Abstrict A fluid flow meter of the variable area flow passage type is provided
wherein the variation in said flow passage area is effected by the
displacement of a control boundary which is supported by an annular
diaphragm element. Said control boundary is shaped and sized so
that the overall displacement required for the flow rate range desired
is within the stroke capacity delivered by the free flexing action
of said diaphragm element, thereby achieving substantially constant
differential pressure operation at a very reasonable value which
is due almost entirely to gravity. The resulting unit is very compact
and stable and is characterized by very smooth linear response between
vertical displacement and fluid flow rate. Said vertical displacement
can be measured by various means but is preferably transduced to
an equally linear, smooth and steady electrical signal by a direct
connection with a specially adapted differential transformer.
Claims 1. A fluid flow measuring device characterized by a highly linear
response and substantially constant differential pressure operation
comprising a main casing with a flexible partition member peripherally
mounted from the lateral walls of said casing, said partition member
having a large open area in the central portion thereof and being
sufficiently flexible to present negligible structural resistance
to measurable vertical motion of its central portion; a rigid, symmetrical
collar-shaped fluid control element concentrically mounted on said
partition member and extending downwardly therefrom; a cooperating
fluid flow control surface on the floor of said casing in directly
opposed uniform alignment to and adjacent to said downwardly extending
collar-shaped element; a fluid inlet into said casing below said
partition member and at a point outside said cooperating fluid control
surface; a fluid outlet from said casing above said partition member;
and means for measuring said vertical motion.
2. The device of claim 1 wherein said means comprises an indicator
rod rigidly mounted from said partition member.
3. The device of claim 2 wherein said indicator rod extends beyond
the main casing into an auxiliary enclosure having means thereon
for visually determining vertical positioning of said rod.
4. The device of claim 2 wherein said indicator rod is connected
to an electrical transducer or signalling device outside of said
main casing.
5. The device of claim 4 wherein said transducer is a differential
transformer and said indicator rod is rigidly connected to the core
of said transformer.
6. The device of claim 1 wherein said downwardly extending collar-shaped
element tapers to a uniform thin edge at the bottom.
7. A flow meter especially suited for accurately measuring gas
flow rates at low pressures comprising a main casing; a flexible
annular diaphragm extending across said casing thereby dividing
same into a lower and an upper section having intercommunication
therebetween solely via the central open area of said diaphragm;
a rigid, heavy plate member mounted on the central portion of said
diaphragm and having a rigid, symmetrical collar-shaped fluid flow
control element concentrically extending therefrom downwardly through
said lower section of the casing; firmly positioned on the floor
of said casing beneath said collar-shaped element, a directly opposed
uniformly aligned fluid flow control surface, the total mass and
positioning of said elements being such that under static conditions
of no fluid flow said control element and said control surface make
contact with each other; a fluid inlet into said lower section at
a point outside said fluid control surface; a fluid outlet from
said upper section; and means for determining the vertical displacement
of the diaphragm mounted elements.
8. A flow meter as described in claim 7 wherein said downwardly
extending collar-shaped element tapers to a uniform thin edge at
the bottom.
9. A flow meter as described in claim 7 wherein said diaphragm
has at least one concentric convolution in the outer portion thereof.
10. A flow meter as described in claim 7 wherein said means comprises
a rigid vertical indicator rod mounted securely from the plate member
on said diaphragm.
11. A flow meter as described in claim 10 wherein said indicator
rod extends beyond the main casing into an auxiliary enclosure having
means thereon for visually determining said vertical displacement
of the diaphragm mounted elements.
12. A flow meter as described in claim 10 wherein said indicator
rod is connected to the iron core of a differential transformer
type of electrical transducer.
13. A flow meter as described in claim 12 wherein said differential
transformer has a dual secondary winding providing a push-pull type
of reinforced electrical signal in response to vertical displacement
of said iron core from its null position.
Description This invention relates to fluid flow meters of the constant differential
pressure type wherein a movable element is provided which, in automatic
response to changes in fluid flow rate, effects corresponding changes
in cross-sectional area of the minimum flow passage thereof. The
flow meter of my invention is especially adapted for accurately
measuring gas flows over a wide range of rates while maintaining
a highly linear relationship which can be extended readily to a
cooperative electrically transduced indicator system of closely
matched linear responsiveness, if desired.
Over the years considerable effort has been directed toward improving
variable area type flow meters to make them more stable and reliable
and more nearly linear and accurate over a wide range of flow rates.
One of the most common forms of apparatus developed in this field
is the so-called rotameter, a device in which a generally roundish
float element is lifted inside of an upwardly expanding tapered
tube to heights directly dependent upon the fluid flow rates therethrough.
Due to their simplicity and responsiveness, rotameters are very
popular but are too fragile and erratic for many applications due
to such problems as float bounce, mechanical resonance and uncertain
or variable alignments.
Most of the more rugged variable area flow meters developed to
date have movable elements which are either inherently stiff or
separately spring biased, or else encumbered with sufficient frictional
effects to dampen their responsiveness and prevent true linearity
of the position vs. flow rate relationship. Examples of this type
of flow meter in which the movable element is deliberately counterbiased
with a back up spring are disclosed in U.S. Pat. No. 3234790 to
Ekstrom and U.S. Pat. No. 1416220 to Long et al. On the other
hand, Ongaro in U.S. Pat. No. 3218853 has advocated the use of
a primarily gravity opposed movable element in such flow meters
provided said movable element is vertically aligned. However, said
patentee also points out the extreme difficulty of eliminating mechanical
interferences and frictional resistances in any such device which
is capable of durable service over a reasonable range of operating
conditions.
The primary object of the present invention is to provide an essentially
constant differential pressure, variable area flow metering device
in which the movable element is truly gravity opposed and mechanical
friction is effectively eliminated thereby achieving a very smooth
linear response between fluid flow rate and displacement of the
movable element. It is further object of my invention to provide
such a device which is durable and can be used continuously to deliver
a smoothly linear mechanical response to flow rates varying over
a wide range (e.g. 10/1 or higher ratios). It is also one of my
objects to provide such an apparatus which is compact and simple
to use and operate.
A secondary object of this invention is to provide a directly coupled
electric transducer which has a smoothly linear output closely matched
to the mechanical displacement of the movable element in the variable
area metering device over its full operating stroke. Finally, it
is a special object of my invention to provide throughout my apparatus
the capability of using corrosion resistant materials of construction
so that it can be used to measure the flow rates of such aggressive
gases as chlorine, oxides of nitrogen or sulfur, anhydrous ammonia
etc.
Further objects and advantages of this invention will be apparent
from the following description including exemplary and preferred
embodiments as illustrated in the accompanying drawings, wherein:
FIG. 1 is a cross-sectional elevation along the center line of
a basic flow meter constructed in accordance with the present invention;
FIG. 2 is a top sectional view taken along line 2--2 of FIG. 1;
FIG. 3 is a cross-sectional elevation along the center line of
another embodiment of flow meter operating on the same general principles
and especially suited for measuring small flow rates;
FIG. 4 is a top sectional view taken along line 4--4 of FIG. 3;
FIG. 5 is a cross-sectional elevation along the center line of
another embodiment of my invention in which a preferred type of
electrical transducer is cooperatively integrated with the basic
mechanical elements of the flow meter;
FIG. 6 is a top sectional view taken along line 6--6 of FIG. 5;
and
FIG. 7 is a sectional view taken in the opposite direction along
line 7--7 of FIG. 5.
Referring now in detail to FIGS. 1 and 2 the main housing 10 of
the meter encloses a generally cylindrical space 12 which is transversely
separated into upper and lower portions by means of thin partition
member or diaphragm 14. The housing 10 is also provided with fluid
inlet opening 16 into the lower portion of space 12 and fluid outlet
passage 18 from the upper portion of space 12.
A large opening 20 is provided in the central area of partitioning
diaphragm 14 in order to permit fluid to pass from lower to upper
sections of space 12. Also, concentric convolution 22 is formed
in the outer portion of diaphragm 14 in order to assure sufficient
unrestrained vertical movement of the inner portions thereof to
give measurable indications of changes in fluid flow over useful
ranges, such as maximum to minimum ratios of about 10/1.
Concentrically mounted on diaphragm 14 to the inside of convolution
22 are relatively thick blocks or plate members 24 and 26 the central
areas of which are also open so as not to interfere with the flow
of fluid up through opening 20. The lowest part of lower plate member
26 is shaped in the form of a downwardly extending concentric collar
or rim 28. This rim 28 is tapered to a precise uniform thin edge
at the bottom in order to provide part of the limiting surface for
the controlling fluid flow passage of the meter. The opposing surface
for the limiting or controlling flow passage is provided by the
horizontal upper face 30 of plug 32 which is mounted in the base
of housing 10 by means of threaded opening 34 using a sealing O-ring
element 36 so that upper face 30 just contacts the lower edge of
rim 28 under static conditions. It will be seen that the controlling
flow passage thus defined between rim 28 and plug face 30 is in
the form of a cylindrically shaped slot, the depth of which increases
directly with the upward movement of plates 24 and 26 mounted on
the freely movable portion of diaphragm 14.
A vertical position indicator rod or mast 38 is centrally mounted
on the top side of upper plate member 24 by means of bridge plate
40. Bridge plate 40 has fluid passage openings 42 in the central
area thereof and the entire diaphragm and plate assembly is held
together by bolts 44. Indicator mast 38 extends up into the free
space inside of transparent extension cover 46 sealably mounted
in the head of main housing 10. Suitable gradations 39 are engraved
on the portion of mast 38 which are visible through extension cover
46 so that the vertical position can be read relative to a bench
mark 48.
Since no counterbiasing springs are used in my flow meter, the
only significant force opposing the flow of fluid therethrough is
the gravitational pull on the mass of elements supported by diaphragm
14. Therefore, the differential pressure across a given meter of
my construction will be substantially constant over a wide range
of fluid flow rates. Said mass can be controlled to some extent
by choice of materials, thickness of diaphragm plates 24 and 26
etc. However, limiting said differential pressure to a very reasonable
value is generally easy because of the rather extensive effective
area of the partitioning diaphragm assembly. This fact renders my
devices ideal for measuring gas flow at low to ordinary pressures.
Even under these sensitive conditions, a very smooth and steady
linear response is achieved between the vertical movement of mast
38 and corresponding flow rates delivered through the meter.
In FIGS. 3 and 4 there is depicted another embodiment of my invention
based upon the same operating principle but illustrating some optional
constructional features of specific interest. In this embodiment,
the limiting or controlling flow passage is defined at a position
well within the vertical opening through the diaphragm plate assembly
instead of at the lower rim of the lower plate member as in the
previously described construction. Specifically, in this case, the
opening through upper diaphragm plate 25 is smaller than the central
openings through diaphragm 14 and lower plate 27 and is shaped as
a precisely tapered, upwardly converging frustoconical section 23
leading to throat section 21 and then diverging rapidly at the top.
Cooperatively aligned with said opening is plug 32 mounted in the
base of housing 10. Concentric extension post 33 in plug 32 extends
vertically up through frustoconical section 23 and is provided with
horizontal shoulder 35 tapered on top to so that a thin edged outer
rim is formed thereon the outside diameter of which is slightly
greater than the diameter of throat section 21. Thus, the limiting
flow passage in this device is in the form of an annular slot which
gradually increases in width in direct proportion to the upward
travel of the inner portion of diaphragm 14 under the lifting force
of the flowing fluid.
Also, in this case upwardly convex concentric convolutions 23 are
provided on either side of downwardly convex convolution 22 in diaphragm
14 in order to ensure a somewhat larger unrestrained vertical movement
of permissible total stroke of the indicator mast 38. The mast 38
of this unit is mounted on top of upper plate 25 by means of concentric,
inverted cup shape bracket 41 having large fluid outlet passages
43 in each lateral quadrant thereof. Four matching lugs 11 of equal
height are provided on the inner floor of housing 10 providing uniform
support for the diaphragm plate assembly under static conditions
while still permitting light contact of the rim of shoulder 35 with
the inner surface of frustoconical section 23. Tests of devices
constructed in accordance with FIGS. 3 and 4 have also shown fully
linear responsiveness. Also, because of the larger stroke provided,
reading the indicator to a given degree of accuracy generally requires
less effort.
Attention is next directed to FIGS. 5 6 7 wherein there is illustrated
the integration of the mechanical features of the present invention
with an electrical transducer system of matched linearity so that
the mechanical motion of the indicator rod is translated directly
into a linearly responsive electrical signal which is more easily
and accurately detected, read and/or recorded. Although the mechanical
arrangements and actions of this embodiment are quite sinilar to
those of FIGS. 1 and 2 a few minor modifications should be noted.
First, in order to provide for larger fluid flow capacities, the
diameter of the thin edged control surface of rim 28 is made considerably
greater and the horizontal floor of main housing 10 itself serves
as the opposing surface of the cylindrical slot under rim 28 wich
constitutes the limiting flow passage. Secondly, the upper face
of the lower diaphragm plate 26 is cut away slightly beginning from
a short distance outside the ring of assembly bolts 44 and tapering
downward to its outer edge, thus taking better advantage of the
flexibility of the larger diameter diaphragm 14 to obtain a along
free and uninhibited vertical stroke while incorporating only one
convolution 22 in the diaphragm 14. Also, in order to adjust the
differential pressure of this unit to the desired value, a ring
of material 29 of different density is built into upper diaphragm
plate 24. For example, ring 29 could be a lead insert or the like
molded into a plastic plate 24 e.g. of polyvinyl chloride composition.
Finally, the mast 38 on top of the diaphragm plate assembly in
this device serves as a connecting rod transmitting the vertical
motion of said assembly to the iron core of a linearly variable
differential transformer which is the preferred form of electrical
transducer for my flow meter. Thus, the upper end of mast 38 is
rigidly connected into the lower end of thin-walled sheath tube
50 within which iron core 52 is carried by means of concentric spacer
caps 54 at either end. The upper end of sheath tube 50 is sealed
by means of plug 56 through which adjusting screw 58 extends and
threadably connects into a tapped opening 60 in upper spacer cap
54 in order to simplify final setting of the vertical position of
iron core 52. These parts associated with iron core 52 (such as
50 54 56 and 58) should be constructed of materials of relatively
low magnetic permeability so as not to affect the armature action
of said iron core. Plastic materials such as nylon, polyolefins,
fluoroplastics and the like are ideal for this purpose but various
metals like tantalum and non-magnetic stainless steels can also
be used. The outer housing 62 of the differential transformer is
provided with a threaded connection 64 for mounting same on top
of main housing 10 and a cooperating O-ring element 66 to insure
a fluid tight seal therebetween. Threaded connection 64 has an internal
bore somewhat larger than the outside diameter of sheath tube 50
so as to permit free vertical movement of same therethrough. The
main body of housing 62 encloses bore or cylindrical well 68 which
is sized to accept coil assembly 70 which has an inner core 72 of
larger diameter than sheath tube 50 so that it can be positioned
around same with the electrical leads 71 and 73 passing out the
side of housing 62 through opening 74. After all of the parts of
the differential transformer are in place, the top of housing 62
can be closed by means of seal cap 76 with the aid of screws 78.
Coil assembly 70 contains a primary winding and at least one secondary
coil. Preferably, two secondary coils of the same configuration
are wound on core 72 equidistantly above and below the center point
thereof or a uniformly wound secondary coil is center-taped dividing
it electrically into upper and lower sections. In this way when
an alternating current is passed through the primary coil by means
of leads 71 the inductance effect in the secondary will be doubled
due to the push-pull effect of iron core 52. In other words, if
the center of iron core 52 is properly adjusted so as to line up
opposite the center point of coil assembly 70 when the diaphragm
assembly is at its lowest position (zero fluid flow), then the induction
effect in the opposing secondary coil sections will be exactly balanced
and the electrical output through leads 73 will be zero. However,
as the iron core 52 is lifted from this null position vertically
up through the coil assembly, the decrease in inductance in the
lower secondary section will be of the same magnitude as the increase
in the upper secondary so that an additive or doubled net electrical
signal is emitted through leads 73 thus insuring a strong, steady
and accurate read-out signal with excellent linearity matching that
of the mechanical stroke produced by the diaphragm assembly as it
responds to the fluid flow rate through the unit. Excellent units
of this type can be built to operate on a very compact overall maximum
mechanical stroke, e.g. a minor fraction of an inch.
One of the prime fields of use for the highly sensitive and linearly
responsive flow meter of this invention is for measuring the flow
of chlorine gas in various chlorination systems, wherein the feed
pressures employed are often below normal atmospheric pressure.
Excellent performance has been experienced in this service provided
the parts which are subject to contact with the chlorine gas are
constructed of suitable materials which include fluorinated polymers,
polyvinyl chloride, acrylonitrile butadienestyrene copolymers, and
the like or metals such as tantalum or silver.
For example, polymers based upon chlorotrifluoroethylene have excellent
properties. Thus, 5 mil thick sheeting of one such polymer sold
under the registered Trade Mark "Kel-F" has been used
with excellent results to fabricate the flexible annular diaphragms
14 used in the flow meters described hereinabove. In molding the
concentric convolutions in these diaphragms, it has been found desirable
to produce very gently rounded contours and to limit the depth of
groove to no more than about half the main contour radius thereof.
Similar materials including elastomeric fluoropolymers such as
those sold under the registered Trade Mark "Viton" can
be used for O-ring elements 36 and 66.
As will be obvious to those skilled in the art, many different
substitutions and modifications can be made in specific elements
of the invention described hereinabove. For example, any suitable
level indicating device, whether mechanical, electrical, magnetic,
electronic etc. can be used to determine the vertical displacement
of the movable element of the limiting fluid flow control passage
of the unit. |