Hukseflux SR20 User Manual

Copyright by Hukseflux | manual v1713 | www.hukseflux.com | [e-mail]

USER MANUAL SR20

Secondary standard pyranometer

Hukseflux

Thermal Sensors

SR20 manual v1713 2/43

Warning statements

Putting more than 12 Volt across the sensor wiring
can lead to permanent damage to the sensor.

Do not use “open circuit detection” when measuring
the sensor output.

SR20 manual v1713 3/43

Contents

Warning stat em e nts 2
Contents 3
List of symbols 4
Introduction 5
1 Ordering and checking at delivery 7

1.1 Ordering SR20 7

1.2 Included items 7

1.3 Quick instrument che ck 8

2 Instrument principle and theory 9
3 Specifications of SR20 12

3.1 Specifications of SR20 12

3.2 Dimensions of SR20 15

4 Standards and recommended practices for use 16

4.1 Classification standard 16

4.2 General use for solar radiation measurement 16

4.3 General use for sunshine duration measurement 16

4.4 Specific use for outdoor PV system perfo rmance testing 17

4.5 Specific use in meteorology and climatology 17

5 Installation of SR20 18

5.1 Site selection and installation 18

5.2 Installation of the sun screen 19

5.3 Electrical connection 20

5.4 Requirements for data acquisition / amplification 21

6 Making a dependable measureme nt 22

6.1 The concept of dependability 22

6.2 Reliability of the measurement 23

6.3 Speed of repair and maintenance 24

6.4 Uncertainty evaluation 24

7 Maintenance and trouble shooting 27

7.1 Recommended maintenance and quality assurance 27

7.2 Trouble shooting 28

7.3 Calibration and che cks in the field 29

7.4 Data quality assurance 30

8 Appendices 32

8.1 Appendix on cable extension / replacement 32

8.2 Appendix on tools for SR20 33

8.3 Appendix on spare parts for SR20 33

8.4 Appendix on standards for classification and calibration 34

8.5 Appendix on calibration hierarchy 35

8.6 Appendix on meteorological radiation quantities 36

8.7 Appendix on ISO and WMO classification tables 37

8.8 Appendix on definition of pyranometer specifications 38

8.9 Appendix on terminology / glossary 39

8.10 Appendix on converting resistance to temperature 40

8.11 EU declaration of conformity 41

SR20 manual v1713 4/43

List of sy m bols

Quantities Symbol Unit
Voltage output U V

Sensitivity S V/(W/m

2

)

Sensitivity at reference conditions S

0

V/(W/m2)
Temperature T °C
Electrical resistance R

e

Ω
Solar irradiance E W/m

2

Solar radiant exposure H W∙h/m

2

Time in hours h h
Temperature co efficient a 1/°C²

Temperature co efficient b 1/°C
Temperature co efficient c -

Resistance of Pt100 R

Pt100

Ω
Pt100 coefficient A
Pt100 coefficient B

Resistance of 10 kΩ thermistor R

thermistor

Ω
Steinhart-Hart coefficient α
Steinhart-Hart coefficient β
Steinhart-Hart coefficient γ

(see also appendix 8.6 on meteorological quantities)

Subscripts

Not applicable

SR20 manual v1713 5/43

Introduction

SR20 is a solar radiation sensor of the highest catego ry in the ISO 9060 classification
system: secondary standard. SR20 pyranometer should be used where the highest
measurement accuracy is required.

SR20 measures the solar radiation received by a plane surface, in W/m

2

, from a 180o
field of view angle. SR20 enables you to attain the highest measurement accuracy and
excels in demanding applications.
The measured quantity, expressed in W/m

2

, is called “hemispherical” solar radiation .
SR20 pyranometer can be employed outdoors under the sun, as well as indoors with
lamp-based solar simulators. Its orientation depends on the application and may be
horizontal, tilted (for plane of array radiation) or inverted (for re flected radiation). In
combination with the rig ht software, also sunshine duration may be measure d.

Using SR20 is easy. It can be connected directly to co mmonly used data logging
systems. The irradiance, E, in W/m

2

is calculated by dividing the SR20 output, a small
voltage U, by the sensitivity S. The sensitivity is provided with SR20 on its calibration
certificate.

The central equation governing SR20 is: E = U/S (Formula 0.1)

SR20’s low temperature dependence makes it an ideal candidate for use under very cold
and very hot conditions. The temperature dependence of every individual instrument is
tested and supplied as a second degree polynomial. This information can be used for
further reduction of temperature dependence during post-processing. In case the
sensitivity is corrected for the instrument body temperature, the optional measurement
equation becomes:

E = U/(S

0

·(a·T² + b·T +c)) (Formula 0.2)

The temperature coefficients a, b, and c can be found on the calibration c ertificate of
each instrument.

SR20 is equipped with an internal temperature sensor. This can be either a Pt100 (T1
version) or a 10 kΩ thermistor (T2 version), as ordered. To calculate temperature in
degrees Celsius from resistance in Ohms, Formula 8.10.1 or 8.10.2 can be used. See the
dedicated chapter in the appendix of this manual for these equations.

The incorporated heater reduces measurement errors caused by early-morning dew
deposition. The instrumen t should be used in accordance with the recommended
practices of ISO, WMO and ASTM.

SR20 manual v1713 6/43

Figure 0.1 SR20 secondary standard pyranometer with its sun screen removed

Suggested use for SR20:

• PV system performance monitoring

• scientific meteorological observations

• reference instrument for comparison

• extreme climates (tropical / polar)

• sunshine dura t io n m easurement

The ASTM E2848 “Standard Test Method for Reporting Photo voltaic Non-Concentrator
System Performance” (issued end 2011) confirms that a pyr anometer is the preferred
instrument for PV system performance monitoring. SR20 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 SR20 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.

SR20’s output is analogue. Model SR20-D2 offers two other types of commonly used
irradiance outputs: digital via Modbus RTU over 2-wire RS-485 and analogue 4-20 mA
output (current loop).

This user manual covers SR20 use. Specifications of model SR20-D2, the digital
secondary standard pyranometer with Modbus RTU and 4-20 mA output, differ
from those of SR20. For SR20-D2 use, please consult the SR20-D2 user manual.

SR20 manual v1713 7/43

1 Ordering and checking at delivery

1.1 Ordering SR20

The standard configuration of SR20 is with 5 metres cable.

Common options are:

• Longer cable (in multiples of 5 m). Specify total cable length.

• Internal temperature sensor. This can be either a Pt100 or a 10 kΩ thermistor.

Specify respectively T1 or T2.

• Five silica gel bags in an air-tight bag for SR20 desiccant holder. Specify order

number DC01.

• VU01 ventilation unit.

1.2 Included items

Arriving at the customer, the delivery should include:

• pyranometer SR20

• 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 and directional response test)

• any other options as ordered

Please store the certificates in a safe place.

SR20 manual v1713 8/43

1.3 Quick instrument check

A quick test of the instrument can be done by using a simple hand held multimeter and a
lamp.

1. Check the electrical resistance of the sensor between the green (-) and white (+) wire.
Use a multimeter at the 1000 Ω range. Measure the sensor resistance first with one
polarity, than reverse the polarity. Take the average value. The typical resistance of the
wiring is 0.1 Ω/m. Typical resistance should be the typical sensor resistance of 100 to
200 Ω plus 1.5 Ω for the total resistance of two wires (back and forth) of each 5 m.
Infinite resistance indicates a broken circuit; zero or a low resistance indicates a short
circuit.

2. Check if the sensor reacts to light: put the multimeter at its most sensitive range of
DC voltage measurement, typically the 100 x 10

-3

VDC range or lowe r. Expose the sensor
to a strong light source, for instance a 100 W light bulb at 0.1 m distance. The signal
should read > 2 x 10

-3

V now. Darken the sensor either by putting something over it or
switching off the light. The instrument voltage output should go down and within one
minute approach 0 V.

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

4. Inspect the instrument for any damage.

5. Ins pect 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).

SR20 manual v1713 9/43

2 Instrument principle and theory

Figure 2.1 Overview of SR20:
(1) cable (standard length 5 metres, optional longer cable)

(2) fixation of sun screen (thumb screw)
(3) inner dome
(4) thermal sensor with black coating
(5) 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

SR20 manual v1713 10/43

SR20’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/m

2

, 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 respon se 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, a 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 glass dome. 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.
Another function of the dome is that it shields the thermopile sensor from the
environment (convection, rain).

• a second (inner) glass dome: For a second ar y sta nd ard 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 se cond dome is a strong
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 b etween 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.

SR20 manual v1713 11/43

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 SR20 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 conten t /

response [arbitrary units]

wavelength [x 10

-9

m]

solar radiation

pyranometer

response

-4%

-2%

0%

2%

4%

0 20 40 60 80

Deviation fr om ide a l cosine behaviour [%]

zenith angle [°]

North

East

South

West

ISO secondary
standard
directional
response limit

SR20 manual v1713 12/43

3 Specifications of SR20

3.1 Specifications of SR20

SR20 is a pyranometer of the highest category in the ISO 9060 classification system:
secondary standard. It measures the solar radiation received by a plane surface from a
180

o

field of view angle. This quantity, expressed in W/m2, is called “hemispherical” solar
radiation. Working completely passive, using a thermopile sensor, SR20 generates a
small output voltage proportional to this flux. It can only be used in combination with a
suitable measurement system.

SR20 has an onboard heater and a temperature sensor. Heating the sensor, measuring
the body temperature and using the correction of the temperature response, all
contribute to the dependability and accuracy of the measurement. However, also when
not using these features, SR20 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 SR20 (continued on next pages)

SR20 MEASUREMENT SPECIFICATIONS:
LIST OF CLASSIFICATION CRITERIA OF ISO 9060*

ISO classification (ISO 9060: 1990)

secondary standard pyranometer

WMO performance level (WM O -No. 8,

seventh edition 2008)

high quality pyranometer

Response time (95 %)

3 s

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

5 W/m

2

unventilated

2.5 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 resp onse

< ± 10 W/m2

Directional resp onse test of individu a l

instrument

report included

Spectral selectivity

< ± 3 % (0.35 to 1.5 x 10

-6

m)

Temperature response

< ± 1 % (-10 to +40 °C)

< ± 0.4 % (-30 to +50 °C) with c or rection in data­processing

Temperature response 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.

SR20 manual v1713 13/43

Table 3.1.1 Specifications of SR20 (continued)

SR20 ADDITIONAL SPECIFICATIONS

Measurand

hemispherical solar radiation

Measurand in SI r a diometry units

irradiance in W/m2

Optional measurand

sunshine duration

Field of view angle

180 °

Measurement range

0 to 4000 W/m2

Sensitivity range

7 to 25 x 10-6 V/(W/m2)

Sensitivity (nominal)

15 x 10-6 V/(W/m2)

Expected voltage output application under natural solar radiat ion: -0.1 to + 50

x 10-3 V

Measurement function / required
programming

E = U/S

Optional measurem e nt function /
required programming for correction of
sensitivity as a f unction of instrument

body temperature

E = U/(S0·(a·T²+b·T+c))

Measurement function / optional

programming for sunshine duration

programming according to W M O guide paragraph

8.2.2

Required readout 1 differential voltage channel or 1 single en ded

voltage channel, input resistance > 106 Ω

Internal temperature sens or measuring the body temperatu r e:

version code = T1 for Pt100 DIN class A,

version code = T2 f or thermistor 10 kΩ at 25 °C

Optional readout 1 temperature channel in cas e the temperature sensor

is used

Rated operating temperatu r e r a nge

-40 to +80 °C

Sensor resistance range

50 to 100 Ω

Required sensor power

zero (passive sensor)

Spectral range

(20 % transmission points)

285 to 3000 x 10-9 m

Standard governing use of the
instrument

ISO/TR 9901:1990 Solar en er gy -- Field pyranometers

-- Recommended practice for u s e
ASTM G183 - 05 Standa r d P r a c tice for Field Use of

Pyranometers, Pyrheliometers an d U V 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, shor t version

Cable connector

M16 straight conne c tor , female thread, 10-pole

Cable connector type

HUMMEL AG 7.810.300.00M s tra ight connector,

female thread, for cable 3 to 6 x 10-3 m, special

version

Connector protection class

IP 67 / IP 69 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 a xis, or 1 x M6 bolt at the centre of
the instrument, c onnection from below u nder the
bottom plate of the in strument

Levelling

bubble level and adjustable lev elling 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 cla s s

IP 67

SR20 manual v1713 14/43

Table 3.1.1 Specifications of SR20 (started on previous pages)

Gross weight including 5 m cable

1.2 kg

Net weight including 5 m cable

0.85 kg

Packaging

box of 200 x 135 x 225 mm

HEATING

Heater operation

the heater is not necessarily switched on;

recommended operation is to activate the heater

when the sun is below the horizon

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 -8 W/m2

CALIBRATION

Calibration traceability

to WRR

Calibration hierarchy from WRR through ISO 9846 and ISO 9847, applying

a correction to ref er e nce conditions

Calibration method

indoor calibration according to ISO 9847, Ty pe IIc

Calibration un certainty

< 1.2 % (k = 2)

Recommended recalibration interva l

2 years

Reference conditions

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

Validity of calibra tion

based on experience the instrument s ensitivity will not
change during storage. Durin g use under exposure to
solar radiation th e instrument “non -stability”

specification is applicable.

MEASUREMENT ACCURACY

Uncertainty of the measurement

statements about the overall measur e m ent

uncertainty can only be made on an individual basis.

See the chapter on uncertainty evaluation

WMO estimate on achievable accuracy
for daily sums ( s ee a ppendix 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 %

VERSIONS / OPTIONS

Digital output via Modbus RTU protocol

and 4-20 mA output (current loop)

option code = D2

for specifications see the SR20-D2 user manual

Longer cable, in mu ltiples of 5 m

option code = total cable length

ACCESSORIES

Ventilation un it

VU01

Separate amplifiers

AC100 and AC420

Hand-held read-out unit

LI19

Bags of silica gel for desiccant

set of 5 bags in an air tight bag

option code = DC01

SR20 manual v1713 15/43

3.2 Dimensions of SR20

Figure 3.2.1 Dimensions of SR20 in x 10

-3

m.

85

M6

M5 (2x)

Ø 150

65

SR20 manual v1713 16/43

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 sho uld 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 spec ification limits.

STANDARDS FOR INSTRUMENT CLASSI F I CAT I O N

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 diff use 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 HEM ISP H ERICAL 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 diff use 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/m

2

.

SR20 manual v1713 17/43

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 F O R SUNSHINE DURATION

WMO

WMO-No. 8; Guide to Meteorological Instrumen ts a nd Methods of Observation, chapter 8,
measurement of s unshine duration, 8.2 .2 Pyranometric Method

4.4 Specific use for outdoor PV system performance testing

SR20 is very well applicable in outdoor PV system performance testing. See also model

SR20-D2 “digital secondary standard pyranometer with Modbus RTU and 4-20 mA

output” and SR12 “first class pyranometer for solar energy test applications”.

Table 4.4.1 Standards with recommendations for i n strument use in PV system
performance testing

STANDARDS ON PV SYSTEM PERFORMANCE TESTING

IEC / ISO STANDARD

EQUIVALENT ASTM STANDARD

IEC 61724; Photovoltaic sy s tem per f or m a nce
monitoring – guidelines for measurement, data
exchange and analysis

COMMENT: Allows pyranometers or reference
cells according to IEC 60904-2 and -6.
Pyranometer reading requi r ed a c c uracy better
than 5% of reading (Par 4.1)

COMMENT: equals JISC 8906 (Ja pa nese
Industrial Standards Committee)

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

COMMENT: confirms that a pyranometer is the
preferred instrum e nt 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 Met hod s of Observation, in which a table is included on “lev el of
performance” of pyranometers. Nowadays WMO conforms itself to the ISO classification
system.

SR20 manual v1713 18/43

5 Installation of SR20

5.1 Site selection and installation

Table 5.1.1 Recommendations for installation of pyranometers

Location

the situation that shadows are cast on th e instruments

is usually not des ir a b le. The horizon sh ould be as free
from obstacles as possible. Ideally there should be no
objects between the course of th e s un and the
instrument.

Mechanical mounting / thermal insu lation preferably use connection by bolts to th e bottom plate

of the instrument. A pyranometer is sensitive to
thermal shocks. Do not mount the instr ument with the
body in direct thermal contact to the mounting plate
(so always use the levelling feet also if th e m ounting
is not horizontal), do not mount the instrument on
objects that become very hot (black coa ted metal
plates).

Instrument moun ting with 2 bolts

2 x M5 bolt at 65 x 10-3 m centre to centre distance

on north-south a xis, connection from bel ow under the
bottom plate of the in strument.

Instrument moun ting with one bolt 1 x M6 bolt at the centre of the instrument,

connection from be low under the bottom plate of the
instrument.

Performing a representativ e
measurement

the pyranometer measures the solar ra diation 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 pa rallel to
the plane of interest.
In case a pyranometer is not mounted horizontally or
in case the horizon is obstructed, the
representativeness of th e loca tion becomes an
important element of the meas urement. 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 w ith the cable exit pointing to the

nearest pole (so the cable exit should point nor th in
the northern hemisphere, south in the southern
hemisphere).

Installation height

in case of inverte d installation, WMO r e c om mends a

distance of 1.5 m between soil surface and sensor
(reducing the effect of sh adows a nd in order to obtain
good spatial averaging).

SR20 manual v1713 19/43

5.2 Installation of the sun screen

SR20’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 SR20’s sun screen

1

2

3

4

5

6

7

8

9

10

11

SR20 manual v1713 20/43

5.3 Electrical connection

In order to operate, a pyranometer should be connected to a measurement system,
typically a so-called datalogger. SR20 is a passive sensor that does not need any power.
Cables generally act as a source of distortion, by picking up capacitive noise. We
recommend keeping the distance between a datalogger or amplifier and the s en s or as
short as possible. For cable extension, see the appendix on this subject.

Table 5.3.1 The electrical connection of SR20 versions T1 and T2. The heater is not
necessarily used. The temperature sensor is not necessarily used.

PIN WIRE SR20-T1 SR20-T2

2 Red Pt100 [+] 10 kΩ thermistor [+]
3 Pink Pt100 [+] 10 kΩ thermistor [+]
6

Blue Pt100 [−] 10 kΩ thermistor [−]

8

Grey Pt100 [−] 10 kΩ thermistor [−]

1

Brown heater heater

4

Yellow heater heater

9

Black ground ground

7

White signal [+] signal [+]

5

Green signal [−] signal [−]

Note 1: Pt100’s of version T1 may be connected in a 3-wire of 4-wire c on f iguration.
Note 2: 10k thermistors of version T2 are usually connected in a 2-wire configurati on .
Note 3: the heater is not necessarily connected. In case it is connected, the polarity of
the connection is not important.
Note 4: signal wires are insulated from ground wire a nd from the sensor body. Insulation
resistance is tested during production and larger than 1 x 10

6

Ω.

Note 5: ground is connected to t he connector, the sensor body and the shield of the wire .

Figure 5.3.1 Electrical diagram of the internal wiring of SR20. The shield is connected to
the sensor body.

SR20 manual v1713 21/43

5.4 Requirements for data acquisition / amplification

The selection and programming of dataloggers is the responsibility of the user. Please
contact the supplier of the data acquisition and amplific ation equipmen t to see if
directions for use with the SR20 are available.
In case programming for similar instruments is available, this can typically also be used.
SR20 can usually be treated in the same way as other thermopile pyranometers.
Pyranometers usually have the same programming as heat flux sensors.

Table 5.4.1 Requirements for data acquisition and amplification equipment for SR20 in
the standard configuration

Capability to measure small voltage
signals

preferably: 5 x 10-6 V uncertainty
minimum requirement: 20 x 10

-6

V uncertainty
(valid for the entire expected temperature range of the
acquisition / am plification equipment)

Capability for th e da ta logger or the
software

to store data, and to per form division by the s e nsitivity to
calculate the solar irradiance.
E = U/S (Formula 0.1)

Data acquisition input resistance

> 1 x 10

6

Ω

Open circuit detec tion
(WARNING)

open-circuit detec tion should not be us ed, unless this is done
separately from the norma l mea s urement by more than 5
times the sensor resp onse time and with a s m all current
only. Thermopile s ensors are sensitive to the current that is
used during open c ir c uit detection. The current will generate
heat, which is measured and will appear as an offset.

SR20 manual v1713 22/43

6 Making a dependable measurement

6.1 The concept of dependability

A measurement with a pyranometer is called “dependable” if it is reliable, i.e. measuring
within required uncertainty limits, for most of the time and if problems, once they occur,
can be solved quickly.

The requiremen t s for a measurement with a pyranometer may be expressed by the user
as:

• required uncertainty of the measurement (see following paragraphs)

• requirements for maintenance and repairs (possibilities for maintenance and repair

including effort to be made and processing time)

• a requirement to the expected instrument lifetime (until it is no longer feasible to
repair)

It is important to realise that the uncertainty of the measurement is not only determined
by the instrument but also by the way it is used.

See also ISO 9060 note 5. In case of pyranometers, the measurement uncertainty as
obtained during outdoor measurements is a function o f:

• the instrument class

• the calibration procedure / uncertainty

• the duration of instrument employme n t un d er natural sunlight (involving the

instrument stability specification)

• the measure m ent conditions (such as tilting, ventilation, shading, instrume nt
temperature)

• maintenance (ma inly fouling)

• the environmental conditions*

Therefore, ISO 9060 says, “statements about the overall measurement uncertainty under
outdoor conditions can only be made on an individual basis, taking all these factors into
account”.

* defined at Hukseflux as all factors outside the instrument that are relevant to the
measurement such as the cloud cover (presence or absence of direct radiation), sun
position, the local horizon (which may be obstructed) or condition of the ground (when
tilted). The environmental conditions also involve the question whether or not the
measurement at the location of measurement is representative of the quantity that
should be measured.

SR20 manual v1713 23/43

6.2 Reliability of the measurement

A measurement is reliable if it measures within required uncertainty limits for most of the
time. We distinguish between two causes of unreliability of the measurement:

• related to the reliability of the pyranometer and its design, manufacturing, calibration
(hardware reliability).

• related to the reliability of the measurement uncertainty (measurement reliability),
which involves hardware reliability as well as condition of use.

Most of the hardware reliability is the responsibility of the instrument manufacturer.
The reliability of the measurement however is a joint responsibility of instrument
manufacturer and user. As a function of user requirements, taking into account
measurement conditions and environmental conditions, the user will se lect an instrument
of a certain class, and define maintenance support procedures.

In many situations the re is a limit to a realistically attainable accuracy level. This is due
to conditions that are beyond control once the measurement system is in place. Typical
limiti ng conditions are:

• the measurement conditions, for instance when working at extreme temperatures
when the instrument temperature is at the extre m e lim its of the rated temperature
range.

• the environmental conditions, for instance when installed at a sub-optimal
measurement location with obstacles in the path of t he sun.

• other environmental conditions, for instance when assessing PV system performance
and the system contains panels at different tilt angles, the pyranometer
measurement may not be representative of irradiance received by the entire PV
system.

The measurement reliability can be improved by ma intenance support. Important aspects
are:

• dome fouling by deposition of dus t, dew, rain or snow. Fouling results in undefined
measurement uncertainty (sensitivity and directional error are no longer defined).
This should be solve d by regular inspection and c leaning.

• sensor instability. Maximum expected sensor aging is specified per instrument as its
non-stability in [% change / year]. In case the sensor is not recalibrated, the
uncertainty of the sensitivity gradually will incr ease. This is s olved b y regular
recalibration.

• moisture condensing under pyranometer domes resulting in a slow change of
sensitivity (within specifications). This is solved by regular replacement of desiccant
or by maintenance (drying the entire sensor) in case the sensor allows this. For non­serviceable sensors like most second class pyranometers, this may slowly develop
into a defect. For first class and secondary standard models (for instance model SR11
first class pyranometer and SR20 secondary standard pyranometer) extra desiccant
(in a set of 5 bags in an air tight bag) is available.

SR20 manual v1713 24/43

Another way to improve measurement reliability is to introduce redundant sensors.

• the use of red undant instruments allows remote checks of one instrument using the
other as a reference, which leads to a higher measurement reliability.

• in PV system performance monitoring, in addition to instruments measuring in the
plane of array, horizontally placed instruments are used for the measurement of
global radiation. Global irradiance data enable the user to compare the local climate
and system efficiency between different sites . These data can also be compared to
measurements by local meteorological stations.

6.3 Speed of repair and maintenance

Dependability is not only a matter of reliability but also involves the reaction to
problems; if the processing time of service and repairs is short, this contributes to the
dependability.

Hukseflux pyranometers are designed to allow easy maintenance and repair. The main
maintenance actions are:

• replacement of desiccant

• replacement of cabling

For optimisation of dependability a user should:

• estimate the expected lifetime of the instrument

• design a schedule of regular maintenance

• design a schedule of repair or replacement in case of defects

When operating multiple instruments in a network Hukseflux recommends keeping
procedures simple and having a few spare instruments to act as replacements during
service, recalibrations and repair.

6.4 Uncertainty evaluation

The uncertainty of a mea surement under outdoor or indoor conditions depends on many
factors, see paragraph 1 of this chapter. It is not possible to give one figure for
pyranometer measurement uncertainty. The work on uncertainty evaluation is “in
progress”. There are several groups aro und the world participating in standardisation of
the method of calculation. The effort aims to work according to the guidelines for
uncertainty evaluation (according to the “G uide to Expression of Uncertainty in
Measurement” or GUM).

SR20 manual v1713 25/43

6.4.1 Evaluation of measurement uncertainty under outdoor conditions

Hukseflux actively participates in the discussions about pyranometer measurement
uncertainty; we also provide spreadsheets, reflecting the latest state of the art, to assist
our users in making their own evaluation. The input to t he assessment is summarised :

1) The formal evaluation of uncertainty should be performed in accordance with ISO 98-3

Guide to the Expression of Uncertainty in Measurement, GUM.

2) The spe cifications of the instrument according to t he list of ISO 9060 classification of

pyranometers and pyrheliometers are entered as limiting values of possible errors, to be
analysed as type B evaluation of standar d uncerta inty per paragraph 4.3.7. of GUM. A
priori distributions are chosen as rectangular.

3) A separate estimate has to be entered to allow for estimated uncertainty due to the

instrument maintenance level.

4) The calibration uncertainty has to be entered. Please note that Hukseflux calibration

uncertainties are lower than those of alternative equipment. These uncertainties are
entered in measurement equation (equation is usually Formula 0.1: E = U/S), either as
an uncertainty in E (zero offsets, directional response) in U (voltage readout errors) or
in S (tilt error, temperature dependence, calibration uncertainty).

5) In uncertainty analysis for pyranometers, the location and date of interest is entered.

The course of the sun is then calculated, and the direct and diffuse components are
estimated, based on a model; the angle of incidenc e of direct radiation is a major factor
in the uncertainty.

6) In uncertainty analysis for modern pyrheliometers: tilt dependence often is so low that

one single typ ical observation may be sufficient.

7) In cas e of special measurement conditions, typical specification values are chosen.

These should for instance account for the measurement conditions (shaded / unshaded,
ventilated/ unventilated, horizontal / tilted) and environmental conditions (clear sky /
cloudy, working temperature range).

8) Among the various sources of uncertainty, some are “correlated”; i.e. present during

the entire measurement process, and not cancelling or converging to zero when
averaged over time; the off-diagonal elements of the covariance matrix are not zero.
Paragraph 5.2 of GUM.

9) Among the various sources of uncertainty, some are “uncorrelated”; cancelling or

converging to zero when averaged over time; the off-diagonal elements of the covariance
matrix are zero. Paragraph 5.1 of GUM.

10) Among the various sources of uncertainty, some are “not included in analysis”; this

applies for instance to non-linearity for pyranometers, because it is already included in
the directional error, and the spectral response for pyranometers and pyrheliometers
because it is already taken into account in the calibration process.

SR20 manual v1713 26/43

Table 6.4.1.1 Prelimi nary estimates of achievable uncertainties of measurements with
Hukseflux pyranometers. The estimates are based on typical pyrano meter pro p erties and
calibration uncertainty, for sunny, clear sky days and well maintained stations, without
uncertainty loss due to lack of maintenance and due to instrument fouling. The table
specifies expanded uncertainties with a coverage factor of 2 and confidence level of
95 %. Estimates are based on 1 s sampling. IMPORTANT NOTE: there is no international
consensus on uncer t ainty evaluation of pyranometer measurements, so this table should
not be used as a formal reference.

Pyranometer
class
(ISO 9060)

season latitude uncertainty

minute totals
at solar noon

uncertainty
hourly totals
at solar noon

uncertainty
daily totals

secondary
standard (SR20)

summer

mid-latitude

2.7 %

2.0 %

1.9 %

equator

2.6 %

1.9 %

1.7 %

pole

7.9 %

5.6 %

4.5 %

winter

mid-latitude

3.4 %

2.5 %

2.7 %

first class

summer

mid-latitude

4.7 %

3.3 %

3.4 %

equator

4.4 %

3.1 %

2.9 %

pole

16.1%

11.4 %

9.2 %

winter

mid-latitude

6.5 %

4.5 %

5.2 %

second class

summer

mid-latitude

8.4 %

5.9 %

6.2 %

equator

7.8 %

5.5 %

5.3 %

pole

29.5 %

21.6 %

18.0 %

winter

mid-latitude

11.4 %

8.1 %

9.9 %

6.4.2 Calibration uncertainty

New calibration procedures were developed in close cooperation with PMOD World
Radiation Center in Davos, Switzerland. The latest calibration method results in an
uncertainty of the sensitivity of less than 1.2 %, compared to typical uncertainties of
higher than 1.7 % for this pyranometer class. See the appendix for detailed information
on calib ration hierarchy.

SR20 manual v1713 27/43

7 Maintenance and trouble shooting

7.1 Recommended maintenance and quality assurance

SR20 can measure reliably at a low level of maintenance in most locations. Usually
unreliable measurements will be detected as unreasonably large or small measured
values. As a general rule this means that regular visual inspection combined with a
critical review of the measured data, preferably checking against other measurements, is
the preferred way to obtain a reliable measurement.

Table 7.1.1 Recommended maintenance of SR20. If possible the data analysis and
cleaning (1 and 2) should be done on a daily basis.

MINIMUM RECOMMENDED PYRANOMETER MAIN TENANCE

INTERVAL

SUBJECT

ACTION

1 1 week data analysis compare measured data to maximum possible / maximum

expected irradiance and to other measurements nearby
(redundant instruments). Also historical seasonal records can
be used as a source for expected values. Analyse night time
signals. These signals may be negative (dow n to - 5 W/m

2

on
clear windless nights), due to zero off s e t a. In case of use with
PV systems, compare daytime measurements to PV system
output. Look for a ny patterns and even ts that deviate from

what is normal or expected.

2

2 weeks

cleaning

use a soft cloth to clea n the dome of the instrument,
persistent stains can be treated with soa py water or alcohol

3 6 months inspection inspect cable quality, inspect connectors, inspec t m ounting

position, inspect cable, clean instrument, clean cable, inspect
levelling, change instrument tilt in cas e this is out of
specification, inspect mounting connection, inspect interior of

dome for condensation

4 2 years desiccant

replacement

desiccant replacement (if applicable). Change in case the blue
colour of the 40 % hu m idity indicator turns pink (indicating
humidity), then replace desiccant. Coat the rubber of the
cartridge with silic one grease or vaseline. Desiccant
regeneration: heating in an oven at 70 °C for 1 to 2 hours.

Humidity indicator r egeneration: he a ting until blue at 70 °C

5 recalibration

recalibration by side-by-side comparison to a higher standard
instrument in th e field according to IS O 9847

6 lifetime
assessment

judge if the instrument should be reliable for another 2 years,
or if it should be replaced

7 6 years parts

replacement

if applicable / necessary replace the parts that are most
exposed to weathering; cable, connector, desiccant holder,

sun screen. NOTE: use Hukseflux approved parts only.

8 internal

inspection

if applicable: open instrument and inspect / replace O-rings;

dry internal cav ity around the circuit boa r d

9 recalibration recalibration by side-by-side comparison to a higher standard

instrument indoors according to ISO 98 47 or outdoors

according to ISO9846

SR20 manual v1713 28/43

7.2 Trouble shooting

Table 7.2.1 Trouble shooting for SR20

The sensor
does not give
any signal

Check the electrical resista nce of the sensor between the green
(-) and white (+) wire. Use a multimeter at the 1000 Ω range. Measure the sensor
resistance first with one polarity, th an reverse the polarity . Take the average
value. The typical resistance of the wiring is 0.1 Ω/m. Typical resistance should be
the typical sensor resistance of 100 to 200 Ω plus 1.5 Ω for the total resistance of
two wires (back and forth) of each 5 m. Infinite resistance indicates a broken
circuit; zero or a low r e s istance indicates a s hort circuit.
Check if the sens or reacts to light: put the multimeter at its most sensitive range
of DC voltage measu r e m ent, typically the 100 x 10

-3

VDC range or lower. Expose

the sensor to strong light source, for instan c e a 100 W light bu lb a t 1 x 10

-1

m

distance. The signal should read > 2 x 10

-3

V now. Darken the sensor either by
putting somethin g over it or switching off the light. The instr ument voltage outpu t
should go down an d within one minute approach 0 V. Check the data acquisition
by applying a 1 x 10

-6

V source to it in the 1 x 10-6 V range. Check the condition of

the connectors (on chassis as well as the cable).

The sensor
signal is
unrealistically
high or low.

Note that night-time signals may be negative (down to -5 W/m

2

on clear windless
nights), due to zer o offset a.
Check if the pyranometer ha s c lea n domes.
Check the location of the pyranometer; a r e there any obstruction s that could
explain the measurement result.
Check the orientation / levelling of th e pyranometer.
Check if the right c alibration factor is entered into the algorithm. Please note th a t
each sensor has its own individual calibration factor, as documented in its
calibration cert ificate.
Check if the voltage reading is divided by the calibration factor in review of the
algorithm. Check the condition of the wiring at the logger.
Check the cable condition looking for cable breaks. Ch e c k the condition of the
connectors (on chassis as well as the cable). Check the range of the data logger;
signal can be negative (this could be out of range) or the amplitude could be out
of range. Check the data acqu is iti on by applying a 1 x 10

-6

V source to it in the

1 x 10

-6

V range. Look at the output. Check if the output is as expected.
Check the data a c q uisition by short circuiting the data acqu is ition input with a 100
Ω resistor. Look at the output. Check if the output is close to 0 W/m2.

The sensor
signal shows
unexpected
variations

Check the presence of strong s ources of electromagnetic radiation (r a d a r , radio)
Check the condition of the shielding.
Check the condition of the sensor cable.
Check if the cable is not moving du r in g the measurement
Check the condition of the connectors (on chassis as well as the cable)

The outer
dome shows
internal
condensation.

In case there is a minor layer of moisture that is hardly visible: r eplace the
desiccant and wait a few days to see if the s ituation improves.
In case of conden sation of droplets: disass emble the instrum ent and dry out the
parts.

The inner
dome shows
internal
condensation

Arrange to send the sensor back to Hukseflux for diagnosis.

SR20 manual v1713 29/43

7.3 Calibration and checks in the field

Recalibration of field pyranometers is typically done by comparison in the field to a
reference pyranometer. The applicable standard is ISO 9847 “International Standard­Solar Energy- calibration of field pyranometers by comparison to a reference
pyranometer”. At Hukseflux an indoor calibration according to the same standard is used.

Hukseflux recommendation for re-calibration: if possible, perform ca l ibration indoor by
comparison to an identical reference instrument, under normal incidence conditions.

In case of field comparison; ISO recommends field calibration to a higher class
pyranometer. Hukseflux suggests also allowing use of sensors of the same model and
class, because intercomparisons of similar instruments have the advantage that they
suffer from the same offsets. It is therefore just as good to compare to pyranometers of
the same brand and type as to compare to an instrument of a higher class. ISO
recommends to perform field calibration during several days; 2 to 3 days under cloudless
conditions, 10 days under cloudy conditions. In gener al this is not achievable. In order to
shorten the calibration process Hukseflux suggests to allow calibration at normal
incidence, using hourly totals near solar noon.

Hukseflux main recommendations for field intercomparisons are:

1) to take normal incidence as a reference and not the entire day.

2) to take a reference of the same brand and type as the field pyranometer or a
pyranometer of a higher class, and

3) to connect both to the same electronics, so that electronics errors (also offsets) are
eliminated.

4) to mount all instruments on the same platform, so that they have the same body
temperature.

5) assuming that the electronics are indepe ndent ly c alibrated, to analyse radiation values
at normal incidence radiation (possibly tilting the radiometers to approximately normal
incidence), if this is not possible to compare 1 hour totals around solar noon for
horizontally mounted in struments.

6) for second class radiometers, to correct deviations of more than ± 10 %. Lower
deviations should be interpreted as acceptable and should not lead to a revised
sensitivity.

7) for first class pyranometers, to correct deviations of more than ± 5 %. Lower
deviations should be interpreted as acceptable and should not lead to a revised
sensitivity.

8) for secondary standard instruments, to correct deviations of more than ± 3 %. Lower
deviations should be interpreted as acceptable and should not lead to a revised
sensitivity.

SR20 manual v1713 30/43

7.4 Data quality assurance

Quality assurance can be done by:

• analysing trends in solar irradiance signal

• plotting the measured irradiance against mathematically generated expected values

• comparing irradiance measurements between sites

• analysis of night time signals

The main idea is that one should look out for any unrealistic values. There are programs
on the market that can semi-automatically perform data screening. See for more
information on such a program http://www.dqms.com.

SR20 manual v1713 31/43

SR20 manual v1713 32/43

8 Appendices

8.1 Appendix on cable extension / replacement

The sensor cable of SR20 is equipped with a M16 straight connector. In case of cable
replacement, it is recommended to purchase a new cable with connector at Hukseflux.
An alternative is to choose for a Do-it-yourself (DIY) approach; please ask for the DIY
connector assembly guide. In case of cable extension, the user may choose purchasing a
new cable with connector at Hukseflux or extending the existing cable himself. Please
note that Hukseflux does not provide support for DIY connector- and cable assembly.
SR20 is equipped with one cable. Keep the distance between d a t a logger or amplifier and
sensor as short as possible. Cables act as a source of distortion by picking up capacitive
noise. In an electrically “quiet” environment the SR20 cable can be extended without
problem to 100 metres. If done properly, the sensor signal, although small, will not
significantly degrade because the sensor resistance is very low (so good immunity to
external sourc e s) and because there is no current flowing (so no resistive losses).
Connector, cable and cable connection specifications are summarised below.

Table 8.1.1 Preferred specifications for SR20 cable replacement and extension

General replacement

please order a new cable with connector at Hukseflux or choose for a DIY
approach. In case of DIY replacement by the user see connector
specifications below and ask for the DIY connec tor a ssembly guide

General cable extension please order a new cable with c onnector at Hukseflux or solder the new

cable conductors and shield to the original se nsor cable and make a
connection, using adhesive-lined heat shrink tubing, with specification s for
outdoor use. Alw a ys connect shield

Connectors used

chassis: M16 panel connector, male thread, 10-pole, HUMMEL AG

7.840.200.000 panel connector, front mounting, sh ort version.
cable: M16 straight connector, female thread, 10-pole. HUMMEL AG

7.810.300.00M straight connector, female thread, for ca ble 3 to 6 x 10

-3

m, special version

Cable

8-wire, shielded, with copper conductors (at Hukseflux 8-wire shielded
cable is used, of which 2 wires are used for signal transmiss ion, 2 for
heating and 2 to 4 for the temperatu r e s ensor)

Conductor resistance

< 0.1 Ω/m

Length

cables should be kept as short as possible, in any case the total cable
length should be less than 100 m

Outer sheath

with specificati ons for outdoor use
(for good stability in outdoor applicati ons)

SR20 manual v1713 33/43

8.2 Appendix on tools for SR20

Table 8.2.1 Specifications of tools for SR20

tooling required for sun screen fixa tion and removal

by hand

tooling required for bottom plate fixation and removal

hex key 2.5 mm

tooling required for desiccant holder fixation and
removal

spanner size 20 mm

tooling required for wire fixation an d r e m oval (internal
wiring inside SR20 body)

screwdriver blade width 2 mm

8.3 Appendix on spare parts for SR20

• Desiccant holder (with glass w indow a nd rubber ring)

• Desiccant (set of 5 ba gs in air tight bag)

• Humidity ind ic a t o r

• Levelling feet (set of 2)

• Static foot

• Sun screen with metal ring and thumb screw

• SR20 cable with connector (specify length in multiples of 5 m)

• O-ring SR20

NOTE: Outer dome, level and sensor of SR20 cannot be supplied as spare parts. In
case of possible damage to the SR20, after repair the instrument must be tested to
verify performance within specification limits. This is required by ISO 9060. Testing
involves verification of the directional response after dome, thermal sensor and level
replacement and verification of the temperature response after thermal sensor
replacement.

SR20 manual v1713 34/43

8.4 Appendix on standards for classification and calibration

Both ISO and ASTM have standards on instrument classification and methods of
calibration. The World Meteorological Organisation (WMO) has largely adopted the ISO
classification system.

Table 8.4.1 Pyranometer standardisation in ISO and ASTM.

STANDARDS ON INSTRUMENT CLASSIFICATI O N AND CALIBRATION

ISO STANDARD

EQUIVALENT ASTM STANDARD

ISO 9060:1990 Solar energy -- Specification
and classification of instruments f or measuring
hemispherical so lar and direct solar radiation

not available
Comment: work is in progress on a new ASTM
equivalent standard

Comment: a standard “Solar energy --Methods
for testing pyranometer an d pyrheliometer
characteristics” has been announced in ISO
9060 but is not yet implemented.

not available

ISO 9846:1993 Solar energy -- Calibration of
a pyranometer using a pyrheliometer

ASTM G167 - 05 Standa r d Test Method for
Calibration of a Pyranometer Using a
Pyrheliometer

ISO 9847:1992 Solar energy -- Calibration of
field pyranometer s by comparison to a
reference pyranometer

ASTM E 824 -10 Standard Test Method for
Transfer of Calibration from Refere nce to Field
Radiometers

ASTM G207 - 11 Standa r d T es t M ethod for
Indoor Transfer of Calibration from Re ference to
Field Pyranometers

ISO 9059:1990 Solar energy -- Calibration of
field pyrheliome te r s b y comparison to a
reference pyrheliometer

ASTM E 816 Standard Test Method for
Calibration of Pyrheliometers by Comparison to
Reference Pyrheliometers

SR20 manual v1713 35/43

8.5 Appendix on calibration hierarchy

The World Radiometric Reference (WRR) is the measurement standard representing the
SI unit of irradiance. It was introduced in order to ensure world-wide homogeneity of
solar radiation measurements and is in use since 1980. The WRR was determined from
the weighted mean of the measurements of a group of 15 absolute cavity radiometers
which were fully characterised . It has an estimated accuracy of 0.3 %. The WMO
introduced its mandatory use in its status in 1979.
The world-wide homogeneity of the meteorological radiation measurements is
guaranteed by the World Radiation Center in Davos Switzerland, by ma intaining the
World Standard Group (WSG) which materialises the World Radiometric Reference.

See http://www.pmodwrc.ch
The Hukseflux standard is traceable to an outdoor WRR calibration. Some small

corrections are made to transfer this calibration to the Hukseflux standard conditions:
sun at zenith and 1000 W/m

2

irradiance level. During the outdoor calibrat io n t he sun is

typically at 20 to 40° zenith angle, and the total irradiance at a 700 W/m

2

level.

Table 8.5.1 Calibration hierarchy for pyranometers

WORKING STANDARD CALIBRATION AT PMOD / WRC DAVOS

Calibration of working standard pyranometers:
Method: ISO 9846, type 1 outdo or. This working standard has an uncertainty “un c e r ta inty of
standard”. The working standard has been calibrated under certain “test conditions of the
standard”. The working standard has traceability to WRR world r a d iom etric reference.

CORRECTION OF (WORKING) STANDARD CALIBRATION TO STANDARDISED

REFERENCE CONDITIONS

Correction fr om “test conditions of th e s tandard” to “reference conditions” i. e . to normal
incidence and 20 °C:
Using known (working) standard pyranom ete r properties: direc tional, non linearity, offsets,
temperature dependence) . This correction has an uncertainty; “ uncertainty of correc tion”.
At Hukseflux we a ls o call the working stan d a r d pyranometer “standar d”.

INDOOR PRODUCT CALIBRATION

Calibration of products, i.e. pyranometers:
Method: according to ISO 9847, Type IIc, which is a n indoor calibration.
This calibration has an uncertainty associated with the m ethod.
(In some cases like the BSRN network the product calibration is with a diff er e nt method; for
example again type 1 ou tdoor)

CALIBRATION UNCERTAINTY CALCULAT ION

ISO 98-3 Guide to the Expression of Uncertainty in Measurem e nt, GUM Determina tion of
combined expanded uncert a in ty of calibration of the product, including uncerta inty of the
working standard, uncertainty of corre c tion, uncertainty of the method (transfer error). The
coverage factor must be determined; at Hukseflux we work with a coverage factor k = 2.

SR20 manual v1713 36/43

8.6 Appendix on meteorological radiation quantities

A pyranometer measures irradiance. The time integrated total is called radiant exposure.
In solar energy radiant exposure is often given in W∙h/m

2

.

Table 8.6.1 Meteorological radiation quantities as recommended by WMO (additional
symbols by Hukseflux Thermal Sensor). POA stands for Plane of Array irradiance. The
term originates from ASTM and IEC standards.

SYMBOL DESCRIPTION CALCULATION UNITS ALTERNATIVE

EXPRESSION

E↓

downward irradiance

E↓ = Eg ↓ + El↓

W/m2

H↓

downward radiant exposur e
for a specified time interval

H↓ = H

g

↓ + Hl ↓

J/m

2

E↑

upward irradiance

E↑ = E

g

↑

+ E

l

↑

W/m2

H↑

upward radiant exposure
for a specified time interval

H↑ = H

g

↑ + Hl ↑

J/m

2

W∙h/m2 Change of

units

E direct solar irradiance

normal to the apparen t

solar zenith angle

W/m2 DNI Direct

Normal

Irradiance

E0 solar constant W/m2

E

g

↓

h

global irradiance;

hemispherical irradiance on
a specified, in this case

horizontal surface.*

E

g

↓

= E cos θh + E

d

↓

W/m2

GHI

Global

Horizontal
Irradiance

Eg ↓ t

global irradiance;
hemispherical irradiance on
a specified, in this case

tilted surface.*

Eg ↓ = E∙cos θt +
E

d

↓ t + Er↑ t ***

W/m2 POA Plane of

Array

Ed ↓

downward diffuse solar
radiation

W/m

2

DHI Diffuse

Horizontal

Irradiance

E

l

↑

, E

l

↓

upward / downward long-

wave irradiance

W/m2
E

r

↑

reflected solar irradiance

W/m2

E* net irradiance

E* = E↓ – E↑

W/m2

T↓

apparent surface
temperature**

ºC or K

T↑

apparent sky

temperature**

ºC or K

SD

sunshine duration

h

θ is the apparent solar zenith angle θ

h

relative to horizontal, θt relative to a tilted surface
g = global, l = long wave, t = tilted *, h = horizontal*
* distinction horizontal and tilted f rom Hukseflux,
** T symbols introduced by Hukseflux,
*** contributions of E

d

↓ t and Er↑ t are Ed ↓ and E

r↑

both corrected for the tilt angle of the

surface

SR20 manual v1713 37/43

8.7 Appendix on ISO and WMO classification tables

Table 8.7.1 Classification table for pyranometers per ISO 9060 and WMO.

NOTE: WMO specification of spectral selectivity is different from that of ISO. Hukseflux
conforms to the ISO limits. WMO also specifies expected accuracies. ISO finds this not to
be a part of the classification system because it also involves calibration. Please note that
WMO achievable accuracies are for clear days at mid latitudes and that the uncertainty
estimate does not include uncertainty due to calibration*.

ISO CLASSIFICATION** T ABLE

ISO CLASS

SECONDARY
STANDARD

FIRST CLASS

SECOND
CLASS

Specification limit
Response time (95 %) 15 s 30 s 60 s
Zero offset a (respons e to 200 W/m2 net

thermal radiation)

+ 7 W/m2 + 15 W/m2 + 30 W/m2

Zero offset b (response to 5 K/h in a m b ient

temperature)

± 2 W/m2 ± 4 W/m2 ± 8 W/m2

Non-stability (change per year) ± 0.8 % ± 1.5 % ± 3 %

Non-linearity (100 to 1000 W/m2)

± 0.5 %

± 1 %

± 3 %

Directional response

± 10 W/m2

± 20 W/m2

± 30 W/m2

Spectral selectivity (350 to 1500 x 10-9 m) ± 3 % ± 5 % ± 10 %
Temperature response (interval of 50 K)* 2 % 4 % 8 %
Tilt response

(0 to 90 ° at 1000 W/m

2

)

± 0.5 % ± 2 % ± 5 %

ADDITIONAL WMO SPECIFICATIONS

WMO CLASS HIGH QUALITY GOOD QUALITY MODERATE

QUALITY

WMO: achievable accuracy for daily sums* 2 % 5 % 10 %

WMO: achievable accuracy for hourly sums*

3 %

8 %

20 %

WMO: achievable accuracy for minute sums* not specified not specified not specified
WMO: resolution

(smallest detectable change)

1 W/m2 5 W/m2 10 W/m2

WMO: spectral selectivity
(300 to 3000 x 10

-9

m)

± 2 % ± 5 % ± 10 %

CONFORMITY TESTING***

ISO 9060

individual

instrument only :
all specs must

comply

group

compliance

group

compliance

* WMO 7.2.1: The estimated uncertainties are based on the following assumptions: (a)
instruments are well-maintained, correctly aligned and clean; (b) 1 min and 1 h figures
are for clear-sky irradiances at solar noon; (c) daily exposure values are for clear days at
mid-latitudes. WMO 7.3.2.5: Table 7.5 lists the expected maximum deviation from the
true value, excluding calibration errors.
** At Hukseflux the expression ± 1 % is used instead of a range of 2 %.
*** an instrument is subject to conformity testing of its specifications. Depending on the
classification, conformity compliance can be proven either by group- or individual
compliance. A specification is fulfilled if the mean value of the respective test result does
not exceed the corresponding limiting value of the specification for the specific category
of instrument.

SR20 manual v1713 38/43

8.8 Appendix on definition of pyranometer specifications

Table 8.8.1 Definition of pyranometer specifications

SPECIFICATION DEFINITION SOURCE

Response time
(95 %)

time for 95 % response. The time in terval between the instant
when a stimulus is subjected to a spec ified abrupt change and the
instant when the response rea c hes and remains within specified
limits around its f inal steady value. The response time is a measure
of the thermal inertia inherent in th e s tabilization period for a final
reading.

ISO
9060­1990
WMO

1.6.3

Zero offset a:
(200 W/m

2

net
thermal
radiation )

response to 200 W/m

2

net thermal r a diation (ventilated) .
Hukseflux assumes that unventilated instr uments have to specify
the zero-offset in unventilated – worst case – conditions.
Zero offsets are a m easure of the stability of the zero-point.
Zero offset a is vis ib le a t night as a negative of fset, the instrument
dome irradiates in the far inf r a red to the relatively cold sky. This
causes the dome to cool down. The pyra nometer sensor irradiates
to the relatively cool dome, causing a negative offset. Z ero offset

a is also assumed to be present during day time.

ISO
9060­1990

Zero offset b:
(5 K/h in ambient

temperature)

response to 5 K/h change in a m bien t temper ature.
Zero offsets are a m easure of the stability of the zero-point.

ISO
9060-

1990

Non-stability
(change per

year)

percentage change in sensitivi ty per year. The dependence of
sensitivity resulting from ageing effects which is a measure of the

long-term stability .

ISO
9060-

1990

Non-linearity

(100 to 1000
W/m

2

)

percentage deviation from the sensitivity at 500 W/m

2

due to the

change in irradia nce within the range of 100 W/m

2

to 1000 W/m2.

Non-linearity has an overlap with direction a l r e s p onse, and

therefore should be handled with car e in uncertainty evalua tion.

ISO

9060­1990

Directional

response

the range of errors caused by a s su m in g that the normal incidence

sensitivity is valid for all directions wh e n m easuring from any
direction a beam ra dia tion whose normal incidence irradiance is
1000 W/m2 . Directional r esponse is a measu r e of the deviations

from the ideal “cosine behaviour” a nd its azimuthal variati on.

ISO

9060­1990

Spectral
selectivity (350
to 1500 x 10

-9

m)

(WMO 300 to

3000 x 10-9 m)

percentage deviation of th e pr odu c t of s pec tr a l absorptance and
spectral transmittance fr om the corresponding mean within 350 x
10

-9

m to 1500 x 10-9 m. Spectral selec tivity is a measure of the

spectral selectivity of the sensitivity.

ISO
9060­1990

Temperature
response

(interval of 50 K)

percentage deviation of the sensitivity due to change in am bie nt
temperature with in an interval of 50 K the temperature of th e

pyranometer body.

ISO
9060-

1990

Tilt response

(0° to 90° at
1000 W/m

2

)

percentage deviation from the sensitivity at 0° tilt (horiz ontal) due

to change in tilt from 0 ° to 90° at 1000 W/m

2

irradiance. Tilt

response describes changes of the sensitivity due to changes of

the tilt angle of the receiving s urface.

ISO

9060­1990

Sensitivity

the change in the response of a mea s uring instrument divided by
the corresponding change in the stimulus.

WMO

1.6.3

Spectral range the spectral range of radiation to which the instrument is

sensitive. For a normal pyranometer th is s hould be in the 0.3 to 3
x 10

-6

m range. Some pyranometers w ith c oloured glass domes

have a limited spectral range.

Hukseflux

SR20 manual v1713 39/43

8.9 Appendix on terminology / glossary

Table 8.9.1 Definitions and references of used terms

TERM DEFINITION (REFERENCE)

Solar energy
or solar
radiation

solar energy is the electroma gnetic energy emitted by the sun. Solar energy is
also called solar radiation and shortwave radiation. The solar radiation incident

on the top of the terrestrial atmosphere is called extra-terrestrial solar radiation;
97 % of which is confined to the spectra l ra nge of 290 to 3 000 x 10

-9

m. Part of
the extra-terrestrial solar r a diation penetrates the atmosph ere and directly
reaches the earth’s surf a ce, while part of it is scattered and / or absorbed by the
gas molecules, aerosol particles, c loud droplets and cloud c r ystals in the
atmosphere. The former is the direct component, the latter is th e diffuse

component of the solar radiation. (ref: WMO, Hukseflux)

Hemispherical
solar radiation

solar radiation received by a plane surface from a 180° field of view a ngle (solid
angle of 2 π sr).(ref: ISO 9060)

Global solar
radiation

the solar radiation received fr om a 180° field of view angle on a horizontal
surface is refe r r e d to a s global radiation. Also called GHI. This includes radiation
received directly from the s olid angle of the sun’s disc, as well as dif fuse sky
radiation that has been scattered in traversing the atmosphere. (ref: WMO)
Hemispherical solar radiation received by a horizontal plane surface.

(ref: ISO 9060)

Plane-of-array
irradiance

also POA: hemispherical solar irradiance in the plane of a PV array.
(ref: ASTM E2848-11 / IEC 61724)

Direct solar
radiation

radiation received from a small solid angle centred on the sun’s disc, on a given
plane. (ref: ISO 9060)

Terrestrial or
Longwave
radiation

radiation not of solar origin but of ter r e s tr ial and atmospheric origin and having

longer wavelengths (3 000 to 100 00 0 x 10

-9

m). In case of down welling El ↓ also
the background radiation from the universe is involved, passin g through the
”atmospheric window”. In case of upwelling E

l

↑, composed of long -wave
electromagnetic energy em itted by the earth’s surface an d by the gases, aerosols
and clouds of the a tm osphere; it is also partly absorbed within the atmosphere.
For a temperature of 300 K, 99.99 % of the power of the terrestrial radiation has
a wavelength longer than 3 000 x 10

-9

m and about 99 per cent longer than

5 000 x 10

-9

m. For lower temperatures, th e s pec tr um shifts to longer

wavelengths. (ref: WMO)

World
Radiometric
Reference
(WRR)

measurement standa r d r ep r esenting the Sl unit of ir r adiance with an uncertainty
of less than ± 0.3 % (see the WMO Guide to Meteorological Ins tr uments and
Methods of Observation, 1983, subclause 9.1.3). Th e reference was adopted by
the World Meteorological Organiza tion (WMO) and has been in effect since 1 July

1980. (ref: ISO 9060)

Albedo ratio of reflected and incoming solar rad ia tion. Dimensionle s s number that varies

between 0 and 1. Typical albedo values are: < 0.1 for water, from 0.1 for wet

soils to 0.5 for dry sand, from 0.1 to 0.4 f or vegetation, up to 0.9 for fresh s now.

Angle of
incidence

angle of radiation r elative to the sensor measured from normal incidence (varies
from 0° to 90°).

Zenith angle angle of incidence of radiation, rela tive to zenith. Equals angle of incidence f or

horizontally mounted instruments

Azimuth angle

angle of incidence of radiation, projected in the plane of the sens or s urface.

Varies from 0° to 360°. 0 is by definition the cable exit direction, also called
north, east is + 90°. (ASTM G113-09)

Sunshine
duration

sunshine duration during a given period is defined as the sum of tha t su b-period
for which the direct solar irradia nce exceeds 120 W/m

2

. (ref: WMO)

SR20 manual v1713 40/43

8.10 Appendix on converting resistance to temperature

SR20 is equipped with an internal temperature sensor. This can be either a Pt100 (SR20­T1 version) or a 10 kΩ thermistor (SR20-T2 version), as ordered.

Both versions require the user to measure the resistance of the temperature sensor and
convert this value to temperature. Many dataloggers have built-in functions to perfor m
such a conversion. In case the user wishes to calculate temperature (in degrees Celsius)
from resistance (in Ohms) himself, there are two distinct procedures:

SR20-T1

T1 versions are equipped with a Pt100 platinum resistance thermometer. It is classified
as class A according to DIN EN 60751. It has a resistance of 100 Ω at a temperature of 0
°C.

To convert resistance in Ω to temperature in °C, one can use the following equation:

 =

+



4󰇡1 

R



100

󰇢

2

with R

Pt100

the resistance in Ω, T the temperature in °C, A and B the Pt100 coefficients

A =

3.908 x 10

-3

B = -5.775 x 10

-7

SR20-T2

T2 versions are equipped with a 10 kΩ thermistor of type 44031RC. It has a resistance of
10000 Ω at a temperature of 25 °C.

To convert resistance in Ω to temperature in °C, one can use the Steinhart-Hart
equation. Measure the resistance of the thermistor and then calculate the temperature
from the resistance.

 =

1

+ ln

(

R



)

+ ln

(

R



)



273.15

with R

thermistor

the thermistor resistance in Ω, T the temperature in °C, α, β and γ the

Steinhart-Hart coefficients

α = 1.0295 x 10

-3

β = 2.391 x 10-4
γ = 1.568 x 10

-7

(Formula 8.10.1)

(Formula 8.10.2)

SR20 manual v1713 41/43

8.11 EU declaration of conformity

We, Hukseflux Thermal Sensors B.V.
Delftechpark 31
2628 XJ Delft
The Netherlands

in accordance with the requirements of the following directive:

2014/30/EU The Electromagnetic Compatibility Directive

hereby declare under our sole responsibility that:

Product model: SR20
Product type: Pyranometer

has been designed to comply and is in conformity with the relevant sections and
applicable requirements of the following standards:

Emission:

IEC/EN 61000-6-1, Class B, RF emission requirements, IEC CISPR11

and EN 55011 Class B requirements
Immunity: IEC/EN 61000-6-2 and IEC 61326 requirements
Report: SR20, 04 January 2014

Eric HOEKSEMA
Director
Delft
20 April, 2016

© 2017, Hukseflux Ther mal Senso rs B.V.

www.hukseflux.com

Hukseflux Thermal Sensors B.V. reserves the rig ht to change specifications without notice.