LI-COR LI-7500, Instruction Manual

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Section 3
Setup and Operation 3-51
Rain/Snow
Flying droplets and flakes in the optical path will affect the performance of the LI-7500, even
if the total light blockage is small enough that the AGC does not reach 100%. The reason is
that the objects are moving, and if a droplet or flake is in the path for a sample measurment,
but out of the path for a reference measurement (or vice versa), it will influence the resulting
reading. Figure 3-4 illustrates a strip chart trace made during a light snow shower. The
occasional spikes are due to blockage changes between sample and reference readings.
Notice that in some cases both CO
2
and H
2
O are affected, and sometimes just one. Also, the
spikes are equally likely to be up or down.