Instrumentation & Measurement Magazine 24-5 - 75

Design of an FBG Based Water
Leakage Monitoring System, Case
of Study: An FBG Pressure Sensor
Vincenzo Marletta
W
ater leakages in water distribution systems due
aging of pipes is a problem affecting most countries
around the world. Official reports from
government-commissioned surveys indicate that water losses
are estimated to amount to an average of around 30% in most
countries, with peaks up to 60% [1]-[3]. Such amounts increase
in urban areas, reaching 70 to 80% in some cities. From
an economic point of view, these losses have a cost that has
been estimated for the European region to an overall of 8 billion
a year [1].
Based on the above depicted background situation, water
leak detection systems are of increasing interest, and many efforts
are being dedicated to the development of new solutions
and methodologies, also made possible by the technological
development and the availability of new low-cost electronics
and sensors. An introductory overview of the water leaks
monitoring in water distribution systems is given in the review
article [4].
A traditional and still adopted approach for leak detection
is based on periodic acoustic surveys using hydrophones.
Such an approach, expensive and time-consuming, is based on
the fact that burst leaks produce noise that propagates along
pipelines and through the ground [5]. Although acoustic based
leak detection methodologies require experienced operators
and provide better results during the nighttime when the water
flow inside the pipes is minimum, they provide the best
accuracy and reliability [5].
An alternative approach is based on the use of accelerometers
for the measurement of leak-related vibrations, typically
below 1 kHz, and propagating along pipes [6]. The frequency
spectrum of vibrations generated by water leaks depends on
many factors, such as the type and the number of cracks on
the pipe, the pressure inside the pipe, the type of surrounding
soil, and the pipe's material [4]. In general, leakage-induced
vibrations in plastic pipes have a lower range of frequencies,
typically below 500 Hz, than in metal pipes where they are located
at higher frequencies [4]-[6].
A commonly adopted technique to detect and localize
water leaks in buried pipes is the cross-correlation between
August 2021
acoustic or vibration signals measured on access points (for
example hydrants or gate valves) on either side of a suspected
leakage. The correlator computes the cross-correlation to evaluate
the time delay between the two acquired signals and to
estimate the position of the leak [7]. A review of real-time sensor-based
methodologies is given in [5].
Many other methodologies adopting different technologies,
e.g., thermography [8], exploitation of hydraulic
transients [9], ground-penetrating radar [10], tracer gas technique
[11], and magnetic fields [12], just to name a few, were
also investigated and adopted by some water management authorities
and service operators.
In recent decades, wired and wireless sensor networks
using different sensing approaches have been largely investigated
and reported in the literature [13], and new
methodologies and signal processing techniques are being
developed [14], [15]. It should be noted that in such an application
context, autonomous or self-powered sensing solutions
would be particularly advantageous [16], [17].
Optical fiber sensor networks [18] have gained large interest
and are recognized as a potentially viable technology
for real-time and continuous monitoring of water distribution
networks [19]. In particular, optical sensors based on Fiber
Bragg Gratings (FBG) show some characteristics like intrinsic
safety (due to the lack of electricity), chemical and electromagnetic
passivity, high sensitivities and precisions, that make
them advantageous [18]. Also, the material biocompatibility,
the capabilities of remote sensing, and the possibility to be
optically multiplexed enabling the realization of sensors networks
with a high number of sensing elements [20] make them
perfectly compatible to be adopted in the monitoring of leakages
in potable water networks. The main drawback indicated
is the need for distribution of optical fiber along with the pipes
network.
An FBG is a microstructure inscribed, by an ultraviolet
source, within the core of an optical fiber, generating a periodic
modulation of the refractive index. If broadband light is
sent through the fiber (with the FBG), a well-defined wavelength
gets reflected while the others can pass. The specific
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