Campbell Scientific EC150 CO2 and H2O Open-Path Gas Analyzer and EC100 Electronics with Optional CSAT3A 3D Sonic Anemometer - Manual (Page 37)

EC150 CO
2
/H
2
O Open-Path Gas Analyzer
Compute the drift in instrument gain using the following equation:
meas meas
actual
zero span
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 dewpoint temperature.
The software calculates span
actual
by taking into account the dewpoint
temperature and current ambient temperature and pressure. The equations
used for this calculation may be found in Appendix D, Useful Equations.
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 8.5, Replacing
CO
2
Scrubber Bottles).
18. 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.
19. Remove the zero air source and replace it with the CO
2
span gas.
20. Allow the gas to flow through the shroud, maintaining a flow between 0.2
and 0.4 LPM. Wait for readings to stabilize.
21. 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.
22. Replace the CO
2
span gas with an H
2
O span gas of known dewpoint.
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 0.2 and 0.4 LPM. Wait for the readings to
stabilize.
23. Enter the known dewpoint (in °C) in the box labeled Span Dewpoint and
press Span.
24. The zero-and-span procedure is now complete. Remove the shroud,
reconnect the EC150 temperature probe, and prepare the site for normal
operation. Verify that readings from the instrument are reasonable.
Record the zero and span coefficients for future reference and to keep
track of the rate of the analyzer drift. Make sure that the coefficients are
between 0.9 and 1.1. Negative or numbers larger than 1.1 are usually an
indication of improper calibration.
NOTE
27