LI-COR LI7200RS, Instruction Manual

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Section 7. Software Reference
Mode Items Sent
6 Diagnostic Value (see below)
7 Trigger Mode
CO
2
dry (µmol/mol)
H
2
O dry (mmol/mol)
Signal Strength (average)
P
total
(kPa)
Cell Temperature (°C)
NOTES: LI-7200/RS Mode 7 requires the Campbell datalogger to broadcast a group
trigger to cause data to be registered. The registered data set is held and not
updated until the next group trigger.
Table 7-1 . Parameter 4 value definitions. (...continued)
Delay Time
The output signal from the optical bench is sampled by a high-speed analog-to-
digital converter and input into a digital signal processor (DSP). This signal is pro-
cessed digitally and gas densities are computed from it. There is a fixed delay in this
process, and an additional user-programmable delay that can be used to make the
instrument output occur on even sampling intervals.
The instrument has a fixed throughput delay of 172 milliseconds for the DAC out-
puts, and 130 ms for the SDM, RS-232, and Ethernet outputs. This delay can be
increased in increments of 1/150 seconds (6.667 ms), to minimize offsets between
the gas analyzer and other sensors.
For example, suppose you are sampling (via SDM output) the LI-7500RS or
LI-7200RS with a Campbell Scientific CR5000 datalogger at 10 Hz (0.1 s). Setting
the delay count of the gas analyzer to 25 yields a total delay of 0.297 seconds, which
means the data will have a delay of 3 execution intervals (0.297 s/0.1 s), which the
logger can allow for when synchronizing the data to the sonic anemometer or
other analog measurements made by the datalogger. Thus, the “unaccounted for”
lag is 0.003s. Without this extra delay, the lag time would be 0.130 seconds, which
is 1 execution interval (0.1 seconds) plus 0.03 seconds unaccounted for.
Similarly, if you are sampling the gas analyzer with the DAC outputs, setting the
delay count to 19 yields a total delay of 0.299 seconds. The lag will be 0.001s.
7-18 LI-7200RS Enclosed CO₂/H₂O Analyzer