50
••••
Measurement options
Document code: SS1-UM-1.05
Finding North, and setting the shade ring
If you are taking measurements which require only the incident total radiation on
the canopy, you can ignore the compass orientation of the BFS and you should set
the shade ring horizontal so that it is not in use.
If, however, you want the Direct and Diffuse components of the incident radiation,
you must elevate the shade ring so that its shadow falls across the Diffuse sensor
(the one directly in line with the shade ring pivots), covering it completely.
If you are moving the BFS frequently, the compass orientation of the BFS need not
be accurately set to north. If you want the shadow to track correctly for longer
periods of time, set the BFS to point true north more carefully. The same instructions
apply if you are using the BFS in the southern hemisphere.
The shadow may not track perfectly throughout the course of a whole day, owing to
limitations imposed by the simple and robust design of the BFS. You should inspect
the BFS from time to time and readjust the ring if necessary.
In overcast conditions, the exact elevation of the shade ring is not critical (and it is
quite difficult to estimate!). If there is any chance of direct sun breaking through,
you must wait for it to do so before setting the elevation.
Levelling the BFS
The BFS is equipped with a miniature bubble level. The tripod supplied has 3-axis
adjustment to facilitate levelling.
It is more important to level the BFS accurately than the probe.
The usual routine for setting up the tripod will be:
•
Orientate the BFS to face true north.
•
Level the BFS.
•
Elevate the shade ring to bring the shadow completely across the Diffuse sensor
(the northernmost one).
Extension cables, and the location of the BFS
Extension cables of 10, 25 and 50 m can be fitted between the BFS and the probe,
which will extend your range of operation from the BFS. There is a trade-off
between range and convenience: the greater your range, the fewer times you need to
re-site the BFS, but the more time you are likely to spend handling the cable.
Extension cables can be joined together. A combination of two shorter cables may
be preferable to one long one.
You should bear in mind that the SunScan will read the BFS and probe
simultaneously. If the different locations are widely spaced apart, the light levels
could momentarily be different - cloud shadows can easily travel at 20 m.s
-1
.
The solution is to be aware of this possibility in fast changing conditions and avoid
taking readings at critical moments.
Very long cable lengths may introduce a small systematic error in BFS readings. Up
to 100 metres, this should not be significant (< 10
µ
mol.m
-2
.s
-1
). At 200 metres it
could add about 20
µ
mol.m
-2
.s
-1
to the readings, which may need subsequent
adjustment .
