Why can the two numbers be so different?
Complete fictional example: 480 valid one-minute means, one at 12 ppm and 479 at zero. Highest minute: 12 ppm. Eight-hour mean: 0.025 ppm. Both bars use the same scale. This is not a real instrument peak or a safety assessment.
One elevated minute. A much lower eight-hour mean.
Fictional complete data: one minute at 12 ppm, 479 minutes at zero.
Highest one-minute mean12 ppm
Mean across all 480 minutes0.025 ppm
0 ppm12 ppm
(12 ppm × 1 min + 0 ppm × 479 min) ÷ 480 min = 0.025 ppm
Same scale: the lower bar is 1/480 of the upper bar.
Interval means only. No instrument algorithm or acceptable exposure is established.