Section 4
Calibration
4-19
You may now wish to check an intermediate vapor pressure. You
will find that the system will equilibrate more quickly with smaller
water vapor changes.
No further span adjustments will be necessary for either absolute or
differential operation. However, if you wish to operate in differential
mode it will be necessary to reset the zero each time a new reference
gas is passed through the LI-6262 reference cell.
Differential Mode H
2
O
Zero Calibration
It is only necessary to set zero in differential mode. The analyzer gain is
set with a span adjustment in Absolute Mode. No further span
adjustments are necessary for operation in Differential Mode because the
calculations automatically correct for analyzer sensitivity changes due to
different reference cell humidities.
1.
Set zero and span in Absolute Mode, as described above.
2.
Flow a reference gas through both the reference cell and sample cell
for 20 min at about 1 liter min
-1
. Reduce the flow to 0.25 liters min
-1
for an additional 20 minutes, or until the displayed water vapor
differential is sufficiently stable.
3.
Enter the reference cell water vapor mole fraction WREF in units of
mmol/mol. WREF is computed from the dew point temperature T as
WREF
1000
e(T)
P
P
=
+ ∆
4.
Adjust the displayed water vapor differential (channels 33, 35 or 37)
to read zero.
Precautions
Pressure influences CO
2
and water vapor infrared absorptance by
altering gas density, and by changing the absorption per mole of gas.
Therefore, careful attention must be paid to pressures in the analyzer.
The barometric pressure should be accurately measured and entered into
the instrument. All LI-COR CO
2
and H
2
O analyzers are calibrated in
terms of mole fraction because this has been found to give the best
results, as theory predicts.
