LI-COR LI-6400, Manual

Page: 75 / 846

Guided Tours
Tour #3: Controlling Chamber Conditions
Using the LI-6400 3-23
sample ( H2OS_mml ) will increase as well, but not as much, since the paper
is not able to evaporate as much water into the more humid air.
7 Change to a low flow rate: 100 µmol s
-1
.
Press F2 then type 100 . (Notice the short cut: when you arenÕt changing con-
trol modes , you can just start typing the target value after you press the func-
tion key. In our case, we didnÕt need to press F again to specify fixed flow
mode.)
8 Note the sample and reference humidities
The reference value hasnÕt changed, but the sample value increased, because
the air is going through the chamber at a much slower rate, thereby having a
longer exposure to the evaporating paper. In fact, you may start getting ÒHigh
Humidity AlertÓ flashing on the center line of the display. If it does, ignore it.
(Warning messages are discussed later, on page 3-26.)
9 Turn the desiccant to full scrub
Observe that the reference and sample humidities fall, and note that a) the
sample value comes down slowly because of the slow flow rate
4
, and b) the
reference goes back to near zero, but c) the sample doesnÕt return to the value
you had in Step 4, because now the flow is slower.
Fixed Humidity Operation
One of the powerful features of the LI-6400 is itÕs ability to operate in a fixed
humidity mode. It does this by actively regulating the flow rate to maintain a
4.
If you have a CO
2
mixer, the reference value will decrease rapidly, because Òex-
cessÓ flow is routed to the reference line (Figure 1-2 on page 1-5).
Points to Remember
¥ At equilibrium, reference humidity is determined solely by the desiccant
tube scrub setting; it doesnÕt depend on flow rate.
¥ At equilibrium, sample humidity is determined by
a) desiccant tube scrub setting
b) flow rate
c) evaporation from the leaf.
¥ The lowest chamber humidities are achieved by high flow and full scrub-
bing, and the highest by low flow and not scrubbing. Between these limits,
any chamber humidity can be achieved by various combinations of flow
rate and scrub setting. (For a picture of this, see Figure 3-39 on page 3-47.)