Campbell Scientific CPEC200 Closed-Path Eddy-Covariance System - Manual (Page 47)

CPEC200 Closed-Path Eddy-Covariance System
6.3.2 Full Manual Control of Zero and Span
In some cases it may be more appropriate to run the zero/span under full
manual control. This allows the user to decide how much time is required for
the zero or span gas to reach equilibrium, or to perform additional status
checking. It also allows the H
2
O to be spanned. The keyboard display has
menus to facilitate manual zero/span control for users that are onsite.
Navigate:
Manual zero/span → Manual Control
6.3.2.1 Getting Ready
There are two tasks to prepare for manual zero/span control. First, enable the
temperature control for the valves and the scrub module (if being used) as
noted above.
Second, measure the pressure offset. The keyboard display has menus to
facilitate manual zero/span control for users that are onsite. Navigate:
Manual zero/span → Manual Control → Pressure Offset
Turn the sample pump off by setting pump_ON = False to enter manual
zero/span mode. Verify mode_status has changed from Normal EC mode to
Manual Zero/Span mode.
If the CPEC200 program is not configured for valve operation,
mode_status will report Standby mode (Pump is Off). See
Appendix A, CPEC200 Diagnostics, for further details.
Tip: Normally a manual zero/span is started when the CPEC200 is in the EC
mode (pump on). In that case, turn the pump off to begin a manual zero/span.
However, if the zero/span sequence is running, stop the sequence by turning
the pump on. This will return the system to normal EC mode. Then turn the
pump off to enter manual zero/span mode.
Set DO_P_offset = True. This will turn all of the valves off and command the
CPEC200 to measure the pressure offset. Watch the value of press_offset.
Turning the pump on or setting DO_P_offset = True will reset this variable to
zero. Within approximately 10 s after turning the pump off and setting
DO_P_offset = True), press_offset will be set to the value of cell_press (with
a 5 s moving average) and DO_P_offset will reset to False. Wait until
DO_P_offset = False before proceeding.
Tip: The purpose of this step is for the CPEC200 to store the ambient pressure
(measured by cell_press when there is no flow in the system) in public variable
press_offset. This allows the CPEC200 to infer the flow in the sample cell
from the pressure rise resulting from the zero/span flow. Because ambient
pressure changes over time (and the pressure sensor may drift over time), this
pressure must be measured each time a zero/span is performed.
6.3.2.2 Checking and Setting the Zero
The keyboard display has menus to facilitate manual zero control for users that
are onsite. Navigate:
Manual zero/span → Manual Control → H2O and CO2 Zero
NOTE
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