(to 0.01 sec) was also recorded to allow synchronization with other indepen- 

 dently recorded data sets. During A0509A - A0915A, the sampling time was 

 taken from the A/D clock, which was set "by hand" (to 0. 1 sec) to Coordinat- 

 ed Universal Time. For the remainder of the project runs, the sampling time 

 was taken from a precision clock installed in the PC and linked via radio (call 

 letters WWV) to Coordinated Universal Time. 



Pre-processing and filtering 



Prior to sampling, the analog signals are low-pass filtered to 20 Hz by a 

 one-pole passive filter. This filter reduces signals at 20 Hz by 3 db (50 per- 

 cent). There is no appreciable alteration of the 10-Hz signal of interest. 



Calibration and zero reference 



The sizes and refractive indices of sediment particles strongly influence 

 light scattering characteristics, and, consequently, OBS must be calibrated 

 with the sediment from the area where they were deployed. OBS calibrations 

 were performed in the laboratory using a recirculating tank from which pump 

 samples of the ambient concentration can be drawn (Downing and Beach 

 1989). The procedure consists of adding sediment, recording sensor output, 

 and drawing a pump sample. This procedure is repeated for increasing sedi- 

 ment concentrations until the range of sensor output in the field is covered. 

 After a calibration run, the water is decanted from each pump sample, and the 

 volume of water is recorded. The sediment is dried, and its weight and vol- 

 ume are determined. The sediment parameters are then used in conjunction 

 with the known volume of water to calculate a mass concentration. Response, 

 typically, is linear over a wide concentration range. Table 6-1 lists the gains 

 and linear regression coefficients calculated for the sensors used during 

 SUPERTANK. 



Offsets, or zero reference values, were determined for each sensor for each 

 run and are included in Table Fl, which is contained in Volume II of this 

 report. These offsets represent "clear" water values measured prior to the 

 start of wave action and are indicative of the background turbidity of the 

 channel. These offsets changed from run to run depending on the previous 

 wave action and on longer time scales as the channel was drained and refilled 

 for logistical reasons. Changes in background turbidity are seen by the 

 instruments as a constant shift. Because the gains of each sensor are different, 

 removal of this constant shift prior to conversion to scientific units is 

 essential. Consequently, the value in the offset column represents an 

 electronic offset seen by the instrument and is in A/D units. The offset value 

 was established for each sensor by taking the mean of the 30-sec 

 (approximate) data segment recorded prior to the official start time of each 

 run. Table Fl provides a listing of these offsets as a function of instrument 

 and run. 



Chapter 6 Suspended Sediment Concentration 



113 



