Abstrict A rate-of-flow meter, particularly for diagnostic spirometry, comprises
a flow tube containing, on diametrically opposite sides, a measuring
stud provided with pressure taps and a displacement body facing
the stud.
Claims What is claimed is:
1. A rate-of-flow meter, particularly for diagnostic spirometry,
comprising:
a. wall means defining a flow tube extending along an axis for
conducting a gas therethrough;
b. a measuring stud and a displacement body, each extending radially
of said axis through said wall means from the exterior to the interior
of said flow tube, said measuring stud and displacement body passing
through said wall means at diametrically opposite sides of said
flow tube such that the respective ends of said measuring stud and
displacement body are in spaced confronting relationship, the space
therebetween being substantially less than the interior cross-sectional
width of said flow tube and the cross-sectional widths of said measuring
stud and displacement body being substantially less than the interior
cross-sectional width of said flow tube such that resistance to
flow through said flow tube is minimized;
c. the confronting end of said displacement body terminating in
at least one knife edge, and the confronting end of said measuring
stud terminating in a longitudinally extending knife edge oriented
in a direction substantially perpendicular to said axis;
d. at least two symmetrically disposed orifices in said measuring
stud at opposite sides of said longitudinally extending edge respectively
and ducts extending through said measuring stud from said orifices
for transmitting the pressure differential on opposite sides of
said edge of said measuring studs; and
e. means for displacing one of said displacement body and measuring
stud relative to the other in a direction normal to said axis whereby
the magnitude of the increase in velocity of flow between said end
of said measuring stud and said end of said displacement body can
be varied.
2. A rate-of-flow meter as defined in claim 1 including means
for displacing the body in the radial direction of the tube for
the purpose of varying the measuring sensitivity of the rate-of-flow
meter.
3. A rate-of-flow meter as defined in claim 1 wherein the displacement
body consists of solid bar material of circular cross section provided
with a sharp edge at its end facing the measuring stud.
4. A rate-of-flow meter as defined in claim 1 wherein the displacement
body consists of solid bar material provided with a sharp edge at
its end facing the measuring stud.
5. A rate-of-flow meter as defined in claim 1 wherein the end
of the displacement body facing the stud carries a sheet metal plate
having a sharp edge of arcuate shape.
6. A rate-of-flow meter as defined in claim 1 further comprising
means for adjusting the measuring stud in the radial direction of
the flow tube.
7. A rate-of-flow meter as defined in claim 1 further comprising
recesses in said measuring stud at opposite sides of said longitudinally
extending edge, said orifices being secreted in said recesses.
8. A rate-of-flow meter as defined in claim 1 wherein the width
of the displacement body normal to the axis of the flow tube is
at least equal to the corresponding width of the measuring stud.
9. A rate-of-flow meter as defined in claim 1 wherein the end
of the displacement body facing the stud carries a sheet metal plate
having a sharp edge of angular shape.
10. A rate-of-flow meter as defined in claim 1 wherein the end
of the displacement body facing the stud carries a sheet metal plate
having a sharp edge of off-angled shape.
11. A rate-of-flow meter comprising:
a. wall means defining a flow tube extending along an axis for
conducting a gas therethrough;
b. a measuring stud and a displacement body forming an integral
unit and means for releasably securing said integral unit in said
flow tube;
c. said measuring stud and displacement body being located, when
said integral unit is secured in said flow tube, at diametrically
opposite sides of said flow tube such that the respective ends of
said measuring stud and displacement body are in spaced confronting
relationship, the space therebetween being substantially less than
the interior cross-sectional width of said flow tube and the cross-sectional
widths of said measuring stud and displacement body being substantially
less than the interior cross-sectional width of said flow tube such
that resistance to flow through said flow tube is minimized;
d. the confronting end of said displacement body terminating in
at least one knife edge, and the confronting end of said measuring
stud terminating in a longitudinally extending knife edge oriented
in a direction substantially perpendicular to said axis; and
e. at least two symmetrically disposed orifices in said measuring
stud at opposite sides of said longitudinally extending edge respectively
and ducts extending through said measuring stud from said orifices
for transmitting the pressure differential on opposite sides of
said edge of said measuring stud.
Description BACKGROUND OF THE INVENTION
This invention relates to a rate-of-flow meter, particularly for
diagnostic spirometry, containing a measuring stud provided with
pressure taps and an adjustable displacement body.
In diagnostic spirometry such rate-of-flow meters are known as
pneumotachographs. Well known is the Fleisch pneumotachograph in
which a theoretically linear relationship between the measured pressure
and the volumetric rate of flow is achieved by maintaining laminar
flow through a plurality of parallel vanes of small cross section
to create the desired pressure differential between the pressure
taps. Objections to this rather complex and expensive instrument
are the difficulty of keeping it clean and of disinfecting it, its
relatively great weight and the rather poor signal strength that
is available for measurement. This signal is provided exclusively
by the pressure drop which arises during flow through the large
number of parallel passages between the vanes. Any increase in signal
strength therefore involves a corresponding increase in flow resistance.
In a respiratory examination of the functioning of the lung it is
desirable that this resistance should be a minimum. Other difficulties
in practice arise because flow through the several channels may
not be uniform and particularly because this may differ when the
breath is inhaled and when it is expelled. Moreover, the measured
pressure differential in laminar flow also depends on the viscosity
of the medium. Finally, obstructions for instance by expectorated
phlegm when coughing may considerably affect the results of the
measurement. Other drawbacks are the relatively large unused capacity
which causes errors when examining gas exchange, and the high demands
on accuracy which the very small measuring signal makes on the efficiency
of the electrical indicating system.
A more recent pneumatometer (German Specification published as-filed
under No. 2044101) which is based on the same physical principle
as the Fleisch pneumotachograph also avails itself of the pressure
drop in laminar flow for making the measurement.
Another proposal relating to the design of a pneumatometer (German
Specification published as-filed under No. 1963349) for generating
the measured pressure avails itself of a flat flow resistor which
completely fills the cross section of the flow tube. Such a flow
resistor may be for instance a fine mesh. However, such resistors
have the major disadvantage that the pressure drop they cause, being
rendered dimensionally indeterminate by reference to a static pressure,
depends in a complicated way upon the Reynold's number, particularly
in the case of screens and nets. Consequently in the description
of pneumatometers based on this working principle (German Specifications
published as-filed under Nos. 1963349 and 2000800) there is
always the qualification that the measuring accuracy that can be
achieved is not very high.
Another rate-of-flow meter used in spirometry comprises two pitot
tubes concentrically inserted in a breathing tube and operating
in contrary directions, the static heads in each being applied to
a differential pressure gauge (German Specification published as-filed
under No. 1153486). In this arrangement the pitot tube which faces
the direction of flow provides the total head whereas that pointing
in the direction of flow approximately represents the static head.
In order of magnitude the pressure difference therefore corresponds
to the static pressure head. A high static head which would be desirable
in the interests of improved measuring accuracy can be achieved
only by a fairly considerable constriction of the cross section
of the breathing tube. Such a constriction naturally represents
an increase in resistance to flow. Consequently the static heads
in a breathing tube cannot be very high. A drawback which appears
particularly when measuring low rates of flow is the dependence
of the pattern of flow through the two pitot tubes upon the Reynold's
number. Owing to this effect the relationship between measured pressure
and volume rate of flow may considerably deviate from the expected
parabolic relationship.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a rate-of-flow
meter which is of uncomplicated design and simple to manufacture,
though complying with the demand for a low idle capacity and for
low resistance to flow, and which will nevertheless provide a relatively
high-strength signal even when the rates of flow of the breath are
low.
Another object is to simplify handling of the instrument in practice,
and to permit it to be cleaned even during use, whilst at the same
time keeping the overall dimensions and its weight low, and giving
the patient more freedom of movement during an examination by connecting
the flow meter directly to the mouthpiece and/or integrally combining
the two components in such a manner that the instrument can be held
without needing a stand or other means of support.
The instrument according to the present invention which achieves
these objects comprises a simple flow tube into which a measuring
stud containing pressure taps projects radially from one side and
faces a displacement body on the other side. This combination of
a measuring stud with a displacement body generates the high velocity
of flow which is necessary for achieving a relatively high pressure
differential, but at the same time it localises the zone of high
velocity within a small part of the tube cross section. The proposed
arrangement thus enables pressure differentials to be achieved which
considerably exceed the static pressure in the breathing tube.
BRIEF DESCRIPTION OF THE DRAWINGS
Some preferred embodiments of the invention will now be described
by way of example and with reference to the accompanying drawings,
in which:
FIG. 1 shows the results of comparative measurements achieved by
the rate-of-flow meter according to the invention and by the known
Fleisch pneumotachograph;
FIG. 2 is a longitudinal section of the rate-of-flow meter;
FIG. 3 is a cross section thereof;
FIGS. 4a, 4b, 4c and 4d show a variety of different forms of construction
of the displacement body;
FIGS. 5a, 5b and 5c illustrate various forms of construction of
the measuring stud;
FIG. 6 is a cross section of an arrangement in which the flow tube
is directly attached to a mouthpiece, and
FIG. 7 shows another embodiment in which the displacement body
and the measuring stud are combined to form a unit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 the measured pressure head.DELTA.pm, the pressure drop.DELTA.pv
and the ratio of pressure drop to measured pressure head are plotted
for each instrument over the rate of flow V. The same hook-up arrangements
were used for both rate-of-flow meters, corresponding to those normally
employed for instance in a conventional plethysmograph chamber.
In other words allowance has been made for the pressure drop in
mouthpiece and pipes. The pneumotachograph according to Fleisch
was of a standard type available on the open market. The absolute
value of the pressure drop is substantially lower throughout the
range of flow rates in a rate-of-flow meter according to the invention
than in a pneumotachograph according to Fleisch. Consequently the
curve representing the plot over the rate of flow of the ratios
of pressure loss and measured pressure differential is also much
better than formerly.
The rate-of-flow meter according to the invention is shown in FIG.
2 and FIG. 3 and comprises a flow tube 1. The measured pressures
are tapped at pressure taps 3 in the end of a measuring stud 5 and
are transmitted through channels 7 and lengths of hose to a differential
pressure gauge. The slidable adjustability of a displacement body
9 in direction of its axis--in FIGS. 2 and 3 the body 9 is a simple
cylinder--is indicated in FIG. 2 by a double-headed arrow. By appropriate
displacement the magnitude of the increase in the velocity of flow
in the neighborhood of the head of the measuring stud 5 can be varied
in a defined way and the sensitivity of the flow meter easily varied.
The proposed combination of a measuring stud 5 with a displacement
body 9 also ensures that the velocity distribution over the cross
section of the tube 1 has no perceptible effect on the result of
the measurement. The change in the velocity distribution due to
the presence of the measuring stud 5 and of the displacement body
9 in the neighborhood of the pressure taps 3 is considerable compared
with the variations in the cross sectional velocity distribution
in the rest of the tube, such as may arise under varying practical
circumstances when the entry conditions into the tube 1 vary. This
effect and the knife edge design of the ends of the measuring stud
5 and of the displacement body 9 causes the pressure differential
which is measured by the proposed rate-of-flow meter in non-dimensional
form to be substantially independent of the Reynold's number. The
theoretically expected parabolic relationship between pressure differential
and rate of flow is therefore substantially achieved. This is of
importance for a subsequent electronic linearisation of the electrically
amplified signal.
For the purpose of further reducing the overall pressure loss the
flow tube 1 may have a cylindrical centre portion containing the
measuring stud 5 and the displacement body 9 and a divergent portion
at each end, like a Venturi, so that the greater part of the energy
of motion in the throat of the tube is recoverable in pressure form.
FIGS. 4a -d illustrate a variety of different forms of construction
of the displacement body 9. Their common feature is the sharply
angled knife edge which may be of arcuate, angular or off-angled
shape, at the end 11 of the displacement body facing the measuring
stud 5.
The end 11 of the displacement body may be a thin sheet metal plate
which laterally embraces the measuring stud 5 (FIG. 4c). The displacement
body may consist of a solid bar material of circular or polygonal
cross-sectional. FIGS. 5a, 5b and 5c illustrate various forms of
construction of the measuring stud. Their common feature is that
below its upper edge (viewed as illustrated in FIGS. 5a, 5b and
5c the stud contains two symmetrically disposed pressure taps 13.
The upper edge of the measuring stud is also sharp edged. This edge
may also be formed by a sheet metal plate (FIG. 5b) let into the
upper end of the stud between the two orifices of the pressure taps
13. Moreover, as illustrated in FIG. 5c these orifices 13 may be
slots and the entries into the bores may be secreted. This will
be particularly desirable when it is possible for the pressure taps
to be blocked or obstructed by contamination with particles of phlegm
or the like. The measuring stud 5 is fitted into the flow tube so
that the line connecting the orifices 13 of the pressure taps is
parallel to the tube axis. The width of the displacement body normal
to the axis of the flow tube may be at least equal to the corresponding
width of the measuring stud.
In order to reduce the unwanted idle capacity of the flow tube
1 it may be directly attached to a mouthpiece 15. A rate-of-flow
meter 16 of this kind is schematically illustrated in FIG. 6. The
mouthpiece is shown pushed into the patient's mouth 12.
In order to prevent the formation of condensate at the measuring
stud 5 and on the displacement body 9 the flow tube 1 may be electrically
heated, for instance by means of a resistance wire.
For industrial applications, for instance for rate-of-flow measurements
in pipes or as a sensor in control systems, the displacement body
9 and the measuring stud 5 may also be integrally combined so that
both can be conveniently fitted as a unit into an existing pipe
line 17 as illustrated in FIG. 7. By suitably matching the dimensions
of the stud and of the displacement body to the diameter of the
pipe it is possible for calibration curves to be applicable to different
sized ducts and pipes, so that special calibration can be dispensed
with. The measuring stud 5 and the displacement body 9 may be contained
in a common holder 14.
The invention may be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The embodiments
are therefore to be considered in all respects as illustrative and
not restrictive. |