PWS – Calibrations

The Davis Vantage PRO 2+, the hardware which forms the basis of the PWS De Wilgen is not a measurement device without flaws. As any measuring device – a thermometer, hygrometer etc… – it requires calibration and a calibration needs to be checked regularly. I use two types of calibration.

    1. Regular checks with the KNMI station Nieuw Beerta (17 km ESE of De Wilgen) and the METAR of airport Eelde and compare these with the general view of temperature and barometric pressure on the site. Obvious deviations would indicate errors of breakdown of devices. If the principle devices of temperature, humidity and barometer still measure but have a deviation, I use the Cumulus software to correct the readout before storage in the database and make a note of it.
    2. To actually formally calibrate the readouts, one needs a second device. For this, many devices are for sale in the range of € 50,- to € 1000,-. I used my own analogue devices for calibration but recently I started to notice larger deviations and anomalies. So I use a second measurement device (UT 330C) with known accuracy, make a measurement series parallel to the Davis and compare these to the actual Davis readouts. Then, using arithmetic and/or graphical techniques, I calculate a correctional offset and activate that in Cumulus. This device – off course – can break down as well and needs to be checked regularly itself. I recently bought a new one and will report on this blog on the new calibration after arrival.

Currently, the following corrections are valid. A plus means Cumulus adds this value to the measured value and thus creates the value the users sees and which is entered in the database, a minus is evidently the inverse. No regressive corrections are made in the database.

Current calibration values
Pressure : + 2 hPa
Temperature + 0.5 °C
Humidity – 6 %


No corrections are made for solar or wind, in case of doubt the sensor is replaced with a new one. Note that solar is extremely difficult to calibrate because of atmospheric interference and wavelength used and would require a significant scientific set up, with measurement devices and theoretical approach, which are beyond the scope of a hobby (price-wise).

Any changes to equipment will be reported here on the blog.

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