Leaf Porometer Operator’s Manual
6. Leaf Porometer Theory
63
From the Leaf Porometer, the two distances are:
d
1
= 3.35 mm
d
2
= 11.43 mm
Cooling of the evaporating surface.
The equations
developed so far assume that the evaporating surface
temperature is equal to the sensor 1 temperature. A more
realistic assumption is that it cools in proportion to the flux
of water vapor, which is proportional to the vapor pressure
difference between 1 and 2. The effect of this cooling is to
reduce the apparent humidity at the evaporating surface.
The actual vapor pressure at the evaporating surface is
(18)
Where
s
is the slope of the saturation vapor concentration
function. The temperature difference can be computed
from an energy balance for the evaporating surface:
(19)
Where
is the latent heat of vaporization, so then
)
(
)
(
1
1
ao
a
atm
a
s
o
T
T
sP
T
e
e
−
−
=
)
(
)
(
2
1
2
1
C
C
g
F
T
T
g
c
H
d
vapor
ao
a
H
p
−
=
=
−
=
λ
λ
