A trip to the Swiss village where the world calibrates its solar monitoring instruments

What does this international comparison of pyrheliometers look like? Who participates ?

According to their website, the IPC is open to all WMO-affiliated or WMO-related institutions, such as national hydrometeorological services. Manufacturers of radiometric equipment like us from OTT HydroMet, calibration and research institutes and other stakeholders can also participate. Everyone can calibrate their absolute cavity radiometer every five years at the IPC, directly on this world standard. The result is a correction factor for your pyrheliometer reading, called the WRR factor. This number is generally very close to unity (=1); for example 0.9989 or more. As OTT HydroMet, we participated with our absolute cavity pyrheliometer PMO-6.

Does this mean that this reference changes from event to event?

Yes. The WRR comes from the World Standard Group, which is a group of measuring instruments, not an absolute standard kept in a laboratory. These instruments could also drift, which would mean that the global standard would drift. So, part of the purpose of the event is to check (using all participants’ instruments) if the WRR has been stable.

What impact does our participation have on our business as a manufacturer of solar irradiance monitoring instruments?

We are not only a manufacturer but also an accredited calibration laboratory. Obviously, we try to calibrate as precisely as possible. Therefore, we calibrate our reference instruments using our absolute cavity pyrheliometer. We need a valid and recent calibration of this instrument, so we go to Davos every five years to get an updated WRR factor for our instrument.

Additionally, we attended the symposium (held on cloudy or rainy days) and participated in discussions on standards, accuracy, instrument intercomparisons and metrology. Furthermore, we had the chance to visit the institute’s research laboratories and test facilities and had fruitful discussions with Swiss radiation scientists on radiation measurements not only in the visible but also in the UV and infrared rays.