Refractometric Fiber Optic Sensor for Detecting Salinity of Water

doi:10.4236/jst.2013.33012

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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

Email: [email protected], [email protected]

Received June 6, 2013; revised July 6, 2013; accepted July 14, 2013

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-

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

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

1sin 1













(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

Reflector

Image of TF

θ2

TF RF

Reflector

Image of TF

θ1

Figure 1. Arrangement of refractometric fiber optic chemi-

cal sensor.

S. S. PATIL, A. D. SHALIGRAM

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

4.2. 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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