Instrumentation & Measurement Magazine 24-5 - 78

Table 3 - The mechanical and temperature strains, the nominal wavelength
of each optical sensor, and the relative variation range
mechanical strain
μm/m
min
FBG1
FBG3
FBG5
FBG2
FBG4
-10000
-1000
max
2000
10000
1000
temperature
°C
min
max
50
50
50
50
50
thermal strain
μm/m
min
-220
-220
-220
-220
-220
max
330
330
330
330
330
maximum Δλ
nm
min
-0.468
-12.871
-1.708
-0.468
-0.468
max
3.182
13.104
1.942
0.701
0.701
1526.5
1543.6
1559.6
1563.1
1565.4
λ0
nm
min
1526.0
1530.7
1557.8
1562.6
1564.9
max
1529.7
1556.8
1561.6
1563.9
1566.2
the quantities to be measured (pressure, flow, and vibrations)
and by the thermal variations ΔT and considering the specifications
of the optical fiber, it is possible to evaluate the band of
variation Δλ of the wavelength of the FBG sensor using the following
relationship:


where:
◗

    
OC
   
max 0  min 0
.
◗ TOC
T T SK
(1)
TT T T T is the thermal variation to
which the FBG sensor is subjected, referred to the reference
value T0
is the thermo-optical coupling coefficient that defines
the contribution of temperature variations to the sensor
strain.
◗ SS S
 

◗ λ0
max max min minTT
S S
 
 is the range of variation
of the mechanical strain generated by the quantity to be
measured net of the thermal contribution.
◗ K is the strain sensitivity (Gauge factor) of the optical
sensor.
is the nominal (central) wavelength of the Bragg
grating.
To compensate for the effect of temperature on the pressure
sensor and on the flow sensor, two further FBG sensors have
been used as " dummy " sensors. They were placed in such a
way to be affected only by the temperature variations and remain
insensitive to the mechanical strains.
The nominal wavelength of each optical sensor, the relative
variation range, the mechanical and the temperature-related
strains are reported in Table 3. It should be noted that the
ranges of the mechanical strains have been estimated by using
suitable analytical models and fixed to be larger than the theoretical
value to assure a sufficient optical separation between
the sensors.
The first three sensors (FBG1, FBG3, and FBG5) were used
to sense the strain generated by the quantity to be measured
in the pressure sensor, the flow sensor, and the vibration sensor,
respectively. The other two FBGs (FBG2 and FBG4) were
used for the sake of temperature compensation. It should be
noted that a safety band of 1 nm between one sensor and the
other was left.
78
Case of Study: The Pressure Sensor
In this section, the design, the modeling, the structure of the
pressure sensor together with the specifics of the material adopted,
and the results of the experimental tests are discussed.
The design of the water pressure sensor started from the specifics
and constraints fixed by the expert partners of the project.
The main specifics required were related to the pressure values
to be measured that ranged from 0 bar to 6 bar, the sensor resolution
of 2 mbar, and the accuracy of 0.1 bar. The constraints
were related to the sensor dimensions, in particular the diameter
of the hydraulic socket and of the fittings to be used that
had to be standard and as small as possible, and the adopted
materials that had to be compliant with the specific regulations
related to the management of potable water to avoid
contamination and be resistant to aggressive environmental
conditions. To this aim, the body of the sensor was constructed
by using standard hydraulic fittings available on the market.
The working principle of the pressure sensor is based on the
measurement of the strain generated by the water pressure on
a steel membrane. A schematization of the section of the sensor's
body is shown in Fig. 2. A 1 in (25.4 mm) high corrosion
resistant stainless steel AISI 316L membrane is placed inside
a cylindrical 0.75 in male to 1 in female adapter between two
hydraulic ring seals used to guarantee the watertight. A 1 in
cylindrical nipple fitting is used to block the membrane and
the seals. A plastic cap (not shown in Fig. 2) is used to close the
top of the fitting.
The strain generated by the water pressure is measured by
the FBG1 glued in the middle of the steel membrane where the
strain reaches its maximum [29]. The FBG has a length of about
10 mm. The FBG2 sensor, realized on the same fiber of FBG1, is
glued on a rectangular piece of stainless steel (the same of the
membrane) that is not subjected to the stress induced by the
pressure but only to any temperature variation and acts as a
" dummy " sensor for thermal compensation. The piece of steel
with the FBG2 was then glued on the inner part of the nipple
fitting during the assembly procedure. The very small spaces
for the positioning of the sensors and the need for a very flexible
optical fiber should be noted.
To design the steel membrane and fix the suitable thickness,
assuring at the same time the robustness and a linear behavior
IEEE Instrumentation & Measurement Magazine
August 2021
λ
nm
Instrumentation & Measurement Magazine 24-5

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 24-5
