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7
FREQUENTLY ASKED QUESTIONS (FAQ's)
The most frequently asked questions are listed below.
1.
Negative output during measurements?
This error is related to the zero offset type A. Normally this zero
offset is present when the dome has a different temperature from the
cold junctions of the sensor. In practice this is always the case when
there is very large and cold object close to the pyranometer. The
emitted heat by the glass dome is attracted from the body (by
conduction in the dome) and from the air (by convection and heat
conductivity). The dome is cooling down too and will attract heat from
the body by conduction and from the sensor by the net infrared
radiation. The latter heat flow is opposite to the heat flow from the
absorbed solar radiation and causes the well known zero
depression. This negative zero offset is always present, however,
hidden within the thermopile signal.
2.
What is the primary entry point for humidity?
The desiccant cartridge and cable gland have equal chances to
transport some moisture. Also the silicon glue of the domes is not
completely watertight.
When care is not taken one can easily make the desiccant
cartridge the primary entry point. See chapter 5 for the
maintenance of the CM 4 Pyranometer.
Note: Water vapour transport through the cable is also possible when
the open end of the cable at the readout device is in a humid
environment.
7 FREQUENTLY ASKED QUESTIONS
37
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