Using the 10 and 20 cm Survey Chambers
6-17
Note that the LI-8100A starts logging data when the chamber is actuated and
starts to close. Raw, or Type 1 records are recorded throughout the entire
observation period. The Elapsed Time (labeled etime on the data output) does
not increment, however, until the chamber is closed. While the chamber is
closing, etime will register -1.
Dead Band
The Dead Band is the time period that starts when the chamber closes
completely, and continues until steady mixing is established and the
measurement begins. The Dead Band requirement changes depending upon
the chamber geometry, system flow rate, and collar and site characteristics.
Testing at LI-COR has indicated that a Dead Band between 10 and 60
seconds generally provides adequate mixing. There may be conditions,
however, where a longer Dead Band is required. Note, too, that collected
data can be recomputed using longer (or shorter) Dead Bands with the
LI8100A File Viewer program.
Obs. Count
You can make repeated observations under the same set of parameters by
setting the Observation Count to reflect the number of times to repeat the
observation. Individual observations are separated by the Pre-purge (below).
Note that in most cases, it may be more desirable from a scientific and/or
statistical standpoint to replicate measurements on multiple soil collars rather
than repeating them on the same collar.
Pre-purge
When making repeated measurements, a delay is required to allow the
chamber air to return to ambient conditions before beginning the next
observation cycle. This delay is referred to as the Pre-purge.
When an observation is complete, the chamber will automatically rise up off
of the soil collar. If the Observation Count (above) is set to 2 or more, the
Pre-purge sets the time during which the chamber is open. Under very still
conditions it may take 2 minutes or more for the chamber air to return to
ambient conditions. Under windy conditions the chamber CO
2
concentration
may return to ambient levels in as little as 20 or 30 seconds. Note, too, that
the Pre-purge begins as soon as the chamber starts to open. Therefore, it is
possible to set a delay time that is too short for the chamber to fully open
before it begins closing again.
Post-purge
The Post-purge is the amount of time during which air continues to flow
through the chamber as it begins to open, after the measurement is complete.
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