Campbell Scientific AP200 CO2/H2O Atmospheric Profile System - Manual (Page 56)

AP200 CO
2
/H
2
O Atmospheric Profile System
6.2.4 Do H
2
O Span
At the Manual Zero/Span menu, select Do H2O Span. This menu guides the
user through the steps to flow the H
2
O span gas and to send the command to
the IRGA to set its H
2
O span.
H
2
O is more difficult to span than CO
2
due to the difficulty of providing a
source of air with known humidity. Therefore, a dewpoint generator, such as
the LI-610 (LI-COR ® , Inc., Lincoln, NE), is used for many applications. An
additional complication is the need to correct the dewpoint temperature for the
difference in pressure between the dewpoint generator and the IRGA sample
cell. The AP200 is capable of making this correction, but three variables must
be set first.
H2OSpanDewPt: This variable is the dewpoint temperature setting (in °C) on
the dewpoint generator. Any value may be entered for H2OSpanDewPt.
However, the IRGA H
2
O span setting will be disabled unless the value is
between 0 and 20. The default is –99 to intentionally disable setting the H
2
O
span until the user enters the value for the dewpoint generator.
PRESS_AMBIENT: This variable is the ambient pressure (kPa), which is
assumed to be the same as the pressure in the dewpoint generator.
SpanCellP: This variable (given in kPa) is the pressure in the IRGA sample
cell during the H
2
O span procedure. The steps below will measure these
pressures as part of the H
2
O span procedure. PRESS_AMBIENT and
SpanCellP are used to calculate a pressure-corrected, span dewpoint
temperature to send to the IRGA (see Appendix I, Useful Equations, for
details).
All three of these variables must be set for the IRGA H
2
O span setting to be
enabled.
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