LI-COR LI-6400, Manual

Page: 168 / 846

Making Measurements
Operational Hints
4-50 Using the LI-6400
4
¥
Match the IRGAs
As the differentials become small, the effect of any offset error is magnified.
Using Closed Mode
We have experimented with using a closed system technique in the LI-6400
to handle low rates. This is accomplished by turning off the pump for 10 or
15 seconds, and measuring the rate of change of CO
2
with time, and H
2
O with
time. The LI-6400 can be programmed to accommodate closed measurements
interspersed at will among normal, open measurements. However, our tests
indicate that the repeatability and accuracy of the closed technique is no better
(and sometimes worse) than using the open method with a low (100 µmol s
-1
)
flow rate. Contact LI-COR for further information.
Humidifying Incoming Air
The LI-6400Õs humidity control balances the leafÕs transpiration with drier in-
coming air to maintain a desired humidity (see
Humidity Control on page
7-7). How dry the incoming air is depends on the manual adjustment knob of
the desiccant tube. The limitations of this approach become apparent when
measuring a small leaf area and/or low transpiration rate when it is desired to
have high humidity in the chamber. Another source of water besides the leaf
is needed.
One solution to this problem if you are using the 6400-01 CO
2
Mixer is to add
a small amount of water (10 ml) to the soda lime tube (Figure 4-14). After
about an hour of subsequent use, the water output becomes quite stable, and
remains so for many hours thereafter. The soda lime used for the experiment
in Figure 4-14 is supplied by W.R. Grace & Co, 5225 Phillip Lee SW, Atlan-
ta, GA 30336 (and is available from LI-COR as part #9960-071).
Caution
When adding water to soda lime, do it slowly, letting the chemical absorb the
liquid, and then let the tube sit for a few minutes before pumping air through
it. The goal is to avoid having any liquid get out of the tube, which can cause
oxidation of downstream metal components. Do not add any more than 10 ml.
of water to the tube.
Another solution is to use a brand of soda lime that has a higher water output.
A good example is that supplied by BDH Laboratory Supplies, Poole, BH15
1TD, England (product number 33115 AX). In situations where you need
fairly dry input air to the chamber, however, this material should be avoided,
as it can saturate the desiccant tube in an hour or two, making frequent chang-
es necessary.