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Measurement of suspended sediment for model experiments using general-purpose optical sensors
Catena 83 (2010) 1–6
Contents lists available at ScienceDirect
Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a
Measurement of suspended sediment for model experiments using general-purpose optical sensors Shinya Ochiai ⁎, Kenji Kashiwaya Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
a r t i c l e
i n f o
Article history: Received 1 February 2010 Received in revised form 22 June 2010 Accepted 22 June 2010 Keywords: Suspended sediment concentration Model experiment Photoelectric sensor Fiber optic sensor
a b s t r a c t A specialized measurement system for measuring suspended sediment concentration in model experiments was developed; it utilizes general-purpose optical sensors to perform measurements. A photoelectric sensor was used for measuring the transmitted light, and a fiber optic sensor was used for measuring the backscattered light. The values of the light absorbance and intensity of backscattered light for turbid standard solutions measured with these sensors show a linear relationship with the turbidity. The calibration lines for sand particles with a large settling velocity were established by using a calibration tank containing upwelling water. The vertical profiles of the sediment concentration measured with the photoelectric sensor corresponded well to those of the sediment concentration measured with a water sampler in an artificial channel. This result suggests that the measurement system using a photoelectric sensor functions satisfactorily. The sediment concentrations measured using the fiber optic sensor correlate well with the concentrations measured through siphon sampling; however, the values measured with the fiber optic sensor are approximately 25% larger than those measured with the siphon. This disagreement may result from the differences in the optical conditions between the calibration tank and the actual channel. Thus, it may be possible to utilize fiber optic sensors calibrated on the basis of siphon sampling for the measurement of sediment concentration. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In order to investigate the sediment transportation in rivers, lakes, and oceans, it is important to measure the turbidity and the amount of suspended sediment in a particular body of water. There are many methods to measure turbidity and sediment concentration, e.g., water sampling, the optical method, and the acoustic method (Wren et al., 2000). Further, many commercial instruments such as turbidity meters and suspended sediment sensors are available for performing such measurements. Previous studies have used the abovementioned instruments for field observations (e.g., Wiberg et al., 1994; Chikita et al., 1996; Langlois et al., 2005), and the need for a technical examination for the proper interpretation of data has been proposed (e.g., Ludwig and Hanes, 1990; Clifford et al., 1995; Bunt et al., 1999). Previous studies have also proposed measuring sediment concentration using new methods, including electrical resistance tomography (Schlaberg et al., 2006) and image processing (Radice et al., 2006) and custom-built devices, including optical sensors (Campbell et al., 2005; Orwin and Smart, 2005) and capacitance sensors (Li et al., 2005). The measurement of suspended sediment concentration is also important for model experiments (e.g., Mateos and Giráldez, 2005; ⁎ Corresponding author. Present address: Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, O 24, Wake-machi, Nomishi, Ishikawa 923-1224, Japan. Tel.: +81 761514440; fax: +81 761515528. E-mail address: [email protected] (S. Ochiai). 0341-8162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2010.06.008
Cloutier et al., 2006; Alexander et al., 2008). However, commercial instruments, because of their size and configuration, are not always suitable for small-scale model experiments. For instance, most field turbidity meters have a large probe. In natural rivers, lakes, and oceans, the disturbances caused by the probe can be ignored. However, it would disturb the water flow in a small-scale experiment, and the large volume of the sensor results in a decrease in the resolution of the measurement. Furthermore, electronic techniques need to be used to design custom-built devices. Therefore, in this study, we attempted to develop a measuring device using low-cost mass-produced instruments that are mainly used for FA (factory automation). Among the methods that have been proposed, the optical method (including the transmitted light and scattered light methods) is the easiest to handle using general-purpose sensors. First, the transmitted light and the scattered light methods carried out using FA instruments are discussed. Second, the examination of the sensors by using an experimental channel is described. 2. Transmitted light method This method is widely used in the adsorption spectrophotometry of chemical substances and in turbidity meters. The intensity of transmitted light in water is given by the Lambert–Beer law: −kcl
I = I0 e
ð1Þ
2
S. Ochiai, K. Kashiwaya / Catena 83 (2010) 1–6
where I is the intensity of the transmitted light: I0, the intensity of incident light; c, the sediment concentration; l, the optical pass length; and k, a constant. If light absorbance A is introduced as A = log10
I0 I
ð2Þ
then, Eq. (1) is rewritten as A = ðlog10 eÞ·kcl = acl
ð3Þ
where a = k log10e = 0.4343k is the extinction coefficient. If the optical pass length is constant, the light absorbance will be proportional to the sediment concentration. If the Lambert–Beer law is valid and the extinction coefficient a is known, the sediment concentration can be estimated from the observed absorbance value. A set of light source and detector is necessary to measure the sediment concentration on the basis of the Lambert–Beer law. In this study, we use a photoelectric sensor, which is one of the FA instruments, along with a laser light source and detector (LV-series, KEYENCE Corp., Japan). This sensor detects objects in the measuring zone from the intensity of the transmitted light. The intensity of the transmitted light can be outputted as a voltage signal. This kind of sensor is available for the measurement of the sediment concentration. A laser light source has an advantage of high spatial resolution because of its high directivity. Among the various models of photoelectric sensors available (shape and width of laser beam, etc.), we adopted one with an amplifier unit (LV-51M) and two sensors (LV-H300: 30mm-width sheet beam; LV-H110: 10-mm-width beam; wave length: 650 nm) (Fig. 1a). The sensor system consists of a pair of transmitter units, a receiver unit and an amplifier unit that converts the intensity of the transmitted light to a voltage signal. LV-H300 and LV-H110 have their own sensitivity range. Therefore, they can cover a wide range of sediment concentrations. When energizing the amplifier unit, the transmitter unit emits a laser beam and the light intensity detected by the receiver unit will be outputted as a voltage in the range 1–4 V. The voltage logger used here is a PLC (KV-700, KEYENCE Corp., Japan); this unit is capable of processing input signals from various sensors, switches, and controlling motors, solenoid valves, etc. 3. Scattered light method Scattered light is also employed for measurements using turbidity meters, variously utilizing either backscattered or side scattered light. Here, we will focus on backscattered light. The intensity of the scattered light is expressed in the following equation (Van de Hulst, 1981; Sutherland et al., 2000): F=
3 VcEQ s 2 ρD
ð4Þ
where F is the flux scattered by the suspended particles; E, the irradiance of the light source; V, the scattering volume; c, the sediment concentration; D, the particle diameter; ρ, the particle density; and Q s, the scattering efficiency. The intensity of the scattered light will be proportional to the sediment concentration if the other variables are constant. A fiber optic sensor, which is one of the FA instruments used here, can be used as a backscatter sensor for measuring the sediment concentration. This particular sensor was originally used to detect objects by means of scattered light. Emitted light from the source is led through the optical fiber and radiated from its end. The scattered light from the objects is also led to a detector through the optical fiber. The advantage of this sensor is that the transmitting part and the receiving part can be freely handled owing to the fiber flexibility. In this study, we adopted an amplifier unit (FS-V20, KEYENCE Corp., Japan) and a waterproof fiber sensor head (FU-91) (Fig. 1b). The
Fig. 1. (a) Photoelectric sensors (LV-series, KEYENCE Corporation, Japan). Front: the transmitter and the receiver unit of LV-H110. Middle: LV-H300. Rear: Amplifier unit LV51M. (b) Fiber optic sensor. The amplifier unit (FS-V20, KEYENCE Corporation, Japan) and waterproof fiber sensor head (FU-91). The end of fiber unit has two holes (transmitting part and receiving part).
amplifier unit includes the light source (red light emitting diode; wave length: 640 nm) and detector. This fiber head unit can be used in water, oils, and chemicals because it is sheathed with Teflon. This unit includes two optical fibers (transmitting and receiving). The end of fiber unit has a transmitting part and a receiving part. The amplifier converts the intensity of the scattered light into a voltage signal (1– 5 V) that is recorded by the voltage logger. 4. Sensor responses to turbidity standard The response characteristics of the abovementioned sensors to a widely used turbidity standard, formazin, were checked to evaluate the response of the sensors to variation in turbidity. The light absorbance and the intensity of the scattered light for various turbidity levels of formazin (expressed in Nephelometric Turbidity Units, NTU) were measured with a photoelectric sensor and fiber optic sensor, respectively. The optical pass length of the transmitted light was 15 cm. The transmitted light intensity of clear water is assumed to be I0 for the LV-H300. The transmitted light intensity of a blackened smooth plastic board is assumed to be the incident light intensity I0 for LV-H110 because the transmitted light intensity of
S. Ochiai, K. Kashiwaya / Catena 83 (2010) 1–6
clear water is out of range for LV-H110. As a result, the zero absorbance exhibited by LV-H110 is considered to be the absorbance of the plastic board. The intensity of the backscattered light measured by the fiber optic sensor is measured in terms of voltage. The turbidity of the formazin solution was also measured by using a turbidity meter (W-22XD, HORIBA, Ltd., Japan) for crosschecking. Fig. 2 shows the light absorbance for various turbidity levels of the formazin solution measured with the photoelectric sensor. The light absorbance is proportional to the turbidity of formazin at comparatively low values, indicating that the Lambert–Beer law is valid in this range. For a high turbidity, the absorbance value did not increase because the transmitted light was very weak. The measurable turbidity ranges for LV-H300 and LV-H110 for the 15-cm pass length are 0 to 60 NTU and 60 to 120 NTU, respectively. Thus, the two sensors will cover a range of 0 to 120 NTU. Fig. 3 indicates the scattered light intensity for various turbidity levels of the formazin solution measured with two fiber optic sensors. This supports the fact that the scattered light intensity (sensor output) is proportional to the turbidity, as given by Eq. (4), the linear relationship breaks down at high values of turbidity. This may be result from the change in the scattering volume V. However, the abovementioned relationship can be closely expressed by a quadratic equation. This implies that the sensor is practically suitable for a turbidity meter. Fig. 3 also suggests that the sensors have their own response characteristics to turbidity, indicating that fiber optic sensors have to be calibrated before every use by using a formazin solution. 5. Calibration lines for sediment suspensions To measure the sediment concentration with photoelectric sensors and fiber optic sensors, calibration lines indicating the relationship between sensor output and sediment concentration must be established. However, owing to the large settling velocity of the sand particles, it is difficult to generate uniformly dispersed sand currents. Therefore, a new calibration tank equipped with a pump was introduced for promoting uniform dispersion (Fig. 4). This system consists of a transparent upright tank (calibration tank) to measure the light absorbance, a reserve tank, and pumps. The sand particles are uniformly dispersed at a flow velocity that is greater than the settling velocity. The optical conditions (material and thickness of the upright tank, optical pass length, etc.) for this calibration tank should be
Fig. 2. Light absorbance for various turbidity levels of formazin solution measured by photoelectric sensor. Circles indicate values measured with LV-H300 and triangles indicate values measured with LV-H110.
3
Fig. 3. Intensity of backscattered light intensity for various turbidity levels of formazin solution measured by fiber optic sensor.
identical to those in the actual measurement (experimental channel etc.). In this study, the optical pass length of this tank is 15 cm. The sediment concentration in the calibration tank, ctank, is given as ctank =
M s s + V u−w Vtank 1− u−w u u
ð5Þ
where Vtank is the volume of the calibration tank; u, the flow velocity in the calibration tank; ws, the settling velocity of the sediments; M, the mass of the sediment in the system; and V, the total volume of water. In this case, the values of Vtank, V, and u are 0.01575 m3, 0.03 m3, and 0.027 m s− 1, respectively. The settling velocity was calculated according to Stokes' law. Calibration lines for sieved silica sand (90–106, 106–125, and 125– 150 μm) and glass beads (125–150 μm) were established for each sensor (Fig. 5). In Fig. 5, the calibration line for LV-H110 is adjusted to the clear water standard. The light absorbance measured with the two sensors is proportional to the sediment concentration, indicating that
Fig. 4. Schematic representation of the calibration tank used to measure calibration lines.
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the Lambert–Beer law is valid for water laden with suspended sand particles. The maximum measurable concentration of the two sensors (LV-H300 and LV-H110) is 6 to 8 kg m− 3 for a 15-cm optical pass length. Fig. 6 shows the calibration lines for the fiber optic sensor. The lines between the sensor output and concentration do not decline at 16 kg m− 3. This suggests that the fiber optic sensor can measure higher concentrations. Although the abovementioned conditions of materials, concentrations, and grain sizes do not completely correspond to range observed in natural environments, these sensors can be adapted for model experiments. The light absorbance and intensity of the scattered light increase as the grain size decreases. The light absorbance and intensity of the scattered light of the glass beads are lower than those of sand particles of the same size. This result suggests that the intensities of the transmitted and scattered light are influenced by differences in the material, grain size, etc., as suggested by previous studies (e.g., Ludwig and Hanes, 1990; Clifford et al., 1995; Bunt et al., 1999). This is an important issue that must be considered for the proper interpretation of data obtained from field observations for which the material and grain size of a sediment sample are unknown. In a model experiment, wherein the sediment materials are usually known and controlled, this problem can be solved by establishing calibration lines for any of the materials used in the experiments.
Fig. 6. Calibration lines for sieved silica sand (90–106, 106–125, and 125–150 μm) and glass beads (125–150 μm) measured with fiber optic sensor. Backscattered light intensity is shown in terms of formazin-equivalent value (NTU).
6. Suspended sediment measurement in the experimental channel After calibration, the sensors were tested in the actual experimental channel. A novel driving mechanism was developed to allow determination of vertical suspended sediment concentration profiles (Fig. 7). This profiler consists of a vertically movable stage with a stepping motor. Two sets of transmitters and receivers for the photoelectric sensor are installed on both the sides of the channel in the aluminium arm, which is fixed onto the movable stage (1-axis stage kit, Originalmind Inc., Japan). The stepping motor (motor: KH42JM2-901 and motor driver: FSD2U2P12-01, Japan Servo Co., Ltd., Japan) used here rotates at 1.8° per pulse (corresponding to a movement of 0.25 mm of the stage movement), and is controlled by the PLC device referred to above.
Fig. 5. Calibration lines for sieved silica sand (90–106, 106–125, and 125–150 μm) and glass beads (125–150 μm) measured with photoelectric sensor. Solid symbols indicate values measured with LV-H300 and open symbols indicate values measured with LVH110.Values of LV-H100 are corrected to clear water standard.
The performance of the vertical profiler with the photoelectric sensor and fiber optic sensor was checked using an experimental channel (Fig. 7) made of transparent plastic (length: 6.5 m ; width and height: 0.15 m). Two tanks are located at the upstream and downstream ends of the channel for pooling water. The upstream tank, which is made of stainless steel, has a structure similar to that of the calibration tank for promoting the uniform dispersion of sand. A water pump is used for transporting water to the upstream tank from the downstream tank. Sand is supplied to the upstream tank by a sand feeder. Photoelectric sensors were installed at 0.75 m and 4.00 m from the upstream end. The sand-turbid water was sampled with a sampler made of “corrugated plastic cardboard” (Fig. 7). This board has parallel layered holes. The water sampler was prepared by cutting this board in a size of 10 cm × 13 cm. The thickness of this sampler was 4 mm and the thickness of the layers was approximately 4.67 mm. This sampler can vertically capture suspended sediments in each hole, as shown in Fig. 7. The sediments in each hole were collected and measured after drying in an oven. This sampler was used 25 cm downstream from the photoelectric sensors. A fiber optic sensor and siphon were installed in the upstream tank to measure the sediment concentration of the water flowing out from the tank. The slope of the channel was set at 0.01. Water discharge was approximately 4.8 l s− 1. The average flow velocity was approximately 0.97 m s− 1. Sieved silica sand (106–125 μm) was used for this experiment. We started the experiment by using clear water (c = 0). Temporal changes in the sediment concentration were measured by supplying sand to the upstream tank at a constant rate (0.009 kg s− 1). Here, the movable stage was driven at a speed of 20 mm s− 1 and the data were recorded every 0.1 s, which corresponds to a 2 mm spatial resolution of the vertical profile. Fig. 8 shows the vertical profiles of the suspended sediment concentrations measured using the photoelectric sensor and water sampling. There are good agreements between the two concentrations except at the uppermost and the lowermost sections. The resolution of the vertical profiles measured with the photoelectric sensor is higher than that of the profiles measured by water sampling. The resolution of a profile can be increased by using a faster sampling rate for the logger. The advantage of this system is the high resolution and non-intrusive nature of the measurement. However, some problems are encountered: vertical profiles obtained with the photoelectric sensor have a large peak
S. Ochiai, K. Kashiwaya / Catena 83 (2010) 1–6
5
Fig. 7. Schematic representation of experimental channel, vertical profiler for suspended sediment concentration and water sampler made of “corrugated plastic cardboard”.
at approximately 40 mm (Fig. 8a) and 33 mm (Fig. 8b) above the bed. These peaks result from scattering at the water surface. The concentration obtained with the sensor near the bed tends to be less than that obtained with the sampler. This may be due to the fact that the photoelectric sensor is incapable of measuring concentrations that are higher than 6–8 kg m− 3. This problem might be overcome by using a photoelectric sensor with a stronger laser. If these problems are overcome, more accurate measurements will become possible. Fig. 9 shows the relationship between the suspended sediment concentration measured with the fiber optic sensor and the siphon in the upstream tank. The concentration measured with the fiber optic
sensor is well related to the siphon sampling. However, values measured with the fiber optic sensor are approximately 25% higher than those measured with the siphon. This disagreement probably results from the difference in optical conditions between the calibration tank and the upstream tank (size, material, etc.). Further work is needed to investigate this problem, although we can use a fiber optic sensor calibrated on the basis of the siphon sampling data. Fig. 10 shows the temporal changes in the suspended sediment concentration measured with the fiber optic sensor in the upstream tank. These data were calibrated using the siphon data. The sediments were supplied at intervals of t = 125 to 500 s. The changes in the
Fig. 8. Results of the comparative experiments. Vertical profiles of suspended sediment concentrations measured using photoelectric sensor (solid lines and solid circles) and water sampling (dashed lines and open circles) at (a) 0.75 m and (b) 4 m from upstream end.
6
S. Ochiai, K. Kashiwaya / Catena 83 (2010) 1–6
Fig. 10. Temporal changes in suspended sediment concentration measured with fiber optic sensor in the upstream tank.
support in designing and constructing the experimental channel. This study was supported by a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science and the 21stCentury COE Program of Kanzawa University. Fig. 9. Relationship between suspended sediment concentrations measured with fiber optic sensor and siphon in the upstream tank.
concentration corresponding to the sediment supply were well recorded by using the fiber optic sensor. These results suggest that general-purpose optical sensors can be applied to the measurement of suspended sediments in a small-scale model experiment. Although it is necessary to carry out further studies to refine the response of these sensors, the high cost performance of these sensors has been found to be effective for model experiments. 7. Conclusions 1. The application of general-purpose optical sensors (photoelectric sensor and fiber optic sensor) for the measurement of suspended sediments shows satisfactory results in terms of their response to turbidity. The values of light absorbance and intensity of backscattered light for turbid standard solutions measured with these sensors show a linear relationship with the turbidity. 2. The calibration lines for sand particles with a large settling velocity were established by using a calibration tank containing upwelling water. Sensor output was found to be influenced by differences in the material and grain size of sand. Therefore, calibration lines should be established for any material used in experiments. 3. The sediment concentrations measured with the photoelectric sensor and the fiber optic sensor corresponded well to those measured by water sampling in an artificial channel; however, there were some disagreements; these are attributed to the differences in the optical conditions between the calibration tank and the upstream tank. Although this problem can be avoided by additionally using other methods such as water sampling, it is necessary to further investigate this issue to refine this measurement system. Acknowledgments We thank our colleagues at the hydrogeomorphological laboratory of Kanazawa University. We also thank Prof. T. Okimura of Kobe University and Murayama Corporation (Kanazawa, Japan) for their
References Alexander, J., McLelland, S.J., Gray, T.E., Vincent, C.E., Leeder, M.R., Ellett, S., 2008. Laboratory sustained turbidity currents form elongate ridges at channel mouths. Sedimentology 55, 845–868. Bunt, J.A.C., Larcombe, P., Jago, C.F., 1999. Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter. Continental Shelf Research 19, 1199–1220. Campbell, C.G., Laycak, D.T., Hoppes, W., Tran, N.T., Shi, F.G., 2005. High concentration suspended sediment measurements using a continuous fiber optic in-stream transmissometer. Journal of Hydrology 311, 244–253. Chikita, K., Smith, N.D., Yonemitsu, N., Perez-Arlucea, M., 1996. Dynamics of sedimentladen underflows passing over a subaqueous sill: glacier-fed Peyto lake, Alberta, Canada. Sedimentology 43, 865–875. Clifford, N.J., Richards, K.S., Brown, R.A., Lane, S.N., 1995. Laboratory and field assessment of an infrared turbidity probe and its response to particle size and variation in suspended sediment concentration. Hydrological Sciences Journal 40, 771–791. Cloutier, D., LeCouturier, M.N., Amos, C.L., Hill, P.R., 2006. The effects of suspended sediment concentration on turbulence in an annular flume. Aquatic Ecology 40, 555–565. Langlois, J.L., Johnson, D.W., Mehuys, G.R., 2005. Suspended sediment dynamics associated with snowmelt runoff in a small mountain stream of Lake Tahoe (Nevada). Hydrological Processes 19, 3569–3580. Li, X., Lei, T., Wang, W., Xu, Q., Zhao, J., 2005. Capacitance sensors for measuring suspended sediment concentration. Catena 60, 227–237. Ludwig, K.A., Hanes, D.M., 1990. A laboratory evaluation of optical backscatterance suspended solids sensors exposed to sand–mud mixtures. Marine Geology 94, 173–179. Mateos, L., Giráldez, J.V., 2005. Suspended load and bed load in irrigation furrows. Catena 64, 232–246. Orwin, J.F., Smart, C.C., 2005. An inexpensive turbidimeter for monitoring suspended sediment. Geomorphology 68, 3–15. Radice, A., Malavasi, S., Ballio, F., 2006. Solid transport measurements through image processing. Experiments in Fluids 41, 721–734. Schlaberg, H.I., Baas, J.H., Wang, M., Best, J.L., Williams, R.A., Peakall, J., 2006. Electrical resistance tomography for suspended sediment measurements in open channel flows using a novel sensor design. Particle and Particle Systems Characterization 23, 313–320. Sutherland, T.F., Lane, P.M., Amos, C.L., Downing, J., 2000. The calibration of optical backscatter sensors for suspended sediment of varying darkness levels. Marine Geology 162, 587–597. Van de Hulst, H.C., 1981. Light Scattering by Small Particles. Dover, New York. Wiberg, P.L., Drake, D.E., Cacchione, D.A., 1994. Sediment resuspension and bed armoring during high bottom stress events on the northern California inner continental shelf: measurements and predictions. Continental Shelf Research 14, 1191–1219. Wren, D.G., Barkdoll, B.D., Kuhnle, R.A., Derrow, R.W., 2000. Field techniques for suspended-sediment measurement. Journal of Hydraulic Engineering 126, 97–104.
Contents lists available at ScienceDirect
Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a
Measurement of suspended sediment for model experiments using general-purpose optical sensors Shinya Ochiai ⁎, Kenji Kashiwaya Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
a r t i c l e
i n f o
Article history: Received 1 February 2010 Received in revised form 22 June 2010 Accepted 22 June 2010 Keywords: Suspended sediment concentration Model experiment Photoelectric sensor Fiber optic sensor
a b s t r a c t A specialized measurement system for measuring suspended sediment concentration in model experiments was developed; it utilizes general-purpose optical sensors to perform measurements. A photoelectric sensor was used for measuring the transmitted light, and a fiber optic sensor was used for measuring the backscattered light. The values of the light absorbance and intensity of backscattered light for turbid standard solutions measured with these sensors show a linear relationship with the turbidity. The calibration lines for sand particles with a large settling velocity were established by using a calibration tank containing upwelling water. The vertical profiles of the sediment concentration measured with the photoelectric sensor corresponded well to those of the sediment concentration measured with a water sampler in an artificial channel. This result suggests that the measurement system using a photoelectric sensor functions satisfactorily. The sediment concentrations measured using the fiber optic sensor correlate well with the concentrations measured through siphon sampling; however, the values measured with the fiber optic sensor are approximately 25% larger than those measured with the siphon. This disagreement may result from the differences in the optical conditions between the calibration tank and the actual channel. Thus, it may be possible to utilize fiber optic sensors calibrated on the basis of siphon sampling for the measurement of sediment concentration. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In order to investigate the sediment transportation in rivers, lakes, and oceans, it is important to measure the turbidity and the amount of suspended sediment in a particular body of water. There are many methods to measure turbidity and sediment concentration, e.g., water sampling, the optical method, and the acoustic method (Wren et al., 2000). Further, many commercial instruments such as turbidity meters and suspended sediment sensors are available for performing such measurements. Previous studies have used the abovementioned instruments for field observations (e.g., Wiberg et al., 1994; Chikita et al., 1996; Langlois et al., 2005), and the need for a technical examination for the proper interpretation of data has been proposed (e.g., Ludwig and Hanes, 1990; Clifford et al., 1995; Bunt et al., 1999). Previous studies have also proposed measuring sediment concentration using new methods, including electrical resistance tomography (Schlaberg et al., 2006) and image processing (Radice et al., 2006) and custom-built devices, including optical sensors (Campbell et al., 2005; Orwin and Smart, 2005) and capacitance sensors (Li et al., 2005). The measurement of suspended sediment concentration is also important for model experiments (e.g., Mateos and Giráldez, 2005; ⁎ Corresponding author. Present address: Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, O 24, Wake-machi, Nomishi, Ishikawa 923-1224, Japan. Tel.: +81 761514440; fax: +81 761515528. E-mail address: [email protected] (S. Ochiai). 0341-8162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2010.06.008
Cloutier et al., 2006; Alexander et al., 2008). However, commercial instruments, because of their size and configuration, are not always suitable for small-scale model experiments. For instance, most field turbidity meters have a large probe. In natural rivers, lakes, and oceans, the disturbances caused by the probe can be ignored. However, it would disturb the water flow in a small-scale experiment, and the large volume of the sensor results in a decrease in the resolution of the measurement. Furthermore, electronic techniques need to be used to design custom-built devices. Therefore, in this study, we attempted to develop a measuring device using low-cost mass-produced instruments that are mainly used for FA (factory automation). Among the methods that have been proposed, the optical method (including the transmitted light and scattered light methods) is the easiest to handle using general-purpose sensors. First, the transmitted light and the scattered light methods carried out using FA instruments are discussed. Second, the examination of the sensors by using an experimental channel is described. 2. Transmitted light method This method is widely used in the adsorption spectrophotometry of chemical substances and in turbidity meters. The intensity of transmitted light in water is given by the Lambert–Beer law: −kcl
I = I0 e
ð1Þ
2
S. Ochiai, K. Kashiwaya / Catena 83 (2010) 1–6
where I is the intensity of the transmitted light: I0, the intensity of incident light; c, the sediment concentration; l, the optical pass length; and k, a constant. If light absorbance A is introduced as A = log10
I0 I
ð2Þ
then, Eq. (1) is rewritten as A = ðlog10 eÞ·kcl = acl
ð3Þ
where a = k log10e = 0.4343k is the extinction coefficient. If the optical pass length is constant, the light absorbance will be proportional to the sediment concentration. If the Lambert–Beer law is valid and the extinction coefficient a is known, the sediment concentration can be estimated from the observed absorbance value. A set of light source and detector is necessary to measure the sediment concentration on the basis of the Lambert–Beer law. In this study, we use a photoelectric sensor, which is one of the FA instruments, along with a laser light source and detector (LV-series, KEYENCE Corp., Japan). This sensor detects objects in the measuring zone from the intensity of the transmitted light. The intensity of the transmitted light can be outputted as a voltage signal. This kind of sensor is available for the measurement of the sediment concentration. A laser light source has an advantage of high spatial resolution because of its high directivity. Among the various models of photoelectric sensors available (shape and width of laser beam, etc.), we adopted one with an amplifier unit (LV-51M) and two sensors (LV-H300: 30mm-width sheet beam; LV-H110: 10-mm-width beam; wave length: 650 nm) (Fig. 1a). The sensor system consists of a pair of transmitter units, a receiver unit and an amplifier unit that converts the intensity of the transmitted light to a voltage signal. LV-H300 and LV-H110 have their own sensitivity range. Therefore, they can cover a wide range of sediment concentrations. When energizing the amplifier unit, the transmitter unit emits a laser beam and the light intensity detected by the receiver unit will be outputted as a voltage in the range 1–4 V. The voltage logger used here is a PLC (KV-700, KEYENCE Corp., Japan); this unit is capable of processing input signals from various sensors, switches, and controlling motors, solenoid valves, etc. 3. Scattered light method Scattered light is also employed for measurements using turbidity meters, variously utilizing either backscattered or side scattered light. Here, we will focus on backscattered light. The intensity of the scattered light is expressed in the following equation (Van de Hulst, 1981; Sutherland et al., 2000): F=
3 VcEQ s 2 ρD
ð4Þ
where F is the flux scattered by the suspended particles; E, the irradiance of the light source; V, the scattering volume; c, the sediment concentration; D, the particle diameter; ρ, the particle density; and Q s, the scattering efficiency. The intensity of the scattered light will be proportional to the sediment concentration if the other variables are constant. A fiber optic sensor, which is one of the FA instruments used here, can be used as a backscatter sensor for measuring the sediment concentration. This particular sensor was originally used to detect objects by means of scattered light. Emitted light from the source is led through the optical fiber and radiated from its end. The scattered light from the objects is also led to a detector through the optical fiber. The advantage of this sensor is that the transmitting part and the receiving part can be freely handled owing to the fiber flexibility. In this study, we adopted an amplifier unit (FS-V20, KEYENCE Corp., Japan) and a waterproof fiber sensor head (FU-91) (Fig. 1b). The
Fig. 1. (a) Photoelectric sensors (LV-series, KEYENCE Corporation, Japan). Front: the transmitter and the receiver unit of LV-H110. Middle: LV-H300. Rear: Amplifier unit LV51M. (b) Fiber optic sensor. The amplifier unit (FS-V20, KEYENCE Corporation, Japan) and waterproof fiber sensor head (FU-91). The end of fiber unit has two holes (transmitting part and receiving part).
amplifier unit includes the light source (red light emitting diode; wave length: 640 nm) and detector. This fiber head unit can be used in water, oils, and chemicals because it is sheathed with Teflon. This unit includes two optical fibers (transmitting and receiving). The end of fiber unit has a transmitting part and a receiving part. The amplifier converts the intensity of the scattered light into a voltage signal (1– 5 V) that is recorded by the voltage logger. 4. Sensor responses to turbidity standard The response characteristics of the abovementioned sensors to a widely used turbidity standard, formazin, were checked to evaluate the response of the sensors to variation in turbidity. The light absorbance and the intensity of the scattered light for various turbidity levels of formazin (expressed in Nephelometric Turbidity Units, NTU) were measured with a photoelectric sensor and fiber optic sensor, respectively. The optical pass length of the transmitted light was 15 cm. The transmitted light intensity of clear water is assumed to be I0 for the LV-H300. The transmitted light intensity of a blackened smooth plastic board is assumed to be the incident light intensity I0 for LV-H110 because the transmitted light intensity of
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clear water is out of range for LV-H110. As a result, the zero absorbance exhibited by LV-H110 is considered to be the absorbance of the plastic board. The intensity of the backscattered light measured by the fiber optic sensor is measured in terms of voltage. The turbidity of the formazin solution was also measured by using a turbidity meter (W-22XD, HORIBA, Ltd., Japan) for crosschecking. Fig. 2 shows the light absorbance for various turbidity levels of the formazin solution measured with the photoelectric sensor. The light absorbance is proportional to the turbidity of formazin at comparatively low values, indicating that the Lambert–Beer law is valid in this range. For a high turbidity, the absorbance value did not increase because the transmitted light was very weak. The measurable turbidity ranges for LV-H300 and LV-H110 for the 15-cm pass length are 0 to 60 NTU and 60 to 120 NTU, respectively. Thus, the two sensors will cover a range of 0 to 120 NTU. Fig. 3 indicates the scattered light intensity for various turbidity levels of the formazin solution measured with two fiber optic sensors. This supports the fact that the scattered light intensity (sensor output) is proportional to the turbidity, as given by Eq. (4), the linear relationship breaks down at high values of turbidity. This may be result from the change in the scattering volume V. However, the abovementioned relationship can be closely expressed by a quadratic equation. This implies that the sensor is practically suitable for a turbidity meter. Fig. 3 also suggests that the sensors have their own response characteristics to turbidity, indicating that fiber optic sensors have to be calibrated before every use by using a formazin solution. 5. Calibration lines for sediment suspensions To measure the sediment concentration with photoelectric sensors and fiber optic sensors, calibration lines indicating the relationship between sensor output and sediment concentration must be established. However, owing to the large settling velocity of the sand particles, it is difficult to generate uniformly dispersed sand currents. Therefore, a new calibration tank equipped with a pump was introduced for promoting uniform dispersion (Fig. 4). This system consists of a transparent upright tank (calibration tank) to measure the light absorbance, a reserve tank, and pumps. The sand particles are uniformly dispersed at a flow velocity that is greater than the settling velocity. The optical conditions (material and thickness of the upright tank, optical pass length, etc.) for this calibration tank should be
Fig. 2. Light absorbance for various turbidity levels of formazin solution measured by photoelectric sensor. Circles indicate values measured with LV-H300 and triangles indicate values measured with LV-H110.
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Fig. 3. Intensity of backscattered light intensity for various turbidity levels of formazin solution measured by fiber optic sensor.
identical to those in the actual measurement (experimental channel etc.). In this study, the optical pass length of this tank is 15 cm. The sediment concentration in the calibration tank, ctank, is given as ctank =
M s s + V u−w Vtank 1− u−w u u
ð5Þ
where Vtank is the volume of the calibration tank; u, the flow velocity in the calibration tank; ws, the settling velocity of the sediments; M, the mass of the sediment in the system; and V, the total volume of water. In this case, the values of Vtank, V, and u are 0.01575 m3, 0.03 m3, and 0.027 m s− 1, respectively. The settling velocity was calculated according to Stokes' law. Calibration lines for sieved silica sand (90–106, 106–125, and 125– 150 μm) and glass beads (125–150 μm) were established for each sensor (Fig. 5). In Fig. 5, the calibration line for LV-H110 is adjusted to the clear water standard. The light absorbance measured with the two sensors is proportional to the sediment concentration, indicating that
Fig. 4. Schematic representation of the calibration tank used to measure calibration lines.
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the Lambert–Beer law is valid for water laden with suspended sand particles. The maximum measurable concentration of the two sensors (LV-H300 and LV-H110) is 6 to 8 kg m− 3 for a 15-cm optical pass length. Fig. 6 shows the calibration lines for the fiber optic sensor. The lines between the sensor output and concentration do not decline at 16 kg m− 3. This suggests that the fiber optic sensor can measure higher concentrations. Although the abovementioned conditions of materials, concentrations, and grain sizes do not completely correspond to range observed in natural environments, these sensors can be adapted for model experiments. The light absorbance and intensity of the scattered light increase as the grain size decreases. The light absorbance and intensity of the scattered light of the glass beads are lower than those of sand particles of the same size. This result suggests that the intensities of the transmitted and scattered light are influenced by differences in the material, grain size, etc., as suggested by previous studies (e.g., Ludwig and Hanes, 1990; Clifford et al., 1995; Bunt et al., 1999). This is an important issue that must be considered for the proper interpretation of data obtained from field observations for which the material and grain size of a sediment sample are unknown. In a model experiment, wherein the sediment materials are usually known and controlled, this problem can be solved by establishing calibration lines for any of the materials used in the experiments.
Fig. 6. Calibration lines for sieved silica sand (90–106, 106–125, and 125–150 μm) and glass beads (125–150 μm) measured with fiber optic sensor. Backscattered light intensity is shown in terms of formazin-equivalent value (NTU).
6. Suspended sediment measurement in the experimental channel After calibration, the sensors were tested in the actual experimental channel. A novel driving mechanism was developed to allow determination of vertical suspended sediment concentration profiles (Fig. 7). This profiler consists of a vertically movable stage with a stepping motor. Two sets of transmitters and receivers for the photoelectric sensor are installed on both the sides of the channel in the aluminium arm, which is fixed onto the movable stage (1-axis stage kit, Originalmind Inc., Japan). The stepping motor (motor: KH42JM2-901 and motor driver: FSD2U2P12-01, Japan Servo Co., Ltd., Japan) used here rotates at 1.8° per pulse (corresponding to a movement of 0.25 mm of the stage movement), and is controlled by the PLC device referred to above.
Fig. 5. Calibration lines for sieved silica sand (90–106, 106–125, and 125–150 μm) and glass beads (125–150 μm) measured with photoelectric sensor. Solid symbols indicate values measured with LV-H300 and open symbols indicate values measured with LVH110.Values of LV-H100 are corrected to clear water standard.
The performance of the vertical profiler with the photoelectric sensor and fiber optic sensor was checked using an experimental channel (Fig. 7) made of transparent plastic (length: 6.5 m ; width and height: 0.15 m). Two tanks are located at the upstream and downstream ends of the channel for pooling water. The upstream tank, which is made of stainless steel, has a structure similar to that of the calibration tank for promoting the uniform dispersion of sand. A water pump is used for transporting water to the upstream tank from the downstream tank. Sand is supplied to the upstream tank by a sand feeder. Photoelectric sensors were installed at 0.75 m and 4.00 m from the upstream end. The sand-turbid water was sampled with a sampler made of “corrugated plastic cardboard” (Fig. 7). This board has parallel layered holes. The water sampler was prepared by cutting this board in a size of 10 cm × 13 cm. The thickness of this sampler was 4 mm and the thickness of the layers was approximately 4.67 mm. This sampler can vertically capture suspended sediments in each hole, as shown in Fig. 7. The sediments in each hole were collected and measured after drying in an oven. This sampler was used 25 cm downstream from the photoelectric sensors. A fiber optic sensor and siphon were installed in the upstream tank to measure the sediment concentration of the water flowing out from the tank. The slope of the channel was set at 0.01. Water discharge was approximately 4.8 l s− 1. The average flow velocity was approximately 0.97 m s− 1. Sieved silica sand (106–125 μm) was used for this experiment. We started the experiment by using clear water (c = 0). Temporal changes in the sediment concentration were measured by supplying sand to the upstream tank at a constant rate (0.009 kg s− 1). Here, the movable stage was driven at a speed of 20 mm s− 1 and the data were recorded every 0.1 s, which corresponds to a 2 mm spatial resolution of the vertical profile. Fig. 8 shows the vertical profiles of the suspended sediment concentrations measured using the photoelectric sensor and water sampling. There are good agreements between the two concentrations except at the uppermost and the lowermost sections. The resolution of the vertical profiles measured with the photoelectric sensor is higher than that of the profiles measured by water sampling. The resolution of a profile can be increased by using a faster sampling rate for the logger. The advantage of this system is the high resolution and non-intrusive nature of the measurement. However, some problems are encountered: vertical profiles obtained with the photoelectric sensor have a large peak
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Fig. 7. Schematic representation of experimental channel, vertical profiler for suspended sediment concentration and water sampler made of “corrugated plastic cardboard”.
at approximately 40 mm (Fig. 8a) and 33 mm (Fig. 8b) above the bed. These peaks result from scattering at the water surface. The concentration obtained with the sensor near the bed tends to be less than that obtained with the sampler. This may be due to the fact that the photoelectric sensor is incapable of measuring concentrations that are higher than 6–8 kg m− 3. This problem might be overcome by using a photoelectric sensor with a stronger laser. If these problems are overcome, more accurate measurements will become possible. Fig. 9 shows the relationship between the suspended sediment concentration measured with the fiber optic sensor and the siphon in the upstream tank. The concentration measured with the fiber optic
sensor is well related to the siphon sampling. However, values measured with the fiber optic sensor are approximately 25% higher than those measured with the siphon. This disagreement probably results from the difference in optical conditions between the calibration tank and the upstream tank (size, material, etc.). Further work is needed to investigate this problem, although we can use a fiber optic sensor calibrated on the basis of the siphon sampling data. Fig. 10 shows the temporal changes in the suspended sediment concentration measured with the fiber optic sensor in the upstream tank. These data were calibrated using the siphon data. The sediments were supplied at intervals of t = 125 to 500 s. The changes in the
Fig. 8. Results of the comparative experiments. Vertical profiles of suspended sediment concentrations measured using photoelectric sensor (solid lines and solid circles) and water sampling (dashed lines and open circles) at (a) 0.75 m and (b) 4 m from upstream end.
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Fig. 10. Temporal changes in suspended sediment concentration measured with fiber optic sensor in the upstream tank.
support in designing and constructing the experimental channel. This study was supported by a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science and the 21stCentury COE Program of Kanzawa University. Fig. 9. Relationship between suspended sediment concentrations measured with fiber optic sensor and siphon in the upstream tank.
concentration corresponding to the sediment supply were well recorded by using the fiber optic sensor. These results suggest that general-purpose optical sensors can be applied to the measurement of suspended sediments in a small-scale model experiment. Although it is necessary to carry out further studies to refine the response of these sensors, the high cost performance of these sensors has been found to be effective for model experiments. 7. Conclusions 1. The application of general-purpose optical sensors (photoelectric sensor and fiber optic sensor) for the measurement of suspended sediments shows satisfactory results in terms of their response to turbidity. The values of light absorbance and intensity of backscattered light for turbid standard solutions measured with these sensors show a linear relationship with the turbidity. 2. The calibration lines for sand particles with a large settling velocity were established by using a calibration tank containing upwelling water. Sensor output was found to be influenced by differences in the material and grain size of sand. Therefore, calibration lines should be established for any material used in experiments. 3. The sediment concentrations measured with the photoelectric sensor and the fiber optic sensor corresponded well to those measured by water sampling in an artificial channel; however, there were some disagreements; these are attributed to the differences in the optical conditions between the calibration tank and the upstream tank. Although this problem can be avoided by additionally using other methods such as water sampling, it is necessary to further investigate this issue to refine this measurement system. Acknowledgments We thank our colleagues at the hydrogeomorphological laboratory of Kanazawa University. We also thank Prof. T. Okimura of Kobe University and Murayama Corporation (Kanazawa, Japan) for their
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