Hukseflux LP02 User Manual

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USER MANUAL LP02

Second class pyranometer

Hukseflux

Thermal Sensors

LP02 manual v1606 2/47

Warning statements

Putting more than 12 Volt across the se nsor wiring
can lead to permanent damage to t h e 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 LP02 7

1.2 Included items 7

1.3 Quick instrument che ck 8

2 Instrument principle and theory 9
3 Specifications of LP02 12

3.1 Specifications of LP02 12

3.2 Dimensions of LP02 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 in meteorology and climatology 17

5 Installation of LP02 18

5.1 Site selection and installation 18

5.2 Installation of the optional sun screen 19

5.3 Electrical connection 19

5.4 Requirements for data acquisition / amplification 20

6 Making a dependable measureme nt 21

6.1 The concept of dependability 21

6.2 Reliability of the measurement 22

6.3 Speed of repair and maintenance 23

6.4 Uncertainty evaluation 24

7 Maintenance and trouble shooting 26

7.1 Recommended maintenance and quality assurance 26

7.2 Trouble shooting 27

7.3 Calibration and checks in the field 28

7.4 Data quality assurance 29

8 LP02-TR 30

8.1 Introduction LP02-TR 30

8.2 Dimensions of LP02-TR 32

8.3 Electrical connection of LP02-TR 34

9 Appendices 37

9.1 Appendix on cable extension / replacement 37

9.2 Appendix on tools for LP02 38

9.3 Appendix on spare parts for LP02 38

9.4 Appendix on standards for classification and calibration 39

9.5 Appendix on calibration hierarchy 40

9.6 Appendix on meteorological radiation quantities 41

9.7 Appendix on ISO and WMO classification tables 42

9.8 Appendix on definition of pyranometer specifications 43

9.9 Appendix on terminology / glossary 44

9.10 EU declaration of conformity 45

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List of sy m bols

Quantities Symbol Unit
Voltage output U V

Sensitivity S V/(W/m

2

)
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
(see also appendix 9.6 on meteorological quantities)

Subscripts

Not applicable

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Introduction

LP02 is a solar ra diation se nsor that is applied in most common s olar radiation
observations. It complies with the seco nd class specifications of the ISO 9060 standard
and the WMO Guide. LP02 pyranometer is widely used in (agro-)meteorological
applications and for PV system performance monitoring.

LP02 measures the solar radiation received by a plane surface from a 180

o

field of view

angle. This quantity, expressed in W/m

2

, is called “hemispherical” solar radia tion. LP02
pyranometer can be employed outdoors under the sun, as well as indoors with lamp­based solar simulators. Its orientation depends on the application and may be horizontal,
tilted (for plane of array radiation) or inverted (for reflected radiation). LP02
pyranometer is a very good alternative to silicon cell (photodiode-based) pyranometers,
which do not comply to the ISO 9060 standard. In combination with the right software,
also sunshine duration may be measured.

Using LP02 is easy. The pyranometer can be connected directly to commonly used data
logging systems. The irradiance in W/m

2

is calculated by dividing the LP02 output, a
small voltage, by the sensitivity. This sensitivity is provided with LP02 on its calibration
certificate.

The central equation governing LP02 is: E = U/S (Formula 0.1)

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

Suggested use for LP02:

• general meteorlogical observations

• agricultural networks

• PV system performance monitoring

Figure 0.1 LP02 second class pyranometer

LP02 manual v1606 6/47

Figure 0.2 LP02 second class pyranometer with LI19 read-out unit / datalogger

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 LP02 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.

Model LP02-TR houses a 4-20 mA transmitter for easy read-out by dataloggers
commonly used in the industry. For more information see the chapter on LP02-TR.

Model LP02-LI19 offers LP02 with handheld read-out unit / datalogger LI19 in a practical
transport case. The LP02-LI19 combination is well suited for mobile measurements and
short term datalogging. For more information see the LP02-LI19 brochure and ask for the
LI19 manual.

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1 Ordering and checking at delivery

1.1 Ordering LP02

The standard configuration of LP02 is with 5 metres cable.

Common options are:

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

• LP02-TR second class pyranometer with 4-20 mA transmitter. Standard setting is 4

mA at 0 W/m

2

and 20 mA at 1600 W/m2. Specify setting and total cable length.

• Adap ted sensitivity range. Specify the lower en higher end of the allowed range in x

10

-6

V/(W/m2).

• Sun screen. Specify order number SCR01.

• For a fast response second class pyranometer see model SR03.

1.2 Included items

Arriving at the customer, the delivery should include:

• pyranometer LP02

• cable of the length as ordered

• product certificate matching the instrument serial number

• any other options as ordered

Please store the certificate 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 200 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 40 to 60

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. If applicable, remove the optional sun screen, using the hex key (see chapter on
installation of the s un screen). Inspect the bubble level.

4. Inspect the instrument for any damage.

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2 Instrument principle and theory

Figure 2.1 Overview of LP02:
(1) cable, (standard length 5 metres, optional longer cable)

(2) cable gland
(3) thermal sensor with black coating
(4) glass dome
(5) sensor body
(6) levelling feet
(7) mounting hole
(8) bubble level

1

2

3

4

5

6

7

8

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LP02’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 “hemisph erical” 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, 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 (conve ction, rain).

• a second (inner) glass dome: For a first class pyranometer, two domes are used, and
not one single dome. This construction pro vides an additional “radiation shield”,
result in g 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 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.

LP02 manual v1606 11/47

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.

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

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3 Specifications of LP02

3.1 Specifications of LP02

LP02 measures the solar radiation received by a plane surface from a 180o field of view
angle. This quantity, expressed in W/m

2

, is called “hemisph erical” solar radiation.
Working completely passive, using a thermopile sensor, LP02 generates a small output
voltage proportional to this flux. It can only be used in combination with a suitable
measurement system. The instrument is classified according to ISO 9060 and should be
used in accordance with the recommended practices of ISO, IEC, WMO and ASTM.

Table 3.1.1 Specifi ca t i on s of LP02 (continued on next pages)

LP02 MEASUREMENT SPECIFICATIO NS:
LIST OF CLASSIFICATION CRITERIA OF ISO 9060*

ISO classification (ISO 9060: 1990)

second class pyranometer

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

moderate quality pyranometer
Response time (95 %)

18 s

Zero offset a (response to 200 W/m2

net thermal radiat ion)

< ± 15 W/m

2

unventilated

Zero offset b (response to 5 K/h

change in ambient temperature)

< ± 4 W/m2
Non-stability

< ± 1 % change per year

Non-linearity

< ± 1 % (100 to 1000 W/m2)

Directional resp onse

< ± 25 W/m2

Spectral selectivity

< ± 5 % (0.35 to 1.5 x 10

-6

m)

Temperature response

< ± 3 % (-10 to +40 °C)

Tilt response

< ± 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 LP02 (continued)

LP02 ADDITIONAL SPECIFICATION S

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 2000 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 radiation: -0.1 to + 50

x 10-3 V

Measurement function / required
programming

E = U/S

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 ended

voltage channel, input resistance > 106 Ω

Optional readout

1 temperature channel in cas e opti onal temperature
sensor is ordered

Rated operating temperatu r e r a nge

-40 to +80 °C

Sensor resistance range

40 to 60 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 and UV Radiom e ter s

Standard cable length (see options)

5 m

Cable diameter

4 x 10-3 m

Cable gland: cable diameter ra nge

accepts cable diameters from 4 to 6 x 10-3 m

Cable replacement cable can be removed and installed by the user

provided that the cable is sealed a t th e sensor side
against humidity ingress. Consult Hukseflux for

instructions or u s e Hukseflux-supplied parts

Mounting 2 x M5 bolt at 65 mm centre-to-centre distance on

north-south ax is

Levelling

bubble level and adjustable lev elling feet are included

Levelling accuracy

< 0.4 ° bubble entirely in ring

IP protection cla s s

IP67

Gross weight including 5 m cable

0.5 kg

Net weight including 5 m cable

0.3 kg

Packaging

box of 170 x 90 x 230 x mm

LP02 manual v1606 14/47

Table 3.1.1 Specifications of LP02 (started on previous pages)

CALIBRATION

Calibration trace a b ility

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, Type IIc

Calibration un certainty

< 1.8 % (k = 2)

Recommended recalibrati on interval

2 years

Reference conditions 20 °C, normal incidence solar radiation, horizontal

mounting, irradiance level 1000 W/m2

Validity of calibra tion

based on experience the instrument sensitivity 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 measurement

uncertainty can only be made on an individual basis.

See the chapter on uncertainty evaluation

VERSIONS / OPTIONS

Adapted sensitivity range

the rated sensitivity range c a n be adapted to

customer requirements (to a lower range only)
option code = lower end - higher end of the allowed

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

4-20 mA transmitter creating a 4-20 mA output signal,

option code = TR,
with adapted housing
standard setting is 4 x 10

-3

A at 0 W/m2 and

20 x 10

-3

A at 1600 W/m

2

for specifications see the chapter on LP02-TR

Longer cable, in multiples of 5 m

option code = total cable length

ACCESSORIES

Sun screen for use on LP02

SCR01

Separate amplifiers

AC100 and AC420

Handheld read-out unit LI19

LP02-LI19 consists of one LP02 pyranometer, one
programmed LI19 handheld read-out unit /
datalogger, two spare batter ies, one USB cable,

software and a transport case

LP02 manual v1606 15/47

3.2 Dimensions of LP02

Figure 3.2.1 Dimensions of LP02 in 10

-3

m.

59

Ø 5

22

65

Ø 78

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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 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

.

LP02 manual v1606 17/47

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 Instruments and Method s of Observation, chapter 8,
measurement of s unshine duration, 8.2.2 Pyranometric Method

4.4 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 includ ed on “level of
performance” of pyranometers. Nowadays WMO conforms itself to the ISO classification
system.

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5 Installation of LP02

5.1 Site selection and installation

Table 5.1.1 Recommendations for installation of pyranometers

Location

The situation tha t s hadows are cast on the

instruments is u sually not desirable. The horizon
should be as free from obstacles as possible. Ideally
there should be no objects between th e c ourse 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 sen s itive 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 mountin g
is not horizontal), do not mount the instrumen t on
objects that become very hot (black coated 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 axis, connection through the
pyranometer flange.

Instrument moun ting with one bolt not applicable

Performing a representativ e
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 su rface
(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 th e loca tion becomes an
important element of the meas urement. See the
chapter on uncertainty evaluation.

Levelling in case of horizontal m ounting only use the bubble

level and levelling feet. The optional sun screen must
be removed for inspection of the bubble level.

Instrument orientation

by convention w ith 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 inverte d installation, WMO recommend s 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).

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5.2 Installation of the optional sun screen

The optional SCR01 sun screen can be installed and removed using a hex key (size 2 x
10

-6

m). See the drawing below.

Figure 5.2.1 Installation and removal of sun screen: Turn the set screw using the hex
key and lift of the sun screen. (1) hex key, (2) sun screen, (3) set scre w

5.3 Electrical connection

In order to operate, a pyranometer should be connected to a measurement system,
typically a so-called datalogger. LP02 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 sensor as
short as possible. For cable extension, see the appendix on this subject.

Table 5.3.1 The electrical connection of LP02

WIRE

COLOUR

MEASUREMENT SYSTEM

Sensor output + White

Voltage input +

Sensor output - Green

Voltage input - or ground

Shield

Black

Analogue ground

Figure 5.3.1 Electrical diagram of LP02. The shield is connected to the sensor body.

1 2 3

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Table 5.3.2 Standard internal connection of LP02 at the internal printed circuit board

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 a mp lification equipment to see if
directions for use with the LP02 are available.
In case programming for similar instruments is available, this can typically also be used.
LP02 can usually be treated in the same way as other thermopile pyranometers.
Pyranometers usually have the same programming as heat flux sensors.

In case of the LP02-TR version, the output is 4 to 20 x 10

-3

A. See the chapter on the
LP02-TR. When using LP02 combined with read-out unit / datalogger LI19, please consult
the LI19 manual as well.

Table 5.4.1 Requirements for data acquisition and amplification equipment for LP02 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 sensitiv ity to
calculate the solar irradiance.
E = U/S (Formula 0.1)

Data acquisition input resistance

> 1 x 106 Ω

Open circuit detec tion
(WARNING)

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

SENSOR

PRINTED CIRCUIT COLOUR CODE WIRE

Plus (+)

+

White

Minus (-) - Green

Shield

SH

Bare metal

TR Not connected unless the sensor

has a trimmed sens itivity or
limited sensitivity range

LP02 manual v1606 21/47

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 employment under natural sunlight (involving the

instrument stability specification)

• the measurement conditions (such as tilting, ventilation, shading, instrument
temperature)

• maintenance (mainly 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.

LP02 manual v1606 22/47

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).

• re lated to the reliability of the measurement uncertainty (measurement reliability),
which involves hardware reliability as well as co ndition 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 select 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
limiting conditions are:

• the measurement conditions, for instance whe n 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 the 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 maintenance support. Important aspects
are:

• dom e fouling by deposition of dust, 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 cleaning.

• 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 sensitiv ity gradually will increase. This is solved by 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 Hukseflux second cl ass pyranometers (for exampl e model
LP02), this may slowly develop into a def ect. For first class and secondary standard
models (for instance model SR11 first class pyranometer) extra desiccant (in a set of
5 bags in an air-tight bag) is available.

LP02 manual v1606 23/47

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

• The use of redundant instruments allow s 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 a nd repair. The main
maintenance actions are:

• replacement of desiccant (not applicable for LP02)

• replacement of cabling

For optimisation of dependability a user should:

• 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.

LP02 manual v1606 24/47

6.4 Uncertainty evaluation

The uncertainty of a mea surement under outdoor or indoor co nditions 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 around 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).

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 the 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 the 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, direc t ional 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 typical observat ion may be sufficient.

7) In cas e of specia l 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.

LP02 manual v1606 25/47

10) Among the various sources of uncertainty, some are “not included i n 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.

Table 6.4.1.1 Preliminary estimates of achievable uncertainties of measurements with
Hukseflux pyranometers. The estimates are based on typical pyranometer properties 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

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 %

(LP02)

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

From 2011 to 2012, calibration of LP02 has been improved. New procedures were
developed in close cooperation with PMOD World Radiation Center in Davos, Switzerland.
Our latest calibration method results in an uncertainty of the sensitivity of less than

1.8 %, compared to typical uncertainties of higher than 3.5 % for this pyranometer

class. See the appendix for detailed information on calibration hierarchy.

LP02 manual v1606 26/47

7 Maintenance and trouble shooting

7.1 Recommended maintenance and quality assurance

LP02 can measure reliably at a low level of maint enance 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 LP02. If possible the data analysis and
cleaning (1 and 2) should be done on a daily basis.

MINIMUM RECOMMENDED PYRANOMETER MAI NTENANCE

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 (down to -5 W/m

2

on
clear windless nights), due to zero offset a. In case of use with
PV systems, compare daytime measurements to PV system
output. Look for a ny patterns and events that dev ia te 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 soapy water or a lcohol

3 6 months inspection inspect cable quality, inspect cable glands, ins pec t mounting

position, inspect cable, clean instrument, clean cable, inspect
levelling, change instrument tilt in case this is ou t of
specification, inspect mounting connection, inspect interior of

dome for condensation

4 desiccant

replacement

desiccant replacement (not applicable for LP02)

5 2 years recalibration recalibration by side-by-side comparison to a higher s ta ndard

instrument in th e field according to ISO 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, c a ble gland, 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 board

9 recalibration recalibration by side-by-side comparison to a higher s ta ndard

instrument indoors according to ISO 9847 or outdo ors

according to ISO9846

LP02 manual v1606 27/47

7.2 Trouble shooting

Table 7.2.1 Trouble shooting for LP02

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 200 Ω range. Measure the sensor
resistance first with one polarity, than rev er se 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 40 to 60 Ω plus 1.5 Ω for the total resistance of
two wires (back and forth) of each 5 m. Infinite resistance indic a tes a broken
circuit; zero or a low r e s istance indicates a short 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 instance a 1 00 W light bulb at 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 instrument voltage out put
should go down an d within one minute approach 0 V.
Check the data a c q uisition by applying a 1 x 10

-6

V source to it in the

1 x 10

-6

V range.

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; are there any obstructions that could
explain the measurement result.
Check the orientation / levelling of the pyranometer.
Check if the right c alibration factor is entered in to the algorithm. Please note that
each sensor has its own individual calibration factor, as documented in its
calibration cert ificate.
Check if the voltage r e a d ing is divided by the calibrati on factor in review of the
algorithm.
Check the condition of the wiring at the logger.
Check the cable condition looking for cable breaks.
Check the range of the data logger; signal can be negative (this could be out of
range) or the amplitude could be ou t of ra nge.
Check the data acquisition by a pplying 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 e xpected.
Check the data a c q uisition by short circuiting the da ta acquisition 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 sources of electromagnetic radiation (radar, radio
etc.)
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

The dome
shows internal
condensation

Arrange to send the sensor back to Hukseflux for diagnosis.

LP02 manual v1606 28/47

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 cal 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 has 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 un d er cloudy conditions. In general th is 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 ndently calibrated, 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 co rr ect 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.

LP02 manual v1606 29/47

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 automatically perform data screening. See for more information
on such a program http://www.dqms.com.

LP02 manual v1606 30/47

8 LP02-TR

8.1 Introduction LP02-TR

As a special version of LP02, Hukseflux offers model LP02-TR: a second class
pyranometer with 4-20 mA transmitter.

LP02-TR is a solar radiation sensor that is applied in most common solar radiation
observations. It complies with the second class sp ecifications of the ISO 9060 standard
and the WMO Guide. LP02-TR pyranometer is widely used in (agro-)meteorological
applications and for PV system performance monitoring. LP02-TR houses a 4-20 mA
transmitt er for ea s y rea d-out by dataloggers commonly used in the industry.

Using LP02-TR is easy. The pyranometer can be connected directly to commonly used
data logging systems. The irradiance in W/m

2

is calculated by using the transmitter’s
output. In LP02-TR’s standard configuration, the 4 to 20 mA output corresponds to a
transmitted range of 0 to 1600 W/m

2

. This range can be adjusted at the factory upon

request.

Figure 8.1.1 LP02-TR second class pyranometer with 4-20 mA transmitter

LP02 manual v1606 31/47

Table 8.1.1 Specifications of LP02-TR

LP02-TR SPECIFICATIONS

Description

second class pyranometer with 4-20 m A transmitter

Transmitted range

0 to 1600 W/m2

Output signal

4 to 20 x 10-3 A

Principle

2-wire current loop

Supply voltage

7.2 to 35 VDC

Options adapted transmitted range

longer cable, in multiples of 5 m

For de finition of pyranometer ISO 9060 specifications see the appendix.

Table 8.1.2 Requirements for data acquisition and amplification equipment with the
LP02–TR configuration

Capability to

- measure 4-20 mA or

- measure currents or

- measure voltages

The LP02-TR has a 4-20 mA output. There are several
possibilities to h a ndle this signal. It is important to r ealise
that the signal wire s not only act to transmit the signal but
also act as power supply.
Some dataloggers have a 4-20 mA input. In that case the
connection can be directly made.
Some dataloggers have the capability to measure currents.
In some cases the datalogger accepts a voltage input.
Usually a 100 Ω precision resistor is used to convert the
current to a voltage ( this will then be in the 0.4 – to 2 VDC
range). This r esistor must be put in the + wire of the sensor.
In the two latter ca s e s the user must check that the low side
of the input channel is connected to ground, and the high
side to a positive voltage in th e r equired range.
See the chapter on LP02-TR’s electrical connections for
wiring diagrams and electrical connections to am - and
voltmeters.

LP02 manual v1606 32/47

8.2 Dimensions of LP02-TR

Figure 8.2.1 Overview of LP02-TR:

(1) cable, (standard length 5 metres, optional longer cable)
(2) cable gland
(3) thermal sensor with black coating
(4) glass dome
(5) sensor body
(6) transmitter hous i ng
(7) levelling feet
(8) bubble level

1

2

3

4

5

6

7

8

LP02 manual v1606 33/47

Figure 8.2.2 Dimensions of LP02-TR in 10

-3

m

106

Ø 78

65

M5

M6

LP02 manual v1606 34/47

8.3 Electrical connection of LP02-TR

In order to operate, a pyranometer should be connected to a measurement system,
typically a so-called datalogger. LP02-TR operates on a supply voltage of 7.2 to 35 VDC.

Table 8.3.1 The electrical connection of LP02-TR

WIRE

COLOUR MEASUREMENT SYSTEM

Sensor output + White

Voltage input +

Sensor output - Green

Voltage input - or ground

Shield

Black

Analogue ground

Figure 8.3.1 Electrical diagram of LP02-TR. The shield is connected to the sensor body.

Table 8.3.2 Standard internal connection of LP02 at the internal printed circuit board

SENSOR

PRINTED CIRCUIT COLOUR CODE WIRE

Plus (+) + White

Minus (-) - Green

Shield SH Bare metal

TR Not connected unless the sensor

has a trimmed sens itivity or
limited sensitivity range

TR

LP02 manual v1606 35/47

Figure 8.3.2 Electrical diagram of the connection of LP02-TR to a typical ammeter or
datalogger with capacity to measure current signals. LP02-TR operates on a supply
voltage of 7.2 to 35 VDC.

Figure 8.3.3 Electrical diagram of the connection of LP02-TR to a typical voltmeter or
datalogger with the capacity to measure voltage signals. Usually a 100 Ω shunt resistor
(R) is used to convert the current to a voltage. LP02-TR operates on a supply voltage of

7.2 to 35 VDC.

TR sensor

white [+]

green [–]

A

ground

7.2 - 35 VDC

ammeter
I = 4 to 20 mA

black

TR sensor

white [+]

green [–]

V

ground

7.2 - 35 VDC

I = 4 to 20 mA

voltmeter
I = U/R

R

black

LP02 manual v1606 36/47

LP02 manual v1606 37/47

9 Appendices

9.1 Appendix on cable extension / replacement

The sensor cable can be removed and installed by the user prov ided that the cable is
sealed at the sensor side against humidity ingress. Please consult Hukseflux for
instructions on cable preparation or use Hukseflux-supplied parts.

LP02 is equipped with one cable. Keep the distance betwee n d a ta 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 LP02 cable can however be extended
without problem to 100 meters. 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).
Cable and connection specifications are summarised below.

NOTE: the body of LP02 contains connector blocks that can be used for the internal
connection of a new cable. See the chapter on electrical connections. Usually it is e a s ie r
to connect a new extended cable inside the pyranometer body cable than to make a good
(weatherproof) connection to an existing cable.

Table 9.1.1 Preferred specifications for cabl e e xt en si on of LP02

General

please consult Hukseflux for instructions or use Hukseflux-supplied parts.

Cable

2-wire, shielded, with copper conductor (at Hukseflux 3-wire shielded
cable is used, of which only 2 wires are used)

Sealing

sealed at the sensor side against humidity ingr es s

Core resistance

< 0.1 Ω/m

Outer diameter

± 4 to 6 x 10

-3

m (to fit cable gland)

Length

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

Outer sheet

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

Connection

either solder the new cable core and shield to the original sensor cable,
and make a waterproof connection using cable s hrink, or use gold plated
waterproof connectors. Always connect sh ie ld.

LP02 manual v1606 38/47

9.2 Appendix on tools for LP02

Table 9.2.1 Specifications of tools for LP02

tooling required for optional sun screen f ixation and removal hex key 2 mm

tooling required for cable gland fixation and r e m oval

spanner size mm

tooling required for wire fixation and removal
(internal wiring inside LP02 body)

screwdriver blade width mm

9.3 Appendix on spare parts for LP02

• Levelling feet (set of 3)

• LP02 cable (specify length in multiples of 5 m), sealed at one end

• Cable gland LP02

• O-ring LP02

LP02 manual v1606 39/47

9.4 Appendix on standards for classification and calibration

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

Table 9.4.1 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 for measu r ing
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 Reference to Fie ld
Radiometers

ASTM G207 - 11 Standa r d T es t M ethod for
Indoor Transfer of Calibration from Referenc e 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 Comparis on to
Reference Pyrheliometers

LP02 manual v1606 40/47

9.5 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 calibration the sun is

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

2

level.

Table 9.5.1 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 “uncertainty of
standard”. The working standard has been calibrated under certain “test conditions of the
standard”. The working standard has traceability to WRR world radiometr ic reference.

CORRECTION OF (WORKING) STANDARD CALIBRATION TO STANDARDISED

REFERENCE CONDITIONS

Correction from “test condition s of the standard” to “referen c e c onditions” i.e. to normal
incidence and 20 °C:
Using known (working) standard pyranometer properties: directional, non linearity, offsets,
temperature dependence) . This correction has an uncertainty; “uncertainty of correction”.
At Hukseflux we a ls o call the working standard pyr anometer “standard”.

INDOOR PRODUCT CALIBRATION

Calibration of products, i.e. pyranometers:
Method: according to ISO 9847, Type IIc, which is an 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 different method; for
example again type 1 ou tdoor)

CALIBRATION UNCERTAINTY CALCULAT ION

ISO 98-3 Guide to the Expression of Uncertainty in Measurement, GU M Determination of
combined expanded uncert a in ty of calibration of the product, including uncertainty 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.

LP02 manual v1606 41/47

9.6 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 9.6.1 Meteorological radiation quantities as recommended by WMO (additional
symbols by Hukseflux Thermal Sensor). POA stands for Plane of Array irradi an ce. 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 from 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

LP02 manual v1606 42/47

9.7 Appendix on ISO and WMO classification tables

Table 9.7.1 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 1 500 x 10-9 m)
(WMO 300 to 3 000 x 10

-9

m)

± 3 % ± 5 % ± 10 %

Temperature response (interval of 50 K)** 2 % 4 % 8 %

Tilt response

(0 to 90 ° at 1000 W/m2)

± 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

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 we use the expression ± 1 % 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 specificatio n 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.

LP02 manual v1606 43/47

9.8 Appendix on definition of pyranometer specifications

Table 9.8.1 Definition of pyranometer specifications

SPECIFICATION DEFINITION SOURCE

Response time
(95 %)

time for 95 % response. T he time interval 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 tim e is a mea s ure
of the thermal iner tia inherent in the stabilizat ion 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 assu mes that unventilated instruments have to specify
the zero-offset in unventilated – worst case – conditions.
Zero offsets are a m easure of the stability of the z e r o-point.
Zero offset a is vis ib le a t night as a negative offset, the instrument
dome irradiates in the far infra red to the relatively cold sky. This
causes the dome to cool down. The pyranometer sensor irradiates
to the relatively cool dome, causing a negative offset. Zero 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 z e r o-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 irradiance with in th e r a nge of 100 W/m

2

to 1000 W/m2.

Non-linearity has an overlap with directional respons e, and

therefore should be handled with care in uncer ta inty evaluation.

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 di rections when measuring from any
direction a beam ra dia tion whose normal incidence ir r adiance is
1000 W/m2 . Directional r esponse is a measure of the devia tions

from the ideal “cosine behaviour” and its az im uthal variation.

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 and the spectr al distribution of irradianc e.

Spectral selectivi ty is a measure of the spectral se lectivity of the

sensitivity.

ISO
9060­1990

Temperature
response

(interval of 50 K)

percentage deviation of the sensitivity due to ch ange in ambient
temperature within an in ter v a l of 50 K the temperature of the

pyranometer body.

ISO
9060-

1990

Tilt response

(0° to 90° at
1000 W/m

2

)

percentage deviation from the sensitivity at 0° t ilt (horizontal) 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 this should 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

LP02 manual v1606 44/47

9.9 Appendix on terminology / glossary

Table 9.9.1 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 inciden t
on the top of the terrestrial atmosphere is called extra-terrestrial so lar 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 atmosphere and directly
reaches the earth’s surf a ce, while part of it is scattered and / or absorbed by the
gas molecules, aerosol particles, cloud dropl ets and cloud crystals in the
atmosphere. The former is the direct c omponent, the latter is the diff use

component of the sola r r adiation. (ref: WMO, Hukseflux)

Hemispherical
solar radiation

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

Global solar
radiation

the solar radiation received from a 180 ° field of view a ngle 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 diffuse 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 centre d on the sun’s disc, on a given
plane. (ref: ISO 9060)

Terrestrial or
Longwave
radiation

radiation not of solar origin but of terrestrial 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, passing through the
”atmospheric window”. In case of upwelling E

l

↑, composed of long -wave
electromagnetic energy emitted by the earth’s surface and 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 radia tion 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 irradian c e with an uncertain ty
of less than ± 0.3 % (see the WMO Guide to Meteor ological Instruments and
Methods of Observation, 1983, subclause 9.1.3). The reference wa s a dopted by
the World Meteorological Organization (WMO ) and has been in effect since 1 July

1980. (ref: ISO 9060)

Albedo ratio of reflected and incoming solar radiation. Dimensionless nu m be r that varies

between 0 and 1. Typical albedo va lues 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 snow.

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, relative to ze nith. Equals angle of incidenc e for

horizontally mounted instruments

Azimuth angle

angle of incidence of radiation, projected in the plane of the sensor surface.

Varies from 0° to 360°. 0 is by definition the cable exit direction, also called
north, east is + 90°. (ASTM G13-09)

Sunshine
duration

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

2

. (ref: WMO)

LP02 manual v1606 45/47

9.10 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: LP02
Product type: Pyranometer

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

Emission: EN 61326-1 (2006)
Immunity: EN 61326-1 (2006)
Emission: EN 61000-3-2 (2006)
Emission: EN 61000-3-3 (1995) + A1 (2001) + A2 (2005).
Report: 08C01340RPT01, 06 January 2009

Kees VAN DEN BOS
Director
Delft
September 09, 2015

© 2016, Hukseflux Thermal Sensors B.V.

www.hukseflux.com

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