Campbell CM 21 User manual

INSTRUCTION MANUAL
CM 21
PYRANOMETER

WARRANTY AND ASSISTANCE
This equipment is warranted by CAMPBELL SCIENTIFIC (CANADA) CORP. (“CSC”) to
be free from defects in materials and workmanship under normal use and service for
twelve (12) months from date of shipment unless specified otherwise. ***** Batteries
are not warranted. ***** CSC's obligation under this warranty is limited to repairing or
replacing (at CSC's option) defective products. The customer shall assume all costs of
removing, reinstalling, and shipping defective products to CSC. CSC will return such
products by surface carrier prepaid. This warranty shall not apply to any CSC products
which have been subjected to modification, misuse, neglect, accidents of nature, or
shipping damage. This warranty is in lieu of all other warranties, expressed or implied,
including warranties of merchantability or fitness for a particular purpose. CSC is not
liable for special, indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. To obtain a Return
Merchandise Authorization (RMA), contact CAMPBELL SCIENTIFIC (CANADA) CORP.,
at [phone]. An RMA number will be issued in order to facilitate Repair Personnel
in identifying an instrument upon arrival. Please write this number clearly on the outside
of the shipping container. Include description of symptoms and all pertinent details.
CAMPBELL SCIENTIFIC (CANADA) CORP. does not accept collect calls.
Non-warranty products returned for repair should be accompanied by a purchase order to
cover repair costs.

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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.
Röntgenweg 1 2624 BD Delft
P.O. Box 507 2600 AM Delft
Holland
Phone [phone]
Fax [phone]
Email info.hol[email protected]
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©2001 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.

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DECLARATION OF CONFORMITY
According to EC guideline 89/336/EEC 73/23/EEC
We Kipp & Zonen B.V.
Röntgenweg 1
2624 BD Delft
Declare under our sole responsibility that the product
Type: CM 21
Name: Pyranometer
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.A Dieterink
President
KIPP & ZONEN B.V.

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TABLE OF CONTENTS
IMPORTANT USER INFORMATION 3
DECLARATION OF CONFORMITY 4
TABLE OF CONTENTS 5
1. GENERAL INFORMATION 7
1.1 INTRODUCTION 7
1.2 PHYSICAL PRINCIPLES OF THE PYRANOMETER 8
1.2.1 Zero offsets 9
1.2.3 Directional error 10
1.2.4 Low temperature dependency of sensitivity 11
2TECHNICAL DATA 13
2.1 SPECIFICATIONS OF CM 21 PYRANOMETER ACCORDING
TO ISO 9060 LISTING 13
2.2 ACCURACY 16
3INSTALLATION 19
3.1 DELIVERY 19
3.2 MECHANICAL INSTALLATION 20
3.2.1 Installation for measurement of global radiation 20
3.2.2 Installation for measurement of solar radiation on inclined surfaces 23
3.2.3 Installation for measurement of reflected global radiation 24
3.2.4 Installation for measurement of diffuse radiation 25
3.2.5 Installation for measurement of albedo 25
3.2.6 Underwater use 26
3.3 ELECTRICAL CONNECTION 27
4OPERATION 31
5MAINTENANCE 33
TABLE OF CONTENTS

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6CALIBRATION 35
6.1 INITIAL CALIBRATION 35
6.2 RECALIBRATION 35
6.3 CALIBRATION PROCEDURE AT KIPP & ZONEN 37
6.3.1 The facility 37
6.3.2 Procedure 37
6.3.3 Calculation 38
6.3.4 Zero offset 38
6.3.5 Traceability to World Radiometric Reference 39
7FREQUENTLY ASKED QUESTIONS (FAQ’s) 41
8TROUBLE SHOOTING 43
9PART NUMBERS / SPARE PARTS / OPTIONS 45
APPENDIX I CLASSIFICATION ACCORDING TO WMO GUIDE 1996 47
APPENDIX II RADIOMETRIC LEVELING 49
APPENDIX III LIST OF WORLD AND REGIONAL
RADIATION CENTRES 51
APPENDIX IV PYRANOMETER MEASUREMENT REPORT 53
APPENDIX V THERMISTOR SPECIFICATIONS 55
APPENDIX VI PT-100 SPECIFICATIONS 57
APPENDIX VII RECALIBRATION SERVICE 59

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1. GENERAL INFORMATION
1.1 INTRODUCTION
The CM 21 pyranometer is designed for measuring the irradiance (radiant-
flux, Watt/m2) on a plane surface, which results from the direct solar
radiation and from the diffuse radiation incident from the hemisphere above.
CM 21 is a high precision pyranometer with strictly selected domes. Because
of the high optical quality of these domes the directional error is reduced to
less than 10 W/m2.
The special features of the CM 21 are:
•High sensitivity, which could result in lower specifications for the
data acquisition system.
•Low impedance, which reduces sensitivity for interference and
noise.
•Low temperature response, which is an advantage when working
under extreme climatological conditions.
•Low non-linearity.
Like the CM 11 pyranometer, the CM 21 complies with the specifications for
the best of three classes, “High quality”, as defined in the 'Guide to
meteorological Instruments and Methods of Observation', sixth edition, 1996,
of the World Meteorological
1.
GENERAL INFORMATION

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Organisation (*WMO) - Geneva - Switzerland. Some specifications of the
CM 21 are twice as good as required.
For measuring the diffuse component of solar radiation only, the direct solar
component can be shielded semi-automatically from the pyranometer by the
Kipp & Zonen shadow ring CM 121. Fully automatic shielding can be done
with the 2AP tracker with shading device.
* The WMO classification is adapted from the international standard ISO 9060 (1990).
Herein “high quality“ class is referred to as “secondary standard”.
1.2 PHYSICAL PRINCIPLES OF THE PYRANOMETER
The pyranometer CM 21 is provided with a thermal detector. This type of
detector responds to the total power absorbed and theoretically it is non-
selective as to the spectral distribution of the radiation. This implies that the
naked thermal detector is also sensitive to long wave infrared radiation
(thermal radiation λ> 3000 nm) from the environment. (e.g. the inner dome)
The radiant energy is absorbed by a black painted disk. The heat generated
flows through a thermal resistance to the heatsink (the pyranometer body).
The temperature difference across the thermal resistance of the detector is
converted into a voltage.
The rise of temperature is easily affected by wind, rain and thermal radiation
losses to the environment ('cold' sky). Therefore the detector is shielded by
two glass domes. These domes allow equal transmittance of the direct solar
component for every position of the sun on the celestial sphere. A dessicator
in the body prevents dew on the inner side of the domes, which can cool
down considerably on clear windless nights.
When the pyranometer is illuminated a temperature difference arise between
the centre and the border of the black absorber. Natural convection inside
the inner dome due to this temperature difference is small and therefore
tilting the CM 21 causes no significant change of sensitivity.

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Figure 1 Construction details of a CM 21 pyranometer
1.2.1 Zero offsets
Zero offset type B
Heat flows in the sensing element e.g. due to rising or falling body
temperature normally cause a spurious voltage., called zero offset. To
compensate for this offset, a second non-illuminated element is installed, in
which the same heat flow will arise. By anti-series arrangement of both
elements the spurious voltage is cancelled out greatly. The white plastic
screen reduces the body temperature variations due to solar radiation and
(cold) rain showers.
Zero offset type A
This zero offset is present when there is a temperature difference between
the inner dome and the cold junctions of the sensor.
In practice a zero offset will be present when there is a clear sky. Because of
the low effective sky temperature (<0ºC) the earth surface emits roughly 100
W/m2longwave infrared radiation upwards. The outer glass dome of a
pyranometer also has this emission and is cooling down several degrees
below air temperature (the emissivity of glass for the particular wavelength
1.
GENERAL INFORMATION

10
region is nearly 1). The emitted heat is extracted from the body (by
conduction in the dome), from the air (by wind) and from the inner dome
(through infrared radiation). The inner dome is cooling down too and will
extract heat from the body by conduction and from the sensor by the net
infrared radiation again. The latter heat flow is opposite to the heat flow from
absorbed solar radiation and causes the well known zero depression at
night. This negative zero offset is also present on a clear day, however,
hidden in the solar radiation signal. Because the temperature difference
between the body and the (clear) sky is higher during day time, the zero-
offset will also be higher.
During indoor measurements with a solar simulator, the inner dome can
become warmer than the pyranometer body due to net thermal radiation
from the lamp housing. A positive zero offset type A is the result.
Zero offset type A can be checked by placing a light and IR reflecting cap
over the pyranometer. The response to solar radiation will decay
with a time constant (1/e) of 5 s, but the dome temperature will go to
equilibrium with a time constant of several minutes. So after half a
minute the remaining signal represents mainly zero offset type A.
Good ventilation of domes and body is the solution to reducing zero offsets
even further. Kipp & Zonen advises the CV 2 for optimal ventilation and
suppression of zero offset type A.
1.2.3 Directional error
The directional error is an individual feature and depends on imperfections of
the glass domes, angular reflection properties of the black paint and
tolerances on the pyranometer construction. The directional error is a
combination of the error in zenith and azimuth directions.
This Kipp & Zonen CM 21 pyranometer is manufactured with great care.
Each pyranometer is constructed with selected glass domes. The glass
domes are matched and positioned with high precision.

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Before leaving the factory, the cosine response of each CM 21 is measured.
At four zenith angles the cosine response versus two azimuth’s is
determined.
These values (40º and 70º) are stated on the calibration certificate,
expressed as percentage deviation from the ideal proportionality and as
absolute values (see appendix IV).
1.2.4 Low temperature dependency of sensitivity
The sensitivity is correlated to the temperature as a consequence of physical
material properties of the sensor. For a given heat flow, the sensitivity of the
pyranometer is a function of the thermal conductivity of the sensor and of the
thermo-electric power of the thermocouple material. Both physical
parameters show temperature dependency.
Due to the sensor construction and applied thermistor compensation circuit
the temperature response is suppressed to a minimum between –20ºC and
+50ºC.
During factory manufacturing the sensitivity of each CM 21 pyranometer is
measured and compensated to provide the temperature dependency
specifications.
The temperature dependency is supplied with each instrument, measured in
8 steps of 10ºC, from +50ºC to –20ºC.

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2 TECHNICAL DATA
2.1 SPECIFICATIONS OF CM 21 PYRANOMETER ACCORDING TO
ISO 9060 LISTING
Spectral range: 305-2800 nm (50% points)
335-2200 nm (95% points)
Sensitivity: between 7 and 17 µV/Wm-2
Impedance: 40 -100 Ohm
Response time: 5 s (95% response)
1.6 s (63% response)?
Non-linearity: < ± 0.2% (< 1000 W/m2)
Spectral selectivity ±2%
Temperature dependence
of sensitivity: < ± 1% (-20ºC to +50ºC)
Directional error: < ± 10 W/m2 ( beam 1000 W/m2)
Tilt error < ± 0.2% (beam 1000 W/m2)
Zero-offset due to FIR < 7 W/m2at 200 W/m2 net thermal
(Ventilated) radiation
Zero-offset due to temp.
changes < 2 W/m2at 5 K/h temp. change
Operating temperature -40ºC to +80ºC
Viewing angle: 2 πsr
2.
TECHNICAL DATA

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Max. irradiance: 0 - 4000 W/m2
Non-stability: < ± 0.5% sensitivity change per year
The specified directional response includes (as relative errors) :
Cosine response: max. ± 2% deviation from ideal at 60º solar zenith
angle in any azimuth direction.
max. ± 6% deviation from ideal at 80º solar zenith
angle in any azimuth direction.
Construction
Receiver paint: Carbon black
Quartz domes: Schott K5 optical glass 2 mm thick, 30 mm and 50 mm outer diameter
Desiccant: Silica gel
Spirit level: Sensitivity 0.1 degree (bubble half out of the ring)
Integral with base of instrument.
Detector surface and base are coplanar
within 0.1º
Materials: Anodised aluminium case
Aluminium levelling screws
Stainless steel screws etc.
White plastic screen, ASA
Drying cartridge, PMMA
Weight: 930 g
Cable length: 10 m
Dimensions: W x H 150 x 95 mm. See figure 2
CM 21 specifications compared against the WMO 1996 qualification classes
are shown in appendix I.

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Figure 2 CM 21 Pyranometer outline dimensions in mm.
92.5 mm
150 mm
6 mm
65 mm
L = 10 m
2.
TECHNICAL DATA

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2.2 ACCURACY
As listed in 2.1 the sensitivity is cross-correlated to a number of parameters,
such as temperature, level of irradiance, angle of incidence, etc.
Normally, the supplied sensitivity figure is used to calculate the irradiances.
If the conditions differ from calibration conditions, errors in the calculated
irradiances must be expected.
For a secondary standard instrument the WMO expects maximum errors in
the hourly radiation totals of 3 %. In the daily total an error of 2 % is
expected, because some response variations cancel out each other if the
integration period is long.
Kipp & Zonen expects max errors of 2 % for hourly totals and 2 % for daily
totals.
These remaining errors can be reduced further if the actual sensitivity of the
pyranometer is used by the conversion of voltage to irradiance. The actual
sensitivity can be calculated when it is a well known function of simply
measured parameters (sometimes called transfer function or sensitivity
function).
This is especially convenient in connection with a programmable data
acquisition system.
For the CM 21 the effect of each parameter on the sensitivity can be shown
separately, because the parameters show less interaction.
The non-linearity error, the sensitivity variation with irradiance, is the same
for any CM 21and negligible.

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The temperature dependence of the sensitivity is an individual function. For
a given CM 21 the curve is somewhere in the region between the curved
lines in figure 3.
Figure 3 The curve of relative sensitivity variation with instrument
temperature of a CM 21 pyranometer is in the shaded region.
The directional error is the summation of the azimuth and zenith error and is
commonly given in W/m2. Figure 4 shows the maximum relative zenith error
in any azimuth direction for the CM 21.
Figure 4 Relative directional error is van CM 21.
2.
TECHNICAL DATA

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Spectral selectivity is the product of spectral absorptance of the black
coating and spectral transmittance of the glass domes (see fig. 5 and 6).
Shifts in the solar spectrum, due to changes from clear to overcast sky, are
mainly in the mid of the spectral range. No significant spectral selectivity
errors have to be expected. E.g. at a sun’s elevation of 30º (air mass 2) only
1 % of the solar radiation has wavelengths below 335 nm and only 1 % has
wavelengths above 2200 nm.
Figure 5 CM 21 Black absorbers relative absorption [%]
Figure 6 1. Relative transmittance of two pyranometer domes.
2. Spectral distribution of solar radiation at sea level. Sun at
zenith (airmass 1).
CM22 Black absorber
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Wavelength [nm]
Relative absorption [%]
CM 21 Black absorber

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3 INSTALLATION
Reading the installation instructions before installation is recommended.
3.1 DELIVERY
Check the contents of the shipment for completeness (see below) and note
whether any damage has occurred during transport. If there is damage, a
claim should be filed with the carrier immediately. In this case, and also if the
contents are incomplete, your dealer should be notified in order to facilitate
the repair or replacement of the instrument.
The CM 21 pyranometer delivery will include the following items:
1. CM 21 pyranometer
2. White sun screen
3. 2 x Mounting bolts + nuts + washers (M5 x 80)
4. 2 x nylon insulators
5. Calibration certificate
6. Temperature dependency data
Unpacking
Keep the original packaging for later shipments!
Although all sensors are weatherproof and suitable for rough ambient
conditions, they do partially consist of delicate mechanical parts. For this
type of equipment, keep the original shipment packaging to safely transport
the equipment to the measurement site.
3.
INSTALLATION

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3.2 MECHANICAL INSTALLATION
The mechanical installation of the pyranometer depends upon the measuring
purpose. Different measuring methods will be explained in the next
paragraphs.
3.2.1 Installation for measurement of global radiation
The following steps must be carefully taken for an optimal performance of
the instrument:
1. Location
Ideally the site for the pyranometer should be free from any obstructions
above the plane of the sensing element, and at the same time the
pyranometer should be readily accessible to clean the outer dome and
inspect the dessicator. If this is not possible, the site should be chosen in
such a way that any obstruction over the azimuth range between earliest
sunrise and latest sunset should have an elevation not exceeding 5º (The
apparent sun diameter is 0.5º). This is important for an accurate
measurement of the direct solar radiation. The diffuse (solar) radiation is less
influenced by obstructions near the horizon. For instance, an obstruction
with an elevation of 5º over the whole azimuth range of 360º decreases the
downward diffuse solar radiation by only 0.8%.
It is evident that the pyranometer should be located in such a way that a
shadow will not be cast on it at any time (for example, by masts or exhaust
pipes). Note that hot (over 200 degrees centigrade) exhausted gas will
produce radiation in the spectral range of the CM 21 Pyranometer.
The pyranometer should be distant from light-coloured walls or other objects
likely to reflect sunlight onto it.
In principle no special orientation of the instrument is required.
3.
INSTALLATION