CHAPTER 8 COMPONENTS OF THE MINI-PAM
diffuser clean. Also, it must be pointed out that there is some
decrease in sensitivity when the sensor is moved from the center of
the measuring field to its periphery. It is advisable to check
calibration regularly by comparison with a standard quantum sensor,
like the LI-190. Any deviation can be corrected by entering a
recalibration factor via menu point 41: LIGHT GAIN. A substantial
increase of the calibration factor from its original value of 1.00
indicates dirt-deposition on the diffuser, which may be reversed by
gentle cleaning using a cotton-tip, moistened with some alcohol. In
addition, it is possible to enter an offset value via menu point 40:
LIGHT-OFFS.
- Thermocouple monitoring leaf temperature
A NiCr-Ni thermocouple is mounted in the perspex tube on
which the studied leaf area is resting. Its tip is forming a loop which
gently presses against the lower surface of the leaf. In this way there
is effective temperature equilibration and the thermocouple is
protected from direct sun radiation. The reference couple is located
on the circuit board, in close proximity to the thermovoltage
amplifier (AD), enclosed in the bottom part of the holder. The
relationship between thermovoltage and temperature is almost linear.
With decreasing temperatures there is a small decline of ∆V/°C.
Calibration was performed at 25 °C. At 0 °C or -15 °C the deviation
amounts to 0.5 or 0.8 °C, respectively. An offset value can be entered
via menu point 42: TEMP-OFFS. The measured temperature is
displayed at the end of the first line of the LCD (...C) when the Leaf-
Clip Holder 2030-B is connected. Temperature resolution is 0.3 °C.
The temperature as well as the PAR data are automatically stored in
the memory after every saturation pulse, together with the
fluorescence data.
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