LI-COR LI-6400, Manual

Page: 140 / 846

Making Measurements
Making Survey Measurements
4-22 Using the LI-6400
4
CO
2
Since photosynthesis is a function of CO
2
, it is important to have the chamber
CO
2
concentrations as consistent as possible.
If you do not have a CO
2
mixer, youÕll have to carry along a buffer volume
to dampen the potentially huge fluctuations in CO
2
that will occur should you
choose to breathe while you work. Buffer volumes are discussed in Air Sup-
ply Considerations on page 4-47. If you use a long tube
4
and a pole to draw
the ÒcleanÓ air from well above your head, you will still need a buffer volume,
albeit a smaller one might suffice. Whatever you use, experiment until you
have fairly steady reference CO
2
concentrations.
Life is much easier with a CO
2
mixer. You only need to decide whether to
control reference or sample CO
2
. If measurement speed is important, then by
all means control on reference. If you want near-ambient values in the sample
cell, set the reference for the right amount above ambient. Try a leaf or two
until you get it right. On the other hand, if you can afford 2 or 3 minutes per
measurement and want consistent sample CO
2
concentrations, try the S op-
tion ( f3 level 2).
Flow / Humidity
Use a fixed flow rate, medium or high, with little or no desiccant scrubbing.
HereÕs the rationale: Fixed flow rate minimizes the time for system equilibra-
tion, once a leaf is installed. Minimal scrubbing along with a high flow rate
means the chamber humidity will be reasonably close to ambient.
There are interactions with CO
2
control. If you are using the CO
2
mixer, the
soda lime must be on full scrub, and that usually means the incoming humid-
ity will be below ambient, even with the desiccant on full bypass. You can
moisten the soda lime (see
Humidifying Incoming Air on page 4-50) and/or
reduce the flow rate a bit to offset this.
Temperature
There are two schools of thought about temperature control and survey mea-
surements: one is that you shouldnÕt use the coolers so that your battery life
is maximized. The other says you should use the coolers to maintain ambient
temperature, so that the chamber doesnÕt get hot being in the sun. You decide.
4.
Make sure its inside diameter is larger than 1/8 inch, to avoid pressure drops
and reduced pump performance.