Environmental Control
CO2 Control
7-14
Using the LI-6400
7
Constant Reference Option
Usually the preferred option is R (constant reference, or incoming, CO
2
con-
centration). When you enter a target concentration, the injector will adjust it-
self to bring the reference cell concentration to that value and hold it.
Depending on how big the adjustment is between the present value and the
one you want, it may take a few seconds or a few minutes to get there (going
up always takes longer than coming down). Once the concentration is
achieved, there is very little that will cause it to change,
2
so maintaining it is
usually no problem. If it does drift, the software will attempt to bring it back
on target.
Constant Sample Option
This option has the advantage of maintaining what the leaf actually ÒseesÓ as
a constant, but it is not as tight a control loop. This is because there are things
that affect this concentration that the control loop
cannot
control, like photo-
synthetic rate of the leaf, and flow rate through the chamber (the latter being
fully in the domain of the humidity controller). All the CO
2
controller can do
is regulate CO
2
concentration coming into the chamber, once those other
things are stable.
■
■
■
■
For best results with the S option:
1
Start out with the R option
Specify a target 20 or 30 µmol mol
-1
above
what you want in the chamber,
and wait for the system to stabilize.
2
Switch to the S option
Once things are stable, the controller can quickly lock in and hold a constant
sample concentration. Subsequent targets can then be specified for the sample
concentration, but if it really looses control, you can always drop back to the
R option and bring things back under control.
Unlike the constant humidity control options, which depend on the leaf pro-
viding lots of water, the S) option doesnÕt depend on
anything
from the leaf,
other than perhaps reasonably stable CO
2
assimilation. In fact, it works well
with no leaf at all (as long as the chamber is closed, of course
3
).
2.
Temperature and pressure changes, bad soda lime, or the soda lime tube not on
full scrub.
3.
Opening the chamber might seem to be a way to really mess up the constant
sample CO
2
option, but usually the subsequent sample cell CO
2
fluctuations are
sufficient to keep the controller from even
trying
to control it.
Summary of Contents for LI-6400
Page 1: ...Using the LI 6400 Portable Photosynthesis System ...
Page 15: ...Part I The Basics ...
Page 16: ......
Page 174: ...Making Measurements Answers to Questions 4 56 Using the LI 6400 4 ...
Page 175: ...Part II Useful Details ...
Page 176: ......
Page 200: ...Standard Tools Power ON Hooks 5 24 Using the LI 6400 5 ...
Page 214: ...Real Time Data Real Time Graphics 6 14 Using the LI 6400 6 ...
Page 234: ...Environmental Control Light Control 7 20 Using the LI 6400 7 ...
Page 244: ...Light Sensor Considerations Gallium Arsenide Phosphide GaAsP Sensor 8 10 Using the LI 6400 8 ...
Page 288: ...Data Logging Making Your Own AutoPrograms 9 44 Using the LI 6400 9 ...
Page 289: ...Part III Working With Files ...
Page 290: ......
Page 312: ...The LPL File System Troubleshooting 10 22 Using the LI 6400 10 ...
Page 340: ...Downloading Files Using a Data Capture Program 11 28 Using the LI 6400 11 ...
Page 375: ...Part IV Configuration Issues ...
Page 376: ......
Page 420: ...Defining User Variables Old Style vs New Style 15 18 Using the LI 6400 15 ...
Page 454: ...Using an Energy Balance Further Reading 17 12 Using the LI 6400 17 ...
Page 455: ...Part V Maintenance Troubleshooting ...
Page 456: ......
Page 572: ...Troubleshooting Useful Information 20 46 Using the LI 6400 20 ...
Page 593: ...Part VI Programming ...
Page 594: ......
Page 622: ...Programming with LPL Compiler Directives 22 28 Using the LI 6400 22 ...
Page 846: ...Index I 16 Using the LI 6400 ...