Campbell Scientific AP200 CO2/H2O Atmospheric Profile System - Manual (Page 47)

AP200 CO
2
/H
2
O Atmospheric Profile System
PC200W, which may be downloaded from www.campbellsci.com . The AP200
uses some constants as “compile switches” to define the state of the system
during compilation.
Variables may be edited while the program is running using either a keyboard
display or a PC connected through PC200W, PC400, or LoggerNet. Variables
that define the operational configuration of the system are defined as system
configuration variables. Any changes to these variables are automatically -
saved in non-volatile memory.
A change to a system configuration variable that affects the
valve switching sequence will not take effect until the sequence
is started.
Other modifications to the AP200 program (for example, to
measure additional sensors) are not recommended without first
consulting a Campbell Scientific application engineer.
5.4.1 System Configuration Variables
Several special variables are used to configure the AP200. These variables are
stored in the sys_conf_var.dat file, so their values will be saved and recalled if
the program is recompiled. They are intended to be set when a new system is
installed, but they may be edited at any time. When a system configuration
variable is changed, the AP200 will write a message to the message_log table
and save the new values in sys_conf_var.dat. This section gives a brief
description of each of these variables and refers the reader to the appropriate
section of the user manual if a lengthy discussion is required.
PRESS_AMBIENT: This is the ambient pressure, in kPa. It depends
primarily on elevation (see FIGURE 4-19), but also varies slightly with
barometric pressure. This variable is used to convert the dewpoint temperature
measured in the IRGA sample cell Td_cell to dewpoint temperature at ambient
pressure Td_ambient. It is also used during a manual H
2
O span operation.
There are three methods to determine the appropriate value for
PRESS_AMBIENT.
• If nominal ambient pressure is known, simply enter the value.
• If elevation is known, determine ambient pressure from FIGURE
4-19.
• Ambient pressure can be measured by turning the AP200 pump off,
waiting for the pressure to stabilize at ambient pressure, and then
using the pressure measured in the IRGA (see Section 6.2.4, Do H
2
O
Span, for details).
PUMP_P_SETPT: This is the setpoint pressure (kPa) for the pump speed
control algorithm. The AP200 will adjust the pumping speed to make the
measured pump pressure, pump_press, match this value. For most applications
it should be set to approximately 35 kPa below ambient pressure (see notes on
variable PRESS_AMBIENT above). The factory default is 200, which will
prevent the AP200 from working properly until PUMP_P_SETPT is set by
the user.
NOTE
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