Hukseflux SHR02 User Manual

Hukseflux

Thermal Sensors

USER MANUAL SHR02

Shadow ring for pyranometers – combined with
a pyranometer forming a diffusometer

Copyright by Hukseflux | manual v1801 | www.hukseflux.com | [e-mail]

Warning statements

Ensure that SHR02 is connected to the protective earth
for proper grounding.

Ensure that the installed pyranometer housing is
connected to the protective earth for proper grounding.

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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 and checking at delivery 8

1.2 What’s in the box 8

1.3 Recommended tooling 10

2 Instrument principle and theory 11

2.1 Instrument overview 11

2.2 Operating principles 12

2.3 Sources of error 13

2.4 Daily adjustment 15

2.5 General usage recommendations 16

3 Standards and recommended practices for use 17

3.1 General use for diffuse solar radiation measurement 17

3.2 Specific use in meteorology and climatology 17

3.3 Pyranometer classification standard 18

4 Specifications 19

4.1 Specifications of SHR02 19

4.2 Dimensions of SHR02 21

5 Installation of SHR02 22

5.1 Assembly 22

5.2 Site selection 28

5.3 On site mounting 29

5.4 Alignment 31

6 Maintenance 39

6.1 Recommended maintenance and quality assurance 39

7 Trouble shooting 40

7.1 Irradiance level too high 40

7.2 Alignment errors 40

8 Appendices 43

8.1 Mounting pattern 45

8.2 Adjustment table: sliding bar settings and corresponding correction factors 46

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List of symbols

Quantities Symbol Unit

Global horizontal solar irradiance E, GHI W/m2
In-plane solar irradiance Gi W/m2
Diffuse Horizontal Irradiance E
Instrument dome radius r m
Shadow ring radius R m
Shadow ring rim height h m
Shadow ring width w m
Shadow ring sliding bar setting x m
Correction factor diffuse radiation component f ­Latitude of installation lat °
Solar declination angle dec °
Hour angle at sunset and sunrise t0 °
Mathematical constant with a value of 3.14159… pi -

Subscripts

in-plane portion i
diffuse portion d
value at a chosen reference condition 0
obscured portion obscured
contribution related to instrument dome dome
contribution assuming symmetrical instrument symmetrical

↓, DHI W/m

d

2

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Introduction

SHR02 is a practical metal shadow ring that helps making diffuse solar irradiance
measurements with pyranometers. The shadow ring, also known as a shadow band,
prevents direct radiation from reaching the pyranometer, so that the shaded pyranometer
measures diffuse radiation only. The combination of a shadow ring and a pyranometer is
called a diffusometer by the ISO 9060:2018 standard. The compact SHR02, combined
with a Hukseflux pyranometer, has several advantages over competing models.

SHR02 is compatible with most Hukseflux pyranometers. Hukseflux pyranometers have
very low zero offsets, so that the diffuse radiation measurement has a better uncertainty
than that of competing pyranometer-shadow ring diffusometers. To avoid problems with
dew and frost deposition, the user should consider using the heated SR25 or the heated
and ventilated SR30 pyranometer.

A diffuse horizontal irradiance (DHI) measurement with a pyranometer-shadow ring
diffusometer is usually combined with a pyranometer-without a shadow ring. The latter
measures the global horizontal irradiance (GHI). Combining GHI and DHI with local solar
position, the direct normal irradiance (DNI) can be estimated.

Most shadow rings are used with the pyranometer mounted horizontally. The incorporated
tilt adjustment stage enables the diffuse irradiance to be measured in a tilted plane. The
height of the ring is adjusted manually by adjusting the sliding bars to correct for the
changing altitude of the sun.

Figure 0.1 SHR02 shadow ring with a Hukseflux SR30 pyranometer, together forming a
diffusometer (a pyranometer is not included in SHR02 delivery).

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Using SHR02 has several advantages:

 relatively small size / light weight
 low-investment alternative for a sun tracker with shading-disc
 high accuracy when used with Hukseflux (low zero-offset) pyranometers
 save costs on expensive external ventilation; compatible with SR30 with internal

ventilation

Suggested use of SHR02:

 meteorological observations
 building energy performance
 solar energy studies

Figure 0.2 Installation of the SHR02 shadow ring with a Hukseflux pyranometer is easy.

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The altitude of the sun varies throughout the year and this influences the shadow cast by
the shadow ring. Aside from adjusting the shadow ring on a regular basis to compensate
for this, it should be taken into account that part of the diffuse radiation obscured by the
shadow ring will change. Hukseflux provides a specifically tuned model that will aid the
user in setting up the SHR02 without the need for in-depth knowledge of diffusometers.
In this user manual a set-up table with required correction factors for the measured
diffuse radiation is provided, for a range of latitudes and declinations. The manual also
provides theory for creating individual models when even higher accuracy is required.

Compatible sensors are model SR30 and SR15 pyranometers. With the optional mounting
adapter, also models SR20, SR22 and SR25 may be used. This user manual covers
installation and use of SHR02 with pyranometer models SR30, SR15, SR20, SR22 and
SR25.

Figure 0.3 Pyranometer example: SR15, combined with SHR02 forming a diffusometer.

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SHR02

Shadow ring for pyranometers, combined with a
pyranometer forming a diffusometer
(pyranometer is not included in SHR02 delivery)

PMA01

SR20 / SR22 / SR25 mounting adapter for
SHR02

1 Ordering and checking at delivery

1.1 Ordering and checking at delivery

The standard configuration of SHR02 is for use with SR15 / SR30 sensor.

Common options are:

 Mounting adapter for SR20 / SR22 / SR25. Specify PMA01

Table 1.1.1 Ordering codes for SHR02

1.2 What’s in the box

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Arriving at the customer, the delivery should include:

1. 1 x base plate

2. 1 x tilt adjustment stage (with engraved rotation scale in °)

with pyranometer mounting plate

3. 1 x central support, horizontal bar connected to pyranometer support arm

4. 2 x sliding bar, with engraved scale in mm

5. 1 x shadow ring

6. 1 x bag with nuts, bolts, set screws:
a. 3 x A4 stainless steel M8 x 120 hexagon head bolt
b. 3 x A4 stainless steel spring
c. 9 x A4 stainless steel M8 washer
d. 3 x A4 stainless steel M8 nut
e. 3 x A4 stainless steel M6 x 35 flat point socket set screw
f. 2 x A4 stainless steel M6 x 16 countersunk socket head cap bolt
g. 4 x A4 stainless steel M5 x 14 black button head cap bolt
h. 1 x A4 stainless steel M5 x 14 button head cap bolt
i. 1 x A4 stainless steel M5 washer
j. 2 x A4 stainless steel M6 x 35 thumbscrew
k. 2 x Plastic M6 washer
l. 2 x A4 stainless steel M5 x 10 socket cap head bolt

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1.3 Recommended tooling

For assembling and mounting the shadow ring, the following tooling is recommended:

 hex key 3 mm
 hex key 4 mm
 2 x spanner 13 mm

For levelling and or adjusting the shadow ring, the following tooling is recommended:

 hex key 3 mm
 hex key 4 mm

Consult your pyranometer user manual for tooling recommended for removal of the
pyranometer feet.

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5

1

2

4

3

6

7

8

9

10

11

2 Instrument principle and theory

2.1 Instrument overview

Figure 2.1.1 below shows SHR02 shadow ring, listing the main functional parts.

Figure 2.1.1 Overview of SHR02:

(1) levelling set screw
(2) thumbscrew for adjustment of sliding bars
(3) screw for adjustment of tilt stage
(4) sliding bar
(5) shadow ring
(6) tilt adjustment stage
(7) screw for latitude adjustment
(8) central support with horizontal bar connected to pyranometer support arm
(9) base plate
(10) bubble level
(11) mounting hole

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Understanding the basic set-up, working principles and operation of a shadow ring
instrument is essential to attain accurate, reliable measurement data. This chapter
describes the basic principles, the main sources of error in data and some practical
considerations regarding installation and operation.

A view angle is an important specification for a shadow ring instrument. The view angle
is determined purely by the geometrical properties of the ring, assuming the sensitive
area of the pyranometer is small compared to the apparent width of the ring. The rims
on the ring not only provide mechanical stability but also minimize the seasonal
dependence of the view angle on the seasonal variations. The chapter on specifications
lists the mean value of the view angle. Due to the ring design, this view angle varies less
than 2 % throughout the year, depending on the declination. Note that the correction
factors provided in Appendix 8.2 of this manual do not assume this angle to be constant,
but take the dependence on the declination into account.

2.2 Operating principles

A shadow ring is used together with a pyranometer to measure diffuse global irradiance.
Direct radiation from the sun is blocked by the ring. To achieve this the entire day, the
ring is set up parallel to the equatorial plane; see Figure 2.2.1. This is practically done by
setting the plane of the ring compared to the horizon under an angle equal 90 ° minus
the local latitude in °.

Figure 2.2.1 The principle of a shadow ring

During the seasons, the sun’s trajectory changes its position in the sky due to variation
in the declination of the earth-sun system. The shadow ring is adjusted regularly to block
the pyranometer from the sun at all times. Adjustable sliding bars are used to put the

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shadow ring in the correct position, compensating for the declination. Figure 2.2.2
schematically explains how a shadow ring is correctly set up. Depending on the time of
the year and the local latitude, adjustments need to be made between every day and
less than only once every 3 weeks.

Figure 2.2.2: Regular adjustment needs to be done to compensate for the position of
the sun in the sky throughout the year.

Most shadow rings are used with the pyranometer mounted horizontally. Using the
incorporated tilt adjustment stage, the pyranometer can also be mounted in a tilted
position. In this case, the diffuse irradiance in the tilted plane is measured. To keep the
pyranometer shaded, the ring must stay aligned with the equatorial plane.

2.3 Sources of error

There are two main sources of error that may affect the accuracy of the diffuse irradiance
component:

 measurement errors
 errors due to unintentional blockage by the ring

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Measurement errors, inherent to the type of pyranometer and data logger used, can
generally be practically eliminated, but unintentional blockage by the ring is inherent to
the instrument operation principle and cannot be avoided. In the next two sections, these
sources of error are explained and, if possible, suggestions to remediate them are made.

2.3.1 Unintentional blockage of diffuse radiation

Seen from the centre of the pyranometer, the ring blocks a band of the sky aligned with
the trajectory of the sun during the day. Thus, not only the direct radiation from the sun
is blocked, but also part of the quantity of interest, the diffuse component. Apart from
other possible errors made, this always causes an underestimation of the diffuse
irradiance component. In other words, the measured diffuse component is a lower bound
for the actual diffuse irradiance component. Depending on the latitude, the declination
and the distribution of diffuse radiation, this underestimation is in the range of 5 % to 20
% or even larger.
In an attempt to improve on this and more closely approach the true value of the diffuse
irradiance, a correction factor can be calculated or estimated. This correction factor, f, is
defined by:

DHI = DHI

with DHI the diffuse radiation and DHI
the shadow ring. In terms of the corrected and uncorrected diffuse radiation the factor f
is given by:

f = 1 - DHI

The simplest possible model to calculate this correction factor assumes a uniform
distribution of the diffuse component and computes the fraction of sky that is blocked by
the ring, and is also known as the Drummond model. This correction factor depends on
the latitude and the declination and is given by:

f =2 (w+h |tan(dec)|) / (pi R) cos3(dec) [t0 sin(lat) sin(dec) + cos(lat) cos(dec) sin(t0)]

(Formula 2.3.1.3)

with R the diameter of the ring, h the height of the rims on the ring, w the width of the
ring, dec the declination, lat the latitude and t0 the hour angle at sunset and sunrise.
Refer to the specification table in Chapter 4 for the instrument dimensions. For
convenience, the table in Appendix 8.2 lists correction factors for a range of latitudes and
declinations. There are more sophisticated correction models possible, but these depend
on the local conditions and require more assumptions and tuning to local conditions.
These models are not discussed here.

/ (1 - f) (Formula 2.3.1.1)

obscured

the part of the diffuse radiation obscured by

obscured

/ DHI (Formula 2.3.1.2)

obscured

2.3.2 Measurement accuracy

A significant error source for diffuse sky radiation measurement is the zero offset a of the
pyranometer, i.e the signal at zero irradiation. Under clear sky conditions, the zero

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irradiance signal may approach 30 W/m2 for a second class pyranometer and may be up
to 7 W/m2 for a secondary standard instrument. Since the diffuse irradiance component
is typically small, this may be up to 22 % for a second class pyranometer and 5 % for a
much more accurate secondary standard instrument under the same conditions. Thus
using pyranometers with a low zero offset, such as SR30 or SR25, will result in an
improved measurement accuracy. These instrument typically have a zero offset < 2
W/m2, resulting in an error in the diffuse component < 1 %. This is the most important
source of instrumentation error, but also the data logger accuracy needs to be
considered.
Since, as mentioned earlier, the diffuse sky radiation from a cloudless sky is small and
may be less than one tenth of the global radiation, relative contribution of pyranometer
measurement errors are large. Therefore, particularly in case of an analogue pyranometer,
the data acquisition should have a high resolution and small zero offset as well.

2.4 Daily adjustment

The position of the sun in the sky changes depending on the time of the year. To keep
the pyranometer shaded by the shadow ring, the position of the ring with respect to the
pyranometer is adjusted using the sliding bars. There are different approaches possible
to make the correct setting. The simplest, most pragmatic method is to centre the cast
shadow around the pyranometer outer dome every day at the time the sun is at its
highest point in the sky, at solar noon. This method does require daily attendance and is
therefore labour intensive, but may tolerate larger errors in the set-up and alignment of
the shadow ring.

An alternative method is to compile a table listing dates and corresponding settings.
There are several levels of sophistication possible within this approach. The table in
Appendix 8.2 lists a table that can be used at all latitudes and should be seen as a
starting point for a pyranometer that is installed horizontally. This table is computed
taking the shading effect of the flanges on the ring into account, and attempts to shade
the outer dome of the pyranometer for the majority of the time. The formula used to
compute this table is:

x

symmetrical

This equation results in a symmetrical shading around the centre of the pyranometer. In
the table, a setting for every 1 ° in declination is computed. In general, there are two
dates during the year at which the declination is the same. To improve on this, the
asymmetrical projection of the half-sphere formed by the pyranometer dome can be
taken into account as well. Since this projection depends on the angle of the sun with the
horizontal this equation depends on the latitude and the declination. Given a setting
x

symmetrical

x

dome

with r the diameter of the dome or half-sphere to be shaded. This correction becomes
particularly significant at large latitudes and small declinations.

= (R + h/2) tan(dec) (Formula 2.4.1)

the amount x

can be added to improve the ring adjustment:

dome

= r / 2 (1 – cos(lat - dec)) / cos(dec) (Formula 2.4.2)

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