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

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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.