CIRAS-3 Operation Manual V. 1.09 160 support@ppsystems.com
e
in
is defined as the partial pressure of water vapor of reference air supplied to the cuvette, but not yet
inside the cuvette, and therefore uninfluenced by the cuvette stirring fans or the leaf itself. e
in
partial
pressure is determined by the Reference H
2
O IRGA.
e
out
is defined as the partial pressure of water vapor in the air inside the cuvette, surrounding the leaf.
This air is both highly mixed by the stirring fans and influenced by transpiration water vapor from the leaf.
e
out
partial pressure is determined by the Analysis H
2
O IRGA. As relates to the calculated values in the
CIRAS-3 display:
=
(
+
)
=
Leaf Temperature
Calculate leaf temperature (T
leaf
) from the energy balance (optional to IR sensor or thermistor method)
(3.1) From Parkinson, 1983, the difference between air and leaf temperature is:
=
×
.×
×
+
[
× (
(
+ )
)
]
where:
H = incident radiation absorbed by the leaf
λ = latent heat of vaporization of water
E = transpiration rate
M
a
= molecular weight of air
C
p
= specific heat at constant pressure
r
b
= boundary layer resistance to water vapor transfer, empirically determined for each cuvette by the
pseudo-leaf (filter paper) method. 0.93 converts it to that for heat transfer.
σ = Stefan Boltzmann constant
T
c
= cuvette air temperature
H is calculated from the photon flux incident on the cuvette (I), taking into account the ratio of infra-red to
PAR in the light source, transmission through the cuvette window (Trans), and reflection/absorption by
the leaf: (H = I x Trans).
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