SR25 pyranometer

Secondary standard pyranometer with sapphire outer dome

Back to overview

SR25 takes solar radiation measurement to the next level. Using a sapphire outer dome, it has negligible zero offsets. SR25 is heated in order to suppress dew and frost deposition, maintaining its high measurement accuracy. When heating SR25, the data availability and accuracy are higher than when ventilating traditional pyranometers. In addition, SR25 needs very low power; it only consumes 1.5 W compared to the usual 10 W for ventilation. The low thermal offsets make SR25 very suitable for measuring diffuse radiation. Patents on the SR25 working principle are pending. SR25 is available with analogue millivolt output and (as SR25-D2) with digital (Modbus over RS-485) and 4-20 mA output .

Best data availability

By keeping the SR25 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).

Best measurement accuracy

SR25 measures the solar radiation received by a plane surface, in W/m2, from a 180o field of view angle. SR25 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.

SR25 pyranometer design

SR25 has a sapphire outer dome, glass inner dome and an internal heater. It employs a state-of-the-art thermopile sensor with black coated surface and an anodised aluminium body. The connector, desiccant holder and sun screen fixation are very robust and designed for long term use. SR25 is delivered in a solid HPRC case. See the image gallery for a visual representation.

Reference for diffuse radiation

SR25 is an excellent reference for diffuse radiation. SR25’s excellent performance was recently confirmed in an independent test* at NREL: Ibrahim Reda, A. Andreas, 2017, Technical Report NREL/TP-1900-68999,  Calibration procedure of Hukseflux SR25 to Establish the Diffuse Reference for the Outdoor Broadband Radiometer Calibration. In this calibration experiment, the NREL lab attributed a 0.5 W/m2 zero offset to the unheated SR25, and was able to determine its sensitivity with an accuracy of around 0.9 %.  Using this SR25 to measure Diffuse Horizontal Irradiance, NREL estimates the contribution of the SR25 to the uncertainty of the Global Horizontal Irradiance as ± (0.09 % + 0.5 W/m2).  * NOTE: the fact that a sensor is used or tested does not constitute a formal endorsement by the user or test institute.

Suggested use

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


  • longer cable, in multiples of 5 metres

See also

Take a look at 'SR25: The making of' for information on the development of SR25.
Take a look at our application note on diffuse solar radiation reference sensor SR25.


Similar products

SR25-D2 pyranometer

Digital secondary standard pyranometer with sapphire outer dome

SR20 pyranometer

Secondary standard pyranometer
SR30-D1 ventilated and heated secondary standard digital pyranometer

SR30 pyranometer

Next level digital secondary standard pyranometer

Are you interested in one of our products?

Are you interested in using one of our products and do you still have some questions open?

Or would you like to get a quote?

Get a quote