Davis VP2 stations use two separate sensors to measure sunshine intensity. The ‘solar’ sensor (part 6450) measures the visible and near IR part of the spectrum while the UV sensor (6490) aims to measure the intensity of the ultraviolet part of the sun’s spectrum.
Both solar and UV sensors are supplied factory fitted as an integral part of the Plus versions of the VP2 stations such as the 6162 model, which provide a very significant price discount relative to buying these sensors separately at a later date. It is possible to buy ISS units (though not complete stations) pre-fitted with just the solar sensor for use with eg Connect and Enviromonitor systems.
Adding either or both of the solar and UV sensors to an existing VP2 ISS requires additional purchase of the 6673 ‘Mounting Shelf’, which provides a fixing point for the sensors on the upper part of the ISS and also allows for levelling of the sensors post installation. While the 6673 shelf/bracket is not strictly essential, in practice it is the simplest mounting solution unless one is a skilled handyman.
If necessary (and it often is, for optimum siting), the solar and/or UV sensors can be mounted away from the ISS by using a Davis extension cable, with due attention to waterproofing any cable joint. An extension cable will have a minimal effect on the sensor reading and for this reason it’s desirable to keep the extension cable as short as possible. If a 2.4m (8ft) cable will do then use that; otherwise the 12m (40ft) cable is the maximum recommended length. Mounting the sensors away from the ISS will require making some type of suitable mounting bracket, though it’s probably still simplest to use the 6673 shelf/bracket and then fashion a secondary bracket to attach the 6673 to eg a pole.
The 6450 solar sensor is a radiometric sensor measuring the energy of the sunshine in watts/square metre across the wavelengths to which it is sensitive. (It differs subtly therefore from photometric sensors which measure light intensity in lux or lumens/sqm in a way which is weighted according to the spectral sensitivity of the human eye. And it is different again from PAR sensors, where the weighting is by the average photosynthetic activity of plants at various spectral wavelengths.)
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UV readings are also expressed in a different format from the ‘solar’ sensor readings: For the solar sensor, straightforward irradiance values in units of W/sqm are recorded. But the prime practical reason for monitoring UV levels is to guide human skin exposure, preventing sunburn and possible skin cancer risks.Therefore UV readings are expressed on a UV Index scale of 0-16 which weights the intensity by the wavelengths of the UV light most likely to cause skin damage. This seems to be explained reasonably well on the relevant Wikipedia page, where one index unit corresponds to an irradiance value of about 25W/sqm. Maximum midsummer UVI readings in the UK rarely exceed 7-8,although substantially higher readings are commonly registered at lower latitudes than the UK.
Davis Application Note #6 also provides further information on interpreting UV readings.
UV readings are technically difficult to make – the sensors need extensive individual calibration and the signal is low in amplitude hence causing noisy readings. This has two consequences. First, UV sensors are expensive because of the prolonged production and calibration process. (You may think that the Davis UV sensor is expensive, but sensitive high-end UV sensors are considerably more so.)
The second consequence is that, without moving to a still more sophisticated and costly sensor design, there is unavoidably some noise and short-term fluctuation in the sensor output, even under dark conditions when of course there should be zero UV readings. When the UV sensor was first introduced, Davis was troubled by support calls reporting overnight UV readings of 0.1 or 0.2 UVI, even occasionally higher, which were simply a consequence of noise in the dark current circuitry. The consensus was that these very low but false readings overnight were more of a concern to users than having good sensitivity at low UV levels. So the decision was taken that there would be a threshold in the displayed UV reading of 0.4 and below which any reading would show as zero and this is the solution that remains in place.
The result is of course that wintertime readings in higher latitudes such as the UK will typically show as zero unless the UVI value is above 0.3.
Is my UV sensor working?
In practice, it’s a common experience that if you install a UV sensor in wintertime then you may not expect to see a significant UV reading until the sun starts to regain greater power, eg during March. Wintertime UV levels even on sunny days can be surprisingly low and therefore it’s very likely if you’re concerned about a zero UV reading on a new station that the reading genuinely is <0.4 UVI. Live UV readings for several places in the UK are available on a page on the DEFRA website.