Abstrict Reduced attitude sensitivity is achieved in a fluid mass flow meter
of the type which determines fluid flow from the temperature difference
between an upstream sensor and a downstream sensor positioned along
the flow path of the fluid externally of a sensing conduit for the
fluid. A housing, taking particular forms, has wall structure spaced
along the sensors adapted to substantially limit the ambient gaseous
atmosphere along the sensors to a thin film. The housing wall structure
further is spaced along portions of the sensing conduit between
the sensors and the input and output ports of the conduit to substantially
limit the ambient gaseous atmosphere along such portions to a thin
film. In one form, such wall structure, along these portions, converges
from both directions along the conduit, to provide support regions
for the conduit. The sensors, specifically, are self-heating coil
elements formed of temperature-sensitive resistance wire wound around
the outside of the sensing conduit. They are incorporated into a
bridge-type circuit for sensing their temperatures.
Claims What is claimed is:
1. A mass 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 self-heating coil element positioned along the flow path
of the fluid externally of the sensing conduit closer to one of
the fluid flow ports and a second self-heating coil element positioned
along the flow path of the fluid externally of the sensing conduit
closer to the other of the fluid flow ports, said coil elements
being formed of temperature-sensitive resistance wire wound around
the outside of the sensing conduit for sensing the temperatures
of said coil elements modified by the fluid flow;
means for heating said coil elements;
means for detecting a temperature differential between said coil
elements; and
a housing having wall structure spaced along said coil elements
to substantially define the thickness of an ambient gaseous atmosphere
along said coil elements, the maximum spacing between said wall
structure, along said coil elements, being less than or equal to
about 270 mils, said wall structure being formed of a metallic material,
or of a polyamide material having a heat conductivity much larger
than air.
2. A mass flow meter as defined in claim 1 wherein said housing
wall structure, along a portion of said conduit between a said coil
element and a said port, converges from both directions along the
conduit to provide a support region for the conduit.
3. A mass flow meter as defined in claim 1 wherein said housing
wall structure is curved along said coils.
4. A mass flow meter as defined in claim 1 wherein said housing
wall structure substantially defines cylindrical surface portions
along said coils.
5. A mass flow meter as defined in claim 4 wherein the axes of
said cylindrical surface portions are substantially perpendicular
to the direction of coiling of said coils.
6. A mass flow meter as defined in claim 1 wherein said housing
comprises a body and a cover which mate to form said wall structure
along said coils.
7. A mass 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 means 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 said first sensor means as
modified by the fluid flow and a second sensor means 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 said second sensor means as modified by the fluid flow;
means for heating said first and second sensor means;
means for detecting a temperature differential between said first
and second sensor means; and
a housing having wall structure spaced along said first and second
sensor means to substantially define the thickness of an ambient
gaseous atmosphere along said first and second sensor means, the
maximum spacing between said wall structure, along said first and
second sensor means, being less than or equal to about 270 mils,
said wall structure being formed of a metallic material, or of a
polyamide material having a heat conductivity much larger than air.
8. A mass flow meter as defined in claim 7 wherein said housing
wall structure, along a portion of said conduit between a said sensor
means and a said port, converges from both directions along the
conduit to provide a support region for the conduit.
9. A mass flow meter as defined in claim 7 wherein said housing
comprises a body and a cover which mate to form said wall structure
along said sensor means.
10. A mass 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 self-heating coil element positioned along the flow path
of the fluid externally of the sensing conduit closer to one of
the fluid flow ports and a second self-heating coil element positioned
along the flow path of the fluid externally of the sensing conduit
closer to the other of the fluid flow ports, said coil elements
being formed of temperature-sensitive resistance wire wound around
the outside of the sensing conduit for sensing the temperatures
of said coil elements modified by the fluid flow;
means for heating said coil elements;
means for detecting a temperature differential between said coil
elements; and
a housing having wall structure spaced along said coil elements
to substantially define the thickness of an ambient gaseous atmosphere
along said coil elements, the maximum spacing between said wall
structure, along said coil elements, being less than or equal to
about 270 mils, said wall structure being formed of material and
structure providing a heat conductivity therethrough much greater
than air.
11. A mass 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 means 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 said first sensor means as
modified by the fluid flow and a second sensor means 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 said second sensor means as modified by the fluid flow;
means for heating said first and second sensor means;
means for detecting a temperature differential between said first
and second sensor means; and
a housing having wall structure spaced along said first and second
sensor means to substantially define the thickness of an ambient
gaseous atmosphere along said first and second sensor means, the
maximum spacing between said wall structure, along said first and
second sensor means, being less than or equal to about 270 mils,
said wall structure being formed of material and structure providing
a heat conductivity therethrough much greater than air.
12. A thermal mass flow meter, comprising a sensor pipe to carry
a fluid flow to be measured therethrough, a heating and temperature-sensing
means mounted on said sensor pipe, a case formed of a polyamide
material having a heat conductivity much larger than air or of a
metallic material and having an opening therethrough, and means
for supporting said sensor pipe in said opening in spaced relationship
with said case, said opening being defined by case wall structure
having a maximum spacing along said heating and temperature-sensing
means of not larger than about 6.858 mm (270 mils).
13. A thermal mass flow meter as set forth in claim 12 wherein
said heating and temperature-sensing means consists of a pair of
windings which are connected both to a power source and to a voltage-detecting
circuit for heating and detecting the temperature.
14. A thermal mass flow meter, comprising a sensor pipe to carry
a fluid flow to be measured therethrough, a heating and temperature-sensing
means mounted on said sensor pipe, a case whose heat conductivity
is much larger than that of air having an opening therethrough,
and means for supporting said sensor pipe in said opening in spaced
relationship with said case, said opening being defined by case
wall structure having a maximum spacing along said heating and temperature-sensing
means of not larger than about 6.858 mm (270 mils).
15. A thermal mass flow meter as set forth in claim 14 wherein
said heating and temperature-sensing means consists of a pair of
windings which are connected both to a power source and to a voltage-detecting
circuit for heating and detecting the temperature.
Description FIELD OF THE INVENTION
The invention pertains to the field of fluid flow sensing and,
more particularly, to fluid mass flow sensing.
BACKGROUND AND SUMMARY OF THE INVENTION
Mass flow meters for gases measure the mass flow rate of a gas
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 fluid. This difference in temperature is
proportional to the number of molecules per unit time flowing through
the tube. Therefore, based on the known variation of 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 and
U.S. Pat. No. 4056975 issued Nov. 8 1977 both having the same
Assignee as herein, are illustrative of the problem.
As discussed in the latter of these patents, at relatively elevated
pressure levels of the gas, the error introduced by free convection
of the gas within the tube becomes relatively dominant. The result,
for such higher pressure levels, is a substantial error due to such
convection when the device is tilted with respect to the direction
of gravity. As discussed in both of these patents, at relatively
lower pressures, the effects of this sort of convection are not
substantial; however, the error introduced by free convection by
the ambient gas outside the tube becomes a dominant source of error
with variations in the attitude of the device with respect to gravity.
In U.S. Pat. No. 3938384 the first-mentioned patent, this sort
of convective effect is addressed by encapsulating the tube with
the coils thereabout, in the vicinity of the coils, with an open
cell foam material. Although, as indicated in the patent, the advantages
of that approach are substantial, it does bring certain detriments.
First, it slows the response of the device as a result of the presence
of the foam material. Second, the calibration of the device can
shift with time as the foam changes its chemical composition or
its degree of contact with the coils and conduit. Third, it reduces
the gain of the device.
A general approach to the convection outside the conduit, of which
the just-mentioned approach may be considered a specific form, involves
the use of various materials to contact the coils in order to keep
convective currents from transferring heat externally from one coil
to the other. This general approach typically is unsatisfactory
for a variety of reasons, the most important one usually being the
reduction of the level of response of the device to changes in flow.
Yet more generally, flow meter devices such as those discussed
above, are commonly enclosed in some type of container to isolate
their sensitive parts from outside air currents and outside localized
sources of heating or cooling. This, of course, is a distinct concern
from the effects of convection immediately adjacent to such sensitive
parts.
Of some, related interest are some propositions that have been
put forward with regard to heat transfer phenomena in gases. As
far back as 1912 I. Langmuir, in "Convection and Conduction
of Heat In Gases", The Physical Review, Vol. 34 No. 6 June
1912 pp. 401-422 proposed that there is a thickness for a layer
of gas on a plane for which loss of heat from the plane through
conduction strongly dominates over loss through convection, and
that this thickness is a constant independent of the temperature
of the plane (at least at a given temperature and pressure for the
surrounding environment). For air, such thickness "B",
at about room temperature and pressure, is proposed to be about
0.43 cm (about 0.17 inch). The analogous situation, then, is said
to apply to a layer of the gas of outside diameter "b"
about a wire of outside diameter "a", wherein, b ln (b/a)=2
B.
The present invention addresses long-standing problems and concerns
with attitude sensitivity in gas mass flow meters stemming from
convective heat transfer outside a tube through which the gas is
directed. It does so while also addressing the goals of high sensitivity
and rapid responsiveness to changes in flow rate.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a mass 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, which includes:
first and second sensors; apparatus for heating such sensors; apparatus
for detecting a temperature differential between the sensors; and
a housing. The first sensor is positioned along the flow path of
the fluid externally of the sensing conduit closer to one of the
fluid flow ports for the purpose of measuring its sensor temperature
as modified by the fluid flow, and the second sensor is positioned
along the flow path of the fluid externally of the sensing conduit
closer to the other of the fluid flow ports for the purpose of measuring
its sensor temperature as modified by the fluid flow. The housing
has wall structure which is spaced along the sensors to substantially
limit the ambient gaseous atmosphere along the sensors to a thin
film. In accordance with more detailed features, the housing wall
structure further is spaced along portions of the sensing conduit
between the sensors and the ports to substantially limit the ambient
gaseous atmosphere along such portions to a thin film. The wall
structure along such a portion may converge from both directions
along the sensing conduit to provide a support region for the conduit.
In accordance with other more detailed features, the housing includes
a body and a cover which mate to form the wall structure along the
sensors.
The sensors may be self-heating coil elements formed of temperature-sensitive
resistance wire wound around the outside of the sensing conduit;
and the housing wall structure may substantially define cylindrical
surface portions along the coils, e.g., having axes which are substantially
perpendicular to the direction of coiling of the coils.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front elevational view, partially in section, of a
mass flow meter device with its cover removed;
FIG. 2 is an enlarged perspective view showing the housing body
and cover of the mass flow meter device of FIG. 1;
FIG. 3 is a side elevational view, partially in section, of the
mass flow meter device of FIG. 1 with the cover in place, the section
being taken along the line 3--3 of FIG. 1;
FIG. 4 is a schematic view showing coils of the mass flow meter
device of FIG. 1 in a bridge-type circuit;
FIG. 5 illustrates the use of the mass flow meter device of FIG.
1 with a primary conduit;
FIG. 6 is a cross-sectional view of an alternative embodiment serving
as a test mass flow meter device.
FIG. 7 is an enlarged perspective view showing an alternative housing
body and cover for the mass flow meter device of FIG. 1.
DETAILED DESCRIPTION
Referring to FIGS. 1-3 there are shown the details of a mass flow
meter 12. By way of introduction, a sensing conduit 14 in a housing
16 has essentially identical upstream 20 and downstream 22 coils
formed of temperature-sensitive resistance wire wound around the
outside of the conduit. The housing has a body 24 and a cover 26
which press-fits into the body. As shown, the housing provides detailed
and carefully designed cavities and wall structure with respect
to various of the internal parts of the device.
The device is adapted to operate with the coils 20 and 22 connected
in a bridge-type electrical circuit. An example of such a circuit,
with the coils 20 and 22 therein, is shown in FIG. 4. The operation
of this and a variety of other bridge-type circuits is well known
to and understood by those knowledgeable in the art. A dc current
source 28 after a switch 30 is closed, with no gas flowing through
the sensing conduit, establishes a base output voltage between two
output terminals 32 and 34 of the circuit. The current through the
coils 20 and 22 heats the wires of the coils to equal levels, establishing
the same temperature in the two coils. With the two bridge resistors
36 and 40 having equal resistances, the base output voltage, then,
of course, is zero and the bridge-type circuit is balanced.
With gas flow, however, the upstream sensor 20 is cooled by the
gas flow, giving up some of its heat to the gas flowing by, and
the downstream sensor 22 is heated, taking some of this heat that
was given to the gas. Therefore, a temperature differential is established
between the coils 20 and 22 resulting in a voltage across the output
terminals 32 and 34. The voltage, of course, is due to the change
in resistance of the coils with temperature.
The temperature difference between the coils, within the range
of the device, is a measure of the number of gas molecules, and
thus of the mass of the gas, flowing through the conduit. The difference
in the resistance of the coils, similarly, is a measure of the difference
in temperature between the coils. 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, the two bridge resistors 36
and 40 should have much larger resistances than the resistances
of the coils; Further, to make the bridge circuit accurate at a
wide variety of temperatures, the current source 28 and the bridge
resistors 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 having a resistance which is proportional
to temperature and for coils, 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 meter device
12 the flow of the gas will become sufficiently fast to cool both
the upstream 20 and the downstream 22 coils.
The sort of interaction between the gas and the coils 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 and 4056975
issued Nov. 8 1977.
By way of further introductory background, in FIG. 5 the input
port 42 and output port 44 of the mass flow meter device 12 are
shown in communication with a primary conduit 45 having a pressure
drop device 46 somewhat schematically shown therein. With the primary
conduit 45 inserted in a gas flow system (having one or more pressure
drop devices for adjusting gas flow), the mass flow meter device
12 can be provided with changes in calibration--i.e., in a ratio
between the mass flow rate through it and through the primary conduit
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 should
be such that the pressure drop versus fluid flow characteristics
in the primary conduit and in the sensing conduit 14 are comparable.
In large part, this means choosing the pressure drop device such
that laminar flow is maintained in both the primary conduit and
the sensing conduit (as opposed to turbulent flow).
With this introduction and background, the aspects and details
of the device 12 of FIGS. 1-3 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 housing 16 and its
relationship to the internal parts of the device 12. An alternative
housing form, shown in FIG. 7 will be treated subsequently.
The housing 16 can be conveniently and advantageously made of metal,
for example a zinc alloy such as that sold under the general designation
Zamac. Another convenient and advantageous alternative is a polyamide
material having relatively high thermal conductivity, for example
a 50% glass or carbon fiber-filled polyamide. For convenience, in
the drawings, the housing is shown as made of the polyamide material
(see, e.g., FIGS. 2 and 3). The sensing conduit 14 disposed in
the housing, should be made of a material which resists corrosion,
such as corrosion resistant stainless steel.
The sensing conduit is advantageously coated with a polyurethane
material to provide good electrical insulation before the coils
20 and 22 are wound around the conduit. Coils made of Balco coated
with a thin layer of an electrically insulating enamel material
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 end of the coil.
In FIG. 1 and also referring to parts of FIG. 4 there is a pair
of leads 48 from the upstream coil 20 one for the junction 50 with
the bridge resistor 36 associated with that coil, and the other
for the junction 52 between the two coils. Similarly, there is a
pair of leads 54 from the downstream coil 22 one for the junction
56 with the bridge resistor 40 associated with that coil, and the
other for the junction 52 between the coils. These leads are conveniently
extensions of the coils, of the same materials as the coils.
The lead from each coil for the coil junction 52 is welded to the
bare end of an otherwise insulated coil junction lead 60 which passes
out of the housing 12 through an exit opening 62 for the lead (FIG.
2). Similarly, the other lead from the pair of leads 48 for the
upstream coil is welded to the bare end of an otherwise insulated
upstream coil-bridge resistor junction lead 64; and the other lead
of the pair of leads 54 from the downstream coil is welded to the
bare end of an otherwise insulated downstream coil-bridge resistor
junction lead 66. There is also then an exit opening 68 for the
upstream coil-bridge resistor junction lead and an exit opening
70 for the downstream coil-bridge resistor junction lead.
In the body 24 of the housing, there are three (like) lead cavities
72 one for each of the three aforementioned leads which exit the
housing; three (like) passage cavities 74 one for the alignment
of each such lead; and a single exit cavity 76 for the three leads,
communicating directly with the exit openings for the three leads.
As shown in FIGS. 1 and 3 an epoxy material 80 is placed in this
exit cavity surrounding these leads. As is also indicated in these
figures, the two pairs of coil leads 48 and 54 pass from cavities
for the coils to the aforementioned lead cavities 72 between the
body 24 and cover 26 of the housing. This does not present a problem
because of the typical extremely small diameter of the wire for
the coils. The diameter is so small that the presence of the leads
between the body and cover has substantially no effect on the press-fit
between the cover and the body.
Turning to the cavities and wall structure closely associated with
the coils 20 and 22 and with the sensing conduit 14 first, the
part of the conduit from the conduit's input port 42 to the upstream
coil 20 essentially is an input side pre-coil portion 82 of the
conduit. Similarly, the part of the conduit from the output port
44 of the conduit to the downstream coil 22 essentially is an output
side post-coil portion 84 of the conduit. Referring to FIG. 2 the
body 24 and cover 26 of the housing have wall structure which defines
a number of cavities closely associated with the coils and the conduit,
which can be identified.
There is a cylindrical form upstream coil cavity 86 which is largely
defined by wall structure 88 taking substantially the form of a
surface portion of a right circular cylinder having an axis 90 which
is perpendicular to the direction of coiling of the upstream coil
20. The ends of the cavity are defined by flat, circular front 92
and back 94 (arbitrarily considering the cover of the housing, the
front and the body, the back) surfaces. There is then an analogous,
mirror-image cylindrical form downstream coil cavity 96 wall structure
98 largely defining the cavity, an axis 100 for such wall structure
and front 102 and back 104 flat, circular surfaces defining the
ends of the cavity. Between these coil cavities, there is a tubular,
mid-coil cavity 105 for the passage of the conduit 14 between the
coil cavities. This mid-coil cavity has a diameter which is larger
than the conduit to avoid touching of the conduit by the wall structure
for the cavity.
The cylindrical form wall structure 88 and 98 along the coils 20
and 22 is curved and oriented the way it is in part to facilitate
the forming of the housing. Efficient formation is considered a
significant concern and advantage when the small size of the various
cavities is appreciated. Some exemplary dimensional information
will be provided subsequently. However, as a general matter, it
is important to appreciate that the wall structure of the housing
along the coils provides spacing to substantially limit the ambient
gas (e.g., air) along the coils to the dimensions of a thin film.
It has been determined that such limitation will limit undesirable
sensitivity of a device such as the meter device 12 to the device's
attitude with respect to the direction of gravity due to convective
heat transfer along the coils. Yet, if the wall structure is too
close, the loss of heat by the coils to the wall structure undesirably
decreases the sensitivity (output levels) of the device.
Now focusing on additional housing cavities and housing wall structure
closely associated with the sensing conduit 14 on the input side
of the device 12 such also serves to describe analogous, mirror-image
cavities and wall structure on the output side. Thus, starting at
the vicinity of the input port 42 of the sensing conduit 14 there
is an essentially right, circular cylindrical seal cavity 106 (FIG.
1) for a generally O-ring type seal 107 which fits tightly about
the conduit and fits tightly in the cavity. That cavity leads to
an elongated essentially right, circular cylindrical input side
transition cavity 108 (FIGS. 1 and 2) having an initial portion
through the base of the housing body 24 and a final portion formed
by wall structure of the housing body and housing cover 26. A small
amount of a potting compound might be injected about the sensing
conduit at the junction of the initial and final portions of the
transition cavity, as a seal between the conduit and housing at
that point.
The transition cavity leads to an input side intermediate cavity
110. There is initial wall structure for that cavity which expands
the cavity to an intermediate portion of the cavity essentially
having the shape of a right, circular cylinder. From that intermediate
section, there is converging wall structure 112 which converges
to an outer support region 114. This is a very short support region
intended to minimize the contact between the housing and the conduit
while at the same time providing support for the conduit.
For ease of construction, this support region along the cover is
flat. Along the inside of the body of the housing, however, the
support region essentially has the shape of a circle which is broken
by a centered slot. The circle typically has a diameter which is
approximately the diameter of the conduit with a center which would
place the circumference of the circle slightly shy of the level
of the base surface 116 inside the housing body 24. The slot typically
has a width somewhat less than the diameter of the conduit. It breaks
through between that base surface level and the interior of the
circle. This shape is convenient for ease of construction and for
gripping the conduit with a minimum of contact.
Leading from the outer support region 114 there is an input side
corner cavity 120. Wall structure for this cavity also includes
converging wall structure 122 which converges to that support region,
just addressed. Thus, wall structure of the housing converges to
the support region from both directions along the conduit. The corner
cavity has relatively short, essentially right, circular cylindrical
portions toward its inner (toward the coils) and outer (away from
the coils) ends, away from the corner region. Further, at its inner
end, there is converging wall structure 124 which converges to wall
structure providing an inner support region 126 for the conduit.
This inner support region 126 is comparable to the outer support
region 114 just described. Thus, there is a flat surface along the
cover and a circular-shaped portion intersected by a slot along
the body of the housing. The dimensional situation, with respect
to the size of the conduit, is the same as that described for the
outer support region. Thus, again, the design provides for efficiency
of construction, good gripping of the conduit 14 and a minimum
of contact between the conduit and the wall structure.
From the foregoing description of the cavities and wall structure
along the input side pre-coil portion 82 of the conduit 14 it is
apparent that, although the cavities are variously shaped for evident
reasons, they are also designed with the particular goal of limiting
the surrounding ambient gas to a thin film while supporting the
conduit with relatively minimal housing-conduit contact.
As previously indicated, the immediately preceding description
with regard to the cavities and sensing conduit at the input side
of the gas flow meter device 12 also serves to describe the analogous
mirror-image structure at the output side. An essentially right,
circular cylindrical housing mid-base 127 and a rectangular-shaped
mid-cover 128 cavity have not yet been noted. They are conventionally
provided in the formation of the housing as a well known technique
to prevent structural sag during formation. The mid-cover cavity
overlaps the exit cavity 76 the passage cavities 74 and bottom
portions of the lead cavities 72 of the housing body 24. In this
respect, the mid-cover cavity also provides some additional clearance
for the leads 60 64 66 and room for any excess epoxy material
80 that may overfill the exit cavity (see FIG. 3).
As indicated above, dimensional relationships for the housing and
conduit of the FIG. 1-3 device are considered to be of some significance.
One embodiment, according to this form and adapted for non-corrosive
gases which should not tend to accumulate corrosion material which
might affect the internal diameter of the conduit over time, has
a sensing conduit with an outer diameter of about 14 mils (wall
thickness of 2 mils). Another such embodiment, particularly adapted
for corrosive gases, has an outer diameter of about 30 mls (wall
thickness of 2 mils). Coils of 125 mils in length are provided in
each embodiment, the diameter of the coil wire being about 0.6 mils.
Essentially the same housing structure is provided for both embodiments
with variations only in the dimensions at the inner 126 and outer
114 support regions for the conduits. The maximum coil cavity wall
separation along the coil in the cavity (employing the conduit-coil
axis as the center line for measuring the separation) occurs at
mid-coil between the junctions of the cavity end walls and cavity
cylindrical-shaped side wall. That separation equals approximately
270 mils. The minimum such separation is at the edge of the coil
in the cavity at the mid-plane of the cavity. This minimum is approximately
130 mls. Wall separation at the middle of the coil and mid-plane
defines the cavity with a diameter of 180-mils. The height of the
cavity (distance between the end walls) is 200-mils.
Some additional interesting computations can be made based on the
foregoing dimensional information. First, for the small conduit
having a 14-mil diameter, with a two-layer coil of 0.6-mil diameter
coil wire, the conduit with the coil thereon might be viewed as
having an outer diameter of about 16 mils. Similarly, for the large
conduit having an outer diameter of 30 mls, such conduit with such
coil thereon might be thought of as having an outer diameter of
about 32 mils. Such outer diameters might then be designated with
the letter "a". Then the coil cavity wall separation (again
employing the conduit-coil axis as the center line for such separation)
might be designated as "b". Further, the places of maximum
and minimum wall separation along the coil are as noted immediately
above; and they are also the places of maximum wall-coil spacing
along the coil. With this background and with the term "spacing"
below referring to such wall-coil spacing, the following calculations
can be made:
______________________________________ Small conduit: maximum (b/a)
= 16.8; minimum (b/a) = 8.13 maximum spacing = 127 mils; minimum
spacing = 57 mils; maximum bln (b/a) = 0.76; minimum bln (b/a) =
0.27. Large conduit: maximum (b/a) = 8.41; minimum (b/a) = 4.06;
maximum spacing = 119 mils; minimum spacing = 49 mils; maximum bln
(b/a) = 0.57; minimum bln (b/a) = 0.18. ______________________________________
Then considering the small and large conduits together and rounding
up for maxima and down for minima, from the above calculations,
the following approximately applies:
______________________________________ Small and large conduits:
______________________________________ maximum (b/a) = 17.0; minimum
(b/a) = 4.0; maximum spacing = 130 mils; minimum spacing = 45 miles;
maximum bln (b/a) = 0.80; minimum bln (b/a) = 0.15. ______________________________________
It is also of some interest to make certain computations at mid-coil
along the mid-plane of the coil cavity (where the 180-mil wall separation
along the coil applies):
______________________________________ Small conduit: b/a = 11.3;
spacing = 82 mils; bln (b/a) = 0.44. Large conduit: b/a = 5.63;
spacing = 74 mils; bln (b/a) = 0.31. ______________________________________
Test data on models intended to somewhat generally simulate the
operation of the meter device of FIGS. 1-3 employing the glass
fiber-filled polyamide housing material, but with certain variant
coil cavity sizes, reveals excellent, low sensitivity to attitude
with respect to the direction of gravity, excellent, fast responsiveness
to changes in flow rate and fully adequate sensitivity. The coil
cavities were oriented with their cylindrical axes parallel to rather
than perpendicular to the direction of coiling of the coils. In
this regard, their ends were sufficiently spaced from the coil ends
to essentially remove any significant effects from coil-housing
interaction in these regions. The simulations, except in one case
noted below, did not include support for the sensing conduit between
the coils (it is noted that such support is also not employed in
the device that has been described). With respect to sensitivity
measurements, the bridge-type circuit was of a type for which a
maximum output (amplified) of about 5 volts is viewed as indicative
of very good sensitivity and in the range of 7.5 volts as indicative
of essentially the greatest sensitivity attainable. The responsiveness
to changes in flow rate was measured by starting at a set flow rate,
increasing the flow rate in essentially a step function, and then
decreasing the flow rate back down to the initial rate in essentially
a step function. The time "t.sub.1 " was then the time
after the initial increase for the output to reach about ninety
percent of its maximum level; the time "t.sub.2 " was
the time after the initial increase for the output to reach about
ninety-eight percent of its maximum value; the time "t.sub.3
" was the time after the decrease for the output to fall to
about ten percent of its maximum value; and the time "t.sub.4
" was the time after the decrease for the output to fall to
about two percent of its maximum value. With this background, with
the coil cavity diameters as indicated below, with the qualification
(as indicated above and also in the chart below) that a center support
was added for one set of data, and with the attitude coefficient
(below) taken to be the maximum of the the +90.degree. and -90.degree.
sensitivity divided by the full scale output and by 90.degree. and
then stated as a percentage by multiplying by 100 the data for
five "runs" covering two different diameters was as follows:
__________________________________________________________________________
Coil Cavity 1 2 1A 2A 2B.sup.1 Diameters (62 mils) (125 mils) (62
mils) (125 mils) (125 mils) __________________________________________________________________________
Full scale output 3.69 5.00 3.69 4.76 4.23 (volts) Attitude +90.degree.
+0.00 +0.05 +0.00 +0.06 +0.03 Sensitivity -0.00 -0.05 -0.00 -0.06
-0.03 (volts) -90.degree. Coefficient 0.000 0.011 0.000 0.014 0.008
.+-. %/degree Response Data: (sec.) t.sub.1 3 3 3 3 4 t.sub.2 11
6 11 6 10 t.sub.3 4 4 4 4 4.5 t.sub.4 16 10 16 10 11 Measured Coil
Resistance (ohms) Upstream 290 290 286 294 -- Downsteam 288 288
287 294 -- 1 Center support added. __________________________________________________________________________
Prior to the foregoing test data, comparable test data for varying
diameters (again, with coil cavities in the parallel orientation)
with the same polyamide housing material and for a rigid urethane
housing material was compiled. At the same time, test data for a
standard commercial device of the Assignee (known as Assignee's
No. 900011-001) with coils encased in open cell foam material, in
accordance with the teachings of U.S. Pat. No. 3938384 referred
to in the Background and Summary of the Invention, was taken ("Standard"
below). Along with this, Assignee's standard commercial device (known
as Assignee's No. 900011-006) that is the same as the former one,
but with the foam material removed, was also tested ("Standard
Uninsulated" below). This earlier testing was done as a rather
preliminary, rough test, but is of some interest. In this regard,
the simulation of the sealing of the space between the housing and
conduit at two points with potting material, referred to earlier,
was not included except in the runs designated 1A and 2A. This was
done then because of instability of readings thought to be due to
looseness of the parts in the housing. However, accurate centering
of the coils was not accomplished in connection with such sealing
and runs. With this background, this earlier test data is as follows:
__________________________________________________________________________
3 5 1 2 (125 mils) Standard 1A 2A Coil Cavity (62 mils) (125 mils)
Rigid 4 Uninsu- (62 mils) (125 mils) Diameters Polyamide Polyamide
Urethane Standard lated Polyamide Polyamide __________________________________________________________________________
Full Scale 3.10.sup.1 4.27 6.07 5.38 5.41 2.69 3.31 Output (volts)
Attitude +90.degree. -0.30 +0.03 +0.2.sup.2 0.00 1.47 0.00 0.02
Sensitivity -90.degree. -0.34 -0.03 -0.2.sup.2 -0.01 -.70.sup.3
-0.02 -0.02 Coefficient 0.12 0.008 0.037 0.002 0.30 0.008 0.007
.+-. %/degree Response Data: (sec) t.sub.1 3.0 3.0 6.0 6.0 5.0 4.0
4.0 t.sub.2 9.0 7.0 30.0 12.5 8.0 14.0 14.0 t.sub.3 4.0 3.0 10.0
9.0 5.5 4.5 4.0 t.sub.4 13.0 11.0 45.0 19.0 11.0 16.0 15.0 Measured
Coil Resistance (ohms) Upstream 289 284 288 306 310 280 290 Downstream
285 284 290 306 311 281 290 __________________________________________________________________________
.sup.1 Zero level unstable. .sup.2 Zero level unstable. .sup.3 Signal
saturated.
It, first of all, is to be noted that some of the indicated and
readily evident difficulties with this test data were overcome with
regard to the test data previously presented. However, this data
is significant in indicating fast response times for a material
such as the polyamide material by comparison with a material such
as rigid urethane. It is also significant in indicating the attainability
of good attitude insensitivity without the disadvantages attendant
to the use of the open cell foam material encapsulating the coils
(and conduit in the vicinity of the coils).
By way of some additional test data, averages for various forms
intended to generally simulate cylindrical coil cavities, was illustrative
of the general decrease in attitude insensitivity with cavity size
(with, however, the general increase in full-scale output with cavity
size). This averaged data for the full-scale output and attitude
sensitivity coefficient versus cavity diameter was as follows: 62
mils-3.2 volts and 0.005%/per degree; 125 mils-4.2 volts and 0.015%/per
degree; 180 mils-5 volts and 0.03%/per degree. The first two involved
coil cavities in the parallel orientation, and the third, coil cavities
in the perpendicular orientation.
FIG. 7 shows an alternative housing form for the flow meter device
of FIGS. 1-3. In this respect, apart from the form of the housing,
the parts of the device and the manner of connection and use of
the device are essentially identical to the parts and manner of
connection and use as described in connection with FIGS. 1-5. Moreover,
the description with respect to the housing of FIGS. 1-3 applies
in essentially identical fashion to the housing of FIG. 7 with the
exceptions which will be particularly focused upon.
First, by way of background, the test data which has been discussed,
along with other related test data, has revealed that the housing
form of FIG. 7 for many potential applications, provides both test
results, including satisfactory amplitude and attitude insensitivity,
and physical stability of structure, which are within the requirements
of such applications. This is of particular significance in that
the added simplicity of construction is advantageous.
Referring only to the areas for which the housing description in
connection with FIGS. 1-3 does not apply, again the input and ouput
sides of the housing body 130 and housing cover 132 with regard
to their internal wall structure and the cavities such structure
provides, are mirror images of one another so that a description
as to the input side in effect also describes the output side.
Thus, referring to the input side, there is a relatively long input
side main cavity 134 having a corner, which replaces a number of
cavities in the housing of FIGS. 1-3 including the cylindrical
form upstream coil cavity 86. This input side cavity 134 has essentially
right, circular cylindrical portions for its inner (toward the coil)
and outer (away from the coil) portions, away from its corner portion.
Leading into the input side main cavity 134 there is an input
side lead-in cavity 136. As to the portion of such cavity defined
by the housing body 130 wall structure, it is essentially identical
to the equivalent portion in the housing of FIGS. 1-3. As to the
portion defined by the housing cover 132 it has essentially a half
right circular cylindrical shape, with the same diameter as the
main cavity 132. Ease of construction, of course, is a consideration
in the "non-mating" construction of such housing body-defined
and housing cover-defined portions of this lead-in cavity.
The cover 132 in the housing form of FIG. 7 also has a generally
rectangular-shaped mid-cover cavity 138 which is significantly larger
in area than the mid-cover cavity 128 of FIGS. 1-3. Thus, in this
case, the mid-cover cavity is large enough to more than overlap
the housing body cavities with which it communicates.
For the small 14-mil diameter conduit and the large 30-mil diameter
conduit, calculations involving "a", "b" and
the wall-coil "spacing", as defined earlier, can be made.
In this case, however, because of the cavity shape adopted, the
calculations involve constants rather than maxima and minima. In
particular, for a 120-mil diameter main input side cavity 134 (and,
of course, the same diameter for its output side mirror image),
which dimension applies with respect to the form of FIG. 7 the
following then approximately applies:
______________________________________ Small conduit: (b/a) = 7.5;
spacing = 52 mils bln (b/a) = 0.24. Large conduit: (b/a) = 3.75;
spacing = 44 mils; bln (b/a) = 0.16. ______________________________________
Then considering the small and large conduits together and rounding
up for maxima and down for minima, from the above calculations,
the following also applies:
______________________________________ Small and large conduits:
______________________________________ maximum (b/a) = 8.0; minimum
(b/a) = 3.7; maximum spacing = 55 mils; minimum spacing = 40 mils;
maximum bln (b/a) = 0.25; minimum bln (b/a) = 0.15. ______________________________________
In FIG. 6 a rough, initial test device as to concept, is shown.
This device was initially tested for adequacy of speed of response
against a standard commercial device of the Assignee's of the type,
referred to above, employing open cell foam encasing material for
the coils of the device. Referring to FIG. 6 there is an upstream
coil 140 and a downstream coil 142. There is a sensing conduit 144
having an input port 146 and an output port 148. Three Mylar (a
polyethylene teraphthalate material) discs are on the conduit at
the center and ends of a cavity which is enclosed by wrapping Scotch
Tape (a cellophane material tape) about the discs. The conduit is
of the smaller outer diameter (14 mil) type referred to earlier.
The coils are also of the type referred to earlier. Two coil leads
from each coil, for connection to a bridge-type circuit, are also
shown. The diameter of the coil cavities was 180 mils, the total
length of the wrapping tape for the cavities was about 300 mils.
For the standard commercial device of the Assignee's, of the type
using open cell foam encasing material for the coils, a given flow
rate was defined as an "open" valve position and a "shut"
valve position was defined as a flow rate of ten percent of this
value. With the given rate then considered the one hundred percent
level (and the "shut" valve level the ten percent level),
the times for the output signal for the standard device (in a given
bridge-type circuit) to fall to the thirty-seven percent and fifteen
percent levels respectively, upon going from an open to shut condition,
were 1.7 seconds and 8.8 seconds, respectively. The comparable times
for the device of FIG. 6 were 0.8 seconds and 4.0 seconds, respectively.
This, of course, was a very favorable result. Going from the shut
to open condition, for the standard device, the times to rise to
the sixty-three percent and ninety percent levels, respectively,
were 0.2 seconds and 5.6 seconds. The comparable times for the device
of FIG. 6 were 0.2 seconds and 2.3 seconds, respectively. Again,
this was a favorable comparison.
The generally desirable properties for the housing material are
low mass, low specific heat and high thermal conductivity--which
of course are not harmonious goals. The test data outlined above
is viewed as indicating that a variety of specific housing materials
should be satisfactory but that the choice can be of substantial
significance. Thus the rigid urethane material is generally unacceptable
due to its low thermal conductivity.
Finally, two points might be noted. First, there is a trade-off
evident from the foregoing with respect to efficiency of production
relating to difficulties associated with very small cavity sizes,
and attitude insensitivity. Second, the goal of relative insensitivity
to attitude, of course, is recognized as important to provide flexibility
in the mounting and orienting of a mass flow meter device. In addition,
it enables the use of such a device on a moving platform in that
the forces of acceleration are analogous to gravitational forces.
It, of course, will be apparent to those skilled in the art that
many modifications and variations may be made in the embodiments
which have been described without departing from the scope or spirit
of the invention. By way of example, the principles applicable to
the specific embodiments also apply equally well to forms of mass
flow meters which employ upstream and downstream sensor coils which
are heated by a heater coil about the conduit, therebetween. In
such case, rather than, for example, the housing of FIGS. 1-3 one
might employ a housing defining three coil cavities, separately
limiting the ambient gaseous atomosphere along each of the three
coils to a thin film. By way of further example, such principles
also apply to forms of mass flow meters 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 hereof shall not be referenced to the disclosed embodiments,
but on the contrary, shall be determined in accordance with the
claims as set forth below. |