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5
MAINTENANCE
Once installed the pyranometer needs little maintenance. The outer dome
must be cleaned and inspected regularly, e.g. every morning. On clear
windless nights the outer dome temperature of horizontally placed
pyranometers will decrease, even to the dew point temperature of the air, due
to IR radiation exchange with the cold sky. (The effective sky temperature can
be 30 °C lower than the ground temperature). In that case dew, glazed frost or
hoar frost can be precipitated on the top of the outer dome and can stay there
for several hours in the morning. An ice cap on the dome is a strong diffuser
and increases the pyranometer signal drastically up to 50% in the first hours
after sunrise. Hoar frost disappears due to solar radiation during the morning,
but should be wiped of as soon as possible manually.
Another periodic check should ensure that the instrument is level and that the
silica gel is still coloured orange. When the orange silica gel in the drying
cartridge is turned completely transparent (normally after several months), it
must be replaced by active silicagel as supplied in the small refill packs. The
content of one pack is sufficient for one complete refill.
Apart from that it is good to visit the pyranometer (or any other sensor)
regularly to check its condition. (desiccant, dirt on dome, levelling of
instrument and condition of cabling)
Some tips to check when changing the desiccant:
Make sure the surfaces of the pyranometer and the
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cartridge that touch the rubber ring are clean (corrosion can do a lot of
harm here and dirt, in combination with water, can cause this)
The rubber ring is normally coated with a silicon grease
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(Vaseline will also do) to make the seal even better. If the rubber ring
looks dry apply some grease to it.
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Check that the metal spring that retains the drying cartridge
MAINTENANCE
35
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