TARGAS-1 Operation Manual V. 1.02
16
The gas sample is of course a mixture of gas molecules, and this can present problems in terms of
accurate detection of concentrations of a specific gas, such carbon dioxide. This effect,
foreign gas
broadening
(FGB), must be corrected to ensure accurate measurement of gas concentrations. With FGB,
the CO
2
gas in the IRGA cell is somewhat diluted by the increased air volume induced by water vapor.
This effect is about 0.1 µmol mol
-1
CO
2
mb
-1
H
2
O. The presence of water vapor also causes an increase
in infra-red absorption, which is detected as an apparent increase in [CO
2
]. This is of a similar magnitude,
but opposite to the dilution effect, and TARGAS-1 automatically corrects these FGB effects.
The TARGAS-1 IRGAs are quite stable owing to their construction, calibration and thermal environment,
but various circumstances can cause apparent changes over time. Some changes may require
recalibration, although one of the strengths of TARGAS-1 is that recalibration is not a routine (annual)
maintenance task. The factory calibration ranges of 0-2000 µmol mol
-1
CO
2
and 0-75 mb water vapor are
ideally suited for most typical applications.
The TARGAS-1 features an Auto-Zero function that corrects for nearly all changes that result in
calibration 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.
System Components for Measurement of Leaf
Gas Exchange
There are 3 main components that make up the TARGAS-1 Portable Photosynthesis System as follows:
TARGAS-1 CO
2
/H
2
O Gas Analyzer
The TARGAS-1 console (2.1 kg) features two, non-dispersive infrared
gas analyzers for CO
2
and H
2
O. It is constructed out of rugged
aluminum with polyurethane, shock absorbing base making it
extremely robust and reliable for use in harsh environmental
conditions. An internal air supply unit provides accurately controlled
reference air to the leaf cuvette and another pump draws the sample
air (analysis) air to the analyzer. Both pumps are user controlled and
accuracy is ensured by two internal electronic flow sensors. It is
