BF5 User Manual v 2.3
Appendix A: Design and Test Summary
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Appendix A: Design and Test
Summary
This appendix gives a brief description of how the BF5 design works, and
a summary of the results of the test program. More detailed versions of
these are available from Delta-T and on the
Software and Manuals DVD
.
Introduction
Measurement of Direct and Diffuse components of solar radiation has
many applications - in modelling the interaction of light with crop canopies,
studying the energy balance of structures, or as a meteorological
indicator. Instruments that make these measurements have generally
been expensive and require considerable attention.
One common approach has been to have two sensors, one measuring
radiation from the whole sky, the other measuring the whole sky apart
from the sun. The shading is generally done using a shade ring, adjusted
to match the track of the sun across the sky for that day, or using an
occluding disk held on a robot arm. Both of these approaches require
accurate alignment to the Earth's axis, and regular adjustment.
Another well established approach is the Campbell-Stokes recorder,
which uses a glass sphere to focus the Direct solar beam onto a recording
chart causing a burn, which indicates direct beam strength.
Design objectives
The aim of the BF2/3/5 design was to measure the Direct and Diffuse
components of incident solar radiation, and provide a measure of
sunshine hours, in a sensor that used no moving parts, and required no
specific polar alignment or routine adjustment. The outputs should be
compatible with electronic dataloggers, and work at any latitude.
How the design evolved
The prime requirement for this design was to create a system of
photodiodes and a shading pattern such that wherever the sun is in the
sky:
at least one photodiode was always exposed to the full solar beam
at least one was always completely shaded
both photodiodes receive equal amounts of Diffuse light from the
rest of the sky hemisphere.