Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The presen-t invention relates in general to
pressure transducers and is concerned, more parti-
cularly, with a new and improved fluid-filled, direct-
sensing pressure transducer. Even more particularly,
the present invention relates to a pressure trans-
ducer adapted for high pressure measurements such as
in the range of 10,000 - 50,000 psi. Moreover, the
present invention relates to an improved method of
forming the pressure transducer and in particular of
interconnecting the transducer sensing element and
capillary tube.
The following is a list of prior art patents
that in general relate to pressure transducers: U. S.
Patents 3,349,623, 4,369,659, 3,678,753, 2,940,313,
2,627,749, 2,326,047, 3,336,555, 2,738,677. Some of
these patents are owned by the assignee herein and
show fluid filled pressure transducers.
U. S. Patents 2,949,313 and 2,627,749 both
illustrate pressure indicators that employ strain
tubes or the like for
'
; 1 -
,t ' ~
aO3 ~
transmitting pressure to a diaphragm on which pressure is
sensed by strain gages.
U~S. patents 3,326,047 and 3,336,555 illustrate pressure
transducers. U.S. patent 3,336,555 illustrates an unfilled
pressure transducer with strain gage sensing. U.S. patent
3,326,047 on the other hand illustrates a fluid filled pressure
transducer employing a pressure sensing capsule having inner
and outer deformable cylindrical walls. The liqu d-filling in
a transducer of this type has a relatively large volume and
thus creates limitations upon the range of pressure
measurements.
U.S. patent 2,738,677 is actually directed to a pressure
indicator particularly for combustion chambers such as in an
internal combustion engine. - -
Liquid-filled pressure transducers owned by the assignee
herein include U.S. patents 3,349,623; 3,678,753; and
4,369,659. The early patent 3,349,623 describes a device
employing an annular sensing chambe~ with associated strain
gages. U.S. patent 3,678,753 is believed to be an improved
form of the earlier version employing a top cap member and
associated disc-shaped compartment defined between the cap
member and the body of the instrument. The transducer
illustrated in U.S. patent 3,678,753 provided an increased
operating pressure range. U.S. patent 4,369,659 describes a
melt pressure transducer preferably for use in pressure
measurements associated with an injection molding machine and
employing a novel temperature compensating filler rod.
Another p-rior art patent is U.S. Patent 3,128,628 to'
Lebow. This patent illustrates a pressure transducer but does
not employ any capillary tube.
In all of the prior art, referred to hereinbefore one of
the main limitations, is the inability to opera~e particularl~
at high pressure levels such as in a pressure range of 10,000 -
50,00Q psi.
Accordingly, one important object of the present invention
is to provide a liquid-filled pressure transducer operating at
high pressu,re ranges.
Another object of the present invention is to provide an
improved pressure transducer that is fluid-filled and that
employs a reduced volume of fluid. This reduced volume of
fluid provides or minimization of temperature effects upon
pressure and furthermore makes for improved diaphragm
constructions.
Accordingly, a further object of the present invention is
to provide an improved liquid-filled pressure transducer in
which diaphragm stresses are reduced.
Still another object of the present invention is to provide
an improved fluid-filled pressure transducer tha't provides for
reduced snout diaphr,agm stresses during applied pressure and
upon exposure to elevated temperature.
Still another object of the present invention is to provide
an improved fluid-filled pressure transducer provided with a
snout piece of a different material, provided for the purpose
of temperature compensation and for improving overall
--3--
1~897~
performance. The snout piece permits the liquid-filled void to
expand at the same rate as the liquid itself during temperature
changes.
A further object of the present invention is to provide an
improved method of construction of a liquid-filled pressure
transducer, particularly an improved technique for
interconnecting the sensing element and capillary tube of the
pressure transducer.
Summary of the Invention
To accomplish the foregoing and other objects features and
advantages of the invention there is provided a pressure
transducer which comprises an elongated frame having an
elongated passage therethrough and a capillary tube extending
through the frame passage and terminating at one end adjacent
to one end of the frame. A coupler closes that one end of the
frame and defines with the frame a chamber in communication
with the capillary tube. A sensing member in accordance with
the invention is disposed about the capillary tube at the other
end of the frame and includes means defining an annular sensing
chamber and fluid communication with the capillary tube. For
this purpose there may be provided a passage transversely in
the capillary tube to enable fluid communication from the
capillary tube to this annular sensing chamber. The sensing
member also includes means defining a recess therein forming a
relatively thin wall adjacent the annular sensing chamber.
This wall has a pressure responsive sensing surface that
--'1--
- ~89~7~
extends substantially parallel to the capillary tube length.
The sensing of pressure may be accomplished by means of a
strain gage arrangement disposed on the pressure responsive
sensing surace of the relatively thin wall. The strain gage
sensing means may be connected in a bridge arrangement. In
accordance with one feature of the invention there is
preferably also provided in the transducer at the diaphragm end
thereof a snout piece of a different material than that of the
frame of the transducer having a relatively low coefficient of
expansion for providing temperature cornpensation.
Furthermore, in accordance with the present invention there
is provided, in a liquid-filled pressure transducer having a
~rame, capillary tube extending through the frame, and a
sensing element having a bore for receiving the capillary tube,
an improved method of securing the sensing element to the
capillary tube in a liquid-tight manner. The method-comprises
the steps of providing a weld prep indentation in the sensing
element adjacent and about the sensing element bore, inserting
the capillary tube in the sensing element and swaging the weld
prep indentation so as to grip the capillary tube with a~ least
a small gripping force. Finally, the area about the capil~ary
tube is welded in the area where the weld prep indentation is
disposed so as to form a liquid-tight well between the
capillary tube and sensing element. The annular indentation is
at least partially formed by an inner ridge adjacent the
capillary tube and in accordance with the step of swaging the
weld prep indentation, this step includes distortion of the
- ~8~
indentation to deflect the inner ridge so as to contact the
capillary tube about the circumference thereof. The formation
of this weld for securing the capillary tube and sensing
element is carried out at both ends of the sensing element
through which the capillary tube extends~
Brief Description of the Drawings
Numerous other objects features and advantages of the
invention should now become apparent upon a reading of the
following detailed description taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a cross-sectional side elevation view of a
preferred embodiment of the fluid filled pressure transducer
constructed in accordance with the present invention;
FIG. 2 is.a cross-sectional view taken along line 2-2 of
FIG. 1 showing further details of the sensing member;
FIG. 3 is a cross-sectional view taken along line 3-3 of
FIG. 2 showing further details;
FIG. 4 is a cross-sectional view taken along line 4-4 of
FIG~ 2 showing still further details o~ the sensing member;
FIG. 5 is a schematic diagram illustrating the strain gage
circuit employed in connection with the pressure transducer of
the invention;
FIG. 6 is an enlarged cross-sectional view of a portion of
a pressure transducer constructed in accordance with the
invention and illustrating the further-use of a snout filler
piece;
6--
.. . . . . . . .
9~
FIG. 7 is a graph illustrating input pressure versus
pressure loss through the diaphragm;
FIG. 8 illustrates the fixture that is employed along ~ith
the method of assembly for interconnection of the capillary
tube and sensing element;
FIG. 9 is a cross-sectional view showing a first step in
the method of assembly of the capillary tube and sensing
element;
FIG. 10 is a fragmentary cross-sectional view illustrating
a subsequent step in which the weld prep indentations have been
swaged against the capillary tube to grip it; and
FIG. 11 is a further cross-sectional view showing
subsequent steps including an illustration of the weld that is
formed annularly about the capillary tube and between the
capillary tube and sensing element.
Detailed Description
Reference is now made to FIGS. 1-4 ~hich show c-omplete
details of one embodiment of a pressure transducer in
accordance with the invention. FIG. 5 shows the schematic
diagram of the strain gage interconnections. FIG. 6 is an
alternate embodiment from the standpoint of illustrating the
preferred use of a filler piece at the diaphragm end of the
transducer. FIG. 7 is a graph of illustrating transducer
performance. FIGS. 8-11 illustrate steps in the method of the
present invention concerned in particular with the securing of
the capillary tube and sensing element in a liquid-tight manner.
Wlth reference to FIGS. 1-~, the pressure transducer
~89~7V
includes a main frame lO, a sensing member 12 proltided at the
top of the frame lO, a capillary tube 14 which extends t~rou~h
the frame, and a diaphragm coupler 16 secured to enclosing the
bottom end of the frame.
The lower section of the frame 10 is constructed in a
similar manner to the construction illustrated in U S. patent
3,678,753. Basically there is an elongated passage 18 that
extends through the main frame and which is for accommodating
the capillary tube 14. The caplllary tube 14 at its bottom end
terminates at a relatively small chamber 20 which is closed by
the diaphragm 16.
At the top end of the frame 10 there is included as part of
the frame a top piece 22 through which the capillary tube 14
extends. The top piece 22 is for supporting the sensing member
12 in the position illustrated in the drawing. The sensing
member 12 may be secured in position within the top piece 22 by
being welded to the top piece.
The capillary tube 14 as noted in, for example, FIG. 3,
extends through a vertical passage in the sensing member 12.
It is noted that in FIG. 3 that the capillary tube 14, in
accordance with one embodiment of the invention, is provided
with an annular recess 26 that defines an annular chamber 28
essentially defined by the recess in the capillary tube and the
inner bore of the passage that extends vertically through the
sensing member 12.
In order to provide fluid communication from the capillary
tube to the chamber 28 there is provided a transverse passage
~397~
30 which is disposed in the position as illustrated in FI5. 3.
The sensing member 12, as indicated previously, includes
vertical passage througA which the capillary tube 14 extends.
As illustrated in FIG. 1, the capillary tube 14 also ex'cend~
beyond the top of the sensing member 12 and is provided at its
top end with some type of a filler cap 32. The sensing member
12 is furthermore constructed by machining a single flat
surface on the outside of its cylindrical body. rrhis is
illustrated in FIGS~ 1-3 by the recess 34 that extends in the
vertical direction in FIG. 3 somewhat less than one third of
the total height of the cylindrical sensing member. As
indicated previously, the cylindrical sensing member is
through-drilled along its full length at its center axis and
fitted closely to the capillary tubing which passes
therethrough. The capillary tubing is preferably TIG welded at
both ends as illustrated by thé welds 38 in FIG. 3O The ends
of the sensing member 12, such as at surface 40 in FIG. 3, are
machined to form weld-preparation surfaces including the cup
shaped indentations so as to aid in maximizing weld penetration
and strength between the capillary tube and the sensing member.
Prior to insertion of the capillary tube, a .015 ~o .020
inch diameter lateral hole 30 is drilled through the center of
the capillary to allow fluid communication between the
capillary tube and the annular chamber 28. The hole 30 may be
drilled approximately 3 inches from one end of the sensing
member and the relative position of the capillary tube and the
sensing member are arranged so that the hole is placed as
... . . . .. . . .. . . . . . . . .
~8~
indicated in FI~. 3 at about the mid distance of the recess
34. The capillary is, o course, welded at both ends of tne
sensing element as illustrated in FIG. 3 to ~orm essentially
upper and lower liquid leak-tight joints.
Hydraulic pressure from inside of the capillary tube bore
is transmitted to the annular sensing element by way of the
hole 30. This fluid communication enables the inside sur~ace
of the sensing member to be pressurized.
The machined recess 34 defines a relatively thin wall 46,
the thickness of which is perhaps somewhat exaggerated in FIG.
3. The wall 46 has a pressure responsive sensing surface 48 to
which the strain gages are secured. Hydraulic pressure under
the machined flat essentially at the annular chamber Z8 creates
high bending stresses across the thin flat wall 46 where the
active strain gages are attached. This action senses the
strained surface along the center line axis. In this regard,
refer to FIG. 2 which illustrates, along the center line o~ the
capillary 14, the active strain gages 51 and 53. Also note in
FIG. 2 the other strain gages 52 and 54 which, with the strain
gages 51 and 53, provide the total strain gage circuit.- The
strain gages 52 and 54 may be considered as the inactive- strain
gages but do sense some compression strains to add to the
electrical sensitivity. However, the strain gages 52 and 54
are used primarily to complete the Wheatstone bridge and to
provide thermal compensation.
- FIG. 2 also shows, associated with the strain gages 51-54,
electrical interconnection tabs. These include tabs 58 and 59
--10--
1~897~ -
to the left in FIG. 2 and tabs 60, 61 and 62 to the right in
FIG. 2. These connection tabs are connected with the strain
gages so as to connect the strain gages in the pattern
illustrated in FIG~ 5. In FIG. 5 the same reference characters
are employed to identiEy the same strain gages illustrated in
FIG. 2. Thus, the circuit interconnection of FIG. 5
illustrates the strain gages 51 - 54 schematically represented
as variable resistances, varying with applied pressure. These
resistances are interconnected in a bridge arrangement as
illustrated in FIG. 5 having excitation inputs at terminals 64
and 65 and having a signal output at terminals 66 and 67.
An electrical input signal is usually applied across the
input terminals 64 and 65 and the pressure responsive voltage
is measured across the outpu~ terminals 66 and 67. Once again,
the active gages are gages 51 and 53 and thus the majority of
the pressure responsive signal is generated by these legs of
the bridge. The gages 52 and 54 primarily provide for
temperature compensation so that if there is a change in
temperature at the sensing member, the bridge is automatically
adjusted by virtue of the temperature of both of the nonact~ve
gages causing essentially a nulling of the bridge.
One of the improved features in accordance with the present
invention is the reduced amount of liquid fill that is employed
in the transducer~ This drastically reduces the fill
displacement and thus controls pressure induced deflections.
This is thus successful in minimizing diaphragm stresses and in
enabling the use of thicker diaphragmsO The reduction in fill
~89~770
is carried out by virtue of -the use of the sensing
member 12 with its substantially minimal fill volume.
The volume is also minimized by virtue of cther struc-
tural elements that are employed including the rela-
tively small snout chamber and capillary tube.
Reference is now also made to FIG. 6 which
shows an enlarged fragmentary view illustrating a
preferred embodiment for the snout end of the device
illustrating the snout filler piece 70 which is pri-
marily used for temperature compensation reasons.
This piece may be constructed of the alloys Kovar*
or Invar**. Alternatively, this may be constructed
of any very low coefficient of expansion material.
It is preferred that it have a coefficient of expan-
sion much less than that of the stainless steel frame.
This fill piece 70 allows the void in chamber 20 to
expand at the same rate as the liquid during tempera-
ture changes. The filler piece 70 illustrated in FIG.
6, functions to reduce internal fill pressure as the
; 20 tip is heated with no applied pressure.
There have been at least two different
liquid fills that have been employed, one being
mercury and the other sodium potassium (NaK). The
NaK filled transducer is a lower pressure range
transducer while the mercury transducer has a pres-
sure range of 10,000 - 50,000 psi. The range of
pressure operation when using a NaK fill is 10,000 -
15,000 psi.
Thus, the filler piece, in combination with
the chamber 20 illustrated in FIG. 6, provides a
thermally compensated internal volume which is
desired. Also, the filler piece as
* A trade mark of Westinghouse Electric and
Manufacturing Company
** A trade mark of Carpenter Steel, a Division of
Carpenter Technology Corporation
- 12 -
, ,:
~ ~ 8 ~
indicated previously is selected to have the~mal properties
that temperature compensate the transducer for differential
thermal expansion coefficients between the internal fluid which
is preferably mercury and the main body of the transducer which
is usually stainless steel. As indicated previously, it is
preferred to have a low temperature coefficient of expansion
particularly in comparison with that of the frame material.
One of the advantages that has been realized with the
improved sensing technique of the present invention is the
capability of now being able to increase the thickness of the
diaphragm 16 while reducing the stresses thereof. It has also
been possible in accordance with this construction to minimize
overall thermal characteristics. Reducing fill displacement
under pressure reduces the diaphragm deflection and the
resulting stresses allows the diaphragm to be increased in
thickness within certain limitations to improve insitu
durability.
In connection with the above, the maximum diaphragm
thickness is related to two independent effects including
internal fluid displacement under applied pressure and secondly
internal pressure generated by thermal expansion of the
contained ~luid fill. In accordance with the invention, the
sensorls small internal volume reduces the amount of fill and
its resulting compression. Moreover, the sensor's low
displacement or deflection under pressure further reduces total
fluid displacement and resulting stresses. Moreover, the
filler piece is adapted to control thermally induced internal
39~
pressure that also further reduces stresses. The combined
improvements outlined above allow diaphragm thickness to be
increased without increasing stresses and controls internal
pressure effects versus temperature. The snout filler piece 70
is used in essence to replace some of the stainless steel at
the tip of the instrument with a low temperature coefficient of
expansion material thus temperature compensating the void 20.
This preferred ~aterial as indicated previously, is Kovar or
Invar.
~ ith regard to the reduction in volume, it is noted that
the internal volume of the sensing member 12 is very small
because the capillary tube fills the passage through the member
with ver~ close tolerance fit. As a matter of fact, in the
illustration of FIG. 3, there is shown a relatively predominant
recess 26 in the capillary tube. However, in another
embodiment that may be employed, the capillary tube need not be
recessed at all but instead one can rely upon the slight
difference in diameter between the bore of the member 12 and
the outer diameter of the capillary tube. In such case, the
annular sensing chamber about the capillary tube actually
extends between top and bottom ends of the sensing member from
weld-to-weldl
By way of example, the amount of volume of liquid in the
capillary tube may be approximately 75 percent of the volume in
the annular space about the capillary tube. The total volume
within the entire transducer including the capillary tube may
be in a range from 1.0 x 10 3 to 2.0 x 10 3" cube. This
-I4-
7~
very small volume compares with a transducer volume on the
order of 3~2 x 10 3n cube for a transducer which is of the
type described in U.S. patent 3~6781753. It can be readily
seen that there is an improvement in the reduction of volume
fill by at least ~ to 1 between the volumes of the transducer
in the prior art and that in the present construction. With
regard to the capillary tube, the inner diameter thereof can
range from 0.005 to 0.010", and the outer diameter thereof can
range from 0.060" to 0.25".
With regard to the filling of the transducer, this is
accomplished at the top end of the capillary tube. In FIG. 1
the capillary tube is shown capped off but before this occurs
the capillary tube and the rest of the voids communicating
therewith in the device are filled with say mercury under
forced pressure so that all of the void areas are filled with
the mercury. The capillary tube is then sealed off to retain
the mercury in the transducer.
Reference is now made to FIG. 7 which is a graph of input
pressure being sensed at the diaphragm versus the pressure lost
at the snout diaphragm. There are actually two curves
illustrated in FIG. 7. The curve Cl is a pressure curve for a
prior art transducer such as the one described in U.S. patent
3,678,753. The curve C2 is the pressure curve for the
transducer in accordance with the present invention. It is
noted that even at high pressure ranges in the area of 40,000
psi that the absorbed pressure is only on the order of about
150 psi; consequently, diaphragm thickness can be increased.
-15~
7~
Increased thickness creates higher absorbed pressures, but its
increased thickness will accommodate the higher pressures
without generating higher stresses than those illustrated in
curve Cl of FIG. 7.
Reference is now made to FIGS. 8-ll in connection with the
teaching of the method of assembly of the transducer of the
present invention and in particular a method of securing
together the capillary tube and sensing element. In this
connection previously, two separate embodiments have been
described, one in FIG. 3 employing an annular chamber 28 about
the capillary tube and another embodiment in which one relies
upon a difference in diameter between the bore of the member 12
and the outer diameter of the capillary tube. However, it has
been found that the capillary tube should not be too closely
fitted in the bore of the sensing element. If the fit is too
tight, then large non-linear calibration errors can occur due
to capillary unevenness, bends in the capillary tube, and
possible burrs at the drilled hole. This may cause the
capillary tube to contact the bore under the strain gages
causing these large non-linear calibration errors. The
technique described in FIGS . 8-11 overcomes these problems.
FIG. 8 shows the fixture that is used in carrying out the
method of the present invention. This fixture includes an
upper mandrel 74 and a lower mandrel 76. The upper mandrel 74
has a bore for receiving a portion of the capillary tube and
also has a channel for receiving the upper crimp block 78.
Similarly, the lower mandrel 76 has a channel for receiving the
-16-
lower crimp block 80. Both of the crimp blocks 78 and 80 have
passages therethrough for receiving the capillary tube in the
manner illustrated in FIG. 8. FIG. 8 also shows the capillary
tube 14 having been inserted in the bore of the sensing element
12.
Now, in FIG. 9 there is illustrated in a fragmentary
cross-sectional view, the capillary tube 14 and the sensing
element 12. The fitting between the capillary tube and the
sensing element is such ~hat there is a gap G illustrated in
FIG. 11 that is on the order of 0.001". FIG. 9 also
illustrates the weld prep indentation 82 which is an annular
indentation disposed about the circumference of the bore in the
sensing element. This indentation defines on the inner side
thereof, an annular ridge 84 as illustrated in FIG. 9. In FIG.
9 the upper mandrel 74 is disposed over the sensing element, .
but is not yet in contact therewith.
FIG. 10 illustrates the upper mandrel 74 having been moved
in the direction of arrows 85 so as to contact the indentation
82 and essentially crimp or swage the ridge 84 against the side
of the capillary tube as illustrated in FIG. lO. .FIG. 10
illustrates only.one of the mandrels 74. However, it is
understood that the operation is such that both mandrels are
brought together at the same time crimping the weld prep
indentations on either end of the sensing element 12.
After the capillary tube has been inserted through this
sensing element into the position illustrated in FIG. 8 and
after the weld indentations have been cr-imped as illustrated in
J -17-
.
FIG. 10, ~hen welding can occur as illustrated in FIG. 11. In
connection with the swagin~ or crimping, this is preferably
controlled so that the force against the capillary is just
enough to hold the sensing element to ~he capillary tube
without it slipping up and down.
With regard to the welding, reference is made to FIG. 11
which shows the weld tip 88 directed at the juncture where ~he
crimping has occurred at the ridge 84 abo~t the periphery of
the capillary tube. FIG. 11 also illustrates the final weld at
90. This is, of course, an annular weld that goes about the
entire circumference of the capillary tube providing a
liquid-tight seal thereabout so that the liquid fill within the
transducer of course, could not be expelled at that location.
The liquid is meant to be held in the annular chamber extending
about the capillary tube in between the inner bore of the
sensing element and the capillary tube.
Having now described a limited number of embodiments of the
present invention, it should now be apparent to those skilled
in the art that numerous other embodiments and modifications
thereof are contemplated as falling within the scope of the
pre.sent invention as defined by the appended claims.
-18-
