Kipp & Zonen CNR 4 User manual

Net Radiometer
CNR 4
Instruction Manual

1
IMPORTANT USER INFORMATION
Reading this entire manual is recommended for full understanding of the use of this product.
The exclamation mark within an equilateral triangle is intended to alert the user to the presence of important
operating and maintenance instructions in the literature accompanying the instrument.
Should you have any comments on this manual we will be pleased to receive them at:
Kipp & Zonen B.V.
Delftechpark 36
2628 XH Delft
P.O. Box 507
2600 AM Delft
The Netherlands
Phone [phone]
Fax [phone]
Email [e-mail]
Web www.kippzonen.com
Kipp & Zonen reserve the right to make changes to the specifications without prior notice.
WARRANTY AND LIABILITY
Kipp & Zonen guarantees that the product delivered has been thoroughly tested to ensure that it meets its published
specifications. The warranty included in the conditions of delivery is valid only if the product has been installed and
used according to the instructions supplied by Kipp & Zonen.
Kipp & Zonen shall in no event be liable for incidental or consequential damages, including without limitation, lost
profits, loss of income, loss of business opportunities, loss of use and other related exposures, however caused,
arising from the faulty and incorrect use of the product.
User made modifications can affect the validity of the CE declaration.
COPYRIGHT©2009 KIPP & ZONEN
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any
form or by any means, without permission in written form from the company.
Manual version: 1107

2
DECLARATION OF CONFORMITY
According to EC guideline 89/336/EEC 73/23/EEC
We: Kipp & Zonen B.V.
Delftechpark 36
2628 XH Delft
The Netherlands
Declare under our sole responsibility that the product
Type: CNR 4
Name: Net-Radiometer
To which this declaration relates is in conformity with the following standards
Imissions EN 50082-1 Group standard
Emissions EN 50081-1 Group standard
EN 55022
Safety standard IEC 1010-1
Following the provisions of the directive
B.A.H. Dieterink
President
KIPP & ZONEN B.V.

3
CONTENTS
IMPORTANT USER INFORMATION ................................................................................................................................... 1!
DECLARATION OF CONFORMITY..................................................................................................................................... 2!
CONTENTS................................................................................................................................................................................. 3!
1!GENERAL INFORMATION ............................................................................................................................................ 4!
1.1!Connecting the CNR 4 ............................................................................................................ 5!
1.1.1!The difference between temperature and sensor connection ........................................... 5!
1.1.2!The radiometer Sensor connector (S)............................................................................... 5!
1.1.3!The radiometer Temperature connector (T) ..................................................................... 7!
1.1.4!The (optional) CNF 4 connector ....................................................................................... 8!
1.1.5!Using the CNR 4 calibration factors................................................................................. 8!
1.1.6!Using the CNR 4 measuring Net Radiation....................................................................... 9!
1.1.6.1!Measuring Solar radiation with the pyranometer........................................................................... 9!
1.1.6.2!Measuring Far Infrared radiation with the pyrgeometer ................................................................ 9!
1.1.6.3!Measuring the CNR 4's body temperature..................................................................................... 9!
1.1.6.4!Calculation of the albedo for solar radiation ............................................................................... 12!
1.1.6.5!Calculation of the Net Solar radiation ......................................................................................... 12!
1.1.6.6!Calculation of the Net (total) radiation........................................................................................ 13!
2!CNR 4 PROPERTIES .....................................................................................................................................................16!
2.1!Properties of the CNR 4 Net-Radiometer ............................................................................... 16!
2.1.1!Specifications of the CNR 4 ........................................................................................... 17!
2.2!Properties of the pyranometer.............................................................................................. 17!
2.2.1!Specifications of the pyranometer ................................................................................. 18!
2.2.2!Spectral properties of the pyranometer ......................................................................... 20!
2.2.3!Directional / Cosine response of the pyranometer......................................................... 20!
2.3!Properties of the pyrgeometer.............................................................................................. 22!
2.3.1!Window heating offset .................................................................................................. 22!
2.3.2!Water deposition on the pyrgeometer window............................................................... 22!
2.3.3!Specifications of the Pyrgeometer ................................................................................. 22!
2.3.4!Spectral properties of the pyrgeometer ......................................................................... 24!
2.3.5!Directional / Cosine response of the pyrgeometer......................................................... 24!
2.4!Properties of the CNF 4 (optional) ventilation unit ................................................................ 25!
2.4.1!CNF 4 specifications...................................................................................................... 25!
2.5!Properties of the Pt-100 and thermistor................................................................................ 26!
3!CALIBRATION ................................................................................................................................................................27!
3.1!Calibration of the pyranometers........................................................................................... 27!
3.2!Calibration of the pyrgeometers........................................................................................... 27!
3.3!Recalibration of pyranometers and pyrgeometers................................................................. 27!
3.4!Checking the Pt-100 and Thermistor .................................................................................... 27!
4!INSTALLATION AND MAINTENANCE ...................................................................................................................28!
4.1!Replacing the drying cartridge ............................................................................................. 28!
4.2!Replacing the Ventilator Filter .............................................................................................. 29!
5!TROUBLE SHOOTING..................................................................................................................................................30!
5.1!Testing the pyranometer ...................................................................................................... 30!
5.2!Testing of the pyrgeometer.................................................................................................. 30!
5.3!Testing the Pt-100................................................................................................................ 31!
5.4!Testing the thermistor ......................................................................................................... 31!
5.5!Testing the Heater ............................................................................................................... 31!
5.6!Testing the Ventilator .......................................................................................................... 31!
6!CMB 1 OPTIONAL MOUNTING BRACKET ...........................................................................................................32!
7!DELIVERY ........................................................................................................................................................................34!
8!RECALIBRATION SERVICE.........................................................................................................................................35!

4
1GENERAL INFORMATION
The CNR 4 is a 4 component net radiometer that measures the energy balance between incoming short-wave
and long-wave Far Infrared (FIR) radiation versus surface-reflected short-wave and outgoing long-wave radiation.
The CNR 4 net radiometer consists of a pyranometer pair, one facing upward, the other facing downward, and a
pyrgeometer pair in a similar configuration. The pyranometer pair measures the short-wave radiation. And the
pyrgeometer pair measures long-wave radiation. The upper long-wave detector of CNR 4 has a meniscus dome. This
ensures that water droplets role off easily and improves the field of view to nearly 180°, compared with a 150° for a
flat window. All 4 sensors are integrated directly into the instrument body, instead of separate modules
mounted onto the housing. Each sensor is calibrated individually for optimal accuracy.
Two temperature sensors, a Pt-100 and Thermistor, are integrated for compatibility with every data logger. The
temperature sensor is used to provide information to correct the infrared readings for the temperature of the
instrument housing. Care has been taken to place the long-wave sensors close to each other and close to the
temperature sensors. This assures that the temperatures of the measurement surfaces are the same and accurately
known. This improves the quality of the long-wave measurements.
The design is very light in weight and has an integrated sun shield that reduces thermal effects on both long-wave
and short-wave measurements. The cables are yellow with waterproof connectors as used with all our new
radiometers. The mounting rod can be unscrewed for transport, like the CNR 2.
An optional ventilation unit with heater is designed as an extension of the sunshield and can be fitted new to the
CNR 4 or retro-fitted later. The ventilation unit CNF 4 is compact and provides efficient air-flow over the domes and
windows to minimize the formation of dew and reduce the frequency of cleaning. The integrated heater can be used
to melt frost.
The CNR 4 specifications when used with CNF 4 comply with the WMO classification of Good Quality
The Net Radiometer, CNR 4, is intended for the analysis of the radiation balance of Solar and Far Infrared radiation.
The most common application is the measurement of Net (total) Radiation at the earth's surface.
The CNR 4 design is such that both the upward facing and the downward-facing instruments measure the energy
that is received from the whole hemisphere (180 degrees field of view). The output is expressed in Watts per square
meter. The total spectral range that is measured is roughly from 0.3 to 42 micrometers. This spectral range covers
both the Solar Radiation, 0.3 to 3 micrometers, and the Far Infrared radiation, 4.5 to 42 micrometers.
Chapter 1 describes how to operate the CNR 4, giving separate attention to the use of the individual pyranometers
and pyrgeometers. More about the physics of the pyranometer and pyrgeometer can be found in chapter 2.
The CNR 4 radiometers all have individual calibration factors. As opposed to the CNR 1 they are not made equal.
The advantage is that the individual sensitivities are more accurate then when made equal with shunt and series
resistors.
For quality assurance of the measurement data, we recommend the recalibration of the CNR 4 as part of a regular
maintenance schedule every two years. More about calibration can be found in chapter 3.
The CNR 4 is intended for continuous outdoor use. It is weatherproof. The materials used in Pyranometer and
pyrgeometer are robust. Contrary to most competitive instruments, plastic domes are not used. Therefore the CNR
4 requires very low maintenance. For optimal results however, proper care must be taken. More about maintenance
can be found in chapter 4. Chapter 5 can be consulted if a problem with the CNR 4 is suspected; this chapter
addresses trouble shooting.
The general user should read chapter 1 and chapter 4.

5
1.1 Connecting the CNR 4
1.1.1 The difference between temperature and sensor connection
CNR 4 has several output signals: two voltages for the pyranometers, two voltages for the pyrgeometers, and two
temperature sensors as standard: a 4 wire Pt-100 connection and a 2 wire connection for the 10k thermistor.
Depending on the used data logger one of the temperature sensors can be used.
The connector with the 4 sensor outputs is indicated with an S on the back of the CNR 4, the temperature connector
is indicated with a T. The sensor connector has 8 pins and 8 wires are coming out of the cable on the other end.
The Temperature connector has also 8 pins but only 6 wires are coming out of the cable.
Exchanging the connectors will not damage the CNR 4. To prevent mix up of the cables after installation it is
advised to mark the cables with a permanent marker or tape showing also S and T.
Figure 1.1 Back of the CNR 4 with the connector for sensor outputs on the left, the temperature connector on the
right and mounting rod in the middle.
1.1.2 The radiometer Sensor connector (S)
CNR 4 sensor connector carries the signals for the two pyranometers and two pyrgeometers. The pin numbers and
wire colours are indicated in the diagram below. The shield of the cable is twisted together and covered with a black
sleeve. If the CNR 4 (rod) is grounded to the mast it is advised not to connect the shielding on the data logger side.
This might cause ground loops and offsets in the signal.

6
Figure 1.2 The sensor connections of the CNR 4. The sensor has four mV outputs, 2 for the pyranometers and 2 for
the pyrgeometers
Ground *
Erde
Terre
Tierra
Red
Rot • Rouge • Rojo
Wire
Kabel
Fil
Cable
White
Weiss • Blanc • Blanco
Brown
Braun • Brun • Marrón
Black
Schwarz • Noir • Negro
Grey
Grau • Gris • Gris
Function
Funktion
Fonction
Función
Pyranometer Upper
Oben • Supérieur • Superior
Pyranometer Lower
Unten • Inférieur • Más bajo
Pyrgeometer Upper
Oben • Supérieur • Superior
Pyrgeometer Lower
Unten • Inférieur • Más bajo
Blue
Blau • Bleu • Azul
Connect with
Anschluss an
Relier à
Conectar con
Yellow
Gelb • Jaune • Amarillo
Green
Grün • Vert • Verde
Housing
Gehäuse
Boîte
Cubierta
Shield
Abschirmung
Protection
Malla
+Hi
+Hi
+Hi
+Hi
- Lo
- Lo
CNR 4 NET RADIOMETER •SENSOR
8 WIRE CABLE • 8-ADRIGES KABEL • CÂBLE 8 FILS • CABLE DE 8 CONDUCTORES
- Lo
- Lo
+
-
-
-
-
+
+
+
* Connect to ground if radiometer not grounded
Mit Erde verbinden, wenn das Radiometer nicht geerdet ist
Reliez à la terre si le radiomètre n'est pas connecté
Conectar a tierra si el radiómetro no lo está
1
2
345
6
7
8
1
2
7
8
5
4
6
3

7
1.1.3 The radiometer Temperature connector (T)
The temperature connector of the CNR 4 carries the signals for the PT-100 and Thermistor. The PT-100 and
Thermistor have identical accuracy, the reason for selecting one or the other is mainly the data logger involved. The
Pt-100 temperature sensor has 4 wires, two for the measuring current and two for measuring the voltage over the
resistor (100 Ohm @ 0 °C) . In this way the measurement accuracy is minimally affected by the cable length. The
thermistor has a higher impedance of 10 kOhm and is therefore less influenced by cable length but has no
compensation for it. The shield of the cable is twisted together and covered with a black sleeve. If the CNR 4 (rod)
is grounded to the mast it is advised not to connect the shielding on the data logger side. This might cause ground
loops and offsets in the signal.
Figure 1.3 The temperature connections of the CNR 4. The instrument has one (4-wire) Pt-100 output and a
Thermistor with a 2-wire connection.
CNR 4 NET RADIOMETER •TEMPERATURE
6 WIRE CABLE • 6-ADRIGES KABEL • CÂBLE 6 FILS • CABLE DE 6 CONDUCTORES
7
8
5
3
4
6
Grey
Grau • Gris • Gris
Combined
Kombiniert
Combiné
Combinado
Pt-100
Standard
Standard
Etalon
Estándar
Combined
Kombiniert
Combiné
Combinado
Brown
Braun • Brun • Marrón
Green
Grün • Vert • Verde
Yellow
Gelb • Jaune • Amarillo
White
Weiss • Blanc • Blanco
Thermistor
Standard
Standard
Etalon
Estándar
Black
Schwarz • Noir • Negro
1
2
345
6
7
8
Ground *
Erde
Terre
Tierra
Housing
Gehäuse
Boîte
Cubierta
Shield
Abschirmung
Protection
Malla

8
1.1.4 The (optional) CNF 4 connector
The optional ventilator CNF 4 for the CNR 1 has separate wires for heating and ventilation. In case the heater is
used also the ventilator should be active. The other way around the ventilator can be used without heating. When
the ventilation unit is mounted later on the CNR 4, the extra bottom plate mounts to the bottom of the CNR 4. The
(S) and (T) cables run on both sides of the ventilator to the back of the CNF 4 housing. The extra cover that comes
with the CNF 4 slides under the CNR 4 cover. The 4 pins connector on the back of the CNF 4 is shown below. The
CNR 4 without CNF 4 is supplied with an extra serial number label. This can be used to put on the bottom of the
CNF 4 when mounted (later) on the CNR 4.
Figure 1.3 The heater and ventilator connections of the CNF 4.
1.1.5 Using the CNR 4 calibration factors
The pyranometer generates a mV signal that is simply proportional to the incoming solar Radiation. The conversion
factor between voltage, V, and Watts per square metre of solar irradiance E, is the so-called calibration constant C
(or sensitivity).
For each pyranometer E = V/C (1.1)
When using the pyrgeometer, you should realise that the signal that is generated by the pyrgeometer represents the
exchange of Far Infrared (thermal) radiation between the pyrgeometer and the object that it is facing. This implies
that the pyrgeometer will generate a positive voltage output, V, when it faces an object that is hotter than its own
sensor housing, and that it will give a negative voltage signal when it faces an object that is colder. This means that
for estimating the Far Infrared radiation that is generated by the object that is faced by the pyrgeometer, usually the
sky or the soil, you will have to take the pyrgeometer temperature, T, into account. This is why a the temperature
sensors are incorporated in the CNR 4's body near the pyrgeometer sensing element, and has therefore the same
temperature as the pyrgeometer sensor surface. The calculation of the Far Infrared irradiance, E, is done according
to the following equation:
For the pyrgeometer only E = V/C + 5.67⋅10-8*T4 (1.2)
In this equation C is the sensitivity of the sensor. Please bear in mind that T is in Kelvin, and not in Celsius or
Fahrenheit.
Red
Rot • Rouge • Rojo
Wire
Kabel
Fil
Cable
Green
Grün • Vert • Verde
Yellow
Gelb • Jaune • Amarillo
Function
Funktion
Fonction
Función
5 Watt ventilator 12 VDC
Ventilator • Ventilateur • Ventilador
10 Watt heater 12 VDC
Heizung • Chauffage • Calentador
Blue
Blau • Bleu • Azul
Connect with
Anschluss an
Relier à
Conectar con
+
-
-
+
2
4
1
3
CNF 4 VENTILATION UNIT
(OPTIONAL • OPTION • OPTION • OPCIONAL)
4 WIRE CABLE • 4-ADRIGES KABEL • CÂBLE 4 FILS • CABLE DE 4 CONDUCTORES
Ground *
Erde
Terre
Tierra
Housing
Gehäuse
Boîte
Cubierta
Shield
Abschirmung
Protection
Malla
1
2
3
4

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.

10
Temperature
[EC]
Resistance
[Ω]
Temperature
[EC]
Resistance
[Ω]
Temperature
[EC]
Resistance
[Ω]
-30
-29
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
88.22
88.62
89.01
89.40
89.80
90.19
90.59
90.98
91.37
91.77
92.16
92.55
92.95
93.34
93.73
94.12
94.52
94.91
95.30
95.69
96.09
96.48
96.87
97.26
97.65
98.04
98.44
98.83
99.22
99.61
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
100.00
100.39
100.78
101.17
101.56
101.95
102.34
102.73
103.12
103.51
103.90
104.29
104.68
105.07
105.46
105.85
106.24
106.63
107.02
107.40
107.79
108.18
108.57
108.96
109.35
109.73
110.12
110.51
110.90
111.28
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
111.67
112.06
112.45
112.83
113.22
113.61
113.99
114.38
114.77
115.15
115.54
115.93
116.31
116.70
117.08
117.47
117.85
118.24
118.62
119.01
119.40
119.78
120.16
120.55
120.93
121.32
121.70
122.09
122.47
122.86
Table 1.1 Resistance values versus temperature in °C of the CNR 4's Pt-100.
The Pt-100 complies with the class A specifications of DIN.

11
Temperature
[EC]
Resistance
[Ω]
Temperature
[EC]
Resistance
[Ω]
Temperature
[EC]
Resistance
[Ω]
-30
-29
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
135200
127900
121100
114600
108600
102900
97490
92430
87660
83160
78910
74910
71130
67570
64200
61020
58010
55170
52480
49940
47540
45270
43110
41070
39140
37310
35570
33930
32370
30890
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
29490
28150
26890
25690
24550
23460
22430
21450
20520
19630
18790
17980
17220
16490
15790
15130
14500
13900
13330
12790
12260
11770
11290
10840
10410
10000
9605
9227
8867
8523
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
8194
7880
7579
7291
7016
6752
6500
6258
6026
5805
5592
5389
5193
5006
4827
4655
4489
4331
4179
4033
3893
3758
3629
3504
3385
3270
3160
3054
2952
2854
Table 1.2 Resistance values versus temperature in °C of the CNR 4's thermistor.
Relatively small errors occur when the CNR 4 is not in thermal equilibrium. This happens for example when the
heater is on, or when the sun is shining. When the heater and ventilator are on, the largest expected deviation
between real sensor temperature and Pt-100 or thermistor reading is less than 0.5 degree.
The internal temperature sensors will not give a good indication of ambient air temperature; at 1000 Watts per
square meter Solar Radiation, and no wind, the instrument temperature can rise a few degrees above ambient
temperature. This will not affect the readings of the CNR 4
The offsets of both pyranometers and pyrgeometers might be larger than 5 Watts per square meter if large
temperature gradients are forced on the instrument (larger than 5 K/hr). This happens for example when rain hits
the instrument. The occurrence of this can be detected using the temperature sensor readout. It can be used as a
tool for quality assurance of your data.

12
1.1.6.4 Calculation of the albedo for solar radiation
The albedo is the ratio of incoming and reflected Solar radiation. It is a figure somewhere between 0 and 1. Typical
values are 0.9 for snow, and 0.3 for grassland. To determine albedo, the measured values of the two pyranometers
can be used. The pyrgeometers are not involved, as they do not measure Solar radiation. Do not use measured
values when solar elevation is lower than 10 degrees above the horizon. Errors in measurement at these elevations
are likely and thus yielding unreliable results. This is due to deviations in the directional response of the
pyranometers.
Albedo = (E lower pyranometer) / (E upper pyranometer) (1.3)
In the above formula, E is calculated according to formula 1.1.
Albedo will always be smaller than 1. Checking this can be used as a tool for quality assurance of your data. If you
know the approximate albedo at your site, the calculation of albedo can also serve as a tool for quality control of
your measured data at this specific site.
1.1.6.5 Calculation of the Net Solar radiation
Net Solar radiation is the incoming Solar Radiation minus the reflected solar radiation. It equals the solar radiation
that is absorbed by the earth's surface.
Net Solar radiation = (E upper pyranometer) - (E lower pyranometer) (1.4)
In this formula E is calculated according to formula 1.1.
Net Solar radiation will always be positive. Checking this can be used as a tool for quality assurance of your
measured data.
Calculation of the Net Far Infrared radiation, soils temperature and sky temperature
Net Far Infrared radiation is, like Net Solar radiation, the part that contributes to heating or cooling of the earth's
surface. In practice most of the time, Net Far Infrared radiation will be negative.
Net Far Infrared radiation = (E upper pyrgeometer) - (E lower pyrgeometer) (1.5)
In this formula E is calculated according to formula 1.2. From this equation the term with T cancels.
The E measured with the pyrgeometer, actually represents the irradiance of the sky (for the upward- facing
pyrgeometer) or the ground (for the downward-facing pyrgeometer). Assuming that these two, ground and sky,
behave like perfect blackbodies (actually this is only in theory), you can calculate an effective "Sky temperature" and
an effective "Ground temperature".
Sky temperature = ((E upper pyrgeometer)/ 5.67⋅10-8)1/4 (1.6)
Ground temperature = ((E lower pyrgeometer)/ 5.67⋅10-8)1/4 (1.7)
As a rule of thumb, for ambient temperatures of about 20 degrees Celsius, you can say that one degree of
temperature difference between two objects results in a 5 Watts per square metre exchange of radiative energy
(infinite objects):
1 degree of temperature difference = 5 Watts per square metre (rule of thumb)

13
1.1.6.6 Calculation of the Net (total) radiation
The Net radiation, NR, can be calculated using all 4 sensor measurement results:
NR = (E upper pyranometer) + (E upper pyrgeometer) - (E lower pyranometer) - (E lower pyrgeometer)
(1.8)
Where E is the irradiance that is calculated for the pyranometer according to equation 1.1, for the pyrgeometer
according to equation 1.2. the terms with T cancel from this equation.
1.2 CNR 4 Performance under different conditions
Below, table 1.3, shows an indication of what you might typically expect to measure under different meteorological
conditions. The first parameter is day and night. At night, the Solar radiation is zero. The second column indicates if
it is cloudy or clear. A cloud acts like a blanket, absorbing part of the Solar radiation, and keeping Net Far Infrared
radiation close to zero. The third parameter is ambient temperature. This is included to show that the "sky
temperature" (column nine) tracks the ambient temperature. Under cloudy conditions this is logical; cloud bases will
be colder than the ambient temperature at instrument level, the temperature difference depends roughly on cloud
altitude.
Under clear sky conditions it is less obvious that sky temperature "adjusts" to the ambient temperature. This can
roughly be attributed to the water vapour in the air, which is a major contributor to the Far Infrared radiation.
Fig. 1.2.1 Different measurement conditions and signals
Fully clouded Sunny, partly clouded Clear and Sunny
Values calculated using:
Werte, errechnet aus Formel:
Calcul des valeurs à l'aide:
Utilizar valores calculados:
Upper
Oben • Supérieur • Superior
Lower
50 ... 120 W/m²
0 ... 50 W/m²
120 ... 500 W/m²
50 ... 200 W/m²
500 ... 1000 W/m²
200 ... 400 W/m²
Unten • Inférieur • Más bajo
Bewölkt
Très nuageux
Totalmente Nublado
Sonnig, teils bewölkt
Ensoleillé, un peu nuageux
Parcialmente nublado
Klarer Himmel und sonnig
Clair et ensoleillé
Cielo despejado
=
0 ... -10 W/m²
-25 ... 25 W/m²
-10 ... -50 W/m²
-25 ... 25 W/m²
-50 ... -150 W/m²
-25 ... 25 W/m²
Upper
Oben • Supérieur • Superior
Lower
Unten • Inférieur • Más bajo
Pyranometer
Pyrgeometer

14
Typical graphs for the pyrgeometers
Figure 1.2.2 partly clouded day for the upper pyrgeometer
Figure 1.2.3 clear day for the downward facing pyrgeometer
Upper%pyrgeometer,%%day%with%alterna3ng%cloud%fields%%%
%"#$%&'(&)&$*!!+,&(-!.!/&012342)#!56.(78! 9&("!:/;!<<=>?!5@A8!
Upwelling%signal%(downward%facing)%pyrgeometer%
%
B#$%&'(&)&$*!+C&(-!.!1&012342)#!56.(78!
9&("!'-!201)$D(&0)!5@A8!

15
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.

16
2CNR 4 PROPERTIES
The CNR 4 consists of two pyranometers, for measuring solar radiation, and of two pyrgeometers for measuring Far
Infrared radiation. Two temperature sensors are available as standard, a Pt-100 and thermistor. The optional
ventilation unit CNF 4 is described in chapter 2.5
2.1 Properties of the CNR 4 Net-Radiometer
The properties of CNR 4 are mainly determined by the properties of the individual sensors. Generally the accuracy
of CNR 4 will be higher than that of competitive Net-Radiometers. The main reasons are that the solar radiation
measurement performed by the pyranometer is accurate, and offers a traceable calibration. Also the optionally
integrated ventilation and heating improve the accuracy significantly. Due to the fact that the Net Solar radiation
can be very intense, 1000 Watts per square metre compared to a typical -100 for the Net Far Infrared radiation, the
accuracy of the solar measurement is very critical. Wind corrections, as applied by less accurate competitive
instruments are not necessary. The robustness of the materials used implies that CNR 4 will not suffer from damage
inflicted by birds. Figure 2.1 depicts a drawing of CNR 4. From a spectral point of view, the pyranometer and
pyrgeometer are complementary. Together they cover the full spectral range: The pyranometer from 0.3 to 3
microns, and the pyrgeometer from 4.5 to 42 microns. The gap between these two produces negligible errors.
Figure 2.1 The dimensions of the CNR 4 with CNF 4 ventilation unit, side view
Figure 2.2 The dimensions of the CNR 4 with CNF 4 ventilation unit, top view
405 mm 347 mm

17
2.1.1 Specifications of the CNR 4
General specifications
Environmental
0 - 100% RH (Relative Humidity)
Definition
Intended for continuous outdoor use
Bubble level sensitivity
< 0.5º (bubble half inside ring)
General Construction
Sensor
Thermopile
Receiver paint
Carbon Black
Desiccant
Silica gel (replaceable)
Housing materials
Anodized Aluminum body
Cable Connectors
Binder series 712
Cable (2 cables)
Color Yellow, Poly Urethane, Halogen free, UV blocking
Cable length (2 cables)
m
10 (standard), 25, 50 (optional)
Weight
kg
0.85 (without cables)
Shock / vibration
IEC 721-3-2-2m2
Operating temperature
°C
-40 to +80
CE
according to EC guideline 89/336/EEC 73/23/EEC
Environmental protection
IP 67
2.2 Properties of the pyranometer
The pyranometer consists of a thermopile sensor and a glass dome both integrated in the CNR 4 body. The
thermopile is coated with a black absorbent coating. The paint absorbs the radiation, and converts it to heat. The
resulting heat flow causes a temperature difference across the thermopile. The thermopile generates a voltage
output. The thermopile and the resistor determine most electrical specifications. The absorber paint and the dome
determine spectral specifications. The thermopile is encapsulated in the housing in such a way that its field of view
is 180 degrees, and that its angular characteristics fulfil the so-called cosine response.

18
2.2.1 Specifications of the pyranometer
Pyranometers Specification
Unit
Value (All indicated values are absolute values)
Spectral range
nm
300 - 2800 (50% points)
Definition
Instrument sensitivity within a specific spectral range
Sensitivity
µV/ W/m²
10 to 20
Definition
Calibration factor
Impedance
Ω
20 to 200, typically 50
Definition
Typical resistance measured at the output
Response time
s
< 18 (95% response)
Definition
Sensor response time
Non-linearity
%
< 1 (from 0 to 1000 W/m2irradiance)
Definition
Maximum deviation from the responsivity at 500 W/m2due to any
change of irradiance within the indicated range.
Temperature dependence of
sensitivity
%
N/S (- 40 °C to -10°C)
< 4 (- 10 °C to +40 °C)
N/S (+40 ºC to +80 ºC)
Definition
Maximum error due to any change of instrument temperature within
the indicated temperature interval.
Tilt error
%
< 1
Definition
Maximum deviation from the responsivity when tilted at any angle
and at 1000 W/m²irradiance .
Zero offset A
W/m²
< 15 (0 to -200 W/m²/ IR net irradiance)
Definition
Caused by cooling of the dome due to sky radiation
Zero offset B
W/m²
< 3 (at 5 K/h temp. change)
< 1 (with CVF 4 installed)
Definition
Response to change in ambient temperature.
Field of view:
Upper detector
Lower detector
180º
150º (due to lower sun shield. To prevent illumination at low zenith
angles)
Definition
Sensor opening angle
Directional error
W/m²
< 20 (angles up to 80°with 1000 W/m²beam radiation)
Definition
Combined zenith and azimuth error from 0°-80 ° with 1000 W/m²
beam radiation

19
Irradiance:
W/m²
0 to 2000
Definition
Measurement range
Non-stability
%
< 1
Definition
Maximum change of sensitivity per year, percentage of full scale
Spectral selectivity
%
< 3% (350 – 1500 nm spectral interval)
Definition
Deviation of the product of spectral absorption and spectral
transmittance from the corresponding mean within the indicated
spectral range
Uncertainty in daily total
%
< 5 (95 % confidence level)
Definition
Achievable uncertainty
International standards
WMO
ISO
Good quality
First Class
Instrument calibration
Indoors, side by side against reference CMP 3 pyranometer
according to ISO 9847:1992 annex A.3.1