LI-COR LI7200RS, Instruction Manual

Page: 173 / 226

Section 8.
Theory of Operation
Relating Absorption to Concentration
The scaling law of Jaimeson et. al., (1963) shows the effect of pressure on infrared
absorption. If the amount of absorber of some gas
u
i
(mol m
-2
) and absorption in a
band are related by some function h
i
(), then
8-1
The subscript i denotes a particular ( i
th
) gas. Pressure is denoted as P
ei
because it is
the equivalent pressure for the i
th
gas. Equivalent pressure is potentially different
from total pressure P if there are gases present other than i that affect how the i
th
gas absorbs radiation.
We rewrite this in terms of number density (mol m
-3
) by introducing a path length
λ
, and noting that u
i
=
ρ
i
λ
. Substituting this into equation 8-1, and solving for the
number density
ρ
i
of gas i yield
8-2
We rewrite equation 8-2 as
8-3
by combining
λ
and the inverse h () functions into a new function f
i
(). The cal-
ibration function f
i
() is generated by measuring a range of known densities
ρ
i
and
fitting a curve to
ρ
i
/ P
ei
plotted against
α
i/ P
ei
. Since gas standards are not available in
8-1 LI-7200RS Enclosed CO₂/H₂O Analyzer