Section 4
Calibration
4-17
The display will show SPAN, and the text in the Calibration window is
greyed out. The span will be set electronically, and the current date will
be entered in the "Last spanned on" field when completed. Repeat for the
H
2
O channel using the LI-610 to provide the airstream with known dew
point.
Calibrating the LI-6262 CO
2
/H
2
O Infrared Gas Analyzer
Preliminary
If present, remove the blue Balston filters from inside the LI-6262 and
replace them with the Gelman disc filters supplied in the spare parts kit.
The Gelman filters will not fit inside the LI-6262 case; they must be
placed in the airstream just before it enters the analyzer. Under no
circumstances should the LI-6262 be operated without filtering the air
before it enters the analyzer.
The protocols suggested here for setting zero and span assume that high
accuracy is desired, so they have long equilibration times. Experience
with CO
2
analyzers causes one to expect rapid and stable responses to
changes in gas concentration; however, this will not be the case with
water vapor.
Working with water vapor presents difficulties due to water vapor
sorption that are not encountered with CO
2
. These problems will cause
the LI-6262 to appear to drift or respond slowly to changes in humidity.
The resolution of the LI-6262 is beyond that obtainable with most other
humidity measuring instruments, even dew point hygrometers; so very
small humidity changes can now be seen, which in the past were largely
unobservable. High sensitivity coupled with water sorption, temperature
variations, pressure effects, etc. may cause complete stability to be
elusive.
We suggest, therefore, that you determine in advance the accuracy that is
required, and then establish the equilibration time necessary to meet your
requirements. The equilibration times given in these protocols are meant
only as starting points.
