LI-COR LI-7500, Instruction Manual

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Section 3
Setup and Operation 3-31
programmable delay that can be used to make the LI-7500 output occur on even sampling
intervals.
NOTE: The delay time applies to all LI-7500 outputs (SDM, DACs, and RS-232).
The LI-7500 has a fixed throughput delay of 240 milliseconds for the DAC output, and 186
ms for the SDM and RS-232 output. This delay can be increased in increments of 1/152
seconds (6.5 ms), to minimize offsets between the LI-7500 and other sensors.
For example, suppose you are sampling the LI-7500 with a Campbell Scientific CR23X
datalogger at 10 Hz (0.1 s). Setting the delay count of the LI-7500 to 17 yields a total delay of
.297 seconds, which means the LI-7500 data will have a delay of 3 execution intervals (0.297
s/0.1 s), which the logger can allow for in synchronizing the data to the sonic anemometer or
other analog measurements made by the datalogger. Thus, the “unaccounted for” lag will be
0.003 seconds. Without this extra delay, the lag time would be .186 seconds, which is 1
execution interval (0.1 seconds) plus .086 seconds unaccounted for. A 0.086 second offset
between the sonic anemometer and the IRGA can reduce the apparent flux in some
circumstances.
Similarly, if you are sampling the LI-7500 with the DAC outputs, setting the delay count to 9
yields a total delay of .299 seconds. The lag will be 0.001 seconds.