LI-COR LI-6400, Manual

Page: 152 / 846

Making Measurements
Matching the Analyzers
4-34 Using the LI-6400
4
Matching the Analyzers
The purpose of matching is to remove offsets between the sample and refer-
ence analyzers caused by small variations in temperature, flow rate, calibra-
tion, drift with time, etc.
Matching the analyzers improves the accuracy of your measurements, espe-
cially when working with low photosynthesis rates. Recall from Equation
(1-15) on page 1-10 that photosynthesis is proportional to the measured CO
2
differential:
(4-1)
If C
r
= 360 µmol mol
-1
and C
s
= 330 µmol mol
-1
, and there is a 1 µmol mol
-1
offset between the IRGAs, then the photosynthetic rate A is in error by 1/30
or 3.3%. If, however, the differential is small (for example C
r
= 360 and C
s
=
355), then the error in photosynthetic rate due to a 1 µmol mol
-1
offset is 1/5
or 20%. Clearly, the smaller the differentials, the more important matching
becomes.
The first step to matching is making the IRGAs see the same air. This is a me-
chanical exercise accomplished by a small valve on the bottom of the cham-
ber/IRGA (Figure 4-4). Outgoing chamber air is sent to the reference cell, and
the air that normally goes to the reference cell is diverted. The second step is
to make the IRGAs read the same, and this is a mathematical operation. The
equations for sample H
2
O and CO
2
(page 14-5 and page 14-6) contain adjust-
ment terms ( W
ms
and C
ms
), and it is these that are changed when matching.
Thus, the sample, not the reference, CO
2
and H
2
O values are adjusted.
A C
r
C
s
Ð
µ