LI-COR LI-6400, Manual

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Making Measurements
Leaks
Using the LI-6400 4-43
¥
Check the seal around leaves and stems
When the gaskets are compressed around thicker leaves and stems, small gaps
will be created. At high flow rates, this may not be a problem, but at lower
rates, be sure to seal them with putty or gum.
If bulk flow leaks are there, they will be a bigger problem at low flow rates,
than at high flow rates.
Diffusion Leaks
CO
2
moves from high concentrations to low concentrations. It does this not
only through air, but also through solids, including many plastics and synthet-
ic rubbers. CO
2
can pass through Bev-a-line tubing (polyethylene, lined with
ethylene vinyl acetate), through gasket material (ethylene and neoprene), and
through O-rings (butyl rubber). It goes through just about everything thatÕs
not glass or metal. There are some thin-film materials that have very low per-
meabilities to CO
2
, however, such as Teflon, Saran (polyvinylidene chlo-
ride), Mylar, and Propafilm
¨
(polypropylene coated with Saran).
Diffusion of CO
2
into or out of the LI-6400 leaf chamber is proportional to
the difference between the inside and outside CO
2
concentrations. It is useful
to think of this diffusion leak as a flux of CO
2
. When the bulk flow rate
through the chamber is high, this diffusion flux will affect the chamber con-
centration very little. When the bulk flow is low, that same diffusion flux will
have a much larger effect. This model is presented in Figure 4-9, and predicts