Hukseflux NR01 User manual

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USER MANUAL NR01 /RA01
NR01 4-component net radiometer
RA01 2-component radiometer
Hukseflux
Thermal Sensors

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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 outputs.

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Contents
Warning statements 2
Contents 3
List of symbols 5
Introduction 6
1Ordering and checking at delivery 9
1.1 Ordering NR01 9
1.2 Included items 9
1.3 Quick instrument check 10
2Instrument principle and theory 12
2.1 General 12
2.2 Pyranometers model SR01 15
2.3 Pyrgeometers model IR01 16
2.4 Typical measurement results 18
2.5 Optional heating 19
3Specifications of NR01 20
3.1 Specifications of NR01 20
3.2 Specifications of pyranometer SR01 23
3.3 Specifications of pyrgeometer IR02 24
3.4 Dimensions of NR01 26
4Standards and recommended practices for use 27
4.1 Classification standard for pyranometers and pyrradiometers 27
4.2 General use for net radiation measurement 28
4.3 General use for sunshine duration measurement 28
4.4 Specific use in meteorology and climatology 29
5Installation of NR01 30
5.1 Site selection and installation 30
5.2 Installation of the sun screens 31
5.3 Mounting NR01 on a tube 31
5.4 Electrical connection of NR01 32
5.5 Requirements for data acquisition / amplification 34
6Making a dependable measurement 35
6.1 The concept of dependability 35
6.2 Reliability of the measurement 36
6.3 Repair and maintenance 37
6.4 Uncertainty evaluation 37
7Maintenance and trouble shooting 41
7.1 Recommended maintenance and quality assurance 41
7.2 Trouble shooting 42
7.3 Calibration and checks in the field 45
7.4 Data quality assurance 47
8RA01 2-component radiometer 48
8.1 Introduction RA01 48
8.2 Specifications of RA01 49
8.3 Dimensions of RA01 52
8.4 Electrical connection of RA01 55
9Appendices 57
9.1 Appendix on cable extension / replacement 57
9.2 Appendix on tools for NR01 58
9.3 Appendix on spare parts for NR01 58

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9.4 Appendix on standards for classification and calibration 59
9.5 Appendix on calibration hierarchy 60
9.6 Appendix on meteorological radiation quantities 63
9.7 Appendix on ISO and WMO classification tables 64
9.8 Appendix on definition of pyranometer specifications 65
9.9 Appendix on terminology / glossary 66
9.10 Appendix on older NR01 models 68
9.11 EU declaration of conformity 69

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List of symbols
Quantities Symbol Unit
Voltage output U V
Sensitivity S V/(W/m2)
Temperature T °C
Longwave and solar irradiance E W/m2
Stefan–Boltzmann constant (5.67 x 10-8) σW/(m2∙K4)
Sunshine duration SD h
(see also appendix 9.6 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
NR01 is a market leading 4-component net radiation sensor, mostly used in scientific-
grade energy balance and surface flux studies. It offers 4 separate measurements of
global and reflected solar and downwelling and upwelling longwave radiation, using 2
sensors facing up and 2 facing down. NR01 owes its popularity to its excellent price /
performance ratio. Advantages include its modular design, low weight, ease of levelling,
and low solar offsets in the longwave measurement. The unique capability to heat the
pyrgeometers reduces measurement errors caused by dew deposition.
NR01 measures the 4 separate components of the surface radiation balance: downward
and upward solar and longwave radiation. The solar radiation sensors are called
pyranometers and the longwave sensors are called pyrgeometers. From these 4 separate
components the net radiation is derived. For calculation of sky- and surface
temperatures, it is necessary to compensate for irradiated heat by the pyrgeometers
themselves (Stefan-Boltzmann law). A Pt100 temperature sensor is included in NR01’s
body for that purpose. Sunshine duration may be estimated according to the WMO
approved pyranometric method.
The solar irradiance is measured by pyranometers model SR01, the longwave radiation is
measured by pyrgeometers model IR01.
In order to prevent condensation of water on the pyrgeometer windows the NR01 has
internal heating close to the pyrgeometers. This keeps the instrument above dew point.
As water blocks longwave radiation, heating will improve the reliability of longwave
radiation measurement, in particular at night, when the risk of condensation is highest.
Solar offsets in the longwave radiation measurement are very low. Features like these
have made NR01 net radiometers popular in energy balance and surface flux studies. In
addition, NR01 net radiometer is practical to mount; it is much lighter than competing
models and a 2-axis levelling assembly is included. The levelling assembly fits a 1 inch
NPS tube (the tube’s recommended outer diameter equals 33.4 x 10-3 m). With the NR01
shim, included in NR01’s delivery, a ¾ inch NPS tube may also be used.
Using NR01 net radiometer is easy. It can be connected directly to commonly used data
logging systems. The irradiance levels in W/m2are calculated by dividing the NR01
outputs, small voltages, by the sensitivities. The longwave irradiance should be corrected
using the instrument body temperature. The sensitivities of all sensors are provided with
NR01 on its product certificate.
NR01 net radiometer has a modular design: it is possible to take the instrument apart
and replace or re-calibrate individual sensors. For this reason it is often selected for use
in large monitoring networks.
Suggested use for NR01:
•energy balance studies
•surface flux measurements
•climatological networks

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The solar irradiance in W/m2is calculated by dividing the SR01 output, a small voltage,
by the sensitivity. The sensitivity is provided with SR01 on its calibration certificate.
The central measurement equation governing solar radiation measurement with
pyranometers is:
E = U/S (Formula 0.1)
The longwave irradiance in W/m2is calculated by dividing the IR01 output, a small
voltage, by the sensitivity and taking in account the irradiated heat by the sensor itself
(Stefan-Boltzmann law). The sensitivity is provided with IR01 on its calibration
certificate.
The central measurement equation governing longwave measurement with pyrgeometers
is:
E = U/S + σ·(T + 273.15)4(Formula 0.2)
From the upward and downward solar radiation, it is possible to calculate net solar
radiation and albedo or surface reflectance. From all 4 solar and longwave components,
the net radiation, sky temperature and surface temperature can be derived.
The instrument should be used in accordance with the recommended practices of ISO,
WMO and ASTM.
Applicable instrument-classification standards are ISO 9060 and WMO-No. 8; Guide to
Meteorological Instruments and Methods of Observation.
Figure 0.1NR01 4-component net radiometer

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RA01 is a single-sided version of NR01. It measures the 2 incoming components of the
surface radiation balance; global solar and downward longwave radiation. RA01 is used
for estimating (not measuring) net radiation, in particular when local surface properties
are not representative, or if system costs need to be reduced. However, when using
RA01 for net radiation estimates, the reflected solar radiation or albedo and the surface
temperature or upwards longwave radiation must be estimated by the user.
This manual is written for NR01 4-component net radiometer. For RA01
2-component radiometer please consult the dedicated RA01 chapter.
Figure 0.2 RA01 2-component radiometer

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1Ordering and checking at delivery
1.1 Ordering NR01
The standard configuration of NR01 is with two cables, each of 5 metres.
Common options are:
•Longer cables (in multiples of 5 m). Specify total cable length. (cable lengths above
20 m in multiples of 10 m)
•Internal temperature sensor. This can be either a Pt100 (standard configuration) or a
10 kΩ thermistor (optional). Specify respectively T1 or T2.
1.2 Included items
Arriving at the customer, the delivery should include:
•NR01 4-component net radiometer, including a 2-axis levelling assembly
•cable of the length as ordered
•including 4 sun screens
•1 x product certificate matching the instrument serial number, including
4 x calibration certificate of the sensors
•1 x hex key (2 mm) for fixation and removal of sun screens
•1 x shim for optionally mounting NR01 on a ¾ inch NPS tube instead of on a 1 inch
NPS tube (tubes are not included)
Please store the certificates in a safe place.

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1.3 Quick instrument check
For checking the instrument see the wiring diagram on the product certificate or in the
chapter on electrical connection of this manual.
1.3.1 General check
1. Inspect the instrument for any damage.
2. remove the sun screens, using the hex key (see chapter on installation of the sun
screen). Inspect the bubble levels.
3. check the instrument serial numbers against the certificates supplied with the
instrument.
4. check the levelling assembly for x- and y-axis by unlocking the 4 x hex bolts for
levelling adjustment.
1.3.2 Check of pyranometers SR01
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 minus (-) and plus (+) 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 lower. 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.
1.3.3 Check of pyrgeometers IR01
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 minus (-) and plus (+) 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
400 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

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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.
1.3.4 Check of the Pt100
1. Check the electrical resistance of the Pt100. The resistance between 2 wires at
opposite ends of the Pt100 should be in the 100 Ω range.
2. Check the electrical resistance of the Pt100. The resistance between 2 wires at the
same end of the Pt100 should be in the 10 Ω range or 0.1 Ohm per metre cable.
1.3.5 Check of the heater
1. Check the electrical resistance of the heater. This should be in the 100 Ω range.
1.3.6 Optional check for trouble shooting: short circuit check
1. Check the electrical resistance between the sensor wires of different sensors; this
should be higher than 1 x 106Ω. Most multimeters cannot measure in this range, so
please use the highest range. Also check between sensors and heater and between
sensors and Pt100. Check the resistance between sensors and body.

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2Instrument principle and theory
2.1 General
NR01 is a 4-component net-radiometer, consisting of 2 pyranometers type SR01, 2
pyrgeometers type IR01, a heater, levelling assembly for x- and y-axis and a Pt100
instrument body temperature sensor. The design is fully modular, which is easy for
servicing and calibration. This chapter describes the instrument measuring principle and
theory.
Pyranometers and pyrgeometers (the latter with additional input of the body temperature
measurement) measure the solar and longwave radiation received by a plane surface, in
W/m2. The most common application of NR01 is measurement of net radiation.
From the 4 individual components measured by NR01, the net radiation and several
other measurands are calculated.
The following equations apply.
For terminology: see also the appendix on meteorological radiation quantities.
For the upfacing and downfacing pyranometers, global and reflected solar radiation:
Eg↓h= U/S (Formula 2.1.1)
Er ↑= U/S (Formula 2.1.2)
For the upfacing and downfacing pyrgeometers downward and upward longwave
irradiance:
El↓= U/S + σ·(T + 273.15)4(Formula 2.1.3)
El↑= U/S + σ·(T + 273.15)4(Formula 2.1.4)
Net radiation and albedo (please note that in the calculation of the net radiation, the
instrument temperature measurement is no longer included):
E* = E ↓– E ↑= Eg↓h- Er ↑ + El↓ - El↑(Formula 2.1.5)
Albedo = Er ↑ / Eg↓h(Formula 2.1.6)
Equivalent blackbody temperatures of the surface and sky:
Tsurface = (El↑/σ)1/4 - 273.15 (Formula 2.1.7)
Tsky = (El↓/σ)1/4 - 273.15 (Formula 2.1.8)

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Figure 2.1.1 Overview of NR01:
(1) upfacing pyrgeometer model IR01
(2) sun screens
(3,4) levelling assembly for x- and y-axis
(5) upfacing pyranometer model SR01
(6) downfacing pyranometer model IR01
(7) 4 x hex bolts for levelling adjustment
(8) downfacing pyrgeometer model IR02
12
3
45
6
7
8

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1
2
32
Figure 2.1.2 Top view of NR01 fitting a 1 inch NPS tube:
(1) levelling assembly for x- and y-axis
(2) cable (cable 1 at the left, at pyranometer side, cable 2 at the right, at pyrgeometer side)
(3) 1 inch NPS mounting tube (not included)
Figure 2.1.3 Top view of NR01 fitting a ¾inch NPS tube (alternative mounting method):
(1) levelling assembly for x- and y-axis
(2) cable (cable 1 at the left, at pyranometer side, cable 2 at the right, at pyrgeometer side)
(3) ¾ inch NPS mounting tube (not included)
(4) shim, included with NR01 delivery
1
2
3
2
4

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2.2 Pyranometers model SR01
SR01’s scientific name is pyranometer. A pyranometer measures the solar radiation
received by a plane surface from a 180° field of view angle. This quantity, expressed in
W/m2, is called “hemispherical” solar radiation. NR01 typically is mounted horizontally.
Measuring in the horizontal plane, downward solar radiation is called global solar
radiation, upward solar radiation is called reflected 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 (convection, rain).
Pyranometers can be manufactured to different specifications and with different levels of
verification and characterisation during production. The ISO 9060 - 1990 standard, “Solar
energy - specification and classification of instruments for measuring hemispherical solar
and direct solar radiation”, distinguishes between 3 classes; secondary standard (highest
accuracy), first class (second highest accuracy) and second class (third highest
accuracy). SR01 is a second-class pyranometer.

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Figure 2.2.1 Spectral response of the pyranometer compared to the solar spectrum. The
pyranometer only cuts off a negligible part of the total solar spectrum.
2.3 Pyrgeometers model IR01
IR01’s scientific name is pyrgeometer. IR01 measures the longwave or far-infra-red (FIR)
radiation received by a plane surface, in W/m2, ideally from a 180 ° field of view angle.
In meteorological terms pyrgeometers are used to measure “downward and upward
longwave irradiance” (WMO definition). In case of IR01 the ideal 180 ° field of view angle
has been reduced to 150 °. This makes it possible to offer an instrument at an attractive
price level, while the loss of accuracy is relatively small.
As secondary measurands, the sky temperature Tsky, and the equivalent surface (ground)
temperature Tsurface 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.
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 2.3.1).
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.
0
0,2
0,4
0,6
0,8
1
1,2
100 1000 10000
relative spectral content /
response [arbitrary units]
wavelength [x 10-9 m]
solar radiation
pyranometer
response

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Figure 2.3.1 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
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.
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.
0,001
0,010
0,100
1,000
1 10 100
spectral irradiance
[x 10-9 W/(m2/m)]
wavelength [x 10-6 m]
downwelling
longwave
solar

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2.4 Typical measurement results
2.4.1 Pyranometer measurement results
Pyranometers theoretically only generate an output when the sun is in their field of view;
i.e, when the sun is above the horizon.
The extraterrestial global solar irradiance (GHI) can serve as the expected range limit to
the local global solar irradiance. This range limit depends depends on time of day,
coordinates and daynumber. Lower values than the range limit will be measured
depending on the local weather. Under some circumstances values may exceed the range
limit, for instance when there is reflection against large cumulus clouds or snow-covered
mountain slopes. Under these circumstances even the extraterrestial solar direct normal
irradiance or solar constant E0of around 1350 W/m2may be exceeded.
Reflected solar irradiance depends largely on the incoming GHI and surface properties.
The study of albedo versus time offers a good method of quality assurance. In case of
constant surface properties, the albedo over the day should not vary very much below
solar zenith angles of 60°. Also in case of constant surface properties the albedo should
show consistent patterns over the day from one day to the next.
The upfacing pyranometer may generate a negative signal at night, as specified by “zero
offset a”. For SR01 the specification is < 15 W/m2 (unventilated).
2.4.2 Pyrgeometer measurement results
Pyrgeometers generate an output both during daytime and during nighttime.
Please note that the output 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
The largest errors in measurement of downward longwave irradiance are made during
clear nights when dew is deposited on the instrument. In that case the output rises from
a large negative value, in the order of of – 100 W/m2 (the correct value) to 0 W/m2(an
incorrect value and a large error). The daytime measurement of downward longwave
may contain a solar offset as specified under “solar offset” as < 15 W/m2 at 1000 W/m2
solar irradiance.

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Table 2.4.2.1 Expected pyrgeometer output U/S for an upfacing pyrgeometer at
different ambient air temperatures, Tambient , and at different cloud conditions. Under clear
sky conditions the U/S is around -100 W/m2while under cloudy conditions it will be close
to 0 W/m2. Also calculated: the sky temperature, Tsky, and the downward longwave
irradiance El↓.
EXPECTED PYRGEOMETER OUTPUT DOWNWARD LONGWAVE IRRADIANCE
Tambient Sky condition U/S Tsky
El↓
[°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
A downfacing pyrgeometer will typically generate a voltage output signal very close to
zero. The ground surface temperature usually is very close to the instrument
temperature, so that the radiative exchange between instrument and surface is low.
Table 2.4.2.2 Expected pyrgeometer output U/S for an downfacing pyrgeometer at
different ambient air temperatures, Tambient , and at different cloud conditions. Under clear
sky conditions as well as under cloudy conditions it will be close to 0 W/m2. Also
calculated: the surface temperature, Tsurface, and the upward longwave irradiance El↑
EXPECTED PYRGEOMETER OUTPUT UPWARD LONGWAVE IRRADIANCE
T
ambient
Sky condition U/S T
surface
El↑
[°C]
[cloudy], [clear]
[W/m2]
[°C]
[W/m2]
-20
cloudy
0
-20
232
-20
clear sky
0
-20
232
0
cloudy
0
0
314
0
clear sky
0
0
314
+30
cloudy
0
+30
477
+30 clear sky 0 +30 477
2.5 Optional heating
A low-power heater is located in the body of the net radiometer at the pyrgeometer side.
The heater is not necessarily switched on; recommended operation is to activate the
heater when there is a risk of dew deposition. In case power is available, many users
choose to keep the heater continuously on.

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3Specifications of NR01
3.1 Specifications of NR01
NR01 is a 4-component net radiometer, consisting of 2 pyranometers type SR01, 2
pyrgeometers type IR01, a heater, levelling assembly for x- and y-axis and a Pt100
instrument body temperature sensor.
Pyranometers and pyrgeometers (the latter with additional input of the body temperature
measurement) measure the solar and longwave radiation received by a plane surface, in
W/m2. For compensation of pyrgeometer emission in the longwave calculation and for
calculation of sky- and surface temperature, a Pt100 temperature sensor is included in
the instrument body. Working completely passive, using a thermopile sensor, the sensors
generate a small output voltage proportional to these fluxes. Optional measurands
include solar albedo and sunshine duration. NR01 can only be used in combination with a
suitable measuring system. The instrument should be used in accordance with the
recommended practices of ISO, IEC, WMO and ASTM.
Table 3.1.1 Specifications of NR01 (continued on next pages)
NR01 GENERAL SPECIFICATIONS
Product type
4-component net radiometer
Included sensors 2 x identical ISO 9060 second class pyranometer
(see separate specification table for model SR01)
2 x identical pyrgeometer with 150 ° field of view
angle
(see separate specification table for model IR01)
Spectral range solar
285 to 3000 x 10-9 m
Spectral range longwave
4.5 to 40 x 10-6 m
Levelling
Bubble level and a levelling assembly for x- and y-
axis are included
Required sensor power
zero (passive sensor)
Temperature sensor
Pt100
Heater on pyrgeometer
12 VDC, 1.5 W (see below for details)
Rated operating temperature range
-40 to +80 °C
Rated operating relative humidity range
0 to 100 %
Required readout 4 x differential voltage channel or 4 x single ended
voltage channel, input resistance > 106Ω
1 x temperature channel for Pt100
Standards governing use of the
instrument
ISO/TR 9901:1990 Solar energy -- Field
pyranometers -- Recommended practice for use
ASTM G183 - 05 Standard Practice for Field Use of
Pyranometers, Pyrheliometers and UV Radiometers
WMO-No. 8, Guide to Meteorological Instruments and
Methods of Observation, seventh edition 2008,
paragraph 7.4 "measurement of total and long-wave
radiation"