Using the 10 and 20 cm Survey Chambers
6-11
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.
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 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 of about 45 seconds is adequate.
IMPORTANT NOTE: The Post-purge function was added to the Windows
Application Software V2.0 to accommodate the use of multiple chambers with
the LI-8150 Multiplexer, where it is important to purge the gas sampling lines
before making the next measurement with that chamber. Because the Post-
purge starts after the measurement is complete, it has slightly different
implications for use in single-chamber mode. As shown in the chart below,
after the first measurement is complete, the Post-purge starts, followed by the
Pre-purge; thus, the Post-purge and Pre-purge become additive. In most
single-chamber applications, the combination of the two delays is excessive.
Note, too, that before the first measurement starts, the chamber will not close
until the Pre-purge has finished; again, in most cases this delay is unwanted,
particularly when moving the chamber from collar to collar. For these
reasons, you may want to use the Post-purge value instead of the Pre-purge;
in other words, set the Pre-purge to zero, and set the Post-purge to 20-30
seconds, or more, depending on the conditions described above at “Pre-
purge”.
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