For CO
2
, The difference in absorptance between 200 and 1000 ppm is typically about 0.1. There
is also a 0.1 absorptance difference between 3500 and 7000 ppm, and between 15,000 ppm and
40,000 ppm. For H
2
O, it is a little harder to get a 0.1 absorptance difference. If you want to do a
secondary span at room temperature, for example, the two span values would have to be 5 °C or
less, and 22 °C, for example.
0
0.1
0.2
0.3
0.4
-30 -20 -10 0 10 20 30 40
H
2
O Absorptance
Dew Point Temp (C)
Typical LI-840A
0.1
You can do multiple secondary spans after a normal span. For example, span at 200, then do a
secondary span at 3000. Then (assuming you didn’t like the result for some reason), you can do
another secondary span at 2000 ppm. As long as the secondary span is well away (in
absorptance) from the previous normal span, it is OK to do as many consecutive secondary spans
at various concentrations as you’d like.
Coefficients Page
The LI-840A uses a double rectangular hyperbola for the CO
2
calibration, and a third order poly-
nomial for H
2
O calibration. The Coefficients page displays these factory-determined calibration
3-29Section 3. Operation
Section 3
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