37, which is used to
control the width of a nec~ 38, is approximately .04
inches. The diameter of the resistive element 22 is
preferably 1.09 inches plus or minus . 0005 inches, and
a width between a line 39 bisecting one of the hinged
windows 36, and another line 41 passing through a slot
34 is approximately .0625 inches plus or minus .005
inches.
One important element of the resistive element 22
of the present invention is the resistance supplied at
low flow rates, since, typically, unhealthy patients
are unable to generate high flow rates. The same
resistive element also functions well at high flow
rates. The resistive element 22 provides good
resistance at various flow rates, regardless of whether
the patient is exhaling or inhaling.
Referring to Figure 5, air tube 12 includes an
open inlet 46 and an open outlet 48. The area
surrounding the open inlet 46 is sized and adapted to
~e fitted into a human being's mouth. This mouthpiece
area is employed by the patient using spirometer 10
(Figure 1) by placing the area 46 into the mouth and
exhaling into hollow space 30 of the air tube 12.
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Turning back to Figure 1, when it is desired to
use air tube 12, it is unpackaged and is coupled to
housing 14. In particular, the air tube 12 is coupled
to the housing tube 51. The housing tube 51 includes
a tab 52, which is adapted to fit within the notch 27
(Figure 2) of the air tube 12. Before the air tube 12
is placed into the housing tube 51, the notch 27 is
aligned with the port 24 (Figure 2) and, as presently
embodied, is manually aligned by the user just before
insertion into the housing tube S1. When the notch 27
is aligned with the port 24, the port 24 will align
with the pressure sensing leg 76, as shown in Figure 6.
More particularly, a fitting of the pressure sensing
leg 76, which preferably comprises a suction cup shape
77 which fits around the port 24 for an airtight fit.
The suction cup shaped fitting 77 preferably comprises
silicone rubber or vinyl, and is adapted to provide a
good fit around the port 24, to thereby attenuate any
leakage of air at this interface. Consequently, breath
from the patient is not introduced into the pressure
sensing leg 76 and contamination of the pressure
sensing leg 76 is avoided.
After the notch 27 of the air tube 12 is placed
within the housing tube 51 and, more particularly,
placed over the alignment tab 52, the distal end 23 of
the collar tube 21 should be flush with the distal end
of the housing tube 51. At this point, spirometer 10 is
ready for use. Note that air tube 12 is longer than
housing tube 51 and, when properly coupled to the
housing tube, extends beyond one end of the housing
tube. The relatively long air tube 12 reduces the risk
of air exhaled from the spirometer user coming into
effective contact with and contaminating the housing.
Figure 6 illustrates the general operation of a
spirometer, shown generally at 10. The following is a
general description of the operation of the spirometer
after the air tube 12 is properly located and
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19
positioned relative to the pressure sensing leg 76. The
calibration method and apparatus of the present
invention will subsequently be discussed in further
detail after the general operational overview now
provided. This general description is applicable using
any spirometer, such as spirometer 10, in accordance
with the present invention. Through port 24 (Figure 2)
communicates with pressure sensing leg 76. As a
further protection against contamination, pressure
sensing leg 76 may be equipped with a filter, although
this is not required. The pressure sensing leg 76
communicates with a differential or "gauge~ type
pressure transducer 80, which may be, for example, a
transducer sold by Motorola under the trademark MPX
2020D. The pressure transducer 80 generates an
electrical signal on a pair of output wires 82 and 84,
which signal is proportional to the differential
pressure between pressure sensing leg 76 and a sensed
atmosphere pressure. This signal is amplified by a
differential amplifier stage 86 and fed into an
analog-to-digital convertor 88 which converts the
amplifier output into digital signals.
The output from convertor 88 is fed to a
microprocessor 90, which is part of control electronics
16. The microprocessor 90 uses calibration data
supplied by coded information on the air tube 12 in
combination with an algorithm stored in a ROM 92 to
perform several calculations on the signal from
convertor 88, and to display the calibrated final
results, e.g., volume and flow rate, on display 94, for
example, a conventional monitor or liquid crystal
display module. Microprocessor 90 is powered by a
power source 91, for example, either a battery or a
connector capable of being coupled or connected to a
source of conventional electric line voltage. Switch
96 can be activated to initiate the operation of the
spirometer through microprocessor 90. The results
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during each measurement may be stored in a RAM 98 for
future reference. An input/output port 100 may also be
provided to allow for changing the programming of the
microprocessor 90. Furthermore, the microprocessor 90
may be programmed so that on command it may download
the results accumulated in RAM 98 through input/output
port 100 to a printer or a computer. Waterson et al
U.S. Patent 5,137,026 provides further details
regarding the operation of a conventional spirometer.
When a patient has concluded one treatment or
diagnostic exercise using the spirometer 10, the
biodegradable air tube 12 is removed from the housing
tube and is disposed of in an environmentally safe
manner.
As shown in Figures 1 and lA, the housing 14 is
structured to be gripped in one hand of the user. For
example, the shaft 102 of housing 14 is configured for
easy hand gripping. In addition, finger indents 104
are provided to make hand holding this device even
easier.
The embodiment shown in Figures 1 and lA includes
control electronics 16 located within the hand held
housing 14. Communication with external computers or
printers can occur through cable 106 which can be
connected to the convertor using a jack 105, such as a
conventional RJ-11 quick connect jack, on housing 14.
As presently preferred, communication can also occur
through an additional infrared data association (IRDA)
link, which is conventional, and operable between the
housing 14 and the external computer or printer. The
electronics in the housing 14 are preferably powered by
a battery pack, such as a conventional rechargeable
nickel-cadmium battery. If such a battery pack is
used, the housing 14 includes a port through which the
battery pack can be charged.
In the embodiment shown in Figures 1 and lA,
microprocessor 90 can be a dedicated microprocessor
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21
including a transparent-overlay keypad structured and
adapted specifically to control the operation of a
spirometer. Alternatively, the microprocessor 90 may be
a component of a general purpose, personal computer
including a full-sized keyboard, video monitor, hard
disk drive and printer. The dedicated microprocessor
is particularly advantageous because of its relative
simplicity, reduced cost and ease of use. In addition,
the shaft 102 of housing 14 includes a tapered portion
107, as shown in Figure lA, which facilitates placing
and maintaining the housing on a flat surface, for
example, between uses.
The embodiment shown iIl Figures 1 and lA is useful
as a completely new spirometer, or the air tube 12 and
housing 14 can be used to retrofit an existing
spirometer. For example, an existing spirometer
includes a hand held unit including a permanent
breathing tube, pressure sensing leg, a pressure
transducer, an amplifier and an analog-to-digital
convertor, and is connected to a dedicated control
system, which functions in a manner substantially
similar to control electronics 16. Simply by replacing
the existing hand held unit with housing 14 and the
components coupled to or disposed in the housing, a
retrofitted spirometer is produced which has many of
the advantages of the present invention. Figure 7
shows a cross-sectional view of the spirometer 10 of
Figure 1, taken along line 7-7 of Figure 1.
Another embodiment is illustrated in Figures 8 and
9. This spirometer, shown generally at 210, is, except
as expressly stated herein, structured in a manner
similar to spirometer 10. Components of spirometer 210
which correspond to components of spirometer 10 have
corresponding reference numerals increased by 200.
The primary differences between spirometer 210 and
spirometer 10 have to do with the configuration of air
tube 212 and the configuration of the housing tube 251.
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Air tube 212 is structured substantially similar to air
tube 12 except that in the region near open outlet 248,
two positioning ports 107 and 108 are provided.
Housing tube 251 is structured to act as a cradle
for air tube 212 rather than surrounding the air tube
212, as does housing tube 51. In addition, housing
tube 251 includes two upwardly extending projections
109 and 110 which are positioned to be received by
positioning ports 107 and 108, respectively, when air
tube 212 is coupled to housing tube 251. With
projections 109 and 110 mated to or received by
positioning ports 107 and 108, the port 224 (not shown)
is properly aligned with the pressure sensing leg 276
(not shown).
As shown in Figures 8 and 9, a transparent-overlay
control keypad 112 of microprocessor 90 is located on
the shaft 302 of housing 214. In addition, this
embodiment preferably comprises greater ROM, and the
display 94 is located on the housing 214 beneath the
transparent-overlay keypad 112. In spirometer 210, the
power source 91 is a battery pack, such as a
conventional rechargeable nickel-cadmium battery, and
is located within housing 214. Port 114 on housing 214
is adapted to provide communication between battery
pack 91 and a conventional battery charger to recharge
the battery pack when needed. I/O port 100 is also
carried by housing 214 and provides convenient
communication between microprocessor 90 and a computer
or printer, when it is desired to download information
from electronic circuitry 111 to such other device. As
with the embodiment of Figure 1, an IRDA optical port
is also disposed on the shaft 302. Spirometer 210 is
a self-contained unit that can be operated by a single
patient.
In order to operate spirometer 210, air tube 212
is coupled to housing tube 251 so that projections 109
and 110 mate with positioning ports 107 and 108,
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respectively. The patient then activates a switch on
the transparent-overlay keypad 112 and uses spirometer
210 for any treatment and/or diagnostic procedure
desired. When it is desired to remove air tube 212 from
housing tube 251, the biodegradable air tube 212 is
simply picked up from the housing tube 212 and can be
discarded in an environmentally acceptable manner.
Referring again to Figure 6, a character
recognition unit 304 is disposed within the housing 14
of the spirometer 10. The character recognition unit
304 preferably comprises a device for recognizing bar-
code-like stripes. The character recognition unit 304
is disposed within the housing 14 to align with a
character sequence 306, preferably bar-code-like
stripes, on the air tube 12, when the air tube 12 is
placed within the housing 14. According to the present
invention, calibration information relating to the air
tube 12 is coded within the character sequence 306.
This coded information is read by the character
recognition unit 304 and is conveyed to the converter
88 via line 308 and then to the microprocessor 90. The
converter 88 preferably comprises eight inputs. Of
these eight, two receive pressure transducer 80
signals, one receives flow tube pressure, and one if
for rhinomanometry (nasal air pressure). As presently
embodied, the character recognition unit 304 is
disposed within the housing 14 of the spirometer 10 to
automatically read the character sequence 306, but,
alternatively, this reading of information from the
character sequence 306 may be performed manually.
Human- readable characters may be disposed next to the
character sequence 306, for example. Additionally, the
reading of information from the character sequence 306
may be performed before, during, or after each reading
by the spirometer 10, according to design preference.
The character recognition unit 304 is preferably
an optical character recognition unit, adapted for
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24
reading a bar code character sequence 306 but,
alternatively, other information conveying techniques
may be implemented. For example, magnetic character
recognition, optical alphanumeric character
recognition, optical symbol recognition, etc. may be
used, so long as calibration information relating to
the air tube 12 is conveyed to the microprocessor 90.
Preferably, the character recognition unit 304
comprises a linear array for recognizing bar-type
codes.
Figure 6A illustrates a cross sectional view taken
along line 6A-6A of Figure 6. As presently embodied, a
light source 310 projects light in the direction of the
arrow Al onto a character sequence 306 disposed on a
surface of the air tube 12. As presently embodied, the
character sequence 306 comprises a bar code label or,
alternatively, a bar code printed directly onto the air
tube 12. The light from the light source 310 reflects
from the character sequence 306 in a direction of the
arrow A2 and enters a self focusing lens array 313.
Light from the self focusing lens array 313 is
subsequently focused onto a linear array of photodiodes
315. The linear array of photodiodes generates an
electrical output, which is subsequently interpreted by
the converter 88 and then by the microprocessor 90
(Figure 6) to discern calibration information contained
within the character sequence 306. According to the
presently preferred embodiment, a wedge shaped black
plastic holder 318 is disposed between the light source
310, and the self focusing lens array 313, and the
linear array of photodiodes 315. The wedge shaped black
plastic holder 318 is adapted for securing these three
elements 310, 313, and 315 thereto for proper alignment
within the housing 14 of the spirometer 10.
A perspective view of the character recognition
unit 304 of the presently preferred embodiment is
illustrated in Figure 10. Light from the light source
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310 is focused onto the character sequence 306 disposed
on the air tube 12. Reflective light is received by the
self focusing lens array 313, which, as presently
embodied, is disposed at an angle 321 of approximately
45 degrees from the light source 310. Both the light
source 310 and the self focusing lens array 313 have
lengths which are substantially parallel to a center
line scan 323 passing through the character sequence
306.
The linear array of photodiodes 315 is disposed
substantially parallel to the self focusing lens array
313, and is adapted for receiving focused light from
the self focusing lens array 313. An extraneous light
stop 325 is disposed over a portion of the self
focuslng lens array 313, and another extraneous light
stop 327 is disposed over the linear array of
photodiodes 315.
Figure 13 illustrates the clip-on light stop 325
adapted for accommodating the self-focusing lens array
313, according to the presently preferred embodiment.
The light stop 325 preferably comprises black plastic,
and may be frictionally fit around the self-focusing
lens array 313 and/or secured thereto using an
adhesive. Alternatively, less expensive light stop
techniques may be implemented, according to design
preference. As mentioned previously with reference to
Figure 6A, both the light source 310 and the self
focusing lens array 313 and, more preferably, also the
linear array of photodiodes 315, are disposed on a
wedge shaped b~ack plastic holder 318. The wedge shaped
black plastic holder 318 provides the correct angle
between the light source 310, and the self focusing
lens array 313 and the linear array of photodiodes 315.
The wedge shaped black plastic holder 318 further
facilitates proper spacing of the light source 310, the
self focusing lens array 313, and the linear array of
photodiodes 315 from each other and from the air tube
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26
12. The wedge shaped black plastic holder preferably
comprises a black color for suppressing light
reflections. The total conjugate focal length 333 of
the self focusing lens array 313 is preferably
approximately 9.4 millimeters, measured from an
internal sensitive surface of the linear array of
photodiodes 315 to the target surface of the character
sequence 306. As presently embodied, the self focusing
lens array 313 comprises a Selfoc~}' lens array,
manufactured by Nippon Sheet Glass Co., Ltd. This self
focusing lens array 313 is positioned midway between
the linear array of photodiodes 315 and the character
sequence 306 so that both the linear array of
photodiodes 315 and the character sequence 306 are at
focal points of the self focusing lens array 313. As
presently embodied, the self focusing lens array 313 is
positioned 2.5 millimeters from the character sequence
306 and 2.5 millimeters from the linear array of
photodiodes 315.
An approximately 1 millimeter wide portion of the
character sequence 306 image along the character
sequence center line 323 is transferred by the self
focusing lens array 313 to the linear array of
photodiodes 315 when the character sequence 306 is
illuminated by the light source 310. As presently
embodied, the self focusing lens array 313 is
approximately 18 to 20 millimeters in length, and
comprises a single row of lenses 336. The self focusing
lens array 313 is preferably slightly longer than the
linear array of photodiodes 315, which is approximately
16 millimeters in length, to insure that the entire
linear array of photodiodes 315 receives an image,
allowing for a plus or minus l millimeter misalignment
and/or end lens damage on the self focusing lens array
313. The two focal points of an exemplary individual
lens 336 of the self focusing lens array 313, which are
not to scale, are shown at 339 and 340.
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The linear array of photodiodes 315 preferably
comprises an intelligent optical sensor manufactured by
Texas Instruments, model number TSL215, and comprising
an array of 128 charge-mode pixels in a 128 X 1 linear
array. The linear array of photodiodes 315 is preferred
over a charge coupled device (CCD) because of ease of
use, among other reasons. The linear array of
photodiodes 315 comprises integrated clock generators,
analog output buffers, and sample and hold circuitry
that would otherwise be required by a CCD circuit. The
focal point 340, for example, is focused approximately
1 millimeter beneath the top surface of the linear
array of photodiodes 315.
As presently embodied, in addition to the
extraneous light stop 327, a clear plastic packaging
344 is disposed over the sensitive surface 346, as
illustrated in Figure 12. The center scan line 323 is
projected onto the sensitive surface 346, as shown by
the line 348. As presently embodied, the focal point
340 (Figure 10) is approximately 1 millimeter beneath
the top surface of the clear plastic packaging 344, and
is projected onto the sensitive surface 346 of the
array.
Light is projected onto the sensitive surface 346
of the linear array of photodiodes 315 when the light
source 310 is activated by the microprocessor 90
(Figure 6). As illustrated in Figure 11, the
microprocessor 90 activates the light source 310 using
the "illumination- on" signal line 350, which is
connected to a parallel port pin 352 of the
microprocessor 90. As presently embodied, the light
source 310 comprises a four element light emitting
diode array of approximately 45 millicandelas
(lumens/ster), having a wavelength of approximately 635
nanometers and being approximately a lambertian source.
The light source 310 is biased with a 20 milliamps of
current on the middle two lamps and 25 milliamps of
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28
current on the end lamps, to provide an even
illumination along the character sequence 306,
according to the present invention. The light source
310 provides approximately 23 microwatts per square
centimeter of illumination, and is positioned
approximately 7 millimeters from the target bar code,
as illustrated by reference numeral 354. The light stop
325 between the light source 310 and the self focusing
lens array 313 suppresses stray light. The present
invention incorporates a 635 nanometer wave length to
roughly match the sensor peak responsivity of the
linear array of photodiodes 315 which is approximately
750 nanometers. The sensitivity obtained in the linear
array of photodiodes 315 is approximately 80~6 of the
100~ maximum linear array sensitivity at 750 nanometers
wave length. The light source 310 has a length of
approximately 16 millimeters. As presently embodied,
the light source 310 is only activated by the
microprocessor 90 during bar code reads, since,
obviously, activation of the light source 310
dissipates power. Both the light source 310 and the
linear array of photodiodes 315 preferably comprise
integrated circuits that are mounted on a flexible PC
board, and form a dihedral angle 321 with respect to
each other of 45~.
Referring to Figure 11, the image integration time
of the linear array of photodiodes 315 begins with a
short pulse on line 360 by the microprocessor 90 into
the serial input pin 362 of the linear array of
photodiodes 315. After approximately 1 to 10
milliseconds, a second serial input pulse is input into
the linear array of photodiodes 315 on line 360. After
this second serial input pulse, the image is read on
the video output pin 364 by clocking the clock pin 366
at between 10 kilohertz and 100 kilohertz, using 129 or
more clock pulses. The resulting signal is placed on
the serial video output line 368. During the above-
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29
mentioned clocking operation, the serial video output,
which comprises an analog voltage, is read by the
analog to digital (A/D) converter 370, which preferably
comprises 12 bit accuracy and a 0 to 5 volt input
range. The analog to digital converter 370 outputs
digital data on data bus 373, which reflects the
amplitude of each video pulse and, consequently, the
darkness of each sensor pixel of the linear array of
photodiodes 315. This digital data on data bus 373 is
subsequently read by the microprocessor 90. The analog
to digital converter 37() is controlled by the
microprocessor 90, and has a conversion time of
approximately 10 microseconds. Accordingly, the linear
array of photodiodes 315 can be clocked at up to 10
microseconds (100 kilohertz).
The linear array of photodiodes 315 is powered by
a 3 terminal voltage regulator 375 to maintain power
supply noise and video array noise at a minimum.
Although the Texas Instruments TSL215 is presently
preferred, a newer Texas Instruments product, the
TSL1402 may be used instead. This later model comprises
twice an many pixels in the same length of 16
millimeters. The model has twice the resolution and
will allow for more digits and more reliability. This
later model is pin compatible, so that the number of
clock cycles can simply be changed from 129 to 257, and
is less susceptible to optical saturation. The TSL1402
further does not require the 40 millisecond initial
pixel charge period, and would provide double the speed
and accuracy.
The character sequence 306 preferably comprises a
bar code having either an Interleaved 2 of 5 ITF
sequence, providing approximately 3 decimal digits of
calibration data plus a check sum digit or,
alternatively, may comprise a straight binary code. The
straight binary bar code is presently preferred, and is
configured to provide approximately five and one half
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digits plus a binary check sum of about six bits. The
binary code will be NRZ (non-return-to-zero) with
constant width bars and spaces, plus a starting mark.
This configuration ensures that the total width of the
code is constant and allows 1 millimeter on each side
for code positioning error. The minimum white and black
bar widths in the bar code are selected to be at least
2 to 3 pixels wide on the linear array of photodiodes
315. Since the linear array of photodiodes has a
spacing of .125 millimeters between photodiodes, the
minimum bar width is approximately twice that width.
This configuration ensures that at least one pixel
position in the video output 368 of the linear array of
photodiodes 315 will go fully low or high, since one
pixel in the array 315 is fully black or white, and not
positioned half way between a black bar and a white
area. The full high or low voltage, in relation to
other voltages in the video output 368 of the linear
array of photodiodes 315, is decoded by software to
positively indicate a bar position.
Since the light source 310 is preferably of
constant intensity, variances in light source
intensity between units and over time are compensated
for by the present invention. For this reason, and to
compensate for sensor efficiency, the light integration
of the linear array of photodiodes 315 is adjusted. The
level of the image video read from the linear array of
photodiodes 315 can be increased by increasing the time
between the serial input pulses on line 360, i.e., the
time of light integration interval. After each bar code
read, if the bar code amplitude data is too low, the
integration time is adjusted up until the amplitude is
sufficient to detect white to black differences. The
overall amplitude of the whole serial video data stream
from each read operation forms a nonlinear curve, due
to changes in light intensity along the light source.
In software, according to the present invention, a
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runnlng differential average or other indicator
indicates the approximate white to black threshold
along the entire video data length. This average will
be used to detect white from black data by software
comparison. High frequency noise is filtered out by
software, and the resulting data stream comprises an
image of the bar code. As presently embodied, this
resulting data stream is decoded by the NRZ binary
method or the interleaved 2 of 5 method, depending on
the code used. This NRZ format changes the bar code
color if the data bits do not change and does not
change the bar code color when the bits do change. The
resulting steam, after being decoded by either the NRZ
binary method or the integrated 2 of 5 method,
comprises the original binary or decimal number that
was originally encoded onto the air tube 12. This
number is then used to calibrate this spirometric flow
sensor.
The linear array of photodiodes 315 must initially
be preconditioned by a 40 millisecond operation period,
before each bar code read, to thereby allow for each of
the 128 pixels to change from white to black or vice
versa, correctly. During this preconditioning period,
the light source remains on, and the data from the bar
code is ignored. Subsequently, several bar code scans
are performed until the correct data is obtained,
judging by the check sum embedded in the bar code.
According, the total read operation is approximately 40
milliseconds plus 5 milliseconds per bar code scan, or
about 100 milliseconds. Each bar scan requires 128
times 10 microseconds minimum time, or 128 times 100
microseconds maximum time. The time is determined by
the required integration time, as mentioned above.
The light source 310 is turned on continually
during all bar code scans, up to 100 milliseconds, and
is not turned off between individual 5 millisecond
scans, since the pixels have to be illuminated
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throughout the integration time. An embedded
microprocessor 16 bit timer is programmed to develop 10
to 100 milliseconds repeated time periods, with each
period generating an interrupt. A timer interrupt
starts a routine that outputs the integration start
pulse if needed, and then outputs 129 clock pulses,
timed by the timer. At each clock pulse, the analog to
digital converter 370 is read by the microprocessor 90
via data bus 373 and stored for later analysis. After
completion of the 129 clock pulses, the timer is
stopped and the data is analyzed by the microprocessor
90 to find the moving white-black threshold level, ~or
each pixel, using continuous filtering and averaging.
The data is then filtered in software and compared to
the moving threshold level, before being converted into
bar codes. In the presently preferred embodiment,
approximately 8 bar code scans are taken and stored at
a time, requiring 8 times 12.5 milliseconds, or 100
milliseconds maximum time, so that the 40 milliseconds
initial pixel charge time does not have to be repeated.
Regarding the self-focusing lens array 313, this
assembly may have to be adjusted to focus exactly on
the character sequence 306 within plus or minus .3
millimeters, unless this is guaranteed by the
manufacturing process. The focal distance may have to
be adjusted in a low light environment, while a
diagnostic program runs on the microprocessor 90 and
continually scans the character sequence 306, outputing
the percentage of read errors from reading the
character sequence 306. This focal distance is
preferably adjusted until the errors are minimized.
Worst case or random bar code examples would preferably
be used for this procedure.
According to the method of calibrating a subject
air tube 12 and placing the calibration information
onto the air tube 12 in the form of a character
sequence 306, a large initial sample lot of air tubes
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12 from a manufacturing line are tested. As presently
embodied, the testing procedure comprises subjecting
each flow tube 12 to an air stream of 7.5 liters per
second in the expiratory direction. A sensor leg,
similar to that shown in Figure 6 at 76, is placed
over the through port 24 ~Figure 2) of the air tube 12,
and this sensing leg is connected to a high-accuracy
pressure sensor. A mechanical resonance filter may be
required in the tube. The measured pressure, in
response to the air stream of 7.5 liters per second in
the expiratory direction, i.s noted for each tube and,
subsequently, a similar measured pressure for the same
air flow rate in the inspiratory direction is obtained
for each air tube 12.
The present invention recognizes that, although
manufacturing differences exist between each air tube
12, the pressure output versus airflow input curve for
each air tube 12 is remarkably similar. More
particularly, this pressure output versus air flow
input curve for each flow tube 12 can be mathematically
modeled by a third order polynomial with fixed
coefficients. The polynomial for each air tube 12
varies by only a single gain factor. Thus, according
to the presently preferred embodiment, the response of
any subject air tube may be calibrated to replicate an
ideal or model response by merely multiplying the
response of the subject air tube by a constant.
Since the pressure output versus air flow input
curve for each air tube 12 varies only by a constant,
the measured pressure of a subject air tube 12 can be
compensated to achieve an ideal pressure output, for
any given air flow rate between 0 and 16 liters per
second. Although the present invention is described in
a particular embodiment where calibration of each
subject air tube can be performed by merely generating
a single calibration constant for each air flow
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34
direction (inspiratory and expiratory), the present
invention is not limited to this exemplary embodiment.
According to the presently preferred embodiment,
after pressure measurements for air flow rates in the
inspiratory direction and the expiratory direction are
obtained for a subject air tube 12, these two pressure
measurements are compared with two corresponding model
pressure measurements. The model pressure measurements
are obtained by averaging pressure measurements of a
large initial sample lot of flow tubes 12 from the
manufacturing line, as presently preferred. A gain
factor is determined, based upon the tube pressure
measurement of the subject air tube 12 and the tube
model pressure measurements. For example, if the model
pressure measurement for the inspiratory direction is
slightly higher than the subject tube pressure
measurement for the inspiratory direction, a correction
factor is generated to increase the pressure
measurement of the subject tube 12 to the model
pressure measurement. This correction factor comprises
a constant in the presently preferred embodiment. A
look-up table having a number of subject-air-tube 12
measurements and corresponding correction factors may
be used, as just one example. As presently embodied,
such a look-up table may comprise a large number of
subject tube pressure measurements according to desired
accuracy, and corresponding correction factors. The
correction factors, as presently embodied, calibrate
each subject tube to a desired accuracy level. Still
further, according to the presently preferred
embodiment, a single binary number is used to represent
both correction factors for any subject air tube 12.
Since the subject air tube 12 is tested for a measured
pressure in both the inspiratory direction and the
expiratory direction, two different correction factors
will be generated, corresponding to the two measured
pressure rates of the subject air tube 12. The single
CA 022~7921 1998-12-09
W 097/48338 PCT~US97/09994
binary number is presently preferred to represent these
two correction factors in a compressed form, and may
also be obtained from a look-up table.
Figures 14 and 15 illustrate perspective views of
a spirometer design according to the presently
preferred embodiment. The air tube 212 is
substantially covered by the housing, and the display
94 and transparent-overlay keypad 112 are larger than
in previously described embodiments.
While this invention has been described with
respect to various specific examples and embodiments,
it is to be understood that the invention is not
limited thereto and that it can be variously practiced
with the scope of the following claims.
