Monitoring technology of salinity in water with optical fiber sensor ...

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 5, MAY 2003

Monitoring Technology of Salinity in Water With Optical Fiber Sensor Yong Zhao, Member, IEEE, Yanbiao Liao, Bo Zhang, Member, IEEE, and Shurong Lai

Abstract—A novel optical fiber sensor used for remote monitoring of salinity in water is proposed. Based on the detection of beam deviation due to the refractive index changes of the salt water, salinity can be measured by a position-sensitive detector. The measurement principle is theoretically described. Preliminary prototype is set up, and experimental results indicate the feasibility of the method. Salinity measurement resolution can reach 0.012‰ within the measurement range from distilled water to salt water with salinity of 50‰. Index Terms—Optical fiber sensor, optical measurement, refractive index, salinity measurement, water quality monitoring.

I. INTRODUCTION

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ONITORING of water quality 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 agriculture and many industrial process. Salinity is a very important parameter for oceanics, marine environment monitoring, seasonal climate prediction, mariculture, and solar engineering. Measurement of salinity is a complex subject that has a long history. Very briefly, it can be concluded that, in the past, the salinity in water was determined by hydrometric methods. In recent years, several new methods and technologies have been proposed for salinity measurement. 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 chemical 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 mille, 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 electromagnetic interferences [3], [4], higher sensitivity, small sensing unit, safety in hazardous or explosive environments, 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 inManuscript received November 8, 2002; revised January 22, 2003. This work was supported by the China Postdoctoral Science Foundation. Y. Zhao is with the Department of Automation Tsinghua University, Beijing 100084, China (e-mail: [email protected]). Y. Liao, B. Zhang, and S. Lai are with the Department of Electronic Engineering, Tsinghua University, Beijing 100084, China. Digital Object Identifier 10.1109/JLT.2003.811318

tensity-modulated fiber-optic sensor for salinity measurement based on radiation loss was developed [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 intensity detected by photodetector. In an earlier work [6], we proposed a new idea to simultaneously measure the temperature and salinity with an optical fiber sensor system. Based on this idea, a practical reflex fiberoptic sensor for measurement of salinity in water was further studied and tested in detail in the present study. In the work reported in this paper, a laser diode with an emitting wavelength of 650 nm and a position-sensitive device (PSD) were used, instead of a broad-band light-emitting diode (LED) light source and a charge-coupled device (CCD). Because of the differential measurement method of PSD, the effect of the light source fluctuation was reduced, resulting in the improvement of the measurement accuracy. In addition, the light beam is visible with a 650-nm light source, which is helpful for adjusting the positions of the optical elements. A preliminary prototype is set up and introduced. Experimental results indicate the feasibility of the developed system. Measurement errors are also discussed and analyzed in detail. Unlike an intensity-modulated fiber-optic sensor, which necessitates a stable light source, this sensor exploits the beam deviation due to optical refraction at the receiving end face of the measurement cell, which is caused by changes in refractive index with different salinity in water [7]–[9]. II. EXPERIMENTAL SETUP AND MEASUREMENT PRINCIPLES The method described in this paper is based on the measurement of the beam deviation due to the refractive angle change, which is nearly proportional to the salinity, using a PSD. A conventional method of measuring the index of refraction is to directly read the critical angle of refraction visually through a microscope, but it is not suitable for long-duration remote measurements. Fig. 1 shows a total schematic configuration of the experimental equipment. The beam from a light source transmits through an incident optical fiber cable having low loss, low noise, and high durability against seawater corrosion. There is only one single-mode fiber in the incident cable, and the geometrical dimension of it is 9/125 m. The deviated light beam, which is modulated by different salinity in water, is reflected in the sensor probe, enters the receiving fibers, and is then measured by the PSD. There are 40 multimode graded-index communication fibers in the receiving fibers cable, and they are ar-

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ZHAO et al.: MONITORING TECHNOLOGY OF SALINITY IN WATER WITH OPTICAL FIBER SENSOR

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Fig. 1. Total schematic diagram of the measurement system.

(a)

(b) Fig. 2. (a) Sensing probe and the beam path. (b) Equivalent transmission beam path.

ranged in linear array. The geometrical dimension of fibers in the receiving array is 62.5/125 m. This reflex sensor structure is in favor of increasing the optical path length and miniaturizing the size of sensor probe. Incident and receiving fibers are attached at one end of the sensor, which makes it easy to implement a remote sensing by adding the length of the fibers. With a view to the absorption and scattering effects in the salt water, a laser diode with an output wavelength of 650 nm is selected as the light source, which also provides good visibility and is convenient for adjusting the ray tracing. The spot size is converged and collimated to less than 0.5 mm by a pigtailed fiber collimator. In front of the light-sensitive surface of the PSD, a narrow-band light filter with the same peak wavelength and a bandwidth of 1.4 nm is used to avoid the effect of most ambient stray light. The dimension of the PSD’s photosensitive area is 2 mm 20 mm, and the position measurement resolution is approximately 0.3 m. Based on the transverse photoelectric effect, the PSD’s output signal is independent of the incident light intensity and only related to positions of the incident beam.

The sensor probe is mainly composed of two parts. One part is called the partitioned water cell, containing a sample water tank in which there exists a wedge-shaped reference cell filled with distilled water. The oblique optical transmission plate on the end of the wedge-shaped reference cell produces an incident angle 45 for the laser beam (as in Fig. 2). The other part is called the reflector element, where a right-angle prism is used to reflect the incident beam. These two parts of the sensor probe are separated by an isolator plate. The incident beam will then sequentially pass through the reference water (refractive index ), the oblique plate (refractive index ), and the sample water (refractive index ), and will then be reflected by the right-angle prism (refractive index ). Except for the prism, the sensor probe is made from the transparent methyl-methacrylate. According to the geometric structure and the ray trace shown in Fig. 2, we denote the PSD’s output signal as the beam deviation when the reference cell is filled with distilled water and the sample cell is filled with the sample water (solid line) and reference water (broken line), respectively. Based on the relation between total beam deviation and the cell geometric structure

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as well as material parameters, there exists the following equations:

(1) and (2)

is the beam deviation before repassing the oblique where plate when the sample cell is filled with measured water, as shown in Fig. 2(b), is the increment after the beam passes the oblique plate, is the beam deviation after the beam passes the oblique plate when the sample cell is filled with distilled water, is the thickness of the oblique plate, is the light path length when first entering the sample water, is that when returning is the total light path length when the sample water, and passing through the isolator plate and the right-angle prism. are selected, and When the parameters , , , and are definite values, as are and . Thus, we can rewrite (1) and (2) as

Fig. 3. Long-term output monitoring results with salinity of 30‰.

(3) . According to the past studies [7]–[9] on the where relation between the refractive index of seawater and its salinity, , where is the salinity of the that is, salt water, the relation between beam deviation (displacement output detected by PSD) and the salinity can be obtained. Therefore, the salinity of the measured water can be obtained from the calibration curve for known salinity. Range and precision of the device in differential refractometer depend on the geometrical factors of the sensing cell as well as of a resolution of the detection system. III. RESULTS AND DISCUSSION A. Stabilization Process Monitoring In case that temperature and ambient stray light are invariable (the temperature was kept within 0.2 C), the outputs of the measurement process are monitored from the time when the measured saltwater with different salinity is filled in the measurement cell. Figs. 3 and 4 are the test results of the output of PSD, where the measured salinity is 30‰ and 40‰, respectively, and the total monitoring time is 3 h. From the monitoring results, we can see that each time after the salt (chemically pure NaCl) is filled in, there exists a fluctuation just like the phenomenon of a damped oscillation. This may be explained as follows: One possible reason is that after some other salt is put into the measured water, the laser beam will be

Fig. 4. Long-term output monitoring results with salinity of 40‰.

distorted by the created fluctuating wave. The other reason is that, after the salt is put into the water, a diffusion effect will occur, which will lead to the scattering of the beam to various directions due to the disordered movements of the salt particles. Therfore, these two causes will result in the positions’ oscillation of the light spot projected on the PSD’s photosensitive surface. Because no stirrer was used to accelerate the dissolving process, it will take a longer time to get the measured water to be stable and in an equilibrium state. In order to avoid the evaporation of water, which will influence the measurement result, an enclosed cover should be added. The measured data as shown in Fig. 5 can be regarded as the measurement stability of the developed system, which indicates a result of 0.9% in the case of a measured salinity of 30‰. B. Examination of Relation Between Salinity and PSD Output In order to examine the salinity measurement characteristics of the developed system, a theoretical calculation and experiment were both conducted. Samples with various concentrations were provided in advance. A series of NaCl solutions of distilled water with salinity from 0‰ to 50‰ at intervals of 5‰ was prepared before the experiments, which were uniform and in an equilibrium state. Thus, much less time is needed for

ZHAO et al.: MONITORING TECHNOLOGY OF SALINITY IN WATER WITH OPTICAL FIBER SENSOR

Fig. 5.

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Stability measurement data of the measured water with salinity of 30‰. Fig. 7. Relationship between the incident angle of light beam and the displacement output.

Fig. 6. Salinity measurement characteristics of calculated and measured data.

stable state in each measurement process as the saline solution is put into the water cell. Fig. 6 shows the relations between the salinity and PSD output signal measured at 20 C obtained by theoretical calculation based on (1)–(3) and experimental measurement, respectively. The structure parameters used in both experimental setup and theoretical calculation are as follows: 45 , the thickness of the the light beam incident angle, 5 mm, , , 40 mm, oblique plate, 40 mm, 55 mm. From the figure, we can see a linear relation between salinity and PSD output signal. However, there exists a bit of deflection between the theoretical curve and experimental curve when the incident angle is 45 . Based on the theoretical analysis and experiments, this can be explained due to the alignment error of the oblique plate; in other words, it is caused by the difference between the theoretical and practical incident angles. Different incident angles will lead to different characteristics, shown in Fig. 7, and it is found that the calcu44.13 agrees lated result in the case of the incident angle well with the experimental result. Unlike the measurement results in Figs. 3 and 5, in which absolute measurement values were plotted, here relative increment values of the PSD’s outputs were drawn in order to describe the salinity measurement characteristics in a more clear way. Measurement resolution is limited by the minimum position resolution of the PSD. Thus,

Fig. 8. Measurement results with temperature change from 24 C to 26.5 C.

the measurement resolution of 0.012‰ can be estimated based on the position resolution of 0.3 m of PSD. C. Repeatability Measurement Repeated experiments were carried out under an invariant condition to examine the measurement repeatability of the system. Unlike the stability measurement of Fig. 5, the repeatability was tested discretely. Measured data of different salinity were recorded for ten times (numbered from #1 to #10), respectively. At each time, four measurement results could be obtained due to the changed standard salinity, as shown in Table I. From the results, a measurement repeatability of 0.3‰ can be obtained. D. Temperature Influence on Measurement Results To avoid the influence of temperature drift on the measurement performance, a reference water cell filled with distilled water was used in the sensor structure. To verify the feasibility of this method, the output of the PSD was examined during a long time (about 10 h) with a temperature change from 24 C to 26.5 C. The measurement results are shown in Fig. 8, from

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TABLE I MEASURED DATA OF THE MEASUREMENT REPEATABILITY (‰)

which we can see that there only exists a slight influence (about 0.5%) on the outputs of the PSD, which is small enough compared with the measurement stability. It is reported [6] that in the case of no reference distilled water was used to compensate the influence of temperature drifts, there would be an influence of about 3% on the measurement results with a temperature variation range of 2.5.

[7] Y. Miyake, “Chemical studies of the western pacific ocean IV: The refractive index of seawater,” Bull. Chem. Soc. Japan, vol. 14, pp. 239–242, 1939. [8] E. M. Stanley, “The refractive index of seawater as a function of temperature, pressure and two wavelengths,” Deep Sea Res., vol. 18, pp. 833–840, 1971. [9] J. S. Rusby, “Measurement of the refractive index of seawater relative to copenhagen standard seawater,” Deep Sea Res., vol. 14, pp. 427–43, 1967.

IV. CONCLUSION The experimental prototype was developed based on the proposed method for salinity measurement. According to the developed system and experimental results, several conclusions can be made. • The sensor system is compact, cost-efficient, and practical. • The salinity measurement resolution of 0.012‰ can be estimated based on the measurement resolution of the PSD. • The measurement repeatability of 0.3‰ is obtained. • Temperature drifts have almost no influence on the measurement results due to the differential measurement method using distilled water for reference. REFERENCES [1] Z. J. Min, J. P. Li, and S. H. Jiang, “Measurement of salt salinity in solar pond by supersonic method,” Acta Eneglae Solaris Sinica, vol. 16, no. 2, pp. 224–228, 1995. [2] F. B. Diniz, K. C. S. de Freitas, and W. M. de Azevedo, “Salinity measurements with polyaniline matrix coated wire electrodes,” Electrochem. Commun., vol. 1, pp. 271–273, 1999. [3] Y. Zhao, P. S. Li, C. S. Wang, and Z. B. Pu, “A novel fiber-optic sensor used for small internal curved surface measurement,” Sens. Actuators A, Phys., vol. 86, no. 3, pp. 211–215, 2000. [4] Y. Zhao, P. S. Li, and Z. B. Pu, “Shape measurement based on fiber-optic technique for complex internal surface,” Measurement, vol. 30, no. 4, pp. 289–295, 2001. [5] Y. Y. Jin, Z. X. Chen, and Q. J. Wang, “Laser optic-fiber technique for measuring the salinity of a solar pond,” Acta Eneglae Solaris Sinica, vol. 15, no. 2, pp. 198–200, 1994. [6] Y. Zhao and Y. B. Liao, “Novel optical fiber sensor for simultaneous undersea temperature and salinity measurement,” Sens. Actuators B, Chem., vol. 86, no. 1, pp. 63–67, 2002.

Yong Zhao (M’00) was born in Shenyang, China, in 1973. He received the M.A. and Ph.D. degrees in precision instrument and automatic measurement with laser and fiber-optic techniques from the Harbin Institute of Technology, Harbin, China, in 1998 and 2001, respectively. He is currently working as a a Postdoctoral Research Fellow at the Department of Electronic Engineering, Tsinghua University, Beijing, China. His current research focuses on the development of fiberoptic sensors and devices, FBG sensors, laser and optical inspection technologies, and their industrial applications. He has authored and coauthored nearly 40 scientific papers, patents, and conference presentations. Dr. Zhao was awarded the first-prize scholarship by the China Instrument and Control Society and the Sintered Metal Corporation (SMC) scholarship by Japan in 2000.

Yanbiao Liao, photograph and biography not available at the time of publication.

Bo Zhang (M’02), photograph and biography not available at the time of publication.

Shurong Lai, photograph and biography not available at the time of publication.