The purpose of the Model 111 is to supply pollutant-free air (zero air) from the ambient
air to allow for proper zeroing, and to provide clean diluent air for spaning ambient air
analyzers. The components to be removed are SO2 NO, NO2, CO and hydrocarbons.
There is no consensus as to what extent zero air should have water vapor removed. Since
many analyzers have longer response times if super dry air (dew point less than -30o
C) is
used for zero and span, and since water vapor is not a pollutant, the Model 111 does not
have a drying system. However, the dew point is reduced as a result of compression of
the ambient air.