26
Figure 3.2. Simple post mount in concrete base.
least affects the data. In the case of a wind mast, the mast should be placed where the
obstruction alters the wind field of non-prevailing winds. Distances from growing vegetation should
be increased to account for any future growth.
3.5.2 Instrument platforms
Instrument stands can be as simple as a vertical post holding a single pyranometer or as complex
as a raised platform that can hold a large number of individual instruments and trackers. In all
cases, the platform must be stable over long periods of tim e, resisting warping by changes in
temperature and humidity, and be immovable in strong wind conditions (to within ±0.05°). In most
climates, wooden platforms should not be used because of their tendency to warp with humidity
and seasonal changes and because of attack by insects. In temperate climates platforms made
of steel or aluminum provide both the necessary stability and durability required for radiation
measurements. In hot climates though, these may be inappropriate because of extreme heating
(both with respect to expansion and ease of access due to heating). Reinforced concrete, or
concrete and steel structures, when expansion is considered, are probably the optimal materials
for the construction of stands, whether they are simple posts or complex platforms.
The base of any post or platform either must be firmly attached to a building or dug into the
ground. In the latter case, the base of the structure should be anchored at a depth below any
material that may be subject to heaving due to frost or water. A local or national building code,
where available, will provide excellent information on both the depth to which posts must be
implanted and the means in which this is best accomplished. If further information is required, a
qualified mechanical or civil engineer familiar with the location should be consulted. Figure 3.2
illustrates a typical post installation for a continental site in well drained soil, while Figure 3.3
illustrates the more complex Canadian platform required to elevate the instruments above the
local horizon.
Along with the structural integrity of the platform, the height of the platform above the surface must
also be carefully considered. As previously mentioned, in built-up areas up-facing sensors can be
located on the top of buildings to overcome local horizons. In more rural areas instruments can be
located as close as 1.5 m above the surface. In the latter case, consideration must be given for
terrain effects such as blowing or accumulating sand or snow. When instruments are placed
higher than approximately 1.5 m, a means of accessing the instrument for cleaning must be
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