Every six minutes the switching valve alternates the sample flows between
the base and reference sample periods. During the base period, sample is
collected normally and the base mass concentration is determined. During
the reference period, the flow is diverted through a chilled filter to remove
and retain the non-volatile and volatile PM. Under normal operation, the
chiller is maintained at a temperature of 4° C. However, under ambient
conditions of high temperature and humidity, operating the chiller at 10° C
is recommended to prevent condensation in the chiller during instrument
operation.