The LI-6400XT has two absolute CO2 and two
absolute H2O non-dispersive infrared analyzers
in the sensor head. Mounting the analyzers in
the sensor head provides a number of advantages
over systems which place the analyzers in the
console:
1. Changes in the leaf dynamics are measured in
real time. Because there is no tubing between
the leaf chamber and the console, there are no
time delays confounding effects of changes in
environmental driving variables such as light,
CO2 mole fraction, etc.
2. CO2 and H2O concentrations are controlled
at the leaf suface, because the fast response
IRGAs are built into the mixing volume of
the leaf curvette. This is important because,
for example, it allows you to easily make
photosynthesis comparisons across treatments
with widely differing rates, at a constant CO2
concentration, thus avoiding biases in your
conclusions.
3. CO2 and H2O concentrations are continuously
measured at both reference and sample
cells, eliminating flow swapping, which provides
fast response times, minimizes noise and
drift, and gives more accurate photosynthesis
measurements.
4. Rapid, automatic control of chamber humidity
is possible, even when the transpiration
rate is changing.
5. Absence of tubing between the chamber and
the analyzers eliminates equilibration times
due to water vapor sorption on the tubing
walls.