27
Figure 3.3. The support structure used to elevate instruments above the
local horizon. The structural steel and concrete support structure at the
Bratt’s Lake Observatory, Meteorological Service of Canada.
provided. This can vary from a permanent deck structure to a simple step ladder, remembering
that the easier the access to the instrument the more likely the instrument will be well maintained.
If the instrument is to be mounted on the roof of a building care must be taken to guarantee that
the instrument will not be blown off during high winds. The secure anchoring of the instrument
stand should be done in consultation with the building manager or engineer. If possible, a
permanent installation with the instrument stand bolted to the building is preferable to the use of
stands set on the roof and secured only by heavy weights.
Depending on the site, further measures may be required to ensure the stability of the
pyranometer platform during high wind conditions. Extra guy-wires or bracketing may be added to
keep the stand from oscillating.
3.5.3 Cables
3.5.3.1 Signal cables
Just as important as determining the best field of view for the instruments, is the routing of the signal
cable from the instrument to the data acquisition system. As most surface-based radiometers are thermopile
instruments, the maximum signal is usually in-the-order of 10 mV for a 1000 W m flux or 10 :V W
-2 -1
m . Such small signals can be affected easily by large line resistance, due to long cable lengths, and
2
electrical interference from other sources, particularly AC power lines running parallel to the signal
lines. Several suggestions follow to aid in the design of the measurement system.
(1) All signal cables should be twisted wire configured as a ground and signal pair sheathed in
a foil wrap. The outer sheathing of the signal cable should be based upon the clim atic regim e
of the station and the overall EMF to which the cable is to be subjected. It is recommended
that cables be m ade of stranded copper for flexibility.
(2) Cable lengths should be kept as short as practically possible. The overall length of the cable
is dependent upon the remoteness of the measurement platform and the type of data acquisition
system being used to sample the signal. Types of data acquisition systems are discussed
in Section 6.
Where long cables are required and the total resistance of the cable is greater than 10 S
(approximately 50 m), a pre-amplifier should be placed at the instrument end of the system.
Extreme care should be taken with this solution because of the temperature dependency
and non-linearity of electronic components.
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