Abstrict A concentric rod and tube sensor for a thermal mass flow controller
or flow meter is disclosed. The flow meter has a flow passage therethrough
for measuring the flow rate of a fluid and includes a heater for
heating the fluid flowing therethrough. The flow meter senses the
temperature rise of the fluid flowing through the flow path heated
by the heater and then generates a signal in response to the temperature
rise of the fluid with this signal being proportional to the mass
flow rate of the fluid flowing through the flow path. The flow meter
includes an inlet bushing having at least one opening therethrough,
an outlet bushing having another opening therethrough, a tube extending
between and being received by the bushings, and a rod disposed concentrically
within the tube. The inlet and outlet bushings receive a respective
end of the tube and the rod and positively hold the rod centered
within the tube thereby to establish and to maintain a concentric
annulus between the outer surface of the rod and the inner surface
of the tube for the flow of fluid through the annulus.
Claims We claim:
1. In a thermal mass flow meter for measuring the flow of fluid
therethrough, said flow meter comprising a meter body having a flow
path therethrough, the latter having an inlet and an outlet, means
for heating said fluid flowing through said flow passage, means
for sensing the temperature rise of said fluid flowing through said
flow passage heated by said heating means, and means responsive
to said temperature rise of said fluid for generating a signal proportional
to the mass flow rate of said fluid flowing through said flow passage,
wherein the improvement comprises: an inlet bushing within said
flow path having at least one opening therethrough, an outlet busing
within said flow path having at least one opening therethrough,
a tube extending between said bushings, a rod disposed concentrically
within said tube, means in said inlet and outlet bushings for positively
holding said rod centered within said tube thereby to establish
and to maintain a concentric annular opening between the outer surface
of said rod and the inner surface of said tube, said annular opening
constituting a portion of said flow path.
2. In a thermal mass flow meter as set forth in claim 1 wherein
each of said bushings has a bore in one end thereof for receiving
a respective end of said tube, and a socket coaxial with respect
to said bore for receiving a respective end of said rod, said bore
and socket constituting said means for positively holding said rod
centered within said tube.
3. In a thermal mass flow meter as set forth in claim 2 wherein
said socket has a conical taper converging away from said rod, said
conical taper being coaxial with respect to said tube received in
said bore of said bushing.
4. In a thermal mass flow meter as set forth in claim 2 wherein
said bushing has a plurality of passages therethrough providing
communication with said flow path at the end of the bushing opposite
said rod and with the concentric annular opening between said rod
and said tube.
5. In a thermal mass flow meter as set forth in claim 4 wherein
said plurality of passages are generally axial with respect to said
rod.
6. In a thermal mass flow meter as set forth in claim 4 wherein
said plurality of passages are generally radial with respect to
said rod.
7. In a thermal mass flow meter as set forth in claim 1 wherein
one of said bushings is in axial engagement with said meter body,
and wherein said flow meter further includes spring means interposed
between said meter body and the other of said bushings thereby to
resiliently bias said other bushing and said tube and said rod into
engagement with said bushings.
8. In a thermal mass flow meter as set forth in claim 1 further
comprising seal means engagable with the outer surface of said tube
thereby to prevent the flow of fluid past the outer surface of said
tube.
9. In a thermal mass flow meter as set forth in claim 1 wherein
said meter body comprises a main body, an inlet body, and an outlet
body, fastener means for positively securing said inlet and outlet
bodies to said main body, said inlet bushing being received within
said inlet body, said outlet bushing being received within said
outlet body, and seal means adjacent the inlet and outlet ends of
said tube sealingly engageable with said tube and said meter body
for preventing the flow of fluid between said meter body and the
outer surface of said tube.
10. In a thermal mass flow meter as set forth in claim 9 wherein
one of said end bodies may be removed from the main body so as to
permit removal and reinsertion of said tube and rod within said
flow meter.
11. In a thermal mass flow meter for measuring the flow of fluid
therethrough, said flow meter comprising a meter body having a flow
path therethrough, the latter having an inlet and an outlet, means
for heating said fluid flowing through said flow passage, means
for sensing the temperature rise of said fluid flowing through said
flow passage heated by said heating means, and means responsive
to said temperature rise of said fluid for generating a signal proportional
to the mass flow rate of said fluid flowing through said flow passage,
wherein the improvement comprises: an inlet bushing within said
flow path, an outlet bushing within said flow path, a tube extending
between said bushings, a rod disposed concentrically within said
tube, means in said inlet and outlet bushings for positively holding
said rod centered within said tube thereby to establish and to maintain
a concentric annular flow passage between the outer surface of said
rod and the inner surface of said tube, each of said bushings having
a plurality of openings therethrough for providing communication
with said flow path at the end of the bushing opposite said rod
and with the annular flow passage between said rod and said tube.
12. In a thermal mass flow meter for measuring the flow of fluid
therethrough, said flow meter comprising a meter body having a low
passage therethrough, the latter having an inlet and an outlet,
means for heating said fluid flowing through said flow passage,
means for sensing the temperature rise of said fluid flowing through
said flow passage heated by said heating means, and means responsive
to said temperature rise of said fluid for generating a signal proportional
to the mass flow rate of said fluid flowing through said flow passage,
wherein the improvement comprises: an inlet bushing within said
meter body, an outlet bushing within said meter body, a tube extending
between and received by said bushings, a rod disposed concentrically
within said tube, means in said inlet and outlet bushings for receiving
a respective end of the tube and of said rod so as to positively
hold said rod centered within said tube thereby to establish and
to maintain a concentric annular flow passage between the outer
surface of said rod and the inner surface of said tube, each of
said bushings having a bore in one end thereof for receiving a respective
end of said tube, and a socket coaxial with respect to said bore
for receiving a respective end of said rod, each of said bushings
having at least one opening therethrough for providing communication
with a portion of said flow passage at the end of the bushing opposite
said rod and with the annular flow passage between said rod and
said tube, said flow meter further including a spring interposed
between said meter body and one of said bushings thereby to resiliently
bias said bushings and said tube and rod into engagement with one
another.
Description BACKGROUND OF THE INVENTION
This invention relates to a thermal mass controller or flow meter,
and more particularly to an improved concentric rod and tube sensor
for such a thermal mass flow meter.
In general, thermal mass flow meters are instruments for accurately
measuring fluid flow rates, preferably gas flow rates. Typically,
such instruments include a flow sensor which generates an output
signal proportional to the flow rate of the fluid flowing through
the thermal mass flow meter and this output signal can be used for
indicating, recording, or controlling the flow rate of the fluid.
Oftentimes, the thermal mass flow meter is combined with an automatic
controller which regulates the flow to a predetermined flow rate
level and thus eliminates the need for continuous monitoring and
manual readjustment of fluid pressure so as to provide a uniform
gas or fluid flow rate. In one type of known thermal mass flow meter,
the sensor is a hollow tube having a heater coil wound therearound.
On the inlet and outlet sides of the heater coil, sensor coils are
provided on the tube. The heater coil generates a uniform amount
of heat which is conducted through the sensing tube to the fluid
flowing through the tube. The sensor coils are connected to a balancing
bridge and thus detect the resulting temperature differential of
the fluid flowing through the sensing tube with the temperature
differential being proportional to mass flow rate. Such thermal
mass flow meters are commercially available from the Brooks Instrument
Division, Emerson Electric Co. of Hatfield, Pa. under the model
number DS-5810.
It will be appreciated that the performance of such thermal mass
flow meters is linked closely to the heat transfer between the heated
portion of the sensing tube and the fluid flowing therethrough.
Further, the heat transfer coefficient of the fluid flowing through
the sensor tube is highly dependent on boundary layer conditions
between the fluid and the surfaces of the sensor tube in contact
with the fluid being measured. Oftentimes thermal mass flow controllers
are utilized to provide a steady flow of gas to a process, such
as is used in the processing of semiconductor materials. Also, such
mass flow controllers are utilized to control carrier gasses in
gas chromatographs. Typically, the flow rates of these gasses are
relatively low, for example, 0-500 sccm.
In such applications, the flow rate of gasses, such as silane,
must be accurately regulated. These gasses have a tendency to plug
or coat small diameter flow passages within the thermal mass flow
meter. However, so as to establish proper boundary layer conditions
for accurately regulating the mass flow rate of the gasses, it is
necessary to maintain close and precise tolerances within the sensing
tube. In an effort to overcome the tendency of small diameter orifices
in prior mass flow meter sensor tubes, a so-called concentric rod
and tube sensor was developed and has been commercially available
from the Brooks Instrument Division, Emerson Electric Co., Hatfield,
Pa. as model number DS-5858.
This concentric rod and tube sensor utilized a relatively thin
wall outer tube which was sealably fitted within the flow passage
through the flow body of the flow meter. The heater coil and sensor
coils were applied to the outer surface of the tube in heat transfer
relation therewith. At spaced locations along the sensor tube, groups
of three inwardly projecting dimples or depressions were formed
spaced around the circumference in the tube at the same diametric
plane. These dimples or depressions engaged the outer surface of
a rod inserted within the tube so as to concentrically locate the
rod within the tube and so as to establish a concentric annular
flow passage between the inner surface of the tube and the outer
surface of the rod. This concentric tube and rod sensing element
provided a straight through measuring tube which effectively controlled
boundary layer conditions of the gas flowing through the tube and
yet permitted the tube and the rod to be readily removed from the
flow meter body for periodic cleaning. This concentric annulus overcame
the problem of prior thermal mass flow meter using small diameter
orifices of becoming plugged.
However, it was difficult to accurately form the depressions in
the tube of this prior flow meter so as to precisely establish the
concentric relationship of the rod within the tube. Also, it was
found during shipping of the flow meter, the rod was oftentimes
not positively held in concentric relation within the tube such
that the rod would be displaced toward one side of the tube resulting
in the annular flow passage becoming eccentric rather than concentric.
This eccentricity of the flow passage could so adversely affect
the calibration of the instrument such that it would not accurately
regulate or control the flow rate of gasses as intended. Also, during
disassembly and reassembly of the instrument for cleaning purposes,
it was sometimes found that calibration would be affected due to
a resulting eccentricity of the flow passage because the rod could
not be precisely maintained in coaxial relation with the sensor
tube upon re-assembly of the rod within the tube. Still further,
it was found that in use, the dimensions of the dimples or depressions
in the sensor tube would vary. This was caused by repeated removal
and insertion of the rod for cleaning purposes placing a strain
on the depressions or dimples which resulted in a permanent deformation
thereof. Thus, after time, it was found that an eccentric flow passage
would result with a corresponding shift in the calibration of the
flow meter.
SUMMARY OF THE INVENTION
Among the several objects and features of this invention may be
noted the provision of a thermal mass controller or flow meter,
as above described, in which the fluid to be measured flows straight
through the flow measuring tube without the necessity of providing
auxiliary passages or the like;
The provision of such thermal mass flow meter in which boundary
layer conditions of the fluid are accurately controlled thereby
to insure repeatability and stability of the calibration of the
flowmeter;
The provision of such a flow meter in which the rod may be readily
removed from and reinserted within the sensing tube for cleaning;
The provision of such a thermal mass flow meter in which the calibration
of the instrument is maintained upon the repeated removal and reinsertion
of the rod from within and into sensor tube;
The provision of such a thermal mass flow meter which is significantly
more resistive to impact and shock loading, such as may be experienced
during shipping or transport of the instrument, than prior flow
meters without adversely affecting the calibration of the flow meter;
and
The provision of such a thermal mass flow meter which accurately
controls and regulates the flow rate of fluids at relatively low
flow rates, which may be used with fluids having a tendency to foul
or plug orifices, which has a long service life, which has a relatively
long service period between calibration of the instrument, and which
is reliable in operation.
Other objects and features of this invention will be in part apparent
and in part pointed out hereinafter.
Briefly stated, this invention relates to a thermal mass flow meter
for measuring the flow of a fluid, the flow meter comprising a meter
body having a flow passage therethrough. The flow passage has an
inlet and an outlet and means is provided for heating the fluid
flowing through the flow passage. Further, means is provided for
sensing the temperature rise of the fluid flowing through the flow
path heated by the heating means. Other means responsive to the
above-stated temperature rise is provided for generating a signal
proportional to the mass flow rate of the fluid flowing through
the flow path. More specifically, the improvement of this invention
comprises an inlet bushing having at least one opening therethrough,
an outlet bushing having at least one opening therethrough, and
a tube extending between the bushings. A rod is disposed concentrically
within the tube. Means is provided within the inlet and the outlet
bushings for positively holding the rod centered within the tube
thereby to establish and to maintain a concentric annulus between
the outer surface of the rod and the inner surface of the tube.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of an improved thermal mass flow
controller or flow meter of the present invention with the body
of the flow meter shown at least in part in vertical cross-section
illustrating a flow path extending through the flow meter body and
further illustrating a concentric rod and tube sensor in accordance
with the improvement of the present invention;
FIG. 2 is a view taken along line 2--2 of FIG. 1 on an enlarged
scale illustrating a first embodiment of the present invention including
a bushing receiving one end of the tube and further receiving one
end of the rod with the bushing positively maintaining the rod in
concentric relationship with the tube, it being understood that
another similar bushing is present at the other end of the tube;
FIG. 3 is an end view of the bushing taken along line 3--3 of FIG.
2;
FIG. 4 is a diametric cross-section of the bushing taken along
line 4--4 of FIG. 3 illustrating an opening for receiving the end
of the tube and illustrating a socket for receiving the end of the
rod;
FIG. 5 is a side elevational view of the tube;
FIG. 6 is a side elevational view of the rod;
FIG. 7 is an outer end view of another embodiment of a bushing
for use with the improvement of this invention;
FIG. 8 is a top plan view of FIG. 7;
FIG. 9 is a vertical cross-sectional view taken along line 9--9
of FIG. 7 illustrating a sensing tube and rod received therein,
similar to FIG. 2;
FIG. 10 is a right end view of the bushing shown in FIG. 9; and
FIG. 11 is left end view of the body of the flow meter shown in
FIG. 1.
Corresponding reference characters indicate corresponding parts
throughout the several views of the drawings.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now to the drawings, and more particularly to FIG. 1
a thermal mass flow meter or controller of the present invention
is indicated in its entirety by reference character 1. This flow
meter may, for example, be an improved flow meter generally similar
to model DS-5858 commercially available from the Brooks Instrument
Division, Emerson Electric Co., Hatfield, Pa., the assignee of the
present invention. Because many of the details of construction and
operation of this flow meter are similar to the above-mentioned
prior commercially available flow meter, and because these details
are generally known to those skilled in the art, this disclosure
will focus primarily on the improvements of this invention to this
previously commercially available flow meter. Thus, the portions
of the flow meter described herein which are common to both the
flow meter of the instant invention and to the prior commercially
available flow meter will only be described in generalities such
as will enable one skilled in the art to understand the environment
of the improvement of the instant invention.
More specifically, flow meter 1 includes a body, as generally indicated
at 3 having a flow path or passage F therethrough with the flow
path having an inlet and an outlet. Body 3 comprises a main body
5 an inlet body 7 (at the left hand end of the flow meter as viewed
in FIG. 1), and an outlet body 9. Flow meter body sections 5 7
and 9 may, for example, be fabricated of a suitable metal alloy,
such as stainless steel or the like. It will also be understood
that inlet and outlet body sections 7 and 9 may be provided with
suitable fittings 8 so as to permit the flow meter to be readily
connected to a fluid circuit. As is conventional, bolts 10 (see
FIG. 11) extend through the inlet and outlet body sections 7 and
9 into threaded openings (not shown) provided in main body section
5 thereby to positively secure the body sections together. A cavity
11 is provided within main body section 5 for purposes as will appear.
As generally indicated at 13 a flow controller valve is provided
for regulating or modulating the flow of fluid (e.g., a gas) through
flow path F. The flow controller may be programmed in accordance
with circuitry (not shown) provided within a cover 15 secured to
the upper face of body 3 or may be remotely controlled by a process
controller or the like. An input/output cable (also not shown) electronically
connects the thermal mass flow controller 1 to remotely located
process control equipment in a manner well-known to those skilled
in the art. It will be appreciated that neither the construction
or operation of flow controller 13 nor the construction or operation
of the electronic circuitry of the mass flow controller of this
invention is, per se, a part of the present invention and therefore
a detailed description of the flow controller and the circuitry
will not, for the purposes of brevity, be herein provided.
More specifically, flow path F comprises an inlet passage 19 in
inlet body 7. A so-called inlet counterbore 21 is formed at the
outlet end of inlet passage 19. Likewise, an outlet passage 23 is
provided in outlet body section 9 and an outlet counterbore 25 is
provided at the inlet end (i.e., the left end) of outlet passage
23.
In accordance with this invention, an inlet bushing, as generally
indicated at 27 is received within inlet counterbore 21 and an
outlet bushing, as generally indicated at 29 is removably received
within outlet counterbore 25. As generally indicated at 31 a concentric
tube and rod flow rate sensor assembly is provided within flow passage
F and is supported by inlet bushings 27 and 29. More specifically,
flow rate sensor assembly 31 comprises an outer tube 33 having a
rod 35 (see FIG. 2) disposed concentrically therewithin. Bushings
27 and 29 constitute means for receiving and holding tube 33 and
rod 35 relative to flow meter body 3 and relative to one another
and for positively maintaining the rod in concentric relation with
the tube thereby to define a concentric annular flow passage, as
generally indicated at 39 (see FIG. 2), with this concentric annular
flow passage constituting the flow path F through cavity 11 within
main body section 5.
It will be understood that by maintaining rod 35 centered within
tube 33 and by positively maintaining the concentric annulus 39
uniform, repeatable, and generally linear measurements of the flow
rate of fluid flowing through flow path F can be reliably measured
or monitored by mass flow meter 1 of the present invention.
As shown in FIG. 1 flow rate sensor 31 includes a heating coil
HC (shown in phantom) surrounding at least in part a portion of
tube 33 with the heating coil being in heat transfer relation with
the tube. On the downstream and upstream sides of the heating coil,
temperature sensing coils SC1 and SC2 (also shown in phantom) are
provided in heat transfer relation with the tube 33. These coils
are bonded to the exterior of tube 33. As is well-known to those
skilled in the art, heating coil HC is energized so as to have a
predetermined thermal output and the temperature difference of the
fluid flowing through annular space 39 between the inner surface
of tube 33 and the outer surface of rod 35 may be determined by
the temperature sensing coils SC1 and SC2. In response to this thermal
differential of the fluid flowing through annular passage 39 circuitry
(e.g., a bridge circuit or the like) within cover 15 may be utilized
to generate a signal proportional to the flow rate of the fluid
flowing through the flow meter. Of course, this output signal may
be utilized to generate an error signal in appropriate control circuitry
(not shown) so that flow controller 13 may be modulated thereby
to regulate the flow of fluid to a predetermined flow rate.
As indicated at 40a, an O-ring seal is provided in a counterbore
in main body 5 so that O-ring 40a sealingly engages the outer surface
of tube 33 and inlet body 7 thereby to positively seal tube 33 relative
to body 3 and to insure that all fluid flowing through flow path
F flows through annular passage 39. Similarly, an outlet O-ring
seal 40b is provided within a counterbore in outlet body 9. When
outlet body 9 is securely bolted to main body 5 outlet O-ring seal
40b sealingly engages the outer surface of tube 33 main body 5
and outlet body 9 thereby to insure that all fluid exiting sensor
assembly 31 is directed into outlet passage 23 of flow passage F
without leakage. A heat sink 41 is provided in cavity 11 of main
body 5 and is in heat transfer relation with tube 33 downstream
from the heating and sensing coils and with main body 5. The heat
sink is held in place within main body 5 by a retainer R.
Referring now to FIGS. 2-4 inlet bushing 27 will be now described
in detail. It will be appreciated that inlet bushing 27 is identical
to outlet bushing 29 and thus only a detailed description of the
inlet bushing is required. As indicated at 42 a blind, concentric
opening or bore is provided in one end of bushing 27 for receiving
and socketing a respective end of tube 33. It is preferred that
opening 42 have a close tolerance, sliding fit with the outer surface
of the end of tube 33 so that opening 42 positively receives and
locates the tube in concentric relation with the bushing. A conical
socket, as indicated at 43 is concentric relative to opening or
bore 42 and the tapered socket converges toward the central longitudinal
axis of bushing 29 in a direction away from opening or bore 42.
As indicated at A, socket 43 is tapered at an angle, for example
30.degree..
Referring now to FIGS. 5 and 6 tube 33 is shown to have a predetermined
length, as indicated by dimension L1. The outer diameter of tube
33 is indicated by diameter D1 while the inner diameter of the tube
is indicated at D2. It will be understood that the tolerances of
the inner bore of the tube and the wall thickness of the tube are
formed to close tolerances. For example, diameter D2 (the inner
diameter of the tube) may be 0.120.+-.0.0005 inches and the wall
thickness of tube 33 is maintained at 0.0025.+-.0.0005 inches. Further,
the concentricity of the inner diameter of the tube is maintained
within 0.002 inches. Further, length L1 of tube 33 may, for example,
be 2.238 inches. As shown in FIG. 6 rod 35 has a length L2 of about
2.38 inches and has a diameter D3 of 0.103+0.000-0.001 inches.
Further, the outer surface of tube rod 35 is finished to have a
relatively smooth surface finish (as is the inner bore B of tube
33). Rod 35 has a concentricity of about 0.001 inches.
Rod 35 is provided with beveled ends 45 tapered to an angle A so
that they precisely mate with conical socket 43 provided in bushings
27 and 29 and such that the conical sockets matingly engage with
the tapered ends 45 of rod 35 so as to precisely locate and maintain
the rod in concentric relation with the bushings and so as to positively
maintain the rod concentric within tube 33. With tube 33 and rod
35 formed to the above-indicated dimensions, annular passageway
39 will have a generally uniform thickness of about 0.0085 inches.
It will be understood, however, that the above-given dimensions
for tube 33 and for rod 35 are merely by way of example for one
particular configuration of sensor 31. These dimensions may vary
considerably, depending on the flow rate ranges of the sensor, the
fluids being sensed, the temperature of the fluid being sensed,
and other variables.
Referring again to FIGS. 2-4 it will be seen that bushing 27 includes
a chamber 49 at the end of opening 42 so as to aid in the insertion
of the end of tube 33 into opening 42. Further, the bushing is provided
with a counterbore 51 on the end thereof opposite opening 42 with
counterbore 51 being of larger diameter than opening 42. Still further,
a plurality (e.g., 6 passages) of manifold openings or passages
53 extend generally longitudinally through the portion of the bushing
between counterbore 51 and the base of opening 42 with at least
a portion of each of the openings 53 being in communication with
annular passage 39 between the inner surface of bore B of tube 33
and the outer surface of rod 35 with these passages 53 constituting
a portion of flow path F. In this manner, the manifold openings
53 direct the fluid from inlet passage 19 through bushing 27 and
into annular flow passage 39. Further, the manifold openings 53
and outlet bushing 29 provide communication between the annular
passage 39 and outlet passage 23.
In accordance with this invention, flow meter 1 may be partially
disassembled so that sensor rod 35 may be readily removed from the
sensor assembly 31 for cleaning of the interior of tube 33 and further
such that the rod may be readily reinserted into the sensor tube
after the latter has been cleaned. The flow meter body may then
be reassembled such that the calibration of flow meter 1 is not
substantially affected by the repeated assembly and disassembly
of the flow meter. To this end, a compression coil spring 55 is
disposed between the outer end of inlet bushing 27 and the base
of inlet counterbore 21 so as to resiliently bias inlet bushing
27 together with sensor assembly 31 toward outlet bushing 29 and
so as to positively maintain both of the bushings in firm engagement
with the ends of tube 33 and so as to further maintain socket openings
43 in positive engagement with the tapered ends 45 of rod 35 thus
to maintain concentricity between the rod and the tube.
In order to disassemble flow meter body 3 for removal of rod 35
it is necessary to remove only bolts 10 joining inlet body section
7 to the main body section 5. With the inlet body section 7 removed,
one may manually grasp the one end of rod 35 and axially withdraw
the rod from sensor tube 33 substantially without disturbing tube
33 or sensor assembly 31. With the rod 35 removed, the tube may
be readily cleaned as by swabbing the interior bore of the tube
with a suitable solvent. Then, the rod 35 is reinserted into tube
33 so that the outlet end of the rod is again received in outlet
bushing 29. The inlet body section 7 is then positioned on main
body section 5 such that rod 35 is pocketed in socket 43 of inlet
bushing 27 and such that tube 33 is received in opening 42 of the
inlet bushing, as shown in FIG. 2. Then, securement bolts 10 are
reinserted and tightened. It will be appreciated that because inlet
seal 40a is positively maintained in place within main body 5 and
because outlet O-ring seal 40b is maintained within its groove in
outlet body 9 the seals are positively maintained upon reassembly
of the unit. Because of the relatively close tolerances of tube
33 rod 35 and bushings 27 and 29 the rod and tube are maintained
in concentric relation, even upon repeated assembly and disassembly.
Thus, the calibration of the flow meter is not adversely affected.
Still further, it will be appreciated that because bushings 27
and 29 are positively held within flow meter body 3 and because
the tube is positively socketed within openings 42 and further
because the tapered ends 45 of rod 35 are socketed within the concentric
socket openings 43 of the bushings, the rod is positively maintained
in concentric relation relative to the tube. Further, spring 55
positively maintains the ends of the rod and tube in proper engagement
with their respective bushings and thus the sensor assembly 31 is
not subject to the adverse effects of shock loading and the like,
such as may be experienced during shipping of flow meter 1.
Referring to FIGS. 7-10 another embodiment of the bushings utilized
to receive tube 33 and to hold rod 35 centered therewithin is shown,
this alternate bushing being indicated by reference character 27'
and with the "primed" reference characters shown in FIGS.
7-10 indicating structural features of bushing 27' similar in construction
and function as the corresponding features of bushing 27 heretofore
described. Among the primary differences between bushing 27 and
27' will be noted that in bushing 27 a plurality (e.g., six) of
axial manifold passages 53 are provided to provided communication
between inlet flow passage 21 and annulus 39 between the inner surface
of tube 33 and the outer surface of rod 35 and so as to provide
communication between the annulus and outlet passage 23. In bushing
27', the manifold passages 53' are shown to be radial passages.
However, it is to be understood that the function of radial manifold
passages 53' is similar to passages 53 heretofore described.
In view of the above, it will be seen that the other objects of
this invention are achieved and other advantageous results obtained.
As various changes could be made in the above constructions without
departing from the scope of the invention, it is intended that all
matter contained in the above description or shown in the accompanying
drawing shall be interpreted as illustrative and not in a limiting
sense. |