LI-COR LI-7500, Instruction Manual

Page: 86 / 155

Section 4
4-4 Calibration
For the water vapor span, a convenient standard to use is a dew point generator such as the
LI-COR LI-610. To avoid condensation problems choose a dew point temperature that is
about 3 to 5 °C below the ambient temperature. Also, since water vapor sorbs and desorbs
from surfaces, allow plenty of time for the reading to stabilize. It is important not to rush
through the water vapor calibration. If it is more convenient, CO
2
and water vapor zero and
span calibrations can be done separately. In general, if reliable calibration standards are not
available or if there is not enough time to do the job properly, it is better to leave the zero and
span settings alone than to rush through the procedure and make incorrect settings.
What Actually Happens
In the LI-7500, the zero and span parameters are set in software via the serial port using a
personal computer (see Section 3). What actually happens when the zero is set is that the
value of Z
co
(or Z
wo
for water) is determined. For example, when CO
2
-free air is in the
optical path of the analyzer,
α
c
should be 0. From Equation 2-16,
α
c
c
co
wc
w
wo
co c d
A
A
X
A
A
Z Z V
= = − +






+
( )

 

0 1