Journal of Sensor Technology, 2013, 3, 70-74
http://dx.doi.org/10.4236/jst.2013.33012 Published Online September 2013 (http://www.scirp.org/journal/jst)
Refractometric Fiber Optic Sensor for
Detecting Salinity of Water
Supriya S. Patil1, Arvind D. Shaligram2
1Department of Electronic Science, Abasaheb Garware College, Pune, India
2Department of Electronic Science, University of Pune, Pune, India
Received June 6, 2013; revised July 6, 2013; accepted July 14, 2013
Copyright © 2013 Supriya S. Patil, Arvind D. Shaligram. This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
ABSTRACT
Salinity is an important property of industrial and natural waters. It is defined as the measure of the mass of dissolved salts
in a given mass of solution. High salinity has an impact on people and industries reliant on water. High levels of salt can
reduce crop yields, limit the choice of crops that can be grown and, at higher concentrations over long periods, can kill
trees and make the land unsuitable for agricultural purposes. Salinity increases the “hardness” of water, which can mean
more soap and detergents have to be used or water softeners installed and maintained. This can also cause scaling in
pipes and heaters. The experimental determination of the salt content by drying and weighing presents some difficulties
due to the loss of some components. The only reliable way to determine the true or absolute salinity of natural water is
to make a complete chemical analysis. However, the method is time consuming and cannot yield the precision necessity
for accurate work. Thus to determine salinity, one normally used method involves the measurement of a physical prop-
erty such as conductivity, density or refractive index. The paper reports the refractometric fiber optic sensor for detec-
tion of salinity of water. The mathematical model is developed for detection of the refractive index of liquid and simu-
lated in MATLAB. The fiber optic sensor probe is developed to measure the refractive index of the solution containing
different amount of salt dissolved in water i.e. different molar concentrations. Experiments are carried out using the
developed probe for these solutions. Experimental results are showing good agreement with the simulated results.
Keywords: Fiber Optic Chemical Sensor; Hardness of Water; Refractometry; Retro-Reflective Type Fiber Optic
Sensor; Salinity of Water
1. Introduction
Water quality monitoring is essential to modern life. Not
only is it a major factor in safeguarding public health, but
also high-quality freshwater is also a key input in agri-
culture and many industrial process. Salinity is a very
important parameter for oceanics, marine environment
monitoring, seasonal climate prediction, mariculture, and
solar engineering. Very briefly, it can be concluded that,
in the past, the salinity in water was determined by hy-
drometric methods. In recent years, several new methods
and technologies have been proposed for salinity meas-
urement. For example, an ultrasonic technique [1] based
on measurement of the travel time of light was presented
to measure the salinity in a solar pond in 1995. A chemi-
cal method [2] based on polyaniline matrix coated wire
electrodes was developed for salinity measurement in a
range from 0.010‰ to 75‰, where salinity is expressed
in grams per kilogram of seawater, that is, in parts per
thousand, or per mile, whereby the ‰ symbol is used.
Fiber-optic sensors are capturing an ever greater share
of the sensor market as industry realizes the numerous
advantages they offer, compared with their conventional
counterparts. The advantages of fiber-optic sensors over
electrical transducers are the inherent immunity to elec-
tromagnetic interferences [3,4], higher sensitivity, small
sensing unit, safety in hazardous or explosive environ-
ments, the possibility of processing the signal at large
distances from the sensor with little degradation, and the
ability to work under high-temperature and high-pressure
conditions. An intensity-modulated fiber-optic sensor for
salinity measurement based on radiation loss was devel-
oped [5]. In the sensor probe, a part of fiber cladding is
removed, and when it is placed into the salt water, some
light will radiate into the liquid. The variations of the
salinity in water will lead to the changes of the light in-
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S. S. PATIL, A. D. SHALIGRAM 71
tensity detected by photodetector. A new idea to simul-
taneously measure the temperature and salinity with an
reflex optical fiber sensor system was suggested [6]. A
fiber optic sensor based on surface Plasmon resonance
for determination of the refractive index is used for mea-
suring degree of salinity of water [7]. A new type of fiber
optic Bragg grating sensor based on the hydrogels is used
for measuring the salinity [8].
This paper discusses the fiber optic sensor based on
liquid refractometry [9] for measuring the salinity of wa-
ter. This technique has the advantage of non-contact type
sensing, requires simple circuitry and as accurate as the
other methods of refractometric sensing. As the salinity
of water increases the amount of salt present in the water
increases which in turn increases the refractive index of
the solution. Thus a method of liquid refractometry is
useful in detection of the variation in the salinity of water.
The developed fiber optic sensor consists of two parallel
fibers with a reflector at a optimized distance. A light is
launched into one of the fibers using high bright RED
LED and the reflected light is collected by the other fiber.
It is then converted to electrical form using the photo-
detector. The sensor is designed using the developed
mathematical model based on ray tracing technique and
simulated in MATLAB. Experiments are carried out for
different amounts of dissolved salt in the distilled water
generally referred as the molar concentrations. The re-
sults are compared with the simulated as well as reported
data [10] and found to be in good agreement with each
other.
2. Basic Principle and Mathematical
Modeling of Sensor
The refractive index and density is related to the by
Gladstone-Dale relation given by,
 
1sum kMnK   ' (1)
where n is the index of refraction and ρ is the density of
miscible liquids that are mixed in mass fraction (M) can
be calculated from characteristic optical constants (the
molar refractivity k in cm3/g). K’ is Gladstone-Dale con-
stant.
As we know,
 
Densitymassvolume m  V
(2)

Concentration CW mM V where mass of solute
(m), molecular weight (MW) and volume of solution (V)
Thus refractive index (n) is given by the equation
W
nK'MC (3)
Thus it is concluded from Equation (3) that as concen-
tration of the solute in the solution increases the refrac-
tive index of the solution also increases.
The variation in the refractive index of the salt solution
due to increase in salinity of water is detected using the
principle of liquid refractometry. The fiber optic sensor
used for measuring the refractive index of liquid is ex-
trinsic. The light carried up to the modulating zone by
transmitting fiber (T fiber). The properties of the incident
light are modulated by the modulator i.e. liquid under test.
The modulated light is carried out to the detector by re-
ceiving fiber (R fiber). The modulating zone is a reflector
kept a distance “Z” from the sensor probe and liquid is
used as a medium.
Figure 1 shows the incident light in the form of cone
of emittance from the transmitting fiber. It get reflected
back in the form of expanding cone of light towards the
receiving fiber. The cone of light depends upon the re-
fractive index of liquid. Figure shows the liquid having
refractive index n1 filled between the gap in probe and
reflector. The angle of emittance θ1 is given by
1
1sin 1
NA
n






(4)
The output power is determined by the number of rays
entered into the receiving fiber and cone of acceptance of
receiving fiber. Now the gap is filled with the liquid
having refractive index n2. Since n2 > n1, θ2 < θ1. This
reduces number of rays entering the receiving fiber after
reflection. This reduces the received output power. Thus
for certain value of the gap distance Z between the sensor
probe and reflector, the received power depends on the
refractive index of liquid.
3. Optimization of Gap Distance Using
Developed Software
Refractometric fiber optic sensor is simulated based on
the operating principle explained in the previous section.
The simulations are carried out using the developed
Z Z
TF RF
n2
Reflector
Image of TF
θ2
TF RF
n1
Reflector
Image of TF
θ1
Figure 1. Arrangement of refractometric fiber optic chemi-
cal sensor.

S. S. PATIL, A. D. SHALIGRAM 72
model based in ray tracing technique for optimizing the
gap length [10]. Distance Z is varied between the sensor
probe and reflector and received light intensity is plotted
as a function of distance Z for each value of refractive
index between 1.33 (pure water) to 1.36 (water with 50%
salinity). The sensitivity is calculated over the given
range of refractive index keeping Z constant. The value
of Z for which the sensitivity is maximum is chosen as
the optimized gap length useful for detection of variation
in the refractive index as shown in Figure 2. The opti-
mized gap length is denoted by dotted line on the graphs
shown in Figure 2. Thus for fixed Z value i.e. Z = 7 mm
the sensor output is a function of the refractive index as
shown in Figure 3.
4. Experimental
4.1. Sensor Prototype
Fiber optic sensor probe consists of light source, detector,
chemical cell etc. The light from source is launched in to
optical fiber and guided to a region and interact with re-
flector through sample solution. After this interaction, it
Figure 2. Sensor response in non linear region.
Figure 3. Received light intensity at Z = 7 mm.
is reflected and collected by the receiving fiber. The
other end of this fiber probe is connected to a detection
and measuring system. The fiber used for the experi-
mentation is a plastic fiber of 488 micrometer core di-
ameter with numerical aperture of 0.47. Cladding thick-
ness (cl) = 0.612 mm. T-R separation with jacket (s) =
0.0 mm, angle between T-R fibers = 0˚. The length of the
fiber = 85 mm. Both transmitting and receiving fibers are
of same type. To the sensing tip end of the fibers a round
cut transparent glass plate is press fitted in order to avoid
damage of polished tip due to the interaction with solu-
tion under test. It consists of a LED and photodiode en-
closed in a brass assembly. An adjustable ring is pro-
vided so as to vary distance between sensor tip and re-
flector. A chemical cell is used to test salinity of solution.
The cell is cylindrical in shape with a mirror fitted at a
centre of bottom. The mirror is used as a reflector. The
total assembly of sensor probe is as shown in the Figure
4.
4.2. Physical Experimentation
The salinity of water is detected using developed sensor
probe and electronic components as shown in the Figure
5. This is the block diagram experimental setup used for
measuring refractive index of a liquid in a chemical cell.
It consists of light source LED with its driving circuit,
photo-detector with signal conditioning circuit, sensor
probe and chemical cell. The experiments are carried out
for fixed distance between probe and reflector. Red LED
is used for experiment having 673 nm wavelength. Photo
detector is a phototransistor L14G3 along with the sens-
ing resistor. The output is buffered and applied to differ-
ential amplifier. Differential amplifier is used to amplify
difference between detector output and the reference vol-
tage. This reference voltage is meant for zero adjust of
instrument. Non-inverting amplifier is used to further
amplify the difference with adjustable gain. Solutions of
LED Photo
detector
T Fiber
Ring R Fiber
Chemical cell
Sensor tip
Glass plate
Figure 4. Developed sensor probe.

S. S. PATIL, A. D. SHALIGRAM 73
0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 molar concentrations are
prepared representing 0%, 10%, 20%, 30%, 40% and
50% salinity of water. The experiment was carried out
for 0% salinity (distilled water) up to 50% salinity using
the developed sensor probe and signal conditioning cir-
cuit. The amount of reflected light received by receiving
fiber depends on the refractive index of the salt solution.
The sensor output is directly proportional to refractive
index of salt solution and hence the concentration of the
salt in the solution.
5. Results and Discussion
The refractive index of water varies from 1.33 to 1.36 as
salinity varies from 0% to 50%. Experiments are carried
out for the salt solution prepared in distilled water. The
solutions are tested in the chemical cell of the sensor
probe. The output of the sensor varies salinity of water as
shown in the Figure 6. The simulated results are shown
by dark line in the figure. The variation shows linear re-
lationship between the variation in salinity of water and
sensor output. The slight deviation from the simulated
curve is due to temperature effects or the added salt may
LED
Driver
Detect
or with
driving
circuit
Display
LED Differential
amplifier
Zero
adjust
Non-Inver
ting
amplifier
Gain
adjust
Figure 5. Experimental Setup.
Figure 6. Response of fiber optic sensor for different molar
concentration of NaCl.
not be totally dissolved in the distilled water.
6. Conclusion
A simple technique is used to measure the salinity of
water using fiber optic sensor based on liquid refracto-
metry. Refractometric fiber optic sensor is modeled ma-
thematically and simulated using the MATLAB. The de-
veloped sensor probe consists of two fibers; one tran-
smitting fiber and other receiving fiber are reflectors at a
fixed gap length. The gap length is optimized using the
developed software. The water solutions are prepared for
different salinities of water volumetrically. A sensor
probe is dipped into the solution and readings are re-
corded. It is observed that the sensor output is the direct
function of salinity of water and hence useful to detect
the salinity of water expressed in percentage. The results
show good agreement with simulated results.
7. Acknowledgements
One of the authors SSP wishes to thank Head, Depart-
ment of Electronic Science, University of Pune for avail-
ing the facilities in the Laboratory. She is also thankful to
UGC for granting a leave for study.
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