Abstrict A peak flow meter has a body forming a chamber along which a piston
can be moved against the force of a spring by a subject exhaling
through an inlet into the chamber. A slot runs along a side of the
chamber to allow the air to escape and a pointer is movable along
the slot by the piston to indicate its maximum displacement. A deflector
at the chamber inlet concentrates the flow through the chamber towards
the side at which the slot is located. By the use of such a deflector
it is possible to measure reliably relatively small peak exhalation
rates.
Claims I claim:
1. A meter for measuring the ventilatory capacity of a subject,
comprising a hollow body, the hollow of said body providing a chamber,
an air inlet passage to the chamber, a piston having an end position
within the chamber adjacent said air inlet passage, resilient biasing
means urging the piston towards said end position in the chamber,
the piston being displaceable from said end position by the subject
blowing through the inlet air passage into the chamber, a boundary
wall of said hollow body defining said chamber extending away from
said inner inlet passage, an air exit slot extending along said
boundary wall of the chamber away from said air inlet passage for
the escape of air from the chamber, said exit slot being increasingly
opened by said displacement of the piston from its end position,
indicator means for indicating the maximum said displacement of
the piston, deflection means in said air inlet passage for concentrating
the air flow towards said first exit slot along said boundary wall
of the chamber.
2. A meter according to claim 1 wherein the air inlet passage has
a cross-sectional area transverse to said displacement of the piston,
and the deflection means comprises a wall member occupying not substantially
less than 50% of said cross-sectional area in a region of the air
inlet passage further from said first side region than said second
side region of the chamber.
3. A meter for measuring the ventilatory capacity of a subject,
comprising: a hollow body, the hollow of said body providing a chamber;
an air inlet passage to the chamber; a piston having an end position
within the chamber adjacent said air inlet passage; resilient biasing
means urging the piston towards said end position in the chamber,
the piston being displaceable from said end position by the subject
blowing through the inlet air passage into the chamber; side walls
of said hollow body defining first and second laterally opposite
side regions of the chamber extending away from said inner inlet
passage; an air exit slot extending along said first side region
of the chamber away from said air inlet passage for the escape of
air from the chamber to be increasingly opened by said displacement
of the piston from its end position; indicator means for indicating
the maximum said displacement of the piston; and deflection means
in said air inlet passage offset laterally away from said first
side region for concentrating the air flow towards said first side
region.
4. A meter according to claim 3 wherein the chamber air inlet
passage has an entry generally in the form of an annulus and said
wall member blocks a sector of said annulus extending not substantially
less than 180.degree. around said annulus.
5. A meter according to claim 4 wherein the chamber inlet comprises
a central boss and an outer periphery surrounding said boss, a convex
face of said boss being presented to the incoming air flow and said
boss and said outer periphery forming said annulus.
6. A meter for measuring the ventilatory capacity of a subject,
comprising: a hollow body, the hollow of said body providing a chamber;
an air inlet passage to said chamber; a tubular mouthpiece connected
to said hollow body for directing air flow to said inlet passage,
said mouthpiece having a longitudinal axis extending in the direction
of said air flow; a piston having an end position within the chamber
adjacent said air inlet passage; resilient biasing means urging
the piston towards said end position in the chamber, the piston
being displaceable from said end position by the subject blowing
through the mouthpiece and through the air inlet passage into the
chamber; side walls of said hollow body defining first and second
laterally opposite side regions of the chamber extending away from
said inner inlet passage; an air exit slot extending along said
first side region of the chamber away from said air inlet passage
for the escape of air from the chamber, said exit slot being increasingly
opened by said displacement of the piston from its end position;
indicator means for indicating the maximum said displacement of
the piston; and deflection means in said air inlet passage for concentrating
the air flow towards said first side region of the chamber, said
deflection means comprising a boundary wall inclined with respect
to the mouthpiece longitudinal axis in said direction of air flow
and toward said first side region of the chamber.
7. A meter according to claim 6 wherein the boundary wall has
respective ends that are leading and trailing in said air flow direction
and said inclination of the boundary wall reduces towards its trailing
end.
8. A meter for measuring the ventilatory capacity of a subject,
comprising: a hollow body, the hollow of said body providing a chamber;
an air inlet passage to the chamber; a piston having an end position
within the chamber adjacent said air inlet passage; resilient biasing
means urging the piston towards said end position in the chamber,
the piston being displaceable from said end position by the subject
blowing through the inlet air passage into the chamber; side walls
of said hollow body defining first and second laterally opposite
side regions of the chamber extending away from said inner inlet
passage; an air exit slot extending along said first side region
of the chamber away from said air inlet passage for the escape of
air from the chamber to be increasingly opened by said displacement
of the piston from its end position; indicator means for indicating
the maximum said displacement of the piston; deflection means in
said air inlet passage for concentrating the air flow towards said
first side region of the chamber; and flow smoothing means in the
path of the air flow past the deflection means.
9. A meter according to claim 8 wherein said flow smoothing means
comprises a series of spaced vanes.
10. A peak flow meter comprising: a body forming a generally cylindrical
chamber; an air inlet passage to one end of the chamber; an air
exit slot along a side wall of said chamber and extending away from
said one end of the chamber; a piston in the chamber having a position
of rest adjacent said one end of the chamber; resilient biasing
means urging the piston towards said rest position, the piston being
displaceable from said rest position by a subject blowing through
said air inlet passage into the chamber, said air exit slot being
increasingly opened to the air blown into the chamber by said displacement
of the piston against the resilient biasing means; indicator means
on the body for indicating the maximum said displacement of the
piston; and a wall member in said air inlet passage forming deflection
means which incline toward said air exit slot for concentrating
the air blown into the chamber towards that side wall in which the
exit slot is situated.
11. A meter for measuring the ventilatory capacity of a subject,
comprising a chamber, an air inlet passage to chamber, a piston
having an end position within the chamber adjacent said air inlet
passage, resilient biasing means urging the piston towards said
end position in the chamber, the piston being displaceable from
said end position by the subject blowing through the inlet air passage
into the chamber, an air exit slot for the escape of air from the
chamber extending along a side of the chamber away from the air
inlet passage to be increasingly opened by said displacement of
the piston from its end position, indicator means for indicating
the maximum said displacement of the piston, the air inlet passage
having an entry generally in the form of an annulus deflection means
in said air inlet passage comprising a wall member situated in a
laterally offset position in said annulus, away from the side of
the chamber in which the exit slot is situated, and concentrating
the air flow towards said side of the chamber in which the exit
slot is situated, said wall member blocking a sector of said annulus
extending not substantially less than 180.degree. around said annulus.
12. A meter for measuring the ventilatory capacity of a subject,
comprising a chamber, an air inlet passage to the chamber comprising
a tubular mouthpiece having a longitudinal axis extending in the
direction of airflow through said mouthpiece, a piston having an
end position within the chamber adjacent said air inlet passage,
resilient biasing means urging the piston towards said end position
in the chamber, the piston being displaceable from said end position
by the subject blowing through the inlet air passage into the chamber,
an air exit slot for the escape of air from the chamber extending
along a side of the chamber away from the air inlet passage to be
increasingly opened by said displacement of the piston from its
end position, indicator means for indicating the maximum said displacement
of the piston, and deflection means in said air passage at the inlet
to the chamber for concentrating the air flow towards said side
of the chamber in which the exit slot is situated, said deflection
means comprising a boundary wall that is inclined with respect to
the mouthpiece longitudinal axis such that said boundary wall slopes
towards the slot in the chamber side, the boundary wall having respective
ends that are leading and trailing in said air flow direction and
said slope of the boundary wall reduces towards its trailing end.
13. A meter for measuring the ventilatory capacity of a subject,
comprising a chamber a boundary wall, an air inlet passage to the
chamber, a piston having an end position within the chamber adjacent
said air inlet passage, resilient biasing means urging the piston
towards said end position in the chamber, the piston being displaceable
from said end position by the subject blowing through the inlet
air passage into the chamber, an air exit slot for the escape of
air from the chamber said air exit slot extending along said boundary
wall, away from said air inlet passage, to be increasingly opened
by said displacement of the piston from its end position, indicator
means for indicating the maximum said displacement of the piston,
deflection means in said air inlet passage at the inlet to the chamber
for concentrating the air flow towards said boundary wall of the
chamber in which the exit slot is situated, and smoothing means
adjacent the deflection means for the air flow passing said deflection
means into the chamber.
Description BACKGROUND OF THE INVENTION
This invention relates to meters for measuring the ventilatory
capacity of a subject, in particular the exhalation capabilities
of a subject.
Meters for obtaining a measure of the peak flow rate of exhalation
are known in which the subject blows into one end of a chamber to
displace a piston along the tube against the force of a spring.
The chamber has an open slot running in the direction of piston
displacement which provides an exit opening for the air being blown
in. The piston is therefore drawn back by the spring when the intensity
of exhalation falls. An indicating member located behind the piston
has a light frictional engagement with the slot and is displaced
by the piston as it moves forward against the spring force. When
the piston moves back, the member remains at the position of maximum
displacement of the piston, so giving an indication of the maximum
flow rate obtained in the exhalation.
Such instruments will be referred to herein as "meters of
the kind described". One example appears in GB 1463814.
Existing meters of the kind described can provide a satisfactory
and consistent measure of the normal subject's breath but have found
to be less suitable for measuring low flow rates, as in the case
of a subject with asthmatic symptoms. The low scale values obtained
on a meter designed for normal subjects are more difficult to read,
which is particularly relevant because meters of the kind described
are often required to be used by patients in their own homes. The
fact that readings can be obtained only in the lower part of the
scale of the instrument is also a significant disadvantage from
the point of view of the self-confidence of the patent.
The metering range can be altered by changing the spring strength
and the dimensions of the instrument to allow for a smaller airflow
rate, but it is found that if this is done to such an extent as
to provide an instrument with a maximum scale reading substantially
less than that required for normal subjects, e.g. 400 l/min instead
of 800 l/min, it is no longer possible to obtain consistent peak
flow measurements: the results will be unduly affected by differences
in the time profile of an exhalation.
It has now been discovered that there is a way in which this problem
can be mitigated or avoided.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a meter for
measuring the ventilatory capacity of a subject comprising a chamber,
a piston within the chamber being displaceable against a resilient
bias by blowing into an inlet air passage to the chamber, an exit
slot located in a side of the chamber for the escape of air from
the chamber being increasingly opened by said displacement of the
piston against its bias, and indicator means for indicating the
maximum said displacement of the piston by the incoming air, said
air passage at the chamber inlet being formed with deflection means
which concentrates the air flow towards that side of the chamber
at which the exit slot is situated.
Typically the chamber is provided with a tubular mouthpiece through
which the subject blows air into the chamber. In such an arrangement
the deflection means preferably take the form of a boundary wall
that is inclined with respect to the longitudinal axis of the mouthpiece
so that in the direction of the air flow it slopes towards the slot.
By way of example, an embodiment of the invention will be described
with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an axial sectional view of a peak flow meter for measuring
the exhalation of a subject,
FIG. 2 is an end view of the retainer member in the mouthpiece
of the meter of FIG. 1 from the direction II in FIG. 1
FIG. 3 is a sectional view of the retainer member on the section
line III--III in FIG. 2 and
FIG. 4 is a partly broken-away isometric view of the retainer member.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The instrument shown comprises a hollow cylindrical body 2 having
a mouthpiece 4 at one end and an apertured closure member 6 at its
opposite end. A support rod 8 extends through the interior of the
body and is attached at one end to the closure member 6 and at the
other to a retainer member 10 fixed in the mouthpiece 4. A piston
12 dividing the interior of the body is freely slidable on the rod.
The piston 12 is attached by a coil spring 14 to the end of the
rod 8 within the retainer member 10 and is drawn by the spring
to the rest position shown, at the inlet end of the body 2 where
it defines with the retainer member an internal chamber 16.
The retainer member 10 at the chamber inlet 16a has openings that
are closed by a thin diaphragm 22 of readily flexible material acting
as a non-return valve.
When the subject blows into the mouthpiece, the diaphragm 22 is
flexed away and air flows through inlet passage 4a into the chamber
formed between the retainer member 10 and the piston 12 as the air
pressure drives the piston away from the retainer. As the piston
moves along the support rod 8 it uncovers a longitudinal slot 24
in the container wall through which the flow can escape. A pointer
26 is mounted in the slot and has a light frictional engagement
with its edges. The displacement of the piston away from the retainer
member entrains the pointer along the slot but the pointer is not
attached to the piston. Thus, when the pressure in the chamber 16
falls as the rate of exhalation through the mouthpiece decreases,
the spring 14 draws the piston back but the pointer 26 is retained
frictionally at the position of maximum piston displacement. A scale
(not shown) along the slot 24 then allows the peak exhalatory flow
to be read off.
The features described so far are similar to those of known meters
of the kind described which may be arranged to record peak flows
up to some 800 l/min. The illustrated meter is however intended
to be able to record satisfactorily a range of peak flows up to
about half that rate. As has already been explained, however, merely
scaling down the dimensions of the known meter and the force of
its spring does not in itself produce a reliable instrument for
such low-range measurements.
The illustrated instrument utilises a novel configuration at the
entry to the chamber 16 to allow more consistent low-range measurements
to be achieved. In this example, the retainer member 10 is formed
with a rim 32 by which it is secured to the mouthpiece and a central
boss 34 securing the rod 8 the annular region between rim and boss
having a series of through-flow apertures 36 separated by radial
ribs 38. The apertures 36 occupy approximately the upper half of
the annular region and the remainder of that region is closed by
an integral web 40 which forms a barrier sloping upwards towards
the chamber 16. The radially outer edge of the web is bounded by
a cylindrical wall 42 and the face of the web towards the mouthpiece
inlet is convex in axial cross-section, as indicated at 40a in FIG.
3.
The retainer member also has a keyway 46 which fits a rib (not
shown) in the mating surface of the mouthpiece to locate it angularly
thereto. The mouthpiece rib may also engage a keyway (not shown)
in the body 2 to locate it angularly relative to the body. In this
way it is ensured that the retainer member is assembled with web
40 in a diametrically opposite position to that of the slot 24 in
the body.
It is found, surprisingly, that with the presence of the flow-deflecting
means at the entry to the chamber 16 a satisfactory smaller scale
version of the known instrument can be provided for a substantially
lower operating range than that of the known instrument. Measurement
of peak flows in a range up to about half that available in the
larger instrument can be made, without there being any sacrifice
of stability of performance, i.e. for a range of different flow
patterns, a similar response is obtained if the flow peak values
are similar. It is also possible to arrange that the instrument
has a substantially linear reading scale.
The precise reasons why such performance can be obtained by the
means described, in contrast to unsatisfactory characteristics resulting
from simply reducing the dimensions of the known instrument, are
not clear. It is believed that the inclined barrier provided by
the web 40 by deflecting the airflow towards the slot, reduces
the degree of turbulence acting on the piston. The presence of the
substantially planar ribs 38 at the through-flow apertures may also
make a contribution by acting as flow-smoothing vanes.
It will be understood that although the illustrated example has
been related to a known form of meter of the kind described, the
invention is capable of wider application to other forms of ventilatory
capacity meters to improve their performance.
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