Hukseflux IR20 User Manual

Copyright by Hukseflux | manual v1604 | www.huksefluxusa.com | [e-mail]

USER MANUAL IR20

Research grade pyrgeometer

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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 statements 2
Contents 3
List of symbols 4
Introduction 5
1 Ordering and checking at delivery 8

1.1 Ordering IR20 8

1.2 Included items 8

1.3 Quick instrument check 9

2 Instrument principle and theory 10

2.1 Pyrgeometer functionality 10

2.2 Solar and longwave radiation 10

2.3 IR20 pyrgeometer design 12

2.4 Typical measurement results 14

2.5 Optional heating 14

2.6 Optional shading 14

2.7 Use as a net radiation sensor 14

3 Specifications of IR20 and IR20WS 15

3.1 Specifications of IR20 and IR20WS 15

3.2 Dimensions of IR20 18

4 Standards and recommended practices for use 19

4.1 Site selection and installation 20

4.2 Installation of the sun screen 21

4.3 Electrical connection 22

4.4 Requirements for data acquisition / amplification 23

5 Making a dependable measurement 24

5.1 The concept of dependability 24

5.2 Reliability of the measurement 25

5.3 Speed of repair and maintenance 26

5.4 Uncertainty evaluation 26

6 Maintenance and trouble shooting 28

6.1 Recommended maintenance and quality assurance 28

6.2 Trouble shooting 29

6.3 Calibration and checks in the field 30

6.4 Data quality assurance 30

7 Appendices 31

7.1 Appendix on cable extension / replacement 31

7.2 Appendix on tools for IR20 32

7.3 Appendix on spare parts for IR20 32

7.4 Appendix on standards for classification and calibration 32

7.5 Appendix on calibration hierarchy 33

7.6 Appendix on meteorological radiation quantities 35

7.7 Appendix on terminology / glossary 36

7.8 EU declaration of conformity 38

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List of symbols

Quantities Symbol Unit

Voltage output U V
Sensitivity S V/(W/m2)
Sensitivity at reference conditions S0 V/(W/m2)
Temperature T °C
Equivalent blackbody radiative temperature T °C
Electrical resistance Re 
Longwave irradiance E W/m2
Stefan–Boltzmann constant (5.67 x 10-8) σ W/(m2∙K4)

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

(see also appendix 7.6 on meteorological quantities)

Subscripts

sky relating to the atmosphere
surface relating to the ground surface
ambient relating to ambient air
body relating to the instrument body
sensor relating to the sensor

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Introduction

IR20 is a research grade pyrgeometer suitable for high-accuracy longwave irradiance
measurement in meteorological applications. IR20 is capable of measuring during both
day and night. In absence of solar radiation, model IR20WS offers even better accuracy
because of its wider spectral range.

IR20 measures the longwave or far-infra-red radiation received by a plane surface, in
W/m2, from a 180 ° field of view angle. In meteorological terms pyrgeometers are used
to measure “downward and upward longwave irradiance” (WMO definition). Longwave
radiation is the part of radiation that is not emitted by the sun. The spectral range of
longwave radiation is not standardised. A practical cut-on is in the range of 4 to 5 x 10-6 m.
IR20 has a dome with a solar blind filter with a cut-on at 4.5 x 10-6 m, making it suitable
for day- and night observations.

Model IR20WS has a wide spectral range with a cut-on at 1.0 x 10-6 m. It offers a
superior accuracy during night-time, when solar radiation is absent.

The main purpose of a pyrgeometer is to measure longwave radiation. As secondary
measurands, the sky temperature T

sky

, and the equivalent surface temperature T

surface

can be measured. Both are so-called equivalent blackbody temperatures, i.e.
temperatures calculated from pyrgeometer data, assuming the source behaves as a
blackbody with an emission coefficient of 1.

Using IR20 is easy. It can be connected directly to commonly used data logging systems.
The irradiance in W/m2 is calculated by dividing the IR20 output, a small voltage, by the

sensitivity and taking in account the irradiated heat by the sensor itself (Planck’s law).

The sensitivity is provided with IR20 on its calibration certificate. Please note that the
IR20 sensitivity is corrected for temperature dependence in the measurement equation
by using 3 additional constants. These coefficients are provided as well.

The central measurement equation governing IR20 is:

E = U/(S0·(a·T² + b·T + c)) + σ·(T + 273.15)4 (Formula 0.1)

S = S0·(a·T² + b·T + c) (Formula 0.2)

The instrument should be used in accordance with the recommended practices of WMO.

Suggested use for IR20 and IR20WS:

 climatological networks
 extreme climates (polar / tropical)
 moving platforms (aircraft, buoys)
 uncertainty assessment (IR20 + IR20WS)
 calibration reference (IR20WS)

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Distinguishing features and benefits of IR20 are:

 correction of temperature dependence by use of the measurement function. This is

far more accurate than temperature compensation in the instrument, especially at
very low and high temperatures. Every pyrgeometer is supplied with temperature
coefficients to enter into the equation.

 high sensitivity. With sufficient input signal a typical datalogger no longer significantly

contributes to the uncertainty of the measurement.

 low thermal-resistance of the sensor. Competing designs need a significant correction

for the difference in temperature between pyrgeometer body and sensor surface. For
IR20 this is not needed.

 fast response time (3 s). A low response time is a benefit for measurements on

moving platforms such as aircraft and buoys.

 on-board heater. Heating prevents condensation of water on the pyrgeometer dome

which, when occurring, leads to very large measurement errors.

 instrument cut-on wavelength (5 %) and the two 50 % transmission points are

displayed on the product certificate for individual sensors.

Figure 0.1 IR20 research grade pyrgeometer with its sun screen removed

More about the instrument principle, theory and specifications can be found in the
following chapters.

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Pyrgeometers are not subject to a classification standard.

Calibration of pyrgeometers is usually traceable to the World Infrared Standard Group
(WISG). This calibration takes into account the spectral properties of typical downward
longwave radiation. As an option, calibration can be made traceable to a blackbody and
the International Temperature Scale of 1990 (ITS-90). This alternative calibration is
appropriate for measurements of upward longwave radiation (IR20 facing down).

See the specific paragraph in this manual about calibration and uncertainty assessment
for more information.

This manual is intended for users of both IR20 and IR20WS. The specifications of
IR20WS are identical to IR20’s except for its spectral range.

Figure 0.1 IR20WS research grade pyrgeometer

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1 Ordering and checking at delivery

1.1 Ordering IR20

The standard configuration of IR20 is with 5 metres cable and a connector.

Common options are:

 Longer cable (in multiples of 5 m). Specify total cable length.
 Five silica gel bags in an air-thight bag for IR20 desiccant holder. Specify order

number DC01.

 Optional calibration to blackbody (ITS-90).
 IR20WS for the special wide spectrum model of IR20.

1.2 Included items

Arriving at the customer, the delivery should include:

 pyrgeometer IR20 or IR20WS
 cable of the length as ordered with connector
 sun screen
 product certificate matching the instrument serial number
 calibration certificate matching the instrument serial number
 temperature dependence report
 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
thermal source.

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 300 to
500 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 heat: put the multimeter at its most sensitive range of
DC voltage measurement, typically the 100 x 10-3 VDC range or lower. Make sure that
the sensor is at 25 °C or lower. Expose the sensor to a heat source at a short distance
from the window of more than 50 °C, for instance a heavy (> 5 kg) painted block of
metal, or a painted metal container holding hot water. Face the side of the container to
avoid condensation of water on the pyrgeometer window. Stir the water to attain
homogeneity. A painted surface will act as a blackbody in the far-infra-red (FIR),
irrespective of the visible colour. The signal should read positive and > 1 x 10-3 V now. In
case of using your hand as a heat source, the signal should be significantly lower.

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

4. Check the electrical resistance of the thermistor. This should be in the 104 Ω range.

5. Check the electrical resistance of the heater. This should be in the 100 Ω range.

6. Inspect the instrument for any damage.

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

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2 Instrument principle and theory

2.1 Pyrgeometer functionality

IR20’s scientific name is pyrgeometer. IR20 measures the longwave or far-infra-red (FIR)
radiation received by a plane surface, in W/m2, from a 180 ° field of view angle. In

meteorological terms pyrgeometers are used to measure “downward and upward
longwave irradiance” (WMO definition).

As secondary measurands, the sky temperature T

sky

, and the equivalent surface (ground)

temperature T

surface

can be measured. Both are so-called equivalent blackbody radiative
temperatures, i.e. temperatures calculated from the pyrgeometer measurement
assuming these are uniform-temperature blackbodies with an emission coefficient of 1.

2.2 Solar and longwave radiation

Longwave radiation is the part of the radiation budget that is not emitted by the sun. The
spectral range of the longwave radiation is not standardised. A practical cut-on is in the
range of 4 to 5 x 10-6 m (see figure 2.2.1). In meteorology, solar- and longwave
radiation are typically measured as separate parameters. The instrument to measure
solar radiation is called pyranometer.

Figure 2.2.1 Atmospheric radiation as a function of wavelength plotted along two
logarithmic axes to highlight the longwave radiation. Longwave radiation is mainly
present in the 4 to 50 x 10-6 m range, whereas solar radiation is mainly present in the

0.3 to 3 x 10-6 m range. In practice, the two are measured separately using
pyrgeometers and pyranometers

0.001

0.010

0.100

1.000

1 10 100

spectral irradiance x 10

-9

W/(m

2

/m)

wavelength x 10-6m

downwelling
longwave
solar

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In the longwave spectrum, the sky can be seen as a temperature source; colder than
ground level ambient air temperature, with its lowest temperatures at zenith, getting
warmer (closer to ambient air temperature) at the horizon. The uniformity of this
longwave source is much better than that in the range of the solar spectrum, where the
sun is a dominant contributor. The “equivalent blackbody” temperature, as a function of
zenith angle, roughly follows the same pattern independent of the exact sky condition
(cloudy or clear). This explains why for pyrgeometers the directional response is not very
critical.

The downwelling longwave radiation essentially consists of several components:

1. low temperature radiation from the universe, filtered by the atmosphere. The
atmosphere is transparent for this radiation in the so-called atmospheric window (roughly
the 10 to 15 x 10-6 m wavelength range).

2. higher temperature radiation emitted by atmospheric gasses and aerosols.

3. in presence of clouds or mist, the low temperature radiation from the universe is
almost completely blocked by the water droplets. The pyrgeometer then receives the
radiation emitted by the water droplets.

The spectral distribution of longwave irradiance varies significantly as a function of the
source composition. A pyrgeometer is relatively insensitive to these variations, but all the
same blackbody calibration tends to differ from WISG calibration by up to 5 %. In
addition there may be effects that are uncompensated for in the calibration (for instance
related to atmospheric water vapour content) in the 5 to 10 W/m2 range. These effects
are still under investigation by the international scientific community. Comparison
between IR20 and IR20WS may serve to investigate this effect.

Upwelling longwave irradiance is measured with downfacing instruments. These are
presumably looking directly at the surface (absorption and emission of the atmosphere is
low over a short distance of around 2 m), which behaves like a normal blackbody.
Hukseflux suggests calibrating downfacing instruments against a blackbody rather than
having WISG as a reference.

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2.3 IR20 pyrgeometer design

Figure 2.3.1 Overview of IR20 pyrgeometer:

(1) cable (standard length 5 metres, optional longer cable)
(2) fixation of sun screen
(3) dome with solar blind filter
(4) sun screen
(5) humidity indicator
(6) desiccant holder
(7) levelling feet
(8) bubble level
(9) connector

By definition a pyrgeometer should not measure solar radiation, and in the longwave

have 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 radiation
hits the sensor perpendicularly (normal to the surface, 0 ° angle of incidence), zero
response when the source is at the horizon (90 ° angle of incidence, 90 ° zenith angle),
and 50 % of full response at 60 ° angle of incidence.

A pyrgeometer should have a so-called “directional response” (older documents mention
“cosine response”) that is as close as possible to the ideal cosine characteristic.

1

2

3

4

5

6

7

8

9

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In order to attain the proper directional and spectral characteristics, a pyrgeometer’s
main components are:

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

0.3 to 50 x 10-6 m range, and has a near-perfect directional response. The coating
absorbs all longwave 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 irradiance exchange
between sensor and source. The sensor not only absorbs, but also irradiates heat as a
blackbody.

 a silicon dome. This dome limits the spectral range from 1.0 to 40 x 10

-6

m (cutting
off the part below 1.0 x 10-6 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 solar blind interference coating deposited on the dome (not for model IR20WS):

this coating limits the spectral range. It now becomes 4.5 to 40 x 10-6 m (cutting off
the part below 4.5 x 10-6 m).

Pyrgeometers can be manufactured to different specifications and with different levels of
verification and characterisation during production. Hukseflux also manufactures lower
accuracy pyrgeometers; see our pyrgeometer model IR02.

Model IR20 has a dome with a solar blind filter with a cut-on at 4.5 x 10-6 m, making it
suitable for day- and night observations.
Model IR20WS has a wide spectral range with a cut-on at 1.0 x 10-6 m. It offers a
superior accuracy under night-time conditions, when solar radiation is absent. See also
the appendix on uncertainty evaluation.

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2.4 Typical measurement results

Please note that the signal generated by an upfacing pyrgeometer usually has a negative
sign. The most important factors determining downward longwave irradiance are:

 ambient air temperature
 sky condition / cloud cover
 atmospheric moisture content

Table 2.4.1 Expected pyrgeometer output U/S at different ambient air temperatures,
T

ambient

, and at different cloud conditions. Under clear sky conditions the U/S is around

-100 W/m2 while under cloudy conditions it will be close to 0 W/m2. Also calculated: the
sky temperature, T

sky

, and the longwave downward irradiance E.

EXPECTED PYRGEOMETER OUTPUT CONDITIONS

T

ambient

Sky condition

U/(S0·(a·T² + b·T + c))

T

sky

E

[°C]

[cloudy], [clear]

[W/m2]

[°C]

[W/m2]

-20

cloudy

0

-20

232

-20

clear sky

-100

-53

132 0 cloudy

0

+0

314 0 clear sky

-100

-24

214

+30

cloudy

0

+30

477

+30

clear sky

-100

+12

377

2.5 Optional heating

A low-power heater is located in the body of the pyrgeometer. The heater is not
necessarily switched on; recommended operation is to activate the heater when there is
a risk of dew deposition.

2.6 Optional shading

One of the larger errors in the daytime measurement of downwelling longwave irradiance
is the offset caused by solar radiation; the “solar offset”. Errors due to solar offset, are of
the order of + 10 W/m2 at 1000 W/m2 global horizontal irradiance. For ultra high
accuracy measurements this offset can be reduced by around 60 % by “shading”, which
means preventing the direct radiation to reach the instrument. Shading is typically done
by using a shading disk on a solar tracker.

2.7 Use as a net radiation sensor

Two pyrgeometers mounted back to back may be used to measure net longwave
radiation. Net longwave radiation is defined as downwelling minus upwelling longwave
irradiance. In case the two instruments are thermally coupled, the body temperatures of
the instruments are identical. In that case the body temperature cancels from the
equation for the net radiation. However for calculation of sky temperature and surface
temperature the instrument temperature still needs to be measured.

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3 Specifications of IR20 and IR20WS

3.1 Specifications of IR20 and IR20WS

IR20 research grade pyrgeometer measures the longwave irradiance received by a plane
surface, in W/m2, from a 180 ° field of view angle. In meteorological terms IR20
measures downward and upward longwave irradiance. Working completely passive, using
a thermopile sensor, IR20 generates a small output voltage proportional to the radiation
balance between the instrument and the source it faces. It can only be used in
combination with a suitable measurement system. The instrument is not subject to
classification. It should be used in accordance with the recommended practices of WMO.
IR20 measures during both day and night. In the absence of solar radiation IR20WS
offers a higher accuracy because of its wider spectral range. For ultra-high accuracy
measurements the user should consider to use the incorporated heater and should
consider “shading” the instrument during daytime.

Table 3.1.1 Specifications of IR20 and IR20WS (continued on next pages)

IR20 & IR20WS SPECIFICATIONS

MEASURANDS

Measurand

longwave radiation

Measurand in SI radiometry units

longwave irradiance in W/m2

Optional measurand

sky temperature

Optional measurand

surface temperature

IR20 VERSUS IR20WS: SPECTRAL RANGE & USE

Spectral range IR20

4.5 to 40 x 10-6 m
(nominal, see product certificate for individual value)

Spectral range IR20WS

1.0 to 50 x 10-6 m
(based on typical material properties only)

IR20WS restrictions for use

only in the absence of solar radiation

Solar offset
(IR20 only, not specified for IR20WS)

< 10 W/m2
(at 1000 W/m2 global horizontal irradiance on the dome)

MAIN SPECIFICATIONS
Field of view angle

180 °

Response time (95 %)

3 s

Sensitivity (nominal)

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

Sensitivity range

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

Rated operating temperature range

-40 to +80 °C

Temperature dependence

< ± 0.4 % (-30 to +50 °C)
using the measurement function

Temperature sensor

10 kΩ thermistor

Required sensor power

zero (passive sensor)

Heater

12 VDC, 1.5 W
(see below for details)

Standard cable length

5 m

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Table 3.1.1 Specifications of IR20 and IR20WS (continued)

ADDITIONAL SPECIFICATIONS

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

< ± 2 W/m2
Non-stability

< ± 1 % change per year

Non-linearity

< ± 0.5 % (100 to 300 W/m2, relative to 200 W/m2
sensor to source exchange)

Measurement range

-1000 to +1000 W/m2
(sensor to source exchange: U/(S0·(a·T² + b·T + c)) )

Tilt dependence

< ± 0.5 % (0 to 90 ° at 300 W/m2)

Sensor resistance range

300 to 500 Ω

Expected voltage output

Application for outdoor measurement of downward
longwave irradiance: -7.5 to 7.5 x 10-3 V

Measurement function / required
programming

E = U/(S0·(a·T² + b·T + c)) + σ·(T + 273.15)4

Measurement function / optional
programming for sky temperature

T

sky

= (El↓/σ)

1/4

- 273.15

Measurement function / optional
programming for surface temperature

T

surface

= (El↑/σ)

1/4

- 273.15

Required readout

1 differential voltage channel or 1 single ended
voltage channel, input resistance > 106 Ω
1 temperature channel

STANDARDS

Standard governing use of the
instrument

WMO-No. 8, Guide to Meteorological Instruments and
Methods of Observation, seventh edition 2008,
paragraph 7.4 "measurement of total and long-wave
radiation"

MOUNTING, CABLING, TRANSPORT

Chassis connector

M16 panel connector, male thread, 10-pole

Chassis connector type

HUMMEL AG 7.840.200.000 panel connector, front
mounting, short version

Cable connector

M16 straight connector, female thread, 10-pole

Cable connector type

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

Connector protection class

IP67 / IP69 K per EN 60 529 (connected)

Cable diameter

5.3 x 10-3 m

Cable replacement

replacement cables with connector can be ordered
separately from Hukseflux

Instrument mounting

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

Levelling

bubble level and adjustable levelling feet are included

Levelling accuracy

< 0.1 ° bubble entirely in ring

Desiccant

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

Humidity indicator

blue when dry, pink when humid

IP protection class

IP67

Gross weight including 5 m cable

1.2 kg

Net weight including 5 m cable

0.85 kg

Packaging

box of 200 x 135 x 225 mm

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Table 3.1.1 Specifications of IR20 and IR20WS (started on previous pages)

HEATING

Heater operation

the heater is not necessarily switched on;
recommended operation is to activate the heater
when there is a risk of dew deposition

Required heater power

1.5 W at 12 VDC

Heater resistance

95 Ω

Steady state zero offset caused by
heating

0 W/m2

CALIBRATION
Calibration traceability

to WISG

Optional traceability

to blackbody (ITS-90 )

Calibration hierarchy

from WISG through Hukseflux internal calibration
procedure involving outdoor comparision to a
reference pyrgeometer

Calibration method

outdoor comparison to a reference pyrgeometer

Calibration uncertainty

< 6 % (k = 2)

Recommended recalibration interval

2 years

Reference conditions

horizontal mounting, atmospheric longwave
irradiance, clear sky nights, 20 °C

Validity of calibration

based on experience the instrument sensitivity will not
change during storage. During use under exposure to
solar radiation the instrument “non-stability”
specification is applicable.
Hukseflux recommends ITS-90 traceable calibration
for upward longwave irradiance measurement.

Characterisation of the dependence of
sensitivity to temperature

temperature coefficients a, b and c of the
measurement equation are determined in an
independent experiment, and reported on the product
certificate

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

Achievable uncertainty (95 % confidence
level) daily totals

± 8 % (Hukseflux’ own estimate)

VERSIONS / OPTIONS

Longer cable, in multiples of 5 m

option code = total cable length

Calibration

optional to blackbody (ITS-90 )

ACCESSORIES

Bags of silica gel for desiccant

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

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3.2 Dimensions of IR20

Figure 3.2.1 Dimensions of IR20 and IR20WS in 10-3 m

68

Ø 150

M6M5 (2x)

65

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4 Standards and recommended practices

for use

Pyrgeometers are not subject to standardisation.

The World Meteorological Organization (WMO) is a specialised agency of the United
Nations. It is the UN system's authoritative voice on the state and behaviour of the
earth's atmosphere and climate. WMO publishes WMO-No. 8; Guide to Meteorological
Instruments and Methods of Observation, in which paragraph 7.4 covers "measurement
of total and long-wave radiation".

For ultra high accuracy measurements, the following manual may serve as a reference:
Baseline Surface Radiation Network (BSRN) Operations Manual, Version 2.1, L. J. B.
McArthur, April 2005, WCRP-121, WMO/TD-No. 1274.
This manual also includes chapters on installation and calibration.

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4.1 Site selection and installation

Table 4.1.1 Recommendations for installation of pyrgeometers

Location

the horizon should be as free from obstacles as
possible. Ideally there should be sources of longwave
irradiance between the course of the sun and the
instrument, only free sky.

Mechanical mounting / thermal insulation

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

Instrument mounting with 2 bolts

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

Instrument mounting with one bolt

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

Performing a representative
measurement

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

Levelling

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

Instrument orientation

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

Installation height

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

Optional shading

for ultra-high accuracy measurements the solar offset
can be reduced by around 60 % by “shading”, which
means preventing the direct radiation to reach the
instrument. Shading is typically done by using a
shading disk on a solar tracker.

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4.2 Installation of the sun screen

IR20’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 8 of the drawing, and remove the cable / connector,
item 9.

Figure 4.2.1 Installation and removal of IR20’s sun screen

1

2

3

5

6

7

8

9

4

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4.3 Electrical connection

In order to operate, a pyrgeometer should be connected to a measurement system,
typically a so-called datalogger. IR20 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 4.3.1 The electrical connection of IR20. The heater is not necessarily used. The
temperature sensor must be used.

PIN

WIRE

IR20

2

Red

10 kΩ thermistor [+]

3

Pink

10 kΩ thermistor [+]

6

Blue

10 kΩ thermistor [−]

8

Grey

10 kΩ thermistor [−]

1

Brown

heater

4

Yellow

heater

9

Black

ground

7

White

signal [+]

5

Green

signal [−]

Note 1: 10 kΩ thermistors are usually connected in a 2-wire configuration.

Note 2: the heater is not necessarily connected. In case it is connected, the polarity of
the connection is not important.
Note 3: signal wires are insulated from ground wire and from the sensor body. Insulation
resistance is tested during production and larger than 1 x 106 Ω.
Note 4: ground is connected to the connector, the sensor body and the shield of the wire.

Figure 4.3.1 Electrical diagram of the internal wiring of IR20. The shield is connected to
the sensor body.

IR2 0 manual v 1604 23/39

4.4 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 amplification equipment to see if
directions for use with the IR20 are available.
In case programming for similar instruments is available, this can typically also be used.
IR20 can usually be treated in the same way as other thermopile pyrgeometers.

Table 4.4.1 Requirements for data acquisition and amplification equipment for IR20 in
the standard configuration

Capability to measure small voltage
signals

preferably: better than 5 x 10-6 V uncertainty
Minimum requirement: 20 x 10-6 V uncertainty
(valid for the entire expected temperature range of the
acquisition / amplification equipment)

Capability for the data logger or the
software

to store data, and to perform division by the sensitivity to
calculate the longwave irradiance.
E = U/(S0·(a·T² + b·T + c)) + σ·(T + 273.15)4 (Formula 0.1)
(see also optional measurands)

Data acquisition input resistance

> 1 x 106 Ω

Open circuit detection
(WARNING)

open-circuit detection should not be used, unless this is done
separately from the normal measurement by more than 5
times the sensor response time and with a small current
only. Thermopile sensors are sensitive to the current that is
used during open circuit detection. The current will generate
heat, which is measured and will appear as an offset.

Capability to measure temperature

a 10 kΩ thermistor must be read-out. Required accuracy of
the readout is ± 0.2 °C, which results in around 1 W/m2
uncertainty of the irradiance measurement.

Capability to power the heater
(OPTIONAL)

IR20 has a 12 VDC, 1.5 W heater on board, which may
optionally be activated to keep the instrument above dew
point. Some users prefer to have the heater on full time,
others prefer to switch it on during nighttime only.

IR2 0 manual v 1604 24/39

5 Making a dependable measurement

5.1 The concept of dependability

A measurement with a pyrgeometer 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 requirements for a measurement with a pyrgeometer 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.

In case of pyrgeometers, the measurement uncertainty as obtained during outdoor
measurements is a function of:

• the instrument properties

• the calibration procedure / uncertainty

• the presence of natural sunlight (involving the instrument specification of solar offset)

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

temperature)

• maintenance (mainly fouling and deposition of water)

• the environmental conditions* (such as temperature, position of the sun, presence of

clouds, horizon, representativeness of the location)

Therefore 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
inverted). 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.

IR2 0 manual v 1604 25/39

5.2 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 pyrgeometer 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 select an instrument
of a certain class, and define maintenance support procedures.

In many situations there 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 when working at extreme temperatures

when the instrument temperature is at the extreme limits of the rated temperature
range.

 the environmental conditions, for instance when installed at a sub-optimal

measurement location with obstacles in the field of view.

 the environmental conditions, for instance when assessing net radiation, the

downfacing pyrgeometer measurement may not be representative of irradiance
received in that particular area.

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

 dome fouling by deposition of dust, dew, rain or snow. With pyrgeometers the most

important source of unreliability is deposition of water on the dome. Water completely
blocks the longwave radiation flux between sensor and sky. In particular at clear
nights this causes very large errors. Water deposition under clear-sky nighttime
conditions can largely be prevented by using the instrument heater. Fouling results in
undefined measurement uncertainty (sensitivity and directional error are no longer
defined). This should be solved 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 sensitivity gradually will increase. This is solved by regular
recalibration.

 moisture condensing under pyrgeometer 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 flat window pyrgeometers (for example model

IR2 0 manual v 1604 26/39

IR02), this may slowly develop into a defect. For model IR20 extra desiccant (in a set
of 5 bags in an air tight bag) is available.

 One of the larger errors in the daytime measurement of downwelling longwave

irradiance is the offset caused by solar radiation; the “solar offset”. Errors due to
solar offset, are of the order of + 10 W/m2 at 1000 W/m2 global horizontal irradiance.
For ultra-high accuracy measurements this offset can be reduced by around 60% by

“shading”, which means preventing the direct radiation to reach the instrument.

Shading is typically done by using a shading disk on a solar tracker.

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

 The use of redundant instruments allows remote checks of one instrument using the

other as a reference, which leads to a higher measurement reliability.

5.3 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 pyrgeometers 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.

5.4 Uncertainty evaluation

The uncertainty of a measurement under outdoor or indoor conditions depends on many
factors, see paragraph 1 of this chapter. It is not possible to give one figure for
pyrgeometer 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 “Guide to Expression of Uncertainty in
Measurement” or GUM).

IR2 0 manual v 1604 27/39

The main ingredients of the uncertainty evaluation for pyrgeometers are:

 Calibration uncertainty, which is in the order of ± 6 % (k = 2) for upfacing

instruments measuring downward longwave irradiance

 Calibration uncertainty, which is larger for other than upfacing instruments; for

downfacing instruments a blackbody calibration seems preferable. Blackbody
calibration will result in a lower sensitivity (S0) than WISG traceable calibraton.

 Errors due to water deposition at clear nights; these completely block the longwave

irradiance exchange between pyrgeometer and may cause the signal U/(S0·(a·T² +
b·T + c)) to change from a large negative value (-100 W/m2) to around 0 W/m2 .
Water deposition at clear nights may largely be avoided by using the on-board heater
of IR20.

 Errors due to solar offset, which is of the order of + 10 W/m

2

at 1000 W/m2 global
horizontal irradiance. This offset can be reduced by around 60% by shading of the
instrument, typically by using a shading disk on a solar tracker. This error is partially
caused by heating of the dome, partially by transmission of solar radiation by the
dome / filter combination. This uncertainty is not taken into account in the WISG
calibration of the reference instrument. With IR20WS, only measuring at night, this
uncertainty does not play a significant role.

 Errors due to the choice of the cut-on wavelength of the pyrgeometer. Depending on

the atmospheric water content, the pyrgeometer will block a variable percentage of
the downward longwave irradiance. This causes an uncertainty of the sensitivity S0.
With IR20, this uncertainty is already taken into account in the WISG calibration of
the reference instrument. With IR20WS this uncertainty does not play a significant
role.

 Errors due to instrument non-stability. This is now estimated at < ± 1% change per

year. The main factor in instrument non-stability is the aging of the pyrgeometer
solar blind filter. For IR20WS this filter is not present. For that reason we expect
IR20WS to have a better non-stability.

 Errors due to the temperature measurement T. For this a 10 kΩ thermistor must be

read-out. Required accuracy of the readout is ± 0.2 °C, which results in around 1
W/m2 uncertainty of the irradiance measurement. To this the uncertainty of the
thermistor itself should be added. In measurement of net radiation, in case the
upfacing and downfacing instruments are thermally coupled, the temperature
measurement (and also its uncertainty) cancel from the equation.

IR2 0 manual v 1604 28/39

6 Maintenance and trouble shooting

6.1 Recommended maintenance and quality assurance

IR20 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 6.1.1 Recommended maintenance of IR20. If possible the data analysis and
cleaning (1 and 2) should be done on a daily basis.

MINIMUM RECOMMENDED PYRGEOMETER MAINTENANCE

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. Look for any patterns
and events that deviate from what is normal or expected

2

2 weeks

cleaning

use a soft cloth to clean the dome of the instrument,
persistent stains can be treated with soapy water or alcohol

3

6 months

inspection

inspect cable quality, inspect cable glands, inspect mounting
position, inspect cable, clean instrument, clean cable, inspect
levelling, change instrument tilt in case this is out of
specification, inspect mounting connection

4 desiccant
replacement

desiccant replacement (if applicable). Change in case the
humidity indicator shows more than 50 %, then replace
desiccant. Coat the rubber of the cartridge with silicone grease
or vaseline. Desiccant regeneration: heating in an oven at 70
°C for 1 to 2 hours. Humidity indicator regeneration: heating
until blue at 70 °C

5

2 years

recalibration

recalibration by side-by-side comparison to a higher standard
instrument in the field

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, cable gland, desiccant holder,
sun screen. NOTE: use Hukseflux approved parts only

8 internal
inspection

if applicable: open instrument and inspect / replace O-rings;
dry internal cavity around the circuit board

9 recalibration

recalibration by side-by-side comparison to a higher standard
instrument at the manufacturer or a reference institute.
Also recalibrate the temperature sensor

IR2 0 manual v 1604 29/39

6.2 Trouble shooting

Table 6.2.1 Trouble shooting for IR20

The sensor does
not give any signal

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.
Check if the sensor reacts to heat: put the multimeter at its most sensitive
range of DC voltage measurement, typically the 100 x 10-3 VDC range or
lower. Make sure that the sensor is at 25 °C or lower. Expose the sensor to a
heat source at a short distance from the window of more than 50 °C, for
instance a heavy (> 5 kg) painted block of metal, or a painted metal
container holding hot water. Face the side of the container to avoid
condensation of water on the pyrgeometer window. Stir the water to attain
homogeneity. A painted surface will act as a blackbody in the far-infra-red
(FIR), irrespective of the visible colour. The signal should read positive and >
1 x 10-3 V now. In case of using your hand as a heat source, the signal should
be significantly lower.Check the data acquisition 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.

Check if the measurement function, including the constant a, b and c has
been implemented properly. Please note that each sensor has its own
individual calibration factor and constants, as documented in its production
certificate.
Check if the pyrgeometer has a clean dome.
Check the location of the pyrgeometer; are there any obstructions / sources
that could explain the measurement result.
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 is usually negative (this could be
out of range) or the amplitude could be out of range.
Check the data acquisition 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 acquisition by short circuiting the data acquisition input with a
100 to 1000 Ω resistor. Look at the output. Check if it 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 during the measurement

The instrument
shows internal
condensation.

Replace the desiccant and wait a few days to see if the situation improves. In
case of condensation of droplets: disassemble the instrument and dry out the
parts.

The instrument
shows persistent
internal
condensation

Arrange to send the sensor back to Hukseflux for diagnosis.

IR2 0 manual v 1604 30/39

6.3 Calibration and checks in the field

Recalibration of field pyrgeometers is typically done by comparison in the field to a
reference pyrgeometer. There is no standard for this procedure.

Hukseflux recommendation for re-calibration: if possible, perform calibration outdoor by
comparison to an identical or a higher class reference instrument, under nighttime as
well as daytime conditions. Use nighttime data only to determine S0. Do not change the
constants a, b and c.

Hukseflux main recommendations for field intercomparisons are:

1) perform field calibration during several days; 2 to 3 days and if possible under
cloudless conditions.

2) to take a reference of the same brand and type as the field pyrgeometer or a
pyrgeometer 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) to analyse downward irradiance values at nighttime only to determine S0.

6) to analyse the daytime data, independently, and look at the residuals between the
calibration reference and calibrated instrument as a function of solar irradiance. The solar
offset can serve as a quality indicator of the pyrgeometer filter condition.

6.4 Data quality assurance

Quality assurance can be done by:

 analysing trends in longwave irradiance signal
 plotting the measured irradiance against mathematically generated expected values
 comparing irradiance measurements between sites
 analysis of daytime signals against solar irradiance

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

IR2 0 manual v 1604 31/39

7 Appendices

7.1 Appendix on cable extension / replacement

The sensor cable of IR20 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.
IR20 is equipped with one cable. Keep the distance between data 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 IR20 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 sources) and because there is no current flowing (so no resistive losses).
Connector, cable and cable connection specifications are summarised below.

Table 7.1.1 Preferred specifications for IR20 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 connector assembly guide

General cable extension

please order a new cable with connector at Hukseflux or solder the new
cable conductors and shield to the original sensor cable and make a
connection, using adhesive-lined heat shrink tubing, with specifications
for outdoor use. Always connect shield

Connectors used

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

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

7.810.300.00M straight connector, female thread, for cable 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 transmission, 2 for
heating and 2 to 4 for the temperature sensor)

Conductor resistance

< 0.1 /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 specifications for outdoor use
(for good stability in outdoor applications)

IR2 0 manual v 1604 32/39

7.2 Appendix on tools for IR20

Table 7.2.1 Specifications of tools for IR20

tooling required for sun screen fixation and removal

by hand

tooling required for bottom plate fixation and removal

hex key 2.5 x 10-3 m

tooling required for desiccant holder fixation and
removal

spanner size 20 x 10-3 m

tooling required for wire fixation and removal (internal
wiring inside IR20 body)

screwdriver blade width 2 x 10-3 m

7.3 Appendix on spare parts for IR20

 Desiccant holder (with glass window and rubber ring)
 Desiccant (set of 5 bags in an air tight bag)
 Humidity indicator
 Levelling feet (set of 2)
 Static foot
 Sun screen with metal ring and thumb screw
 IR20 cable with connector (specify length in multiples of 5 m)
 O-ring IR20

NOTE: The dome of IR20 and IR20WS cannot be supplied as spare part.

7.4 Appendix on standards for classification and calibration

Unlike pyranometers, pyrgeometers are not subject to a system of classification.
At Hukseflux we distinguish between normal pyrgeometers, like our model IR02, and

“research grade” pyrgeometers, like IR20 and IR20WS. The term “research grade” is

used to indicate that this instrument has the highest attainable specifications.

IR2 0 manual v 1604 33/39

7.5 Appendix on calibration hierarchy

Hukseflux pyrgeometers are traceable to the World Infrared Standard Group (WISG).
WISG is composed of a group of pyrgeometers. The calibration hierarchy of Hukseflux
IR20 is from WISG through Hukseflux internal calibration procedure involving outdoor
comparison to a reference pyrgeometer.

The WISG group of instruments is maintained by World Radiation Center (WRC), in
Davos Switzerland. An absolute sky-scanning radiometer provides the absolute longwave
irradiance reference. Comparisons between the reference and the WISG are performed
on a regular basis to maintain the WISG and supervise its long-term stability. It is
essential that these intercomparisons take place under various sky conditions, but the
predominant condition is a clear sky, which means that the validity of WISG calibration is
a clear-sky spectrum. Typical exchange between pyrgeometer and sky is in the -70 to

-120 W/m2.

At Hukseflux in an independent lab experiment, the detector properties are determined
as a function of temperature, resulting in temperature coefficients a, b, and c.

WRC works with a measurement equation involving additional constants k1, which
corrects for thermal resistance of the thermopile sensor, k2 , which corrects for emissivity
that does not equal 1.

E = (U / S) (1+ k

1 ∙

σ·(T + 273.15)

3

) + k2 σ·(T + 273.15)4 (Formula 7.5.1)

Pyrgeometers are calibrated by WRC in two ways:

- by nighttime comparison to WISG, to determine S. A typical uncertainty of S is 4.2%.

- by calibration under a blackbody to determine k1 and k2.

There also is a third constant, k3 , correcting for the difference between dome
temperature and thermopile temperature.

The Hukseflux measurement equation for IR20 is:

E = U/(S0·(a·T² + b·T + c)) + σ·(T + 273.15)4 (Formula 0.1)

In other words, Hukseflux ignores k1 and assumes that k2 = 1. It is Hukseflux’ opinion
that by the design of the instrument, the influence of these parameters is negligible.

IR2 0 manual v 1604 34/39

Table 7.5.1 Calibration hierarchy for pyrgeometers

WORKING STANDARD CALIBRATION AT PMOD / WRC DAVOS

Calibration of working standard pyrgeometers traceable to WISG.
A typical uncertainty of S0 is 4.2% (k = 2).

CORRECTION OF (WORKING) STANDARD CALIBRATION TO STANDARDISED
REFERENCE CONDITIONS

Correction from “test conditions of the standard” to “reference conditions” :
No corrections are applied.
Reference conditions are: horizontal mounting, atmospheric longwave irradiance, clear sky
nights, 20 °C.

OUTDOOR PRODUCT CALIBRATION AT HUKSEFLUX

Calibration of products, i.e. pyrgeometers:
Outdoor side by side comparison to a reference pyrgeometer.

CALIBRATION UNCERTAINTY CALCULATION

ISO 98-3 Guide to the Expression of Uncertainty in Measurement, GUM Determination of
combined expanded uncertainty of calibration of the product, including uncertainty of the
working standard, uncertainty of correction, uncertainty of the method (transfer error). The
coverage factor must be determined; at Hukseflux we work with a coverage factor k = 2.
Hukseflux specifies a calibration uncertainty of < 6% (k = 2).

IR2 0 manual v 1604 35/39

7.6 Appendix on meteorological radiation quantities

A pyrgeometer measures longwave irradiance. The time integrated total is called radiant
exposure.

Table 7.6.1 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↓ = E

g

↓ + E

l

↓

W/m2

H↓

downward radiant exposure
for a specified time interval

H↓ = Hg ↓ + H

l

↓

J/m2
E↑

upward irradiance

E↑ = E

g

↑ + El ↑

W/m2

H↑

upward radiant exposure
for a specified time interval

H↑ = H

g

↑ + Hl ↑

J/m2

W∙h/m2

Change of
units

E

direct solar irradiance
normal to the apparent
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.*

Eg↓ = E cos θh + Ed↓

W/m2

GHI

Global
Horizontal
Irradiance

E

g

↓

t

global irradiance;
hemispherical irradiance on
a specified, in this case
tilted surface.*

E

g

↓ = E∙cos θ

t

+

E

d

↓ t + E

r ↑ t

***

W/m2

POA

Plane of
Array

E

d

↓

downward diffuse solar
radiation

W/m2

DHI

Diffuse
Horizontal
Irradiance

E

l

↑, El ↓

upward / downward
longwave irradiance

W/m2
Er ↑

reflected solar irradiance

W/m2

E*

net irradiance

E* = E↓ – E↑

W/m2

T

surface

equivalent blackbody
radiative temperature of
the surface**

ºC

T

sky

equivalent blackbody
radiative temperature of
the sky**

ºC

SD

sunshine duration

H

θ is the apparent solar zenith angle θh relative to horizontal, θt relative to a tilted surface

g = global, l = longwave, t = tilted *, h = horizontal*
* distinction horizontal and tilted from Hukseflux,
** T symbols introduced by Hukseflux,
*** contributions of E

d

↓ t and E

r↑ t

are E

d

↓ and E

r↑

both corrected for the tilt angle of the

surface

IR2 0 manual v 1604 36/39

7.7 Appendix on terminology / glossary

Table 7.7.1 Definitions and references of used terms

TERM

DEFINITION (REFERENCE)

Solar energy
or solar
radiation

solar energy is the electromagnetic 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 spectral range of 290 to 3 000 x 10-9 m. Part of
the extra-terrestrial solar radiation penetrates the atmosphere and directly
reaches the earth’s surface, while part of it is scattered and / or absorbed by the
gas molecules, aerosol particles, cloud droplets and cloud crystals in the
atmosphere. The former is the direct component, the latter is the 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 angle (solid
angle of 2 π sr).(ref: ISO 9060)

Global solar
radiation

the solar radiation received from a 180 ° field of view angle on a horizontal
surface is referred to as global radiation. Also called GHI. This includes radiation
received directly from the solid 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 centred 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 100 000 x 10-9 m). In case of downwelling E

l

↓ also

the background radiation from the universe is involved, passing through the

”atmospheric window”. In case of upwelling E

l

↑, composed of longwave

electromagnetic energy emitted by the earth’s surface and by the gases, aerosols

and clouds of the atmosphere; 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, the spectrum shifts to longer
wavelengths. (ref: WMO)

World
Radiometric
Reference
(WRR)

measurement standard representing the Sl unit of irradiance with an uncertainty
of less than ± 0.3 % (see the WMO Guide to Meteorological Instruments and
Methods of Observation, 1983, subclause 9.1.3). The reference was adopted 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 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 for vegetation, up to 0.9 for fresh snow.

Angle of
incidence

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

Zenith angle

angle of incidence of radiation, relative to zenith. Equals angle of incidence 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, west is + 90°.

Sunshine
duration

sunshine duration during a given period is defined as the sum of that sub-period
for which the direct solar irradiance exceeds 120 W/m2. (ref: WMO)

IR2 0 manual v 1604 37/39

WISG

World Infra Red Standard Group. Group of pyrgeometers, maintained by PMOD
Davos Switzerland that forms the reference for calibration of pyrgeometers.
WISG is traceable to international standards through an absolute sky scanning
radiometer. WISG has been formally recognised by the World Meteorological
Organisation WMO as “interim WMO Pyrgeometer Infrared Reference”.

Sky
temperature

equivalent blackbody radiative temperature of the sky; i.e. the temperature
calculated from pyrgeometer data measuring downwelling longwave radiation,
assuming the sky behaves as a blackbody with an emission coefficient of 1.

Surface
temperature

equivalent blackbody radiative temperature of the surface; i.e. the temperature
calculated from pyrgeometer data measuring upwelling longwave radiation,
assuming the ground behaves as a blackbody with an emission coefficient of 1.

IR2 0 manual v 1604 38/39

7.8 EU declaration of conformity

We, Hukseflux Thermal Sensors B.V.
Elektronicaweg 25
2628 XG 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: IR20 and IR20WS
Product type: Pyrgeometer

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
20 April, 2016

© 2016, Hukseflux Thermal Sensors B.V.

www.hukseflux.com

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