218 



experiment did not call for this capability, however. This measurement 

 became available strictly on an opportunity basis. Two of these sensors were 

 used: one attached to the profiler, sensing scattering approximately 16 cm 

 above the LDV sample volume, and the other attached to the fixed member of 

 the profiler, 122 cm above the bed. 



Instrument calibration coefficients 



LDV. As described in the previous section, the calibration factor is 

 uniquely determined for a given geometric configuration of the transmitted 

 beams. For the LDV as configured for SUPERTANK, the calibration factor 

 was 1.6 kHz/cm/sec. The frequency resolution of the signal processor was 

 1.465 kHz/bin, i.e., 0.92 cm/sec per digitization step. The maximum measur- 

 able velocity was 128 bins, or 1.2 m/sec. 



BASS. The calibration factor for the BASS depends on the change in tran- 

 sit time across the 15-cm path of the acoustic beams and gain settings in the 

 electronics converting the delays to stored digitized bits. The manufacturer 

 has specified a calibration factor for the BASS to be 0.0207 cm/sec per 

 digitization step. The calibration factors were confirmed through tow-tank 

 measurements at QUEST'S 18-m facility. 



Pressure sensor. The static calibration of the pressure sensor was 

 performed by submerging the sensor in water at different depths. The result- 

 ing calibration factor was 0.028 cm per digitization step. 



OFS. No calibration was performed for the OFS at the time of report 

 preparation. The calibration for optical scattering devices depends upon the 

 size spectra producing the scattering. For this reason, we apply an empirical 

 calibration coefficient chosen to ensure that the LDV concentration 

 measurements are continuously matched to OFS data. 



Key instrument specifications and calibration coefficients are summarized 

 in Table 11-1. 



Experiment Procedures 



Sequence of events. To obtain the vertical velocity distribution, the LDV 

 sensor head must be positioned at a known distance from the sand bed. This 

 is achieved as follows. The LDV sensor head is first positioned at a large 

 distance (~50 cm) above the bed. Profiling down under computer control, 

 the photomultiplier current (scattering intensity) is monitored. As the sensor 

 head approaches the bed (from an earlier indication) the profiler moves the 

 sensor head in progressively shorter segments. When the sensor head reaches 

 the sand bed, intense scattering from the bed causes a sudden increase in the 

 photomultiplier current; this is recognized by the software as the location of 



Chapter 1 1 LDV in the Bottom Boundary Layer 



