between a denser sampling rate and the rate at which the internal binary 

 computations for the averaging are done most efficiently. 



At the time of execution of the data collection program within PCI, the 

 first task is to read the output of the installed WWV time circuit. This precise 

 time is stored as the first four bytes of the data file. Immediately following 

 this the data stream from the ACP microprocessor is accessed, since the ACP 

 is already actively taking data. Timing for sampling is provided by the sys- 

 tem microprocessor. Each 1 10-byte data segment for the profile represents a 

 second of elapsed time counting from the time taken from the WWV output. 



BASS. Controlling circuitry within the BASS electronics canister can be 

 programmed to sample up to 5 Hz, but for SUPERTANK deployment, 4 Hz 

 was judged to be adequate. For each of the four BASS frames, the four 

 transducer pairs were sampled sequentially, from A to D looking down from 

 the upper ring of a BASS frame (see Figure 9-7) at four 0.25-sec intervals. 

 The two measurements for each transducer pair are subtracted, and the differ- 

 ence in travel time between transducers is a flow velocity along that acoustic 

 beam path. For each 0.25 sec, the 16 two-byte hexadecimal data words for 

 each sample are sent to the data stream along with header and checksum 

 bytes. 



Similar to the case for the ACP, at the initiation of data collection, the 

 WWV time is read and stored to the data file as the first four bytes. After 

 that, each sample record represents an elapsed time of 0.25 sec. Timing for 

 each of the four BASS sensor arrays is controlled by its own circuit board, 

 and overall timing is controlled by the BASS microprocessor. 



Pressure sensor. The processing circuitry of the ACP is also programmed 

 to control the sampling rate of the pressure sensor. The sampling rate at 

 SUPERTANK was set to 2 Hz, although samples can be collected at up to 

 20 Hz. The passive nature of the pressure sensor means that a sample can be 

 obtained at any moment, and thus the instantaneous pressure is timed with the 

 ACP. Twice a second, at 0.5-sec intervals, the output of the pressure trans- 

 ducer was sampled and stored in a buffer until the whole ACP data record 

 was ready to be sent to the output data stream. 



Pre-processing and filtering 



Signals from the ARMS instruments were sampled, digitized, and trans- 

 ferred to storage under the control of two separate microprocessors. BASS 

 was controlled by a Tattletale IV manufactured by Onset Corporation, and the 

 ACP and pressure sensor were controlled by a CDP18S601 manufactured by 

 RCA Corporation. The dual microprocessor setup proved to be flexible in 

 being able to independently access the instrumentation via separate communi- 

 cation ports on PCI during SUPERTANK. 



' °" Chapter 9 The Ohio State University Measurements 



