Liquid leak sensor

A liquid leak detecting sensor in the form of a coaxial cable assembly is provided in which means are provided for enhancing the passage of leaks from the outer surface of the outer conductor into the sensing layer which separates the inner and outer conductors. The change in permittivity of the sensing layer caused by the leak enables rapid detection of the location of the leak along the cable.

BACKGROUND OF THE INVENTION 
This invention relates to a coaxial cable type of sensor for detecting the 
leakage of a liquid and, more particularly, to a liquid leakage sensor 
capable of detecting the leakage of a liquid more quickly than sensors of 
the prior art and capable of reducing variations in the time required for 
the detection thereof. 
Heretofore, oils such as crude oil, petroleum, gasoline, kerosene, organic 
solvents, chemicals and the like have been transported from one place to 
another through underground piping. Various methods have been proposed for 
detecting leakage in pipes in these liquid transportation systems. One of 
the methods proposed, for example, comprises installing along a pipeline a 
pair of conductors separated from each other by an insulating material 
which is porous to the liquid being transported and allows passage of the 
liquid therethrough causing the electrical characteristics between the 
conductors to change when the liquid leaks out of the pipe and enables one 
to detect the place where the liquid is leaking. 
In the aforementioned method, a liquid leakage sensor comprising a pair of 
coaxial conductors has been used as shown in FIG. 1. Therein, an internal 
conductor 1 and an external braided metallic conductor 3 which is porous 
to the liquid to be detected is shown, the conductors being arranged 
coaxially, and a porous insulator 2 of continuously porous 
polytetrafluoroethylene (PTFE) and the like is provided as a detector 
layer between the conductors. A protective polyester braid 5 is provided 
and acts as a protective layer on the periphery of the external braided 
conductor 3. In the liquid leakage sensor thus constructed, a leak of a 
liquid such as petroleum passes through the external braid 5 and porous 
PTFE insulator 4, through the openings in the braid of conductor 3, and 
permeates through the internal porous PTFE insulator 2 used as a detector 
layer, causing the relative permittivity of the detector layer 2 to 
change. Such a detector was the subject of a prior patent application of 
the present inventors, which was published as Japanese Patent Application 
Laid-Open Gazette No. 54-15435 and registered as Patent No. 984263. Such a 
liquid leakage sensor was capable of detecting liquid leakage by measuring 
the change in permittivity or by means of a pulse reflection distortion 
based on the change in the permittivity. The liquid leakage sensor thus 
contructed offered capabilities allowing for the position and range of 
leakage to be detected, the types of oils to be distinguished to a certain 
degree, and oil and water to be selectively differentiated. However, the 
prior art detector still has drawbacks in that 20 to 100 minutes can be 
required to detect oil leakage and there are variations in detection time 
in the longitudinal direction of the cable. 
In view of the aforementioned problems, it is an object of the present 
invention to provide a liquid leakage sensor capable of detecting leakage 
quickly and minimizing variations in detection time in the longitudinal 
direction. 
SUMMARY OF THE INVENTION 
An improved liquid leakage sensor is provided comprising at least two 
electrical conductors in substantially parallel relationship with each 
other and having a porous detector layer between the conductors and 
adjacent to both, the permittivity of the detector layer being measurably 
changed by the entry of liquid into the detector layer, at least one of 
the conductors being permeable to the liquid, the improvement comprising 
the permeable conductor constructed of at least one electrical conducting 
member and at least one member which enhances passage of liquid through 
the permeable conductor. The permeable conductor may be a braided 
construction braided over the detector layer in coaxial construction with 
respect to the other conductor and, in this embodiment, the member which 
enhances passage of liquid is constructed of a continuously porous plastic 
material and the braided construction is a mixed braid of metallic wires 
and porous plastic strands preferably porous polytetrafluoroethylene 
strands. In another embodiment, the permeable conductor may be a spirally 
wrapped construction wrapped over the detector layer in a coaxial 
construction with respect to the other conductor and the permeable 
conductor is preferably constructed as a metal wire helically wound around 
the detector layer adjacent to a tape of a porous plastic helically wound 
about the detector layer, the tape preferably being 
polytetrafluoroethylene. The member which enhances passage of liquid may 
also be a granular substance. The conductors may be arranged coaxially and 
the member which enhances passage of liquid may be a cylindrical metal 
member having openings allowing liquid to pass therethrough, the openings 
being filled with the granular substance which preferably is 
polytetrafluoroethylene fine powder.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS WITH 
REFERENCE TO THE DRAWINGS 
A liquid leak detecting sensor in the form of a coaxial cable assembly is 
provided in which means are provided for enhancing the passage of leaks 
from the outer surface of the outer conductor into the sensing layer which 
separates the inner and outer conductors. The change in permittivity of 
the sensing layer caused by the leak enables rapid detection of the 
location of the leak along the cable. 
The present invention attempted to remedy the shortcomings of the prior art 
sensors and found that the storage of leakage liquid in openings in the 
external braided metallic conductor layer was attributed to the gaps among 
the metal wires constituting the braid and formed the determining cause of 
the variations in detection time and caused long detection times. 
In a liquid leakage sensor comprising a plurality of conductors arranged 
substantially parallel with each other and a porous detector layer 
sandwiched between the conductors for electrically separating one from the 
other, the permittivity of said detector layer being caused to change by 
liquid leakage, an improvement is provided comprising providing the 
external braided conductor whose layer is porous to the liquid to be 
detected with a component which enhances passage of the liquid into the 
detector layer so that the storage of the leaking liquid in the braided 
conductor in its initial as well as later stages may be substantially 
prevented. Because the leaking liquid arriving at the detector layer 
quickly permeates through the detector layer, the time required for 
detecting the liquid leak is largely shortened and the time for detecting 
a liquid leak in the near or far stages of the detector becomes roughly 
constant depending upon the quality of the member having a component which 
enhances passage of the liquid into the detection layer. As a result, 
variations (lack of uniformity) in the time required for detecting liquid 
leaks are minimized. 
Referring to the drawings, the present invention will be described in 
detail. The following are preferred embodiments of the disclosed invention 
and various changes and modifications may be made in the invention without 
departing from the spirit and scope thereof. 
FIG. 2 is a perspective view, partly in cross-section, for illustrating a 
first example of the present invention. In this example, the arrangement 
comprising internal conductor 1 of a silver-plated copper wire 3 mm in 
diameter, a detection layer 2 prepared by winding unsintered continuously 
porous expanded PTFE tape, 2.175 mm thick, around the internal conductor 1 
by helically rolling the tape 1.5 times, about the conductor 1 along its 
length, and a conductor layer 10 which is porous to the liquid to be 
detected and having a component member for enhancing passage of the liquid 
prepared by making a combination of two strands twisted together, twisting 
16 of these into a bundle and alternately braiding the bundles to obtain a 
braid comprising a bundle formed with a combination of fine copper wires 
11 which are each 0.18 mm in diameter and a continuously porous PTFE tape 
12 which is 0.06 mm thick and 3.18 mm wide, which was obtained by 
stretching extruded PTFE tape three times and sintering the tape, the 
conductor layer 10 being provided on the periphery of the detector layer 
2. Essentially, the outer conductor layer 10 comprises a mixed braid of 
metallic fine wires 11 and porous PTFE strands 12. Subsequently, 
protective layer 4 of an unsintered, continuously porous PTFE tape 
helically wound 1.5 times over the conductor layer 10 to a thickness of 
0.8 mm and a protective over-braid 5 formed with a net of polyester thread 
having 0.6 mm diameter strands was provided thereon to complete the liquid 
leakage sensor according to this embodiment. 
Another liquid leakage sensor as a second example was made which was 
similar in construction to the first example except that the member 12 for 
enhancing passage of the liquid was replaced with one formed with a 
filament made of unsintered continuous PTFE tape extended 1.5 times, the 
tape being 0.12 mm thick and 3.18 mm wide. 
The following table shows the test results obtained by measuring the time 
required for detecting kerosene leakage with the above-described examples 
1 and 2 of liquid leakage sensors and a conventional sensor having an 
external braided conductor 3 totally formed with 32 braided fine copper 
wires. 
______________________________________ 
Measuring/Liquid Conventional 
item: sensor used: 
Example 1: 
Example 2: 
sensor: 
______________________________________ 
Characteristic 
50 50 50 
impedance (.OMEGA.) 
Detection time (min)*: 
5.07 6.51 80.90 
Variations in 0.28 0.75 20.40 
detection time for 
every 10 m (min): 
______________________________________ 
*Time required until characteristic impedance decreases by 5.OMEGA. at 
30.degree. C. 
As is apparent from this table, the liquid leakage sensors according to the 
present invention provided excellent results in that they were capable of 
saving considerable time required for detecting liquid leaks and reducing 
variations in the time required to detect leaks along the longitudinal 
direction of the sensors. Moreover, the amount of leakage of 
electromagnetic waves and attenuation did not increase after the 
improvements. Such improvements were observed when expanded, sintered 
continuously porous PTFE thread instead of the continuously porous PTFE 
tape was used as the member 12 for enhancing passage of the liquid. 
Although the member 12 for enhancing passage of the liquid was formed with 
a tape or thread, i.e., a PTFE strand and a copper wire each were made 
into bundles in the above-described examples and they were alternately 
braided in parallel, the conductor 10 may be formed of a braid using 
another mixture of metal wire such as a copper wire and PTFE strands. It 
is also possible to change the shape or size of the PTFE strand and the 
metal wire, or the number of them. 
Preparation of the member for enhancing passage of the liquid from a strand 
facilitates and economizes the provision of such a member. Moreover, use 
of a continuously porous plastic material as the strand is further 
effective for this purpose because both the peripheral portion and the 
inside of its construction may be used as a passageway for the liquid. 
Among the continuous porous plastic materials, continuously porous PTFE is 
stable physically and chemically and is preferred. Unsintered, partially 
sintered and sintered materials may be made of continuously porous PTFE 
and any one of these may be used. The sintered material is most suitable 
for the purpose of the present invention. 
Preparation of the continuously porous PTFE and the production of its 
unsintered, partially sintered and sintered materials have been disclosed 
in Japanese Patent No. 51-18991, Japanese Patent Application Laid-Open, 
Gazette Nos. 50-22881 and 53-99955. 
In addition, a thermoplastic fluorocarbon resin such as FEP, PFA or a color 
pigment may be premixed in with the PTFE. Use of the reinforced plastic 
may be effective to some extent and such a plastic material can be 
produced at less cost. Use of a strand made of sintered PTFE may enhance 
its mechanical strength and stabilize its physical and chemical 
properties. 
As shown in FIG. 3, it is also possible to wind a conductor 21 such as a 
copper wire on the periphery of the detection layer 2 without using the 
braided conductor layer 10 and to arrange the tape member 22 for enhancing 
passage of the liquid formed with a strand adjacent to the conductor 21 as 
shown. In this case, the preparation of the conductor layer 20 porous to 
the liquid is easier than the case of forming a braid. In FIG. 3, like 
elements are given like reference characters of FIG. 2. 
In the above-described sensor it is desirable that the members for 
enhancing the liquid passage provided on the conductor layer porous to the 
liquid be, in view of durability and detection time and the like, various 
continuously porous plastic tapes or threads as those made of the 
continuously porous PTFE. In addition, natural fibers such as cotton yarn 
and reinforced plastic tapes or threads may be used to produce liquid 
leakage sensors which are less costly. Moreover, the continuously porous 
PTFE is preferred as the continuously porous member for enhancing the 
liquid passage when reliability and short detection time are important 
considerations. 
As shown in FIG. 4, in the case of a liquid leakage sensor having a coaxial 
construction, a cylindrical metal body 31 made of copper and having a 
number of small openings for enhancing passage of the liquid may be used 
as the conductor porous to the liquid. Further, a conductor layer 30 
porous to the liquid so constructed that the openings are filled with 
granular plastic members 32 for enhancing passage of the liquid may also 
be used. In this case, like elements are given like reference characters. 
Although other granular members 32 for introducing the liquid having 
various properties may be usable, unsintered PTFE fine resin powder is 
preferred in view of its durability and processibility. Although not 
shown, the openings for introducing the liquid may be filled with the 
granular members 32 for introducing the liquid after winding a metal strip 
such as a copper tape having a number of openings spirally around the 
periphery of the detection layer 2 without using the cylindrical body 31. 
When the granular members 32 are used, selective permeability is provided 
and this makes it unnecessary to separately install a protective or 
selective layer permeable to the liquid and it becomes possible to improve 
the liquid pressure resistance of the sensor. 
In the liquid sensor of FIG. 1, gaps among the metal wires of the external 
braided conductor 3 may be filled with the granular members 32 for 
introducing the liquid. On the other hand, instead of the strand in the 
example of FIG. 3 or together therewith, the granular members for 
introducing the liquid may be used to fill the openings. In particular, it 
is preferred to use the PTFE fine powder resin as the member for 
introducing the liquid in view of its ease of workability. As in the case 
of the first and second examples where gaps are provided over the whole 
conductor layer porous to the liquid prepared from the braided mixture of 
metal wire and strand, the gaps may be filled with particulate grains 
including PTFE fine resin powder. 
In the above examples, liquid leakage sensors each having a coaxial 
construction have been described. However, as shown in FIG. 5, a pair of 
ribbon-like metal conductors 41 having a number of openings for 
introducing the liquid may be arranged in parallel so as to sandwich a 
porous detection layer 2 therebetween wherein conductor layer 40 is 
prepared by filling the openings therein with the granular member 32 and 
wherein a continuously porous insulator 4 and a protective braid 5 are 
installed on the periphery thereof. In FIG. 5, like elements are given 
like reference characters of FIG. 2. 
For the detection and protective layers employed in the liquid leakage 
sensors according to the present invention, materials having a porous 
microstructure provided with a number of fine continuous pores as an 
internal structure prepared from plastics such as polyethylene and 
polypropylene by the known extrusion/stretching, salt/leaching and solvent 
volatilization methods and the like may be used in addition to the 
expanded, continuously porous PTFE tape. 
As above described, a liquid leakage sensor is provided comprising a 
plurality of conductors arranged substantially parallel with each other 
and having a detector layer sandwiched between the conductors, the 
permittivity of the detector layer being caused to change by a liquid 
leakage, the improvement comprising providing a conductor whose layer is 
porous to the liquid being detected with a member for enhancing passage of 
the liquid into the detector layer, so that the time required for 
detecting a liquid leak is shortened and variations in the time required 
for detecting the liquid leak are minimized. Moreover, because the time 
required can be shortened, it becomes possible to increase the thickness 
of a protective layer to be provided on the periphery of the conductor 
layer porous to the liquid and, when the liquid leakage sensor is covered 
with water, prevent (improve in water pressure resistance) reduction in 
electrical insulating resistance which is apt to occur between the 
conductor and the outside, whereby conductor erosion is minimized while 
the mechanical protective capability can be improved. In this case, the 
reduction in insulating resistance and conductor erosion will be further 
effectively provided by using a conductor precoated with plastics and the 
like for insulation. 
While the invention has been disclosed herein in connection with certain 
embodiments and detailed descriptions, it will be clear to one skilled in 
the art that modifications or variations of such details can be made 
without deviating from the gist of this invention, and such modifications 
or variations are considered to be within the scope of the claims 
hereinbelow.