Campbell CNR4, Instruction Manual, Page: 32 / 70

CNR4 Net Radiometer
22
4 / 1
8
10 67 . 5




⋅
=
−
r Pyrgeomete lower E
e Temperatur Ground
(8-7)
As a rule of thumb, for ambient temperatures of about 20 degrees Celsius, one
can say that one degree of temperature difference between two objects results
in a 5 W/m 2 exchange of radiative energy (infinite objects):
1 degree of temperature difference = 5 W/m 2 (rule of thumb)
8.7 Calculation of Net (Total) Radiation
In the four separate components mode, net radiation, R
n
, can be calculated
using the individual sensor measurement results:
R
n
= {(E upper Pyranometer) - (E lower Pyranometer)}
+ {(E upper Pyrgeometer) - (E lower Pyrgeometer)}
(8-8)
Where E upper/lower pyranometers are calculated according to Equation 8-1 ,
and E upper/lower pyrgeometers are calculated according to Equation 8-2 . The
terms with T cancel each other out.
9. Troubleshooting and Maintenance
9.1 Troubleshooting
If there is no indication as to what may be the problem, start performing the
following “upside-down test”, which is a rough test for a first diagnosis. It can
be performed both outdoors and indoors. Indoors, a lamp can be used as a
source for both short-wave and long-wave radiation. Outdoors, one should
preferably work with a solar elevation of more than 45 degrees (45 degrees
above horizon) and under stable conditions (no large changes in solar
irradiance, and preferably no clouds).
1. Measure the radiation outputs in the normal position. Record the
measured values when the signals have stabilized, i.e. after about three
minutes.
2. Rotate the instrument 180 degrees, so that the upper and the lower sensors
are now in the reverse orientation as to the previous position.
3. Measure the radiation outputs once more. Record the measured values
when the radiometers have stabilized.
4. The computed net radiation values in rotated position should be equal in
magnitude but only differing in sign. In a rough test like this, deviations of
± 10 % can be tolerated. If deviations greater than this are encountered,
additional testing is warranted.
9.1.1 Testing the Pyranometer
As a first test, check the sensor impedance. It should have a nominal value as
indicated in the specifications. Zero, or infinite resistance, indicates a failure in
hardware connection.