Abstrict A flow meter is adapted to measure the flow rate of a fluid flowing
in the interior of a sensing conduit. The flow meter includes one
sensor element disposed along the flow path of the fluid externally
of the sensing conduit closer to one fluid flow port of the conduit,
another sensor element disposed along the flow path of the fluid
externally of the sensing conduit closer to the other fluid flow
port of the conduit, an elongated structure disposed along the flow
path of the fluid internally of the sensing conduit and away from
the two sensor elements for reducing the fluid flow space in the
sensing conduit, apparatus for heating the sensor elements, and
apparatus for detecting a temperature differential between the sensor
elements. The elongated structure has an end that engages the sensing
conduit at a fluid flow port. The elongated structure may be a wire
that is pre-bent or straight prior to its insertion into the sensing
conduit, or may be a tube.
Claims What is claimed is:
1. A flow meter for measuring the flow rate of a fluid flowing
in the interior of a sensing conduit having first and second fluid
flow ports, comprising:
a first sensor element disposed along the flow path of said fluid
externally of said sensing conduit closer to said first fluid flow
port and a second sensor element disposed along the flow path of
said fluid externally of said sensing conduit closer to said second
fluid flow port;
an elongated structure disposed along the flow path of said fluid
internally of said sensing conduit and away from said first and
second sensor elements for reducing the fluid flow space in said
sensing conduit;
means for heating said sensor elements; and
means for detecting a temperature differential between said sensor
elements.
2. The flow meter claimed in claim 1 wherein said elongated structure
further comprises:
a first end disposed along the fluid flow path terminating away
from said first sensor element.
3. The flow meter claimed in claim 2 wherein said elongated structure
further comprises:
a second end engaged with said sensing conduit substantially at
said first fluid flow port.
4. The flow meter claimed in claim 3 wherein said second end of
said elongated structure is rolled outward sufficiently to engage
a portion of the outer perimeter of said sensing conduit.
5. The flow meter claimed in claim 1 wherein said elongated structure
is manufactured from a material which allows it to be both permanently
bent at one or more locations and flexible enough to allow it to
be removed from said sensing conduit without deforming said sensing
conduit.
6. The flow meter claimed in claim 3 wherein said elongated structure
is bent between said first and second ends such that said elongated
structure is spring biased outwardly when disposed within said sensing
conduit for retaining said elongated structure in position.
7. The flow meter claimed in claim 1 wherein said elongated structure
is bent in one or more locations prior to insertion in said sensing
conduit.
8. The flow meter claimed in claim 1 wherein each sensor element
comprises a heating and temperature-sensing coil.
9. The flow meter claimed in claim 3 wherein said first end of
said elongated structure terminates greater than or equal to about
one-quarter the length of said first sensor element, away from said
first sensor element.
10. The flow meter claimed in claim 3 wherein said first end of
said elongated structure terminates greater than or equal to about
the length of said first sensor element, away from said first sensor
element.
11. The flow meter claimed in claim 1 wherein said elongated structure
comprises a wire.
12. The flow meter claimed in claim 1 where said elongated structure
comprises a tube.
13. A flow meter for measuring the flow rate of a fluid flowing
in the interior of a sensing conduit having a pair of fluid flow
ports, comprising:
a first sensor element positioned along the flow path of said fluid
externally of said sensing conduit closer to one of said fluid flow
ports for measuring the temperature of said first sensor element
as modified by said fluid flow and a second sensor element positioned
along the flow path of said fluid externally of said sensing conduit
closer to said other of said fluid flow ports for measuring the
temperature of said second sensor element as modified by said fluid
flow;
an elongated structure positioned along the flow path of said fluid
internally of said sensing conduit and terminating adjacent to said
first sensor element for reducing the flow space in said sensing
conduit;
means for heating said sensor elements; and
means for detecting a temperature differential between said first
and second sensor elements.
14. The flow meter claimed in claim 13 wherein said elongated
structure is deformed such that it is biased against the inside
of said sensing conduit for retaining said structure in position
within said sensing conduit.
15. The flow meter claimed in claim 13 wherein said elongated
structure is deformed in one or more locations prior to insertion
in said sensing conduit.
16. The flow meter claimed in claim 13 wherein said elongated
structure is manufactured from a material which allows it to be
both permanently deformed in one or more locations and flexible
enough to allow it to be removed from said sensing conduit without
deforming said sensing conduit.
17. The flow meter claimed in claim 13 wherein each sensor element
comprises a heating and temperature-sensing coil.
18. The flow meter claimed in claim 13 wherein said elongated
structure comprises a first end terminating greater than or equal
to about one-quarter the length of said first sensor element, away
from said first sensor element.
19. The flow meter claimed in claim 18 wherein said first end
of said elongated structure terminates greater than or equal to
about the length of said first sensor element, away from said first
sensor element.
20. The flow meter claimed in claim 13 wherein said elongated
structure comprises a wire.
21. The flow meter claimed in claim 13 wherein said elongated
structure comprises a tube.
22. A flow meter, comprising:
a sensing conduit to carry a fluid flow to be measured therethrough;
a sensor element mounted on said sensing conduit; and
an elongated structure positioned along the flow path of said fluid
internally of said sensing conduit and terminating adjacent to said
sensor element for reducing the fluid flow space in said sensing
conduit.
23. The flow meter claimed in claim 22 wherein said elongated
structure is deformed such that it is biased against the inside
of said sensing conduit for retaining said elongated structure in
position within said sensing conduit.
24. The flow meter claimed in claim 22 wherein said elongated
structure is deformed in one or more locations prior to insertion
in said sensing conduit.
25. The flow meter claimed in claim 22 wherein said elongated
structure is manufactured from a material which allows it to be
both permanently deformed in one or more locations and flexible
enough to allow it to be removed from said sensing conduit without
deforming said sensing conduit.
26. The flow meter claimed in claim 25 wherein said elongated
structure is manufactured from a stainless steel material.
27. The flow meter claimed in claim 26 wherein said sensor comprises
a heating and temperature-sensing coil.
28. The mass flow sensor claimed in claim 27 wherein said elongated
structure comprises a first end, terminating greater than or equal
to about one-quarter the length of said first sensor element, away
from said first sensor element.
29. The mass flow sensor claimed in claim 28 wherein said first
end of said elongated structure terminates greater than or equal
to about the length of said first sensor element, away from said
first sensor element.
30. The flow meter claimed in claim 29 wherein said elongated
structure comprises a wire.
31. The flow meter claimed in claim 30 wherein said elongated
structure comprises a tube.
32. The flow meter claimed in claim 1 wherein said first and second
sensor elements are mounted on said conduit.
Description FIELD OF THE INVENTION
The invention relates generally to the field of sensing and measuring
fluid flow.
BACKGROUND OF THE INVENTION
Mass flow meters for gases measure the mass flow rate of a gas,
ideally, independently of gas temperature or pressure. Forms of
such devices which operate on heat transfer principles have become
widely adopted. A common commercial form incorporates a small diameter
tube which has two coils of wire wound on the outside in close proximity
to each other. The coils are formed from a metallic material having
a resistance which is temperature-sensitive.
In a bridge-type electrical circuit, the coils can then be heated
by an electrical current to provide equal resistances in the absence
of flow of the gas and a balanced condition for the bridge-type
circuit--e.g., a null output signal.
Then, with the gas flowing within the tube, within the relevant
measuring range of the device, the temperature of the upstream coil
is decreased by the cooling effect of the gas and the temperature
of the downstream coil is increased by the heat from the upstream
coil transmitted by the gas. This difference in temperature is proportional
to the number of molecules per unit time flowing through the tube.
Therefore, based on the known variation in resistance of the coils
with temperature, the output signal of the bridge circuit provides
a measure of the gas mass flow.
In various circumstances, forms of heat transfer phenomena can
introduce substantial error in the measurements of these mass flow
meter devices. U.S. Pat. No. 3938384 issued Feb. 17 1976 U.S.
Pat. No. 4056975 issued Nov. 8 1977 and U.S. Pat. No. 5191793
issued Mar. 9 1993 all having the same owner as the Assignee herein,
illustrate some of the problems.
One particular problem is sensor calibration shift due to changes
in the mounting orientation of the sensor of the mass flow meter
and to the effects of changing inlet pressure. In particular, referring
to FIG. 1 when the mass flow sensor 10 is mounted at certain orientations,
particularly, the more it is oriented vertically, a phenomenon commonly
known as thermal siphoning caused by thermal gradients present inside
the sensor tube 12 occurs. Initially, when the gas begins to flow
within the sensor tube 12 the temperature of the upstream coil
14 is decreased by the cooling effect of the gas and the temperature
of the downstream coil 16 is increased by the heat from the upstream
coil 14 transmitted by the gas. As the gas inside the sensor tube
12 increases in temperature and rises, however, cooler gas from
the bypass element 18 rushes into the sensor tube 12 to replace
the warmer, rising gas, causing a forced convection current 20.
This forced convection current 20 becomes superimposed on the natural
convection current pattern otherwise established within the sensor
tube 12 without any vertical orientation, thus causing shifts in
zero (change in the null output without a change in actual flow
rate through the mass flow meter--i.e., shifting the output to non-zero)
and span (the flow rates covered by the relevant measuring range
of the meter up to the maximum intended flow rate). As a result,
the actual flow measurement thus becomes a function of the inlet
pressure and of the nature of the process gas. The thermal siphoning
effects on zero and span increase with increasing inlet pressure
and gas density.
The Grashof Number (Gr), generally representing free convection
heat transfer around the sensor tube, is commonly used to measure
the severity of the thermal siphoning problem. The Grashof Number
Gr, a dimensionless value, is given by the equation:
where
g=gravitational constant
.rho.=gas density
.beta.=thermal coefficient of volumetric expansion
T.sub.1 =sensor tube wall temperature
T.sub.0 =ambient temperature
D=internal diameter of sensor tube
.mu.=gas viscosity
In general, when the Grashof Number Gr exceeds a certain critical
value, determined by the various factors, thermal siphoning occurs.
In general, the convection current becomes superimposed on the natural
convection pattern, resulting in shifts in zero and span.
One way to minimize the thermal siphoning effect is to actually
reduce the internal diameter of the sensor tube. Although this will
generally reduce the effects of thermal siphoning, manufacturing
a tube having such a small diameter can be difficult and, thus,
impractical.
Another way to reduce the thermal siphoning effect is to insert
a single wire through the entire length of the sensor tube. Although
this approach reduces the internal diameter of the tube, severe
linearity problems can be created.
The present invention addresses thermal siphoning in a way that
is simple to manufacture and particularly advantageous.
SUMMARY OF THE INVENTION
The present invention provides a flow meter for measuring the flow
rate of a fluid flowing in the interior of a sensing conduit having
first and second fluid flow ports. The flow meter includes a first
sensor element disposed along the flow path of the fluid externally
of the sensing conduit closer to the first fluid flow port and a
second sensor element disposed along the flow path of the fluid
externally of the sensing conduit closer to the second fluid flow
port, an elongated structure disposed along the flow path of the
fluid internally of the sensing conduit and away from the first
and second sensor elements for reducing the fluid flow space in
the sensing conduit, apparatus for heating the sensor elements,
and apparatus for detecting a temperature differential between the
sensor elements.
In another aspect, the flow meter includes a first sensor element
positioned along the flow path of the fluid externally of the sensing
conduit closer to one of the fluid flow ports for measuring the
temperature of the first sensor element as modified by the fluid
flow and a second sensor element positioned along the flow path
of the fluid externally of the sensing conduit closer to the other
of the fluid flow ports for measuring the temperature of the second
sensor element as modified by the fluid flow, an elongated structure
positioned along the flow path of the fluid internally of the sensing
conduit and terminating adjacent to the first sensor element for
reducing the flow space in the sensing conduit, apparatus for heating
the sensor elements, and apparatus for detecting a temperature differential
between the sensor elements.
In still another aspect, the present invention provides a flow
sensor including a sensing conduit to carry a fluid flow to be measured
therethrough, a sensor element mounted on the sensing conduit, and
an elongated structure positioned along the flow path of the fluid
internally of the sensing conduit and terminating adjacent to the
sensor element for reducing the fluid flow space in the sensing
conduit.
The foregoing and additional features and advantages of this invention
will become apparent from the detailed description and accompanying
drawing figures that follow. In the figures and the written description,
numerals indicate the various features of the invention, like numerals
referring to like features throughout for both the drawing figures
and the written description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front, schematic, cross-sectional view showing a conventional
mass flow sensor device mounted in a vertical orientation;
FIG. 2 is a front, cross-sectional view of a mass flow sensor device,
including a sensing conduit having a wire disposed therein, constructed
in accordance with the present invention;
FIG. 3 is an enlarged view showing the wire of the mass flow sensor
device of FIG. 2 prior to the inserting and positioning of the wire
in the sensing conduit;
FIG. 4 is schematic view showing sensor coils of the mass flow
sensor device of FIG. 2 in a bridge-type circuit;
FIG. 5 illustrates, somewhat schematically, the use of the mass
flow sensor device of FIG. 2 with a primary conduit;
FIG. 6 is a front, cross-sectional view of an alternative embodiment
of a mass flow sensor device, including a sensing conduit having
a wire disposed therein, constructed in accordance with the invention;
and
FIG. 7 is a front, cross-sectional view of an alternative embodiment
of a mass flow sensor device, including a sensing conduit having
a tube disposed therein, constructed in accordance with the invention.
DETAILED DESCRIPTION
Referring to FIG. 2 there is shown generally a mass flow sensor
30 including a sensing conduit 32 having a wire 34 disposed therein,
for measuring the flow rate of a fluid flowing in the interior 36
of the sensing conduit 32 constructed in accordance with the present
invention. As will be further noted and hereinafter more fully described,
a single wire 34 inserted in the upstream portion 38 of the sensing
conduit 32 reduces the effective internal flow diameter for that
portion, thereby reducing the magnitude of the Grashof Number Gr,
described in Eq. (1). Arrangements in accordance with the present
invention may also include a wire inserted in the downstream portion
39. When the mass flow sensor 30 is oriented with a vertical aspect
and with the upstream coil above, there generally is more enhanced
performance with a wire mounted in the upstream portion 38 as opposed
to the downstream portion. Additionally, however, arrangements in
accordance with the present invention may also be adapted to include
a wire in both the upstream and downstream portions 38 and 39 of
the sensing conduit 32 with neither wire coming into contact with
the thermal sensing area 42 for the sensing conduit 32.
In accordance with an advantage of the present invention, the thermal
siphoning effect is reduced, thereby allowing for improved sensing
for gas flow measurement. The sensing conduit 32 in a housing 40
has a thermal sensing area 42 having disposed therein generally
identical upstream and downstream coils 44 and 46 formed of temperature-sensitive
resistance material wound around the outside of the sensing conduit
32. In accordance with an advantage of the present invention, the
wire, which may be pre-bent as shown in FIG. 2 or straight as represented
in FIG. 6 prior to insertion, is of a length such that when inserted
in place within the sensing conduit 32 it does not come into contact
with the thermal sensing area 42 in the sensing conduit 32 thereby
eliminating the possibility of upsetting the balanced and symmetrical
heat transfer through the wall of the flow conduit. Linearity problems
which may be caused by a wire contacting the thermal sensing area
42 are thus minimized.
The mass flow sensor 30 is adapted to operate with the upstream
and downstream coils 44 and 46 connected in a bridge-type circuit.
A simple example of such a circuit 48 with the coils 44 and 46
disposed therein, is illustrated in FIG. 4. The present invention
is not limited, however, to the bridge-type electrical circuit 48
illustrated in FIG. 4. Rather the present invention may be adapted
to operate with a variety of other bridge-type circuits known in
the art. Referring to FIG. 4 as is well known by one skilled in
the art, in operation, a dc current source 50 after a switch 52
is closed, with no gas flowing through the sensing conduit (not
shown), establishes a base output voltage between two output terminals
33 and 35 of the circuit 48. The current flows through the coils
44 and 46 and heats the wires of the coils 44 and 46 to equal levels,
establishing the same temperature in the two coils 44 and 46. With
the two bridge resistors 58 and 60 having equal resistances, the
base output voltage, then, of course, is zero and the bridge-type
circuit 48 is balanced.
With gas flow, however, the upstream coil 44 is cooled by gas flow,
giving up some of its heat to the gas flowing by, and the downstream
coil 46 is heated, taking some of this heat that was given to the
gas. Therefore, a temperature differential is established between
the coils 44 and 46 resulting in a voltage across the output terminals
33 and 35. The voltage is due to the change in resistance of the
coils 44 and 46 with temperature.
The temperature difference between the coils 44 and 46 within
the range of the mass flow sensor 30 is a measure of the number
of gas molecules, and thus of the mass of the gas, flowing through
the sensing conduit 32. The difference in the resistance of the
coils 44 and 46 similarly, is a measure of the difference in temperature
between the coils 44 and 46. With the output voltage determined
by this difference in resistance, the output voltage then becomes
a measure of the mass flow rate of the gas.
To provide an output voltage which is proportional to the difference
between the resistances of the coils 44 and 46 the two bridge resistors
58 and 60 should have much larger resistances than the resistances
of the coils 44 and 46. Further, to make the bridge circuit 48 accurate
over a wide variety of temperatures, the current source 50 and the
bridge resistors 58 and 60 should incorporate temperature independence
for the variety of temperatures.
As indicated, bridge-type circuits, including variations to implement
independence of various environmental conditions, are well known
and well understood. There, of course, is a concern here for coil
material for the circuit 48 having a resistance which is proportional
to temperature and for coils 44 and 46 of this material, providing
temperature differences which are proportional to mass flow rate.
In this regard, typically, well outside the range of the mass flow
rate sensor, the flow of the gas will become sufficiently fast to
cool both the upstream and the downstream coils 44 and 46.
The sensing conduit is advantageously made of a stainless steel
material coated with a polyurethane material to provide good electrical
insulation before the coils 44 and 46 are wound around the conduit.
Coils, e.g. made of Balco, coated with a thin layer of an electrically
insulating enamel known as Pyre-ml (these being designations of
Amax Metals), are convenient and satisfactory. Each coil might typically
include two layers, e.g., an inner layer wound from the outside
in and an outer layer wound on top of the inner layer from the inside
out, with the two leads for connection to a bridge-type circuit,
as in FIG. 4 leading from the outside of the coil.
As illustrated in FIG. 2 the sensing conduit 32 is disposed in
a housing 40 which may be of an ordinary type, or of a special
type, such as a housing having a wall structure closely spaced along
the sensors to limit attitude sensitive convection outside the conduit
within the housing. Typically, the housing may have a base having
walls that are shown in the cross-sectional view in FIG. 2 and
a cover (not shown) that press-fits to close the cavity in the base
for the U-shaped portion of the sensing conduit. One skilled in
the art will recognize that the present invention may be adapted
to operate with a variety of housing structures known in the art,
e.g., made of polymers, fiber-filled polymers, such as glass or
carbon-filled, metals, as well as other materials.
The sort of interaction between the gas and the coils 44 and 46
of concern here is also well understood, as indicated by, e.g.,
the Assignee's prior U.S. Pat. Nos. 3938384 issued Feb. 17 1976
4056975 issued Nov. 8 1977 and 5191793 issued Mar. 9 1993.
The last of these provides examples of the above-referenced special
types of housing.
By way of further introductory background, in FIG. 5 somewhat
schematically, the input and output ports 62 and 64 of the mass
flow sensor 30 are shown in communication with a primary conduit
66 having a pressure drop device 68 therein. In a typical actual
(non-schematic) arrangement (not shown), the sensor 30 is mounted
so that the end of the sensing conduit 32 near the input port 62
of the conduit is disposed in a transverse upstream opening to the
fluid flow through the primary conduit 66. Similarly, the end of
the sensing conduit near the output port 64 of the conduit is disposed
in another, transverse downstream opening to such fluid flow. Referring
to the somewhat schematic representation of FIG. 5 with the primary
conduit 66 inserted in a gas flow system (having one or more pressure
drop devices 68 for adjusting gas flow), the mass flow sensor 30
can be provided with changes in calibration--i.e., in a ratio between
the mass flow rate through it and through the primary conduit 66
and system. This sort of technique, also discussed in the aforementioned
patents, is also well known and well understood. In accordance with
well understood gas flow principles, the pressure drop device 68
should be such that the fluid flow characteristics in the primary
conduit 66 near the sensing conduit input and output ports 62 and
64 and in the sensing conduit 32 are proportional. In large part,
this means choosing the pressure drop device such that laminar flow
is maintained in both the primary conduit 66 in this vicinity,
and in the sensing conduit 32 (as opposed to turbulent flow).
With this introduction and background, the aspects and details
of the device of FIGS. 2 through 6 which are of primary concern
for present purposes, can be focused upon and understood in their
context. Such details, to a substantial degree, concern the wire
34 disposed in the interior 36 and upstream portion 38 of the conduit
32 outside of the thermal sensing area 42 for reducing the fluid
flow area, within the upstream portion of the conduit, thereby minimizing
the thermal siphoning effect, and related alternative aspects.
Referring now more specifically to FIGS. 2 and 3 the wire 34
which in the embodiment shown, has a circular cross-section, is
illustrated in further detail. As will further be noted and hereinafter
more fully described, the wire 34 inserted in the sensing conduit
32 where it is present, reduces the effective internal flow diameter,
thereby reducing the magnitude of the Grashof Number Gr. This effective
internal diameter can be considered to be the internal diameter
which a circular tube, having a clear opening, would have in order
to provide the same cross-sectional area for gas flow. As a result,
the thermal siphoning effect is reduced, thereby allowing for improved
sensing for gas flow measurement.
In the embodiment of FIG. 2 the wire 34 is bent at two places
utilizing conventional manufacturing techniques prior to insertion
into the upstream 38 interior portion 36 of the sensing conduit
32. The wire 34 is thus pre-bent to be resiliently spring biased
outward so that when the wire 34 is inserted in the sensing conduit
32 the bends provide sufficient force to retain the wire 34 in
position. One end of the wire 34 is disposed in an upper region
70 of the upstream portion 38 of the sensing conduit 32 outside
of the thermal sensing area 42. The other end of the wire 34 the
exposed end, extends out of the sensing conduit 32 and then is rolled
outwardly sufficiently to engage a portion of the outer perimeter
72 of the sensing conduit 32 thus further retaining the wire 34
in position as illustrated in FIGS. 2 and 3. The result is greater
stability of position of the wire against the effects of differential
pressure that will arise within the sensing conduit 32. The wire
34 in the present invention, more generally, thus may be bent at
one or more points prior to insertion to form one or more corresponding
convex contact points which engage the inner circumference of the
sensing conduit 32 when the wire 34 is inserted.
Referring in particular to FIG. 3 showing the wire of FIG. 2 prior
to insertion and positioning in the sensing conduit 32 the wire
34 is bent at two locations 74 and 76 each having an approximate
angle of 120.degree.. One skilled in the art will recognize that
the present invention is not limited to the two-bend or angle configuration
illustrated in FIGS. 2 and 3. Rather, in certain situations, such
as for a sensing conduit having a very small diameter, a wire having
a single bend may be preferred. Correspondingly, in other situations,
such as for a sensing conduit having a very large diameter, a wire
having multiple bends may be preferred. For the latter situation,
a wire may be bent into an elongated zigzag pattern, e.g., four
or more substantially straight wire sections separated by, e.g.,
three or more bends prior to insertion.
The wire 34 is preferably manufactured from a material which allows
it to be both permanently bent at one or more locations and also
flexible enough to allow it to be removed from the sensing conduit
32 without permanently damaging or deforming the sensing conduit
32. Stainless steel materials, generally, and alloys such as that
sold under the name Hastelloy and that sold under the name Monel,
may typically be used as the wire material.
In a typical configuration, particularly adapted for non-corrosive
gases, which do not tend to accumulate corrosion material which
might affect the internal diameter of the sensing conduit 32 over
time, the sensing conduit 32 has an outer diameter of approximately
14 mils (wall thickness of approximately 2 mils). The wire 34 then,
is of a length such that when inserted in place within the sensing
conduit 32 it does not reach the thermal sensing area 42 in the
sensing conduit 32. In such a configuration, a diameter for the
wire of approximately 7 to 8 mils is convenient and effective. For
a conduit length from the input port to the upstream edge of the
upstream coil of approximately 1.1 inches, a total wire length of
approximately 0.75 inch, approximately 0.7 inches of which are disposed
inside the sensing conduit 32 having two bends, each having an
approximate angle of 12.degree., is convenient and effective. However,
this may readily vary, depending upon specific requirements and
considerations.
In another typical configuration, particularly adapted for corrosive
gases, the sensing conduit 32 has an outer diameter of approximately
30 mils (wall thickness of approximately 2.5 mils). Once again,
the wire 34 is of a length such that when inserted in place within
the sensing conduit 32 it does not come into contact with the thermal
sensing area 42 in the sensing conduit 32. In such a configuration,
a diameter for the wire of approximately 16 to 19 mils is convenient
and effective. For a conduit length from the input port to the upstream
edge of the upstream coil of approximately 1.1 inches, a wire length
of approximately 0.75 inches, approximately 0.7 inch of which are
disposed inside the sensing conduit 32 having two bends, each having
an approximate angle of 120.degree., is convenient and effective.
Again, however, this may readily vary.
As an example, coils 44 and 46 of approximately 0.125 inch in length
can conveniently be provided in each of the above configurations,
with the diameter of the coil wire being approximately 0.6 mil.
As indicated, one skilled in the art will recognize that the present
invention is not limited to the dimensions or geometrical configurations
specifically referred to above. Rather, the insertion of a wire,
partially through a sensing conduit, may be adapted for sensing
conduits of different diameters and lengths, and coils or other
sensors of varying lengths.
Referring to FIG. 6 in accordance with another embodiment, the
wire 78 initially generally unbent, is precut to length such that
when inserted into the upstream portion 38 of the sensing conduit
32 one end of the wire 78 engages the radiused portion 80 of the
sensing conduit 32 and is bent upon engagement. As a result, the
forced bend along the radiused portion 80 acts as a spring element
and provides sufficient force to hold the wire 78 in place within
the interior 36 of the sensing conduit 32. While disposed in the
upper region of the upstream portion 38 of the sensing conduit 32
the wire 78 does not, however, come into contact with the thermal
sensing area 42. The other end of the wire 78 the exposed end,
extends downwardly and then is rolled outwardly sufficiently to
engage a portion of the outer perimeter 72 of the sensing conduit
32 thus retaining the wire 78 in position as illustrated in FIG.
6. The wire 78 is thus disposed along the upstream portion 38 of
the sensing conduit 32 and is bent upon engagement with the radiused
portion 80 of the sensing conduit 32.
In accordance with the present invention, the non-pre-bent wire
78 illustrated in FIG. 6 although generally easier to prepare and
install, may not be as solidly attached within the sensing conduit
32 as the pre-bent wire 34 illustrated in FIG. 2. In certain circumstances,
looseness of the wire in the sensing conduit 32 of course, could
cause variations in sensing accuracy.
In a typical configuration of the embodiment illustrated in FIG.
6 particularly adapted for non-corrosive gases which do not tend
to accumulate corrosion material which might affect the internal
diameter of the sensing conduit 32 over time, the sensing conduit
32 has an outer diameter of approximately 14 mils (wall thickness
of approximately 2.0 mils). The wire 78 then, is of a length such
that when inserted in place within the sensing conduit 32 it does
not reach the thermal sensing area 42 in the sensing conduit 32.
In such a configuration, a diameter for the wire of approximately
7 to 8 mils is convenient and effective. For a conduit length from
the input port to the upstream edge of the upstream coil of approximately
1.1 inches, a total wire length of approximately 1.1 inches, approximately
1.05 inches of which are disposed inside the sensing conduit 32
is convenient and effective.
In another typical configuration, particularly adapted for corrosive
gases, the sensing conduit 32 has an outer diameter of approximately
30 mils (wall thickness of approximately 2.5 mils). Once again,
the wire 78 is of a length such that when inserted in place within
the sensing conduit 32 it does not come into contact with the thermal
sensing area 42 in the sensing conduit 32. In such a configuration,
a diameter for the wire of approximately 16 to 19 mils is convenient
and effective. For a conduit length from the input port to the upstream
coil of approximately 1.1 inches, a total wire length of approximately
1.1 inches, approximately 1.05 inches of which are disposed inside
the sensing conduit 32 is convenient and effective. Again, however,
this may readily vary.
As before, coils 44 and 46 of approximately 0.125 inch in length
can be provided in each of these configurations, with the diameter
of the coil wire being approximately 0.6 mils.
In accordance with the present invention, the wire illustrated
in FIGS. 2 or 6 is disposed along the flow path of the fluid internally
of the sensing conduit 32 and away from the coils 44 and 46 for
reducing the fluid flow space in the sensing conduit 32. The wire
should typically terminate greater than or equal to about one-quarter
the length of coil 44 or 46 away from the outer edge of coil 44
or 46. However, terminating greater than or equal to about the length
of the coil 44 or 46 away from the outer edge coil 44 or 46 is
another convenient and effective alternative.
In accordance with the present invention, when the wire illustrated
in FIGS. 2 or 6 is inserted into the sensing conduit 32 the internal
diameter of the sensing conduit 32 is effectively reduced along
the portion in which the wire is disposed, thereby reducing the
magnitude of the Grashof Number Gr. As a result, thermal siphoning
can be suppressed. When the wire is not inserted in the mass flow
sensor 30 the forced convection current from a non-horizontal orientation,
typically becomes a significant factor, superimposed on the natural
convection pattern otherwise established within the sensing conduit
without the non-horizontal orientation, resulting in shifts in zero
and span. The actual flow measured then becomes a function of the
inlet pressure and the nature of the process gas. However, when
the wire is inserted in the mass flow sensor 30 of the present invention,
thus effectively reducing the internal flow area along the portion
where the wire is present, the value of the Grashof Number Gr can
readily be reduced by a factor of approximately 2.7. Problems caused
by thermal siphoning, thus, may be substantially reduced.
Referring to FIG. 7 in accordance with another embodiment of the
invention, a tube 84 rather than a wire as illustrated in FIGS.
2 and 6 is disposed in the interior 36 and upstream portion 38
of the conduit 32 outside of the thermal sensing area 42 for reducing
the fluid flow area, within the upstream portion 38 of the conduit
32 thereby reducing the thermal siphoning effect. The tube 84 has
a stepped, increased outer diameter portion at the lower end 86
of the conduit 32 (not shown), thus allowing it to be interference
or press fitted into the interior 36 of the conduit 32 when inserted.
The tube 84 having a hollow center portion, is configured with
this increased diameter portion at one end 86 thereof utilizing
conventional manufacturing techniques prior to insertion into the
sensing conduit 32. The increased diameter at the lower end of the
tube 84 provides sufficient force to retain the tube 84 in position.
Simply for purposes of illustration, the step to the increased diameter
is assumed to occur at a step location 87 along the tube and to
continue to the end of the downstream end of the tube. The tube
84 terminates in the upper region 70 of the upstream portion 38
of the sensing conduit 32 before the radiused portion of the conduit
32 and outside of the thermal sensing area 42. The other end 88
of the tube 84 the end not having the increased outer diameter,
extends downwardly and is not interference fitted. The result of
the interference fit at one end is good stability of position of
the tube 84 against resistance to the effects of differential pressure
that will arise within the sensing conduit 32. One skilled in the
art will recognize that the tube may be retained within the sensing
conduit by other means, such as by mechanically engaging the tube
at the lower end to the conduit.
The configuration shown in FIG. 7 is particularly beneficial for
a sensing conduit having a relatively large diameter. Arrangements
in accordance with the present invention may also include a tube
inserted in the downstream portion 39. When the mass flow is oriented
with a vertical aspect and with the upstream coil above, there generally
is more enhanced performance with a tube mounted in the upstream
portion 38 as opposed to the downstream portion. Additionally,
however, arrangements in accordance with the present invention may
also be adapted to include a tube in both the upstream and downstream
portions 38 and 39 of the sensing conduit 32 with neither tube
coming into contact with thermal sensing area 42 for the sensing
conduit 32.
The tube 84 is preferably manufactured from a material which allows
it to be accurately configured at a desired diameter and also flexible
enough to allow it to be removed from the sensing conduit 32 without
permanently damaging or deforming the sensing conduit 32. Stainless
steel materials may typically be used as the tube material.
In a typical configuration, particularly adapted for either corrosive
or non-corrosive gases, the sensing conduit 32 might have an outer
diameter of approximately 40.0 mils (wall thickness of approximately
2.5 mil). The tube 84 then, would be of a length such that when
inserted in place within the sensing conduit 32 it does not reach
the thermal sensing area 42 in the sensing conduit 32 and, in fact,
would not reach the radiused portion of the sensing conduit. In
such a configuration, an outer diameter for the tube 84 of approximately
34 mils with an increased diameter at the lower end 86 of approximately
36 mils would be convenient and effective. However, this may readily
vary, depending upon specific requirements and considerations.
As an example, coils 44 and 46 of approximately 0.125 inch in length
can conveniently be provided in the above configuration, with the
diameter of the coil wire being approximately 0.6 mil. As indicated,
one skilled in the art will recognize that the present invention
is not limited to the dimensions or geometrical configurations specifically
referred to above. Rather, the insertion of a tube, partially through
a sensing conduit, may be adapted for sensing conduits of different
diameters and lengths, and coils or other sensors of varying lengths.
It, of course, will be appreciated by those skilled in the art
that many modifications and variations may be made in what has been
specifically described without departing from the scope or spirit
of the invention. By way of example, the applicable considerations
also apply equally well to forms of flow sensors which employ upstream
and downstream sensor coils which are heated by a heater coil about
the conduit, therebetween. By way of further example, such considerations
also apply to forms of flow sensors having thermocouples as upstream
and downstream sensors to measure temperature differences along
a conduit which is internally heated by electrical current. These
other forms, as indicated, are merely exemplary. Accordingly, the
scope of the invention shall not be referenced to the disclosed
embodiments, but on the contrary, shall be determined in accordance
with the claims that follow.
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