SR25-D2 pyranometer

SR25-D2 pyranometer, digital secondary standard pyranometers with sapphire outer dome by Hukseflux
SR25-D2 pyranometer, digital secondary standard pyranometers with sapphire outer dome by Hukseflux
SR25 pyranometer with sapphire outer dome
SR25 next level pyranometer for diffuse measurement
Digital secondary standard pyranometer with sapphire outer dome

SR25-D2 represents the next level in solar radiation measurement. Using a sapphire outer dome, it has negligible zero offsets. SR25-D2 is heated in order to suppress dew and frost deposition, maintaining its high measurement accuracy. When heating SR25-D2, the data availability and accuracy are higher than when ventilating traditional pyranometers. In addition, SR25-D2 needs very low power; heating only consumes 1.5 W compared to the usual 10 W for ventilation. The low thermal offsets make SR25-D2 very suitable for measuring diffuse radiation. Patents on the SR25-D2 working principle are pending.

Specifications
Measurand
  • hemispherical solar radiation
ISO 9060:2018 classification
  • class A (secondary standard)
IEC 61724-1:2017 compliance
  • class B
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Specifications

Measurand
  • hemispherical solar radiation
Application
  • National Meteorological Networks
  • PV monitoring commercial and utility scale
  • meteorology / climatology
  • other
Heater
  • yes
ISO 9060:2018 classification
  • class A (secondary standard)
IEC 61724-1:2017 compliance
  • class B
Output
  • 4-20 mA current loop
  • Modbus RS-485
Zero offset a
  • 1 W/m² unventilated
Calibration traceability
  • to WRR
Calibration registers
  • accessible to users
Spectral range
  • 285 to 3000 x 10⁻⁹ m
Rated operating temperature range
  • -40 to +80 °C
Temperature response
  • <± 0.4 % (-30 to +50 °C)
Temperature response test report of individual instrument
  • included
Directional response test report of individual instrument
  • included
Standard cable length
  • 5 m
Backwards compatibility
  • SR25-D1
Digital output
  • :
Output
  • irradiance in W/m² and instrument body temperature in °C
Communication protocol
  • Modbus / over 2-wire RS-485
Transmission mode
  • RTU
Rated operating voltage range
  • 5 to 30 VDC
Power consumption (sensor)
  • < 75 x 10⁻³ W at 12 VDC
Heater
  • 1.5 W at 12 VDC
4-20 mA output
  • :
Output
  • irradiance in W/m²
Transmitted range of 4-20 mA output
  • irradiance in W/m²
Rated operating voltage range of 4-20 mA output
  • 5.5 to 40 VDC
Power consumption
  • :
main supply
  • < 75 x 10⁻³ W at 12 VDC
4-20 mA current loop
  • < 240 x 10⁻³ W at 12 VDC
Heater
  • 1.5 W at 12 VDC
Options

• longer cable, in multiples of 5 metres

Manuals & Downloads
SR25-D2 pyranometer brochure
SR25: The making of

Description

SR25 pyranometer, spectrally flat class A pyranometers with sapphire outer dome

Best data availability

By keeping the SR25-D2 outer dome free of dew and frost with help of the internal heater, data availability is highly increased over traditional pyranometers, whether these are ventilated or not. The application photo in the image gallery shows the clear difference between SR25 (on the left side of the photo) versus a non-heated pyranometer without sapphire dome (on the right).

sapphire outer dome on SR25 pyranometer

Best measurement accuracy

SR25-D2 measures the solar radiation received by a plane surface, in W/m², from a 180 ° field of view angle. SR25-D2 offers the best measurement accuracy: the specification limits of two major sources of measurement uncertainty have been greatly improved over competing pyranometers: “zero offset a” and temperature response. Test certificates for temperature response and directional response are included: all sensors are tested individually for ISO 9060 compliance.

Suggested use

• all situations where ventilated pyranometers are employed
• all networks with regular instrument exchange
• PV system performance monitoring
• indoor PV testing with solar simulators
• airborne measurements
• diffuse measurements
• environments with dew
• environments with frost

Frequently asked questions

How does a pyranometer work?

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. 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).

•    a second (inner) glass dome: For secondary standard and first class pyranometers, two domes are used, and not one single dome. This construction provides an additional "radiation shield", resulting in a better thermal equilibrium between the sensor and inner dome, compared to using a single dome. The effect of having a second dome is a strong reduction of instrument offsets.

•    a heater: in order to reduce the effect of dew deposition and frost on the outer dome surface, most advanced pyranometers have a built-in heater. The heater is coupled to the sensor body. Heating a pyranometer can generate additional irradiance offset signals, therefore it is recommended to activate the heater only during night-time. Combining a heater with external ventilation makes these heating offsets very low.

Why use a pyranometer?

There are good reasons why pyranometers are the standard for solar radiation measurement in outdoor PV system performance monitoring. 

The purpose of outdoor PV testing is to compare the available resource to system output and thus to determine efficiency. The efficiency estimate serves as an indication of overall performance and stability. It also serves as a reference for remote diagnostics and need for servicing.

The irradiance measurement for outdoor PV performance monitoring is usually carried out with pyranometers. Some standards suggest using PV reference cells. Reference cells are (with some minor exceptions) unsuitable for proof in bankability and in proof of PV system efficiency. Pyranometers are and will remain the standard for outdoor solar energy monitoring.

From a fundamental point of view:

  • Pyranometers measure truly available solar irradiance (so the amount of available resource). This is the parameter you need to have for a true efficiency calculation.
  • Reference cells measure only that part of solar radiation that can be used by cells of identical material and identical packaging (flat window), so the yield of a certain PV cell type. This is not a measurement that can be used in an efficiency calculation and in fact leads to several percentage points error in efficiency estimates.

The International Energy Agency (IEA) and ASTM standards for PV monitoring recommend pyranometers for outdoor PV monitoring. PV reference cells do not meet IEC 61724-1 class A requirements for irradiance measurement uncertainty: their directional response makes them systematically overestimate daily radiant exposure in J/m2 (or W·hr/m2 ) by more than 2 %, larger on hourly basis. 

How do I choose a pyranometer?

Choosing the right pyranometer for your application is not an easy task. We can offer assistance. But first, you should ask yourself the following questions:

  • are there standards for my application?
  • what level of accuracy do I need?
  • what will be the instrument maintenance level?
  • what are the interfacing possibilities?

When discussing with Hukseflux, our recommendation for the best suited pyranometer will be based on:

  • recommended pyranometer class
  • recommended maintenance level
  • estimate of the measurement accuracy
  • recommended calibration policy
  • recommended interface

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). From second class to first class and from first class to secondary standard, the achievable accuracy improves by a factor 2.

The ISO 9060 - 1990 standard is up for revision. The new 2018 version of the standard will be slightly different from the 1990 version. The new version of ISO 9060 includes three instrument accuracy classes A, B and C, and a special extension of every class “Spectrally Flat”, which is recommended for Plane of Array (POA), albedo, and reflected solar measurements.

Our pyranometer selection guide offers practical guidelines for choosing a pyranometer. The application of pyranometers in PV system performance monitoring according to IEC 61724-1 is highlighted as an example. Sensors specific for diffuse radiation and meteorological networks are also addressed in this selection guide.

What is the difference between a pyrheliometer and a pyranometer?

A pyranometer measures hemispherical solar radiation. When measuring in the horizontal plane this is called Global Horizontal Irradiance (GHI). When measuring in “plane of array”, next to PV panels, this is called plane of array POA irradiance.

A pyrheliometer is used to measure Direct Normal Irradiance (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. DNI is composed of the solar irradiance within the extent of the solar disk (half-angle 0.266 ° ± 1.7 %) plus some circumsolar radiation.

SR25-D2 pyranometer
SR25-D2 pyranometer, digital secondary standard pyranometers with sapphire outer dome by Hukseflux
  • With sapphire outer dome
  • Best data availability
  • Best measurement accuracy
  • Industry standard outputs allow for easy data acquisition, easy read-out and error-free instrument exchange.
Would you like a personalised quote?

or contact us: info@hukseflux.com