Section 4
Calibration
4-5
4.3 Absolute Mode Calibration
The procedure for setting the zero and span potentiometers in absolute mode
is described in Steps 1 thru 5 above for differential mode calibration.
4.4 H
2
O Absolute Mode Caution
When using the LI-6262 to measure water vapor in absolute mode, note the
following:
When the LI-6262 is configured as described in Section 2.2 for absolute
operation, air flow created by movement of the chopping shutter disc is
sufficient to purge the reference cell of CO
2
. This air flow is not, however,
sufficient to remove all water vapor present in the reference cell.
An external pump, such as the 6262-04 Reference Pump from LI-COR (see
Using the 6262-04 Reference Pump
below), or the LI-670 Flow Control Unit,
is necessary to provide an adequate flow of dry air through the desiccant
circuit. A flow rate of approximately 500 ml per minute is sufficient to
purge the reference cell.
The flow of dry air through the reference cell and scrubber tube assembly
will also greatly affect the performance and maintenance requirements of the
external soda lime/magnesium perchlorate mixture. An inherent property of
soda lime is that a small amount of water vapor must be present for the
chemical to scrub CO
2.
Ordinarily, this water vapor is present in the soda
lime due to normal diffusion through the air hoses; however, Mg(ClO
4
)
2
reduces the airstream water vapor concentration to near zero. Prolonged
exposure of the soda lime to dry air reduces its ability to scrub CO
2
.
It is therefore recommended that the external soda lime/desiccant be
replaced every 1-2 days when operating the LI-6262 for absolute H
2
O
measurements.
If this is a problem, an alternative is to maintain a low flow
of CO
2
-free air from a compressed tank that goes through the chopper and
reference cell and is then vented to the atmosphere.
