Zero/Span
Part of the QAQC process for many of the continuous analyzers is the daily zero/span cycle. Each day, usually in the early hours of the morning, the normal sample port is closed to the analyzer and, for about twenty minutes, zero air is fed to the analyzer. Zero air is air from a cylinder or air that has been filtered so that it contains almost no pollutant. The analyzer should read zero. If the analyzer does not read zero, then the data is adjusted by a corresponding amount.
This common procedure is known as baseline correction. After the zero air comes the span gas. Span gas is air with a known high concentration of pollutant. The analyzer should output a reading that is within ten percent of the known concentration. If it does not, then the analyzer must be calibrated. After the zero and span gases have been passed through the analyzer, the sample port is again opened. About ten minutes is allowed for the analyzer to recover before readings are accepted again as real.
http://wbea.org/content/view/24/63/#zero

Span-Gas - A gas of known concentration that is used routinely to calibrate the output level of an analyzer. See “Calibration Check Standard”.
http://www.env.gov.bc.ca/wsd/data_searches/field_sampling_ma...
Suppression ratio: The ratio of the lower-range value to the span. If range is 20-100 and therefore span is 80 and LRV is 20, the suppression ratio is 20/80 = 0.25 or 25%.
http://www.omega.com/literature/transactions/volume3/glossar...
In realitate, insa, pentru etalonarea analizoarelor se folosesc butelii etalon cu o concentratie cunoscuta de gaz. Pentru gazele de ardere, amestecurile de gaze etalon cele mai bune au o eroare relativa a concentratiei etalon de gaz de 1%.
http://www.emisii.ro/Erori monitorizare emisii, recomandari....