5-2
User Calibration
CO
2
and H
2
O Gas Standards
Tanks of compressed air with known CO
2
concentration and specified accuracy
are readily available from commercial suppliers. Such tanks can provide
convenient standards for checking the CO
2
span of the LI-8100A; however, tanks
are not available that contain a known water vapor concentration for checking the
span of the water vapor channel. The LI-610 Portable Dew Point Generator can be
used to provide air with a known dew point for the primary calibration of new
analyzers. The span of the analyzer in the LI-8100A is stable when the zero is set
on a regular basis, and soil CO
2
flux measurements are not very sensitive to errors
in the water vapor measurement. For these reasons, it is recommended not to alter
the span of the water vapor channel unless an LI-610 or similar equipment is used
to produce an accurate span gas.
CO
2
and H
2
O Zero
Either CO
2
-free air or CO
2
-free nitrogen can be used as a zero gas. It is best to use
a column filled with a chemical CO
2
scrubbing agent downstream of the tank,
however, as even "CO
2
-free" gases at times contain 10 or 20 ppm of CO
2
. Note
that the scrubber may add significant H
2
O vapor, so you may want to use a water
vapor scrubber downstream from the CO
2
scrubber.
A suitable CO
2
zero scrub is soda lime (LI-COR part number 9964-090) and an
H
2
O scrub is Drierite (LI-COR part number 622-04299). When using chemical
scrubbers, it is important to make sure that the chemicals are fresh. Always be sure
to have a particulate filter installed in the air flow path between the chemicals and
the instrument.
Compressed cylinders may be at pressures of several thousand pounds per square
inch; before using them for calibration, they should be fitted with a regulator to
reduce the pressure down to a range of around fifteen pounds per square inch.
CO
2
Span(s)
To set either the primary or secondary CO
2
span, a cylinder of CO
2
in air, verified
to be accurate to within at least 1%, would be a suitable choice. Be cautious with
a new cylinder, as the stated value of the calibration cylinder may be significantly
different from the actual gas concentration (i.e., mislabeled). Choose a gas with a
CO
2
concentration slightly greater than the highest concentration encountered
during measurements.
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