Measuring and testing – Liquid level or depth gauge – Float
Reexamination Certificate
1998-03-05
2001-08-07
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Float
C073S29000R, C073S305000, C073S319000, C338S033000
Reexamination Certificate
active
06269695
ABSTRACT:
TECHNICAL FIELD
This invention relates to liquid level sensors and, in particular, to analog liquid level sensors.
BACKGROUND ART
A large number of sensing approaches and transducer types have been developed for the determination of the level of liquids in open or closed vessels or containers. Not only is the knowledge of the level itself important, but other measurements can be inferred from the level. For example, if the container geometry and dimensions are additionally known, the volume of the liquid can be determined. If additionally, the density of the liquid is known, its mass can be calculated.
Relatively high technology methods to sense liquid level by measurement of properties such as distance, time, and pressure include: sonic and ultrasonic surface reflection, optical surface reflection, optical transmission, rotation of polarized light, capacitive property sensing, dielectric conduction between electrodes, inductive property sensing, mechanical resonance damping, float device containing a magnet affecting pulse reflection of a ferromagnetic strip, float containing an end of a linear variable differential transformer, pressure sensor at bottom of the tank, differential pressure sensor bubbling tube at bottom of the tank, long vertical float buoyancy force, long vertical negative buoyancy force, sight gauge with multiple optical level discrimination, multiple thermistors, tank sonic resonance spectra correlations, and the like. These methods typically have cost and complexity as their major detriment.
Liquid level is generally sensed by one of two methods: obtaining a discrete indication if a predetermined level has been reached (i.e. point sensing) or obtaining an analog representation of the level as it changes (i.e. continuous sensing). One commonly employed apparatus for continuously sensing the level of liquid, such as a fuel, in fuel storage tanks is the use of a float mechanism.
Relatively low technology methods currently being used to sense liquid levels by measurement of various properties use techniques such as: ohmic conduction between electrodes, variable force sensitive resistive conductors, float moving a plunger, and float on a mechanical pivot arm moving a wiper contact element across a wire wound or thick film resistor. The ohmic conduction method is prone to detrimental accuracy impairing contamination and electrolytic effects on the electrodes. Variable force sensitive resistors inherently suffer from poor accuracy and are seldom used. The float moving a plunger method is capable of good accuracy and precision but is cumbersome and requires some additional sensing method to convert position into a sensor signal. The method by which a float rotating a mechanical pivot arm to move an electrical wiper contact across a variable resistor type of liquid level sensor is of primary commercial interest because of its large number of existing and potential applications, relative simplicity, and low cost.
The following listed features pertaining to liquid level sensing via floats on pivot arm mechanisms, however, are less than desirable:
Requires a relatively large volume of space for pivot arm motion;
Often requires specific orientation to allow for pivot arm motion;
Cumbersome to install and remove;
Large size;
High weight;
High parts count;
Mechanism if often flimsy—heavier mechanism requires a larger float;
Mechanism has inherent modes of mechanical vibration—potential failures;
Mechanism is inherently poor in mechanical impact testing—potential failures;
Relatively large pivot arm movement is reduced to smaller motion of the wiper arm on the wire wound or film resistor—loss of resolution and accuracy;
Mechanical play, articulation, and tolerance of numerous components introduces hysteresis and adversely affects accuracy;
Wear of articulating mechanical components increases modes of failure;
Nonmetallic substrate of the wire wound or thick film resistor acts as a thermal insulator allowing heat build up in events of an incorrect electrical connection—a potential source of ignition for a dangerous incendiary incident;
Small size of the wire wound or thick film resistor promotes concentration of heat generation and higher temperatures in events of an incorrect electrical connection—an additional potential source of ignition for a dangerous incendiary event;
Location of the articulating electrical wiper contact with the wire wound or thick film resistor can be above liquid level such that mechanical play can potentially produce a source of sparking and/or heat production—a safety concern;
Nonmetallic substrate of the wire wound or thick film resistor offers no shielding from spurious magnetic fields that can cause signal noise;
Nonmetallic substrate of the wire wound or thick film resistor offers no shielding from spurious electric fields that can cause signal noise.
The above detrimental points and limitations present numerous opportunities for improvements.
U.S. Pat. No. 4,779,460 discloses a sensor and system for measuring the level of a liquid and which includes a linear potentiometer having a resistance circuit element and a mechanism for changing the resistance of the circuit element including a conductor formed from a semiconducting composition having a resistance inversely proportional to the horizontal force it experiences.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved analog liquid level sensor including a printed circuit having a resistance which varies at different positions on the printed circuit wherein the sensor is provided for a container having a volume/depth relationship and wherein the sensor has a resistance-to-displacement profile matched to the container's volume/depth relationship.
Another object of the present invention is to provide an analog liquid level sensor for a container which is simple to assemble, install and thereafter remove from the container.
Still another object of the present invention is to provide an analog liquid level sensor including a support structure and a printed circuit having a resistance formed on the support structure wherein the support structure acts as a heat sink in order to dissipate any heat build up.
Yet still another object of the present invention is to provide an analog liquid level sensor including at least one metallization pattern having graduations formed on a printed circuit having a resistance for reduced electrical noise characteristics and improved hysteresis.
In carrying out the above objects and other objects of the present invention, in a system for measuring the level of a liquid in a container having a volume/depth relationship, an analog liquid level sensor is provided. The sensor includes a support structure adapted to be received and retained within the container, a pair of electrically spaced terminals, and a printed circuit formed on the support structure and electrically coupled between the terminals and having a resistance which varies at different positions on the printed circuit between the terminals. The sensor also includes a float assembly connected to the support structure to reciprocate in a generally vertical direction relative to the support structure in response to the level of the liquid in the container. The float assembly includes a hollow buoyant housing and a contact assembly supported within the buoyant housing in fixed vertical relationship thereto for making electrical connection with the printed circuit for changing the effective resistance of the printed circuit between the terminals. The terminals provide an electrical signal which is a function of the generally vertical position of the contact assembly along the support structure. The sensor has a resistance-to-float-displacement profile matched to the volume/depth relationship of the container.
Preferably, the support structure is a dielectric coated metal bar which acts like a heat sink.
Also, preferably, the printed circuit includes a resistor pattern and a metallization pattern having metallized graduations formed on opposite sides of the metal bar for r
Cesternino Kimberly
Strom Peter H.
Brooks & Kushman P.C.
Loo Dennis
Nartron Corporation
Williams Hezron
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