Remote monitoring of water salinity by using side-polished fiber-optic U-shaped sensor

Dragan Z. Stupar1, Jovan S. Bajić1, Ana V. Joža1, Bojan M. Dakić1, Miloš P. Slankamenac1, Miloš B. Živanov1, Edvard Cibula2

1Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovića 6, Novi Sad, Serbia drstupar@uns.ac.rs, bajic@uns.ac.rs, annajozza@yahoo.com, dakicb@uns.ac.rs, miloss@uns.ac.rs, zivanov@uns.ac.rs

2University of Maribor, Faculty of Electrical Engineering and Computer Science, Smetanova 17, Maribor, Slovenia edvard.cibula@uni-mb.si

Abstract — In this paper, a remote water salinity measurement system based on a simple and low-cost intensity based side-polished fiber-optic U-shaped sensor is presented. The sensor system utilizes a side-polished U-shaped configuration in order to maximize the sensitivity and expand the measurement range. The implemented salinity sensor is made of a multimode plastic optical fiber, and sensor determines the salinity by measuring the refractive index. Measurement resolution and uncertainty of proposed salinity sensor are 0.001 and 0.002, respectively. Wireless electronics based on ZigBee protocol is also implemented. Therefore, the sensor has the possibility of wireless measurements. The main advantages of this sensor are simplicity, lightness and flexibility. This sensor is also electrically safe and immune to electromagnetic interferences. In LabVIEW software package client and server applications are implemented, which gives a possibility of remote monitoring of water salinity over the internet.

Keywords — fiber-optic sensor, U-shaped, water salinity.

I. INTRODUCTION Water salinity sensors are essential for numerous

applications such as in chemical and biological systems. Salinity sensors are used for measurement of water quality and salinity levels in seawater desalination to supplement industrial water use in large coastal cities. There are several measurement methods for determination of salinity. One of the most common methods is determination of salinity through measurement of electrical conductivity, temperature and pressure (practical salinity measurement). Second method, which can be also very frequently encountered in practice, is determination of salinity by measuring refractive index of solution (absolute salinity measurement). Terms “absolute salinity measurements” and “practical salinity measurements” are reported in [1].

Recently, some remote and contactless methods for seawater salinity determination have appeared. Determination of surface salinity from remotely sensed ocean color is reported in [2]. Remote method for monitoring sea surface salinity from space based on microwave sea surface salinity measurements is given in [3].

Fiber-optic sensors [4] due to their advantages over conventional sensors, such as immunity to electromagnetic interference, small size and flexibility,

multiplexing capability and remote measurements, have found application in various kinds of measurements of parameters such as temperature, pressure [5, 6], strain [7], small displacements [8-10], tilt [11], salinity [12-14], and many others. Fiber-optic salinity sensors are commonly based on intensity modulation, which is used for measuring refractive index [15] of liquid, a solution with higher degree of salinity (higher concentration of NaCl) has higher refractive index. Increase of refractive index attenuates the optical power transmitted by the fiber, which is measured by photodetector. Commonly used fiber-optic sensors for refractive index measurements are sensors based on bending loss in optical fiber [16], tapered sensors [13], sensors based on reflection [17, 18], sensors based on interferometry [19] etc.

In this paper, an intensity modulated side-polished fiber-optic U-shaped sensor with strongly bent multimode plastic optical fiber is proposed. In this approach, salinity detection is based on measurement of refractive index of solutions with sodium chloride (NaCl). The presented sensor system measures the output voltage on the photodetector. Signal on the photodetector is influenced by the interaction of the evanescent wave produced in the U-shaped sensor, and solution which forms surrounding of sensor. The basic sensing electronics, which consists of light source and photodetector is attached to digital microcontroller based circuit with ZigBee communication module that provides opportunity of wireless measurement for proposed salinity sensing system. For data acquisition and processing in LabVIEW software package client and server applications are implemented. These applications provide the ability to remotely monitor water salinity over the internet. The complete salinity measurement system offers simplicity, reliability and remote and long-term measurement capability. Implemented system is cheap and with some small modifications it can be competitive in the market.

II. PRINCIPLE OF OPERATION Bending of optical fibers causes loss of optical power

and reduces sensor performance, which is disadvantage in optical communications. In the field of fiber-optic sensors, the losses due to bending of optical fibers are used as operating principle of sensors in many fields. Following the theory of wave guiding in fibers, with total reflection the light penetrates a few micrometers at the core-cladding (or core-air) interface [20]. This area is called the evanescent field. The operation of the

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implemented sensor is based on the loss of total internal reflection or absorption of evanescent wave when the probe is immersed into the liquid solution whose refractive index (salinity) is measured. When the sensing part of optical fiber is in the air, the majority of the light rays from light source will undergo total internal reflection and will be guided along the optical fiber to the end of the fiber. When the sensing part of optical fiber is immersed into a liquid with a refractive index of 1.33 or higher, cladding-mode rays in fiber will be absorbed by liquid. In this case the condition for total internal reflection at the fiber-water interface is not satisfied. The intensity of the light at the output end of the fiber will be reduced.

III. EXPERIMENTAL SETUP U-shaped sensor probe is shown in Fig. 1. A

multimode PMMA plastic optical fiber with 1.5 mm diameter and 0.5 numerical aperture is used. Plastic optical fibers are used for the fabrication of the sensor because they are cheap, robust and easy to handle. The light emitted from the light source is launched on the input end of the fiber. The output end of the fiber is connected to photodetector. The fabrication of sensing part of the fiber is done by removing a protective part of the plastic optical cable. Optical fiber is then strongly bent in order to form a U-shaped probe. Bent part of the optical fiber is placed in a heat shrinkable tube which is heated until fiber remained U-shaped. Afterward, optical fiber cladding part of U-shaped probe is polished with polishing paper to obtain side-polished part of sensing probe. Bending radius and polishing depth of the sensor are 3.5 mm and 0.3 mm, respectively.

U-shaped sensor for refractive index measurement has characteristic with minimum at cladding refractive index value. To avoid these drawbacks, and get unambiguous characteristic of the sensor, optical fiber is polished on convex side of U-shaped plastic optical fiber. In this way refractive index measurement range is expanded

a) b) Fig. 1. a) Drawing of side-polished fiber-optic U-shaped sensor with sensor dimensions, b) Photograph of implemented side-polished fiber-optic U-shaped sensor.

Implemented sensor is tested in several liquid solutions

with different refractive indices. Different concentration of glycerine in water [17] is used for measurement of sensor characteristic (output voltage depending on the refractive index). Table 1 shows liquid solutions used for sensor characterization and their refractive indices.

TABLE I WEIGHT RELATIONSHIP BETWEEN REFRACTIVE INDEX AND WATER-

GLYCERINE SOLUTION [17] Liquid Refractive

index Water (100%) 1.33 Water (75%) + glycerine (25%)

1.36404

Water (75%) + glycerine (25%)

1.39809

Water (25%) + glycerine (75%)

1.43532

Glycerine (100%) 1.47399

Fig. 2 shows a block diagram of the laboratory experimental setup for salinity measurement. For laboratory tests sensor is immersed in solutions with various water-salt concentrations. As a light source and photodetector, LED and photodarlington are used. High optical power red LED with low power consumption, IF E97 with the peak wavelength at 660 nm, and photodarlington IF-D93 produced by Industrial Fiber Optic, Inc are used. Photodarlington provides a very high optical gain, eliminating the need for the post amplification. The integrated design of the IF-D93 makes it a simple, cost-effective solution in a variety of applications. Optical response of the IF-D93 extends from 400 to 1100 nm, making it compatible with a wide range of visible and near-infrared LEDs and other optical sources. This includes 650 nm visible red LEDs used for optimum transmission in PMMA plastic optical fiber (POF). Both source and detector are housed in a “connector-less” style plastic fiber optic package, to which POF can be easily connected.

Fig. 2. The block diagram of the laboratory experimental setup for water salinity measurements based on the fiber-optic U-shaped sensor: PD – photodarlington, LED – light emitting diode, µC – microcontroller, A/D – analog to digital converter.

As microcontroller (µC), ATtiny13 from Atmel is used. This microcontroller is used because it is appropriate for the desired application tasks, and it is very cheap and in small package. µC drives LED, measures a signal from photodarlington by using 10 bit A/D converter, and communicates with a PC via ZigBee communication. The device is supplied by a 9V battery. Power supply for whole device is stabilized to 3.3 V with step-down voltage stabilizer.

An indicator for battery level that measures level of battery charge and provides numerical information is realized with one A/D converter channel. The battery

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level is displayed as a voltage level. When the battery discharges to 8 V, user is informed that there is a need to recharge the battery.

For ZigBee communication XBee-PRO module produced by MaxStream Inc., is used. The modules were designed to meet ZigBee/IEEE 802.15.4 standards and support the unique needs of low-cost, low-power wireless sensor networks. The modules have indoor range up to 100 m and outdoor range with line-of-sight up to 1500 m. The power supply voltage for these modules can be in the range from 2.8 to 3.4 V. XBee-PRO modules provide a possibility to build an easy to configure network, with a high data rate, up to 250000 Baud/s. For the configuration of the XBee-PRO module, X-CTU software is used. Connection of the XBee-PRO module to the microcontroller is done by using 4 wires: Power-Supply (3.3V), Ground, TX and RX. XBee-PRO module is configured to operate in Transparent Mode. When operating in this mode, the modules act as a wireless serial communication (UART). In this mode it is necessary that all modules have the same PAN-ID, both a module which sends data broadcast, and all other modules that receive data. In this work, communication with two modules is realized, but in order to obtain a wireless sensor network, communication with more modules can be easily implemented. ZigBee modules used in this work provide secure wireless transmission of measured data from the sensor electronics to the computer with software. It should be mentioned that wireless communication does not affect the measured data. Communication without ZigBee communication modules is also possible, but then small hardware modifications are necessary. Proposed solution without ZigBee modules consists of sensor and electronics attached to measurement station, where sensor is connected to electronics through a long lead-in and lead-out parts of the plastic optical fiber.

The potential errors arising from the connector imperfections, misalignment of light sources and detectors and other effects that are not related to the water-salt solution variations are reduced by simple calibration. The calibration button is implemented in hardware, but sensor calibration is software based. When calibration button is pressed and sensing probe is in the air, the microcontroller sends value of A/D converter to the client application. Client sends data to the server application, which calculates the calibrated sensor characteristic. From the calibration value and normalized sensor characteristic in relation to the air, it is easy to obtain the measured refractive index (salinity). Calibration procedure at the sensor level provides only adjustment of the sensor sensitivity. Software based calibration of sensor proposed here eliminates the influence of the potential errors arising from the connector imperfections, misalignment of light sources and detectors and other effects that are not related to the water-salt solution variation.

To obtain valid measurements, software calibration must be done. The value provided on the sensor output is first captured in the air. Afterwards, output of the sensor is captured in glycerine-water solutions given in Table 1. To eliminate errors which arise from effects that are not related to the water-salt solution variations, values obtained by capturing sensor output when the water-glycerine solutions are used as media are divided by

value obtained by capturing sensor output when the air is medium. In this way is introduced normalization in respect to the air. The normalization introduced here with software calibration provides measurements which are only sensitive to refractive index changes.

Once done the procedure of calculating the normalized sensor characteristic enables the software calibration. Before measurement it is only necessary to capture sensor output value in the air. After that, by simple multiplying sensor output value for the air with normalized sensor characteristic, real values are obtained.

In Fig. 3 the block diagram of the hardware and software realizations for remote salinity measurements is presented. As we can see from the block diagram, proposed system is designed for wireless and long-term remote monitoring.

zigbeeserial

antenna

antenna

serialzigbee PC

client

InternetTCP/IP PC

server

remote monitoring over the internet

Sensor with μC

Fig. 3. The block diagram of the hardware and software realizations for remote measurements of water salinity.

Client and server applications are implemented in LabVIEW software package. The client application receives measured data from XBee-PRO module, and graphically displays and records the measured data. Measured values can be displayed, analyzed, processed and recorded on a PC and distributed over the internet. The client sends received data to server over the internet using TCP/IP protocol. These data from server are available for remote monitoring.

IV. RESULTS AND DISCUSSION The measured relative attenuation of light in optical

fiber in relation to the air, depending on the liquid solution in which the sensor is immersed is shown in Table 2.

TABLE II THE RELATIVE ATTENUATION OF LIGHT IN RELATION TO THE AIR DEPENDING ON THE LIQUID IN WHICH THE SENSOR IS IMMERSED

Liquid Relative attenuation in relation to the air

Water (100%) 0.203451 Water (75%) + glycerine (25%) 0.132243 Water (75%) + glycerine (25%) 0.073242 Water (25%) + glycerine (75%) 0.050863 Glycerine (100%) 0.044759

As we can see from data from Table 2 increase in the

refractive index reduces output value provided by sensor.

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In Fig. 4 the relative attenuation of light in relation to the air, depending on the refractive index of liquids is shown.

1.32 1.34 1.36 1.38 1.40 1.42 1.44 1.46 1.48

0.040.060.080.100.120.140.160.180.200.22

Nor

mal

ized

inte

nsity

Refractive index

Normalized intensity with respect to AIR

Fig. 4. Relative attenuation of light in relation to the air depending on the refractive index of liquids.

From Fig. 4 we can see that characteristic is unambiguous in the measured refractive index range. It can be seen that the range of measured refractive index extends from 1.33 to almost 1.5. As it can be seen the refractive index characteristic is almost linear (linear fitting factor R2=0.99) in range from 1.33 to 1.4. This linear region is very suitable for salinity measurements. In Fig. 5 dependence of the refractive index on the salinity is shown.

0 5 10 15 20 251.33

1.34

1.35

1.36

1.37

1.38

Ref

ract

ive

inde

x

Concentration of salt in water-salt solution [%] Fig. 5. Refractive index depending on concentration of salt in water-salt solution.

From Fig. 5 it can be noted that characteristic is almost linear in the salinity range from 0 to 25%. The measured results from Fig. 5 are well matched with the results given in [21].

In Fig. 6 the implemented LabVIEW client application is shown. The produced application receives data from sensor electronics via ZigBee communication, processes, displays, records and distributes data to server application over internet. Data stored on server are available to everyone who has internet connection and permission to access the measured data. Obtained refractive index resolution for proposed measurement system is 0.001. The measurement uncertainty of the captured data is 0.002.

Fig. 6. Implemented client application with measurement results.

In Fig. 7 the final version of described salinity sensor system is shown. As can be seen, the whole system is packaged in small box which is suitable for portable measurements, also.

Fig. 7. Photograph of wireless measurement system for water salinity monitoring.

V. FUTURE WORKS Our future research will focus on the realization of

fiber-optic sensors for monitoring quality of water, air and soil, as well as sensors for measuring UV radiation. As a logical extension of this work follows a series of experiments for the realization of fiber-optic sensors for testing liquids whose application is primarily to protect the environment. The main goal is to realize terrain sensor system which will collect information from a variety of wireless sensors, process and store data to be sent over internet to the control station.

VI. CONCLUSION A simple and low-cost fiber-optic water salinity sensor

based on intensity modulation is presented. The sensor has an approximately linear characteristic for the refractive indices in range from 1.33 to 1.4. The implemented sensor refractive index resolution is 0.001 and measurement uncertainty is 0.002.

Wireless communication electronics based on ZigBee standard is developed for this sensor. The function of

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mentioned wireless electronics is to enable communication between the sensing part of the system and a PC.

Client and server applications are designed on a PC in LabVIEW software package. These applications display, record and distribute the measured signal over the internet.

The implemented sensor system is suitable for long-term monitoring of water salinity, such as, for monitoring salinity in some industrial processes. The main advantage of developed measurement system is the fact that it is cheap, simple and has the possibility of remote measurement.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the funding provided by the Ministry of Education and Science of the Republic of Serbia under projects “Development of methods, sensors and systems for monitoring quality of water, air and soil”, number III43008 and “Optoelectronic nanodimension systems – route towards applications”, number III45003. This work was enabled by the COST Action TD1001: Novel and Reliable Optical Fibre Sensor Systems for Future Security and Safety Applications (OFSeSa).

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