Most national and international standards require averaging of measurements over a 6-minute time frame. The regulatory limit must be complied with in any of those averages, i.e. over any 6-minute period. That is why it is important for continuous measurement systems to provide averages uninterruptedly over 6 minutes, applying the rule of “new sample – new average”, in other words each time a new sample enters the window, the oldest sample leaves, and a new average is calculated. Where the sampling frequency is 1 second, for example, a new average must be calculated every second; the sliding window moves one second at a time. It is clearly seen on the graph that it is not the same to measure with a fixed 6-min window than to use a sliding window. In that example, an intermediate window from 2 to 2 will exceed the values of both fixed windows from 0 to 6.
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