Particulate matter monitoring systems that employ tapered element oscillating microbalances (TEOM) technology are “gravimetric” instruments that draw (then heat) ambient air through a filter at a constant flow rate, continuously weighing the filter and calculating near real-time mass concentrations of particulate matter.
The TEOM monitor technique relies upon an exchangeable filter cartridge seated on the end of a hollow tapered tube. The wider end of the tube is fixed. As the air passes through the filter, particulate is deposited. The filtered air then passes through the tapered tube to a flow controller. The tapered tube with the filter on its end is maintained in oscillation in a clamped-free mode. The frequency of oscillation is dependent upon the physical characteristics of the tapered tube and the mass on its free end.
As particulate deposits land on the filter, the filter mass change is detected as a frequency change in the oscillation of the tube. The mass of the particulate matter is thus determined inertially, i.e. directly. When this mass change is combined with the flow rate through the system, the monitor yields an accurate measurement of the particulate concentration in real time. The major advantage of this method is that any changes in aerosol characteristics will not influence the accuracy of the mass measurement.