Delta-T SPN1 User Manual

Also see for SPN1: Technical note

Delta-T SPN1 User Manual Manual pdf 20 page image

Accuracy and errorsOverall accuracy limits are given in the specifications, which give theexpected performance in normal use. There are some specificcircumstances which may show unexpected results.Spectral responseBecause the SPN1 spectral response runs from 400nm upwards, it missesout some of the blue end of the solar spectrum, and this may show anunder-reading of the diffuse component under very clear blue skies, or athigh altitudes.Cosine responseIn relative terms, the cosine response error increases when the sun isclose to the horizon. This may appear as an overall sensitivity error inclear-sky conditions when the sun is very low. This is true for all cosine-corrected sensors.Errors due to inaccurate levelling also show up in these conditions – whilea 0.5° levelling error has very little effect when the sun is high in the sky,or under overcast conditions, it can give a 5% output error when the sun isbright and 10° above the horizon.OffsetsMost thermopile pyranometers show a negative output during the night,due to radiative cooling of the earth into space. The construction of theSPN1 includes three separating elements between the atmosphere andthe thermopiles, so this effect is minimal. The electronics within the SPN1will only measure and output positive signals, so the output should nevergo below zero. In general, there will be a small positive output (<3W.m-2)in dark conditions, due to the effects of noise in the system.The SPN1 is sensitive to fast changes in temperature, and these willcreate a positive error on cooling, or a negative error on warming. Thismay be visible if you move the sensor from a warm room into a coldatmosphere outside, until the sensor reaches ambient temperature.Thermopile matchingThe SPN1 outputs are based on 7 individual sensor readings. Thesensors are closely matched at calibration, but will never be exactlyidentical in all respects. These small variations will sometimes show up assmall steps in the output time series, as the shadowmask shades orexposes different sensors.20 z Accuracy and errors SPN1 User Manual v1.0