Method of making a sensor cable

A sensor cable for detecting the presence of electrically conductive liquids, e.g. water. The cable has decreased sensitivity to contamination and condensation than conventional cables which are covered by braids. First and second elongate conductors are positioned in first and second channels which partially surround the conductor and are part of first and second insulating support members; each channel has at least one shoulder which extends outwardly from the channel beyond the conductor. The channels are positioned so that when the cable is placed on a flat substrate in any position, neither the first nor the second conductor contacts the flat surface and at least one first shoulder and at least one second shoulder make intermittent contact with the flat surface. In a preferred embodiment, the first and second conductors follow a generally spiral path down the length of the cable. When the cable is immersed in an electrically conductive liquid, the liquid provides an electrical connection between the first and second conductors.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to methods of making sensor cables. 
Introduction to the Invention 
Sensor cables and assemblies comprising sensor cables are wellknown. Such 
sensors may be used to detect changes in variables along an elongate path, 
e.g. the presence of a liquid such as water or an organic solvent, the 
attainment of a predetermined temperature or pressure, the presence or 
absence of light or another form of electromagnetic radiation or a change 
in the physical position of a movable member, e.g. a valve in a chemical 
process plant or a window in a building fitted with a burglar alarm 
system. Changes of this kind are referred to in this specification by the 
generic term "event". Reference may be made, for example, to U.S. Pat. 
Nos. 1,084,910 (Stephenson), 2,581,213 (Spooner), 2,691,134 (Ford), 
3,248,646 (Brazee), 3,382,493 (Loper et al), 3,470,340 (Hakka), 3,564,526 
(Butts), 3,800,216 (Hamilton), 3,991,413 (Berger), 4,278,931 (Huggins), 
4,400,663 (May), and 4,580,477 (Sugibuchi), British Patent Nos. 182,339, 
1,355,176, and 1,481,850, German Offenlegunschriften Nos. 3,001,150 and 
3,225,742, European Patent Application Publication Nos. 133,748, 144,211, 
160,440, 160,441, 164,838, 191,547, 250,776, and 253,085, and copending, 
commonly assigned application Ser. Nos. 838,725 (Lahlouh et al, filed Mar. 
11, 1986), now U.S. Pat. No. 4,926,165 (issued May 15, 1990). Ser. No. 
202,278 (Kamas, filed Jun. 3, 1988) now U.S. Pat. No. 4,922,183 (issued 
May 1, 1990), Ser. No. 256,874 (Wasley et al, filed Oct. 12, 1988) now 
U.S. Pat. No. 4,926,129 (issued May 15, 1990), and Ser. No. 372,179 (Masia 
et al, filed Jun. 27, 1989), now U.S. Pat. No. 4,015,958 (issued May 14, 
1991). The disclosure of each of the patents, applications, and 
publications referred to above is incorporated herein by reference. 
SUMMARY OF THE INVENTION 
This invention provides a novel method of making a sensor cable, said 
method comprising (1) providing an elongate core member whose outer 
surface comprises a deformable insulating material; (2) wrapping a first 
elongate conductor spirally around the core member so that a first spiral 
portion of said deformable material is deformed by pressure exerted on it 
by the first elongate conductor and thus provides (i) a first channel 
which partially surrounds the first conductor and (ii) at least one first 
shoulder extending outwardly beyond the first conductor; and (3) wrapping 
a second elongate conductor spirally around the core member so that it is 
spaced apart from the first conductor and so that a second spiral portion 
of said deformable material is deformed by pressure exerted on it by the 
second elongate conductor and thus provides (i) a second channel which 
partially surrounds the second conductor and (ii) at least one second 
shoulder extending outwardly beyond the second conductor; the first and 
second elongate conductors having exposed surfaces which, if the product 
of step (3) is immersed in an electrically conductive liquid, are 
contacted by the liquid, the liquid thus providing an electrical 
connection between the first and second conductors.

DETAILED DESCRIPTION OF THE INVENTION 
Sensor cables made by the method of this invention can be used to detect 
the presence of electrically conductive liquids. Such liquids are most 
commonly electrolytes, i.e. liquids in which an electrical connection is 
made by means of ions. Suitable liquids include water, aqueous acids, 
aqueous bases, and other ionic solutions in which the charge-carrying 
entities are ions. 
The sensor cable comprises a first elongate conductor and second elongate 
conductor, each of which has an exposed surface which can be contacted by 
the electrically conductive liquid when the cable is immersed in the 
liquid. The first and second conductors may be the same or different in 
composition, construction, and size. For most applications, and for ease 
of manufacture, it is preferred that the first and second conductors be 
same. Particularly preferred are conductors in which a metal core, e.g. a 
solid or stranded metal wire or metal braid made from copper, nickel, 
tin-plated copper, metal alloys such as Copel.TM., or other suitable 
material, is electrically surrounded by a conductive polymer. The 
conductive polymer, i.e. a composition which comprises a polymeric matrix 
in which is dispersed a particulate conductive filler, preferably 
completely surrounds the metal core and is in good electrical and physical 
contact with it. Any conductive polymer composition may be used, although 
for many applications it is preferred that the polymer be selected for its 
solvent and chemical resistance to materials with which it may come in 
contact. A useful polymer for many applications is polyvinylidene 
fluoride. Any suitable conductive filler may be used, e.g. carbon black, 
graphite, metal, metal oxide, particles of conductive polymer, or a 
mixture thereof. In addition, the conductive polymer composition may 
contain inert fillers, crosslinking agents, plasticizers, lubricants, or 
other process aids. The appropriate resistivity level of the composition 
will vary depending on the application, but is commonly in the range 0.1 
to 20,000 ohm-cm, particularly 1 to 1,000 ohm-cm, especially 1 to 250 
ohm-cm. If the polymer is one which swells on use, the resistivity of the 
conductive polymer is measured prior to swelling. For some applications 
the stability of the conductor can be improved and its sensitivity to 
vapor and temperature can be decreased by crosslinking the conductive 
polymer on one or both conductors. Crosslinking may be achieved by 
irradiation or chemical means. Suitable levels of irradiation are 2 to 30 
Mrads, particularly 5 to 15 Mrads, e.g. about 7.5 Mrads. The polymer may 
be crosslinked to the same level throughout its thickness or to different 
levels. Depending on the application, the size of the metal core and the 
thickness of the conductive polymer coating can vary. In order to have 
adequate flexibility, it is preferred that the outer diameter of the first 
and the second conductors be 0.010 to 0.500 inch (0.025 to 1.27 cm), 
particularly 0.020 to 0.200 inch (0.051 to 0.508 cm), especially 0.025 to 
0.100 inch (0.064 to 0.254 cm), e.g. 0.025 to 0.060 inch (0.064 to 0.152 
cm). 
The first elongate conductor is positioned in a first channel of the one 
member. The first channel, which may be of any suitable shape, partially 
surrounds the first conductor and allows exposure of the first conductor 
to the liquid. At least one, and preferably two, first shoulders extend 
outwardly beyond the first conductor in order to prevent the first 
conductor from protruding from the channel and being abraded during 
installation and use. The extent to which the shoulder protrudes beyond 
the conductor may be small, e.g. 0.002 to 0.020 inch (0.005 to 0.051 cm), 
although the design of the shoulder will determine the extent of the 
protrusion. Any distance which is sufficient to protect the conductor is 
acceptable. 
The second conductor is positioned in a second channel in the core member 
in a similar manner to the first conductor. At least one, and preferably 
two, shoulders extend outwardly beyond the second conductor to protect the 
second conductor. The dimensions of the second conductor and the second 
channel may be the same or different from those of the first conductor and 
first channel. The core member has an outer surface comprising a 
deformable insulating material. Particularly preferred is a core member in 
which the outer surface comprises a polymer which has a softening point at 
a temperature T.sub.s. This softening point T.sub.s can be measured by a 
Vicat test as the temperature in which an indentor under a fixed load 
penetrates a specified distance into the material. The polymer can be a 
thermoplastic, e.g. polyvinylidene fluoride, or an elastomer, e.g. 
thermoplastic elastomer (TPR), or a blend of materials depending on the 
physical and thermal properties desired for the application. Deformation 
of the material is preferably achieved by heating to a temperature above 
T.sub.s. For many thermoplastic materials it is desirable to heat the 
material to a temperature which is above T.sub.s but is below the melting 
point T.sub.m, i.e. the temperature at the peak of the DSC curve measured 
on the material. This allows the material to soften in order to be 
deformed but prevents the material from melting and dripping off the core 
member. In some cases, it is possible to crosslink the outer surface of 
the core material to a low level thus preventing it from flowing off the 
core member, but still allowing it to be deformed. Heating can be achieved 
by any suitable means, e.g. radiant heat, microwave heating, or induction 
heating. For many applications, it is desirable that the core member 
comprise a central support member, e.g. a metal wire or a polymer fiber, 
which is surrounded by the deformable material. This central support 
member provides physical reinforcement of the core member, and, if it is 
conductive as in the case of a wire, can be used as part of an electrical 
circuit to detect faults or breaks in one of the conductors or any other 
elongate components. 
The first and the second conductors are spaced apart from one another. If 
an electrically conductive liquid contacts both the first and the second 
conductors, an electrical connection is made between them. Therefore, if 
the sensor is used to detect a liquid with a given electrical 
conductivity, the distance between the first and second conductors 
controls the minimum size of the leak which can be detected. For most 
applications it is preferred that leaks be detected when they are 
relatively small in order to minimize damage. Thus although the distance 
between the wires is dependent on the size of the first and second 
elongate insulating support members, the wires are generally spaced apart 
at their closest point by less than 12 inches (30.5 cm), preferably less 
than 6 inches (15.24 cm), particularly less than 1.0 inch (2.54 cm), 
especially less than 0.5 inch (1.27 cm), most especially less than 0.1 
inch (0.25 cm), e.g. 0.025 to 0.100 inch (0.064 to 0.25 cm). 
The first and second conductors follow a generally spiral path down the 
length of the cable, wrapped around the core member. In this 
specification, the term "spiral" means any form of progression of the 
conductor down the length of the cable, whether the pitch is constant or 
varies, and whether the progression is regular or irregular. If the outer 
surface of the core member is heated to a temperature sufficient to deform 
the deformable material, when the first and second conductors are wrapped 
around the core member, they become embedded into the deformable material 
and form first and second channels. By managing the temperature of the 
deformable material, the tension on the first and second conductors, and 
the rate of the wrapping, the depth of the channels and the distance 
between them can be controlled. It is necessary to have conditions which 
allow the conductors to penetrate the deformable material to a depth 
greater than the diameter of each conductor in order to provide first and 
second shoulders extending beyond each conductor. Such shoulders are 
formed by the displacement of the deformable material as the conductors 
press into it. This technique, in which the conductor "carves" the 
channel, allows the conductors to be positioned securely within each 
channel and prevents them from sliding out. By controlling the rate at 
which the conductors are wrapped, the distance required for one complete 
spiral wrap of a conductor (i.e. the pitch) can be regulated and the 
resistivity of the cable controlled. In addition, when the core member 
comprises a central support member, the central support member can act as 
a barrier to control the position of the conductor in the channel, 
preventing it from moving too far into the core member. While the two 
conductors can be wrapped around the core member in separate operations, 
it is often most efficient to wrap both simultaneously. To improve the 
ease of deformation of the deformable material, the conductors, the 
deformable material, or both can be heated prior to or during the wrapping 
operation. 
Sensor cables made by the method of this invention does not have a 
preferred orientation and can be used in any attitude. Thus when a product 
of step (3) of the method, in any attitude, is placed upon a flat surface, 
neither the first conductor nor the second conductor contacts the flat 
surface and at least one first shoulder makes intermittent contact with 
the flat surface and at least one second shoulder makes intermittent 
contact with the flat surface. In addition, the distance between the 
exposed surface of the first conductor and the flat surface varies between 
a minimum and a maximum along the length of the cable, and the distance 
between the exposed surface of the second conductor and the flat surface 
varies between a minimum and a maximum along the length of the cable. In a 
preferred construction, the positions of the first and second conductors 
are balanced, i.e. the cable can be bent equally easily in any direction. 
For many embodiments, the first and second conductors are equidistant from 
the central axis of the conductor. Thus if the core member has a generally 
circular shape, the first conductor and the second conductor are on 
opposite sides of the core member diameter rather than adjacent to one 
another. 
For some applications, it is useful to monitor for the presence, and in 
some cases, the exact location of the leak. By the use of the proper 
electronic components connected to the first and second conductors and to 
one or more elongate insulating wires, the exact location of the 
electrical connection produced at the site of the leak can be determined. 
A preferred embodiment is a four-wire system in which a first elongate 
insulating wire acts as a return wire to a voltage meter and a second 
elongate insulating wire acts as an auxiliary wire. If elongate insulating 
wires are present, they comprise a central wire which is surrounded by an 
insulating material, e.g. polymer. A first, as well as a second, 
insulating wire can be wrapped around the core member separately or at the 
same time as the one or both of the first and second conductors are 
wrapped. Alternatively, if the central support member is an insulated 
wire, it can be used in place of one of the first and second insulating 
wires. It is preferred that the first and second insulating wires are 
balanced, i.e. that they form part of a symmetrical cable, equi-spaced 
from one another and from each of the first and second conductors. 
Documents which describe suitable electronics and methods of detecting the 
location of a leak are European Patent Application Publication Nos. 
133,748, 144,211, 160,440, 160,441, 164,838, 250,776, and 253,085, and 
copending, commonly assigned application Ser. Nos. 838,725 (Lahlouh et al, 
filed Mar. 11, 1986), now U.S. Pat. No. 4,926,165 (issued May 15, 1990), 
Ser. No. 202,278 (Kamas, filed Jun. 3, 1988), and Ser. No. 256,874 (Wasley 
et al, filed Oct. 12, 1988) now U.S. Pat. No. 4,926,129 (issued May 15, 
1990), and particularly application Ser. No. 372,179 (Masia et al, filed 
Jun. 27, 1989) now U.S. Pat. No. 5,015,958 (issued May 14, 1991). The 
disclosure of each of these patent applications and publications is 
incorporated herein by reference. When cables made by the method of the 
invention are used to monitor the presence of an electrically conductive 
liquid which is adjacent to a substrate, the cable can be positioned on 
the substrate so that neither the first conductor nor the second conductor 
contacts the substrate, the distance between the exposed surface of the 
first conductor and the substrate and the distance between the exposed 
surface of the second conductor and the substrate vary between a minimum 
and a maximum along the length of the cable, and at least one first 
shoulder and at least one second shoulder make intermittent contact with 
the substrate. The signal system, which comprises at least one first 
insulating wire and possibly one second insulating wire, provides a signal 
when the presence of the liquid adjacent to the substrate results in 
electrical connection between the first and second conductors. 
The invention is illustrated by the drawing in which FIG. 1 shows a plan 
view of a sensor cable 1 made by the method of the invention. A core 
member 3 is wrapped in a spiral pattern with a first elongate conductor 5 
which is a locating wire, a first elongate insulating wire 9 which is a 
continuity wire, a second elongate conductor 7 which is a source wire, and 
a second elongate insulating wire 11 which is a signal wire. Each of the 
wires is embedded into the core member 3 to a depth sufficient to prevent 
the wires from protruding above the surface of the core member. 
FIG. 2 is a cross-sectional view of the sensor cable 1 along line 2--2 of 
FIG. 1. In this embodiment, deformable polymeric material comprises the 
core member 3 and surrounds a central support member 13 which comprises a 
center conductor 15 and an insulating polymeric layer 17. The first 
conductor 5 and second conductor 7 are embedded into the core member 3. 
Each conductor 5,7 comprises a center conductor 19, either a solid or a 
stranded wire, surrounded by a layer of conductive polymer 21. The first 
insulating wire 9 which is the continuity wire comprises a center 
conductor 23 surrounded by an insulating polymer layer 25 and the second 
insulating wire 11 which is the signal wire comprises a center conductor 
27 surrounded by an insulating polymer layer 29. 
The invention is illustrated by the following example. 
EXAMPLE 
An elongate conductor wire was prepared by extruding a 0.011 inch (0.028 
cm) layer of carbon-filled polyvinylidene fluoride over a first 30 AWG 
(0.010 inch/0.025 cm diameter) solid Copel.TM. conductor to give an outer 
diameter of approximately 0.032 inch (0.081 cm). An insulating wire was 
prepared by extruding a first layer of polyethylene and a second layer of 
polyvinylidene fluoride over a 24 AWG (0.025 inch/0.064 cm diameter) 
stranded tin-plated copper wire to give an outer diameter of approximately 
0.054 inch (0.137 cm). The polymer layers were then irradiated to 10 to 15 
Mrad. A central support member was prepared by extruding two layers of 
ethylene/tetrafluoroethylene copolymer to a total of 0.008 inch (0.020 cm) 
over a 16 AWG (0.060 inch/0.152 cm) diameter stranded nickel-plated copper 
wire to give an outer diameter of approximately 0.077 inch (0.196 cm). 
Using a 1.5 inch (3.8 cm) extruder, a core member was prepared by extruding 
an 0.060 inch (0.152 cm) layer of thermoplastic elastomer (TPR.TM. 5490, 
available from BP Performance Polymers) over one central support member. 
The resulting core member had an outer diameter of 0.195 to 0.201 inch 
(0.495 to 0.511 cm). The plastic of the core member was softened by 
passing the core member through a 3-foot (91 cm) long radiant heater 
heated to 580.degree. C. at a rate of 9 to 10 feet/min (2.74 to 3.05 
m/min). The softened core member then travelled 2.5 feet (76 cm) through 
ambient air before entering a wrapping head. Two elongate conductor wires 
and two insulating wires were wrapped at an equal spacing (approximately 
0.157 inch/0.40 cm from wire center to wire center) in a spiral pattern 
around the carrier rod at a pitch of about 0.400 inch (1.02 cm). The 
resulting cable had the pattern of a first conductor, a first insulating 
wire (the continuity wire), a second conductor, and a second insulating 
wire (the signal wire). The tension of each wire was adjusted to a level 
at which each wire was forced into the softened deformable polymer of the 
core member to a depth sufficient to prevent any protrusion of the wire 
above the surface of the core member. The resulting sensor cable had a 
maximum diameter of approximately 0.250 inch (0.635 cm).