Experiment Apparatus 



Instrument description 



Acoustic concentration profiler (ACP). The Simrad Mesotech Model 810 

 is a transceiver that transmits and receives 5-MHz underwater sound. The 

 sound pulse is generated every one-hundredth sec. Backscattered sound 

 energy is detected at the sensor and sampled at 250 kHz. The time delay 

 between transmission and reception of the pulse is proportional to the distance 

 from the sensor to sediment particles. Backscattered intensity is related to the 

 sediment concentration in the ensonified volume. Spatial resolution of the 

 measurement is limited by the pulse length and sampling rate. Temporal 

 resolution is related to the pulse repetition rate and to the number of profiles 

 averaged to produce statistically meaningful data. 



Optical backscatter sensor (OBS). The D & A Instrument Co. OBS-1 

 consists of a high-intensity infrared emitting diode (TRED), a detector (four 

 photo diodes), and a linear, solid-state temperature transducer. The IRED 

 produces a beam with half-power points at 50 deg (0.87 rad) in the axial plane 

 of the sensor and at 50 deg (0.87 rad) in the radial plane. The detector 

 integrates IR scattered between 140 and 160 deg (2.44 and 2.79 rad). The 

 integrated intensity is linearly proportional to the sediment concentration in the 

 vicinity of the sensor. This sensor measures the time series of sediment con- 

 centration at one level. 



Electromagnetic current meter (EMCM). The Marsh-McBirney Model 

 OEM521 current meter consists of an alternating current electromagnet within 

 a 3.8-cm-diam ball and two pairs of electrodes. The water flowing past the 

 ball interacts with the magnetic field generated by the magnet and produces a 

 voltage that is proportional to the velocity of the water. The two pairs of 

 electrodes detect the voltage and resolve the velocity vector into two 

 components. 



Pressure sensor. The Trans Metrics P21 pressure transducer is the 

 bonded strain gauge type, where strain gauges detect strains caused by 

 pressure. It measures absolute pressure. 



Pore pressure sensor. The Druck miniature pore water pressure 

 transducer type PDCR 81 is a wet-dry differential pressure gauge with a 

 porous filter plate at the wet end to resist the effective stress of bed soil. The 

 wet end is exposed to bed soil and the dry end is kept dry in the housing. 

 Back pressure is established by compressing the dry air in an underwater 

 housing with a miniature latex balloon, which is installed inside the housing 

 and inflated by hydrostatic water pressure. The strain of the diaphragm 

 caused by the difference in pressure between the ends is detected by strain 

 gauges. 



Chapter 10 Intermittent Near-Bed Sediment Suspension 



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