The concentrations in the classroom with displacement ventilation showed larger variations than these in the room with mixing ventilation. This was expected for short time variations, considering the smoothing effect of a mixing system, but less expected for daily or weekly means.In general the between day standard deviations were large, 25-50 %, similar to values reported at considerably lower concentration levels (e.g. Stridh et al. 2000). This large variability makes system performance characterizations and comparisons quite uncertain. Weekly means did not notably decrease the uncertainty of characterisation as their standard deviation was only slightly smaller (about 10 %).Photometer data was studied for the effect of the length of the measuring period for a one day concentration estimation. The relative standard deviations of the order of 40-60 % for 5 minute periods was about halved when using 60 minute periods.
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