Conductive resin structure for connecting metal conductors.

The invention relates to the connection of metal electrical conductors by means of a conductive resin, and to oil leakage detection sensors and apparatus incorporating such a connection.

Electrically conductive resins have been developed in recent years and, depending on the application involved and the location of use, are being used increasingly to make electrical connections between metal conductors. For example, it is known to use connecting structures of this type in the detecting parts of oil leakage detection sensors designed to check for oil leakage from oil tanks, piping in chemical plants, or aqueous or organic fluid carrying pipelines.

One example of a conductive resin which may be used in the sensing part of an oil leakage detection sensor is carbon-filled polytetrafluoroethylene (PTFE), which is endowed with electrical conductivity by the admixture of powdered carbon in a PTFE insulation material. PTFE resins have the property of repelling water while allowing the permeation of oil. In addition, carbon-­filled PTFE resins have the property of exhibiting a change in electrical resistance when permeated by oil, and it is this property which is utilised by the sensor in detecting oil leakage.

A known form of oil leakage detecting sensor of this type comprises a pair of conductive metal lead wires which are mounted so that they are separated from each other. In the detecting part of the sensor, these lead wires are electrically connected to each other by a resistor which consists of an electrically conductive resin, such as carbon-filled PTFE. This resistor is usually in the form of a connecting band extending between the wires. The respective ends of the resistor are bonded to the circumferential surfaces of the lead wires by means of a conductive adhesive, with each end covering approximately half of the circumferential surface of the corresponding lead wire. Furthermore, these lead wires and the resistor are covered by an insulating substrate, which may comprise a PTFE resin.

The respective lead wires of the oil leakage detecting sensor are connected to a detection circuit, so that a closed circuit is formed by the lead wires, the resistor, and the detection circuit. When oil contacts the detecting part of the sensor, the oil permeates through the insulating substrate and penetrates into the resistor. As a result, the electrical resistance of the resistor changes, so that an accompanying electrical change occurs in the detection circuit, whereby the leakage of oil can be detected.

In the above conventional structure, different materials, such as the lead wires, which are made of metal, and the resistor, which is made of a resin material, are bonded directly to each other by means of an adhesive. Accordingly, the bonding strength is weak, which can lead to improper or inconsistent contact, and in some cases to an interruption of electrical continuity owing to peeling of the bonded area. As a result, such conventional structures have lacked reliability.

Further, since the resistor is in contact with only about half of the circumferential surface of each of the lead wires, the area of contact is small, so that there is a large increase in electrical resistance in the connecting part. In the case of an oil leakage detecting sensor as described, this results in reduced sensitivity.

In view of the problems encountered with the above described electrical connecting structure, according to the invention there is provided a connecting structure connecting at least two metal electrical conductors comprising: <ul id="ul0001" list-style="none"><li>(a) a covering layer of electrically conductive resin surrounding each metal conductor over at least a portion of its length;</li><li>(b) a band of electrically conductive resin interconnecting the covering layers of said metal conductors, the band at least partially wrapping around and attaching to the covering layer of each conductor; and</li><li>(c) a hydrocarbon-penetrable jacket surrounding said covered metal conductors and said conductive resin band.</li></ul>

Since the entire circumference of the connection area of each metal conductor is covered by its resin covering layer, the joint between each metal conductor and the corresponding covering layer is secure despite the fact that the materials involved are different. Furthermore, the contact area is also large. Accordingly, a good electrical connection is obtained. In addition, since both the covering layers and the connecting band are made of a resin material, sufficient mechanically strength can be obtained in the joining of the covering layers and the connecting band. As a result, a connecting structure is obtained which is favourable both in terms of electrical characteristics and in terms of mechanical strength.

According to a further aspect of the invention, there is provided an oil-leakage detection apparatus comprising: <ul id="ul0002" list-style="none"><li>(a) means for detecting a change in the electrical resistance of an electrical circuit;</li><li>(b) an electrical circuit connected to said resistance-change detecting means; and</li><li>(c) an oil detecting sensor including electrically-conductive resin covered metal electrical conductors connected to the electrical circuit, an electrically conductive resin band connecting said covered metal conductors, and an oil-penetrable jacket surrounding said covered metal conductors and said band.</li></ul>

An embodiment of the connecting structure in accordance with the invention will now be described, by way of example, in the context of an oil-leakage detecting sensor and with reference to the accompanying drawings in which:-­ <ul id="ul0003" list-style="none"><li>Figure 1 is a perspective view of the oil-leakage detecting sensor; and</li><li>Figure 2 is a cross-section of the oil-leakage detecting sensor along line II-II of Figure 1.</li></ul>

The oil leakage detecting sensor <u style="single">1</u> shown in the drawings has a pair of conductive carbon-filled PTFE resin-covered conductors <u style="single">2</u> which are arranged so that they are separated from each other. Each conductor <u style="single">2</u> comprises a metal lead wire <u style="single">3</u> and a covering <u style="single">4</u> which comprises a conductive carbon-filled PTFE resin and which surrounds the entire circumference of the lead wire <u style="single">3</u> in a tightly adhering state. Accordingly, the connection between the lead wire <u style="single">3</u> and the covering <u style="single">4</u> in each conductor <u style="single">2</u> is extremely strong mechanically. Also, since the area of contact is large, this connection is also favourable from an electrical standpoint. The method used to cover each lead wire <u style="single">3</u> with its covering <u style="single">4</u> may be the same as the method used generally to manufacture PTFE resin-covered or coated wires, and a description of the method is therefore omitted here.

The tip portion of each of the conductors <u style="single">2</u> forms a connection area <u style="single">A</u> in which the coverings <u style="single">4</u> of the two conductors are electrically connected to each other via a band-shaped resistor or connecting piece <u style="single">5</u> which is formed from conductive carbon-filled PTFE resin. Each of the ends of the band-shaped resistor <u style="single">5</u> is joined by thermal fusion to approximately half of the circumferential surface of the covering <u style="single">4</u> of a respective one of the conductors <u style="single">2</u>. As is known, joints formed by thermal fusion are excellent in terms of mechanical strength. Such a connection method is also extremely favourable from an electrical standpoint.

Although conductive carbon-filled PTFE resin-covered wires are used in this example as described above, it would also be possible to provide the covering layer <u style="single">4</u> only in the connection area <u style="single">A</u> of each lead wire <u style="single">3</u>.

The conductors <u style="single">2</u> and resistor <u style="single">5</u> are covered by an insulating substrate <u style="single">6</u> formed of porous PTFE resin, so that one end of each conductor is embedded in the insulating substrate <u style="single">6</u>, while the other end of the conductor <u style="single">2</u> projects from the insulating substrate <u style="single">6</u>. The projecting portions of the conductors <u style="single">2</u> serve as terminals for connection to a detection circuit (not shown), for which purpose the covering layers <u style="single">4</u> are stripped away at the tips of the projecting portions of the conductors <u style="single">2</u> to expose the lead wires 3 as shown in Figure 1 for connection to the detection circuit. After the lead wires <u style="single">3</u> have been connected to the detection circuit, the conductors <u style="single">2</u> and naked lead wires <u style="single">3</u> projecting from the insulating substrate <u style="single">6</u> may be covered by insulation moulded from an appropriate insulating material.

In the oil leakage detecting sensor <u style="single">1</u> as described above, the joints between the lead wires <u style="single">3</u> and the covering layers <u style="single">4</u> and the joints between the covering layers <u style="single">4</u> and the resistor band <u style="single">5</u> are extremely strong mechanically. Accordingly, the joint areas tend not to be damaged even if the sensor <u style="single">1</u> is subjected to vibration or external forces. Further, these joints are also extremely favourable in terms of electrical characteristics. Accordingly, there are no problems of improper contact or interruption of electrical continuity as in the case of sensors employing a conventional connection structure. The electrical resistance of the connecting structure can also be made smaller than is possible in the case of a conventional device, so that the sensitivity of the oil leakage detecting sensor <u style="single">1</u> is also improved.

The operating principle of the oil leakage detecting sensor of this example is the same as that of a conventional oil leakage detecting sensor, in that the electrical resistance of the resistor <u style="single">5</u> and the covering layers <u style="single">4</u> in the connection areas <u style="single">A</u> changes when the sensor is permeated by oil, and this change in electrical resistance is detected by the detection circuit.

It will of course be appreciated that the invention is not limited to the specific example described above, and that various other configurations may be used. For example, there is only one pair of lead wires <u style="single">3</u> in the oil leakage detecting sensor described above, whereas it would be possible, if desired, to install two pairs of lead wires <u style="single">3</u> or an even larger number of lead wires <u style="single">3</u>. The conductive resins which may be used in the present invention are not limited to conductive carbon-­filled PTFE resins. Conductive metal powder could be used as the filler, and porous polyethylene, porous polypropylene, and porous rubber are examples of other resins which may be used. Also, the connecting structure described may be used in other devices besides oil leakage detecting sensors.