Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Prior U. S. patents 4,074,184 granted 2/14/78,
4,063,153 granted 12/13/77 and 4,082,994 granted 4/4/7~,
all of common assignment with the application describe
inventions for measuring respective fractions of mixed
phased fluids, particularly gas and conductive liquid or
solids entrained in liquids utilizing means partlcularly
suited for flow which develops a conductance under a voltage
field or alternatively utilizing capacitance techniques for
non-conductive fluids.
However, there remains a need for means for
measuring liquid content of a mixed stream of liquid and
gas in situations when the liquid fraction of a stream may
be very small (less than 10%) -- a condition occurring, for
instance, in some steam flows, which may be measured.
It is therefore the object of the invention to
provide commercially practical apparatus meeting such need
reliably and economically.
In accordance with the present invention, the
amount of liquid is measured by employing its dielectric
characteristics. There are two distinct advantages of
employing a dielectric capacitive measurement of the amount
of water present as a means of establishing respective
fractions of water and gas. First, the water does not have
to form a conductive lattice between electrodes as is
required in conductive or impedance techniques. Second,
the dielectric constant of water is very high when compared
with most other materials. When the direct measurement of
the capacitive component of water is attempted, the rather
large conductive component usually causes amplifier over-
load. The practice of the present invention involves means
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to automatically cancel out the conductive component toprevent amplifier overloading.
In accordance with a particular embodiment, a
method of measuring liquid content of mixed phase fluid
in a container wherein a conductive liquid contains voids
therein comprises: making a capacitive current measurement
between two spaced (in cross-section of the fluld container)
electrodes, and cancelling out a conductive (electrically)
component of the current measurement through the fluid
inherently picked up in making the capacitive measurement
so that the capacitive measurement is not masked by the
conductive measurement and wherein the measured current is
produced by application of an oscillating voltage to the
electrodes and the output capacitive current component
~or a voltage derivative thereof) is shifted into phase
with the oscillating voltage.
From another aspect, and in accordance with the
invention, an apparatus for practicing the method of the
invention comprises: means for applying an oscillating
voltage to a mixed phase medium including relatively con-
ductive liquid and relatively non-conductive second phase,
first means for detecting current produced by said voltage
in the medium as a current signal with mixed and varying
- conductive and capacitive components, second means apply-
ing such current to a summing junction together with a valid
transfer function derivative of the conductive component,
third means for producing such valid transfer function,
fourth means for converting a current output of the summing
junction to a voltage and applying components thereof to
separate means for indicating conductive and capacitive
components of the original measured component.
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Other objects, features and advantages of the
invention will be apparent from the following detailed
description of preferred embodiments thereof, taken in
connection with the accompanying drawing, in which:
FIGURE 1 is a block diagram of a circuit to
accomplish the automatic cance]lation
of the conductive components;
FIGURES lA-lC are voltage vs. time traces taken
for different points of the circuit of
Figure 1, as mentioned below, and
FIGURE 2 is a schematic of a variant embodiment
of the invention,
The Figure 1 circuit comprises a sinusoidal oscil-
lator 20 w~ich drives one plate, e.g., 12 of a sensor 10 so
that a current (IG + jB ), due to bo-th the conductive and
capacitive components, appears at the opposed plate 14. The
sensor 10 and electrical circuit components, unless stated
otherwise herein, can be essentially described in the above
cited U, S. patents.
The oscillator output also is fed to a comparator
22 to detect the axis crossing of the sensor excitation
voltage (see Figure lC), Also, the oscillator is fed to
one input of multipliers 23 and 36. The current from the
sensor plate enters the summing junction (~) input of an
amplifier 24 which translates the current to a voltage
signal with some effective gain, A second voltage gain
stage indicated at 26 further amplifies the signal. A
FET switch circuit 2~, driven by the comparator 22, (see
Figure lA) commutates the said amplified voltage signal in
synchronism with the oscillator into an integration circuit
30 ~rectification is also achieved)~ The output of the
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averaging circuit taken off at point 32 of the circuit is
proportional to the conductive current IG (in phase component)
of the sensor current. The signal from the amplifier 26 is
also fed to an integrator 34 which shifts the phase of the
signal by 90, therefore, the capacitive (quadrature) currenk
signal is shifted into phase with the output of the
oscillator (see Figure lB).
The output of the integrator enters multiplier
circuit 36 which feeds an averager 38. The output of the
averager 38 taken at point 40 is a signal proportional to
the capacitive current IB (quadrature current) of the
sensor. Since the conductive current of the sensor usually
far exceeds (in water) the capacitive current, the ampli-
fiers would be driven into saturation unless some means of
nulling the conductive current is employed. To accomplish
this, the proportional conductive component output of signal
available at 32 is fed back to the second input of the
multiplier 23. The output of the multiplier is an inversion
of the oscillator input to the multiplier, but varying in
amplitude. The output of the multiplier is a current, and
as such is fed directly into the summing junction (~) as a
signal nearly nulling the conductive component of the sensor
current~ The averaged output (at 32) is now an error signal
proportional to the conductive component IG. The signal
levels in the amplifiers are now reduced to non-saturation
levels. Thus, it is possible to measure a relatively small
value of capacitance in the presence of a rather large con-
ductance.
After considerable experimentation, it became
evident that unless the plates were bridged by water, the
field would tend to locate in the gap, thus reducing the
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apparent capacitive output due to the presence of the water.
A logical output derived from the conductive output would
allow an operator to provide further gain at the capacitive
output.
A further refinement is to drive the sensor input
to ground, eliminating one active plate, as shown in Figure
2 where a current transformer IXF extracts a measured
current from a grounded sensing electrode GE for application
to the summing junction ~ Also, a driven shield ~S is
extended to form an electrode around the driven electrode
DE to focus the sensing field SF.
It is evident that those skilled in the art, once
given the benefit of the foregoing disclosure, may now make
numerous other uses and modifications of, and departures
from the specific embodiments described herein without de-
parting from the inventive concepts. Consequently, the
invention is to be construed as embracing each and every
novel feature and novel combination of features present
in, or possessed by, the apparatus and techniques herein
disclosed and limited solely by the scope and spirit of
the appended claims.
