Hukseflux SR20 User Manual

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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.

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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

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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

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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.

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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.

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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.

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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).

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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

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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