U.S. patent number 3,876,916 [Application Number 05/321,912] was granted by the patent office on 1975-04-08 for capacitor for sensing contaminated oil.
Invention is credited to Donald S. Stoakes.
United States Patent |
3,876,916 |
Stoakes |
April 8, 1975 |
CAPACITOR FOR SENSING CONTAMINATED OIL
Abstract
Apparatus to determine the dielectric characteristics of a fluid
dielectric material including a rigid mounting with a planar
mounting surface, a base electrode on a stem secured to the
mounting surface and having a broad disc-like portion at one end of
the stem and defining a flat electrode face, a ring electrode
concentric with the flat face of the base electrode and having an
annular wafer-like shape and a mounting projection extending from
the outer edge of the wafer-like ring electrode to the planar
surface of the rigid mounting and affixed thereto, and a solid
insulating media between the base and ring electrodes and forming
the remainder of a container wall adjacent the electrodes, the
insulating media being formed of a stable low dielectric material
which is substantially insensitive to temperature changes.
Inventors: |
Stoakes; Donald S.
(Minneapolis, MN) |
Family
ID: |
23252588 |
Appl.
No.: |
05/321,912 |
Filed: |
January 8, 1973 |
Current U.S.
Class: |
361/280;
361/300 |
Current CPC
Class: |
H01G
4/04 (20130101) |
Current International
Class: |
H01G
4/04 (20060101); H01G 4/018 (20060101); H01g
005/16 (); H01g 007/00 () |
Field of
Search: |
;317/246,247,248,249R
;324/61R,60 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Attorney, Agent or Firm: Palmatier; H. Dale
Claims
What is claimed is:
1. A sensor for determining the dielectric characteristics of a
fluid medium, comprising:
means defining a rigid mounting lying in a plane,
a rigid metallic base electrode having a generally flat electrode
face lying substantially parallel to said plane and also having an
annular edge at the periphery of the face, said base electrode
being secured to said rigid mounting,
a rigid ring electrode of the same material as the base electrode
and spaced from the base electrode, the ring electrode having an
inwardly facing annular electrode face conforming to the size and
shape and orientation of the annular edge of the base electrode
face, said annular electrode face lying substantially perpendicular
to the flat face of the base electrode and having an annular end
edge adjacent the flat electrode face and uniformly spaced from the
flat electrode face around the periphery thereof, said ring
electrode having a width, in a direction outwardly from the annular
electrode face, equal to the distance from the center of the flat
electrode face to the annular edge thereof, said ring electrode
also being secured to said rigid mounting, and
fluid sealing insulator means disposed between the base and ring
electrodes and closing the space between the annular edges of the
electrodes, the insulator means being formed of a solid insulating
low dielectric material with characteristics of minimum change of
dielectric constant in response to changes in temperature, and said
insulating material having strength characteristics considerably
weaker than the material of the electrodes to allow limited
relative movement between the electrodes during temperature induced
expansion and contraction, and said insulating means cooperating
with the electrodes in defining an open ended fluid
medium-confining chamber with the base electrode forming one end of
the chamber and the ring electrode forming a portion of the
peripheral wall of the chamber.
2. The sensor according to claim 1 and the area of the annular
electrode face being substantially equal to the area of the flat
electrode face.
3. The sensor according to claim 2 and the area of the flat
electrode face being at least as large as the area of the annular
electrode face.
4. The sensor according to claim 2 and the flat electrode face of
the base electrode being circular, and said annular electrode face
also being circular.
5. The sensor according to claim 2 and the base electrode having a
mounting stem extending toward and secured to the rigid mounting
and having a thickness significantly less than the distance across
the flat electrode face.
6. The sensor according to claim 2 and the ring electrode being
substantially flat and lying substantially parallel with the flat
electrode face of the base electrode.
7. The sensor according to claim 6 and said ring electrode having a
mounting projection at the outer periphery of the electrode and
extending to said rigid mounting.
8. The sensor according to claim 7 and said mounting projection
comprising a cylindrical wall formed integrally of and in one piece
with the flat portion of the ring electrode.
9. The sensor according to claim 7 and the base electrode having a
mounting stem extending to said rigid mounting and having a
thickness significantly less than the distance across the flat
electrode face, the space between the stem of the base electrode
and the mounting projection of the ring electrode being filled with
insulating medium including a portion of said fluid sealing
insulator means.
10. The sensor according to claim 1 wherein the sensor is oriented
to position the open-ended chamber in upright position with the
base electrode forming the bottom end of the chamber, and
insulating means defining a peripheral wall above the ring
electrode to extend the chamber upwardly and facilitate confining
an increased depth of the fluid medium.
11. The sensor according to claim 1 and a housing having a flow
passage therethrough and through which said fluid medium may flow,
said housing embracing the sensor and providing open flow
communication between said passage and said open-ended fluid medium
confining chamber for renewing the fluid medium in the chamber with
the fluid medium from the passage.
12. The sensor according to claim 1 and the base electrode having a
beveled side wall adjacent said annular edge, said beveled side
wall facing obliquely away from the ring electrode.
13. The sensor according to claim 1 and said base electrode being
movable toward and away from the ring electrode to vary the spacing
therebetween and to vary the capacitance of the sensor.
14. The sensor according to claim 13 and said base electrode having
a disc portion defining said flat electrode face and also having a
threaded stem extending to and being threaded into the rigid
mounting to facilitate adjustment of the base electrode relative to
the ring electrode.
15. A sensor for determining the dielectric characteristics of a
fluid medium, comprising:
means defining a rigid mounting lying in a plane,
a rigid metallic base electrode having a generally flat electrode
face lying substantially parallel to said plane and also having an
annular edge at the periphery of the face, said base electrode
engaging and being secured to said rigid mounting at the plane,
a rigid ring electrode of the same material as the base electrode
and spaced from the base electrode, the ring electrode having an
inwardly facing annular electrode face conforming to the size and
shape and orientation of the annular electrode of the base
electrode face, said annular electrode face lying substantially
perpendicular to the flat face of the base electrode and said
annular electrode face being uniformly spaced from the flat
electrode around the periphery thereof; said ring electrode having
a thickness in a direction across the annular electrode face, and
also having a width in a direction outwardly from the annular face
of such magnitudes as to cause the area of the annular electrode
face to change corresponding to the change of the base electrode
face with change in temperature and proportionately to the change
in spacing between the electrode faces of the base and ring
electrodes as the electrodes extend from or contract toward the
reference plane of the rigid mounting in response to such change in
temperature, and fluid sealing insulator means disposed between the
base and ring electrodes and closing thespace between the
peripheries of the electrodes, the insulator means being formed of
a solid insulating low dielectric material with characteristics of
minimum change of dielectric constant in response to changes in
temperature, and said insulating material having strength
characteristics considerably weaker than the material of the
electrodes to allow limited relative movement between the
electrodes during temperature induced expansion and contraction,
and said insulating means cooperating with the electrodes in
defining an open ended fluid medium-confining chamber with the base
electrode forming one end of the chamber and the ring electrode
forming a portion of the peripheral wall of the chamber.
16. The sensor according to claim 15 and said annular electrode
face having an area at least as small as the area of the flat
electrode face of the base electrode.
Description
BACKGROUND OF THE INVENTION
Reference is made to U.S. Pat. No. 3,746,974 entitled "Oil
Permittivity Sensor" .
When the dielectric characteristics of a material can be
determined, significant conclusions can be reached about the
material itself. Low dielectric materials are essentially
non-conductive of electrical current and include many liquid
materials such as various oils and petroleum products including
lubricating oil, hydraulic fluids, kerosene and gasoline, and other
liquids such as alcohol, molten plastics such as polyethylene, ABC,
styrene, and other liquid materials such as molten glass, printing
ink, molten rubber, etc. Low dielectric gases include many common
gaseous materials such as methane, natural gas, automobile and
diesel exhaust gases, combustion flue gases, air, Freon, and the
sulfites and sulfates.
It has been found that as many such dielectric materials are used,
impurities and contaminants will be picked up in the material. By
sensing and measuring the dielectric characteristics of these
materials, the presence and the relative quantity of such
impurities or contaminants can be determined. Of course, the
dielectric characteristics of samples being measured will be
compared to predetermined normal characteristics so that proper
conclusions can be drawn as to the nature of the samples being
tested.
It should further be noted that high dielectric materials, which
are relatively conductive, exhibit the same general characteristics
such that the dielectric characteristics of the material will vary
with the purity or impurity of the material. To determine the
dielectric characteristics of high dielectric material will permit
significant conclusions as to the nature of the material and the
contaminants which may be contained therein.
IMPORTANT CONSIDERATIONS RELEVANT TO THE PRESENT INVENTION
The dielectric constants for various materials vary extremely
widely. For certain materials, the dielectric constant is 1, and
for other materials the dielectric constant is as high as 12,000.
For any particular percentage change in the dielectric constant,
the actual change in the dielectric constant of a low dielectric
material will be very significantly smaller than the actual change
in the dielectric constant of a high dielectric material.
This concept becomes extremely significant when measuring
dielectric changes in low dielectric liquids such as lubricating
and hydraulic oils. It has been found through correlated laboratory
tests that often a 5 percent change in a lubricating oil's
dielectric constant represents the entire range of measurement,
from a new and unused oil to an oil containing such oxides and
contaminants that the oil is unfit for further use; and, similarly,
in the case of hydraulic oils, frequently a 2.5 percent change in
the dielectric constant is the full range of change from new and
unused oil to an oil so contaminated that it is unfit for further
use as a hydraulic oil. The dielectric constants of hydraulic and
lubricating oils are 2.0 and 2.2, respectively, in new and unused
condition, and therefore the changes in the actual dielectric
constants of 0.05 and 0.11, respectively, represent the total
dielectric measurement ranges of these oils. These relationships
emphasize that the sensor for determining the dielectric
characteristics of the fluid must be extremely sensitive and stable
so that results can be relied upon.
Furthermore, temperatures of the oil may vary widely in test
conditions, particularly where lubricating oil in an engine is
being monitored as it recirculates. The minute changes in
dielectric characteristics must be measured even though ambient
temperatures of the air at the exterior of the engine may vary
100.degree.F., and temperatures of the dielectric material being
sampled will vary as much as 300.degree.F.
By contrast, to the hydraulic oil, a 2.5 percent change in the
dielectric constant of water is an increment approximately 39 times
larger than the increment of change of the hydraulic oil. It is
important that the sensor be able to detect these large changes in
dielectric characteristics as well as the extremely minute
changes.
Whereas in the prior art, it is asserted that the dielectric
constant of lubricating oil decreses 400 percent from 100.degree.F.
to 200.degree.F., that assertion, which has been popularly
accepted, is false. The dielectric constant of oil does not change
significantly with the temperature of the oil, within the range of
15.degree.F. to 350.degree.F.
BRIEF SUMMARY OF THE INVENTION
The present invention is a sensor which has maximum sensitivity to
low dielectric material and maximum stability throughout wide
changes in temperatures.
The sensor has two electrodes insulated from each other to form a
capacitor. One electrode, which may be referred to as the base
electrode, has a flat horizontal electrode surface, the edge of
which has a certain shape such as a circle; and the base electrode
is ordinarily ungrounded for applying a signal or voltage to it.
The ungrounded base electrode is formed of metal, such as aluminum
or beryllium copper, and extends downwardly from the electrode
surface to a horizontal reference plane lying parallel to the
electrode surface. The base electrode is supported at said
reference plane.
The second or ring electrode is ordinarily grounded and is formed
of the same material as the ungrounded base electrode. The ring
electrode is supported at the same reference plane from which the
undergrounded base electrode is supported. The grounded ring
electrode has an inner annular electrode surface of uniform width
lying normal to the spaced from the flat electrode surface of the
base electrode, the flat and annular electrode surfaces having
similar surface areas. The annular surface of the ring electrode
has the same shape and orientation as the edge of the flat
electrode surface and is uniformly spaced from said edge.
Accordingly, the annular electrode surface lies normal to the
reference plane and the end edges of the annular electrode surface
lie parallel to the reference plane.
The ring electrode extends horizontally outwardly in all directions
from the annular surface and has a depending leg portion spaced
outwardly from the ungrounded base electrode and extending down to
the reference plane at which the ring electrode is supported.
A rigid insulator of a material with stable characteristics, such
as mica filled fluorocarbon, surrounds the sides of the base
electrode and underlies the ring electrode so as to leave both the
flat and annular electrode surfaces exposed and confronting each
other and cooperatively defining a sample chamber or space to
confine a quantity of the dielectric material, the characteristics
of which are to be determined.
In one form, the sample chamber is increased in height, above the
ring electrode by an impervious wall to materially increase the
depth of the sample of dielectric liquid.
In another form, the sensor may be wholly confined in a
non-metallic flow line as for lubricating oil in an engine, so that
the entire quantity of oil in the lubrication system is continually
being sensed as to its characteristics.
Another aspect of our invention is a monitoring apparatus to
continuously sense and determine the dielectric characteristics of
a dielectric material. A capacitance bridge incorporates a pair of
capacitance sensors previously described, wherein one of the
sensors monitors and senses the continuously change test sample of
low dielectric material as the material circulates and recirculates
during use. The second sensor continuously monitors and senses the
characteristics of an unused sample of such material to provide a
norm against which a comparison is made in the bridge circuit.
Although the second sensor is virtually unaffected by the
temperature of the sample, the unused sample is maintained at the
same temperature as the continuously changing test sample.
Furthermore, the utilization of the second sensor readily
facilitates initial balancing of the capacitive bridge circuit and
eliminates other capacitors which may be severely affected by
temperature.
Because the flat and annular electrode surfaces are oriented normal
to each other, the sensor has a minimum of static capacity.
Supplying oil between the electrodes to constitute the dielectric
will have the maximum effect upon the capacity of the sensor; and
there will be a maximum change in the dielectric characteristics
between new and used oil. Maximum capability is thereby
achieved.
The electrode surfaces confront each other obliquely and the
electrodes are closest to each other only along spaced and
juxtaposed edges.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a section view taken on an upright plane through a sensor
embodying the present invention.
FIG. 2 is a schematic circuit diagram of a preferred circuit for
use in connection with the sensor of FIG. 1.
FIG. 3 is a section view taken on an upright plane through a
modified form of sensor.
FIG. 4 is a transverse section view through another form of sensor
embodied in an apparatus for continually sensoring and mointoring
the dielectric characteristics of a liquid such as the lubricating
oil of an internal combustion engine.
FIG. 5 is a detail section view taken approximately at 5--5 of FIG.
4.
FIG. 6 is a bottom plan view of the apparatus of FIG. 5 and being
partly broken away and shown in section along a broken line 6--6 as
illustrated in FIG. 5.
FIG. 7 is a schematic circuit diagram of a preferred form of bridge
circuit for use in connection with the apparatus of FIGS. 4 --
6.
DETAILED DESCRIPTION OF THE INVENTION
The form of sensor illustrated in FIG. 1 embodies all of the
essential characteristics of the sensor for determining the
dielectric characteristics of a fluid. The sensor is indicated in
general by numeral 50 and has a base electrode 51 and a ring
electrode 52, both of which are formed of the identical material
which is preferably a heat treated beryllium copper compound with a
low coefficient of linear expansion, but other metals, including
aluminum, are acceptable. Both the base electrode and ring
electrode are supported by and anchored securely to a rigid
mounting panel 53 which has very stable characteristics so as to
hold the electrode 51 and 52 in stationary relation with respect to
each other; and the mounting panel 53 may be an epoxy resin sheet
or a fiberglass panel. The upper surface 53.1 of the mounting panel
defines a reference plane from which both the electrodes will
expand or contract with temperature variations.
The sensor 50 also includes an insulator 54 between all portions of
the electrodes 51, 52, with the exception of the active capacitor
faces thereof. The insulator 54 is preferably a low dielectric
material with stable dielectric characteristics and expansion and
contraction characteristics over wide temperature changes, and a
typical material may be mica filled fluorocarbon.
More specifically, with respect to the sensor 50, it will be noted
that the base electrode 51 has a substantially flat electrode face
51.1 which lies substantially parallel to the top surface or
reference plane 53.1 of the mounting. In this particular form, the
electrode face 51.1 is circular in shape and has a circular
peripheral edge, but the face might have some other regular or
irregular shape. The principal portion of the electrode 51, is
substantially disc-shaped, and the peripheral side 51.2 of the
disc-shaped portion of the electrode is substantially cylindrical
in shape. This side 51.2 may, in some forms of sensor, be beveled
or tapered to be somewhat conical in shape in a downwardly
convergent direction.
The base electrode 51 has a mounting stem 51.3 of substantially
reduced thickness as compared to the diameter of the circular
electrode face. The stem 51.3 is threaded and is threadably mounted
in an internally threaded socket sleeve 55 which is carried on a
rigid with the mounting plate 53. In this form, a sealing O-ring or
gasket 56 seals the upper portion of stem 51.3 to the insulation
block 54 to prevent any migration of the dielectric material being
tested.
The ring electrode 52 has a flat wafer-shaped portion 52.1 which
lies parallel to the electrode face 51.1 and to the upper surface
or reference plane 53.1 of the mounting panel. The upper and lower
surfaces of the substantially annular wafer-shaped portion 52.1 of
the ring electrode are preferably smooth and parallel to each
other. The ring electrode 52 also has a mounting projection in the
form of a cylindrical wall 52.2 extending from the annular
wafer-shaped portion 52.1 of the ring electrode to the upper
surface or reference plane 53.1 of the mounting panel. The
cylindrical wall 52.2 is formed integrally of and in one piece with
the annular wafer-shaped portion of the ring electrode, and if
desired, the projection portion 52.2 could be in the form of spaced
legs around the periphery of the ring electrode instead of being a
continuous cylindrical wall. It will be recognized that the lower
edge of the cylindrical mounting projection 52.2 bears against the
mounting panel 53, and is secured thereto by screws 56 which are
screwed into tapped apertures in the ring electrode 52.
The ring electrode 52 defines an annular electrode face 52.3 which,
in the form illustrated, is cylindrically shaped and of the same
diameter as the flat electrode face 51.1; and the annular electrode
face 52.3 is positioned in spaced and coaxial relation to the flat
electrode face 51.1. In the event that the flat electrode face 51.1
has a shape other than circular, then the annular electrode face
52.3 would also have that similar shape, identical to the shape of
the peripheral edge of the flat electrode face 51.1, and the
annular electrode face 52.3 would also have the same size and
orientation as the flat electrode face 51.1.
The annular electrode face 52.3 must be oriented substantially
perpendicular to the flat electrode face 51.1 and to the top
surface or reference plane 53.1 of the mounting panel.
The relative spacing between the peripheral edge of the flat
electrode face 51.1 and the lower peripheral edge of the annular
electrode face 52.3 is fixed and controlled by the electrodes 51
and 52 themselves in their relationship with the mounting panel 53,
and the insulation 54, which is considerably weaker in its strength
characteristics than the metal of the electrodes, does not have any
appreciable effect on the physical relationship maintained between
the two electrodes.
The flat and annular electrode faces 51.1 and 52.3 preferably have
approximately the same areas, but the annular electrode face 52.3
may have an area somewhat less than the area of the flat electrode
face 51.1. It is highly preferable that the width of the annular
wafer-shaped portion 52.1 of the ring electrode 52, as measured
across the lower surface in a direction outwardly from the annular
electrode face 52.3 to the cylindrical mounting projection 52.2 be
the same as the distance from the center of the flat electrode face
51.1 to the peripheral edge thereof, which in this situation
wherein the flat electrode face 51.1 is circular, is equal to
one-half the diameter thereof.
It is important that there is no space between the periphery of
base electrode 51 and the insulation 54, and further that the
insulation engage and seal against the lower surface of the ring
electrode 52. The insulation 54 is shaped to have an opening
receiving the mounting stem 51.3 and to define a wall 54.1
coextensive with the annular electrode face 52.3 and extending
between the annular electrode face and the peripheral edge of the
flat electrode face 51.1. The insulation 54 is relatively yieldable
as compared to the substantially rigid electrodes 51, 52 which have
rather high strength characteristics in relation to the low
strength characteristics of the insulation. It will be noted that
the coextensive annular electrode face 52.3 and the wall 54.1 of
the insulation cooperatively define the periphery of a container or
chamber, the bottom of which is formed by the flat electrode face
51.1 of the base electrode. The periphery of this container is
extended upwardly by an overlying plastic shroud 57 which seals
downwardly against the ring electrode 52 and has a central opening
coextensive with the annular electrode face for the purpose of
increasing the depth of the chamber or compartment which will
confine the sample of fluid material being tested.
In one embodiment of the sensor 50, the diameters of the flat
electrode face 51.1 and the cylindrical annular electrode face 52.3
may be 0.5 inches. The width or height of the cylindrical annular
electrode face 52.3 will be approximately 0.125 inches. The
internal diameter of the cylindrical mounting projection 52.2 of
the ring electrode 52 will be 1.0 inches; and the spacing between
the edges of the electrodes is uniformly 0.044 inches around the
entire periphery thereof.
The sensor 50 is peculiarly adapted to be self-compensating to
temperature variations as to resist any changes of inherent
capacity by virtue of the physical relationships or changes of
physical relationships of the parts of the sensor itself. The
principal physical characteristics which control capacity are the
surface areas of the flat electrode face 51.1 and the annular
electrode face 52.3; and the average distance between the capacitor
plates or electrode faces, and, as in the present situation wherein
the electrode faces are circular and cylindrical, the average
distance between these electrode faces will be from a point on the
annular electrode face midway along the length of it to a point on
the flat electrode face located a distance inwardly from the
annular edge thereof equal to one-third the diameter. As
temperature changes, all portions of the sensor 50, and
particularly all portions of the electrodes will change to the same
temperature. The diameter and area of the circular flat electrode
face 51.1 will enlarge with increased temperature; the diameter of
the cylindrical annular electrode face 52.3 will enlarge
identically with the enlargement of the diameter of the flat
electrode face because the width of the annular wafer-shaped
portion 52.1 of the ring electrode, measured at the lower surface
of the wafer-shaped portion 52.1, is the same as half the diameter
of the disc-shaped portion of the electrode 51. The width or height
of the annular electrode face 52.3 will also enlarge with increased
temperature to the extent that the enlarged area of the annular
electrode face 52.3 will remain the same as the enlarged area of
the flat electrode face 51.1.
The height of the cylindrical mounting projection or wall 52.2 of
ring electrode 52 will also enlarge with an increase in
temperature, and the distance from the top surface or reference
plane 53.1 of the mounting panel to the annular wafer portion 52.1
of the ring electrode will accordingly increase. In a similar
manner, the length of the base electrode 51, from the top surface
or reference plane 53.1 of the mounting panel to the electrode face
51.1 will enlarge with an increase in temperature. As a result, the
location of the flat electrode face 51.1 relative to the location
of the cylindrical electrode face 52.3 will change slightly, but
only to the extent as to proportionately offset the change in the
electrode surface area of the sensor.
With respect to the capacity of the sensor, the capacity is
determined by the basic formula:
C.sub.p = K A/kd
where C.sub.p is the active sensor capacity, K is the dielectric of
the liquid material being measured, A is the area of either of the
electrode surfaces if the areas thereof are equal, otherwise A is
the area of the smaller of the electrode surfaces, k is a constant
which varies with units of measure, and d is the average distance
between the areas of the electrode faces exposed to the dielectric
material under measure.
Whereas the area of the annular electrode face 52.3 will be at
least as small as the area of the flat electrode face, the area (A)
will be the area of the annular electrode face 52.3 which may be
expressed:
A = .pi.DT
where D is the diameter of the ring electrode face 52.3 and T is
the length of the annular electrode face 52.3 in direction
perpendicular to the reference plane 53.1.
The average distance (d ) between the flat and annular faces of the
base and ring electrodes is measured between a point midway the
height of the ring electrode face 52.3 and a point on the flat
electrode face 51.1 located a distance inwardly from the periphery
thereof equaling one-third the diameter. The average distance (d)
may be expressed therefore as follows:
d =.sqroot. (T/2 + X).sup.2 + (D/3).sup.2
where X is the spacing between the peripheral edge of the flat
electrode face 51.1 and the nearest edge of the ring electrode
52.3.
Through the use of these basic concepts, it can be readily
determined that there is no change in the active capacity of the
sensor (C.sub.p), even with a temperature change of as much as
300.degree.F. and wherein the base and ring electrodes are assumed
to be formed of aluminum.
Although there is shunt capacity between the surfaces of the
electrodes 51 and 52 which are not exposed to the dielectric
material being measured, changes in temperature of the sensor does
not produce any discernible change in the shunt capacity as the
temperature is changed. The shunt capacity (C.sub.s) is
proportional to the fraction A/d wherein A is the area of the
smaller of the inactive surfaces of the base and ring electrodes,
and in this situation, the peripheral areas of the base electrode;
and wherein d is the average distance between the inactive
electrode areas of the two electrodes. This average distance, as
relates to the inactive surface areas is the distance from the
point on the ring electrode where the inner periphery of the
cylindrical mounting projection or wall 52.2 meets with the flat
lower surface of the wafer portion 52.1 thereof, and to a point on
the base electrode half way along the cylindrical side surface 51.2
thereof. It can be determined that temperature changes of as much
as 300.degree.F. produce no change in the shunt capacities
(C.sub.s) in this sensor 50.
The sensor 50 may be used to determine the dielectric
characteristics of high dielectric material as well as low
dielectric material. In the event that a high dielectric liquid is
to be tested, one of the electrode surfaces will be coated with an
insulating material such as a thin film of Teflon. The sensor 50
may then be employed in the same way as has been described in
connection with determining the dielectric characteristics of low
dielectric material.
The circuit illustrated in FIG. 2 provides a capacitance bridge
whereby the change in capacitance in the sensor 50 may be detected
and measured. In the circuit, a mercury type battery 60 of five to
twelve volts has one side connected to ground 65 and the other side
connected to an on-off single pole switch 62 through which power
may be supplied into the oscillator 61. The oscillator must be
fairly stable at a fixed frequency between 2.5 and 10.0 MHz, and
also provides a constant 5.0 to 10.0 VAC output under no load
conditions. The capacitor 63, as well as the capacitors 79 and 80,
is a 0.02 uF, 16 VDC ceramic disc type condensor. Capacitor 63
couples the output of oscillator 61 into the measuring output at
point 64 and provides DC isolation between point 64 and ground 65.
The coil 67, as well as coils 76 and 77, is a molded powdered iron
core, 2.5 mH choke providing low resistance path for DC measuring
currents while offering high impedance to AC current at the
oscillator frequency. Coil 67 is connected between point 64 and
ground 65. The coil 67 may be replaced by a 500 to 1,50. ohm
resistor with only a minimum effect upon circuit operation. The
coils 68 and 71 may be molded, fixed or variable inductance coils
whose value depend primarily upon the capacitance value of sensor
50 and of capacitor 72. If the sensor is fixed in the relative
positioning of the base electrode and ring electrode, the coil 68
will be a variable inductance coil, in order to provide for initial
circuit tuning; and if the sensor 50 is slightly variable whereby
the base electrode may be adjusted slightly with respect to the
ring electrode, the coil 68 may be a fixed inductance coil. Coil 68
and sensor 50 form a series resonant circuit condition which
produces a maximum AC voltage in proportion to the impedance value
of sensor 50 between the ground 65 and the midpoint 69 between coil
68 and sensor 50. It will be recognized that the ring electrode is
connected to ground 65 and the base electrode is connected to the
point 69 in the circuit.
The capacity of the capacitor 72 must be equal in value with the
capacity of sensor 50 with a pure state dielectric material
contained in the sensor, and if the sensor is fixed, the capacitor
72 is fixed, and if the sensor 50 is adjustable or variable, then
the capacitor 72 will be variable. The capacitor 72 is connected at
one side to ground 65 and at the other side through a circuit
connection to the coil 71 which forms a resonant circuit condition
with the capacitor 72 which produces a maximum AC voltage, in
proportion to the impedance value of capacitor 37, between ground
65 and point 70 which is midway between the capacitor 72 and coil
71.
A pair of diodes 73 and 74 are respectively connected to points 70
and 69 to provide a low impedance path in one direction only for AC
electron current flow from point 70, through diode 47 and through
capacitor 75 and diode 48 to the point 69. Capacitor 75 which is
connected in series with the diodes 73 and 74 is of the same value
and type as capacitor 63 and provides a low impedance path for AC
current and a DC blocking action for DC voltages rectified by
diodes 47 and 48.
Coils 76 and 77 are respectively connected to the midpoints of the
circuits between diode 74 and condensor 75 and diode 73 and
condensor 75, and the coils 76 and 77 are interconnected with each
other through a resistor 81 which is a wire wound resistor and has
a center tap connected to one side of a galvanometer 78, the other
side of which is connected directly to the ground. The capacitors
79 and 80 are respectively connected to opposite ends of the wire
wound resistor 81 and to ground 65.
The resistor 81 is a low wattage type, between 50 anad 2,000 ohms,
whose purpose can be either to provide a zeroing adjustment or a
measuring scale for DC electron currents flowing in opposite
directions between points 70 to ground 65 and between ground 65 to
circuit point 69 through meter 78. The meter 78 is a 25 to 200
microamp DC galvanometer with a zero center or offset zero scale
which may function either as a measuring scale or as an indicator
and thereby display any differences in DC currents flowing through
it in opposite directions.
To calibrate the circuit of FIG. 2 for a particular range of liquid
dielectric material being tested in sensor 50, the pure state of
the liquid dielectric material, representing a midpoint in the
measurement range, is placed in the sensor 50, filling it to the
top. Next a vacuum tube voltmeter, set to 0 to 5.0 VAC scale, is
connected between points 66 and ground 65 and switch 62 is shorted
across with a jumper wire. Resistor 81 is also shorted across with
a jumper wire between the underground ends of capacitors 79 and 80.
While observing the vacuum tube voltmeter, either the sensor 50 or
the inductor 68, whichever is variable, is adjusted until the
vacuum tube voltmeter reading is an absolute minimum AC voltage.
Then, while observing both the vacuum tube voltmeter and meter 78,
either the capacitor 72 or inductor 71, whichever is the variable,
is adjusted until the vacuum tube voltmeter reads a still further
absolute minimum AC voltage, and simultaneously, the meter 78
pointer is directly on its zero indication point. Thereafter,
remove the vacuum tube voltmeter connections from point 66 and
ground 65 and remove both jumper wires from across switch 62 and
resistor 81. When the pure state or unused dielectric material is
removed from the sensor 50, the circuit completely calibrated and
ready for use.
In one example of operation, the meter 78 of FIG. 2 may be
considered as a measuring scale. A sample of low dielectric liquid
material in its pure and unused state is placed in the sensor 50 as
in the previous calibration process and switch 62 is closed to
energize the oscillator. Any deviation of the meter 78 pointer away
from zero is noted and adjusted to an exact zero by rotating the
arm of the adjustment resistor or potentiometer 81. The reference
standard of pure and unused dielectric material is then removed
from the sensor 50 which is wiped clean. Thereafter, and without
further adjusting anything in the circuit, another sample of the
same type and brand of liquid, but used, will be placed in the
sensor 50.
If the sample then in the sensor 50 contains a contaminant not
previously contained in the reference sample, the meter 78 will
indicate a positive deviation in direct proportion to the
percentage change in the amount of contaminant in the liquid test
sample. Likewise, if the second test sample had contained less
contaminants than the original reference sample, the meter would
indicate a negative deviation.
The meter scale may be divided into increments of measure
representing percent, dielectric constant, or mere numbers for
convenience of deviation measurements. Of course, the meter may be
supplemented or replaced by a chart recorder to record the
readout.
In the event lubricating oil of an engine is being sampled, the
normal meter deviation will be a positive measurement, indicating
the normal buildup of oxidized particles. However, if there is a
negative deviation on the meter, then it will be determined that
there is some other contaminant being found in the oil which might
be a small quantity of fuel.
In summary, with respect to the sensor 50 and the bridge circuit
illustrated in FIG. 2, the dielectric characteristics of a liquid
material can be determined by sensing and measuring the material in
its pure and unused state and subsequently sampling the same
material after it has been used for a period of time. The nature of
the change in the dielectric characteristic can permit the
conclusion of the nature of the material for performing the
function it is desired to perform. The sensor 50 is insensitive to
temperature changes, even up to a temperature change of
300.degree.F. It is important that the remainder of the components
in the circuit of FIG. 2 be maintained at fairly constant
temperature because the value of the other circuit components such
as the variable capacitor 72 may change.
In FIG. 3, another form of sensor 50' is illustrated. This sensor
has a disc-shaped base electrode 51' and a ring electrode 52'. A
quantity of stable insulating material 54' confines the sides of
base electrode 51' and separates the base electrode at its sides
from the ring electrode 52'.
In this form illustrated in FIG. 3, the peripheral sidewall of the
base electrode 51' is tapered so as to be somewhat conically
shaped, converging in a downward direction. The base electrode 51'
is secured as by adhesive or mechanical means to the top surface or
reference plane of a rigid mounting panel 53'. Although this form
of sensor has no stem on the base electrode, both the base
electrode and the ring electrode are supported by and secured to
the top surface or reference plane of the mounting panel. The ring
electrode has all the characteristics of that described in
connection with the ring electrode of FIG. 1 with the exception
that the cylindrical mounting projection of ring electrode 52' and
extending from the annular wafer-shaped portion thereof to the
mounting panel 53' is somewhat shorter than the corresponding
cylindrical mounting projection of sensor 50 in FIG. 1.
The beveled or conically tapered side of the base electrode 51
leaves only a rather sharp edge at the periphery of the flat
electrode face thereof so that the stray capacity between the base
electrode 51' and the ring electrode 52' is considerably reduced.
The conically tapered side of the base electrode does not squarely
confront any of the lower surfaces of the ring electrode 52', but
is disposed at extremely sharp angles with respect to all of the
lower surfaces of the ring electrode 52' so as to reduce the stray
capacity.
The form of the invention illustrated in FIGS. 4 - 7 includes a
dual sensor unit indicated in general by numeral 84, including a
pair of sensors 85 and 86. The sensors 85 and 86 are identical to
each other. These sensors have base electrodes 85.1 and 86.1, and
ring electrodes 85.2 and 86.2 which define inwardly facing
cylindrical annular electrode faces 85.3, 86.3 in perpendicular and
spaced relation with the flat electrode faces 85.4, 86.4 of the
base electrodes. The base and ring electrodes of sensors 85 and 86
are constructed and arranged substantially identically with the
electrodes of the sensor 50 described and illustrated in connection
with FIG. 1, with a few exceptions as will be pointed out.
The base electrodes 85.1, 86.1 have conically shaped side surfaces
85.5 which converge in a direction away from the ring electrodes
and converge toward the rigid mounting panel 87 which is common to
both the sensors 85, 86. In this dual unit 84, the base and ring
electrodes of both sensors 85 and 86 are suspended in depending
relation from the mounting panel 87. The cylindrical mounting
projections or walls 85.6, 86.6 of the ring electrodes, bear
against the lower surface or reference plane 87.1 of the mounting
panel 87 and are clamped thereagainst by screws 88 which are
screwed into tapped apertures in the cylindrical wall portions
85.6, 86.6 of the ring electrodes.
Screws 88 also serve to clamp the sensing unit 84 to a rigid
mounting bracket 89 which has enlarged openings 89.1 adjacent each
of the sensors 85, 86.
The mounting stems 85.7, 86.7 of the base electrodes bear against
the lower surface or reference plane 87.1 of the rigid mounting
panel 87 and are clamped thereagainst by the reduced threaded ends
of mounting studs 90 which are screwed into tapped apertures
extending axially of the mounting stems.
The insulators 85.8, 86.8 which separate the base and ring
electrodes and cooperate therewith in defining a chamber to contain
the liquid dielectric material to be measured, have a conical
exterior shape to be spaced from substantial portions of the
cylindrical mounting projections or walls 85.6, 86.6 whereby to
define annular air spaces for the purpose of reducing stray
capacity because of the low dielectric constant (1.0) of air. The
insulators 85.8, 86.8 are molded integrally of the ring electrodes
85.2, 86.2 which are apertured at 85.2' and 86.2' to receive small
plugs of the insulator and thereby affixedly position the
insulators with respect to the ring electrode.
A single plastic molding 91 defines housing 91.1 and 91.2 which
enclose sensors 86 and 85 respectively. Each of the housings has a
longitudinal flow passage 91.3 extending therethrough in a
direction transversely of the axes of the base and ring electrodes.
The flow passages 91.3 in the plastic molded housings have threaded
nipples or pipe fittings 92 threaded into the housings for flow
communication and to facilitate connection to pipe fittings. The
housings are provided with enlarged transverse bores 91.4 extending
transversely of the flow passages 91.3 and oriented in concentric
alignment with the respective sensors 85, 86 so as to provide open
flow communication between the flow passages 91.3 and the sensors
85, 86.
The plastic molding 91 defining the housings which are sealed to
sensors 85, 86, is affixed to the mounting bracket 89 and panel 87
by clamping bolts 93.
The plastic molding has cylindrical openings to receive the ring
electrodes 85.2, 86.2 and to snugly seal against the exterior or
lower flat surfaces thereof and the peripheral cylindrical wall
surfaces of the mounting projections 85.6, 86.6. As a result, the
sensors 85, 86 are exposed only to the dielectric material which is
flowing in the passage of the particular housing 91.1, 91.2.
It is intended that the sensor unit 84 be employed for the purpose
of continually sensing and monitoring the dielectric
characteristics of a low dielectric liquid material in use, such as
the lubricating oil of an internal combustion engine. The flow
passage through housing 91.2 will be connected at the fittings 92
to the flow line for the recirculating lubricating oil of the
engine so that oil at engine temperature will be continuously
supplied and constantly changed at the sensor 85. A quantity of low
dielectric material, and in this case lubricating oil, in new and
unused condition is supplied into the flow passage of housing 91.1
so as to be constantly exposed to the sensor 86 and to fill the
chamber adjacent the base and ring electrodes thereof. Closure caps
92.1 are applied to confine the quantity of new and unused liquid
low dielectric material at the sensor 86 to provide a norm to which
reference will be made in comparing the characteristics of
recirculating oil being sensed and monitored by the sensor 85.
Because the sensor 86 is mounted in close proximity with the sensor
85, and from the same mounting bracket, the sample of oil in the
sensor 86 will be substantially the same temperature as the
recirculating sample of lubricating oil being continuously exposed
to sensor 85.
The mounting post or studs 90 also carry a fiberglass circuit board
94, which will carry substantial portions of the capacitance bridge
circuit illustrated in FIG. 11. The fiberglass panel 94 will carry
a pair of variable coils 68 and 71 with rotary adjustment pins to
vary the inductance thereof. The circuit board and the components
thereon and also the coils 68, 71 are enclosed within and confined
by a housing 95 which is clamped downwardly against the mounting
bracket 89 and sealed thereto by gasket 95.1.
With respect to the bridge circuit illustrated in FIG. 7 which is a
part of the sensor unit 84, substantially the entire bridge circuit
is the same as illustrated in FIG. 2, and the same numerals on all
of the identical components and circuit points are repeated in FIG.
7 to show the very substantial similarity. The principal difference
in the circuit of FIG. 11 is the addition of a fail-safe circuit to
indicate that the circuit is operating properly. In this regard, a
single pole double throw switch 96.5 is inserted between the meter
78 and the wiper of potentiometer 81. The second pole of the switch
96.5 is connected directly to a current limiting resistor 96.4 of
approximately 27,000 ohms. The resistor 96.4 is connected in series
with a diode 96.3, identical in characteristics to diodes 73, 74,
and the diode 96.3 rectifies the negative half cycle of AC voltage
appearing at point 64 at the oscillator frequency. The diode 96.3
is connected in series with a coil 96.1 which is identical to coil
67, and coil 96.1 is connected directly to point 64 of the circuit.
The midpoint between the diode 96.3 and coil 96.1 is connected to
ground 65 through a capacitor 96.3 and is identical to capacitors
79, 80 as to block DC current to ground, but effectively pass AC
voltages of the oscillator frequency to ground 65. The combined
purpose of coil 96.1, diode 96.3 and resistor 96.4 is to provide a
fail-safe indication on meter 78 when switch 96.5 is placed in test
position. Once the circuit is calibrated, the AC voltage appearing
across circuit point 64 to ground 65 remains nearly constant and
serves as an indication the circuit functioning properly.
Also in this circuit, the supply of voltage is provided at terminal
60.1, and, as in the circuit of FIG. 2, a voltage of 5 to 10 volts
DC is supplied to the oscillator 61. The variable resistor 96
between the power supply terminal 60.1 and the oscillator 61 will
vary the input voltage supplied to the oscillator. Variable
resistor 96 is a 100 ohm wire wound resistor.
In this circuit of FIG. 7, the sensor 85 is exposed to the flowing
liquid dielectric material, as indicated by the arrows. This sensor
85 replaces the sensor 50 of FIG. 2. The variable capacitor 72 of
FIG. 2 is replaced by the other sensor 86 of unit 84 which contains
a small pure state sample of the unused dielectric liquid which is
being recirculated and monitored by sensor 85. The unit 84 provides
a direct comparison of the dielectric constant of the liquid
dielectric material being sensed and monitored. It may be desirable
that the meter and test switch 96.5 be located at a position remote
from the unit 84, and in the case of a stationary combustion engine
as used in power generating plants, the meter may be several miles
away.
As hereinbefore explained, neither of the sensors 85 or 86 will
vary the effective capacity thereof by reason of a change of
temperature. The use of the second sensor 86 provides a constant
reference to the norm against which the comparison is made. The
variable inductance coils 68, 71 are exposed to identical
conditions within the cover 95 of the unit and after they are
originally adjusted to balance the system will remain in balance
with each other.
In the use of the sensors as described herein, there is no
requirement as to the orientation of the sensors with respect to
the vertical. The sensors must only be completely exposed over
their entire electrode surface areas to the dielectric material,
the dielectric characteristics of which are being measured. In the
sensing of the dielectric characteristics of a high dielectric
material, the electrode face of one of the electrodes, or of both
of the electrodes may be very thinly coated with an insulating
material, preferably at a thickness of 0.005 to 0.010 inches.
It will be seen that we have provided a new and improved sensor for
determining the dielectric characteristics of a fluid dielectric
material with a high degree of accuracy so that conclusions can be
drawn as to the nature of the dielectric material and any
contaminants that may be contained therein. The sensor is
characterized by a base electrode with a flat electrode face which
forms one end of a container or chamber wherein the fluid
dielectric material is confined. A ring electrode with an annular
electrode face is disposed normal to the flat electrode face of the
base electrode, and is spaced from the peripheral edge of the base
electrode face. Both the base electrode and the ring electrode are
mounted on and secured to a ridge mounting at a common reference
plane; the diameters of the flat and cylindrical electrode faces
are identical, and the width of the wafer-shaped portion of the
ring electrode is the same as half the diameter of the flat
electrode face. In certain forms of the sensor, the base electrode
may be adjustable slightly with respect to the ring electrode for
initial tuning. Both the flat electrode face and the annular
electrode face are oriented parallel to the reference plane of the
rigid mounting. An insulator of stable low dielectric material
which is substantially insensitive to temperature changes and is of
such strength as to yield to the substantial strength of the
similar metal in the base and ring electrodes, is provided between
the ring and base electrodes to confine the liquid dielectric
material in the chamber or container and also to minimize stray or
shunt capacity between the base electrode and the ring electrode. A
capacitance bridge which is highly sensitive to change in capacity
of the sensor is used for detecting changes in the capacity
produced by variances in the dielectric characteristics of the
fluid dielectric material. In a continuous monitoring form for
monitoring the characteristics of a supply of liquid during use, a
second sensor containing a quantity of the same nature of liquid,
but in a pure and unused state, is utilized as a reference or norm
against which the comparison is made to the characteristics of the
material being used and recirculated.
* * * * *