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The particle size selective cyclone of the pDR-1200 provides the user with two important capabilities: a) to measure the particulate matter concentration of a specific aerodynamic size fraction, and b) to determine the mass median size of a particle population. These two applications will be discussed in what follows.For both these applications, a variable measured flow rate pump is required,such as the MIE model pDR-PU (for which a separate instruction manual is provided). 14.1 Size Fractionated Monitoring The pDR-1200 can be used to monitor a specific particle size fraction below a selectable cut off equivalent aerodynamic diameter. The particle size cut point can be selected by adjustment of the sampling flow rate.The higher this flow rate through the cyclone the smaller the cut off particle diameter. Figure 7 is a graph showing the dependence of the particle cut off size in micrometers as a function of the sampling flow rate in liters per minute. The cut off size is the particle aerodynamic diameter at which the collection efficiency of the cyclone is 50%, or conversely,the size at which the cyclone transmission is 50%. For example, to obtain a particle size cut off of 2.5 pm (i.e., PNL.$), the required sampling flow rate is 4 liters/minute. A that flow rate only particles smaller than(approximately) 2.5 µm are allowed to pass into the pDR-1200 sensing stage, to be monitored and then to be collected on the filter. As can be seen on Fig. 7, the lowest particle size cut for the GK 2.05 cyclone included with the pDR-1200 is about 1 µm, and the largest is about 12µm. For particle size classification outside this range,consult with MIE. 14.2 Particle Sizing The selectable particle size capability of the cyclone,in combination with the concentration measuring capability of the photometric system of the pDR-1200 permits the user to determine the mass median aerodynamic particle diameter of an aerosol, i.e., of the airborne particle population being sampled. One simple procedure to determine the median particle size is as follows (please refer to the graph of Fig. 7): • Remove cyclone from its black attachment cup and set cyclone aside • Start pump and sample aerosol at a flow rate between 2 and 4 liters/minute • Press ON key on pDR-1200 panel and after about one minute key NEXT and then ENTER • After an elapsed time (ET) of about one minute,read and note TWA concentration • Shut off pump 32