Capacitive liquid level sensor

A non-intrusive fluid level detector for detecting a predetermined level of liquid in a container, comprising:

• container means for containing the liquid, said container means fabricated from an electrically insulating material;
• at least two conductive plates situated in the same plane and mounted to said container means, said at least two conductive plates being cooperative with liquid in the container means to produce a sensor capacitor having a capacitance which varies in magnitude in accordance with the level of liquid contained within the container means;
• detection circuitry coupled to said sensor capacitor and responsive to a difference between a voltage decay signal derived from said sensor capacitor and a reference voltage decay signal derived from a reference capacitor to provide an output signal when a predetermined voltage difference is detected therebetween;
• and
• indicating means coupled to said detection circuitry and responsive to said output signal to provide an indication of predetermined level of liquid contained within the container means.

A non-intrusive fluid level detector for detecting a predetermined level of liquid in a container comprising:

• container means for containing the liquid, said container means fabricated from a metallic, electrically conductive material, said container means including a window of non-conductive material as an integral portion of a wall of said container means;
• a single capacitance plate for mounting to said window of non-conductive material, said capacitance plate being cooperative with said electrically conductive receptacle and the liquid in the container means to produce a sensor capacitor having a capacitance which varies in magnitude in accordance with the level of liquid contained within the container means;
• detection circuitry coupled to said sensor capacitor and responsive to a difference between a voltage signal derived from said sensor capacitor and a reference voltage to provide an output signal when a predetermined voltage difference is detected;
• and
• indicating means coupled to said detection circuitry wherein said indicating means is responsive to said output signal to provide an indication of a predetermined level of liquid contained within the container means.

A non-intrusive capacitive liquid level sensor for detecting a predetermined level of liquid in a container, comprising:

• non-conductive container means for storing liquid;
• at least two conductive plates situated in the same plane wherein said at least two conductive plates are cooperative with the liquid in the container means to provide a sensor capacitor having a capacitance which varies in magnitude with respect to the amount of liquid present between said at least two conductive plates;
• detection circuit means coupled to said sensor capacitor, said detection circuit means including:
• an oscillator circuit coupled to said sensor capacitor for providing an oscillating voltage signal to said sensor capacitor, said sensor capacitor providing a sensor voltage decay signal as the voltage across said sensor capacitor decays over time in response to a negative transition in the oscillating voltage signal;
• a reference capacitor circuit coupled to said oscillator circuit for receiving said oscillating voltage signal and providing a reference voltage decay signal, as the voltage across said reference capacitor decays over time in response to a negative transition in the oscillating voltage signal;
• a voltage comparator circuit coupled to said sensor capacitor and to said reference capacitor circuit for receiving and comparing said reference voltage decay signal and said sensor voltage decay signal and providing an output signal when a predetermined voltage difference is detected therebetween;
• and
• indicating means coupled to said detection circuit means, responsive to said output signal to provide an indication of the presence or absence of a predetermined level of liquid in the container means.

A non-intrusive capacitive liquid level sensor for detecting a predetermined level of liquid in a container, comprising:

• electrically conductive container means for storing liquid, said container means including, as an integral portion of a wall of said container means, a window of non-conductive material;
• a single capacitance plate for mounting to said window of non-conductive material, wherein said single capacitance plate is cooperative with said electrically conductive container means and the liquid therein to provide a sensor capacitor having a capacitance which varies in magnitude with respect to the amount of liquid present in said container means;
• detection circuit means coupled to said sensor capacitor, said detection circuit means including:
• an oscillator circuit coupled to said sensor capacitor for providing an oscillating voltage signal to said sensor capacitor, said sensor capacitor providing a sensor voltage decay signal as the voltage across said sensor capacitor decays over time in response to a negative transition in the oscillating voltage signal;
• a reference capacitor circuit coupled to said oscillator circuit for receiving said oscillating voltage signal and providing a reference voltage decay signal, as the voltage across said reference capacitor decays over time in response to a negative transition in the oscillating voltage signal;
• a voltage comparator circuit coupled to said sensor capacitor and to said reference capacitor circuit for receiving and comparing said reference voltage decay signal and said sensor voltage decay signal and providing an output signal when a predetermined voltage difference is detected therebetween;
• and
• indicating means coupled to said detection circuit means and responsive to said output signal to provide an indication of the presence or absence of a predetermined level of liquid in the container means.

A non-intrusive liquid level sensor for detecting a predetermined level of liquid contained within a receptacle, comprising:

• a single capacitance plate mounted to said receptacle, said capacitance plate being cooperative with the liquid in the receptacle and the residual ground of the receptacle to produce a sensor capacitor having a capacitance which varies in magnitude in accordance with the level of liquid contained within the receptacle;
• detection circuitry coupled to said capacitance plate and responsive to a difference between a voltage signal derived from said sensor capacitor and a reference voltage signal to provide an output signal when a predetermined voltage difference is detected;
• and
• indicating means coupled to said detection circuitry and responsive to said predetermined voltage difference as detected by said detection circuitry to provide an indication of a predetermined level of liquid contained within the receptacle.