Introduction

THE GALAI INSTRUMENT is one of the first time-of-flight instruments used to characterize the size of powder grains (Galai Production Ltd., Ha Hameak, Israel). For a time this instrument was widely sold in the United States by the Brinkmann Company, so it is sometimes referred to as the Brinkmann size analyzer. Figure 1 shows the basic principles of the Galai instrument. Light from a laser is focused to a fine beam, and the beam is rotated by the rotating optical wedge. As the laser tracks a circular path in the cell containing the particles to be characterized, the size of the profile is measured by the time required for the laser beam to track across the profile of the powder grain. The instrument determines when a particle is intercepted at its maximum width, and it is calibrated using standard fine particles. The instrument also incorporates a video camera to view the particles being characterized at right angles to the rotating laser (Ref 1).

Fig. 1 Galai particle size analyzer. (a) Basic layout of the Galai instrument. (b) Laser beam tracing a circular path within the cell and the logic of the instrument rejecting any particles that are off center or out of focus. (c) Laser beam blocked from reaching the photodetector

Figure 2 demonstrates the accuracy of the time-of-flight, or time-of-transit method, by the agreement in the measurement of the powder-size distribution measured by the Galai instrument, with the known size distribution of a standard powder prepared and distributed by the European Commission on Standard Materials, Brussels, Belgium. The sample studied in Fig. 2 is BCR-67.

Fig. 2 Data for BCR-67, standard quartz test powder, obtained form the Galai particle size analyzed compared with known size distribution

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