Weather data is now (from March 2003) being collected by a Davis Vantage Pro wireless weather station. The accuracy seems to be very good, perhaps because the sensors are now placed in a much better, and at about 280m slightly higher, more exposed location. More about this system as time permits, for starters this page has a comparison of the two systems performance during May 2003, and this will be the link to notes on the VP.
Previously (1992-2003) data was only collected using a Davis Weather Monitor II. This was situated at a height of around 277 m, facing gently south. While the sensors seemed reliable and I have a high opinion of the Davis Weather Monitor II product I use - it's 10 years old and still going strong - the limitations cables provide means it's sensors exposure to the elements was not perfect and again I cannot make claims about accuracy of the data.
The WMII thermometer sensor was prone to slight over exposure in the afternoon on still sunny days from May-September, which could lead to higher maximum temperatures being recorded than in a less sheltered more representative place. A new sensor shield was installed in February 2002 in an effort to reduce this effect (*). At other times temperatures were I think quite close to what might be expected. The tipping bucket rain gauge was in an unorthodox site but again I think gave a fair indication of rainfall (probably under reading if anything), if regularly calibrated. The system did have wind and humidity measures but they either get rattled to bits by Dartmoor's winter storms (and I'm wobbly at height...) or destroyed by the often weeks of near saturated conditions.
Any data pre 1992 was collected using accurate thermometers sited on a north wall and later, from 1987, in a much better exposed Stevenson Screen.
* unfortunately it didn't much, the site was simply to sheltered...