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US 5,859,536 AGranted
Moisture sensor having low sensitivity to conductance changes
AssigneeOliver Haugen
InventorPaul Stockton
Life of this patent
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Abstract
A sensing device includes a pair of sensing electrodes disposed within a medium and a circuit connected to the sensing electrodes via impedance matching networks for producing an output signal which varies in response to a capacitive change in the medium. The circuit includes a first circuit portion including the sensing electrodes and a second circuit portion including the oscillator. The first and second circuit portions are tuned to match their impedance for more accurately measuring the capacitive changes.
Claims20
A moisture sensing device for measuring the moisture content of a medium, comprising:
- a pair of sensing electrodes disposed within said medium;
- a circuit connected to said pair of sensing electrodes for producing an output signal which varies in response to a change in the dielectric constant of said medium, said circuit comprising
- an oscillator providing a signal having a constant frequency;
- an electrode circuit portion including said sensing electrodes and said medium, said electrode circuit portion having an output resistance sufficiently low to be insensitive to a chance in conductance of the medium, and an output reactance which varies substantially in response to the change in the dielectric constant of said medium, said electrode circuit portion providing the output signal;
- and
- a matching circuit portion, coupled between the oscillator and the electrode circuit portion, the impedance of the matching circuit portion approximately matching the impedance of the electrode circuit portion at the constant frequency, if the medium has a predetermined condition, wherein a measurable mismatch occurs when the dielectric constant of said medium varies from the dielectric constant at the predetermined condition, indicating a change in moisture content of said medium.
3 more elements
A method of measuring a water content of a medium, comprising:
- providing a moisture sensing device including a pair of sensing electrodes disposed within said medium and an impedance matching circuit which includes a conductance in parallel with the sensing electrodes, said impedance matching circuit having an output resistance sufficiently low to be insensitive to a change in conductance of the medium, and an output reactance which varies substantially in response to the change in the dielectric constant of said medium;
- operating an oscillator at a constant frequency, the oscillator being coupled to the sensing device by the impedance matching circuit;
- disposing said electrodes in said medium;
- measuring an output signal of the impedance matching circuit;
- and
- detecting a mismatch in said impedance matching circuit, based on the output signal of the sensing electrodes, if a moisture content of said medium varies over time, the mismatch indicating a change in the dielectric constant of said medium.
3 more elements
A moisture sensor comprising:
- a polymeric housing;
- a printed circuit board mounted within said housing;
- a pair of sensing electrodes electrically connected to said printed circuit board and disposed through said housing into a medium to be measured;
- said printed circuit board comprising:
- an oscillator which operates at a constant frequency;
- and
- a circuit connecting the sensing electrodes to the oscillator, said circuit having an output resistance sufficiently low to be insensitive to a change in conductance of the medium, and an output reactance which varies substantially in response to the change in the dielectric constant of said medium, the circuit providing an output signal that indicates a matched condition at the constant frequency when the medium has a predetermined dielectric constant, the output signal indicating a measurable mismatch when the dielectric constant of said medium varies from the dielectric constant at the predetermined condition, indicating a change in moisture content of said medium.
4 more elements
Dependencies are inferred from the claim text.