LI-COR LI7200RS, Instruction Manual

Page: 176 / 226

Section 8. Theory of Operation
Equation Summary
H
2
O
In the atmosphere, the absorption of radiation by water vapor is not significantly
influenced by any other gas, so the effective pressure for water vapor
P
ew
is simply
the total pressure P .
8-12
H
2
O absorptance,
α
w
, is
8-13
where b
1
, b
2
, and b
3
are constants (
CO
2
SD1
,
SD2
, and
SD3
on the calibration
sheet) and V
c
is cooler voltage. Uncorrected absorptance,
α
*
w
, is given by
8-14
where A
w
and A
wo
are the raw signals for the water absorption and reference bands,
X
cw
is the cross sensitivity factor for CO
2
on water vapor (
H
2
O XS
on the calibration
sheet), A
c
and A
co
are raw signals from the CO
2
absorption and reference bands,
Z
wo
is the zeroing coefficient (
H2O Zero
on the calibration sheet), and Z
w
is the
zero drift coefficient (
H
2
O Z
on the calibration sheet), A
c
and A
co
are the raw signals
for the CO
2
absorption and reference bands, and T
block
is the block temperature.
Mole density of H
2
O,
ρ
w
, is given by
8-15
The coefficients for the 3rd order polynomial f
w
( ) are given on the calibration
sheet. The polynomial has the form Ax + Bx
2
+ Cx
3
, where x =
ɑ
w
S
w
/ P . S
w
is the
span for water vapor.
CO
2
The absorption of radiation by CO
2
molecules is influenced by several other gases,
including O
2
and H
2
O. Since the concentration of H
2
O is most variable, it must be
8-4 LI-7200RS Enclosed CO₂/H₂O Analyzer