Using the LI-8150 Multiplexer
10-39
period. At moderate to low CO
2
fluxes an Observation Length of 90 to 120
seconds is usually adequate.
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.
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 (30
seconds is a good starting point if you are new to the system). 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
LI-8100A Data Analysis (File Viewer) program.
The Post-purge Time is the amount of time during which air continues to flow
through the chamber as it begins to open, after the Observation is complete. This
is important in certain cases where environmental factors may influence the
amount of CO
2
or moisture that is present in the gas sampling lines. For example,
in hot, moist conditions, you may want to increase the Post-purge Time to ensure
that the gas sampling lines are purged of moisture that may condense in the lines,
before the next measurement using that chamber is started. In most cases, a Post-
purge Time of about 45 seconds is adequate.
When an observation is complete, the chamber will automatically rise up off of the
soil collar. If a Post-purge Time is designated, air will continue to flow through the
port whose Observation has just completed (post-purge). After the Purge Time has
finished, air flow switches to the next port in the sequence for the Pre-purge.
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