LI-COR LI7200RS, Instruction Manual

Page: 109 / 226

Secondary H
2
O Span
1
Zero the H
2
O reading (see Zero H2O on
page 4-7).
2
Span the H
2
O reading at a dew point that is just
below the ambient temperature (see Span H2O
on page 4-7).
3 Flow a very low dew point through the tubing.
4
Click on the Span 2 tab, and when stable, click
Span 2 H
2
O
.
It may take another 10 minutes or so to
ensure equilibrium is reached when
changing from one concentration to
another, due to water present in the
tubing, etc.
NOTE: You can reverse the concentrations if you wish. The normal span can be
the high dew point, and the secondary the low dew point. Also, multiple secondary
spans can follow a normal span. Simply make sure that the concentration dif-
ference between the normal span and any secondary span is large.
What Actually Happens
In the LI-7200/RS, the zero and span parameters are set in software (see Manual on
page 7-7). 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 8-17,
so
4-1
Similarly,
Section 4. User Calibration
4-9 Secondary H2O Span