Measuring and testing – Liquid level or depth gauge – Immersible electrode type
Reexamination Certificate
1999-06-18
2001-02-13
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Immersible electrode type
C073S30400R, C073S298000, C073S301000
Reexamination Certificate
active
06186001
ABSTRACT:
This invention is in the field of measuring the content level in containers, such as bins and tanks, and particularly sensors for such measurement.
BACKGROUND
It is often necessary to determine the quantity of liquid stored in a tank or the quantity of granular material, such as grain, stored in a bin. Conventionally this is accomplished by climbing to the top of the container and looking in. Remote sensors are used as well which employ various techniques including photo-electric and ultra-sonic techniques which are costly. Changes in resistance of an element as the container level changes has been measured and correlated to bin level, however foreign material and corrosion can cause changes in resistance of the contact points and so is not dependable. Liquid tanks can also use sight glasses mounted on the tank exterior.
A dependable and economic level sensor which can be mounted on the interior of a liquid or particulate container would be beneficial.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a sensor for determining the level of material in a container by changing the capacitance thereof in response to the action of the material on the sensor.
It is a further object of the present invention to provide such a sensor that is suitable for use with liquid or particulate material.
The invention accomplishes its objects providing in one aspect a container level sensor comprising an elongate strip member for mounting in a container in a generally upright orientation, said strip member having a measurable capacitance that changes as the level of material stored in said container changes.
In a second aspect the invention provides an apparatus for measuring the level of material in a container comprising a sensor comprising an elongate strip member for mounting in said container in a generally upright orientation, said strip member having a measurable capacitance that changes as the level of material stored in said container changes; and a circuit for detecting the change in capacitance.
As capacitance varies with the proximity of the elements of the capacitor, no physical contact is required. Problems with poor contact are eliminated. Two capacitor elements will have a given capacitance when separated along their entire length, and this capacitance will be reduced as the elements are brought closer together at one end, and will be progressively reduced as the elements are progressively brought closer together along their length. Providing a separating force between flexible elements that is small enough to be overcome by the pressure of material in the container allows a measuring circuit to determine at what point along the length of the strip member the separating forces have been overcome, and therefore determine the level of material in the container. The sensor will work with either liquid or particulate material, as either will exert some force that is greater than the empty space above the material.
The strip member could comprise a first elongate electrically conductive strip insulated from the material and from the container; a second elongate conductive strip insulated from the material, from the container and from the first strip; at least one of the first and second strips being flexible and mounted spaced from the other the strip such that the force of the material on the flexible strip causes the space between the first and second strips to be reduced so as to change the capacitance of the capacitor defined by the first and second strips; and a cover to prevent the material from entering the space between the first and second strips.
The cover could be a flexible membrane between the two strips on each side, or could enclose the two strips entirely. It could be made of any material suitable to prevent the material in the container from entering the space between the strips and thereby preventing the strips from moving toward one another in response to the pressure of the material.
Each of the strips could conveniently comprise a laminate of an electrically conductive foil and an insulating plastics material. The foil could be covered on both sides by the insulating plastic material. The foil material also provides flexible first and second strips, which will flex when subjected to pressure from the material.
Conveniently, the space between the first and second strips could be maintained by opposing field magnets attached along the length of each strip wherein the force of the material on the flexible strip overcomes the magnetic force of the magnets to reduce the space between the strips and thereby reduce the capacitance of the strip member. The uncompressed space can conveniently be defined and maintained by having the cover enclose the two strips thereby preventing the magnetic forces from pushing the strips beyond the desired spacing.
Alternatively the first and second strips could be separated by a pair of resilient spacer members arranged along the edges of the strips leaving an open area between the spacer members. The spacer members could be formed from foam or the like, and could comprise a strip of foam with adhesive on each side for attachment respectively to each of the strips.
Other spacing devices or materials could be used as well, and all such devices and materials are considered to fall within the scope of the invention.
The strip member could be attached to the container wall, as by magnets or mechanical fasteners, or anchored so as to extend from substantially the bottom of the container to the top thereof at a location inside the walls of the container.
For measuring the level of particulate material, the apparatus could further comprise at least a second strip member laterally spaced from the first strip member; and a circuit for detecting the change in capacitance of the second strip member. This would provide level measurements at two locations such that the surface contour of the particulate material in the container can thereby be calculated and the quantity of the particulate material in the container can be closely estimated. Further sensors could be added, allowing for further definition of the surface contour and more accurate estimation of the quantity.
For example in a grain bin, one strip member could be attached to the wall and the other near the centre of the bin. Liquid containers would of course require only one sensor.
REFERENCES:
patent: 4756076 (1988-07-01), Dyben et al.
patent: 4890492 (1990-01-01), Andrejasich et al.
patent: 5554937 (1996-09-01), Sanders et al.
Eaves, Jr. James C.
Greenebaum Doll & McDonald PLLC
Loo Dennis
Williams Hezron
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