Optical backscatter point sensor (OBS)
This article is a summary of sub-section 188.8.131.52 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas. This article gives an introduction of an optical backscatter (OBS) point sensor, which can be used to measure turbidity and the concentration of suspended load.
The OBS is an optical sensor for measuring turbidity and suspended solids concentrations by detecting infra-red light scattered from suspended matter (see Figures 1A and 1B). The response of the OBS sensors strongly depends on the size, composition and shape of the suspended particles (see Figure 2). Battisto et al. (1999) show that the OBS response to clay of 2 um is 50 times greater than to sand of 100 um of the same concentration. Hence, each sensor has to be calibrated using sediment from the site of interest (see Figures 1 and 2). The measurement range for sand particles (in water free of silt and mud) is about 1 to 100 kg/m3. The sampling frequency generally is 2 Hz.
The OBS sensors consist of a high intensity infra-red emitting diode (IRED), a detector (four photodiodes), and a linear, solid state temperature transducer (Downing et al., 1981). The OBS sensor measures infra-red radiation scattered by particles in the water at angles ranging from 140° to 165°. Infra-red radiation from the sensor is strongly attenuated in clear water (more than 98% after travelling just 0.2 m), (D&A instruments, 1989). Therefore, even bright sunlight does not interfere with measurements made in shallow water. The diameter of the sensor is about 0.02 m (see Figure 1); the length is about 0.05 m. The IRED produces a beam with half power points at 50o in the axial plane of the sensor and 30o in the radial plane.The detector integrates IR-light scattered between 140o and 160o. Visible light incident on the sensor is absorbed by a filter. Sensor components are potted in glass-filled polycarbonate with optical-grade epoxy. The sensor gain of the OBS has to be adjusted in order to match the highest output voltage expected from the OBS during the measurements with the input span of the data logger. Undesirable results will be obtained if the gain is not correctly adjusted. When the gain is too high, data will be lost because the sensor output is limited by the supply voltage and will “saturate” before peaks in sediment concentration are detected. If the gain is too low, the full resolution of the data logger will not be utilized.
The performance of the OBS-sensor is claimed to be superior to most other in-situ turbidity sensors, because of: small size and sample volume, linear response and wide dynamic range, insensitivity to bubbles and phytoplankton, ambient light rejection and low temperature coefficient and low cost.
The OBS sensors are about the same size (or larger) as the length of gradients in the sand concentration being measured. This may cause hydrodynamic noise in the output signal because the turbulent flow around the sensor redistributes the particles in the water and increases the variation of sediment concentration above natural levels. Furthermore, the volume sampled by the OBS sensors depends on how far the IR beam penetrates into the water. This decreases as sediment concentration increases and so the sample volume is constantly varying with concentration which may also cause random noise in the output signal. From limited tests performed by the manufacturer it appeared unlikely that the random noise would exceed 30% of the mean signal in situations with high concentrations of coarse sediment. The manufacturer recommends post-processing the data with a low-pass filter to reduce the random noise in the output signal. Other noise in the output signal may be caused by electronic noise or environmental conditions. According to specifications, the electronic noise is insignificant for most applications. Some causes for environmental noise are: biofouling, excess in suspended sediment resulting from scour around instrument structures and cables moving in front of the OBS sensor with the currents.
Experiments have shown that the sensor gain varies with particle size. Ranging from mud (< 10 um) to sand (> 200 um) the gain decreases approximately by a factor 10. Hatcher et al. (2000) have used OBS sensors measuring at wavelengths of 442, 470, 510, 589, 620 and 671 nm with source beams originating from colour LED’s (six channel OBS; multi-spectral OBS) which can be used to measure concentrations of sediment mixtures (multiple grain sizes). This makes it possible to measure spectral responses of suspended particle concentrations across the optical range of wave lengths. Using the differential response of the backscatter coefficient of the suspended constituents at six wave lengths, an accurate estimation of concentration of mixtures can be obtained. This method is based on the simultaneous solution of linear equations that relate output of optical backscatter sensors to concentrations of various constituents of suspended sediments (see Green and Boon, 1993). The basic requirements are:
- linear sensor response to concentration of a particular sediment size,
- different sensor response to different sediment sizes and
- grain shielding and multiple scattering should be negligible.
The OBS sensors often show a reasonably steady offset concentration, which is related to the background concentration of relatively fine sediments (silt and mud). It is common practice to subtract this offset value from the original time series data. The offset can be defined as the minimum value of the data record (burst) or as the 1% to 5% lowest value of the signal. For example, Battisto et al (1999) found that the most appropriate cut-off voltage at the Duck site (USA) was 1% to 5% of the signal values.
Calibration results from Duck site, USA
Battisto et al. (1999) have made a comparison between OBS and pump sampler concentrations measured in the surf zone at the Duck site (USA) during October 1997. For this study, OBS sensors were calibrated separately using sand and mud collected at the Duck site. OBS voltage gain associated with mud was found to be an order of magnitude larger than that for sand. Based on this calibration, Battisto et al. show that the concentration of particles smaller than 63m pumped at the Duck site during October 1997 correspond to the lowest 1% to 5% of the output voltage recorded by the OBS sensors (background turbidity). The intake tubes of the pump sampler were positioned approximately 0.1 to 0.2 m above the bed. Calibrated OBS response above this background turbidity level was consistent with pumped sand concentration as long as corrections were made for
- varying size of suspended sand,
- the precise time of pump sampling,
- apparent noise in the OBS records.
Corrections for the smaller size of the suspended sand relative to that used during calibration resulted in a decrease of the OBS sand concentration by about 50%. Accounting for signal noise resulted in a decrease of the OBS sand concentration by about 0.05 to 0.2 kg/m3. Despite these corrections the OBS concentrations are considerably larger (factor 2 to 5) than the pump concentrations for sand concentrations smaller than 1 kg/m3. Hence, OBS data are unreliable for c<1 kg/m3.
Summaries of the manual
- Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas
- Chapter 1: Introduction, problems and approaches in sediment transport measurements
- Chapter 2: Definitions, processes and models in morphology
- Chapter 3: Principles, statistics and errors of measuring sediment transport
- Chapter 4: Computation of sediment transport and presentation of results
- Chapter 5: Measuring instruments for sediment transport
- Chapter 6: Measuring instruments for particle size and fall velocity
- Chapter 7: Measuring instruments for bed material sampling
- Chapter 8: Laboratory and in situ analysis of samples
- Chapter 9: In situ measurement of wet bulk density
- Chapter 10: Instruments for bed level detection
- Chapter 11: Argus video
- Chapter 12: Measuring instruments for fluid velocity, pressure and wave height
- Light fields and optics in coastal waters
- General principles of optical and acoustical instruments
- Optical Laser diffraction instruments (LISST)
- Acoustic point sensors (ASTM, UHCM, ADV)
- Acoustic backscatter profiling sensors (ABS)
- Chung, D.H. and Grasmeijer, B.T., 1999. Analysis of sand transport under regular and irregular waves in large-scale wave flume. Report R99-05, Department of Physical Geography, University of Utrecht.
- Connor, C.S. and De Visser, A.M., 1992. A laboratory investigation of particle size effects of an optical backscatterance sensor. Marine Geology, Vol. 108, p. 151-159.
- Van de Meene, J.W.H., 1994. The shoreface connected ridges along the central Dutch coast, The Netherlands, Doctoral Thesis, Utrecht University, Department of Physical Geography, The Netherlands.
- Rijn, L. C. van (1986). Manual sediment transport measurements. Delft, The Netherlands: Delft Hydraulics Laboratory
- Battisto, G.M., Friedrichs, C.T., Miller, H.C. and Resio, D.T., 1999. Response of OBS to mixed grain size suspensions during Sandy Duck’97. Coastal Sediment Conference 99, ASCE, New York. pp. 297-312.
- Downing, J.P., Sternberg, R.W. and Lister, C.R.B., 1981. New Instrument for the Investigation of Sediment Suspension Processes in the Shallow Marine Environment. Marine Geology, 42, p. 19-34.
- D and A Instruments, 1989. Optical Backscatterance Turbidity Monitor. Instruction Manual Tech. Note 3, 2428, 39th Street, N.W., Washington, D.C., 20007, USA.
- Hatcher, A., Hill, P., Grant, J. and Macpherson, P., 2000. Spectral optical backscatter of sand in suspension: effects of particle size, composition and colour. Marine Geology, Vol. 168, p. 115-128.
- Green, M.O. and Boon, J.D., 1993. The measurement of constituent concentrations in non homogeneous sediment suspensions using optical backscatter sensors. Marine Geology, Vol. 110, p. 73-81.
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