CIRAS-3 Operation Manual V. 1.09
71
While CO2d and/or H2Od=0.0 is ideal, it is common to see a small differential even under the best of
circumstances. With experience, you will be in the best position to decide what is or is not an acceptable
differential, and to take action to try to correct it. Whether an empty-chamber differential is acceptable or
not is often dependent on the scale of gas exchange rates expected over the course of your
measurements. A small differential will hardly be noticed when it occurs along with high photosynthetic
rates, but could present a significant problem if the focus of your data is miniscule rates of gas exchange,
such as occur near light compensation points and with dark respiration. If during this test CO2d >0.5 you
can try waiting a little longer in case this is being caused by small fluctuations in CO
2
control. In theory, a
CO2r fluctuation of even 0.1 µmol mol
-1
will result in a small transient differential. If CO2d remains >0.5
and stable, try running
Diff Bal (F7)
. Other causes could be imperfect gaskets or improper tension
adjustment of the cuvette. Keep in mind also that CO
2
in air surrounding the cuvette is more likely to
enter the Analysis gas stream if strong gradients exist. For example, suppose your CO
2
control value is
390 µmol mol
-1
and the test location is indoors where the CO
2
concentration is 600-800 µmol mol
-1
. CO
2
in the surrounding air can enter the chamber through any existing leak. Cuvette flow rate is another
factor that influences the magnitude of detectable leaks. Higher flow rates create a slight chamber
overpressure that can slow or prevent CO
2
in surrounding air from diffusing through the gaskets. From
this you might deduce that the most rigorous leak test involves creating a large gradient of CO
2
between
the air inside and outside of the leaf chamber, while supplying the lowest flow rate to the chamber.
Survey Measurements – Some Tips
There are a few basic points to consider before you begin recording data. In many instances, the leaf
blade/vegetation may extend beyond the foam gasket surrounding the leaf area opening. Try
approaching the leaf from the side with the cuvette head open to avoid accidental injury to the leaf.
Approach the leaf from its side margin, not the leaf tip, to avoid crushing it in the rear section of the leaf
chamber. Try to orient the cuvette to the leaf and do not severely twist the petiole. Use a tripod if
F1
F3
F2
F4
F6
F5
F7
F8
