angle scatter

90° scattering

angle scatter

collects 30-170°

collects 15-150°

droplets passing through the laser beam scatter light that is focused on a ring diode array detector. Each detector is radially optimized for a particular band size. The output signal is digitized and converted into a particle size distribution by means of the integral computer. Depending on the configuration of the instrument, the size range that can be measured is 0.5-300 mm for dry powders and 0.5-1800 mm for sprays. Results are presented in 32 size classes. The instrument can be arranged to synchronize the spray and data collection. This has been specifically developed to analyze metered-dose inhalers. The data are expressed in terms of volume percentiles. However, other statistical diameters are calculated. The data are manipulated so that many of the parameters that are of interest are automatically available to the investigator.

Laser Doppler Velocimetry

Jet Acceleration

Laser Doppler velocimetry may be used in a decelerating flow where the aerosol particles are traveling at low speed [129]. However, accelerating the particles through a jet or nozzle seems the most popular method among instrument manufacturers. The principle of using a nozzle to accelerate particles and laser Doppler velocimetry to measure the particle velocity near the nozzle exit is seen as a rapid method for determining the aerodynamic diameter [130,131]. This technique eliminates the time-consuming analysis required to obtain the same parameter by cascade impactor or impinger.

Figure 18 Principle of operation of a Fraunhofer diffraction laser particle sizer. (With permission of Malvern Instruments, Inc.)

The TSI Model APS34 powder-sizing system, shown in Fig. 19, measures powders dispersed in air and measures aerodynamic diameter. It is combination of the TSI Model 3433 disperser and Model APS33B aerodynamic particle sizer. The Model APS33B aerodynamic particle sizer, as its name suggests, can be used independently to size aerosol powders and sprays. A time-of-flight measurement technique is used where larger particles have a greater inertia and, thus, longer transit times than smaller particles. Particles accelerate and cross a two-spot laser velocimeter. A photomultiplier collects the scattered-light pulses generated by particles crossing the laser beams and activates a high-speed digital clock. There are 58 channels that collect transit times and calculate the size distribution. It can accurately measure particles from 0.5 to 30 mm. The operating flowrate is 5 L/min.

The Malvern API Aerosizer (Malvem Instruments Ltd., Southborough, MA) operates on the principle of supersonic flow in a jet, followed by laser Doppler velocimetry to measure the aerodynamic diameter of particles in the size range from 0.5 to 200 mm using 50 channels. The operating flowrate is 6 L/min. The Atcor Net-2000 is a similar device for determining aerodynamic diameter, except that it is capable of sizing particles up to only 5.0 mm in diameter at a flowrate of 0.1 ft3/min (2.83 L/min).

The Insitee particle velocimeters include PCSV-E and PCSV-P, which provide two independent measurement volumes. The PCSV-E has three configurations, is nonintrusive, and can span a flow of stream up to 1.5 in diameter. The PCSV-P extends measurement to a larger scale. Both operate using a low-power He-Ne laser that illuminates particles, and a photomultiplier then measures the scattered light from each particle. The amplitude of the scattered-light signal is dependent on the particle size as well as the trajectory. A deconvolution algorithm solves the sizing ambiguity and is capable of sizing particles from 0.2 to 200 mm.

Acoustic Vibration

Laser Doppler velocimetry has been combined with acoustic excitation to allow the derivation of the relaxation time for particles, from which the aerodynamic diameter can be calculated [132-136]. The particle relaxation time is derived from the velocity amplitude of the aerosol particle and that of the medium while the aerosol is subjected to acoustic excitation of a known frequency. A differential laser Doppler velocimeter is used to measure the velocity amplitude of the particle, and a microphone is used to measure the velocity amplitude of the medium. The aerodynamic diameter of the particle can be derived from the relaxation time and the known particle density. The method can be applied to real-time in situ measurement of the size distribution of an aerosol containing both solid and liquid droplets in the diameter range of 0.1-10 mm.

Figure 19 Principle of operation of laser Doppler velocimetry (LDV) instrument. (With permission TSI, Inc.)
Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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