CIRAS-3 Operation Manual V. 1.09
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drifts. Auto Zero minimizes effects on span (gas sensitivity), of sample cell contamination, lamp ageing,
changes in detector sensitivity, amplifier gains and reference voltages. Measurements are ratioed to the
Zero reading before IR absorbance is determined. From the relationship between absorbance and
concentration determined in the factory for each instrument, and the current calibration factor, the sample
concentration is determined.
We overcome short-term drifts by use of a second mode, called Differential Balancing or Diff Bal. Using
Diff Bal temporarily diverts
only
Reference air through all cells (Reference and Analysis). If existing
offsets are detected between the Reference and Analysis cells while measuring the same Reference gas
sample, appropriate correction factors are calculated to equalize the readings of each cell pair. This way,
you can have confidence that a reported differential between the Reference and Analysis cell pair is real
and not artificial.
An overview of the gas circuit design of CIRAS-3 configured for leaf-level photosynthesis is shown in the
schematic on the following page. Sample air, denoted as AIR IN, entering the console and is first
pumped and its flow rate metered, then directly “conditioned” by passing through CO
2
and water vapor
absorbent chemicals. At this point CO
2
can be added to the gas stream in a precise mixture from the CO
2
source cartridge, while existing water vapor can be removed from the gas stream or allowed to remain at
its current partial pressure, measured in mb. One portion of the mixed air (AIR OUT) is then passed
along downstream to be measured at the Reference IRGAs. The other portion is sent to the leaf chamber
before returning to the Analysis IRGAs. REF (Reference) and AN (Analysis) air are drawn into the IRGAs
at precisely controlled flow rates by respective REF and AN pumps. The IRGAs are contained in a
rugged, sealed case and taken together form the thermally-stable optical bench.
