SUPERTANK Calibration 



Mobile OBS Array 



Concentration (g/l) 



Figure 7-2. Calibration curves for mobile OBS array 



Calibration of the capacitance wave gauge was a fairly simple process. 

 Markings were made on the mast in 3-in. (7.6-cm) increments. The mast was 

 then immersed in still water to the midpoint of the sensing lead, and a trim 

 pot on the circuit board was adjusted so that the output was V. A second 

 trim pot allowed the gain to be adjusted to a chosen value of 0. 1 V/in. 

 (0.04 V/cm). The calibration was verified by immersing the sensing lead to 

 different levels and measuring the output to ensure that the gain was constant 

 over the length of the sensing lead. 



Sampling and time reference 



The sampling rate chosen for the mobile OBS array was 10 Hz (identical to 

 that for the stationary OBS arrays). Ten samples per second was acceptable 

 resolution for the changes in sand concentration and kept the size of the data 

 records within the storage limitations of the PC used in data acquisition. 



A computer time standard system was installed to link the PC to Coordi- 

 nated Universal Time. The system consisted of a circuit board with a 10-Mhz 

 radio receiver, a high-precision clock, and a powered antenna and cable. The 

 clock was sampled with every data collection pass to ensure proper operation 

 of the data collection system. 



The capacitance wave gauge was sampled at a rate of 16 Hz. The A/D 

 converter received a voltage in the +5-V range, after which the voltage repre- 

 sentation was converted to inches of displacement from the still-water level, 

 prior to being saved to disk. 



128 



Chapter 7 Sediment Suspension Measurements from a Platform 



