Solar / UV Interface

It is important to measure solar radiation because this is the primary energy source for many biophysical processes on the Earth, but this is more difficult than measuring temperature or rainfall, for example. However, it is possible to make reasonably accurate measurements of solar radiation with relatively simple equipment, provided care is taken with the design, siting and calibration of the sensor. The WeatherDuino system is well-suited to making routine measurements of solar radiation at low cost and it also offers a platform for experimenting with more advanced solar sensors.

In common with many other weather stations, the WeatherDuino system uses a silicon sensor to measure the Sun's radiant energy rather than a much more expensive thermopile. This means that not all the Sun's energy is measured, as silicon devices are only sensitive over the wavelength range 0.4 -1.1 micrometres (approx), whereas the energy from the Sun reaching the Earth's surface is distributed over 0.3 - 3.0 micrometres (approx). Precise intercalibration between a silicon solar sensor and a thermopile is never going to be possible because of this discrepancy, but fortunately the amount of solar energy is the 'missing' part of the electromagnetic spectrum is relatively small, especially in areas with a lot of water vapour in the atmosphere.

On the plus side, it is worth noting that a silicon solar sensor is uniquely suited to measuring the solar energy available to a PV solar power generation system, so for many practical purposes the approach adopted by the WeatherDuino project has much to commend it.

The Solar Radiation / UV interface can be used with commercially available silicon photodiode pyranometers, provided their output voltage does not exceed 1.1V, for example the Apogee SP-110 Pyranometer. Although much cheaper than a professional thermopile pyranometer, commercial silicon photodiode pyranometers are still expensive, probably costing far more than the rest of the weather station combined. For this reason the WeatherDuino project is primarily designed to use a simple solar sensor based on a design by Charles G. Wright which uses a silicon photovoltaic cell, see Measuring Solar Radiation.

Despite its simplicity, a solar radiation sensor based on a photovoltaic panel works well, and its readings are comparable to those from expensive solicon photodiode pyranometers.

For UV index, we have chosen to use the UVM-30A UV sensor.

Usually the Solar Radiation / UV interface is supplied as a kit that includes the fully assembled interface, plus a tested miniature photovoltaic panel. The op amp gain is pre-calibrated for the supplied photovoltaic panel, allowing solar radiation readings up to 1500 W/m² .
Optionally it can be supplied without the photovoltaic panel.
The UVM-30A UV sensor for UV Index isn't included. This sensor is easy to find, costs around 5€, and can be bought from several sellers on eBay or other sources.

The easiest way to measure solar radiation data with your weather station is by using the Solar / UV interface with a ready made pyranometer, such as the Apogee SP-110.
For those who like a challenge, the use of the photovoltaic panel is a cheap alternative to a commercial pyranometer.
In the WeatherDuino support forum there is an extensive article on this topic which we strongly recommend you read: Measuring Solar Radiation based on Photovoltaic Cells.

Example of a commercial pyranometer that uses a miniature photovoltaic panel.

The Apogee SP-110 Self-Powered Pyranometer.

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