Aerosol Instrumentation (Inertia-Based)

:: Section 6

Time-of-Flight

Time-of-flight instruments can provide real-time high-resolution measurement of aerodynamic particle size and size distribution over a wide size range. As shown in the following animation, air is accelerated in a converging nozzle to a high velocity (>100 m/s) at the nozzle exit. Two narrowly focused laser beams are positioned in the jet about 100 μm apart. Small particles can keep up with the accelerating air in the nozzle and exit with approximately the same velocity as the air. As a particle passes through a laser beam, a brief pulse of scattered light is detected by a photomultiplier tube. The time interval between the two pulses is used to determine the average velocity of the particle. The larger or heavier the particle, the lower its velocity in the timing zone. With suitable calibration, the particle's aerodynamic diameter can be determined based on the time it travels through the beams.

Figure 4. Schematic diagram of time-of-flight instrument