is taken with the sensors connected normally and one taken with the sensor 

 connections reversed, it takes slightly more than 200 msec to obtain one ve- 

 locity from each transducer pair. The travel time of the acoustic pulse is 

 1.33 nsec per cm/sec of velocity component. The two measurements are 

 subtracted and then digitized by an analog-to-digital (A/D) converter to give a 

 signed 12-bit word with a scale factor of 0.03 cm/sec per bit. The full-scale 

 range of a sensor pair is 120 cm/sec (hexadecimal 0FFF). 



The difference of the two measurements is obtained to remove electronic 

 drift in the amplifiers, comparators, integrators, and A/D converter. The 

 output on the signal line is a 9,600-baud RS232 UART in which the digitized 

 range of each velocity is from -4,096 bits to +4,095 bits (hexadecimal F000 

 to 0FFF where 0000 is zero and FFFF is -1). The first hexadecimal digit, 

 or F, represents the sign of the velocity component. If the signal from one 

 transducer is not received within a window of time, corresponding to 

 5 percent of 1,500 m/sec, a flag with hexadecimal value 8000 is written to the 

 output. The cause of such a null response is the attenuation of sound by an 

 acoustic inhomogeneity such as a bubble or other object within the sampling 

 volume. 



Each measurement from a transducer pair must be adjusted to account for a 

 zero offset which is caused by capacitance in the cables from the transducers 

 and the BASS electronics canister. The offset is different for each configura- 

 tion of the cables and is obtained by stilling the medium within the sampling 

 volume and taking measurements for several minutes. Changes in temperature 

 and pressure affect the offset values by a maximum of 0.3 cm/sec. 



Pressure sensor. The pressure transducer has a 0- to 5-V analog output 

 that is sampled for 5. 12 msec and digitized to an 8-bit number by an A/D 

 converter separate from that used for the ACP signals. The 0- to 5-psi (0- to 

 34.5-kPa) range of the device has an overall accuracy of 0.0125 psi 

 (0.086 kPa), but is digitized to 0.0195 psi (0.134 kPa) per bit increments. 

 The sampling sequence is the same as that for the ACP since it is controlled 

 by the same microprocessor. 



Experiment Procedures 



Sequence of events 



A typical wave run, including the preparations needed to collect the data, 

 is described: 



a. The offshore moveable carriage is moved into place. The position 

 along the length of the channel is chosen in consultation with other 

 members of the offshore group so that sensors will be aligned in 

 shore-parallel positions taking corroborative data. 



Chapter 9 The Ohio State University Measurements 



163 



