Calibration
CIRAS-2 Operator's Manual Version 2.04
- 143 -
Calibration
CIRAS-2 is a very stable instrument and should NOT require
recalibration.
This is because the ZERO function corrects for nearly all changes that give rise to drifts in calibration.
The common cause of a drift in CIRAS-2 readings is an incorrect ZERO due to either exhausted
chemicals or to leaks on the ZERO columns from rolled or damaged O-rings etc. This is therefore easily
corrected.
The second cause, though fortunately less common, is liquid water through the cells, affecting their
reflectivity and therefore the linearity.
The only reason to re-calibrate CIRAS-2 is to use your own laboratory standard, rather than relying on our
calibration. Regularly
check
the CIRAS-2 calibration as changes signify instrument problems. For CO
2
,
use the internal checking against the mixture generated by CIRAS-2 or a standard gas and for H
2
O, check
against an H
2
O calibrator or other standard source. Another quick check for CO
2
is to use ambient air
drawn from a position away from any CO
2
source such as cars, boilers, breath etc. Ambient air is in the
range 340-360ppm. CO
2
and H
2
O calibrations and checking are done separately but they may be done
consecutively in any order. The procedure for checking or calibrating is similar.
The contents of the absorber columns must
NOT
be exhausted. The analyzer must have been switched
on for at least 20 minutes minimum.
Required equipment/supplies -
A standard gas for CO
2
calibration or .if using the CIRAS-2 internal standard, then a CO
2
cylinder
should have been recently inserted.
A source of known water vapour content for H
2
O calibration. (We strongly recommend using our
Water Vapour Calibrator supplied with the CIRAS-2).
**** WARNING ****
It is absolutely critical that the analyzer ZERO columns
(columns 1 & 2) are fresh before performing CO
2
/H
2
O
calibrations. If the chemicals are not fresh, it will cause
the ZERO to drift, thus causing an inaccurate calibration.
External connections for CO
2
Calibrations
CO
2
calibrations using the internal standard
Connect the cuvette air supply AIR OUT and the REF inlet through a T-piece with 30cms of vent pipe on
the third leg. The CO
2
calibration concentration should be set to 0000. On entering calibration CIRAS-2
will switch on the air supply to 300 ml/minute and will fully open the CO
2
control valve.
CO
2
calibrations using a gas cylinder
The gas cylinder must
not
be connected directly to CIRAS-2. Instead, connect through a T-piece. One
leg goes to the REF inlet, the 2
nd
leg to the gas cylinder, and the remaining leg vents to atmosphere
