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.

Abstract: A liquid level detector for detecting liquid level height in a vessel includes a housing adapted to float in the liquid, first and second spaced, fixed stops disposed in the housing, a pendulum having a end pivotally mounted in the housing wherein the pendulum is movable between the stops in response to a change in orientation of the housing due to a change in liquid level height in the vessel, an electrical switch disposed of in the housing adjacent to the first stop and actuable by the pendulum and a cable for tethering an end of the housing to the vessel.

Abstract: A process instrument enclosed in a hermetically sealed housing for indicating and controlling a measured variable, such as the level in a tank. The instrument includes magnetically operated electrical switches actuable between open and closed positions in response to the measured variable. The electrical switches used are capable of switching current levels on the order of 10 Amps.

Abstract: A level detecting device for detecting the level of liquid in a tank includes a displacer or float displaceable in response to the level of the liquid in the tank, an enclosure tube disposed on the tank and sealed thereagainst so that the exterior of the enclosure tube is isolated from the liquid in the tank and a variable transformer for detecting the displacement of the float. The transformer includes at least two windings disposed adjacent the outside of the enclosure tube and an elongate core disposed within the enclosure tube and coupled to the float so as to be movable therewith, the amount of magnetic coupling between the windings being dependent upon the position of the core in the enclosure tube. Means are provided for generating an output signal representing the amount of magnetic coupling between the windings to derive an indication of the level of the liquid in the tank.

Abstract: A level sensor for sensing the level of viscous, electrically conducting materials in a tank includes a variable capacitor, the capacitance of which varies in response to changing material level height in the tank. The variable capacitance is connected through a charging resistor to a comparator to form an astable multivibrator which develops an output signal having a period dependent partly upon the capacitance of the variable capacitor. The charging resistor has a resistance which is small with respect to the resistance of the material so that build up or coating of material on the variable capacitor has little or no effect on the measurements. The frequency of the output signal from the multivibrator is divided to derive a signal of relatively low frequency which may be coupled to a remote location by a two wire conductor.

Abstract: A capacitance sensing circuit utilizes an operational amplifier connected in a comparator configuration. The capacitance to be measured is connected to one input of the operational amplifier and is also connected to the output of the operational amplifier through a resistor to form a free-running multivibrator. The period of the multivibrator output depends upon the value of capacitance being measured. A switching transistor is connected to the output of the operational amplifier to cause a changing current output signal. The output signal is obtained as a change of current in a current sensor connected to the collector of the transistor.