CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
12.3.3 AUTO-ZERO (menu point 2)
The MINI-PAM, like any other chlorophyll fluorometer, is not
absolutely selective for chlorophyll fluorescence but also shows a
small signal when no plant sample is in contact with the fiberoptics.
This false signal originates from traces of scattered measuring light
which reach the photodetector despite the blocking filters. Most
importantly, this signal is not dependent on properties of the
investigated sample and, therefore, is constant as long as the
measuring light intensity (menu point 50) and the gain (menu point
49) are not changed.
The false signal can be automatically subtracted from all
measured fluorescence signals by the AUTO-ZERO function (menu
point 2). For this purpose, the sample is removed and in menu
position 2 AUTO-ZERO is carried out via SET. Thereafter the signal
(F) without a sample fluctuates around 0 and sample specific
fluorescence can be assessed. Any changes in GAIN (menu point 49)
or MEAS-INT (menu point 50) lead to corresponding changes in the
offset voltage caused by the false signal. Therefore, in this case
AUTO-ZERO has to be repeated. If this is not done and a new
YIELD-determination is made via START, there is the warning?
NEW OFFSET?, which reminds the user to first carry out AUTO-
ZERO (without sample) in order to determine YIELD correctly. If
the user prefers to keep the old offset value, the warning can be
overruled simply by pressing SET (while in menu position 1).
Any false signal which is not compensated by AUTO-ZERO
(menu point 2) or manually by ZERO-OFFS (menu point 44) will
lead to underestimation of Y (∆F/Fm' or Fv/Fm). Normally, i.e. with
a leaf at 10-15 mm distance from the fiberoptics, the error is small
(approximately 2 %). The error can increase considerably, when
samples with low chlorophyll content and unfavorable geometries
are assessed. In such cases, the signals can be made large by
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