CIRAS-3 Operation Manual V. 1.09
192
CO
2
Calibration
The CO
2
gas analyzers should not require recalibration due to our built-in, innovative “Auto-Zero”
function. During normal operation, the CIRAS-3 periodically passes “CO
2
free” air through the CO
2
gas
analyzers ensuring long term stability, accuracy and calibration of the gas analyzers. This function
corrects for nearly all changes that can affect calibration including cell reflectivity, changes in electronics,
etc. The most common cause of a drift in CIRAS-3 readings is an incorrect Zero due to either exhausted
chemicals, leaks on the Zero columns from pinched or damaged O-rings or the Zero absorber assembly
is not properly seated in the console manifold. Another likely cause of drift in calibration is contamination
of the sample cell (liquid water) which is more serious though fortunately less common. We do
recommend periodic checks of the gas analyzers by simply passing an accurate CO
2
gas mixture through
the instrument. Another simple way to check of the CO
2
analyzers is to sample ambient air in an area
away from any CO
2
influences or disturbances (i.e. roadways, air conditioning/ventilation systems,
individuals breathing, etc.). We know that the typical ambient CO
2
concentration is approximately 375-
400 µmol mol
-1
. To check the CO
2
analyzers using ambient air as your reference:
Power up the CIRAS-3 console on its own (no leaf cuvette attached and no CO
2
cartridge installed) and
allow the instrument to warm up for 30 minutes.
1. Ensure that all desiccants are fresh.
2. Go to
Settings (F2)
and at “Settings File”, select Analyzer Only and OK.
3. Press Accept to return to the main measurement screen and observe the CO2r and CO2a values.
Both values should be approximately 390 µmol mol
-1
or close to your local ambient CO
2
concentration. If
the values are close to what you would expect then the analyzers are more than likely fine. If the
readings are far off from the expected ambient CO
2
concentration, a local or factory recalibration is
recommended.
If CO
2
recalibration is required, an accurate gas mixture is required. Refer to
on page 99
for instructions.
It is critical that the Zero columns are fresh before performing CO
2
recalibration
. It
is always best to recalibrate the analyzer using a gas mixture that is slightly above your normal
measurement range. For instance, if you routinely make measurements around 390-450 µmol mol
-1
, use
a gas calibration mixture around 500 µmol mol
-1
.
