LI-COR LI-6400, Manual

Page: 78 / 846

Guided Tours
Tour #3: Controlling Chamber Conditions
3-26 Using the LI-6400
3
10 Enter a target thatÕs too wet
Press
f2
and enter a wet target, such 5 mmol mol
-1
above the original value
used back in Step 4 (e.g. 25, since we used 20). Soon the message
>> FLOW: Need
¯
SCRUB or drier target <<
will be flashing on the center of the display. Notice that the flow rate is about
30 µmol s
-1
if you have a CO
2
mixer, or near zero if you donÕt. Change the
desiccant to full bypass and wait; the target humidity may or may not be
achieved, and in fact you may even get ÒHigh Humidity AlertÓ messages
while you are waiting.
11 Return to the original target
WeÕll end this experiment by returning the target to the original value, and the
desiccant to the mid-range setting.
About Warning Messages
While in New Measurements mode, there are a handful of situations that
will cause OPEN to alert you to possible problems. Experiment #2 illus-
trated two. The complete list of possible causes is in
New Measurements
Mode Warning Messages on page 20-7. When such a situation occurs, a
warning message is displayed on the middle label line on the display, as
illustrated in Figure 3-20.
If you wish to disregard the displayed message and get it out of the way,
press
ctrl Z to turn it off (hold the ctrl key down and press Z ). Note that this
is a toggle: if you press ctrl Z again, the message will re-appear. Also, note
that each time you re-enter New Measurements mode, OPEN resets this
flag, so that warnings will again be displayed.
Points to Remember
¥ Sample humidity is a balance between what is coming from the leaf, and
the flow from the console: how dry (desiccant scrub setting) and how fast
(flow rate).
¥ If you ask for humidities outside what can be achieved, given a desiccant
tube scrub setting and leaf transpiration rate, youÕll get a warning message.
¥ These flow warnings can be remedied by adjusting the scrub knob, or
changing the target value, or sometimes just by waiting.