Flow sensing device

A device for sensing fluid flow comprising a housing having an inlet and an outlet and defining a fluid flow channel between the inlet and the outlet, a first member disposed in the fluid flow channel and including at least one rigid portion, and a second member disposed in the fluid flow channel upstream from the first member and including at least one flexible portion, said flexible portion being elastically flexible between an initial position spaced from the rigid portion and with its periphery generally spaced from the periphery of the channel to define a first effective flwo area and a final position adjacent the rigid portion and with its periphery generally spaced from the periphery of the channel to a second effective flow area which exceeds the first effective flow area.

TECHNICAL FIELD 
The present invention relates to devices for sensing the flow of a fluid. 
In particular, it relates to flow sensing devices which develop a 
differential pressure in response to fluid flow. 
DISCLOSURE OF THE INVENTION 
Fluid is a generic term which encompasses both liquids and gases. Flow may 
be defined as the volume of fluid passing a location in a certain amount 
of time and may be expressed in terms such as gallons per minute or cubic 
feet per second. For many mechanical and chemical systems, it is important 
to be able to accurately sense fluid flow at some point in the system. 
Many devices for sensing fluid flow are commonly available. For example, 
one type of device includes a rigid flow obstruction, such as a rigid 
plate with a hole in it, located within a pipe. The diameter of the hole 
is smaller than the inside diameter of the pipe. In accordance with 
certain physical laws, the pressure of the fluid flowing through the flow 
obstruction is less than the pressure of the fluid flowing through the 
larger diameter pipe upstream from the flow obstruction. The difference 
between these two pressures is known as the differential pressure and the 
value of the differential pressure is related to the flow, i.e., a large 
flow yields a large differential pressure while a small flow yields a 
small differential pressure. Typically, this type of device further 
includes an arrangement for sensing the differential pressure, relating 
the differential pressure to the flow, and displaying the value of the 
flow. 
While flowing sensing devices with rigid flow obstructions are effective 
and reliable, they nonetheless have several undesirable characteristics. 
For example, due to the relationship between flow and differential 
pressure in these devices, low flows are difficult to accurately sense. 
Consequently, the range of flows which these devices can usefully sense is 
rather small, e.g., the maximum flow may be only about 3 to 4 times the 
minimum flow. 
A general object of the present invention is to provide an improved flow 
sensor. Specific objects include providing a flow sensor which has a wide 
flow range and yet is extremely reliable and effective. 
In accordance with the invention, a flow sensing device is provided which 
comprises a housing which has an inlet and an outlet and defines a fluid 
flow channel between them, a first member which is disposed in the fluid 
flow channel and includes at least one rigid portion, and a second member 
which is disposed in the fluid flow channel upstream from the first member 
and includes at least one flexible portion. The flexible portion is 
elastically flexible between an initial position spaced from the rigid 
portion and with its periphery generally spaced from the periphery of the 
channel to define a first effective flow area within the channel and a 
final position adjacent the rigid portion and with its periphery generally 
spaced from the periphery of the channel to define a second effective flow 
area within the channel which exceeds the first effective flow area. At 
low flows, the flexible portion flexes elastically from its initial 
position, generating a differential pressure which allows the low flow 
greater than a predetermined velocity, to be accurately sensed. At high 
flows, the flexible portion is maintained securely against the rigid 
portion, allowing the high flow to be accurately sensed. Consequently, the 
flow range for the flow sensing device of the present invention is much 
larger than many conventionally available devices.

BEST MODE FOR CARRYING OUT THE INVENTION 
As shown in FIGS. 1 and 2, a first exemplary flow sensing device 10 
constructed and operated according to the present invention generally 
comprises a housing 11, a flow obstruction assembly 12, and a pressure 
sensing arrangement 13. The housing 11 has an inlet 14 and an outlet 15 
and defines a fluid flow channel 16 between the inlet 14 and the outlet 
15. While the housing may be fabricated from any suitably impervious 
material and fashioned in any appropriate configuration, the housing 11 of 
the first exemplary flow sensing device 10 is fabricated from aluminum and 
fashioned in a generally cylindrical configuration. Further, although the 
flow channel may be variously configured, the flow channel 16 of the first 
exemplary flow sensing device 10 extends coaxially between the inlet 14 
and the outlet 15 and has a uniform, circular cross section. Consequently, 
the general direction of fluid flow through the channel 16 is parallel to 
the channel axis A. 
The flow obstruction assembly 12 is mounted to the housing 11 in the fluid 
flow channel 16 and, in accordance with one aspect of the invention, 
generally includes a rigid member 20 disposed downstream from a flexible 
member 21. Although the flow obstruction assembly may assume any suitable 
configuration, in the exemplary flow sensing device 10, the rigid member 
20 comprises a plate having a generally U-shaped cross section while the 
flexible member 21 comprises a thin circular disc. The thinness of the 
flexible member 21 may be uniform or non-uniform, e.g., may taper toward 
the edge. 
The rigid member 20, which may be fabricated from aluminum, is preferably 
mounted symmetrically within the flow channel 16 with the bight 22 of the 
U-shaped rigid member 20 lying generally normal to the direction of flow 
and the legs 23 extending downstream oblique to the direction of flow. The 
rigid member 20 may be fixed within the flow channel 16 by any appropriate 
means, e.g., by welding the rigid member 20 to the housing 11. In the 
first exemplary flow sensing device 10, the rigid member 20 is attached to 
a pin 24. The pin 24 is attached at both ends to the housing 11 and 
extends along the downends stream side of the bight 22 perpendicularly 
through the channel axis A. 
The thin circular disc of the flexible member 21 preferably is fashioned 
from stainless steel and has a diameter somewhat less than the inside 
diameter of the housing 11. The diameter and thinness of the flexible disc 
member 21 may be selected to yield a desired response to flow, thinner and 
larger diameter members 21 being more responsive to lower flows. The 
flexible disc member 21 of the first exemplary flow sensing device 10 is 
preferably mounted coaxially within the channel 16 normal to the direction 
of flow and proximate the rigid member 20. For example, the flexible 
member 21 may be joined to the bight 22 of the rigid member 20 by a staple 
25 which extends through the rigid member 20 into the pin 24, fixing the 
rigid member 20 to the pin 24 as well. The wings 26 of the flexible disc 
member 21, i.e., the portions of the flexible disc member 21 away from the 
bight 22 of the rigid member 20, remain free to flex between an initial 
position I normal to the direction of flow and a final position II 
adjacent the legs 23 of the rigid member 20. 
The pressure sensing arrangement 13 may comprise any suitable, well-known 
pressure transducer capable of sensing a differential pressure within the 
flow channel 16 caused by the flow obstruction assembly 12 and providing 
an electrical signal which corresponds to the differential pressure. For 
example, in the first exemplary flow sensing device 10, the pressure 
sensing arrangement 13 may include a conventional pressure transducer 30 
mounted to the outside of the housing 11 and two apertures 31, 32 in the 
housing 11 which allow fluid pressure to be communicated between the 
channel 16 and the pressure transducer 30. The first aperture 31 is 
located upstream from the flow obstruction assembly 12, preferably 
slightly ahead of the flexible member 21 and in the plane defined by the 
channel axis A and the pin 24. 
The second aperture 32 may be located in various locations near the flow 
obstruction assembly 12 where the pressure drops due to fluid flow around 
the obstruction assembly 12. In the first exemplary flow sensing device 
10, the second aperture 32 is preferably disposed slightly behind the pin 
24 but in the same plane and on the same side of the housing 11 as the 
first aperture 31. The pressure transducer 30 may be connected to any 
conventional device for correlating the differential pressure with the 
flow and displaying the flow. For example, the electrical signal output by 
the pressure transducer 30 may be supplied to a microprocessor 33 which 
correlates the signal with the appropriate flow and drives a display 34, 
such as an LED display, to indicate the flow. 
A second exemplary flow sensing device 40 is shown in FIG. 3. It is similar 
to the first exemplary flow sensing device 10 except that a larger pin 41 
serves as both the mounting pin 24 and the U-shaped rigid member 20 of the 
first exemplary flow sensing device 10. The larger pin 41 may have any 
suitable configuration with an upstream surface sufficient to limit the 
curvature of the flexible member 21 to a value which will delay failure 
due to fatigue caused by flexing. For example, in the second exemplary 
flow sensing device 40, the larger pin 41 has a circular cross section 
with a large enough diameter to limit the minimum bend radius of the 
flexible disc member 21, delaying fatigue. The center of the flexible 
member 21 may be mounted directly the pin 41, for example, by means of the 
staple 25, the wings 26 remaining free to flex between an initial position 
I normal to the direction of flow and a final position II adjacent much of 
the upstream surface of the pin 41. 
The preferred mode of operation of both exemplary flow sensing devices 10, 
40 is similar. For example, the first exemplary flow sensing device 10 may 
be installed in a hydraulic or pneumatic system by means of the threaded 
fittings on the inlet 14 and the outlet 15. Fluid flowing from the inlet 
14 to the outlet 15 past the flow obstruction assembly 12 develops a 
differential pressure within the channel 16. In the exemplary flow sensing 
device 10, the flow obstruction assembly 12 is mounted in the center of 
the channel 16 and forces the fluid toward the periphery of the channel 
16. This not only limits stress on the flexible member 21 but also allows 
for higher flows in a specific envelope at an equal pressure drop than 
conventional orifice devices. 
For low flows, the wings 26 of the flexible member 21 flex elastically 
downstream from the initial position I. The effective flow area around the 
flow obstruction assembly 12 increases as the wings 26 flex from the 
initial position I toward, the final position II at some predetermined 
flow velocity, attaining a maximum at the final position II. Further, the 
force required to elastically flex the wings 26 increases with the 
magnitude of flexion from the initial position I. Consequently, at low 
flows, the exemplary flow sensing device 10 acts as a variable area flow 
sensor in which small changes in flow produce easily detectable changes in 
differential pressure, allowing very low flows to be accurately sensed. 
The low end of the useful flow range is defined at least in part by the 
effective flow area with the flexible member in the initial position I, 
e.g., as determined by the diameter of the flexible disc member 21, and by 
the modulus of elasticity of the flexible member, e.g., as determined by 
the thickness of the flexible disc member. 
For high flows, the wings 26 are maintained in the final position II 
adjacent the legs 23 of the rigid member 20. The rigid member 20 not only 
limits the minimum bend radius of the wings 26 but also dampens any 
flutter or angular distortion which may be caused, for example, by 
tubulent flow, flow surges, or fluid hammer. Unlike the first exemplary 
flow sensing device 10, the end portions of the wings 26 of the second 
exemplary flow sensing device 40 extend beyond the pin 41. Thus, the 
second exemplary flow sensing device 40 may be more susceptible to flutter 
but is also more easily and less expensively manufactured. Nonetheless, at 
high flows, both exemplary flow sensing devices 10, 40 act as a variable 
head flow sensor, allowing high flows to be accurately sensed. Since a 
flow sensing device according to the present invention acts as a variable 
area flow sensor at low flows and a variable head flow sensor at high 
flows, it is anticipated that the flow sensing device may have a useful 
flow range of about 500 to 1. 
The differential pressures developed by flow past the flow obstruction 
assembly 12 are communicated to the pressure transducer 30 by means of the 
first and second apertures 31, 32. With the second aperture 32 in the 
preferred location slightly behind and in line with the pin 24, it is 
protected from eddy currents which may be caused by the pin 24 or the 
rigid member 20. In response to the differential pressure, the pressure 
transducer 30 generates an electrical signal proportional to the 
differential pressure. This electrical signal may be supplied to a 
mechanism which correlates the signal to the flow and displays the flow, 
such as the microprocessor 33 and display 34 shown in FIG. 1. 
Although the present invention has been described in terms of two exemplary 
embodiments, it is not limited to these embodiments. Alternative 
embodiments and modifications which would still be encompassed by the 
invention may be made by those skilled in the art, particularly in light 
of the foregoing teachings. Therefore, the following claims are intended 
to cover any alternative embodiments, modifications, or equivalents which 
may be included within the spirit and scope of the invention as defined by 
the claims.