9
1.1.6
Using the CNR 4 measuring Net Radiation
In the CNR 4 all components are measured separately. This implies that you should connect all individual
radiometers and one of the temperature sensors. The two pyranometers will measure the solar radiation, both
incoming and reflected, the two pyrgeometers will measure the Far Infrared radiation. For proper analysis of the
pyrgeometer measurement results, they must be temperature corrected using the temperature measurement. The
following paragraphs describe how you should treat the instrument, and how different parameters like net Solar
radiation, net Far Infrared radiation, soil temperature, sky temperature, and Net (total) radiation can be calculated.
Because all radiometers have different sensitivities it is not possible to interconnect the outputs to get the total Net
Radiation.
1.1.6.1
Measuring Solar radiation with the pyranometer
Measuring with a pyranometer can be done by connecting two pyranometer wires, + and -, to a voltmeter. Incidental
light results in a positive signal. The pyranometer mounting plate and ambient air should be at the same
temperature, as much as possible. Conversion of the voltage to irradiance can be done according to equation 1.1.
This is sometimes done in the data logging system itself, sometimes during evaluation in the user's software.
Measuring with the upward-facing pyranometer, the so-called global (solar) radiation is measured. The downward-
facing pyranometer measures the reflected solar radiation. When calculating the Net radiation, the Reflected
radiation must be subtracted from the global radiation. See 1.1.6.5.
1.1.6.2
Measuring Far Infrared radiation with the pyrgeometer
A measurement with the pyrgeometer can be performed by connecting two pyrgeometer wires, + and -, to a voltme-
ter. A signal radiating from a source which is warmer than the pyrgeometer results in a positive signal.
To measure the Far Infrared irradiances with the two pyrgeometers, separately the Pt-100 output is required. The
formula 1.2 is used to calculate the Far Infrared irradiance of the sky and of the ground.
With the downward-facing pyrgeometer, you would generally measure the Far Infrared radiation that is emitted by
the ground. In contrast, the upward-facing pyrgeometer is generally used to measure the Far Infrared radiation from
the sky. As the sky is typically colder than the instrument, you can expect negative voltage signals from the upward-
facing pyrgeometer.
1.1.6.3
Measuring the CNR 4's body temperature
The CNR 4 has two temperature sensors built in as standard. The main reason to choose between the Pt-100 or the
Thermistor is the connected data logger.
Some data loggers have inputs for thermistors some only for thermistors some have both.
Check carefully the correct sensor regarding to the data logger. There is no difference in accuracy.
The Pt-100 however can be used in 4 wire mode as is therefore compensated for longer wires.
The Thermistor has itself higher impedance (10k) and is less susceptible for longer wires, but can not be
compensated for it.
To obtain a signal from the Pt-100, a current of about 1 mA is fed through two wires on either side of the PT-100.
The voltage that is generated must be measured using the other pair of wires which are connected in parallel with
the PT-100.This is known as a 4-wire measurement. Measuring in this manner eliminates errors during measure-
ment, which would be produced by additional wire length. Some systems have a 3-wire connection. In this case omit
one current lead and follow the instructions of your measurement system manual. Table 1.1 states the Pt-100
resistance values as a function of temperature. Please note that for use in formula 1.2, you must use Kelvin, not
degrees Celsius or Fahrenheit. Most data acquisition systems have standard readout and conversion for Pt-100's. .
The thermistor resistance values as a function of temperature are indicated in table 1.2.
