Point level switch
First Claim
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1. An instrument for sensing level of a material comprising:
- a variable capacitor having an electrode adapted to be immersed in a material, the capacitance of the electrode being a function of material level;
an oscillator circuit operable to develop an output signal having a period which is dependent upon an input capacitance;
a switching circuit connected to said variable capacitor and to said oscillator circuit for selectively connecting said electrode to said oscillator circuit to measure said variable capacitance;
a determining circuit connected to said oscillator circuit to determine the period of said output signal and including self-test means controlling operation of said switching circuit to disconnect said electrode from said oscillator circuit for testing existence of an abnormal electrode condition.
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Abstract
A point level sensor senses capacitance of a material in a vessel to determine level. The capacitance is input to a comparator which is part of an oscillator circuit developing a square wave signal having a period corresponding to capacitance. A microprocessor includes an internal counter which determines the period of four pulses to determine an average period. The microprocessor drives relay circuits providing discrete outputs according to specific precalibrated levels to be sensed. The point sensor includes a self-test mode which operates once per second to test for shorting of the probe, expected frequency response and an open probe.
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Citations
20 Claims
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1. An instrument for sensing level of a material comprising:
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a variable capacitor having an electrode adapted to be immersed in a material, the capacitance of the electrode being a function of material level; an oscillator circuit operable to develop an output signal having a period which is dependent upon an input capacitance; a switching circuit connected to said variable capacitor and to said oscillator circuit for selectively connecting said electrode to said oscillator circuit to measure said variable capacitance; a determining circuit connected to said oscillator circuit to determine the period of said output signal and including self-test means controlling operation of said switching circuit to disconnect said electrode from said oscillator circuit for testing existence of an abnormal electrode condition. - View Dependent Claims (2, 3, 4, 5, 6, 8, 9, 10, 11)
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7. The instrument of claim wherein said oscillator circuit and said switching circuit are contained in a first housing and said determining circuit is contained in a second housing.
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12. An instrument for sensing level of a material in a vessel comprising:
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a variable capacitor having a probe adapted to be immersed in a material, the capacitance of the probe being a function of material level in the vessel; an oscillator circuit connected to said probe operable to develop an output pulse signal having a period which is dependent upon the capacitance of the probe; a determining circuit connected to said oscillator circuit to determine the period of said output signal and including a counter means for counting an amount of time required to receive a plurality of pulses and dividing said time amount by a factor related to the number of said pulses to determine an average pulse period. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20)
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Specification