IRGASON Integrated CO
2
/H
2
O Open-Path Gas Analyzer and 3D Sonic Anemometer
10. Remove the H
2
O span gas, and connect a zero air source (no CO
2
or H
2
O)
to the inlet tube of the shroud. As described in step 8, use a pressure
regulator and flow controller so that zero air flows through the shroud
between 0.4 and 0.6 l/min. Wait for the measurement readings to stabilize
(this may require several minutes) and write down the reported values for
CO
2
and H
2
O concentrations. If the readings remain erratic, ensure that
flow of the zero air is sufficient and the shroud is correctly seated on the
snouts.
11. Examine the measurements that were written down for span CO
2
, span
H
2
O, and zero air. Compute the drift in instrument gain using the
following equation:
measmeas
actual
zerospan
span
gain
−
=
where,
• span
actual
= known concentration of the span gas
• span
meas
= measured concentration of the span gas
• zero
meas
= measured concentration in zero gas.
Note that in the zero-and- span window of ECMon, span
actual
is reported to
the right of the box where the user enters the span dew-point temperature.
The software calculates span
actual
by taking into account the dew-point
temperature and current ambient temperature and pressure. The equations
used for this calculation may be found in Appendix B. If drift (offset or
gain) for CO
2
or H
2
O is excessive, it may be time to replace the desiccant
and CO
2
scrubber bottles (see Section 9.5, Replacing Desiccant and
Scrubber Bottles).
12. With zero air still flowing and measurements stabilized, click on the Zero
CO
2
and H
2
O button in the ECMon zero-and-span window.
Air flow into the shroud should be close to the recommended
rate. If the flow is too low, the shroud will not be properly
flushed; if it is too high, the air pressure within the shroud will be
too high, and the analyzer will not be zeroed and spanned
properly.
NOTE
13. Remove the zero air source and replace it with the CO
2
span gas. Allow
the gas to flow through the shroud, maintaining a flow between 0.4 and 0.6
l/min. Wait for readings to stabilize.
14. In the zero-and-span window, enter the known concentration of CO
2
(in
ppm) in the box labeled Span Concentration (dry) and press Span.
15. Replace the CO
2
span gas with an H
2
O span gas of known dew point.
Allow the gas to flow through the shroud; higher flows may be desired for
a couple of minutes to more quickly establish equilibrium before resuming
a flow between 0.4 and 0.6 l/min. Wait for the readings to stabilize.
31
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