Note: Descriptions are shown in the official language in which they were submitted.
10841i7
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~ Background of the Invention
.~ This invention relates to an improved method and
apparatus for the RF measurement of conductive liquid levels in
a vessel, and in particular, to the measurement of flow rates
~ . -
through flow channels by measuring the level of the conductive
`i liquid in a flow channel such as a flume or weir.
, United States patent 3,269,180 which issued in August 1966 to Schreiber discloses a sensing element or probe for
measuring the flow rate through a flow channel. In order to
properly correlate the flow rate with the head height level of
liquid in the flow channel, the probe electrode of the Schreiber
patent is characterized such that connection of the probe to an
appropriate electronic unit will produce an output which is
linear with the flow rate.
In probe electrodes of the type shown in the Schreiber
patent, accumulation of a coating is a very substantial problem.
For example, if the coating accumulates on the probe of Figure
; lb where the conductive backing or guard shield is connected
to ground, the capacitance of the coating will be resistively
coupled around the sides of the probe to ground thereby producing
an erroneous reading of the head height and thus the flow rate.
Another very significant problem with probes of this
type is that water or other conductive fluids flowing through
; the channel will permeate the insulation of the probe and this
permeation is greatly increased with the temperature of the
liquid if the bond between the probe electrode and the insulation
is not tight, the liquid will permeate the insulation and
delaminate the probe electrode and insulation. This in turn
creates voids or air gaps at the electrode which will adversely
affect the level measurements and create distortion and curling ~;
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1~84117 ~-
of the probe.
In United States patent 3,781,672 - Maltby et al and
3,706,980 - Maltby, both of which are assigned to the assignee
of this invention, systems are disclosed for immunizing capa-
citance measuring probes from the effects of coatings. This is
accomplished by providing a guard shield which is exposed to
the materials being measured and driven at the same potential as
the probe electrode so as to maintain the accumulated coating
at substantially the same potential as the probe electrode and
thereby eliminating its effects on any capacitance measurement.
However, the guard element of Figures lb and lf of the Schreiber
patent could not be driven at the same potential as the probe
electrode where the probes are mounted on the wall or an other -
wise grounded support member of the flow channel since the
guard element would be grounded. Even if it could be driven at
the potential of the probe electrode, this would not eliminate
the adverse effects of the coating since the driven guard
electrode which is at the rear of the probe would not be
closely coupled to the coating at the front of the probe due
to the presence of a rather thick insulation from back-to-front
of the probe. As a result, the capacitance of the coating
would be resistively coupled to the wall of the flow channel
which is effectively coupled to ground through the conductive
liquid in the flow channel and would thereby enter into the
capacitance measurement.
United States patent 3,729,994 which issued on May 1,
1973 to Klug, like the Schreiber patent, discloses a curved and
characterized probe for measuring the flow rate through a flow
channel. However, unlike the Schreiber patent, the Klug patent
does not disclose a conductive backing or a guard electrode of
1~841~ :
; any kind other than a dielectric medium intended to immunize
the probe electrode from any changes in capacitance through the
- rear of the probe. The probe electrode is insulated from the
conductive liquid within the flow channel at the front of the
probe by *Teflon, presumably of sufficient thickness so as to
avoid "cut-through" by the materials and debris flowing in the
flow channel. However, Teflon has a relatively low dielectric
constant of approximately 2.2 which would provide less than
the optimal capacitive coupling of any coating to a guard elec-
trode if a sufficient *Teflon thickness were utilized to avoid
"cut-through".
The Klug patent also discloses a probe electrode com-
prising a metallic woven wire mesh which is embedded in a poly-
ester reinforced glass fiber conduit. Typically, the probe
electrode would be embedded in the glass fiber while the glass
fiber is in a liquid state (as by a spray-on process) and there
would be no heat curing of the probe. As a result, the bond
or lackofabond between the mesh and the glass fiber permits
the formation of tiny voids which collect water. The nature of
the mesh is not specified, e.g., the gauge of the mesh and size
~;~ of the openings are not specified.
United States patent 2,852,937 which issued on -
September 25, 1958 to Naze discloses a probe adapted to be
mounted on the wall of a container for measuring the level of a
;~ conductive liquid within the container. The probe includes a
probe electrode and a shield electrode located behind and ex-
tending somewhat laterally outwardly beyond the lateral extrem-
ities of the probe electrode. However, the shield electrode is
not closely capacitively coupled to the conductive liquid. The
insulation itself comprises *Teflon which, in combination with
the spacing of the shield from the surface of the insulation,
*Trade Mark
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substantially precludes any close coupling of a coating to the
shield.
United States patent 3,324,647 which issued on June 13,
1967 to Jedynak discloses a pair of isolator plates behind and
extending laterally outwardly beyond a probe electrode. There is
no suggestion that either of the isolator plates is driven at the
same potential as the probe electrode nor is there any suggestion
of a close coupling between a coating on the surface of the probe
and the isolator plates.
Other prior art techniques involve the use of a mesh
embedded in a thermoplastic material for use as a conveyor belt.
Summary of the Invention
It is an object of this invention to provide an
improved method and apparatus for measuring the level of a
conductive liquid in a vessel or container.
It is a more specific object of this invention to
provide an improved method and apparatus for measuring level or
flow rates of a conductive liquid or other fluid in a vessel or
flow channel such as a weir or flume.
It is a further specific object of this invention to
provide an improved method and apparatus for measuring the level
of the fluid in a vessel where the measuring probe may be mount-
ed flush with the wall of the vessel so as not to adversely
affect or be adversely affected by the flow or movement of the
fluid in the vessel.
It is a still further specific object of this inven-
tion to provide an improved method and apparatus for measuring
~ .
the level of the fluid in a vessel where the measuring probe
is mounted flush with the wall of the vessel so as not to collect
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fibrous materials at the probe which may adversely affect the
measurement or adversely affect the flow of the fluid.
It is a further object of this invention to provide an -~
improved method and apparatus wherein the probe may be mounted ;
on the wall of the vessel without any adverse effects from an ; -
accumulated coating of the fluid on the probe. -
It is a further object of this invention to provide an
improved method and apparatus wherein the probe generally con-
forms with the curve at the wall of the vessel or flow channel.
It is also an object of this invention to provide an
improved method and apparatus wherein the probe may be readily
installed within an existing vessel or flow channel.
I~ is also an object of this invention to provide an ~ `~
improved method and apparatus wherein changes in the dielectric
constant of the insulation material in the measuring probe will
not adversely affect the measurement of a conductive liquid
level.
' It is a further object of this invention to provide
an improved method and apparatus wherein the probe is capable -
of detecting the presence of an unwanted insulating coating on
the probe and eliminate the effects of that coating on the level
measurement. ~ ;~
It is a further object of this invention to provide
an improved method and apparatus wherein the foregoing objects
are achieved without sacrificing the ability of the probe to
resist the abrading effect of materials which may pass through
a flow channel.
It is also an object of this invention to provide an
improved method and apparatus wherein a probe may be readily
calibrated.
1~341~
In accordance with these and other objects, an RF probe
system includes a probe adapted to be mounted against a surface
in a vessel for measuring the level of a substantially conductive
liquid therein where the surface when covered with a coating of
the liquid is effectively grounded through the conductive liquid.
The probe comprises a conductive probe electrode extending
longitudinally along the probe adjacent the front thereof and
conductive guard electrode means extending longitudinally along
the probe. The guard electrode means includes lateral portions
extending laterally outwardly beyond the lateral extremities of
the probe electrode. Interior solid insulation means are
located between and insulate the probe electrode and the rear
portion of the guard electrode. Exterior solid insulation means
cover the conductive guard electrode means and the conductive
probe electrode such that the conductive liquid is closely
capacitively coupled to the probe electrode at the lateral
portions of the conductive guard electrode means. Preferably,
the coupling between the lateral portions of the guard electrode
and the conductive liquid is achieved by permitting the lateral
portions to extend laterally outwardly a distance of at least
six times greater than the thickness of the solid insulation~
means covering the guard electrode means divided by the di- -~
electric constant of the solid insulation means. The probe
system further comprises means for maintaining the potential of
the guard electrode means at substantially the same potential
as the probe electrode.
In accordance with this invention there is provided a
probe system for measuring the level of a liquid comprising: a
vessel having a wall; a probe including a conductive probe
electrode extending longitudinally along the probe and insul-
ation covering said probe electrode; and track means in the
vessel adapted to removably receive and guide the probe along a
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predetermined path.
In accordance with another aspect of this inventionthere is provided a method of making a probe for measuring fluid
levels comprising the following steps: applying conductive sur-
faces to opposite sides of an insulating substrate; etching one
of said surfaces so as to form a longitudinally extending probe
electrode between a pair of guard electrodes on one of said
sides of said substrate; and applying insulation to the exterior
of said conductive surfaces.
In accordance with another aspect of this invention
there is provided a probe for use in RF liquid level measuring ;
systems comprising: a conductive probe electrode extending
longitudinally along the probe; conductive guard electrode means
including lateral portions extending laterally outwardly beyond
the lateral extremities of said probe and a rear portion extend-
ing behind said probe electrode means; interior insulation means
located between said probe electrode and said rear portion of
; said guard electrode means; and exterior insulation means con-
tacting said probe electrode and said lateral portions of said
guard electrode and forming an exposed frontal surface of the
probe adapted to directly contact the liquids; said lateral
portions extending laterally outwardly from the lateral extremi-
ties of said probe electrode a substantially greater distance
than the minimum thickness of said exterior insulation means in
contact with said probe electrode between said lateral
extremities.
In the preferred embodiment of the invention, the guard
electrode comprises a rear portion juxtaposed to and disposed
behind the probe electrode.
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In order to permit the probe to be utilized in measuring the flow
rate of the liquid through a flow channel, the probe electrode may be char-
acterized so as to have a frontal lateral dimension increasing with increasing
distance from one longitudinal end of the probe. The lateral portions of the
guard electrode means then extend laterally outwardly beyond the lateral
extremities of the probe electrode a greater distance with decreasing distance
from one longitudinal end of the probe.
In accordance with other specific objects of the invention, the
system comprises track means having a longitudinal groove for receiving the
probe and permitting movement of the probe through the groove. The track
which may be flexible can be mounted on the wall or other support member of
the vessel or flow channel. In the alternative, the track may be formed with- `
in the wall or support member. The track serves the dual function of providing -
support for a probe which is preferably very thin while also permitting removal
of the probe from the vessel for replacement, repair or calibration purposes. - ;
In a particularly preferred embodiment, the track extends along a
curved portion of a flow channel such that the probe itself is curved to con-
form with the curvature in the wall of the flow channel. In order to accom-
modate this curvature, the probe may comprise a flexible laminated structure
which may be formed by appropriately etching a conductive surface on one side
of an insulating substrate so as to form the probe electrode and lateral
guard electrodes while the conductive surface on the opposite side of the
' insulating substrate serves as the rear guard electrode. The exterior insula-
tion may comprise two sheets which are heat sealed along the lateral extremi-
ties thereof.
In further accordance with the objects of the invention, the
; exterior insulation means may comprise a high molecular weight polymer of
vinylidene fluoride having a dielectric constant in excess of 4 so as to assure
close capacitive coupling of a conductive coating at the lateral po~tions of
the guard electrode means while at the same time providing resistance to
abrasion in a flow channel.
In further accordance with this invention, the system may comprise
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adjustable mounting means for use in mounting a probe in various positions
on a surface or a wall of the vessel. The mounting means may permit removal
of the probe therefrom for calibration purposes and replacement of the probe
on the mounting means in a predetermined position. In addition, indicia may
be longitudinally placed along the probe under a transparent outer insulation
so as to assist in the calibration. -
: ~ - .
In accordance with certain other objects of the invention, the
probe system may comprise means for heating the probe for assisting in the
; dispersal of a coating such as grease which may build up on the exterior of
the probe. The means for heating may comprise the guard electrode itself
which may have a generally serpentine shape so as to serve as a resistor. In
the alternative, a separate heating element may be utilized at the rear of
the probe adjacent the rear portions of the guard electrode means. Where the ;
probe is inserted into the track, the heating means may be sandwiched between
the base of the longitudinal groove in the track and the probe.
In accordance with certain other objects of the invention, addition-
al electrode means may be provided in the probe and covered by the exterior
insulation for measuring changes in the dielectric constant of the exterior
insulation means. Such an additional electrode means in combination with
suitable compensating means will eliminate~ the effects of any change in the
dielectric constant of the exterior insulation means on the measurement of the
liquid level.
,
It is a further object of this invention to provide a method and
apparatus wherein undesirable voids or air gaps between a probe electrode
; and insulation of the probe are substantially eliminated.
It is a more specific object of this invention to provide a method
and apparatus wherein any delamination between a probe electrode and the probe
insulation which would create such air gaps is substantially eliminated.
It is a still more specific object of this invention to provide a
method and apparatus wherein the delamination between the probe electrode and
the probe insulation is substantially eliminated even when the probe is
submerged in hot water for a long period of time.
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108411`7
It is a still further specific object of this invention to provide
à method and apparatus wherein a uniformity in the laminated probe is achieved.
It is another specific object of this invention to provide a method
and apparatus wherein distortion and curling of the laminated probe are avoided.
In accordance with these and other objects of the invention, the
probe comprises conductive probe electrode means extending longitudinally along
the probe and having a multiplicity of openings extending through the probe
electrode means from one side to the other. Heat cured insulation means extend
longitudinally along the probe electrode means on opposite sides thereof. The
insulation means extends through the openings and applies compressive forces
to the electrode means over the surface area thereof.
In accordance with this invention, the compressive forces on the
probe electrode means should exceed 100 psi and preferably exceed 500 psi.
In order to obtain the desired compressive forces, the average mini-
mum thickness of the insulation means as measured from the probe electrode
means between adjacent openings and the exterior surface of the insulation
means is greater than 10% of the average minimum spacing between openings in
the probe electrode. Preferably, the average minimum thickness is greater than
the average minimum spacing with the maximum compressive forces achieved when
the average minimum thickness is at least equal to one-and-one-half the average
minimum spacing.
In order to assure the desired compressive forces, the probe elec-
trode means must be surrounded by sufficient insulation. This requires a
certain minimum thickness as measured from the probe electrode means to the
exterior surface of the insulation means. Similarly, it requires a certain
minimum cross-sectional dimension of the openings in the probe electrode means.
However, the average maximum cross-sectional dimension of the openings in the
probe electrode means should be less than twice the average minimum thickness
of the insulation so as to assure the proper fringing effect.
In-a particularly preferred embodiment of the invention, the probe
electrode comprises a plurality of conductive filaments. These filaments may
be woven into a mesh. In another embodiment of the invention, the probe
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electrode means comprises a sheet of conductive material including a multi-
plicity of etched openings where the edges of the openings are rounded off.
In order to assure that adequate compressive forces are attained,
it is important that the heat curing temperature exceed the operating tempera-
ture of the probe. In this connection, a heat curing temperature in excess
of 100F. is desirable and preferably a temperature in excess of 212F.
In a particularly preferred embodiment of the invention, the insula-
tion means comprises a thermoplastic material. The thermoplastic material may -~
comprise a fluorocarbon resin which is a crystalline, high molecular weight i~ ~;
polymer of vinylidene fluoride having a dielectric constant in excess of 4.
When such a thermoplastic material is utilized, the curing temperature will
be at least 370F. which represents the gel point for the material.
In accordance with another important aspect of the invention, pres-
sure is applied to the insulation material prior to heating to the curing ;
temperature. In a particularly preferred embodiment of the invention, this
pressure is achieved by evacuating any air between the insulation means and
the probe electrode. The fluid pressure is then applied to the insulation
means.
Brief Description of the Drawings
Figure 1 is a top plan view of a flow channel having a restriction -
therein with flow rate sensing probes mounted in a channel;
Figure 2 is a side view of the flow channel of Figure l;
Figure 3 is a sectional view of a flow rate sensing probe construct-
ed in accordance with this invention;
Figure 4 is a frontal view taken along line 4-4 of Figure 3;
Figures 5(a-g~ are schematic representations of probes and probe
potentials utilized in explaining the advantages of a probe such as that shown
in Figures 3 and 4;
Figure 6 is a sectional view of another sensing probe constructed
in accordance with this invention;
Figure 7 is a frontal view taken along line 7-7 of Figure 6;
Figure 8 is a sectional view of a track for mounting the probe of
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~084117
Figures 6 and 7;
Figure 9 is a frontal view of the track shown in Figure 8 mounted on
the wall of a flow channel;
Figure 10 is a sectional view of another sensing probe constructed
in accordance with this invention;
Figure 11 is a frontal view taken along line 11-11 of Figure 10;
Figures 12(a-e) depict a method of making the probe of Figures 10
and 11; `~ :~
Figure 13 is a sectional view of the probe of Figures 10 and 11
mounted in a track attached to a flow channel;
Figure 14 is a sectional view of the probe of Figures 10 and 11
mounted in a track cast within the wall of a flow channel;
Figure 14a is a sectional view of the probe of Figures 10 and 11
mounted in a track cast from the wall of the flow channel itself;
Figure 15 is a sectional view of the probe and track in Figure 13
mounted on a mechanical support within a flow channel;
Figure 16 is a sectional view of the probe of Figures 10 and 11 with
an integral heating element;
Figure 17 is a sectional view of the probe of Figure 16 mounted in
a track adapted to be mounted on the wall of the flow channel; .~
Figure 18 is a sectional view of an alternative guard electrode for -
the probe of Figures 10 and 11 which may be utilized as a heating element;
Figure 19 is a sectional view of a probe similar to that of Figures
10 and 11 including an additional electrode measuring variations in the di-
electric current of the exterior insulation;
Figure 20 is a sectional view of means for mounting a probe in a
precalibrated position on the curved wall of a flow channel;
Figures 21(a~b) are schematic representations utilized in explaining
the dual characterization of probes mounted along a curved wall of a flow
channel;
Figures 22 and 23 are sche~atic diagrams of a circuit in which the
various probe embodiments of the invention may be utilized;
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1084~17
Figure 24 is a block diagram of circuitry which may be utilized in :~
conjunction with a probe of the type shown in Figure 19;
Figure 25 is a schematic diagram utilized to explain the importance
of a rear guard electrode;
Figure 26 is a sectional view of a probe embodying another important
aspect of the invention; -
Figure 27 is an enlarged fragmentary sectional view of the probe in . :
Figure 26 taken along line 27-27 of Figure 28;
Figure 28 is a partially broken away plan view of the fragmentary
10portion of the probe shown in Figure 27; ~:
Figure 29 is a sectional view of a probe similar to that shown in
Figure 26 representing yet another embodiment;
Figure 30 is an enlarged fragmentary sectional view of the probe in ;
Figure 29 taken along line 31-31 of Figure 31; ` ,
Figure 31 is a partially broken away plan view of the fragmentary :.
portion of the probe shown in Figure 30;
Figure 32 is a sectional view of a press apparatus utilized in making
the embodiments of Figures 26-28;
Figure 33 represents a first step in the method of utilizing the
20apparatus of Figure 32;
Figure 34 represents a second step in a method of utilizing the
apparatus of Figure 32;
Figure 35 represents a third step in the method of utilizing the
apparatus of Figure 32;
Figure 36 depicts the relationship of the probe elements in the step
shown in Figure 33;
Figure 37 depicts the relationship of the elements in the step shown
in Figure 34;
Figure 38 depicts the relationship of the elements after the applica-
30tion of heat;
Figure 39 is a schematic representation of an insulated electrode .
filament utilized to explain the invention;
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`` ~084~7
':
~ F~gure 40 is a graph depicting pressures generated on the filament
of Figure 39 under various conditions; and
Figure 41 is a schematic representation of an insulated section of
a planar electrode utilized to explain the invention.
Detailed Description of the Preferred Embodiments ~-
As shown in Figures 1 and 2, flow rate sensing probes 10 and 12 are
mounted with the longitudinal axis of the probe extending vertically into the
grounded liquid flowing within a flow channel 14 having a restriction 16
therein. As the flow rate of the liquid through the channel increases and
decreases, the head height within the flow channel rises and falls.
A preferred embodiment of an RF admittance sensing probe adapted to
be utilized as the probe 12 mounted on a grounded wall 15 of the flow channel
; 14 in Figure 1 will now be described in detail with reference to Figures 3 and
4. As shown in Pigures 3 and 4, the probe comprises an insulating substrate
18 having a conductive probe electrode 20 extending longitudinally along the ;~
front of the probe. The probe further comprises a conductive guard electrode
22, adapted to be driven at the same potential as the probe electrode 20,
which extends longitudinally along the back of the probe with the substrate
1~ between the probe electrode 20 and the guard electrode 22.
In accordance with one important aspect of the invention, the conduc-
tive guard electrode 22 extends laterally outwardly from behind the probe
electrode 20 at opposite lateral extremities thereof and around to the front
of the probe 12 so as to closely couple any coating of conductive liquid which
accumulates on the probe 12 to the potential of the guard electrode 22 and
precludes resistive coupling of the conductive coating around the sides of the
probe to the wall 15 of the flow channel which is effectively grounded. The
close coupling of the coating at the sides of the probe 12 near the guard
electrode 22 is assured by providing an exterior solid insulation means in
the form of a sleeve 24, which may be of the heat-shrunk type, such that the
capacitance Cg from the guard electrode to the surface of the probe is sub-
stantial with respect to, i.e., at least 50% as great as, the capacitance Cp
of the insulating cover 24 from the frontal surface of the probe electrode 20
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1~84117
., ' :.
to the frontal surface of the insulating cover 24. Note that due to the uni-
form thickness of the sleeve 24, the capacitance through the sleeve 24 per
unit area at the guard electrode 22 will be equal to the capacitance through
the sleeve 24 per unit area at the probe electrode 20.
By eliminating the resistive coupling around the probe, the capa-
citance to ground as measured by the probe electrode 20 may be measured in
accordance with the technique disclosed in United States patent 3,746,975 -
Maltby, assigned to the assignee of this invention, which eliminates the
effects of the coating accumulated on the probe. The basis for this measuring ~`
technique disclosed in patent 3,746,975 is that a coating, after a certain
length, will act as an infinite transmission line whose characteristic terminal
impedance is predictable. The resistive and capacitive components are measur-
ed, and the effects of the coating can be computed and eliminated. The above
is true assuming the resistive coupling extends longitudinally along the
sensing element of the probe to the conductive liquid being measured rather
than around the probe to a grounded wall. Since the conductive guard elec-
trode 24 with its frontal portion 26 closely coupled to the coating will pre- ;
vent resistive current from flowing around the probe, the coating will appear
as a model of the infinite transmission line enabling the use of the technique
disclosed in the aforesaid 3,746,975 patent.
In further accordance with this invention, the probe electrode 20 is
characterized, i.e., the lateral dimension of the probe electrode decreases
with decreasing distance from one longitudinal end of the probe as shown in
Figure 4. Furthermore, the frontal portion 26 of the probe is also character-
ized in that the lateral dimension of the frontal portion increases with
decreasing distance from the longitudinal end of the probe. As a result of
the characterized nature of the frontal portion of the guard electrode, the
resistive coupling of the coating on the probe increases as the head height of
the liquid decreases so as to further reduce the effects of the resistive
coating on the capacitance measurement.
It will be understood that the probe electrode may be characterized
so as to accurately read the flow in the particular channel in which the probe
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lS mounted. In this connection, the probe may be characterized by the
following equation
b = kl~H~a 1) ~1)
where
b = the width of the probe;
kl = a constant;
~ = a constant depending upon the shape of the flow channel;
H = the height of the stream above the zero flow level; and ~
the flow through the channel may be expressed by the equation ;; ~ -
Q 1 ~2)
For a full understanding of the importance of the guard electrode 22
with its frontal portions 26, reference may be made to Figures 5(a-g) which
depict probes having coatings and the effects of the co~atings on the probe
measurement. As shown in Figure 5a, a non-characterized probe electrode p hav- -
ing an insulated cover is placed in a conductive liquid Q. As the level of
the conductive liquid Q falls, a coating c is left on the probe electrode p.
As shown in Figure 5a, the probe electrode p is mounted away from the conduc-
tive wall of the vessel or flow channel, i.e., the probe p is mounted in the
position of the probe 10 shown in Figure 1.
As shown in Figure 5b, the potential or voltage of the coating c in
Figure 5a represented by the line VC is substantially constant along the width
or lateral dimension of the probe p and only slightly below ~a potential drop
d) the potential or voltage vp of the probe itself. Since the liquid Q is
conductive, and the liquid Q is contained in a grounded vessel or flow channel,
the potential level of the liquid Q is represented by the line vQ which is
maintained at ground. It will be seen from Figure 5b that the coating itself
` has very little effect on the probe measurement since the potential seen by
the probe electrode p above the liquid level Q is substantially the same as
the potential vp of the probe itself as long as the probe is not mounted again-
~: 30 st a grounded wall or support member in the vessel or flow channel. ~
Figure 5c depicts the mounting of the probe p against a wall w such ~ -
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1084~17
that a coating of the liquid Q extends across the probe electrode p and along
the wall w. Since the coating c is again conductive, the coating c provides
a resistive path to ground from the probe electrode p to the grounded liquid Q.
As shown in Figure 5d, the coating c of Figure 5c which provides a
path to ground having a relatively small resistance produces a very substantial
coating error since the probe potential vp as shown in Figure 5d remains the -
same but the coating potential VC droops considerably toward ground at the
lateral extremities (a potential drop of 3d). Thus, a worse coating error
results when the probe is mounted along the wall of the vessel than when the
probe is mounted away from the wall for the same thickness of coating c and ~ -
the same width or lateral dimension of the probe p.
From Figure 5e, it will be appreciated that a probe of infinite `
width may be mounted along the wall of the vessel or flow channel without a
substantial coating error. As shown in Figure 5e, the coating potential VC
is substantially constant and only slightly below the probe potential vp (a
drop of d) along the entire width of the probe. However, from a practical
standpoint, it is not possible to give the probe infinite width. This is par- ~
ticularly true where the probe electrode must be characterized as in the case - -
of the probe shown in Figures 3 and 4 so as to provide an indication of flow
; 20 rate through a flow channel. In such a case, the probe electrode becomes ex~
tremely narrow at the lower end thereof.
By providing a guard electrode g as shown in Figure 5f at the lateral
extremities of the probe electrode p and the wall w, the advantages of a
relatively wide probe electrode are achieved without making the probe elec-
trode p itself excessively wide. Since the guard electrode g is driven at sub-
stantially the same potential as the probe electrode p, the potential VC as
;~ shown in Figure 5g droops across the entire width of the probe electrode p plus
the guard electrode g. However, the actual droop at the lateral extremities
; of the probe electrode p is not great, i.e., the potential VC at the lateral
extremities is no more than 25% of the potential drop between VC at the center
of the probe p and the potential vp of the probe electrode itself. As shown
in Figure 5g, the potential VC at the probe electrode p is substantially con-
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1084117 ~ `
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stant and only slightly below the potential vp of the probe elec- - -
trode p itself thereby minimizing the coating error. Preferably,
the droop (i.e., the drop in the potential v from the center
of the probe electrode p to the lateral extremity) is less than ~
the voltage drop between v and v at the center of the probe. -
From the foregoing, the following conclusions may be
reached. First, the sensing or probe electrode p should be as
wide as possible so that the capacitive coupling through the in-
sulation covering the probe electrode is as high as possible.
In addition, the guard electrode g should extend between the
lateral extremities of the probe electrode p and any adjacent wall
so as to be as well coupled to the coating as possible without
loading. For purposes, of this invention, the guard electrode g
is considered closely coupled to the conductive liquid when the
capacitive coupling is at least 50% of the capacitive coupling
between the liquid and the probe electrode p. In general, it
-~ has been found that the lateral portions which extend laterally
. ~
outwardly a substantially greater distance than the minimum
thickness of the exterior insulation provide the necessary ~ -
close coupling. Typically, the distance is greater than the
spacing between the lateral portions of the guard electrode
~` and the probe electrode plus the thickness of the guard electrode..~' Preferably, the distance is at least six times greater than the
thickness of the exterior insulation covering the guard electrode ` ,-
divided by the dielectric constant of the exterior insulation.
`, Of course, for the characterized probes of Figures 3 and 4, the -~-
~ lateral extension will vary as a function of distance from the end`; of the probe. However, the distance of "six times" is applicable
over the entire measuring length of the probe or at least a substan-
tial portion thereof. -
- 18 -
, . .:, . . .. ::. . .. :. .:. ~ :,.. . .
1084117 ~:
Another embodiment of a probe constructed in accordance ;~
with the principles of this invention is disclosed in Figures 6
and 7. As shown there, a backing or guard electrode 32 comprises -~
a conductive substrate which is relatively thick, as measured
from front-to-back, as compared with a probe electrode 34 and ;~
intermediate insulation material 36. In the embodiment of
Figures 6 and 7, the thick guard electrode substrate 32 provides
the mechanical strength for the probe, which mechanical strength -~
is provided by the thick insulating substrate 18 in the probe of
Figures 3 and 4. Note that any coating which accumulates on
the probe in Figures 6 and 7 adjacent frontal portions 40 of the
guard electrode will be closely coupled to the potential of the ~;
guard electrode due to the fact that the only insulation between
the coating and the guard electrode 32 is a sleeve 38 and frontal
portions 40 of the guard electrode 32. As a result, the capa-
citance Cg through the frontal portions 40 is not substantially
less than the capacitance Cp to the probe electrode 34 through
the cover 38. The guard electrode substrate 32 has a curved ~-~
' frontal surface so as to permit the use of a heat-shrunk tube
for the sleeve 38 which will minimize the air spaces between the
guard electrode 32 and the sleeve 38.
In accordance with another important aspect of the in-
vention, the conductive guard electrode 32 encloses a heating coil
42 which is adapted to heat the overall probe structure to a tem-
perature so as to prevent the accumulation of a coating, e.g.,
grease, on the probe. Where the supply of power available is
limited, it may be desirable to only activate the heating element
42 if a predetermined thickness of grease has accumulated on the
probe. This may be accomplished by providing an additional sensing
electrode 44 as shown in Figure 7 which is covered by the sleeve
38. By locating the electrode 44 beneath the longitudinal end of
- 19 ' ~'.
. . ~ . . . ~ ,
1~84117
the probe electrode 34, the sensing electrode 44 will be in a posi-
tion to detect a grease build-up even though the level of the liquid
in the flow channel is relatively low. By periodically dispersing
any grease build-up, the indicia on the probe remain visible.
Figure 8 discloses a track 48 having a surface 46 for
mounting on the wall 52 of a flow channel as shown in Figure 9. The
track includes a concave recess 50 adapted to receive the probe of
Figures 6 and 7. When the track is mounted such that the longitudinal
axis of the concave recess 50 extends vertically up the wall 52 of
the flow channel, the probe of Figures 6 and 7 may be moved vertically
through the track to assist in calibrating the probe.
Another embodiment of the probe constructed in accordance
with the principles of this invention is disclosed in Figures 10
and 11. This probe, which is relatively inexpensive to produce,
comprises a laminated structure including an interior insulation sub-
strate 60 with inner adhesive layers 62 and 64 which bond a probe ~ `
electrode 66 and frontal guard electrode 68 to the front of the
` substrate 60 and a rear guard electrode 70 to the rear of the substrate
60. The foregoing structure is then sandwiched between two sheets
of exterior insulation 72 and 74 with outer adhesive layers 62 and
64 in between which are heat sealed to one another along edges 78.
As shown in Figure 11, the probe electrode 66 is characterized and
^ separated from the frontal guard electrode 68 by a slight space 80. A
probe of this type may be very thin, i.e., less than 3/8 inch thick
, and preferably less than 1/16 inch thick.
A method for making the probe of Figures 10 and 11 will now
be described with reference to Figures 12(a-f). The bare substrate 60
as shown in Figure 12a is first coated with the inner adhesive layers
62 and 64 such as a polyester adhesive film as shown in Figure 12b.
Next, a copper film 70 and a copper film 67 are applied over the adhe-
sive layers 62 and 64 as shown in Figure 12c. At this time, the film
- 20 -
~0~34117
.~ .
67 may be etched to form the probe electrode 66 and the frontal guard
electrode 68 as shown in Figure 12d. The sheets 72 and 74 are then
applied over outer adhesive layers 62 and 64 and the entire structure
is laminated under heat and pressure as shown in Figure 12e. The
edges of the sheets 72 and 74 are finally heat sealed to each other
along the lateral extremities to complete the probe.
One particular advantage of a probe such as that shown in
Figures 10 and 11 is its flexibility so as to permit it to be mounted
in a curved track which conforms with the curvature of the wall in a
vessel or flow channel, i.e., extends vertically and horizontally.
In this connection, the substrate 60 may comprise *Mylar. *Mylar
provides the necessary flexibility and support for the probe structure
while at the same time having a sufficiently low dielectric constant
to insulate the probe electrode 66 from the rear guard electrode
70.
., .
Preferably, the exterior insulation sheets 70 and 72 are
characterized by high strength so as to resist abrasion and cut-through
due to floating debris. At the same time, it is preferable that the
sheets 70 and 72 have a very high dielectric constant so as to effec-
tively capacitively couple the potential of the frontal guard electrode
68 to the accumulated coating on the probe adjacent thereto. In this
connection, it has been found that a fluorocarbon resin such as
crystalline, high molecular weight polymer of vinylidene fluoride
having a dielectric in excess of 4 is particularly well suited for
use as the exterior insulation of the probe. One particularly good
material for this purpose is the high molecular weight polymer of
vinylidene fluoride which is supplied by Pennwalt Corporation under
the trade name of *Kynar. Generally, *Kynar is characterized by a
dielectric constant of 8 with a good deal of abrasion or cut-through
resistance.
*trade mark
- 21 -
~8~117
~ The main advantage of using a high performance 1uorocarbon resin
such as *Kynar in a probe such as that shown in Figures 10 and 11 is that the
combination of its mechanical and electrical characteristics enable the system
to ignore coatings which are developed on the probe. The degree to which the
coatings can be ignored is dependent upon the measuring technique as described
in the aforesaid patent 3,746,975 and also the capacity per square inch of the
probe insulation. The higher the capacitive coupling of the probe, the smaller
the error due to coatings which are deposited on the probe element. In
general, the capacitance through insulating material may be expressed by the
following equation:
K AK
c = tl ~3)
where ~ r
- Ka = a constant;
, A = the surface area of the insulation;
K3 = the dielectric constant; and
~ t = the thickness of the insulation.
,~ Because *Kynar is tough, i.e., resistant to abrasion and "cut~
through", it can be very thin. For example, *Kynar can be less than one-half
as thick as *Teflon and still provide the same resistance to "cut-through".
In addition, the high dielectric constant of 8 for *Kynar as compared with
;~ approximately 2.2 for~Teflon permits the *Kynar to provide a capacitive coup-
::
ling 3.6 times higher than the same thickness of *Teflon. The combined effect
of the reduced thickness of *Kynar as compared with *Teflon and the larger -~
dielectric constant of *Kynar as compared with *Teflon, allows a *Kynar cover-
ed probe to have a coupling capacity at least 7 to 8 times higher than a
*Teflon covered probe. Since the increased ability of the probe to ignore
coatings is equal to the square root of the increase in the coupling capacity,
a probe made with *Kynar will reduce errors due to coating by more than one-
third of that encountered by a probe made with *Teflon.
From the foregoing, it will be understood that the probe itself
should be particularly thin so as to avoid disturbing the flow or movement of
*trade marks
- 22 -
:- . :,:: .::, .: . ~. . .,,: ,
: ~ : . . ,: .~ .
~841~7
the liquid in a vessel. However, a thin probe may be very difficult to mount
since it may be so flexible so as to be incapable of self-support, i.e.,
incapable of resisting deflection when turned at right angles with respect
to the flow or movement of liquid in the vessel. Generally, such an incap-
ability exists when the ratio of probe width to thickness is greater than 20
to 1. The addition, it is generally desirable to mount the probe so as to
- facilitate its removal for replacement, repair and calibration.
In accordance with the mounting requirements, the flexible probe of r
Figures 10 and 11 may be inserted into a track 82 such as that shown in Figure
- 10 13 which has an opening 84 which generally conforms with the shape of the
~; removable probe. The track 82 may then be mounted on the wall 15 of the flow `~
channel even though the wall lS is curved since the track may be flexible as
~, well as the probe. Note that the opening 84 in the track 82 is adapted to be
;j substantially filled by the probe so as to eliminate any cracks and crevices
~-, in which materials might accumulate.
:, :
~( It is also possible to cast a recessed track 86 into the wall 15 as
, . . .
shown in Figure 14 where the opening 88 through the track conforms with the
shape of the removable probe. As an alternative, the wall 15 may itself be
formed or molded so as to provide an opening 88 which acts as a recessed track
for the probe as shown in Figure 14a. In Figures 14 and 14a, it is particular-
ly desirable to limit the thickness of the probe where the wall 15 is relative-
ly thin. As shown in Figure 15, the track 82 is mounted on a mechanical
support member 90 in a vessel or container. Since the removable probe would
include guard electrodes, the support member 90 may be effectively grounded
as in the case of the wall 15 in the embodiment of Figures 13 and 14. The
mechanical support member 90 is tapered at the ends thereof so as to minimize
the effect of the support member 90 on the flow through a flow channel.
Figure 16 shows a probe of the type shown in Figures 10 and 11 except -
that a discrete heater element 92 is included in the laminated structure be-
hind the guard electrode 70 but between the insulating sheets 72 and 74. If
the heater element 92 which may comprise a suitable resistive heating element
is made flexible, the entire structure shown in Figure 16 may be inserted into
_ 23 -
,, .: . :, : ~
~1~8~117
the tracks 82 and 86 shown in Figures 13, 14 and 15 as shown in Figure 17.
As an alternative, the heating element 92 may be located outside and behind
the laminated probe structure within the longitudinal opening 94 of a track
96.
As a still further alternative, the rear guard electrode 70 of the
probe in Figures 10 and 11 may be of a generally serpentine shape so as to
provide a combined guard electrode and heating element for the probe. Such a
serpentine guard electrode 94 is shown in Figure 18 in combination with a `
source of AC power 96 which is coupled to the guard electrode 94 through an ~ -
isolation transfor~er 97 with a low capacitive coupling. The guard electrode
94 is also connected to a source of guard potential not shown. As utilized
herein, a low capacitive coupling is intended to mean a capacitive coupling
s with an impedance which is high relative to the impedance of the guard ter- ,
minal to which the guard electrode is connected.
It will of course be appreciated that the capacitance through the
~ exterior insulation of a probe constructed in accordance with this invention
;j, is the capacitance which is measured by the probe since the liquid or mater-
i ials whose level is being measured is substantially conductive. Thus, any
A variation in the dielectric constant of the insulation with temperature will
produce an error. In accordance with one important aspect of the invention, ~ -
the probe of the type shown in Figures 10 and 11 includes a compensating elec- -
trode 98 which is positioned below the lower end of the probe electrode 66
and insulated from the guard electrode by an appropriate spacing which may be
achie~ed by appropriately etching the conductive film which forms the frontal
guard electrodes 68 and the probe electrode 66. By measuring the change in -
the dielectric constant of the exterior insulation 72 not shown in Figure 19
and utilizing that to compensate and correct the measurement of the capacitance
through the insulation 72 juxtaposed to the probe electrode 66 below the level
of the conductive liquid, accurate measurements of the liquid level may be
achieved. Note that the compensating probe electrode 98 is located in a
; position which will always remain below the liquid in the vessel or flow
channel.
- 24 -
!
1~8~ 7
` In accordance with another important aspect of the invention, a
probe of the type shown in Figures 10 and 11 may be precalibrated outside of
the flow channel and then returned to the flow channel and readily mounted in
the appropriate position. As shown in Figure 20, a mounting apparatus on the
wall of the flow channel includes a base plate 100 which supports a mounting
bracket 102 on a threaded stud 104. The bracket 102 which is attached by a
suitable means to the upper end of the probe 106 may be readily removed from
the stud 104 by removing a wing nut 108. Once the probe 106 is removed from
the flow channel, it may be inserted into a suitable calibrating vessel so as
to establish an appropriate zero level. The probe 106 with the mounting
- bracket 102 may then be returned to the stud 104 and positioned at the approp-
riate zero level by moving the nuts 110 to the appropriate position on the
stud 104. The wing nut 108 may then be secured to the stud 104 so as to clamp
the bracket 102 in the appropriate position. In order to assist in position-
ing the probe 106 in the zero level position, suitable indicia such as those
shown on the probe in Figures 4 and 7 may be utilized. The base plate 100 of
the mounting means may be attached to the wall 15 by suitable means such as a
threaded fastener 112. Note that the probe 106 extends into a track 114 hav-
ing a curved longitudinal groove and the probe 106 must therefore be flexible
enough to follow this groove when inserted into the track. In order to assure
that the probe 106 is not raised or lowered with respect to the wall 15, a
wire seal 111 is provided which extends between and through openings in the
bracket 102 and the lowermost nut 110.
As indicated in the foregoing, it is oftentimes desirable to mount
a probe so as to conform with a curvature of a wall in a flow channel or other
vessel. However, where the probe does follow the curvature of the wall in a
flow channel, the probe must be characterized differently from a probe which
extends vertically. Consider, for example, the characterization necessary for
a probe which is mounted flush against the wall of a circular pipe as schemat-
ically depicted in Figure 21a where the probe 120 extends along half of the
pipe 122 and the height H of the liquid within the pipe corresponds with the
10~411~
`.
submerged length H1. R corresponds to the radius of the pipe. The flow
rate through the pipe may be expressed by the equation
Q = K4Ha (4)
s where K4 is a constant and the width of the probe may be expressed by the
equation ;
b K5H
where K5 is a constant, keeping in mind that the relationship between H and H
for a circular pipe is
H1 = rArc cos ~RRH) ~ I2~R]. (5)
L 360
Similarly, for a Leopold Logco flume as shown in Figure 21b where H
represents the height of the stream above the zero level and Hl represents
the length of the probe covered by the stream, the flow rate may be expressed ~ i
.,~
by the equation `
Q = K6Hl-547 (6)
where K6 is a constant and the width of the probe may be expressed by the -~
equation t-
b = K7H.547 ~;
where K7 is a constant and where
Hl = [ Arc cos ( R ~ I2~R~ ~7) ;
360
below the straight walls of the flume. Assuming that the semicircular portion
of the flume has a radius of 3 inches and the width b of the probe is 1 inch
at a height H equal to 5 inches, the probe would be characterized as follows:
'~ `
_ 26 -
,, !
~ 1~84117 .;
..
Hl W :
o o ~ :
. .30 .142
-` .59 .272
1.12 .407
1.64 .516
2.15 .582
2.65 .584
3.15 .757
4.15 .85
5.15 1.0
This compares with the characterization for a straight probe in the Leopold
Logco flume which follows: -
__
H W
. . __ ---- . .
O O
.25 .194
.5 .283
1.0 .414
1.5 .517
2.0 .605
2.5 .684
3.Q .757
4.0 .85
5.0 1.0
- 27 -
1~1 34117
To readily see that the characterizations are different the relationship -~
between H and W and Hl and Wl can be normalized as shown below:
.~ H W Hl
. ' '
,' O O o
-~ .050 .194 .058 .142 ~-
., -
.100 .283 .115 .272
.2Q0 .414 .217 .407 -
.300 .517 .318 .516 --
.400 .605 .417 .582
.500 .684 .515 .684
.600 .757 .612 .757
.800 .850 .806 .850
1 . 00 1 . 000 1 . 001 . 000 ' ~'
Figure 22 illustrates a circuit in which the capacitance between
the probe electrodes, e.g., the probe electrode 66 of Figures 10 and 11, and
ground is measured. The circuit includes a fixed frequency RF oscillator 130
which drives a bridge network 132 through a transformer 134 where the secon~
dary of the transformer forms one side of the bridge 132. The capacitance
sensed by the probe electrode is represented by a variable capacitor 136 which
is connected between a capacitor 138 and ground. Any variation in the capa-
- -
citor 136 which represents the change in the level of the liquids being measur-
ed produces a signal across a span capacitor 140 and the signal across the
span capacitor 140 may be applied to an amplifier 142 to generate a guard
potential and an output terminal 144 which is equal to the potential of the
probe electrode.
In some instances, the guard electrode may be driven at a potential
which does not equal the potential of the probe electrode at all times. This
may be achieved by connecting the junction of the span capacitor 140 and the
secondary of the transformer 134 to the guard electrode at an output terminal
148. The potential at the terminal 148 will equal the potential at the probe
- 28 -
~08~117
electrode when the bridge is balanced. In the circuit of Figure 23, the
potential for the guard electrode is obtained at the output terminal 150 of
an amplifier 152 which is connected across the probe electrode 136.
Where the probe includes a compensating electrode 98 as shown in
~ the probe of Figure 19, additional circuitry must be provided in order to pro- -
- vide a compensated output signal. Such circuitry is shown in Figure 24 where-
in a probe unit 154 represents the circuitry shown in Figure 22 and the
output from the probe unit 154 represents the signal across the span capacitor ~ -
140. A similar circuit wherein the measured probe capacitance 136 is replaced
by the capacitance between the compensating electrode 98 of the probe in
Figure 19 and ground is shown as a compensating unit 156 in Figure 24. The
output from the probe unit 154 and the output from the compensating unit 156
are then applied to a divider 158 which produces a compensated output signal
which is unaffected by changes in the dielectric constant of the probe exter-
ior insulation.
In the foregoing, a good deal of emphasis has been placed upon the
significance of the frontal guard electrodes or frontal portions of the guard ~ :
electrode. However, the rear guard electrode or rear portion of the guard
electrode is also of considerable importance as will be clear from the follow-
ing numerical example for the probe of Figure 25. As shown there, the probe
comprises front exterior insulation 160 having a capacitance Ca where the
insulation 160 covers a probe electrode 162. Interior insulation 164 separ-
ates the probe electrode 162 from a guard electrode 166 where the capacitance
Cb represents the capacitance through the insulation 164. Rear insulation
168 covers the guard electrode 166. As also shown in Figure 25, the thickness
through the insulation 160 is da and the thickness through the insulation 164
is db-
Assume the probe of Figure 25 is to measure liquid levels in a
parshall flume and the probe has an active length of approximately 24 inches.
Also assume that the probe is linear and is one inch wide.
For such a probe, the capacitance
- 29 _
.... .. . . ... ; ,
1084117 :
.....
C = 8d 9 = 235 9 (8) ~ ~
where ~ ;
C = capacity in pf; -
K8 = constant;
; A = the area in square inches;
d = the spacing in inches; and
Kg = dielectric constant of insulation. --
Assume the dielectric constant Kg of the material is 2.5 and the
thickness da and db of the insulation 160 and 164 is .020 inches. From the
foregoing, the capacitance Ca may be calculated by the following equation: -
Ca = 29.4L ~9) -
where L is the length of the probe which is covered by the liquid in the flume.
Assume the probe is to be used for a 4 inch maximum head, corres- --
ponding to a full scale reading, such that
a = (29-4) (4) = 117.5 pf. ~lO)
By similar calculations, the capacity Cb equals
b = (-235)(2 5)(1)(24l (ll)
or
Cb = 705 pf (12)
Cb is the standing capacity of the probe and is higher than Ca as
calculated above because 24 inches of the probe contributes to Cb where only -
4 inches contributes to Ca. i ~
If it is assumed that the dielectric constant Kg of the insulation ~ !;
164 varies 5% over a 70F temperature range, the variation in Cb would be
35.3 pf. and this would be equivalent to a 30% shift in the zero of the system.
If it were desirable to keep the error due to the standing capacity to 3%,
insulation thickness db would have to be ten times thicker to that Cb equals
70.5 pf. and a 5% change in the dielectric constant Kg would produce a 3.5 pf.
error, representing approximately a 3% error in the standing capacity.
Therefore, if the guard electrode 166 is driven at ground potential,
~; there is a limit as to how thin the probe may be made. In the above numerical
- 30 -
iO84117
' '
example, the probe would be limited to a thickness of .25 inches. Moreover,
if the active length of the probe were longer, the probe would have to be
even thicker and increases in thickness of the probe may be undesirable where
the probe is intended to be mounted flush against the wall of a flume or
vessel so as to minimize interference with the flow through the vessel or
flume. However, by driving the guard electrode 166 at guard potential, the
capacitance Cb drops out of the measuring equation and the probe can be made
as thin as the particular application requires. It should be appreciated
that the use of a rear guard electrode is particularly important where the
guard is removably mounted in a track since it is virtually impossible to
prevent the collection of a liquid behind the probe in such a track.
A further embodiment of the probe which is constructed in accordance -~
with the principles of this invention is disclosed in Figure 26. The probe
of Figure 26 again comprises a laminated structure including a probe electrode
200, a frontal guard electrode 202 and a rear guard electrode 204. The
electrodes are encapsulated within insulation comprising a heat curable in-
sulation material 208 which applies compressive forces to the probe electrode
200.
; In accordance with one very important aspect of the invention, the
probe electrode 200 and the guard electrodes 202 and 204 comprise a multipli-
city of openings which are formed by a wire mesh 210 which is shown in some-
what greater detail in Figure 27. By utilizing a heat curable material for
the insulation 208 which shrinks on cooling and providing a multiplicity of
openings 212 in the mesh 210 as depicted in Figure 28, compressive forces are
applied over the surface area of the probe electrode 200, 202 and 204 includ-
ing the surface area at the openings. These compressive forces exceed 100
psi of surface area preferably 500 psi under normal operating conditions.
In the probe structure of Figures 26-28, each wire in the mesh 210
is surrounded by the insulation material so as to mechanically grip each wire
in the mesh 210. In other words, all of the interstices in the mesh 210 are
filled with the insulation material. This assures that water or other liquid
whose flow rate or depth is being measured will not collect in pockets or
- ~084117
,
voids adjacent the probe electrode 200. It will of course be appreciated
that any collection of water adjacent the probe electrode 200 would change
the calibration of the probe since the dielectric constant of water is 80
times the dielectric constant of air.
The mechanical grip and compressive forces around the wires of the
mesh 210 is achieved as the heat curable material contracts about the wires
of the mesh 210 when the probe is cooled after heat curing. In a particular-
ly preferred embodiment, the insulation material comprises a thermoplastic
material which forms a fusion bond through the-openings 212 formed by the mesh
~210 at the time of heat curing. Subsequently, the thermoplastic material is ;
cooled to achieve the compressive forces. Kynar is a highly preferred thermo-
plastic material for use as the thermoplastic insulation 208. However, other
fluorocarbon resins may be utilized including but not limited to FEP ~fluoro-
nated ethylene propylene).
It has also been found that the grip around the wires in the mesh
210 is maximized by providing a multiplicity of substantially uniformly spaced
openings extending along the length and breadth of the probe electrode 200 as
well as the guard electrodes 202 and 204. It is also important that each fila- ~~ ;~
ment or wire be surrounded by an adequate amount of insulation. In this con-
nection, it has been found that the average maximum cross-sectional dimension
do of the openings is greater than 10% of the average diameters of the wires
minimum spacing d5 between openings in said probe electrode. Moreover, it is
important that the distance d5 between adjacent openings-relative to the thick-
ness of the insulation should not become too large. In this connection, it
has been found that the thickness ti of the insulation as measured from said
probe electrode to the exterior of the probe should be greater than 10% of the
distance ds. Preferably thickness ti is greater than d5 with the optimum
achieved when t,~ 2d5 so as to produce a good mechanical grip on the electrode.
In order to ensure the proper RF field on the electrode, the minimum cross-
sectional dimension do of the openings should be no more than twice the-thick-
ness ti of the insulation. Finally it has been found that the avoidance of
sharp edges or corners at the openings in the electrodes produces a better
- 32 -
1084~17
mechanical grip on the electrode. For this reason, a wire mesh such as that
shown in Figures 26-28 is particularly desirable for use as an electrode since
thè openings are formed by strands or filaments of the mesh which are substan-
tially circular in cross-section.
Figures 29-31 show another embodiment of the invention wherein a mul-
tiplicity of openings 214 are formed in a probe electrode 216 and guard elec-
trodes 218. The insulation 220 again forms a continuum through the openings
214 as best shown in Figure 30.
However, the openings in the probe electrode 216 and the guard
electrodes 218 are not formed by a wire mesh but rather formed by etching con-
ductive sheets which form the probe electrode 216 and the guard electrodes 218.
As best shown in Figure 30, this etching achieves a natural roundness of the
metal at the edges of the openings 214 thus assuring a maximized mechanical
grip. Of course, the appropriate distance d5, thickness ti opening dimension
d may be carefully controlled by well-known photographic techniques utilized
in the printed circuit art. Preferably, the dimension do and the thickness
- ti are equal to at least 25% of the distance d5 (assuming no roundness at the
openings) and preferably greater than d5 where the optimum is achieved when d
and ti are greater than dS.
The etching of the openings 214 may be done simultaneously with the
shaping of the electrodes 216 and 218 as discussed with reference to Figures
12(a-e). It will therefore be understood that the probe electrode 216 may be
characterized in a similar manner to that shown in Figure 11. This of course
is also true with respect to the electrode 200 and the adjacent guard electrode
202 of Figure 26 although characterization may there be achieved by a mechani-
cal cutting technique or etching.
Apparatus for forming the laminated structures shown in Figures 26 -~
and 29 will now be described with reference to Figure 32. As shown there, the
fixture comprises a base plate 222 having a channel 224 receiving the elements
226 of the probe. Pads 228 are positioned above and below the elements 226
within the channel 224. Preferably, the pads comprise a highly polished
surface such as that provided by FEP Teflon so as to produce a smooth laminated
- 33 -
1~841~
surface which greatly impedes the build-up of any coating on the probe. An
insert 230 which transmits pressure to the laminated structure is located
immediately above the upper pad 228.
In accordance with one important aspect of this invention, uniform
pressure is applied throughout the length of the laminated structure 226 with-
in the chamber 222. This is accomplished by providing a flexible gasket 232
which extends above the insert 230 so as to form a substantially air-tight ~ -
chamber. The gasket 232 is sealed in place by bolts 236 which extend through
compression gaskets 234 and the sealing gasket 232 into holes within the base
222. Evacuation of the chamber beneath the gasket 232 is accomplished through
a vacuum port 238 in the base 222.
In order to avoid trapping any air bubbles within the laminated ~ -
structure 226, the chamber in which the structure 226 is located is evacuated
before any pressure is applied to the laminated structure 226. This is accom-
plished by providing a bridge member 240 which is attached to a support member
244 by bolts 242. The member 244 is in turn attached to an insert 230 by
bolts 245. The bridge member 240 is supported above the base 222 by bolts 246
which come to rest on a compression gasket 236. Once the chamber within which
the laminated structure 226 is located is evacuated through the port 238 and
all air bubbles have been removed, the bolts 242 may be removed so as to permit
removal of the bridge 240 while atmospheric pressure uniformly applies an
appropriate laminating pressure to the elements of the structure 226. The
entire fixture may then be placed in an oven for heating to the gel point of
- the thermoplastic insulation material so as to form a desired fusion bond
through the multiplicity of openings in the electrodes of the probe.
Figure 33 depicts the elements of a probe structure prior to lamina-
tion sandwiched between pads 228 located within the channel 224 of the base
222. As shown, channel 224 has not yet been evacuated nor has any pressure
yet been applied to the elements to be laminated. More particularly, air
spaces exist between each wire mesh 210 and thermoplastic sheets 250 which
will form the insulation 208 of Figure 26. In order to assure that the mesh
210 remains properly located within the laminated structure, it may be desirable
- 34 -
,. . .: : . - :
1084117
to tack the mesh 210 in place on, for example, the central thermoplastic sheet
250.
As shown in Figure 34, the gasket 232 is sealingly applied to the
top of the base 222 so as to define a substantially air-tight chamber within ~ ~-
the channel 224. In order to prevent the formation of air bubbles between the
components 210 and 250, the bridging member 240 supports the insert 230 by
virtue of the bolts 246. The fixture is retained in the condition shown in
Figure 34 until such time as all of the air has been evacuated through the -
vacuum port 238.
Once the air has been completely evacuated from the chamber within
the channel 224, the bridge 240 is removed and atmospheric pressure forces the
elements of the probe to a compressed position as shown in Figure 35. It is
important to note that the pressure applied to the probe elements through the
insert 230 occurs prior to any heating of the probe elements. It has been
; found to be particularly important to apply this pressure prior to heating so
that the subsequent relief of internal stress during heating will not cause
shrinking and distortion of the plastic. Moreover, it has been found that the ;
very uniform pressure applied to the insert by virtue of atmospheric pressure
insures a uniformity throughout the laminate by preventing any swimming of the
various mesh electrodes 210. It will of course be appreciated that the probe
may be extremely long and this uniformity of pressure would be extremely dif-
ficult to achieve by mechanical means. However, by using the fluid pressure
provided by the atmosphere, uniformity of pressure is rendered a certainty
and a uniformity of the laminate results.
The entire fixture shown in Figure 35 with atmospheric pressure
bearing down on the insert 230 is next inserted into a heated press or oven
so as to form a fusion bond of the thermoplastic sheets through the openings
in the mesh electrodes 210. The temperature of the oven or press should assure
that the thermoplastic sheets 250 reach a temperature at least equal to the
gel point of the thermoplastic sheets. In the case of Kynar sheets 250, a
temperature of 370F is sufficient to form a fusion bond through the openings
in the mesh electrodes 210. Such a temperature is below the 550 melting point
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of FEP so as to permit the use of FEP as the pads 228. On the other hand,
if FEP is utilized as the thermoplastic sheet material, the temperature of the
sheets must equal or exceed 550F and the pads 228 must comprise another highly
polished surface such as stainless steel.
In Figure 36, the sheets of thermoplastic material 250 and the mesh
electrodes 210 are shown as spaced apart by air as in the case of Figure 33.
In Figure 37, the air has been evacuated and pressure has been applied corres-
ponding generally with Figure 35. In Figure 38, heat has been applied to the
structure so as to form a fusion bond through the openings in the electrodes
210 thus assuring the mechanical grip on the strands in the wire mesh.
By utilizing the foregoing technique for laminating probes, it has
been found that the resulting probes are particularly resistant to delamination.Even when the probe is inserted into water at relatively high temperatures, `
the probe resists any delamination which might otherwise result due to the
collection of water in air pockets adjacent to the probe electrodes. In this
connection, it is important to note that the liquid in which the probe is
inserted must have a temperature of less than the curing temperature of insula-
tion. It is therefore important to heat cure at temperatures no lower than
120F. which represents the maximum ambient temperature a probe is likely to
encounter and preferably in excess of 212F., the boiling point of water. In
other words, the curing temperature of the insulation material should be higher
than the temperature of the liquid in which the probe is immersed.
The manner in which the compressive forces are developed around the
probe electrodes will now be described with reference to Figure 39 which re-
presents a filament 300 of a wire mesh electrode surrounded by insulation 302
where a represents the internal radius of the insulation 302 and b represents
the external radius of the insulation 302.
Assume the following:
P = internal radial pressure in psi (longitudinal pressure = O)
~a = change in internal radius due to P
~b = change in external radius due to P
E = modulus of elasticity; and
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~ .
v = Poisson's ratio
For the model shown in Figure 39;
Aa = P Ea ( ~ + v) (1)
ab = P E ~ ) ~2)
a (b _ + v) (3)
b - a
2 2 (4)
a (~ + v)
It is believed that the model expressed by Equation (4) represents
the pressure which is applied to the wire 300 as the insulation 302 shrinks
around the filament 300. It will be noted that Poisson's ratio v may be
10expressed by the equation
: v = ( 2G ) (5)
where G = the modulus of elasticity in shear. Poisson's ratio v may also be
, ~
expressed as .
v = 3~6~ (6)
where K = the bulk modulus of elasticity. It will therefore be understood that
the denominator of Equation (4) is a constant and the pressure exerted on the
wire 302 is therefore a function of the product of shrinkage aa at the internal
radius of the insulation 302 and the modulus of elasticity of the insulation E.
Reference will now be made to Figure 40 wherein the pressure P exert-
ed on the wire 300 by Kynar insulation 302 is depicted assuming a specific
shrinkage of approximat~ely 1.5%. The upper curve 304 of the graph shows that
the pressure increases almost linearly as the insulation becomes larger but
begins to reach a limiting value as the ratio of b/a is about 4. In other
words, from a pressure standpoint, it would not help to have the insulation
thicker than approximately 3 times the radius of the wire or one and one-half
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,~ .
~ 8411~ ~
times`the diameter of the wire. However, the lower cur~e 306 of the graph -~
clearly indicates that increases in ratio of b/a up to 2 produces very sub-
stantial increases in the pressure applied to the wire 300 by the insulation ;~-
302.
The model for the round wire therefore shows that any thickness of
insulation will produce positive pressure on the wire as long as there is some
shrinkage of the insulation. This shrinkage, as prevously explained, can be
maintained as long as the insulation is cured at a temperature higher than the ~:
maximum operating temperature. However, since many thermoplastic materials ~ ~`
which are particularly suitable for use as the insulation 302 will relax after -
initial tension, a sufficient amount of pressure must be applied initially so
as to assure adequate pressure and grip on the wire 300 even after relaxation. ~^~
Referring to Figure 41, consideration will now be given to sheet-
like conductive electrodes having openings etched therein so as to leave any
electrode portion 308 which is substantially rectangular in cross-section and
surrounded by insulation material 310. The pressure exerted on the electrode
portion 308 which is a function of insulation thickness is different from the -
case with a round wire because of the rectangular nature of the electrode
portion 308. More particularly, the metal electrode 308 will prevent any sub-
stantial movement at the corners 312 of the electrode and place the insulation -
materials 310 in lateral tension along surfaces 314 and 316 as depicted by
arrows 318 and 320 respectively. Because of the lateral tension depicted by
the arrows 318 and 320, the insulation 310 will resist any deflection required
to exert pressure on the metal electrode surfaces 314 and 316. For very thin
insulation, the pressure in the radial direction is not high enough to deflect
the insulation because it is in lateral tension. As a result there is essen-
tially no pressure on the flat surfaces of the electrode 308. As the
insulation thickness increases, the radial pressure becomes high and eventually
is enough to overcome the effects of the insulation being in tension and the
3Q pressure is transferred to the surfaces 314 and 316 of the electrode 308.
It will be understood that the probes shown in Figures 26 and 29 may
be utilized in conjunction with the circuitry disclosed in Figures 22 and 23.
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Indeed, it is intended that the unknown capacitance 136 represents the capa-
citance from the probe electrode 200 of the probe shown in F~gure 26 and
ground or the capacitance between the probe electrode 216 of the probe shown
in Figure 29 and ground.
It will also be understood that various heat curable insulation
~aterials may be utilized which are characterized by shrinkage after heat
curing. It will further be understood that heat curing may be accomplished
by an exothermic reaction within the insulation material rather than the
application of external heat.
Although specific embodiments of the invention have been shown and
described and certain modifications suggested, it will be understood that
other modifications may be made without departing from the true spirit and
scope of the invention as set forth in the appended claims.
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