Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2003-04-10
2004-04-27
Hofsass, Jeffery (Department: 2636)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S612000, C340S618000, C340S623000, C073S29000R, C073S291000, C073S30400R
Reexamination Certificate
active
06727822
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of sensors and, in particular, to a liquid level sensor apparatus and method of using thereof.
BACKGROUND OF THE INVENTION
The use of devices to indicate the level of liquid within a tank or container is well known in the art. The need for knowing whether a liquid in a container is above or below a predetermined level is found in many very different situations. Typical of those situations where the liquid level must not be permitted to fall below a predetermined level are found in automobile radiators, windshield washer containers or oil in an engine crankcase. The situation where the level can be too high is also common such as found with a recreational vehicle holding tank. Applications for such equipment is also found in industry where frequently the liquid in question in extremely corrosive as experienced with chemical reagent reservoirs which must be filled once the chemical has fallen below a particular amount in order to keep a process running.
A typical float-type of liquid level sensor is disclosed in U.S. Pat. No. 4,386,337, issued to Todd on May 31, 1983. This device is said to be useful for measuring the level of oil in an engine crankcase or transmission fluid in a reservoir. An electrically conductive float disposed within an encapsulator and movably responsive to the liquid level is provided. This movement is then determined. The problem associated with sludge buildup resulting in shorts bypassing the proper electrical pathways is not directly discussed. While Todd does suggest that by having his encapsulator with open upper and lower ends provides for self-flushing of debris, it is unclear how this can inhibit or stop the problems associated with residue formation, particularly as found in older systems.
The type of devices used to measure the liquid level also varied substantially. The use of floats, changes in capacitance, optical readers, weight measurements of the liquid are just a few examples of techniques that have been adopted to serve as an indicator of the surface level of a liquid. Some devices have attempted to utilize non-invasive measures, that is, a sensor that is outside of the liquid container.
Representative of this genre is U.S. Pat. No. 4,749,988, issued to Berman et al, on Jun. 7, 1988. The '988 discloses the use of a pair of conductive band electrodes oriented in parallel so that the gap between the electrodes corresponds to the liquid surface level to be measured. The change in capacitance between the electrodes as the liquid level rises and falls serves as the indicator. While this device solves the corrosive problem found in processing industries using materials in liquid form, its use is limited to containment vessels that have sufficient outside room to put the sensor in place. Further, this type of sensor is expensive to manufacture in that it must be shaped to correspond to the particular shape of the vessel being used. Also, the thickness and material used for the vessel can also appreciably impact the accuracy and sensitivity of the device. Whether this will also solve the problem of residue buildup on the inside of the container is not known as Berman et al. does not discuss this issue.
U.S. Pat. No. 5,315,872, issued to Moser on May 31, 1994, discloses a liquid level sensor for an electrically conductive liquid. The device requires the use of an electrically conductive material for constructing the tank. Thus, many commonly used containers are unsuitable for use with this device without having substantial additional expense. A voltage source supplies an electrical charge to the tank walls, with ground potential supplied to the liquid contained therein. The capacitance between the tank walls and liquid varies with the volume of liquid in the tank. The problem of false “full” readings is not discussed.
One of the potentially simplest and least expensive methods for the measurement of the level of a liquid is the use of an electrically conductive liquid as a part of a resistive circuit to ascertain the level. Representative of this genre is found in U.S. Pat. No. 4,277,773, issued to Blatnik on Jul. 7, 1981. This reference discloses a device for measuring the level of cooling liquid within a radiator. The radiator is grounded and an insulated probe is installed in the radiator and measures the level via interconnecting the sensor with ground to obtain a conductivity measurement. A second sensor is provided to measure a hot liquid level in the same manner. While this device is simple and inexpensive to build, no protection against false “full” readings due to the accumulation of conductive sludge or other residue is disclosed or suggested.
U.S. Pat. No. 5,719,556, issued to Albin et al. on Feb. 17, 1998, discloses still another variation of the liquid level sensor that utilizes the measurement of resistance as an indicator. Albin et al do recognize the problem of shorts from the sensor tip to ground via a conductive path through sludge. However, the inventors claim that merely insulting the sensing rods except at the tip eliminate this problem. Further, their device is quite complicated and expensive requiring the use of alternating current, an oscillator, and filters as well as other circuitry in order to achieve their objectives.
Therefore, a simple conductive-type liquid level sensor for conductive liquids that is easy and inexpensive to manufacture, can be attached in a variety of tanks and containers without substantial modification of those tanks and prevents false “full” indications due to residue buildup is not found in the prior art.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, the invention includes a liquid level sensor apparatus for determining the level of a conductive liquid held within a container having inside walls, one of which is an attachment wall, wherein the container is subject to the accumulation of residue as a consequence of the conductive liquid contained therein. The liquid level sensor includes a sensor having an attachment end and a sensor end, wherein the attachment end of the sensor is connected to the container on the attachment wall such that the sensor end is able to contact the conductive liquid held therein at a conductive liquid level. The sensor has a voltage disposed at the sensor end of the sensor. The liquid level sensor also includes a ground connection having an electrical polarity, the ground connection being in contact with the conductive liquid; a circumferential ring attached around the sensor adjacent to the attachment end of the sensor, wherein the circumferential ring having a bias voltage having the opposite polarity of the ground connection and having a voltage that corresponds to the voltage of the sensor end of the sensor; a float having a top and a bottom and predetermined dimensions, the float being buoyant on the conductive liquid; and a conductive tip attached to the top of the float, the conductive tip being conductive with the sensor end when in at least one predetermined position. The bottom of the float is in contact and buoyant on the conductive liquid and the conductive liquid has a level wherein when the level rises, the float rises, and when the conductive tip reaches the at least one predetermined position, a circuit is completed to the ground connection which indicates a condition to the sensor tip and wherein when the conducting tip does not reach the at least one predetermined position, and wherein when the container contains residue on the container walls the bias voltage on the circumferential ring prevents a false indication of the condition by preventing completion of the circuit from the sensor tip to the ground connection.
Implementation of this aspect of the present invention may include one or more of the following. Where the conductive tip is a metal ball. Where the conductive tip is a metal flat valve. Where the liquid level sensor also includes a capacitor that is connected between the ground connection and the circumferential ring; and a diode t
Chamberlin Edward R.
Linsley Paul
Hofsass Jeffery
Pham Lam
Ritchie William B.
Standex International Corporation
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