Repeat for the H
2
O channel using an airstream of known dew point. Note: Purge the cell with the
H
2
O span gas for up to 15 minutes before spanning H
2
O. The LI-610 Portable Dew Point Generator is
capable of providing an airstream with a known water vapor concentration.
Two Point Span (Span 2 Tab)
Suppose you have two non-zero calibration gasses (e.g. CO
2
at 300 ppm and 1500 ppm), and you
use one of them (say, 1500 ppm) to set the span. If you then flow the other gas through the LI-
840A, you would expect it to read within 1% of the expected value (in our example, 300 ±3
ppm). If it does not, you can adjust the calibration for this by performing a Secondary Span, by
entering the span gas concentration and clicking the Span CO
2
(Span 2 tab). After that, the 300
ppm tank should read 300, and the 1500 tank should still read 1500. This can also be performed
with H
2
O if needed.
Section 3
3-26 Section 3. Operation
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