Currently there is a lot of interest in the history of scientific instruments and their measurements. The pyranometer is still one of Kipp & Zonen’s core products today, but it dates back to the early years of the last century. Who better to ask than our senior scientist Leo van Wely to reflect on this subject? Leo just retired, but is more than happy to keep involved with his passion for 33 years.
Who built the first pyranometers?
This article is entitled “The Pyranometer – An Instrument for Measuring Sky Radiation” and is the first published mention of the name ‘pyranometer’. It describes the development, starting in 1912, of instruments to measure the whole sky radiation. The starting point was the already successful electrically compensated pyrheliometer of Professor K. Ångström, invented in 1893.
The sixth prototype by Abbot and Aldrich is described in the article and was the final ‘production’ instrument offered for sale. It used a glass dome and mechanically driven shadow band to measure the total (global) sun and sky radiation, and the diffuse sky radiation – from which the direct sun radiation could be calculated. By removing the glass dome the night-time sky, or terrestrial, radiation could be measured.
The principle works on the difference in temperatures caused by radiation falling on two blackened manganin strips with the same area but a thickness ratio 10:1, the thicker strip acting as a heat-sink. The temperatures are measured by two thermocouples and the difference voltage signal is shown by a sensitive mirror galvanometer. Electrical heating is then applied to balance out this signal to zero and the power required is proportional to the received radiation. This interesting construction self-compensates for the effects of changing instrument temperature and non-linearity.
Abbot and Aldrich succeeded with their design and the Smithsonian offered to make instruments available ‘at cost’ of $150 (about $15,000 today) to researchers. However, it was a product of their time. It was manually operated and the electrical power correction to null the signal could only be applied under steady state conditions, taking about one minute. There was also a problem that they described in “On the Use of the Pyranometer” as “drifting of zero”. Their solution was to read out the galvanometer at the end of the first swing. All this was acceptable for scientific work, but not for continuous monitoring.
Where does Kipp & Zonen fit in?
Since 1913, Kipp & Zonen had been manufacturing, for the measurement of thermal radiation and light, a fast and sensitive thermopile (made up of many thermocouples) designed by Dr. Moll of Utrecht University. However, it was not until a meteorological conference at Utrecht in 1923 that Dr. Moll drew the attention of some scientist to this thermopile. Professor L. Gorczynski of the Polish Meteorological Institute decided to construct a pyrheliometer and a pyranometer using modified Moll-Gorczynski thermopiles.
Because of the thermopile characteristics the instruments could be small, light, low cost, have a continuous voltage output signal, and not require any external electrical power or control systems. These prototypes proved to be successful and Kipp & Zonen became the manufacturer of the instruments in 1924.
Where are they now?
The ideas of Ångström, Abbot and Aldrich are not completely forgotten. There are a number of instruments on the market that use a rotating shadow band for global and diffuse radiation measurements in one unit, and electrically compensated Absolute Cavity Radiometers are still the most accurate pyrheliometers available for scientific use.
About Leo van Wely
I was first contacted by Kipp & Zonen in 1979 when they suspected a problem with the domes of their CM 5 pyranometer. I was a physics engineer with the Optics Group of Delft Technical University, working on the construction of a tunable pulsed dye laser, and they thought that I might be able to find out what was wrong with the pyranometer domes.
In fact domes, detectors and optics kept me busy for the next 32 years! I’ve always been very passionate about my work and it has always been a pleasure for me at Kipp & Zonen. Although I retired at the end of 2011, my interest in solar radiation measurements continues.