LI-COR LI7200RS, Instruction Manual

Page: 175 / 226

Cross Sensitivity
Because the instrument 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 cal-
ibration, and is corrected in software. Equation 8-6 is written as
8-9
where X
ji
is the cross sensitivity response of gas j on gas i (determined during cal-
ibration), and A
j
and A
jo
are the absorbed and non-absorbed power for gas
j . Equation 8-7 becomes
8-10
Zero Drift
Even though the detector and filters are temperature controlled in the LI-7200/RS,
the detector is subject to slight temperature drift as ambient temperature changes.
This error is directly related to the detector cooler control voltage, which is meas-
ured, and thus provides a mechanism for a software "fine tuning".
The zero term z
i
is computed from
8-11
where T
block
is the reference temperature for the T
air
thermocouples in the sensor
head (°C), Z
i
is the slope of the relationship between T
block
and z
i
(determined dur-
ing calibration), and Z
io
is the zero factor determined when setting the zero.
Section 8. Theory of Operation
8-3 Cross Sensitivity