The CO
2
span slope value is first determined at the factory by setting the span at 200 ppm, and
doing an secondary span close to 20000 ppm. This value should remain valid for some time.
Things that are likely to make it change include changing the source and/or changing the
detector, a dirty optical cell, and possibly even taking the optical bench apart for cleaning.
Considerations for performing a secondary span
l Zero first.
l After zeroing, do a (normal) span.
l The span and secondary span concentrations should be as far apart as possible and they should bracket the con-
centration range of interest to you. (e.g. 200 and 20000, or 300 and 1500, etc.). “Far apart” means in absorptance, not
concentration. We recommend absorptance differences of at least 0.1 when choosing concentrations for setting span
and secondary span.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
100 1000 10000 100000
CO
2
Absorptance
CO
2
(ppm)
Typical LI-840A
0.1
0.1
0.1
Section 3
3-28 Section 3. Operation
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