Contamination capacitance probe system

A contamination capacitance probe system is disclosed that capacitively mtors whether or not two or more fluids or materials that have to remain separated have been mixed or mingled and that provides first and second control signals respectively representative of the unmingled and mingled conditions. The system includes a capacitance sensor constituted by spaced-apart first and second capacitor plates of preselected size and spacing. The plate size and spacing is selected to render negligible any effect that the different materials or fluids or materials might have on the capacitance of the sensor. Circuit means are disclosed that includes a capacitance to frequency convertor, a frequency to voltage convertor, and a Schmidt trigger operative to provide the first and second control signals. The contamination capacitance probe system of the present invention is operable with any two or more fluids or materials having different dielectric constants and that are either immiscible or miscible.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention is directed to the field of measurement and testing, and 
more particularly, to a contamination capacitance probe system. 
(2) Description of the Prior Art 
In many applications different systems and subsystems employ or are exposed 
to two or more fluids or materials where their mutual separation is 
essential to achieving the intended control or other action, and where 
their co-mingling effects an undesirable contamination. Examples are water 
or other contaminate in the fuel of a fuel burning system; metal, water or 
other contaminate in the motor oil of an internal combustion engine, and 
seawater in an hydraulic ballast and buoyancy compensation unit of an 
unmanned undersea vehicle, among others. 
Whenever the fluids or materials that should be kept separated are caused 
to mingle, due to a failure of some component, system, subsystem or other 
cause, the ability of the component, system or subsystem or other device 
to perform its function or intended design mission becomes compromised. In 
the hydraulic ballast and buoyancy compensation unit of an unmanned 
undersea vehicle, for example, the failure of a rubber bladder that is 
controllably expanded and contracted by pumping hydraulic oil thereto to 
displace different quantities of seawater into and out of the ballast tank 
of the undersea vehicle could prevent the undersea vehicle from trimming 
to a positive buoyancy state and therewith prevent its surface recovery. 
Resistance-type and capacitance-type sensors are known that measure changes 
in level of a fluid or material in which they are immersed or with which 
the are brought into contact. The utility of these heretofore known 
sensors, however, is limited to sensing level changes of a single fluid or 
material but are generally incapable of responding to and providing a 
signal representative of two or more fluids or materials having been mixed 
or mingled. In the case of seawater and oil, for example, the presence of 
the oil, that acts to coat such prior art level sensors both of the 
resistance-type and capacitance-type, effectively prevents their ability 
to respond to the presence of the seawater and oil after they have been 
mixed or mingled. 
SUMMARY OF THE INVENTION 
It is accordingly the principal object of the present invention to provide 
a contamination capacitance probe system that capacitively monitors 
whether or not two or more fluids or materials that have to remain 
separated have been mixed or mingled and that provides first and second 
control signals respectively representative of the unmingled and mingled 
conditions. The contamination capacitance probe system of the present 
invention is operable with any two or more fluids or materials having 
different dielectric constants that are either immiscible or miscible. 
In accord with one feature thereof, a capacitance sensor is disclosed 
having a preselected plate size and plate spacing selected to render 
negligible any effect that would otherwise arise from coating or other 
operational impediment of the capacitance sensor by oil or other fluid or 
material. 
In accord with another feature thereof, circuit means are disclosed that 
respond to the capacitance sensor's capacitance to provide the control 
signals that respectively indicate either that the two or more fluids or 
materials remain separated or have been mixed or mingled. In the preferred 
embodiment, oil and seawater are the two fluids.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, generally illustrated at 10 is a block diagram of 
a typical applications environment in which the contamination capacitance 
probe of system the present invention has exemplary utility. A system, 
subsystem or other device is schematically illustrated in dashed outline 
12 that operatively employs two or more fluids or materials to achieve 
some intended control action or other effect in such manner that the two 
or more fluids or materials are normally kept separated, but which, due to 
failure of some system or subsystem or other cause, are undesirably caused 
to be mixed or mingled, whereby the intended control action or other 
effect might become compromised, as may be found, for example, in a gas 
tank subject to water contamination, water or other contamination of motor 
oil in an internal combustion engine , and seawater contamination of the 
hydraulic ballast and buoyancy compensation unit of an unmanned undersea 
vehicle, among others. 
Member 14 schematically illustrates the component of device 12 where the 
mingling or mixing of the two or more fluids or materials exhibits itself, 
and may be, for the examples given, the gas tank, gas feed line, or 
hydraulic oil reservoir, respectively of a motor vehicle, internal 
combustion engine, and hydraulic ballast and buoyancy compensation unit of 
an undersea vehicle. 
Indicia 16, 18, respectively marked "N", "U" for "nominal" and "undesired", 
schematically represent the two or more fluids or materials that should be 
kept separated but which, when copresent, are indicative of a possible 
failure condition. The fluids 16, 18 may be gas and water; oil and water, 
and oil and seawater, for the examples given. 
A capacitive sensor 20 to be described is positioned within the member 14 
at a location thereof selected to expose the sensor 20 to the fluid or 
material, whether the nominal fluid or material 16, the undesired fluid or 
material 18, or a combination of the two or more fluids or materials 16, 
18. A circuit 22 to be described is coupled to the capacitance sensor 20. 
The circuit 22 responds to the two or more fluids or materials 16, 18 
having been mingled or mixed to provide a signal indicative thereof. 
A processor 24 or other controller is coupled to the circuit 22 and is 
responsive to the signal indicative of the two or more fluids or materials 
having been mixed or mingled to provide a control action, as schematically 
illustrated by an arrow 26. Such a control action may, for example, be to 
compensate the device 12 for any effects that may arise from the 
undesirable mixing or mingling of the two or more fluids or materials 16, 
18, or to provide an alarm indicating such mixing or mingling of the 
fluids, among others. 
Referring now to FIG. 2, generally designated at 30 is a block diagram 
illustrating a novel capacitance contamination probe system constructed in 
accordance with a preferred embodiment of the present invention. The probe 
system 30 includes a capacitor generally designated at 32 consisting of 
two metallic plates 34, 36. The plates 34, 36 of the capacitor 32 have a 
preselected size and spacing selected to render negligible any effect that 
would otherwise arise from coating or other operational impediment of one, 
or both, of the plates 34, 36 by the two o more fluids or materials 16, 18 
(FIG. 1) having been mingled or mixed. In one preferred embodiment for 
detecting the presence of seawater and oil, the plates 34, 36 are about 
ten (10) centimeters by ten (10) centimeters in size and are spaced apart 
about one and one fourth centimeters (1.25), which has been found to 
render negligible the effects of oil coating the plates that arises when 
oil, which is the nominal fluid between the plates, is wholly or partially 
displaced by seawater, which is the undesired fluid between the plates, as 
the seawater flows between the plates due to failure of a rubber bladder, 
not shown, that controls a buoyancy tank, not shown, of an unmanned 
undersea vessel, not shown. In this embodiment, the plates 32, 34 are 
disposed at the bottom of an oil reservoir, not shown, of an oil pumping 
system, not shown, that is connected to the rubber bladder. The rubber 
bladder displaces different amounts of seawater, depending on the quantity 
of oil pumped into and out of it, and therewith controls the buoyancy of 
the buoyancy tank. 
A capacitance to frequency converter 38 is connected to the plates 34, 36 
of the capacitor 32. The capacitance to frequency converter 38 is 
responsive to the capacitance between the plates 34, 36 to provide a 
frequency signal representative thereof. In one presently preferred 
embodiment, where the capacitor 32 is nominally exposed only to oil, but 
in the case of a failure condition is undesirably exposed to seawater, 
which is immiscible in the oil, the capacitance defined between the plates 
34, 36 undergoes a shift in value. The dielectric constant of seawater is 
on the order of thirty-five (35) times greater than that of hydraulic oil. 
The capacitance to frequency converter 38 is responsive to change in 
capacitance and varies its output frequency accordingly. In the presently 
preferred embodiment, the capacitance to frequency converter 38 is 
implemented with a "555" capacitance responsive oscillator timing chip 
commercially available from National Semiconductor, although any other 
suitable capacitance to frequency converter may be employed as well 
without departing from the inventive concept. The capacitance to frequency 
convertor is operative to output a first frequency typically 11,000 Htz., 
when oil alone is present between the plates 32, 34 of the capacitor 30, 
and a second predetermined frequency, typically 345 Htz., when seawater is 
present therebetween due to a failure condition having arisen that mixes 
the seawater and oil in the hydraulic oil reservoir. As seawater is denser 
than hydraulic oil, the seawater displaces the oil nominally found between 
the plates, and therewith changes the dielectric constant of the capacitor 
30. 
A frequency-to-voltage converter 40 is coupled to the frequency signal 
output of the capacitance to frequency converter 38. The frequency to 
voltage converter 40 produces a voltage output signal having an amplitude 
that is directly proportional to the particular frequency of the variable 
frequency output signal of the capacitance to frequency converter 48. The 
frequency to voltage converter 40 preferably is a Model 4702 converter, 
commercially available from Teledyne Philbrick, although any other 
suitable frequency to voltage converters may be employed without departing 
from the inventive concept. 
The frequency to voltage converter is operative to output a first 
predetermined voltage, typically 11.5 volts, when the first predetermined 
frequency is being output from the capacitance to frequency convertor 38, 
and a second predetermined voltage, typically 0.1 volts, when the second 
predetermined frequency is being output therefrom. 
A Schmitt trigger 42 is coupled to the frequency to voltage converter 40. 
The Schmitt trigger 42 threshold is selected to lie between the first and 
second predetermined voltages output by the frequency to voltage 
convertor. In response to the voltage signal output by the frequency to 
voltage convertor 40 exceeding the predetermined threshold, the Schmidt 
trigger 42 produces an output signal that is representative that the two 
or more fluids or materials have been undesirably mixed or mingled. 
A processor 44 or other control device is coupled to the Schmitt trigger 
42. In response to an output signal from the Schmitt trigger 42, the 
processor 44 provides an output control signal 46. The output control 
signal 46 may indicate or compensate for the failure that resulted in the 
two or more fluids or materials having become mixed or mingled. 
Many modifications of the presently disclosed invention will become 
apparent to those skilled in the art without departing from the inventive 
concept. The contamination capacitance probe system could be used for many 
applications besides the sensing of seawater in oil of the exemplary 
application. For example, by varying the dimensions of the capacitor 
plates, and by tuning both the capacitance responsive oscillator timing 
chip frequency and the frequency to voltage converter, a wide variety of 
different contamination types may be sensed. Furthermore, the Schmidt 
trigger, that provides a good/bad output signal, may be replaced by 
circuitry that provides a continuously variable or more finely graded 
output signal, such as would be provided by a bank of comparators or other 
circuitry. The output control signal could also be used to sound an alarm 
bell, among other things.