Other Timeof Flight Measurement Systems

Another system that uses time-of-transit measurements to obtain particle size information is manufactured by Lasentec (Ref 2). This system is often used in on-line monitoring of material. Figure 3 shows the basic system used in their equipment. In technical literature (Ref 2) Lasentec states:

"The focused beam will cross the particle structure on a straight line between any two points on the edge of the structure. The distance between two points is a chord length. Thousands of chords are typically counted per second. The number of chords measured over a specific time period are sorted by chord length into a 38 channel distribution. The resulting chord length distribution tracks changes in particle geometry, a function of the shape and dimension of the particles, and particle structures as they naturally exist in process."


Sapphire window

Fig. 3 Lasentec focused beam reflectance measurement system

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Inspection — zone

Sapphire window

Fig. 3 Lasentec focused beam reflectance measurement system

Two time-of-flight instruments that study the size of fine particles in the aerosol format are available from TSI Inc., St. Paul, MN, and Amherst Process instruments Inc., Amherst, MA. Figure 4(a) shows the basic system of the instrument manufactured by TSI, known as the aerodynamic particle sizer (APS) analyzer. After the powder to be characterized is made into an aerosol, the aerosol is fed into the equipment. As shown in the Fig. 4(a), some of the aerosol under study is filtered to create a clean air sheath, which confines the aerosol being created to the center of the measurement zone. The two laser beams are 125 ,i,!m apart and —200 t-'m downstream from the nozzle orifice. The air feed to the interrogation zone is used to accelerate the particles, which respond to the accelerating forces at varying rates depending on their mass. The time-of-flight between the two laser beams is measured and used to calculate the particle velocity. The instrument is able to measure particles in the region of 0.5 to 30 /'m. The instrument is calibrated using standard polystyrene spheres. Figure 4(b) shows typical results for the instrument.

Fig. 4 The TSI aerodynamic particle sizer. (a) Basic layout. (b) Results from a mixture of three sizes of polystyrene latex spheres. (c) Results for a therapeutic aerosol

The parameter of the powder grains measured by this instrument is known as the aerodynamic diameter of the particles. The aerodynamic diameter of a particle is defined as the size of a sphere of unit density having the same velocity as the particle under the same flow conditions. This aerodynamic diameter differs from diameters measured in other methods of particle size, and data need to be interpreted in a knowledgeable manner. Several publications deal with the relationship between particle sizes measured by time-of-flight and other well-known methods of size analysis, such as microscope image analysis and diffractometers (Ref 3).

The TSI Inc. system works at subsonic velocities and uses one photocell. In 1994, TSI Inc. announced an advanced version of their instrument, the Model 3320 aerodynamic particle size spectrometer, using double crested optics. TSI Inc. claims that the new model sizes particles from 0.5 to 20 /'m aerodynamic size and can operate at 1000 particles/cm3.

Figure 5 shows the system used in the Aerosizer, manufactured by Amherst Process Instruments Inc. Note that one of the problems in using instruments such as a time-of-flight aerosol spectrometer is ensuring that the powder under study is efficiently dispersed as an aerosol (Ref 4). The Amherst instrument has an auxiliary piece of equipment, the Aero-Disperser, for dispersing powders that enable the investigator to vary the shear rate applied to the powder being aerosolized (Fig. 6). The equipment is used to carry out investigations of various shear rates applied to powder as it is aerosolized, until further increase in the shear rate does not result in a difference in distribution measurements. The aerosol being interrogated is surrounded by sheath air as the particles under study are accelerated into the interrogation zone. As shown in Fig. 5, the interrogation zone differs from that of the APS equipment in that two photocells are used to measure the optical energy scattered by the particles as they move through the two beams. A sophisticated electronic editor distinguishes which signal arriving at the photodetectors is associated with the initial signal from a given particle as it enters the first laser beam. The Aerosizer operates at sonic velocities and can count fine particles at a very high rate. The equipment measures the aerodynamic diameter and is calibrated using particles of known size. The output can be either a measure of the number of particles of a given aerodynamic size (Fig. 7a) or as a transformation into geometric size by volume (Fig. 7b). The geometric size by volume is a transformation of the aerodynamic diameter and takes into account the density of the particles being measured.

Fig. 5 Schematic diagram of the inspection zone of the Amherst Process Instruments Inc. Aerosizer time-offlight particle sizer

Fig. 6 Aero-Disperser used with the Aerosizer for aerosol preparation using controlled shear rates to disperse the sample and ensure that there is no agglomeration








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Fig. 7 Output of measurements for the size distribution of an irregular iron powder. (a) Number size distribution of the iron powder. (b) Volume size distribution of the iron powder

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