Note: Descriptions are shown in the official language in which they were submitted.
CA 02239~48 1998-06-04
SPECIFICATION
OPTICAL FUEL VAPOR DETECTOR AND FUEL LEAK MONITORING
SYSTEM UTILIZING POLYMER THIN FILM
TECHNICAL FIELD
5This invention relates to a fuel vapor detector for
optically detecting the existence and/or the concentration
of a vaporized fuel such as gasoline, light oil, kerosine,
heavy oil and so on, which is particularly suitable for use
in a fuel leak detector and fuel leak monitoring system for
detecting a fuel leak as early as possible.
BACKGROUND ART
A float type sensor is well known for detecting a
fuel leak in a fuel tank or the like which may be installed
underground in a service station area or the like. The
float type sensor has a float which rises in response to a
fuel leaking from the tank and activates a switch when the
amount of the leaked fuel exceeds a preset value to thereby
determine a fuel leak. In addition, several methods for
detecting a fuel leak have been proposed as illustrated
below.
Published Japanese translation of PCT International
publication for Patent Application No. 3-503674 discloses a
computerized automatic system for detecting the volume of a
leaked liquid including measurements of pressure and
temperature as well as measurements of level (liquid
surface) and temperature to detect a liquid leak from an
underground storage container, wherein an electro-mechanical
level sensor is employed for measuring a liquid level.
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Laid-open Japanese Patent Application No. 2-233393
discloses a leaked oil detector intended to eliminate
disadvantages encountered in the detection of a leaked oil
by a leaked oil display, wherein a water-floatable oil
detecting means is disposed in a gas detecting tube buried
near an underground tank, and the oil detecting means is
electrically connected to an alarming means.
Laid-open Japanese Patent Application No. 6-201510
discloses a leaked oil measuring apparatus for accurately
detecting leaked oil in a tank such as a gasoline tank which
may experience high temperatures, wherein a pressure is
applied to the tank itself, and a change in external
pressure (applied pressure) is measured by means of a
diaphragm type silicon pressure sensor.
These known methods have a disadvantage that an
initial leak detection is impossible. In other words, the
occurrence of fuel leak can be determined only after a
leaked fuel has been accumulated to a certain amount. As a
result, the methods are defective in that the leak may be
found too late, and consequently, because of its electrical
measuring principles, the risk of explosion is potentially
involved.
To overcome the disadvantage of the prior art methods,
a fuel vapor detector for optically sensing a fuel vapor has
already been proposed as illustrated in Fig. 16. In this
fuel vapor detector, a light beam sent from a light source
100 through an optical fiber 102 is incident on a polymer
thin film 106 formed on a substrate 104. The light beam
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reflected by the polymer thin film 106 is sent to and
detected by a light detector 110 through another optical
fiber 108. The polymer thin film 106 reacts with a fuel
vapor passing through a passage 112, or adsorbs or absorbs
the fuel vapor, so that, as a result of such interaction,
the polymer thin film 106 exhibits a change in thickness
and/or refractive index. Since the fuel vapor is optically
detected utilizing the characteristics inherent to the
polymer thin film 106 as mentioned above, a fuel leak can be
advantageously found in early stages. Also advantageously,
the fuel vapor detector is intrinsically safe because the
optical fibers 102, 108 are used (because free from
electrical power).
In the fuel vapor detector illustrated in Fig. 16, an
16 interference enhanced reflection method (hereinafter
referred to as the "IER method") is utilized. Specifically,
light reflected on the surface of the polymer thin film 106
has a phase relationship with light reflected on the surface
of the substrate 104 supporting the polymer thin film 106,
and they interfere with each other. Thus, since the
reflectivity of the polymer thin film 106 or the intensity
of the reflected light changes as the thickness and/or the
refractive index of the polymer thin film 106 changes, the
existence and/or the concentration of a fuel vapor can be
detected as a function of the intensity of the reflected
light.
However, since the two optical fibers 102, 108 are
used as illustrated in Fig. 16, a plurality of collimators
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or connectors must be disposed between the light source 100
and the optical fiber 102, between the optical fiber 102 and
the polymer thin film 106, and between the optical fiber 108
and the light detector 110, the fuel vapor detector has a
problem of a complicated structure and an increased cost.
Another problem to be considered is a higher
likelihood of fuel leak due to increasingly introduced oil-
immersed pumps for purposes of reducing a cost in a service
station. Conventionally, in a service station, each of oil
supply machines is provided therein with a suction pump
corresponding to an associated type of oil for sucking the
oil. However, a system for feeding a fuel to respective oil
supply machines by equipping a tank with a single oil-
immersed pump has been employed in order to reduce a cost in
a service station. However, when an oil-immersed pump is
installed in a service station, the pressure of a fuel
becomes higher than before so that once a leak occurs, a
larger amount of fuel is likely to effuse in a shorter time
period to result in serious environmental contamination.
Also, with any pump concerned, if a fuel leak occurs in an
llnm~nned state such as at night, provision must have been
made so that a service man can be immediately sent to the
site. There is a strong need for the realization of a
remote monitoring system which satisfies requirements as
mentioned above.
DISCLOSURE OF THE INVENTION
This invention has been made in view of the problems
mentioned above, and it is a general object of this
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invention to provide a fuel vapor detector using no optical
fiber or using one optical fiber for detecting the existence
and/or the concentration of a fuel vapor, which is
inexpensive and simple in structure. More specifically, it
is an object of this invention to provide a fuel vapor
detector which is intrinsically safe, simple in structure,
easy to manufacture, highly reliable, inexpensive, and
suitable for reduction in size. Further, it is another
obJect of this invention to provide a fuel leak monitoring
system which is capable of remotely monitoring a fuel leak
utilizing a fuel vapor detector as mentioned above.
To achieve the above objects, this invention provides
a fuel vapor detector for detecting at least one of the
existence and concentration of a fuel vapor. The fuel vapor
detector comprises, as illustrated in Fig. 1:
a light source unit 2 having a light emitting
element;
a sensor unit 10 including a sensor element 8 made of
a polymer thin film 4 formed on a substrate 6, where the
polymer thin film 4 exhibits a change in at least one of a
thickness and a refractive index due to a contact with the
fuel vapor, and positioned such that light from the light
source unit 2 is incident normal to the sensor element 8;
a light transmitting/outputting unit 12 disposed
between the light source unit 2 and the sensor unit 10 for
transmitting light from the light source unit 2 so that the
light is incident on the sensor unit 10 and for outputting
reflected light reflected by the sensor element 8; and
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a light detector unit 14 for receiving the reflected
light from the sensor element 8 to generate a signal
corresponding to the reflected light.
The fuel vapor detector is preferably installed in an
underground tank, a thump, surroundings of an oil immersed
pump, a ground tank, an oil refinery, an oil transporting
line, an oil transporting tanker, and so on.
This invention detects the existence or the
concentration of a fuel vapor by measuring a change in the
reflection characteristic of the sensor unit 10, making use
of the characteristics of the polymer thin film 4 which
exhibits a change in at least one of the thickness and the
refractive index due to a contact with a vapor under
detection. As a result of an interaction with a fuel vapor,
the polymer thin film 4 experiences physical changes such as,
for example, swelling. Also, such swelling causes the
polymer thin film 4 to change the thickness and the
refractive index which are optical parameters inherent
thereto. Since such changes result in a change in the
optical property of the polymer thin film 4, a fuel vapor
can be detected by measuring the reflection characteristic
of the polymer thin film 4.
To realize this, in this invention, light from the
light source 2 is incident normal to the sensor unit 10.
The light is reflected by the sensor element 8 to cause the
light to propagate through the same path as when it was
incident thereto. Then, the light is reflected by the light
transmitting/outputting unit 12 in a direction different
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from that of the propagation path to introduce the light
into the light detector unit 14 which is forced to generate
an electrical signal corresponding to the reflected light
from the sensor element 8.
It should be particularly noted in this invention
that by appropriately selecting a polymer material
constituting the polymer thin film 4, it is possible to
selectively or non-selectively detect the existence of a
fuel vapor such as gasoline, light oil, kerosine, heavy oil
or the like. Moreover, since the polymer thin film 4 has
the reflection characteristic corresponding to the
concentration of a fuel vapor, the fuel vapor detector of
this invention may serve as a concentration meter for a fuel
vapor.
In this invention, an IER method, for example, is
employed for detecting a change in thickness and/or
refractive index of the polymer thin film 4. As mentioned
above, the IER method utilizes the optical interference
characteristic of a thin film structure. Light reflected by
the surface of the polymer thin film 4 has a phase
relationship with light reflected from the interface between
the polymer thin film 4 and a reflecting surface of the
substrate 6, and they interact with each other. The
reflectivity of the sensor element 8 largely depends on the
thickness and/or the refractive index of the polymer thin
film 4. In other words, as the thickness and/or the
refractive index of the polymer thin film 4 changes, the
reflectivity of the polymer thin film 4 or the intensity of
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light reflected therefrom also changes. In this way, the
existence and/or the concentration of a fuel vapor can be
detected as a function of the intensity of reflected light
in accordance with the IER method.
As described above, while the IER method is sensitive
to a change in thickness of the film, this invention may
attach more importance to the influence of the thickness of
the polymer thin film 4 than the reflectivity of the same,
provided that a material having a refractive index not
substantially different from the reflective index of a fuel
vapor is used as the polymer thin film 4 employed in this
invention. This is a unique advantage of this invention
over the prior art.
Another point to be emphasized for a comparison with
the prior art relates to the thickness of the polymer thin
film 4. Fig. 2 illustrates a graph which plots the
reflectivity of the polymer thin film 4 having a refractive
index equal to 1.5 formed on a substrate 6 made of silicon,
to which light is incident at an incident angle of 0~, as a
function of the thickness of the polymer thin film 4.
Polarized light and non-polarized light used herein have a
wavelength of 633 nm.
According to this graph, the thickness of the polymer
thin film 4 suitable for the IER method is preferably
adjusted depending on a particular concentration range of a
fuel vapor in the following manner. First, when a fuel
vapor concentration is low, the reflectivity changes little,
so that the polymer thin film 4 adjusted to have a thickness
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corresponding to a minimum value or a maximum value of the
IER curve would not provide a sufficient change in
reflectivity. It is therefore understood that the thickness
is preferably not a value near any multiple of ~/4ncos~
corresponding to the minimum value or the m~Ximum value of
the reflectivity, where ~ is the wavelength of incident
light, n is the refractive index of the polymer thin film 4,
and H is a light propagation angle within the polymer thin
film 4. When the fuel vapor concentration is relatively
high, on the other hand, the reflectivity largely changes,
so that the polymer thin film 4 is preferably adjusted to a
thickness corresponding to the minimum value or the m~ximum
value of the IER curve in order to take a large signal span.
While the polymer thin film 4 may have a thickness in a
range of 10 nm to 10 ~m, a thickness not more than 1 ~m is
preferable in view of a high speed response.
Materials for the polymer thin film 4 preferably
include a homopolymer or a copolymer having a recurring unit
represented by the following chemical formula (I):
CH2
I
X-C-Rl (I)
I
where X represents -H, -F, -Cl, -Br, -CH3, -CF3, -CN,
or -CH2-CH3:
R1 represents _R2 or -Z-R2;
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Z represents -O-, -S-, -NH-, -NR2'-, -(C=Y)-, -(C=Y)-
Y-, -Y- ( C=Y ) -, - ( SO2 ) -, -Y - ( SO2 ) -, - ( SO2 ) -Y -, -Y - ( SO2 ) -Y -,
-NH-(C=O)-, -(C=O)-NH-, -(C=O)-NR2'-, -Y'-(C=Y)-Y'-, or -O-
(C=O)-(CH2)n-(C=O)-O-;
6 Y independently represents O or S;
Y' independently represents O or NH;
n represents an integer ranging from 0 to 20; and
R2 and R2' independently represent hydrogen, a
straight-chain alkyl group, a branched-chain alkyl group, a
cycloalkyl group, an unsaturated hydrocarbon group, an aryl
group, a saturated or unsaturated hetero ring, or
substitutes thereof. It should be noted that R1 does not
represent hydrogen, a straight-chain alkyl group, or a
branched alkyl group.
Preferably, in the foregoing recurring unit (I):
X represents H or CH3;
Rl represents a substituted or non-substituted aryl
group or -Z-R2;
Z represents -O-, -(C=O)-O-, or -O-(C=O)-; and
R2 represents a straight-chain alkyl group, a
branched alkyl group, a cycloalkyl group, an unsaturated
hydrocarbon group, an aryl group, a saturated or unsaturated
hetero ring, or substitutes thereof.
A polymer used as the polymer thin film 4 may be a
polymer consisting of a simple of the above-mentioned
recurring unit (I), a copolymer consisting of another
recurring unit and the above-mentioned recurring unit (I),
or a copolymer consisting of two or more species of the
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recurring unit (I). The recurring units in the copolymer
may be arranged in any order, and a random copolymer, an
alternate copolymer, a block copolymer or a graft copolymer
may be used by way of example. Particularly, the polymer
thin film 4 is preferably prepared from polymethacrylic acid
esters or polyacrylic acid esters. The side-chain group of
the ester is preferably a straight-chain or branched alkyl
group, or a cycloalkyl group with the number of carbon
molecules ranging preferably from 4 to 22.
Polymers particularly preferred for the polymer thin
film 6 are listed as follows:
poly(dodecyl methacrylate);
poly(isodecyl methacrylate);
poly(2-ethylhexyl methacrylate);
poly(2-ethylhexyl methacrylate-co-methyl methacrylate);
poly(2-ethylhexyl methacrylate-co-styrene);
poly(methyl methacrylate-co-2-ethylhexyl acrylate);
poly(methyl methacrylate-co-2-ethylhexyl methacrylate);
poly(isobutyl methacrylate-co-glycidyl methacrylate);
poly(cyclohexyl methacrylate);
poly(octadecyl methacrylate);
poly(octadecyl methacrylate-co-styrene);
poly(vinyl propionate);
poly(dodecyl methacrylate-co-styrene);
poly(dodecyl methacrylate-co-glycidyl methacrylate);
poly(butyl methacrylate);
poly(butyl methacrylate-co-methyl methacrylate);
poly(butyl methacrylate-co-glycidyl methacrylate);
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poly(2-ethylhexyl methacrylate-co-glycidyl methacrylate);
poly(cyclohexyl methacrylate-co-glycidyl methacrylate);
poly(cyclohexyl methacrylate-co-methyl methacrylate);
poly(benzyl methacrylate-co-2-ethylhexyl methacrylate);
poly(2-ethylhexyl methacrylate-co-diacetoneacrylamide);
poly(2-ethylhexyl methacrylate-co-benzyl methacrylate-co-
glycidyl methacrylate);
poly(2-ethylhexyl methacrylate-co-methyl methacrylate-co-
glycidyl methacrylate);
poly(vinyl c;nn~m~te);
poly(vinyl c;nn~m~te-co-dodecyl methacrylate);
poly(tetrahydrofurfuryl methacrylate);
poly(hexadecyl methacrylate);
poly(2-ethylbutyl methacrylate);
poly(2-hydroxyethyl methacrylate);
poly(cyclohexyl methacrylate-co-isobutyl methacrylate);
poly(cyclohexyl methacrylate-co-2-ethylhexyl methacrylate);
poly(butyl methacrylate-co-2-ethylhexyl methacrylate);
poly(butyl methacrylate-co-isobutyl methacrylate);
poly(cyclohexyl methacrylate-co-butyl methacrylate);
poly(cyclohexyl methacrylate-co-dodecyl methacrylate);
poly(butyl methacrylate-co-ethyl methacrylate);
poly(butyl methacrylate-co-octadecyl methacrylate);
poly(butyl methacrylate-co-styrene);
poly(4-methyl styrene);
poly(cyclohexyl methacrylate-co-benzyl methacrylate);
poly(dodecyl methacrylate-co-benzyl methacrylate);
poly(octadecyl methacrylate-co-benzyl methacrylate);
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poly(benzyl methacrylate-co-benzyl methacrylate);
poly(benzyl methacrylate-co-tetrahydrofurfuryl
methacrylate);
poly(benzyl methacrylate-co-hexadecyl methacrylate);
poly(dodecyl methacrylate-co-methyl methacrylate);
poly(dodecyl methacrylate-co-ethyl methacrylate);
poly(2-ehtylhexyl methacrylate-co-dodecyl methacrylate);
poly(2-ethylhexyl methacrylate-co-octadecyl methacrylate);
poly(2-ethylbutyl methacrylate-co-benzyl methacrylate);
poly(tetrahydrofurfuryl methacrylate-co-glycidyl
methacrylate);
poly(styrene-co-octadecyl acrylate);
poly(octadecyl methacrylate-co-glycidyl methacrylate);
poly(4-methoxystyrene);
poly(2-ethylbutyl methacrylate-co-glycidyl methacrylate);
poly(styrene-co-tetrahydrofurfuryl methacrylate);
poly(2-ethylhexyl methacrylate-co-propyl methacrylate);
poly(octadecyl methacrylate-co-isopropyl methacrylate);
poly(3-methyl-4-hydroexystyrene-co-4-hydroxystyrene);
poly(styrene-co-2-ethylhexyl methacrylate-co-glycidyl
methacrylate).
In the methacrylate ester polymers or copolymers
listed above, acrylate may be substituted for methacrylate.
The polymers may be crosslinked on their own, or they may be
crosslinked by introducing into the polymer a compound that
has crosslinking reactive groups. Such crosslinking
reactive groups appropriate for the purpose include, for
example, an amino group, a hydroxyl group, a carboxyl group,
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an epoxy group, a carbonyl group, a urethane group, and
derivatives thereof. Other examples may include maleic acid,
fumaric acid, sorbic acid, itaconic acid, cinnamic acid, and
derivatives thereof. Materials having chemical structures
capable of forming carbene or nitrene by irradiation of
visible light, ultraviolet light, or high energy radiation
may also be used as crosslinking agents. Since a film
formed from crosslinking polymer is insoluble, the polymer
forming the polymer thin film 4 may be crosslinked to
increase the stability of the detector. There is no
particular limits to the crosslinking method, and methods
utilizing irradiation of light or radioactive rays may be
used in addition to known crosslinking methods, for example,
a heating method.
In the fuel vapor detector according to this
invention, preferably, the reflecting surface of the
substrate 6 for supporting the polymer thin film 4 is
sufficiently flat such that a reflecting surface of the
substrate 6 reflects light, and the substrate itself
preferably has a high reflectivity. An example of the
substrate 6 may be a silicon wafer. The polymer thin film 4
may be formed on the surface of the substrate 4 by a spin
coat method or any other coating method used in common.
The light source unit 2 may be implemented by a
simple or a combination with a collimator or the like of any
light emitting element such as a laser diode, a light
emitting diode or the like for emitting visible light or
infrared rays. The light transmitting/outputting unit 12
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may be implemented by a glass plate, a beam splitter, a
polarizing beam splitter, a non-
polarizing beam splitter or a half mirror, and preferably by
a beam splitter. The light detector unit 14 may be formed
of either a photodiode, a phototransistor or a
photomultiplier tube, and a photodiode is preferably used.
The light transmitting/outputting unit 12 may be
connected to the sensor unit 10 through an optical fiber.
As the light source unit 2 for this case, a laser diode or a
light emitting diode is suitable. A light beam emitted from
the light transmitting/outputting unit 12 is preferably
introduced into the optical fiber through a collimator. The
collimator used herein is preferably a connector having a
collimator lens, a SELFOC lens or the like available in the
market. The optical fiber is preferably a single mode
optical fiber, a multi-mode optical fiber, an optical fiber
light waveguide formed of a single mode optical fiber, or an
optical fiber light waveguide formed of a multi-mode optical
fiber. Since light exiting from an optical fiber has a
rather wide angle, the light is preferably converged by a
lens before it is incident on the sensor unit 10. A lens
for this purpose is preferably a glass spherical convex lens,
a glass aspherical convex lens, a plastic spherical convex
lens, a plastic aspherical convex lens, a quartz spherical
convex lens, a quartz aspherical convex lens, a SELFOC lens,
a ball lens or the like.
It goes without saying that any optical element need
not be positioned between the light transmitting/outputting
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unit 12 and the sensor unit 10. In this case, the degree of
freedom is increased. For example, light can be measured by
the sensor unit 10 spaced from the light
transmitting/outputting unit 12 by a desired distance. A
light source unit 2 for this case is preferably a laser
diode. A collimator lens may be preferably used to
collimate light from the light source unit 2. While a glass
spherical convex lens, a glass aspherical convex lens, a
plastic spherical convex lens, a plastic aspherical convex
lens, a quartz spherical convex lens, a quartz aspherical
convex lens, a SELFOC lens, a ball lens or the like may be
used as such a collimator lens, a quartz aspherical convex
lens is preferred.
In one embodiment of this invention, the sensor unit
10 comprises a housing having a chamber, and the sensor
element 8 having the polymer thin film 4 formed on a
reflecting surface of the substrate 6 is positioned in this
chamber. The chamber is provided with a fuel vapor inlet
port or a fuel vapor intake port for interacting a fuel
vapor with the polymer thin film 4, and provided with a fuel
vapor exhaust port for exhausting a fuel vapor in the
chamber to the outside, if necessary. A cover may be
attached to the fuel vapor inlet port or the fuel vapor
intake port for blocking stray light, dust, mist or the like
from introducing and transmitting a fuel vapor. This cover
is preferably formed, for example, of a sintered metal or a
separating membrane of Teflon.
At a position of the housing of the sensor unit 10
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opposite to the sensor element 8, a window and a glass plate
covering the window may be provided as a light input/output
unit for passing therethrough light from the light source
unit 2 and reflected light from the sensor element 8.
However, when the light transmitting/outputting unit 12 is
connected to the sensor unit 10 through an optical fiber, a
leading end of the optical fiber may be extended to the
chamber such that the leading end opposes the sensor element
8, instead of forming the window through the housing. In
addition, a collimator lens may be provided at a position of
the housing of the sensor unit 10 opposite to the sensor
element 8, if necessary, as a light input/output unit for
passing therethrough light from the light source unit 2 and
reflected light from the sensor element 8.
In practice, the light source unit 2, the light
transmitting/outputting unit 12 and the light detector 14
unit may be integrated into a light emitting/light receiving
unit, accommodated in a casing, and installed in an
explosion-proof safe area. In this case, an integrated
light emitting/light receiving laser having an additional
hologram (see Laid-open Japanese Patent Application No. 6-
52588) may be advantageously utilized. The sensor unit 10,
in turn, may be installed at any site where fuel leak is
highly likely to occur, for example, in a double-shell
underground tank, an oil tube, an oil thump, a double-shell
tank, and so on. In this event, the positional relationship
between the casing containing the light source unit 2, the
light transmitting/outputting unit 12 and the light detector
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unit 14 and the sensor unit 10 is adjusted such that light
emitted from the light source unit 2 is incident normal to
the sensor unit 10, and reflected light therefrom propagates
back the same path.
In one embodiment, the light detector unit 14
comprises photoelectrical transducing means for generating
an electrical signal in accordance with the amount of
reflected light from the sensor element 8, and means for
comparing the electrical signal with a predetermined value
to notify the existence and/or the concentration of a fuel
vapor as the result of the comparison.
Furthermore, this invention provides a fuel leak
monitoring system characterized in that:
at least one of the so far described fuel vapor
detectors is provided such that the sensor unit is installed
at a monitored site and the light emitting/light receiving
unit is disposed in an alarm controller;
the alarm controller has a determination circuit for
determ,n'ng whether or not a fault has occurred based on an
electrical signal output from the light emitting/light
receiving unit, wherein the fault includes a trouble in the
sensor unit and the existence of a fuel vapor at a site
where the sensor unit is installed; and
the alarm controller and the remote location are
coupled by bi-directional communicating means,
whereby the occurrence of a fault is monitored from
the remote location.
In this fuel leak monitoring system, the alarm
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controller further comprises:
a memory for storing data indicative of a
determination result by the determination circuit; and
communicating means for communicating data stored in
the memory to the remote location at predetermined time
intervals when the determination result does not indicate
the occurrence of a fault, and for immediately communicating
to the remote location when the determination result
indicates the occurrence of a fault.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagram schematically illustrating the
configuration of a fuel vapor detector according to this
invention;
Fig. 2 is a graph illustrating the reflectivity of a
polymer thin film formed on a substrate;
Fig. 3 is a diagram schematically illustrating the
structure of a first embodiment of the fuel vapor detector
according to this invention;
Fig. 4 is a diagram schematically illustrating the
structure of a second embodiment of the fuel vapor detector
according to this invention;
Fig. 5 is a graph illustrating changes over time of
the magnitude of a signal generated by the fuel vapor
detector of Fig. 4;
Fig. 6 is a diagram schematically illustrating the
structure of a third embodiment of the fuel vapor detector
according to this invention;
Figs. 7(A) and 7(B) are diagrams each schematically
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illustrating a structure for coupling a sensor unit to an
end of an optical fiber in the fuel vapor detector of Fig.
6;
Fig. 8 is a diagram schematically illustrating an
analog circuit for processing electrical signals fetched
from a photodiode in the fuel vapor detectors of Figs. 3, 4
and 6;
Fig. 9 is a diagram schematically illustrating the
structure of a fourth embodiment of the fuel vapor detector
according to this invention;
Fig. 10 is a graph illustrating changes over time of
the magnitude of a signal generated by the fuel vapor
detector of Fig. 9;
Fig. 11 is a diagram schematically illustrating the
structure of a fifth embodiment of the fuel vapor detector
according to this invention;
Fig. 12 is a diagram schematically illustrating the
structure of a sixth embodiment of the fuel vapor detector
according to this invention;
Fig. 13 is a schematic diagram of a fuel leak
monitoring system using the fuel vapor detector according to
this invention;
Fig. 14 is a block diagram schematically illustrating
the configuration of an alarm controller in Fig. 13;
Fig. 15 is a graph illustrating an output voltage of
an analog determination circuit in Fig. 14 together with a
gas alarm threshold and a trouble alarm threshold; and
Fig. 16 is a diagram schematically illustrating the
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structure of an example of a conventional fuel vapor
detector.
BEST MODE FOR CARRYING OUT THE INVENTION
This invention will hereinafter be described in
connection with several embodiments thereof in specific
forms, however, this invention is not limited to these
embodiments.
Fig. 3 is a diagram schematically illustrating the
structure of a first embodiment of a fuel vapor detector
according to this invention. A laser diode 20, a beam
splitter 22 and a sensor unit 10 are positioned such that
light from the laser diode 20 passes through the beam
splitter 22 and is incident normal to a sensor element 8 in
the sensor unit 10. A light beam emitted from the laser
16 diode 20 is split into two by the beam splitter 22, and one
of the light beams is received by a first photodiode
(reference channel photodiode) 24 as a reference signal.
The other light beam is incident on the sensor element 8,
and reflected off a surface of a polymer thin film 4 and off
an interface between the polymer thin film 4 and a substrate
6. The reflected light mutually interferes with each other
to produce reflected light having an intensity corresponding
to at least one of the thickness and the refractive index of
the polymer thin film 4. This reflected light propagates
back the same path as the going path, enters the beam
splitter 22, and is bent perpendicularly by the beam
splitter 22 to be received by a second photodiode (signal
channel photodiode) 26 as a detected signal.
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The second photodiode 26 is used to monitor the
thickness of the sensor element 8, and the first photodiode
24 is used to monitor fluctuations in light output of the
light source unit 2 or the like to compensate for the output
of the second photodiode 26. The respective photodiodes 24,
26 are connected to current-to-voltage converter circuits
24', 26' for producing voltage outputs.
The sensor unit 10 comprises a housing 30 which is
formed with a chamber 28 for communicating a fuel vapor to
the inside. The sensor element 8 having the polymer thin
film 4 formed on a reflecting surface of the substrate 6 is
positioned in place within the chamber 28 by an appropriate
means such that light from the light source unit 2 is
incident normal to the polymer thin film 6. A window 32 is
formed through a side of the housing 30 which faces the
sensor element 8. A glass plate 34, transmitting light
emitted from the light source unit 2 and light reflected by
the sensor element 8, is fitted in the window 32 to
constitute a light input/output unit. The housing 30 of the
sensor unit 10 is further provided with a fuel vapor inlet
port 36 for introducing a fuel vapor into the chamber 28 to
promote interaction of the fuel vapor with the polymer thin
film 4, and a fuel vapor exhaust port for exhausting the
fuel vapor in the chamber 28 to the outside.
For actually fabricating the sensor element 8, 8.5
grams of poly(benzyl methacrylate-co-2-ethylhexyl
methacrylate) was dissolved in cyclohexanone to produce a
solution having a total weight of 100 grams. The solution
- 22 -
CA 02239~48 1998-06-04
was spin-coated on a substrate made of silicon wafer at 2900
rpm to form a polymer thin film. The polymer thin film was
dried at 60~C in a reduced pressure environment for one hour,
and then the thickness of the polymer thin film, when
measured using a three-wavelength automatic ellipso-meter
"Auto EL IV NIR III" manufactured by Rudolph Research Co.
was approximately 330 nm. This silicon wafer substrate was
diced into 10 mm x 10 mm squares to produce sensor elements
8. To ex~m~ne the performance of a fuel vapor detector
using this sensor element 8, the sensor element 8 is set in
a chamber 28 in parallel to and opposite to the glass plate
34 as illustrated in Fig. 34. When a light source for
emitting light at wavelength of 670 nm was used as the laser
diode 20, the output of the second photodiode 26 after
16 current-to-voltage conversion was approximately 890 mV when
nitrogen was introduced into the chamber 28. However, a
signal of approximately 960 mV was generated from the second
photodiode 26 after current-to-voltage conversion with good
reproductivity when a gasoline vapor having a relative
concentration of 0.4 was introduced into the chamber 28. It
was revealed from this that the sensor element 8 using the
polymer thin film 4 made of the aforementioned material has
a larger sensitivity for a gasoline vapor than for nitrogen.
Fig. 4 is a diagra~m~ schematically illustrating the
structure of a second embodiment of the fuel vapor detector
according to this invention. This second embodiment differs
from the first embodiment of Fig. 3 in that a light source
is composed of a laser diode 20 and a collimator lens 40 in
- 23 -
CA 02239~48 1998-06-04
combination, a glass plate 42 is used in place of the beam
splitter 22, and a window 32 is provided with a glass plate
44 having an additional interference filter of the same
wavelength as that of light emitted from the laser diode 20.
A light beam emitted from the laser diode 20 and
passing through the collimator lens 40 is split into two by
the glass plate 42 as a light splitting means, and one of
the light beams enters a first photodiode 24. The other
light beam is incident normal to a sensor element 8 through
the glass plate 44 having the interference filter. The
light beam reflected by the sensor element 8 returns along
the same path, and is reflected on the glass plate 42 in a
direction different from the path, along which it has been
travelling, and received by a second photodiode 26.
For example, a light source for emitting light at
wavelength of 830 nm was used for the laser diode 20, a
glass plate with an interference filter at 830 nm was used
for the glass plate 44, and the collimator lens 40 was
position at 10 meters from the glass plate 42. Then, a
previously adjusted 1 vol% gasoline vapor was introduced
into a chamber 28 through an inlet port 36 to observe
changes in intensity of reflected light from the sensor
element 8. The magnitude of an output signal from the
second photodiode 26 was 300 mV after current-to-voltage
conversion when air was introduced. When gasoline was
introduced after exhausting the air from the chamber 28, an
output signal having a magnitude of 670 mV was generated
from the second photodiode 26 after current-to-voltage
- 24 -
CA 02239~48 1998-06-04
conversion. Similarly, when lvol% light oil vapor was
prepared and similar experiments were made, an output signal
having a magnitude of 350 mV was generated from the second
photodiode 26 after current-to-voltage conversion. Fig. 5
illustrates changes over time in magnitude of the output
signal generated in the experiments. It was revealed from
the foregoing that the fuel vapor detector of Fig. 4 could
also be used as a fuel leak detector.
Fig. 6 is a diagram schematically illustrating the
structure of a third embodiment of the fuel vapor detector
according to this invention. The third embodiment differs
from the first embodiment of Fig. 3 in that a highly
directive light-emitting diode 46 is used as a light source,
a beam splitter 22 and a sensor unit 10 are coupled by an
optical fiber 48, and connectors 50, 52 having a collimator
are connected to both ends of the optical fiber 48. Fig.
7(A) illustrates a structure for coupling the optical fiber
48 to the sensor unit 10, where a connector 52 with a
collimator is mounted to cover a window 32 formed through a
housing 30 of the sensor unit 10. One end of the optical
fiber 48 is connected to this connector 52. For example,
the light emitting diode 46 emits light at wavelength of 660
nm, and the optical fiber 8 is a multi-mode optical fiber
having a length of 50 meters.
In Figs. 6 and 7(A), a light beam emitted from the
light emitting diode 46 is split by the beam splitter 22
into two, one of which is detected by a first photodiode 24
as a reference signal for compensating for fluctuations of
- 25 -
CA 02239~48 1998-06-04
the light emitting diode 46, and the other of which is
introduced into the connector 50 with a collimator and
incident on the optical fiber 48. The light from the
optical fiber 48 is collimated by the connector 52 with a
collimator, incident normal to a sensor element 8 and
reflected there to again pass through the connector 52 with
a collimator, and propagates through the optical fiber 48,
and is reflected by the beam splitter 22 to be received by a
second photodiode 26.
Instead of the structure of Fig. 7(A), an end of the
optical fiber 48 may be positioned to be in contact with the
chamber 28 without forming a window through the housing 30,
as illustrated in Fig. 7(B).
Now, Fig. 8 is used to describe an analog circuit
which receives electrical signals from the first and second
photodiodes 24, 26 illustrated in Figs. 3, 4 and 6 to output
an electrical signal indicative of the existence of a fuel
vapor. A reference signal Iref from the first photodiode 24
is converted to a reference signal Vref and amplified by the
current-to-voltage converter circuit 24', while a detected
signal Idet from the second photodiode 26 is converted to a
detecting signal Vdet and amplified by the current-to-
voltage converter circuit 26'. Both the signals are input
to a compensating circuit 54. The compensating circuit 54
uses the reference signal Vref to compensate for the
detected signal Vdet with respect to fluctuations of the
light emitting diode 46. The compensating circuit 54
outputs a detected signal Vcom which has been compensated
- 26 -
CA 02239~48 1998-06-04
for fluctuations of the light emitting diode 46. This
signal Vcom is lead to a differential circuit 56 which
generates a value derived by subtracting a signal V_base
corresponding to a zero concentration from the signal Vcom,
6 i.e., a voltage Vdif corresponding to a difference from the
zero concentration. The voltage Vdif is compared with a
comparison level V_th in a comparator circuit 58. The
comparator circuit 58 outputs a voltage Vout at high level
when Vdif exceeds V_th and at low level when Vdif does not
exceed V_th. This voltage Vout is utilized to sense the
existence of a fuel vapor.
For example, a hexane vapor atmosphere having a
relative concentration of 0.4 was introduced into the sensor
unit 10 according to the third embodiment illustrated in Fig.
6, and sensing was attempted with the circuit of Fig. 8. At
a room temperature (19~C), a signal at 5.95 V was generated
as the detected signal Vdet. On the other hand, when the
sensor unit 10 was free of hexane vapor, the detected
voltage Vdet was at 5.84 V. Between the two voltages, there
is a voltage difference or a span of 0.11 V, from which it
is understood that the fuel vapor detector illustrated in
Fig. 6 is sufficiently capable of sensing the existence of
hexane vapor.
Fig. 9 schematically illustrates the structure of a
sensor unit in a fourth embodiment of the fuel vapor
detector according to this invention, which is an embodiment
suitable for the sensing of diffused vapor. Fig. 9
illustrates the structure of a sensor unit 10' in this
- 27 -
CA 02239~48 1998-06-04
embodiment, where the sensor unit 10' is used in place of
the sensor unit 10 in Fig. 6. A housing 30 of the sensor
unit 10' is provided with fuel vapor intake ports 60, 62 for
taking a diffused vapor into a chamber 28, and a sintered
metal filter is attached to each of the fuel vapor intake
ports 60, 62 to prevent mist from intruding into the chamber
28. One end of an optical fiber 48 is connected to a
connector 52 with a collimator which covers a window 32 of
the housing 30. The optical fiber 48 is, for example, a
multi-mode optical fiber having a length of 50 meters. A
light beam exiting from one end of the optical fiber 48 is
converged by a glass spherical lens 64 to be incident normal
to a sensor element 8.
Actually, a fuel vapor detector using the sensor unit
10' of Fig. 9 was placed in the air of a diffusion bath
having a volume of 200 liters, and changes in intensity of
reflected light from the sensor element 8 were observed for
a condition in which the diffusion bath was filled with air,
and for a condition in which the diffusion bath was enclosed
after 1 cc of gasoline in liquid state had been dripped onto
the bottom of the diffusion bath. While the magnitude of
the detected signal Vdet was 5.84 V when the diffusion bath
was filled with air, the magnitude of the detected signal
Vdet at 6.54 V was generated when the diffusion bath was
enclosed after the gasoline liquid had been dripped into the
diffusion bath as mentioned above. Fig. 10 illustrates
changes in magnitude of the detected signal Vdet in this
case. When 1 cc of light oil in liquid state was poured
- 28 -
CA 02239~48 1998-06-04
into the diffusion bath in a similar manner, the magnitude
of the detected signal at 5.90 V was generated. It was
revealed from the foregoing results that the fuel vapor
detector of the fourth embodiment could also be used as a
fuel leak detector.
Fig. 11 schematically illustrates the structure of a
sensor unit in a fifth embodiment of the fuel vapor detector
according to this invention, which is an embodiment suitable
for the sensing of diffused vapor. A sensor unit 10"
illustrated in Fig. 11 may be used in place of the sensor
unit 10' in Fig. 9. An end portion of an optical fiber 48
passes through a side wall of a housing 30 of the sensor
unit 10", and its leading end opposes a sensor element 8
through a SELFOC lens 66. This results in elimination of
the connector 52 with a collimator, so that the sensor unit
10" is reduced in size. Similar to the sensor unit 10' of
Fig. 9, the housing 30 is provided with fuel vapor intake
ports 60, 62, and a sintered metal filter is fitted in each
of the fuel vapor intake ports 60, 62, such that the sensor
element 8 is in contact with external air.
Actually, the sensor unit 10" having the sensor
element 8 of 3 mm~ x 10 mm in size positioned at 2 mm from
the lower end of the SELFOC lens 66 was placed in the air of
a diffusion bath having a volume of 200 liters, and changes
in intensity of reflected light from the sensor element 8
were observed for a condition in which the diffusion bath
was filled only with air, and for a condition in which the
diffusion bath was enclosed after 1 cc of gasoline had been
- 29 -
CA 02239~48 1998-06-04
dripped onto the bottom of the diffusion bath in a liquid
state. While the magnitude of the detected signal Vdet was
6.95 V when the diffusion bath was filled with air, the
detected signal Vdet of magnitude at 7.78 V was generated
when the gasoline liquid existed on the bottom of the
diffusion bath. It was revealed from the foregoing results
that the fuel vapor detector according to the fifth
embodiment illustrated could be used as a fuel leak detector.
Fig. 12 is a diagram schematically illustrating the
structure of a light emitting/light receiving unit in a
sixth embodiment of the fuel vapor detector according to
this invention. The light emitting/light receiving unit in
this embodiment is characterized by an integrated structure
comprising an integrated light emitting/light receiving
laser 70 having an additional hologram (see Laid-open
Japanese Patent Application No. 6-52588), and a fiber
receptacle FC connector 72 having a collimator lens, one end
of which is connected to an optical fiber for propagating
laser light between the integrated light emitting/light
receiving laser 70 and a sensor unit 10.
In Fig. 12, the integrated light emitting/light
receiving laser 70 comprises, in an integrated form, a laser
diode 74 having an oscillation wavelength of, for example,
780 nm; a photodiode (not shown) for monitoring laser light
emitted from the laser diode 74; a light receiving
photodiode 76 for detecting reflected light; and a hologram
78 for transmitting laser light emitted from the laser diode
74 and for deflecting reflected light from a sensor element
- 30 -
CA 02239~48 1998-06-04
8 from its traveling direction so that the reflected light
is incident on the light receiving photodiode 76. These
elements are supported on an appropriate base.
The integrated light emitting/light receiving laser
70 is fixed to one end of a cylinder 80 made of aluminum,
and a collimator lens 82 for collimating light transmitting
the hologram 78 is fixed in place within the cylinder 80 by
an appropriate means. A lower end of the fiber receptacle
FC connector 72 is secured to an upper end of the cylinder
80. The optical fiber 84, one end of which is linked to the
sensor unit 10, has the other end drawn to the inside of the
fiber receptacle FC connector 72. The other end of the
optical fiber 84 is at a position at which laser light
collimated by the collimator lens 82 is converged by a
collimator lens 86.
When the light emitting/light receiving unit
illustrated in Fig. 12 is compared with the structure
illustrated in Fig. 1, the laser diode 74 corresponds to the
light source unit 2; the hologram 78 to the light
transmitting/outputting unit 12 and the light receiving
photodiode 76 to the light detector unit 14, respectively.
Since the fuel vapor detector of the sixth embodiment
is structured as described above, laser light emitted from
the laser diode 74 transmits the hologram 78, is collimated
by the collimator lens 82 and converged at the other end of
the optical fiber 84 by the collimator lens 86, propagates
through the optical fiber 84 to the sensor unit 10, and is
reflected by the sensor element 8. The laser light
- 31 -
CA 02239~48 1998-06-04
reflected by the sensor element 8 propagates back along the
same path, passes through the collimator lens 82, and is
incident on the hologram 78. The hologram 78 deflects the
traveling direction of the incident light so that it is
forced to be incident on the light receiving photodiode 76.
The existence of a fuel vapor can be detected by measuring
the magnitude of an output signal from the light receiving
photodiode 76 at this time.
Actually, a diffusion type sensor unit (for example,
the one illustrated in Fig. 9) connected to the light
emitting/light receiving unit of the structure illustrated
in Fig. 12 through an optical fiber of 50 meters in length
was placed in the air in a diffusion bath having a volume of
200 liters. 1 cc of gasoline in liquid state was dripped
onto the bottom of the diffusion bath, and the diffusion
bath was enclosed, and the magnitude of a detected signal of
the light receiving photodiode was observed for a gasoline
vapor, with a detected signal at 2780 mV being generated.
The magnitude of the output signal of the light receiving
photodiode was 1435 mV when the sensor unit was placed in
the air. It was revealed from these results that the sixth
embodiment can also be used as a fuel leak detector.
One important application of the fuel vapor detectors
so far described in detail is a system for remotely
monitoring leak of fuel vapor. Fig. 13 illustrates an
outline of such a system for remotely monitoring leak of
fuel vapor. In Fig. 13, sensor units 10l, 102, 103, 104 of a
structure illustrated in any of Figs. 7, 9, 11 and 12 are
- 32 -
CA 02239~48 1998-06-04
disposed at sites where a fuel vapor is likely to leak, and
these sensor units are connected to an alarm controller 90
through optical fibers 481, 482, 483, 484, respectively. The
alarm controller 90 is installed at an appropriate site
where a fuel vapor is likely to leak, for example, near a
service station, an oil supply station or the like, and are
connected to a data processing apparatus 94 installed in a
remote monitoring center 92 through arbitrary lines 96 such
as telephone lines, dedicated lines, wireless lines,
satellite lines or the like. The data processing apparatus
94 is for example an operation processing apparatus such as
a personal computer or the like. Relay stations 98 may be
installed in the middle of the line 96 as required. It goes
without saying that a plurality of the alarm controllers 90
may be connected to the monitoring center 92 such that a
plurality of different sites can be collectively monitored
at the same time.
Fig. 14 is a block diagram illustrating an exemplary
structure of the alarm controller 90 which has three sensor
units 101, 102, 103 each connected to one end of an optical
fiber 481, 482 or 483. In the figure, the alarm controller
90 comprises three alarm control units 1001, 1002, 1003
connected to the other ends of the optical fibers 481, 482,
483: a microprocessor 102 for generally controlling the
operation of the alarm controller 90; a modem 104 connected
to the telephone line 96; a memory 106 for storing monitored
data; an external input terminal 108 through which external
information is input; and an alarm unit 100 for generating
- 33 -
CA 02239~48 1998-06-04
alarm.
The three alarm control units 100l, 1002, 1003, which
are in the same structure, each have a light emitting/light
receiving unit, an analog determination circuit and a
display unit. The light emitting/light receiving unit sends
laser light to the optical fiber to irradiate the sensor
unit with the laser light, and receives reflected light from
the sensor unit and transduces the received light to an
electrical signal. The electrical signal is compared with a
gas alarm threshold value and a trouble alarm threshold
value, respectively, in the analog determination circuit for
determining the presence or absence of a fuel leak and
whether a trouble has occurred in the sensor unit, the
optical fibers, the light source or the like. A
determination result in the analog determination circuit is
either "Normal", "Fuel Leak Has Occurred" or "Trouble in
Sensor Unit, Optical Fiber, Light Source or the Like". The
microprocessor 102 always displays the determination result
on the display unit to enable a field manager to know the
situations at monitored sites. The determination result may
be printed out as required.
The microprocessor 102 temporarily stores the
determination result in the analog determination circuit in
the memory 106. If the determination result shows that the
sensor units 101, 102, 103 are normally operating, and no
fuel leak has occurred, the microprocessor 102 reads data
indicative of the determination result from the memory 106
at predetermined time intervals, and sends this data to the
- 34 -
CA 02239~48 1998-06-04
monitoring center 92 together with a field identification
code through the modem 104. In this event, the
microprocessor 102 may also send external data (for example,
the amount of fuel stored in a tank, a power interruption
occurring time, a power interruption recovery time, and so
on) input from the external input terminal 108 to the data
processing apparatus 94 in the monitoring center 92, through
the modem 104 and the telephone line 106. The contents of
data thus communicated to the monitoring center 92 may be
set by sending instructions from the data processing
apparatus 94 to the microprocessor 102 using an appropriate
input means in the field. In addition, the values of the
gas alarm threshold and the trouble alarm threshold may be
set or changed similarly by inputting new values in the
field, or by sending instructions from the data processing
apparatus 94 to the microprocessor 102 through the telephone
line 96.
On the other hand, if the determination result of the
analog determination circuit shows that fuel is leaking or a
trouble has occurred in any of sensor units, optical fibers,
light source and so on, the microprocessor 102 immediately
activates the alarm unit 110 to generate alarm for prompting
the field manager to take appropriate actions, and notifies
the data processing apparatus 94 of the occurrence of the
fault through the modem 104 and the telephone line 96. In
this way, the data processing apparatus 94 displays a
message for notifying the fault, activates an alarm lamp or
a buzzer, and so on to prompt the manager to take
- 35 -
CA 02239~48 1998-06-04
appropriate actions. It is therefore possible to find the
occurrence of a fault at a remote location in early stages.
Fig. 15 is a graph illustrating an example of changes
in the output voltage from the analog determination circuit
over time together with the gas alarm threshold at 2.5 volts
and the trouble alarm threshold at 0.5 volts. When the
output voltage of the analog determination circuit exceeds
the gas alarm threshold or lowers below the trouble alarm
threshold, it is determined that leaked fuel or a fault such
as a trouble in any of sensor units, optical fibers, light
source and so on has occurred.
INDUSTRIAL USABILITY
As will be apparent from the foregoing detailed
description of this invention made with reference to several
embodiments thereof, the fuel vapor detector according to
this invention is simple in construction and inexpensively
manufacturable while highly reliable, and can intrinsically
safely detect the existence and/or the concentration of a
fuel vapor. Further, since the sensor units may be disposed
at sites where a fuel is more likely to leak and the sensor
units are connected to light emitting/light receiving units
through optical fibers such that the occurrence of a fault
can be determined utilizing the outputs of the light
emitting/light receiving units, any fault can be safely
detected in early stages.
Furthermore, since data generated by the sensor units
can be communicated to a remote location, monitored sites
can be always kept monitored, including even during the
- 36 -
CA 02239~48 1998-06-04
night when monitored sites are unattended, thus making it
possible to rapidly attend to any fault whenever it occurs.
