Overview
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 shade-free 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 very slight 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 using or making some type of suitable mounting bracket, For which there are two options:
- Buy one or two 6670 Universal Mounting brackets. Each bracket will allow one sensor to be mounted to a pole;
- Continue to use the 6673 shelf/bracket and then fashion a DIY secondary bracket to attach the 6673 to eg a pole or post.
Solar sensor
The Davis solar sensor is a relatively simple sensor and rarely gives trouble provided the sensor is cleaned from time to time. If the readings are noisy or unexpectedly erratic then it’s worth checking the cable and especially the plug and socket joint where the cable plugs into the SIM board inside the ISS. It may be that one of the wires is not making good contact. If this fails to cure the issue then it may be that the sensor itself or the input circuitry on the SIM board are becoming faulty, especially if the parts are relatively old (eg 10 years plus). In this case, replacement of the faulty part is likely to be the only answer.
Understanding data from the 6450 solar sensor
The 6450 solar sensor is a radiometric sensor measuring the intensity of the sunshine in watts/square metre across the wavelengths to which it is sensitive. The watts/sqm unit indicates the energy being received per second on every horizontal square metre of ground at the station location. (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.)
Note that the solar sensor measures total or global solar irradiance, that is sunlight coming from the whole sky. This is made up of direct sunlight due to the sun shining directly on the sensor plus indirect sunlight from sunlight scattered by clouds and other particles in the atmosphere. On a clear day then a high proportion of the total irradiance will be due to direct sunlight; but, conversely, on a heavily overcast day all of the sunlight will be indirect.
The Davis Weatherlink software provides two separate records of sunshine – ‘solar radiation’ and ‘solar energy’. Solar radiation is what the sensor actually measures and is the total sunshine intensity at the moment of taking the reading. Obviously this reading can vary from moment to moment with clouds or haze in the sky. It will of course also depend on the time of day and the time of year.
Solar energy (sometimes expressed as total solar energy) is an estimate of the total solar energy received aggregated across some period of time. For weather records, this period will typically be the complete day. And so a high value of energy in a day will reflect a day when the sun was shining brightly for much of the day. Obviously, away from the equator, total energy values are going to be much higher in summer than in winter due to the longer day length and the higher elevation of the sun in the sky..
Davis unfortunately chose a rather obscure unit – the Langley – to record solar energy in Weatherlink. So 1 Langley = 11.622 Watt-hours per square meter, or in other words 10 Langleys would represent the total energy from weak sunshine at 116.2 W/sqm for 1 hour. Personally, I think it makes much more sense to express solar energy in Wh/sqm or kWh/sqm, because the values are then much more immediately comparable with the spot readings of solar radiation intensity.
In the past, the only way of recording sunshine energy for a day was as sunshine hours using a Campbell-Stokes recorder. There is no accurate conversion possible between sunshine hours and solar energy as measured in kWh/sqm – these are two quite different concepts. That said, it is possible to attempt a rough conversion but be aware that this is a complicated topic with no easy answers. Weatherlink does offer a very crude method of estimating sunshine hours, but in my opinion this method or algorithm is so crude as to be largely meaningless. There are more sophisticated conversion algorithms available that are implemented in software like CumulusMX and Weather Display and which have much better accuracy than Weatherlink for this sunshine hours parameter, but ultimately sunshine hours and solar energy are two distinct concepts with no direct equivalence.
UV Sensor
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.
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