It is recommended that you perform the zero calibrations first, followed by the span calibrations.
To zero, flow a dry, CO
2
-free gas through the LI-840A and make sure the optical cell is com-
pletely purged. Press the Zero CO
2
button under the Zero tab.
The display will show ZERO, and the text in the Calibration window is grayed out. The zero will
be set electronically, and the current date will be entered in the "Last CO
2
zero performed on:"
field when completed. Repeat for the H
2
O channel. Note: Purge the cell with the H
2
O zero gas for
up to 15 minutes before zeroing H
2
O. Water vapor molecules are “sticky” and it may take up to 15
minutes to reach equilibrium in the optical cell.
Section 3
3-24 Section 3. Operation
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