Abstrict A flow meter including a variable area obstruction which is mounted
in a conduit. Inputs to a differential pressure transducer are positioned
in the conduit on opposite sides of the obstruction to measure fluid
pressure differential across the obstruction. In one embodiment,
a sleeve is secured within the conduit thereof substantially separated
from the conduit so as to prevent fluid condensation on the sleeve,
and to create a dead end pocket between the conduit and the sleeve
for collecting liquids from the conduit end preventing them from
contacting the obstruction. The variable area obstruction comprises
an elastic membrane that includes first, second and third leaves
which extend into the flow stream and are compliant to fluid flow
so that an increase in flow rate increases the deflection of the
leaves and increases the through flow area through the obstruction.
The first leaf is secured at a fixed end adjacent a first portion
of the interior surface of the conduit and extends in a first direction
to a free end preferably positioned adjacent a second portion of
the interior surface of the conduit opposite the first portion.
The second and third leaves are located adjacent opposite sides
of the first leaf and extend from their fixed ends toward their
free ends in directions which are generally opposite to the first
direction.
Claims What is claimed is:
1. A flow meter comprising:
a conduit for communicating fluids therethrough and having a first
port positioned at a first location in said conduit and a second
port positioned at a second location in said conduit, wherein said
first and second ports are adapted for connection to a transducer;
and
an obstruction secured within the conduit between the first location
and the second location, said obstruction comprising:
first and second independently flexible leaves which are each secured
along a portion of their body adjacent to an inner conduit surface
and which each extend in parallel configuration to a free end within
the conduit;
a third independently flexible leaf which is secured along a portion
of its body adjacent to an inner conduit surface and extends within
the conduit in a direction opposite to said first direction so as
to lie between the first and second leaves and in parallel configuration
with them;
wherein the first, second and third leaves extend initially in
a plane transverse to the flow of fluid in said conduit, each of
said leaves being compliant to said fluid flow and deflected in
response to one parameter of said fluid flow so that said obstruction
to said fluid flow varies in response to said one parameter.
2. A flow meter as defined in claim 1 wherein the first, second
and third leaves are deflected in different planes in response to
the flow of fluid in the conduit.
3. A flow meter as defined in claim 1 wherein the third leaf is
substantially trapezoidal in configuration.
4. A flow meter as defined in claim 3 wherein the first and second
leaves are substantially asymmetrical in configuration.
5. A flow meter as defined in claim 1 wherein the inner conduit
surface is substantially insulated from influence of ambient temperature
present outside of the conduit.
6. A flow meter as defined in claim further comprising a sleeve
secured within said conduit so that fluids within said conduit flow
through said sleeve, with a portion of said sleeve being separated
from contact with the conduit and wherein said inner conduit surface
comprises an inner surface of said sleeve.
7. A flow meter as defined in claim 6 wherein the sleeve is positioned
with respect to the conduit so as to form a dead end pocket between
the sleeve and the conduit, so that liquids on inner surfaces of
the conduit will collect therein without contacting the obstruction.
8. A flow meter as defined in claim 7 wherein one of the first
and second ports is located within a dead end pocket, so as to be
substantially protected from fluid turbulences localized around
and downstream of the obstruction.
9. A flow meter as defined in claim 1 wherein a free end of the
third leaf is located between and substantially adjacent to the
first and second ports.
10. A device for measuring fluid flow through a conduit, comprising:
an obstruction disposed across the interior of the conduit, said
obstruction comprising a first flexible leaf secured along a portion
of its body in a position adjacent to an inner surface of the conduit
and extending in a first direction to a free end thereof within
the conduit, and second and third flexible leaves located adjacent
opposite sides of the first flexible leaf, said second and third
flexible leaves each having a fixed end and a free end, and being
secured at their fixed ends within the conduit and extending therefrom
toward their free ends in directions which are generally opposite
to the first direction, wherein the first, second and third leaves
extend initially in a plane transverse to the flow of fluid in said
conduit, each of said leaves being compliant to said fluid flow
and deflected in response to one parameter of said fluid flow so
that said obstruction to said fluid flow varies in response to said
one parameter; and
a transducer having a first input connected in fluid communication
with the interior of the conduit on a first side of the obstruction,
and a second input connected in fluid communication with the interior
of the conduit on a second side of the obstruction, for sensing
pressure differential between said first and second sides of the
obstruction.
11. A device for measuring fluid flow as defined in claim 10 wherein
the first, second and third leaves are deflected in different planes
in response to the flow of fluid in the conduit.
12. A device for measuring fluid flow as defined in claim 10 wherein
the obstruction comprises a diaphragm formed of a thin sheet of
metal.
13. A device for measuring fluid flow as defined in claim 12 wherein
the thin sheet of metal has a thickness of about 0.001 inch.
14. A device for measuring fluid flow as defined in claim 12 wherein
the metal comprises stainless steel.
15. A device for measuring fluid flow as defined in claim 10 wherein
the obstruction comprises a diaphragm including a partially cutout
portion defining said first, second and third flexible leaves.
16. A device for measuring fluid flow as defined in claim 10 wherein
the free end of the first leaf is located between and substantially
adjacent to the first and second ports.
17. A device for measuring fluid flow as defined in claim 10 wherein
the free end of the first leaf is located in close proximity to
the inner surface of the conduit so that materials will pass therethrough
without substantial clogging of a passage formed by deflection of
the first leaf.
18. A device for measuring fluid flow as defined in claim 17 wherein
the leaves occupy an area of the diaphragm corresponding to a substantial
portion of the interior cross sectional area of the conduit.
19. A device for measuring fluid flow as defined in claim 18 wherein
said substantial portion is about 94% of the interior cross sectional
area of the conduit.
20. A device for measuring fluid flow as defined in claim 15 wherein
the cutout portions define a gap between adjacent portions of the
diaphragm of about 0.001 inch.
21. A device for measuring fluid flow as defined in claim 10 wherein
the second and third leaves are secured along a portion of their
body adjacent to an inner surface of the conduit.
22. A device for measuring fluid flow as defined in claim 21 wherein
the first leaf is substantially trapezoidal in configuration.
23. A device for measuring fluid flow as defined in claim 22 wherein
the second and third leaves are substantially asymmetrical in configuration.
24. A device for measuring fluid flow as defined in claim 10 wherein
the second and third leaves are secured along a portion of their
body to a portion of the first leaf.
25. A device for measuring fluid flow as defined in claim 10 wherein
a portion of the first leaf is substantially circular in configuration.
26. A device for measuring fluid flow as defined in claim 25 wherein
the second and third leaves each define a portion of the first leaf,
and are oriented in asymmetrical configuration with respect to each
other.
27. A device for measuring fluid flow as defined in claim 26 wherein
the obstruction is disposed within the conduit in a configuration
such that the substantially circular portion of the first leaf is
offset with respect to a center of a circular conduit interior configuration,
so as to place the free end of the first leaf in close proximity
to an interior surface of the conduit.
28. A flow meter comprising:
a conduit for communicating fluids therethrough and having a first
port positioned at a first location in said conduit and a second
port positioned at a second location in said conduit, wherein said
first and second ports are adapted for connection to a transducer;
and
an obstruction mounted in the conduit between the first location
and the second location, said obstruction comprising:
a first flexible leaf secured along a portion of its body in a
position adjacent to an inner surface of the conduit and extending
in a first direction to a free end thereof within the conduit, a
portion of the first leaf being substantially circular in configuration,
and second and third flexible leaves which are integral with the
first leaf so as to form a portion thereof, and which are located
adjacent opposite sides of the first flexible leaf, said second
and third flexible leaves extending from fixed ends on the first
leaf toward free ends in directions which are generally opposite
to the first direction;
wherein the first, second and third leaves extend initially in
a plane transverse to the flow of fluid in said conduit, each of
said leaves being compliant to said fluid flow and deflected in
response to one parameter of said fluid flow so that said obstruction
to said fluid flow varies in response to said one parameter.
29. A flow meter as defined in claim 28 wherein the first, second
and third leaves are deflected in different planes in response to
the flow of fluid in the conduit.
30. A flow meter as defined in claim 28 wherein the obstruction
comprises a diaphragm including a partially cutout portion defining
said first, second and third flexible leaves.
31. A flow meter as defined in claim 28 wherein the free end of
the leaf is located in close proximity to the inner surface of the
conduit so that materials will pass therethrough without substantial
clogging of a passage formed by deflection of the first leaf.
32. A flow meter as defined in claim 28 wherein the leaves occupy
an area of the diaphragm corresponding to a substantial portion
of the interior cross sectional area of the conduit.
33. A flow meter as defined in claim 28 wherein the obstruction
is disposed within the conduit in a configuration such that the
substantially circular portion of the first leaf is offset with
respect to a circular conduit interior configuration, so as to place
the free end of the first leaf in close proximity to an interior
surface of the conduit.
34. A flow meter as defined in claim 28 further comprising a sleeve
secured within said conduit so that fluids within said conduit flow
through said sleeve, with a portion of said sleeve being separated
from contact with the conduit and wherein said inner conduit surface
comprises an inner surface of said sleeve.
35. A variable area diaphragm for use in the conduit of a flow
meter, comprising:
a membrane formed of an elastic material;
a peripheral portion of the membrane configured to define a securing
border for anchoring the membrane to the conduit body;
a central portion of the membrane bounded by the securing border
and including a partially cutout portion defining a first flexible
leaf having a fixed end thereof adjacent to the securing border
and extending in a first direction to a free end thereof within
the central portion of the membrane, the partially cutout portion
also defining second and third flexible leaves located adjacent
opposite sides of the first flexible leaf, said second and third
flexible leaves each having a fixed end and a free end, and extending
from their fixed ends toward their free ends in directions which
are generally opposite to the first direction, wherein the first,
second and third leaves extend initially in a common plane, and
when installed in the conduit, said leaves are compliant to fluid
flow within the conduit so as to be deflected in response to one
parameter of the fluid flow, so that an obstruction to the fluid
flow created by the leaves varies in response to said one parameter.
36. A variable area diaphragm as defined in claim 35 wherein the
first, second and third leaves are deflected in different planes
in response to the flow of fluid in the conduit.
37. A variable area diaphragm as defined in claim 35 wherein the
membrane comprises a thin sheet of metal.
38. A variable area diaphragm as defined in claim 37 wherein the
thin sheet of metal comprises stainless steel, having a thickness
of about 0.001 inch.
39. A variable area diaphragm as defined in claim 35 wherein the
cutout portions define a gap between adjacent portions of the membrane
of about 0.001 inch.
40. A variable area diaphragm as defined in claim 35 wherein the
leaves occupy an area of the membrane corresponding to a substantial
portion of the central portion of the membrane.
41. A variable area diaphragm as defined in claim 35 wherein the
fixed ends of the second and third leaves are located adjacent to
the securing border and are independent of the first leaf.
42. A variable area diaphragm as defined in claim 41 wherein the
first leaf is substantially trapezoidal in configuration.
43. A variable area diaphragm as defined in claim 42 wherein the
second and third leaves are substantially asymmetrical in configuration.
44. A variable area diaphragm as defined in claim 35 wherein the
fixed ends of the second and third leaves are integral with the
first leaf, and are independent of the securing border.
45. A variable area diaphragm as defined in claim 35 wherein a
portion of the first leaf is substantially circular in configuration.
46. A variable area diaphragm as defined in claim 45 wherein the
second and third leaves each define a portion of the first leaf,
and are oriented in asymmetrical configuration with respect to one
another.
47. A variable area diaphragm as defined in claim 46 wherein the
central portion of the membrane is substantially circular in configuration,
and wherein the substantially circular portion of the first leaf
is offset with respect to the center of the substantially circular
central portion of the membrane, so as to place the free end of
the first leaf in close proximity to a portion of the securing border.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to flow meters, and in particular
to a variable area obstruction in a conduit for use with a pressure
transducer, particularly in medical applications, for measuring
bi-directional flow rate of fluids such as respiratory gas in the
conduit.
2. Prior Art
Long term monitoring of respiratory air flow in critical care patients
and in patients during anesthesia is very important for correctly
assessing the patient's condition and for selecting the course of
his or her future treatment.
The conditions under which this monitoring occurs are not always
ideal. For example, in order to monitor this respiratory air flow,
a flow meter is typically installed in a breathing conduit in close
proximity to a patient. Thus, the flow meter is exposed to a flow
of warm air at body temperature that is 100% saturated with water
vapor and contains airborne droplets of water, saliva or mucus.
These airborne droplets can collect in the flow meter and adversely
affect its operation. In addition, since the ambient temperature
is always lower than body temperature, water vapor normally condenses
on inner surfaces of conduits installed in breathing circuits. The
water droplets formed by this condensation glide along the inner
walls of the conduit, and frequently collect in, and impede operation
of, the flow meter.
Of course, because of the necessity of very accurate and reliable
measurements under these circumstances, it is important that the
flow meter which is selected meet all of the requirements for proper
operation under critical care conditions. In particular, these conditions
require use of a very light weight flow meter, with small dead space,
with a wide measuring range, and with an accuracy that is not affected
by the presence of fluid, including mucus, produced by the patient.
When used in applications where a wide range of breathing flow
rates are to be measured, it is very desireable that the flow meter
response, as reflected in its output signal, be substantially linear,
so that the sensitivity of the meter and accuracy of its readings
is the same at low fluid flow rates as at high fluid flow rates.
Alternatively, when very low flow rates are being measured, then
it is often desireable to produce a flow meter response which changes
substantially in response to small changes in the flow rate, thereby
enabling more accurate monitoring of the measured flows. In addition,
sterility requirements in critical care applications, such as operating
room, require that the flow meter be disposable. This dictates a
very low production cost, while still necessitating a high sensitivity
and accuracy for each individual flow meter.
There are a number of different types of flow meters that are well
known among those in the technology and which are acceptable for
short term use in applications such as diagnostic pulmonary measurements,
due to their linear output signal with respect to the measured flow
characteristics. One such device is generally referred to as the
Fleisch pneumotachograph, which is the most widely used flow meter
for medical applications. This device directs the flow of gas through
a bundle of long, small diameter tubes which laminarize the flow.
Under laminar flow conditions, the pressure differential is linearly
proportional to flow rate. Accordingly, the output signal is generally
linear in its characteristics. The sensitivity also remains substantially
the same, throughout the range of measurement. However, this device
cannot be used for the long term monitoring of respiratory air flow,
since moisture or mucus collecting on the bundle of small diameter
tubes produces a significant, adverse effect on the output signal
of the device. Also, because of the linear response produced throughout
the entire range of measurement, this device is not suited for application
where a non-linear response is desireable at low flow rates, with
a linear response produced at higher flow rates.
Another problem associated with the Fleisch pneumotachograph is
its mechanical complexity, which in turn requires a high production
cost. The high production cost makes the unit essentially non-disposable.
Because it is not disposable, the requirement that the device be
cleaned and sterilized after each patient use adds additional cost
to the use of this device. The problems encountered in use of the
Fleisch pneumotachograph are also experienced in the use of other
popular flow meters, such as the ultrasonic, hot wire and turbine
pneumotachographs.
Other types of flow meters may be of simple design, but are either
too bulky for use in critical care applications, or they produce
a non-linear output signal in a flow range where a linear signal
is desired, or they have a limited measuring range. These types
of flow meters include the fixed orifice, venturi tube and pitot
tube meters. More specifically, the fixed orifice flow meter provides
an output signal defining a curve which gets progressively steeper
as the flow rate increases. This is true since the device operates
under turbulent flow conditions, where the pressure differential
is proportional to the square of flow rate. This characteristic
is undesirable because of drastically reduced sensitivity at low
flow rates. Thus, fixed orifice flow meters are generally used in
applications having a limited range of flow rates which are likely
to be encountered.
Variable obstruction flow meters have also been provided, which
combine the simplicity and low cost of the fixed orifice flow meters
with better low end sensitivity and linear characteristics that
typically have been available only from pneumotachographs. Nevertheless,
many existing variable area obstruction designs still suffer from
certain drawbacks such as accumulation of moisture or liquids in
areas immediately adjacent the variable obstruction. These flow
meters have also been known to suffer from resonant vibration or
flutter of the leaves which comprise the variable obstruction at
low flow rates corresponding to resting breathing of the patient.
Furthermore, reduced sensitivity is often experienced at low flow
rates due to relatively large leakage area in the obstruction produced
by the gaps between the flexible leaves which are apparent when
the leaves are in their resting condition. The large leakage area
is usually a result of the type of material selected for use as
the obstruction, and the manufacturing process selected for producing
the obstruction. The material utilized for the obstruction will
also determine the obstructions' predisposition to errors caused
by unwanted deflection of the leaves due to gravitational or inertial
forces.
Based on the above, it would be an important improvement in the
art to provide a flow meter having a wide measuring range and accuracy,
and which is substantially not affected by the presence of moisture
or mucus produced by a patient. It would be a further improvement
in the art to provide one embodiment of such a flow meter in which
the pressure differential across the obstruction is approximately
linearly proportional to the flow rate of fluid in the conduit at
very low flow rates as well as at higher flow rates. It would also
be an improvement in the art to provide another embodiment of such
a flow meter in which the pressure differential across the obstruction
varies non-linearly with respect to changes in fluid flow rate in
the conduit at very low flow rates, but is approximately linearly
proportional to such flow rate changes at higher flow rates. It
would be a still further improvement in the art to provide such
a flow meter wherein the size of the flow passage in the variable
area obstruction is minimized at zero flow condition. It would be
a still further important improvement in the art to provide such
a flow meter which has a light, simple and inexpensive structure,
permitting it to be disposable, while being manufactured in a manner
that permits mass production and also maintains the high level of
sensitivity and accuracy in each unit produced. A still further
improvement in the art would be to provide such a flow meter where
the active area of the variable obstruction is substantially the
same as the full cross sectional area of the fluid conduit, and
the elements of the variable obstruction are shaped in such a manner
that they, at maximum deflection, leave a substantially unobstructed
passage along the fluid conduit wall.
BRIEF SUMMARY OF THE INVENTION
The present invention comprises a bi-directional flow meter for
measuring flow of fluids as a function of pressure differential
generated by a variable area obstruction. The flow meter includes
a variable area obstruction mounted perpendicular to the fluid flow
direction in a cylindrical sleeve installed concentrically within
a cylindrical conduit, and a differential pressure transducer having
its two sensing ports located on the opposite sides of the obstruction
and in communication with the fluid in the conduit.
The cylindrical sleeve extends within the conduit in a configuration
that is substantially separated from the walls of the conduit so
that it is enveloped by air exhaled from the patient and will approach
that air temperature. This avoids fluid condensation on the walls
of the sleeve, and creates a dead end adjacent each sensing port.
These dead end pockets prevent turbulences localized around and
downstream of the variable area obstruction from affecting static
pressure readings produced by the differential pressure transducer.
In a preferred embodiment, the variable area obstruction includes
three elastic flexible leaves which extend across the diameter of
the flow passage in a configuration which is perpendicular to the
direction of fluid flow. The leaves are an integral part of a very
thin metal diaphragm which forms the variable area obstruction.
The leaves are configured such that the outer most leaves are of
asymmetrical shape with their fixed ends defining bases which are
anchored to the metal diaphragm in asymmetric orientation adjacent
the inner surface of the sleeve. In the absence of fluid flow in
the conduit, the outer most leaves extend in a common plane, with
their free flexing ends being pointed and curved to substantially
conform to the sleeve bore. The center leaf is preferably of trapezoidal
configuration, with its fixed end or base being substantially centered
about a center line running from its free end to the base. The base
is secured adjacent the inner surface of the sleeve along a portion
of the inner surface which is opposite to the sensing port. In the
absence of fluid flow in the conduit, the center leaf extends toward
its free end in a direction opposite to that of the outer leaves,
so as to lie between the outer leaves and in parallel configuration
with them. The free end of the center leaf is defined by an arch
which extends adjacent to the inner surface of a portion of the
sleeve which is adjacent to the sensing ports.
The leaves deflect under the influence of fluid such as respiratory
gas moving within the conduit, such that an increased flow rate
increases the deflection of the leaves and, thus, reduces the resistance
of the obstruction to the flow. Because of the angular orientation
of the fixed portion of the outer leaves with respect to the center
line, those outer leaves twist, as well as bend, about their fixed
ends, in response to fluid flowing within the sleeve. In contrast,
fluid flow through the sleeve does not cause substantial twisting
of the center leaf, but just bending about the fixed end thereof.
Accordingly, each of the leaves is deflected in a different plane
from the others in a manner which brings each of the leaves closer
to the inner sleeve surface as the fluid flow rate increases. Because
of this shape and disposition of the leaves, which uses substantially
all of the sleeve cross sectional area for the active variable obstruction,
the pressure differential across the obstruction changes substantially
linearly with any change in the rate of flow.
In another preferred embodiment, the variable area obstruction
also includes three elastic flexible leaves. In this embodiment,
a primary leaf is formed with its fixed end or base being substantially
centered about a center line running from the center of its free
end to the base. The base is secured adjacent the inner surface
of the sleeve along a portion of the inner surface which is opposite
to the sensing ports. This primary leaf defines a portion of a substantially
circular configuration, wherein the diameter of the circular portion
of this leaf is smaller than the inner diameter of the sleeve. The
circular portion of the primary leaf is offset with respect to the
center of the conduit, so that the free end of the primary leaf
is substantially adjacent to a portion of the inner surface of the
sleeve in a location which is substantially opposite to the center
of the fixed end of the primary leaf. The free end of the center
leaf is positioned adjacent to the inner surface of a portion of
the sleeve which is near the sensing ports.
A pair of secondary leaves of asymmetrical shape are defined on
the primary leaf by a pair of cut lines extending from each side
of the base of the primary leaf to termination points on the distal
portion of the primary leaf and in a direction angled inward toward
the center of the free end of that leaf. Thus, in response to fluid
flowing through the sleeve within the conduit, the primary leaf
initially flexes about its base, and then the secondary leaves begin
to flex about locations on the primary leaf between the termination
of the cut lines and the free end of the primary leaf. The primary
leaf is the first to respond to fluid flow, with the secondary leaves
subsequently beginning to flex. Accordingly, at low flow rates the
pressure differential across the obstruction changes in a nonlinear
fashion with respect to any change in the rate of any fluid flow
through the conduit. However, as fluid flow rate increases to the
point that the secondary leaves begin to flex, such flexing causes
further changes in the pressure differential across the obstruction
to be substantially linear with respect to any further change in
rate of fluid flow through the conduit.
The leaves are formed by etching a very narrow continuous gap through
the thickness of the diaphragm, creating a gap between the leaf
edges and adjacent structures which is very small, in order to minimize
leakage of the fluid through the obstruction. Due to this minimal
leakage path through the obstruction, sensitivity and accuracy at
very low flow rates are drastically improved. The upper range linearity
is also improved since the active flexing area of the obstruction
is, in the preferred embodiment, approximately 94% of the total
conduit area. Therefore, since the leaves are configured in such
a manner that they, while deflected, leave a substantially unobstructed
passage along the fluid conduit wall, choking of the fluid flow
happens at much higher flow rates than occurs in flow meters of
other designs. In addition, use of a chemical milling or chemical
etching technique in manufacturing the variable area obstruction
from a single piece of thin material such as stainless steel allows
the gaps between leaves to be formed with very high accuracies,
and at very low costs. Thus, the flow meter described herein is
of a type which has a light, simple and very inexpensive structure,
which is substantially unaffected by the water vapor and liquid
present in respiratory air flow, and which can be mass produced
while maintaining high accuracy and reliability in reproduction
of each unit.
These and other objects and features of the present invention will
become more fully apparent from the following description and appended
claims taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view of one preferred embodiment
of the variable area obstruction flow meter of the present invention.
FIG. 2 is a transverse sectional view taken along lines 2--2 of
the flow meter of FIG. 1 illustrating the configuration of the
leaves in the obstruction diaphragm when the leaves are not deflected.
FIG. 3 is a side sectional view of the obstruction diaphragm, taken
along lines 3--3 of FIG. 2 but with the leaves in partially deflected
configuration as illustrated in FIG. 1.
FIG. 4 is a transverse sectional view taken along lines 2--2 of
the flow meter of FIG. 1 illustrating the configuration of the
leaves in the obstruction diaphragm with the leaves in partially
deflected configuration, as in FIG. 3.
FIG. 5 is a graphical illustration of flow rate versus pressure
differential as measured by a flow meter of the type illustrated
in FIGS. 1-4.
FIG. 6 is a graphical illustration of flow rate versus pressure
differential, showing predicted results based upon theoretical characteristics
as defined by mathematical formulas representing the flow meter,
and also showing actual test results produced by a prototype of
the flow meter illustrated in FIGS. 1-4.
FIG. 7 is a front elevational view of another preferred embodiment
of an obstruction diaphragm for use in the variable area obstruction
flow meter of the present invention, illustrating the obstruction
diaphragm when the leaves are not deflected.
FIG. 8 is a side sectional view of the obstruction diaphragm, taken
along lines 8--8 of FIG. 7 but with the leaves in a partially deflected
configuration.
FIG. 9 is a front elevational view of the obstruction diaphragm
of FIG. 8 illustrating the configuration of the leaves in the obstruction
diaphragm with the leaves in partially deflected configuration,
as in FIG. 8.
FIG. 10 is a graphical illustration of flow rate versus pressure
differential, showing test results produced by the variable area
obstruction flow meter of the present invention, using the obstruction
diaphragm of FIGS. 7-9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention is best understood by reference to the figures wherein
like parts are designated with like numerals throughout.
Referring initially to FIG. 1 a flow meter generally indicated
at 10 is interposed in a conduit 12 so as to permit the through
flow of fluid through the conduit 12 and the flow meter 10 in the
direction of arrows 14. The flow meter 10 comprises a first section
16 which is substantially tubular in configuration, and which defines
a first port 18 for communicating fluid between the bore of the
first section 16 and the interior of conduit 12.
Extending through the body of the first section 16 is a first pressure
port 20. A first tube 22 is connected to the pressure port 20 so
as to permit communication of fluid from the first section 16 through
the first pressure port 20 and the tube 22 to a first sensor 24
of a differential pressure transducer 26. The differential pressure
transducer may comprise any of numerous such transducers which are
well know and commercially available in the technology. For example,
one preferred embodiment of a differential pressure transducer for
use with the present invention comprises a Model NPH-8-015DH manufactured
by Nova Sensors, Inc.
Extending inwardly from the first section 16 and, preferably, adjacent
to the first pressure port 20 is a first flange or base 28 that
supports an inner conduit wall 23 which defines a substantially
concentric cylindrical sleeve configuration that extends parallel
to the wall of section 16 between the first base and a free edge
directed toward the first port 18. The conduit wall 23 base 28
and parallel wall of section 16 form a dead end pocket 25 that extends
adjacent to the first pressure port 20.
First base 28 is configured to be received, in mating relationship,
with a second flange or base 30 extending inwardly from a second
section 32. The bases 28 and 30 are secured together along their
adjacent surfaces by chemical or mechanical bonding or by other
conventional fastening means.
Preferably, second section 32 is configured as substantially a
mirror image of the first section 16 and includes a second pressure
port 36 which extends through the surface of the second section
32 so as to provide for communication of fluid from the second section
32 through the second pressure port 36 and through the bore of
a second tube 38 connected to the second pressure port 36 to a
second sensor 40 which is connected to the differential pressure
transducer 26. Fluid is communicated between the interior of the
second section 32 and the interior of conduit 12 via a second port
42.
Again, in mirror image configuration with respect to first section
16 the base 30 in the second section 32 supports an inner conduit
wall 35 which defines a substantially concentric cylindrical sleeve
configuration that extends parallel to the wall of section 32 between
the second base 30 and a free edge directed toward the second port
42. The conduit wall 35 base 30 and parallel wall of section 32
form a dead end pocket 37 that extends adjacent to the second pressure
port 36.
Portions of the adjacent surfaces of the bases 28 and 30 are preferably
constructed so as to receive and secure an outer portion of a variable
area obstruction 44 such as that illustrated in FIG. 2. The variable
area obstruction 44 comprises a membrane or diaphragm that preferably
includes at least three flexible leaves, including a first outer
leaf 46 a center leaf 48 and a second outer leaf 50. The leaves
46 48 and 50 are disposed so as to extend within the bore of sections
16 and 32 interior of the conduit walls 23 and 35 and, in the absence
of fluid flow therethrough, to lie in a plane transverse to the
direction of fluid flow therein. The leaves 46 48 and 50 deflect
under the influence of moving fluid pressure in such a manner that
an increased flow rate increases the deflection of the leaves 46
48 and 50. For example, in the embodiment of FIG. 1 fluid flow
through the flow meter 10 from port 42 to port 18 would cause a
deflection of the leaves 46 48 and 50 in the direction and manner
illustrated in FIG. 1.
In operation, the inner conduit walls 23 and 35 define a conduit
bore which, because it is substantially separate from the walls
of sections 16 and 32 is substantially free from condensation.
Specifically, the configuration of the bases 28 and 30 and inner
conduit walls 23 and 35 provide an inner conduit surface adjacent
the leaves 46 48 and 50 which is insulated from the ambient temperature
communicated through the walls of sections 16 and 32. In this configuration,
the inner conduit walls 23 and 35 are enveloped by exhaled air,
causing them and the leaves 46 48 and 50 to assume the temperature
of that exhaled air. Thus, condensation does not occur on those
surfaces and, therefore, condensed liquids are not transmitted from
the walls 23 and 35 to the leaves 46 48 and 50. Accordingly, interference
in the leaf operation due to condensation is substantially avoided.
Of course, the inner conduit or sleeve formed by bases 28 and 30
and inner conduit walls 23 and 35 could also be formed by other
structural configurations to produce an insulated inner conduit
surface. For example, the bases 28 and 30 which, in the illustrated
embodiment, extend about the central periphery of the sleeve, could
be located in a different position. Alternatively, other insulating
material could be provided to support the sleeve, and could be located
in at least a portion of the area between the walls of section 16
and 36 and the inner conduit walls 23 and 35.
In addition to avoiding the problem of condensation on bore surfaces
adjacent to the leaves 46 48 and 50 the separation of the inner
conduit walls 23 and 35 from the conduit walls of sections 16 and
32 prevent condensation from other portions of the flow meter 10
from influencing the operation of the leaves 46 48 and 50. In particular,
liquid adhering to the inner walls of sections 16 and 32 and gliding
along the conduit surfaces in other areas of the system will not
reach the area occupied by leaves 46 48 and 50 since this liquid
will become trapped in one of the end pockets 25 or 37. Those end
pockets 25 and 37 also extend adjacent the pressure ports 20 and
36 so as to separate those pressure ports from direct communication
with the through flowing fluid within the flow meter 10 and thus
preventing turbulences localized around and downstream of the leaves
46 48 and 50 from affecting the static pressure readings produced
in the pressure transducer 26.
The configuration of the variable area obstruction 44 can be more
clearly understood by reference to FIG. 2. In particular, the variable
area obstruction 44 preferably comprises a diaphragm constructed
of an elastic material such as very thin stainless steel shim. In
one preferred embodiment, the obstruction 44 is approximately 0.001
inches in thickness. The outer perimeter of the obstruction 44 is
substantially circular, and includes at least one indexing cutout
52 for use in properly positioning the obstruction 44 between the
sections 16 and 32. Corresponding indexing guides such as matched
ridges and indentations or detents on adjacent surfaces of bases
28 and 30 can be provided to register the cutouts in proper positions.
A support border 53 is sandwiched between the mating surfaces of
flanges 28 and 30 to structurally secure the obstruction 44 in position
between sections 16 and 32. With the variable area obstruction 44
secured between sections 16 and 32 the approximate position of
the interior surface of the inner conduit walls 23 and 35 with respect
to the obstruction 44 is indicated by a dashed conduit bore surface
line 54.
The first outer leaf 46 center leaf 48 and second outer leaf 50
are formed in the variable area obstruction 44 by etching a very
narrow continuous gap 56 through the thickness of the variable area
obstruction 44 along the path indicated in FIG. 2. The width of
the gap 56 is exaggerated for clarity. In one preferred embodiment,
the gap is approximately 0.001 inches wide, resulting in a leakage
area, when the leaves 46 48 and 50 are in the unflexed state, of
approximately 0.0023 square inch. This is at least a twenty fold
improvement over the leakage area provided by other commercially
available variable area flow meters. The circular outside periphery
of the obstruction 44 with indexing cutouts 52 is also formed during
the etching operation. The etching operation can comprise any of
the well known chemical or electro-chemical etching techniques which
are extremely accurate, and do not stretch or deform the material
along the edges. Because of this etching technique, the variable
area obstruction devices can be mass produced with substantially
identical properties.
The leaves 46 48 and 50 define a configuration which provides
a substantially linear change in the pressure difference measured
across the variable area obstruction 44 by the transducer 26 with
respect to the change in flow rate through the bore of sections
16 and 32. To do this, the leaves 46 48 and 50 provide a very small
amount of leakage area in the absence of fluid flow, while defining
a very limited obstruction to fluid flow at high flow rates. This
response is achieved as a result of the leaf configuration illustrated
in FIG. 2. In particular, the leaves 46 48 and 50 are arranged
so that they extend across the diameter of the conduit bore defined
by the inner conduit walls 23 and 35 within sections 16 and 32 in
an alternating, intertwining manner.
In one preferred embodiment, the obstruction 44 is secured between
sections 16 and 32 so that the leaves 46 48 and 50 lie within the
conduit formed by inner conduit walls 23 and 35 in an orientation
wherein the free end of leaf 48 extends toward pressure ports 20
and 22. Generally, the width of the leaves at their free end is
smaller than that at their fixed end.
The center leaf 48 is of trapezoidal configuration, with its fixed
base defined by points 58 and 60 located adjacent the conduit bore
surface line 54 with the base extending generally along line 54
between points 58 and 60. The free end of leaf 48 defines an arch
extending generally adjacent the conduit bore surface line 54 between
points 62 and 64. In the illustrated embodiment, the distance between
points 58 and 60 is twice the distance between points 62 and 64.
The first and second outer leaves, 46 and 50 respectively, are
of asymmetrical shape with their free flexing ends terminating in
a point at points 58 and 60 and curved along their outward edges
to lie adjacent, near their points, to the conduit bore surface
line 54. The fixed ends of the first and second outer leaves, 46
and 50 are anchored along those portions of the conduit bore surface
line 54 connecting points 62 and 66 and 64 and 68 respectively.
Thus, the bases of the outer leaves 46 and 50 are angularly oriented
with respect to a center line extending longitudinally along the
center of leaf 48.
In the illustrated configuration, the outer leaves 46 and 50 respond
to fluid flow through the obstruction 44 by both bending and twisting
about their fixed ends, so that as fluid flow increases, the curved
outer portions of leaves 46 and 50 tend to remain adjacent to the
inner surface of sections 16 and 32. This characteristic of the
leaves can be more fully appreciated by reference to FIG. 3. At
the same time, the center leaf 48 responds to such fluid flow by
bending in a symmetrical manner about its fixed end, with substantially
no longitudinal twisting of that center leaf.
With the center leaf 48 extending upwardly within the flow meter
10 as illustrated, airborne droplets of water, saliva and mucus
will sink downward to the base of the center leaf 48 and will, therefore,
not impede the free movement of the free end of that center leaf
48. This permits the use of the flow meter in a breathing circuit
which can be very close to the patient, while minimizing adverse
effects of airborne liquids on device operation.
In the configuration described above, the center leaf 48 occupies
approximately 44% of the area of the obstruction 44 located within
the circular area defined by the conduit bore surface line 54. The
outer leaves 46 and 50 each represent approximately 25% of that
total area. Thus, the active, or flexible portion of the variable
area obstruction 44 which includes the leaves 46 48 and 50 comprises
approximately 94% of the area defined within the conduit bore surface
line 54. Accordingly, as the rate of fluid flow increases, the variable
area obstruction 44 is able to respond by providing a very limited
obstruction to the through passage of fluid, while providing a very
minimal fluid leakage path during periods of low fluid flow rate.
FIG. 3 illustrates a sectional view of the variable area obstruction
44 with the leaves 46 and 48 partially deflected. From this view,
it can be more fully appreciated that deflection of the center leaf
48 produces substantially no twisting, but bending occurs about
the fixed end portion of that leaf. In contrast, it is noted that
the first outer leaf 46 experiences both bending and twisting about
its fixed end, between points 64 and 68. Since the second leaf 50
(not shown) is asymmetrical with respect to leaf 46 it will bend
and twist about its fixed end in a different direction. Accordingly,
the leaves 46 48 and 50 lie in different planes, with respect to
each other, when they are deflected. This condition results in progressively
faster increases in the flow area provided by the variable area
obstruction 44 at higher flow rates. This provides for the substantially
linear relationship between the change in differential pressure
measured by transducer 26 with respect to the change in fluid flow
rate through the variable area obstruction 44.
By reference to FIG. 4 one may view and more fully appreciate
the process whereby the increasing flow area is provided through
the variable area obstruction 44 as the leaves 46 48 and 50 deflect
in response to fluid flow as illustrated in FIG. 3. In particular,
it is noted that the gap 56 defining the opening between the leaves
46 48 and 50 widens in response to increased rate of fluid flow
through the variable area obstruction 44 to define the through
passage for the fluid. It is also noted that the twisting of leaves
46 and 50 along with their bending motion, draws them toward the
interior surface of the first section 16 as the fluid flow rate
increases. The same situation would be true with respect to the
second section 32 if the direction of fluid flow were reversed.
The performance of the flow meter 10 is graphically illustrated
in FIG. 5. In particular, trace 70 represents the ideal characteristic
of a flow meter producing a substantially linear relationship between
the change in pressure measured through the meter and the change
in flow rate. The actual characteristic trace 72 relating to the
flow meter 10 is very close to the ideal characteristic trace 70.
The trace lines illustrated in FIG. 5 cover a range of flow rates
extending from 0.016 liters per second to 2.6 liters per second.
The linearity of the trace line 72 at lower flow rates is a result
of the minimal leakage path through the obstruction 44 which is
present at those low flow rates. Linearity at the upper end of the
flow rates is also improved over other devices, because the active
flexing area of the obstruction 44 in the flow meter 10 comprises
approximately 94% of the total area within the conduit bore surface
line 54. Accordingly, choking of the flow occurs at much higher
levels of flow than in devices which do not provide such a large
active flexing area of the obstruction.
FIG. 6 illustrates a comparison of the theoretical results represented
by theoretical characteristic trace 74 as compared with the actual
results determined in testing of a prototype, represented by the
actual characteristic trace 76. The prototype flow meter that produced
the actual characteristic trace 76 was constructed pursuant to criteria
and design parameters listed below:
1. Maximum flow rate required for this specific application was
three liters per second.
2. Maximum pressure differential at this flow rate was to be approximately
10 cm H.sub.2 O.
3. Material for the variable area obstruction 44 was 0.001 inch
thick, full hard stainless steel 300 Series.
4. Size of the active area of the variable area obstruction 44
was to be inscribed within a circle of 0.600 inches diameter.
5. The variable area obstruction 44 would be of the three-leaf
design, with the leaf unsupported length L=0.582 inch.
The sizing of the variable area obstruction 44 is a complex task.
There are many factors which influence the flow versus pressure
drop relationship. Secondary turbulences, caused by deflection of
the air stream by the flexing leaves and interference of individual
streams of air as they merge after passing though the variable area
obstruction, create an additional pressure drop which is not accounted
for by formulas. The prototype using the variable area obstruction
44 as illustrated in FIGS. 2-4 in accordance with the criteria
and design parameters set forth above, produced the following results
when tested with ambient air:
______________________________________ Flow Rate Pressure Differential
Liter/Sec. cm H.sub.2 O ______________________________________ 0.12
0.21 0.19 0.50 0.42 1.50 0.69 2.60 1.02 4.00 1.365 5.40 1.735 6.95
2.160 8.70 2.585 10.50 ______________________________________
The actual characteristic trace 76 of FIG. 6 illustrates a graphical
representation of the above results.
In order to build flow meters of different sizes, incorporating
the principles of the present invention, a formula which expresses
the basic theoretical relationship between flow rate, length of
the leaves and pressure differential was developed. In particular,
the following formulas established the relationship between flow
velocity and pressure differential across an obstruction. Flow rate
as a function of variable flow area may then be defined, and this
information may then be expressed as a function of fluid pressure
and leaf dimensions.
In text "Formulas for Stress and Strain" by R. J. Roark
and W. C. Young, Fifth Edition, McGraw-Hill, 1975 pg. 98 case 2a,
it is noted that deflection of a flat beam with one end fixed, the
other end free, loaded with uniform continuous load is defined by
the following: ##EQU1## It is also known that: ##EQU2## Substituting
(2) and (3) into (1) gives the following relationship: ##EQU3##
Where y=deflection at free end
1=unsupported length of the beam
E=modulus of elasticity of the beam material
I=sectional moment of inertia at fixed end
t=thickness of the beam
W=unit loading in Lbs per linear inch
p=fluid pressure in cm H.sub.2 O
As the leaf deflects, it forms a triangular flow area along its
sides which is proportional to the deflection and the leaf length
such that:
Where
A=flow area
KA=area coefficient
From fluid mechanics it is known that: ##EQU4## Where v=flow velocity
g=acceleration of gravity
KR=resistance coefficient
Q=flowrate in liters per second
From (6) and (7), ##EQU5##
Substituting (4) into (5) and then (5) into (8) one obtains the
following: ##EQU6##
If all coefficients and constants are condensed into one, the relationship
may be expressed as: ##EQU7##
The theoretical characteristic trace 74 of FIG. 6 is a graphical
representation of the flow rate Q as a function of pressure differential
p using Equation 10. From testing the prototype, it was determined
that the relationship between the flow rate Q and the pressure differential
p is substantially linear. Accordingly, the exponent related to
pressure (p) must be equal to 1.0 and not 1.5 as Equation 10 indicates,
for the configuration of the variable area obstruction 44 illustrated
in FIGS. 2-4. This difference appears to be due to the additional
turbulences encountered around the variable area obstruction 44
which were not addressed by the formula. Thus, the modified formula
reads: ##EQU8##
The numerical value of K for the prototype incorporating the variable
area obstruction 44 of FIGS. 2-4 is determined by substituting the
test data for that prototype into Equation 11. Thus, from the prototype
dimensions and test results defined above, 1=0.582 inch; t=0.001
inch; Q=1.735 liters/sec. (a value lower than the maximum was selected
to ensure performance within the linear range); p=6.95 cm H.sub.2
O. With this information, the value of K is determined to be: ##EQU9##
Thus, a formula has been developed which can be used for sizing
the flow meter for other flow rates and pressure differentials,
as follows: ##EQU10##
For example, for the parameters: Q max=3 liters/sec.; p=9 cm H.sub.2
O; t=0.001 inch. the diameter of the active area of the variable
area obstruction 44 is calculated as follows: ##STR1##
Since the length of the leaf 1=0.9253.times.D, where D is the diameter
of the active variable obstruction area: ##EQU11##
There are many appliCations wherein pressure differentials need
to be measured at flow rates of one-half liter per second and below
across an obstruction. A high degree of accuracy in accomplishing
the measurement at these flow rates is very desirable. Accordingly,
another preferred embodiment of the invention provides for a substantially
linear relationship between the change in pressure differential
and change in flow rate at higher flow rates, while having increased
sensitivity of the pressure differential with respect to changes
in flow rates at low flow rates. This embodiment of the invention
is illustrated in FIGS. 7-9.
In particular, FIG. 7 illustrates another preferred embodiment
of a variable area obstruction generally indicated at 80 for use
in the flow meter of FIG. 1. The variable area obstruction 80 is
constructed in a manner substantially the same as that of the variable
area obstruction 44 illustrated in FIGS. 2-4 with the exception
of the configuration of the leaves in the active flexing area bounded
by the conduit bore surface line 54.
Within the active flexing area, the variable area obstruction 80
comprises a primary leaf 82 with its fixed base extending generally
along line 54 between points 84 and 86. From points 84 and 86 the
parameter of the primary leaf 82 is defined by the very narrow,
continuous gap 56 which is etched in the obstruction 80. In particular,
from points 84 and 86 the gap 56 travels along a substantially
straight, inwardly directed line to points 88 and 90 respectively,
defining a short neck between the base and points 88 and 90. Points
88 and 90 are connected to each other by the gap 56 in a manner
which forms a substantially circular configuration, within the diameter
of the conduit bore surface line 54. The circular portion of the
primary leaf 82 is offset with respect to the center of the area
within the conduit bore surface line 54 so that the free end 92
of the primary leaf is substantially adjacent to a portion of the
conduit bore surface line 54 at a location which is substantially
opposite to the center of the fixed end of the primary leaf 82.
A pair of secondary leaves 94 and 96 of asymmetrical shape are
defined on the primary leaf 82 by extensions of the gap line 56.
In particular, secondary leaf 94 is defined by a portion of the
primary leaf 82 which extends outwardly from the center of the primary
leaf 82 and lies on the opposite side of a substantially straight
extension of the gap 56 running from point 88 to a termination point
98 on the distal portion on the primary leaf 82. Likewise, secondary
leaf 96 is defined by that portion of the primary leaf 82 extending
outward from the center of the primary leaf, and lying on the other
side of a substantially straight portion of the gap 56 extending
from the point 90 to a termination point 100 on the distal portion
of the primary leaf 82.
The substantially straight gap segments between points 88 and 98
and points 90 and 100 respectively, are directed generally inwardly
from the points 88 and 90 to the points 98 and 100 respectively,
toward the center of the free end of the primary leaf 82. Thus,
the free flexing ends of secondary leaves 94 and 96 terminate at
points 88 and 90 respectively. The secondary leaves 94 and 96 comprise
a base which is generally defined by the region of the primary leaf
82 which extends generally downwardly and outwardly from the points
98 and 100 respectively, to the gap 56.
The secondary leaves 94 and 96 are arranged so that their fee flexing
ends extend in a direction opposite to that of the primary leaf
82 with the base portion of the primary leaf 82 interposed between
the free ends of the secondary leaves 94 and 96 so that the leaves
are arranged with their free ends oriented in alternately opposing
directions.
In the configuration illustrated in FIG. 7 the primary leaf 82
responds to fluid flow through the obstruction 80 by bending about
its fixed end in the region defining a neck, between points 84 and
88 and 86 and 90 respectively. The primary leaf 82 does not experience
substantial twisting.
The secondary leaves 94 and 96 respond to fluid flow through the
obstruction 80 by both bending and twisting about their fixed ends,
so that as fluid flow increases, and as the primary leaf 82 bends
about its fixed end, the secondary leaves 94 and 96 bend and twist
about their fixed ends so their free ends are deflected in a generally
inwardly direction, such that the curved outer portions of the secondary
leaves 94 and 96 tend to remain adjacent to the inner surface of
sections 16 and 32.
In the configuration illustrated in FIG. 7 the initial response
to fluid flowing through the conduit at lower flow rates is that
the primary leaf 82 initially flexes about its fixed end. No substantial
independent flexing is exhibited by the secondary leaves 94 and
96 at the low flow rates. The flow rates creating this response
vary depending on the size of conduit and other variables as was
discussed in connection with the embodiment of FIGS. 2-4. However,
for example purposes, low flow rates through a conduit of about
0.60 inch diameter would be in the range of 0 to 0.5 liters per
second.
As the flow rate increases further, the secondary leaves 94 and
96 begin to flex about their fixed locations on the primary leaf.
Accordingly, at low flow rates, the pressure differential across
the obstruction changes at a rate which is greater than the change
in the rate of fluid flow, thus creating a non-linear response at
low flow rates. However, as flow rates through the obstruction 80
increase to the point that the secondary leaves 94 and 96 begin
to flex, that additional flexing increases the rate at which an
opening is created through the obstruction 80 so that the further
changes in the pressure differential across the obstruction 80 are
substantially linear with respect to further changes in the rate
of fluid flow through that obstruction 80.
With the circular portion of the primary leaves 82 offset with
respect to the center of the area defined by the conduit bore surface
line 54 a bending region is created at the neck adjacent the base
of the primary leaf 82. Sufficient space is also thereby provided
between the inner surface of sections 16 and 32 and the active flexing
area of the obstruction 80 so that the primary leaf 82 and the
secondary leaves 94 and 96 can respond to increased fluid flow by
providing a very limited obstruction to the through passage of fluid,
while still providing a very minimal fluid leakage path during periods
of low fluid flow rate. In fact, as with the prior embodiment, the
active, or flexible portion of the variable area obstruction 80
including leaves 82 94 and 96 comprises approximately 94% of the
area defined within the conduit bore surface line 54.
By reference to FIG. 8 the response of the primary and secondary
leaves with respect to fluid flow can be more fully appreciated.
In particular, when there is substantially no fluid flow through
the variable obstruction 80 the leaves 82 94 and 96 lie substantially
parallel in plane 101 which is substantially transverse to the
direction of fluid flow indicated at 83.
The leaf position at low fluid flow rates is generally illustrated
at 102 by phantom lines. In this configuration, it is noted that
the primary leaf 82 is bending, without substantial twisting, about
the neck region defined at the fixed end of the primary leaf 82
but no substantial bending or flexing of the secondary leaves 94
and 96 is experienced. The leave position at higher rates of fluid
flow through the obstruction 80 is illustrated generally at 104.
In this condition, the primary leaf 82 is more fully flexed in a
bending configuration about the neck adjacent its fixed end. The
secondary leaves 94 and 96 (not shown) also both bend and twist
about their fixed ends, in the region, respectively, between point
98 and the adjacent gap 56 and point 100 and the adjacent gap 56.
As with the leaves in the embodiment of FIGS. 2-4 the leaves 82
94 and 96 each lie in different plains with respect to one another,
when flexed in response to higher fluid flow rates as illustrated
in FIG. 8. This condition results in progressively faster increases
in the through flow area provided by the variable area diaphragm
obstruction 80 at higher flow rates. However, at low flow rates,
the embodiment of FIGS. 7 and 8 provides for a non-linear relationship
between the parameters, wherein the change in pressure differential
across the obstruction 80 is greater than the change in rate of
fluid flow through the variable area obstruction 80.
Referring now to FIG. 9 the increase in size of the passage way
through the variable area obstruction 80 defined by the gap 56 is
illustrated as it appears at a higher fluid flow rate, corresponding
to the condition generally indicated at 104 in FIG. 8.
FIG. 10 provides a graphical illustration of the reverse parabolic
configuration which characterizes a plot of flow with respect to
pressure differential through the variable area obstruction 80 at
flow rates at or less than about one-half liter per second. It is
noted that the pressure differential .DELTA.p0 to .DELTA.p1 experienced
in the low flow rate region defined between Q0 and Q1 is in the
range of about two times as sensitive as the pressure differential
.DELTA.p2 to .DELTA.p3 measured in response to the same flow rate
changes in the higher flow regions between Q2 and Q3 in the graph
of FIG. 10.
In summary, the invention described herein comprises a significant
improvement over the prior art by providing a flow meter having
a configuration which, in one preferred embodiment, provides a substantially
linear relationship between the change in pressure differential
across the obstruction and changes in fluid flow rate across that
obstruction. Another embodiment is provided wherein a substantially
non-linear relationship exists between change in pressure differential
and change in fluid flow through the obstruction at lower flow rates,
with a substantially linear relationship existing between these
elements at higher flow rates. The invention also overcomes other
long existent problems in the industry by (1) providing a flow meter
having a wide measuring range and accuracy, which is substantially
not affected by the presence of moisture or mucus produced by a
patient; (2) providing such a flow meter wherein the size of the
flow passage in the variable area obstruction is minimized at zero
flow condition; (3) providing such a flow meter that has a light,
simple and inexpensive structure which permits it to be disposable
while being manufactured in a manner that allows mass production
while maintaining the high level of sensitivity and accuracy in
each unit; and (4) providing such a flow meter where the active
area of the variable obstruction is substantially the same as the
full cross sectional area of the fluid conduit, and wherein the
elements of the variable area obstruction are shaped in such a manner
that they, at maximum deflection, leave a substantially unobstructed
passage along the fluid conduit wall.
The invention may be embodied in other specific forms without departing
from its spirit or essential characteristics. The described embodiments
are to be considered in all respects only as illustrative and not
restrictive. The scope of the invention is, therefore, indicated
by the appended claims rather than by the foregoing description.
All changes which come within the meaning and range of equivalency
of the claims are to be embraced within their scope. |