Abstrict A fluid flow meter has a housing with a fluid inlet and a fluid
outlet and defining therebetween a fluid flow path. The flow meter
has a flexible membrane having a inlet end mounted at an inlet region
of the fluid flow path and a outlet end mounted at an outlet region
of the fluid flow path. The flexible membrane produces undulating
motion in response to fluid flow in the fluid flow path. Fluid flow
rate is measured by sensing the undulating motion of the membrane.
Performance of the meter is enhanced by laminating a layer of material
to the membrane.
Claims What is claimed is:
1. A fluid flow meter, comprising:
a housing having a fluid inlet and a fluid outlet and defining
therebetween a fluid flow path;
a flexible membrane having an inlet end mounted at an inlet region
of the fluid flow path and an outlet end mounted at an outlet region
of the fluid flow path, the membrane being designed to produce an
undulating motion along an active portion in response to fluid flow
in said fluid flow path; and
a device responsive to the undulating motion of the membrane for
measuring the fluid flow;
the membrane having:
at least a first layer of flexible material laminated to the membrane
at the inlet end of the membrane;
at least a second layer of the flexible material laminated to the
membrane at the outlet end of the membrane; and
wherein the first layer of flexible material is longer than the
second layer of flexible material.
2. A fluid flow meter as in claim 1 further comprises a third
layer of flexible material laminated to the membrane at the inlet
end of the membrane on a reverse side of the membrane.
3. A fluid flow meter as in claim 2 further comprises a fourth
layer of flexible material laminated to the membrane at the outlet
end of the membrane on a reverse side of the membrane.
4. A fluid flow meter of claim 1 wherein at least one of said
first and second layers has an antistatic surface.
5. A fluid flow meter of claim 1 wherein said membrane has an
antistatic surface.
6. A fluid flow meter of claim 1 wherein at least one of said
first and second layers contains piezoelectric material.
7. A fluid flow meter of claim 1 wherein said first and second
layers have similar thermal characteristics as the membrane.
8. A fluid flow meter of claim 1 wherein the housing and the membrane
have similar coefficients of thermal expansion.
9. A fluid flow meter of claim 1 wherein the membrane has a thickness
of about 0.0015 inch, wherein the first layer of flexible material
has a thickness of about 0.002 inch and extends over the active
portion of the membrane by about 0.5 inch and the second layer of
flexible material has a thickness of about 0.002 inch and extends
over the active portion of the membrane by about 0.25 inch, said
first and second layers being on the same side of the membrane.
10. A fluid flow meter of claim 1 wherein said membrane has a
thickness of about 0.0015 inch, wherein the first layer of flexible
material has a thickness of about 0.002 inch and extends over the
active portion of the membrane by about 0.5 inch and the second
layer of flexible material has a thickness of about 0.002 inch and
extends over the active portion of the membrane by about 0.25 to
0.5 inch, said first and second layers being on opposite sides of
the membrane.
Description FIELD OF THE INVENTION
The present invention relates in general to volumetric fluid flow
measurement and in particular to an improved membrane for use in
a volumetric fluid flow meter. More particularly, the present invention
is related to a membrane with lamination for improving performance
of the meter, for example, by improving the durability of the membrane
and its response to low fluid flow rate.
BACKGROUND OF THE INVENTION
Fluid flow meters, such as those disclosed by Dov Ingman in U.S.
Pat. Nos. 4920794 and 5069067 both of which are incorporated
herein by reference, measure flow rate of a fluid by measuring undulation
of a flexible membrane in response to passage of the fluid through
a flow chamber. The flexible membrane spans between two points along
the flow path of the fluid and has a length which is long enough
to undulate and divide, with a pair of opposing walls, the flowing
fluid into discrete quanta, each with a determinable volume. By
detecting the motion of the membrane, the movement of the fluid
quanta and therefore the flow rate of the fluid can be measured.
Among the factors affecting the performance of the above described
flexible membrane fluid flow meters are the physical properties
of the membrane.
For example, an ideal membrane would be one that is formed from
material with very low mass and a high modulus of elasticity. Theoretically,
with a zero mass membrane, the resonant frequency of the membrane
would be infinite and would therefore not resonate or flutter because
the actual frequency of motion induced in the membrane by the fluid
flow can never reach infinity. In addition, an ideal membrane should
require only negligible energy to move in response to the fluid
flow so that it can measure very low or near zero flow rates.
The mass of a membrane is proportional to its thickness. However,
the stiffness of a membrane is proportional to the cube of its thickness.
Therefore, decreasing the thickness of the membrane to reduce its
mass would decrease the stiffness at a greater rate. If the membrane
is too thin and the stiffness is reduced too far, a portion of the
membrane will lie limply on a membrane face under gravitational
force. As a result, a limit to the possible reduction in membrane
thickness is the point where the stiffness is reduced to below the
level required to maintain membrane shape against the force of gravity.
Moreover, although it is desirable to have the thinnest membrane
capable of maintaining its shape against gravity, membrane thickness
is generally desirable in order to minimize leakage through the
gaps between the edges of the membrane and the side walls of the
flow chamber. If the membrane can be made thicker, the required
manufacturing tolerances to maintain the effective seal between
the membrane edges and the side walls can be relaxed, thereby reducing
manufacturing costs.
Thus, the ideal membrane would be relatively thick to minimize
sealing problems, of low density to minimize mass and stiff enough
to enable it to support its own weight. In other words, the membrane
thickness is a trade off between the membrane mass, the required
stiffness for supporting its own weight and the required tolerances
for maintaining an effective seal against leakage.
Another factor affecting the performance of a flexible membrane
fluid flow meter is its durability. One factor affecting the durability
of a membrane is the short operating life which could result from
membrane fatigue due to repetitive flexing thereof at the clamping
points.
SUMMARY OF THE INVENTION
The present invention provides a fluid flow meter which has a housing
with a fluid inlet and a fluid outlet and defining therebetween
a fluid flow path. The fluid flow meter has a flexible membrane
with an inlet end mounted at an inlet region of the fluid flow path
and an outlet end mounted at an outlet region of the fluid flow
path. The membrane is designed to produce undulating motion along
an active portion in response to fluid flow in the fluid flow path.
The fluid flow meter also has a device responsive to the undulating
motion of the membrane for measuring the fluid flow. In accordance
with the present invention, the membrane has at least one laminated
portion for improving performance of the meter.
In another aspect, the present invention also provides a method
for improving performance of a fluid flow meter. The fluid flow
meter has a membrane mounted between an inlet and outlet for dividing
fluid flow through the meter into discrete quanta. Fluid flow is
measured by measuring undulating motion along an active portion
of the membrane. The method includes the steps of supporting the
membrane with at least one layer of flexible material laminated
to the active portion of the membrane, sensing the undulating motion
of the membrane, and measuring volume of fluid flowing through the
meter based upon the undulating motion of the membrane.
In still another aspect, the present invention provides a method
for measuring flow rate of a fluid, including the step of defining
a fluid flow path by a first pair of opposing walls which are separated
by a predetermined first distance and a second pair of opposing
walls which are separated by a predetermined second distance. The
method also includes the step of providing a membrane, with a predetermined
thickness, which undulates and contacts the first pair of opposing
walls in response to fluid flow in the flow path. The method also
includes the step of laminating an additional layer of flexible
material to a portion of the membrane based upon one or more dimensions
of the membrane.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a fluid flow meter in which a flexible membrane
with one or more laminated portions is used to measure fluid flow;
FIG. 2 is a side view of a membrane depicting preferable portions
thereof which can be laminated to enhance performance of the fluid
flow meter;
FIG. 3 is a top plan view of the membrane shown in FIG. 2 and depicts
preferable portions thereof which can be laminated to enhance performance
of the fluid flow meter;
FIG. 4 is a side view of a membrane structure in which an additional
layer of flexible material is laminated at each end of the membrane,
with the additional layers of flexible material being laminated
on the same side of the membrane;
FIG. 5 is a side view of a membrane structure in which an additional
layer of flexible material is laminated at each end of the membrane,
with the additional layers of flexible material being laminated
on opposite sides of the membrane;
FIG. 6 is a side view of a membrane structure in which a laminated
portion is provided at the inlet end of the membrane by laminating
an additional layer of flexible material to the membrane, and in
which a laminated portion is provided at the outlet end of the membrane
by laminating two layers of flexible material, one on each side
of the membrane; and
FIG. 7 is a side view of a membrane structure in which a laminated
portion is provided at each end of the membrane, each by laminating
two layers of flexible material, one on each side of the membrane.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a fluid flow meter 10 with a housing 12 having an
inlet aperture 14 a central flow chamber 16 and an outlet aperture
18. The central flow chamber 16 has a first pair of opposing walls
11 and 13 defining a predetermined height (H) and a second pair
of opposing walls (see FIG. 3 reference numerals 15 and 17) defining
a predetermined width (W). The first pair of walls 11 13 and the
second pair of walls 15 17 together define a fluid flow path in
the meter 10. Optionally, an inlet chamber 20 is provided between
the inlet aperture 14 and the central flow chamber 16 for removing
unwanted particles, such as debris and moisture, from the fluid
before the fluid enters the central flow chamber 16. Optionally,
an outlet chamber 24 is also provided between the central flow chamber
16 and the outlet aperture 18 for removing unwanted particles in
the event fluid flows through the meter 10 in a reversed direction.
A sensor assembly 26 is mounted within the fluid flow meter 10.
The sensor assembly 26 includes a sensor membrane 28 with a thickness
(T) which is mounted between an inlet membrane mounting assembly
30 and an outlet membrane mounting assembly 32. The length (L) of
the sensor membrane 28 between the inlet membrane mounting assembly
30 and the outlet membrane mounting assembly 32 defines an action
portion that undulates in response to fluid flow through the central
flow chamber 16. The sensor membrane 28 is made of flexible material
such as polyester and has sufficient length to maintain two or more
points of contact simultaneously with the walls 11 and 13 when it
undulates in response to the fluid flow.
The height (H) between the first pair of opposing walls 11 13
the length (L) of the active portion and the thickness (T) of the
sensor membrane 28 are selected to optimize the responsiveness of
the sensor membrane to the fluid flow. That is, they are selected
so that the sensor membrane 28 is very flexible and yet would support
its own weight.
Preferably, the sensor membrane 28 is made of or coated with an
antistatic substance (not shown) to prevent build-up of static electricity
and/or with a hydrophobic substance (not shown) to prevent build
up of moisture.
The inlet and outlet membrane mounting assemblies 30 and 32 are
preferably made of material, such as stainless steel or glass-filled
polyester, which has similar thermal characteristics as the sensor
membrane 28.
A guide block and/or a guide extension (not shown), such as described
in the above patents to Dov Ingman, may be provided at the inlet
and outlet membrane mounting assemblies 30 and 32 to improve the
flexure of the sensor membrane 28.
The inlet and outlet membrane mounting assemblies 30 an 32 can
also include clamping means described by Dov Ingman in a U.S. patent
application filed Nov. 23 1992 with Ser. No. 07/979996 and which
is incorporated herein by reference.
It is preferable that the thermal expansion of the housing 12 of
the meter 10 matches the thermal expansion of the sensor membrane
28.
In operation, fluid enters the meter 10 through inlet aperture
14 and passes through inlet chamber 20 where debris are deposited
or filtered. The fluid then passes through the central flow chamber
16 and the outlet chamber 24 and exits through the outlet aperture
18. When the fluid flows through the central flow chamber 16 it
causes the sensor membrane 28 to undulate and contact the walls
11 and 13 of the central flow chamber 16. The undulating motion
of the sensor membrane 28 can be detected by one of the many methods
described in the prior art, such as by passing a light beam across
the central flow chamber 16 through a path which can be broken or
cut off by the undulating motion of the sensor membrane 28 or by
providing a thin layer of piezoelectric material (not shown) on
the sensor membrane 28 and detecting changes in the electrical potential
generated by the piezoelectric material due to the undulating motion
of the sensor membrane 28.
Referring to FIGS. 2 and 3 in accordance with the present invention,
performance (i.e., response to fluid flow characteristics such as
density, temperature and flow rate of the fluid) of the sensor membrane
28 is enhanced by increasing the stiffness of at least one portion
of the sensor membrane 28 so that the overall mass of the sensor
membrane 28 is not substantially affected. The stiffness is increased
based upon design parameters of the meter 10 such as the thickness
and the length of the active portion, or the modulus of elasticity
of the sensor membrane 28 or the amplitude of undulation of the
sensor membrane 28 as defined by the height (H) of the flow path.
In the presently preferred embodiment, the stiffness is increased
preferably by increasing the thickness of at least one portion of
the sensor membrane 28 which in turn is increased preferably by
laminating the sensor membrane 28 with one or more additional layers
of flexible material at one or more of the positions, such as positions
34 36 38 and 40 shown in FIG. 2.
Laminating the sensor membrane 28 with one or more additional layers
of flexible material helps to support the sensor membrane 28 against
collapsing, and allows a reduction in thickness in the active portion
(i.e., the portion which undulates) of the sensor membrane 28 while
maintaining (or even increasing) the appropriate stiffness of the
sensor membrane 28.
Laminating the sensor membrane 28 at the inlet and/or outlet mounting
assemblies 30 32 also improves the performance of the meter 10
by reinforcing the sensor membrane 28 against fatigue failure at
the mounting points of the sensor membrane 28 and thereby increasing
the durability of the sensor membrane 28.
Lamination at the outlet end of the membrane increases the bending
moment of the sensor membrane 28 about the outlet mounting point
and can thereby reduces reflection of waves by the outlet membrane
mounting assembly 32. Reflection of the waves is undesirable because
it distorts the undulation caused by the fluid flow. However, it
needs to be noted that the bending moment of the sensor membrane
28 should preferably not be increased to a point where it would
force the fluid flow back towards the inlet.
Lamination of the sensor membrane 28 at each of the portions 34
36 38 and 40 is achieved by affixing, such as by using an adhesive
(e.g., an adhesive with product number SP23 from Sun Process, Elk
Grove Village, Ill.) or other bonding techniques (e.g., thermal
bonding), one or more layers of flexible material to the sensor
membrane 28. A lamination should preferably extend from and be clamped
by the inlet and outlet membrane mounting assemblies 30 and 32.
Referring to FIG. 3 the width at the ends 35 and 37 of the sensor
membrane 28 is usually narrower than the width of the active portion
because it has been determined that such narrowing of the ends 35
and 37 reduces flutter both along the flow path as well as transverse
thereto. An additional layer which is laminated with the sensor
membrane 28 should preferably have the same shape (i.e., with a
tapered end) as the sensor membrane 28.
Each additional layer is preferably made of similar material as
the sensor membrane 28. If an additional layer is made with other
material, it is preferable that such other material has similar
characteristics (e.g., coefficient of thermal expansion) as the
sensor membrane 28.
The thickness and length of an additional layer laminated at any
one of the positions 34 36 38 and 40 may and may not be equal
to the thickness and length of the additional layers at other positions.
The portion(s) of the sensor membrane 28 to be laminated, and the
length and thickness of the additional layer(s) laminated thereto
are selected based upon such factors as the thickness (T) of the
sensor membrane 28 as well and the length (L) of the active portion
of the sensor membrane 28 and height (H) of the fluid flow path,
which in turn are determined based upon factors such as the lowest
flow rate and/or highest flow rate required to be measured by the
meter 10 and the desired durability of the sensor membrane 28.
FIG. 3 also shows the second pair of walls 15 and 17 and a sensor
33 connected to the sensor membrane 28 through the inlet sensor
mounting assembly 30. The sensor 33 senses undulation of the sensor
membrane 28 to measure the fluid flow rate.
Referring again to FIG. 2 performance of the meter 10 is further
improved by providing an antistatic surface on the additional layer(s)
laminated to the sensor membrane 28. Materials having an antistatic
surface or made with antistatic material are available off-shelf.
FIG. 4 depicts a membrane structure 42 which is one implementation
of the present invention. The membrane structure 42 includes a flexible
sensor membrane 28 with a first layer 44 of flexible material laminated
at position 34 shown in FIG. 2 and a second layer 46 of flexible
material laminated at position 38.
To detect undulating motion of the sensor membrane 28 an additional
layer of piezoelectric material 45 47 can be added on one or both
of the first and second layers 44 and 46. Alternatively, one or
both of the first and second layers 44 and 46 can contain piezoelectric
material. In that case, the additional layer 45 and/or 47 can be
omitted. If other means is used for such detection, a layer of piezoelectric
material is not needed.
According to one specific implementation of the membrane structure
42 the thickness (T) of the sensor membrane 28 is about 0.0015
inch. A thickness of about 0.0015 inch is the preferred membrane
thickness for a fluid flow path with a length (L) of about 4 inches
and a height (H) of about 0.25 inch, which are the preferred dimensions
for measuring a gas flow with a maximum flow rate of 250 cubic feet
per hour (CFH) and a minimum flow rate of 0.25 CFH, which is the
range of gas consumption of an average household. The edges 29 (see
FIG. 3) of the sensor membrane 28 are each separated from the corresponding
one of walls 15 and 17 by a gap of between 0.0015 to 0.003 inch.
A larger gap may allow the sensor membrane 28 to undulate more freely,
but sensitivity in measuring low flow rate will be reduced thereby.
The sensor membrane 28 is made from a polyester film with antistatic
coating, such as Product Number 142A302 or Product Number 142J302
from Dupont Corporation. The first layer 44 of flexible material
has a preferred thickness of about 0.002 inch and extends over the
active portion of the sensor membrane by a length (L1) of about
0.5 inch. The second layer 46 of flexible material has a preferred
thickness of about 0.002 inch and extends over the active portion
of the sensor membrane by a length (L3) of about 0.25 inch. The
width of each of the first and second layers, 44 and 46 is the
same as the width of the sensor membrane 28. Both the first layer
44 and the second layer 46 are made with the same material as the
sensor membrane 28 and are laminated with the sensor membrane 28
by means of adhesive or other bonding techniques including thermal.
FIG. 5 depicts a membrane structure 48 which is another implementation
of the present invention. The membrane structure 48 includes a flexible
sensor membrane 28 with a first layer 50 of flexible material laminated
at position 34 shown in FIG. 2 and a second layer 52 of flexible
material laminated at position 40 shown in FIG. 2. Both the first
layer 50 and the second layer 52 are made with the same material
as the sensor membrane 28 and are laminated to the sensor membrane
28 by means of adhesive or other bonding techniques including thermal.
To detect undulating motion of the sensor membrane 28 an additional
layer of piezoelectric material (not shown) can be added on the
surface of one or both of the additional layers. Alternatively,
one or both of the additional layers can contain piezoelectric material
and the additional piezoelectric layer can then be omitted.
According to one specific implementation of the membrane structure
48 the sensor membrane 28 has thickness (T) of approximately 0.0015
inch and an active portion (L) of about 4 inches. The height (H)
of the flow path is about 0.25 inch. The first layer of flexible
material 50 has a preferred thickness of about 0.002 inch and extends
over the active portion of the sensor membrane 28 by a length (L1)
of about 0.5 inch. The second layer of flexible material 52 has
a preferred thickness of about 0.002 inch and extends over the active
portion of the sensor membrane 28 by a length (L4) of about 0.25
to 0.5 inch. The width of each of the first and second layers, 50
and 52 is the same as the width of the sensor membrane 28.
FIG. 6 depicts a membrane structure 54 which is another implementation
of the present invention. The membrane structure 54 includes a flexible
sensor membrane 28 with a first layer 56 of flexible material laminated
at position 36 shown in FIG. 2 a second layer 58 laminated at position
38 shown in FIG. 2 and a third layer 60 laminated at position 40
shown in FIG. 2. The layers 56 58 and 60 are made with the same
material as the sensor membrane 28 and are laminated to the sensor
membrane 28 by means of adhesive or other bonding techniques including
thermal.
To detect undulating motion of the sensor membrane 28 an additional
layer of piezoelectric material (not shown) can be added on the
surface of one or both of the additional layers. Alternatively,
one or both of the additional layers can contain piezoelectric material
and the additional piezoelectric layer can then be omitted.
According to one specific implementation of the membrane structure
54 the sensor membrane 28 has a thickness (T) of approximately
0.0015 inch and an active portion (L) of about 4 inches. The fluid
flow path has a height (H) of about 0.25 inch. The first layer 56
has a preferred thickness of about 0.002 inch and extends over the
active portion of the sensor membrane 28 by a length (L2) of about
0.25 inch. The second and third layers 58 and 60 each has a preferred
thickness of about 0.002 inch. The second layer 58 extends over
the active portion of the sensor membrane 28 by a length (L3) of
about 0.50 inch. The third layer 60 extends over the active portion
of the sensor membrane 28 by a length (L4) of about 0.25 inch.
FIG. 7 depicts a membrane structure 62 which is another implementation
of the present invention. The membrane structure 62 includes a flexible
sensor membrane 28 having first and second layers 64 and 66 of flexible
material laminated at positions 34 and 36 respectively shown in
FIG. 2 and third and fourth layers 68 and 70 flexible material
laminated at positions 38 and 40 respective shown in FIG. 2. The
additional layers 64 66 68 70 are made with the same material
as the sensor membrane 28 and are laminated to the sensor membrane
28 by means of adhesive or other bonding techniques including thermal.
To detect undulating motion of the sensor membrane 28 an additional
layer of piezoelectric material (not shown) can be added on the
surface of one or both of the additional layers. Alternatively,
one or both of the additional layers can contain piezoelectric material
and an additional piezoelectric layer can then be omitted.
According to one specific implementation of the membrane structure
62 the sensor membrane 28 has thickness (T) of approximately 0.0015
inch and an active portion (L) of about 4 inches. The fluid flow
path has a height (H) of about 0.25 inch. The first layer 64 has
a thickness of about 0.002 inch and extends over the active portion
of the sensor membrane 28 by a length (L1) of about 0.5 inch. The
second layer 66 has a preferred thickness of about 0.002 inch and
extends over the active portion of the sensor membrane 28 by a length
(L2) of about 0.25 inch. The third layer 68 has a preferred thickness
of about 0.002 inch and extends over the active portion of the sensor
membrane 28 by a length (L3) of about 0.125 inch. The fourth layer
70 has a thickness of about 0.002 inch and extends over the active
portion of the sensor membrane 28 by a length (L4) of about 0.25
inch.
It will be appreciated by persons skilled in the art that the present
invention is not limited to what has been shown and described hereinabove.
For example, a laminated portion of the membrane can have more than
one additional layer. In addition, the different additional layers
at the same laminated portion of the membrane can further have different
thicknesses and different lengths. In addition, the additional layers
at the same laminated portion of the membrane can even be made of
different material. Therefore, it will be understood that various
modifications can be made to the invention without departing from
the scope thereof, which is defined by the following claims. |