LI-COR LI7200RS, Instruction Manual

Page: 110 / 226

Section 4. User Calibration
4-2
When the span is set, the value of S
c0
(or S
w0
for water) is determined. For
example, if there is a known CO
2
density
ρ
'c in the optical path and the measured
absorptance is
α
’c , then from equations 8-19 and 8-8, we can write
so
4-3
We rewrite this in terms of a known mole fraction C’
f
instead of density.
4-4
If we assume that the CO
2
span gas is dry, then P
ec
= P , so
4-5
Similarly, for a known H
2
O mole fraction W’
f
and resulting measured absorptance,
ɑ
'
w
,
4-6
Whenever the CO
2
span is set, the instrument saves two values that are used if and
when a secondary CO
2
span is performed. These values are
4-7
When a secondary span for CO
2
is performed at a mole fraction C’
f
with measured
absorptance
ɑ
’
c
, then the span slope term S
c1
is computed from
4-10 LI-7200RS Enclosed CO₂/H₂O Analyzer