VU01 ventilation unit

VU01 ventilation unit
Ventilation unit for SR20, SR20-D2, SR22, IR20 & IR20WS

VU01 is a high-quality ventilation unit for use with pyranometers and pyrgeometers. Its purpose is to improve the dependability of the measurement. Measurement accuracy improves because offsets are reduced. Reliability benefits from prevention of dew and frost formation and quick evaporation and sublimation of water and snow. ISO/TR 9901 “Solar Energy - Field Pyranometers - Recommended practice for use” recommends use of ventilators where high accuracy and reliability are required. VU01 is designed in agreement with the recommendations of ISO/TR 9901 and BSRN.

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Compatible instruments
  • SR20, SR20-D1, SR20-D2, SR22, SR25, SR25-D2, IR20, IR20WS
Compliance with standards
  • ISO/TR 9901, BSRN operations manual
Rated operating power
  • 7.8 W at 12 VDC (unheated)
Rated operating voltage range
  • 10.8 to 13.2 V
Rated operating temperature range
  • -40 to +70 °C
Fan status signal
  • high / low output (optional use)
  • 5 and 10 W at 12 VDC (optional use)
Increase of air temperature
  • 0.5 °C at 0 W heating, 1.0 °C at 5 W heating, 1.5 °C at 10 W heating
Zero offset a reduction SR20
  • 50 % at 0 W heating
Offset SR20 by heating
  • -2 W/m² at 5 W heating and -4 W/m² at 10 W heating
Offset IR20 by heating
  • 0 W/m² at 5 W heating and 0 W/m² at 10 W heating
Cable resistance
  • 0.3 Ω/m (2 x 0.15 Ω/m)
Voltage drop at 10 W heating power
  • 0.12 V/m
Footprint diameter
  • 0.17 m
Standard cable length
  • 5 m (see options)
  • longer cable, in multiples of 5 metres. Consult Hukseflux.
  • pack of 5 dust filters (VU01F)
Manuals & Downloads


VU01 ventilation unit with SR20 pyranometer (cut out image)


VU01 may be used in combination with Hukseflux’ SR20, SR20-D1, SR20-D2, SR22, SR25 and SR25-D2 secondary standard pyranometers and IR20 and IR20WS research grade pyrgeometers. VU01 provides an airflow over the instrument body and dome and has two heaters on board to heat the air. The use of heating (5 W or 10 W) is optional. The VU01 ventilation unit runs on 12 VDC voltage.

VU01 ventilation unit for pyranometers

Improved measurement dependability

Ventilation promotes thermal equilibrium between all components of radiometers, and thereby reduces zero offsets. Dew and frost formation is prevented. Additional heating will promote evaporation of water droplets and sublimation of snow. It should be noted that heating in combination with pyranometers will lead to increased thermal offsets, so heating is only recommended if necessary.

VU01 ventilation unit for pyranometers


Using VU01 is easy. The footprint of VU01 is small, which prevents accumulation of snow. The ventilator power is relatively high in order to increase the airflow. The ventilator is generally used continuously; the heater is typically controlled by the datalogger. Ventilation is typically used with high accuracy measurement requirements. In such cases the need for a high level of instrument maintenance and dome cleaning still exists; evaporating droplets of water might leave stains on the instrument domes. The bottom of VU01 provides access to the removable dust filter. For more information on VU01 please download the product brochure or ask for the user manual.

Suggested use

  • PV system performance monitoring
  • scientific meteorological observations
  • extreme climates (tropical / polar)

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.

VU01 ventilation unit
VU01 ventilation unit
  • VU01 provides an airflow over the instrument body and dome and has two heaters on board to heat the air.
  • VU01 is designed, using high-quality materials, in agreement with the recommendations of ISO/TR 9901 and BSRN.
  • The footprint of VU01 is small, which prevents accumulation of snow
  • Dew and frost formation is prevented.
Would you like a personalised quote?

or contact us: