PP Systems CIRAS-2, Руководство оператора

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System Setup and Operation
CIRAS-2 Operator's Manual Version 2.04
- 46 -
First, we convert the radiation falling on the leaf into the energy absorbed. We measure the visible quanta
(PAR) and using the TRANS factor (see more at
Energy Factor on page 49) convert this to energy
absorbed. The value of TRANS depends on the energy distribution with wavelength of the source; the
position of the PAR sensor (inside or outside the cuvette); the transmission characteristics of the cuvette
windows; the reflection characteristics of the leaf surface; and the absorption characteristics of the leaf.
From the humidity measurements and flow rate, we can determine the energy lost in evaporation. From
the measurements of the boundary layer in the cuvette we can calculate the conductive heat exchanges
between the cuvette and air. By assuming that the cuvette wall temperature is the same as air
temperature we can calculate the radiation exchanges. Using all this information, we can calculate
average T
leaf
. See Calculations in Appendix 2. on page 153 for a detailed description.
Thermistor
Not recommended -
we do however make it available for use with all “Manual” and some “Automatic” type
Leaf Cuvettes. This option is not available with the PLC6 (U) or PLC5 (B) Automatic Broad Leaf Cuvettes
as these cuvettes have an infrared temperature sensor fitted (See IR below).
Direct Leaf Temperature Measurement - Common Problems
A leaf temperature sensor making physical contact against the underside of the leaf will have at least 50%
of its area exposed to the air rather than the leaf. If it is covered to insulate it from the air, then the area of
leaf that is also covered will not transpire nor will there be energy exchanges with the air. It will therefore
be at a different temperature than other areas of the leaf.
Very small sensors have to be inserted in the leaf to affect an accurate and direct measurement. But this
will cause local damage to the leaf, affecting transpiration and therefore the temperature. Heat conduction
along the wires that are in the air, to/from the sensor also has a significant effect. IR photography of
leaves also shows significant patchiness in leaf temperature.
Infrared (IR)
Only the PLC6 (U) and PLC5 (B) Automatic Broad Leaf Cuvette have this option available as it features an
IR sensor fitted to the base of the cuvette head.
If we can determine the net radiation exchanges between the leaf and a sensor of known temperature
then we can calculate the leaf temperature. The sensor consists of a thermopile with an upper blackened
absorbing surface facing the underside of the leaf. The temperature of the base of the thermopile is
measured. If the thermopile and leaf are at the same temperature then there will be no net energy
exchange, therefore no temperature gradient across the thermopile and no signal. If the leaf is cooler
then the thermopile surface will also cool relative to the base and vice-versa. The thermopile output is
calibrated against leaf simulants of known temperature so we can determine the leaf temperature.
The
one limitation of this method is that the field of view of the sensor must be completely filled with
leaf material.
Given that the area of the PLC6 (U) and PLC5 (B) Automatic Leaf Cuvette is very small,
most broad leaf types will completely fill the chamber making this method of leaf temperature
measurement the most accurate and recommended choice.
Please note: If the leaf does not completely cover the entire window area, (e.g. the PLC6 (U) 18mm
diameter circular opening in the center of the window), this method of leaf temperature measurement
MUST NOT BE USED. We then recommend that you select the Energy Balance method.
Leaf Area
User Specified. Cuvette/head plate selection defaults a defined Leaf Area value, for example, the PLC6
(U) cuvette will default to 2.5 cm
2
(18 mm diameter insert). The size and shape of the leaf may be
important in your decision as to which cuvette head plate to use (1.75 cm
2
, 2.5 cm
2
, 4.5 cm
2
). Leaf area
also has a significant effect on final calculations of photosynthesis. However, if recalculation of data is
required, this can be done fairly easily. See
Recalculate Data on page 81.