Kipp&Zonen CNR 4 Net Radiometers - Manual (Page 16)

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It is assumed that when ambient temperature varies, the Net Far Infrared radiation remains roughly the same,
independent of ambient temperature. The resulting measured values of the pyrgeometer's and pyranometer's are
stated in columns 4 to 7. These are indicative figures only, they depend strongly on other circumstances; the
pyrgeometer results, of course, change with the sensor temperature. This is indicated in column 8. During the day,
the Pt-100 reading may rise due to solar heating, up to 10 degrees above ambient temperature. During the night,
the sensor temperature may be lower than the ambient temperature due to Far Infrared radiative cooling. The latter
two effects do not influence the end result of the calculations of Sky T and ground T. Therefore they are not taken
into account in the table. Actually in column 4 you might expect to see "0 to -50" for all positions that are showing
"0", in column 5 the "0" values may in reality be "-20 to +20". The resulting sky temperature is indicated in column
9. Under cloudy conditions this sky temperature is equal to ambient temperature. Under clear conditions the sky
temperature is lower than the ambient temperature.
The ground temperature in column 10 is assumed to be equal to the ambient temperature. In practice it may be
higher during the day, due to solar heating. Ground temperature may be lower than ambient during the night, due
to Far Infrared radiative cooling. The sky and the ground temperature can be calculated from the measured values
of the sensors using formulas 1.6 and 1.7.
day
night
Cloudy
clear
+20 ºC
- 20 ºC pyrgeo
Up
pyrgeo
low
pyrano
up
pyrano
low
Pt 100
thermistor
sky T
ground
T
d cloud 20 0 -20 - 20 0-500 0-150 20 20 20
d cloud -20 0 -20 - 20 0-500 0-150 -20 -20 -20
d clear 20 -100* -20 - 20 0-1300 0-400 20 1* 20
d clear -20 -100* -20 - 20 0-1300 0-400 -20 -53* -20
n cloud 20 0 -20 - 20 0 0 20 20 20
n cloud -20 0 -20 - 20 0 0 -20 -20 -20
n clear 20 -100*** -20 - 20 0** 0 20 1*** 20
n clear -20 -100*** -20 - 20 0** 0 -20
-
53***
-20
Table 1.2.2 Typical output signals of CNR 4 under different meteorological conditions. Explanation can be found in
the text.
* Values may suffer from the so-called window heating offset; the sun heats the pyrgeometer window causing a
measurement error of + 10 Watts per square metre (maximum).
** Values may suffer from negative Infrared offsets, caused by cooling off of the pyranometer dome by Far Infrared
radiation. The maximum expected offset value is 15 Watts per square metre.
*** Values may suffer from dew deposition. This causes the pyrgeometer-up values to rise from -100 to 0 Watts per
square metre.
1.3 Quality assurance of data
Because of the fact that separate sensors are used in the CNR 4, there are possibilities to check the quality of the
data by analysing the signals. For this, you can use the measurement results of the temperature, the albedo and the
net-solar radiation. If the values that are obtained for these quantities exceed certain values, this can be an
indication that something is wrong. For more details we refer to the paragraphs 1.2, table 1.2.1 and 1.2.2.