OPENÕs System Variables
Computed Variables
Using the LI-6400
14-9
Computed Variables
Computed variables donÕt have sensors of their own, but instead derive from
measurements of multiple sensors. There is no relative humidity sensor, for
example, but we can compute relative humidity from water mole fraction,
pressure, and temperature.
Humidity Variables
Vapor Pressure
The water IRGAs measure vapor concentration in mmol mol
-1
. To convert
this to vapor pressure, convert to mol mol
-1
and multiply by the total pressure
P
(kPa). Thus, given a reference measurement of water
W
r
, the reference va-
por pressure
e
r
(kPa) is
ID: -53
(14-17)
while in the sample cell, the vapor pressure
e
s
(kPa) is
ID: -54
(14-18)
Relative Humidity
Relative humidity is the ratio of vapor pressure to saturation vapor pressure.
The reference and sample relative humidity
h
r
and
h
s
(in percent) are given by
ID: -14
(14-19)
ID: -15
(14-20)
Saturation Vapor Pressure
The saturation vapor pressure function
e()
used by OPEN is from
Buck (1981)
3
:
e
r
W
r
P
1000
------------
=
e
s
W
s
P
1000
------------
=
h
r
e
r
e T
a
(
)
-------------
=
h
s
e
s
e T
a
(
)
-------------
=
3.
Buck, A.L. (1981) New equations for computing vapor pressure and enhance-
ment factor. J. Appl. Meteor. 20:1527-1532.
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 ...