LI-COR LI-7500, Instruction Manual

Page: 21 / 155

Section 2
2-4 Theory of Operation
ρ
i ei i
i
io
i
i
ei
P
A
A
z
S
P
= −







 

f 1
2-7
Note the zeroing term z
i
and the span adjustment term S
i
in Equation (2-7).
Cross Sensitivity
Because the LI-7500 uses one detector for measuring A
c
, A
co
, A
w
, and A
wo
, (the absorbed and
non-absorbed power for CO
2
and H
2
O, respectively), there is a slight cross-sensitivity
between gases due to imperfections in the detector's frequency (time) response. This varies
from detector to detector, but is measured during calibration, and is corrected in software.
Equation 2-6 is written as
α
i
i
io
ji
j
jo
A
A
X
A
A
= − + −

 















1 1
2-8
where X
ji
is the cross sensitivity response of gas j on gas i (determined during calibration), and
A
j
and A
jo
are the absorbed and non-absorbed power for gas j. Equation 2-7 becomes