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Plastic fiber evanescent sensor in measurement of turbidity
Sensors and Actuators A 285 (2019) 1–7
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Plastic fiber evanescent sensor in measurement of turbidity S. Yeoh, M.Z. Matjafri, K.N. Mutter, Ammar A. Oglat ∗ School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia
a r t i c l e
i n f o
Article history: Received 1 August 2018 Received in revised form 8 October 2018 Accepted 29 October 2018 Keywords: Turbidity Optical fiber sensor Tracepro software Water quality monitoring
a b s t r a c t The construction and working principles of a plastic fiber sensor for examining the level of turbidity is studied in this paper. This work focuses on designing an inexpensive turbidity sensor that incorporates a pair of multimode fibers (MMF) that are attached side by side and their beveled tips are mounted vertically. The efficiency of different beveled angles is evaluated by simulation with Tracepro software. The reflected signal is collected by immersing the sensor head into a water mixture and analyzed for various concentration. It was found that there is a linear increment of output intensity when concentration of mixture is increased. The turbidity sensor is tested with real samples that are collected from lake, river and coastal areas to demonstrate its consistency with commercial apparatus in natural compounds. The results showed that the proposed sensor structure is able to produce reliable results in a dynamic range of detection from 0 to 1000 Nephelometric Turbidity Unit (NTU) to facilitate practical field measurements. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Turbidity is defined as the decrease in transparency or clarity of a solution due to the presence of suspended and colloidal materials such as sand, clay, silt, organic and inorganic matter, algae or other micro-organisms [1]. High level of turbidity in natural water and wastewater brings risk to sustaining the ecological processes and prolonged exposure can be detrimental to human health. A common method in measuring turbidity is a portable device called turbidimeter that measures the intensity of light transmitted through the suspended particles; it is often reported in the unit, NTU [2,3]. Measurement of turbidity often involves disturbance in background colour, interference and scattering of suspended particles that affect the accuracy of collected signals. Continuous turbidity monitoring is expensive, time-consuming and prone to errors in highly turbid water [4]. A turbidity sensor operates by detecting the light that scatters off the suspended particles present in the water. The monitoring of turbidity is necessary for different purposes, sustaining the aquatic ecosystem [5,6] and domestic water quality system [7]. Several studies have been demonstrated to detect various physical changes of polluted water using fiber optic sensors [8–12]. Common online instruments that offers in-situ monitoring are applied in most turbidity sensors for measurement in streams, rivers, lakes and oceans [2]. In this application, plastic optical fiber (POF) has the
∗ Corresponding author. E-mail address: [email protected] (A.A. Oglat). https://doi.org/10.1016/j.sna.2018.10.042 0924-4247/© 2018 Elsevier B.V. All rights reserved.
advantage of high transmission in the visible-near infrared (VISNIR) range, high mechanical stability and resistance to chemical degradation. Hands on and simple designs without involving high budget equipment were attempted by mechanically polishing the fiber to make bundle sensors [2,3,8–11,13–17]. Theoretical studies on wave propagation of side by side fibers based on evanescence have been reported in journals and literature [14–18]. Utzinger and Richards-Kortum showed that the incident angle on the exit surface of fiber reaches the critical angle, c the light is total internally reflected and leave through the side wall of the emitting fiber [11]. The electromagnetic (EM) field scattered by the randomly located suspended particles within the system is categorised into an average component that travels along a definite direction, i.e., the coherent beam and a fluctuating component that travels in all different directions, i.e., diffuse field. The effective EM response depends not only on the frequency, of the exciting field but also on its wave vector, k [19]. There are reports on evanescent wave fiber optic sensor for the purpose of pollutants detection [12,20,21]. The sensing mechanism of evanescent wave is very sensitive to changes in the surrounding medium of the fiber core. The substrate in vicinity from the fiber head acts as a medium that allows frustrated total internal reflection (FTIR). The transmission of light through FTIR depends on o , polarization gap, gap width and RI of medium. The backscattered light wave interferes with the total internal reflected light wave in opposite direction and the evanescent wave pattern localized on the surface of beveled fiber [22]. Therefore, no coupling optics is required in the sensing region that makes miniarization possible in integrated optics. Evanescent wave sensor is preferred due to its accurate measurement on highly absorbing and highly
2
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
Fig. 1. Configuration for POF sensor setup.
scattering media due to its small effective path length. This sensing mechanism incorporated in fiber optics is a solution worth researching on for wastewater system in confined spaces and narrow sewer pipes. In this paper, a real-time measurement is demonstrated to measure sucrose and formazin solution in different geometrical configurations achieved from the simulation results. The monochromatic light propagating in the optical system are employed for turbidity analysis of a transparent medium through backscattered light intensity. The turbidity sensor constructed using 1 mm POF is suitable for high concentrations of suspended particles or with relatively small particle sizes. Light rays coupled into the fiber core within its maximum acceptance angle are totally reflected at the interface of the fiber core and some loss at the cladding. A portion of the optical energy were dissipated through the interface between the fiber core and the cladding via an evanescent field that essentially decays exponentially with the distance from the core-cladding interface. This evanescent leakage may be used to couple optical energy into or out of the fiber core, or alternatively, to perturb the guided optical energy in the fiber core. Further experiment is performed in turbidity measurement of real water samples collected from lake, river and sea. This sensor employs MMF with strong signal and does not involve any external optical reflectors to minimize optical interference and optical error. 2. Methods 2.1. Sensor structure and sensing principle methodology The turbidity probe is assembled by aligning the stripped section of 5 cm long from the tip of a pair of MMFs in parallel. The unjacketed sections are fixed firmly side by side with epoxy glue to minimise motion of fibers at the sensing tip section. Epoxy was chosen for its strong adhesion and durability with the least interaction to the light wave. As shown in Fig. 1, the turbidity probe was immersed into a random turbid solution, approximately 5 mm below the surface of the solution. The surface tip of a fiber is cleaved at an angle, o so that the guided ray in the fiber undergoes total internal reflection (TIR). In this experiment, the three
measured angles with respect to the core axis are of 35◦ , 45◦ and 55◦ . The diagram also shows the apparatus involved to assemble turbidity probe which consist of NIR laser diode light source with the wavelength of 808 nm, QE65000 spectrometer as detector, MMFs, plastic cuvette (1 cm length), cuvette holder and Newport fiber optic mounts to give flexibility in immersing turbidity probe into samples. The International Standardization Organization (ISO) method 7027 requires the use of light source greater than 800 nm to minimise interference and absorption by the ambient light and water compound. This region wavelength is less susceptible to the effects of particle colour and it yields better sensitivity response of turbidity variation in the smaller particle size range. It gives an accurate determination of the usually low absorption coefficient in the NIR spectral region as compared to mid-IR spectral region which suffers from high transmission losses. The transmission of light in soluble and suspended solid solution is operated in two configurations as shown in Fig. 2(a) and (b). The geometrical configuration in Fig. 2(a) is arranged parallel to the fiber axis so as to provide a direct reflective path towards the direction of receiving fiber and vice versa for configuration of refractive probe as illustrated in Fig. 2(b). The sucrose solution is prepared via dilution method with distilled water within a range of concentration from 0 to approximately 50 Brix, with resolution of 0.5 Brix. The samples are ® calibrated using pocket refractometer Atago Co.,LTD (Tokyo, Japan). The variation of absolute sugar concentrations will be used to study the change in absolute intensity arising from the evanescent field of light in the sensing region in two different configurations. Next, the experiment is proceeded with preparation of 45 samples of formazin standard solution with 20 NTU interval per sample. With the turbidity probe attached to the vertical mount, the fiber tip was motion downwards until it was fully immersed into the cuvette so that the transmitted light in the fiber could interact with the sample. The black plastic cuvette was ensured to be free from scratches and rinsed with distilled water each time the sample is replaced. Reference point was collected with distilled water as sample in the cuvette. Detection of output intensity was collected by the receiving fiber back to QE65000 spectrometer to process information with software in computer. The measurement range studied in this experiment is 0–1000 NTU. The reproducibility of
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
3
Fig. 2. Schematic fiber configuration of (a) reflective probe and (b) refractive probe showing evanescent wave propagates in opposing direction.
Fig. 3. Rays propagating out from the fiber with ni 1.492 into the cuvette containing air of nt 1.000 where o is beveled a) perpendicular b) 75◦ c) 60◦ d) 45◦ e) 30◦ f) 15◦ to the surface.
Table 1 Simulated results for different o in the external medium of air and water. External medium
Air
Oblique angle, o (degree)
Flux Emitted (W/mm2 )
Incident Ray
Flux Emitted (W/mm2 )
Water Incident Ray
0 15 30 45 60 75
0.0000 0.0000 0.0466 0.2183 0.0120 0.0000
0 12 2845 26649 8482 6
0.0002 0.0035 0.0039 0.0045 0.0055 0.0050
14 217 274 479 533 686
4
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
Fig. 4. Rays propagating out from the fiber with ni 1.492 into the cuvette containing water of nt 1.333 where o is beveled a) perpendicular b) 75◦ c) 60◦ d) 45◦ e) 30◦ f) 15◦ to the surface.
Fig. 5. Intensity distribution against the concentration of sucrose using reflective probe.
Fig. 6. Intensity distribution against the concentration of sucrose using refractive probe.
the fiber sensor was verified in real sample application. Real water samples of different locations were collected that may vary in settling time and absorption of different organic matters. A total of six water samples were collected from two man-made lakes, Lake A (Tasek Aman Damai, USM, 5◦ 21 N 100◦ 17 E) and Lake B (Tasek Harapan, USM, 5◦ 21 N 100◦ 18 E); rivers, River A (Sungai Keluang, 5◦ 18 N 100◦ 17 E) and River B (Sungai Pinang, 5◦ 24 N l00◦ l9 E); as well as seas, Sea A (North Strait, 5◦ 25 N 100◦ 19 E) and Sea B (South Strait, 5◦ 19 N 100◦ 18 E). The samples were validated using Lovibond Turbidimeter immediately after sample preparation. The entire experiment was carried out in a fully air-conditioned dark room. 3. Results and discussions Propagation of light from the fiber (ni = 1.492) into the cuvette containing air (nt = 1.000) is simulated to evaluate the ray patterns of the designed fiber sensor. Fig. 3 shows the cross-sectional ray
Fig. 7. Correlations of respective relative intensity in the range of 0 – 1010 NTU for POF1, POF2 and POF3 turbidity sensors.
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
tracing images for probe bevel angle, o beveled a) perpendicular b) 75◦ c) 60◦ d) 45◦ e) 30◦ f) 15◦ to the surface (relative to the core axis). For o larger than 45◦ as shown in Fig. 3(a–c), weak multiple internal reflections occurred at the tip of emitting fiber and eventually refracted out into the external medium, some were loss at the surface and some were deflected towards the direction of receiving fiber. Transmitted rays into the external medium became weaker as o decreased to c while rays were mostly reflected towards the receiving fiber. As shown in Fig. 3(d–f), where o is equal to or less than 45◦ , the direction of light was totally internal reflected and turned through 90◦ at each surfaces of the fiber. It is observed that similar raytrace is simulated when the external medium is replaced with water as compiled in Fig. 4. The reflected intensity in receiving fiber is much weaker compared to air as external medium due to the propagated ray were scattered randomly by water molecules and absorbed by the surface of cuvette. Due to smaller difference in RI between fiber and water in this case, Fresnel reflection decreases in theory. TIR can only be achieved at c = 63.3◦ where evanescent field strength is maximum at c . Hence, a larger number of rays were refracted into the water medium while smaller amount of rays reflected as TIR at the tip leaving through the side cladding of the fiber. The simulation results above clarified the experimental optical geometry in light propagation contributed by TIR where the refracted rays dispersed in the water medium are eliminated. Table 1. tabulates the flux emitted and number of incident rays for different o . The flux emitted is found to be highest when o is 45◦ with a relatively large amount of incident rays. Whereas for o < 15◦ , the flux emitted is completely insignificant. When the
5
Fig. 8. Linear slopes of relative intensity in the range 0 – 110 NTU for three turbidity sensors.
external medium is replaced with water, the flux detected for all angles have the same number of decimal places due to reduction in RI mismatch between the probe tip and the external medium. The flux emitted is found to be highest when o is 60◦ , though at o = 75◦ has higher incident rays, the rays are slightly weaker. Fresnel equations and appropriate coherent wave from Snell’s law describes the relative amount of transmission and reflection. As the data suggested, the light exiting the emitting fiber will be largely refracted and scattered by the suspended particles when air is replaced with water.
Fig. 9. Comparative turbidity in (a) lake (b) river and (c) sea water samples determined by POF2 and turbidimeter.
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S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
As expected from Snell’s law, the light ray is bent away from the normal of the fiber interface towards the direction of the refracted ray. As the angle of refraction i is drawing towards c , the refracted angle also increases and more rays travel along the surface between the two media. The response obtained by successively changing the concentration of the sucrose solution for reflective (configuration of Fig. 2a) and refractive probe (configuration of Fig. 2b) with o = 35◦ are as plotted in Figs. 5 and 6 respectively. The refractive probe is found unsuitable for detection of a wide range of turbidity such as this study. The correlations of respective relative intensity in the range of 0–1010 NTU for POF1 (o = 35◦ ), POF2 (o = 45◦ ) and POF3 (o = 55◦ ) turbidity sensors is shown in Fig. 7. The result shows that there is a linear relationship between intensity received in the receiving fiber as a function of turbidity. Referring to the simulation results in Fig. 3, more rays were deflected towards the direction of receiving fiber as o becomes smaller. Ray paths undergoing refraction with external medium and evanescent wave are mostly detected by the receiving fiber of POF2 and POF3. Whereas, for POF1 rays are largely refracted away and evanescent waves are dissipated into the external medium. For comparison purposes, the relative intensity, Irelative is obtained by converting the intensity In to a common scale relative to Iavg for each POF through Eq. (1). Irelative =
Iavg − In × 100% Iavg
Fig. 10. Predicted turbidity by POF2 turbidity sensor in comparison with the actual turbidity of sample collected from Lake A.
(1)
Where In is the collected raw data and Iavg is the average data. It was found that the turbidity sensors have different minimum signal collection; thus, the experiment is narrowed down to 0–110 NTU, as shown in Fig. 8 POF3 has the highest gradient of all; however, it has a poor turbidity resolution. The geometrical structure of POF2 enables evanescent wave that exists on the fiber core surface to detect suspended particles. The advantage of this turbidity sensor is the wide measurement range with a good approximation of low RMSE 2.3812 NTU. Using the mathematical relation, we can determine the turbidity of the water by measuring the optical power of the light reflected from the sensing probes. In the proposed fiber-optic sensor system, the precondition for operating the turbidity sensing is that the low limit of the turbidity must be greater than pure water. Turbidity detected by turbidity sensor is lower than the measured readings by turbidimeter. The direct contact of turbidity sensor and samples solution with absorbing properties exhibit by organic suspended particles is to be taken into account. Since, increase of suspended particles decreases the returned output intensity, samples of highly absorbing suspended particles would cause slight variation in the baseline of regression equation. Fig. 9 shows the turbidity in real water samples determined by POF2 compared to readings taken from turbidimeter. The measurement principle of turbidimeter has different sensing method and operating wavelength. It is expected to work well for samples with low absorbing properties but it has limitation at high concentration. Certainly, the measurement inconsistency of the two methods should not affect the application of turbidity sensor to monitor changes in suspended solids of one specific ecosystem over a wide turbidity range. The plastic turbidity sensor has the ability to register changes of small resolution of turbidity as shown in Figs. 10 and 11 with R2 0.8421 for Lake A and 0.8783 for Lake B. The R2 for the sample for Lake A is low due to the difference in RI, shape, and size of particles compared to the calibration standard. The measurements performed are in reference to turbidimeter of different setting in sensing method and wavelength, thus the equivalent analysis method is a comparison study for the real sample. The results analyzed suggested that the direct contact turbidity sensor are influenced by suspended particles exists in different biodiversity aquatic ecosystem and causes different efficiency due to
Fig. 11. Predicted turbidity by POF2 turbidity sensor in comparison with the actual turbidity of sample collected from Lake B.
organic and inorganic compounds and domestic sewage affects the turbidity evaluation. 4. Conclusion This paper has presented the overall design concept of optical instruments for water turbidity measurement. Using simulation, the angle o between 30◦ to 60◦ is found best to retrieve light scattered in water samples. Based on the validation results, POF2 has the largest value of R2 (0.9947) with a relatively small value of RMSE (2.38 NTU). POF2 is found to function best in lake water with R2 for Lake A was 0.8421 with RMSE 3.2011 and R2 of Lake B was 0.8783 with RMSE 6.2680. The turbidity sensor developed in this research has contribution especially in field work purposes due to its simplicity in setup and is economical to monitor water quality. Acknowledgment This research study was supported by Prof. Dr. Mohammad Zubir Mat Jafri, under the grant number 304/PFIZIK/6315023. We thank our colleagues from Universiti Sains Malaysia, School of physics who provided insight and expertise that greatly assisted the research, although they may not agree with all of the interpretations of this paper. References [1] Agency, U.S.E.P, Turbidity, Available from:, 2012 https://archive.epa.gov/ water/archive/web/html/index-18.html. [2] R. Harmel, et al., Cumulative uncertainty in measured streamflow and water quality data for small watersheds, Trans. ASABE 49 (3) (2006) 689–701.
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7 [3] R. Harmel, et al., Practical guidance for discharge and water quality data collection on small watersheds, Trans. ASABE 49 (4) (2006) 937–948. [4] F. Druine, et al., In situ high frequency long term measurements of suspended sediment concentration in turbid estuarine system (Seine Estuary, France): Optical turbidity sensors response to suspended sediment characteristics, Mar. Geol. 400 (2018) 24–37. [5] L. Parra, et al., Design and development of low cost smart turbidity sensor for water quality monitoring in fish farms, Aquac. Eng. 81 (2018) 10–18. [6] Ó. Sampedro, J.R. Salgueiro, Turbidimeter and RGB sensor for remote measurements in an aquatic medium, Measurement 68 (2015) 128–134. [7] A.A. Azman, et al., A low cost nephelometric turbidity sensor for continual domestic water quality monitoring system, in: 2016 IEEE International Conference on Automatic Control and Intelligent Systems (I2CACIS), 2016. [8] M. Shenoy, Optical fibre probes in the measurement of scattered light: Application for sensing turbidity, Pramana 82 (1) (2014) 39–48. [9] A.F. Omar, M.Z.M. Jafri, Optical System in Measurement of Water Turbidity: Design and Analytical Approach (Penerbit USM), Penerbit Usm, 2015. [10] A. Omar, M. MatJafri, Development of optical fiber sensor for water quality measurement, AIP Conference Proceedings (2008). [11] R.L.A. Shauri, et al., Optical Transmission-Based Water Turbidity Measurement System, Universiti Putra Malaysia, 2006. [12] P. Chevalier, et al., In-water fiber-optic evanescent wave sensing with quantum cascade lasers, Sens. Actuators B Chem. 262 (2018) 195–199. [13] K.I. Oberg, R. Hodyss, J. Beauchamp, Simple optical sensor for amine vapors based on dyed silica microspheres, Sens. Actuators B Chem. 115 (1) (2006) 79–85.
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[14] U. Utzinger, R.R. Richards-Kortum, Fiber optic probes for biomedical optical spectroscopy, J. Biomed. Opt. 8 (1) (2003) 121–147. [15] T.E. Clark, M.W. Burrell, Thermally switched coupler, in: Fiber Optic Smart Structures and Skins, International Society for Optics and Photonics, 1989. [16] Y.-F. Li, J.W. Lit, Temperature effects of a multimode biconical fiber coupler, Appl. Opt. 25 (11) (1986) 1765–1769. [17] Y. Murakami, S. Sudo, Coupling characteristics measurements between curved waveguides using a two-core fiber coupler, Appl. Opt. 20 (3) (1981) 417–422. [18] R. Reif, O. A’Amar, I.J. Bigio, Analytical model of light reflectance for extraction of the optical properties in small volumes of turbid media, Appl. Opt. 46 (29) (2007) 7317–7328. [19] E. Gutiérrez-Reyes, A. García-Valenzuela, R.G. Barrera, Overview of an effective-medium approach to the reflection and refraction of light at a turbid colloidal half-space, Phys. Status Solidi B 249 (6) (2012) 1140–1147. [20] V.B. Llorente, et al., Optochemical fiber sensor for Toluidine Blue detection in high turbidity media, Sens. Actuators B Chem. 216 (2015) 316–320. [21] T. Okazaki, et al., Fundamental study on the development of fiber optic sensor for real-time sensing of CaCO3 scale formation in geothermal water, Anal. Sci. 31 (3) (2015) 177–183. [22] C. Wang, H. Chen, J. Ma, The fiber-optic evanescent wave fluorescence sensor with a novel sensing architecture for determining penicillin G, Electronics and Optoelectronics (ICEOE), 2011 International Conference on (2011).
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Plastic fiber evanescent sensor in measurement of turbidity S. Yeoh, M.Z. Matjafri, K.N. Mutter, Ammar A. Oglat ∗ School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia
a r t i c l e
i n f o
Article history: Received 1 August 2018 Received in revised form 8 October 2018 Accepted 29 October 2018 Keywords: Turbidity Optical fiber sensor Tracepro software Water quality monitoring
a b s t r a c t The construction and working principles of a plastic fiber sensor for examining the level of turbidity is studied in this paper. This work focuses on designing an inexpensive turbidity sensor that incorporates a pair of multimode fibers (MMF) that are attached side by side and their beveled tips are mounted vertically. The efficiency of different beveled angles is evaluated by simulation with Tracepro software. The reflected signal is collected by immersing the sensor head into a water mixture and analyzed for various concentration. It was found that there is a linear increment of output intensity when concentration of mixture is increased. The turbidity sensor is tested with real samples that are collected from lake, river and coastal areas to demonstrate its consistency with commercial apparatus in natural compounds. The results showed that the proposed sensor structure is able to produce reliable results in a dynamic range of detection from 0 to 1000 Nephelometric Turbidity Unit (NTU) to facilitate practical field measurements. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Turbidity is defined as the decrease in transparency or clarity of a solution due to the presence of suspended and colloidal materials such as sand, clay, silt, organic and inorganic matter, algae or other micro-organisms [1]. High level of turbidity in natural water and wastewater brings risk to sustaining the ecological processes and prolonged exposure can be detrimental to human health. A common method in measuring turbidity is a portable device called turbidimeter that measures the intensity of light transmitted through the suspended particles; it is often reported in the unit, NTU [2,3]. Measurement of turbidity often involves disturbance in background colour, interference and scattering of suspended particles that affect the accuracy of collected signals. Continuous turbidity monitoring is expensive, time-consuming and prone to errors in highly turbid water [4]. A turbidity sensor operates by detecting the light that scatters off the suspended particles present in the water. The monitoring of turbidity is necessary for different purposes, sustaining the aquatic ecosystem [5,6] and domestic water quality system [7]. Several studies have been demonstrated to detect various physical changes of polluted water using fiber optic sensors [8–12]. Common online instruments that offers in-situ monitoring are applied in most turbidity sensors for measurement in streams, rivers, lakes and oceans [2]. In this application, plastic optical fiber (POF) has the
∗ Corresponding author. E-mail address: [email protected] (A.A. Oglat). https://doi.org/10.1016/j.sna.2018.10.042 0924-4247/© 2018 Elsevier B.V. All rights reserved.
advantage of high transmission in the visible-near infrared (VISNIR) range, high mechanical stability and resistance to chemical degradation. Hands on and simple designs without involving high budget equipment were attempted by mechanically polishing the fiber to make bundle sensors [2,3,8–11,13–17]. Theoretical studies on wave propagation of side by side fibers based on evanescence have been reported in journals and literature [14–18]. Utzinger and Richards-Kortum showed that the incident angle on the exit surface of fiber reaches the critical angle, c the light is total internally reflected and leave through the side wall of the emitting fiber [11]. The electromagnetic (EM) field scattered by the randomly located suspended particles within the system is categorised into an average component that travels along a definite direction, i.e., the coherent beam and a fluctuating component that travels in all different directions, i.e., diffuse field. The effective EM response depends not only on the frequency, of the exciting field but also on its wave vector, k [19]. There are reports on evanescent wave fiber optic sensor for the purpose of pollutants detection [12,20,21]. The sensing mechanism of evanescent wave is very sensitive to changes in the surrounding medium of the fiber core. The substrate in vicinity from the fiber head acts as a medium that allows frustrated total internal reflection (FTIR). The transmission of light through FTIR depends on o , polarization gap, gap width and RI of medium. The backscattered light wave interferes with the total internal reflected light wave in opposite direction and the evanescent wave pattern localized on the surface of beveled fiber [22]. Therefore, no coupling optics is required in the sensing region that makes miniarization possible in integrated optics. Evanescent wave sensor is preferred due to its accurate measurement on highly absorbing and highly
2
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
Fig. 1. Configuration for POF sensor setup.
scattering media due to its small effective path length. This sensing mechanism incorporated in fiber optics is a solution worth researching on for wastewater system in confined spaces and narrow sewer pipes. In this paper, a real-time measurement is demonstrated to measure sucrose and formazin solution in different geometrical configurations achieved from the simulation results. The monochromatic light propagating in the optical system are employed for turbidity analysis of a transparent medium through backscattered light intensity. The turbidity sensor constructed using 1 mm POF is suitable for high concentrations of suspended particles or with relatively small particle sizes. Light rays coupled into the fiber core within its maximum acceptance angle are totally reflected at the interface of the fiber core and some loss at the cladding. A portion of the optical energy were dissipated through the interface between the fiber core and the cladding via an evanescent field that essentially decays exponentially with the distance from the core-cladding interface. This evanescent leakage may be used to couple optical energy into or out of the fiber core, or alternatively, to perturb the guided optical energy in the fiber core. Further experiment is performed in turbidity measurement of real water samples collected from lake, river and sea. This sensor employs MMF with strong signal and does not involve any external optical reflectors to minimize optical interference and optical error. 2. Methods 2.1. Sensor structure and sensing principle methodology The turbidity probe is assembled by aligning the stripped section of 5 cm long from the tip of a pair of MMFs in parallel. The unjacketed sections are fixed firmly side by side with epoxy glue to minimise motion of fibers at the sensing tip section. Epoxy was chosen for its strong adhesion and durability with the least interaction to the light wave. As shown in Fig. 1, the turbidity probe was immersed into a random turbid solution, approximately 5 mm below the surface of the solution. The surface tip of a fiber is cleaved at an angle, o so that the guided ray in the fiber undergoes total internal reflection (TIR). In this experiment, the three
measured angles with respect to the core axis are of 35◦ , 45◦ and 55◦ . The diagram also shows the apparatus involved to assemble turbidity probe which consist of NIR laser diode light source with the wavelength of 808 nm, QE65000 spectrometer as detector, MMFs, plastic cuvette (1 cm length), cuvette holder and Newport fiber optic mounts to give flexibility in immersing turbidity probe into samples. The International Standardization Organization (ISO) method 7027 requires the use of light source greater than 800 nm to minimise interference and absorption by the ambient light and water compound. This region wavelength is less susceptible to the effects of particle colour and it yields better sensitivity response of turbidity variation in the smaller particle size range. It gives an accurate determination of the usually low absorption coefficient in the NIR spectral region as compared to mid-IR spectral region which suffers from high transmission losses. The transmission of light in soluble and suspended solid solution is operated in two configurations as shown in Fig. 2(a) and (b). The geometrical configuration in Fig. 2(a) is arranged parallel to the fiber axis so as to provide a direct reflective path towards the direction of receiving fiber and vice versa for configuration of refractive probe as illustrated in Fig. 2(b). The sucrose solution is prepared via dilution method with distilled water within a range of concentration from 0 to approximately 50 Brix, with resolution of 0.5 Brix. The samples are ® calibrated using pocket refractometer Atago Co.,LTD (Tokyo, Japan). The variation of absolute sugar concentrations will be used to study the change in absolute intensity arising from the evanescent field of light in the sensing region in two different configurations. Next, the experiment is proceeded with preparation of 45 samples of formazin standard solution with 20 NTU interval per sample. With the turbidity probe attached to the vertical mount, the fiber tip was motion downwards until it was fully immersed into the cuvette so that the transmitted light in the fiber could interact with the sample. The black plastic cuvette was ensured to be free from scratches and rinsed with distilled water each time the sample is replaced. Reference point was collected with distilled water as sample in the cuvette. Detection of output intensity was collected by the receiving fiber back to QE65000 spectrometer to process information with software in computer. The measurement range studied in this experiment is 0–1000 NTU. The reproducibility of
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
3
Fig. 2. Schematic fiber configuration of (a) reflective probe and (b) refractive probe showing evanescent wave propagates in opposing direction.
Fig. 3. Rays propagating out from the fiber with ni 1.492 into the cuvette containing air of nt 1.000 where o is beveled a) perpendicular b) 75◦ c) 60◦ d) 45◦ e) 30◦ f) 15◦ to the surface.
Table 1 Simulated results for different o in the external medium of air and water. External medium
Air
Oblique angle, o (degree)
Flux Emitted (W/mm2 )
Incident Ray
Flux Emitted (W/mm2 )
Water Incident Ray
0 15 30 45 60 75
0.0000 0.0000 0.0466 0.2183 0.0120 0.0000
0 12 2845 26649 8482 6
0.0002 0.0035 0.0039 0.0045 0.0055 0.0050
14 217 274 479 533 686
4
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
Fig. 4. Rays propagating out from the fiber with ni 1.492 into the cuvette containing water of nt 1.333 where o is beveled a) perpendicular b) 75◦ c) 60◦ d) 45◦ e) 30◦ f) 15◦ to the surface.
Fig. 5. Intensity distribution against the concentration of sucrose using reflective probe.
Fig. 6. Intensity distribution against the concentration of sucrose using refractive probe.
the fiber sensor was verified in real sample application. Real water samples of different locations were collected that may vary in settling time and absorption of different organic matters. A total of six water samples were collected from two man-made lakes, Lake A (Tasek Aman Damai, USM, 5◦ 21 N 100◦ 17 E) and Lake B (Tasek Harapan, USM, 5◦ 21 N 100◦ 18 E); rivers, River A (Sungai Keluang, 5◦ 18 N 100◦ 17 E) and River B (Sungai Pinang, 5◦ 24 N l00◦ l9 E); as well as seas, Sea A (North Strait, 5◦ 25 N 100◦ 19 E) and Sea B (South Strait, 5◦ 19 N 100◦ 18 E). The samples were validated using Lovibond Turbidimeter immediately after sample preparation. The entire experiment was carried out in a fully air-conditioned dark room. 3. Results and discussions Propagation of light from the fiber (ni = 1.492) into the cuvette containing air (nt = 1.000) is simulated to evaluate the ray patterns of the designed fiber sensor. Fig. 3 shows the cross-sectional ray
Fig. 7. Correlations of respective relative intensity in the range of 0 – 1010 NTU for POF1, POF2 and POF3 turbidity sensors.
S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
tracing images for probe bevel angle, o beveled a) perpendicular b) 75◦ c) 60◦ d) 45◦ e) 30◦ f) 15◦ to the surface (relative to the core axis). For o larger than 45◦ as shown in Fig. 3(a–c), weak multiple internal reflections occurred at the tip of emitting fiber and eventually refracted out into the external medium, some were loss at the surface and some were deflected towards the direction of receiving fiber. Transmitted rays into the external medium became weaker as o decreased to c while rays were mostly reflected towards the receiving fiber. As shown in Fig. 3(d–f), where o is equal to or less than 45◦ , the direction of light was totally internal reflected and turned through 90◦ at each surfaces of the fiber. It is observed that similar raytrace is simulated when the external medium is replaced with water as compiled in Fig. 4. The reflected intensity in receiving fiber is much weaker compared to air as external medium due to the propagated ray were scattered randomly by water molecules and absorbed by the surface of cuvette. Due to smaller difference in RI between fiber and water in this case, Fresnel reflection decreases in theory. TIR can only be achieved at c = 63.3◦ where evanescent field strength is maximum at c . Hence, a larger number of rays were refracted into the water medium while smaller amount of rays reflected as TIR at the tip leaving through the side cladding of the fiber. The simulation results above clarified the experimental optical geometry in light propagation contributed by TIR where the refracted rays dispersed in the water medium are eliminated. Table 1. tabulates the flux emitted and number of incident rays for different o . The flux emitted is found to be highest when o is 45◦ with a relatively large amount of incident rays. Whereas for o < 15◦ , the flux emitted is completely insignificant. When the
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Fig. 8. Linear slopes of relative intensity in the range 0 – 110 NTU for three turbidity sensors.
external medium is replaced with water, the flux detected for all angles have the same number of decimal places due to reduction in RI mismatch between the probe tip and the external medium. The flux emitted is found to be highest when o is 60◦ , though at o = 75◦ has higher incident rays, the rays are slightly weaker. Fresnel equations and appropriate coherent wave from Snell’s law describes the relative amount of transmission and reflection. As the data suggested, the light exiting the emitting fiber will be largely refracted and scattered by the suspended particles when air is replaced with water.
Fig. 9. Comparative turbidity in (a) lake (b) river and (c) sea water samples determined by POF2 and turbidimeter.
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S. Yeoh et al. / Sensors and Actuators A 285 (2019) 1–7
As expected from Snell’s law, the light ray is bent away from the normal of the fiber interface towards the direction of the refracted ray. As the angle of refraction i is drawing towards c , the refracted angle also increases and more rays travel along the surface between the two media. The response obtained by successively changing the concentration of the sucrose solution for reflective (configuration of Fig. 2a) and refractive probe (configuration of Fig. 2b) with o = 35◦ are as plotted in Figs. 5 and 6 respectively. The refractive probe is found unsuitable for detection of a wide range of turbidity such as this study. The correlations of respective relative intensity in the range of 0–1010 NTU for POF1 (o = 35◦ ), POF2 (o = 45◦ ) and POF3 (o = 55◦ ) turbidity sensors is shown in Fig. 7. The result shows that there is a linear relationship between intensity received in the receiving fiber as a function of turbidity. Referring to the simulation results in Fig. 3, more rays were deflected towards the direction of receiving fiber as o becomes smaller. Ray paths undergoing refraction with external medium and evanescent wave are mostly detected by the receiving fiber of POF2 and POF3. Whereas, for POF1 rays are largely refracted away and evanescent waves are dissipated into the external medium. For comparison purposes, the relative intensity, Irelative is obtained by converting the intensity In to a common scale relative to Iavg for each POF through Eq. (1). Irelative =
Iavg − In × 100% Iavg
Fig. 10. Predicted turbidity by POF2 turbidity sensor in comparison with the actual turbidity of sample collected from Lake A.
(1)
Where In is the collected raw data and Iavg is the average data. It was found that the turbidity sensors have different minimum signal collection; thus, the experiment is narrowed down to 0–110 NTU, as shown in Fig. 8 POF3 has the highest gradient of all; however, it has a poor turbidity resolution. The geometrical structure of POF2 enables evanescent wave that exists on the fiber core surface to detect suspended particles. The advantage of this turbidity sensor is the wide measurement range with a good approximation of low RMSE 2.3812 NTU. Using the mathematical relation, we can determine the turbidity of the water by measuring the optical power of the light reflected from the sensing probes. In the proposed fiber-optic sensor system, the precondition for operating the turbidity sensing is that the low limit of the turbidity must be greater than pure water. Turbidity detected by turbidity sensor is lower than the measured readings by turbidimeter. The direct contact of turbidity sensor and samples solution with absorbing properties exhibit by organic suspended particles is to be taken into account. Since, increase of suspended particles decreases the returned output intensity, samples of highly absorbing suspended particles would cause slight variation in the baseline of regression equation. Fig. 9 shows the turbidity in real water samples determined by POF2 compared to readings taken from turbidimeter. The measurement principle of turbidimeter has different sensing method and operating wavelength. It is expected to work well for samples with low absorbing properties but it has limitation at high concentration. Certainly, the measurement inconsistency of the two methods should not affect the application of turbidity sensor to monitor changes in suspended solids of one specific ecosystem over a wide turbidity range. The plastic turbidity sensor has the ability to register changes of small resolution of turbidity as shown in Figs. 10 and 11 with R2 0.8421 for Lake A and 0.8783 for Lake B. The R2 for the sample for Lake A is low due to the difference in RI, shape, and size of particles compared to the calibration standard. The measurements performed are in reference to turbidimeter of different setting in sensing method and wavelength, thus the equivalent analysis method is a comparison study for the real sample. The results analyzed suggested that the direct contact turbidity sensor are influenced by suspended particles exists in different biodiversity aquatic ecosystem and causes different efficiency due to
Fig. 11. Predicted turbidity by POF2 turbidity sensor in comparison with the actual turbidity of sample collected from Lake B.
organic and inorganic compounds and domestic sewage affects the turbidity evaluation. 4. Conclusion This paper has presented the overall design concept of optical instruments for water turbidity measurement. Using simulation, the angle o between 30◦ to 60◦ is found best to retrieve light scattered in water samples. Based on the validation results, POF2 has the largest value of R2 (0.9947) with a relatively small value of RMSE (2.38 NTU). POF2 is found to function best in lake water with R2 for Lake A was 0.8421 with RMSE 3.2011 and R2 of Lake B was 0.8783 with RMSE 6.2680. The turbidity sensor developed in this research has contribution especially in field work purposes due to its simplicity in setup and is economical to monitor water quality. Acknowledgment This research study was supported by Prof. Dr. Mohammad Zubir Mat Jafri, under the grant number 304/PFIZIK/6315023. We thank our colleagues from Universiti Sains Malaysia, School of physics who provided insight and expertise that greatly assisted the research, although they may not agree with all of the interpretations of this paper. References [1] Agency, U.S.E.P, Turbidity, Available from:, 2012 https://archive.epa.gov/ water/archive/web/html/index-18.html. [2] R. Harmel, et al., Cumulative uncertainty in measured streamflow and water quality data for small watersheds, Trans. ASABE 49 (3) (2006) 689–701.
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