Campbell Scientific CSAT3B CSAT3B Three-Dimensional Sonic Anemometer - Page 56

CSAT3B Three-Dimensional Sonic Anemometer TABLE 8-1. Overview of CSAT3B Operating Modes. Mode 1 2 3 4 Measurement Trigger Filters Output Prompt Source Rate Enabled Bandwidth (Hz) Source Rate Datalogger 1 to 100 Hz1 No - Datalogger 1 to 100 Hz* CSAT3B 100 Hz Yes 5, 10, 20, 25 Datalogger 1 to 100 Hz* CSAT3B 100 Hz No - CSAT3B 10, 20, 50, 100 Hz CSAT3B 100 Hz Yes 5, 10, 20, 25 CSAT3B 10, 20, 50, 100 Hz 1 The exact rate is determined by the scan interval set in the CRBasic program of the datalogger. NOTE The options for datalogger-prompted output require use of SDM or CPI communications, while the options for unprompted output to a PC require use of RS-485 or USB communications. 8.2.1 Measurement Trigger A measurement trigger is the actual command to initiate a sonic measurement and can be driven by either a datalogger or the CSAT3B internal timer. If the trigger is given by a datalogger as in Mode 1, then no data filtering is done (bandwidth is wide open) since each trigger will initiate a single new measurement (single-measurement regime). The datalogger can trigger measurements at rates from 1 to 100 Hz. If the trigger is given by the internal CSAT3B timer as in Modes 2 ˗ 4, the measurements will be self-triggered at a rate of 100 Hz. This results in an oversampled measurement regime where the 100 Hz samples may be optionally processed through a filter before being downsampled or decimated and output. NOTE If a datalogger fails to provide a trigger and the CSAT3B has not been configured for unprompted output, the CSAT3B will enter into a lost-trigger state where it will output measurements at 1 Hz based on its internal timer. If the CSAT3B is in a lost-trigger state while connected with Device Configuration Utility, real-time anemometer data will flash on the screen at one second intervals. This is an indication that the settings should be checked. It does not indicate a problem with the CSAT3B hardware. Once a trigger is received by the CSAT3B, or once unprompted output is enabled, normal operation will ensue. 8.2.2 Data Filter The optional data filter (Modes 2 and 4) takes the 100 Hz sample points that were self-triggered by the CSAT3B and runs them through a low-pass (highcut) filter, resulting in a new filtered output at the same 100 Hz rate. The degree to which the data are filtered is determined by the user-selected filter bandwidth. This is selected from the Device Configuration Utility in the Measurement Settings tab. The bandwidth is the frequency at which a signal is filtered to (1-1/e) of its original power. In cases where unaliased spectra from the CSAT3B are to be 46