Data collection began approximately 45 sec before the wave generator was 

 started, so that still-water offsets for the wave gauge and OBS array could be 

 determined. The video camera was manually started at the beginning of the 

 run and did not require any further adjustment. While the run was under 

 way, the sensors could be monitored in real-time using graphical displays of 

 their output. 



After the wave generator was shut down at the end of each run, data were 

 collected for an additional 45 sec to measure the wash load. The distance 

 from the bed to the bottom of the OBS array was measured again. Once the 

 sensor elevation was recorded, electrical connections were undone. The OBS 

 array was raised to avoid other instruments or sandbars, and the carriage was 

 moved to the seaward end of the channel in preparation for beach profile 

 surveying. 



Calibration and zero reference 



Calibration of the OBS was accomplished by immersing the individual 

 sensors into a tank containing a sand-water mixture. The mixture was stirred 

 continuously, to promote uniform turbidity in the region of the sensor. 

 Sensor data were collected starting at a relatively low concentration, from 

 which a mean sensor reading could later be determined. A pump sample was 

 then drawn from the tank, before sand was added to increase the concentra- 

 tion. This process was then repeated, incrementally increasing the 

 concentration in the calibration tank. Each sample was decanted and the 

 volume of water recorded. The concentration at each incremental level was 

 determined by drying the sand and measuring the weight with a laboratory 

 balance. With this information, the calibration parameters for the individual 

 OBS were established. The OBS has been shown to be highly linear, so a 

 simple regression analysis provided the gain and offset for each sensor. Table 

 7-1 and Figure 7-2 present the calibration constants and curves, respectively. 



1 Table 7-1 

 Mobile OBS Array Calibration Constants 



1 SENSOR* 



GAIN |g/l/V) 



OFFSET |g/l) 



1 AAA 



49.83 



3.32 



BBB 



16.81 



-0.19 



CCC 



16.00 



0.08 



DDD 



22.57 



0.10 



EEE 



16.38 



-0.14 



1 " Sensor AAA is closest and sensor EEE is furthest from the bed. 



Chapter 7 Sediment Suspension Measurements from a Platform 



127 



