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This product has limited availability. It will be replaced by: SR30-L. Some accessories, replacement parts, or services may still be available when it is no longer available.
VU01-L Ventilation Unit for SR20-T2-L and SR20-D2-L


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

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Benefits and Features

  • Reduces zero offsets by promoting thermal equilibrium of all radiometer components
  • Prevents dew and frost formation
  • Optional heating promotes evaporation of water droplets and sublimation of snow
  • Removable dust filter
  • Meets recommendations of ISO/TR 9901 for required high accuracy and reliability


VU01-L Ventilation Unit
VU01-L Ventilation Unit
VU01-L Ventilation Unit
VU01-L Ventilation Unit, cross-section view
VU01-L Ventilation Unit
VU01-L Ventilation Unit
31394 mounting kit (sold separately)
31394 mounting kit and pipe (sold separately)
31394 mounting kit and pipe(sold separately)

Detailed Description

The VU01 uses a 7.8 W fan and inlet filter to draw clean air over the pyranometer’s outer dome. The fan should operate continuously to reduce dust and dirt settling, to dissipate raindrops, and to stabilize the dome temperature.

The VU01 has dual 5 W heaters, which are used to raise the temperature of the dome slightly above ambient temperature and prevent the formation of dew and frost. Relays are recommended to control and source the appropriate power to the ventilator and heaters separately.

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 the 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.


Using the VU01 is easy. The footprint of the VU01 is small, which prevents accumulation of snow. The ventilator power is relatively high to increase the airflow. The ventilator is generally used continuously; the heater is typically controlled by the data logger. 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 the VU01 provides access to the removable dust filter.

For more information on the VU01, download the manufacturer's product brochure or ask for the user manual.


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)
Heater 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
  • -4 W/m2 at 10 W heating
Offset IR20 by Heating
  • 0 W/m2 at 5 W heating
  • 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 (6.69 in.)
Maximum Cable Length 5 m (17 ft)
  • 5 and 10 W (at 12 Vdc for heater)
  • 7.8 W (at 12 Vdc for vent)


Note: The following shows notable compatibility information. It is not a comprehensive list of all compatible or incompatible products.

Data Loggers

Product Compatible Note
CR1000 (retired)
CR200X (retired)
CR206X (retired)
CR211X (retired)
CR216X (retired)
CR3000 (retired)
CR5000 (retired)
CR9000X (retired)


Product Compatible Note

Mounting Equipment

Product Compatible Note
CM256 -VN option
CM261 -VM option

Additional Compatibility Information

The VU01 may be used in combination with Hukseflux’s SR20, SR20-D1, SR20-D2, SR22, SR25, and SR25-D2 secondary standard pyranometers, as well as with IR20 and IR20WS research-grade pyrgeometers. The 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.

Mounting Considerations

The VU01 mounting stand can be used to fasten the VU01 to a vertical or horizontal crossarm. The VU01 is not compatible with the CM245 or CVF4 mounting stands.

Power Considerations

The 10 W of heater current drain is approximately 833 mA at 12 Vdc, and the 7.8 W ventilator’s current drain is approximately 650 mA at 12 Vdc. These power requirements are large compared to many Campbell Scientific products. Because of this, the VU01, depending on total system configuration, may not be powered by the data logger alone and should be connected to a 12 V power supply such as the CH150/200 and controlled using the Crydom relays.

The Crydom relay allows the heater's power to be controlled by the data logger program, thus reducing power consumption. For example, the data logger program can turn on a heater only when the irradiance level falls below 20 W/m2 or, if a measurement of air humidity is available, the dew point can be calculated and the relay can be toggled if the air falls to within 1ºC of the sensor body temperature.

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