Patent ID: 6016697
Filing Date: 2000-01-25
Classification: G01F,G05D

Abstract:
A capacitive level detector for detecting the level of a contained liquid, the detector comprising:an elongate capacitive probe sensor having a first end for being submersed in a contained liquid and a second end for being positioned above the surface of said liquid, said probe sensor having a capacitance corresponding to the extent of the probe's immersion in the liquid, and having a central axis extending the length of said sensor, wherein said length is defined by the distance between the first and second ends;a capacitive liquid reference sensor adjacent the first end of said probe sensor, said liquid reference sensor for being substantially continuously submersed in the contained liquid and having a first capacitance when the sensor is submersed in the liquid and a second capacitance which is different from the first capacitance when the sensor is exposed to gas;a capacitive gas reference sensor adjacent the second end of said probe sensor, said gas reference sensor for being substantially continuously exposed to the gas above the surface of the contained liquid and having a first capacitance when the sensor is exposed to gas and a second capacitance which is different from the first capacitance when the sensor is submersed in liquid; anda controller connected to said probe sensor and said reference sensors to determine level based on the capacitances of the sensors, said controller having a memory for storing calibration values corresponding to capacitances of said sensors when placed in a calibration or identical medium, wherein said calibration values are determined by the controller in accordance with the following equations: ##EQU9## where: V.sub.lrefcal is a measured voltage corresponding to the capacitance of the liquid reference sensor when placed in the calibration or identical medium,V.sub.grefcal is a measured voltage corresponding to the capacitance of the gas reference sensor when placed in the calibration or identical medium,V.sub.probecal is a measured voltage corresponding to the capacitance of the probe sensor when placed in the calibration or identical medium,R.sub.lg is a calibration ratio of the liquid reference sensor capacitance to the gas reference sensor capacitance, andR.sub.gp is a calibration ratio of the gas reference sensor capacitance to the probe sensor capacitance.