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Measurement options
Document code: SS1-UM-1.05
You should not expect to have to do this very often. The photodiodes and light
measurement circuits are very stable.
The source of the apparent variability mentioned above is the not quite ideal cosine
and spectral response of the sensors.
Restoring the factory calibration
At any time after carrying out the
Recalibrate
option you can restore the original
factory calibration. You do not have to set up the probe or BFS in uniform light.
♦
On the Workabout, simply access the
Calibrate
procedure (
∪
∪
∪
∪
+C
) and select
Restore Factory calibration
.
You will briefly see a message confirming that this has been done.
Effect of the shade ring on the BFS
If the above matching tests are carried out with the shade ring elevated (but not
actually casting a shadow on either BFS sensor), you will get different results.
The shade ring blocks about 7% of the diffuse sky light, and none of the direct beam.
In bright sun conditions it will have no significant effect on LAI readings. In
completely overcast conditions it might contribute a small error of up to 0.1 to the
LAI value.
Comparing the calibration with other PAR sensors
You can carry out matching comparisons between the probe and BFS and any other
PAR quantum sensor. You cannot reset the probe values to it, but you can annotate
the readings and retain the comparison information in the SunScan data files.
Routine maintenance and cleaning
The probe diffuser is Perspex. Clean it when necessary with a damp cloth, moistened
with mild detergent, or with IPA (iso propyl alcohol).
The BFS diffusers are also made of Perspex. Clean them very carefully, especially
the groove around the diffusers. Use a fine soft brush to remove any dirt or dust
from the groove - it could affect the accuracy of the cosine response of the sensors.
The "spread" measurement
The "spread" value is a measure of the
relative
variation of the light along the probe.
This is a useful parameter in light profiling measurements: it is the value of the
standard deviation of the 64 photodiode readings, divided by their mean.
You can check the probe
uniformity
of calibration at any time by taking a reading in
uniform light. The spread value should be 0.00 or 0.01.
