Section 3
3-6
Theory of Operation
In this section we have assumed that the sample and reference cells are
optically identical. Strictly speaking, of course, that is not true. This can be
illustrated by setting zero with zero gases in both the sample and reference
cells and then placing a single span gas in both cells. A small zero offset
will usually be observed, illustrating any minor optical differences between
the cells that may exist. Nevertheless, the treatment given here works very
well in practice. Small optical anomalies of whatever origin can be
corrected empirically by setting zero and span.
3.3 Calculating CO
2
Concentration
Mole fraction CO
2
, C (
µ
mol/mol, displayed using FCT 22) comes directly
from equation 3-8. The reference concentration value C
r
(
µ
mol/mol), should
be entered via FCT 59. If the reference air is not dry then c
r
should be the
actual CO
2
mole fraction in the presence of water vapor. See sections 3.5
and 3.6.
CO
2
differential
∆
C,
µ
mol/mol (FCT 23) is C - C
r
.
CO
2
partial pressure p
c
(Pa, displayed using FCT 24) is computed from C
and total pressure P (kPa) by
p
cP
c
=
1000
3-10
The CO
2
weight fraction c
g
(
µ
g/g, displayed at FCT 26) is computed as
C
c
M
g
=
44
3-11
where M = molecular weight of air (g/mol), weighted for water vapor.
= 29(1 - w/1000) + 18w/1000
w = mole fraction of water vapor (mmol/mol).
3.4 Calculating H
2
O Concentration
The calculation of water vapor w (mmol/mol, displayed using FCT 32) is
performed using an equation similar to equation 3-8. However, we have
found that the pressure dependence is different from that used in equation 3-
12. We have found empirically that P
o
/P)
0.9
gives more accurate pressure
correction than P
o
/P. Thus,
