Hukseflux SR25-D2 User Manual

Copyright by Hukseflux | manual v1603 | www.huksefluxusa.com | [e-mail]

USER MANUAL SR25-D2

Digital secondary standard pyranometer
with sapphire outer dome

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Warning statements

Putting more than 30 Volt across the sensor wiring
of the main power supply can lead to permanent
damage to the sensor.

Putting more than 40 Volt across the sensor wiring
of the current loop (4 to 20 mA) can lead to
permanent damage to the sensor.

For proper instrument grounding: use SR25-D2 with
its original factory-made SR25-D2 cable.

Using the same Modbus address for more than one
device will lead to irregular behaviour of the entire
network.

Your data request may need an offset of +1 for each
SR25-D2 register number, depending on processing
by the network master. Consult the manual of the
device acting as the local master.

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Contents

Warning statements 2
Contents 3
List of symbols 5
Introduction 6
1 Ordering and checking at delivery 9

1.1 Ordering SR25-D2 9

1.2 Included items 9

1.3 Quick instrument check 10

2 Instrument principle and theory 11
3 Specifications of SR25-D2 14

3.1 Specifications of SR25-D2 14

3.2 Dimensions of SR25-D2 18

4 Standards and recommended practices for use 19

4.1 Classification standard 19

4.2 General use for solar radiation measurement 19

4.3 General use for sunshine duration measurement 19

4.4 Specific use for outdoor PV system performance testing 20

4.5 Specific use in meteorology and climatology 20

5 Installation of SR25-D2 21

5.1 Site selection and installation 21

5.2 Installation of the sun screen 22

5.3 Electrical connection of SR25-D2: wiring diagram 23

5.4 Grounding and use of the shield 23

5.5 Using SR25-D2’s 4 to 20 mA output 24

5.6 Connecting to an RS-485 network 26

5.7 Connecting to a PC 28

6 Communication with SR25-D2 29

6.1 PC communication: Sensor Manager software 29

6.2 Network communication: function codes, registers, coils 34

6.3 Network communication: getting started 42

6.4 Network communication: example master request to SR25-D2 43

7 Making a dependable measurement 46

7.1 The concept of dependability 46

7.2 Reliability of the measurement 47

7.3 Speed of repair and maintenance 48

7.4 Uncertainty evaluation 48

8 Maintenance and trouble shooting 51

8.1 Recommended maintenance and quality assurance 51

8.2 Trouble shooting 52

8.3 Calibration and checks in the field 53

8.4 Data quality assurance 54

9 Appendices 56

9.1 Appendix on heating SR25-D2 56

9.2 Appendix on cable extension / replacement 59

9.3 Appendix on tools for SR25-D2 60

9.4 Appendix on spare parts for SR25-D2 61

9.5 Appendix on standards for classification and calibration 62

9.6 Appendix on calibration hierarchy 63

9.7 Appendix on meteorological radiation quantities 64

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9.8 Appendix on ISO and WMO classification tables 65

9.9 Appendix on definition of pyranometer specifications 66

9.10 Appendix on terminology / glossary 67

9.11 Appendix on floating point format conversion 68

9.12 Appendix on function codes, register and coil overview 69

9.13 EU declaration of conformity 72

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List of symbols

Quantities Symbol Unit

Voltage output U V
Sensitivity S V/(W/m2)
Temperature T °C
Solar irradiance E W/m2
Solar radiant exposure H W∙h/m2
Time in hours h h

Temperature coefficient a 1/°C²
Temperature coefficient b 1/°C
Temperature coefficient c -

Output of 4-20 mA current loop I A
Transmitted range of 4-20 mA output r W/m

2

(see also appendix 9.7 on meteorological quantities)

Subscripts

Not applicable

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Introduction

SR25-D2 digital secondary standard pyranometer takes solar radiation measurement to
the next level. Using a sapphire outer dome, it has negligible zero offsets. SR25-D2 is
heated in order to suppress dew and frost deposition, maintaining its high measurement
accuracy. SR25-D2 offers two types of commonly used irradiance outputs in W/m2: digital
via Modbus RTU over RS-485 and analogue 4-20 mA (current loop). Patents on the SR25
working principle are pending.

SR25-D2’s digital output, provided via the industry standard Modbus protocol, allows for
easy data acquisition and error-free instrument exchange. SR25-D2 offers analogue 4 to
20 mA (current loop) output too. The instrument is also available with analogue millivolt
output (as SR25).

This user manual covers SR25-D2 use. Specifications of SR25-D2 differ from
those of model SR25 with analogue millivolt output. For SR25 use, consult the
SR25 user manual.

By keeping the SR25-D2 outer dome free of dew and frost with help of the internal
heater, data availability is highly increased over traditional pyranometers, whether these
are ventilated or not. SR25-D2 offers the best data availability. In addition, SR25-D2
needs very low power; heating only consumes 1.5 W compared to the usual 10 W for
ventilation. The low thermal offsets make SR25-D2 very suitable for measuring diffuse
radiation. SR25-D2 offers the best measurement accuracy: the specification limits of two
major sources of measurement uncertainty have been greatly improved over competing
pyranometers: “zero offset a” and temperature response.

SR25-D2 has the following distinguishing features and benefits:

 sapphire outer dome: negligible zero offsets
 internal heater: because of dew and frost suppression by heating, better data

availability and accuracy than ventilated instruments

 digital output: easy implementation & servicing
 test certificates for temperature response and directional response included: all

sensors tested individually for ISO 9060 compliance

Figure 0.1 SR25-D2 digital secondary standard pyranometer with sapphire outer dome

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SR25-D2 has a sapphire outer dome, glass inner dome and an internal heater. It
employs a state-of-the-art thermopile sensor with black coated surface and an anodised
aluminium body. The connector, desiccant holder and sun screen fixation are very robust
and designed for long term use. SR25-D2 uses a high-end 24-bit A/D converter. All parts
are specified for use across SR25-D2’s entire rated operating temperature range.
SR25-D2 offers two types of outputs: digital output via Modbus RTU over 2-wire RS-485
and analogue 4-20 mA output (current loop).

For communication between a PC and SR25-D2, the Hukseflux Sensor Manager software is
included. It allows the user to plot and export data, and change the SR25-D2 Modbus
address and its communication settings.

Figure 0.2 User interface of the Sensor Manager

SR25-D2 is designed for use in SCADA (Supervisory Control And Data Acquisition)
systems, supporting Modbus RTU (Remote Terminal Unit) protocol over RS-485. In these
networks the sensor operates as a slave. SCADA systems are often implemented in
photovoltaic solar energy (PV) systems and meteorological networks. Using SR25-D2 in a
network is easy. Once it has the correct Modbus address and communication settings and
is connected to a power supply, the instrument can be used in RS-485 networks. A
typical network will request the irradiance and temperature data in registers 3+4 and 7
every 1 second, and eventually store the averages every 60 seconds. How to issue a
request, process the register content and convert it to useful data is described in the
paragraphs about network communication. The user should have sound knowledge of the
Modbus communication protocol when installing sensors in a network.

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The instrument should be used in accordance with the recommended practices of ISO,
WMO and ASTM.

Suggested use for SR25-D2:

 all situations where ventilated pyranometers are employed
 all networks with regular instrument exchange
 PV system performance monitoring
 indoor PV testing with solar simulators
 airborne measurements
 diffuse measurements
 environments with dew
 environments with frost

The ASTM E2848 “Standard Test Method for Reporting Photovoltaic Non-Concentrator
System Performance” (issued end 2011) confirms that a pyranometer is the preferred

instrument for PV system performance monitoring. SR25-D2 pyranometer complies with
the requirements of this standard. For more information, see our pyranometer selection
guide.

WMO has approved the “pyranometric method” to calculate sunshine duration from

pyranometer measurements in WMO-No. 8, Guide to Meteorological Instruments and
Methods of Observation. This implies that SR25-D2 may be used, in combination with
appropriate software, to estimate sunshine duration. This is much more cost-effective
than using a dedicated sunshine duration sensor. Ask for our application note.

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1 Ordering and checking at delivery

1.1 Ordering SR25-D2

The standard configuration of SR25-D2 is with 5 metres cable.

Common options are:

 Longer cable (in multiples of 5 m). Specify total cable length.
 Five silica gel bags in an air-thight bag for SR25-D2 desiccant holder. Specify order

number DC01.

 Adapted transmitted range for 4-20 mA output. Standard setting is 4 mA at 0 W/m

2

and 20 mA at 1600 W/m2. Specify preferred range setting.

1.2 Included items

Arriving at the customer, the delivery should include:

 pyranometer SR25-D2
 sun screen
 cable of the length as ordered
 calibration certificate matching the instrument serial number
 product certificate matching the instrument serial number

(including temperature response test report and directional response test report for
the individual instrument)

 Hukseflux Sensor Manager software on a USB flash drive
 any other options as ordered

Please store the certificates in a safe place.

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1.3 Quick instrument check

A quick test of the instrument can be done by connecting it to a PC and installing the
Sensor Manager software. See the chapters on installation and PC communication for
directions.

1. At power–up the signal may have a temporary output level different from zero; an
offset. Let this offset settle down.

2. Check if the sensor reacts to light: expose the sensor to a strong light source, for
instance a 100 W light bulb at 0.1 m distance. The signal should read > 100 W/m2 now.
Darken the sensor either by putting something over it or switching off the light. The
instrument irradiance output should go down and within one minute approach 0 W/m2.

3. Remove the sun screen, (see chapter on installation of the sun screen). Inspect the
bubble level.

4. Inspect the instrument for any damage.

5. Inspect if the humidity indicator is blue. Blue indicates dryness. The colour pink
indicates it is humid: in the latter case replace the desiccant (see chapter on
maintenance).

6. Check the instrument serial number as indicated by the software against the label on
the instrument and against the certificates provided with the instrument.

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2 Instrument principle and theory

Figure 2.1 Overview of SR25-D2:

(1) cable (standard length 5 metres, optional longer cable)
(2) fixation of sun screen (thumb screw)
(3) glass inner dome
(4) thermal sensor with black coating
(5) sapphire outer dome
(6) sun screen
(7) humidity indicator
(8) desiccant holder
(9) levelling feet
(10) bubble level
(11) connector

1

2

3

4

5

6

7

8

9

10

11

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SR25-D2’s scientific name is pyranometer. A pyranometer measures the solar radiation
received by a plane surface from a 180° field of view angle. This quantity, expressed in
W/m2, is called “hemispherical” solar radiation. The solar radiation spectrum extends
roughly from 285 to 3000 x 10-9 m. By definition a pyranometer should cover that
spectral range with a spectral selectivity that is as “flat” as possible.

In an irradiance measurement by definition the response to “beam” radiation varies with
the cosine of the angle of incidence; i.e. it should have full response when the solar
radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0° angle
of incidence), zero response when the sun is at the horizon (90° angle of incidence, 90°
zenith angle), and 50 % of full response at 60° angle of incidence.
A pyranometer should have a so-called “directional response” (older documents mention
“cosine response”) that is as close as possible to the ideal cosine characteristic.

In order to attain the proper directional and spectral characteristics, SR25-D2
pyranometer’s main components are:

 a thermal sensor with black coating. It has a flat spectrum covering the 200 to 50000

x 10-9 m range, and has a near-perfect directional response. The coating absorbs all
solar radiation and, at the moment of absorption, converts it to heat. The heat flows
through the sensor to the sensor body. The thermopile sensor generates a voltage
output signal that is proportional to the solar irradiance.

 a high-end 24-bit A/D converter. The analogue thermopile voltage is converted by the

instrument electronics to a digital signal. In this process also the temperature
dependence of the thermopile is compensated.

 a sapphire (outer) dome. The high thermal conductivity of the sapphire outer dome

ensures excellent thermal coupling between body and outer dome, even when the
pyranometer is heated. As a result, both zero offset a and heating offset are very low.

 a second (inner) dome made of glass. This dome limits the spectral range from 285

to 3000 x 10-9 m (cutting off the part above 3000 x 10-9 m), while preserving the
180° field of view angle. For a secondary standard pyranometer, two domes are

used, and not one single dome. This construction provides an additional “radiation
shield”, resulting in a better thermal equilibrium between the sensor and inner dome,

compared to using a single dome. The effect of having a second dome is a further
reduction of instrument offsets.

Pyranometers can be manufactured to different specifications and with different levels of
verification and characterisation during production. The ISO 9060 - 1990 standard, “Solar
energy - specification and classification of instruments for measuring hemispherical solar

and direct solar radiation”, distinguishes between 3 classes; secondary standard (highest

accuracy), first class (second highest accuracy) and second class (third highest
accuracy).

From second class to first class and from first class to secondary standard, the achievable
accuracy improves by a factor 2.

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Figure 2.2 Spectral response of the pyranometer compared to the solar spectrum. The
pyranometer only cuts off a negligible part of the total solar spectrum.

Figure 2.3 Directional response of a SR25-D2 pyranometer of 4 azimuth angles,

compared to secondary standard limits

0

0.2

0.4

0.6

0.8

1

1.2

100 1000 10000

relative spectral content /

response

[arbitrary units]

wavelength [x 10

-9

m]

solar radiation

pyranometer
response

-4%

-2%

0%

2%

4%

0 20 40 60 80

Deviation from ideal cosine behaviour

[%]

zenith angle [°]

North

East

South

West

ISO secondary
standard
directional
response limit

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3 Specifications of SR25-D2

3.1 Specifications of SR25-D2

SR25-D2 measures the solar radiation received by a plane surface from a 180o field of
view angle. This quantity, expressed in W/m2, is called “hemispherical” solar radiation.
SR25-D2 offers irradiance in W/m2 as a digital output and as a 4-20 mA output. It must
be used in combination with suitable power supply and a data acquisition system which
uses the Modbus communication protocol over RS-485 or one that is capable of handling
a 4-20 mA current loop signal. The instrument is classified according to ISO 9060 and
should be used in accordance with the recommended practices of ISO, IEC, WMO and
ASTM.

SR25-D2 has an onboard heater and a temperature sensor. All contribute to the
dependability and accuracy of the measurement. However, also when not using this
feature, SR25-D2 still complies with the secondary standard requirements. The
instrument should be used in accordance with the recommended practices of ISO, IEC,
WMO and ASTM.

Table 3.1.1 Specifications of SR25-D2 (continued on next pages)

SR25-D2 MEASUREMENT SPECIFICATIONS:
LIST OF CLASSIFICATION CRITERIA OF ISO 9060*

ISO classification (ISO 9060: 1990)

secondary standard pyranometer

WMO performance level (WMO-No. 8,
seventh edition 2008)

high quality pyranometer
Response time (95 %)

3 s

Zero offset a (response to 200 W/m2
net thermal radiation)

1 W/m

2

unventilated

1 W/m

2

ventilated

Zero offset b (response to 5 K/h
change in ambient temperature)

< ± 2 W/m2
Non-stability

< ± 0.5 % change per year

Non-linearity

< ± 0.2 % (100 to 1000 W/m2)

Directional response

< ± 10 W/m2

Directional response test of individual
instrument

report included
Spectral selectivity

< ± 3 % (0.35 to 1.5 x 10

-6

m)

Temperature response

< ± 0.4 % (-30 to +50 °C)

Temperature response test of
individual instrument

report included
Tilt response

< ± 0.2 % (0 to 90 ° at 1000 W/m2)

*For the exact definition of pyranometer ISO 9060 specifications see the appendix.

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Table 3.1.1 Specifications of SR25-D2 (continued)

SR25-D2 ADDITIONAL SPECIFICATIONS

Measurand

hemispherical solar radiation

Measurand in SI radiometry units

irradiance in W/m2

Optional measurand

sunshine duration

Field of view angle

180 °

Output definition

running average over 4 measurements, refreshed
every 0.1 s

Recommended data request interval

1 s, storing 60 s averages

Measurement range

-400 to 4000 W/m2

Zero offset steady state

< ± 0.5 W/m2 at 20 °C
< ± 0.8 W/m2 (-40 to + 80 °C)

Zero offset dynamic / during power up

< 10 W/m2 (nominal)

Measurement function / optional
programming for sunshine duration

programming according to WMO guide paragraph

8.2.2

Internal temperature sensor

Analog Devices ADT7310 digital SPI temperature
sensor

Rated operating temperature range

-40 to +80 °C

Spectral range
(20 % transmission points)

285 to 3000 x 10-9 m

Standard governing use of the
instrument

ISO/TR 9901:1990 Solar energy -- Field pyranometers

-- Recommended practice for use
ASTM G183 - 05 Standard Practice for Field Use of
Pyranometers, Pyrheliometers and UV Radiometers

Standard cable length (see options)

5 m

Cable diameter

5.3 x 10-3 m

Chassis connector

M16 panel connector, male thread, 10-pole

Chassis connector type

HUMMEL AG 7.840.200.000 panel connector, front
mounting, short version

Cable connector

M16 straight connector, female thread, 10-pole

Cable connector type

HUMMEL AG 7.810.300.00M straight connector,
female thread, for cable 3 to 6 x 10-3 m, special
version

Connector protection class

IP67 / IP69 K per EN 60 529 (connected)

Cable replacement

replacement cables with connector can be ordered
separately from Hukseflux

Mounting

2 x M5 bolt at 65 x 10-3 m centre-to-centre distance
on north-south axis, or 1 x M6 bolt at the centre of
the instrument, connection from below under the
bottom plate of the instrument

Levelling

bubble level and adjustable levelling feet are included

Levelling accuracy

< 0.1° bubble entirely in ring

Desiccant

two bags of silica gel, 0.5 g, (35 x 20) mm

Humidity indicator

blue when dry, pink when humid

IP protection class

IP67

Gross weight including 5 m cable

2.05 kg

Net weight including 5 m cable

0.85 kg

Packaging

HPRC casing of 255 x 225 x 165 mm

HEATING

Heater

the heater is not necessarily switched on;
recommended operation is to continually power the
heater (see appendix 9.1)

Required heater power

1.5 W at 12 VDC (the heater is not necessarily active)

Heater resistance

95 Ω

Steady state zero offset caused by
heating

0 to -2.5 W/m2

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Table 3.1.1 Specifications of SR25-D2 (started on previous pages)

CALIBRATION
Calibration traceability

to WRR

Calibration hierarchy

from WRR through ISO 9846 and ISO 9847, applying
a correction to reference conditions

Calibration method

indoor calibration according to ISO 9847, Type IIc

Calibration uncertainty

< 1.2 % (k = 2)

Recommended recalibration interval

2 years

Reference conditions

20 °C, normal incidence solar radiation, horizontal
mounting, irradiance level 1000 W/m2

Validity of calibration

based on experience the instrument sensitivity will not
change during storage. During use under exposure to
solar radiation the instrument “non-stability”
specification is applicable.

Adjustment after re-calibration

via a PC, as power user with the Sensor Manager
software. Request “power user” status at the factory
for sensitivity adjustment and access to the calibration
history data.

MEASUREMENT ACCURACY AND RESOLUTION

Uncertainty of the measurement

statements about the overall measurement
uncertainty can only be made on an individual basis.
see the chapter on uncertainty evaluation

WMO estimate on achievable accuracy
for daily sums (see appendix for a
definition of the measurement conditions)

2 %

WMO estimate on achievable accuracy
for hourly sums (see appendix for a
definition of the measurement conditions)

3 %

Irradiance resolution

0.05 W/m2

Instrument body temperature resolution

7.8 x 10-3 °C

Instrument body temperature accuracy

± 0.5 °C

DIGITAL

Digital output

irradiance in W/m2
instrument body temperature in °C

Rated operating voltage range

5 to 30 VDC

Power consumption main supply

< 75 x 10-3 W at 12 VDC

Communication protocol

Modbus over 2-wire RS-485
half duplex

Transmission mode

RTU

System requirements for use with PC

Windows Vista and later, USB or RS-232 (COM) port
and connector, RS-485 / USB converter or RS-485 /
RS-232 converter

Software requirements for use with PC

Java Runtime Environment – software
available free of charge at http://www.java.com

User interface on PC

Hukseflux Sensor Manager software
supplied with the instrument on a USB flash drive.
for available software updates, please check
http://www.hukseflux.com/page/downloads

4 TO 20 mA

4 to 20 mA output

irradiance in W/m2

Transmitted range

0 to 1600 W/m2

Output signal

4 to 20 x 10-3 A

Standard setting (see options)

4 x 10-3 A at 0 W/m2 and
20 x 10-3 A at 1600 W/m

2

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Table 3.1.1 Specifications of SR25-D2 (started on previous pages)

Principle of 4 to 20 mA output

2-wire current loop. note: 2 additional wires are
needed for the main supply of the sensor

Rated operating voltage range of 4 to 20
mA output

5.5 to 40 VDC
Power consumption of main supply

< 75 x 10-3 W at 12 VDC

Power consumption of 4 to 20 mA
current loop

< 240 x 10-3 W at 12 VDC (see chapter on using
SR25-D2’s 4-20 mA output)

BACKWARDS COMPATIBILITY

SR25-D2 and SR25-D1

SR25-D2 is the successor of model SR25-D1.
SR25-D2 is completely backwards compatible with
SR25-D1: SR25-D1 users can use SR25-D2 without
the need to change settings or wiring

VERSIONS / OPTIONS

Adapted transmitted range 4 to 20 mA

can be adjusted at the factory upon request

Longer cable, in multiples of 5 m

option code = total cable length

ACCESSORIES

Ventilation unit

VU01

Bags of silica gel for desiccant

set of 5 bags in an air tight bag
option code = DC01

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3.2 Dimensions of SR25-D2

Figure 3.2.1 Dimensions of SR25-D2 in x 10-3 m.

85

M6

M5 (2x)

Ø 150

65

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4 Standards and recommended practices

for use

Pyranometers are classified according to the ISO 9060 standard and the WMO-No. 8
Guide. In any application the instrument should be used in accordance with the
recommended practices of ISO, IEC, WMO and / or ASTM.

4.1 Classification standard

Table 4.1.1 Standards for pyranometer classification. See the appendix for definitions of

pyranometer specifications, and a table listing the specification limits.

STANDARDS FOR INSTRUMENT CLASSIFICATION

ISO STANDARD

EQUIVALENT
ASTM STANDARD

WMO

ISO 9060:1990
Solar energy -- specification and
classification of instruments for
measuring hemispherical solar and
direct solar radiation

Not available

WMO-No. 8; Guide to
Meteorological Instruments
and Methods of Observation,
chapter 7, measurement of
radiation, 7.3 measurement
of global and diffuse solar
radiation

4.2 General use for solar radiation measurement

Table 4.2.1 Standards with recommendations for instrument use in solar radiation

measurement

STANDARDS FOR INSTRUMENT USE FOR HEMISPHERICAL SOLAR RADIATION

ISO STANDARD

EQUIVALENT
ASTM STANDARD

WMO

ISO/TR 9901:1990
Solar energy -- Field
pyranometers -- Recommended
practice for use

ASTM G183 - 05
Standard Practice for Field
Use of Pyranometers,
Pyrheliometers and UV
Radiometers

WMO-No. 8; Guide to
Meteorological Instruments
and Methods of Observation,
chapter 7, measurement of
radiation, 7.3 measurement
of global and diffuse solar
radiation

4.3 General use for sunshine duration measurement

According to the World Meteorological Organization (WMO, 2003), sunshine duration
during a given period is defined as the sum of that sub-period for which the direct solar
irradiance exceeds 120 W/m2.

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WMO has approved the “pyranometric method” to estimate sunshine duration from

pyranometer measurements (Chapter 8 of the WMO Guide to Instruments and
Observation, 2008). This implies that a pyranometer may be used, in combination with
appropriate software, to estimate sunshine duration. Ask for our application note.

Table 4.3.1 Standards with recommendations for instrument use in sunshine duration
measurement

STANDARDS FOR INSTRUMENT USE FOR SUNSHINE DURATION

WMO

WMO-No. 8; Guide to Meteorological Instruments and Methods of Observation, chapter 8,
measurement of sunshine duration, 8.2.2 Pyranometric Method

4.4 Specific use for outdoor PV system performance testing

SR25-D2 is very well applicable in outdoor PV system performance testing. See also
Hukseflux model SR20-D2 “digital secondary standard pyranometer with Modbus RTU
and 4-20 mA output”.

Table 4.4.1 Standards with recommendations for instrument use in PV system
performance testing

STANDARDS ON PV SYSTEM PERFORMANCE TESTING

IEC / ISO STANDARD

EQUIVALENT ASTM STANDARD

IEC 61724; Photovoltaic system performance
monitoring – guidelines for measurement, data
exchange and analysis

COMMENT: Allows pyranometers or reference
cells according to IEC 60904-2 and -6.
Pyranometer reading required accuracy better
than 5% of reading (Par 4.1)

COMMENT: equals JISC 8906 (Japanese
Industrial Standards Committee)

ASTM 2848-11; Standard Test Method for
Reporting Photovoltaic Non-Concentrator
System Performance

COMMENT: confirms that a pyranometer is the
preferred instrument for outdoor PV testing.

Specifically recommends a “first class”

pyranometer (paragraph A 1.2.1.)

4.5 Specific use in meteorology and climatology

The World Meteorological Organization (WMO) is a specialised agency of the United
Nations. It is the UN system's authoritative voice on the state and behaviour of the
earth's atmosphere and climate. WMO publishes WMO-No. 8; Guide to Meteorological

Instruments and Methods of Observation, in which a table is included on “level of
performance” of pyranometers. Nowadays WMO conforms itself to the ISO classification

system.

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5 Installation of SR25-D2

5.1 Site selection and installation

Table 5.1.1 Recommendations for installation of pyranometers

Location

the situation that shadows are cast on the instruments
is usually not desirable. The horizon should be as free
from obstacles as possible. Ideally there should be no
objects between the course of the sun and the
instrument.

Mechanical mounting / thermal insulation

preferably use connection by bolts to the bottom plate
of the instrument. A pyranometer is sensitive to
thermal shocks. Do not mount the instrument with the
body in direct thermal contact to the mounting plate
(so always use the levelling feet also if the mounting
is not horizontal), do not mount the instrument on
objects that become very hot (black coated metal
plates).

Instrument mounting with 2 bolts

2 x M5 bolt at 65 x 10-3 m centre to centre distance
on north-south axis, connection from below under the
bottom plate of the instrument.

Instrument mounting with one bolt

1 x M6 bolt at the centre of the instrument,
connection from below under the bottom plate of the
instrument.

Performing a representative
measurement

the pyranometer measures the solar radiation in the
plane of the sensor. This may require installation in a
tilted or inverted position. The black sensor surface
(sensor bottom plate) should be mounted parallel to
the plane of interest.
In case a pyranometer is not mounted horizontally or
in case the horizon is obstructed, the
representativeness of the location becomes an
important element of the measurement. See the
chapter on uncertainty evaluation.

Levelling

in case of horizontal mounting only use the bubble
level and levelling feet. For inspection of the bubble
level the sun screen must be removed.

Instrument orientation

by convention with the cable exit pointing to the
nearest pole (so the cable exit should point north in
the northern hemisphere, south in the southern
hemisphere).

Installation height

in case of inverted installation, WMO recommends a
distance of 1.5 m between soil surface and sensor
(reducing the effect of shadows and in order to obtain
good spatial averaging).

SR2 5-D2 ma nual v1 603 22/73

5.2 Installation of the sun screen

SR25-D2’s sun screen can be installed and removed by using the dedicated thumb screw.
See item 2 of the drawing below. The thumb screw can be turned without tools for
fixation or loosening of the sun screen, as visualised below. Once the thumb screw has
turned the sun screen loose, the screen can be lifted off manually. After removal the user
may inspect the bubble level, item 10 of the drawing, and remove the cable / connector,
item 11.

Figure 5.2.1 Installation and removal of SR25-D2’s sun screen

1

2

3

4

5

6

7

8

9

10

11