Abstrict A flow meter for measuring the velocity of fluid flow in a conduit
is disclosed, characterized in that an electromagnetic sensor probe
is mounted within an opening contained in the conduit for generating
electrical signals as a function of the velocity of fluid flow.
The probe has a debris shedding head portion which protrudes into
the interior of the conduit and includes an electromagnet for generating
an electromagnetic field in the fluid, the axis of the field extending
normal to the direction of fluid flow. The probe further includes
a plurality of electrodes arranged beyond the poles of the electromagnet
and adjacent the fluid for producing the electrical signals in response
to the flow of fluid through the electromagnetic field. Signal processing
apparatus is connected with the sensor probe for converting the
electrical signals into a flow velocity output signal.
Claims What is claimed is:
1. A flow meter for measuring the velocity of fluid flow in a conduit,
comprising
(a) a generally cylindrical electromagnetic sensor probe means
for generating electrical signals as a function of the velocity
of fluid flow within the conduit, said probe means including a probe
head and a body portion; and
(b) means for mounting said probe means within an opening contained
in the conduit with a portion of said probe head protruding into
the interior of the conduit and with the longitudinal axis of said
probe means extending normal to the direction of fluid flow in the
conduit, the probe head portion protruding into the interior of
the conduit defining a debris shedding surface facing the fluid
flow and arranged at an angle other than normal to the direction
of fluid flow, whereby debris is prevented from accumulating thereon;
(c) said probe means further including
(1) electromagnetic means having a pair of poles for generating
in the fluid an electromagnetic field the axis of which extends
normal to the direction of flow; and
(2) a plurality of electrodes mounted in said probe head portion
for producing said electrical signals in response to the flow of
fluid through said electromagnetic field, said electrodes being
arranged beyond the poles of said electromagnetic means and comprising
at least a pair of sensing electrodes symmetrically arranged relative
to said field axis along a line transverse to the direction of fluid
flow within the conduit.
2. Apparatus as defined in claim 1 and further comprising
(d) processing means connected with said sensor probe means for
converting said electrical signals into a flow velocity output signal.
3. Apparatus as defined in claim 2 wherein said electrodes are
embedded within said debris shedding surface of said sensor probe
head portion.
4. Apparatus as defined in claim 3 wherein said electrodes are
recessed from said debris shedding surface of said sensor probe
head portion.
5. Apparatus as defined in claim 4 wherein said mounting means
comprises a compression seal and ball valve assembly.
6. Apparatus as defined in claim 3 wherein said sensor probe head
portion includes a pair of spaced parallel, generally parabolic
portions extending from said probe body portion to define a flow
trough therebetween, the flow through said trough being arranged
parallel to the fluid flow in the conduit, and further wherein one
of said sensing electrodes is arranged at the apex of each of said
parabolic portions.
7. Apparatus as defined in claim 6 wherein said plurality of electrodes
further comprises a ground electrode arranged between said parabolic
portions and adjacent the periphery of said probe head portion.
8. Apparatus as defined in claim 3 wherein said debris shedding
surface of said sensor probe head portion has, in longitudinal cross-section
relative to a plane containing the longitudinal axes of both the
conduit and said probe means, an oblique substantially wedge-shaped
configuration to define, in the direction of fluid flow in the conduit,
from the front portion of said head portion to the rear portion
thereof, a diverging acute angle (.alpha.) between the inner wall
surface of the conduit and a line tangent to said debris shedding
surface.
9. Apparatus as defined in claim 8 wherein said diverging angle
is between 25.degree. and 35.degree..
10. Apparatus as defined in claim 9 wherein said plurality of
electrodes further comprises a ground electrode embedded within
said oblique debris shedding surface of said sensor probe head portion
adjacent the front peripheral portion thereof.
11. Apparatus as defined in claim 10 wherein a pair of said sensing
electrodes are spaced by an angle (.beta.) of between 180.degree.
and 220.degree. relative to the axis of said probe means in the
direction of said ground electrode.
12. Apparatus as defined in claim 10 wherein a pair of said sensing
electrodes are spaced by an angle (.DELTA.) of between 90.degree.
and 110.degree. relative to the axis of said probe means in the
direction of said ground electrode.
13. Apparatus as defined in claim 3 wherein said debris shedding
surface of said sensor probe head portion has a dome-like configuration,
the transverse thickness of which is less than the longitudinal
thickness thereof in the direction of fluid flow from the front
portion of said head portion to the rear portion thereof.
14. Apparatus as defined in claim 13 wherein said sensing electrodes
are centrally arranged on opposite sides of said dome-like head
portion, and further wherein said plurality of electrodes includes
a ground electrode arranged on the front portion of said probe head
portion adjacent the periphery thereof.
15. Apparatus as defined in claim 6 wherein said processing means
includes means for modifying said electrical signals to correct
nonlinearities in Reynolds number.
16. Apparatus as defined in claim 15 wherein said modifying means
includes
(1) multiplier means for squaring a portion of said electrical
signals; and
(2) adder means for adding said squared portion of said signal
to said original signal, thereby to provide a first order correction
for changes in fluid flow profiles resulting from increasing velocity.
17. Apparatus as defined in claim 16 wherein said processing means
further includes means for adjusting said electrical signals from
said electrodes in accordance with the mean velocity of the fluid
flowing through the conduit.
Description FIELD OF THE INVENTION
The present invention relates to a fluid flow meter having an electromagnetic
sensor probe which protrudes into a conduit and generates electrical
signals as a function of the velocity of fluid flow within the conduit.
BRIEF DESCRIPTION OF THE PRIOR ART
Electromagnetic flow meters of the Faraday type are well-known
in the patented prior art as evidenced by the patents to Marsh U.S.
Pat. No. 3885443 Darby U.S. Pat. No. 3898881 and Marsh U.S.
Pat. No. 4015471 all of which are assigned to the assignee of
the instant invention.
Probe type electromagnetic flow and velocity meters are disclosed
in the patents to Griswold U.S. Pat. No. 3161047 and Zanker et
al. U.S. Pat. No. 3668931 respectively. Generally, these prior
probe type flow meters protrude from the hull of a ship and are
used to measure the velocity of the ship passing through a body
of water. More particularly, the probes generally extend into the
body of water perpendicularly to the boat hull at a significant
distance to obtain an accurate signal from the electrodes which
are arranged between the poles of the electromagnet and the boat
hull. An inherent drawback of the prior probe type electromagnetic
flow meters is that they are not suitable for use for measuring
fluid flow in a conduit. As noted above, the prior flow meters must
extend a significant distance into the fluid whose flow is being
measured to obtain a proper signal from the electrodes owing to
their arrangement relative to the field generated by the electromagnet.
Because of this excessive protrusion, debris within the fluid accumulates
on the probe and distorts fluid flow, causing errors in the electrical
signals corresponding to flow velocity which are generated by the
electrodes.
In order to overcome these disadvantages, an electromagnetic probe
type flow meter of the type disclosed in the patent to Cushing U.S.
Pat. No. 4125019 was developed. In the Cushing device the sensor
probe has the sensing electrodes mounted on the sides thereof, and
the probe extends to a considerable degree into a conduit with the
longitudinal axis of the probe arranged at an acute angle relative
to a direction perpendicular to the direction of fluid flow, whereby
debris is shed from the probe. While this type of flow meter normally
operates satisfactorily, it suffers inherent limitations in accuracy
since it senses flow profiles in a plane which is not perpendicular
to the direction of fluid flow, whereby the sensed flow profiles
are distorted.
The present invention was developed in order to overcome the above
and other drawbacks of the prior devices by providing a probe type
electromagnetic flow meter wherein a debris shedding portion of
the probe, having a unique arrangement of sensing electrodes relative
to the poles of the electromagnet, protrudes into a conduit for
sensing flow profiles in a plane normal to the direction of fluid
flow, and further wherein the signals corresponding to fluid flow
velocity are compensated for non-linearities in the flow profiles.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to
provide a flow meter for measuring the velocity of fluid flow in
a conduit, including an electromagnetic sensor probe for generating
electrical signals as a function of the velocity of fluid flow within
the conduit. The probe includes a debris shedding sensing head portion
and a body portion, and probe mounting apparatus is provided for
mounting the probe within an opening contained in the conduit with
the longitudinal axis of the probe extending normal to the direction
of fluid flow in the conduit and with the sensing head portion protruding
into the interior of the conduit. The probe further includes an
electromagnet for generating an electromagnetic field in the fluid
with the axis of the field extending normal to the direction of
flow. A plurality of electrodes are mounted in the probe sensing
head portion for producing the electrical signals in response to
the flow of fluid through the electromagnetic field. The electrodes
are arranged beyond the poles of the electromagnet relative to the
conduit wall and include at least a pair of sensing electrodes symmetrically
arranged relative to the field aixs along a line transverse to the
direction of fluid flow.
According to another object of the invention, the flow meter further
includes a processing device connected with the sensor probe for
converting the electrical signals into a flow velocity output signal.
According to yet another object of the invention, the flow meter
processing apparatus includes a mean velocity adjustment device
for producing a mean velocity signal corresponding to the electrical
signals and circuitry for modifying the mean velocity signal to
correct for nonlinearities in the Reynolds number.
According to a further object of the invention, the portion of
the sensing head which protrudes into the conduit is configured
to shed debris and to minimize the distortion of the electrical
signals owing to eddies produced in the fluid by the protruding
probe.
According to a preferred embodiment of the invention, the configuration
of the probe sensing head enables the flow meter to sense flow velocities
in opposite directions.
BRIEF DESCRIPTION OF THE FIGURES
Other objects and advantages of the subject invention will become
apparent from a study of the following specification when viewed
in the light of the accompanying drawing, in which:
FIG. 1 is a partial sectional view of the flow meter with the sensor
probe mounted in an opening in the wall of a conduit;
FIG. 2 is a partial sectional view illustrating the electromagnetic
field generated by the sensor probe mounted in the conduit;
FIGS. 3a, 3b, and 3c are rear, side, and top views, respectively
of the preferred configuration of the sensor probe head portion;
FIG. 4a is an exploded plan view of the sensor probe mounting assembly;
FIG. 4b is a plan view of the sensor probe mounting assembly with
the sensor probe in its operative position;
FIG. 5 is a block diagram of the flow meter signal processing apparatus;
FIG. 6 is a detailed circuit diagram of the signal modifier of
the flow meter signal processing apparatus;
FIGS. 7a-7c are rear, side, and top views, respectively of an alternate
configuration of the sensor probe head portion, the side view being
a partial sectional view;
FIG. 7d is a top view of the sensor probe head portion of FIGS.
7a and 7b showing a different configuration of the sensing electrodes;
and
FIGS. 8a-8c are rear, side, and top views, respectively of a third
configuration of the sensor probe head portion.
DETAILED DESCRIPTION
Referring first more particularly to FIG. 1 the subject invention
relates to a flow meter 2 for measuring the velocity of fluid flow
in a conduit 4. The flow meter includes an electromagnetic sensor
probe 6 mounted in an opening 8 in the conduit wall for generating
electrical signals as a function of the velocity of fluid flow within
the conduit. The electrical signals are delivered to a signal processor
10 via a cable 12 where the electrical signals are converted to
a flow velocity output signal which represents the velocity of fluid
flow in the conduit.
The flow meter sensor probe 6 includes a debris shedding head portion
6a which protrudes into the conduit, and a generally cylindrical
body portion 6b connected with the head portion. As will be developed
in greater detail below, apparatus is provided for mounting the
sensor probe within the conduit opening with the longitudinal axis
of the probe being arranged normal to the direction of fluid flow
within the conduit to obtain accurate flow profiles.
As shown in FIGS. 2 and 3 the sensor probe head portion 6a includes
a magnet and a plurality of electrodes. Referring first to FIG.
2 centrally arranged within the interior of the sensor probe head
portion is an electromagnet 14 having poles S and N on opposite
ends thereof. The electromagnet establishes an electromagnetic field
16 in the fluid flowing through the conduit with the axis of the
field extending through the conduit in a direction normal to the
direction of fluid flow. As shown in FIG. 3 a plurality of electrodes
are embedded in the remote outer surface of the protruding portion
of the sensor probe head portion beyond the poles of the electromagnet.
While the electrodes are preferably mounted flush with the outer
surface of the head portion, the electrodes may be recessed slightly
as shown in FIGS. 8a-8c to eliminate contamination from grease or
oil. The plurality of electrodes preferably comprise a pair of sensing
electrodes 18 and a ground electrode 20. The sensing electrodes
are symmetrically arranged relative to the electromagnetic field
axis along a line transverse to the direction of fluid flow. The
sensor probe is formed of any suitable material such as a synthetic
plastic material which is light-weight, durable, inexpensive, and
resistant to corrosion.
The apparatus for mounting the flow meter sensor probe in an opening
contained in the conduit will be described with reference to FIGS.
4a and 4b. The sensor probe head portion 6a is threadably connected
with the generally cylindrical probe body portion 6b and the cable
12 to the signal processor extends from the probe body portion.
An end cap 22 is adapted to fit over the end of the probe body portion.
The sensor probe head portion is inserted into a compression seal
assembly 24 having a compression nut 26. Tightening of the nut compresses
the seal assembly into sealing engagement with the sensor probe
head portion. The compression seal assembly includes a lower restraining
plate 28 and threaded retaining rods 30 pass through aligned openings
in the end cap and lower restraining plate, respectively. Tightening
of the nuts 32 serves to draw the end cap toward the lower restraining
plate to further connect the probe assembly with the compression
seal assembly.
The lower end of the compression seal assembly 24 is threadably
connected with a ball valve assembly 34 including a knob 35 for
opening and closing the valve in a manner similar to a stop cock.
An opening is cut into the wall of the conduit by conventional means
(not shown) and a threaded pipe nipple 36 is threadably connected
with the pipe nipple, whereby the sensor probe head portion 6a protrudes
within the conduit 4 as shown in FIG. 4b. The degree of protrusion
of the sensor probe head portion into the conduit is controlled
via the upper captive nuts 33. Turning the captive nuts 33 in opposite
directions extends and retracts the sensor probe head portion, respectively,
relative to the conduit. The limited protrusion of the sensor probe
head portion into the conduit further prevents debris from accumulating
on the probe. A clamping assembly 38 as shown in FIG. 4b is also
provided to secure the mounting assembly in position adjacent the
opening in the conduit and to maintain the longitudinal axis of
the probe normal to the direction of fluid flow.
Energization of the electromagnet in the sensor probe head portion
establishes an electromagnetic field in the fluid, the axis of the
field being arranged normal to the direction of fluid flow. In accordance
with Faraday's Law, a voltage is generated in the fluid as the fluid
moves through the electric field, the voltage being directly proportional
to the velocity of the fluid. The sensing electrodes embedded in
the surface of the sensor probe head portion sense the generated
voltage and produce electrical signals which are a function of the
flow velocity. The electrical signals are delivered to the signal
processor via the electric cable.
The electrical signals produced by the sensor probe of the subject
invention are more accurate representations of fluid flow velocity
than the signals produced by prior electromagnetic sensor probes
for a number of reasons. Of primary importance is the arrangement
of the sensing electrodes relative to the electromagnetic field.
As noted above, the longitudinal axis of the probe is arranged normal
to the direction of fluid flow, as is the longitudinal axis of the
electromagnetic field. Owing to this arrangement, flow profiles
are taken in a plane perpendicular to the direction of fluid flow,
resulting in very accurate flow profile. The sensing electrodes
are arranged in an "end fire" configuration wherein the
electrodes are placed beyond the poles of a sinusoidal electromagnetic
field rather than midway between the poles. This design allows the
sensing of fluid flow velocity to occur at the remote end of the
sensor probe head portion for minimum protrusion in the conduit.
Moreover, the probe senses the velocity of fluid at a spaced distance
therefrom. This characteristic of the invention is significant since
nonlinearities in the electrical signals produced by the sensing
electrodes are diminished. More particularly, the protruding probe
head produces turbulence and drag in the fluid about the probe surface.
When sensed by the electrodes, this flow/sensor interaction produces
signals that do not give an accurate measure of the free stream
velocities. With the "end fire" electrode configuration,
however, the interference of the eddies is reduced since the electrodes
are able to sense the velocity of fluid flow at a spaced distance
from the probe head well beyond the near surface of the probe head
portion.
Referring now to FIG. 5 the signal processor 10 will be described.
A power supply 40 provides power to the signal processor 10. The
signal processor includes a signal generator 41 which delivers switching
signals to a magnet driver 42 which in turn supplies an alternating
current to the electromagnet 14 to generate an electromagnetic field.
The signal generator 41 also delivers operating signals to a detection
amplifier 43 of the signal processor. The flow of fluid through
the electromagnetic field generates a voltage proportional to the
flow velocity. The voltage is sensed by the sensing electrodes 18
as indicated schematically by the arrow 44. In response to the sensed
voltage, the electrodes generate electrical signals which are amplified
and delivered to the detection amplifier of the signal processor
where the electrical signals are extracted from electrical background
noise.
The electrical signals corresponding to the velocity flow are adjusted
in a conventional manner by an adjustment device 46 to produce a
signal that represents the mean velocity which is delivered to a
signal modifying apparatus 48 to provide a Reynolds correction as
will be developed in greater detail below. For example, for normal
well-developed flow, the mean velocity V is equal to the point velocity,
when the point velocity is measured at 1/8 of the diameter of the
conduit. If the flow conditions are not normal or are unknown, the
sensor probe is used to profile the conduit to determine the mean
velocity V. This is accomplished by sensing the velocities at various
points along the diameter of the conduit with the probe and then
averaging the various sensed velocities. In either case, after the
mean velocity is calculated, it is recorded and the sensor probe
is retracted to its normal measurement position wherein the sensor
probe head portion protrudes into the conduit and the cylindrical
body portion extends on the outside of the conduit. The flow meter
is then adjusted to read the calculated mean velocity V. The adjustment
is made because the velocity profiles in a conduit are such that
the velocity of a fluid near the conduit wall is less than the mean
velocity.
The signal processor 10 further includes a signal modifying apparatus
48 for modifying the mean velocity signal in a dynamic sense to
correct nonlinearities in Reynolds number. More specifically, fluid
flow velocity profiles change with changes in velocity and/or viscosity
in a fixed diameter conduit. Variances in the velocity profile reflect
variances in Reynolds number and result in errors in fluid flow
determination by point measurement techniques. To provide correction
for this nonlinear phenomenon, a portion of the mean velocity signal
is supplied to a multiplier 50 such as the National Semiconductor
Model LH 0094 where the signal is squared. The squared portion of
the mean velocity signal is then supplied to an adder 52 where it
is combined with the original velocity signal to produce a modified
mean velocity signal having a first order correction. Additional
modification of the mean velocity signal can be performed by analog
circuitry or by a digital microprocessor if high accuracies in the
measuring of the mean velocity signal are desired.
Since the actual flow velocity Q of a fluid flowing in a conduit
having an area A is given by the equation
it is necessary to provide an input signal representing the area
of the conduit. A conduit area signal device 54 is provided by which
the conduit area is entered into the signal processor for combination
with the modified mean velocity signal to provide the actual flow
velocity Q. For example, where the conduit is cylindrical in shape,
the area of the conduit is given by the equation
where r=radius of the conduit. Accordingly, the conduit area signal
means produces an area signal for a conduit containing a fluid flow
whose velocity is to be measured in accordance with the radius of
the conduit.
The resulting flow velocity signal is delivered to a display device
56 for display.
A schematic representation of the mean velocity signal modifying
apparatus of the signal processing device is provided in FIG. 6.
A portion of the mean velocity signal from the amplifier is supplied
to a first operational amplifier 58 the output of which is delivered
to the multiplier circuit 50 where the signal is squared. Since
the flow meter may be used in pipes of different diameters, the
mean velocity signal modifying apparatus is made diameter sensitive.
More particularly, it is known that the velocity profile near the
wall of a conduit varies with Reynolds number which is a function
of velocity, conduit diameter, and fluid viscosity. Assuming the
fluid viscosity to be constant, a correction for non-linearities
in the velocity profile owing to conduit diameter must be introduced
into the mean velocity adjustment. The correction is a function
of the ratio of the probe insertion depth (i.e. the distance from
the conduit wall where the electrodes sense flow velocity) to the
conduit internal diameter. A selector 60 comprising a plurality
of resistors, switches S.sub.1 -S.sub.5 and a second operational
amplifier 62 automatically produces the appropriate correction or
compensation for Reynolds number non-linearities in accordance with
the manual input--via the switches--of pipe diameter. The output
from the selector is delivered to the adder 52 which includes a
third operational amplifier 64 such as the National Semiconductor
Model LM 324. At the adder 52 the squared portion of the velocity
signal is combined with the original mean velocity signal to produce
a velocity output signal compensated for nonlinearities in Reynolds
number.
Referring now once again to FIGS. 3a-3c, the preferred configuration
of the sensor probe head portion will be described. The remote portion
of the sensing head portion 6a (i.e. the portion furthest from the
probe body portion 6b) comprises a pair of spaced, parallel, generally
parabolic portions 70 which define a trough 72 therebetween. When
mounted in an opening in the conduit wall 4 the parabolic portions
70 and the trough 72 protrude into the interior of the conduit while
the electromagnet 14 arranged within the sensing head portion is
arranged adjacent the conduit opening. The parabolic portions 70
are arranged generally parallel with the direction of fluid flow
so that the fluid may flow directly through the trough 72 with little
or no impedance in flow owing to the parabolic portions. The sensing
electrodes 18 are arranged at the apices of the parabolic portions,
respectively, as shown in FIGS. 3a and 3b, and the ground electrode
20 is arranged between the parabolic portions in the base of the
trough adjacent the periphery of the sensing head portion as shown
in FIG. 3c. With this preferred sensing head portion configuration,
all of the fluid which flows through the trough contributes to the
electrical signal produced by the sensing electrodes as does the
fluid just outside of the trough. Since these fluids are spaced
from the eddies produced at the probe surface, they do not contribute
nonlinearities to the electrical signal as do the fluids which flow
against the probe surfaces. Moreover, this probe head configuration
is suitable for use in sensing the velocity of a bidirectional flow
since the electrodes are capable of producing signals from fluids
both upstream and downstream of the probe. Finally, owing to the
parabolic configuration of the head portion, debris from the fluid
does not collect on the probe.
Referring now to FIGS. 7a-7d, an alternative configuration of the
probe sensing head portion 106a is shown. The outer surface of the
sensor probe head portion 106a has, in longitudinal cross-section
relative to a plane containing the longitudinal axes of both the
conduit and the probe, an oblique substantially wedge-shaped configuration
to define, in the direction of fluid flow in the conduit, an acute
angle .alpha. relative to the inner wall surface of the conduit
4 as shown in FIG. 1 which diverges from the front portion of the
probe sensing head to the rear portion thereof. More particularly,
the angle .alpha. is an acute angle between the conduit inner wall
surface and a line substantially tangent to the surface of the probe
heat portion. The diverging angle .alpha. may be in the range between
25.degree. and 35.degree. and is preferably 30.degree.. With such
a configuration, the sensor probe head portion is particularly adapted
for shedding debris which may be contained in the fluid flowing
through the conduit. Because the electrodes are embedded in the
surface of the probe head portion and owing to the oblique configuration
thereof, no obstructions are provided in the conduit to which debris
may attach. The probe is thus self-cleaning to insure that accurate
measuring of fluid flow is continuously afforded since the electrodes
are always free of debris. As shown in FIGS. 7b, 7c, and 7d, the
ground electrode 120 is embedded within the oblique outer surface
of the sensor probe head portion adjacent the front peripheral portion
thereof. In the arrangement of FIGS. 7a, 7b, and 7c, the sensing
electrodes 118 are arranged toward the rear portion of the sensing
probe head portion and are spaced by an angle .beta. of between
180.degree. and 220.degree. relative to the longitudinal axis of
the probe in the direction of said ground electrode. With such an
arrangement of the sensing electrode, the electrical signal produced
by the electrodes contains a large number of non-linearities due
to flow disturbance of the sensor and variations due to changes
in Reynolds number. Accordingly, in an alternate arrangement shown
in FIG. 7d, the sensing electrodes are positioned in the front portion
of the probe sensing head and are spaced by an angle .DELTA. of
between 90.degree. and 110.degree. relative to the probe axis in
the direction of the ground electrode. The electrical signal produced
by the sensing electrodes arranged in FIG. 7d is more linear than
that produced by the electrode configuration of FIG. 7c since the
electrodes are placed where they are less affected by the sensor
interference with the flow and variations due to changes in Reynolds
number.
In a third embodiment shown in FIGS. 8a-8c, the outer surface of
the sensor probe head portion 206a has a dome-like projection 80
which protrudes into the interior of the conduit. The transverse
thickness of the projection is less than the longitudinal thickness
thereof. More particularly, as shown in FIG. 8a, the sides of the
projection are tapered, whereas in FIG. 8b, the front and back portions
of the projection define a generally parabolic curve. The sensing
electrodes 218 are centrally arranged on opposite sides of the projection
and may be recessed as shown in the drawing, if desired. The ground
electrode 220 is arranged in the front portion of the sensing head
portion adjacent the periphery thereof as shown in FIG. 8c. The
dome-like projection is capable of shedding debris in the same manner
as the probe head portions shown in FIGS. 3 and 7.
While in accordance with the provisions of the Patent Statues the
referred forms and embodiments of the invention have been illustrated
and described, it will be apparent to those skilled in the art that
various changes and modifications may be made without deviating
from the inventive concepts set forth above. |