CIRAS-3 Operation Manual V. 1.09 197 support@ppsystems.com
Boundary Layer Determination (r
b
)
Gas mixing efficiency in leaf cuvettes is tested in the factory by measuring the boundary layer resistance
(r
b
) over an area of exposed wet filter paper, which simulates a leaf. This is based on the method of
Parkinson (1985) and Parkinson & Day (1990).
A close approximation of r
b
is needed for three important gas exchange calculations used in CIRAS
firmware: leaf temperature derived from the Energy Balance method, stomatal conductance (g
s
) and sub-
stomatal CO
2
concentration (C
i
). Energy Balance is perhaps less important in the PLC3 (U) cuvette, due
to its built-in IR sensor used for determining leaf temperature. It is always needed for the calculation of g
s,
the inverse of stomatal resistance (r
s
):
(
)
=
(
×
)
This is essentially the total leaf resistance less r
b
, with a correction for the proportion of stomata
distributed between the adaxial and abaxial leaf surfaces. Boundary layer resistance is relatively weak as
compared to the resistance of the stomata to transfer of gas molecules. For a thorough discussion of leaf
external and internal resistances see e.g. P. S. Nobel, 2009, Physicochemical and Environmental Plant
Physiology, 4
th
Edition. Boundary layer resistance is also a factor in the von Caemmerer and Farquhar
(1981) equation determining leaf conductance for CO
2
transport (g
c
), where r
s
and r
b
are associated with
different H
2
O:CO
2
diffusion coefficients in air and in the leaf boundary layer, and subsequently used in the
calculation of C
i
:
(
)
=
(
. ×
)
+
(
. ×
)
×
For a PLC3 Universal cuvette with the circular insert, the area of filter paper is 18 mm diameter (2.5 cm
2
projected area), exposed on both surfaces. For a PLC53 (N/C) the filter paper area is 3 cm x 1 cm (3 cm
2
projected area), forming the center part of a 1 cm wide simulated leaf which is suspended across the
entire width of the cuvette, exposed on both surfaces.
The factory-calculated r
b
value is always provided with your system documentation. The r
b
value should
be recalculated if it is lost and periodically checked as the cuvette is used. An appropriate typical value
can be used temporarily, with the understanding that any data must be recalculated using the correct r
b
value.
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