Capacitive level sensor

A main sensor electrode part 3 and a reference sensor electrode part 4 are deposited in patterns over the surface of a support member 10. A lead-out part 4b of the reference sensor electrode part 4 is extended through an unplated passage 11 of the main sensor electrode part 3 to the upper edge of the support member 10 to form there a terminal part 4a. A portion of the main sensor electrode part 3 is also extended to the upper edge of the support member 10 to form there a terminal part 3a. The support member 10 is deposited all over its surface with an insulating film 12 covering the both sensor electrode parts 3 and 4.

TECHNICAL FIELD 
The present invention relates to a capacitive level sensor which is 
suitable for use with a tank for a nonconductive liquid or a tank for a 
conductive liquid (of low electrical resistance) such as water. 
As shown in FIG. 1, a conventional capacitive level sensor of this kind is 
composed of a pair of opposed electrodes, that is, an outer tube 1 made of 
metal (referred to also as a common electrode) and an inner tube 2 made of 
metal and disposed concentrically therewith, and the inner tube 2 is made 
up to a main sensor electrode part 3 and a reference sensor electrode part 
4. The main sensor electrode part 3 and the reference sensor electrode 
part 4 have the same radius and are mechanically secured to the outer tube 
1 by support members 5 made of an insulating material. Cables 6 and 7 are 
connected to the main sensor electrode part 3 and the reference sensor 
electrode part 4, respectively. 
Such a level sensor is disposed with the reference sensor electrode part 4 
positioned at the bottom of a tank on the inside thereof, whereby the 
height of the liquid level H or liquid level h in the range from the lower 
and to the upper end of the main sensor electrode part 3 can be detected. 
When oil or similar liquid enters the space defined between the outer tube 
or common electrode 1 and the inner tube 2, the electrostatic capacitance 
between the main sensor electrode part 3 and the common electrode 1 
changes in proportion to the liquid level h. Letting the length of the 
main sensor electrode part 3 in its axial direction (in the vertical 
direction) be represented by L, the electrostatic capacitance between the 
main sensor electrode part 3 and the common electrode 1 when the liquid 
level is h (which electrostatic capacitance will be referred to simply as 
the electrostatic capacitance of the main sensor electrode part) by Csh, 
the electrostatic capacitance when the space between the main sensor 
electrode part 3 and the common electrode 1 is filled with air (that is, 
when h=0) by Cso and the specific inductivity of the liquid by .epsilon., 
the following equation holds true: 
EQU Csh=Cso(L-h)/L+.epsilon.Csoh/L. 
Therefore, the liquid level h is given by 
EQU h=L(Csh-Cso)/{Cso(.epsilon.-1)}. 
The specific inductivity of the liquid is given by .epsilon.=Cr/Cra, where 
Cra is the electrostatic capacitance when the space between the reference 
sensor electrode part 4 and the common electrode 1 is filled with air and 
Cr is the electrostatic capacitance when the space between the reference 
sensor electrode part 4 and the common electrode 1 is filled with the 
liquid. Cso and Cra are both constants that depend on the sizes and 
constructions of the main sensor electrode part 3 and the reference sensor 
electrode part 4. The specific inductivity e can be preobtained by 
premeasuring the electrostatic capacitance Cra of the reference sensor 
electrode part 4 in the state of no liquid being filled in the tank and 
the electrostatic capacitance Cr in the state of the reference sensor 
electrode part 4 being submerged in the liquid. The height h of the liquid 
level H can be obtained by measuring the electrostatic capacitance Csh of 
the main sensor electrode part 3 by means of an instrument not shown. 
Since such a capacitive level sensor requires, as the tubular electrode 2, 
the two electrically insulated sensor electrode parts 3 and 4, it is 
necessary in the prior art to fix these sensor electrode parts 3 and 4 to 
the outer tube 1 through the support members 5; thus, the conventional 
level sensor has disadvantages of requiring many support members 5 and 
involving work of securing the sensor electrode parts to the outer tube 
through the support members. Another problem is the use of the cables 6 
and 7 that are connected to the sensor electrode parts 3 and 4, 
respectively. 
An object of the present invention is to provide a capacitive level sensor 
which overcomes these problems and, though simple-structured, ensures the 
same performance as is obtainable with the prior art. 
DISCLOSURE OF THE INVENTION 
To solve the above-mentioned problems, according to the present invention, 
in a capacitive level sensor of type that has first and second electrode 
means disposed opposite each other with a space defined therebetween which 
can be filled with liquid whose level is to be measured and the first 
electrode means is made up of a main sensor-electrode part and a reference 
sensor electrode part, the first electrode means comprises: support means 
disposed opposite and spaced apart from the second electrode means and 
having at least one surface; the main sensor electrode part and the 
reference sensor electrode part deposited by electroless plating or 
printing over the said surface of the support means, the main and 
reference sensor electrode parts being spaced apart in the vertical 
direction; an insulating film deposited all over the said surface and 
covering the main sensor electrode part and the reference sensor electrode 
part; a terminal part formed by extending one part of the main sensor 
electrode part to the upper edge of the support means; and a lead-out part 
of the reference sensor extended up to the upper electrode part extended 
through a vertical unplated passage in the main sensor electrode part up 
to the upper edge of the support member to form a terminal part at the 
upper end. 
The construction mentioned above permits pattern plating or printing of the 
main sensor electrode part and the reference sensor electrode part and 
precludes the necessity of the cables of the sensor electrode parts 3 and 
4 needed in the prior art, and hence provides level sensors of this kind 
with ease and at low cost.

BEST MODE FOR CARRYING OUT THE INVENTION 
A description will be given, with reference to FIG. 2, of an embodiment of 
the capacitive level sensor according to the present invention. The parts 
corresponding to those in FIG. 1 are identified by the same reference 
numerals and no description will be given of them. In this embodiment, a 
cylindrical support member 10 made of an insulating material such as epoxy 
resin is disposed upright, and the main sensor electrode part 3 and the 
reference sensor electrode 4 described previously in respect of FIG. 1 are 
deposited on the outer peripheral surface of the cylindrical support but 
spaced apart in the vertical direction (i.e. in the axial direction of the 
support member.) These sensor electrode parts 3 and 4 can be formed by 
depositing a conductive material through use of a known electroless 
plating or printing method. 
A part of the main sensor electrode part 3 is removed axially thereof to 
form an unplated passage 11 and a part of the upper edge of the reference 
sensor electrode 4 is extended up to form a lead-out portion 4b, which is 
further extended through the unplated passage 11 up to the upper edge of 
the support 10 to form there a terminal part 4a. A part of the upper 
marginal edge of the main sensor electrode part 3 is also extended to the 
upper edge of the support member 10 to form a terminal part 3a. The 
support member 10 is coated all over its outer peripheral surface with an 
insulating film 12 covering both sensor electrode parts 3 and 4. The 
insulating film 12 can be formed by coating or painting a resin material. 
In this way, the inner tube 2 is constructed. 
With the construction shown in FIG. 2, the main sensor electrode part 3 and 
the reference sensor electrode part 4 can simultaneously be deposited by 
electroless plating or printing on the surface of the support member 10 
made of an insulating material and the inner tube 2 can be completed by 
depositing the insulating film 12 through mere painting. Since the main 
sensor electrode part 3 and the reference sensor electrode part 4 forming 
the inner tube 2 are formed on the common cylindrical support member 10, 
the number of support members 5 for securing the inner tube 2 to the outer 
tube 1 can also be made smaller than in the prior art example of FIG. 1, 
facilitating the work. 
While in the FIG. 2 embodiment the main sensor electrode part 3 and the 
reference sensor electrode part 4 are formed over the outer peripheral 
surface of the cylindrical support member 10, they can also be formed over 
its inner peripheral surface and it is apparent that the same effect could 
be obtained in such a case. Alternatively, the tubular common electrode 1 
may be disposed inside of the inner tube 2, and such a construction is 
shown in FIG. 3 but no description will be repeated. 
In the case where the support member 10 is made of conductive material such 
as a metal plate, an insulator 13 is deposited over the surface of the 
support member 10, and the afore-mentioned sensor electrode parts 3 and 4 
are deposited over the insulator surface as depicted in FIG. 4. In this 
instance, those portions of the support member where the terminal parts 3a 
and 4a are formed are also covered with the insulator. The illustrated 
inner tube shown in FIG. 4 is identical in construction with that in the 
FIG. 2 embodiment except the above. Such an inner tube 2 is secured by 
support members to the inside of the outer tube (the common electrode) 1 
described previously with reference to FIG. 1. Although in the above the 
common electrode and the sensor electrode part are described to be formed 
by the outer tube 1 and the inner tube 2, the individual electrodes may 
also be flat electrodes disposed opposite each other. 
As described above, according to the capacitive level sensor of the present 
invention, the main sensor electrode part 3 and the reference sensor 
electrode part 4 of the conventional inner tube (i.e. the sensor electrode 
part) are deposited in desired patterns; hence, the inner tube 2 can be 
fabricated with ease. Moreover, the reference sensor electrode part 4 has 
the lead-out part 4b, which is extended through the unplated passage 11 of 
the main sensor electrode part 3 to the upper edge of the support member 
10 to form there the terminal part 4a, and the lead-out part and the 
terminal part can be formed at the same time that the main sensor 
electrode part 3 and the reference sensor electrode part 4 are formed by 
deposition--this precludes the necessity of using cables, and hence 
reduces the manufacturing costs of the device. Besides, since the patterns 
(the sensor electrode parts 3 and 4) are covered with the insulating film 
12, the level sensor of the present invention can be used regardless of 
whether the liquid to be measured is conductive or not.