Campbell Scientific CSAT3B CSAT3B Three-Dimensional Sonic Anemometer - Page 58

CSAT3B Three-Dimensional Sonic Anemometer NOTE Instructions, for details on the instruction. When the trigger is executed, it will also prompt the CSAT3B to output the most recent measurement in the buffer. This means that the data collected in the datalogger will be delayed by exactly one scan interval or timestamp. The earlier generation CSAT3 had a delay of two scan intervals. The program code can correct for this delay before storing the data to a data table, or the raw time series data can be saved and aligned with other data during post-processing of the data. Once the one scan delay is accounted for, this method of triggering and prompting the CSAT3B with a datalogger is the best way to achieve optimum synchronicity with other fast-response sensors. Because the measurement was triggered according to the datalogger's clock, the timing is known exactly. If the datalogger makes measurements of other fast-response sensors via analog measurement, SDM, or CPI, then the measurements from the CSAT3B and other sensors can be synchronized precisely. Mode 2 In the case where the CSAT3B is self-triggered and filtered, the output data can still be collected by a datalogger (Mode 2). In this case, the datalogger will prompt the CSAT3B for an output each time it executes the CSAT3B() CRBasic instruction in the program scan. When the datalogger prompt is received by the CSAT3B, it will output the most recent 100 Hz filtered sample in its buffer to the datalogger. This filtered data is delayed by a certain time interval (given in TABLE 8-2) and should be taken into account when aligning with other fast-response sensors. There are additional synchronicity considerations. Because the measurement was not triggered by the datalogger according to its own clock, the sample received by the datalogger may have a small synchronicity error between −5 and +5 milliseconds with respect to the datalogger's timestamp. The actual error depends on the operational mode and the output rate, as shown in TABLE 8-3. Modes 3 and 4 If the CSAT3B measurements are to be self-triggered and output to a PC, as in Modes 3 and 4, an unprompted output operating mode should be selected. Available output rates for the CSAT3B are 10, 20, 50, or 100 Hz. In unprompted mode, the CSAT3B will downsample or decimate the 100 Hz buffer data (unfiltered or filtered, depending on whether the user has selected Mode 3 or Mode 4, respectively) to output at the appropriate rate. The unprompted output record is an ASCII string of comma-delimited data terminated by a carriage return and contains the following seven data fields: 1) ux - x-axis wind speed in meters per second (m·s-1) 2) uy - y-axis wind speed in meters per second (m·s-1) 3) uz - z-axis wind speed in meters per second (m·s-1) 4) Ts - Sonic temperature in degrees Celsius (°C) 5) Diagnostic word 6) Record counter 7) Signature The record counter is a decimal value that is incremented each record until reaching a maximum value of 63, at which point it starts back over at zero. The counter may be used to ensure each record is not a duplicate of the last and that a record has not been omitted. 48