Campbell Scientific CPEC200 Closed-Path Eddy-Covariance System - Manual (Page 44)

CPEC200 Closed-Path Eddy-Covariance System
In addition to identifying the most appropriate use of manual versus automatic
and remote versus onsite calibration, there is one additional option to consider:
whether to simply check the zero/span, or to set the zero/span.
Checking the zero/span allows the user to track the performance of the EC155,
apply gain and offset corrections in post processing, and decide when to
actually set the zero/span. Setting the zero/span involves sending commands to
the EC155 to update its internal zero/span parameters.
Campbell recommends setting the zero/span in the EC155 as this
will maintain better accuracy than applying corrections in post
processing. See the EC155 CO
2
and H
2
O Closed-Path Gas
Analyzer Manual for details.
The recommended approach for zero/span involves both monitoring and setting
the zero/span according to the following steps:
1. Measure the H
2
O span with a dewpoint generator.
2. Measure the CO
2
span.
3. Measure the CO
2
and H
2
O zero.
4. Set the CO
2
and H
2
O zero in the EC155.
5. Measure the CO
2
span again (after zero has been set).
6. Set the CO
2
span in the EC155.
7. Measure the H
2
O span again (after zero has been set).
8. Set the H
2
O span in the EC155.
Steps 1, 7, and 8 require the use of the dewpoint generator and
must be omitted for remote operations, including the automated
zero/span.
6.2 Automatic Zero and Span
The automatic zero/span sequence consists of eleven steps, listed in TABLE
6-1. If CHECK_ZERO = True, the CPEC200 will periodically cycle through
the sequence as listed. If CHECK_ZERO = False, the CPEC200 will
continue to sample atmospheric air for eddy covariance measurements (no
automatic zero/span).
mode_status is the text variable that reports the status of the sequence. Pump
on/off is reported in public variable pump_ON. Public variable
valve_number shows which valve is active. The status of valve_number is
displayed to the user in two manners; in the public and output tables as the
actual numeric value of an integer from 0 to 6, or as text on the datalogger
keypad. TABLE 6-1 gives both the datalogger keypad text and the
corresponding numeric value in the public and output tables. The duration of
each step is also given in TABLE 6-1. Some of the steps have a duration that
cannot be changed, while others are set by the configuration variables
SPAN_TIME and ZERO_TIME.
Step 2 and steps 4 through 9 are optional, as determined by the
configuration variables. If a step is disabled, it is skipped.
NOTE
NOTE
NOTE
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