Section 4
4-20
Calibration
Span adjustments can be used to correct the CO
2
or H
2
O mole fractions if
pressure is entered incorrectly, or if a nominal value is used; however,
subsequent conversions to partial pressure will be incorrect if they are
computed from an incorrect barometric pressure.
The span of all LI-COR infrared gas analyzers will vary with barometric
pressure changes because infrared absorptance varies with pressure.
Therefore, an electronic barometer should be used to provide accurate
barometric pressure data to the LI-6262 (or other analyzer) if continuous
recordings of CO
2
and/or H
2
O vapor concentrations over long periods of
time are required. Note also, that some zero shift can be expected with
changes in analyzer temperature.
Pressure in the analyzer optical bench varies with gas flow rate. It
follows that span settings should be made at the same flow rate as
measurements, whenever possible. Further, the calculations in the
LI-6262 use the same pressure for both sample and reference cells.
Therefore, flow rates and total pressures should be the same in the two
cells.
In practice, flow dependent over-pressures in the sample and reference
cells will be small when flows are low (about 1 liter min
-1
, or less) and
the analyzer is vented directly to the atmosphere. However, large
pressure differences between the two cells that could occur with high flow
rates (> 5 liters min
-1
) cannot be accommodated by span adjustments or
on-board calculations. Presumably, large pressure differentials between
sample and reference cells could be included in calculations outside the
LI-6262, but this has not been tested.
Calibrating the LI-1600 Steady State Porometer RH Sensor
Initial Setup
1.
Set the LI-610 dew point to 0.05 °C.
2.
Remove the LI-1600 console from the case as described in Section 5
of the LI-1600 instruction manual. For your safety, do not operate
the LI-1600 from AC power while performing this procedure.
