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2.6. Maintenance
Once installed the pyrgeometer needs little maintenance. The dome/window must be cleaned and
inspected regularly, ideally every morning. On clear windless nights the dome/window temperature of
horizontally placed pyrgeometers will decrease, even to the dew point temperature of the air, due to
infrared radiation exchange with the cold sky. (The e ective sky temperature can be 30°C lower than
the ground temperature). In this 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
di user and increases the pyrgeometer signal drastically, up to 100% 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.
The dome of the pyrgeometer can be ventilated continuously by a heated blower to keep the
dome/window above the dew point temperature. The need for heating strongly depends upon local
climatological circumstances. Generally, heating is advised during cold seasons when frost and dew
can be expected. The Kipp & Zonen CV 2 ventilation unit is specially designed for unattended operation
under most weather conditions and has a choice of heating levels.
Note that the CGR 3 pyrgeometer cannot be used with the CV 2 ventilation unit.
A periodic check is to ensure that the pyrgeometer is level and that the silica gel desiccant 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 fresh silica gel as supplied in the small refill
packs. The content of one pack is su cient for one complete refill. At the same time check that the
pyrgeometer mounting is secure and that the cable is in good condition.
Some tips when changing the desiccant:
-
Make sure the surfaces of the pyrgeometer and the drying cartridge that touch the rubber o-ring
are clean (corrosion can do a lot of harm here and dirt, in combination with water, can cause this);
-
The rubber o-ring is coated with a silicon grease to improve the seal. If the rubber o-ring looks dry
apply some grease to it (Vaseline will also do);
-
Check that the drying cartridge is tightly threaded into the pyrgeometer body.
It is very di cult to make the pyrgeometers hermetically sealed; so, due to pressure di erences between
the inside and the outside of the instrument, there will always be some exchange of (humid) air.
The pyrgeometer sensitivity changes with time and with exposure to radiation. Calibration every two years
is advised. Further information about Kipp & Zonen recalibration services can be found in Appendix V.
CGR 4 Manual
Page 15
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