Campbell Scientific EC150 CO2 and H2O Open-Path Gas Analyzer and EC100 Electronics with Optional CSAT3A 3D Sonic Anemometer - Manual (Page 33)

EC150 CO
2
/H
2
O Open-Path Gas Analyzer
natural elements. A zero-and span procedure should be performed after
installation of the instrument to give appropriate baseline readings as a
reference. A zero-and-span procedure should also be performed occasionally
to assess drifts from factory calibration. In many cases, a zero and span can
help resolve problems that are being experienced by the user during operating
the EC150. For example, a zero-and-span procedure should always be
performed on the analyzer after changing the internal chemicals. Before
performing a zero-and-span procedure, clean the windows of the EC150 as
described in Section 8.3, Cleaning Analyzer Windows.
After the first several zero-and-span procedures, the rate of drift in gain and
offset (explained later in this section) should be analyzed to better determine
how frequently the zero-and-span procedure should be performed once the
instrument has been put into service.
The first part of the procedure listed below simply measures the CO
2
and H
2
O
span and zero without making any adjustments. This allows the CO
2
and H
2
O
gain factors to be calculated. These gain factors quantify the state of the
analyzer before the zero-and-span procedure and, in theory, could be used to
correct recent measurements for drift. The last part of the zero-and-span
procedure adjusts internal processing parameters to correct subsequent
measurements.
If the zero-and-span procedure is being performed off site (for example, in a
laboratory), be sure to mount the EC150 on the zero-and-span stand (refer to
FIGURE 7-1). This will ensure the analyzer is in the correct upright
orientation and has the correct optical alignment.
The zero-and-span procedure must be performed correctly and not rushed.
Allocate at least one hour (preferably more) for the procedure. Ensure that the
readings are stable and all sensors are properly connected and functioning.
It is conceivable that there are circumstances in which both a zero and a span
cannot be performed by the user. In these instances, it is recommended that the
user attempt to perform a zero of the instrument even if spanning is not
possible or inconvenient. The information gained through zeroing the
instrument can help troubleshoot problems that may be encountered during
field operations.
The water vapor measurement is used in the CO
2
concentration
calculations to correct instrument and pressure broadening effects.
To achieve good CO
2
calibration, it is imperative to maintain a
reasonable water vapor calibration.
Resting the analyzer on its side during the zero-and-span
procedure may result in measurement inaccuracy.
7.2 Zero and Span Procedure
This section gives instructions for performing a zero-and-span procedure, and
should be referred to any time a zero-and-span procedure is undertaken.
Check and then set the EC150 zero and span according to the following steps:
NOTE
CAUTION
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