Hukseflux DR15-A1, DR20-A1 User Manual

USER MANUAL

Hukseflux

Thermal Sensors

DR20-A1 & DR15-A1

Analogue spectrally flat Class A & B pyrheliometers, with heating

Warning statements

Putting more than 12 Volt across the signal wiring can lead to permanent damage to the sensor.

Do not use “open circuit detection” when measuring

the sensor output.

For proper instrument grounding: use DR20-A1 and DR15-A1 with its original factory-made cable.

Disconnect power while performing service or maintenance.

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Contents

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

1.1 Ordering DR20-A1 and / or DR15-A1 8

1.2 Included items 9

1.3 Quick instrument check 9

2 Instrument principle and theory 10

2.1 Why you need a “spectrally flat” pyrheliometer 12

2.2 Operating modes: heating 13

3 Specifications of DR20-A1 and DR15-A1 15

3.1 Specifications 15

3.2 Dimensions of DR20-A1 and DR15-A1 19

4 Standards and recommended practices for use 20

4.1 Classification standard 20

4.2 General use for solar radiation measurement 20

4.3 General use for sunshine duration measurement 21

4.4 Specific use in meteorology and climatology 21

5 Installation of pyrheliometers 22

5.1 Site selection and installation 22

5.2 Mounting 23

5.3 Electrical connection 24

5.4 Grounding and use of the shield 25

6 Making a dependable measurement 26

6.1 The concept of dependability 26

6.2 Reliability of the measurement 27

6.3 Speed of repair and maintenance 28

6.4 Uncertainty evaluation 28

6.5 Definition of the measurand 28

6.6 Contributions from circumsolar radiation 29

6.7 Instrument classification, and approaches to uncertainty evaluation 30

6.8 Evaluation of measurement uncertainty under outdoor conditions 30

6.9 Calibration uncertainty 32

7 Maintenance and trouble shooting 33

7.1 Recommended maintenance and quality assurance 33

7.2 Trouble shooting 34

7.3 Calibration and checks in the field 35

7.4 Data quality assurance 36

8 Appendices 38

8.1 Appendix on cable extension / replacement 38

8.2 Appendix on tools for DR20-A1 and DR15-A1 39

8.3 Appendix on spare parts for DR20-A1 and DR15-A1 39

8.4 Appendix on standards for classification and calibration 40

8.5 Appendix on calibration hierarchy 41

8.6 Appendix on requirements for solar tracking 42

8.7 Appendix on meteorological radiation quantities 43

8.8 Appendix on ISO and WMO classification tables 44

8.9 Appendix on ISO 9060:1990 classification no longer valid 46

8.10 Appendix on definition of pyrheliometer specifications 47

8.11 Appendix on terminology / glossary 48

8.12 Appendix on converting resistance to temperature 50

8.13 Appendix on literature references 51

8.14 EU declaration of conformity 52

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List of symbols

Quantities Symbol Unit

Voltage output U V Solar irradiance E W/m

Sensitivity S V/(W/m2)

(see also Appendix 8.7 on meteorological quantities)

Subscripts

Not applicable

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Introduction

DR20 and DR15 are high-accuracy direct (normal incidence) solar radiation sensors, or pyrheliometers. DR20 complies with the Class A, and DR15 with the Class B specifications of the ISO 9060:2018 standard. Both pyrheliometers offer analogue millivolt outputs, and have superior window heating that leads to high data availability.

Hukseflux model DR15 pyrheliometer is an ISO 9060 spectrally flat Class B (old ISO classification “first class”) instrument. It replaces the models DR01, DR02 and DR03. DR20 is a new Class A instrument. It has a better temperature response than DR15. Both instruments offer the highest accuracy and highest data availability, featuring window heating at low offsets. At the same heating power, the heating has been improved with a factor 4, at a 4 times lower offset!

DR20 and DR15 are applied in high-accuracy measurements of the solar radiation received by a plane surface from a 5 ° full field of view angle. This quantity, expressed in W/m2, is

called “direct” solar radiation or DNI (direct normal irradiance). It is necessary to keep the

instrument pointed at the sun by using a two-axis tracker.

DR20 / DR15 pyrheliometers feature a precision ground and polished quartz window, a compact sized collimated tube and a thermopile sensor with black coated surface. DR20-A1 and DR15-A1 can be connected directly to commonly used data logging systems. They offer analogue outputs in the millivolt range.

High data availability is attained by heating of the front window. This suppresses dew and frost deposition at a very low power consumption: DR20 / DR15 needs only 1 W to keep its window free from dew and frost.

Figure 0.1 DR20-A1 Class A and DR15-A1 Class B pyrheliometers: the external housing of these instruments is identical

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A pyrheliometer is used in tracker-mounted operation. Using DR20 / DR15 pyrheliometer is easy. It can be connected directly to commonly used data logging systems. The irradiance, E, in W/m2 is calculated by dividing the DR20 / DR15 output, a small voltage U, by the sensitivity S. The sensitivity is provided with DR20 / DR15 on its calibration certificate.

The central equation governing DR20 and DR15 is:

E = U/S (Formula 0.1)

The temperature dependence of every individual DR20 pyrheliometer is tested and supplied as a second degree polynomial. This information can be used for further reduction of temperature dependence during post-processing. In case the sensitivity is corrected for the instrument body temperature, the optional measurement equation becomes:

E = U/(S0·(a·T² + b·T +c)) (Formula 0.2)

The temperature coefficients a, b, and c can be found on the calibration certificate of each DR20 instrument.

Both DR20 and DR15 are equipped with an internal temperature sensor. This can be either a Pt100 (T1 version) or a 10 kΩ thermistor (T2 version), as ordered. To calculate temperature in degrees Celsius from resistance in Ohms, Formula 8.12.1 or 8.12.2 can be used. See the dedicated chapter in the appendix of this manual for these equations.

Figure 0.2 Application of DR20-A1 and DR15-A1 pyrheliometers, here with SR15-A1

pyranometers, in a typical solar radiation monitoring station

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Suggested use for DR20 / DR15:

• solar energy surveys

• solar resource assessments

• meteorological networks

• sites with dew and frost problems

A pyrheliometer can also be used to measure 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.

Applicable instrument classification standards are ISO 9060 and WMO-No. 8. Calibration is traceable to WRR (World Radiometric Reference). As required by ISO 9060:2018 for Class A classification, each DR20 is supplied with test results for the individual instrument:

• sensitivity, response time and temperature response

DR15 certificates include sensitivity and response time only. The instrument should be used in accordance with the recommended practices of ISO, WMO and ASTM.

Figure 0.3 DR20 / DR15 pyrheliometer side view

Using DR20 and DR15 pyrheliometers offer significant benefits over the use of competing models. The pyrheliometers offer the highest accuracy and highest data availability, featuring heating at low offsets. The advantages of having a heater are demonstrated in Chapter 2.2 on heating.

DR20 / DR15 is available with 5 m cable. No other lengths are offered. See the Appendix for cable specifications.

Various tracking solutions can be offered by Hukseflux. Please contact us for more information on solar trackers.

1 Ordering and checking at delivery

1.1 Ordering DR20-A1 and / or DR15-A1

DR20 / DR15 pyrheliometers are available in several versions, each with 5 metres cable. No other lengths are offered.

Table 1.1.1 Ordering codes for DR20 / DR15

VERSIONS OF DR20 / DR15 (part numbers)

DR20-A1-T1

analogue spectrally flat Class A pyrheliometer, with heating and Pt100 temperature sensor

DR20-A1-T2

analogue spectrally flat Class A pyrheliometer, with heating and 10 kOhm thermistor

DR15-A1-T1

analogue spectrally flat Class B pyrheliometer, with heating and Pt100 temperature sensor

DR15-A1-T2

analogue spectrally flat Class B pyrheliometer, with heating and 10 kOhm thermistor

CABLE FOR DR20 / DR15, with female M12-A connector at sensor end, pigtails of 0.15 m and conductors with ferrules

‘-05’ after DR20 /DR15 part number

cable length: 5 m

Figure 1.1.1 Front and back view of DR20 / DR15 pyrheliometer

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1.2 Included items

Arriving at the customer, the delivery should include:

• pyrheliometer DR20-A1

• 5 metre cable, if ordered

• product certificate matching the instrument serial number, including:

o calibration certificate, including sensitivity and response time o temperature response test report

• any other options as ordered

• pyrheliometer DR15-A1

• 5 metre cable, if ordered

• product certificate matching the instrument serial number, including:

o calibration certificate, including sensitivity and response time

• any other options as ordered

Please store the certificates in a safe place.

1.3 Quick instrument check

A quick test of the instrument can be done by using a simple hand-held multimeter and a lamp.

1. Check the electrical resistance of the sensor between the green (-) and white (+) wire.

Use a multimeter at the 1000 Ω range. Measure the sensor resistance first with one

polarity, than reverse the polarity. Take the average value. The typical resistance of the cable is 0.1 Ω/m. Typical resistance should be the typical sensor resistance of 50 to 150

Ω plus 1.5 Ω for the total resistance of two wires (back and forth). 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 the front window. The signal should read > 2 x 10-3 V now. Darken the sensor either by putting something over it or switching off the light. The instrument voltage output should go down and within one minute approach 0 V.

3. Inspect the instrument for any damage. Check if the sight is straight and aligned.

4. Optional: inspect the electrical resistance range of the internal temperature sensors

(for Pt100 in the 100 Ω range, for 10 kΩ thermistors in the 10 000 Ω range, in case of 3 wire or 4 wire connections of the Pt100 in the < 10 Ω range between connections at one

end).

2 Instrument principle and theory

Figure 2.1 Overview of DR20 / DR15 pyrheliometer. Versions DR20-A1 and DR15-A1 have an identical external housing.

(1) sights (2) aperture tube (3) protection cap (4) window assembly with heater (5) connector (6) cable (length 5 metres)

DR20 / DR15’s scientific name is pyrheliometer. It measures the solar radiation received by a plane surface from a 5 ° full field of view angle. In addition to the full field of view angle, (the angle from the centre of the sensor to the edge of the aperture window) WMO recommends in WMO manual 7.2 a slope angle (the angle from the side of the sensor to the edge of the window aperture) of 1 °. The opening angle and slope angle together with the sensor surface define the so-called acceptance function (see appendix on terminology).

A pyrheliometer should measure ‘direct’ solar radiation, also called direct normal irradiance or DNI. DNI is defined as the solar radiant flux collected by a plane unit surface normal to the axis pointing towards the centre of the sun, within an optical angular aperture. This aperture is characterised by the acceptance function (ref: Blanc et al. (2014), see appendix on terminology and appendix on literature references). DNI is composed of the solar irradiance within the extent of the solar disk (half-angle

0.266 ° ± 1.7 %) plus some circumsolar radiation.

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Summarising, DR20-A1 and DR15-A1 are radiometers designed to measure DNI (i.e. including some circumsolar irradiance). The instruments comply with the WMO recommended parameters for the view-limiting geometry: a full opening angle of 5 °, and a slope angle of 1 °, and therefore a limit angle of 4 °.

The solar radiation spectrum extends roughly from 285 to 3000 x 10-9 m. By definition a pyrheliometer should cover that spectral range with a spectral selectivity that is as “flat” as possible.

For a correct measurement, DR20 / DR15 should be pointed at the sun. It is usually mounted on a solar tracker. For tracking requirements see the appendix on solar tracking.

In order to attain the proper directional and spectral characteristics, DR20 / DR15 pyrheliometer’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. The coating absorbs all solar radiation and, at the moment of absorption, converts it to heat. The heat flows through the sensor to the instrument body. The thermopile sensor generates a voltage output signal that is proportional to the solar irradiance.

• a quartz glass window. This window limits the spectral range from 200 to 4000 x 10

-9

m (cutting off the part above 4000 x 10-9 m).

• an aperture tube. The most important components of this tube are two apertures,

one at the detector and the other at the front window. These determine the openingand slope angle.

• a heater incorporated in the window assembly. This reduces measurement errors

caused by (early-morning) dew deposition. The heater is not necessarily switched on; recommended operation is to always run the heater.

Pyrheliometers can be manufactured to different specifications and with different levels of verification and characterisation during production. The ISO 9060:2018 standard,

“Solar energy - specification and classification of instruments for measuring hemispherical solar and direct solar radiation”, distinguishes between 4 classes: Class AA

(highest accuracy), Class A (second highest accuracy), Class B (third highest accuracy) and Class C (fourth highest accuracy).

Due to the required spectral properties, a Class AA pyrheliometer can only be made with a cavity-type sensor. For this reason, commercially available thermopile pyrheliometers with a flat detector and a window can only be Class A.

From Class B to Class A, the achievable accuracy improves by a factor 2.

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2.1 Why you need a “spectrally flat” pyrheliometer

The new ISO 9060, 2018 version defines classes AA, A, B and C. The standard also adds a new subclass, called "spectrally flat". The term "spectrally flat" may be added to the name of the class (A, B, C) if the instrument has a spectral selectivity of less than 3 % (guard bands 2 %) in the (350 to 1500) x 10-9 m range.

The vast majority of users needs to use instruments meeting the requirements of this spectrally flat subclass. Why? Only spectrally flat instruments measure with high accuracy, also when a cloud obscures the sun. Ordinary instruments, classified solely as Class A, B or C and not spectrally flat, only measure accurately under clear sunny skies. Compliance with the spectrally flat subclass also means the instrument complies with the WMO guide and keeps continuity with the 1990 version of ISO 9060.

The spectral error of ISO 9060:2018 of a normal (spectrally variable, not flat) pyrheliometer of Class A, B or C is defined as "Clear sky direct normal irradiance spectral error". This error is valid under a clear sky on a sunny day. This is not the common spectrum in normal applications in solar renewable energy, and it is also not the common spectrum in meteorological applications. Even for the most common situations, for example when a cloud obscures the direct sun or under a cloudy sky, the measurement error with an ordinary Class A, B or C pyrheliometer is undefined. This is why almost all users need a "spectrally flat" pyrheliometer.

Summarising, specifying "spectrally flat" is essential because this ensures:

• you can measure accurately not only with clear-blue-sky, but also when a cloud

obscures the sun.

• you comply with WMO; spectrally flat Class A and B instruments comply with the

WMO spectral requirements of good quality pyrheliometers. Ordinary instruments do not comply with WMO requirements.

• you can use the normal standardised ISO and WMO calibration procedures, and can

benefit from relatively low-cost indoor calibrations. For ordinary Class A, B and C instruments this is not possible.

• you comply with the old ISO 9060 version of 1990, attaining continuity of

performance and specifications.

• you can perform uncertainty evaluations with negligible (zero) spectral errors under

all conditions, because these are calibrated out.

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2.2 Operating modes: heating

A feature of both DR20 and DR15 is its built-in heater in the front window assembly. This is effective against dew and frost deposition.

In standard operating mode, the heater is [ON], powered by a 12 VDC source. When no power is available, the heater is [OFF].

As the heater is a resistive element with a nominal resistance of 144 Ω, the user can change the heating power by adjusting the supply voltage. We define a high power mode with a heater voltage of 24 VDC. This is only necessary in extreme conditions, for example to counter severe frost.

Table 2.2.1 gives an overview of these settings and our recommendations for use.

Table 2.2.1 Possible user scenarios for the heater

Operating mode

heater

status

power use

(nominal)

comment

standard

[ON]

12 VDC, 1 W

recommended settings

no power

[OFF]

none

high power

[ON]

24 VDC, 4 W

only necessary in extreme conditions

Heating does not affect the classification specifications and the measurement accuracy.

DR20 / DR15 pyrheliometers offer the highest accuracy and highest data availability, featuring heating at low offsets and low power consumption. The advantages of having a heater are demonstrated in the following graphs:

0.5

1.5

2.5

3.5

4.5

0 1 2 3 4 5

steady state offset [W/m²]

heating power [W]

DR01, DR02, DR03

DR15, DR20

DR30

100

200

300

400

500

600

700

800

8:45 9:15 9:45 10:15

irradiance [W/m²]

time [hh:mm]

DR30 with heating DR30 without heating

Figure 2.2.1 The offset of the latest models DR20, DR15 (and also the digital DR30) when heating, is much improved relative to the older DR01, DR02 and DR03 models. DR20 and DR15 have a 12 VDC, 1 W heater which produces a negligible offset. The older models had offsets of the order of 1 W/m2 at the same heating level. In addition, the temperature of the front window of DR20 and DR15 is 4 times higher than that of the older models, at the same heating power.

Figure 2.2.2 Comparison of the measured data with and without heating on a typical winter morning, using model DR30 which has the same heating as DR20 and DR15. The unheated DR30 has dew on its front window and strongly underestimates the incoming irradiance. At around 10:30 the dew evaporates. Real measured data from Delft, the Netherlands.

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3 Specifications of DR20-A1 and DR15-A1

ISO classification (ISO 9060:2018)

DR20-A1: spectrally flat Class A pyrheliometer DR15-A1: spectrally flat Class B pyrheliometer

ISO classification (ISO 9060:1990)

DR20-A1: first class pyrheliometer DR15-A1: first class pyrheliometer

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

DR20-A1: good quality pyrheliometer DR15-A1: good quality pyrheliometer

Response time (95 %)

4 s (nominal)

Zero off-set a (response to 5 K/h change in ambient temperature)

< ± 1 W/m2 Complete zero off-set including a)

< ± 1 W/m²

Non-stability

< ± 0.5 % change per year

Nonlinearity

< ± 0.2 % (100 to 1000 W/m2)

Clear sky direct normal irradiance spectral error

< ± 0.2 %

Temperature response

DR20-A1: < ± 0.5 % (-10 to +40 °C) DR20-A1: < ± 0.4 % (-30 to +50 °C) with correction in

data processing

DR15-A1: < ± 1.0 % (-10 to +40 °C)

Temperature response test of individual instrument

DR20-A1: included DR15-A1: not included

Additional signal processing errors

0 W/m²

Tilt response

< ± 0.2 % (0 to 90 ° at 1000 W/m2)

Full field of view angle

5 °

Slope angle

1 °

Limit angle

4 ° (follows from full field of view angle and slope angle)

3.1 Specifications

DR20 and DR15 pyrheliometers measure the solar radiation received by a plane surface from a from a 5 ° full field of view angle and a 1 ° slope angle, i.e. the acceptance function recommended by WMO. This quantity, expressed in W/m2, is called ‘direct’ solar radiation or direct normal irradiance (DNI). DNI can be used to calculate sunshine duration. Working completely passive, using a thermopile sensor, DR20 / DR15 generates a small output voltage proportional to the DNI. It can only be used in combination with a suitable measurement system and a solar tracker to keep it continuously aimed at the sun. The window assembly contains a heater to prevent dew deposition. The instrument is classified according to ISO 9060 and should be used in accordance with the recommended practices of ISO, WMO and ASTM.

Table 3.1.1 Specifications of DR20 / DR15 (continued on next pages)

-A1 AND DR15-A1 MEASUREMENT SPECIFICATIONS:

DR20 LIST OF CLASSIFICATION CRITERIA OF ISO 9060*

*For the exact definition of pyrheliometer ISO 9060 specifications see the appendix.

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Table 3.1.1 Specifications of DR20 / DR15 (continued)

ADDITIONAL SPECIFICATIONS

Measurand

direct solar radiation with the acceptance function recommended by WMO

Measurand in SI radiometry units

irradiance in W/m2

Optional measurand

sunshine duration

Measurement range

0 to 4000 W/m-2

Spectral range (50 % transmission points)

200 to 4000 x 10-9 m Sensitivity range

10 to 30 x 10-6 V/(W/m2)

Sensitivity (nominal)

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

Rated operating temperature range

-40 to +80 °C

Rated operating relative humidity range

0 to 100 %

Sensor resistance range

50 to 150 Ω

Required sensor power

zero (passive sensor) heater requires power (12 VDC, 1 W) or (24 VDC, 4 W)

Expected voltage output

application under natural solar radiation: -0.1 to + 40 x 10-6 V

Measurement function / required programming

E = U/S

Optional measurement function / required programming for correction of sensitivity as a function of instrument body temperature

DR20-A1: E = U/(S0·(a·T²+b·T+c))

Measurement function / required programming for sunshine duration

if E > 120 W/m2 then SD = 1, else SD = 0

Required readout

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

Optional readout

1 resistance measurement channel for the temperature sensor

Total sensor length including cap

277 x 10-3 m

Cable length

5 m

Cable diameter

4.8 x 10-3 m

Chassis connector

M12-A straight male connector, male thread, 8-pole

Chassis connector type

M12-A

Cable connector

M12-A straight female connector, female thread, 8pole

Cable connector type

M12-A

Connector protection class

IP67 (connected)

Cable replacement

replacement cables with connector can be ordered separately from Hukseflux

Mounting

mounting with 1 or 2 clamps around the pyrheliometer tube Ø 38 x 10-3 m, clamp to clamp distance of 120 x 10-3 m max. Contact the factory for solar tracker compatibility.

IP protection class

IP67

Desiccant

3 bags of silica gel, 1 g, (45 x 24 x 4) x 10-3 m

Desiccant replacement

> 5 year interval, typically replaced during recalibration, ask the manufacturer for instructions

SHIPPING

Gross weight including 5 m cable

1.3 kg

Net weight including 5 m cable

1.1 kg Packaging

box of (430 x 105 x 105) x 10-3 m

+ 37 hidden pages