Delta Electronics SS1-UM-1.05 Finding North, and setting the shade ring, Levelling the BFS

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 ∝ mol.m-2.s-1). At 200 metres it could add about 20 ∝ mol.m-2.s-1to the readings, which may need subsequent adjustment .

50 • Measurement options Document code: SS1-UM-1.05